Symposium Organizers
G.Malcolm Stocks Oak Ridge National Laboratory
Blas Uberuaga Los Alamos National Laboratory
Anter El-Azab Florida State University
RR5: Poster Session
Session Chairs
Anter El-Azab
George Stocks
Blas Uberuaga
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
RR1: Electronic and Magnetic Defects
Session Chairs
Tuesday PM, April 26, 2011
Room 2022 (Moscone West)
9:00 AM - RR1.1
Micro-mechanisms of Strain Partitioning Across the Interfaces in Two-phase Composites at Submicron Length Scale.
R. Barabash 1 , H. Bei 1 , Y. Gao 1 , G. Ice 1 , O. Barabash 1
1 Materials Science and Technology Div., Oak Ridge National Laboratory, Oak Ridge TN, Tennessee, United States
Show AbstractRenewed interest in composite materials is driven by the fact that their mechanical properties can be superior to those of individual constituent phases. Interfaces between the phases are the key elements responsible for the unique micro-mechanisms of plastic deformation in composites. In this study the depth-dependent strain distributed in the two phases and partitioned across the composite interfaces is directly measured at submicron length-scale using X-ray microdiffraction and compared to a detailed micromechanical stress analysis. It is shown that indentation-induced deformation in the composite material is distinct from deformation expected in a single-phase material. This difference arises in part from residual thermal strains in both phases of the composite in the as-grown state. Interplay between residual thermal strains and external mechanical strain results in a complex distribution of dilatational strain in the fibers and matrix and is distinct in different locations within the indented area. Several examples are discussed including NiAl/Mo; NiAl/Cr and Ni/Mo composites. Research supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. X-ray microbeam measurements were performed at 34-ID-E at the Advanced Photon Source. The use of the APS was supported by the Scientific Users Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
9:15 AM - RR1.2
Charge State of Point Defects in Uranium Dioxide Studied by Density Functional Theory with Hybrid Functional for Correlated Electrons.
Jean-Paul Crocombette 1 , Chartier Alain 2
1 SRMP, CEA, Saclay France, 2 LM2T, CEA, Saclay France
Show AbstractPoint defects in uranium dioxide are key elements to understand the micro-structural and chemical evolutions of the nuclear fuel. We present electronic structure simulations of these defects. We specifically address the problem of their charge states which cannot be tackled by usual Density Functional Theory (DFT) calculations within Local Density Approximation (LDA) or Generalized Gradient Approximation which predict that UO2 is metallic.We use a novel method, the Hybrid Functional for Correlated Electrons that we believe offers advantages over the common LDA+U correction. In this method a portion of exact exchange is added to the DFT terms but only to the correlated f electrons. This restriction compared to standard hybrid functional approaches makes the computer time of the calculations comparable to standard calculations, thus enabling to study defects, while the method does not rely on a U parameter as in LDA+U calculations. We present the basics of this approach, showing that it is able to reproduce the insulating gap of UO2, and then turn to the properties of defects. We found that point defects (vacancies and interstitials of U and O) are indeed charged: oxygen interstitials and uranium vacancies bearing a negative (-2 and -4 respectively) charge while oxygen vacancies are neutral or positively charged depending on the position of the Fermi level. The calculated formation energies of disconnected oxygen Frenkel pairs and Schottky defects made of the association of charged defects are in very good agreement with experimental estimates of thes formation energies.A Brouwer diagram based on a point defect model for stoichiometry variations in UO2+/-x is built. It predicts that oxygen vacancies are in a +1 charge state in the understoichiometric oxide. This charge state differs from the one assumed in the traditional (fully ionic) picture of UO2 but it is confirmed by the obtained variation of the deviation from stoichiometry with oxygen pressure which is consistent with experiments.The migration energies of the defects in their various charge states are calculated. Assemblies of oxygen interstitials are also considered, especially the cuboctahedral clusters that exist in the overstoichiometric oxide.
9:30 AM - RR1.3
Effect of Defect on the Piezoelectric Response of Zinc Oxide Nanobelts.
Kasra Momeni 1 , Anjana Ashtana 1 , Abhishek Prasad 2 , Yoke Yap 2 , Reza Yassar 1
1 Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan, United States, 2 Physics, Michigan Technological University, houghton, Michigan, United States
Show AbstractPiezoelectric response of one-dimensional zinc oxide nanostructures have been investigated in the literature using PFM. It was shown while piezoresponse of zinc oxide nanorodes are not frequency dependent, piezoresponse of zinc oxide nanobelts is inversely related to the applied frequency. It can be concluded that the piezoelectric response of one-dimensional zinc oxide nanostructures depends on their crystallography and presence of internal defects. In this paper, dependency of piezoelectric response of zinc oxide nanobelts on defects is systematically investigated using Piezoresponse Force Microscopy (PFM) and Transmission Electron Microscopy (TEM). Zinc oxide nanobelts were grown on Si substrate without the use of gold catalyst by Chemical Vapor Deposition (CVD) method. The as grown ZnO nanobelts are annealed at 600 °C for 30 min. Piezoelectric response and crystalline structure of as grown and annealed ZnO nanobelts are investigated using PFM and TEM, respectively. Local and integral piezoelectric response of as grown and annealed ZnO nanobelts were investigated using PFM. Integral response of as grown and annealed ZnO nanobelts show proportional dependency and independency to the applied electric field, respectively. Although local piezoelectric response of as grown ZnO nanoblets show presence of piezoelectric domains in their crystalline structure, but we could not find any piezoelectric domain in the annealed ZnO nanobelts. A model was proposed to interpret the piezoresponse of ZnO nanobelts based of Maxwell-Wagner relation. This model can interpret both frequency dependence and independence of piezoresponse of ZnO one-dimensional nanostructures based on presence of defects in their crystalline lattice. Presence of planar defects in as grown ZnO nanobelts are shown using TEM.
9:45 AM - RR1.4
Evolution of Electronic Defects in Fused Silica Following CO2 Laser Heating.
Rajesh Raman 1 , Manyalibo Matthews 1 , John Adams 1 , Stavros Demos 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractLocalized laser energy deposition in optical components remains a problem in high average laser power systems such as those designed to perform laser micromachining, microlithography, and laser-driven inertial confinement fusion. Deposition of even a fraction of this energy in component optics has been shown to lead to the generation of electronic defects and microscopic damage sites [1]. These sites can grow rapidly in size to macroscopic dimensions upon irradiation by subsequent laser pulses [2], setting a finite lifetime for the optic. One suggested means of increasing this lifetime is to reduce defect concentration and reflow material by raising the local temperature via localized CO2 laser treatment [3]. However, details of the physical mechanisms involved in thermal defect healing and damage threshold enhancement are poorly understood. To better understand this phenomenon, we characterized microstructural changes occurring in fused silica surface damage sites following increasing levels of CO2 laser irradiance. This task was performed using 1) edge-illuminated light scattering microscopy for the visualization of morphologic changes of the damage site, 2) photoluminescence microscopy to map the changes in the defect populations, and 3) synchrotron-based infrared spectro-microscopy to investigate the modification in the molecular bond network. To provide an estimate of the time-dependent surface temperature distribution induced during CO2 laser treatment, the axisymmetric (2D) heat-flow equation was solved [4].Electronic defect concentrations decreased slightly for laser heat treatments from ~1300 to 1800 K, then dramatically from 1800 to 2000 K, the latter consistent with a large increase in damage threshold [3]. The sharp reduction in light scattering as a function of heat treatment temperature suggests that capillary action is a major driving force in annealing fused silica via CO2 laser heating. Structurally, the vibrational spectrum of defective silica was similar to that of ion-bombarded silica [5] with an additional band near ~930 cm-1 that progressively diminished with laser heat treatment. Additionally, the TO asymmetric stretch mode of SiO2 shifted 5-10 cm-1, indicating slight densification caused by CO2 laser treatment. Above the SiO2 boiling point (2500 K), the majority of defective material is expected to have been removed by evaporation. Optical spectroscopic methods may be suitable for in situ characterization of the evolution of fused silica microstructure subject to CO2 laser heating.[1] L.Skuja, J. Non-Cryst. Sol. 239, 16-48 (1998)[2] M.A. Norton, L.W. Hrubresh, Z.L. Wu, et al., Proc. SPIE 4347, 468 (2001)[3] R.M. Brusasco, B.M. Penetrante, J.A. Butler, et al., Proc. SPIE 4679, 40-47 (2002)[4] S.T. Yang, M.J. Matthews, S. Elhadj, et al., J. Appl. Phys. 106, 103106 (2009)[5] N. Bibent, A. Faivre, G. Ferru, et al., J. Appl. Phys. 106, 063512 (2009)Prepared by LLNL under Contract DE-AC52-07NA27344.
10:00 AM - RR1.5
Observation of Near-infrared Luminescent Defect Centers in Ion Implanted Silicon.
Supakit Charnvanichborikarn 1 , Byron Villis 2 , Brett Johnson 2 , Jennifer Wong-Leung 1 , Jeffrey McCallum 2 , Jim Williams 1 , Chennupati Jagadish 1
1 Electronic Materials Engineering, Research School of Physics and Engineering, Canberra, Australian Capital Territory, Australia, 2 Centre for Quantum Computer Technology, University of Melbourne, Melbourne, Victoria, Australia
Show AbstractDifferent types of optically active defects are formed by a combination of ion implantation and thermal annealing. Low-temperature photoluminescence measurements have identified the formation of silicon-interstitial clusters (such as W-line and X-line), extended defects (R-line), and dislocations (D-bands) for various processing conditions. Further investigation was performed using silicon wafers with a range of boron and phosphorus doping concentration. Our results show that the luminescence intensity from these silicon-interstitial type defects decreases with increasing boron concentration [1]. In accordance with the previous experimental observation [2] and the existing theoretical prediction [3], we suggest that there is a possible competing formation between boron-interstitial clusters and the optically active silicon-interstitial defects. The effects of other processing parameters will also be discussed in view of enhancing defect luminescence in silicon. The outcomes of this study provide new insight into defect interactions in silicon and are important for silicon process modelling as well as the development of defect-driven silicon optoelectronic devices [4].
[1] S. Charnvanichborikarn, B. J. Villis, B. C. Johnson, J. Wong-Leung, J. S. Williams, and C. Jagadish, Appl. Phys. Lett. 96, 051906 (2010).[2] T. E. Haynes, D. J. Eaglesham, P. A. Stolk, H.-J. Gossmann, D. C. Jacobson, and J. M. Poate, Appl. Phys. Lett. 69, 1376 (1996).[3] L. Pelaz, G. H. Gilmer, H.-J. Gossmann, C. S. Rafferty, M. Jaraiz, and J. Barbolla, Appl. Phys. Lett. 74, 3657 (1999).[4] J. Bao, M. Tabbal, T. Kim, S. Charnvanichborikarn, J. S. Williams, M. J. Aziz, and F. Capasso, Opt. Express 15, 6727 (2007).
10:15 AM - RR1.6
Defect Mediated Room Temperature Ferromagnetism and Controlled Switching Characteristics in ZnO Thin Films.
