Xinghang Zhang Texas A&M University
Oliver Kraft Karlsruhe Institute of Technology
Michael Demkowicz Massachusetts Institute of Technology
Meimei Li Argonne National Laboratory
T1: Radiation Damage in Nano-Composites
Monday PM, November 29, 2010
Room 206 (Hynes)
9:30 AM - **T1.1
Development of Ultra-high Strength Radiation Damage-tolerant Nanocomposites via Atomic-scale Design of Interfaces.
Amit Misra 1 Show Abstract
1 , LANL, Los Alamos, New Mexico, United States
The combination of high strength and high radiation damage tolerance in nanolaminate composites can be achieved when the individual layers in these composites are only a few nanometers thick and therefore these materials contain a large volume fraction associated with interfaces. These interfaces act both as obstacles to slip, as well as sinks for radiation-induced defects. The morphological and phase stabilities of these nano-composites under ion irradiation are explored as a function of layer thickness, temperature and interface structure. Using results on model systems such as Cu-Nb, Ag-V, etc we highlight the critical role of the atomic structure of the incoherent interfaces that exhibit multiple states with nearly degenerate energies in acting as sinks for radiation-induced point defects. Reduced radiation damage also leads to a reduction in the irradiation hardening, particularly at layer thickness of approximately 5 nm and below. The strategies for design of radiation-tolerant structural materials based on the knowledge gained from this work will be discussed. This research is funded by DOE, Office of Basic Energy Sciences.
10:00 AM - T1.2
Non-saturable Sinks at Grain Boundaries in Nanocrystalline Mo - Molecular Dynamics Simulations.
Yongfeng Zhang 1 2 , Hanchen Huang 2 , Paul Millett 1 , Michael Tonks 1 , Dieter Wolf 1 , Simon Phillpot 3 Show Abstract
1 Fuel modeling and simulation, Idaho National Lab, Idaho Falls , Idaho, United States, 2 Mechanical Engineering, University of Connecticut, Storrs, Connecticut, United States, 3 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
The defect accumulation in nanocrystalline Mo under electron irradiation is studied using molecular dynamics (MD) simulations. The nanocrystalline Mo is simulated using a bi-crystal model with <100> ∑29 twist boundaries as representation of high angle/energy grain boundaries. By assigning a kinetic energy of 40 eV to randomly selected Primary Knock-on Atoms (PKA), electron irradiation is simulated under 2000K until the defect concentration reaches steady state (in < 100 ns). The development of defect concentration obtained from MD simulations agrees well with the prediction of rate theory with the parameters derived from MD simulations. With regard to the constant sink strength used in rate theory, this result suggests that high angle/energy grain boundaries are non-saturable sinks for point defects under electron irradiation. In supporting, structural analysis of both short range order by pair correlation function and long range order by planar structure factor shows no change in the grain boundary structure.
10:15 AM - T1.3
On the Interaction of Radiation-induced Point Defects with Semicoherent Heterophase Interfaces.
Michael Demkowicz 1 , Kedarnath Kolluri 1 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
We use atomistic modeling to investigate the relation between atomic structure of semicoherent heterophase interfaces and their interaction with radiation-induced point defects. Using Cu-Nb interfaces as a model system, we show that misfit dislocations present at such interfaces play a governing role in point defect absorption and migration. Interactions between point defects and the interface misfit networks can be described quantitatively within dislocation theory. Implications of these insights for predicting point defect interactions with other semicoherent interfaces like Cu-V are discussed.This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
10:30 AM - T1.4
Modeling He Behavior at Cu-Nb Interfaces Using a Predictive Potential.
Abishek Kashinath 1 , Michael Demkowicz 1 Show Abstract
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
We use a force difference matching technique to construct a potential describing He interactions with Cu and Nb. This potential correctly predicts He defect energies despite not having been fitted to them. Its form can be motivated directly from electronic charge density distributions obtained from density functional theory (DFT) calculations. We apply this potential to the study of He trapping, migration, and clustering at Cu-Nb interfaces. The influence of implanted He on the mechanical properties of Cu-Nb multilayer nanocomposites will be discussed.
10:45 AM - T1.5
Deformatio in Trilayer Nanometallic Multilayers with Increased Interface Area for Point Defect Sinks.
David Bahr 1 , IIoannis Mastorakos 1 , Aikaterini Bellou 1 , Amit Misra 2 , Hussein Zbib 1 Show Abstract
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
he proper design of nanoscale multilayer metallic (NMM) composites, can lead to materials with remarkable mechanical and electrical properties, and with good resistance to harsh environments. When designed to have high interface-to-volume ratio and chemically and morphologically stable interfaces, they can posses high resistance to fatigue damage as well as high tolerance to irradiation damage. Understanding the physical origin of these phenomena and their relation to the interface structure, chemical composition and morphology, as well as to the underlying dislocation mechanisms, is critical in designing such nanocomposites with desired properties for various applications. NMM composites are typically made of bimetallic systems with either coherent (like the fcc/fcc CuNi systems with cube-on-cube orientation) or incoherent interfaces (as the fcc/bcc CuNb system with the Kurdjumov-Sachs orientation). The coherent interface has continuous atomic arrangement and slip systems, while an incoherent interface has discontinuous slip systems. In general, coherent systems are more ductile while incoherent systems are stronger. In addition, studies on CuNb revealed that incoherent interfaces can also act as sinks for radiation-induced defects, making these systems candidates for use in radiation environments. However, at layer thicknesses less than 5 nm there appears to be a degradation of streingth in these mateirals. Therefore, ff the fraction of interfaces could be increased while maintaining the strength of 5 nm layers, the material should be more capable of withstanding radiation environments. In this respect, we studied trimetallic NMM systems that combine the two types of interfaces. These systems possess the superior properties of the two types of bimetallic systems discussed above. Guided by results from molecular dynamics (MD) investigations as well as dislocation dynamics analyses (DD), multilayer thin films of CuNbNi with various layer thickness where fabricated and tested using nanoindentation. The results show that the trimetallic system is stronger than the CuNi system, exhibits significant strain hardening, displays moderately ductile behavior, and can be as strong as the CuNb system at an optimum layer design - the optimum design is a trilayer CuNiNb system with a layer thickness scheme of 3nm/5nm/5nm respectively. The presentation will focus on both similarities between the simulation and experiments, as well as using information from the simulation to identify a hardening mechanism present in the trilayer film which does not appear in the bilayer films; cross slip in the CuNi layer which spans both FCC films.
11:30 AM - **T1.6
Radiation Resistant Materials Through Self-organization.
Nhon Vo 1 , Xuan Zhang 1 , Brad Stumphy 1 , Yinon Ashkenazy 1 , Daniel Schwen 1 , Pascal Bellon 1 , Robert Averback 1 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States
While the strategy for developing radiation resistant materials has long been known, i.e., providing a high density of traps or unbiased sinks for point defects and gas atoms, finding alloys that are able to retain their initial microstructures after prolonged exposures to the extreme conditions envisioned for advanced nuclear power sources has been a difficult challenge. Nanocrystalline materials, for example, provide the necessary high density of sinks, but they are generally unstable to coarsening at high temperatures and levels of irradiation. At Illinois, we have explored the concept of using alloys that self-organize on a mesoscopic length scale under irradiation and form stable, two-phase structures. In this way, the nanoscale structure becomes the steady state of the system, with no tendency to coarsen. We illustrate this idea using a series of immiscible, binary Cu alloys. We show that such alloys are stable under irradiation to doses over 100dpa and temperatures as high as 0.85Tm (Tm = melting temperature). We also show that this concept can be vastly broadened by the additions of ternary and quaternary alloying elements. Computer simulations are presented to help provide an understanding of the kinetic response of these materials to irradiation.
12:00 PM - T1.7
Effect of Misfit Dislocation Density on the Radiation Damage Resistance of Cr-Mo/MgO Interfaces.
Richard Kurtz 1 , Renee Van Ginhoven 1 , Howard Heinisch 1 , Alan Joly 1 , Tiffany Kaspar 1 , Shutthanandan Vaithiyalingam 1 , Chongmin Wang 1 , Brian Wirth 2 Show Abstract
1 , Pacific Northwest National Laboratory, Richland, Washington, United States, 2 , University of Tennessee, Knoxville, Tennessee, United States
The interaction of radiation with materials controls the performance, reliability, and safety of many structures in nuclear power systems. Improvements in radiation damage resistance may be attainable if methods can be found to manipulate interface properties to give optimal interface stability and point defect recombination capability. Ion irradiation experiments, a combination of physical measurements and atomistic modeling are being used to study the effect of misfit dislocation density on radiation damage resistance. Epitaxial thin films of metallic Cr, Mo, and their alloys are deposited on MgO(001) substrates. By controlling the composition of Cr-Mo alloys, the lattice mismatch with MgO can be adjusted so that the misfit dislocation density varies over a wide range. A variety of damage states are produced by ion irradiation of these films at room temperature using 1 MeV Au+ ions to fluences ranging from 1016 to 1021 ions/m2. The transparent MgO substrates allow second harmonic generation (SHG) spectroscopy to be used to investigate defect concentrations near interfaces in real-time. High-resolution transmission microscopy (HRTEM) and Rutherford backscattering spectroscopy (RBS) measurements are used to examine mixing, defect structures, defect distribution, and interface evolution following irradiation. Positron annihilation spectroscopy (PAS) is used to study the efficiency of defect trapping. Multiscale modeling methods combined with TEM, PAS, and SHG theory are being developed to relate signal response to interface structure before and after irradiation. The integration of these methods and the results of the initial irradiation experiments are described.
12:15 PM - T1.8
In-situ Transmission Electron Microscopy Observation of KeV-ion Irradiation of Multi-walled Carbon Nanotubes and Single-walled Boron Nitride Nanotubes.
Amelia Liu 1 , Raul Arenal 2 Show Abstract
1 School of Physics, Monash University, Clayton, Victoria, Australia, 2 Laboratoire d'Etude des Microstructures, CNRS-ONERA, Chatillon France
Ion beam irradiation of nanotubes (NTs) has been investigated as a scalable technique to alter atomic structure, and so tune the electronic, mechanical and acoustic properties [1 and references therein]. Studies of the nature of the ion-induced defects in these materials and the evolution of the structure over time are important to enable the use of this tool, and also ascertain the stability of NT-based devices in high-radiation environments. In the eV - keV energy-range, it is generally thought that single vacancies and subsequent adatoms from knock-on damage are the predominant form of defect to arise from ion irradiation. After the initial collision event, the atomic re-arrangements and defect accumulation lead to many of the reported phenomena such as stable vacancy and Stone-Walls defects, amorphization, ion beam welding and diameter shrinkage .We report on an in situ transmission electron microscope (TEM) ion irradiation experiment conducted at the IVEM-Tandem facility at Argonne National Laboratory. The TEM was operated at a voltage of 100 keV and images and video-rate data were collected using a Gatan Orius 1000 CCD camera (4008 x 2672). Multi-walled carbon NTs (MWNTs) and single-walled boron nitride NTs (SWBNNTs) were irradiated with 30 keV Kr+ ions (1.25 e10 ions/cm2/sec) to doses of up to 1e15 ions/cm2 at room temperature. All regions of both specimens displayed ion-beam disordering and eventual amorphization, at roughly the same dose (5e14 ions/cm2). This suggests that the accumulation of defects was the major mechanism for structural change, and that the ion beam interactions were similar for both materials.Diameter shrinkage was also observed for both MWCNTs and SWBNNTs, and at high ion dose, this was responsible for the eventual breakage of the NTs. The irradiation of certain areas was observed using video-rate image capture. In a particular SWBNNT and at low ion dose, a catastrophic ion scission event (< 33 msec) was seen, suggestive of a collision cascade. Thus we suggest that ballistic interactions might occur with a small probability between ions at this energy and NTs . Multi-wavelength ex situ Raman measurements were also carried out on these samples at various stages of the irradiation and we correlate these results with the TEM images. These results suggest that ballistic interactions need to be considered for NT-based devices in high-radiation environments.The in situ ion irradiating TEM was conducted at the IVEM-Tandem facility at Argonne National Laboratory in the Electron Microscopy Center. We thank P. Baldo and E. Ryan for their assistance in these measurements. A. V. Krasheninnikov and F. Banhart. Nature Materials, 6, 723, (2007) A. V. Krasheninnikov, K. Nordlund, M. Sirvio, E. Salonen and J. Keinonen. Phys. Rev. B, 63, 245405, (2001) A. C. Y. Liu and R. Arenal, to be published.
12:30 PM - **T1.9
Nanostructured Engineering Alloys for Nuclear Application.
