Electric Field-induced Processes in Ternary Oxides

Abstract

When a multicomponent material such as ternary oxides, which are used throughout the industry in the form of solid oxide fuel cells, multilayered ceramic capacitors, thermoelectrics and so on, are put under working conditions (i.e. under driving forces) degradative phenomena of kinetic unmixing and/or decomposition arises. These phenomena have their origin in the difference of diffusion coefficient (or mobility) of the constituent cations. Even if the compound is in its thermodynamically stable regime, this type of unmixing and decomposition may still occur, regardless of the compound being an electrolyte or a semiconductor. As it is caused by the diffusion coefficient difference, the phenomena are termed "kinetic."

Many works have been done for kinetic unmixing under Po2 gradient. Fewer works have been done for kinetic decomposition under Po2 gradient and unmixing under electric field. No literature reports results for kinetic decomposition under electric field. Work by Yoo et al. on BaTiO3 showed neither unmixing nor decomposition even when large electric field was applied. One must therefore wonder (i) whether kinetic unmixing and decomposition occur under electric field and (ii) if they occur, would they occur always

or is there a certain threshold for these phenomena to occur. This kind of analysis was never done under electric field. Also, electric field does not affect only these reactions. Rather, it can affect other reactions as well, such as formation reaction, which is the reverse reaction of decomposition, acting as a secondary driving force (the primary driving force being the concentration gradient of a component). The formation kinetics under electric field should also be found.

To answer these questions, a model ternary system was selected. Observations were made on the system NiTiO3, which is the only intermediate compound in the NiO-TiO2 system at 1300oC in air. Constant current was applied to NiTiO3 pellets and the applied current values were converted into applied voltage by conductivity measurement. Different levels of current were applied to the pellets. For specimens exposed to voltage less than a threshold value, only kinetic unmixing was observed while for pellets exposed to voltage higher than the threshold value both kinetic unmixing and decomposition were observed. The investigation was done by electron probe microanalysis to obtain the concentration profile between the anode and the cathode. X-ray diffraction was also done at the electrodes to check the existence of NiO and TiO2. Both the EPMA and XRD results confirmed kinetic unmixing and decomposition of NiTiO3.

The master equation for estimating the threshold voltage (or critical decomposition voltage) Ud is important in estimating the degradative behavior of the compound and is here derived. Two conditions were applied, (i) steady state and (ii) closed circuit condition. The resulting master equation showed that the diffusion coefficient ratio of cations A and B is the critical factor in determining Ud, as expected. To obtain this ratio, a diffusion couple of NiO-TiO2 was made, meanwhile applying Pt inert markers at the starting interface of NiO/TiO2. After heat treatment, the ratio of distance from the inert markers to the respective interfaces (NiO/NiTiO3, NiTiO3/TiO2) corresponds to the diffusion coefficient ratio. From this diffusion couple a diffusion coefficient ratio of DB/DA=0.21 0.07 was obtained. The range of Ud from the kinetic unmixing/decomposition experiment and the range expected from the master equation coincided within error bounds, indicating that the master equation is indeed valid. Also, the possible reasons for the anomalous behavior of BaTiO3 mentioned above are suggested.

For the effect of electric field on the formation reaction, the master equation was derived by Korte et al., but experimental confirmation was not given. Therefore in this thesis quantitative analysis of the formation kinetics under electric field and microstructure characteristics are introduced. It is shown that the thickness and microstructure of the system is greatly affected by electric field, a fact which can be used in industries.