Final-Stage Sintering and Grain Growth in Oxides

Abstract

The atomic transport processes that result in densification during the sintering of oxides are controlled primarily by volume diffusion or by grain-boundary diffusion. Sintering can be described phenomenologically in terms of diffusion of atomic species from internal surfaces (i.e., grain boundaries) to pores. The flux of atoms from the grain boundaries is a function of the appropriate rate-limiting diffusion coefficient and the chemical-potential gradient. The purpose of this paper is to show that changes in the chemical-potential gradient can account for the densification of oxides that have been sintered to theoretical density.

The densification of Y₂O₃ doped with ThO₂ is described since the addition of 5 mole percent or more of ThO₂ to Y₂O₃ allows the densification to proceed to theoretical density. The requirement of 5 mole percent dopant for successful densification casts doubt on a mechanism entailing the concentration of point defects, and it has been shown by indirect experimental techniques that the thoria solute is segregated at the yttria grain boundaries.

Grain growth occurs during densification, and this geometric change in the microstructure causes a decrease in the chemical-potential gradient, which is the driving force for sintering. Grain-growth kinetics of Y₂O₃ at 2000°C follow the theoretical grain-growth law, D² - D ²_o = kt (where D_o is the initial grain size, D is the grain size after time t, and k is a constant), and show a decrease in the grain-boundary mobility as the concentration of the thoria in solid solution increases. When the amount of thoria exceeds the solid-solubility limit, the grain-growth kinetics follow a cubic-grain-growth law and can be described in terms of a grain-growth process that is controlled by the coalescence of the second phase.

If the assumption is made that the velocity of the average grain boundary is proportional to dD/dt, where D is the average diameter of the yttria grains, the effect of the thoria solute on the grain growth of yttria can be described in terms of an impurity drag as described by Cahn for low-velocity grain growth with a low driving force . Thus the inhibition of the grain growth or the impurity drag of the ThO₂ solute in the yttria grain boundaries causes the chemical-potential gradient in doped samples to be greater than in undoped samples, and this inhibition of the grain growth allows the pores to remain on grain boundaries and allows the densification to proceed to completion, i.e., to theoretical density.