Peter Hosemann 1 2 , Andrew Nelson 2 , Erich Stergar 1 , Joris Van den Bosch 2 , Stuart Maloy 2 , Osman Anderoglu 2 Show Abstract
1 Nuclear Engineering, University of California Berkeley, Berkeley, California, United States, 2 material science and engineering, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
In advanced nuclear applications, materials experience a wide range of extreme environments. High temperature and a corrosive environment are present in addition to a high dose radiation field causing displacement damage in the material. In recent times it has been shown that nanostructured ferritic alloys (NFA’s) such as advanced oxide dispersed strengthened (ODS) steels are suitable for this environment as they tolerate high dose irradiation without significant changes in microstructure or relevant mechanical properties. Ion beam irradiation is a fast and cost effective way to induce radiation damage in materials so fundamental studies of the materials behavior under radiation can be explored. However, ion beam irradiation has limited penetration depth and small scale mechanical testing has to be performed in order to measure the changes in mechanical properties of materials of interest. In addition to characterizing changes in mechanical properties, focused ion beam based sample preparation can be used to prepare Transmission Electron Microscopy (TEM) and Local Electrode Atom Probe (LEAP) samples of a shallow ion beam irradiated material allowing for microstructural characterization. Small scale mechanical testing such as nanoindentation and micro compression testing with TEM characterization of ion beam irradiated materials allows a full assessment of the materials’ behavior under radiation environment. In this work several different ODS materials have been irradiated using proton and combined proton and He beams up to 1 dpa. Nanoindentation and TEM investigations were performed in order to assess the changes in properties of these alloys due to irradiation.The same techniques were applied to intermetallic nanostructured alloys in order to investigate the effectiveness of the metal-intermetallic interphase to provide defect sinks for He and radiation damage. It was found that irradiation can cause the formation of intermetallic particles even at room temperature while increasing the material strength significantly without reducing its ductility to a unacceptable level for engineering applications.
T2: Radiation Damage and Synthesis of Nano-Materials under Harsh Conditions
Monday PM, November 29, 2010
Room 206 (Hynes)
2:30 PM - **T2.1
Recent Progress in Developing Irradiation Tolerant Nanostructured Ferritic Alloys.
G. Robert Odette 1 2 , Takuya Yamamoto 1 , Nicholas Cunningham 1 , Yuan Wu 2 , Erich Stergar 2 , Auriane Etienne 2 Show Abstract
1 Mechanical Engineering Department, UC Santa Barbara, Santa Barbara, California, United States, 2 Materials Department, UC Santa Barbara, Santa Barbara, California, United States
Nanostructured ferritic alloys (NFAs) have the potential to make transformational contributions to developing advanced sources of fission and fusion energy. NFAs are Fe-Cr based ferritic stainless steels that contain an ultrahigh density of Y-Ti-O nanofeatures (NFs). The NFs provide for both outstanding high temperature properties and remarkable tolerance to irradiation damage, including mitigation of the degrading effects of transmutation product helium. Indeed, the results of recent in-situ injection irradiation experiments suggest that the NFs can be tailored to effectively transform helium from a liability to an asset. A detailed model demonstrating the physical basis for providing extraordinary irradiation tolerance is described. The model tracks the transport and fate of helium in realistic alloy microstructures and describes the role of the resulting helium bubbles in mediating the accumulation and consequences of simultaneous displacement damage. The talk also briefly outlines recent progress on determining the structures and compositions of the NFs and their long term, high temperature thermal stability.
3:00 PM - T2.2
Ultrafine and Nanocrystalline Tungsten as a Radiation-tolerant Plasma-facing Material in Nuclear Fusion Devices.
Osman El-Atwani 1 2 , Mert Efe 1 , Dat Quach 4 , Bryan Heim 3 , Dan Rokusek 3 , Eric Stach 1 2 , Jean Paul Allain 1 2 3 Show Abstract
1 Materials Engineering, Purdue University, Lafayette, Indiana, United States, 2 Birck Nanotechnology Center, Purdue University, Lafayette, Indiana, United States, 4 Chemical Engineering and Materials Science, University of California Davis, Davis, California, United States, 3 Nuclear Engineering, Purdue University, Lafayette, Indiana, United States
Tungsten is one of the primary material choices in the upcoming International Tokamak Experiment Reactor (ITER) to be built in in Cadarache, France. As a plasma-facing material in the divertor region, tungsten and its ability to withstand > 1-5 MW/m2 heat-flux and particle-flux deposition will be a major technological for future plasma-burning devices. Although tungsten has a high erosion threshold, helium products will be a pre-dominant impurity in burning plasmas. Therefore, tungsten will be exposed to helium fluences > 10^18 cm2, which is known to induce helium bubbles and cavities in the tungsten surface structure. This can lead to enhanced surface erosion and generation of particle dust, a serious issue for ITER and other future burning plasma fusion device. As a possible solution to this problem, fine, ultrafine and nanocrystalline grained tungsten can be of higher radiation tolerance. In this project, Micro-powders of tungsten (average size of 1µm) were sintered using Spark Plasma Sintering (SPS) at different sintering conditions. High relative density of low impurity and high Vickers Hardness of the consolidated samples were obtained. The samples showed a multi-modal distribution similar to the powder distribution, and the average grain size increased as the sintering temperature increased. The samples, which have different grain sizes and distributions, are irradiated with helium ions at high fluencies (≥ 10^18 ions/cm2) for radiation tolerance investigation. Bulk samples are also irradiated at the same conditions for investigating the effect of large grain area on irradiation tolerance. In addition, tungsten powders with a core-shell structure (tungsten-tungsten carbide) are prepared via carburization of tungsten powders. These powders are believed to lead higher densities at low average grain size upon sintering with SPS at high temperatures. Irradiation tolerance investigation is also performed on consolidated samples from the core-shell powders. The samples are characterized with Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Transmission Electron Microscopy. Surface analysis is perfoemed using In-Situ Ion Scattering Spectroscopy and X-ray Photoelectron Spectroscopy under ultrahigh vacuum conditions.
3:15 PM - T2.3
Dose and Size Dependent Radiation Damage in Cu/V Nanolayers.
Xinghang Zhang 1 , E. Fu 1 , H. Wang 2 , L. Shao 3 Show Abstract
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 2 Electrical Engineering, Texas A&M University, College Station, Texas, United States, 3 Nuclear Engineering, Texas A&M University, College Station, Texas, United States
We present layer thickness and dose (0.5-12 dpa) dependent He ion radiation damage in sputtered Cu/V nanolayers with individual layer thickness, h, varying from 1 to 200 nm. Layer interfaces retain under all radiation conditions. Peak bubble density increases monotonically with dose, and it is much lower in Cu/V 2.5 nm than that in Cu/V 50 nm multilayers. A similar radiation hardening trend is observed in all multilayers irradiated at different doses, i.e., radiation hardening decreases with decreasing layer thickness. Mechanisms through which interface interacts with radiation induced point defects and the consequence on the evolution of microstructures and mechanical behavior are discussed.
3:30 PM - T2.4
Microstructure and Mechanical Properties of He Irradiated Fe Film.
Kaiyuan Yu 1 , Haiyan Wang 2 , Xinghang Zhang 1 Show Abstract
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 2 Electrical Engineering, Texas A&M University, College Station, Texas, United States
Sputter-deposited Fe films prepared on various substrates were irradiated by 100 keV helium ions. SRIM simulation was utilized to simulate the radiation profile. Transmission electron microscopy (TEM) studies revealed that Fe films consisted of nanocrystalline grains on the scale of tens of nanometers. Radiation induced defects clusters, such as He bubbles, were examined with TEM experiments. Nanoindentation was performed to probe hardness evolution of irradiated Fe films. This study provides insight on grain size dependent radiation damage in Fe films.
3:45 PM - T2.5
Effect of Low Energy Proton Irradiation on Single Layer InAs/GaAs Quantum Dot Heterostructure.
R. Sreekumar 1 , Sudipta Das 1 , Saumya Sengupta 1 , Subhananda Chakrabarti 1 , S. Gupta 2 Show Abstract
1 Center of Nanoelectronics, Department of Electrical Engineering , Indian Institute of Technology Bombay, Mumbai, Maharashtra, India, 2 Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
Optoelectronic devices those work under extreme conditions such as in outer space, are required to be more tolerant since the radiations can cause degradation of their efficiency. Recent studies have revealed that the tolerance of quantum dot (QD) based devices is higher than their quantum well counterpart towards energetic radiations due to the 3D confinements of carries in QD. This opened up a new scope of application based on QD devices for outer space/satellites, where these devices are exposed to energetic radiations (that consists of 91% protons, 8% helium and 1% heavy ions). In the present study we report enhanced photoluminescence (PL) efficiency (nearly an order) for the first time without any post thermal annealing treatment on InAs/GaAs QD, employing proton irradiation. Molecular beam epitaxial grown self assembled InAs/GaAs QD samples are exposed to protons of energy 50 keV with fluence ranging from 5x1012 to 2x1015 ions/cm2. PL analysis reveals the enhancement of PL efficiency of about an order (~10 fold at 145 K) up on irradiation. The PL emission increases with proton fluence up to 2x1013 ions/cm2 and beyond that PL efficiency degrades. Optimized dose of proton stream probably reduces non-radiative recombination of carriers by annihilating defects inside the dots and hence the enhancement of PL intensity. Moreover PL emission persists up to room temperature for the samples which are irradiated with fluence up to 2x1014 ions/cm2. Interestingly the sample irradiated with 2x1013 ions/cm2 exhibits only an enhancement in PL of 1.8 fold at 9 K, but with increasing temperature, PL emission increases and attains ~10 fold increase with respect to un-irradiated sample at 145 K. A blue shift in emission wavelength of only about ~17 nm was also observed on proton irradiation. Structural analysis carried out using X-ray diffraction reveals the FWHM of the reflection from InAs/GaAs QD increases from 0.0638 to 0.0981 on increasing the proton fluence from 2x1013 to 2x1015 ions/cm2. This indicates that the size of the InAs/GaAs QD is reduced on proton irradiation causing the blue shift in PL emission peak. Un-irradiated sample exhibits two activation energies, 4 meV (defect related) and 162 meV (thermal release). The activation energy related to the thermal release increases to 337 meV for the sample irradiated with 2x1013 ions/cm2 fluence and then decreases to 137 meV which is irradiated with 2x1015 ions/cm2 fluence. This result concludes that optimum proton irradiation meditates the defects of the un-annealed sample resulting in the enhancement of quantum confinement of the carriers. But beyond the optimized dosage, excess irradiation of proton itself creates defects, and thereby reducing the quantum confinement. This study proves that the InAs/GaAs system is not only tolerant to 50 keV protons but also their optoelectronic property improves if subjected to optimized flux of proton radiation. DST, India is acknowledged.
4:30 PM - T2.6
Coming Full Circle: The Application of Microtechnology Techniques to Evaluate Bulk Materials.
David Read 1 , Nicholas Barbosa 1 Show Abstract
1 Materials Reliability Division, National Institute of Standards and Technology, Boulder, Colorado, United States
Testing the effect of specialized or harsh environments, for example, nuclear or fusion reactor conditions, on the mechanical properties of bulk structural materials is difficult because of the environment of interest and often limited by the availability of specimen material. The quest for the capability to test smaller specimens in harsh environments is ongoing, because advances in this capability can address both of these issues. We have recently demonstrated the adaptation of apparatus and techniques from the “micro” world to the tensile testing of very small specimens of bulk materials. We tested stainless steel 302 in the full hard condition in the form of tensile specimens with gauge sections 300 μm long by 70 μm wide by 25 μm thick. Individual specimens with ‘bow-tie’ geometry were produced from foil nominally 25 μm thick by photofabrication. Wedge grips with a taper matching that of the specimen grip sections were etched into a compliant silicon frame, which had been produced from a silicon wafer by bulk micromachining techniques. The force, which had a range of about 2 N for these specimens, was applied to the frame externally, by a three-axis actuator with a smallest measurable step of 50 nm. It was measured using a custom load cell, which was calibrated by use of dead weights. The strain was measured by sub-pixel digital image correlation, using images with a resolution of approximately 0.8 μm per pixel. Yield strength, ultimate tensile strength, Young’s modulus, and a measure of total elongation were extracted from the stress-strain records. Results compared favorably with vendor data and with modulus and hardness measurements by nanoindentation. For these specimens, the greatest source of uncertainty in the measured properties was the measurement of the cross-sectional area. This uncertainty is of the order of 5 percent. Possible errors in the strain measurements, particularly relating to out-of-plane displacements, were investigated carefully. Out-of-plane displacements were limited to below our measurement capability for such displacements. The final Young’s modulus values were obtained by interrupting the classic monotonic loading pattern of a tensile test with unloading and reloading steps. The resulting modulus values were somewhat lower than values typically quoted for bulk 300-series stainless steels. The particular values obtained are attributed to deviations of the microstructure of the present specimen material relative to the more ideal material usually used for Young’s modulus measurements. This technique appears promising for further development of “in situ” testing and for situations where the supply of specimen material is extremely limited.
4:45 PM - T2.7
A Fast 3D Phase Field Simulation for Irradiation Effects.
