Vacancy Diffusion Coefficient Research Articles

Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial La1-xSrxCoO3-δ Films.

Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO3-δ and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO2.5. This is emerging as a paradigmatic example of the power of electrochemical gating (using, e.g., ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO3 films are challenging with respect to stability, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P → BM transformations across almost the entire La1-xSrxCoO3 phase diagram (0 ≤ x ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all x, including x ≤ 0.50, where both P and BM phases are highly stable. Under small compressive strain, the transformation threshold voltage decreases from approximately +2.7 V at x = 0 to negligible at x = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low x. The P → BM threshold voltage is thus tunable, via both composition and strain. At x = 0.50, voltage-controlled ferromagnetism, transport, and optical transmittance are then demonstrated, achieving Curie temperature and resistivity modulations of ∼220 K and at least 5 orders of magnitude, respectively, and enabling estimation of the voltage-dependent Co valence. The results are analyzed in the context of doping- and strain-dependent oxygen vacancy formation energies and diffusion coefficients, establishing that it is thermodynamic factors, not kinetics, that underpin the decrease in the threshold voltage with x, that is, with increasing formal Co valence. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications.

Effect of an external electric field on the diffusion of oxygen ions in zirconium dioxide doped with yttrium oxide. Molecular dynamics study

The influence of an external electric field on the character of ion motion, diffusion coefficients, and diffusion activation energies in cubic ZrO2 doped with Y2O3 with varying dopant content, temperature, and electric field strength has been studied by the molecular dynamics method. The diffusion coefficient of oxygen vacancies for models with different Y2O3 contents in the temperature range 873–1273 K without an imposed field decreases with an increase in the molar fraction of yttrium oxide in the crystal, while the dependence of the diffusion coefficient of O2− ions has a maximum (2.4∙10−11 m2/s) with an yttrium oxide content of 3 mol%. The activation energy of ionic conductivity, estimated in the absence of a field on the basis of diffusion of O2− ions, increases from 0.45 eV at a Y2O3 content of 4.6 mol% to 0.61 eV at a dopant content of 15.7 mol%. An external electric field of 0.02–0.08 V/Å noticeably changes the frequency of jumps of oxygen ions. At the electric field strength of 0.08 V/Å, a dopant content of 4.6 mol%, and a temperature of 1073 K, the О2− diffusion coefficient increases to 1.41∙10−9 m2/s. The dependence of diffusion coefficients on the field strength is close to quadratic.

Microstructural evolution and corrosion behavior of Ti–6Al–4V alloy fabricated by laser metal deposition for dental applications

In this study, microstructural evolution and corrosion behavior of Ti–6Al–4V (TC4) alloy fabricated by laser metal deposition (LMD) were investigated in simulated saliva at 37 °C as a function of scanning strategies. One-way scanning possessed a higher corrosion resistance than cross-scanning, but both of those were inferior to the wrought one. Compared with conventional wrought counterpart, the higher content of β phase improved the corrosion resistance. Results of Mott–Schottky and potentiostatic tests indicated that the passive film upon the surfaces of all the samples exhibited an N-type semiconductor structure and instantaneous nucleation. Of these, the LMD-ed parts displayed a lower donor concentration, higher vacancy diffusion coefficient, and thicker passive film than the wrought samples. XPS analysis revealed that the double-layer structure of passive film leads to a dense protective layer on the surface while the oxide contents of the three were different.

Hyperdynamics simulations with ab initio forces.

By applying the locally optimal rotation method to deal with the lowest eigenvalue of a Hessian matrix, we have efficiently incorporated the hyperdynamics method into the ab initio scheme. In the present method, we only need to calculate the first derivative of the potential and several more force calls in each molecular dynamics (MD) step, which makes hyperdynamics simulation applicable in ab initio MD simulations. With this implementation, we are able to simulate defect diffusion in silicon with boost factors up to 105. We utilized both direct MD and the hyperdynamics method to investigate diffusion of lithium atoms and silicon vacancies in silicon. We identified the complex diffusion process. The obtained diffusion coefficients of Li atoms and Si vacancies are in good agreement with the direct MD results.

