November 29-December 4, 2015 | Boston
Meeting Chairs: T. John Balk, Ram Devanathan, George G. Malliaras, Larry A. Nagahara, Luisa Torsi
It was theorized, that at high compressions, the increased zero-point oscillations in hydrogen would destabilize the lattice and form a ground fluid state at 0 K1. Theory also predicts this fluid would have very unusual properties governed by quantum effects2,3. By combining Raman spectroscopy and in-situ high temperature techniques at high pressures, we have investigated the behavior of dense hydrogen above 200 GPa and above 300K, P-T conditions previously inaccessible in experiments. The novel data lead to a significant revision of the phase diagram of hydrogen above 200 GPa and suggest unusual physics in the molecular liquid and solid phases.1.E. Brovman, Y. Kagan, A.Kholas, Soviet Physics JETP. 35, 783 (1972).2.E. Babaev, A. Sudbo, N.W. Ashcroft, Nature. 431, 666 (2004).3.E. Babaev, A. Sudbo, N.W. Ashcroft, Phys. Rev. Lett. 95, 105301 (2005).
Layered superconducting compounds have been the focus of immense interest due to the discovery of superconductivity in cuprate and iron-based systems. Discovery of Fe based pnictide superconductors viz. REFeAsO/F (RE=rare earths) with superconductivity in more than 55K has attracted a lot of attention. Primarily, because this is the only class of superconductors besides infamous high Tc cuprates (HTSc), which falls outside the popular BCS (Bardeen Cooper Schrieffer) theoretical limit of 40K. Moreover both Fe pnictides and Cuprates superconductors do have striking similarities viz. the semimetallic/insulating magnetic ground states, very high upper critical fields and the layered structures. Seemingly the role of CuO2 planes in cuprates is played by the FeAs superconducting layers in pnictides. Within the same structure of REFeAsO, when FeAs layer is replaced by BiS2 a new superconductor REOBiS2 emerged very recently, though with lower Tc of around 5K. The similar layered structure and existence of superconductivity in BiS2 layer mimicking the FeAs superconducting block of REFeAsO system has been a recent point of debate and it will be addressed in the presentation and comment on the fact about how the new FeAs and BiS2 based superconductors are thought provoking for both experimentalists and theoreticians outside the HTSc cuprates mystery of more than two decades by now.Discovery of superconductivity in BiS2 layers of Bi4O4S3 and ReO/FBiS2 systems has attracted tremendous interest of both experimentalists and theoreticians from condensed matter physics community. Comparing the role of various Re in ReO/FFeAs and ReO/FBiS2, one finds that though Tc of (La/Pr/Nd)O0.5F0.5BiS2 is around 2.1K/3.5K/5K respectively, the same for (La/Pr/Nd)O0.8F0.2FeAs is 26K/43K/50K. It appears that chemical pressure plays an important role in superconductivity of these layered BiS2 and FeAs superconductors. We review our efforts on synthesis and physical properties of title compounds from very beginning and present the new results related to impact of hydrostatic pressure on their superconductivity. The Tc of ReO/FBiS2 compounds is enhanced to fivefold for just above 1GPa external pressure, accompanied with normal state semi-metallic to metallic transition in normal state, which is thought provoking for solid state physics community.
Uranyl peroxides are a diverse family of compounds that form in environmentally relevant conditions. They are of great importance due to the prevalence of the uranyl (UO22+) cation in nuclear waste. This oxidized form of uranium is a common constituent of nuclear waste, and peroxide forms in waste from the alpha-radiolysis of water. Understanding the properties of uranyl peroxides is considered an important piece of the nuclear waste management pie. Uranyl peroxides assemble into diverse nano-cages. These clusters consist of 20 to -68 uranyl units in various cage topologies. Their properties have been characterized extensively in ambient P-T conditions; uranyl peroxides form spontaneously in wide pH ranges, and can form a multitude of structures due to the uranyl cation's ability to incorporate a variety of ligands in its equatorial bonding positions. Much study has been devoted to characterizing their crystal structures and properties in solution. These structures often crystallize readily and persist for extended time periods in situ. Here, several uranyl peroxides have been studied in the solid-state at elevated pressures to determine their strengths, bulk moduli, and structures at high pressures. The aforementioned properties have been found to depend heavily on the compositions of the uranyl peroxides in question, and their ambient structures. Two uranyl peroxides in particular have been fully characterized: U60 and U24Py12. U60 clusters are of special interest because they are topologically identical to carbon fullerenes, forming isometric crystals with the formula: Li68K12(OH)20[UO2(O2)(OH)]60(H2O)310. In diamond anvil cell experiments, the fullerene U60 topology is stable up to pressures of 10 GPa, above which the structure collapses irreversibly. A pressure-induced tetragonal structure (a = 36.6764(2), c = 35.1672(4) Å) forms in the range of 4 to 10 GPa, as determined by in situ synchrotron XRD and Raman spectroscopy. Above 10 GPa, the U60 decomposes into smaller uranyl peroxides, as confirmed by ESI-MS. U24Py12 clusters form tetragonal crystals at ambient pressures with the formula: Na8[(UO2)24(O2)24(P2O7)12]. Single crystals of U24Py12 undergo reversible pressure-induced amorphization at 17 GPa. Based on Raman spectroscopy, the uranyl units persist to 50 GPa, at which point the Raman signal is imperceptible. Following pressure quenching, the original tetragonal structure of U24Py12 recovers, as evidenced by in situ XRD and Raman spectroscopy. In contrast to the topologically-identical C60, which collapses at pressures above 30 GPa, but retains its hexagonal structure, U60 loses its long-range periodicity at much lower pressures and ultimately forms smaller clusters. U24Py12 retains its chemical and structural integrity upon pressure quenching. The persistence of uranyl peroxide nano-cage structures during pressure experiments is a unique result, showing the strength of these compounds in extreme conditions.
The combination of transition metal and light elements such as B, C, N, and O is an effective approach to developing novel superhard multifunction materials. With this philosophy, compounds such as OsB2, ReB2, FeB4, PrN2, IrN2, OsN2, Re3N, and Re2N were successfully designed for the past decade. TM nitrides play a special role within these materials, for their high melting point, high chemical stability, corrosion resistance, high hardness, semiconductor property, and superconductivity with relatively high transition temperature. Thus we systematically studied several kinds TM nitrides theoretically and experimentally.The structure, stoichiometry, phase transition, mechanical properties, hardness, and electronic structure of typical transition metal - nitrogen systems, such as Re-N, Tc-N, W-N, Nb-N, and FeN are investigated extensively by means of first-principles density functional theory. And some of them were synthesized by high pressure and high temperature technique and the high pressure characters of them were also studied. More than a dozen of new nitrides such as Re3N2, ReN3, ReN4, TcN, Tc2N3, TcN2, TcN3, TcN4, W3N2, NbO-WN, W5N6, Nb2N, NbN2, NbN3, and NbN4 with high hardness have been theoretically designed. The polycrystalline ε-NbN sample has been synthesized, and the incompressibility and rigidity have been measured for the first time. Our works on typical transition nitrides established the relations among their physical properties such as the N concentration, bonding, and hardness. New approaches for designing novel superhard multifunctional materials are therefore covered throughout the research.
As experimentally established, all the alkali metals and heavy alkaline earth metals (Ca, Sr and Ba) become progressively less conductive on compression, at least up to some critical limit over a broad pressure range. Of these metals, Li and Na clearly undergo pressure-induced metal-insulator transitions, which may also be called reverse Mott transitions. Here, using group theory arguments and first-principles calculations, we show that such transitions are controlled by symmetry principles not previously recognized. The valence bands in the insulating states are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multi-center chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as re-entrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of semimetallic phases with flat surface states. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been identified or reported (e.g., Li, Na, and Ca).
Materials are being designed and engineered for ever superior mechanical and operational properties, such as steels for lighter cars and energy-absorbing behavior in an accident, and titanium aluminides for lighter airplane turbine blades. The manufacturing of such materials may involve processes at extreme conditions, under high pressure or high temperature. Examples are high-pressure torsion and near net-shape forging. Therefore, it becomes eminently important to know and understand the phase diagrams of such materials at extreme conditions. Structural changes may open processing windows, while elevated mechanical properties are conserved under less extreme conditions. Here, we present first phase diagram studies on high-manganese steels and on titanium aluminides by in-situ synchrotron X-ray diffraction in a large-volume cell.
Metallic glass (MG) is an important new category of materials with many unique properties, but very few rigorous laws are known to define its ‘disordered&’ structure. Recently we found that under simple compression, the volume (V) of a MG changes precisely with the 2.5 power of its principle diffraction peak position (1/q1). In the present study, we determine the V and q1 of a Ce68Al10Cu20Co2 MG by in-situ high-pressure x-ray microscopy and diffraction, and find that the 2.5 power law holds even through and during its polyamorphic transition. The transition is, in effect, equivalent of a continuous compositional change of 4f-electron-localized Ce (γ-like) to 4f-electron-itinerant Ce (α-like), indicating the 2.5 power law is general for tuning with composition and pressure. We further reexamine the previously reported compositional power law exponent of 2.3 on seven different MGs, and find indeed the more precise determination also yields the exponent of 2.5. The exactness and universality of the 2.5 power law imply that the structural change in various MGs may strictly follow a general rule of packing which is imposed by efficient filling of the space without identifiable symmetry.
Pressure categorically and profoundly alters all materials. With the recent experimental breakthroughs after the turn of century, especially the new synchrotron facilities and nanotechnology coupled with diamond-anvil cell, sweeping changes of structural, electronic, magnetic and bonding properties have been uncovered and often come as a total surprise. Fundamental breakthroughs have been observed across the board in simple elements, organic compounds, stoichiometry of compounds, free electron metals, strongly correlated systems, superhard materials, materials, metallic glasses, nanomaterials, and deep-Earth minerals. We are witnessing a new era of materials science and applications with the added pressure dimension.
The thermal behaviour of materials exposed to high pressure process during manufacturing or transformation cannot be understood and/or predicted if their physicochemical properties are determined under atmospheric pressure conditions. The high pressure calorimetry is used to simulate the actual condition thus providing critical data on the materials&’ behaviour.Based on the MicroDSC technology, Setaram Instrumentation has designed a calorimeter operating up to 1 000 bars and between -45°C and 120°C. A dedicated high pressure panel is attached to the HP MicroDSC to cover the pressure range either under controlled pressure scanning or isobaric conditions. A review of recent work perfroemd with this technique will be presented to illustrate the effectiveness of directly acquiring calorimetric data at high pressure.For example polymers are often processed at elevated temperatures and pressures (ex: high pressure injection molding, extrusion) and under different gases (foaming) with significant effects on their melting points or glass transition temperatures. High pressure calorimetry can be also applied in the food industry for the investigation of the oxidative stability of oils; the modification of glass transition temperature of carbohydrates; to polymorphic changes in fatty compounds, etc.. Pressure-induced solid-solid transitions are also of key interest in the pharmaceutical industry. Gas hydrates or clathrate formation and dissociation, and wax appearance temperature in crude or diesel oils represent another pressure dependant phenomena which has been successfully studies using high pressure calorimetry as well
Metal organic frameworks (MOFs) are microporous materials consisting of metal ions and organic linkers which have a huge variety of applications like gas storage, separation, catalysis, drug delivery etc. Zeolitic imidazolate frameworks (ZIF) are a class of MOFs where the metal atoms (Mn+=Zn2+, Co2+) are tetrahedrally coordinated to the N atoms of imidazolate derived linkers (IM=C3N2H3-) subtending an angle of 1450 at M-IM-M centre analogous to Si-O-Si angle in zeolites to form porous architecture. ZIF-8, a prototypical ZIF has been widely studied due to its exceptional chemical, thermal stability and its gas adsorption properties which makes it a potential candidate for many practical applications. ZIF-8 posses exceptionally low shear modulus (< 1 GPa) which results in pressure induced mechanical failure upon application of shear stresses when used in practical applications. We have used Brillouin spectroscopy to study the pressure dependence of the acoustic modes (or elastic properties) of ZIF-8 in different pressure transmitting mediums (guests) and under non-hydrostatic conditions. Our study shows the pressure induced flexibility and dynamics of ZIF-8 as well as a huge increase in the acoustic velocities on applying pressure, illustrating the role of guest in enhancing the elastic properties. On applying pressure the size of windows connecting the pores of ZIF-8 increases (gate opens) letting more guests to enter the pores. The pressure transmitting medium plays an important role in this gate opening behaviour. The interaction between the framework and guest which results in different pressure dependent behaviour in different mediums is investigated using high pressure Raman spectroscopy. We have also studied the temperature dependent elastic properties of ZIF-8 on adsorption of various gases.
Metal-organic frameworks (MOFs) of the general formula [cat][M(HCOO3)], with M = divalent cation and cat = amine cation, have received a lot of attention in recent years because of their CO2 sorption capacity, magnetic, ferroelectric and multiferroic properties, and glassy behavior.[1-4] In this respect, the first report on the discovery of multiferroic properties in dimethylammonium (DMA) MOFs of the formula [(CH3)2NH2][M(HCOO)3] (M = Mn, Ni, Fe, Co) was published in 2009.5 This discovery promoted broad interest in the properties and phase transition mechanism in these compounds, as well as led to many efforts to synthesize novel amine-templated metal formate frameworks. Raman scattering and IR spectra have been performed on azetidinium zinc formate, [(CH2)3NH2][Zn(HCOO)3] under the Temperature- and pressure variation. Raman spectra reveal distinct anomalies in mode frequencies and bandwidths near 250 and 300 K. These anomalies were attributed to structural phase transitions associated with the gradual freezing of ring-puckering motions of the azetidinium cation. Pressure-dependent studies revealed a pressure-induced transition near 0.4 GPa. Raman spectra indicate that the structure of the room-temperature intermediate phase observed near 0.4 GPa is the same as the monoclinic structure observed at ambient pressure below 250 K. The second phase transition was found near 2.4 GPa. This transition has strong first-order character and is associated with strong distortion of both the zinc formate framework and azetidinium cations. The last phase transition was found near 7.0 GPa. This transition leads to lowering of the symmetry and further distortion of the zinc formate framework, whereas the azetidinium cation structure is weakly affected.References(1) Rossin, A.; Chierotti, M. R.; Giambiastiani, G.; Gobetto, R.; Peruzzini, M. CrystEngComm 2012, 14, 4454minus;4460.(2) Zhang, W.; Xiong, R. G. Chem. Rev. 2012, 112, 1163minus;1195.(3) Wang, Z.; Zhang, B.; Otsuka, T.; Inoue, K.; Kobayashi, H.; Kurmoo, M. Dalton Trans. 2004, 2209minus;2216.(4) Jain, P.; Dalal, N. S.; Toby, B. H.; Kroto, H. W.; Cheetham, A. K. J. Am. Chem. Soc. 2008, 130, 10450minus;10451.