Daniel Schwen 1 , Arnoldo Badillo 2 1 , Robert Averback 1 , Pascal Bellon 1 Show Abstract
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Laboratory for Thermal Hydraulics, Paul Scherrer Institut, Villingen Switzerland
A phase field model for the simulation of alloys under irradiation is presented. The kinetic equations of the model are derived from an atomistic description, using a coarse graining in space and time to define the continuous variables. As opposed to phenomenological approach in current phase field models our model possesses absolute time and space scales, which are important when studying irradiation effects with well defined intrinsic scales, such as collision cascades.Some irradiation effects cannot be captured by simply continuum variables. This is especially true for the primary recoil spectrum, which plays a key role in microstructural evolution and it would be missed. It requires either to include fluctuations or to retain some information at discrete level. Our approach is combining continuous field variables with discrete objects such as defect clusters, which influence the defect concentration and diffusion in their neighborhood. Different methods of coupling discrete objects and continuous fields are explored.The algorithm is adapted to modern massively data-parallel hardware (GPUs), making large three dimensional simulation feasible. We present applications of the model in pure systems to study the evolution of isolated and clustered point defects, as well as irradiation-induced segregation and irradiation-induced precipitation in alloy systems.
5:00 PM - T2.8
Simple Model for Radiation Induced Creep in Nano-crystalline Metal.
Yinon Ashkenazy 1 2 , Robert Averback 2 , Pascal Bellon 2 Show Abstract
1 Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem Israel, 2 Department of Materials Science and Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois, United States
While creep and radiation damage in nano-grained samples have been widely studied before, the different time scales involved in these processes present a challenge in simulating them in unison. A simple atomistic model is proposed for describing creep of a nano-crystalline sample under irradiation. The model is based on introducing defects at grain boundaries and simulating short-time relaxations following this using Molecular Dynamics simulation. A similar model was used successfully in the past for simulating radiation induced viscous flow [S. G. Mayr, Y. Ashkenazy, K. Albe, and R. S. Averback, Phys Rev. Lett. 90, 055505 (2003)]. We demonstrate that by varying stress and temperature one can achieve conditions under which radiation induced creep (RIC) dominates sample compliance to external load rather then the usual thermally induced mechanism. Based on our simulation results we introduce a simple analytical model to describe this RIC which extends the standard creep equations by including an additional dose dependency.We further study changes in this effect by contrasting model FCC and BCC metals (Copper and Tungsten).
5:15 PM - T2.9
Nanostructure Evolution on Silicon Surfaces: Silicon Nanodots via Irradiation of Thin Gold Coating Films on Si.
Osman El-Atwani 1 3 , Dan Rokusek 2 , Bryan Heim 2 , Jean Paul Allain 1 2 3 Show Abstract
1 Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 3 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States, 2 Nuclear Engineering, Purdue University, West Lafayette, Indiana, United States
Deciphering self-organization mechanisms of nanodots on silicon via low-energy ion beam irradiation is critical to manipulate functionality in these nanostructured systems. It is postulated that irradiation on pure silicon surfaces doesn’t lead to the stability of nanostructures but rather smoothing at near-normal incidence angles. To induce nanopatterning, recent results have shown the need for a minimum level of impurities on silicon [1-3]. This work presents the study of how impurities on silicon can affect the nanostructure formation of gold-coated silicon surfaces of known thickness and concentration irradiated at energies near the damage threshold. Studies include impact energies ranging from 25-200 eV, and current densities between 10-300 uA/cm^2 and normal incidence with argon ions. To probe the incident energy deposition, controls will compare argon irradiation with Xe and Kr ions. Results show that nanostructures evolve over the silicon surfaces. Pure silicon irradiated with identical impact conditions do not result in nanostructure formation. This work shows how the concentration of gold is correlated to the final structure of the nanodots. In-situ LEISS (low-energy ion scattering spectroscopy) and XPS (X-ray photoelectron spectroscopy) were used to determine the surface concentration of species present on the surface. SEM, and cross section FIB studies on the samples were performed for morphology study. EDX were used for further chemical analysis in the sub-surface region. Atomic force microscopy is used to determine the characteristic length of the dots. References J.A. Sánchez-García et al., Nanotechnology 19, 355306 (2008). G. Ozaydin et al., Appl. Phys. Lett. 87, 163104 (2005). G. Kästle et al., Adv. Funct. Mater. 13, 853 (2003).
5:30 PM - T2.10
Mass Redistribution Causes the Structural Richness of Ion-irradiated Surfaces.
Charbel Madi 1 , Eitan Anzenberg 2 , Karl Ludwig 2 , Michael Aziz 1 Show Abstract
1 , Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts, United States, 2 Physics, Boston University, Boston, Massachusetts, United States
Energetic particle irradiation of solids can cause surface ultra-smoothening, self-organized nano- and microstructures in surface topography or bulk composition, or degradation of the structural integrity of nuclear reactor components. The resulting structures have previously been attributed to the effects of the removal of target atoms by sputter erosion. Here we show that surface stability or instability is determined by the effects of ion impact-induced atomic redistribution, and that the effect of sputter erosion is essentially irrelevant. We study the transition, with increasing ion beam incidence angle and ion beam energy, from stability to instability of a flat silicon surface under high-radiation dose of argon ions. We use Grazing Incidence Small Angle X-Ray Scattering to characterize in situ, in real time, the surface response as the control parameters ion beam energy and incidence angles are varied. Non-local processes such as stress buildup in the ion-stimulated surface layer generated during the ion irradiation process are also investigated in real time using Multi-beam Optical Stress Sensor. A model based on the effects of impact-induced redistribution of those atoms that are not sputtered away both explains the observed ultra-smoothening at low angles, and also drives its own instability at higher angles.The negligibility of sputter erosion effects for the morphological stability of flat surfaces has practical implications. It indicates the inadequacy of a low sputter yield as a morphological stability design criterion for walls exposed to energetic particle irradiation. Because the negligibility of erosive effects does not prevent impact-induced redistributive effects from causing morphological instabilities, it may be important to consider impact-induced redistributive effects in developing a more robust stability criterion for plasma-facing wall materials.
5:45 PM - T2.11
X-ray-induced Sintering of Colloidal Particles.
Byung Mook Weon 1 2 , Ji Tae Kim 1 , Jung Ho Je 1 Show Abstract
1 X-ray Imaging Center, Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Colloidal nanoparticles are widely studied in an emerging variety of applications as one of self-assembling building blocks. High-flux X-ray photons are often used for analysis and synthesis of nanoparticles. However, little is known about the response of nanoscale colloidal particles to environments of high-radiation dose. Here we study a significant morphological change of colloidal particles under continuous irradiation of X-ray photons. From our studies on soft matter systems in the past few years, we have knowledge about non-thermal photonic ablation of polymers with X-ray photons. In this study we find a new phenomenon of X-ray-induced sintering for colloids; that is, the irradiation of X-ray photons can be a highly efficient route to photonic sintering of colloidal particles through rapid chain scission. We observe nanoscale sintering phenomena of PMMA spheres in real time with high resolution transmission X-ray microscopy (TXM) at the photon energy of 8 keV on the 32-ID-C microscopy beamline of the Advanced Photon Source (APS) at the Argonne National Laboratory. The key mechanism of non-thermal sintering is the X-ray-induced chain scission via photodecomposition, which effectively diminishes the glass temperature and the viscosity of PMMA particles. High resolution imaging from TXM offers direct evidence that contact neck grows with time by 1/6 power scaling of time. This result is in a good agreement with a typical scaling for viscoelastic sintering in theory. Conventional sintering using heat and pressure is often unsuitable to nanoscale colloids because of difficulty in temperature control and thermal damage. In practice, to reduce thermal damage, it is required to lower sintering temperature or to reduce sintering time. X-ray photons can effectively induce sintering via photodecomposition without thermal damage in nanoscale spatial resolution. Our findings may help new understanding of radiation response of colloidal nanoparticles and also open a new photonic sintering of nanoparticle assembly without thermal damage.
Xinghang Zhang Texas A&M University
Oliver Kraft Karlsruhe Institute of Technology
Michael Demkowicz Massachusetts Institute of Technology
Meimei Li Argonne National Laboratory
T7: Poster Session: Nanostructured Materials in Harsh Environments
Wednesday PM, December 01, 2010
Exhibition Hall D (Hynes)
T5: Stability of Nano-Materials at Very High Temperature or Stress
Wednesday PM, December 01, 2010
Room 206 (Hynes)
9:30 AM - **T5.1
Mechanical Mixing and Self-organization in Alloys Subjected to Severe Plastic Deformation.
Nhon Vo 1 , Elvan Ekiz 1 , Wenjun Cai 1 , Yinon Ashkenazy 1 , Mohsen Pouryazdan 2 , Horst Hahn 2 , Robert Averback 1 , Pascal Bellon 1 , Daniel Schwen 1 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States, 2 , Karlruhe Institute of Technology, Karlsruhe Germany
Materials subjected to severe plastic deformation often undergo non-equilibrium microstructural evolutions and phase transformations, for instance during extrusion or sliding wear. Severe plastic deformation is also employed to synthesize non-equilibrium alloys, e.g., high-energy ball milling for nano-ODS steels.The kinetics of this mixing can be directly measured by high-pressure torsion (HPT) testing at low temperature of two-phase alloys comprised of moderately immiscible elements, such as Cu and Ag, or Cu and Co. The chemical mixing forced by dislocation-based plasticity in these materials can be superdiffusive. In such a case, the dynamical competition between forced mixing and thermally activated decomposition may lead to self-organization at the nanoscale. In contrast, in the case of highly immiscible alloy systems, such as Ni-Ag, full mixing cannot be achieved even by deformation at cryogenic temperatures.We show that these experimental observations can be explained using atomistic simulations. In particular, in highly immiscible systems, the motion of dislocations promotes chemical decomposition at small scale, leading to the formation of layered structures, even at temperatures where thermally activated diffusion is negligible. The existence of this novel mechanism is confirmed experimentally by performing ball milling on ternary alloys such as Ag-Cu-Ni with specific initial microstructures.We also show that alloy systems comprised of elements that crystallize on different lattices, such as Cu-Nb and Cu-Mo show more complex behaviors, including mixing-induced amorphization.
10:00 AM - **T5.2
Thermal Stabilization of Nanocrystalline Grain Size by Solute Segregation.
Carl Koch 1 , Ronald Scattergood 1 , Brian VanLeeuwen 1 , Kristopher Darling 2 , Mark Atwater 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Weapons and Materials Research Directorate, U.S.Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States
This talk will review the thermal stabilization of nanocrystalline grain sizes in metals by the "thermodynamic stabilization" method of solute segregation to grain boundaries. This approach, which has been pioneered by Weissmuller and subsequently addressed by Kirchheim and others assumes that appropriate solute additions can segregate to the nanocrystalline grain boundaries, lower their free energy, and therefore the driving force for grain growth. The enthalpy of segregation in previous models has been assumed to be equal to the elastic enthalpy. This approach is equivalent to treating the system as an ideal solution, that is, with an enthalpy of mixing of zero. We have recently proposed a modification of the Wynblatt and Ku model of surface segregation to grain boundary segregation. This includes both an elastic strain energy term and a chemical component. Predictions of this model are experimentally addressed for several alloy systems including Fe-Zr, Pd-Zr, and Cu-Zr. It appears that this model provides good predictive powers for the thermal stabilization of nanocrystalline microstructures to elevated temperatures.
10:30 AM - T5.3
Atomic and Meso-scale Modeling of the Solute Induced Stabilization of Nanostructured Al and Cu.
Hongli Dang 1 , Yojna Purohit 1 , Lipeng Sun 1 , Ronald Scattergood 1 , Carl Koch 1 , Donald Brenner 1 Show Abstract
1 Department of Material Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Materials with nanometer-scale grains are prone to grain growth, which limits their service temperatures and lifetimes. Grain growth can be slowed either kinetically, for example by grain boundary pinning, or thermodynamically by lowering the Gibb's free energy of the grain boundaries through the segregation of otherwise immiscible solute atoms to the grain boundaries. To better understand the driving forces for solute-induced stabilization we have been using a first principles based disclination structural units model to calculate energies of tilt grain boundary in copper and aluminum with and without Pb and Zr solute atoms, respectively. In this approach grain boundaries are modeled as disclination dipole walls with energies given as a weighted sum of individual disclination energies determined from special low-sigma structures, elastic terms and disclination core energies. Predictions of the model using bulk shear moduli and Poisson's ratios in the elastic and core energy terms are found to be comparable to energies for fully atomistic simulations. For Al-Pb no relation between grain boundary energies in pure Al and the degree of stabilization due to Pb doping was found. Our model parameterized to density functional theory calculations predicts a ~50% reduction in the energy anisotropy for <100> tilt grain boundaries in Al-Pb with respect to angle due to doping compared to the pure system, while calculations using an analytic potential yield no appreciable reduction in the energy anisotropy. The implications of these results for using solute atoms to stabilize nanostructured metals at high temperatures will be discussed.
10:45 AM - T5.4
Theoretical Model on Thermal Annealing of Self-assembled InAs/GaAs Quantum Dots and Its Experimental Validation.
