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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Post Irradiation Examination of SiC Tube Subjected to Simultaneous Irradiation and Radial High Heat Flux

Silicon carbide (SiC) based composite is considered among leading candidate materials for accident tolerant fuel (ATF) cladding in light water reactors (LWRs) because of its outstanding irradiation and oxidation resistance, as well as low neutron absorption. Under normal operation conditions, the synergism between neutron irradiation and a high radial heat flux though the cladding thickness is expected to produce significant stress. This stress is attributed to a significant through-thickness temperature gradient and the resulting differential swelling strain due the highly temperature-dependent point-defect swelling of SiC. Although modeling of the in-pile stress state of SiC cladding has been conducted using finite element analysis, the analysis is challenging because of the complex stress state that depends upon swelling, irradiation creep, and thermal conductivity, all of which are both temperature and dose-dependent. These integral models have not yet been verified by experiments. In order to experimentally validate the multi-physics thermo-mechanical models of ATF SiC cladding during operation, SiC tube specimens have been irradiated under a high radial heat flux. This paper reports preliminary results from the post-irradiation examination (PIE) in on-going project supported by the U.S. Department of Energy's Nuclear Science User Facility program. The objectives of the PIE are (1) determination of the irradiation temperature distribution within the tube specimens, and (2) evaluation of the state and magnitude of the stress within the tube specimens arising from the temperature gradient. This paper mainly presents findings for objective (1)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Effect of Cu on the passivity of Ti–xCu (x = 0, 3 and 5 wt%) alloy in phosphate-buffered saline solution within the framework of PDM-II

The effect of copper content on the passivity of Ti–xCu alloys in a phosphate-buffered saline electrolyte (pH 7.4), in the steady-state condition, was examined using potentiostatic polarization, electrochemical impedance spectroscopy, and Mott-Schottky analysis. The study demonstrates that the oxide layer forms on the Ti–xCu alloys surface have n-type semiconducting character, and the steady-state thickness of the oxide layer is observed linearly and depends on the applied potential. The study observations are in line with the predictions of the Point Defect Model (PDM-II), which provides a physiochemically realistic description of the oxide layer formation on Ti–xCu alloys. The observations reveal that the Cu additions result in a decrease in the charge transfer resistance and capacitances associated with the hydroxide and barrier layer, an increase in the donor density, and reduce the electrochemical resistance of the Ti–xCu alloys. The study also exhibits that the decrease in the steady-state current density after the initial addition of 3 wt% of Cu is attributed to the progressive substitution of copper on the cation sublattice of the film resulting in enhanced electrostatic interaction between the immobilized copper $Cu^{x'}_{Ti}$, and the mobile cation interstitials $Cu^{+}_{i}$, which carries the excess current over and that conveyed by oxygen vacancies $V^{••}_o$ in the barrier layer. Moreover, it is also observed that over a hundred-year implant period, about 0.006 cm (0.06 mm) of Ti–3Cu alloy is predicted to lose due to steady-state corrosion in the PBS solution under the given set of conditions, it is further seen that, as the Cu amount increased from 3 wt% to 5 wt% in the Ti–xCu alloys, the steady-state corrosion rate decreases.

Electrochemical Impedance Spectroscopy↗
DOE OSTI2018

Self-compensation induced vacancies for significant phonon scattering in InSb

Here, phonon scattering by point defects via mass differences and strain fluctuations could effectively reduce the lattice thermal conductivity. The atomic mass difference can be maximized by introducing the vacancies thus leading to a significant phonon scattering. Usually, the vacancies are introduced by tuning the stoichiometry or forming solid solution with certain compound that contains intrinsically high concentration of vacancies. In this work, we demonstrate that vacancies can be effectively induced by the self-compensation effect via chemical doping. Indium (In) vacancies in InSb were induced by Te-doping and a substantial reduction in thermal conductivity was observed. Room temperature lattice thermal conductivity of the melted and then hot-pressed InSb (without In vacancies) is ~ 14.5 W m –1 K –1 but only ~ 3.8 W m –1 K –1 for InSb 0.96 Te 0.04 (with In vacancies), a reduction of ~ 74%. The advantage of using this strategy for phonon engineering lies in the fact that a substantial reduction in thermal conductivity can be achieved even when the dopant concentration is rather low. Since the self-compensation effect is widely observed in different compounds, it indicates that the vacancy engineering strategy used here is also applicable to a variety of other materials to effectively reduce the lattice thermal conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effects of TiO{sub 2} addition and electron irradiation on superconducting and mechanical properties of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} (Bi-2212) superconductor

