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Final report. Defects and transport in mixed oxides

Description: New results on the point defect chemistry of (Ni{sub x}Fe{sub 1-x}){sub 3-delta}O{sub 4} and on the cation tracer diffusion in this spinel solid solution are presented and discussed. The equation system for the defect chemistry of perovskites of the type A{sub 1-x}B{sub 1+x}O{sub 3-delta} have been worked out and used to derive Kr{umlt o}ger-Vink diagrams. The deviation from stoichiometry, delta, in LA{sub 1-x}Mn{sub 1+x}O{sub 3-delta} has been measured at 1100, 1200, and 1300 degrees Celsius as a function of the oxygen activity and the composition variable x. At high and low oxygen activities, the data were fit by taking into account the electrostatic interaction between the charge defects by making use of the Debye H{umlt u}ckel theory.
Date: December 13, 2001
Creator: Dieckmann, R {umlt u}diger
Partner: UNT Libraries Government Documents Department

Point Defects in Binary Laves-Phase Alloys

Description: Point defects in the binary C15 NbCrQ and NbCoz, and C 14 NbFe2 systems on both sides of stoichiometry were studied by both bulk density and X-ray Iattiee parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000"C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr2 Laves phase alloys after quenching from 1400"C. However, there are essentially no thermal vacancies in NbFe2 alloys after quenching from 1300oC. Anti-site hardening was found on both sides of stoichiometry for all the tie Laves phase systems studied, while the thermal vacancies in NbCr2 alloys quenched from 1400'C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds and the thermal vacancy softening is unique to the Laves phase. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.
Date: January 11, 1999
Creator: Liaw, P.K.; Liu, C.T.; Pike, L.M. & Zhu, J.H.
Partner: UNT Libraries Government Documents Department

Tight-binding molecular dynamics simulations on point defects diffusion and interactions in crystalline silicon

Description: Tight-binding molecular dynamics (TBMD) simulations are performed (i) to evaluate the formation and binding energies of point defects and defect clusters, (ii) to compute the diffusivity of self-interstitial and vacancy in crystalline silicon, and (iii) to characterize the diffusion path and mechanism at the atomistic level. In addition, the interaction between individual defects and their clustering is investigated.
Date: January 23, 1995
Creator: Tang, M.; Diaz de la Rubia, T. & Colombo, L.
Partner: UNT Libraries Government Documents Department

GaN: Defect and Device Issues

Description: The role of extended and point defects, and key impurities such as C, O and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.
Date: November 9, 1998
Creator: Pearton, S.J.; Ren, F.; Shul, R.J. & Zolper, J.C.
Partner: UNT Libraries Government Documents Department

Te Inclusions in CZT Detectors: New Method for Correcting Their Adverse Effects

Description: Both Te inclusions and point defects can trap the charge carriers generated by ionizing particles in CdZnTe (CZT) detectors. The amount of charge trapped by point defects is proportional to the carriers’ drift time and can be corrected electronically. In the case of Te inclusions, the charge loss depends upon their random locations with respect to the electron cloud. Consequently, inclusions introduce fluctuations in the charge signals, which cannot be easily corrected. In this paper, we describe direct measurements of the cumulative effect of Te inclusions and its influence on the response of CZT detectors of different thicknesses and different sizes and concentrations of Te inclusions. We also discuss a means of partially correcting their adverse effects.
Date: October 25, 2009
Creator: Bolotnikov, A.E.; Babalola, S.; Camarda, G.S.; Cui, Y.; Egarievwe, S.U.; Hawrami, R. et al.
Partner: UNT Libraries Government Documents Department

Influence of point defects on grain boundary motion.

Description: This work addresses the influence of point defects, in particular vacancies, on the motion of grain boundaries. If there is a non-equilibrium concentration of point defects in the vicinity of an interface, such as due to displacement cascades in a radiation environment, motion of the interface to sweep up the defects will lower the energy and provide a driving force for interface motion. Molecular dynamics simulations are employed to examine the process for the case of excess vacancy concentrations in the vicinity of two grain boundaries. It is observed that the efficacy of the presence of the point defects in inducing boundary motion depends on the balance of the mobility of the defects with the mobility of the interfaces. In addition, the extent to which grain boundaries are ideal sinks for vacancies is evaluated by considering the energy of boundaries before and after vacancy absorption.
Date: September 1, 2010
Creator: Foiles, Stephen Martin
Partner: UNT Libraries Government Documents Department