The Passivity of Pure Nickel in Alkaline Solution under Different Temperatures: Electrochemical Verification and First-Principles Calculation

Properties of a passive film formed on pure nickel in an anaerobic alkaline solution are investigated by a first-principles calculation and electrochemical experiments in this work, and both results show agreement with each other. The formation energy of nickel vacancies is lower than that of oxygen vacancies in the NiO film, which is consistent with the deduction of the point defect model in which nickel vacancies impart p-type semiconducting character to the passive film, as confirmed by Mott–Schottky analysis. The density of nickel vacancies (approximately 1021 cm−3) in the passive film increases with temperature but decreases with the film-formation potential. The thickness of the passive film increases linearly with the film-formation potential, as verified both by Auger electron spectroscopy and electrochemical impedance spectroscopy. The diffusion coefficient of the nickel vacancies increases from 10−18 to 10−16 cm2/s as the temperature rises from 298 to 348 K, respectively, in experiments using high-field equations and first-principles calculation.

Frequency-Resolved X-Ray Absorption Spectroscopy: A Synchrotron-Based Technique for Extracting Kinetic and Transport Parameters

Worldwide energy infrastructures are shifting to renewable, but intermittent sources, requiring new solutions for energy storage and conversion. Solid oxide fuel cells are attractive for their high energetic efficiencies, but poor performance in oxygen electrodes contributes to high material costs preventing their widespread adoption. Improving the performance of these technologies is often hindered by inadequate knowledge of localized kinetic and transport behavior. Spatial averaging across the electrode structure makes these properties inaccessible to typical bulk electrochemical characterization techniques like DC polarization and AC impedance spectroscopy. Conversely, chemically sensitive operando and in situ techniques spatially resolve local behavior but struggle to separate physics by timescale. We aim to bridge this gap by extending on previous work using synchrotron-based operando micro X-ray absorption spectroscopy (µ-XAS)1–3 with steady periodic voltage perturbations to isolate and spatiotemporally resolve physics.We introduce this technique – called frequency-resolved X-ray absorption spectroscopy (fr-XAS)– by showing one-dimensional oxygen vacancy distribution images in a patterned thin film La0.6Sr0.4CoO3-δ electrode. Oxygen vacancies in the electrode are compensated by Co oxidation state, resulting in Co K-edge absorption shifts. At a fixed incident X-ray energy, the edge shifts appear as vertical changes, providing a direct measure of electrode oxidation state in the irradiated volume. Scanning the X-ray beam across the electrode and measuring the absorption shift at each point over time (with Fourier analysis to accurately distinguish shifts caused by the voltage perturbation from noise in the signal) allows us to construct the oxygen vacancy distribution images. The oxygen chemical potential is equilibrated at the electrode/electrolyte interface controlled by the voltage with gradients extending away from the interface determined by surface exchange and ionic transport processes. Changing the perturbation frequency separates observed physics by timescale and allows for directly extracting kinetic and transport parameters.Initial results show good quantitative agreement with a one-dimensional Gerischer model, and extracted vacancy diffusion coefficients and surface exchange rates are within a factor of 10 from literature values on porous electrodes.4,5 Impedance results from the same sample are obfuscated by extraneous processes past the point of reasonable interpretation, thereby highlighting the power of fr-XAS to extract meaningful parameters despite extraneous factors. These promising results demonstrate proof-of-concept for fr-XAS and provide motivation for the community to adapt this framework for other material systems relevant to energy storage and conversion technologies.(1) Fujimaki, Y.; Watanabe, H.; Terada, Y.; Nakamura, T.; Yashiro, K.; Hashimoto, S.; Kawada, T.; Amezawa, K. Direct Evaluation of Oxygen Chemical Potential Distribution in an SOFC Cathode by In Situ X-Ray Absorption Spectroscopy. ECS Trans. 2013, 57 (1), 1925–1932. https://doi.org/10.1149/05701.1925ecst.(2) Amezawa, K.; Fujimaki, Y.; Nakamura, T.; Bagarinao, K. D.-; Yamaji, K.; Nitta, K.; Terada, Y.; Iguchi, F.; Yashiro, K.; Yugami, H.; Kawada, T. (Invited) Determination of Effective Reaction Area in a Mixed-Conducting SOFC Cathode. ECS Trans. 2015, 66 (2), 129–135. https://doi.org/10.1149/06602.0129ecst.(3) Amezawa, K.; Fujimaki, Y.; Mizuno, K.; Kimura, Y.; Nakamura, T.; Nitta, K.; Terada, Y.; Iguchi, F.; Yugami, H.; Yashiro, K.; Kawada, T. (Invited) Triple Phase Boundary Reaction in a Mixed-Conducting SOFC Cathode. ECS Trans. 2017, 77 (10), 41–47. https://doi.org/10.1149/07710.0041ecst.(4) Lu, Y.; Kreller, C.; Adler, S. B. Measurement and Modeling of the Impedance Characteristics of Porous La1 − x Sr x CoO3 − δ Electrodes. J. Electrochem. Soc. 2009, 156 (4), B513–B525. https://doi.org/10.1149/1.3079337.(5) Søgaard, M.; Hendriksen, P. V.; Mogensen, M.; Poulsen, F. W.; Skou, E. Oxygen Nonstoichiometry and Transport Properties of Strontium Substituted Lanthanum Cobaltite. Solid State Ionics 2006, 177 (37), 3285–3296. https://doi.org/10.1016/j.ssi.2006.09.005.

Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

The absorber layers in perovskite solar cells possess a high concentration of mobile ion vacancies. These vacancies undertake thermally activated hops between neighboring lattice sites. The mobile vacancy concentration N0 is much higher and the activation energy EA for ion hops is much lower than is seen in most other semiconductors due to the inherent softness of perovskite materials. The timescale at which the internal electric field changes due to ion motion is determined by the vacancy diffusion coefficient Dv and is similar to the timescale on which the external bias changes by a significant fraction of the open-circuit voltage at typical scan rates. Therefore, hysteresis is often observed in which the shape of the current–voltage, J–V, characteristic depends on the direction of the voltage sweep. There is also evidence that this defect migration plays a role in degradation. By employing a charge transport model of coupled ion-electron conduction in a perovskite solar cell, we show that EA for the ion species responsible for hysteresis can be obtained directly from measurements of the temperature variation of the scan-rate dependence of the short-circuit current and of the hysteresis factor H. This argument is validated by comparing EA deduced from measured J–V curves for four solar cell structures with density functional theory calculations. In two of these structures, the perovskite is MAPbI3, where MA is methylammonium, CH3NH3; the hole transport layer (HTL) is spiro (spiro-OMeTAD, 2,2′,7,7′- tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9′-spirobifluorene) and the electron transport layer (ETL) is TiO2 or SnO2. For the third and fourth structures, the perovskite layer is FAPbI3, where FA is formamidinium, HC(NH2)2, or MAPbBr3, and in both cases, the HTL is spiro and the ETL is SnO2. For all four structures, the hole and electron extracting electrodes are Au and fluorine doped tin oxide, respectively. We also use our model to predict how the scan rate dependence of the power conversion efficiency varies with EA, N0, and parameters determining free charge recombination.

The role of diffusion in the aging process of electrolytic iron

Changes in the structure and properties of electrodeposited iron during its aging are caused by migration of vacancies. Determination of the effective diffusion coefficient of vacancies taking into account the grain structure of the coating remains a relevant method of studying the processes associated with this migration. The aim of the work was to establish the relations between the structural features of electrodeposited iron and relaxation of internal stresses during aging and annealing.The size of the grain was studied using scanning and transmission electron microscopes, internal stresses were determined on an original installation. Time parameters of internal stress relaxation were calculated using the Comsol Multiphysics program.In the course of the work, it was found that the magnitude of the effective diffusion coefficient of vacancies is significantly affected by the size of the grain, which, in its turn, affects the character of relaxation of internal stresses in the iron coating. The rate of internal stress relaxation during aging is faster when the grain size is smaller.

Accelerated kinetic Monte Carlo: A case study; vacancy and dumbbell interstitial diffusion traps in concentrated solid solution alloys.