Sudhakar Nori 1 , Siddhartha Mal 1 , John Prater 2 , Jagdish Narayan 1
1 Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science Division, Army Research Office, Durham, North Carolina, United States
Show AbstractWe present here a detailed description of the structural, chemical, electrical, and magnetic properties of undoped ZnO thin films grown under different conditions as well as the films that were annealed in various environments and irradiated with an UV laser. The observed room temperature ferromagnetism (RTFM) in these films can be explained qualitatively within the frame work of a defect assisted mechanism leading to the plausible cause of ferromagnetism in ZnO. These defects can be created in a controlled way by pulsed laser irradiation and vacuum annealing, and subsequently removed by oxygen annealing. This induced ferromagnetism can be turned on and off by defect creation and annealing in a systematic way. Samples prepared in low oxygen partial pressure or subsequently annealed in vacuum have always been strongly magnetic. Oxygen annealed films displayed a sequential transition from the ferromagnetic to the diamagnetic state as a function of the annealing temperature. Reversible switching of RTFM and n-type conductivity have been demonstrated either by annealing in different environments or by a novel laser irradiation treatment. Enhancements in both the electrical conductivity and magnetic moment have been controlled precisely with laser pulses, without altering the wurtzite crystal structure and n-type semiconducting characteristics of the ZnO thin films. EPR data were found to be in good agreement with the magnetization and conductivity measurements. Our SIMS and EELS studies conclusively rule out the presence of any external ferromagnetic ions or impurities. Correlation between structural, electrical, and magnetic properties has been established and discussed in terms of defects and defect complexes.References:1. J. Narayan and B. C. Larson, Journal of Applied Physics, 93, 278 (2003) 2. M. Venkatesan, C. B. Fitzgerald and J. M. D. Coey, Nature, 430, 630 (2004)3. S. Mal, S. Nori, C. Jin, J. Narayan, S. Nellutla, A. I. Smirnov and J. T. Prater, Journal of Applied Physics, 108, 073510 (2010)4. S. Mal, J. Narayan, S. Nori, J. T. Prater and D. Kumar, Solid State Communications, 150, 1660 (2010)
10:30 AM - RR1.7
A Theoretical Study of the Magnetic Structure of Bulk Iron with Radiation Defects.
Yang Wang 1 , Don Nicholson 2 , G. Malcolm Stocks 2 , Aurelian Rusanu 2 , Markus Eisenbach 2 , Roger Stoller 2
1 Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractA fundamental understanding of the radiation damage effects in solids is of great importance in assisting the development of improved materials with ultra-high strength, toughness, and radiation resistance for nuclear energy applications. In this presentation, we show our recent theoretical investigation on the magnetic structure evolution of bulk iron in the region surrounding the radiation defects. We applied the locally self-consistent multiple scattering method (LSMS), a linear scaling ab-initio method based on density functional theory with local spin density approximation, to the study of the magnetic structure in a low energy cascade in a 10,000-atom sample for a series of time steps for the evolution of the defects. The primary damage state and the evolution of all defects in the sample were simulated using molecular dynamics with empirical, embedded-atom inter-atomic potentials. We will show a detailed noncollinear magnetic structure in the vicinity of the defects, and will discuss the importance of thermal effect on the magnetic structure evolution.Acknowledgments: This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
10:45 AM - RR1.8
First Principles Magnetic and Electronic Structure in Large Non-periodic Dislocation Models.
Kh. Odbadrakh 1 , Donald Nicholson 1 , A. Rusanu 1 , Yang Wang 2 , G. Stocks 1
1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States, 2 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractDislocations are central to the deformation behavior of crystalline Fe. The local magnetic moments contribute to the interactions between dislocations and between dislocations and other defects. The magnetic changes are most pronounced near the dislocation core and decay as the strain field induced by the dislocation decreases with distance. We have implemented open boundary conditions in the Locally Self-consistent Multiple Scattering (LSMS) method in order to calculate the local moments in large simulated dislocation structures. The boundary conditions for these simulations include free surfaces and ridged moving blocks used to apply shear forces. The LSMS is used in a manner that allows application of the LDA-DFT to themagnetic and electronic structure within a sub-volumeof the much larger simulation cell. The influence of various boundary conditions on the electronic /magnetic structure are taken into account by modifying the Madelung procedure to treat periodicity in 1,2, and 3 dimensions.Acknowledgements:This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
RR5: Poster Session
Session Chairs
Anter El-Azab
George Stocks
Blas Uberuaga
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
6:00 PM - RR5.1
An Atomistic Simulation on Sharp Corner Mediated Dislocation Nucleation in Nanoscale Crystal Copper: Anisotropy Aspects.
Yu Sun 1 , Satoshi Izumi 1 , Shotaro Hara 1 , Shinsuke Sakai 1
1 Department of Mechanical Engineering, The University of Tokyo, Tokyo Japan
Show AbstractIn the recent years, since the greater requirements for using metallic elements at a sub-micron scale have emerged, dislocation nucleation at free surface and stress concentrations has begun to attract more attention with regard to the role it plays as a governing factor in macroscopic behavior.In the present work, we have investigated the nucleation of 90° and 30° partial dislocation from a sharp corner in an f.c.c. crystal copper by means of reaction pathway analysis. The anisotropy aspects of dislocation nucleation revealed by the results have shown that the stress-dependent activation energy of 30° partial dislocation is approximately twice over the counterpart of 90° partial dislocation, and that the maximum inelastic displacement for the former is also higher. Moreover, the shape of the saddle-point configuration of 30° partial dislocation is similar to a half-ellipse whereas in the case of 90° partial dislocation it is more like a semi-circle, reflecting the different Peierls barriers influenced by the Burgers vectors. Further study of the surface reconstruction demonstrated that although the nucleation of 30° partial dislocation was enhanced by surface reduction, it was still more energy-unfavorable than the 90° partial dislocation. These results suggest that the higher Peierls barrier is responsible for the larger activation energy of 30° partial dislocation nucleation. We believe that the anisotropy aspects of dislocation nucleation exhibited by our simulation provide useful insight into the incipient plastic deformation of nanoscale metals.
6:00 PM - RR5.11
Atomistic Properties of ThO2 Based Nuclear Fuels.
Rakesh Behera 1 , Chaitanya Deo 1
1 Nuclear and Radiological Engineering Program, George W. Woodruff School of Mechanical Engineering , Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractThorium nuclear fuel cycles are promising for their intrinsic proliferation resistance and greater thorium abundance. However, thorium has not been exploited commercially compared to uranium based fuels for nuclear applications. The current study attempts to develop an atomic level understanding of ThO2 based fuels using atomic level simulations. The overall accuracy of the predicted properties and fundamental mechanisms depend on the description of the atomic interactions defined in the model. In this study we have developed a few different atomic interactions based on previous UO2 potentials. This presentation will focus on the predicted bulk properties, defect stability and thermal properties of ThO2 using the developed interatomic potentials. Transferability of the potentials to study radiation damage and thermal conductivity will be checked. The properties of a mixed oxide fuel system (ThO2-UO2) will be addressed using the above models, with comparison to the available experimental results.
6:00 PM - RR5.12
On the Void-Surface Reaction Kinetics in Irradiated Materials—The Void Growth Problem Revisited.
Thomas Hochrainer 1 , Srujan Rokkam 2 , Anter El-Azab 1
1 Scientific Computing, Florida State University, Tallahassee, Florida, United States, 2 Mechanical Engineering, The Florida State University, Tallahassee, Florida, United States
Show AbstractNucleation and growth of voids is a key technical problem in irradiated materials. Classical studies of void growth under irradiation treat the growth process within the framework of a homogenized (effective medium theory) chemical rate theory, where it is assumed that the growth process is diffusion limited. Over the past few years, a new approach for modeling void nucleation and growth based on the phase field framework has emerged. In this framework, the void/matrix interface is modeled as a diffuse interface, which facilitates casting the entire void growth problem in a spatially resolved fashion. Development of quantitative phase field models requires connecting specific parameters of the diffuse interface model to the physical parameters of a more explicit ‘sharp interface’ formulation of void surface motion. We present a sharp interface model for void surface motion driven by fluxes of vacancies and interstitials to and from the surface based on an accurate representation of the defect kinetics at the surface. The model is based on transition state theory and features the void surface as a material discontinuity which can act both as defect sink and source. Detailed balance at the surface recovers the equilibrium concentration of point defects in the bulk. Results obtained for simple examples are used to validate the model for different levels of point defect supersaturation. Further, we discuss the implications of our model for the development of diffuse interface phase field models. This research was supported by the US Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center.
6:00 PM - RR5.14
Core Properties of Mixed Dislocations in BCC Iron.
Mathilde Miguras 1 , Emmanuel Clouet 1
1 SRMP, CEA Saclay, Gif-sur-Yvette France
Show AbstractWe used atomic simulations with the Mendelev's empirical potential [1] to study core properties of mixed dislocations in bcc iron. All studied dislocations have a 1/2[111] Burgers vector and are gliding on a {110} plane. The lowest core energy is obtained for the screw orientation. Variations of the core energy for a character lower than ~30° agree with a kinked dislocation description, whereas the core energy is quite constant beyond. The Peierls barriers, under zero stress, have also been calculated. The highest barrier is obtained for a mixed dislocation with a character ~109.5°. This high barrier is proposed to arise from the fact that the dislocation screw component did not spread in the glide plane for this orientation, in contrast with other dislocation characters.The estimation of the Peierls stress from these different barriers shows that the screw orientation is the hardest to make glide. The ~109.5° mixed orientation is also associated with a high Peierls stress.[1] M. I. Mendelev et al., Philos. Mag. 83, 3977-3994 (2003).
6:00 PM - RR5.15
Formation Criterion of Multiple-component Single-phase Solid-solution High Entropy Alloys.
Hongbin Bei 1 , Easo George 1
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractAlloys with multiple components normally exhibit complicated microstructures because of their tendency to form intermetallic compounds and other phases with different compositions or crystal structures. Recently, multi-component high-entropy alloys (HEAs) have been discovered consisting of five or more metallic elements with approximately equal atomic concentrations which appear to be stabilized as solid solutions because of their high entropies of mixing. Here we report on our study of phase stability in multi-component HEAs CoCrFeNiM (M = Al, Cu, Mn, Nb, Ti) using a combination of thermodynamic modeling and experiments. We propose a simple criterion to identify compositions of single-phase solid solution HEAs. Using this criterion, we confirm experimentally the existence of several equi-atomic HEAs.Research was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences.
6:00 PM - RR5.16
Effect of Precipitates in Cold Rolled and Post-heat-treated Ni-30Cr Alloy.
Kuk Hyun Song 1 , Han Sol Kim 1 , Won Yong Kim 1
1 , KITECH, Incheon Korea (the Republic of)
Show AbstractIn order to investigate the effect of precipitates in Ni-30Cr alloy used as heat resistance material, this work was carried out. To achieve the goals, the material was cold rolled to 90% in thickness reduction, recrystallized at 700 degrees for 30 min and post-heat-treated at 550 degrees up to 100 hours. At recrystallized material after cold rolling, the material showed significantly refined grain structure than that of initial state, which resulted in the increase in mechanical properties such as microhardness and tensile strength. Furthermore, the application of post-heat-treatment on recrystallized Ni-30Cr alloy led to the notable increase in mechanical properties. Consequently, microhardness and tensile strength showed more high values, over than 50% in fraction, than those of recrystallized state, which mainly resulted from the precipitates dispersed at microstructure during the post-heat-treatment. Accordingly, finely distributed intermetallic compounds such as Ni2Cr and CrNi3 have a notable effect on increase in mechanical properties, directly. In this study, we evaluated the effect of precipitates which affected on mechanical properties, in detail.
6:00 PM - RR5.17
Precipitates Formation and Its Impact in Friction Stir Welded and Post-heat-treated Inconel 718 Alloy.