Molybdates and tungstates systems with A2(XO4)3 formula (A = a trivalent ion from Al to Ho; X = Mo6+, W6+) have been investigated due to their optical, ferroelastic, ferroelectric and negative thermal expansion (NTE) properties.1,2,3 Particularly, NTE systems are widely studied as potential fillers in order to control thermal expansion in composites. The Sc2(WO4)3/ZrO2 and Sc2(WO4)3/Cu composites, for example, show almost zero thermal expansion when the volume fraction of Sc2(WO4)3 is as high as 25 and 70%, respectively.4 In general, the easy of XO4 rotation in open framework systems is at the origin of its specific properties. In particular, the relation of these rotations with the low/negative thermal expansion of such structure is well established (see e.g. the NASICON and LAS frameworks, see e.g. J. Mol. Struct. 270 (1992) 407-416, Solid State Ionics 9&10 (1983) 845-850, 73 (1994) 209-220, etc. and refs herein) , i.e., NTE properties in framework structures are intrinsically related to the existence of low energy Rigid Unit Modes (RUMs) or transverse anharmonic vibrations of the two-fold coordinated oxygen atoms. In this work we report results of high-pressure Raman experiments (P < 8 GPa) on In2-xYxMo3O12 for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure-induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In2(MoO4)3 and In1.5Y0.5(MoO4)3, respectively, resulting in the change of structure from monoclinic P21/a to a more denser structure. The pressure-induced amorphization started at 5 GPa and 3.4 GPa for In2Mo3O12 and In1.5Y0.5Mo3O12, respectively. The amorphization process takes place in two stages in the case of In1.5Y0.5Mo3O12 phase, while for In2Mo3O12 it is not complete until the pressures as high as 7 GPa. Our results also suggest that with increase of ionic size of the A3+ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A2(MoO4)3 system.References E. T. Keve, S. C. Abrahams and J. L. Bernstei, Journal of Chemical Physics. 1971, 54, 3185-&. Y. Wang, T. Honma and T. Komatsu, Materials Chemistry and Physics. 2012, 133, 118. J. S. O. Evans, T. A. Mary and A. W. Sleight, Journal of Solid State Chemistry. 1997, 133, 580. Q. Q. Liu, X. N. Cheng and J. Yang, Materials Technology. 2012, 27, 388.
The discovery in ZrW2O8  of strong and isotropic negative thermal expansion (NTE), also known as thermomiotic behavior , has raised interest in searching for materials with similar NTE properties. The NTE phenomenon has been the subject of manystudies with the aim to find suitable materials for applications that require controllable, low, or zero thermal expansion coefficients .Among the NTE materials, the family of A2M3 O12 (A = trivalent cation, M = W/Mo) compounds has been widely investigated, since many members exhibit such a phenomenon . The NTE behavior in Y2Mo3O12 is known but the mechanism is not yet satisfactorily understood . The importance of understanding the pressure-induced amorphization in these materials comes from the fact that this phenomenon is related to NTE .In this work we investigate the vibrational properties a high-pressure Raman spectroscopic study of Y2Mo3O12 and Y2Mo3O12middot;xH2O (x < 3). Our goal is to achieveinsight concerning the role of MoO4 units and O-O bonds on the structural packing and related NTE phenomenon. An analysis of vibrational modes suggests that the anhydrous material experiences a phase transformation from orthorhombic to a lower symmetry phase (probably monoclinic) about 0.3 GPa, and to a highly disordered phase above 2.4 GPa. The structural transformation to the high-pressure disordered phase is not reversible and suggests the onset of a pressure-induced amorphization process. The vibrational mode dependence on pressure is discussed considering lattice dynamics calculations.Reference J. Evans, Z. Hu, J. Jorgensen, D. Argyriou, S. Short, A. Sleight, Science 275 (1997) 61. C.P. Romao, K.J. Miller, C.A. Whitman, M.A. White, B.A. Marinkovic, in: Jan Reed-ijk, K. Poeppelmeier (Eds.) Comprehen. Inorg. Chem. 4 (2013) 127. J. Evans, T. Mary, A. Sleight, Physica B 241 (1997) 311. B.A. Marinkovic, P.M. Jardim, R.R. de Avillez, F. Rizzo, Solid State Sci. 7 (2005)1377. S. Sikka, J. Phys. Condens. Matter 16 (2004) S1033.
Germanates have wide applications as structural analogs in the study of silicate crystals and glasses, particularly in the field of high-pressure research. At high pressures and temperatures, (Mg,Fe)(Si,Ge)O3 undergoes a sequence of transitions from a pyroxene (px) to a perovskite (pv), followed ultimately by a transition to a CaIrO3-type post-perovskite (ppv) structure. In silicates, this sequence of transitions is crucial for understanding the structure and dynamics of planetary interiors, but the transition pressures and temperatures are difficult to achieve (e.g. ~125 GPa and 2500 K pv agrave; ppv in MgSiO3). Magnesium iron germanates are useful analogs because they undergo these phase transitions at much lower pressures and temperatures than in the silicates (e.g. ~63 GPa and 1800 K pv agrave; ppv in MgGeO3), enabling improved characterization of the nature and properties of the transitions. In order to investigate the effect of Fe2+ on the pv agrave; ppv phase boundary, and on the compressibilities of pv and ppv, polycrystalline px with compositions of (MgxFe1-x)2Ge2O6 (x = 1, 0.9, 0.8, 0.6, 0.5) were synthesized and characterized using x-ray diffraction, Raman, Mössbauer, and microprobe analysis. The px samples were found to exhibit a linear increase in lattice parameters with iron content. Raman peaks became broader, and shifted linearly to lower frequency with increasing iron. Mössbauer data for (Mg0.5Fe0.5)GeO3 and (Mg0.8Fe0.2)GeO3 px showed that the iron is high-spin Fe2+ that is concentrated primarily in the M1 site. Laser heated x-ray diffraction diamond anvil cell compression experiments were performed at beamline 13-ID-D of the Advanced Photon Source. Pv and ppv were synthesized from all px compositions upon compression and heating. The pressure-volume equations of state of MgGeO3 and (Mg0.8Fe0.2)GeO3 pv and ppv were determined, showing a change in volume between the compositions but negligible effect of Fe2+ on compressibility. Future work will focus equation of state measurements of other compositions in order to further determine the effect of Fe on the compressibility of the pv and ppv phases, on the exact temperature and pressure range for the pv agrave; ppv phase transition in each composition, and on the nature of the two phase loop as a function of composition, pressure and temperature.
Discovery of efficient and inexpensive photocatalytic materials is required for the practical implementation of solar based H2 production through water splitting. The challenge here lies in designing new semiconductor materials which can absorb solar light and can also have the band-edge positions matching with water redox couple. One possible material design strategy is to tune the properties of existing inexpensive transition metal oxide catalysts to improve solar energy absorption while preserving the redox reactivity. Based on this strategy, some theoretical predictions suggest that the bandgap of photocatalyst MnO can be tuned for visible light activity by doping Zn in place of Mn. However, the required doping concentrations are as high as 50 at% which would be possible only through high pressure and high temperature (HPHT) synthetic techniques. Here, we synthesize rocksalt Mn0.5Zn0.5O using HPHT technique in multi-anvil apparatus starting with 1:1 mixture of rocksalt MnO, wurtzite ZnO. As synthesized material exhibits narrower bandgap compared to its parent oxides and can be a potential visible light photocatalyst for water splitting applications.
Microstructure of commercial pure Mg, AZ31 and AZ91D Mg alloys were remarkably changed by multi-pass equal-channel angular pressing (ECAP) plus aging treatment, to investigate the effect of various substrate states on stress corrosion cracking(SCC) of the fine-grained Mg alloys with better mechanical properties. The slow-stain-rate tensile results in distilled water showed that pure Mg was not susceptible to SCC and the susceptibility increased with Al content. The more high-angle grain boundaries after ECAP seriously hampered the crack initiation of SCC of the fine-grained Mg alloys, and the uniform distribution of broken second phases changed localized corrosion to general corrosion. After combining ECAP with pre-solution and post-aging, the smaller and even-distributed precipitation particles in the Mg matrix can effectively inhibit the anodic dissolution process and prevent the cracks from propagating. Moreover, the fine precipitates in the grain boundary can act as trapping sites for atomic hydrogen to nucleate hydrogen bubbles, thereby reducing hydrogen concentration below a critical value to retard hydrogen embrittlement. Therefore, the post-ECAP aging Mg alloys have the higher stress corrosion resistance than that processed by ECAP.
Carbon nanotubes (CNTs) have been recognized as potential candidate for reinforcements in lightweight metals. A composite consisting of CNTs embedded in an Al-matrix might work as an ultra-low-resistive material with the potential of having a room-temperature resistivity far below Al, Cu and Ag. While several advances have been made in developing Al-CNT composites, three major challenges: (1) interfacial bond strength between CNT and the Al matrix, (2) homogeneous dispersion of the CNTs in the Al matrix and impurity (CNTs) scattering centers, continue to limit progress in Al-CNT composites. Several conventional methods including powder metallurgy, melting and solidification, thermal spray and electrochemical deposition have been used to process Al and CNT to form composites. We present preliminary results that address these challenges and demonstrate the fabrication of ductile high strength Al-CNT composite wires of 1.0mm diameter with ~12% ± 2% reduction in the electrical resistivity of Al-CNT composite using CNT-hybrid as reinforcement and an inductive melting technique that takes advantage of the induced eddy current in the melt to provide in-situ stirring.
Multilayer optical structures are photonic devices that have unique spectral properties determined by the thickness and composition of the individual layers. These spectral properties are also driven by time-resolved material states in the multilayer, and due to this intrinsic sensitivity the spectral response of these structures can be altered by external perturbations. With nanometer to micrometer dimensions, near instantaneous optical response, and potential micron level spatial sensitivity, these structures have significant potential to form the basis for mesoscale time-resolved experimental diagnostics. Such tools are needed to effectively probe the dynamic behavior of heterogeneous materials, to correlate the influence of mesoscale structural characteristics with observed shock-compressed states.Initial computational and experimental results have shown that the magnitude of dynamic loading can be directly correlated to the altered spectral response of the multilayer structures. A coupled computational-experimental study is being performed to fully characterize the optical and mechanical behavior of multilayer optical structures, applied to both homogeneous and heterogeneous material systems. A mechanistic analysis of the spectral response of the structures to dynamic loading will be presented, along with computational simulations illustrating the observable spectral effects of 1D shock loading. Results from fabrication of specific multilayer designs and laser-driven 1D shock loading experiments will be shown and compared to the simulation results. Simulations of heterogeneous loading in mesoscale materials will also be presented, illustrating the potential spatial sensitivity of the multilayer structures in various applications. Additional applications and challenges associated with spatially resolved data collection methodology will be discussed.
Measuring and understanding the mechanical, physical, and chemical changes induced during dynamic loading ofheterogeneous systemswithmeso-scale variations,is critically important for designing impact resistant material systems.The dynamic loading response ofheterogeneous systemsis difficult to acquire and differs significantly from the continuum behavior. Furthermore, the information obtained is oftenlimited. Understanding the shock behavior ofheterogeneous systems with meso-scale variations supports a variety of engineering applications, such as high impact collisions and thedetonation of heterogeneous explosives.In this study, we investigated the use of various optical structures such asmultilayers (Bragg reflectors) and optical mirror cavities (OMC)asmeso-scale in-situ diagnostic tools for measuring compression and stress in real time under dynamic loading. Their optical properties such as reflectance and transmission were simulated using OpenFilters software and the design criteria were determined. Poly(9-vinylcarbazole) (PVK) and poly vinyl alcohol (PVA) dissolved in their respective solvents were spin-coated sequentially to form multilayers witha tunable reflectance peak from 300-900nm. Inorganic SiO2/Al2O3 multilayers or cavity structuresmay also be fabricated using ion-assisted deposition (IAD). Various OMC structures such as Ag/SiO2/Ag, and Ag/Al2O3/Agwere fabricated with a sharptransmission peak of FWHM less than 10 nm, a critical requirement for sensor applications. The mechanical responses of fabricated optical structures under dynamic loading were recorded by using a streak system to measuretheirtime-resolved spectral response.The generated shock impacts the optical structures with a pressure pulse width of 25 nanoseconds at pressures of 1-10 GPa. Theoretical predictions show a well-defined shift to shorter wavelengths that is proportional to the impact velocity and which for the OMC structure has been substantiated experimentally. The results of these studies are being used to design sensor couponscapable of measuring the dynamic response of heterogeneous systems at themeso-scale. Additionally, the collected results increase the understanding and differentiation between thermal and strain effects onthe system
High-current electrodes of vacuum and gas-discharge switching devices (pseudospark switches, high power thyratrons, reed switches) are made of refractory materials: Mo, Ta, W alloys, etc. having high fusion temperature in order to increase their life time. The prospective direction of erosion resistance increase is the use of multilayer coatings, meeting the requirements of local heat resistance, high heat conductivity and electroconductivity, generally consisting of refractory metal layers, a high heat conductivity layer and an intermediate layer equalizing thermomechanical stresses. For maximization of erosion resistance it is necessary to calculate thermal field and mechanical stresses inside the multilayer coating structure for conditions of local thermal effect on the film structure.The paper presents the results of numerical calculation of characteristics of electrodes erosion-resistant coatings of vacuum and gas-discharge switching devices with W-Ti-Cu and W-Mo-Cu structures at local thermal effect equivalent to the influence of the channel form discharge. The calculation of thermal effect on W-Ti-Cu and W-Mo-Cu film structures is carried out in axially symmetrical approximation taking into account temperature change of materials characteristics using COMSOL Multiphysics.The calculations show that in W-Ti-Cu structure the greatest temperatures gradient is located in Ti layer that is caused by its small heat conductivity, and the maximum thermomechanical stresses - on border of W-Ti layers. In W-Mo-Cu structure the temperatures gradients in W and Mo layers are approximately identical that is caused by similar heat conductivity factors, and the maximum thermomechanical stresses are concentrated on border of Mo-Cu layers. Comparison of W-Ti-Cu and W-Cu structures shows that Ti layer reduces more than 2 times the arising thermomechanical stresses.The dependence calculation of the maximum value of interlaminar mechanical stresses at various layers thickness of W-Ti-Cu, W-Mo-Cu coatings shows that:- in W-Ti-Cu structure the increase in W layer thickness within 5-50 microns results in more than 10 times decrease of thermomechanical stresses on W-Ti border. The dependences of mechanical stresses between W-Ti and Ti-Cu layers on titan layer thickness have minima the sum of which corresponds to Ti layer thickness of 50 mu;m.- in W-Mo-Cu structure the increase of W layer thickness within 5-100 mu;m leads to insignificant decrease (less than 1.5 times) of thermomechanical stresses on Mo-Cu border and to stresses decrease on W-Mo border approximately by 5 times. The growth of Mo layer thickness within 5-100 microns leads to insignificant decrease (less than 1.5 times) of thermomechanical stresses on W-Mo border and to stresses decrease on Mo-Cu border approximately twice.In general, W-Ti-Cu structures provide approximately two times smaller level of mechanical stresses in comparison with W-Mo-Cu structure at similar coatings thickness.