Bacillus atrophaeus Outer Spore Coat Assembly and Ultrastructure

Description: Our previous atomic force microscopy (AFM) studies successfully visualized native Bacillus atrophaeus spore coat ultrastructure and surface morphology. We have shown that the outer spore coat surface is formed by a crystalline array of {approx}11 nm thick rodlets, having a periodicity of {approx}8 nm. We present here further AFM ultrastructural investigations of air-dried and fully hydrated spore surface architecture. In the rodlet layer, planar and point defects, as well as domain boundaries, similar to those described for inorganic and macromolecular crystals, were identified. For several Bacillus species, rodlet structure assembly and architectural variation appear to be a consequence of species-specific nucleation and crystallization mechanisms that regulate the formation of the outer spore coat. We propose a unifying mechanism for nucleation and self-assembly of this crystalline layer on the outer spore coat surface.
Date: November 21, 2005
Creator: Plomp, M; Leighton, T J; Wheeler, K E; Pitesky, M E & Malkin, A J
Partner: UNT Libraries Government Documents Department

MOLECULAR DYNAMICS SIMULATIONS OF DISPLACEMENT CASCADES IN MOLYBDENUM

Description: Molecular dynamics calculations have been employed to simulate displacement cascades in neutron irradiated Mo. A total of 90 simulations were conducted for PKA energies between 1 and 40 keV and temperatures from 298 to 923K. The results suggest very little effect of temperature on final defect count and configuration, but do display a temperature effect on peak defect generation prior to cascade collapse. Cascade efficiency, relative to the NRT model, is computed to lie between 1/4 and 1/3 in agreement with simulations performed on previous systems. There is a tendency for both interstitials and vacancies to cluster together following cascade collapse producing vacancy rich regions surrounded by interstitials. Although coming to rest in close proximity, the point defects comprising the clusters generally do not lie within the nearest neighbor positions of one another, except for the formation of dumbbell di-interstitials. Cascades produced at higher PKA energies (20 or 40 keV) exhibit the formation of subcascades.
Date: September 8, 2003
Creator: Smith, Richard Whiting
Partner: UNT Libraries Government Documents Department

Atomistic models of point defects in plutonium metal.

Description: The aging properties of plutonium (Pu) metal and alloys are. driven by a combination of materials composit ion, p rocessing history, and self-irradiat ion effects . Understanding these driving forces requires a knowledge of both t h ermodynamic and defect properties of the material . The multiplicity of phases and the small changes in tempe rat u re, pressure, and/or stress that can induce phase changes lie at the heart of these properties . In terms of radiation damage, Pu metal represents a unique situation because of the large volume chan ges that accompany the phase changes . The most workable form of the meta l is the fcc (S-) phase, which in practice is stabi l ized by addit io n of a ll oying el eme n ts s u c h as Ga or Al. The thermodynamically stable phase at ambient conditions is the monoclinic (a-) phase, which, however, is 2 0 % lower i n volume th an the S phase . In stabilized Pu metal, there is an in t er play between th e n atu ral swe l li n g tendencies of fcc metals and the volume-contraction tendency of the u n d erlyin g thermodynamicall y stable phase. This study exp lores the point d efect pr operties that are necessary to model the long-term outcome of this interplay.
Date: January 1, 2003
Creator: Valone, S. M. (Steven M.); Baskes, M. I. (Michael I.); Uberuaga, B. P. (Blas Pedro) & Voter, A. F.
Partner: UNT Libraries Government Documents Department

Comparison of binary collision approximation and molecular dynamics for displacement cascades in GaAs.

Description: The predictions of binary collision approximation (BCA) and molecular dynamics (MD) simulations of displacement cascades in GaAs are compared. There are three issues addressed in this work. The first is the optimal choice of the effective displacement threshold to use in the BCA calculations to obtain the best agreement with MD results. Second, the spatial correlations of point defects are compared. This is related to the level of clustering that occurs for different types of radiation. Finally, the size and structure of amorphous zones seen in the MD simulations is summarized. BCA simulations are not able to predict the formation of amorphous material.
Date: October 1, 2011
Creator: Foiles, Stephen Martin
Partner: UNT Libraries Government Documents Department

Development of Nanostructured Materials with Improved Radiation Tolerance for Advanced Nuclear Systems

Description: This project will explore the fundamental mechanisms through which interfaces in nanolayered structures and grain boundaries of bulk nanomaterials are able to attract and rapidly eliminate point defects and unwanted foreign species. Candidate materials that will be studied include both nanostructured multilayer composites synthesized by magnetron sputtering and structural bulk nanomaterials produced by severed plastic deformation, equal channel angular extrusion.
Date: August 12, 2009
Creator: Zhang, Zinghang & Hartwig, K. Ted
Partner: UNT Libraries Government Documents Department