Kuk Hyun Song 1 , Han Sol Kim 1 , Won Yong Kim 1
1 , KITECH, Incheon Korea (the Republic of)
Show AbstractIn order to investigate the formation of precipitates such as MC carbides and intermetallic compounds in the friction stir welded and post-heat-treated Inconel 718 alloy, this work was carried out. Furthermore, the microstructural and mechanical properties of welds and post-heat-treated material were evaluated to identify the effect on precipitates formed during post-heat-treatment. Friction stir welding (FSW) was performed at a rotation speed of 200 rpm and welding speed of 150 mm/min; heat treatment was performed after welding at 720 degrees for 8 hours in vacuum. As a result, the grain size due to FSW was notably refined from 5–20 micron in the base material to 1–3 micron in the stir zone; this was accompanied by dynamic recrystallization, which resulted in enhancements in the mechanical properties as compared to the base material. In particular, applying heat treatment after FSW led to improvements in the mechanical properties of the welds—the microhardness and tensile strength increased by more than 50% and 40% in fraction, respectively, as compared to FSW alone.
6:00 PM - RR5.18
Microstructure and Thermal Properties of Spray Formed Al-25%Si Alloy.
Han-Sol Kim 1 , Kuk-Hyun Song 1 , Won-Yong Kim 1
1 Eco Materials & Processing Department, Korea Institute of Industrial Technology, Incheon Korea (the Republic of)
Show AbstractThe microstructures and thermal properties of hyper eutectic Al-25%Si alloy produced by spray forming were investigated. In order to compare the thermal properties and microstructure permanent mold casting was also carried out in air. The spray formed Al-25%Si alloy is typically composed of refined and uniformly distributed Si dispersoids. This microstructural feature is analyzed quite different to Al-25%Si alloy prepared by permanent mold casting. It is found that spray forming plays an effective role to reduce the size of Si dispersoids of the present Al-25%Si alloy. In the temperature range from 293 to 673K, the coefficient of thermal expansion of spray formed alloy was lower than that of permanent casting alloy. Details will be discussed in relation with the results obtained.
6:00 PM - RR5.19
Atomistic Modeling on Defects in Multicrystalline Silicon.
Hiroshi Mizuseki 1 , Ambigapathy Suvitha 1 , Ryoji Sahara 1 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku Univ., Sendai, Miyagi, Japan
Show AbstractMulticrystalline silicon (mc-Si) is widely used as a solar cell material because of its low production cost, even though the energy conversion efficiency of mc-Si solar cells is lower than that of single-crystalline Si solar cells due to the random orientations of the crystal grains in the former. Optimization of the grain-boundary structures of mc-Si is a key issue to achieving high efficiency, because these regions act as recombination centers for carriers in solar cell materials. Multicrystalline Si with artificially-controlled grain orientations has been proposed as a means of reducing the number of electrically-active grain boundaries that lead to undesirable carrier recombination[1]. In the present study, we applied the spherical model [2] as proposed by Lee and Choi. To make a spherical model including a grain boundary under a given misorientation, the right cluster is rotated about the [110] or the [112] orientation and the left cluster is rotated about the [110] or the [112] orientation. The [110] and [112] axes correspond to the preferred growth directions. These geometries correspond to a grain boundary in multi-crystalline silicon under dendrite growth conditions. In this case, the prepared grain boundary is located at the center of the spherical model. To perform structural relaxation, we apply a Monte Carlo (MC) method based on the Tersoff potential [3] for the silicon system. Moreover, we used DFT method to understand relationship between sigma value and impurity precipitation. First we study the dopant position and the nature of interaction between the grain boundary and transition metal, such as copper, iron, nickel and chromium. Finally we have studied the electronic changes that occurred up on doping the transition metal impurities in the grain boundary regions using Sigma grain boundaries of polycrystalline silicon. Sigma 3(111), Sigma 5(210), Sigma 9(122) grain boundaries of silicon were constructed using GB studio [4]. The segregation energy for the impurities in Sigma 3(111) follows the order of Fe greater than Cu, Ni, and Cr at the substitutional site and Cr greater than Cu, Fe, and Ni, at the interstitial site. The calculated values were positive, indicating that segregation is not favored in the Sigma 3(111) grain boundaries. When the metal impurity is placed at the substitutional site, a new state in the fundamental gap was observed in the density of states, the band gap is reduced, which may have an effect on the solar cell performance [5]. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.[1] N. Usami et al., Jpn. J. Appl. Phys. 45 (2006) 1734.[2] B.-J. Lee and S.-H. Choi, Modelling Simul. Mater. Sci. Eng. 12, 621 (2004).[3] J. Tersoff, Phys. Rev. B39 (1989) 5566.[4] H. Ogawa, Mater. Trans. 47 (2006) 2706.[5] A. Suvitha et al., Jpn. J. Appl. Phys. 49 (2010) 04DP02.
6:00 PM - RR5.20
Multiscale Approach to Study Defect Evolution in Vanadium.
Ning Wang 1 , Huiqiu Deng 1 2 , Nengwen Hu 1 , Wangyu Hu 1 , Fei Gao 2
1 Department of Applied Physics, Hunan University, Changsha 410082 China, 2 , Pacific Northwest National Laboratory, PO Box 999, Richland 99352, Washington, United States
Show AbstractVanadium-based alloys are one kind of promising candidate materials in fusion reactions due to their excellent low activation and high radiation resistance properties. In the present paper, molecular dynamics (MD) and object kinetic Monte Carlo (OKMC) methods had been combined to study the defect evolution generated by displacement cascades in pure metal vanadium. The configurations and locations of the surviving defects were generated by MD, while the binding and migration energies of small self-interstitial atoms (SIAs), vacancies or their clusters were obtained with nudged elastic band method. Larger space and longer time annealing was carried out using OKMC. It was found that almost all of the single interstitials were formed in the most stable configure of <111> dumbbells and migrated along the <111> direction. The simulation temperature and the recoil energy are the key factors that decide the cascade debris. The lower migration energies of SIAs than vacancies lead to different recovery stages during annealing simulations. SIAs and vacancies began to recombine when the temperature was as low as 4K. At higher temperature the vacancies migrated more easily, which lead to the formation of large vacancy clusters.
6:00 PM - RR5.23
Microstructure Evolution of Pure Cu Processed by Surface Rolling.
Qingsong Mei 1 , Koichi Tsuchiya 2 , Kaneaki Tsuzaki 2
1 International Center for Young Scientists, National Institute for Materials Science, Tsukuba Japan, 2 , National Institute for Materials Science, Tsukuba Japan
Show AbstractPure Cu (99.97%) was deformed by surface rolling (SR) at room temperature. The microstructure variation with the distance from the treated surface (DFS) was investigated by electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). It was found that SR can induce substantial microstructure refinement in the surface layer up to a DFS as large as ~ 400 micronmeter, including an ultrafine-grained layer ~200 micronmeter thick. With the decrease of DFS, the structure length scale decreases gradually from ~100 micronmeter in the matrix to ~ 100 nm at the topmost surface (with a minimum of ~ 10 nm); meanwhile the fraction of the high angle grain boundaries (HAGB) increases. Detailed microstructure analysis shed light on how the microstructure refinement was mediated by the evolution of dislocation structures under various stress and strain levels.
6:00 PM - RR5.25
A Peierls Perspective on Mechanisms of Atomic Friction.
Yanfei Gao 1 2
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractStick-slip behavior observed from nanoscale asperity friction experiments are often modeled by the one-degree-of-freedom Tomlinson model, which is unable to explain the effects of lattice structure and interface defects, or by molecular simulations which suffer temporal limitations in modeling the velocity- and temperature-dependent frictional behavior. A Peierls-type model developed in this work views the atomic frictional process as the initiation and gliding passage of dislocations with diffused cores on the interface. As a consequence of loss-of-ellipticity instability, the occurrence of stick-slip behavior relies on the interaction among interface slip field, contact stress fields, and existing defects. The friction stress for commensurate interface under large contact area can be approximated from the Rice model of screw dislocation nucleation from a planar crack tip. The spatially inhomogeneous nature of rate-limiting processes and the coupling effects between contact size and interface incommensurability are successfully determined, which cannot otherwise be tackled in the Tomlinson model.Financial support for this work was provided by the National Science Foundation and by the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
6:00 PM - RR5.26
Effects of Different Selenium Supply Levels on Defect Formation in Non-vacuum Deposited Cu(In,Ga)Se2 Thin Films and Their Photovoltaic Performances.
Qing Cao 1 , Oki Gunawan 1 , Matthew Copel 1 , David Mitzi 1
1 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractFor non-vacuum based Cu(In,Ga)Se2 (CIGS) thin-film deposition processes, control over Se supply during high-temperature annealing, which transforms solution-deposited precursors to crystalline CIGS grains, is critical for getting device quality photovoltaic (PV) materials. Using the hydrazine-based CIGS deposition approach, the influences of different levels of Se supply, i.e. the amount of molten Se available during CIGS grain growth, on materials and device properties are characterized via temperature dependent optoelectrical characterizations, quantum efficiency measurements, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and admittance spectroscopy. Although Se supply level does not significantly affect grain structure or crystal orientation of the resulting CIGS films, higher Se supply suppresses the formation of an ordered defect compound layer near the CIGS surface by reducing Se loss from the CIGS lattice, leading to significantly better PV performance. This result is important for understanding the correlation among process conditions, material defects, and PV performances of CIGS, as well as the future development, especially scaling-up, of non-vacuum based PV-targeted CIGS deposition approaches.
6:00 PM - RR5.27
Characterization of Oxide Precipitates in NiAl Based Oxide Dispersion Strengthened (ODS) Alloy.
Hyon-Jee Voigt 1 , Yong Deog Kim 1 2 , Zuhair Munir 3 , Brian Wirth 2
1 Nuclear Engineering, UC Berkeley, Berkeley, California, United States, 2 Nuclear Engineering, University of Tennessee, Knoxville, Tennessee, United States, 3 Chemical Engineering and Materials Science, UC Davis, Davis, California, United States
Show AbstractRecently, an attempt to produce a new generation of NiAl based oxide dispersion strengthened (ODS) alloy has been made using mechanical alloying methods, with the objective of improving the high temperature strength and creep resistance. These target properties are expected if high number density of stable nano-meter scale clusters are formed, akin to those recently observed to improve creep strength and radiation resistance in nano-structured ferritic alloys (NFAs). To better understand and aid the currently on-going NiAl based ODS alloys development process, computational methods are also employed with the objective of understanding the size, structure, number density, and thermal stability of oxide precipitate in addition to the experimental methods. A lattice Monte Carlo (LMC) method with ab-initio derived interaction energies is used to describe the oxide precipitates in NiAl alloy and the results are compared to the experimental analysis results. The improved mechanical properties will be explained in terms of the size, structure, and number density of the precipitated oxide particles. The thermal stability will be discussed using the oxide particle structure and the mechanical properties as a function of thermal annealing condition.
6:00 PM - RR5.29
Wang-Landau Sampling for First Principles Finite Temperature Magnetism of Defects in Fe.
Aurelian Rusanu 1 , Don Nicholson 1 , Kh. Odbadrakh 1 , G. Brown 2 , M. Eisenbach 1 , G. Stocks 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Florida State University, Tallahassee, Florida, United States
Show AbstractDeploying recent developments in computational statistical mechanics, Wang-Landau method, with the first principles multiple scattering electronic structure code (LSMS) to evaluate the energy of constrained magnetic states we describe from first-principles the thermodynamic and magnetic properties of Fe. The combined Wang-Landau LSMS method (WL-LSMS) utilizes parallelism at multiple levels to accelerate sampling of the energy landscape. At an early stage of the calculation, long before sampling is complete the WL-LSMS generates configurations distributed evenly over the energy spectrum. These configurations and their energies can be used in a least-squares fitting procedure to obtain a simple, e.g. Heisenberg, model that can benefit convergence of the sampling procedure. The least-squares procedure and its use in accelerating the first principles sampling of the energy landscape will be presented. The approach will be demonstrated for Fe systems and will address the effects of defects on magnetic properties.This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. Calculations performed at the National Center for Computational Sciences (NCCS).