Titanium Oxide (TiO{sub 2}) compounds having very high melting point with lower heat capacity, is an excellent candidate for reinforcement of brittle materials such as superconductor ceramics. In addition to high melting point, the TiO{sub 2} is also capable of establishing flux pinning centers in bismuth-based superconductors such as the Bi-2212. To further enhance the flux pinning properties, irradiation is one of the techniques that can be used to re-create the required point defects. In this study, the effects of TiO{sub 2} addition and electron irradiation on Bi-2212 superconductor were studied. TiO{sub 2} added Bi-2212 superconductor samples with 5%, 10% and 15% weight percentage addition respectively, were prepared using the conventional solid-state reaction method. The samples were irradiated with electron beam with radiation dose of 100 KGray. Characterization was performed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The transition temperature (T{sub c}) and the critical current density (J{sub c}) of the samples were also measured. The XRD patterns for all the samples show well-defined peaks all of which could be indexed on the basis of a Bi-2212 phase structure. In addition, the XRD patterns indicate that electron irradiation did not change the structure of Bi-2212 superconducting phase. Results of SEM micrographs show disorientation in the texture of the microstructure for samples that are subjected to electron irradiation. The grains are seen to align randomly with higher degree of orientation. With regard to TiO{sub 2} additions, only small TiO{sub 2} addition sustained the superconducting properties upon irradiation. Addition of more than 5% weight percentage of TiO{sub 2} degrades the superconducting properties of the irradiated samples. Formation of weak-links may result in higher grain boundaries orientation within the superconducting grains and thus deteriorates the inter-grains connectivity and resulted in lower T{sub c} and degradation of J{sub c}. The strength of the superconductor reduced significantly when subjected to electron irradiation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Non-random walk diffusion enhances the sink strength of semicoherent interfaces

Clean, safe and economical nuclear energy requires new materials capable of withstanding severe radiation damage. One strategy of imparting radiation resistance to solids is to incorporate into them a high density of solid-phase interfaces capable of absorbing and annihilating radiation-induced defects. Here we show that elastic interactions between point defects and semicoherent interfaces lead to a marked enhancement in interface sink strength. Our conclusions stem from simulations that integrate first principles, object kinetic Monte Carlo and anisotropic elasticity calculations. Surprisingly, the enhancement in sink strength is not due primarily to increased thermodynamic driving forces, but rather to reduced defect migration barriers, which induce a preferential drift of defects towards interfaces. The sink strength enhancement is highly sensitive to the detailed character of interfacial stresses, suggesting that ‘super-sink’ interfaces may be designed by optimizing interface stress fields. Lastly, such interfaces may be used to create materials with unprecedented resistance to radiation-induced damage.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Direct Observation of Sink-Dependent Defect Evolution in Nanocrystalline Iron under Irradiation

Crystal defects generated during irradiation can result in severe changes in morphology and an overall degradation of mechanical properties in a given material. Nanomaterials have been proposed as radiation damage tolerant materials, due to the hypothesis that defect density decreases with grain size refinement due to the increase in grain boundary surface area. The lower defect density should arise from grain boundary-point defect absorption and enhancement of interstitial-vacancy annihilation. In this study, low energy helium ion irradiation on free-standing iron thin films were performed at 573 K. Interstitial loops of a 0 /2 [111] Burgers vector were directly observed as a result of the displacement damage. Loop density trends with grain size demonstrated an increase in the nanocrystalline (<100 nm) regime, but scattered behavior in the transition from the nanocrystalline to the ultra-fine regime (100–500 nm). To examine the validity of such trends, loop density and area for different grains at various irradiation doses were compared and revealed efficient defect absorption in the nanocrystalline grain size regime, but loop coalescence in the ultra-fine grain size regime. Lastly, a relationship between the denuded zone formation, a measure of grain boundary absorption efficiency, grain size, grain boundary type and misorientation angle is determined.