Hard coatings endure extreme temperature and wear during their usage lifetime. Gaining fundamental insights into their mechanical deformation at service temperatures is essential to develop better, more wear resistant coatings and to evaluate their performance. Nanoindentation is well suited to probe the mechanical properties of hard coatings in service conditions, particularly high temperatures. However, till date, this versatile technique has mostly been limited to ambient conditions. Determining the mechanical properties of hard coatings and thin films, particularly at low depths penetration depths (less than 100nm) and high temperatures requires high measurement stability and low compliance of the system so that the artifacts of measurements are kept to a minimum.This poster will present test results of a novel vacuum high temperature nanoindentation system that enables mechanical property measurements of thin films and hard coatings up to 700 °C. The system, based on the Ultra Nanoindentation Tester (UNHT), utilizes active surface referencing consisting of two independent indentation axes: one for surface referencing and the other for indentation. This design minimizes thermal drift and results in almost zero compliance that makes it attractive for such challenging measurements. Independent indenter, reference and sample heating coupled with tip-sample surface temperature tuning results in drift rates of less than 5nm/min at 700 °C. High temperature indentation test results on nitride hard coating will be presented as a case study.
Ytterbium disilicate is of interest as a potential environmental barrier coating for aerospace applications, notably for use in next generation jet turbine engines. In such an extreme environment, the transport of oxygen and water vapor through these coatings is undesirable if high temperature corrosion is to be avoided. In an effort to understand the diffusion process in these materials, we have performed kinetic Monte Carlo simulations of vacancy-mediated and interstitial oxygen diffusion in Ytterbium disilicate. Oxygen vacancy site energies, vacancy formation energies, heats of solution and migration barrier energies were computed using Density Functional Theory. We have found that, in the case of vacancy-mediated diffusion, many potential diffusion paths involve large barrier energies, but some paths have barrier energies smaller than one electron volt. However, computed vacancy formation energies suggest that the intrinsic vacancy concentration is small. In the case of interstitial diffusion, migration barrier energies are typically smaller, but the heat of solution is positive, indicating that the concentration of interstitial oxygen is likely to be small. It is therefore expected that the material is unlikely to exhibit significant oxygen permeability.
High performance coatings that are utilized in aerospace environments are designed to withstand extreme temperature changes as well as harsh operational environments. Due to the high cost of these coatings, they are expected to have a lifespan of at least 15 years. This increased lifecycle is intended to reduce maintenance cost and downtime. However, many of these high performance coatings are failing long before the manufacturer&’s stated life span, triggering expensive and time-intensive reworks and repainting. Results from past engineering investigations have suggested that urea linkages from over-indexing with excess isocyante contribute to coating accelerated degradation. Coatings are typically over indexed to ensure complete reaction with the incorporated polyol and thus form more durable coatings. In this study, we have synthesized control analogous coatings that were overindexed from 1.1 to 2 -NCO:-OH. These coatings were then subjected to accelerated weathering and analyzed with both bulk thermal and surface characterization techniques. Thermal and surface analysis of weathered coatings illustrated changes not only in the mechanical integrity, but also in the chemical makeup that were correlated with the index value of the coating. The effects excess urea linkages had on coating degradation will be discussed in detail.
Carbon nanotube/polymer composites have been studied extensively for applications in which increased thermal stability or increased flammability is desired. However, the behavior of such materials under more extreme environments has not been studied. In our work, we have examined carbon nanotube composite behavior when subjected to laser pulse heating (LPH) with a 1064 nm variable pulse laser. The heating rates in these studies are five orders of magnitude higher than traditional thermal stability studies. In previous work, we have shown that the carbon nanotube composites form a protective nanotube network on the surface of the composite, which reduces heat input to the underlying polymer and slows mass loss. We have recently studied the interaction of the laser beam with the plume formed above the nanotube composites and correlated this interaction to time-resolved mass loss data. In addition, we have performed time-resolved studies on the formation of the protective nanotube network during the millisecond time domain of the laser pulse. The formation and subsequent behavior of the network layer was studied as a function of laser power and nanotube content in the composites.References:Stephen F. Bartolucci, Karen E. Supan, Jeffrey M. Warrender, Christopher E. Davis, Lawrence La Beaud, Kyler Knowles, Jeffrey S. Wiggins, “Laser-Induced thermo-oxidative degradation of carbon nanotube/polypropylene nanocomposites”, Composites Science and Technology, 105 (2014) 166-173.
Cu-Ag alloy with high strength and high conductivity is widely used as coil material for high-field AC pulsed and DC resistant magnets. The property of Cu-Ag material is closely related to microstructure during solidification and heat treatment. In this study, a static magnetic field was superimposed to directional solidification of Cu-6wt.%Ag alloy. Applying magnetic field during solidification increased both primary dendrite arm spacing and length of proeutectic Cu, increased solubility of Ag in proeutectic Cu, but decreased volume fraction of eutectic colony. These increased the resistivity and the hardness of alloy. The modeling of resistivity showed that the changes were as a result of changes in the solubility of Ag in proeutectic Cu. The solidified samples were cold rolled. By comparison with the sample without the magnetic field, the sample solidified with magnetic field had lower volume fraction of off-eutectics, lower hardness at lower reduction, but had higher hardness at higher reduction. It was thought to be from the role of more precipitates of Ag induced during rolling. Ageing at 300°C promoted the Ag atoms to precipitate from proeutectic Cu matrix. This optimized the combination of strength and conductivity of alloys.
Equiatomic FeMnNiCoCr which crystallizes as a face-centered cubic solid solution is one of the few examples of high-entropy alloys which can be tailored by means of classical metallurgical processes like cold-working and subsequent recrystallization as well as hot-working. Thus, regarding this new class of materials, FeMnNiCoCr has been the most investigated high-entropy alloy, so far. Recently, Otto et al. [Acta Materialia 61 (2013) 5743-5755] revealed an increasing mechanical strength together with an increase of elongation to failure by decreasing test temperature down to 77 K. The observed change of mechanical properties was accompanied with planar slip in the entire temperature range during the initial stages of deformation as well as the activation of deformation twinning at 77 K in later stages beyond a tensile strain of 20 %.We present quasistatic tensile and compressive tests at low temperatures down to 9 K of this FeMnNiCoCr alloy which was produced by arc-melting, homogenization and recrystallization subsequent to a cold-deformation process. Work hardening rate as a function of flow stress of FeMnNiCoCr at low temperature is compared to well-understood materials like single-phase copper alloys or TWIP steels exhibiting different amounts of deformation by twinning. In contrast to the aforementioned classical materials, an outstanding ability for work hardening at low temperature must be noted for FeMnNiCoCr. The appearance of twinning induced plasticity, which plays an important role for the work hardening behavior during low temperature deformation, is evaluated by means of orientation imaging microscopy. The orientation dependence of deformation under compression and tensile loading is stressed.
In the thermal spray, temperature and velocity of the in-flight particles can be controlled by selecting thermal spray parameter such as the appropriate combustion gases. For example, theoretical studies indicated that the combustion gas species and their ratios in a high-velocity oxygen fuel (HVOF) affect the temperature and velocity of the in-flight particles, which in turn determine the collision energy when they impinge on the substrate. Therefore, the choice of combustion gases is very important and it determines the particle collision energy.Metal-EDTA complexes have low decomposition temperatures and various metal oxides with different morphologies can be obtained by pyrogenic decomposition of the metal EDTA complexes. Spray drying and laser deposition were used to synthesize metal oxides with metal-EDTA complexes in previous studies. Recently, we developed a technique for synthesizing a thick metal oxide layer from a metal-EDTA complex using commercial flame-spraying apparatus. High-rate deposition of metal oxides from the metal-EDTA complexes was accomplished with the assistance of combustion gas mixtures of C2H2-O2 or H2-O2. In particular, the metal-EDTA complexes&’ species and their diameters affected the crystal structures and morphologies. For example, a Er2O3 film with 9% cross-sectional porosity was synthesized on a stainless steel (SUS) substrate from a EDTAbull;Erbull;H complex. A Y2O3 layer with 15% porosity was also formed on a SUS substrate under similar conditions. In this study, the combustion gas species selected were C2H2-O2 and H2-O2 mixtures. With the assistance of these combustion gases, we synthesized Y2O3 films on stainless steel substrates from the EDTAbull;Ybull;H using the flame thermal spray technique and investigated the effects of the combustion gas species. We measured the temperature and velocity of the in-flight particles for each kind of combustion gas. The crystal structure, surface, cross-sectional morphology, and elemental distribution of the films were then analyzed.In this study, yttria (Y2O3) films were deposited on stainless steel substrates from yttriumbull;hydrogenbull;ethylenediaminetetraaceate (EDTAbull;Ybull;H) complex by flame spraying. The gas mixtures of acetylene-oxygen (C2H2-O2) or hydrogen-oxygen (H2-O2) were used for the combustion flame and the effects of the combustion gas species on Y2O3 films were investigated. Experiments revealed that the particles propelled in the H2-O2 flame had lower temperature and higher velocity compared with the particles in the C2H2-O2 flame. The existence of Y2O3 crystalline phases and complete decomposition of the Y-EDTA complexes were confirmed. The porosity of the film was 25% when the H2-O2 flame was used and 32% when the C2H2-O2 flame was used. In addition, the Y2O3 films showed excellent adherability in tape tests. The H2-O2 flame is thus considered suitable for fabricating dense Y2O3 films.
Among the alternative thermal barrier coating materials, gadolinium zirconate (Gd2Zr2O7, GZ) seems to be the most efficient one in terms of its low thermal conductivity (0.65 Wm-1K-1), high phase stability (above 1200°C) and better resistant to hot corrosion than YSZ. But, thermal shock behaviour of the GZ is poor due to its low coefficient of thermal expansion (10.4x10-6 K-1) and high tendency to react with TGO layer. In this study, CYSZ/GZ multilayered and functionally graded thermal barrier coatings were produced by high-velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) processes in 2, 4 and 8 layered. Microstructure, thermal conductivity and thermal shock behaviour of the coatings were evaluated. The results indicated that the lowest thermal conductivity value and higher thermal shock resistance were achieved with functionally graded thermal barrier coatings.
Pure molybdenum is one of the most important refractory metal and its alloys are used for high temperature applications in a variety of industries. However, application temperature is limited by the beginning of recrystallization, which can be inhibited by addition of fine grained structure and the formation of TiC and ZrC in the grain boundaries of molybdenum in TZM alloys (molybdenum alloy containing 0.5-0.8 wt% titanium, 0.08-0.1 wt% zirconium and 0.016-0.02 wt% carbon). In this study, B4C-TZM composites having 0-30 vol% B4C were sintered by spark plasma sintering (SPS). The prepared composites were then characterized in terms of their densification, microstructure and mechanical properties.
Pure molybdenum (Mo) is an important refractory metal, having high wear and heat resistance properties, electrical (5.5 × 10#8209;8 Omega; m) and thermal (139 W m#8209;1 K#8209;1) conductivity. In this study, pure molybdenum powders were sintered by spark plasma sintering (SPS) technique. Sintering process was conducted under various temperatures and periods (1650°C, 1700°C, 1725°C and 3 min, 6 min, 9 min) in order to investigate the effect of sintering parameters on sintering behavior, microstructure and mechanical properties.The linear shrinkage of the specimens depending on temperature and time were recorded during sintering process. Dense molybdenum metals with a relative density of more than 97.5% were obtained at temperature of 1650°C with a holding time of 3 minutes. Relative density values decreased with increasing sintering temperature and period. Microstructure analysis revealed that, during sintering process molybdenum carbide layers are formed on the surfaces of the specimens as a result of carbon diffusion from the graphite sheets. Molybdenum carbide layers have thickness of more than 500µ at 1725°C. Microhardness results of surfaces were ~14-15GPa while the results of the substance metals were ~2-2.1GPa. These results proved that during sintering process a substance layer of molybdenum metal and a carbon rich layer of molybdenum carbide is formed.
As a new topological material, ZrTe5 exhibits novel properties. Pressure is believed to be effective for studying the topological phase transitions experimentally. In this work, we have performed resistance and ac magnetic susceptibility measurements for ZrTe5 single crystal at various pressures up to 68.5 GPa. We show that, accompanied by complete suppression of the high temperature resistance anomaly near 120 K, superconductivity is induced at a critical pressure of 6.2 GPa. The superconducting transition temperature Tc initially increases with pressure then decreases slightly with a maximum of 4.0 K at 14.6 GPa. At pressures above 21.2 GPa, a second superconducting phase with the maximum Tc of about 6.0 K manifests and coexists with the original one. We also performed the theoretical calculations of pressure-induced variations of both crystal and electronic band strucrues to explore the origin of superconductivity in the pressurized ZrTe5.