Srujan Meesala 1 , Kaustab Ghosh 1 , Saumya Sengupta 1 , Subhananda Chakrabarti 1 Show Abstract
1 Center of Nanoelectronics, Department of Electrical Engineering , Indian Institute of Technology Bombay, Mumbai, Maharashtra, India
Self assembled InAs/GaAs quantum dots (QDs) have been gaining increasing importance over the last years due to potential applications in high-performance QD lasers and photodetectors. Post-growth rapid thermal annealing (RTA) is commonly used to improve the optical properties of QDs. However, too high temperature annealing introduces excessive In/Ga interdiffusion leading to degradation of the QD structure. In view of this problem, we for the first time present theoretical modeling and simulations on the interdiffusion process in a single-layer InAs/GaAs QD heterostructure subjected to annealing and the corresponding variation in its full PL spectrum. A quantitative correlation is established with experimental PL results from the same samples. The potential of the model is highlighted in predicting the annealing induced changes in QD materials and evaluating its thermal stability for specific device applications. Single layer InAs/GaAs QDs were grown by solid source MBE. HAADF-STEM data from the as-grown sample indicate a truncated pyramid type QD structure whose dimensions were incorporated in our simulations. The grown samples were subjected to ex situ rapid thermal annealing (RTA) at different temperatures (650 - 850 C) for 30s duration. Fickian diffusion is used to model In/Ga interdiffusion in the QD on annealing. Our simulation results showed the gradual desorption of indium from the QD commencing at 650°C followed by total dissolution of the QD in the wetting layer at 850°C. We further studied the band profile of the QD heterostructure, where we obtained a reduction in carrier confinement potential with increase in annealing temperature. Subsequently, PL ground state energies of QD annealed at different temperatures are calculated by solving the Schrödinger equation separately for electrons and holes. Our theoretical calculations, which illustrate the blueshift of PL emission wavelength on annealing, are in good agreement with our experimental results with a maximum deviation of 10 nm. PL spectra of the entire ensemble of QDs, annealed at different temperatures, are calculated from a lognormal distribution of QD heights derived from AFM observations on the as-grown sample. We obtained a close correlation between calculated and experimental variation in full-width-at-half-maximum (FWHM) of the spectrum, which increased for annealing up to 750°C annealing followed by a decrease. This can be explained by considering two competing effects i.e an inverse dependence of the annealing-induced blueshift on dot size and homogenization of the dot size distribution, which we have studied in extended simulations. In conclusion, the simplicity of the model along with its multiple useful features, including computation of material interdiffusion, QD band profiles and full PL spectrum, makes it a demanding tool in studying and predicting annealing effects on QD heterostructures. Financial assistance from DST is being acknowledged.
11:30 AM - **T5.5
Thermal Stability of Nanocrystalline Metals.
Rainer Birringer 1 Show Abstract
1 , Universitaet des Saarlandes, Saarbruecken Germany
The high density of grain boundaries in nanocrystalline (nc) materials provide a strong driving force for coarsening or other irreversible processes. The occurence of such processes depends on the availibility of sufficient thermal activation. In nc Pd, fast grain growth has been observed even at room temperature. Therefore, developing strategies to enhance the stability of nc materials is a necessary prerequisite for applications. This talk will briefly review kinetic approaches to thermally stabilize nc materials and then focus on thermodynamic stabilization of nanocrystallinity.
12:00 PM - T5.6
Stability of Nanostructures Formed on Dislocations.
Mark Jhon 1 , Daryl Chrzan 2 3 , Andreas Glaeser 2 3 Show Abstract
1 Computational Materials Science and Engineering, Institute of High Performance Computing, Singapore Singapore, 2 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Rod-like microstructures are well known to be unstable at elevated temperatures. They are susceptible to various types of morphological instabilities. First, they may be susceptible to the Rayleigh instability, where rods break up into discrete particles. Second, they may coarsen, where larger rods will grow at the expense of smaller ones. Often, rod-like nanostructures are formed around a dislocation. Examples of such systems include hollow-core dislocations and nanowires. A continuum theory is presented that examines the effect of the axial dislocation on the thermal stability of such structures. It is found that in the case of a hollow-core dislocation, sufficiently small defects are stable to both Rayleigh break-up as well as coarsening. This suggests that under some conditions, nanowires nucleated on dislocations can be thermodynamically stable.
12:15 PM - T5.7
Thermal Stability of Nanopores in Palladium Alloys and Their Hydrides.
Benjamin Jacobs 1 , David Robinson 1 , Markus Ong 1 , Mary Langham 1 , Ilke Arslan 2 Show Abstract
1 , Sandia National Labs, Livermore, California, United States, 2 , University of California Davis, Davis, California, United States
Nanoporous palladium and palladium alloy powders are of potential value for hydrogen isotope storage applications in radiation and in high temperature environments. These materials can improve charging and discharging kinetics for hydrogen due to their high surface area, and can help the helium decay product to escape when tritium is stored. The nanoporous palladium and palladium alloy powders, with diameters on the micrometer-scale and perforated by 3 nm pores, were synthesized in a scalable fashion by reduction of palladium salts in a concentrated aqueous surfactant. Using in situ heating in a TEM, we show that the pores of pure nanoporous palladium are not stable above 150° C. When palladium is alloyed with rhodium or platinum, the pore thermal stability is dramatically increased to temperatures as high as 400° C. The enhanced stability is related to the pore density, particle microstructure, and surface purity. High-resolution electron microscopy and tomography were used to visualize pore density and order, which is a strong function of palladium content; other metals such as platinum, rhodium, and palladium alloys produce more uniform pores. EDS and EELS element mapping showed that the alloy particles have a graded core-shell structure with palladium making up the majority of the core and the majority of the second element making up the shell. The pore thermal stability and core-shell structure were also analyzed before and after hydrogen exposure at elevated temperatures. In situ XPS analysis showed evidence of some surface-to-bulk migration of palladium and rhodium in a reducing and oxidizing environment, respectively. This migration was minimal, and TEM element mapping showed that the elemental constituents of the bulk particle remained the same. The addition of rhodium and platinum enhanced the hydrogen storage properties relative to pure nanoporous palladium particles.
12:30 PM - T5.8
Thermal Stability of Nanocomposite Metals: In situ Observation of Anomalous Residual Stresses Relaxation During Annealing under Synchrotron Radiation.
Jean-Baptiste Dubois 1 2 , Ludovic Thilly 1 , Pierre-Olivier Renault 1 , Florence Lecouturier 2 , Marco Di Michiel 3 Show Abstract
1 Pprime Institute, University of Poitiers, Futuroscope France, 2 , LNCMI, Toulouse France, 3 , ESRF, Grenoble France
The thermal stability of nanocomposite metals (a nanostructured copper matrix embedding niobium nanotubes) is investigated via time-resolved in situ annealing under synchrotron high-energy x-rays. The diffraction peak profile analysis demonstrates that internal-stress relaxation begins in the Nb nanotubes at a temperature far below the bulk recrystallization temperature and follows size-specific regimes originating from a proximity effect with the nanostructured Cu matrix: the increased Cu-Nb interface surface disrupts internal-stress relaxation processes, leading to larger thermal resistance.
12:45 PM - T5.9
Multilayer Antidiffusion Barrier Schemes for Schottky and Ohmic Contact Metallisations to InAlN/GaN HEMTs.
Eliana Kaminska 1 , Iwona Pasternak 1 , Michal Borysiewicz 1 , Marek Guziewicz 1 , Anna Piotrowska 1 , Elzbieta Dynowska 1 2 , Rafal Jakiela 2 , Valery Kolkovski 2 , Marie-Antoinette di Forte-Poisson 3 Show Abstract
1 Department of Micro- and Nanotechnology of Wide Bandgap Semiconductors, Institute of Electron Technology, Warsaw Poland, 2 , Institute of Physics, Polish Academy of Sciences, Warsaw Poland, 3 , Alcatel-Thales III-V Lab, Marcoussis France
Power III-N HEMT devices that operate at high frequencies require that the metallization schemes applied for Schottky and ohmic contacts be stable at temperatures elevated due to heating from power dissipation. One of the issues of this stability is to inhibit the interdiffusion between the contact layer and the Au (Cu) overlayer introduced for bonding and interconnections purposes. The most common solution to this problem is to deposit a thin-film antidiffusion barrier in-between the main contact and mounting layers. In the search for an effective antidiffusion barrier, conducting amorphous or near-amorphous compounds with a very high crystallisation temperature have been considered as the most promissing ones .
The reported work focuses on developing antidiffusion barriers capable to increase the thermal stability of metal contacts above 400oC. In the chosen approach, such an antidiffusion barrier consists of several bilayers of materials with different crystalline structures. It has been demonstrated that an interface between such materials effectively blocks the atomic interdiffusion . Three groups of materials were studied as the bilayers, namely: ZrN and ZrB2, TaSiN and TiN and finally TiC and TiN. The materials were deposited by means of room temperature sputtering from elemental and compound targets in inert Ar and reactive Ar+N2 atmospheres.The metallisation schemes containing the antidiffusion barriers were studied by means of X ray diffraction, secondary ion mass spectrometry profiling and scanning electron microscope imaging. Contact structurisation was performed by means of UV-photolithography and lift-off technique. Electrical characterization of metal/semiconductor contacts involved I-V characteristics for rectifying contacts and specific resistance by circular transmission line method. The measurements were performed for the as-deposited samples and samples after aging tests, which were conducted at temperatures up to 800oC in Ar flow.
We show that by properly designing the configuration of multilayer barrier systems, the thermal stability of Zr-, Ta- and Ti-based nitrides, borides and carbides can be enhanced above their crystallisation temperatures. The use of multilayer antidiffusion barriers effectively encapsulates the metal/III-N system, preventing the decomposition of the semiconductor surface under annealing and effectively supressing the interaction between the III-N material and the top Au (Cu) overlayer.
The research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy POIG 01.01.02-00-108/09 MIME and by the EC under the project 'Materials for Robust Gallium Nitride' CP-IP 214610-2 MORGaN.
 M.-A. Nicolet, Vacuum 59 (2000) 716.
 J. Jasinski, E. Kaminska, A. Piotrowska, A. Barcz, M. Zielinski, Mat. Res. Soc. Symp. 622 (2000) T6.34.1
T6: Mechanical Response of Nanomaterials Under High Stress
Wednesday PM, December 01, 2010
Room 206 (Hynes)
2:30 PM - **T6.1
Nanocomposite Metallic Conductors for High Field Magnets.
Ludovic Thilly 1 , Florence Lecouturier 2 , Jean-Baptiste Dubois 1 2 , Vanessa Vidal 3 , Pierre-Olivier Renault 1 Show Abstract
1 Pprime Institute, University of Poitiers, Futuroscope France, 2 , LNCMI, Toulouse France, 3 , Institut Clément Ader, Albi France
The development of reinforced conductors for the winding of high-field pulsed magnets requires multi-functional materials with high electrical conductivity and high strength. To obtain fields over 80T, a yield stress of the order of 2GPa (at 77K) is needed. It is now established that only copper-based nanocomposite metallic wires can exhibit reasonably low electrical resistivity with such elevated elastic limit: among the candidates for this application are copper/silver (Cu/Ag) and copper/niobium (Cu/Nb) nanocomposite wires.After considering the specifications for high-field magnets, the different available nanocomposite wires will be reviewed in terms of fabrication (usually based on severe plastic deformation processes), microstructure and properties.A special attention will be devoted to the Cu/Nb system that has been studied in-depth with complementary characterization techniques at the microscopic and macroscopic level (in-situ tensile tests in the TEM, nanoindentation, in-situ tensile tests under high energy synchrotron beam) to shed light on the role of the microstructure in the recorded extreme properties.
3:00 PM - **T6.2
Shock Response and Recovery of Cu-Nb Nanolayer Composites.
Timothy Germann 1 , Ruifeng Zhang 1 , Richard Hoagland 1 , Shengnian Luo 1 , Amit Misra 1 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Large-scale classical molecular dynamics (MD) simulations and laser-launched flyer plate experiments have been used to study the shock response of Cu-Nb nanolayered composites. At a layer thickness of 5 nm, the hardness of such metallic multilayers (as measured by quasistatic indentation or compression tests) reaches a maximum due to the difficulty of dislocation transmission across the interfaces. We observe a similar strengthening effect under dynamic shock loading, both in the MD simulations and in post mortem examinations of shock-recovered samples subjected to ~20 GPa shock loading. The MD simulations provide insight into the dislocation nucleation and transmission processes that occur under compression, as well as the subsequent annihilation upon release. The Cu-Nb interfaces serve both as dislocation sources during shock compression, and dislocation sinks upon release, and thus lead to a greater degree of recovery as compared to either constituent Cu or Nb single crystal.
3:30 PM - T6.3
Nano-chemo-mechanics of Hydrogen Embrittlement under Extreme Conditions.
Shan Huang 1 , David McDowell 1 , Ting Zhu 1 Show Abstract
1 School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Challenges associated with a hydrogen economy are substantial, ranging from hydrogen generation, storage and transportation. Hydrogen in metallic containment systems such as high-pressure vessels and pipelines causes the degradation of their mechanical properties that can result in sudden and unexpected catastrophic fracture. A wide range of hydrogen embrittlement phenomena was attributed to the loss of cohesion of interfaces (between grains, inclusion and matrix, or phases) due to interstitially dissolved hydrogen. However, this concept and associated models have not been made sufficiently predictive, due to a lack of fundamental understanding of the chemo-mechanical processes of embrittlement. Here, by combining the atomistic simulation, thermodynamic theory of interfacial embrittlement, and electron theory of alloying, we analyze how the structure of grain boundaries influences the propensity of chemisorption of hydrogen, and the consequent effects on the cohesive strength of grain boundaries. Our results highlight the collective effects of multiple segregation sites in grain boundary embrittlement. Implications of our results for efficient hydrogen storage and transport at high pressures are discussed by linking to a recent experiment showing that susceptibility to hydrogen embrittlement in metallic materials can be markedly reduced by grain boundary engineering.