Vacancy and self-interstitial atomic diffusion coefficients in concentrated solid solution alloys can have a non-monotonic concentration dependence. Here, the kinetics of monovacancies and ⟨100⟩ dumbbell interstitials in Ni-Fe alloys are assessed using lattice kinetic Monte Carlo (kMC). The non-monotonicity is associated with superbasins, which impels using accelerated kMC methods. Detailed implementation prescriptions for first passage time analysis kMC (FPTA-kMC), mean rate method kMC (MRM-kMC), and accelerated superbasin kMC (AS-kMC) are given. The accelerated methods are benchmarked in the context of diffusion coefficient calculations. The benchmarks indicate that MRM-kMC underestimates diffusion coefficients, while AS-kMC overestimates them. In this application, MRM-kMC and AS-kMC are computationally more efficient than the more accurate FPTA-kMC. Our calculations indicate that composition dependence of migration energies is at the origin of the vacancy's non-monotonic behavior. In contrast, the difference between formation energies of Ni-Ni, Ni-Fe, and Fe-Fe dumbbell interstitials is at the origin of their non-monotonic diffusion behavior. Additionally, the migration barrier crossover composition-based on the situation where Ni or Fe atom jumps have lower energy barrier than the other one-is introduced. KMC simulations indicate that the interplay between composition dependent crossover of migration energy and geometrical site percolation explains the non-monotonic concentration-dependence of atomic diffusion coefficients.

Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at −20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (∼30 nm in diameter) inside a ceria grain is reduced to the C-type CeO1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transportare determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO2, Ce0.94Sm0.06O1.97, Ce0.89Sm0.11O1.945, and Ce0.8Sm0.2O1.9, respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.

Influence of alkaline-earth metal substitution on structure, electrical conductivity and oxygen transport properties of perovskite-type oxides La0.6A0.4FeO3-δ (A = Ca, Sr and Ba).

Structural evolution, electrical conductivity, oxygen nonstoichiometry and oxygen transport properties of perovskite-type oxides La0.6A0.4FeO3-δ (A = Ca, Sr, and Ba) were investigated. La0.6Ca0.4FeO3-δ (LCF64) and La0.6Sr0.4FeO3-δ (LSF64) show a phase transformation in air at elevated temperature, i.e., from orthorhombic (Pnma) to rhombohedral (R3[combining macron]c) and from rhombohedral to cubic (Pm3[combining macron]m), respectively, while La0.6Ba0.4FeO3-δ (LBF64) remains cubic over the entire temperature range from room temperature to 1000 °C. The different phase behaviour of the solids is interpreted to reflect the decreased tendency for octahedral tilting with increasing alkaline-earth-metal dopant ion radius. The electrical conductivity of LSF64 is 191 S cm-1 in air at 800 °C, decreasing to a value of 114 S cm-1 at a pO2 of 0.01 atm, and found over this pO2 range roughly twice as high as those of LCF64 and LBF64. Failure to describe the data of electrical conductivity using Holstein's small polaron theory is briefly discussed. Chemical diffusion coefficients and surface exchange coefficients of the materials in the range 650-900 °C were extracted from data of electrical conductivity relaxation. Data of oxygen nonstoichiometry was used to calculate the vacancy diffusion coefficients from the measured chemical diffusion coefficients. The calculated migration enthalpies are found to decrease in the order LCF64 (1.08 ± 0.04 eV) > LSF64 (0.95 ± 0.01 eV) > LBF64 (0.81 ± 0.01 eV). The estimated ionic conductivities of the materials, at 900°C, are within a factor of 1.4.

Achieving outstanding combination of strength and ductility of the Al-Mg-Li alloy by cold rolling combined with electropulsing assisted treatment

Outstanding combination of strength and ductility of the Al-Mg-Li alloy could be achieved using the efficient and energy saving route of cold rolling combined with two steps of electropulsing assisted treatment (EAT), namely, the electropulsing assisted recrystallization annealing (EARA) and electropulsing assisted ageing (EAA). It was found that the EARA tremendously promoted the recrystallization behavior of the cold rolled alloy, leading to the grain refinement and texture weakening. Besides, the following EAA accelerated the coarsening of the strengthening δ′ phase. As a result, there was a remarkable improvement in strength and ductility, among which the increase in strength was mainly due to the combined effect of precipitation of δ′ phase and grain refinement, while the increase in elongation was mainly attributed to the grain refinement and texture modification. The rapid recrystallization during EARA was substantially attributed to the promotion effect of electropulsing on dislocation climb, which further resulted from not only the higher diffusion coefficient of vacancies under the application of electropulsing but also the another diffusion flux of vacancies induced by electro-migration.