Nitrogen-rich organic compounds may offer distinct advantages over conventional energetic materials for applications relating to gas generators, low-signature propellants, and additives to pyrotechnics and explosives. We have performed plane-wave, density functional theory calculations of TAGzT, an energetic, nitrogen-rich salt, up to 40 GPa, and report the pressure dependences of polarizability, x-ray diffraction patterns, and dipole moments. These results are compared to those we obtain experimentally from Raman Spectroscopy,1 and x-ray diffraction analysis and infrared spectroscopy. Our results suggest TAGzT does not undergo any phase transitions within this pressure range. Mulliken and Hirshfeld population analysis of TAGzT at ambient and high pressure yields the change of charge distribution with an increase in pressure. We report and discuss this trend at the meeting. Also, we report trends in the pressure-induced modifications of both bond lengths and angles of TAGzT, and reveal how hydrogen bonding contributes to the stability of TAGzT under pressure.1K.D. Behler, J.A. Ciezak-Jenkins, R.C. Sausa, J. Phys. Chem A. 117(8), 1737 (2013)
New Molybdo- Cuprates of 1212 type with composition Mo0.5Cu0.5Sr2RECu2O7.5 (RE = Rare Earth) have been prepared by High Pressure and High Temperature (HPHT) synthesis. Their crystal structures were characterized by combining the X-Ray/neutron powder diffraction and electron diffraction techniques. All the materials show tetragonal symmetry, crystallizing in the P4/mmm space group (S.G.). Structural analysis using the joint refinement of RT NPD and RT XRD showed that the chain oxygens are randomly distributed in the two different oxygen site, which are not completely filled and the defect induced by oxygen vacancies in fact makes the chain fragmented and disordered. A remarkable microdomain texture has been identified using the electron microscopy. The microstructure of these compounds is interpreted by a well-known diagonal cell radic;2ap × radic;2ap × 3ap, distributed in three dimensional domains, as confirmed by the SEAD and HRTEM. X-ray photoelectron spectroscopy (XPS) studies show the predominance of the non magnetic MoVI state over the MoV one. At the same time oxidation state of copper is found to be dominated by CuII. The presence of higher amount of nonmagnetic MoVI is found to be a reason for the absence of the magnetic interaction in these materials. Specific heat studies indicate the magnetic ordering at 2.13 K for Mo0.5Cu0.5Sr2GdCu2O7.5 material is due to the 3-dimentional AFM ordering of the Gd ion. The heat capacity of the Mo0.5Cu0.5Sr2HoCu2O7.5 material shows a flat maximum at 5 K due to the separation of the two lowest-lying singlet levels of HoIII ions at that temperature.
The rare-earth nitrides with formula LnN (Ln rare-earth element) constitute a series of compounds all crystallizing in the rock-salt (B1 type) structure. While transition metal nitrides have been studied widely at high pressure, the corresponding series using lanthanoids have not. They have high hardness and mechanical strength and are also of interest owing to their magnetic, electronic and optical properties[1-3]. The systematic change in lanthanoid properties as the 4f shell is filled also makes these systems ideal for comparison with theory and many such DFT studies already exist in literature[4-8].In this work, we present the result of high pressure exploration of the phase behavior of the LnN with Ln = Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er. Powders of all phases have been measured up to at least 80 GPa, using synchrotron x-ray diffraction. In the whole series, only La, Pr, Ce and Tb have previously been explored experimentally[7,9-11]. The findings are compared to our own theoretical calculations and trends throughout the series are highlighted.References: C.-G. Duan,et al, J. Phys.: Condens. Matter. 19 315220, 2007 P. Larson, et al, Phys. Rev. B, 75 045114, 2007 C.M. Aerts,et al, Phys. Rev. B 69 045115, 2004 A. Svane, et al, J. Phys.: Condens. Matter 10 5309-5325, 1998 Hao A. et al. J. Phys. Chem. Solids 74 1504-1508, 2013 Shabara et al. Results Phys. 1 30-35, 2011 Jakobsen et al. Solid State Commun. 121 447-452, 2002 Gupta D.C. et al. J. Mol. Model. 19 5343-5354, 2013 Schneider et al. J. Appl. Phys. 111 093503, 2012 Cynn et al. J. Phys.: Conf. Ser. 215 012010, 2010 Olsen et al. J. Alloys Compd. 533 29-32, 2012
Polymorphs of SiO2 and their transitions have long been a focus of great interest and intense theoretical and experimental pursuits. Compressing single-crystal coesite SiO2 under hydrostatic pressures of 26~53 gigapascal at room temperature, we discover a new polymorphic phase transition mechanism of coesiteto post-stishovite, by means of single-crystal synchrotron X-ray diffraction experiment and first-principles computational modeling. The transition features formation of multiple previously unknown triclinic phases of SiO2 on the transition pathway as structural intermediates.The metastable phases are similar in volume and degenerate in free-energy, but distinct in structures and x-ray diffraction patterns. Coexistence of the low-symmetry phases results in extensive splitting of the original coesite X-ray diffraction peaks that appear as a dramatic peak broadening and weakening, thus resembling an amorphous material. This work shed light on the transition mechanism of SiO2 crystal under high pressures, and clarifies the issue of the pressure-induced amorphization of coesite.
Diamond anvil cells (DAC) made it possible to explore extreme energy-density regimes that are of relevance for planetary interiors and extreme physics and chemistry. How different is chemistry rules at high pressure have been a topic of many recent studies 1, 2 that found a number of empirically derived principles that govern the modification of chemical bonds in conditions of increased density. DAC experiments in the Mbar pressure range have shown a variety of novel phenomena in elementary materials and compounds 3, 4. First principles structure search methods 5, 6 have been very instrumental in new material predictions that often possess unusual chemical bonding and stoichiometry. In this study, we explore a synergy between theory and experiment to investigate emerging chemistry laws at extreme conditions. We will give examples of recent studies of K-Cl 7, C-N 8, N-H 9, Na-He 10, Mg-O 11, Na-H 12 systems that revealed the formation of novel compounds that become thermodynamically stable at high pressures and can be synthesized in the pressure-temperature range of their thermodynamic stability. These materials revealed a wide range of metastability holding a promise for synthesis of materials with unique physical and chemical properties for a variety of applications. This investigation has a direct relevance for planetary interiors as the compounds synthesized in this study can exist as alternative planetary ices or mantle/core minerals. I thank A. R. Oganov, S. Lobanov, and E. Stavrou for contributing greatly to this work.References1. C. T. Prewitt and R. T. Downs, Rev. Mineral. 37, 283 (1998).2. W. Grochala et al., Angew. Chem. Int. Ed. 46, 3620 - 3642 (2007).3. Y. M. Ma et al., Nature 458 (7235), 182-185 (2009).4. W. Zhang et al.,Science 342 (6165), 1502-1505 (2013).5. A. R. Oganov and C. W. Glass, Journal of Chemical Physics 124 (24), 244704 (2006).6. C. J. Pickard and R. J. Needs, Journal of Physics: Condensed Matter 23 (5), 053201 (2011).7. W. Zhang, et al, arXiv:1405.3007 [cond-mat.mtrl-sci] (2014).8. E. Stavrou et al., arXiv:1412.3755 (2014).9. A. F. Goncharov, et al., J. Chem. Phys.142, 214308 (2015).10. X. Dong, et al., arXiv:1309.3827 [cond-mat.mtrl-sci] (2014).11. S. Lobanov et al., Sci. Rep. Under review (2015).12. V. V. Struzhkin et al., Nat Commun. Under review. (2014).
CrAs was observed to possess the bulk superconductivity under high pressure conditions. To understand the superconducting mechanism and explore the correlation between the structure and superconductivity, the high-pressure structural evolution of CrAs was investigated using the angle dispersive X-ray diffraction (XRD) method. The structure of CrAs remains stable up to 1.8 GPa, while the lattice parameters exhibit anomalous compression behaviors. With increasing pressure, the lattice parameters a and c both demonstrate a non-monotonic change, and the lattice parameter b undergoes a rapid contraction ~ 0.2minus;0.35 GPa, which suggests that a pressure-induced isostructural phase transition occurs in CrAs. Above the phase transition pressure, the axial compressibilities of CrAs present remarkable anisotropy. A schematic band model was used to address the anomalous compression behavior of CrAs. The pressure-related structural changes occurred concurrently with the appearance of bulk superconductivity, shedding light on the superconducting mechanism in CrAs.
Metastable polymorphs of nanocrystalline silicon offer unique opportunities for future energy applications by combining interesting possibilities of quantum confinement with multiple exciton generation. In this work we have explored polymorphism in silicon nanoparticles (SiNPs) under various pressure-temperature pathways using synchrotron XRD and Raman spectroscopy. Hydrogen / oxide surface terminated SiNPs were synthesized using a plasma-enhanced chemical vapor deposition method, where size was controlled by varying reactor pressure and flow. At room temperature, diamond-structured SiNP's transition to the high-pressure hexagonal phase directly near 20 GPa, bypassing the beta-tin transformation observed in bulk Si. Upon decompression at room temperature, the hexagonal phase transforms directly to an amorphous phase without loss of nanoscale morphology, indicating irreversibility to the diamond structure. Interestingly, the transition between the hexagonal phase and amorphous phase is completely reversible at ambient temperature through application of pressure cycling, indicating local structural correlations between these phases. High-temperature annealing and pressure cycling provide different transformation pathways than those obtained at room temperature. The effect of temperature pathway and nanoparticle size on polymorphism will be discussed.
Nanocrystal growth in high pressure supercritical water fluid is relatively unexplored. In our work, we studied coarsening of nano anatase (TiO2) using in-situ synchrotron X-ray diffraction. It was found that upon heating under high pressure (up to 550 oC and ~ 8 GPa), ~ 3 nm anatase particles transformed to nano TiO2(II) and rutile in water, which then appeared to be in quasi-equilibrium that was responsive to temperature and pressure perturbations. The results suggest that the nanophase transformation and coarsening are likely limited by the process thermodynamics and the supercritical water fluid may have mediated the nanophase equilibrium via dissolution-precipitation mechanism.
In order to facilitate the development of novel micro-electro-mechanical components as well as non-destructive pulsed field magnets for fundamental research under extreme magnetic fields, specialized conductor materials are needed. Therefore, a combination of high mechanical strength with an at least good electrical as well as thermal conductivity with a good ductility is needed. A crucial point for this kind of development is an efficient restructuring and refinement of the microstructure during the production of semi-finished parts such as wires and tapes. In the present study, we will provide a summary on two different ways of refining copper-based alloys in order to increase mechanical strength while keeping electrical conductivity on high level.Inspired by thin film deposition methods enabling a severe increase of growth twin density of the microstructure, a macroscopic approach of producing a severely twinned microstructure of single-phase copper alloys (Cu and CuAl) by deformation due to either lowering the deformation temperature or decreasing the stacking fault energy is discussed. The peculiarities of deformation twinning with respect to a highest possible volume fraction of refined material are stressed. By analysing the orientation dependence of deformation twinning with respect to the state of stress of various deformation methods, suitable production routes are evaluated. The amount of twinned material within the microstructure is determined by means of local and global texture analyses of the according texture components.In comparison to that, state of the art high strength conductor materials based on CuAgZr alloys are refined by means of dynamic recrystallisation during hot-work prior to severe cold-work. The recrystallisation behaviour is analysed with respect to temperature, strain and strain rate dependence as well as the mechanism of dynamic recrystallisation in multi-phase CuAgZr alloys. The consequences for homogeneity of semi-finished parts, precipitation of Ag, and properties are discussed in detail.
Cu-Nb wire made by severe plastic deformation (SPD) is currently used by the magnet lab for building high strength pulsed magnets. The use of SPD allows refinement of Cu-Nb microstructure such that increases in UTS, exceeding that of the rule of mixtures, are obtained. Due to the immiscibility of Cu and Nb, reasonable conductivity is maintained during the SPD process. An alternate SPD method for producing refined grain structures in Cu-Nb composites is accumulative roll bonding (ARB). Literature indicates that ARB Cu-Nb plates achieve UTS values (>1.3 GPa) that are comparable or higher when compared with Cu-Nb wires of similar geometry size. The high strength has served as the catalyst for the bulk of the research surrounding ARB Cu-Nb composites while the corresponding conductivity of these same materials has been understudied. Results from a recent LANL study conducted on ARB Cu-Nb at room temperature indicate that %IACS decreases with layer thickness (h) and is in the range of 20-45% IACS when h are on the order of 20-200 nm.ARB panels made thus far at LANL have not been optimized for high strength and high conductivity as desired for a magnet coil application. The conductivity values obtained during the seed study were obtained from sheets that were 50 vol% Nb. Research on nanolamellar Cu-Nb bimetallic composites has revealed several desirable properties including high strength, good ductility, high radiation damage resistance, and resistance to deformation at high strain rates tied to both the interfacial density and the interfacial structure. An accumulative roll bonding method for fabricating these composites in bulk sheet form is utilized to make multilayers with layer thicknesses ranging from 95 um to 10 nm and with accompanying rolling strains of 3 to 12.21. Characterization methods such as neutron diffraction and electron microscopy are used to provide microstructural information such as grain and layer morphology as well as orientation information such as texture and interface plane normal distributions. The evolution of these measures is collected as a function of increasing strain. Results are compared with rolling studies of single phase Cu, single phase Nb, and Cu-Nb composites. An atypical texture and microstructural evolution takes places at layer thicknesses <100 nm, likely due to the density of interfaces and the character of those interfaces. The effects of heating, joining, and cross-rolling on the microstructure evolution will be presented. These results will be placed in the context of developing a materials-by-design approach for maximizing conductivity while maintaining the other desirable properties for Cu-Nb nanolamellar composites.
Until 2014 high field Bitter magnets around the world were generating fields in the mid-30 tesla range, with a modest outer magnet diameter of around 60 centimetres, except of course for very wide bore magnets as developed in the 1990s. In the last years HFML in Nijmegen, NHMFL in Tallahassee and CHMFL in Hefei started the development of one metre Bitter magnet systems aiming at fields towards 40 tesla. HFML realised a 37.5 T magnet in March 2014, followed by Hefei with a 38.5 T magnet later that year and NHMFL will soon realise a 40 T-class magnet. At the same time there a hybrid magnet programmes running aiming at 45 T (HFML, CHMFL). All these coils have in common that there are additional coils operating at elevated stress levels that tend to exceed the strength of copper and copper-alloys that are available for these coil dimensions. In an attempt to increase the feasibility of these stress-limited coils we have established contact with European manufacturers to find a copper-alloy with moderate strength (UTS ge; 500 MPa) and a good conductivity (ge; 90% IACS). In this presentation we present the results of mechanical and electrical tests on a copper-silver alloy (1% Ag in Cu) on 410x410x0.44 mm sheets that were done to validate this material for use in high field magnets.
Copper-based high strength and high electrical conductivity nanocomposite wires reinforced by Nb nanotubes are prepared by severe plastic deformation, applied with an Accumulative Drawing and Bundling process (ADB), for the windings of high pulsed magnets. The ADB process leads to a multi-scale Cu matrix containing up to N=854 (52.2 106) continuous parallel Nb tubes with diameter down to few tens nanometers. After heavy strain, the Nb nanotubes exhibit a homogeneous microstructure with grain size below 100 nm. The Cu matrix presents a multi-scale microstructure with multi-modal grain size distribution from the micrometer to the nanometer range.The use of complementary characterization techniques at the microscopic and macroscopic level (in-situ tensile tests in the TEM, nanoindentation, in-situ tensile tests under neutrons and high energy synchrotron beam) shed light on the role of the multi-scale nature of the microstructure in the recorded extreme mechanical properties and strong stability in extreme environment.[Acta Mat, 57 (2009), p3157; Acta Mat, 58 (2010), p6504; Adv. Eng. Mat., 14-11 (2012), p998].
