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.     

Electrode materials, interface processes and transport properties of yttria-doped zirconia

The kinetics of the oxygen exchange reactions at the electrodes of a galvanic cell using yttria-doped zirconia single crystals (9.5 mole-% Y₂O₃) as solid electrolyte and Pt or Ag as electrode materials was studied by complex impedance spectroscopy. The electrode resistance when using silver was found to have negligible values over the temperature range 180 – 900 °C. In agreement with these results, oxygen sensors were tested successfully at temperatures as low as 200 °C. According to the performance of silver as electrode material, an electrochemical method was developed to determine the oxygen diffusion coefficient in doped zirconia. The results obtained, compared to those of conductivity and oxygen tracer diffusion measurements, have allowed us to obtain information both on the structure of the defects in yttria-doped zirconia and on the correlation factor. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

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     

Composition dependence of the ionic diffusion coefficients in yttria-stabilized zirconias

The diffusion coefficient of the O²⁻ ions in yttria-stabilized zirconia (1−x)ZrO₂·xYO_1.5 has been investigated as a function of the concentration of Y³⁺ ion over a range x = 0.131–0.261 by using quasielastic light scattering. The diffusion coefficients were deduced from the analysis of the temperature dependence of the scattering intensity at 3 GHz. It is concluded that the diffusion coefficient reaches its maximum value at x = 0.165. The composition dependence of the scattering intensity suggests that the concentration of the moving carrier decreases as the Y³⁺ content increases in spite of an increase of the oxygen vacancies.     

Oxygen electrodes of zirconia electrolytes: fundamentals and application to analysis of oxygen containing gases

The kinetics of the oxygen exchange reaction and the reduction of NO at La_0.8Sr_0.2CoO₃−, La_0.8Sr_0.2MnO₃− and Ag-electrodes on stabilized zirconia (8mol% Y₂0₃=YSZ) has been studied by means of electrochemical methods (impedance, I-U characteristics). For La_0.8Sr_0.2CoO₃ electrodes the oxygen exchange was found to proceed via the bulk of the electrode with a rate limiting oxygen exchange at the electrode surface. Electrodes based on La_0.8Sr_0.2MnO₃ change their electrode characteristics with the applied potential. At low cathodic polarization the electrode reaction is limited to the three-phase boundary electrode/YSZ/gas. At high cathodic potentials oxygen vacancies are created and consequently additional oxygen is exchanged via the electrode bulk. Furthermore, a significant NO reduction was observed which indicate a reaction with the oxygen vacancies at the electrode surface. For Ag a rate limiting transport of oxygen atoms through the bulk of the electrode was found. As a consequence the oxygen concentration at the electrode surface remains nearly constant. In this context, the observed inactivity for the NO reduction of Ag-electrodes may be explained. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

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.     

Doping effect of Y ions on the microstructural and electromagnetic properties of Mn-Zn ferrites

Mn-Zn ferrites doped with different contents of Y³⁺ ions were prepared by conventional two-step synthesis method. The microstructure and electromagnetic properties of the as-prepared Mn-Zn ferrites were investigated. It was found that all the samples consisted of ferrite phases of typical spinel cubic structure, and when Y³⁺ ion content was upto 1.5 mol%, yttriumirongarnet (Y₃Fe₅O₁₂) phase with garnet structure was detected. With increasing doping content of Y³⁺ ions, the lattice constant and grain size increased, and after an increase to its maximum value, the sample apparent and relative densities dropped down. Through the analysis of magnetic properties, it was revealed that the saturation magnetization, and both the real and imaginary parts of permeability of the as-prepared samples raised with increasing doping content of Y³⁺ ions but decreased with more Y³⁺ ions, while their coercivity showed an opposite change trend; and the Curie temperature increased monotonously. The measurement of dielectric properties indicated that the dielectric constant of the doped Mn-Zn ferrites presented a rise with increasing Y³⁺ ion content, and dropped down gradually when more Y³⁺ ions were doped, while the dielectric loss tangent would decrease with Y³⁺ content upto 1.5 mol%, but after that, it increased.     