A molecular dynamics simulation study of defect production in vanadium

Description: We performed molecular dynamics simulations to investigate the process of defect production in pure vanadium. The interaction of atoms was described by the EAM interatomic potential modified at short range to merge smoothly with the universal potential for description of the high energy recoils in cascades. The melting point of this EAM model of vanadium was found to be consistent with the experimental melting temperature. The threshold energies of displacement events in the model system are also consistent with experimental minimum threshold in vanadium, and its average was found to be 44 eV. We evaluated the efficiencies of defect production in the displacement events initiated by recoils with kinetic energy up to 5 keV, and found that the probability of cluster formation is smaller than that of simulated events in fcc metals reported in the literature.
Date: January 23, 1995
Creator: Morishita, K. & Diaz de la Rubia, T.
Partner: UNT Libraries Government Documents Department

Interaction between point defects and edge dislocation in BCC iron

Description: We present results of atomistic simulations of the interaction between self interstitial atoms and vacancies with edge dislocations in BCC iron. The calculations are carried out using molecular dynamics with an energy minimization scheme based on the quasi-Newton approach and use the Finnis-Sinclair interatomic potential for BCC iron developed by Ackland et al. Large anisotropy in the strain field of self interstitials is observed and it causes strong interaction with edge dislocations even when the defect is located on the dislocation glide plane. For vacancies, the relaxation volume is smaller and much more isotropic, which results in a far weaker interaction with the dislocation. A temperature dependent capture radius for vacancies and self interstitials is extracted from the simulations. The difference between the capture radii of vacancies and self interstitials is used to define the sink strength of the dislocation. Large deviations are observed from the predictions of elasticity based on treating point defects as isotropic dilatational centers. Further, the capture radius of edge dislocations in BCC iron is observed to be small and is of the order of l-3 nm for self interstitials.
Date: October 12, 1998
Creator: Diaz de la Rubia, T. & Shastry, V.
Partner: UNT Libraries Government Documents Department

Structural defect control and photosensitivity in reactively sputtered germanosilicate glass films

Description: The optical performance of refractive index structures induced in photosensitive (PS) glasses ultimately depends on the index modulation depth attainable. In germanosilicate materials, the photosensitive response is linked to the presence of oxygen-deficient germanium point defect centers. Prior efforts to increase PS in these materials, e.g., hydrogen loading, rely on a chemical reduction of the glass structure to enhance the population of oxygen deficient centers and thus increase the saturated refractive index change. We have previously reported the development of highly photosensitive, as-deposited germanosilicate glass films through reactive atmosphere (O{sub 2}/Ar) sputtering from a Ge/Si alloy target. The present work details our investigation of the effect of substrate temperature during deposition on the material structure and propensity for photosensitivity. Using optical absorption/bleaching, Raman, electron paramagnetic resonance (EPR) and selective charge injection techniques we show that the predominate defect states responsible for the PS response can be varied through substrate temperature control. We find that two regimes of photosensitive behavior can be accessed which exhibit dramatically different uv-bleaching characteristics. Thus, the corresponding dispersion of the refractive index change as well as its magnitude can be controlled using our synthesis technique. Tentative defect models for the photosensitive process in materials deposited at both ambient temperature and at elevated substrate temperatures will be presented.
Date: February 1, 1997
Creator: Potter, B.G. Jr.; Simmons-Potter, K.; Warren, W.L. & Ruffner, J.A.
Partner: UNT Libraries Government Documents Department

Effect of point defects and disorder on structural phase transitions

Description: Since the beginning in 1986, the object of this project has been Structural Phase Transitions (SPT) in real as opposed to ideal materials. The first stage of the study has been centered around the role of Point Defects in SPT`s. Our intent was to use the previous knowledge we had acquired in the study of point defects in non-transforming insulators and apply it to the study of point defects in insulators undergoing phase transitions. In non-transforming insulators, point defects, in low concentrations, marginally affect the bulk properties of the host. It is nevertheless possible by resonance or relaxation methods to study the point defects themselves via their local motion. In transforming solids, however, close to a phase transition, atomic motions become correlated over very large distances; there, even point defects far removed from one another can undergo correlated motions which may strongly affect the transition behavior of the host. Near a structural transition, the elastic properties win be most strongly affected so as to either raise or decrease the transition temperature, prevent the transition from taking place altogether, or simply modify its nature and the microstructure or domain structure of the resulting phase. One of the well known practical examples is calcium-stabilized zirconia in which the high temperature cubic phase is stabilized at room temperature with greatly improved mechanical properties.
Date: June 1, 1997
Creator: Toulouse, J.
Partner: UNT Libraries Government Documents Department

Simplified models of growth, defect formation, and thermal conductivity in diamond chemical vapor deposition

Description: A simplified surface reaction mechanism is presented for the CVD of diamond thin films. The mechanism also accounts for formation of point defects in the diamond lattice, an alternate, undesirable reaction pathway. Both methyl radicals and atomic C are considered as growth precursors. While not rigorous in all details, the mechanism is useful in describing the CVD diamond process over a wide range of reaction conditions. It should find utility in reactor modeling studies, for example in optimizing diamond growth rate while minimizing defect formation. This report also presents a simple model relating the diamond point-defect density to the thermal conductivity of the material.
Date: April 1, 1996
Creator: Coltrin, M.E. & Dandy, D.S.
Partner: UNT Libraries Government Documents Department

