Oxygen Self-diffusion Coefficients Research Articles

Molecular dynamics study of oxygen transport and thermal properties of mixed oxide fuels

Evaluation of thermal and transport properties of actinide mixed oxide fuels is a problem of high importance for the development of new generation reactors. Molecular dynamics provides a powerful tool, that can be used for this goal. In this work molecular dynamic simulations have been performed with the purpose to evaluate thermal and diffusion properties of UO 2, PuO 2 and (Pu 0.3U 0.7)O 2 oxide nuclear fuels. Specific heat, thermal conductivity and thermal expansion of these materials were estimated in the temperature range of 300–2500 K and compared with available recommendations. Migration energies of the anion defects via the interstitial and vacancy mechanisms have been estimated together with oxygen self-diffusion coefficients. The presence of a strong interaction between point defects and Pu atoms in (Pu 0.3U 0.7)O 2 oxide has been revealed and this interaction alters the diffusivity of point defects in comparison with the pure uranium and plutonium dioxides.

Determination of potential energy functions and calculation transport properties of oxygen and nitric oxide via the inversion of reduced viscosity collision integrals at zero pressure

The potential energy functions of oxygen and nitric oxide have been determined via the inversion of reduced viscosity collision integrals at zero pressure and fitted to obtain the analytical potential forms. The potentials reproduce viscosity, self-diffusion coefficient, and second virial coefficient of oxygen and nitric oxide in excellent accordance with the experimental data over a wide range of temperature. We have also derived very accurate equations for viscosity, self-diffusion coefficient, and second virial coefficient of O 2 and NO at different temperatures. Comparisons of O 2 O 2 potential with experimental potentials of Perugia group and ESMSV-type potential and ab initio potentials (MCRI-1/B3 and CCSD (T)/MCRI mixed model) and NO NO potential with the recently determined potential by means of ab initio electronic structure calculations, CASSCF/CASPT2 (18/14)/6-311G(2d) have been also included.

Mechanisms and dynamics of crystal growth, viscous flow, and self-diffusion in silica glass

We analyzed extensive literature data on crystal growth rate and viscosity in the temperature range between $1.1\phantom{\rule{0.3em}{0ex}}{T}_{g}$ (glass transition temperature) and the melting point of silica, ${T}_{m}$. We selected data for one silica glass type, having similar impurity contents, and confirmed that the normal growth model describes quite well the experimental growth rate data in this wide undercooling range. We then calculated effective diffusion coefficients from crystal growth rates, ${D}_{u}$, and from viscosity, ${D}_{\ensuremath{\eta}}$ (through the Eyring equation), and compared these two independent diffusivities with directly measured self-diffusion coefficients of silicon and oxygen in the same silica glass for which viscosity was measured. Our results show that silicon (not oxygen) controls the diffusion dynamics involved in both crystal growth and viscous flow in undercooled silica. This study not only unveils the transport mechanism in this important glass-forming material, but also validates the use of (easily measured) viscosity to account for the unknown transport term of the crystal growth expression in a wide range of undercoolings.

Predicting ionic conductivity of solid oxide fuel cell electrolyte from first principles

First-principles quantum simulations complemented with kinetic Monte Carlo calculations were performed to gain insight into the oxygen vacancy diffusion mechanism and to explain the effect of dopant composition on ionic conductivity in yttria-stabilized zirconia (YSZ). Density-functional theory (DFT) within the local-density approximation with gradient correction was used to calculate a set of energy barriers that oxygen ions encounter during migration in YSZ by a vacancy mechanism. Kinetic Monte Carlo simulations were then performed using Boltzmann probabilities based on the calculated DFT barriers to determine the dopant concentration dependence of the oxygen self-diffusion coefficient in (Y2O3)x(ZrO2)(1−2x) with x increasing from 6% to 15%. The results from the simulations suggest that the maximum conductivity occurs at 7–9mol% Y2O3 at 600–1500K and that the effective activation energy increases at higher Y doping concentrations in good agreement with previously reported literature data. The increase in the effective activation energy for migration arises from the higher-energy barrier for oxygen vacancy diffusion across an Y–Y common edge relative to diffusion across one with a Zr–Y common edge of two adjacent tetrahedra. The binding energies between oxygen vacancies and dopants were extracted up to the fourth nearest-neighbor interaction. Our results reveal that the binding energy is the strongest when the vacancy is in the second nearest-neighbor position relative to the Y dopant atom. The methodology was also applied to scandium-doped zirconia (SDZ). Preliminary results from quantum simulations of SDZ suggest that the effective activation energy for vacancy diffusion in SDZ is lower than that of YSZ, in agreement with experimental observations. The agreement with experimental studies on the two systems analyzed in this paper supports the use of this technique as a predictive tool on electrolyte systems not yet characterized experimentally.