High modulus, high strength, corrosion resistant Co-Ni based alloy MP35N® has important applications in medical, chemical, marine applications. It is also an excellent reinforcement material for ultrahigh field pulsed magnet. In addition, due to its superior mechanical properties at cryogenic temperatures, it may be used in superconducting magnets as a reinforcement material.For applications in a superconducting magnet, accurate data of material physical properties, especially its thermal properties is crucial. Therefore in this paper, we report our measurement results of physical properties of MP35N® sheet metal, including magnetization, thermal conductivity, specific heat capacity and resistivity at 2 - 300 K using a physical property measurement system (PPMS). These properties are compared with those of other Ni based alloys such as Hastelloy C-276® and Haynes 242®.The effect of the aging heat treatment on its magnetic properties is discussed.
High field magnets have become powerful tools to study materials. High magnetic fields (HMF) are only achievable through the use of specialized function materials, like high-strength conductors and insulators, and in some cases, high-strength structural materials. When high performance conductors are used, both mechanical properties and electrical conductivity must be taken into consideration. At any given level of electrical conductivity, raising the level of mechanical strength potentially raises the strength of the magnetic field.Understanding the performance of materials in a high field magnet is made more complex by the variety of changes in material properties that can be induced by exposure to cryogenic conditions, high operation temperatures, high strain rates, and HMF operations in general. Many of these changes have been studied, but are not well understood. Other changes, like the magnetoplastic effect, are not familiar to some researchers and must be fully investigated. In order to avoid catastrophic failure and to increase the available choice of materials and processing method, so as to allow for building larger magnets with large size conductors , we need to understand the behavior of materials used to build high field magnets and to develop new approaches to attaining the required properties.One important approach to increasing HMF beyond what is now possible is to improve the properties of various composite materials used as both conductors and structural support. Typical conductors for high field magnets are Cu-based metal-metal composites. To achieve high mechanical strength, these composites are fabricated by cold deformation, which introduces high densities of interfaces along with lattice distortions.During the operation of a magnet, mechanical load, high magnetic field, extreme temperatures and other stressors are imposed on the materials, causing them to be further “processed”. The composite conductors in a magnet, for example, may undergo high temperatures, which reduce lattice distortions. At the same time, HMF may increase lattice distortion, leading to a complex change in interface characteristics. Both the mechanical properties of the conductors, like the tensile and yield strength, and the electric conductivity of the composites are closely connected to changes in lattice distortion and interface density.Understanding change in material properties helps us assure that materials can operate in optimized conditions during most of magnets&’ service life. Maximizing service life is critical, given the high cost of building and operating high field magnets. The goal of this presentation is to show the understanding we have so far developed of changes that occur in the properties of selected materials under HMF and to show how those changes relate to the microstructure of these materials and consequently to the service life of high field magnets.
The national High Magnetic Field Lab designs, builds and operates high-field resistive magnets which require very high-strength, high-conductivity materials in sheet and other forms. The material used most often is Cu24w%Ag sheet between 0.3 mm and 0.7 mm thick. It has an ultimate strength greater than 850 MPa with an electrical conductivity >70% of the International Annealed Copper Standard (%IACS). In addition pure copper is used as well as CuZr and CuBe. Attaining higher fields is dependent on finding materials with better combinations of strength and conductivity. Material requirements for next-generation magnets will be presented.
We have been developing mono-coil to generate pulsed high magnetic fields. The highest field so far is 85.8 T. The field was generated by 11 layers mono-coil with bore size of 18 mm. The coil was made by winding wire of Cu-6wt%Ag insulated by Kapton and its cross section is 2.15 mm x 3.15 mm. The wire has a tensile strength of about 1.1 GPa and its conductivity is about 80 % IACS. The coil is reinforced only from the outside.The coil has been used as a user-coil and offering 75 T fields for usual experiment every 30 minites. Lifetime of 75 T-field seems to exceed 100 shots but is still unclear. It is a present subject to search for a manufacturing process of Cu-Ag wire to keep a good quality to generate 85 T without any failure.I will talk about advantage of Cu-Ag wire to generate pulsed high magnetic fields.
The Dresden High Magnetic Field Laboratory (HLD) is a user facility which provides external users and in-house researches with the possibility to perform experiments in pulsed magnetic fields using a large variety of experimental techniques . The HLD develops, manufactures, and operates the non-destructive pulsed magnets for the field range up to 90 T and beyond. Here I will discuss various issues of material requirements and the choice of materials for pulsed magnet designs. The pulsed-field magnets experience huge mechanical stresses arising due to the strong Lorentz forces, the large magnetic and electric fields, and last but not least, the thermal shock during the pulse. This results in significant material challenges. In particular, the conductor should possess a number of contradicting properties, such as high electrical conductivity, high ultimate tensile strength (UTS), high ductility, high specific heat, low fatigue, and low magnetoresistance. The properties of the reinforcing materials are very important as well. Most of the pulsed-field magnets are stress limited, with a stress level of the materials very close to the UTS. Therefore, the correct choice of materials and a careful magnet design are crucial for a reliable magnet operation. Very often the vital information about the material properties can be obtained from the post-mortem analysis of the failed magnets and the comparison with numerical simulations. I will report on our long-standing experience obtained at the HLD with our pulsed magnets. Most likely, with the existing materials the peak field for the non-destructive pulsed magnets of the order of 100 T is very close to the physical and practical limit. Obviously, in future stronger high-conductive wires and fiber reinforcement with higher strength may provide the possibility for even higher magnetic fields. Finally, I will briefly present some scientific results obtained in the pulsed magnetic fields at the HLD.This work was done in cooperation with T. Foerster, D. Gorbunov, E. Kampert, P.T. Cong, Y. Skourski, Z. Wang, S. Zvyagin, T. Herrmannsdoerfer, and J. Wosnitza. We acknowledge the support of the HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL) and Deutsche Forschungsgemeinschaft via SFB 1143.
Mechanical properties of the CuNb micro-composite conductor have been studied by uniaxial tensile test, three-point bending test, tension-tension fatigue test and SEM observations. It is found that the conductor is transversely homogenous with Poisson&’s ratios of 0.35 in the transverse direction. The tensile strength of the conductor is around 765 MPa at the strain of 2% and obvious necking occurs after the strain of 5%. The rate dependency of the conductor is slight with the strength increasing about 30 MPa from the straining rate of 5E-5 to 2E-2. According to the bending test, the conductor is cyclically stable without any hardening/softening effect during loading cycles. In the uniaxial tension-tension fatigue test, it is found that the nonlinear unloading behavior is caused by the internal Bauschinger effect in the conductor. The ratcheting deformation of the conductor is influenced by both the peak stress and stress ratio in a way that a higher peak stress leads to a non-stop increase in the ratcheting strain till failure while a lower peak stress induces a stable ratcheting strain without any increase. On the other hand, a higher stress ratio restrains the increase in the ratcheting strain provided that the peak stress is the same. In general, the effect of stress ratio on the ratcheting deformation seems less prominent than that of the peak stress. SEM observations prove that there are mainly three modes of fatigue failure at different peak stress levels and stress ratios: failure by necking, shear failure and nominal failure. The first mode happens at higher peak stress with microvoid coalescence dominating the fracture morphology, presenting obvious ductility in both macroscopic and microscopic scale. The mode of nominal fracture has a fracture surface full of interfacial cracks rendering the failure mode more brittle than the first one. The mode of shear failure at intermediate peak stress can be seen as a mixture of the above two modes with visible tortuous dimples and fatigue striations. Given the same peak stress, a higher stress ratio would result in a more brittle failure which is a mixture of the neighboring two of the above three modes. As a result of the different failure modes, the S-N curve of the conductor is nonlinear in log-log scale in a broad range of peak stress. In a macroscopic point of view, the differences in failure modes can be attributed to ratcheting deformation and accumulative cyclic deformation. The ratcheting deformation is responsible for the failure by necking while the accumulative cyclic deformation causes nominal failure. The shear failure can be seen as a combined effect of both deformation types.
The Pulsed Field Facility of the National High Magnetic Field Laboratory (NHMFL) at Los Alamos National Laboratory (LANL) has successfully provided its users routine access to magnetic field up to 100 Tesla. From a scientific point of view, higher magnetic fields enable unprecedented research in quantum matter, such as research on topological insulators and electronic structure determination. So the 2014 MagSci Report from the National Research Council suggested a long term goal of development of non-destructive pulsed magnets that can reach 150 T. Clearly, better and stronger conductors and reinforcement materials will be required to realize that challenging goal. This presentation will provide an overview on some requirements for the materials to realize such a magnet by showing how the improvements of materials can help to reduce the stored energy, size and cost of the magnet. Some initial efforts to relate high strength materials to better magnet design will also be discussed in this talk.
One of the authors developed the high strength and high conductivity Cu-24wt % Ag alloy as a conductor material for high field magnets twenty years ago.Wire and sheet of the alloy have been used as a conductor material for pulsed magnets or water cooled magnets of the high magnetic field facilities of each country.However, the alloy required large quantities of Ag addition to realize high strength. The cost performance and workability of the alloy were not good for that. So, we investigated possibility of low Cu-Ag alloy for decreasing in material cost and improving in workability.We succeeded in the development of the Cu-6wt% Ag alloy by the new heat treatment which is superior to the characteristic of the Cu-24wt% Ag alloy even if the amount of Ag content is decreased in 1/4.At present, we make a lot of high field pulsed magnets by using the Cu-6wt%Ag wire manufactured industrially, and do that magnetic field experiment and are getting good results at the ISSP, the university of Tokyo.We will talk about the characteristic, new heat treatment method and the manufacturing process of the conductor material for the Cu-6wt%Ag alloy.
The French National Laboratory for Intense Magnetic Field (LNCMI) is involved since several years in the research and development of high performance novel conductors for high field pulsed magnets, in order to improve the quality of user magnets. This R&D activity is performed thanks to collaboration with academic and industrial partners. To succeed in producing very high magnetic fields, the development of high strength conductors able to resist both to the heating and to electrodynamic forces is required.First, “in-house” copper/stainless steel macrocomposite conductors for wire wound pulsed field magnets generating magnetic fields up to 90 Tesla, will be reviewed. Mechanical and electrical properties are tailored to the magnet requirements by selecting the volume fraction of stainless steel reinforcement and the work-hardening state at the end of the drawing process.A special focus will be devoted to the multiscale copper/niobium nanocomposite wires dedicated to the production of magnetic field over 90 Tesla. The process based on severe plastic deformation will be detailed. Their macroscopic physical properties will be discussed.Then, new innovative developments on carbon nanotube-copper composite wires thanks to collaboration with CIRIMAT (F) will be presented.
Time-resolved X-ray absorption spectroscopy (TR-XAS) uses pulsed laser as excitation source, interrogates with stroboscopic X-ray pulse snapshots. It is a powerful tool to study the correlation between electronic and structural dynamics linked to molecular and interfacial electron transfer processes that are fundamental to energy conversion in a broad range of scientific disciplines, including solar energy conversion, catalysis, geochemistry, fuel cells, and electrochemical (battery) energy storage.We have applied TR-XAS to study Ruthenium and Osmium polypyridyl complexes and their derivatives which have been extensively used as photosensitizers in solar-cells, molecular electronics and light emitting devices. Photoexcitation of those photosensitizers leads to long lived metal-to-ligand charge transfer (MLCT) states. If bonded to proper semiconductor nanocrystals, the photoexcited Ru and Os complexes inject electrons to semiconductors, resulting in interfacial-charge transfer state. We have measured TR-XAS spectra of photoexcited Ruthenium and Osmium complexes under various sample environment including liquid solution, nanoparticle liquid suspension and solid thin film. Experimental results were compared with simulations/theoretical calculations, quantitative information on the molecular structure, electronic configuration and molecular orbital energies of ground and excited states have been revealed. The chances in the Metal-ligand distances have been directly characterized and rationalized by the interplay among important factors governing the metal to ligand bonding, covalence, steric hindrance and π-backbonding. These works have demonstrated the great potential of time-resolved X-ray spectroscopy to study fundamental structural-functional correlations in solar electricity and fuel generation for both homogenous systems and heterogeneous interfacial systems.
Subsurface modification of materials with nanosecond pulsed lasers provides a route for rapid energy deposition deep in a material with simultaneous constraint by the surrounding unaffected material. An application of such extreme processing conditions concerns modifications created subsurface in Si for use as part of a wafer dicing process. Such modifications, when closely spaced, guide crack propagation in a highly controlled manner when the sample is subsequently cleaved, allowing for accurate sectioning of wafers. Suitable modifications may be induced using a nanosecond laser with a short to mid IR wavelength which is focussed subsurface to produce an intensity gradient and thus achieve selective, intensity-dependent absorption.We illustrate in this presentation that the modifications themselves result from rapid heating beyond the melting point of Si, followed by rapid solidification. Due to the constraint of the surrounding material, pressure variations accompany the temperature variation as the material expands and phase transforms. The resultant morphology has been characterised by micro-Raman spectroscopy, SEM and TEM with the aim of understanding the entire modification process and elucidating routes by which the dicing process might be improved. Thus far, the laser-induced modifications have been found to contain a range of features including: (1) lattice defects related to imperfect epitaxial solidification of molten zones created by laser heating, (2) voids induced by volume contraction of molten Si due to its higher density than the solid, (3) the observation of melt-quenched amorphous Si and (4) the formation of pressure-induced high density Si phases such as the rhombohedral (r8) and body centred cubic (bc8) phases. The origin of these features will be discussed based on two key processes: the redistribution of mass within the melt volume, and the rapid radial solidification of Si from the melt. In particular, we show that there is an orientation dependence of radial growth that can lead to defects at the intersection of liquid-solid interfaces. Furthermore, interesting crystalline-amorphous-crystalline structures can evolve radially as a result of changes in liquid-solid interface velocity arising from large differences in thermal conductivity of crystalline and amorphous Si.