Investigation of defects in highly photosensitive germanosilicate thin films

Description: Germanosilicate glasses exhibit a significant photosensitive response which has been linked to the presence of oxygen-deficient germanium point defects in the glass structure. Based on this correlation, highly photosensitive thin films have been engineered which demonstrate the largest reported ultraviolet-induced refractive index perturbations (An) in an as-synthesized material. Our thin-film fabrication process avoids the use of hydrogen sensitizing treatments and, thus, yields stable films which retain their predisposition for large photosensitivity for over one year of storage. Understanding the nature of the defects in such films and their relationship to charge trapping and enhanced photosensitivity is of paramount importance in designing and optimizing the materials. Toward this end, our films have been studied using electron paramagnetic resonance (EPR), capacitance-voltage, and optical bleaching and absorption spectroscopies. We find experimental evidence suggesting a model in which a change in spin state and charge state of isolated paramagnetic neutral Ge dangling bonds form either diamagnetic positively or negatively charged Ge sites which are largely responsible for the charge trapping and photosensitivity in these thin films. We present experimental data and theoretical modeling to support our defect model and to show the relevance of the work.
Date: February 1, 1997
Creator: Simmons-Potter, K.; Potter, B.G. Jr. & Warren, W.L.
Partner: UNT Libraries Government Documents Department

Point Defect Cluster Formation in Iron Displacement Cascades Up to 50 keV

Description: The results of molecular dynamics displacement cascade simulations in iron at energies up to 50 keV and temperatures of 100, 600, and 900K are summarized, with a focus on the characterization of interstitial and vacancy clusters that are formed directly within the cascade. The fraction of the surviving point defects contained in clusters, and the size distributions of these in-cascade clusters have been determined. Although the formation of true vacancy clusters appears to be inhibited in iron, a significant degree of vacancy site correlation was observed. These well correlated arrangements of vacancies can be considered nascent clusters, and they have been observed to coalesce during longer term Monte Carlo simulations which permit short range vacancy diffusion. Extensive interstitial clustering was observed. The temperature and cascade energy dependence of the cluster size distributions are discussed in terms of their relevance to microstructural evolution and mechanical property changes in irradiated iron-based alloys.
Date: November 30, 1998
Creator: Stoller, R.E.
Partner: UNT Libraries Government Documents Department

Point Defects in Binary Laves-Phase Alloys

Description: Point defect mechanisms in the binary C15 NbCr{sub 2} and NbCo{sub 2}, and C14 NbFe{sub 2} systems on both sides of stoichiometry was studied and clarified by both bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000 C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. However, thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr{sub 2} laves phase alloys after quenching from 1400 C. These could be completely eliminated by annealing at 1000 C. Anti-site hardening was found on both sides of stoichiometry for all three Laves phase systems studied. Furthermore, the thermal vacancies in NbCr{sub 2} alloys after quenching from 1400 C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds, while the thermal vacancy softening is unique to the Laves phase. Both the anti-site defects and thermal vacancies do not significantly affect the fracture toughness of the Laves phases.
Date: November 30, 1998
Creator: Liaw, P.K.; Liu, C.T.; Pike, L.M. & Zhu, J.H.
Partner: UNT Libraries Government Documents Department

Influence of Neutron Energy Spectrum on Primary Damage Formation

Description: Displacement cascade formation in iron has been investigated by the method of molecular dynamics (MD) for cascade energies up to 40 keV. The results of these simulations have been used to obtain effective, energy-dependent cross sections for two measures of primary damage production: (1) the number of surviving point defects expressed as a fraction of the those predicted by the standard secondary displacement model by Norgett, Robinson, and Torrens (NRT),and (2) the fraction of the surviving interstitials contained in clusters that formed during the cascade event. The primary knockon atom spectra for iron obtained from the SPECTER code have been used to weight these MD-based damage production cross sections in order to obtain spectrally-averaged values for several locations in commercial fission reactors, materials test reactors, a DT fusion reactor first wall, and a pulsed spallation neutron source. An evaluation of these results indicates that neutron energy spectrum differences between the various environments do not lead to significant differences between the average primary damage formation parameters. This conclusion implies that the displacement damage component of radiation damage produced in a high energy spallation neutron source should be well simulated by irradiation in a fission reactor neutron spectrum, and that differences in nuclear transmutation production may be a greater source of uncertainty in the prediction of material performance in the planned National Spallation Neutron Source.
Date: December 31, 1997
Creator: Stoller, R. E.
Partner: UNT Libraries Government Documents Department