3:45 PM - T6.4
A Coupled Quantum-continuum Method to Predict Embrittlement in Materials due to Impurities at the Crack Tip.
Arun Nair 1 , Derek Warner 1 , Richard Hennig 2 Show Abstract
1 School of Civil and Environmental Engineering, Cornell University, Ithaca, New York, United States, 2 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Dislocation nucleation plays a key role in the deformation of nano-structured and nano-dimensioned metals in severe and harsh environments. An important tool for gaining insight into dislocation processes has been atomic-scale computer modeling. However, atomistic simulations of deformation processes have long been plagued by the challenge of accurately and efficiently describing the complexities of multispecies bonding. In the case of metals, this has lead to the majority of the atomistic modeling effort focusing on pure elemental metals in a vacuum, rather than more technologically relevant problems involving alloys with impurities and surface oxides in realistic environments. At the root of the challenge is a trade-off between accuracy and computational expense. At one end of the spectrum lies Kohn-Sham Density Functional Theory (KSDFT). Although KSDFT can produce the interatomic forces originating out of many multi-element bonding situations to reasonable accuracy, its computational expense is severely limiting. Most studies are restricted to less than 1,000 atoms. At the other end of the spectrum lie empirical interatomic potentials. While these are computationally much less expensive, scaling to millions of atoms, they often struggle to accurately capture multi-element bonding.Here we use a concurrent multi-scale approach to address this long-standing challenge. We couple an atomistic region whose forces are calculated via Kohn-Sham Density Functional theory to a continuum region described by linear elasticity. Each domain in the simulation framework is governed by its own energy functional with the constraint that the forces be zero across the domain interface. The KSDFT domain is solved using a plane-wave basis set and pseudo-potentials, while the finite element method is employed for the continuum domain. This approach enables us to carry out the chemistry in the atomistic region accurately described via KSDFT and also to examine large simulation cell sizes and thus properly account for the long-range elastic fields associated with defects such as dislocations. The method has been used to predict the critical stress intensity factor for dislocation nucleation from a crack tip in aluminum using both GGA and LDA exchange-correlation functionals. The results are compared to popular empirical potentials and Rice’s model. Similarly, critical stress intensity factor is predicted for crack propagation in aluminum for brittle orientation. In this talk, we will also discuss the application of the above method to study the embrittlement of aluminum in the presence of various impurities such as oxygen and hydrogen at the crack tip. The competition between dislocation nucleation and its competing mechanism, crack-tip propagation, in the presence of impurities will be discussed.
4:30 PM - **T6.5
Coupled Grain Boundary Motion in Nanocrystalline Structures.
Mario Velasco 1 , Christian Brandl 2 , Helena Van Swygenhoven 1 Show Abstract
1 NUM/ASQ, Paul Scherrer Institut, Villigen Switzerland, 2 Physics and Chemistry of Materials, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Molecular dynamics computer simulations performed on nanocrystalline fcc metals demonstrated the dislocation mediated plasticity by dislocations being emitted from grain boundaries (GB) and absorbed in surrounding grain boundaries after propagation through the grain with our without involvement of cross-slip . Such dislocation mechanisms require also a number of grain boundary accommodation mechanisms. The GB structure is found to be crucial in determining when and where dislocations are emitted. Furthermore the local stress concentrations in the GB can pin dislocations or induce dislocation cross-slip.On the other hand, there are many experimental observations of stress driven grain coarsening, and often coarsening is suggested to be related to coupled GB motion - a mechanism that has been addressed by molecular dynamics in bi-crystal geometries. Here we present molecular dynamics studies of grain boundary migration occurring in nanocrystalline GB networks in which we have incorporated special GB orientations that are known to facilitate coupled GB motion. The occurrence of this migration will be discussed in terms of the grain size. A detailed study of the atomistic configuration will show how triple junctions and neighboring grain boundaries accommodate this migration.
5:00 PM - T6.6
High-strain Rate Behavior of Nanostructured Niobium Processed by Severe Plastic Deformation to Very Large Strains.
Zhiliang Pan 1 , Weihua Yin 1 , Suveen Mathaudhu 2 , Laszlo Kecskes 2 , Qiuming Wei 1 Show Abstract
1 Mechanical Engineering, UNC-Charlotte, Charlotte, North Carolina, United States, 2 WMRD, US ARL, Aberdeen Proving Ground, Maryland, United States
Understanding the behaviors of nanostructured materials in extreme environments, such as during high-strain-rate (dynamic) deformation is important for a number of applications such as high-speed machining, impact, and penetration phenomena. There have been extensive efforts investigating the mechanical behavior of nanostructured materials at quasi-static loading rates (strain rate less than 1.0 s^-1), but little has been reported on the dynamic behavior of such materials.In this work, we have investigated the high-strain rate, uni-axial compressive behavior of nanostructured niobium (Nb), a refractory metal with body-centered cubic lattice structure. The material was processed via route C (180 degree rotations about the billet long axis) equal channel angular extrusion (ECAE) up to 24 passes. The microstructure of the processed billets was analyzed using electron back-scattering diffraction (EBSD). Microstructural analysis includes grain size and texture information.To examine the mechanical properties of the ECAE-nanostructured Nb, we have performed mechanical testing within a wide range of strain rates: from quasi-static loading to high-rate (strain rate >10^3 s^-1) loading. Strain rate sensitivities were measured by strain rate jump tests under quasi-static loading. It was found that nanostructured Nb exhibits reduced strain rate sensitivity (~0.02) in comparison with the coarse-grained counterpart (~0.04). Unlike many other nanostructured bcc metals such as Fe and W, nanostructured Nb exhibits homogeneous plastic flow under high-rate uni-axial compression. Flow softening is primarily caused by uniform adiabatic temperature rise of the whole specimen. The experimental results were discussed by considering a mechanistic model of dynamic deformation of viscoplastic materials.
5:15 PM - T6.7
Surface Modification of 316L Stainless Steel by a Low Temperature Severe Plastic Deformation Linear Raking Process.
Giovanni Facco 1 , Andreas Kulovits 1 , Jorg Wiezorek 1 Show Abstract
1 Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh , Pennsylvania, United States
A linear plane-strain machining process using a wedge shaped tool is used to apply a severe plastic deformation by removing a prescribed amount of material from the surface. This processing, referred to as linear raking, is a novel technique to modify the morphology and microstructure of complex multi component alloy systems, which can be easily up-scaled. Here we apply high strains of cold-deformation at high rates by linear raking to 316L stainless steel. This can either lead to the formation of a nano-structured sub-surface layer by dynamic recovery and recrystallization or to the introduction of uncommonly high dislocation densities. Both a high grain boundary density and a high dislocation density can act as sinks for radiation induced point defects, thereby improving the irradiation tolerance of the modified 316L steel. We use electron backscatter diffraction (EBSD) in the scanning electron microscope, as well as the automated acquisition and indexing of diffraction patterns in the transmission electron microscope to characterize the micro- and nano-scaled structure changes in response to the linear raking processing. We used hardness measurements to monitor changes in mechanical properties. We discuss the influence of changes in average grain size, grain size population and grain boundary character on mechanical properties, in order to establish linear raking as a method for grain boundary engineering in materials susceptible to irradiation assisted stress corrosion cracking (IASCC). We plan to use alloys processed by linear raking for susceptibility studies to evaluate improvements in IASCC properties. We acknowledge use of the facilities of the Materials Micro-Characterization Laboratory of the Department of Mechanical Engineering and Materials Science, University of Pittsburgh, and support by a grant from the Nuclear Regulatory Commission, NRC-38-09-935.
5:30 PM - T6.8
Reactive Ni-Al Nanoparticle Interfaces under Extreme Mechanical Environments.
Hansohl Cho 1 , Krystyn Van Vliet 2 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Nickel and aluminum have been widely used and investigated for a variety of extreme-environment applications, owing to the strongly exothermic mixing of these elemental metals. There is thus an increasing interest in development of nanostructured Ni and Al interfaces for energetic and structural applications that can benefit from the ultrafast initiation of mixing and high kinetic energy dissipation under extreme thermal and mechanical environments. Specifically, while the high speed mechanical impact induced exothermic alloying process is of great importance in the applications such as reactive armor, the chemomechanics underlying the atomistic details of the alloying initiation and propagation have not been established. Such understanding is required in the design of polymer-nanoparticle composites that can effectively dissipate mechanical energy under extreme rates. Here, we employ nonequilibrium molecular dynamics simulations to consider the critical conditions required for impact induced mixing and energy dissipation of Ni and Al nanoparticles. We compare high velocity (km/s) collisions between these particles of 3 nm to 10 nm diameter, and highlight the role of surface faceting in the thermal, chemical and mechanical responses under the high strain rates. These simulations enable predictions of the final adiabatic temperature (1700 to 1900K), which quantifies the extent of kinetic energy release during the reactions. This energy dissipation depends strongly on particle radius and surface roughness, and proceeds over ns timescales. We then show how these computational results directly enable further predictions of how particle oxidation and incorporation within polymers nanocomposites will alter the critical conditions required for full energy dissipation under extreme impact loading.
T7: Poster Session: Nanostructured Materials in Harsh Environments
Thursday AM, December 02, 2010
Exhibition Hall D (Hynes)
9:00 PM - T7.10
Temperature-dependent Mechanical Properties of Polycrystalline Nanomaterials.
Sangil Hyun 1 , Youngho Park 2 , Yongsoo Choi 2 3 Show Abstract
1 , KICET, Seoul Korea (the Republic of), 2 , Hankyung National University, Ansung Korea (the Republic of), 3 , Johns Hopkins University, Baltimore, Maryland, United States
Polycrystalline materials are generally known to have strong dependence of the material properties on their nanoscale morphology such as grain size. For example, silicon carbide, which is widely applied in nuclear reactors as high-temperature structural material, naturally possesses many grains and pores inside. The Hall-Petch effect states that mechanical strength of nanocrystalline materials can substantially vary in the wide range of grain size, which is attributed to the competition between intergranular and intragranular deformations. Thus, it has been pursued to predict the mechanical properties depending on their morphology in systematic manners. In the present work, we employed classical molecular dynamics simulations to investigate the morphology-dependent mechanical properties of polycrystalline silicon carbide, silicon, and copper. The degradation of materials property under high thermal load was investigated particularly in terms of the grain size and shape. It is shown that the Hall-Petch effect can be temperature-dependent. The degradation mechanism was analyzed to suggest optimal microstructures of general polycrystalline materials to improve mechanical performance for high-temperature applications.
9:00 PM - T7.11
The Fabrication and Characterization of Ga2O3/(La,Sr)MnO3 Nanowire Arrays with Excellent High Temperature Stability.
Hui-Jan Lin 1 , Haiyong Gao 1 , Pu-Xian Gao 1 Show Abstract
1 Materials Science and Engineering, University of Connecticut, Storrs, Connecticut, United States
Ga2O3/(La,Sr)MnO3 (LSMO) nanowire arrays have been synthesized on Si substrates. Using carbothermal vapor deposition method, Ga2O3 nanowire arrays have been successfully grown first, through control of temperatures and atmospheres to form different dimensionality and density of Ga2O3 nanowire arrays. Sputtering deposition was used to form mesoporous perovskite LSMO materials on the Ga2O3 nanowire arrays to form Ga2O3/LSMO heterostructured nanowire arrays. X-ray diffraction analysis and electron microscopy were systematically conducted to characterize the core-shell structured nanowires arrays. The newly fabricated Ga2O3/LSMO structure shows an ultra-large surface area per unit volume, but with high thermal stability high temperature, which could be useful in harsh high temperature conditions, such as combustion engine and power plants.
9:00 PM - T7.12
The High Temperature Oxidation Properties of the IN738LC Surface Coated SiO2 Protective Layer by Combustion CVD.
Youngman Kim 1 , Kyoung-Soo Park 1 Show Abstract
1 Materials Science and Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Gas turbines need to be operated at high temperatures to increase their fuel efficiency and the turbine materials should be able to endure the harsh environments of high temperature oxidation. Thermal Barrier Coatings (TBC’s) on the surfaces of turbine blades are commonly used to protect gas turbine materials by reducing the level of thermal conduction and protecting from high temperature oxidation. Sometimes thermally grown oxides may form between TBC and base material, which may cause instability of turbine blades including spalling. Protective coatings of oxides on TBC may help to improve the lifetime of turbine blade materials. In this study, SiO2 was deposited on an IN738LC alloy by combustion chemical vapor deposition(CCVD), and then oxidized at 1,150, 1,250 and 1,350 C by a flame in air. The uncoated IN738LC specimens were also oxidized under the same condition for comparison. Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction were used to examine the changes on the surface. In case of uncoated IN738LC specimens, Cr existed at relatively high concentrations on the surface until 1,250 C, and produced a Cr2O3 layer that protected the surface from oxidation but TiO2 which do not help to improve oxidation properties was observed at 1,350 C. However, a larger amount of Cr existed on the specimens coated with SiO2 until 1,350 C compared to uncoated IN738LC specimens. Si might help to produce a thin Cr2O3 layer on the surface at higher temperatures, and suppress the increase in TiO2 layer thickness. The above mentioned phenomena were discussed in terms of chemical affinity of oxygen between the main elements of IN738LC, including Cr and Ti.