42 ENGINEERING↗
DOE OSTI2016

First-principles study of inert gas incorporation and migration in zirconium nitride

Uranium-Molybdenum alloy based fuels are the most promising fuel for the future high performance research reactors using low enriched uranium (LEU). However, when used in dispersion with aluminum, significant amount of interaction products form at the interface between the UMo fuel particle and the surrounding Al during in-pile tests which leads to non-acceptable swelling. To reduce the swelling in U-Mo/Al dispersion fuel at high burnups, different diffusion barrier coatings, e.g., ZrN, were deposited on the U-Mo particles to avoid the interaction between fuel particles and the Al matrix. These diffusion barrier coatings are very effective in reducing the formation of interaction layer and therefore the fuel swelling. So far, ZrN is selected as the candidate coating material for the European Dispersion Fuel program due to the superior thermophysical properties of ZrN. The thermodynamic and kinetic stability of ZrN coating with respect to Al and U-Mo has been studied by the current authors using density functional theory calculations. So far there are very few studies of the diffusion behaviors of inert gases in ZrN. A recent publication on irradiated (Pu,Zr)N reveals that the majority of fission-induced Xe is retained in the material, while He is release to the fuel pin to a large extent. This raises the questions on the underlying differences of inert gas behavior in nitride fuel. Therefore, the purpose of this study is to understand the incorporation and migration behavior of inert gases He, Kr and Xe in ZrN using density functional theory (DFT) based first-principles calculations. Using density-functional theory calculations we provided a systematic study of the incorporation and migration of fission gases in ZrN. We investigated the formation energies of several defects (vacancies, interstitials, divacancies, Frenkel pairs, and Schottky defects) and the incorporation energies of inert gases in these defects. Results show that the formation energies of point defects are strongly affected by the chemical potentials of N and Zr species. The energetically most favorable incorporation site for fission gases is found to be the first nearest neighbor Schottky defect, in which the fission gas atom sits at somewhere between the Zr and N vacancies. The predicted solution energies of rare gases, i.e, He, Kr and Xe, in ZrN, is almost linearly dependent on the atom size of inert gases. We compared the solution energy Xe in ZrN with several other nuclear materials with the same B1 structure (MX), e.g., TiN, UN and UC. The solution energies of Xe in MX are closely related to the size of vacancy sites. The potential migration mechanisms of Xe in ZrN include interstitial and vacancy-assisted diffusion. The calculations show that vacancies essentially act as Xe trapping sites. The migration behavior of Xe in N and Zr divacancy and Schottky defects were compared with that through tetrahedral interstitials. It is found that the migration barrier is much lower with the interstitial mechanism than those with the vacancy mechanism. Therefore, at low temperatures Xe diffusion in ZrN is mostly through the interstitial mechanism or existing grain boundaries while at high temperature volume diffusion could be preferred, i.e., diffusion with Zr and N vacancies. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2017

Development of High Electronic Quality CdTe Films Required for 1 Volt V OC CdTe Cells

The main goal of this project was to develop an understanding of the role of point defects and demonstrate control of the doping process in thin films of CdTe, using a deposition process that can create conditions favorable for extrinsic (mainly group V) dopant incorporation. The project’s two key objectives were to demonstrate: (a) doping concentrations in p-CdTe above 10 16 cm -3 , and (b) lifetimes greater than 2 ns. Device modeling suggests that both these conditions are required to achieve V OC ’s of 1.0 Volt in thin film CdTe cells a long-sought objective of the CdTe thin film community. The process utilized for the deposition of the CdTe thin films is unique in that it can be used to create Cd- or Te-rich conditions (or stoichiometric) in order to favor the formation of certain native defects (i.e. Cd or Te vacancies). All significant accomplishments clearly pointed to the impact of deposition conditions on the formation of defects in CdTe thin films.