Non-stoichiometric oxides relevant to solid oxide fuel cells (SOFCs), electrolyzers, or gas sensors are frequently subjected to extreme environments that include elevated temperatures (>550 °C) and significant oxygen activity gradients. Many mixed conducting electrodes and membranes will release or absorb oxygen under these variable operating conditions, leading in turn to significant volumetric dilation (chemical expansion). In short, these materials “breathe” dynamically, with attendant changes in electrical properties. In situ measurements now provide an opportunity to understand this dynamic, electro-chemo-mechanical coupling in extreme environments relevant to SOFC thin films. Measurement of chemical expansion of thin films has been limited typically to X-ray diffraction techniques, which quantifies only the lattice parameter (not overall film strain) with timescales unsuitable to the kinetics of oxygen exchange. Therefore, we developed a novel approach to rapid, direct detection of chemical expansion of thin film non-stoichiometric oxides at temperatures up to 650°C. In this technique, the oxygen activity is modulated via electrical bias, while amplitude and phase lag of film expansion are detected with sub-nanometer displacement resolution and second-scale time resolution. The expansion signal is measured on the model SOFC cathode material, Pr0.1Ce0.9O2-δ (PCO) thin films, and compared to our previously derived defect equilibria models. The role of substrate vs. film thickness is explored, along with estimation of activation energies related to the kinetics of defect transport using this technique. This method is expected to provide facile access to chemical expansion measurements under high temperature conditions for non-stoichiometric oxide thin films, enabling improved material design for systems subjected to extreme temperature or redox cycling.
New material phases formed under non-equilibrium conditions at pressures above 100 GPa and temperatures exceeding 104 K, the conditions of the warm dense matter (WDM), have become accessible using micro-explosions triggered by ultra-short sub-1 ps pulses tightly focused into micro- volume with cross sections comparable with the wavelength of light. Ultra-fast quenching of the WDM conditions where potential energy of the interaction between electrons and nuclei is comparable with the kinetic energy of electrons, opens new material formation pathways and possibility to recover metastable phases of exotic high temperature and pressure materials. Direct laser writing was used to pattern large areas by closely packed arrays of the microexplosion modified sites for structural characterisation of the minute volumes of nano-materials with transmission electron microscopy, diffraction and synchrotron X-ray diffraction. Challenges and methods for precise light energy delivery to the micro-explosion site is discussed as well as possibilities to use different light intensity distributions for the explosion and implosion experiments. The method of ultrafast-laser induced confined microexplosion is demonstrated for modification and creation of new phases in case of bcc-Al inside sapphire, valence change of Fe-ions in olivine, formation of new tetragonal bt8 and st12 phases of silicon, Ge and O separation in GeO2 glass and molecular oxygen formation inside voids at the site of microexplosion inside glasses.
The coupled electromagnetic-thermo-mechanical response of energetic aggregates under laser irradiation and high strain rate loads has been investigated for various aggregate sizes and binder volume fractions. The thermo-mechanical behavior of the RDX crystals are modeled with a dislocation density-based crystalline plasticity formulation and the estane binder is modeled with finite viscoelasticity through a nonlinear finite element approach that couples electromagnetic (EM) wave propagation with laser heat absorption, thermal conduction, and inelastic deformation. Material property and local behavior mismatches at the crystal-binder interfaces resulted in geometric scattering of the EM wave, electric field and laser heating localization, high stress gradients, dislocation density and crystalline shear slip accumulation. It is shown how the coupled electromagnetic and thermo-mechanical behavior leads to hot spot formation and dynamic fracture. This investigation indicates the complex interactions between EM waves and mechanical behavior, for accurate predictions of laser irradiation, hot spot formation, and crack nucleation in heterogeneous materials subjected to extreme loading conditions.
Blast-induced traumatic brain injury (bTBI) is the signature cause of deaths and disabilities in modern combat and has been linked to post-traumatic stress disorder, memory loss, and chronic traumatic encephalopathy. Understanding shockwave-induced physical and chemical changes of impact-absorbing materials is an important step toward the rational design of materials that mitigate the damage. In this work, we report a series of network-forming ionic liquids (NILs) that possess an intriguing shockwave absorption property upon laser-induced shockwave. Microstructure analysis by X-ray scattering suggests presents nano-segregation of alkyl side chains and charged heads in NILs. Further post-shock observations indicate changes in the low Q region suggesting that the soft alkyl domain in NIL plays an important role in absorbing shockwaves. Most surprisingly, we observed a shock-induced ordering in the NIL with longest hexyl side chain. These results indicate that both nano-segregation structure and shock-induced ordering contribute to NIL&’s shockwave absorption performance.
We are investigating novel mesoscale sensors for studying the shock compression response of highly heterogeneous granular materials. One-dimensional photonic crystals based on multilayered optical structures composed of alternating layers of high and low refractive index dielectric materials have the potential for use as mesoscale sensors, because of their spectral sensitivity to deformation. Shock compression of highly heterogeneous granular materials is often dominated by effects of particle-level interactions, which can significantly alter the bulk response and make it difficult to correlate the mesoscale characteristics with measured shock-compressed states. The effects of such mesoscale heterogeneities are difficult to monitor in real time due to the lack of available diagnostics typically employed with shock compression experiments. We are focusing on novel optical sensors as in-situ diagnostics for spatial and temporal probing of mesoscale processes associated with shock compression of inert/reactive granular/particulate materials. We are exploring multilayer optical structures including Diffractive Bragg Reflectors (DBR) and Optical Micro-Cavities (OMC), which have unique spectral properties that are a function of the individual layer thickness and their refractive index. Their spectral response can be altered by external perturbations to yield near instantaneous optical response with micron scale spatial sensitivity. We used OpenFilters software to determine the reflectance and transmission of the optical structures, to develop the design criteria and fabricate organic and inorganic multilayered and microcavity structures. Ion-assisted deposition (IAD) was then used to fabricate various OMC structures such as Ag/SiO2/Ag, and Ag/Al2O3/Ag. The optical and mechanical behavior of multilayer optical structures was characterized using a coupled computational-experimental study. The results from laser-driven 1D shock loading experiments performed on OMC structures will be described along with their comparison with predictions from simulations. The potential of the OMC structures for applications as sensors for probing the mesoscale response of heterogeneous materials will also be discussed in this presentation.*Funding for this research was provided by DTRA under Grant No. HDTRA-1-12-1-0052
TZM alloys possess high temperature strength, high creep resistance, low coefficient of thermal expansion, high thermal conductivity and with all these properties, TZM alloys manage to resist extreme conditions. TZM alloys (Mo-0.5Ti-0.1Zr-0.02C) contain TiC, ZrC and complex carbides which inhibit recrystallization at high temperatures and also improves the working conditions. In this study, under influence of different sintering temperature, time and pressure the densification and sintering behavior of Spark Plasma sintered TZM alloy were investigated on spark plasma system. The sample was sintered at 1575 °C for 2.5 min under 40 MPa exhibited the highest relative density, ~96%. Microstructural characterization was performed by using FESEM and it was observed that TiC and ZrC dispersed through the microstructure. The average size of precipitates were measured as 3.5µm and 7µm for TiC and ZrC respectively.
Recently a kind of new-type martensitic transformation called as confined martensitic transformation (CMT) with the transformation kinetics obviously different from that found in traditional martensitic transformation (TMT) has been experimentally evidenced in shape memory alloys. For the TMT, the collective motion of martensitic variants controls the transformation kinetics, while the growth of martensites is confined in the CMT due to three intrinsic physical factors, i.e. (1) the irregular distribution of point defects, (2) the disordered stacking of multiple martensitic structures, (3) the nano-scale distribution of disorder structure or inhomogeneous chemical composition. This talk will summarize our new experimental findings on magnetic field-driven new functional behaviors in shape memory alloys such as NiFeGaCo, NiMnGa and NiMnInCo system, which exhibit CMT. The in-situ experimental investigations on evolution of the complex crystallographic structures and transformation kinetics under high magnetic field by the high-energy X-ray and neutron diffraction techniques will be given. The in-depth understanding of the new physical mechanism on CMT related to partial phonon softening in the above-mentioned interesting alloy systems will be also presented. The financial support by the National Basic Research Program of China (973Program) under Grant No. 2012CB619405 is acknowledged.
The extreme high magnetic field environment, generally coupled with elevated temperature, provides an enabling disruptive technology for making significant major science and technological advances in developing the next generation of novel structural and functional materials. High magnetic field processing (HMFP) literally impacts all materials and can result in significant changes in phase equilibria and phase transformation kinetics resulting in performance improvements up to 300%, and represents an entirely new synthesis/catalysis tool. Additionally, coupling HMFP with an induction field results in a non-contact electromagnetic acoustic/ultrasonic transducer (EMAT) process that can dramatically impact the solidification microstructures of any conductive media leading to wrought property performance in as-cast materials and new nanoparticle reinforced composities. This presentation will have three thrusts: 1) highlight several technologically significant examples of our HMFP/EMAT processing research, 2) summarize ORNL capabilities to do supportive, predictive ab-initio supercomputing Local Spin Density Functional modeling validated by in-situ, real-time characterization of materials under the HMFP environment using our world-class neutron diffraction facilities (the SNS), and 3) discuss our efforts with industrial partners directed towards the near-term commercialization of this technology detailing the world&’s first commercial-scale prototype HMFP facility at ORNL.
Cation diffusion is the rate-limiting unit process that determines the performance of pyrrhotite (Fe1-xS) phases for applications like corrosion barrier layers  and diffusion-driven phase-change memories . However, an accurate characterization of this crucial step remains lacking because of the influence of pyrrhotites&’ magneto-structural phase transition on cation diffusion is unknown. Recent radiotracer measurements that show significantly lower mobilities for Fe ions at low temperatures around 150 °C than at temperatures above 320 °C attribute it to the different magnetic states of the pyrrhotite crystal at these temperatures .In this study, we corroborate this ‘magnetic diffusion anomaly&’ based on the first-reported direct quantification of migration barriers and defect formation energies in different magnetic phases of pyrrhotite using hybrid Hartree-Fock/Density Functional Theory calculations. Specifically, ionic diffusion in antiferromagnetically-ordered pyrrhotite is found to be slower, with an activation barrier of 1.3 eV, compared to 0.6 eV for pyrrhotite crystals where local magnetic moments on Fe ions are ferromagnetically coupled. Fe vacancy formation enthalpies follow an identical trend with values for the antiferromagnetic crystals being 1.6 eV higher than that for ferromagnetic ones. We further show that a simple Heisenberg Hamiltonian model of the dominant magnetic interaction, the Fe-S-Fe superexchange  can explain the difference in ionic migration barriers and vacancy formation energies between the magnetic phases of pyrrhotite.This mechanistic description of magnetic contributions to ionic transport will not only enable more accurate characterization of pyrrhotite performance in corrosion barriers and future memory devices, but is also broadly applicable to the study of ionic mobilities in other related superexchange-driven antiferromagnetic materials like NiO and MnO.References H. Vedage, T.A. Ramanarayanan, J.D. Mumford, S.N. Smith, Corrosion 49 (1993) 114. T. Takayama, H. Takagi, Applied Physics Letters 88 (2006). F. Herbert, A. Krishnamoorthy, L. Rands, K. Van Vliet, B. Yildiz, Physical Chemistry Chemical Physics 17 (2015) 11036. I. Lyubutin, C.-R. Lin, S.-Z. Lu, Y.-J. Siao, Y. Korzhetskiy, T. Dmitrieva, Y. Dubinskaya, V. Pokatilov, A. Konovalova, Journal of Nanoparticle Research 13 (2011) 5507.
Extreme conditions of low temperature, high pressure, and high magnetic field are essential ingredients for the study of strongly correlated electron systems as well as many other topics, and instrumental developments in these directions have been at the origin of numerous discoveries and advances in our understanding of the physics of these systems. In some cases higher magnetic fields than the static magnetic fields produced in superconducting magnets, or in resistive magnets available in dedicated facilities, are desirable. However combining low temperatures and high pressure with pulsed magnetic field presents specific challenges. We have developed a pressure cell optimized for the use in pulsed magnetic fields up to 60T, and pressures up to at least 4.5 GPa, at temperatures down to 1.5K. Heating effects due to the field pulse are reduced by the use of electrically insulating materials for the elements that are close to the sample. We have used this technique to investigate the field induced magnetic instabilities in the heavy fermion antiferromagnetic system CeRhshy;2Si2In an Ising antiferromagnet like CeRh2Sishy;2 an external magnetic field will destroy the antiferromagnetic order, and induce a polarized paramagnetic state. These two phenomena can occur concomitantly with a 1st order metamagnetic transition, or antiferromagnetic order can be destroyed at a lower field than the transition to a polarized state, with 2 characteristic fields, HC and HM. In CeRh2Si2 these two fields coincide, implying that HC>HM. At ambient pressure this occurs at a field of 26T which increases when pressure is applied1. In the paramagnetic compound CeRu2Si2, doping with Rh induces antiferromagnetic order and a decoupling of HC and HM is found2. Pressure is a clean way to tune an antiferromagnetic cerium compound to its critical point. Pressure will increase HM and decrease HC, and perhaps induce this decoupling of the characteristic fields.We report magnetoresistance measurements on a single crystal of CeRh2Si2. A continuous increase of HM with pressure is found, exceeding 60T at 1.5 GPa. For P<
C only one anomaly is seen, implying that HC>HM. However close to the critical pressure a broad shoulder develops on the low field side of HM. We suggest that this shoulder is likely a sign of the decoupling of the 2 fields, and present the obtained phase diagram. Measurements at different temperatures also allow us to extract the coefficient of the quadratic temperature dependence of the resistivity, so giving information on the evolution of the effective mass versus field and pressure.References T. Hamamoto, K. Kindo, T. C. Kobayashi, Y. Uwatoko, S. Araki, R. Settai and Y. Onuki, Physica B 281, 64-65 (2000). D. Aoki, C. Paulsen, H. Kotegawa, F. Hardy, C. Meingast, P. Haen, M. Boukahil, W. Knafo, E. Ressouche, S. Raymond and J. Flouquet, Journal of the Physical Society of Japan 81 (3) (2012).