9:00 PM - T7.13
Nanodiamond-polyaniline Nanocomposite: Corrosion Inhibition Application of New Frontier Films.
Humberto Gomez 1 2 4 , Manoj Ram 1 2 , Farah Alvi 1 2 3 , Ashok Kumar 1 2 Show Abstract
1 Department of Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 2 Nanotechnology Research and Education Center, University of South Florida, Tampa, Florida, United States, 4 Departamento de Ingenieria Mecanica, Universidad del Norte, Barranquilla Colombia, 3 Department of Electrical Engineering, University of South Florida, Tampa, Florida, United States
Nanodiamonds (NDs) particles have gained worldwide attention due to their inexpensive large scale synthesis with narrow size (4 to 5 nm) distribution, facile surface functionalization, biocompatibility, quantum information processing, magnetotometry, novel imaging and IR-fluorescence applications. It is anticipated that attractive properties of NDs composite with other materials could be exploited for the development of new biocompatible material and devices (non-photo-bleaching fluorescence). The nanocomposite polymeric materials have also received much attention as a new form of support for electrocatalysts, biosensor, sensor and thermal applications because of their high accessible surface area, low resistance and high stability. It is also known that polyaniline (PANI) is one of the most studied materials due to its unique electrochemical, chemical and physical properties as well as high electrical conductivity and good environmental stability in doped and pristine (undoped) states. The PANI composites possess a variety of unique properties such as electrical, mechanical, and structural properties because of the combined effect due to the close incorporation between PANI with inorganic or organic components at a molecular and atomic level.In this work, we have extended the method to the preparation of ND-PANI nanocomposite films. The choice of NDs and PANI was motivated by a wide range of technological applications of both components, i.e. transparent electrodes for photovoltaic, solar cell, electroluminescent, and corrosion inhibition films. The ND-PANI nanocomposite were synthesized using wet chemical precipitation technique using various ratio of ND to aniline monomer. The ND-PANI films were characterized by UV-Vis, FTIR, electrochemistry, impedance, Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and electrical conductivity techniques. Current–voltage characteristics of nanodiamond-PANI nanocomposite show the ohmic junction. The electrochemical investigation on ND-PANI revealed the wider potential values with independent redox characteristics of polyaniline and nanodiamond. The ND-PANI films was investigated for corrosion protection of mild and stainless steel, aluminum, copper, and in NaCl, H2SO4, HCl solutions. The ND-PANI (2:1) ratio coating had long-term corrosion resistance on both uncoupled steel, copper and aluminum due to the passivation effect of NDs-PANI on the metal surfaces. The imdedance at 0.1 Hz remained constant for prolonged period. Immersion tests and salt spray tests on coated steel–copper galvanic couple showed that NDs-PANI coating offered better protection than simple emeraldine form of polyaniline over metal surfaces.
9:00 PM - T7.15
Carbon Onion sp2/sp3 Bond Hybridization Ratio Investigations and Relation to Fundamental Properties under Heavy Ion Irradiation.
Raed Alduhaileb 1 , Virginia Ayres 1 , Benjamin Jacobs 1 , Xudong Fan 1 , Kaylee McElroy 1 , Joshua Myers 1 , Atsushi Hirata 2 Show Abstract
1 Electrical & computer Engineering, Michigan State University, East Lansing, Michigan, United States, 2 Mechanical Engineering, Tokyo Institute of Technology, Tokyo Japan
Nano-carbon materials such as carbon onions, carbon nanotubes, C60, and related fullerenes are under investigation as nano-property enabled solid lubricants that maintain performance in harsh environments. Nano-carbons have all demonstrated low friction coefficients in terrestrial and more challenging vacuum environments. To date, the best overall lubrication performance across air and vacuum environments has been achieved with carbon onions, which are multiple layer concentric fullerene shells. These are one of the least explored fullerene systems; therefore, an investigation of fundamental properties was performed. Carbon onions have a known topographical evolution from a spherical to a polyhedral multi-layer structure as a function of increasing synthesis temperature. This structural evolution has been associated with a change of the sp2/sp3 bond hybridization ratio based on the appearance of HRTEM images. In the present work, the sp2/sp3 bond hybridization ratio was precisely investigated using electron energy loss spectroscopy (EELS). Results to date indicate an approximately 20% sp2 component increase for polyhedral carbon onions. The carbon onions were exposed to heavy ion irradiation at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University and their post-radiation properties were similarly characterized. Experiments were performed with primary beams including 16O, 48Ca and 39Ar at 140 MeV/nucleon and 70 MeV/nucleon to test the influence of charge-to-mass ratio on coupling between heavy ion species and carbon onions, while maintaining energy conditions comparable to those encountered in space, nuclear reactors and particle collider environments. Results to date indicate that carbon onions may respond to heavy ion irradiation with self-healing and self-annealing reactions that are parallel to those observed at increased synthesis temperatures.
9:00 PM - T7.16
Time-dependent Static Water Contact Angle Variation on Tungsten Nanorods Coated with Teflon.
Khedir Khedir 1 , Ganesh Kannarpady 1 , Hidetaka Ishihara 1 , Justin Woo 1 , Steve Trigwell 1 , Charles Ryerson 1 , Alexandru Biris 1 Show Abstract
1 Nanotechnology Center, University of Arkansas at Little Rock, Little Rock, Arkansas, United States
Tungsten as a robust material is used in harsh environment such as aerospace applications and high machinery engines. But the high surface energy of this material gives very high water adhesion property and consequently ice accumulates on the surface during severe weather conditions. Therefore, surface modification to obtain water repellency is highly desired to overcome the technical barrier for various applications. We have successfully fabricated tungsten nanorods (WNRs) of different morphologies, using glancing angle deposition technique GLAD, by RF magnetron sputtering method. Subsequently, the as-deposited WNRs were coated with Teflon (PTFE) using effusion cell. The static water contact angle (SWCA) as high as 1380 was obtained on such a chemically modified surface. However, the time-dependent SWCA, measured at room temperature, was found to be highly dependent on the volume and size of the water droplets. It was observed that the SWCA decreases exponentially with time. However, the decrease was significantly slower with the increase in the size of water droplet. SEM and AFM were used to characterize the surface topography and XPS was used to analyze the chemical modification of the surface.
9:00 PM - T7.17
Parylene-C Encapsulation of Carbon Nanotube Field Effect Transistors for Harsh Environments.
Selvapraba Selvarasah 1 2 , Xinghui Li 1 , Ahmed Busnaina 2 , Mehmet Dokmeci 1 2 Show Abstract
1 Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Center for High Rate Nanomanufacturing, Northeastern University, Boston, Massachusetts, United States
Carbon nanotubes (CNTs) are extremely sensitive to molecular species in the environment and hence require proper passivation especially under harsh environments. Parylene-C, a room temperature deposited stress-free polymer is being utilized for encapsulation layer for electronics packaging and in implantable devices, yet its applications in nanotube electronics is nascent. In this paper, we present a parylene-C encapsulation approach for carbon nanotube field effect transistors (CNTFETs). We have found that a thin (1µm) parylene-C coating, not only fixes the position of the CNTs but also reduces the turn on resistance of transistors. Furthermore, parylene-C acts as an ultra thin pin hole free barrier layer for the carbon nanotube transistors. Utilizing thicker parylene-C layers, such as 3µm and 10µm thick parylene-C films, we found that the CNTFETs displayed a significant reduction in their electrical characteristics and hysteresis width due to desorption of oxygen and water molecules. Our latest findings conclude that parylene-C is a very promising material for encapsulation of CNTFETs against various environmental conditions and can be readily utilized in the passivation of CNT devices such as FETs, p-n diodes, and logic circuits.
9:00 PM - T7.18
First-principles Molecular Dynamics Study of Melting of Intermediate-size Al Nanoclusters.
Joongoo Kang 1 , Su-Huai Wei 1 , Yong-Hyun Kim 2 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Because Al nanoclusters can be used as nano-catalysts and phase-change nanomaterials for thermal energy storage, it is important to understand in details the melting process of Al nanoclusters . Although several simple phenomenological models with some fitting parameters have been proposed to explain the diverse melting behaviors of Al nanoclusters, the microscopic and fundamental understanding is still lacking.Using first-principles molecular dynamics simulations, we reveal the microscopic origin of the diverse melting behaviors of Al_n clusters with n around 55. The conceptual link between the degree of symmetry (e.g., Td, D2d and Cs) and solidity of atomic clusters is quantitatively demonstrated through the analysis of the configuration entropy. The size-dependent abrupt change of melting behavior is explained by the catastrophic symmetry reduction (Td → D2d → Cs). In particular, the sudden drop of the melting temperature Tm and appearance of the dip at N = 56 are due to the Td-to-D2d symmetry change, triggered by the surface saturation of the tetrahedral Al55 with Td symmetry. We also revealed a novel flexible solid state in which only thermal fluctuation induces rapid, collective structural transformations of the low-energy tetrahedral cluster — such as correlated diffusion of two distant vacancies or a dislocation row — without losing its structural order . The emergence of the collective motions from waste heat is solely due to effective excitation of soft phonon modes at nanoscale. Each collective transformation is followed by another randomly chosen amongst 12 different configurations, resulting in a random mixture or flow of surface atoms as random colour shuffle of a nano-Rubik’s cube. The flexible solid is in a novel atomistic “lattice-liquid” state (termed as half-solidity) because it has both solid and liquid characteristics.  Breaux et al., Phys. Rev. Lett. 94, 173401 (2005) J. Kang and Y.-H. Kim, ACS Nano 4, 1092 (2010).
9:00 PM - T7.19
Effect of Salt Water on Conventional and Nanophased Carbon/Epoxy Composites.
Mohammad Hossain 1 , Kazi Imran 1 , Mahesh Hosur 1 , Shaik Jeelani 1 Show Abstract
1 Mechanical Engineering & T-CAM, Tuskegee University, Tuskegee, Alabama, United States
Exploring oil and gas from the depth of the ocean is currently very expensive. Steel is normally used for this drilling purpose. Fiber reinforced polymeric (FRP) composites might be used to recover oil and gas from more depth economically due to their high strength and stiffness to weight ratio. Average density of FRP composite and steel is 0.8 and 6.9 g/cm3, respectively. As the weight of composite materials is much lower than steel, it can save cost. Other than the cost-effectiveness, FRP composites have better corrosion resistance to salt water, and good fatigue and mechanical properties that are comparable to steel. The biggest challenge for this study is to predict the long term effect of sea water on the performance of these conventional and nanophased FRP composites. Moisture absorption decreases the properties of composites which are dominated by the matrix or interface. Moisture weakens the bonding between the fiber and matrix and also softens the matrix materials. For this purpose, conventional and nanophased carbon/epoxy composite panels were fabricated by vacuum assisted resin transfer molding (VARTM) and their mechanical and thermal properties were determined to be used as baseline data using the state-of-the-art testing techniques available at T-CAM. Mechanical characterization was accomplished performing the flexural tests and thermal performance of these composites was studied using dynamic mechanical analysis (DMA). A nanoscale inclusion such as nanoclay was dispersed into the matrix by using the magnetic stirring and three-roll mixing methods. High vacuum was used to remove the produced bubble from the mixture. The percent of nanaclay loading was varied ranging from 1 to 3 wt.%. The effect and optimized amount of nanoclay loading was explored comparing the baseline data of the conventional and nanophased composites. Composite panels are exposed to the salt water for different length of time varying from one week to four weeks. The amount of moisture absorbed during this exposure time can be calculated by taking the weight difference of composite materials before and after expose at a time increment of 5 or 10 days. The effect of salt water on these materials would be presented in the seminar.
9:00 PM - T7.2
Effect of MOCVD Grown Al2O3 Coatings in Improving the Performance of Cemented Carbide Cutting Tool Inserts.
