Calcia-stabilized Zirconia Solid Electrolytes Research Articles

Influence of Bi2O3 additive on the electrical conductivity of calcia stabilized zirconia solid electrolyte

A few compositions of Bi2O3-doped calcia stabilized zirconia (Zr0.88−xBixCa0.12O2−δ; x=0.0, 0.02, 0.05, 0.10) were prepared via solid state reaction route. Addition of Bi2O3 decreased the sintering temperature of calcia stabilized zirconia (CSZ) and also augmented the growth of zirconia grains. Dense ceramic systems were formed in a temperature range of 1200–1300°C. X-ray Rietveld refinement analysis of the system revealed the formation of cubic zirconia phase at room temperature. Microstructural studies of the sintered pellet depicted highly dense grain morphology. XPS spectra of the compositions confirmed the elemental confinements and various characteristic valence states of the constituents. Thermal expansion coefficient of the Bi2O3 doped CSZ system was found to be compatible with the other components of solid oxide fuel cells (SOFC). Furthermore, conductivity analysis evidenced an Arrhenius type thermally activated ionic conduction above 300°C. The electrical conductivity of the system was found to increase with bismuth content up to x=0.05, thereafter decreased due to less solubility of Bi3+ into zirconia matrix. A high conductivity ∼0.019Scm−1 was achieved for 5mol% Bi2O3 calcia-codoped cubic zirconia system at 760°C. Thus, cost effective bismuth codoped calcia stabilized zirconia might be a possible alternative to yttria stabilized zirconia (YSZ), due to its ionic conductivity comparable to that of the YSZ.

Influence of Interfaces on the Electrical Properties of Sintered Calcia-Stabilized Zirconia Solid-Electrolytes

Precursor powders of 15 mol% calcia-stabilized zirconia solid electrolytes synthesized by a freeze-drying method, and a sol-gel method, were used to prepare oxygen sensors. Sensor elements were fabricated via uniaxial pressing of the calcined powders and subsequently sintered at 1650 {degree sign}C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to analyze the microstructure, including the interfaces, grain boundary phases and impurity distribution, etc. The electrical response of the gas sensors to oxygen and the complex impedance in air were investigated at various operating temperatures. The correlations between grain microstructure and the impedance spectra characteristics of the sensors as well as oxygen sensing characteristics are discussed.

Free energy of formation of zircon and hafnon

The free energy of formation of zircon (ZrSiO4) from its oxides was determined between 1100 and 1300 K by an electrochemical method, in which values of μO₂ defined by the two assemblages Fe2SiO4-Fe-SiO2 (fayalite-iron-quartz) and Fe2SiO4-Fe-ZrO2-ZrSiO4 were each measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in μO₂ between these two assemblages corresponds to the reaction ZrO2 + SiO2(qz) = ZrSiO4. The results, when analyzed using calorimetric data for the entropies and high-temperature heat capacities of ZrSiO4, ZrO2, and SiO2(quartz), yields Δf,oxH0 298K = -24.0 ± 0.2 kJ/mol for ZrSiO4, in good agreement with the calorimetric value of Ellison and Navrotsky (1992). ZrSiO4 is predicted to decompose to ZrO2 plus SiO2 (cristobalite) at 1938 K, assuming a temperature of 1430 K for the martensitic phase transition between the tetragonal and monoclinic forms of ZrO2 (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol. The same experimental approach was used also to determine the free energy of formation of hafnon (HfSiO4). The entropy of hafnon (S0 298K = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δf,oxH0 298K = -25.0 ± 0.2 kJ/mol).

Influence of Interfaces on the Electrical Properties of Sintered Calcia-Stabilized Zirconia Solid-Electrolytes

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Synthesis, processing and characterization of calcia-stabilized zirconia solid electrolytes for oxygen sensing applications

Precursor powders of calcia-stabilized zirconia (CSZ) solid electrolytes have been synthesized by a sol–gel method. The phase evolution of the precursor powders after thermal treatments at different temperatures were analysized by X-ray diffraction technique. Disc-shaped sensor elements were fabricated via uniaxial pressing of the calcined powders and subsequently sintered at 1650 °C. Scanning electron microscopy (SEM) was used to analyze the microstructure of the sintered pellets. Platinum electrodes were applied to the sintered elements to produce potentiometric/electrochemical gas sensors. The electrical response of the gas sensors to oxygen and the complex impedance of the sensors in air were measured at various temperatures. Impedance analyses indicate that the sensor cell with 15 mol% CaO has much lower resistance (the sum of bulk and grain-boundary resistance) than the sensor cell with 22 mol% CaO. This is also reflected by the EMF responses of both sensor cells to various oxygen concentrations in the testing gas. The EMF deviation from the theoretical value of the CSZ sensor cell with 22 mol% CaO was larger than that of the CSZ sensor cell with 15 mol% CaO. The corrrelations between material compositions, microstructures of the sintered pellets and the electrical properties of the sensors are discussed.

Thermodynamic Properties of K 2U 4O 12( s) and K 2U 4O 13( s) by EMF and Calorimetric Measurements

The coexistence of K 2U 4O 12( s) and K 2U 4O 13( s) in the K–U–O system up to 1200 K was established. The EMF measurements over the above phase field were carried out using a calcia-stabilized zirconia solid electrolyte galvanic cell in the temperature range 1053 to 1222 K. The equilibrium oxygen partial pressure above the phase fields was calculated to be ln p(O 2) kPa±0.09=−31452/ T+25.065. The enthalpy increment values of K 2U 4O 12( s) and K 2U 4O 13( s) were measured using a high temperature Calvet calorimeter by drop method and the thermodynamic values were suggested for the compounds.