14 SOLAR ENERGY↗
DOE OSTI2014

Diffusion of n-type dopants in germanium

Germanium is being actively considered by the semiconductor community as a mainstream material for nanoelectronic applications. Germanium has advantageous materials properties; however, its dopant-defect interactions are less understood as compared to the mainstream material, silicon. The understanding of self- and dopant diffusion is essential to form well defined doped regions. Although p-type dopants such as boron exhibit limited diffusion, n-type dopants such as phosphorous, arsenic, and antimony diffuse quickly via vacancy-mediated diffusion mechanisms. In the present review, we mainly focus on the impact of intrinsic defects on the diffusion mechanisms of donor atoms and point defect engineering strategies to restrain donor atom diffusion and to enhance their electrical activation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Room temperature corrosion behaviour of plastically deformed AISI 304 stainless steel by friction stir welding in neutral and acidified chloride solutions

Severe plastic deformation of 304L stainless steel plate was carried out by friction stirring under an isothermal tool temperature at 825°C. The friction stirred zone showed extensive grain refinement, high fraction of low angle grain boundaries due to dislocation rearrangement, Σ3 and Σ9 special grain boundaries and discrete Fe–Cr–Mn-rich particles. Electrochemical polarisation measurements were done on the base metal (BM) and friction stir welded (FSW) specimens in the neutral and acidified 3.5% NaCl solutions. The BM showed marginally better pitting resistance in the neutral chloride solution than the FSW because of a larger fraction of special grain boundaries, and larger grain size. Whereas in acidified 3.5% NaCl solution, the FSW showed better corrosion resistance than the BM. Furthermore, the surface film of the FSW had a lower impedance and a lower defect concentration than the BM indicating higher diffusivity of point defects in the FSW.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The role of native defects and their diffusivity in CdTe

CdTe is a leading thin-film technology for solar-cell applications. Basic features such as the optimum band gap of 1.5 eV and simple manufacturing process have been driving the research on improving the current record efficiency of 22% for CdTe solar cells toward the theoretical limit of 32%. Low p-type doping and short carrier lifetimes in the CdTe absorber have been suggested as the main limiting factors and attributed to defect compensation and carrier recombination. Understanding the role of point defects and impurities in CdTe is crucial to solving these problems. Despite many years of research, the sources of compensation and the microscopic recombination mechanisms are still unclear. By using hybrid density functional calculations with spin-orbit coupling and large supercells, we investigate the electronic properties, the formation energies, and the diffusivities of the native defects in CdTe. As possible sources of compensation for p-type conductivity, we discuss the structural and electronic properties of Cd interstitial, Te vacancies, and TeCd antisites. The stability of each defect is discussed in terms of both the defect formation energy and the migration barrier. We also address possible mechanisms that lead to non-radiative recombination.

14 SOLAR ENERGY↗
DOE OSTI2013

Atomistic Studies of Cation Transport in Tetragonal ZrO2 During Zirconium Corrosion

Zirconium alloys are the major fuel cladding materials in current reactors. The water-side corrosion is one of the major degradation mechanisms of these alloys. During corrosion the transport of oxidizing species in zirconium dioxide (ZrO2) determines the corrosion kinetics. Previously it has been argued that the outward diffusion of cation ions is important for forming protective oxides. In this work, the migration of Zr defects in tetragonal ZrO2 is studied with temperature accelerated dynamics and molecular dynamics simulations. The results show that Zr interstitials have anisotropic diffusion and migrate preferentially along the [001] or c direction in tetragonal ZrO2. The compressive stresses can increase the Zr interstitial migration barrier significantly. The migration barriers of some defect clusters can be much lower than those of point defects. The migration of Zr interstitials at some special grain boundaries is much slower than in a bulk oxide. The implications of these atomistic simulation results in the Zr corrosion are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Modeling intrinsic defects in LiNbO{sub 3} within the Slater-Janak transition state model

Intrinsic point defects in LiNbO{sub 3}, i.e., isolated Nb antisites and Li as well Nb vacancies, are investigated from first-principles within the Slater-Janak transition state model. Thereby the electronic structure of the investigated defects is calculated with hybrid exchange-correlation functionals. This approach allows for the calculation of charge transition levels without comparing the total energies of differently charged supercells. The obtained results are in agreement with previous hybrid density-functional theory calculations based on total-energy differences. Li and Nb vacancies can be formed in the V{sub Li}{sup −} and V{sub Nb}{sup 5−} charge states only, as long as the host is not strongly p-type or n-type, respectively. Nb{sub Li} antisites may capture one or two electrons, forming the defect states often referred to as small bound polaron and bi-polaron.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Strain enhanced defect reactivity at grain boundaries in polycrystalline graphene

Grain boundaries dominate the property of polycrystalline graphene. We report first-principles calculations and classical molecular dynamics simulations that reveal enhanced defect reactivity induced by an inhomogeneous strain field at grain boundaries. Strained carbon bonds located at heptagons and pentagons can accumulate interstitials and single vacancies, respectively. We find that recombination of vacancies and interstitials can occur locally at grain boundaries, which serve as effective sinks, resulting in efficient annealing of defects. The enhanced defect reactivity indicates that grain boundaries may be manipulated by point defects.