We performed a series of resistivity, heat capacity and ultrasound speed measurements of a MnSi single crystal at high pressures and strong magnetic fields. The forms of the resistivity and heat capacity curves at ambient pressure clearly indicate a first order nature of the magnetic phase transition in MnSi. Application of high pressure shows fast degradation of the first order features of the phase transition. The heat capacity and temperature derivative of resistivity demonstrates two notable features of the phase transition that disappear on pressure increasing. They are a sharp peak marking the first order phase transition and a shallow maximum, situated slightly above the critical temperature and pointing to the domain of prominent helical fluctuations. The shoulders of the heat capacity curves shrink with decreasing temperature suggesting that they arise from classical fluctuations. The sharp peak and shoulder at the heat capacity disappear simultaneously probably signifying the existence of a tricritical point and confirming the fluctuation nature of the first order phase transition in MnSi.The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2-40 K and magnetic fields to 7 Tesla. The magnetic phase diagram of MnSi appears to depend somewhat on the experimental setups, which is related to a difference in demagnetization factors arising due to the disc shape of the sample.The magnetic phase transition in MnSi in zero magnetic field is signified by a quasidiscontinuity in the c11 elastic constant, which varies significantly with magnetic field. It is notable that the region where the c11 discontinuity almost vanishes closely corresponds to the extent of skyrmion phase along the magnetic to paramagnetic transition. This implies that the c11 elastic constant is almost continuous through the transition from the skyrmion to paramagnetic phases. A recovery of the discontinuity of c11 and enhanced sound absorption occur at the crossing of the phase transition line and the line of minima in c11. The powerful fluctuations at the minima of c11 make the mentioned crossing point similar to a critical end point, where a second order phase transition meets a first order one.This suggestion seemingly contradicts to the above proposal of a tricritical point, following from the high pressure measurements and should be resolved in future studies.The skyrmion domain in the case of a “perpendicular” setup with a smaller demagnetization factor has a reduced temperature range, which suggests that the magnetic field inhomogeneity plays an important role in the skyrmion occurrence and, hence, opens a way of skyrmion manipulation.
Semiconductor nanostructures including quantum dots (QDs) and nanowires exhibit interesting linear and non-linear optical and dynamic properties that are important for many emerging applications including light energy conversion, solid state lighting, and optical imaging. At low excitation intensity, the excitons or charge carriers decay in a linear manner or follow first-order kinetics. At high excitation intensities, multiple excitons or electron-hole pairs are generated in one nanostructure, and non-linear decay or higher-order kinetic processes become important, such as Auger recombination or exciton-exciton annihilation. The threshold for observing non-linear processes is strongly dependent on the density of trap states in the nanostructure, with higher density of trap states corresponding to a higher threshold for observing non-linear processes. This is attributed to the fact that trap states need to be saturated before non-linear exciton-exciton annihilation can occur or become dominat. A high density of trap states results in faster trapping and recombination of electrons and holes, thereby requiring more photons to generate more excitons to saturate the trap states and lead to bandedge excitons for non-linear decays. Several examples of semiconductors nanostructures, including II-VI, IV, and III-V, studied using ultrafast laser techniques will be discussed and compared to gain insight into the non-linear decay processes and their implications in technological applications.
Ultrafast optical processes can occur in the intense electromagnetic near-fields of plasmonic nanostructures due to a variety of mechanisms, including, for example, nonlinear absorption effects, excitation annihilation effects, and energetic charge generation. When plasmonic metal nanostructures are combined with excitonic semiconductor materials to create hybrid structures, it can become possible to harness excitations created in the enhanced optical near-fields for energy conversion or nanophotonic applications. Furthermore, the creation of localized intense fields can create a high concentration of energetic charge that can catalyze multielectron processes related to the production of solar fuels. Because plasmonic particles generally have strong energy dissipation channels that can compete with utilization of energetic charge, such as thermalization of photoexcited electrons or damping through electron-phonon coupling, charge separation to the semiconductor must occur on an ultrafast time scale in order to effectively compete with these damping mechanisms. In this talk, I discuss ultrafast pump-probe measurements on hybrid metal nanoparticle - semiconductor systems and describe the impact of energetic charge separation from the metal to the semiconductor. Two types of structures, either metal-core/semiconductor shell or coupled metal nanostructures with a thin semiconductor spacer, are described. The results are analyzed in terms of the degree of hot electron generation in these nanostructures under varying conditions. The prospects for efficient use of energetic charge in these nanostructures are discussed. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
It is well known that silicon can transform into novel metastable crystalline phases, that are denser than the equilibrium diamond cubic phase, by the application of high pressure and often high temperatures under near equilibrium conditions such as in a diamond anvil cell. The recently developed method of ultra-short laser-induced confined microexplosions1 can potentially extend the range of possible new phases by initiating a highly non-equilibrium plasma state deep inside a bulk material that introduces local pressures that can exceed 1TPa and temperatures exceeding 105K. We have recently shown that ultra-high quenching rates from such a laser-induced plasma state can help to overcome kinetic barriers to the formation of new metastable silicon phases, while the surrounding pristine crystal confines the affected material and preserves it for further study2. In this presentation we have focussed a fs laser through a thick transparent silicon dioxide layer onto an underlying silicon substrate to produce confined micro-explosions within silicon and examined the microstructure and residual phases that are observed in the laser affected material. The structures of the observed new phases are determined from detailed analysis of electron diffraction patterns along with comparison with structures produced using an ab initio random structure search method. Such analyses have revealed six new energetically competitive phases, four tetragonal and two monoclinic. We show the presence of the tetragonal phases bt8 and st12, which have been predicted theoretically previously, but to date have not been observed experimentally. Additionally, the presence of the previously reported Si-VIII phase (of which the structure had not been determined) and the observation of two of our other predicted tetragonal phases are also indicated in the modified silicon zone. In some cases different laser conditions (absorbed energy) are shown to favour particular end phases. All of these new silicon end phases appear to be quite stable at room pressure and temperature. These findings pave the way for new materials with possible novel and exotic properties.1. E. G. Gamaly, S. Juodkazis, H. Misawa, B. Luther-Davies, L. Hallo, P. Nicolai and V. T. Tikhonchuk, Phys. Rev. B73, 214101 (2006).2. L. Rapp, B. Haberl, C. J. Pickard, J. E. Bradby, E. G. Gamaly, J. S. Williams and A. V. Rode, Nature Communications, in press (2015).
This talk will be based on recent publication on Formation of monatomic metallic glasses through ultrafast liquid quenching, Nature , Vol.512, 177 (2014). It has long been conjectured that any metallic liquid can be vitrified into a glassy state provided that the cooling rate is sufficiently high. Experimentally, however, vitrification of single-element metallic liquids is notoriously difficult. True laboratory demonstration of the formation of monatomic metallic glass has been lacking. Here we report an experimental approach to the vitrification of monatomic metallic liquids by achieving an unprecedentedly high liquid-quenching rate of 10^14Ks. Under such a high cooling rate, melts of pure refractory body-centred cubic (bcc) metals, such as liquid tantalum and vanadium, are successfully vitrified to form metallic glasses suitable for property interrogations. Combiningin situtransmission electron microscopy observation and atoms-to-continuum modelling, we investigated the formation condition and thermal stability of the monatomic metallic glasses as obtained. The availability of monatomic metallic glasses, being the simplest glass formers, offers unique possibilities for studying the structure and property relationships of glasses. Our technique also shows great control over the reversible vitrification-crystallization processes, suggesting its potential in micro-electromechanical applications. The ultrahigh cooling rate, approaching the highest liquid-quenching rate attainable in the experiment, makes it possible to explore the fast kinetics and structural behaviour of supercooled metallic liquids within the nanosecond to picosecond regimes.
Si24 is a new orthorhombic allotrope of silicon known to have a quasi-direct bandgap near 1.3 eV. Synthesis of Si24 is accomplished by a novel two step synthesis technique. In the first step, the precursor Na4Si24 is synthesized by reacting elemental sodium and silicon at high pressure of 8 GPa. As synthesized, Na4Si24 is a degenerate semiconductor. In the second step the precursor Na4Si24 is degassed by heating so that some of the sodium atoms are evacuated and thus the material approaches a non-degenerate semiconductor.In this work we study the role of the sodium dopants in determining the electronic and optical properties of Na4Si24. This is carried out by electron spin resonance (ESR) experiments as a function of temperature. ESR is a useful tool to investigate paramagnetic species, especially when present in dilute concentrations. In this case, except perhaps at very low temperatures, we are looking at the conduction electron resonance for the paramagnetic component to the magnetic susceptibility. The room temperature ESR spectrum acquired on a Na4Si24 sample reveals a peak close to g=2 with a width of about 7 Gauss. This conduction electron resonance appears as a result of essentially all of the electrons from the sodium being in the conduction band at room temperature. The spectrum at 80 K is very similar to that at 300 K, as is generally expected for conduction electron resonance. With further cooling down the sample below liquid Helium temperatures, it is likely that the electrons will condense onto individual sodium donor sites and thus lead to a hyperfine interaction with Na nuclei. This hyperfine spectrum can be interpreted to obtain the s- and p-character of the electronic wave function as well as the fraction of the wave function that is on the Na and the fraction on the Si backbone. The temperature dependence of resonance line parameters, in the temperature range 4 K to 300 K will be discussed.
Photomultiplier tube (PMT) is one of the primary components of water Cherenkov neutrino detection for the Long Baseline Neutrino Experiment (LBNE). Water-Cherenkov detector uses photomultiplier tube (PMT) glass bulbs, which creates a unique experimental condition for glass science. The detector houses an enormous (200 × 106 kg) high purity water tank held at constant temperature of 14°C, exerting a maximum hydrostatic pressure of approximately 890 kPa on thousands of bulbs of diameters in the range of 25-50 cm semi-hemispherical PMT bulbs lined in the inner walls of the detector. The PMTs in water-Cherenkov detector convert weak optical signal (Cherenkov radiation, wavelength asymp; 390 nm, Pavel Alekseyevich Cherenkov, Nobel Prize in Physics, 1958) produced from the interaction of a neutrino and water molecule. The detection of neutrinos requires reliable and stable PMTs for at least 20 years time period under the detector environments. We have investigated glass formulations, mechanical properties (Vickers indentation, four-point bend test, and ring-on-ring biaxial flexural strength test), optical transmission, and chemical strengthening of these glasses. We have also performed simulated accelerated testing of selected glasses in the environment relevant to neutrino detection for their chemical durability and mechanical properties. The simulated static high purity water test at 14°C and 25°C, is conducted in accordance to product consistency test (PCT) - B to compare the results with previous works with other glasses. In our work, we found coupling effects of pH and temperature on the chemical durability of the glasses. The ion release behavior suggests preferential ion release behavior of sodium (Na) and boron (B). The present talk summarizes the progress we have made and presents the prospects of the PMT glasses in future neutrino detector experiments.
Tungsten-Copper composites are materials that offer an interesting combination of thermophysical and mechanical properties: a coherent copper matrix - ensuring a high conductivity as well as an acceptable ductility- can be combined with the inherent hardness and low thermal expansion of tungsten in order to obtain a material with tailored thermal management and mechanical properties. Furthermore, these composites can easily be fabricated by liquid copper infiltration. For these reasons, tungsten-copper composites are of direct interest as heat sink materials used in highly loaded components of future nuclear fusion reactors where tungsten plasma facing components have to be joined to a high thermal conductivity material, such as copper.The aim of this study is the presentation of the microstructural and mechanical properties of novel tungsten-copper composites as a function of temperature. For this purpose, hardness and elastic modulus have been measured using instrumented and Vickers indentation and as well as resonance frequency analysis, respectively. Flexural strength and fracture toughness of the materials have been determined by means of 3-point bending tests on smooth and laser notched samples in the temperature range between room temperature and 800 °C under high vacuum atmosphere. Finally, a series of tensile tests have been conducted in the same temperature range and atmosphere.
Plasma facing components (PFCs) in nuclear fusion reactors operate under particularly extreme environments, resulting in the implantation of helium (He) atoms into these materials that impacts significantly their surface morphological and near-surface structural evolution. Tungsten (W) is an important PFC material due to its thermomechanical properties. In tungsten, such interstitial He atoms are very mobile and aggregate to form clusters of different sizes. The smallest of these helium clusters, containing n He atoms with n = 1-7, also are mobile and their diffusional transport mediates the evolution of surface morphology and the sub-surface helium gas bubble structure and dynamics.In this presentation, we report the results of a systematic atomic-scale analysis of the kinetics of small mobile helium clusters near low-Miller-index tungsten surfaces, aiming at a fundamental understanding of the near-surface dynamics of helium-carrying species in plasma-exposed tungsten. These small mobile helium clusters are attracted to the surface and migrate to the surface by Fickian diffusion and drift due to an elastic interaction force that constitutes the dominant contribution to the thermodynamic driving force for surface segregation. As the clusters migrate toward the surface, trap mutation (TM) and cluster dissociation reactions are activated at rates higher than in the bulk. TM produces W surface adatoms and immobile complexes of helium clusters surrounding W vacancies located within the lattice planes at a short distance from the surface. These reactions are identified and characterized in detail based on analysis of a large number of molecular-dynamics (MD) trajectories for each such mobile cluster near W(100), W(110), W(111), and W(211) surfaces. TM is found to be the dominant cluster reaction for all cluster and surface combinations, except for the clusters with n = 4 and n = 5 near W(100) where cluster partial dissociation following TM dominates. We find that there exists a critical cluster size, n = 4 near W(100), W(111), and W(211) and n = 5 near W(110), beyond which multiple W adatoms and vacancies are formed in TM reactions.The identified cluster reactions are responsible for important structural, morphological, and compositional features in plasma-exposed tungsten, including surface adatom populations, near-surface immobile helium-vacancy complexes, and retained helium content in the tungsten, which are observed in our large-space-scale MD simulations of He implantation and resulting near-surface evolution in tungsten. These features are expected to strongly influence the amount of hydrogen re-cycling and tritium retention in fusion tokamaks. The results of our study also contribute to the parameterization of continuum transport-reaction models for the coarse-grained modeling of such cluster dynamics.
Corrosion in aqueous systems containing CO2 is by far the most prevalent form of attack encountered for low carbon pipeline steel in upstream operations. CO2 is very soluble in water, where it forms carbonic acid. The corrosivity of the aqueous phase will be dependent on CO2 partial pressure (one to few bar), temperature (can exceed 100°C), solution chemistry (a few percent weight salt concentration), and in situ pH (typically 4-7). Microscopic localized corrosion, pitting, can form under this environment following a partial failure of a FeCO3 layer by chemical dissolution. Cracks can initiate from these pits because of the enhanced stress intensity around the pit and the aggressiveness of the chemistry inside the pit. This is especially true for fatigue loading conditions. The dividing point between pitting and long crack growth, the pit-to-crack transition, is of significant interest because a large percentage of the total lifetime of the pipeline steel can be consumed before this transition occurs. A systematic parametric study focusing on the effect of both mechanical and environment factors on the pit-to-crack transition has been performed. Artificial, but prototypic, pits were produced in X65 pipeline steel by a galvanostatic method in an NaCl solution. Fatigue loading by four-point bending at different frequencies and stress levels were used to induce a pit-to-crack transition. An alternative current potential drop (ACPD) technique was used to monitor the transition and subsequent crack growth. An autoclave system has been constructed in order to simulate the environment in pipelines at temperatures up to 125°C in a chloride-containing, CO2 saturated solution. Preliminary results shows that crack initiation occurs from “precursors” that form due to corrosion at the base of pits around the pit during corrosion fatigue. The underlying driving force of pit-to-crack transition and the roles of mechanical and environmental factors are being analyzed.