Ceria: Relation among thermodynamic,electronic hole and proton properties

The proton solubility and the hole conductivity of the rare earth doped ceria have been examined in their relations to the thermodynamic properties of doped ceria under the assumption that the hypothetical species, LnOOH and LnOO (Ln = Rare earth), can be regarded as constituents for representing protons and holes in the fluorite lattice. Focus is made on the dopant dependence, the host dependence and the temperature dependence in the rare earth doped zirconia(or ceria) fluorite lattice. The chemical potentials of the rare earth dopant are less stabilized in the ceria-based oxides than in the zirconia-based ones. The proton solubility in the ceria-based, zirconia-based, and ceria–zirconia solid solutions has been well interpreted in terms mainly of the hydroxidation energy and the stabilization energy of LnO_1.5 in the fluorite lattice. Since the dopant dependence of the stabilization energy of LnO_1.5 is stronger than the hydroxidation energy, the proton solubility becomes high in the smaller dopants. To account for less dopant-dependent behavior in the hole conduction, the peroxidation energy is assumed to have about the same dopant dependence as the stabilization energy. The calculated temperature dependences of proton solubility and hole concentration were compared with available experimental data; it has been suggested that holes and protons in ceria reach to saturation levels with lowering temperature. Some discussions are made on the possible explanation on recently observed anomalous hole conductivity in nano-size Ce_0.8Gd_0.2O_1.9 in terms of plausible effects of miscibility gap, associated Gd enrichment, and simultaneous formation of Ce³⁺ and holes.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.     

Improvement of the electrical properties of TiO2-doped Bi5Nb3O15 thin films by the addition of MnO2

The leakage current density of a 1.0 mol% TiO₂-doped Bi₅Nb₃O₁₅ (TB₅N₃) film was high, and the breakdown electric field was low. This could be attributed to the presence of intrinsic oxygen vacancies and free electrons. The electrical properties of the TB₅N₃ film improved upon the addition of MnO₂ because of the formation of extrinsic oxygen vacancies, which caused the number of intrinsic oxygen vacancies to decrease in order to maintain the equilibrium concentration of oxygen vacancies in the film. However, the electric properties degraded when the MnO₂ content exceeded 15.0 mol% because of the formation of interstitial oxygen ions and holes. The dielectric constant (?_r) of the TB₅N₃ film slightly decreased upon the addition of a small amount of MnO₂. The TB₅N₃ film with 15.0 mol% MnO₂, which exhibited a small leakage current density of 2.5 × 10^?11 A/cm² at 0.15 MV/cm and a high breakdown electric field of 0.47 MV/cm, still maintained a large ?_r of 118 with a small loss tangent of 2.0% at 100.0 kHz.     

Diffusion of hydrogen and oxygen in the high temperature proton conductor srceo3 investigated by dynamic conductivity measurements

In SrCeO₃ the oxygen vacancy concentration at elevated temperatures depends on the oxygen partial pressure in the surrounding atmosphere. Changes of the oxygen vacancy concentration are accompanied by change in the hole concentration and therefore can be measured by electrical conductivity measurements. We apply a dynamic method for studying the diffusion of oxygen vacancies by measurements of the time dependence of the electrical conduction after a sudden change in the oxygen partial pressure has taken place.

In doped samples and under wet conditions the protonic charge carriers are mainly incorporated by a dissociative absorption of water and lead to a noticeable protonic conduction. The proton diffusion in SrCe_0.95 Y_0.05O₃ is determined by relaxation measurements at a polarised sample in wet and hydrogen containing atmosphere. The electrochemical cell discussed here is characterised by a combination of one reversible and one (hydrogen) ion blocking electrode. From the time dependence of the depolarisation process the hydrogen diffusivity is extracted.     

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：     

Local structure analysis of YSZ by Y-89 MAS-NMR

The local coordination structure around Yttrium ions in yttria stabilized zirconia (YSZ) has been investigated by ⁸⁹Y MAS-NMR. The NMR spectrum showed multiple peaks corresponding to yttrium ions in different coordination numbers. The compositional dependence of spectra was observed. Yttrium ions of different oxygen coordination number were quantified. The oxygen vacancy concentration around the cations was determined. It was found that the vacancies were distributed around Zirconium ions in lower Y₂O₃ concentrations, and the vacancy concentration located to Yttrium began increasing at concentrations above 10 mol% Y₂O₃. The local structure change was able to be directly observed by ⁸⁹Y NMR measurements.     