Chemistry↗
DOE OSTI2018

The defect chemistry of U O 2 ± x from atomistic simulations

Control of the defect chemistry in UO 2±x is important for manipulating nuclear fuel properties and fuel performance. For example, the uranium vacancy concentration is critical for fission gas release and sintering, while all oxygen and uranium defects are known to strongly influence thermal conductivity. Here the point defect concentrations in thermal equilibrium are predicted using defect energies from density functional theory (DFT) and vibrational entropies calculated using empirical potentials. Electrons and holes have been treated in a similar fashion to other charged defects allowing for structural relaxation around the localized electronic defects. Predictions are made for the defect concentrations and non-stoichiometry of UO 2±x as a function of oxygen partial pressure and temperature. If vibrational entropy is omitted, oxygen interstitials are predicted to be the dominant mechanism of excess oxygen accommodation over only a small temperature range (1265 K to 1350 K), in contrast to experimental observation. Conversely, if vibrational entropy is included oxygen interstitials dominate from 1165 K to 1680 K (Busker potential) or from 1275 K to 1630 K (CRG potential). Below these temperature ranges excess oxygen is predicted to be accommodated by uranium vacancies, while above them the system is hypo-stoichiometric with oxygen deficiency accommodated by oxygen vacancies. Our results are discussed in the context of oxygen clustering, formation of U 4 O 9 , and issues for fuel behavior. In particular, the variation of the uranium vacancy concentrations as a function of temperature and oxygen partial pressure will underpin future studies into fission gas diffusivity and broaden the understanding of UO 2±x sintering.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

A three dimensional field formulation, and isogeometric solutions to point and line defects using Toupin's theory of gradient elasticity at finite strains

In this study, we present a field formulation for defects that draws from the classical representation of the cores as force dipoles. We write these dipoles as singular distributions. Exploiting the key insight that the variational setting is the only appropriate one for the theory of distributions, we arrive at universally applicable weak forms for defects in nonlinear elasticity. Remarkably, the standard, Galerkin finite element method yields numerical solutions for the elastic fields of defects that, when parameterized suitably, match very well with classical, linearized elasticity solutions. The true potential of our approach, however, lies in its easy extension to generate solutions to elastic fields of defects in the regime of nonlinear elasticity, and even more notably for Toupin's theory of gradient elasticity at finite strains (Toupin Arch. Ration. Mech. Anal., 11 (1962) 385). In computing these solutions we adopt recent numerical work on an isogeometric analytic framework that enabled the first three-dimensional solutions to general boundary value problems of Toupin's theory (Rudraraju et al. Comput. Methods Appl. Mech. Eng., 278 (2014) 705). We first present exhaustive solutions to point defects, edge and screw dislocations, and a study on the energetics of interacting dislocations. Then, to demonstrate the generality and potential of our treatment, we apply it to other complex dislocation configurations, including loops and low-angle grain boundaries.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Defects activated photoluminescence in two-dimensional semiconductors: interplay between bound, charged and free excitons

Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by a-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Intrinsic Defect Engineering of Cuprous Oxide to Enhance Electrical Transport Properties for Photovoltaic Applications

Intrinsic point-defect species in cuprous oxide films are manipulated based on their thermodynamic properties via the implementation of a controlled annealing process. A wide range of electrical properties is demonstrated, with a window suitable for high-quality solar cell devices. A variation of carrier concentration over two orders of magnitude is demonstrated. Minority carrier lifetime is investigated by means of microwave photoconductance decay measurements, demonstrating a strong correlation with carrier concentration. Spectrally resolved photoluminescence yields are analyzed to provide insight into lifetime limiting mechanisms as a function of Cu2O processing parameters. Hall measurements of carrier mobility and concentration are taken at room temperature to provide insight into the effect of these processing conditions on net ionized defect concentration.

14 SOLAR ENERGY↗