Preparation of Sintered Oxide with Hollandite-Type Structure (K_xM_xTi_8-x, O₁₆)(M=Al, Fe, Co, Cr)and Its Ionic Conduction

Abstract

The ionic conduction in sintered oxides with hollandite-type structure, Ka, MrTi8s016 (M=Al, Fe, Co, Cr), was investigated. These oxides have tetragonal lattice and contain tunnels along the c axis. In the case of KzAlsTi8x0, 6, well sintered products were obtained when K2CO₃, r-Al2O^2- and TiO₂ were fired at 1200°C for 5 hr after prefired at 950°C for 10 hr. On the basis of the powder X-ray diffraction patterns, fluorescence X-ray analyses and lattice constants, the composition range where the tetragonal single phase of KxA4Ti8_s016 appeared was determined to be 1.6 <=X<=2.0.
Tablets about 11 mm in diameter and 2 mm in thickness were prepared from the sintered oxides with the compositions 1.4<=x<=2.2. The conductivities of the specimens were measured by using 10 kHz A. C. impedance bridge in the temperature range from 200° to 400°C. Gold plates of 0.05 mm thick were used as electrodes. The conductivities increased from 10-5 to 10-s mho. cm^-1 with increase in x of the single phase. The charge carrier was clarified to be potassium ion from analyses of D. C. polarization curves and of the weight change of the tablets before and after the electrolysis and the qualitative analysis of cathode products. The conductivities of K2M2T40, 8 (M=Fe, Co, Cr) measured by the similar method as in the case of KrA1xTi8, 016 showed that the conduction was mixed ionic and electronic character. The potassium-ion conduction in the hollandite type structure was discussed.