6:00 PM - RR5.3
Molecular Dynamics Simulation of the Degradation of the Thermal Conductivity in Silicon Carbide with Irradiation Induced Point Defects.
Jean-Paul Crocombette 1
1 SRMP, CEA Saclay, Gif/Yvette Cedex France
Show AbstractThe high thermal conductivity of silicon carbide is a key property in view of its possible use for future fusion or fission nuclear reactors. However SiC thermal conductivity is known to exhibit a huge decrease with irradiation even at high temperatures where irradiation produces basically only point defects. To understand this effect, we calculated by empirical potential molecular dynamics the thermal resistivity associated with point defects of various kinds : carbon and silicon vacancies, interstitials and antisites.We found that the semi-empirical law stating that the additional thermal resistivity (compared to defect free material) due to point defects is proportional to their concentration is indeed valid up to very large concentrations of defects ( ~10 %). The resistivity of the defects vary greatly with the defect nature with more than an one order of magnitude of difference between the weakly resistant defects (carbon antisites) and the strongly resistant ones (silicon vacancies and interstitials). We are thus able to predict the additional thermal resistivity of SiC from the concentrations of defects. As an example of application we considered as a first approximation that high temperature irradiation mainly results in an unknown concentration of vacancies (interstitials being more mobile). We are then able to build from our calculated thermal resistances a scale to estimate the concentration of vacancies in samples irradiated under various conditions from the measured thermal conductivities. The defect concentrations obtained are quite high but are consistent for low temperature irradiation with the threshold concentrations needed to trigger amorphization.
6:00 PM - RR5.30
Degradation Mechanisms of Lithium-iron-phosphate as Cathode Materials for Rechargeable Batteries.
Yixu Wang 1 , Sergey Verlinski 1 , Hsiao-Ying Shadow Huang 1
1 Mechanical & Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractThe need for development and deployment of reliable and efficient energy storage devices, such as lithium-ion rechargeable batteries, is becoming increasingly important due to the scarcity of petroleum. Lithium-ion batteries operate via an electrochemical process in which lithium ions are shuttled between cathode and anode while electrons flowing through an external wire to form an electrical circuit. In this work, we provided an overview of commercially available cathode materials for Li-ion rechargeable batteries and focused on characteristics that give rise to optimal energy storage systems for future transportations. The study showed that the development of lithium-iron-phosphate (LiFePO4) batteries promises an alternative to conventional lithium-ion batteries, with their potential for high energy capacity and power density, improved safety, and reduced cost. However, current prototype LiFePO4 batteries have been reported to lose capacity over ~3000 charge/discharge cycles or degrade rapidly under high discharging rate. In this study, we hypothesized that the mechanical and structural failures were attributed to dislocations formations. Numerical models and crystal visualizations were provided to further understand the stress development due to lithium movements during charging or discharging. This work will contribute to the fundamental understanding of the mechanisms of capacity loss in lithium-ion battery materials and help the designing of better rechargeable batteries, and thus leads to economic and environmental benefits.
6:00 PM - RR5.31
Evolution of Copper-rich Precipitates in Reactor Pressure Vessel Steels under High-dose Irradiation.
Mikhail Sokolov 1 , Michael Miller 1 , Randy Nanstad 1
1 , ORNL, Oak Ridge, Tennessee, United States
Show AbstractCurrent fleet of nuclear power plants is poised for operating life time extension. It means that main structural components, including reactor pressure vessel, will be subject to higher neuron exposure than originally planned. This raises serious concerns regarding our ability to predict reliability of reactor pressure vessel steels at such high doses. In this study, several representative reactor pressure vessel steels (RPVS) were irradiated at high doses to study degradation of mechanical properties and related microstructural changes of RPVS. It is well known that copper-rich precipitates are key microstructural features that are responsible for radiation hardening of RPV steels. In this study, the evolution of copper-rich precipitates (CRP) is studied by means of small-angle neutron scattering and atom-probe tomography. These techniques are used to measure number density, volume fraction, and radius of precipitates. Evolution of these microstructural features is compared to degradation of fracture toughness and hardening of these steels.
6:00 PM - RR5.32
Crystalline and Amorphous Models of Highly Damaged Fe.
Madhu Ojha 2 1 , Donald Nicholson 1 , Takeshi Egami 2 1
2 , University of Tennessee, Knoxville, Tennessee, United States, 1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show AbstractThe structure of irradiated material near the primary knock on atom shortly after impact is largely unknown. Molecular dynamics simulations with classical force fields are largely responsible for our current understanding. We find that the simulated pair distribution function in the highly defected region of the cascade resembles that of the liquid. The energy and structure of the highly defected material is similar to that of amorphous phases. Analysis of defected crystalline and amorphous structural models of the highly defected volume will be presented together with the related magnetic/electronic structure, and local stresses. In particular the fluctuation over time of local moments, electronic density of states, and stress will be discussed. Results are taken from MD based on classical force fields and VASP.Acknowledgements:This work was supported by the Office of Basic Energy Sciences, Materials Science and Engineering Division and the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
6:00 PM - RR5.4
Observation of Dislocation Networks in Pre-strained Directionally Solidified NiAl-Mo Alloys.
J. Kwon 1 , L. Yang 1 , H. Bei 2 , E. George 2 , M. Mills 1
1 Materials Science and Engieering, The Ohio State University, Columbus, Ohio, United States, 2 Materials Science and Technology, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractRecent experimental results (Bei et al. 2008) indicate that directionally solidified (DS) Mo-alloy single crystal micropillars behave like a dislocation free material, exhibiting yield strengths comparable to the theoretical strength. Pre-strained Mo micropillars (4-8%), on the other hand, show stochastic strength distribution until, at large pre-strain (11%), they exhibit deterministic bulk-like behavior. In order to understand the origin of this mechanical behavior in terms of development of dislocation networks at different pre-strain levels, transmission electron microscopy (TEM) analysis was conducted for NiAl-Mo alloys.DS NiAl-Mo composites consisting of well-aligned [100] Mo fibers embedded in a [100] NiAl matrix were pre-strained from 0% to 14.3% along the [100] direction. Samples were then extracted parallel to the [100] direction using focused ion beam (FIB) techniques and their cross-sections were examined via TEM.Preliminary TEM results showed that as-grown samples (without pre-straining) do not contain dislocations in the Mo fiber, consistent with the previous results (Bei et al. 2008). In the NiAl matrix, on the other hand, <100>-type dislocations exist along the [100] Mo fiber axis, and wrap around the axis, presumably arising from the lattice mismatch and different thermal expansion coefficients. With increased pre-strain, <111>-type dislocations are present inhomogeneously in the Mo fiber and the dislocation density is increased. Pre-existing dislocations in the NiAl matrix are replaced by entangled dislocation configurations. The dislocations in the Mo fibers appear to be transmitted from the NiAl matrix. The inhomogeneous dislocation distribution in the Mo fiber at intermediate pre-strain (2.2-5.3%) can be related to the stochastic distribution of strength values. As the dislocation density becomes more homogeneous at larger pre-strain (> 12%), the Mo microcrystals take on bulk-like mechanical behavior. Research sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, “Center for Defect Physics,” an Energy Frontier Research Center (JK and MJM) and the Materials Sciences and Engineering Division (HB and EPG).
6:00 PM - RR5.7
Evaluation of Yittria Segregation in Polycrystalline 10 mol% Yittria Doped Zirconia (10YSZ).
Muhammad Idris 1 2 , Janusz Nowotny 3 , Kathryn Prince 4 , Sean Li 1
1 School of Materials Science and Engineering, The University of New South Wales, Sydney, New South Wales, Australia, 2 School of Materials Science and Engineering, University Malaysia Perlis, Kubang Gajah, Perlis, Malaysia, 3 Solar Energy Technologies, School of Natural Sciences, University of Western Sydney, Penrith South DC, New South Wales, Australia, 4 , Australian Nuclear Science & Technology Organisation, Menai, New South Wales, Australia
Show AbstractThe present work applied secondary ion mass spectroscopy (SIMS) to assess the segregation of yttria in yttria-stabilized zirconia (10 mol% doped yittria – 10YSZ) annealed at temperature 1073 – 1673 K in p(O2) = 75 kPa. The essential effects, which influence the quantification of SIMS intensity data, is briefly discussed including (i) the matrix effect, (ii) charging and (iii) surface topography. The impact of these effects on the intensity of secondary ion spectra, including the host lattice cation, 90Zr, as well as the bulk concentration of dopant species, 89Y, is discussed. The effect of oxygen activity and temperature on 89Y segregation is discussed. The obtained SIMS data are compared with the XPS data reported earlier by Hughes.
6:00 PM - RR5.8
Measuring ZnO c-Axis Polarity With X-ray Photoelectron Diffraction.
Jesse Williams 1 , Naoki Ohashi 1 , Igor Pis 2 3 , Masaaki Kobata 2 , Keisuke Kobayashi 2 , Aimo Winkelmann 4
1 International Center for Materials Nanoarchitectinocs, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 NIMS Beamline station at Spring-8, National Institute for Materials Science, Sayogun, Hyogo, Japan, 3 Department of Surface and Plasma Science, Charles University, Prague Czech Republic, 4 , Max-Planck-Institut für Mikrostrukturphysik, Weinberg Germany
Show AbstractIn the present study, we use x-ray photoelectron diffraction (XPD) to measure the polarity of ZnO non-destructively. The growth direction of thin-film ZnO is in the crystallographic c-axis, and the polarity of the c-axis is defined with the (0001) direction has a Zn-terminated surface while the (000-1) direction has an O-terminated surface. The c-axis polarity affects material parameters such as chemical resistance to surface etchants such as acids and interface electronic band structure. Current methods used to distinguish (0001) from (000-1) polarity are acid etching, convergent beam electron diffraction (CBED), and coaxial impact collision ion scattering spectroscopy (CAICISS). Etching and CBED are both destructive techniques while CAICISS requires high quality single crystals.The XPD measurements are made using synchrotron radiation as well as a Cr Kα x-ray source that produces hard x-rays. Both x-ray sources facilitate bulk measurement. The XPD patterns are obtained by measuring the angular dependence of photoelectron intensity for a defined energy. XPD patterns of single crystal ZnO and oriented polycrystalline ZnO thin-films were measured using the Zn 2p3/2 and the O 1s peaks. The resulting diffraction patterns are unique for a given polarity indicating that XPD is a viable method for such polarity determination.
6:00 PM - RR5.9
Characterization of Nanoporous TiO2 Surface Defects by Temperature Dependent Electron Transport Studies on Dye Sensitized Solar Cells.
