MECHANICALLY-ACTIVATED SYNTHESIS AND MIXED CONDUCTIVITY OF TbMO4-DELTA (M = Zr, Hf) CERAMICS

Terbium hafnate and zirconate ceramics with submicron grain sizes were prepared by mechanically-activated synthesis. XRD and electron diffraction and IR absorption spectroscopy showed that TbZrO4-delta has a disordered fluorite-type structure, while TbHfO4-delta is partially ordered, containing pyrochlore microdomains. The oxygen ion transference numbers, determined by the modified emf technique under oxygen/air gradient, vary in the range 0.08-0.26 at 873-1123 K, increasing with temperature. The activation energies for ionic and p-type electronic transport are 82-83 and 29-40 kJ/mol, respectively. Ionic conduction becomes dominant in reducing atmospheres, but tends to decrease at low pO2. Oxygen partial pressure dependencies of the Seebeck coefficient can be described by a model common for oxide phases with mixed ionic and electron-hole conductivity. Due to partial cation ordering, terbium hafnate exhibits lower ionic and hole transport as compared to terbium zirconate. The average thermal expansion coefficients of TbMO4-delta (M = Zr, Hf) ceramics in air, calculated from dilatometric data, are (11.5-12.4) x 10exp(-6)/K at 600-1200 K and (18.4-20.3) x 10 exp(-6)/K at 1200-1420 K. 30 refs.