Abdul Sathar 2 , Arshiyan Shariff 2 , Mohommed Saif 2 , Piyush Jaiswal 1 , Sukanya Dhar 2 , S. Shivashankar 1 Show Abstract
2 Department of Mechanical Engineering, Reva Institute of Technology and Management , Bangalore, Karnataka, India, 1 Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka, India
Chemical vapour deposition (CVD) methods have been proven to be excellent for the deposition of alumina (Al2O3), and has been the industrial process of choice in the deposition of alumina coatings on cemented carbide cutting tool inserts for improving hardness and wear-resistance. This process uses aluminium chloride (AlCl3) as the aluminium precursor, with the disadvantage that the process requires high temperatures (~1000oC), as well as that it releases HCl, making it corrosive (thus requiring abatement). The deposition temperatures for CVD can be reduced significantly by using metalorganic (MO) precursors. Such a MOCVD also has the advantage of harmless inorganic and organic by-products. To evaluate the MOCVD process, MOCVD-coated tools need to be compared with industrially used CVD-coated tools, to investigate their efficiency during cutting operations. In the present effort, low-pressure MOCVD, with aluminium acetyl acetonate as precursor has been used to coat alumina on to cemented carbide cutting tools, at temperatures as low as 700oC, in oxygen ambient.This study presents the comparison between three CVD multilayer-coated cemented carbide cutting tool inserts (TiN/TiC/WC, CVD-coated Al2O3 and MOCVD-coated Al2O3 on TiN/TiC/WC) employed in the dry turning of mild steel. The influence of the top layer and the cutting parameters (depth of cut and cutting speed) on the cutting performances was investigated. Turning tests were conducted for cutting speeds varying from 14 to 47 m/min, for a depth of cut varying from 0.25 to 1 mm, at the constant feed rate of 0.2 mm/min. The axial, tangential, radial forces were measured using a lathe tool dynamometer using different cutting parameters, and the machined work pieces were subjected to surface roughness test. The results indicate that the cutting parameters, such as depth of cut and speed, have significant influence on cutting forces and surface roughness values. The MOCVD-coated inserts, for most of the cases examined, produced a smoother surface finish, while requiring lower cutting forces, indicating that it is the best insert, followed in performance by the CVD-coated one. The superior performance of MOCVD-alumina is attributed as to the greater hardness is expected from the MOCVD-coating because of the co-deposition of carbon along with the oxide, due to the very nature of the precursor used. In MOCVD-grown alumina-carbon composite films, the carbon matrix is expected to restrict the growth of the oxides to limit the grain size to nanometer scale, which may lead to enhanced mechanical properties in harsh environment of cutting applications.
9:00 PM - T7.21
Photoacoustics and Nanoindentation Studies of the Mechanical Properties of Colloidal Silica Particles after MeV Ion-induced Shape Deformation.
Juan-Carlos Cheang-Wong 1 , Ulises Morales 1 , Rosalba Castaneda-Guzman 2 Show Abstract
1 , Instituto de Física, Universidad Nacional Autónoma de México, México, D.F., Mexico, 2 , CCADET, Universidad Nacional Autónoma de México, México, D.F., Mexico
Submicrometre-sized colloidal silica particles are being intensively studied due to their potential applications in catalysis, intelligent materials, optoelectronic devices and coating technology. The properties of these SiO2 particles depend on their size, size distribution and shape, which in turn determine the different roles they can play as electronic substrates, electrical and thermal insulators, photonic bandgap crystals, masks for lithographic nanopatterning, etc, in technologically expected nanodevices. Ion irradiation induces damage and structural changes in solids due to energy losses of multi-MeV heavy ions via ionization events and atomic collisions occurring in the near-surface region of the irradiated sample. Spherical silica particles were prepared using the Stober process, and deposited onto silicon wafers. The samples were then irradiated at room temperature with MeV Si ions and after the irradiation the spherical silica particles turned into oblate particles, as a result of the increase of the particle dimension perpendicular to the ion beam and the decrease in the parallel direction. The objective of the present work is to study not only the shape deformation of spherical SiO2 particles, but also the structural and mechanical changes induced by the ion irradiation. The size, size distribution and shape of the silica particles were determined by scanning electron microscopy. Photoacoustics measurements were performed in order to study the structural characteristics and anisotropies of the samples, because these structural differences can be initially deduced from the phonon vibration spectra. The photoacoustic measurements are carried out using a pulsed laser to excite the samples and piezoelectric sensors to detect the vibration signals. Our results revealed significant structural differences between the spherical and the deformed silica particles. Also, indentation studies were performed in order to characterize the mechanical properties of the particles.
9:00 PM - T7.3
Electrochemical Nanopatterning on Copper Surface Using a Conductive AFM Cantilever Tip.
Gyudo Lee 1 , Kihwan Nam 1 , Suho Jeong 1 , Huihun Jung 1 , Bumjoon Choi 1 , Sang Woo Lee 1 , Dae Sung Yoon 1 , Kilho Eom 2 , Taeyun Kwon 1 Show Abstract
1 Biomedical Engineering, Yonsei University, Wonju, Kangwon, Korea (the Republic of), 2 Mechanical Engineering, Korea University, Seoul Korea (the Republic of)
In this paper, we present technique to fabricate nanopatterns on Cu thin films via an electrochemical nanomachining (ECN) using an atomic force microscope (AFM). A conductive AFM cantilever tip (Pt/Ir5 coated) was used to form an electric field between tip and Cu substrate with applying a voltage pulse, resulting in the generation of an etched nanopattern. In order to precisely construct the nanopatterns, an ultra-short pulse was applied onto the Cu film through the AFM cantilever tip. The line width of the nanopatterns (the lateral dimension) increased with increased pulse amplitude, on-time, and frequency. The tip velocity effect on the nanopattern line width was also investigated. The line width is decreased with increasing tip velocity. Experimental results were compared with an equivalent electrochemical circuit model representing an ECN technique. The study described here provides important insight for fabricating nanopatterns precisely using electrochemical methods with an AFM cantilever tip.
9:00 PM - T7.4
Electrical and Structural Properties of Aligned Aluminum Doped Zinc Oxide Nanorod Arrays via a Novel Sonicated Sol-gel Immersion.
Uzer Noor 1 , Mohd Mamat 1 , Musa Zahidi 1 , Zuraida Khusaimi 2 , Rusop Mahmood 1 Show Abstract
1 , CEES, Universiti Teknologi Mara, Shah Alam Malaysia, 2 , IOS, Universiti Teknologi Mara, Shah Alam Malaysia
Aligned Aluminum (Al) doped zinc oxide (ZnO) nanorod arrays were prepared on seeded catalyst-coated glass substrate using novel technique of sonicated sol-gel immersion method at different annealing temperatures. The diameter sizes of synthesized hexagonal-shaped nanorods are found to be in the range of 50 nm to 150 nm. The annealing treatment up to 500 °C did not give any significance effects on the nanorods morphology as observed by field emission electron microscope (FESEM). The micro-Raman spectra reveal that the crystallinity of the nanorod arrays were improved at higher annealing temperature as shown by the E2 (high) peak intensity enhancement. This result supported by XRD spectra where the XRD peak intensity for annealed nanorods is higher than as-grown nanorods. XRD spectra reveal Al doped ZnO nanorod arrays grew preferentially along (002) plane or c-axis indicating high quality of the ZnO crystal. Al metal forms Ohmic contacts with Al doped ZnO nanorod arrays which indicate through a linear current-voltage (I-V) characteristic. The conductivity of nanorod arrays increased with annealing temperature up to 500 °C with a maximum conductivity ~1.72 x 10-2 Scm-1.
9:00 PM - T7.5
Ultrafast Laser Formation of Periodic Structures at a Si-Glass Interface.
Ryan Murphy 1 2 , Ben Torralva 2 , Steven Yalisove 2 1 Show Abstract
1 Applied Physics Program, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
We will present results showing the presence of Laser Induced Periodic Structures (LIPS) at a silicon-glass (Si-SiO2) interface formed after irradiation by an ultrafast laser. The laser used was a commercially available Clark MXR Ti:Sapphire 800 nm laser pulsed at 150 fs with a repetition rate of 1KHz and maximum pulse energy of 800 μJ. The output is a Gaussian beam and was focused to a 40 μm beam diameter. Micro-tubes are formed at the compressively stressed interface by passing the beam through the glass and irradiating the silicon underneath with 25% overlapping pulses. LIPS are seen through the glass in the Si, have approximately the same periodicity as the incident laser wavelength, and are visible after irradiation by both single and multiple passes over the same tube. LIPS are commonly observed in laser irradiated materials but what’s unusual is that the orientation of the LIPS at the interface does not follow the conventional description of LIPS. They are parallel to the incident electric fields and perpendicular to the length of the micro-tubes. Dependence of LIPS orientation on the glass thickness and laser fluence will be presented.
9:00 PM - T7.7
On Orbit Atomic Oxygen and Ultra Violet Light Resistance of Clay Polyimide Nanocomposites.
Godfrey Sauti 1 , Jae-Woo Kim 1 , Jin Ho Kang 1 , Cheol Park 1 , Sharon Lowther 2 , Peter Lillehei 2 , Sheila Thibeault 2 , Nathan Stowe 3 Show Abstract
1 , National Institute of Aerospace, Hampton, Virginia, United States, 2 , NASA Langley Research Center, Hampton, Virginia, United States, 3 , Virginia Tech, Blacksburg, Virginia, United States
Polymeric materials provide a tough, flexible, lightweight, relatively easy to manufacture, scalable and therefore potentially low cost option for construction materials for use in aerospace applications as well as in many terrestrial applications where they are exposed to harsh environments. However, the resistance of polymeric materials to degradation when subjected to irradiation with intense ultraviolet light and/or exposure to atomic oxygen is generally not very high. We report increases in the ultraviolet light and atomic oxygen resistance of clay polyimide nanocomposites flown in low earth orbit on the International Space Station. The results obtained indicate that well dispersed nanoclay platelets reduce the rate of degradation, with the composites retaining high optical transparency, suffering reduced erosion loss, smaller changes in the solar absorptivity/thermal emissivity ratio as well as retaining thermal and mechanical integrity. The current results suggest that these composites offer a route to improving the properties of polymer-based materials for aerospace applications. The samples in this study were mounted on the International Space Station (ISS) as part of the Materials International Space Station Experiment (MISSE) 3 and 5.
9:00 PM - T7.8
Simulation of Multiple Collision Cascades in Cu-Nb Interfaces.
Liang Zhang 1 , Michael Demkowicz 1 , Alfredo Caro 2 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
We use molecular dynamics (MD) to investigate the stability of Cu-Nb interfaces subjected to continuous irradiation under athermal conditions. Multiple collision cascades with various primary knock-on atom (PKA) energies are performed in FCC Cu, BCC Nb, and a Cu-Nb bilayer. The PKA energy and direction distributions used in MD are calculated from the statistics of subcascade formation modeled by SRIM. To assess the dependence of radiation-induced interfacial intermixing on the miscibility of Cu and Nb in the liquid state, we compute the complete Cu-Nb phase diagram using the Hamiltonian switching technique. Implications of this work on the morphological stability metal-metal nanocomposites under irradiation are discussed. *This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
9:00 PM - T7.9
Structure, Strength and Mechanisms of Plastic Deformations and Fraction of Nanocrystalline Materials.
Nina Noskova 1 Show Abstract
1 , Institute of Metal Physics, Yekaterinburg Russian Federation
Investigations into the structure of nanocrystalline pure metals, alloys, and composites suggest that nanocrystalline Pd and Ni metals produced by the sublimation method have nanograins of a sufficiently perfect volume and a nonequilibrium state at nanograin boundaries. Nanocrystalline Mo and W metals produced by the method of intensive plastic deformation (IPD) have "wider" grain-boundary interlayers with a high density of defects than those in fcc metals. Finally, nanocrystalline Ti and Co synthesized by the IPD method have two types (phase and grain) of interfaces, leading, in turn, to different densities of defects in them. As compared to the spectrum of grain misorientations in a usual polycrystal, the spectrum of nanograin misorientations is displaced towards small angles.From HRTEM studies of the nanograin structure it can be conjectured that the crystal lattice of a nanograin and a precipitating nanophase can be free from defects, contain defects, and be elastically stressed. According to those studies, the crystal lattice of nanograins in alloys with internal elastic distortions is distorted most at nanograin boundaries as it is in pure metals. Only nanophases of the smallest size (4-6 nm) probably are free of defects. Regarding grain boundaries of nanograins and nanophases, they can have different structures, such as a quasi-amorphous structure, a highly distorted crystalline structure, or a structure with misfit dislocations, or a transition layer having a different chemical composition. It has been found that the structure of nanophases and boundaries in a nanophase alloy has a number of specific features. It has been established that an alloy in the nanocrystalline state generally has a main phase, which determines the physical properties of the alloy, and phases showing themselves as metastable or associated ones. It should be remembered that the phase composition changes (the initial phase dissolves and new phases appear) and initial phases do not transform simultaneously to nanosized phases during IPD. An "in situ" electron microscopic examination of the deformation and the fracture of nanocrystalline pure Ni, Cu, Mo, and Ti metals and alloys based on Fe and Al allowed developing a concept for an alternation of the mechanisms responsible for plastic deformation and fracture of nanocrystalline pure metals, solid solutions, and multiphase alloys depending on the average size of nanograins (nanophases) and the character of the nanograin size distribution. It has been found that depending on the average size of nanograins and the presence of particles of a phase, the deformation mechanism is implemented by dislocation modes of deformation at 100 to 70 nm, dislocation-rotational modes at 60 to 30 nm, and rotational modes of deformation at <30 nm, involving the process of the impurity mass transfer.
Xinghang Zhang Texas A&M University
Oliver Kraft Karlsruhe Institute of Technology
Michael Demkowicz Massachusetts Institute of Technology
Meimei Li Argonne National Laboratory
T8: Deformation of Nano-Materials at High Temperature or Strain
Helena van Swygenhoven
Thursday AM, December 02, 2010
Room 206 (Hynes)
9:30 AM - **T8.1
Measurement of 5-Parameter Interface Character Distribution in ARB Composites.