Measurements of chemical diffusion coefficients of mixed conducting solids using point electrodes-investigations on Cu2S

The use of point electrodes for measuring chemical diffusion coefficients of mixed conducting solids with prevailing electronic conductivity is described and applied to low temperature Cu₂S. The electrochemical cell consist of the sequence of phases Pt/mixed conductor/Pt-point electrode. Applying small dc voltages to the cell leads to a steady state composition gradient within characteristic times that depend on the radius of the point electrode and on the chemical diffusion coefficient. The composition change after changing electrical voltage or current is followed by measuring the ohmic resistance change as a function of time, using ac nethods in the case of switching off. The method requires that the electronic conductivity depends on the usually small, but variable deviations of the composition of the solids from a definite stoichiometric composition. The measurements on the low temperature phase of Cu₂S give chemical diffusion coefficients ranging between 6.10^-6 cm²/s and 1.10^-7 cm²/s at 60°C.     

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.     

Rare earth oxide doping in oxide cathodes

The effect on life performance and poisoning with O₂ by doping oxide cathodes with rare earth oxides and pseudo rare earth oxides, notably yttria, is qualitatively explained in terms of electrolysis of BaO during emission of electrons. Doped cathodes show less electrolysis and consume therefore less Ba during life: consequently, doped cathodes have a better life performance. However, the lower Ba-production makes doped cathodes more sensitive to oxygen poisoning. The experimentally found relation between conductivity and yttria concentration was the motive to propose a new model for the crystal imperfections in BaO. In this new imperfection model most Y³⁺-ions will combine with barium vacancies, therefore, the increase of the conductivity is modest and also the effect on the position of the Fermi level is modest. By assuming a combination of bulk and surface conductivity, the agreement between experiment and theory can be improved further.     

Conductivity of Fe2O3-doped YSZ

Effect of 2 mol.% Fe₂O₃ addition on conductivity of yttria stabilized zirconia ceramics (YSZ, 6–12 mol.% Y₂O₃) was established. For Fe doped material, it was shown that conductivity demonstrates maximum at 10 mol.% Y₂O₃, and its value increases by 1.4 times. Conductivity maximum corresponds with the minimal volume of crystal lattice. Discussion of results was performed in terms of the model that assumes the control effect of hydrostatic pressure on oxygen ions diffusion.     

Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite

An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La_0.67Ca_0.33MnO₃/YSZ and La_0.67Ca_0.33MnO₃/Fe₃O₄. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe₃O₄ is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La_0.67Ca_0.33MnO₃ and the impurity doped La_0.67Ca_0.33MnO₃ composites. Compared with pure La_0.67Ca_0.33MnO₃, a possible interpretation is presented by considering the influences of YSZ and Fe₃O₄ on the structure of grain boundaries and/or surfaces of La_0.67Ca_0.33MnO₃grains.     

Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

Effect of Y2O3 addition on the conductivity and elastic modulus of (CeO2)1 − x(YO1.5)x

The conductivity and elastic modulus of (CeO₂)_1 − x(YO_1.5)_x for x values of 0.10, 0.15, 0.20, 0.30, and 0.40 were investigated by experiments and molecular dynamics simulations. The calculated conductivity exhibited a maximum value at approximately 15 mol% Y₂O₃; this trend agreed with that of the experimental results. In order to clarify the reason for the occurrence of the maximum conductivity, the paths for the transfer of oxygen vacancies were counted. The numerical result revealed that as the content of Y₂O₃ dopant increases, the number of paths for the transfer of oxygen vacancies decreases, whereas the number of oxygen vacancies for conductivity increases. Thus, the trade-off between the increase in the number of vacancy sites and the decrease in the vacancy transfer was considered to be the reason for the maximum conductivity occurring at the Y₂O₃ dopant content of approximately 15 mol%. The calculated elastic modulus also exhibited a minimum value at approximately 20 mol% Y₂O₃, which also agreed with the experimental results. It was shown that the Y–O–Y bonding energy increased with the increasing content of Y₂O₃ dopant. Thus, the trade-off between the increase in the number of vacancy sites and that in the Y–O–Y bonding energy was considered to be the reason for the minimum elastic modulus occurring at the Y₂O₃ dopant content of approximately 20 mol%.