Computer simulation of defects and oxygen transport in yttria-stabilized zirconia

We have used molecular dynamics simulations and energy minimization calculations to examine defect energetics and oxygen diffusion in yttria-stabilized zirconia (YSZ). Oxygen vacancies prefer to be second nearest neighbors to yttrium dopants. The oxygen diffusion coefficient shows a peak at 8 mol% yttria consistent with experimental findings. The activation energy for oxygen diffusion varies from 0.6 to 1.0 eV depending on the yttria content. The Y_Zr′–V_O^··–Y_Zr′ complex with a binding energy of − 0.85 eV may play an important role in any conductivity degradation of YSZ.     

Electrochemical transient investigations on the diffusion of minority charge carriers in YSZ doped by transition metal oxides

The voltage relaxation of galvanic cells with zirconia based electrolytes polarised between an inert Pt electrode and a Pt/air electrode is analysed to obtain the diffusion coefficients of holes and electrons. The hole diffusion coefficient can be reduced by replacing zirconium with guest ions of different size, e.g. Nb⁵⁺ and Ti⁴⁺. The TZP phase with 3 mol% Y₂O₃ of dopant has a higher hole diffusion coefficient than the CYZ phase doped with 8 mol% Y₂O₃. 1 and 3 mol% p-type MnO_1.5 doping increases the conductivity of holes in CYZ to a large extend, but does not influence the diffusivity. This indicates that the doping increases the hole conductivity through an increased concentration of holes. In the case of 10 and 15 mol% MnO_1.5 doped Z3Y, the electronic conductivity is dominant. The chemical diffusion coefficients which are related to the oxygen vacancies were determined by GITT. The results show that the chemical diffusion coefficient of oxygen vacancies is much larger than that for holes in zirconia.     

Bi2O3-Y2O3系高氧离子导体的离子电导和电子电导的研究

用交流电桥法研究了Bi₂O₃-Y₂O₃体系含22.5—30mol％Y₂O₃烧结试样在po₂值由1至10^-21atm范围内氧离子的电导率,实验证明该种材料的氧离子电导率比同温度下ZrO₂基固体电解质高若干倍;用这种材料作为固体电解质组成氧浓差电池,电池电动势和理论电动势的比值E/E₀等于1或接近1,说明这种材料几乎为纯氧离子导体,p型电子空穴导电性很小;用库伦滴定抽氧法测定了含Y₂O₃ 27.5mol％样品的电子导电特征氧分压,其值为lgp_e＇=(-767000/T)+655,电子导电性极小。可期望为一种新型氧离子导体材料。 关键词：     

Oxygen diffusion studies on (Y2O3)2(Sc2O3)9(ZrO2)89

The oxygen tracer diffusion coefficient (D?) has been measured for 9 mol% scandia 2 mol% yttria co-doped zirconia solid solution, (Y₂O₃)₂(Sc₂O₃)₉(ZrO₂)₈₉, using isotopic exchange and line scanning by Secondary Ion Mass Spectrometry, as a function of temperature. The values of the tracer diffusion coefficient are in the range of 10^? 8–10^? 7 cm² s^? 1 and the Arrhenius activation energy was calculated to be 0.9 eV; both valid in the temperature range of 600–900 °C. Electrical conductivity measurements were carried out using 2-probe and 4-probe AC impedance spectroscopy, and a 4-point DC method at various temperatures. There is a good agreement between the measured tracer diffusion coefficients (D?, Ea = 0.9 eV) and the diffusion coefficients calculated from the DC total conductivity data (D_σ, Ea = 1.0 eV), the latter calculated using the Nernst–Einstein relationship.     

Defect structures and migration mechanisms in oxide pyrochlores

Using computer simulation techniques the defect structure and oxygen ion migration mechanism of oxide pyrochlores (eg. Gd₂Zr₂O₇) was investigated in order to explain the decreased activation enthalpy for oxygen ion conductivity as a function of order. Shell model potentials were found to be necessary in order to obtain sufficiently accurate physical properties for the pyrochlore compound. The oxygen Frenkel defect consisting of ‘a split 48f vacancy’ and 8b interstitial appeared to be the most stable instrinsic defect, but vacancies related to extended defect structures may play an important role in the diffusion mechanism too. The migration mechanism of oxygen ions is mainly based on 48f-48f jumps and involve 0.9 eV barrier energy, comparable with the experimental activation enthalpies of 70–85 kJ/mol.     