Chemical Diffusion in TiO<sub>2</sub>

Chemical diffusion in TiO2-x within n - p transition regime has been studied. In this regime several point defects such as oxygen vacancies, • • O V , titanium interstitials, • • • i Ti and • • • • i Ti , electrons, ' e , and electron holes • h are present with comparable concentrations. Based on defect structure in TiO2-x the relationship between chemical diffusion coefficient and self-diffusion coefficients of both titanium and oxygen were deduced.

Thermodynamic and Structural Properties for the FeO–SiO<SUB>2</SUB> System by using Molecular Dynamics Calculation

Molecular dynamics (MD) simulation has been widely used as a very useful method for the calculation of thermodynamic, structural and transport properties for the molten slags and fluxes at high temperatures. In this study, MD simulation using the Born-Mayer-Huggins type pairwise potential with partial ionic charges has been used to calculate the thermodynamic, structural and transport properties for the FeO–SiO2 system. The calculated structural properties such as pair distribution functions and fractions of bonding types of oxygen (bridging, non-bridging and free oxygen) with silicon atoms in FeO–SiO2 melts were in good agreement with previously measured and estimated results, and also the self-diffusion coefficients of iron, silicon and oxygen have been calculated at various temperatures and compositions. The enthalpy, entropy and Gibbs energy of mixing for the FeO–SiO2 system were calculated based on the thermodynamic and structural parameters obtained from MD simulation. The phase diagram for the FeO–SiO2 system estimated by calculated Gibbs energy of mixing shows good agreement with observed result in the range from pure iron to fayalite, and the liquid–liquid immiscibility region in the FeO–SiO2 system has also been assessed by MD calculation.

Kinetics and mechanisms of crystal growth and diffusion in a glass-forming liquid

Extensive data on the viscosity, covering 15 orders of magnitude, and crystal growth rate, covering seven orders of magnitude, of liquid diopside (CaO.MgO.2SiO(2)) were collected in a wide range of undercoolings from 1.10T(g) to 0.99T(m) (T(g) is the glass transition temperature and T(m) the melting point). The raw growth rate data were corrected for the increased interfacial temperature produced by the heat released during crystallization. A detailed analysis confirms that growth mediated by screw dislocations reasonably explain the experimental data in these wide ranges of temperatures and growth rates. Effective diffusion coefficients were calculated from crystal growth rates and from viscosity, and were then compared with measured self-diffusion coefficients of silicon and oxygen in diopside melt. The results show that oxygen and silicon control the diffusion dynamics involved in crystal growth and viscous flow. This study not only unveils the transport mechanism in this complex liquid, but also validates the use of viscosity (through the Stokes-Einstein or the Eyring equations) to account for the kinetic term of the crystal growth expression in a wide range of temperatures.

Mobility and diffusion of oxygen isotopes in YBa2Cu3O6+x : Monte carlo simulation

The self-diffusion (D0) and isotope diffusion (D*) coefficients of oxygen in YBa2Cu3O6+x are calculated as functions of the temperature (600–1200 K) and the oxygen content (0<x<1). The Monte Carlo simulation is performed with due regard for both the interaction of oxygen ions at lattice sites in the basal planes of YBa2Cu3O6+x and the interaction between a jumping ion at a saddle point and the environment. Equilibrium thermodynamic characteristics (including the phase diagram and the heat capacity) are calculated in terms of the Hamiltonian of interaction between oxygen ions at the lattice sites. It is found that an increase in the oxygen content leads to a decrease in the diffusion coefficients D0 and D*, an increase in the effective activation energies for diffusion by 0.3–0.5 eV, and a decrease in the Haven ratio from 1 to ∼0.5.

