Charge and mass transport in oxide single crystals of fluorite structure
Abstract
The transport of charge and mass in oxide single crystals with fluorite structure were investigated employing electrical conductivity and electrochemical measurements. Based on the four probe conductivity measurements of compositions 9.4, 15, 18, and 21 m% Yttria stabilized Zirconia with fluorite structure and 3 m% Yttria Tetragonal Zirconia, with a closely related tetragonal structure in the temperature range of 450-1000$\sp\circ$C, it was found that 9.4 m% Yttria stabilized Zirconia exhibited the highest ionic conductivity. The isothermal ionic conductivities as a function of oxygen vacancy concentration was found to be in agreement with the model proposed by Honhke. The lower temperature ($<$450$\sp\circ C$) ionic conductivity, obtained using the impedance spectroscopic technique, showed a departure from the high temperature extrapolation in $Log(\sigma\sb{i}T)\ vs.$ 1000$/T$ plots. This was attributed to dopant-vacancy associates formation. The contribution of grain boundaries to the total resistance of pure and dense 3 m% Yttria - Tetragonal Zirconia and 8 m% Yttria stabilized Zirconia ceramic samples, was evaluated using the impedance spectroscopic technique. It was observed that grain boundaries made a marginal contribution to the total resistance, only below ${\approx}500\sp\circ$C. The partial minority charge carrier conductivities in high oxygen partial pressures, ($1-10\sp{-8} atms$)(hole conductivities), and in the temperature range of $700-1000\sp\circ$C, were characterized in 3 m% Yttria - Tetragonal Zirconia, 9.4 m% Yttria stabilized Zirconia and in another fluorite oxide system, 10 m% Calcia Doped Ceria, employing two different permeation measurements. Using the form $$\sigma\sb{p}=\sigma\sbsp{p}{0}e\sp{-Ep\over kT}P\sbsp{O\sb{2}}{+{1\over 4}},$$activation energies of 1.67, 1.70 and 1.36 eV were obtained for hole conductivity for 3 m% Yttria - Tetragonal Zirconia, 9.4 m% Yttria stabilized Zirconia and 10 m% Calcia Doped Ceria. The hole diffusivities were determined from the time dependence of non steady state permeation. Results indicated that the holes move by a thermally activated hopping type mechanism. Using the form, $$D\sb{h}=D\sbsp{h}{o}e\sp{-Q\sb{p}\over kT},$$ activation energies of 1.31, 1.55 and 1.15 eV were obtained for hole diffusivities for 3 m% Yttria - Tetragonal Zirconia, 9.4 m% Yttria stabilized Zirconia and 10 m% Calcia Doped Ceria. The hole concentrations were estimated combining the measured hole conductivities and diffusivities. Using a form, $$p=p\sp{0}e\sp{-E\sbsp{p}{p}\over kT}P\sbsp{O\sb2}{+{1\over 4}},$$ activation energies of 0.46, 0.04, and 0.36 eV were obtained for 3 m% Yttria - Tetragonal Zirconia, 9.4 m% Yttria stabilized Zirconia and 10 m% Calcia Doped Ceria.
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