Определение эффективного коэффициента диффузии вакансий в ультрадисперсном электролитическом железе и его влияния на режимы термической обработки

Определение эффективного коэффициента диффузии вакансий в ультрадисперсном электролитическом железе и его влияния на режимы термической обработки

Single-Component Solid Electrolyte Interphases on Lithium

The ability to utilize Li metal anodes in rechargeable batteries would be an enabling breakthrough in the quest to increase energy density and meet current aggressive performance targets (> 350 Wh/kg) for electric vehicles. However, Li anodes face several persistent challenges:1 ,2 (1) Large volume expansion upon cycling, leading to rapid mechanical degradation; (2) Poor Coulombic efficiencies during cycling, which leads to loss of active Li and formation of so-called “dead Li”; and (3) Evolution of dendritic structures upon Li deposition, which grow rapidly and ultimately short-circuit the cell. The origins of these problems can be traced back to the chemical, ionic, and mechanical properties of the solid electrolyte interphase (SEI)3- consisting of ionically conductive, physically blocking solid inorganic and organic phases derived from the reaction of Li with the electrolyte - and the electrochemical processes that break down this layer. After decades of study, however, the native SEI, which is highly chemically nonuniform and nanoscopically thin (< 100 nm), remains notoriously difficult to experimentally probe. Consequently, there remains a need for improved understanding of which Li-derived phases are desirable or undesirable in the SEI, their precise roles, and how to manipulate or modify the SEI properties for improved function and stability. In this talk, we present our recent studies of single-component artificial interfaces on Li, which we have conducted to gain deeper insights into the possible roles of common interfacial constituents found in nearly all Li SEI.4,5We have developed synthetic approaches based on gas-phase reactions with oxide- and fluoride-donating reactants that permit introduction of single phases with high spatial uniformity. We first show results of controllable growth (10 – 100’s of nm thickness) of targeted ionic materials Li2O and LiF, which is achieved through judicious selection of the reactant gas composition and reaction conditions. The formed layers are conformal, compact, polycrystalline, and chemically stabile in carbonate electrolytes, and thus serve as model all-ionic interphases useful for further qualitative and quantitative study. Following the establishment of these “ionically enriched” Li anodes, we subject these interphases to electrochemical measurements including electrochemical impedance, galvanostatic cycling (relevant to practical battery operating conditions), as well as softer electrochemical techniques such as chronoamperometry, which allow the minimal overpotentials and current densities driving deposition through the layers to be determined. We find evidence that there exist maximum sustainable overpotentials and current densities in the all-ionic layers, above which degradation mechanisms are activated within the films, leading to breakage. Using defect and transport modeling applied to the electrochemical results, we are able to quantify transport parameters including Li+vacancy carrier concentrations and diffusion coefficients, and compare these to the bulk materials. Our results show that interfacially-formed Li2O and LiF have unique properties when they are formed on Li metal. Current understanding of SEI behavior, which is largely based on knowledge of bulk properties of Li2O and LiF formed under drastically different thermochemical conditions, should be updated to reflect the unique structural, chemical and electrochemical properties occurring at interphases derived directly from Li. Tikekar, M. D.; Choudhury, S.; Tu, Z. Y.; Archer, L. A., Nat Energy 2016, 1, 1-7. Lin, D.; Liu, Y.; Cui, Y. Nat. Nanotechnol. 2017, 12, 194–206. Peled, E. J. Electrochem. Soc. 1979,126(12), 2047. Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H. Solid State Ionics 2002, 148(3–4), 405–416. Wang, C.; Meng, Y. S.; Xu, K. J. Electrochem. Soc. 2019, 166(3), A5184–A5186.