About ionic conductivity/diffusion relationships in yttria doped zirconia

The values of the oxygen self-diffusion coefficients (measured using18O tracer) are compared to the ionic conductivity (measured by impedance spectroscopy) of 9.5 mol% yttria doped zirconia single crystals in the temperature range 240–800 °C in air.

The influence of oxygen partial pressure on recovery creep in magnetite single crystals

Creep tests of magnetite single crystals have been performed in compression at high temperatures (between 1000 and 1340°C). The main goal was two-fold: First of all, the determination of the dependence of steady state creep rate as a function of the oxygen partial pressure in the external atmosphere. Secondly, the comparison of this dependence with that of oxygen self-diffusion coefficient with the partial oxygen pressure. This second one had been reported in literature previously. Both are quantitatively very similar. This fact has allowed us to discuss some aspects of the minority point defects in the oxygen sub-lattice of this oxide.

Oxygen self-diffusion in cerium oxide doped with Nd

Polycrystalline Ce0.77Nd0.23O1.885having a relative density in excess of 98% was prepared. Oxygen diffusion experiments were performed for the temperature range from 750 to 1100 °C, in an oxygen partial pressure of 6.6 kPa. The concentration profile of18O in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). The oxygen self-diffusion coefficient obtained using secondary ion mass spectrometry was expressed by D = 6.31 × 10−9exp(−53 kJ mol−1/RT) m2s−1and was in the extrinsic region. The oxygen diffusion coefficient of Ce0.77Nd0.23O1.885was larger than that of Ce0.8Y0.2O1.90; it was close to those of Ce0.6Y0.4O1.80and Ce0.69Gd0.31O2−δ. The oxygen diffusion coefficient obtained by the tracer method at 700 °C agreed with that calculated from the electrical conductivity in Ce0.77Nd0.23O1.885. The activation energy of the surface exchange coefficient was 94 kJ mol−1, and the values of the surface exchange coefficient were similar to those of stoichiometric CeO2and ThO2.

Kinetic parameters influencing the performance of IT-SOFC composite electrodes

Composite electrodes have made important contributions to the development of intermediate temperature solid oxide fuel cells (IT-SOFC). At a macroscopic level the separate electronic and ionic components can usefully be considered as a combined mixed conductor. This allows available kinetic data for oxygen self-diffusion ( D * cm 2 s −1) and surface exchange ( k cm s −1) coefficients to be evaluated so that possible rate-determining processes can be identified. For LSM/YSZ and LSCF/CGO composite electrodes it appears that the k values for YSZ and CGO have to be enhanced by at least two orders of magnitude to account for observed cathode ASR values. It is suggested that ‘spillover’ reactions involving the electronic component could be responsible for the increased rate of O 2− injection into YSZ and CGO. Additional strategies to further enhance the surface exchange rate for CGO at 500°C, such as doping with redox cations (e.g. Tb 4+/Tb 3+, Nb 5+/Nb 4+), changing the reduction enthalpy for Ce 4+⇒Ce 3+, and promotion with noble metal catalysts, are also discussed.

Self-diffusion in α-Fe2O3 natural single crystals

Measurements of18O self-diffusion in hematite (Fe2O3) natural single crystals have been carried out as a function of temperature at constant partial pressure aO2=6.5·10−2 in the temperature range 890 to 1227 °C. The aO2 dependence of the oxygen self-diffusion coefficient at fixed temperature T=1150 °C has also been deduced in the aO2 range 4.5·10−4 - 6.5·10−1. The concentration profiles were established by secondary-ion mass spectrometry; several profiles exhibit curvatures or long tails; volume diffusion coefficients were computed from the first part of the profiles using a solution taking into account the evaporation and the exchange at the surface. The results are well described by $$D_O \left( {{{cm^2 } \mathord{\left/ {\vphantom {{cm^2 } s}} \right. \kern-\nulldelimiterspace} s}} \right) = 2.7 \cdot 10^8 a_{O_2 }^{ - 0.26} \exp \left( { - \frac{{542\left( {{{kJ} \mathord{\left/ {\vphantom {{kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}}} \right)}}{{RT}}} \right)$$

Diffusion Mechanism of Oxide Ions in Mn-Zn-ferrites.

