Sintering of thoria

Isothermal sintering of thoria stabilised with 7.5% (by weight) yttria (YST) has been carried out in air at 14500C, 14750C, 15000C, 15250C and 15500C. The activation energies for densification have been calculated at densities between 65% and 85% of the theoretical density. The densification process follows a law of the type rho=rho0+ K 1n t, indicative of a lattice diffusion mechanism for densification and an effect of simultaneous grain growth during densification. The observed grain growth kinetics approximately follow a law of the type G3 is proportional to t. The diffusion coefficient for the rate controlling species for sintering (believed to be thorium ions moving through the lattice) has been calculated. The activation energy for rate controlled sintering (RCS) has been found to have a similar value to that for isothermal sintering. Different densification rates during RCS does not cause any change in the grain size/density ratio. A comparison of microstructure and grain size obtained during the isothermal sintering and RCS shows no significant difference. Depending on the controlling mechanisms which operate, a computer simulation has predicted that the use of high temperature isothermal sintering may produce finer grain size at equivalent density and RCS may be better for producing dense materials with finer grain size. (U.K.)