Doh-Kwon Lee

Nonstoichiometry and defect structure of Mn-doped BaTiO3−δ

Abstract Oxygen nonstoichiometry (δ) of 1 m/o Mn-doped polycrystalline BaTiO3−δ has been measured as a function of oxygen partial pressure in the range of 10−16≤PO2/atm≤0.1 at elevated temperatures (900≤T/°C≤1100) by a solid-state coulometric titration technique. The extent of nonstoichiometry of Mn-doped BaTiO3 is much larger than that of undoped BaTiO3, which is attributed to defect-chemical role of Mn as acceptors. The nonstoichiometry isotherms indicate that Mn-ions change their valence from +4 (or MnTix) to +3 (or MnTi′) to +2 (or MnTi″) with decreasing PO2.

Oxygen nonstoichiometry of undoped BaTiO3−δ

Abstract Oxygen nonstoichiometry ( δ ) of “undoped”, polycrystalline BaTiO 3− δ (but with two different nominal purity) has been measured as a function of oxygen partial pressure in the range of 10 −15 ≤ P O 2 /atm≤0.1 at elevated temperatures (1073≤ T /K≤1273) by a solid-state coulometric titration technique. Each isotherm varies sine-hyperbolically against log( P O 2 ) with an inflexion point therein (corresponding to the electronic stoichiometric composition or δ =0). The nonstoichiometry variation is attributed to oxygen vacancies compensated mostly by background and/or intrinsic acceptors in the higher P O 2 region and by electrons in the lower P O 2 region. The purity effect on nonstoichiometry is discussed. The intrinsic electronic excitation equilibrium constant is evaluated and compared exhaustively with the reported. The relative partial molar enthalpy and entropy of component oxygen are also evaluated as functions of the nonstoichiometry.

Electrochemical Determination of the Oxygen Permeability of La1 − x Ca x CrO3 − δ ( x = 0.27 )

The oxygen permeability of La 0.73 Ca 0.27 Cr 0.98 O 3-δ was measured in the ranges of temperature and oxygen partial pressure, 1173 ≤ T/K ≤ 1373 and 10 -18 ≤ p O2 /atm ≤ 1, respectively, by using electrochemical permeation cells of which all the mechanical and electrochemical leakage sources were eliminated by cell design and operation. The ionic partial conductivity was derived therefrom and turned out to vary isothermally as σ ion p O2 m with the oxygen exponent m changing from m 0 to -1/2 to +1/2 as p O2 increases in its range examined. The ionic carrier was identified, via electrotransport experiments, as being oxide anions over the entire p O2 range, thus suggesting the anti-Frenkel disorder as being the majority ionic defects. The ionic conductivity, and the oxygen vacancy diffusivity and redox equilibrium constant therefrom are exhaustively compared with the literature data.

Chemical diffusion in complex oxides with an emphasis on BaTiO3

Chemical diffusion has long been studied mostly on binary oxides, but practical interest in it is increasing with more complex oxides these days, e.g., perovskite oxides that are finding a variety of electroceramic and electrochemical applications. In this paper, we compile the chemical diffusivity data of oxygen as measured on a prototype perovskite BaTiO3 and review the connotation of their variation with oxygen activity in the light of the chemical or ambipolar diffusion theory. The chemical diffusivity of BaTiO3 is compared with those of other perovskite oxides of mostly electrochemical concern and of typical binary transition metal oxides. The chemical diffusivity is further compared with those of the compounds such as Ag2S, Ag2Te, YSZ and Fe3O4, that have dissimilar crystal structures, but with a similar defect structure to BaTiO3, in order to get a further insight into the inner workings of chemical diffusivity particularly in the stoichiometric regime of complex oxides. Finally, possible complication with nonstoichiometry re-equilibration kinetics due to cation mobility is pointed out with reference to what has been observed from LiNbO3 under an oxygen activity gradient.

Surface reaction kinetics in oxygen nonstoichiometry re-equilibration of BaTiO3−δ

Abstract We report the (bare) surface redox-reaction rate constant k that was determined, along with the chemical diffusivity D , by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO 3− δ as a function of oxygen activity in its range of −16≤log a O 2 ≤0 at elevated temperatures of 800–1100 °C. It takes a value in the range of −4 k /cm s −1 )≤−1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition ( δ ≈0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of k against oxygen activity.