Oxygen self-diffusion coefficients in a manganese zinc ferrous ferrite single crystals were determined at various oxygen partial pressures by means of a secondary ion mass spectrometer. Temperature dependence of diffusivity of oxide ions consisted of three parts. The characteristics at middle temperature range above 5 hPa oxygen partial pressure exhibited a behavior as an extrinsic one and was described as;D=4.8×10-6⋅P0.35O2 exp(-137kJ/mol/RT)cm2⋅s-1This activation energy corresponded to the barrier for the migration of oxide ions in spinel structure. The dependence of oxygen self-diffusion on oxygen partial pressure exhibited a positive sign (slope value: 0.35). This phenomenon suggests that the oxygen ions diffuse in Mn-Zn ferrite through interstitial sites above 5 hPa of the oxygen partial pressure.

Simulated features of the air-hydrate formation process in the Antarctic ice sheet at Vostok

AbstractA recently developed theory of post-nucleation conversion of an air bubble to air-hydrate crystal in ice is applied to simulate two different types of air-hydrate formation in polar ice sheets. The work is focused on interpretation of the Vostok (Antarctica) ice-core data. The hydrostatic compression of bubbles is the rate-limiting step of the phase transformation which is additionally influenced by selective diffusion of the gas components from neighboring air bubbles. The latter process leads to the gas fractionation resulting in lower (higher) N2/O2 ratios in air hydrates (coexisting bubbles) with respect to atmospheric air. The typical time of the post-nucleation conversion decreases at Vostok from 1300-200 a at the beginning to 50-3 a at the end of the transition zone. The model of the diffusive transport of the air constituents from air bubbles to hydrate crystals is constrained by the data of Raman spectra measurements. The oxygen and nitrogen self-diffusion (permeation) coefficients in ice are determined at 220 K as 4.5 × 10−8 and 9.5 × 10−8 mm2 a−1, respectively while the activation energy is estimated to be about 50 kJ mol−1. The gas-fractionation time-scale at Vostok, τF ∼300 a, appears to be two orders of magnitude less than the typical time of the air-hydrate nucleation, τz ∼30-35 ka, and thus the condition for the extreme gas fractionation, τF ≪ τz is satisfied. Application of the theory to the GRIP and GISP2 ice cores shows that on average, a significant gas fractionation cannot be expected for air hydrates in central Greenland. However, a noticeable (statistically valid) nitrogen enrichment might be observed in the last air bubbles at the end of the transition.

The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids

Differences between the δ18O values of Si- and Fe-rich immiscible liquids in the system Fe2SiO4-KAlSi2O6-SiO2 (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition of 18O into the Si-rich liquid is consistent with previous experience with the preferential partition of 18O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally coexisting crystals. The effects of Soret (thermal) diffusion on the distribution of oxygen isotopes in silicate liquids above the solvus in the system Fa-Lc-Q under conditions of an imposed temperature gradient of ca. 250 °C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a result of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in 18O by up to 4.7 permil, and the highest δ18O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from the Soret experiments are in accord with those from other isotope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxygen needed to produce variations in the δ18O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely.

Oxygen Self-Diffusion in Cerium Oxide

Polycrystalline cerium oxide was prepared by sintering and used for oxygen diffusion experiments by the gas phase analysis. Lattice constant measurements confirmed the stoichiometric condition of the sample. Although Si was contained in the specimens as an impurity, it was considered that point defects were not created by Si, because both Si and Ce have the same valence. The oxygen diffusion coefficient obtained in the present work could be represented by the equation: Doxy=0.364exp(-322kJ⋅mol-1/RT)m2⋅s-1. This result is similar to the oxygen self-diffusion coefficients in other stoichiometric oxides with fluorite structure. This agreement with literature values was considered to be a confirmation that the diffusion coefficient obtained in this work is actually that of oxygen in a stoichiometric CeO2 lattice.