Studies on the oxygen potentials of (U yTh 1− y)O 2+ x solid solutions

The oxygen potentials of (U) y Th 1− y )O 2+ x solid solutions ( y varying from 0.54 to 0.9) were measured by using a gas equilibration method, employing H 2 + H 2O, CO + CO 2 and CO 2 + H 2 gas mixtures. The measurements covered an oxygen potential range of −450 to −220 kJ mol −1 and the O M ratios varied between 2.000 and 2.035. The attainment of equilibrium as well as the actual oxygen potentials realised in the gas mixture were determined by using an oxygen probe based on calcia stabilized zirconia solid electrolyte. The results of the measurements indicate that the oxygen potentials of (U y Th 1− y )O 2+ x solid solutions are a function of both the uranium valence and the U/U + Th ratio. Partial molar enthalpies and entropies of solution of oxygen in the solid solutions were also computed from the temperature dependance of the oxygen potentials.

酸化鉄または二酸化珪素存在下における硫酸亜鉛の分解圧

The decomposition pressure of zinc sulphate in the absence or presence of ferric oxide or silica has been measured by e.m.f. method, using calcia stabilized zirconia solid electrolyte, and thermogravimetry in the temperature range 806 to 1088K.The results obtained are as follows;1) In the absence of ferric oxide or silica, zinc sulphate decompose to form zinc oxide through basic zinc sulphate, with the increase of temperature. These decomposition pressure measured well agree with those of other investigators.2) In the presence of ferric oxide, zinc sulphate directly decompose to form zinc ferrite, ZnFe204, without the formation of basic sulphate. The decomposition pressure and standard free energy change were obtained as follows;log (pSO3 /atm) =(12717 ± 106)/T+ 11.070 ± 0.18 (882 to 1015K)log (pSO3 /atm) =-(9499 ± 79)/T + 7.886 ± 0.13 (1015 to 1076K)Δ°/cal =58180 + 480 (50.64 ± 0.83) T (882 to 1015K)ΔG°/cal =43460 + 360 (36.08 ± 0.59) T (1015 to 1076K)3) In the presence of silica, zinc sulphate directly decompose to form zinc silicate, Zn2SiO4, without the formation of basic zinc sulphate. The decomposition pressure and standard free energy change were obtained as follows;log (pSO3 /atm) =-(10832 ± 105)/T+ 9.088 ± 0.15 (903 to 1015K)log (pSO3 /atm) =-(8968 ± 92)/T+7.266 ± 0.13 (1015 to 1088K)ΔG°/cal =49560 ± 480 -(41.58 ± 0.68) T (903 to 1015K)ΔG°/cal =41030 ± 420-(33.24 ± 0.59) T (1015 to 1088K)

Thermodynamics of the oxidation of rare earth oxysulfides at high temperatures

The thermodynamics of the oxidation of rare earth oxysulfides to their respective oxysulfates was investigated at 1000 to 1500 K using oxygen concentration cells of the type: Pt (s)∣RE2O2S (s), RE2O2SO4 (s), RE2O3 (s)∣CSZ∣Air(p02 = 0.21 atm)∣Pt (s) where CSZ represents the calcia stabilized zirconia solid electrolyte and RE − La, Pr, Nd, Sm, Eu, Gd, Tb, and Dy. It was found that the stability of oxysulfates with respect to their respective oxysulfides decreases as the atomic number of the rare earth increases. The standard free energies of oxidation of oxysulfides to oxysulfates for the above rare earths were used to estimate the standard free energies of oxidation of the oxysulfides of higher atomic number rare earths.

Determination of the standard free energies of formation of Ce 2O 2S and Y 2O 2S at high temperatures

The formation of rare earth oxysulphides is of importance in the rare earth treatment of iron and steel and the desulphurization of gaseous fuels with rare earth oxides. Phase stability diagrams governing the formation of cerium oxide, oxysulphide and sulphides in these processes are constructed from the limited thermochemical and experimental thermodynamic data available in the literature. In this paper the results of a galvanic cell technique using a calcia-stabilized zirconia solid electrolyte are reported, from which the standard free energies of formation of Ce 2O 2S and Y 2O 2S are calculated: ΔG f o ( Ce 2 O 2 S) = −354300 ± 3000 cal mol − at 1068 K and Δ f o ( Y 2 O 2 S) = −340100 ± 3000 cal mol −1 at 1072 K.

Thermodynamics of lead silica-fluoride melts

Abstract Activities of PbO in PbO-PbF2 and PbO-PbF2-SiO2 melts were determined in the temperature range 1050–1250K using a galvanic cell incorporating a calcia-stabilized zirconia solid electrolyte. Compositions investigated included slag mixtures containing up to 30 mole % PbF2 for the PbO-PbF2 binary, and up to 20 mole % PbF2 for the PbO-PbF2-SiO2 ternary. Results for the binary were in agreement with previously published values. An electron microprobe was used to examine for slag-electrolyte interactions, and necessary restrictions on run times and temperatures were noted. For melts in which the mole fraction of silica exceeded about 0.2, the activity of PbO was raised by the addition of PbF2; the reverse was true at lower silica contents. The results are discussed with reference to the anionic constitution of the melts described in terms of O2− and F− ions and an array of silicate and silico-fluoride ions the chain members of which have the general formula SinO3n+1−m Fm (2n+2−m)−, where 1 ≤ n ≤ ∞ and ...