Measurement of Oxygen Potential Distribution on the Surface of a Mixed Conducting Electrode

Introduction Mixed ionic and electronic conductors (MIEC) are widely used as cathodes of intermediate temperature SOFCs. With a mixed conducting cathode, electrode reaction proceeds mainly through oxygen incorporation reaction at gas/electrode double phase boundary (DPB) followed by diffusion of oxide ion in the electrode particles. However, in certain conditions, the reaction through TPB (gas / electrode / electrolyte triple phase boundary) via surface diffusion may not be negligible. The contributions of TPB and DTB paths to the total process remain unclarified, but should be understood in a quantitative way to optimize the electrode design. An experimental method to evaluate the behavior of oxygen “on” and “in” the MIEC electrode is required. Kawada et al. proposed a potentiometric method to measure oxygen potential “on” the surface of a polarized electrode using a porous electrolyte that was made softly in contact with the surface of an electrode [1]. They explored the method to a position-resolved measurement to evaluate the surface diffusion of oxygen. Although it appeared promising, the reliability and reproducibility of the method was left unestablished. On the other hand, Hendriksen et al. used the similar technique to measure the local oxygen potential “in” the solid, by tightly pushing a pointed cone of an electrolyte on the surface of a MIEC [2]. The difference of those results could be from the way how the probe touches the sample surface, which makes the uncertainty of this technique. This paper revisits the experiments of oxide ion probes aiming at establishing the method and its interpretation. Discussion will be made on possibility of using this technique for studying surface reaction mechanism in detail. Methods 2.1 Oxide ion probe We made an oxide ion probe by coating a Pt wire with the porous Ce0.9Gd0.1O1.95 (GDC). The probe was made in contact with the MIEC electrode La0.6Sr0.4CoO3-δ (LSC64). In order to make the contact force as constant as possible, the probe was slowly brought into contact with the electrode surface and pressed until the error of the potentiostat disappeared and the measurement could be started. 2.2. Measurement condition The test cell was composed of GDC electrolyte, LSC64 thin film as Working Electrode (WE), LSC64 porous as Counter Electrode (CE), and Pt paste as Reference Electrode (RE). Measurements were made by three terminal electrochemical impedance spectroscopy method. Temperature dependence was measured from 723 K to 873 K. Oxygen partial pressure dependence was measured from 100ppm to 100%. The frequency response of the probe was evaluated by dividing the probe potential by the cell current. Results and Discussion Temperature and partial pressure dependences of the WE impedance and probe response are shown in Fig.1. At 873 K, the frequency response of the probe and WE were close to each other although the probe response did not include high frequency resistance which could be assigned to the contribution of the electrolyte. The WE and probe responses similarly increased as the oxygen partial pressure became lower. On the other hand, temperature dependence was different between WE and probe responses. WE resistance increased more remarkably than probe response when the temperature was decreased. A possible cause for the difference in the responses of the probe and WE is the resistance of oxide ion diffusion through the film. The oxide ion conductivity of LSC64 was estimated from the vacancy diffusion coefficient reported in [3] and vacancy concentration obtained from the chemical capacitance of the film. The roughly estimated diffusion resistance was 0.14 Ω at 873 K, 8.81 Ω at 773 K, and 32.7 Ω at 723 K, which are much smaller than the reaction resistance of WE. Additionally, the high frequency response of the impedance did not show 45° line in the Nyquist plot. So Thus, it can be said that the difference between the probe and WE is not due to the contribution of diffusion. Another possible factor is the oxygen incorporation resistance. Budiman et al. suggested the existence of a barrier layer just below the surface of the LaNi0.6Fe0.4O3-δ electrode [4]. A similar but less critical barrier layer could exist on LSC64. This also suggests the oxide ion probe measures at least the above the barrier layer, that is, almost the surface of the electrode.

Study of Oxygen Vacancy Diffusion in Yttria-doped Ceria and Yttria-stabilized Zirconia by Statistical Moment Method

Oxygen vacancy diffusion in yttria-doped ceria (YDC) and yttria-stabilized zirconia(YSZ) are investigated using statistical moment method, including the anharmonicity effects of thermal lattice vibrations. The expressions of oxygen vacancy-dopant association energy and oxygen vacancy migration energy are derived in an explicit form. Calculation of the vacancy migration energy enable us to evaluate the important role of dopant cation on the oxygen vacancydiffusion. The dependences of the vacancy activation energies and diffusion coefficients in YDC and YSZ systems on dopant concentration are also discussed in detail. The calculated results are in good agreement with the other theoretical and experimental results.

