SINTERING KINETICS STUDY OF ZIRCONIA POWDER COMPACT - ANALYSIS OF A FLOW MODEL

Zirconia exhibits crystallographic phase transformation around 1200-degrees-C (monoclinic reversible tetragonal), making sintering studies of its powder compacts difficult and subject to error. To study the pure sintering of the material compact, a modification of the sintering model proposed by Frenkel and valid for non-crystalline viscous solids has been proposed, in which the viscosity term is presumed to be linearly related to time; such a hypothesis is derived from the assumption that the transformation proceeds through generation of a non-crystalline phase in the transformation region and the viscosity is presumed to have a linear dependence on time. This relates shrinkage to time as lambda = t/alpha + betat where lambda is the relative linear shrinkage, t is time, and alpha and beta are constants. A plot of alpha1 (shrinkage rate at zero time) and T-1 (where T is temperature) gives rise to two different slopes corresponding to pre-transformation and post-transformation regions of sintering. The activation energy calculated at the pre-transformation temperature is around 30 kcal/mol.