Mariyappan Shanmugam 1 , Mahdi Farrokh Baroughi 1
1 , South Dakota State University, Brookings, South Dakota, United States
Show AbstractDye sensitized solar cells (DSSCs) use large surface area nanoporous TiO2 photoelectrodes to adsorb dye molecules onto the porous network. The large surface area of the nanoporous TiO2 possesses surface states which are basically defect centers which can trap an electron from the conduction band of the TiO2. Since TiO2 is used as an electron transport layer in DSSCs, the performance is greatly depends on the density and activity of TiO2 surface states on photoelectrons. Life time of the photoelectrons should be increased in order to improve the collection efficiency at the transparent conducting oxide. But, large density of TiO2 surface states trap photoelectrons from the conduction band of TiO2 which consecutively recombine with a hole present either at the dye or electrolyte. This major photoelectron recombination mechanism is governed by the TiO2 surface states since they initiate the process by trapping. Several methods have been proposed to suppress the effect of TiO2 surface states on photoelectrons to improve DSSC performance. This paper presents characterization of TiO2 surface states using temperature dependent current-voltage characteristics of the DSSCs under dark condition. A reference (no metal oxide interface treatment), Al2O3 and HfO2 treated DSSCs were fabricated as reported in our earlier work. The DSSCs were fixed in a holder arranged inside an oven in which the temperature can be varied. Additionally a thermometer was attached at the DSSC holder inside the oven to measure the temperature on the DSSC precisely. Temperature dependent I-V characteristics of the DSSCs were measured in the temperature range from 30 to 85°C with the step of 5°C using Agilent 4155C Semiconductor parameter analyzer. DSSCs with Al2O3 and HfO2 treated photoelectrodes showed activation energies 1.44 and 1.62 eV respectively while the DSSC without any metal oxide treatment showed 1.08 eV. These activation energies of electron transport can directly correlated with the performance of the DSSCs. Increasing the energy gap between the surface state which are close to the redox potential of the electrolyte and conduction band of the TiO2 reduce the probability of electron trap and hence the reduced probability of electron back injection to the electrolyte/dye. Basically, growth of Al2O3 and HfO2 layers on TiO2 surface changed the energy dependence of the TiO2 surface states which are close to the redox potential of the electrolyte. Thus, electron transfer from the surface states to electrolyte is suppressed.References[1] Gerrit Boschloo and Anders Hagfeldt, J. Phys. Chem. B 109 (2005) 12093-12098[2] Nikos Kopidakis et al, Physical Review B, 73 (2006) 45326-45333[3] Jenny Nelson et. al., Physical Review B 63(2001) 205321-205329. [4] Xin-Tong Zhang et al. Solar Energy Materials & Solar Cells 81 (2004) 197–203.[5] Andreas Kay, and Michael Gratzel, Chem. Mater., 14 (2002) 2930-2935.
Symposium Organizers
G.Malcolm Stocks Oak Ridge National Laboratory
Blas Uberuaga Los Alamos National Laboratory
Anter El-Azab Florida State University
RR6: Interfaces: Grain Boundaries
Session Chairs
Wednesday AM, April 27, 2011
Room 2022 (Moscone West)
9:00 AM - RR6.1
First Principles Approach for Dislocation/Twin Boundary Interactions in Ti with Chemistry Changes.
Maryam Ghazisaeidi 1 , Dallas Trinkle 1
1 , University of Illinois, Urbana, Illinois, United States
Show AbstractPredictive models of mechanical properties of materials depend on accurate understanding of the defects. Interaction of gliding dislocations with twin boundaries influences the plastic deformation of titanium. In addition, solutes greatly affect both strength and twining in Ti. To understand the underlying atomic-scale mechanisms, we model a [1-210] screw dislocation interacting with a Ti (10-12) twin boundary using density functional theory. Flexible boundary conditions are used to calculate the accurate geometry by coupling the isolated dislocation to the surrounding medium through the interfacial lattice Green's function. We compute the dislocation core structure and transmission stresses through the boundary and investigate the effects of oxygen and aluminum solutes on the core structure and mobility of the dislocation in the twin boundary. This is a computationally tractable approach that marks the first ab initio study of isolated line defects in interfaces. The results provide first principles data for core structure and transmission mechanisms of a screw dislocation interacting with a twin boundary in the presence of solutes.
9:15 AM - RR6.2
Study of Grain Growth in SrTiO3 Using X-ray Tomography.
Melanie Syha 1 , Michael Baeurer 2 , Wolfgang Rheinheimer 2 , Erik Lauridsen 3 , Wolfgang Ludwig 4 , Daniel Weygand 1 , Michael Hoffmann 2 , Peter Gumbsch 1
1 izbs, Karlsruhe Institute of Technology, Karlsruhe Germany, 2 IKM, Karlsruhe Institute of Technology, Karlsruhe Germany, 3 Metals in 4D, Risø National Laboratory for Sustainable Energy, Roskilde Denmark, 4 , European Synchrotron Radiation Facility, Grenoble France
Show AbstractRecent experimental investigations on grain growth kinetics in SrTiO3 revealed a puzzling phenomenon: a decrease of several orders of magnitude in the effective growth rate with increasing temperature occuring at two distinct transition temperatures [1].The determination of possible origins for this behavior requires a detailed analysis of grain growth in SrTiO3 with a special emphasis on the connection between grain morphology and interface properties in the different temperature regimes. Conventional two dimensional analysis however is not conclusive in the identification of topological quantities that are crucial for the understanding of the growth behavior of any 3D grain boundary network. Thus, SrTiO3 structures from the three different growth regimes were subjected to X-Ray Diffraction Contrast Tomography (DCT) [2] measurements. The DCT method, here applied for the first time to a perovskite material, allows to map the 3D grain structure and the crystallographic orientation of each grain simultaneously and hence enables the investigation of topological quantities such as number of neighbors, misorientation and inclination distributions and more with reasonable statistics. In order to investigate the grain growth kinetics, DCT measurements were performed on identical samples before and after a heat treatment, giving access to the local relation between interface orientation and effective interface mobility. Furthermore, TEM and SEM investigations [3] of grain and pore shapes were used to identify the temperature dependency of the interfacial energies.[1]M.Bäurer, D.Weygand, P.Gumbsch and M.J.Hoffmann: Grain growth anomaly in strontium titanate, Scripta Mat. 2009[2]W. Ludwig, P.Reischig, A.King, M.Herbig, E.M.Lauridsen, G.Johnson, T.J.Marrow and J.Y.Buffiere: Three-dimensional grain mapping by x-ray diffraction contrast tomography and the use of Friedel pairs in diffraction data analysis, Rev. Scientific Instr. 2009[3]M.Bäurer, H.Störmer, D.Gerthsen and M.J.Hoffmann:Linking Grain Boundaries and Grain Growth in Ceramics, Advanced Engineering Materials, 2010
9:30 AM - RR6.3
Grain Boundary Energy Function for FCC Metals.
Vasily Bulatov 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractTo enable meso-scale simulations of GB networks under thermal coarsening and/or irradiation conditions, it is desirable to express the GB energy as an explicit function of five macroscopic degrees of freedom. The very existence of such a function has been controversial [1,2] but recent computer simulations suggest that, at least for one specific type of materials (metals) and crystallography class (FCC), such a function should exist [3,4]. In this development we focus on the global topology of the functional 5-space of the GB energy function and examine positions and connectivity of its singular points – cusps. By construction, the GB energy function reflects all relevant symmetries of the bi-crystal, its functional form is universal for FCC crystallography but contains material-specific parameters that can be fit to energies measured or computed for just a handful of special boundaries. The resulting function provides a high quality fit to the extensive data sets of GB energies computed by Olmsted et al for Al, Ni, Cu and Au (388 CSL boundaries for each metal) [3,4]. [1] A. P. Sutton and R. W. Balluffi, Acta metall. 35, (1987) 2177-2201. [2] D. Wolf, Handbook of Materials Modeling, S. Yip (ed.) (Springer, 2005), 1953-1983. [3] D. L. Olmsted, S. M. Foiles, and E. A. Holm, Acta mater. 57 (2009) 3694–3703.[4] E. A. Holm, D. L. Olmsted, and S. M. Foiles, Scripta mater. 63 (2010), 905-908.
9:45 AM - **RR6.4
Computing Excess Free Energies of Interfaces at High Temperature.
Mike Finnis 1
1 , Imperial College London, London United Kingdom
Show AbstractTraditional approaches of statistical mechanics and molecular dynamics, whether using quantum mechanically based total energies or empirical potentials, are challenged by the problem of calculating the structural and thermodynamic properties of interfaces in materials at high-temperature. I describe here our recent developments in the computational study of grain boundaries, solid-solid interfaces and crystal-melt interfaces. There are two generic problems in all these areas of application: to sample the phase space of configurations, in order to obtain the high-temperature excess free energy, and to find collective variables with which to describe phase transitions. Two examples will be illustrated, a) the use of a genetic algorithm to determine low-energy solid-solid interface structures , and b) the use of metadynamics to obtain the free energy of the crystal-melt interface.
10:15 AM - RR6.5
Theoretical Approach to the Effect of the Grain Boundaries on the Atomic Diffusion in α-Al2O3 by First Principles Calculation and Molecular Dynamics Simulation.
Nobuaki Takahashi 1 , Teruyasu Mizoguchi 1 , Tetsuya Tohei 1 , Tsubasa Nakagawa 2 , Takahisa Yamamoto 1 3 , Yuichi Ikuhara 1 3 4
1 Materials Engineering, The University of Tokyo, Tokyo Japan, 2 , National Institute for Materials Science, Tsukuba Japan, 3 , Japan Fine Ceramics Center, Nagoya Japan, 4 , Tohoku University, Sendai Japan
Show AbstractHigh temperature mechanical properties of polycrystalline materials are strongly related to the atomic transportation phenomena, namely diffusion. α-Al2O3 is one of the most utilized high temperature structural materials and the mechanical properties are known to be dominated by a short-circuit diffusion along grain boundaries (GBs). So many investigations focused on oxygen self-diffusion in Al2O3 have been performed. However, its atomistic migration mechanism has not been revealed especially about the atomic diffusion at the GBs. In this study, to discuss the origin of GB diffusion mechanism of Al2O3, we investigated oxygen vacancy formation energetics at GBs by first principles projector augmented wave calculation and effective atomic diffusion paths along GB by classical molecular dynamics (MD) simulation. In this study, three different GBs, Σ11 (10-11), Σ13 (10-14) and Σ37 (-1018) were employed. The supercell containing 200 (Σ11), 240 (Σ13) and 480 (Σ37) atoms was used for the defect formation energy calculations by first-principles calculation. One oxygen atom was removed from the supercell to introduce a vacancy. On structure optimizations, all atoms in the supercell were allowed to relax. For the classical molecular dynamics (MD) simulations, vacancies were randomly introduced from the extended supercell. After the adequate equilibration of the systems, oxygen trajectories were investigated by the statistical MD simulations using the empirical pair-potential under NEV unsembles.We calculated the oxygen vacancy formation energies of 10 sites in Σ13 GB. It was found that there is clear site-dependency of the vacancy formation energy, and the vacancies are more preferably formed at the GBs than that in bulk. Furthermore, It was found that the strains tend to decrease as increasing the distance from the GB, and simultaneously, the formation energies of the oxygen vacancy also increase. This tendency was confirmed in the results about other two boundaries. This indicates that the defect energetic at the GB is closely related to these structural distortions at the GB. From the MD simulation, it is revealed that the oxygen will migrates along the GBs only via the specific sites quite selectively. For the several “effective” migration paths and a “not effective” path predicted by MD simulation, the migration barriers were calculated by the first principles calculation with nudged elastic band method. The result was that the values of migration barrier for the formers were about 0.5~1.1 eV whereas that for the latter was about 2.9 eV, which was consistent with the result of MD simulation. In the presentation, we will show the comparison between these calculation results and the previous experimental data about the oxygen GB diffusivity and refer to the several possibilities about the GB diffusion mechanism in Al2O3.
10:30 AM - RR6.6
In Situ TEM Observations of Deformation Twinning in Single Crystal Mg.