Radiation induced grain boundary segregation in ferritic/martensitic steels

Abstract The radiation induced segregation of Cr at grain boundaries (GBs) in Ferritic/Martensitic steels was modeled assuming vacancy and interstitialcy diffusion mechanisms. In particular, the dependence of segregation on temperature and grain boundary misorientation angle was analyzed. It is found that Cr enriches at grain boundaries at low temperatures primarily through the interstitialcy mechanism while depletes at high temperatures predominantly through the vacancy mechanism. There is a crossover from Cr enrichment to depletion at an intermediate temperature where the Cr:Fe vacancy and interstitialcy diffusion coefficient ratios intersect. The bell-shape Cr enrichment response is attributed to the decreasing void sinks inside the grains as temperature rises. It is also shown that low angle grain boundaries (LAGBs) and special Σ coincidence-site lattice (CSL) grain boundaries exhibit suppressed radiation induced segregation (RIS) response while high angle grain boundaries (HAGBs) have high RIS segregation. This different behavior is attributed to the variations in dislocation density at different grain boundaries.

Effects of temperature and dopant concentration on oxygen vacancy diffusion coefficient of yttria-stabilized zirconia

Oxygen vacancy diffusion coefficient in yttria-stabilized zirconia (YSZ) is investigated by statistical moment method. Vacancy-dopant association energy, vacancy migration energy are calculated as a function of temperature and dopant concentration. Oxygen vacancy diffusion coefficient increases with the increasing temperature but decreases with the dopant concentration. This degradation of the diffusion coefficient arises from the limitation for oxygen vacancy migration across cation barriers with the increasing dopant concentration. Calculated results are in good agreement with other theoretical and experimental results.

Temperature dependence of vacancy concentration and void growth mechanism in Al with constant hydrogen concentration: A first-principles study

In this study, a formulation that yields the chemical potential of solute hydrogen and thermal equilibrium concentrations of various coexisting vacancies, including atomic vacancy and vacancy–hydrogen complexes under the conditions of constant solute hydrogen concentration and temperature, is first proposed. The chemical potential of solute hydrogen and vacancy concentrations in Al are then obtained by evaluating the temperature-dependent physical properties in the equations via density functional theory and lattice vibration analyses. We found that hydrogen disbanded from vacancy–hydrogen complexes to form empty vacancy and interstitial hydrogen at the critical temperature approximately 200–300 K, depending on the solute hydrogen concentration. Finally, we revealed the operation temperatures of two principal void growth mechanisms by using the hydrogen pressure in the voids that are estimated from the chemical potential of hydrogen, vacancy concentrations, and diffusion coefficient of atomic vacancy. The first mechanism is plastic deformation growth, which is caused by high pressure inside the void at lower temperature. The other one is vacancy absorption growth mechanism, which is caused by absorbing diffusive vacancy in the lattice at higher temperature.

Enhancement of electrical and magnetic properties of nanocomposite BaTiO3 - NiZnFe2O4 irradiated by Gamma ray

The effects of the gamma ray irradiation on the, microstructural and magnetic properties of the composite (1-x) (Ni0.8Zn0.2Fe2O4) + x (BaTiO3) where x = 0%, 20%, 40%, 60%, 80% and 100 %BT content have been investigated. All samples were exposed to gamma radiation dose in range 310 kGy with dose rate 1.4 kGy/h at room temperature using 60Co radioactive source at irradiation cell (medical sterilizer type CM-20). From the X-ray diffraction analysis, it was found obviously that the lattice parameters and average grain size have been affected, for all irradiated samples. The initial permeability at Curie temperature for all samples were sharply decreases after irradiation. The calculated values of diffusion coefficient of Oxygen vacancies, the concentration of Oxygen vacancies and the activation energy of diffusion process were calculated before and after irradiation. From the results we conclude that the material after irradiation can be used for the Gamma ray detection.