The mechanism of ionic conductivity in stabilized cubic zirconia

The electron-density functional method (in the gradient approximation) and the pseudopotential method are used to study the mechanism of ionic conductivity in the cubic phase of zirconia stabilized with magnesium or yttrium. The oxygen-ion migration in the stabilized zirconia is shown to be a two-stage process, which consists in the formation of active oxygen vacancies and in oxygen-ion jumps from one active vacancy to another. The total activation energy of these processes is calculated to be 1.0–1.5 eV, which agrees with experimental data.     

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. Electrical conductivity measurements in polycrystals, exhibiting a grain boundary contribution to the ionic conductivity, are furthermore discussed in the frame of a “brick/boundary model”. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Oxygen diffusion in SrZrO3

Oxygen diffusion coefficients in SrZrO₃ polycrystals were determined using the isotopic exchange method with ¹⁸O as oxygen tracer. Diffusion treatments were performed at different temperatures between 1173 K and 1473 K. Oxygen diffusion profiles were established by secondary ion mass spectroscopy (SIMS). Classical diffusion equations were used to fit experimental results and to determine bulk diffusion (D_vol) and surface exchange (k) coefficients of oxygen in SrZrO₃ polycrystalline materials. From these values, bulk diffusion and grain boundary diffusion coefficients as well as oxygen surface exchange coefficients were determined. The activation energy of oxygen diffusion in the bulk is 2.1 eV, while for the diffusion in the grain boundary, 1.8 eV was found. The surface exchange reaction has an activation enthalpy of 1.2 eV.     

Computer modelling of ion migration in zirconia

Defect structure and migration pathways of cations in cubic zirconia (ZrO₂) have been calculated using two computer modelling techniques. The first is based on the Mott–Littleton method, which considers defects to be embedded in an otherwise perfect crystal, and the second is the supercell approach, which allows finite defect concentrations to be modelled. Using the first approach, migration pathways for both intrinsic and dopant cations have been calculated. Activation energies ranging from 3.1 to 5.8?eV have been calculated assuming a vacancy mechanism. For highly charged dopants a curved pathway was found to be favoured over a straight pathway. The effect of stabilizer concentration on the properties of the system investigated has been analysed using the supercell method; 3 × 3 × 3 and 4 × 4 × 4 supercells containing 3–40?mol% calcia (CaO) or yttria (Y₂O₃) have been constructed assuming a random distribution of both dopant cations and oxygen vacancies. After relaxation the oxygen vacancies were found to be located adjacent to the zirconium cations in the CaO-doped system, while remaining randomly ordered in the Y₂O₃-doped system. Also cation vacancies were created, and after relaxation they were surrounded in all systems (CaO-stabilized ZrO₂ and Y₂O₃-stabilized ZrO₂) on average by 2.7 oxygen vacancies.     

Electrical conductivity of yttria-stabilized zirconia with cobalt addition

Yttria-stabilized zirconia is the most developed solid electrolyte for use in high-temperature solid oxide fuel cells. Commercial yttria-stabilized zirconia powders reach high densification at temperatures higher than that of the usual anode materials. Reduction of the sintering temperature of the solid electrolyte could allow for co-firing of both ceramic components, thereby reducing production costs. The main purpose of this work was to study the effect of small cobalt additions on densification and on electrical conductivity of 8 mol% yttria-stabilized zirconia. Linear shrinkage results show that the onset temperature for shrinkage decreases with increasing cobalt content. Impedance spectroscopy measurements reveal that the electrical conductivity depends on the sintering profile. For specimens sintered at 1400 °C for 0.1 h the electrical conductivity of grains and grain boundaries are almost unchanged with that of 8YSZ. In contrast, for specimens sintered at the same temperature but for 0.5 h of soaking time, the electrical conductivity is higher in 0.025 mol% samples and is lower for 1 mol% Co doped 8YSZ. Degradation of the microstructure by increased porosity was obtained for high additive contents.     

Electrical properties of ZrO2-In2O3-Y2O3 and its application to a membrane for gas separation

Electrical properties of In₂O₃-doped yttria-stabilized zirconia (In-YSZ) were investigated. The solubility limit of In₂O₃ in YSZ (10 mol% Y₂O₃) is 17.5 mol%. The total conductivity depended on the concentration of In₂O₃. The activation energy of In-YSZ was higher than that of YSZ. From the oxygen partial pressure (P_o2) dependence of the total conductivity of In-YSZ, the electronic conductivity increased with increasing In₂O₃ concentration at low oxygen partial pressures and at high temperature. From the results, we discussed the applicability of In-YSZ to a membrane for hydrogen production from direct water splitting at high temperature.     