Anthony Rollett 1 , J. Ledonne 1 , N. Mara 2 , S. Suwas 3 , S. Lim 1 , I. Beyerlein 2 , S. Lee 1 Show Abstract
1 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 , Indian Institute of Science, Bangalore India
10:00 AM - T8.2
High Temperature Creep Strength in a Nanodispersion-strengthened Ferritic Alloy by Heavy Plastic Deformation.
David Morris 1 , Maria Munoz-Morris 1 Show Abstract
1 Physical Metallurgy, CENIM, CSIC, Madrid Spain
Processes of severe plastic deformation have been investigated for a wide range of ductile alloys over the past decade, generally with an objective of refining the microstructural scale, for example the grain size, but have hardly been considered for intermetallics. This presentation discusses processing of a boride-containing Fe3Al alloy using a technique of multidirectional, high-strain and high-temperature forging.Iron aluminides with relatively low Al contents can be regarded as Al-rich ferritic steels with outstanding oxidation-corrosion properties. However, as for many ferritic steels, they show poor creep resistance at temperatures above about 600C. The deformation processing leads to a material with large grain size and refined dispersion of thermally-stable boride particles. These particles produce a considerable increase in creep strength under conditions of stresses and low strain rates at temperatures around 700C. This high-strain forging technique can be seen as an intermediate processing method between conventional wrought metallurgy and mechanical-alloying powder metallurgy, whereby an initially coarse and inhomogeneous dispersion of second phase is refined and made somewhat more homogeneous, and can be considerd as a useful processing technique for a wide range of particle-containing materials.
10:15 AM - T8.3
Multiscale Modeling of Deformation of Ultra-fine Grained Metals at High Homologous Temperatures.
Alexander Hartmaier 1 , Naveed Ahmed 1 Show Abstract
1 Interdisciplinary Centre of Advanced Materials Simulation (ICAMS), Ruhr-University Bochum, Bochum, NRW, Germany
Two-dimensional dislocation dynamics (2D-DD) and diffusion kinetics simulations are employed to study the mechanisms of plastic deformation of ultra-fine grained (UFG) metals at high homologous temperatures. Besides conventional plastic deformation by dislocation glide within the grains, we also consider grain boundary mediated deformation and recovery mechanisms based on the absorption of dislocations into grain boundaries. This requires solving the diffusion equation and coupling it to dislocation motion within the model. The material is modeled as an elastic continuum that contains a defect microstructure consisting of a preexisting dislocation population, dislocation sources, and grain boundaries. The temperature dependence of the mechanical response of the model material to an external load is calculated and the contributions of different deformation mechanisms to the total deformation are monitored. We find that at low homologous temperatures the behavior of the model material is well described by a classical Hall-Petch law. At high homologous temperatures, instead, we find a pronounced grain boundary softening and, moreover, a high strain rate sensitivity of the model-material. These numerical findings qualitatively agree very well with experimental results known from the literature. Thus, we conclude that grain boundary sliding although its immediate contribution to plastic slip is only marginal, enables recovery processes at the grain boundary. Hence, grain boundary diffusion is the rate-limiting factor for recovery and plastic deformation of UFG metals and in consequence causes the pronounced strain-rate sensitivity of UFG metals and also the observed tendency towards grain boundary softening at high temperatures.
10:30 AM - **T8.4
Extreme High Temperature Stability and Creep Resistance of a Silicon Based Nanocomposite.
Amiya Mukherjee 1 Show Abstract
1 Chemical Engineering & Materials Science, University of California, Davis, California, United States
New materials that can withstand higher temperatures and stresses without failure are required for the next generation of efficient turbines for steam and nuclear power plants and for future fusion power applications. The availability of materials with high strength at elevated temperatures is critical for the further development of power generation systems with improved efficiency. Hence, new materials that can withstand higher temperatures and stresses without failure are required for the next generation of efficient turbines and heat exchangers for most power plants and for future fusion power systems.Silicon nitride (Si3N4) ceramic matrix is an ideal matrix to address this requirement. It has one of the strongest covalent bonding strengths. At elevated temperature, dislocation activity in this matrix is minimal because of very strong Peierls-Nabarro force. Hence, its creep resistance should make it a superior candidate for elevated temperature structural material. However, sintering of silicon nitride is a difficult process. It needs a sintering additive like alumina or yittria. There is always a Si02 coat on the silicon nitride particles which interacts with the sintering aids to produce a low viscosity oxynitride glass. In creep and other rheological tests, this oxynitride glass degrades (severely) the intrinsic creep resistance of the silicon nitride at high temperatures.We present in this work the processing, characterization and creep properties of a silicon nitride/silicon carbide nanocomposite processed with no sintering additives. This was achieved by using a liquid polymer precursor (polyureasilazane), pyrolysis and subsequent sintering in Spark Plasma Sintering equipment. This yielded a 30 nm Si3N4 and a 25 nm SiC phase nano ceramic composite. The truly astoundingly high temperature strength of this nanocomposite will be discussed in the context of currently available elevated temperature deformation mechanisms and stability criterion.
11:30 AM - T8.5
Effect of Precipitate Morphology on Chemical Mixing During Severe Plastic Deformation.
Nhon Vo 1 , Robert Averback 1 , Pascal Bellon 1 , Yinon Ashkenazy 1 Show Abstract
1 Materials Science and Engineering, University of Illinois, Urbana Champaign, Urbana, Illinois, United States
The effect of precipitate morphology on atomic mixing during severe plastic deformation (SPD) was investigated using molecular dynamics (MD) computer simulations. Different initial precipitate morphologies in fcc A75B25 alloys with heats of mixing ranging from 0 to 21 kJ/mol were cyclically deformed to strains up to ε = 200. For highly immiscible alloys (~16 kJ/mol), the precipitates always acquired a platelet structure during shear, independent of their initial microstructures. We attribute this tendency to dislocation localization on the interface and enhanced local atomic rearrangements. We also examined SPD in Cu-Nb for which bcc precipitates reside in an fcc matrix. In this case the shear-induced mixing depends strongly on the specific interface, with Kurdjumov-Sachs (KS) interfaces behaving very differently than non-KS interfaces. The role of temperature in the mixing mechanism is also discussed.
11:45 AM - T8.6
Relevance of Interface Structure to Interface Properties: Manipulating Interface Properties via Tunable Potentials.
Xiang-Yang Liu 1 , Richard Hoagland 1 , Michael Demkowicz 2 , Xian-Ming Bai 1 , Blas Uberuaga 1 , Michael Nastasi 1 , Amit Misra 1 , John Hirth 1 Show Abstract
1 , Los Alamos National Lab, Los Alamo, New Mexico, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Nanolayered Cu-Nb composites exhibit high strength and enhanced radiation damage tolerance. To understand the relevance of interface structure to interface properties in fcc-bcc systems, tunable potentials offer a fairly simple way to selectively vary parameters independently. In this work, the parameterization of the EAM interatomic potential in fcc-bcc system is modified to understand the interface properties. We first change the dilute heats of mixing between Cu and Nb and investigate the effect on interface structure and defect formation energies near interface. To understand the interface behavior in different lattice misfit environment, the relative lattice constants between Cu and the bcc crystal are varied. Interface dislocation analysis based on Frank-Bilby formulation is to be presented, together with atomistic simulation result. Defect-interface interactions are studied with molecular dynamics (MD) and accelerated MD method, to predict the radiation damage tolerance of these interface systems.
12:00 PM - T8.7
A Hybrid MD-kMC Algorithm to Study Diffusion in the Presence of Fields: Creep in Nanostructured Materials.
Enrique Martinez 1 , Alfredo Caro 1 Show Abstract
1 MST-8, LANL, Los Alamos, New Mexico, United States
A new hybrid Molecular Dynamics-kinetic Monte Carlo algorithm has been developed in order to study the basic mechanisms taking place in diffusion in concentrated alloys under the action of chemical and stress fields. Parallel implementation of the k-MC part based on a recently developed synchronous algorithm [J. Comp. Phys. 227 (2008) 3804-3823] resorting on the introduction of a set of null events aiming at synchronizing the time for the different subdomains, added to the parallel efficiency of MD, provides the computer power required to evaluate jump rates ‘on the flight’, incorporating in this way the actual driving force emerging from chemical potential gradients, and the actual environment-dependent jump rates. The time gain has been analyzed and the parallel performance reported. The algorithm is tested on simple creep experiments on nanostructured materials such as CuNb multilayer’s under irradiation.
12:15 PM - T8.8
Spark Plasma Sintering of ZrB2-SiC-ZrC Ultra-high Temperature Ceramics.
Alexandra Snyder 1 , Lia Stanciu 1 Show Abstract
1 Materials Engineering, Purdue University, West Lafayette, Indiana, United States
Spark plasma sintering (SPS) was investigated as a processing technique for improving the properties of ultra-high temperature ceramics. Secondary and ternary phases of SiC and ZrC were added to ZrB2 in order to restrain grain growth and improve oxidation behavior at high temperatures. Four different compositions of these ternary ceramics were prepared by SPS at 1800C. The sample containing the smallest amount of SiC exhibited the highest density while the presence of a higher percentage of ZrC led to an increase in density. These trends were reversed for Rockwell hardness values, with increasing hardness proportional to increasing SiC content and decreasing ZrC content. A higher percentage of SiC in the initial powder enhances grain growth inhibition and contributes to higher values for thermal conductivity.
12:30 PM - T8.9
A Chemo-thermo-mechanically Coupled Theory Accounting for Large Elastic-viscoplastic Deformation, Diffusion, and Swelling due to a Chemical Reaction: Application to Thermal Barrier Coatings.
Kaspar Loeffel 1 , Shawn Chester 1 , Lallit Anand 1 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Over the past 40 years, natural gas combined cycle (NGCC) turbine technology has evolved to dominate large-scale power generation. The use of advanced single-crystal materials and thermal barrier coatings (TBCs) ensures that the critical first-stage blades will stand up to high firing temperatures, while meeting maintenance interval standards. As the turbine blades have to withstand this environment, especially the behavior of the TBCs - insulating ceramic coatings deposited on bond-coated superalloys, used to lower the metal temperature - has to be carefully modeled. The long-term durability of TBCs is one of the problems limiting their use. After a certain amount of service life, the coatings usually fail by spallation of the ceramic topcoat at or near the interface that the bond-coat forms with the thermally grown oxide (TGO). Bond-coat oxidation thus plays an important role in TBC failure. Our overall aim is to develop a simulation-based design capability to predict the performance (stability and durability) of thermal barrier coatings on single-crystal turbine blades.In order to establish such a model, we specifically developed a chemo-thermo-mechanically coupled constitutive framework for the bond coat to account for (i) large elastic-viscoplastic deformations, (ii) diffusion of the oxygen, (iii) swelling due to the oxidation reaction, and (iv) transient heat conduction. We then implemented the constitutive model into the finite element package ABAQUS.
12:45 PM - T8.10
On the Role of Misfit Strain in Reactive Properties of Ni/Al Multilayers.
Joshua Crone 1 , Jaroslaw Knap 1 , Peter Chung 1 , Betsy Rice 1 Show Abstract
1 , U.S. Army Research Laboratory, Aberdeen, Maryland, United States
Reactive multilayers are nanostructured films that undergo a highly exothermic self-propagating high temperature synthesis (SHS) reaction when triggered by an external stimulus such as shock, heat, or electrical spark. As with reactive powders, the mechanical and reactive properties are highly tunable. Due to this tunability, reactive materials are used in applications ranging from material synthesis and welding to localized heat sources and parasitic weight reduction in munitions. In this work, we use molecular dynamics (MD) to study the effects of morphology, by way of misfit strain, on the reactive properties of nickel aluminum (Ni/Al) reactive multilayers. Misfit strains can vary significantly within multilayer systems, especially when prepared by rolling. The resulting variation in strain energy and misfit dislocation density can greatly affect the energy balance and reaction mechanisms of the system. We model Ni/Al bilayers with respect to varying m/n interface coincidence, where every mth Al atom coincides with every nth Ni atom. The misfit dislocation density, and consequently the misfit strain, is controlled by the m/n interface coincidence parameter. We define ignition temperature (Tig) as the lowest temperature at which a reaction is observed and compute its value under isochronal heating conditions using constant pressure and constant stress ensembles. Results from both ensembles suggest Tig decreases significantly as the misfit strain increases from 0.6% to 6.9%. The magnitude of the observed drop in Tig varies from 350 K in constant pressure simulations to 175 K in constant stress simulations. Both ensembles also reveal that the total energy release is nearly 25% greater in systems with large misfit strains. In constant pressure simulations the reaction rate is found to decrease with increasing misfit strain. However, in constant stress simulations the reaction rate of the high misfit strain systems increases significantly, reversing the previously observed trend and suggesting that reaction rates increase with increasing misfit strain when the multiaxial nature of stress is considered. Our results are indicative of the importance of microstructure in Ni/Al reactive multilayers. Moreover, they suggest a potential use of material morphology in tuning the reactive properties of reactive multilayers.