Qian Yu 1 2 , Andrew Minor 1 2 , Raj Mishra 3
1 MSE, University of California Berkeley, Berkeley, California, United States, 2 , National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , General Motors Research and Development Center, Warren, Michigan, United States
Show AbstractMagnesium is a lightweight metal that would be much more useful for structural applications than it currently is if not for its complicated and anisotropic plastic deformation behavior. The HCP structure of Mg has limited dislocation slip systems and thus twinning generally accompanies dislocation plasticity. Recently we have run a series of in situ mechanical tests in a TEM on pure Mg where we have quantitatively measured and characterized the deformation behavior in compression, tension and bending in samples oriented along [0001]. Both tension and bending are free of boundary constrains, but compression is subjected to a contact surface. Homogenous nanotwinning is found to dominate deformation and contribute to the high plasticity in tension and bending. Alternatively, heterogeneous nucleation and growth of a single large twin was observed in compression. Through these quantitative in situ TEM experiments we are able to provide significant insight into the twinning mechanisms and the origins of the high strength and plasticity in small Mg samples.
10:45 AM - RR6.7
Thermal Transport in Ceria Thin Films Having Engineered Microstructure.
Marat Khafizov 1 , David Hurley 1 , In-Wook Park 2 , John Moore 2 , Jianliang Lin 2 , Ryan Deskins 3 , Anter El-Azab 3
1 Material Science and Engineering, Idaho National Laboratory, Idaho Falls, Idaho, United States, 2 , Colorado School of Mines, Golden, Colorado, United States, 3 , Florida State University, Tallahassee, Florida, United States
Show AbstractUnderstanding thermal conductivity of nuclear fuels is important for predictive modeling of nuclear fuel performance. Here, we present our studies of phonon thermal transport in ceria as a surrogate material for oxide fuels. Polycrystalline ceria thin films having engineered microstructure were deposited on silicon substrates using RF magnetron sputtering. Thermal transport properties were analyzed using laser based time-resolved thermal wave microscopy (TRTWM). In TRTWM, the sample is heated by a strongly focused, amplitude modulated ultrashort laser pulse train. Thermal transport in the material is measured by a time delayed probe pulse that senses small temperature induced changes in reflectivity. Thermal wave profiles are recorded by scanning the probe beam across the surface. Thermal properties of material are extracted from the analysis of the measured temperature profile using a continuum based model. We perform temperature and grain size dependent measurements and develop a model to determine phonon scattering mechanisms governing thermal conductivity in ceria based on Boltzmann transport equations.
RR7: Defect Aggregation and Clustering I
Session Chairs
Wednesday PM, April 27, 2011
Room 2022 (Moscone West)
11:30 AM - RR7.1
Defect Characterization in Swift Heavy Ion and Proton Irradiated CeO2.
Peng Xu 1 , Tony Schulte 1 , Hunter Henderson 2 , Billy Valderrama 2 , Michele Manuel 2 , Jian Gan 3 , Allen Todd 1 3
1 , University of Wisconsin, Madison, Wisconsin, United States, 2 , University of Florida, Gainesville, Florida, United States, 3 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Show AbstractDefect production and evolution play an important role in determining mechanical, chemical, and thermophysical properties of irradiated materials. The focus in this study is to characterize radiation induced defects and defect evolution in CeO2, a fluorite type material which serves as a surrogate for UO2, the common light water reactor (LWR) fuel used worldwide. Due to the similarities in their structures and physical properties, the knowledge learned in radiation response of CeO2 is helpful in advancing the current understanding of the radiation damage mechanisms in fluorite type compounds such as UO2. To simulate fission fragments damage, irradiation tests have been completed using 300 and 1000 MeV Au ions to fluences from 1010 to 5x1012 /cm2. To study the nucleation and accumulation of defects and fission gas bubbles at intermediate temperatures, irradiation tests will be conducted using 2.6 MeV protons at temperatures ranging from 500 to 900 °C and doses up to 1 dpa. The structure and microstructure evolution of the irradiated CeO2 as a function of ion fluences and temperatures will be characterized using various techniques such as Raman spectroscopy, glancing angle x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), aberration corrected scanning transmission electron microscopy (STEM), drift corrected energy dispersive spectroscopy (EDS), and high energy x-ray scattering using the advanced photon source (APS). Structure recovery under thermal annealing will be discussed.
11:45 AM - RR7.2
Investigations on Vacancy Clustering and Formation of Stacking Fault Tetrahedra over Experimental Time Scales.
Hao Wang 1 2 , David Rodney 1 , Dongsheng Xu 2 , Rui Yang 2
1 SIMAP-GPM2, Institut National Polytechnique de Grenoble, St Martin d’Heres France, 2 , Institute of Metal Research, Shenyang China
Show AbstractVacancy clusters, particularly in the form of stacking fault tetrahedra (SFTs), significantly alter the mechanical behavior of metals and alloys. This is especially relevant under condi-tions of irradiation where defect clusters are observed in high concentrations. However, the atomic-scale mechanisms by which such clusters are produced remain unclear. In the pre-sent investigation, molecular dynamics (MD) is combined with accelerated dynamics to reveal both the short-term transformations and long-term evolutions of vacancy clusters under conditions of local supersaturation induced by dislocation interactions. MD simula-tions show that SFTs, together with various forms of vacancy clusters, form within a few ns upon annihilation of edge dislocation dipoles. This process is enhanced by the low mi-gration barriers of vacancies along the defected channels that constitute dislocation cores. Accelerated dynamics, based on the activation-relaxation technique and metadynamics, is employed to explore the energy landscape of defected crystals and to predict their long-term structural evolutions. Here we show how the vacancy clusters produced at the end of MD simulations evolve into larger, more structured SFTs over experimental time scales.
12:00 PM - RR7.3
Multiscale Modeling of Fe Systems under Irradiation.
Ioannis Mastorakos 1 , Ngoc Le 1 , Hussein Zbib 1 , Mohammad Khaleel 2
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 2 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractStructural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production, thus experiencing severe degradation of mechanical properties. The development of structural materials for use in such a hostile environment is predicated on understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the behavior of Fe systems (namely α-Fe, Fe-Cr and Fe-Ni) under irradiation. To achieve that we use molecular dynamics (MD) simulations to investigate dislocation mobilities, dislocation – voids and dislocation – prismatic loops interactions. The information collected from the atomistic simulations is organized in a form suitable for the dislocation dynamics (DD) in order to study the phenomena involved at a higher length scale, where the systems are larger and contain higher dislocation densities and irradiated induced defects. Furthermore, the role the grain boundaries play on the hardening of irradiated a-Fe is investigated using MD simulations. Critical rules including critical stress dependencies on grain boundary size and type are derived.
12:15 PM - RR7.4
Nonequilibrium Thermodynamics of Void Microstructure Evolution and Swelling under Irradiation.
Srujan Rokkam 1 , Anter El-Azab 2 3 , Thomas Hochrainer 2
1 Department of Mechanical Engineering , Florida State University, Tallahassee, Florida, United States, 2 Department of Scientific Computing, Florida State University, Tallahassee, Florida, United States, 3 Materials Science Program, Florida State University, Tallahassee, Florida, United States
Show AbstractIrradiation induced voids and associated swelling is a key problem of fuel and structural materials performance in nuclear reactor. Void formation and a myriad of other phenomena occurring in materials exposed to irradiation are sensitive to dynamics of point defects and their interaction with other microstructural entities. We formulate a gradient theory based diffuse interface model in which the void nucleation and growth and, in turn, swelling processes are treated simultaneously in a spatially resolved fashion. Using the principles of irreversible thermodynamics and a gradient theory approximation we obtain the temporal evolution equations for our system which are reminiscent of classical Cahn-Hillard type equations for defect concentration fields, and an Allen-Cahn type equation for void order parameter. The processes of void nucleation and dynamics of void growth are obtained in terms of the evolution of non-conserved order parameter field. Irradiation induced point defects are modeled as a stochastic source in the Cahn-Hilliard equations, introducing vacancies and interstitials in spatially segregated fashion similar to the nature of displacement cascade. The model accounts for mutual interactions between point defects, interactions between point defects and extended defects, effects of applied stress, cascade and thermally induced fluctuations. Using two dimensional numerical simulations, we demonstrate our model capabilities for void nucleation, growth and swelling resulting from cascade-induced defects. Contrary to the populist belief, we observe that the swelling of the material can occur prior to the nucleation of voids, through the migration of interstitials to the material surface. It is observed that void nucleation in fact restricts the amount of swelling. Our model predictions pertaining to the dependence of nucleation and swelling on various material parameters, dose rate and temperature are presented. This research was supported by the US Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center.
12:30 PM - RR7.5
Irradiation-induced Void Microstructure Evolution in Stoichiometric Oxides.
Thomas Hochrainer 1 , Anter El-Azab 1 , Abdel Hassan 2
1 Scientific Computing, Florida State University, Tallahassee, Florida, United States, 2 Materials Science Program, The Florida State University, Tallahassee, Florida, United States
Show AbstractVoid formation and growth is one of the most technologically relevant and intriguing problems in the performance of nuclear fuels. We present a theoretical framework for point defect dynamics and microstructure evolution in irradiated oxides based on non-equilibrium thermodynamics. The model consists of a set of balance laws of defect concentrations, electrical charge density and stress. The obtained field equations describe the evolution of both anion and cation vacancy and interstitial densities in space and time. Special emphasis is put on the boundary conditions at free surfaces like that of voids or bubbles. Describing the motion of free surfaces, which move due to defect fluxes, requires modeling the reaction of defects with the surface and the adjacent atmosphere. In oxide crystals the surface motion is restricted by the stoichiometry of the crystal near the surface. We propose a formulation for the growth kinetics of voids and gas filled bubbles due to reactions at the surface based on transition state theory. Stoichiometry requirements lead to constraints on defect fluxes to, from and across the surface. This makes the evolution of the slow defect species being diffusion limited while the evolution of the faster species is limited by surface reactions. Results obtained for simple examples are used to validate the model for different levels of point defect supersaturation. This research was supported by the US Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center.
RR8: Dopants and Impurities
Session Chairs
Wednesday PM, April 27, 2011
Room 2022 (Moscone West)
2:30 PM - RR8.1
A Molecular Dynamics Study on the Morphological Stability of Cu-Nb Interfaces under Continuous Irradiation.
Liang Zhang 1 , Michael Demkowicz 1 , Enrique Martinez 2 , Alfredo Caro 2
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWe use molecular dynamics to study the evolution of Cu-Nb interface morphology upon irradiation to more than 5 displacements per atom (dpa) under athermal conditions. As damage accumulates, an intermediate non-crystalline layer forms between the neighboring fcc Cu and bcc Nb and grows by consuming the fcc Cu layer at a rate about five times larger than that of bcc Nb layer. We investigate dependence of this behavior on interface crystallography and the liquid phase miscibility of the neighboring elements.**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.
2:45 PM - RR8.2
Effects of Defects and (N, H)-codoping on Photocatalytic Ability of TiO2: A Combined Computational and Experimental Study.