锆掺杂二氧化铀中氧缺陷扩散机制的密度泛函计算研究

应用基于量子力学的密度泛函计算和过渡态搜寻的CI-NEB方法,研究了锆掺杂前后二氧化铀晶格中氧空位和氧间隙本征缺陷的扩散机理,计算了扩散路径和扩散能垒。计算结果表明,锆掺杂使得氧空位缺陷<100>方向的扩散能垒降低了0.40 eV,氧间隙交换机制的扩散能垒降低了0.07 eV。锆掺杂后,氧空位远低于氧间隙缺陷的扩散能垒。最后分析了扩散过程中氧原子和金属原子之间的键长,说明锆掺杂导致点缺陷扩散能垒降低与晶格畸变密切相关。Oxygen vacancy and interstitial diffusion mechanisms in uranium dioxide doped with zirconium are investigated by the density functional theory calculations. The migration pathways and barriers are identified using the climbing-image nudge elastic band (CI-NEB) method. It is found that the vacancy migration barrier along the <100> direction decreases by about 0.40 eV, while the indirect interstitial migration barrier decreases by about 0.07 eV in the zirconium doped uranium dioxide. The oxygen vacancy migration barrier is far lower than the oxygen interstitial migration barrier in the uranium dioxide doped with zirconium. Based on the analysis of bond length of local structures during the migration of oxygen atoms, it is concluded that the lattice distortion may be responsible for the reduction of oxygen migration barrier.     

Ionic and electronic conduction of oxygen ion conductors in the Bi2O3−Y2O3 system

The ionic conduction of sintered samples of Bi₂O₃?Y₂O₃ containing 20–30 mol% Y₂O₃ has been investigated by means of ac conductivity experiments and EMF measurement of an oxygen concentration cell using the specimen tablet as electrolyte. Ac conductivity was measured at a frequency of 10 kHz under oxygen partial pressures ranging from 1 to 10^-21 atm. The results show that these materials possess high ionic conduction. The conductivities for samples containing 22.5–30 mol% Y₂O₃ are many times higher than those of stabilized zirconia-based solid electrolyte at corresponding temperatures. The ratio of E_meas./E_calc. of an oxygen concentration cell Pt∣O₂(air)∣Bi₂O₃?Y₂O₃∣O₂(pure oxygen)∣Pt is close to 1 which shows that the materials containing 22.5 to 30 mol% Y₂O₃ are nearly pure ionic conductors. The p-type conductivity is negligible at higher P_O2 values. The n-type conduction for a sample containing 27.5 mol% Y₂O₃ was investigated using the Coulomb titration technique in which the following cell was used: Pt Rh∣O₂(air)∣Bi₂O₃?Y₂O₃∣[O]_sn∣W.log P_é=-767000/T+665. P_é is equal to 2.6×10^-61 atm at 800°C. The n-type conductivity is also very small. Thus these materials are good oxygen ionic conductors.     

Electrical properties of modified TZP(+TiO2,FeOx)

1 to 24 mol% TiO₂ and FeO_X were added to 3.2 mol% Y₂O₃ doped ZrO₂ (Z3Y) to obtain tetragonal zirconia polycrystals (TZP) with modified electronic properties. The materials were prepared by coprecipitation which allows to obtain fine, homogeneous and sinteractive powders. The solubility of TiO₂ in Z3Y can reach 24 mol%, while the maximum solubility of FeO_x is about 8 mol%. The impedance results show a decrease of the bulk and total conductivity of titania doped TZP with increasing titania concentration, while those of FeO_x doped TZP show only minor changes. The Hebb-Wagner polarization method was applied to evaluate the partial hole and electron conductivities. Three models are presented to interpret the polarization curves. 1.6 mol% FeO_x doped TZP has higher hole conductivity in air, while titania doped TZP has higher electronic conductivity at low oxygen partial pressures as compared to pure Z3Y. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept.1994     

Oxygen vacancy formation and migration in Sr- and Mg-doped LaGaO₃ : a density functional theory study

