Cesium dioxide and yttrium oxide systems: Powder synthesis, sintering characteristics, and dopant effects.

Abstract

New methods have been successfully developed to synthesize high yield, nanocrystalline $\rm CeO\sb2$ and $\rm Y\sb{2}O\sb3$ powders. These powders were highly sinterable. The lowest temperatures required to reach full density for these powders were below 0.45 of their melting points. The sintering characteristics of these nanocrystalline powders were studied and their microstructural evolution during sintering was examined. The normalized pore size distributions exhibited a universal characteristic that is only density dependent and can be described by a simple particle network model. From these distributions, a method was established to determine the critical normalized pore size, beyond which pores are thermodynamically stable. This size is dependent on the dihedral angle which was determined in the present study. At high densities, pores are subcritical and they shrink following the conventional sintering theory. In this regime, the size effect on the sintering rate, in the form of Herring's scaling law, was verified for nanocrystalline powders assuming grain boundary diffusion to be the dominate transport mechanism. At low densities when the majority of pores are supercritical, particle coarsening was found to contribute to increasing packing density even without any space filling at the interparticle neck. Since nanocrystalline $\rm CeO\sb2$ powders coarsen rapidly, this mechanism was responsible for the remarkable sinterability of some $\rm CeO\sb2$ compacts which had a density as low as 18%. Dopants covering a wide range of charges and sizes have been chosen to investigate their effect on grain boundary mobility. In both $\rm CeO\sb2$ and $\rm Y\sb{2}O\sb3$ systems, grain boundary mobility is controlled by cation diffusion, and cations diffuse by an interstitial mechanism that can be enhanced by the presence of oxygen vacancies, as in acceptor doping, or suppressed by the presence of oxygen interstitials, as in donor doping. At high dopant concentrations, a solute drag mechanism may operate that can suppress grain boundary mobility. This was verified by using acceptor dopants. Grain boundary mobility is further influenced by dopant-defect interactions which are charge and size dependent. Severely undersized dopants have a tendency to markedly enhance grain boundary mobility, due to the destabilizing distortion of the surrounding lattice that apparently facilitates defect migration. This accounted for some anomalous effects that were observed for dopants such as Sc, Ti, and Nb. Dopant effects on sintering of CeO$\sb2$ and $\rm Y\sb{2}O\sb3$ have also been studied. Their major effects can be summarized as due to the altering of dihedral angle, and the changed rate of coarsening or diffusion. By exerting such influences, dopants were found to affect the microstructural evolution during sintering even at the very early stage. The most beneficial dopants are those which increase dihedral angle and enhance kinetics, particle coarsening notwithstanding. An example of this kind was found in Ti doped $\rm Y\sb{2}O\sb3$.