Nuclear fuel cladding is thin-walled tubes used to clad UO2 fuel in light water nuclear reactors. Zirconium based alloy have been generally used for nuclear fuel cladding due to their fairly good resistance to corrosion from water at elevated temperatures and a very low absorption cross-section of thermal neutrons. After Fukushima nuclear reactor accident, however, the development of new concept of fuel cladding is required because the aggressive oxidation and significant heat production of existing zirconium-based alloys in a high-temperature steam environment could significantly increase the risk of explosion caused by hydrogen gas.In this study, we aim to develop surface coated zirconium cladding with FeCrAl alloy to enhance safety of nuclear reactor during accident. FeCrAl alloy has various excellent properties such as excellent formability, high strength, and oxidation resistance at high temperature. For FeCrAl coating, cold spray technique was used and coated layer was formed at room temperature in air environment. To investigate efficiency of the FeCrAl alloy coating to act as oxidation resistant barrier, high temperature oxidation test as well as metallurgical investigation were carried out. FeCrAl coated zirconium-based alloy oxidized at 1200°C in a steam environment showed much lower weight gain compared to bare zirconium-based alloy samples. The microstructure of the oxide layers formed on coated zirconium tubes during high temperature oxidation test is characterized systematically by using various analyzing tools.
As good candidate for the conductors used in high field magnets, Cu-Nb composite wires fabricated by accumulative drawing and bundling (ADB) have drawn many attentions due to the high strength and high electrical conductivity. The Nb filaments significantly improve the mechanical strength of the composite wire, while reduce its electrical conductivity. Strain also has influence on both strength and conductivity. In this paper, microstructures of the wires with different strains were characterized by scanning electron microscopy (SEM). Electrical conductivities of the wires were measured by four-wire method. The influences by the content and size of Nb filaments and by the strains on the electrical conductivity were investigated. Results showed that by adding extra Cu core in the center of the Cu-Nb composite wires can get a good combination of strength and electrical conductivity.
Zr-4 is widely used as fuel cladding materials in water-cooled reactors, due to its low thermal neutron capture cross section and adequate ductility and corrosion resistance. However, degradation of zircaloy by waterside corrosion in harsh conditions severely limits the burnup and thus cycle life of nuclear fuels. There are mainly two types of waterside corrosion: oxidation and hydrogen up-take. Both of them will lead to severe degradation of mechanical performance during reactor operation. What&’s worse, such kind of corrosion will be dramatically accelerated during accident conditions due to higher temperatures, leading to catastrophic safety problems. For example, the waterside oxidation is one of the main causes of the Fukushima Nuclear Accident in 2011.Extensive researches have been done to understand the corrosion mechanism of zirconium; however, several key aspects of knowledge related to the corrosion mechanisms of zircaloy still remain unclear. So far, most microstrutural analysis of corrosion behaviors of zircaloy is performed ex-situ, making it difficult to know what is taking place during corrosion. We performed in-situ environmental transmission electron microscope (E-TEM) study of zircaloy-4 corrosion in air gas environments at ~500#730;C. This enables direct observation of the microstructure evolution during reaction, with a very high resolution (~0.8 Angstrom). The TEM sample was made by twin-jet polishing and thus there was no effect of radiation damage during sample preparation. The preliminary analysis of the results has shown that brittle oxide was formed during corrosion, and there were preferable sites for crack nucleation and propagation. What's more, compared to our experiment in pure oxygen, the oxide formed in air is more porous. Our study may help discover new corrosion mechanisms of zircaloy, and thus provide insights on prediction and prevention of zircaloy corrosion in nuclear reactors.
One of the major challenges in refining natural gas for energy production is the elimination of hydrogen sulfide (H2S) and carbon dioxide from gas streams. H2S is not only highly toxic, but also contributes towards the major economic challenge of pipeline corrosion. Currently, H2S is removed by absorption processes that utilize amine-based fluids, pressure swing adsorption, or cryogenic distillations.1 An alternative separation technology utilizes semipermeable polymeric membranes, which can be advantageous because of their smaller footprints and reduced energetic costs. With pressures up to 1000 psi and varying concentrations of H2S content, these membrane-based separations must survive harsh operating conditions. Thus, any material used must be resistant to physical and chemical degradation while demonstrating high permeabilities and good separation performance. Recent literature demonstrated crosslinked cellulose acetate membranes with unusually high CO2 and H2S permeabilities while retaining favorable separation performance.2 This prior work was expanded upon by investigating the role of silane crosslinkers on the cellulose acetate polymer to further optimize gas separation properties. Altering the silane structure promoted unique steric effects and differing polymer chain spacing between these systems, which varied polymeric free volume and had a pronounced effect on gas permeabilities. A series of silane-based crosslinkers were introduced into the CA structure and thermally and structurally characterized. The membranes&’ effectiveness for removing H2S at high pressures will also be discussed, which was determined by gas transport testing under industrially relevant feed compositions.(1) Baker, R. W. Ind. Eng. Chem. Res.2002, 41, 1393.(2) Achoundong, C. S. K.; Bhuwania, N.; Burgess, S. K.; Karvan, O.; Johnson, J. R.; Koros, W. J. Macromolecules (Washington, DC, U. S.)2013, 46, 5584.
Porous ceramics have been widely used under extreme environments due to their high strength, good thermal shock resistance, and excellent corrosion resistance.1 Recently, silicon aluminum oxynitride (SiAlON) ceramic, a solid solution of Si3N4 with AlN, SiO2, and Al2O3, attracted our interest because of its superior mechanical and physical properties for applications under extreme environments (i.e., high temperature, high pressure, excellent mechanical wear, and low PH). In particular, β-SiAlON which has the chemical formula Si6-zAlzOzN8-z where z varies between 0 - 4.2 has been extensively developed as an engineering ceramic due to its exceptional fracture toughness.2 However, in spite of its many unique properties, porous SiAlON production has not been scaled up sufficiently to meet industrially relevant quantities due to its high synthesis cost and the difficulty of manufacturing articles. Here, we report on a scalable two-step carbothermal reduction nitridation (CRN) method to synthesize mechanically robust SiAlON ceramic materials with controlled porosity levels. The morphologies and chemical compositions of the synthesized porous SiAlON ceramics were characterized using SEM, XRD, EDAX, and microprobe analysis. The facture toughness and flexural strength of the engineered porous SiAlON ceramics will be presented along with their microhardness and will be compared with data on state-of-the-art porous ceramic materials.Reference:1. R. W. Rice, Porosity of ceramics. Marcel Dekker, Inc., New York, 1998.2. K. H. Jack and W. I. Wilson, Nature, 238 28-29 (1977).
Due to the size- and shape-dependent properties, nanoparticles have been successfully used as functional building blocks to fabricate multi-dimensional (D) ordered assemblies for the development of ‘artificial solids&’ (e.g., metamaterials). At ambient pressure, entropy driven self-assembly of monosized or binary nanoparticles generally results in polycrystalline 2- or 3D close-packed arrangements, and extensive efforts have been made to develop structural perfection of nanoparticle arrays or ‘single crystal-like&’ domain structures with precise long range order for their definite advantages for electron transport. To date, fabrications of ordered nanoparticle assemblies have been relied on specific interparticle chemical or physical interactions such as van der Waals interactions, dipole-dipole interaction, chemical reactions, and DNA-templating, etc. Recently we have discovered a pressure-induced assembly method in which an external pressure has been utilized to engineer nanoparticle assembly and to fabricate new nanoparticle architectures without relying on specific nanoparticle interactions. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle arrays can be manipulated to reversibly shrink, allowing fine-tuning of interparticle separation distance. Under a uniaxial pressure field, nanoparticles are forced to contact and coalesce, forming hierarchical nanostructures. Depending on the orientation of the initial nanoparticle arrays, 1-3D ordered nanostructures including nanorod, nanowire, and nanoporous network can be fabricated through the pressure-induced self-assembly method. Guided by computational simulations, we were able to rationalize the pressure-induced self-assembly of nanoparticle arrays for predictable nanostructures. Moreover, we discovered for the first time a transition from an ordered polycrystalline nanoparticle mesophase to quasi-single crystalline nanoparticle lattices induced by PDA process. Exerting pressure-dependent control over the structure of nanoparticle arrays provides a unique and robust system to understand collective chemical and physical characteristics and to develop novel electronic and photonic behavior for energy transduction related applications.Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
High pressure is a useful tool to study, manipulate and ultimately control exotic properties in energy materials. Among many properties, the insulator to metal transition (IMT) of VO2 has been the topic of many theoretical and experimental investigations. This phase transition property makes VO2 an attractive material for smart windows. Indeed, traditional smart windows and solar cells cannot be combined into one device for energy saving and electricity generation. A VO2 film can respond to the environmental temperature to intelligently regulate infrared transmittance while maintaining visible transparency, and can be applied as a thermochromic smart window. VO2 is a monoclinic crystalline structure, which is insulating and transparent to infrared light, but it becomes a tetragonal crystalline structure that is metallic and reflective to infrared light in the metallic phase.However, the nature of the low-temperature insulating phase of VO2 is still not well understood. The evolution of the local structure of VO2 was investigated across the pressure induced insulator to metal transition by means of pair distribution function measurements. We found that the V-V interatomic distances remain well distinguished in the high pressure metallic phase, demonstrating that the IMT is not a Peierls-type transition. A clear octahedra symmetrization is observed which suggests a close correlation between structural modifications and the metallization process.
We report on high-pressure and high-temperature synthesis of mesoporous aluminosilica with crystalline pore walls. Mesoporous aluminosilica/carbon composite was used as starting material for high-pressure synthesis. The mesoporous aluminosilica was synthesized using amphiphilic Pluronic P123 triblock copolymers as structure directing agent and aluminosilica precursor via sol-gel route followed by pH adjustment. It was observed that the aluminosilica/carbon nanocomposite was crystallized at a pressure of 2 GPa and a temperature of 550 oC. Combustive removal of carbon phase in air leads to the formation of mesoporous aluminosilica material as confirmed by transmission electron microscopy and nitrogen adsorption technique. The synthesized material are hydrothermally stable at least up to 800 oC for 2h without any pore shrinkage indicating its potential to use in hydrocarbon cracking.
Microgravity condition provides a particular opportunity to produce an extraordinary microstructure and optimized mechanical properties. In this paper, fine Al-Ag-Ge alloy particles were prepared under microgravity condition provided by containerless drop tube. Their micromechanical properties were analyzed by means of nanoindentation and frictional sliding techniques. As particle diameter decreased, the thickness of surface (Ge) layer formed by Marangoni motion decreased, and the distribution of (Ge) phase became more homogenous. The microstructure transformed from dendrites plus pseudobinary eutectic, to pseudobinary eutectics, and eventually to anomalous ternary eutectic. The microhardness and yield strength of the alloy droplets were evidently improved particularly with the decrease of particle size, mainly attributed to the microstructure refinement and the homogenous distribution especially of the hardening (Ge) phase, and their increasing trends categorized into three regimes were in accord with the microstructural transition. Consistently, the increase of strain hardening exponent while the decrease of normalized pile-up height and friction coefficient, all showed a linear correlation with particle size.
Ternary transition metal nitrides have received attention as substitutes for binary nitrides due to their excellent coating properties such as high hardness and oxidation resistance. Among them, Ti1-xZrxN is more stable solid solution than other ternary nitrides because it consists of face-centered cubic lattice of TiN and ZrN. The effects of Zr substitution to TiN lattice have been investigated focused on mechanical properties with thermal stability. Thermal shock testing method using pulsed laser ablation can simulate practical conditions of the hard coatings since repeated thermo-mechanical loads apply to the coating surface and result in cracking or delamination.Thermal shock behaviors of Ti1minus;xZrxN coatings by pulsed laser ablation were investigated in terms of local atomic structure. Increase of lattice parameter and hardness of the coatings was identified according to Zr addition. Microstructural observation for the degraded surfaces and cross-sections after the thermal shock showed that cracks and delamination of the coating/substrate systems were inhibited as the ZrN formation increased. Extended x-ray absorption fine structure analysis revealed that the increase of hardness attributed to lattice distortion caused by Zr substitution, and the decrease of interatomic bonding length increased thermal conductivity which improved thermal shock resistance of the coatings.
The microstructure and properties of refractory high-entropy alloys based on compositions drawn from the Mo-Nb-Ti-V-Zr system are investigated using a combination of experimental measurements, semi-empirical and first-principle computations. These multi-principal component alloys are attracting increasing attention for applications in the aerospace, biomedical, and energy production and storage industries. The current study focuses on refractory systems, with the objective of designing an alloy resistant to both high temperatures and radiation damage for possible service in generation III+ and IV reactors. Samples were produced by arc-melting and characterized by X-ray diffraction and electron microscopy before and after intense ion bombardment. Disordered body-centered cubic crystal structures were observed, in some cases with significant coring due to dendritic solidification. Ab-initio methods were also used to model the response to displacive radiation damage on an atomistic level. Split interstitials dominate the interstitial type observed in the modeled systems and all vacancies display unfavorable formation enthalpies. It is thought that the high configurational entropy may promote superior point defect recombination compared to other materials allowing for a good resistance to radiation damage.
The operation of existing fission reactors provides nearly 20% of the electricity in the US. Radiation induced swelling causes dimensional distortion and embrittlement, and is a life-limiting materials issue for structural materials in nuclear power reactors. To extend the lifetime of current nuclear power plants, it is necessary to develop radiation tolerant material without significant structural changes and serious thermal/mechanical degradation under harsh environments. In our previous efforts, a new kind of radiation tolerant materials: amorphous silicon oxycarbide (SiOC) has been developed. The material showed no evidence of crystallization up to a temperature of 1200 °C and an annealing time of 2.0 hours. SiOC alloys irradiated with damage level up to 20 displacement per atom (dpa) at both room temperature and 600 °C, the highest temperature tested so far, showed no indication of crystallization or void formation. The highlights of our previous work support the hypothesis that some amorphous alloys are radiation-indifferent; within an envelope of irradiation conditions, radiation-induced damage anneals out as fast as it is created, allowing these alloys to persist indefinitely in an externally driven steady-state, with time-invariant structure and properties. To step toward realistic application, an amorphous SiOC/crystalline Fe composite has been developed and examined its irradiation stability. Our preliminary results showed that the structure of Fe/SiOC composite materials remained unchanged after ~1 dpa irradiation. The results demonstrate that Fe/SiOC crystalline/amorphous interfaces and Fe grain boundaries act as efficient defects sinks, promoting interstitials and vacancies recombination.