Oxygen vacancy formation and migration in La0.9 Sr0.1 Ga0.8 Mg0.2O3δ (LSGM) with various crystal symmetries (cubic, rhombohedral, orthorhombic, and monoclinic) are studied by employing first-principles calculations based on density functional theory (DFT). It is shown that the cubic LSGM has the smallest band gap, oxygen vacancy formation energy, and migration barrier, while the other three structures give rise to much larger values for these quantities, implying the best oxygen ion conductivity of the cubic LSGM among the four crystal structures. In our calculations, one oxygen vacancy migration pathway is considered in the cubic and rhombohedral structures due to all the oxygen sites being equivalent in them, while two vacancy migration pathways with different migration barriers are found in the orthorhombic and monoclinic symmetries owing to the existence of nonequivalent O1 and O2 oxygen sites. The migration energies along the migration pathway linking the two O2 sites are obviously lower than those along the pathway linking the O1 and O2 sites. Considering the phase transitions at high temperatures, the results obtained in this paper can not only explain the experimentally observed different behaviours of the oxygen ionic conductivity of LSGM with different symmetries, but also predict the rational crystal structures of LSGM for solid oxide fuel cell applications.     

Determination of the Oxygen Diffusion Coeficient in Polycrystalline Li-Ti Ferrites

Two independent methods, the isotope method based on nuclear microanalysis and the method based on measuring the electronic-conductivity activation energy, are used to determine the grain-boundary diffusion and volume diffusion of an oxygen isotope ¹⁸O in a polycrystalline lithium-titanium ferrite at the thermal annealing temperature 1073 K. A comparative analysis is conducted of the potential of the methods in studying oxygen diffusion in the material concerned. It is shown that the technique for obtaining the diffusion parameters from the electronic conductivity measurements allows a comparatively precise determination of both the volume and grain-boundary diffusion coefficients of oxygen in polycrystalline ferrites.     

The effect of alumina addition on the conductivity, microstructure and mechanical strength of zirconia — yttria electrolytes

The effect of alumina additions on the ionic conductivity and mechanical properties of 3 and 8 mol% Y₂O₃ - ZrO₂ compositions has been investigated. Such materials are of interest for use in fuel cells and other similar applications. Sintered specimens were characterised by XRD, SEM, impedance spectroscopy, four — probe DC conductivity and mechanical strength measurements. Alumina additions had no affect on the conductivity degradation behaviour at 1000 °C and the activation energy but resulted in an increase in the strength of 8 mol% Y₂O₃ - ZrO₂ composition by about 25%. However, a significant drop in the ionic conductivity well above what could be accounted for by the decrease in the volume fraction of the conducting phase, was observed.     

Oxide-ion conductivity of (Ba1−xLax)2In2O5+x system based on brownmillerite structure

(Ba_1−xLa_x)₂In₂O_5+x, whose end member is Ba₂In₂O₅, is an oxygen-deficient perovskite oxide showing high oxide-ion conductivity. In order to clarify the reason why the high oxide ion conductivity appeared in this system, the electrical conductivity was measured as a function of temperature and La content. With an increasing La content, the discontinuous jump of ion conductivity in the Arrhenius plot, which is related to the disordering of the oxygen vacancies, disappeared for the sample with x0.2. Above x=0.12, the ion conductivity linearly increased with La content, while the activation energy remained constant with respect to the La content. Moreover, the conductivity for x=0.6 was 0.042 (S/cm) at 1073 K, which exceeded that of 8 mol% yttria-stabilized zirconia. The higher oxide-ion conductivity of this system could be dominated by the amount of mobile oxygen ions.     

Effect of Sulfur Addition on the Redox State of Iron in Iron Phosphate Glasses

Hyperfine Interactions - Mössbauer, IR, and electrical conductivity were investigated in iron phosphate glasses, 40Fe2O3–60P2O5 (in mol%), containing sulfur ranging from 0 to 2, 4, 6,...     

Absorption and diffusion of oxygen in the Ti<Subscript>3</Subscript>Al alloy

The absorption and diffusion of oxygen in the Ti₃Al alloy are studied by the projector augmented wave within the density functional theory. The highest absorption energies are shown to correspond to the sites in the octahedra formed by six titanium atoms, and the presence of aluminum in the nearest neighbors leads to a substantial decrease in the binding energy of oxygen in the alloy by approximately 1.5 eV. The energy barriers of oxygen diffusion between various interstices in the crystal lattice of the alloy are estimated, and the preferred migration paths in the (0001) plane and the [0001] direction are determined. It is found that the migration barrier from the most preferred octahedral O1 site to distorted tetrahedral Ti-site (2.42 eV) is a key barrier and limits the oxygen diffusion in the alloy. The calculated temperature diffusion coefficient of oxygen in the Ti₃Al alloy and the activation energies determined in two directions agree with the experimental data.