Hui Pan 1
1 , Institute of High Performance Computing, Singapore Singapore
Show AbstractIn this work, we systematically studied the effects of the defects and (N, H) codoping on the photocatalytic properties of TiO2 by a combined first-principles calculation and experimental approach. Three TiO2 polymorphs, anatase, rutile, and brookite were considered in our study.For the effects of defects, three typical defects, oxygen vacancy, titanium interstitial, and titanium vacancy, were considered in three TiO2 polymorphs. A defect band is formed by removing oxygen atom from or inserting interstitial Ti atom into the TiO2 lattice, which is responsible for the improvement of photocatalytic ability due to the enhanced visible-light absorption. Our calculation further revealed that the defect formation energy increases as following brookite, anatase, and rutile, indicating defects are much easy to be created in brookite TiO2. The relatively high defect density and wide defect band contribute to the better photocatalytic performance of brookite TiO2. Experimentally, the brookite-rich titanium oxide (TiO2) films were synthesized by mild oxidation of Ti foils in air. XRD and Raman spectroscopy show that the onset of brookite formation occurs at 500 °C, and the material is characterized by a strong absorption band in the visible spectral range. Photocurrent density measurements of water splitting reveal that the brookite-rich TiO2 exhibits the highest photocatalytic performance among the different forms of TiO2 produced by oxidation of Ti foils. With increasing oxidation temperature transformation to the rutile phase accompanied by declining visible range photoactivity is observed. The effects of hydrogen, nitrogen and (H, N) doping on the photocatalytic activity of TiO2 were investigated by first-principles calcualtions. Our calculations revealed that n-type TiO2 was formed by the introduction of hydrogen interstitial or substitution, although interstitial state is stable for hydrogen in all of the TiO2 structures, revealing that hydrogen is one of important sources for the n-type character of TiO2. Nitrogen substitution/interstitial in TiO2 is phase-dependent: interstitial for anatase and rutile TiO2, while substitution for brookite TiO2. Our calculation further predicted that nitrogen substitution is enhanced in the presence of hydrogen interstitial bonded to nitrogen due to the reduced formation energy. The narrowing of the bandgap by the (H, N) codoping results in the improvement of photocatalytic performance of TiO2 because of the enhancement of absorption in visible-light.These observations have important implications in human searching for clean energy: the new brookite-rich and (N, H)-codoped TiO2 film may find its future applications in solar energy conversion, hydrogen generation via water splitting, and environmental catalysis under visible light, which is the most abundant in solar spectrum.
3:00 PM - **RR8.3
Calculating Defect Energies in Dynamically Stabilised Materials.
Graeme Ackland 1 , Derek Hepburn 1 , T. Peter Klaver 1
1 Physics, University of Edinburgh, Edinburgh United Kingdom
Show AbstractIn recent years quantum mechanical calculations have given a new method of reliably calculating material properties such as point defects and substitutional energies in structural materials. An international effort is gradally piecing together the various energies and interactions which will form the basis of a multiscale modelling approach to eventually enable us to understand the behaviour of multicomponent alloys. The computation required for this enterprise is enormous, and this is without considering temperature effects - most calculations are still done at T=0.Several important technological materials, such as austenitic steels and beta-titanium alloys, have crystal structures which are unstable at 0K. Such materials provide a unique challenge for calculation, as the entropic contribution simply cannot be ignored. In this talk I will describe some of the methodologies one can apply to this problem, and describe pitfalls which may be encountered along the way. In particular, I will describe an extensive series of calculation in austenitic iron with vacancies, interstitials and interaction with various defects including Ni, Cr, C, N etc.
3:30 PM - RR8.4
First-principles Lattice Stabilities of Selected Fission Products in UO2.
Dongwon Shin 1 , Theodore Besmann 1
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPredicting the solubility of the fission products (FP) in UO2 has been of importance in obtaining a better understanding of the thermochemical behavior of nuclear fuels during irradiation. However, exploring phase equilibria solely from experimental investigations has its limitations. In this regard, computational thermodynamic modeling, the so-called the CALPHAD (CALculation of PHAse Diagram) approach using a compound energy formalism (CEF), allows the efficient prediction of phase stability in complex multi-component systems by extrapolating the Gibbs free energy functions of individual phases in the constituent systems. In the present study, the solubility of major FPs Sr, Ba, Y, La, Pr, Nd, Zr, Mo, and Te, in the CaF2 structure of UO2 has been assessed via the CALPHAD method. In the CEF, the lattice stability of a FP ---energetics of FP in the CaF2 structure--- is required to predict the solubility of a FP in UO2, however the values for the hypothetical phase cannot be measured experimentally. Thus we have used first-principles calculations based on density functional theory (DFT) to evaluate the lattice stability of FPs at 0K. We then used the frozen phonon method to obtain finite-temperature properties of FPs in the CaF2 structure to evaluate the temperature dependence of the lattice stabilities. The relevant pseudo-binary UO2-FP oxide systems have been then calculated with the evaluated lattice stability from DFT calculations and compared with experimental data, where available.Research supported by the US Department of Energy Office of Nuclear Energy, Nuclear Energy Advanced Modeling and Simulation Program.
Symposium Organizers
G.Malcolm Stocks Oak Ridge National Laboratory
Blas Uberuaga Los Alamos National Laboratory
Anter El-Azab Florida State University
RR11: Properties of Point Defects II
Session Chairs
Thursday AM, April 28, 2011
Room 2022 (Moscone West)
11:30 AM - **RR11.1
Fully Ab Initio Description of Point Defect Formation and Properties at Extreme Temperatures.
Joerg Neugebauer 1 , Roman Nazarov 1 , Blazej Grabowski 1 , Fritz Koermann 1 , Johann von Pezold 1 , Hickel Tilmann 1
1 Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
Show AbstractAn accurate theoretical description of defect formation energies or properties at extreme temperatures is challenging due to the various entropic contributions related e.g. to electronic, harmonic and anharmonic or magnetic excitations. Consequently in most theoretical studies on point defects only configurational entropy has been considered. While such an approximation may be valid at low or moderate temperatures, going to extreme temperatures (e.g. close to the melting temperature) other entropy contributions may dominate and even give rise to qualitative changes. The combination of accurate first principles calculations with mesoscopic/macroscopic thermodynamic and/or kinetic concepts has quickly advanced in the past few years and allows now to tackle these fundamental questions. Key to these studies is the highly accurate determination of free energies and subsequently free energy surfaces. In the first part of the talk it will be shown how efficient sampling strategies together with highly converged density-functional theory calculations allow an accurate determination of all relevant temperature dependent free energy contributions such as electronic, harmonic, anharmonic, magnetic and structural excitations. Using these results to construct coarse grained models it became for the first time possible to study and explore the effect temperature has on the various excitation mechanisms and to identify which mechanisms dominate at extreme temperatures. The flexibility and the predictive power of this approach will be discussed in the second part of the talk for a few examples relevant to the design and understanding of high-strength and/or high-temperature stable metallic alloys: Determination of defect energies as critical parameters in mechanism maps, chemical trends in alloy composition or unraveling defect related mechanisms causing failure in these materials.
12:00 PM - RR11.2
Quantum Monte Carlo Calculations of Defects in Aluminum.
Randolph Hood 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractWe use first-principles fixed-node diffusion quantum Monte Carlo to calculate the energetics of point defects in bulk FCC aluminum. Aluminum has been well studied experimentally as a "simple" metal prototype for investigating the effects of radiation damage such as void formation and helium embrittlement. Often accuracies at the level of milli-electronvolts are required, which is not achieved even for the simple case of pairs of vacancies in aluminum, using common density functionals. Perhaps surprisingly, even single vacancy energies are not reliable in many simple structural materials. Specifically we present benchmark results of the vacancy formation energy, the vacancy migration energy, the self-interstitial formation energies for an aluminum atom at an octahedral and a tetrahedral site, the binding energy of a nearest-neighbor divacancy, the formation energies of a helium interstitial at the octahedral and the tetrahedral sites and the formation energy of a helium atom at an aluminum vacancy site. Also presented are results for the bulk properties of aluminum - the equilibrium lattice constant, the cohesive energy, and the bulk modulus. These calculations bring a new level of rigor to the study of defects in metals.This work was supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, under award number ERKCS99, and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
12:15 PM - RR11.3
Defect Formation, Electronic Properties and Empirical Interatomic Potentials of YPO4 Waste Form.
Fei Gao 1 , Haiyan Xiao 2 , Ram Devanathan 1 , Shenyang Hu 1 , Yulan Li 1 , Xin Sun 1 , Mohammad Khaleel 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Show AbstractXenotime (YPO4) is an orthophosphate that occurs in nature with significant concentrations of U and Th, and has been suggested to be an alternative to monazite or zircon in geochronology studies. Also, xenotime has shown stability in nature over millennial time scales while retaining from ppm levels to a few percent of actinides, and is considered candidate a ceramic waste form. Recent progress in theoretical approaches to calculate structural and bonding properties, electronic properties and defect formation in xenotime and some pyrochlores is reviewed, which will provide significant insights into the radiation resistance behavior of these materials. Ab initio calculations of YPO4 demonstrate that the formation energies of vacancies are much higher than those of interstitials, and the O vacancy forms the most stable configuration for all the vacancies considered. The O-O split interstitial is the most stable defect configuration, with a formation energy of only 2.53 eV that is much lower than those of other cation interstitials. These results demonstrate that oxygen defects may provide a driven force leading to local cation disorder, which, in turn, causes radiation-induced amorphization. The defect properties and configurations calculated by ab initio method are compared with those obtained using recently developed interatomic potentials, which will provide extra data training sets to validate xenotime potentials. Based on ab initio calculations of defect formation and their electronic properties, the possible approaches to develop new interatomic potentials for xenotime and pyrochlores will be discussed.
12:30 PM - RR11.4
Defect Production by Low Energy Recoils in Y2Ti2O7.
William Weber 1 2 , Haiyan Xiao 1
1 Materials Science & Engineering, University of Tennesee, Knoxville, Tennessee, United States, 2 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractAb initio molecular dynamics (AIMD) is a promising simulation method for studying defects and radiation effects that provides details on defect generation by low-energy recoil events with ab initio accuracy. AIMD simulations have been carried out to determine the threshold displacement energy, Ed, along specific directions and the corresponding defect configurations in Y2Ti2O7. The threshold displacement energies are anisotropic for each recoil atom. The minimum Ed is found to be 27 eV for Y atoms along the <100> direction, 31.5 eV for Ti atoms along the <100> direction, 14.5 eV for O48f atoms along the <110> direction and 13 eV for O8b atoms along the <111> direction. For the three directions considered, the average Ed values are 35.1, 35.4, 17.0 and 16.2 eV for Y, Ti, O48f and O8b atoms, respectively. After energetic recoil events for anions, the vacant 8a sites are occupied by displaced oxygen atoms either from 48f or 8b positions in all cases, resulting in no net increase in oxygen vacancies. The AIMD results reveal that low energy recoil events can lead to cations occupying vacant 8a sites and cation interstitials occupying bridge sites between two neighboring cations along the <010> direction, neither of these cation defect configurations is observed in molecular dynamics studies. A systematic study of defect formation energies, based on ab initio methods, indicates that titanium interstitials located at 8a sites, at bridge sites along the <010> direction and in split configurations along the <111> direction are all stable configurations, with defect formation energies of 1.2 eV. For Y interstitials, the most stable configuration is the bridge site. These cation interstitial configurations should play important roles in driving irradiation-induced amorphization in Y2Ti2O7.
RR12: Defect Aggregation and Clustering II
Session Chairs
Thursday PM, April 28, 2011
Room 2022 (Moscone West)
2:30 PM - RR12.1
Phase-field Simulation of Interstitial Loop Evolution in Fe-Cr Alloys under Irradiation.
Yulan Li 1 , Shenyang Hu 1 , Xin Sun 1 , Fei Gao 1 , Charles Henager 1 , Moe Khaleel 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractRich microstructure evolution such as the nucleation, growth and coalescence of interstitial loops, voids, and second phase takes place in irradiated materials. This microstructure evolution leads to structural instability such as swelling and cracking as well as thermo-mechanical property change. In this talk, we present our simulation and prediction on the evolution kinetics of interstitial loops by phase-field approach. Fe-Cr alloy, which is extensively used as a structural material in nuclear reactors, is chosen as a model system. We will discuss the construction of thermodynamic and kinetic models used for the model system. The influence of elastic interactions among interstitial loops and defects, the generation rates of vacancies and interstitials, and the recombination rate between vacancies and interstitials on the stability, morphology and evolution kinetics of interstitial loops will be studied and discussed. The simulation results will be compared with existing experimental observations and numerical calculations from other theoretical methods.
