The electrical resistivity of some castable zirconias

Some properties of zirconates and stannates with the pyrochlore structure

“…The behaviour of ceria-zirconia compounds is complicated because cerium easily changes its valency between three and four. Consequently a 1 : 1 ceria-zirconia material undergoes changes of phase during equilibration following a change of oxygen partial pressure (Fehrenbacher et al 1964, Meadowcroft 1968, Leonov et al 1966. A pyrochlore cerium zirconate phase (trivalent cerium) can be formed in a reducing atmosphere, whereas in an oxidizing atmosphere, such as air, three phases are present: tetragonal zirconia, a cubic phase rich in ceria, and the pyrochlore phase (Asquiedge et al 1969).…”

“…On the other hand, although praseodymium is also multivalent, praseodymium zirconate is stable enough to form and maintain its structure over a wide range of oxygen partial pressures (Fan Fu-Kang et al 1965). Only the electrical properties of this material have yet been considered and are discussed elsewhere (Meadowcroft 1968). Whinfrey et al (1960) reported that there are many rare-earth stannates (M2Sn207) with the pyrochlore structure, namely Nd, Sm, Pr, La, Gd, Er, Eu, Yb and Y.…”

“…The resistivity of the materials was measured using a four-probe technique, in various oxygen partial pressures in argon, and at temperatures up to 1800 K (Meadowcroft 1968).…”

“…They also limit the applicability of the material, because, after a few cycles of oxygen partial pressure variation, a sample begins to break up, a process presumably connected with the expansion associated with the oxidation. For component oxides initially fired in air, subsequent variations in the oxygen partial pressure between 1 and lo-* bar have not been found to cause cracking (Meadowcroft 1969). The resistivity results for two such materials are shown in figure 5, one 'cerium' zirconate (B) which was fired in nitrogen, the other also an equimolar composition of pure ceria and zirconia but fired in air.…”

“…The following requirements must be met for all applications of refractory ceramics in an open cycle M H D generator : resistance to oxidation, reduction, corrosion, erosion and thermal shock; low volatility, and adequate mechanical strength. For general ceramic applications the electrical conductivity is not important, but for electrodes it must be at least 1 to 10 Q-1 m-1 at temperatures above that of the rear ceramic-to-metal contact (Meadowcroft 1968), whereas for insulators the conductivity must be much lower than this. In addition, for these electrical applications, materials must not be damaged by the passage of current.…”