Ionic conductivity of perovskite LaCoO3 measured by oxygen permeation technique

Oxygen permeation measurement is demonstrated, not only for a mixed oxide ionic and electronic conductor, but also as a new alternative to determine ambipolar conductivities, which can be usually reduced to be partial conductivities (either ionic or electronic). As a model system and an end member of an important oxygen permeable La1-xSrxCoO3- dense membrane system, LaCoO3 dense ceramic was measured with respect to the thickness, temperature and oxygen partial pressure dependencies of its oxygen permeability. Within the thickness range used (down to 0.041cm), the oxygen permeation of LaCoO3 was found to be purely controlled by bulk diffusion with activation energies between 260–300kJmol-1. Its ionic conductivity and oxygen self-diffusion coefficient as functions of oxygen partial pressure were also derived from permeability data. PO2-0.46 and PO2-0.31 relations were found in the high pressure (1.01–10-2bar) and low pressure (10-2.8–10-3.4bar) ranges, respectively.

Diffusion in silicate melts: I. Self diffusion in CaO[sbnd]Al 20 3[sbnd]SiO 2 at 1500°C and 1 GPa

Self diffusion coefficients of calcium (DCa), aluminum (DAl), silicon (DSi), and oxygen (DO) were measured in molten CaOAl 20 3SiO 2 at 1500°C and 1 GPa over a range of melt compositions, using the isotope tracer method. For all but one composition, the measured self diffusion coefficients decrease in the order DCa > D Al > D O > D Si, with D Ca ∼ 4–19 D Si , D Al ∼ 2D Si, and D O ∼ 1–2 D Si. The relative uncertainties, based on replicated experiments, are 8% for DCa, 27% for DAl, 28% for DSi, and 18% for DO. Although the self diffusion coefficients of calcium, aluminum, silicon, and oxygen increase with the decrease of melt viscosity, they do not obey the Stokes-Einstein equation or the Eyring equation. The compositional dependence of the self diffusion coefficients of calcium, aluminum, and oxygen were parameterized in terms of melt viscosity (Pa · s) via the power law relation: DCa = (115.141/viscosity 0.389) × 10 −12m 2s −1DA1 = (47.302/viscosity 0.800) × 10 −12m 2s −1DO = (33.080/viscosity 0.722) × 10 −12m 2s −1The compositional dependence of the self diffusion coefficients of Si was parameterized in terms of the degree of melt polymerization (NBO/T) DSi = [0.669 exp(4.195 NBO/T)] × 10 −12m 2s −1There is an abrupt change in the diffusion behavior of Al and Si between melts of more and less polymerized compositions, occurring at a NBO/T value of approximately 0.6, which may be indicative of a major change in melt structure. The hypothesized structural transition, the nature of which remains to be identified, may control other aspects of diffusion such as the sign of the activation volume for self diffusion of network-forming cations and the magnitude of the activation energy.The self diffusion coefficients of calcium, aluminum, silicon, and oxygen were used to calculate electrical conductivities of molten CaOAl 2O 3SiO 2 at 1500°C and 1 GPa using the generalized NernstEinstein relation. The estimated electrical conductivity implies that calcium, silicon, and oxygen contribute nearly equally to the total conductance: 31 % from calcium, 29% from silicon, and 30% from oxygen, which is quite different from what one would expect for alkali-bearing melts in which Na and K are believed to be the dominant conducting species.

Self-diffusion in α‒Al2O. IV. Oxygen grain-boundary self-diffusion in undoped and yttria-doped alumina polycrystals

Abstract Oxygen self-diffusion coefficients were determined in polycrystals of α-alumina either ‘undoped’ or doped with 500wt.ppm (225molppm) Y2O3 in the temperature range 1460-1720°C, with 18O2 and secondary-ion mass spectrometry. Oxygen grain-boundary diffusion coefficients obey the relations for ‘undoped’ alumina and for ‘yttrium-doped’ alumina. The comparison between the two materials indicates that yttrium addition decreases the oxygen grain-boundary diffusion. For both materials, it was observed that the activation energy of grain-boundary diffusion is greater than the activation energy of the bulk diffusion. Similar results were obtained in single crystals for diffusion in the bulk and in subboundaries. These results were interpreted as being due to impurity segregation along boundaries and led to a model.