2:45 PM - RR12.2
Development and Application of Phase-field Crystal Model for Defects in Fe.
Donald Nicholson 1 , J. Dantzig 2 , Sarma Gorti 1 , Bala. Radhakrishnan 1
1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States, 2 , University of Illinois, Urbana, Illinois, United States
Show AbstractThe Phase-Field Crystal (PFC) model provides a representation of the density as a continuous function, whose spatial distribution evolves in time at diffusional, rather than vibrational time scales. PFC provides a tool to study defect interactions at the atomistic level but over longer time scales than those achievable with MD. We examine the behavior of the PFC model with the goal of relating the PFC parameters to physical parameters of real systems, derived from MD simulations based on either classical force fields or on Hellmann-Feynman forces from density functional calculations. Direct comparison of MD and PFC results for Fe diffusion rates and melting points as a function of vacancy concentration determines free energy functional and motilities that yield system specific behavior. We present simulations of the mechanical response in the PFC model in various modes of deformation, with particular attention to the interaction of defects such as dislocations and vacancies. Acknowledgements:This work was supported by the Center for DefectPhysics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
3:00 PM - RR12.3
Point Defect Evolution Produced by Primary Damage in BCC Iron using Kinetic Monte Carlo Simulations.
Haixuan Xu 1 , Yuri Osetsky 1 , Roger Stoller 1
1 Materials Science and Technology Division, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show AbstractThe long-term evolution of mobile point defects created in primary damage in BCC iron is investigated using the kinetic Monte Carlo (KMC) simulations. The source term was an extensive MD cascade database with PKA energies from 0.1 keV up to 200 keV. It is found that around two thirds of the total self interstitial atoms formed in the primary damage escape from the cascade region and most of them are in the form of clusters. Furthermore, the diffusion mechanism of interstitial clusters has a significant effect on the fraction of defects that survived intra-cascade annealing and escape. The effects of temperature, simulation box size, dissociation of interstitial clusters, evaporation of vacancies, and rotation of interstitials are evaluated and their relative importance are discussed. Nevertheless, standard KMC techniques are insufficient to fully model the process; the evolution of complex interstitial clusters or the interaction between interstitial clusters with dislocations, interfaces, or grain boundaries cannot be accurately described. To overcome these limitations, the framework of a new method is developed. The features of this method will be discussed in comparison of the existing methods. This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCS99.”
3:15 PM - RR12.4
The Energetics of Hydrogen-vacancy Clusters in BCC Iron.
Erin Hayward 1 , Chaitanya Deo 1
1 Nuclear Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractHydrogen and helium are present in steels due to transmutation and implantation in both fusion and fission reactors. In an irradiated environment, an excessive number of vacancies are created which can coalesce into voids. Hydrogen trapped inside these defects may provide internal pressure leading to the stabilization of these voids. The resulting bubbles negatively affect the mechanical properties of reactor structural materials, causing swelling and embrittlement. To test this theory, molecular dynamics and Monte Carlo simulations are conducted to characterize the thermodynamic properties of hydrogen-vacancy clusters in bcc iron. Clusters of up to 50 hydrogens and 10 vacancies are studied, with ratios of hydrogen atoms to vacancies of up to 10. Our results show that the presence of hydrogen does have stabilizing effects on clusters of vacancies. Vacancies are more tightly bound to clusters containing high concentrations of hydrogen, while neighboring iron atoms are less tightly bound. However, the hydrogen atoms are not strongly bound to the clusters. At 0K, the binding energy falls from approximately 1 eV to 0 eV as the ratio is changed from 0 to 7. As the temperature is raised, the release stages of hydrogen in bubbles are described and compared to experiment. Some experimental evidence suggests that the presence of helium may assist in the trapping of hydrogen in voids. Additional atomistic simulations are performed in which the ratio of hydrogen to helium is varied within a void. Interatomic potentials used include exisiting EAM and pair potentials, as well as a potential for H-He that we have fit to quantum Monte Carlo data. The evolution of these bubbles and the conditions under which they may exist are discussed.
3:30 PM - **RR12.5
Complexity of Small Self-interstitial Clusters in α-iron.
Francois Willaime 1 , Mihai-Cosmin Marinica 1 , Normand Mousseau 2
1 , CEA, Gif-sur-Yvette France, 2 , Université de Montréal, Montréal, Quebec, Canada
Show AbstractThe structure of interstitial-type defects formed by the clustering of self-interstitial atoms (SIAs) produced under irradiation is a challenging question in iron. In this metal, a competition is expected for the orientation of the dumbbells in the cluster between the <110> orientation, as for the isolated point defect, and the <111> or <100> orientations of the nanometer size loops. Moreover, configurations made of non-parallel dumbbells, mostly with the <110> orientation, have been evidenced from molecular dynamics simulations. Totally different mobilities and hence defect kinetic evolutions are expected from these various structures, which cannot be resolved by experimental techniques.In this study, we used the activation relaxation technique, an eigenvector following method for systematic search of saddle points and transition pathways on a given potential energy surface, to determine the lowest energy structures of clusters with 2 to 5 SIAs in iron modelled by a Mendelev-type potential with an improved parameterization. The most relevant structures are then investigated by DFT calculations. For a few of them, an unusual discrepancy between results obtained with different methods is observed; it is attributed to a lack of transferability of some pseudopotentials. In addition to the triangular structure of the di-interstitial and the ring-like structure of the tri- and quadri-interstitials, new low energy structures which are expected to play a key-role in iron base systems under irradiation are proposed. Their magnetic and vibrational properties are presented.
RR13: Dislocation Structure II
Session Chairs
Thursday PM, April 28, 2011
Room 2022 (Moscone West)
4:30 PM - RR13.1
Screw Dislocations in Zirconium: An Ab Initio Study.
Emmanuel Clouet 1
1 SRMP, CEA Saclay, Gif-sur-Yvette France
Show AbstractPlasticity in zirconium is thought to be controlled by screw dislocations gliding in the prismatic planes of the hexagonal close-packed structure. This prismatic and not basal glide is observed for a given set of transition metals like zirconium and is known to be related to the number of valence electrons in the d band.We used ab initio calculations based on the density functional theory to study the core structure of screw dislocations in zirconium. Screw dislocations are found to dissociate in two partial dislocations, each with a pure screw character. The dissociation distance observed in the ab initio calculations agrees well with the one predicted by anisotropic elasticity theory using the calculated stacking fault energy.The Peierls barrier have also been calculated for a screw dislocation gliding in a prismatic plane. A low value, ~0.3 +/- 0.1 meV/Å, has been obtained. The Peierls stress deduced from this barrier is compatible with experimental data.
4:45 PM - RR13.2
Atomistic Study of Dislocation Activity during Nanoindentation in bcc and fcc Metals.
Yury Osetskiy 1 , Roger Stoller 1
1 Materials Science and Technology, ORNL, Oak Ridge, Tennessee, United States
Show AbstractAtomic-scale processes occurred during nanoindentation is a rear case that can be studied by both experimental and modeling techniques. Such comparative investigation is in progress in CDP under ORNL EFRC acidity. In this paper we present results of extensive molecular dynamics study of materials deformation during nanoindentation process in Fe and Cu. We have used different indentation surfaces namely ½{110} and {100} in Fe and ½{111} and {100} in Cu and spherical rigid indenters of diameter from 5 to 40 nm at indentation speeds of 2 m/s and 10 m/s in crystals of up to 1.1x108 atoms.Plastic deformation at early stages or by small size indenter occurred mainly by emission of glissile dislocation loops. Large indenters may create complex three-dimensional structures that affect strongly the deformation process. Dislocation junctions of different structures and mobility were observed and classified from the point of view of their effects to nanoindentation process. A higher indentation loading rate was observed to promote the formation of shear loops on a nucleated circular dislocation in Fe.This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCS99.”
5:00 PM - RR13.3
Modelling and Characterization of Dislocation Microstructure in High Temperature α-Fe.
Steve Fitzgerald 1 , Sen Xu 2 , Martha Briceno 2 , T. Britton 2 , Sylvie Aubry 3 , Michael Jenkins 2 , Sergei Dudarev 1 , S. Roberts 2
1 Theory and Modelling, Culham Centre for Fusion Energy, Abingdon United Kingdom, 2 Department Materials, University of Oxford, Oxford United Kingdom, 3 Department of Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractReactor materials for future fusion and advanced fission power generation will be required to operate in an environment involving both a broad range of temperatures and intense neutron and ion irradiation. The options available for the materials’ constituent elements are restricted by the criteria of low activation. The austenitic steels often used for high temperature applications have low resistance to irradiation. Steels exhibiting ferritic-martensitic or ferritic microstructure show significantly greater resistance to irradiation. However, these steels exhibit a sharp reduction in tensile strength and other critical mechanical properties at temperatures above ~500C. Recent theoretical work [1,2] has attributed this to the increasing level of elastic anisotropy of the ferritic matrix, which is associated with vicinity of the -γ phase transition (occurring in pure iron at 912C). This has profound and unexpected effects on the dislocation behaviour which controls mechanical deformation. In this presentation we discuss recent transmission electron microscope observations of the dislocation microstructure in -Fe performed over a variety of relevant temperatures, irradiation and deformation conditions. We observe dislocation structures that are not explicable within the framework of conventional isotropic-elasticity-based dislocation theory, yet can be fully understood once the intrinsic anisotropy of the crystal is taken into account. The experimental results are compared with computer simulations performed using a recently-developed fully anisotropic discrete dislocation dynamics code [3] (based on the isotropic code ParaDiS [4]), and point the way towards an understanding of the deformation behaviour of Fe and ferritic steels under these technologically pertinent conditions. 1. SPF and SLD, Proc R Soc London A464 (2098) 2549 (2008)2. SPF, Phil Mag Lett 90 (3) 209 (2010)3. SPF and SA, J. Phys. Cond. Mat. 22, 295403 (2010)4. VV Bulatov et al, Nature 440 (7088) 1174 (2006)
5:15 PM - RR13.4
SEM-Based In-Situ Tensile Testing of Single Crystal Molybdenum Alloy Fibers.
Kurt Johanns 1 , Andreas Sedlmayr 2 , Pardhasaradhi Sudharshan Phani 1 , Oliver Kraft 2 , Easo George 1 3 , George Pharr 1 3
1 Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee, United States, 2 Institute for Materials Research II, Karlsruhe Institute of Technology, Karlsruhe Germany, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractIn-situ tensile tests have been performed inside a dual-beam FIB-SEM on as-grown and pre-strained single crystal molybdenum alloy fibers. The fibers had square or rectangular cross-sections with edge lengths between 400 and 500 nm and the tensile specimens had gage lengths greater than 10 μm. In contrast to previously observed dislocation nucleation strengths in pillar compression tests of 10 GPa, a wide scatter of yield strengths between 1 and 10 GPa was observed in the as-grown fibers in tension. However, the pre-strained fibers exhibited yield strengths (≈1 GPa) and stress-strain behavior similar to those observed in compression. Deformation was dominated by inhomogeneous plastic events possibly including the formation of Lüder’s bands. These deformation events were qualitatively different from fiber to fiber for a given amount of pre-strain. As a result, the load-displacement behavior could be categorized into three types: linear elastic to failure, stable necking, and discrete plasticity. A simple stochastic model incorporating the measured dislocation density has been used to explain the stochastic nature of the as-grown fiber yield strengths as well as the deterministic behavior of the pre-strained fiber yield strengths.The work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.