Solid Solution Additives and the Sintering of Ceramics
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SOLID
SOLUTION
ADDITIVES
AND
THE
MOHAMED N. RAHAMAN AND CHING-LI HU University of Missouri-Rolla, Ceramic Rolla, Missouri 65401
SINTERING
OF
Engineering
CERAMICS
Department,
ABSTRACT The use of solid solution additives has been shown to be very effective for the formation, by conventional sintering, of ceramic materials with high density and with controlled grain size. However, the number of systems for which such additives have been successfully found remains quite small, and the role of the additives is fairly well understood in only two or three of these. This paper describes the initial part of a systematic study into the effects of solid solution additives on the sintering of ceramics. Cerium oxide was chosen as a model host powder for this work because it has appreciable solubility for many additives. A combination of kinetic data and microstructural observations indicate that the sintering and grain growth are influenced significantly by the additive size but less significantly by the additive charge. The density versus grain size relationship is almost independent of the additive below relative densities of = 0.90 but depends strongly on the additive above this density. The data are interpreted in terms of the effect of the additives on the densification to coarsening ratio. INTRODUCTION The sintering of powders remains the most recognized process for the formation of ceramics with the high density and controlled grain size normally required for many technological applications. Following the work of Coble [1], in which small additions of MgO (0.25 wt%) to A1 2 0 3 produced, by sintering, polycrystalline bodies with theoretical density, the use of solid solution additives has been shown to be very effective for achieving this requirement. However, the number of systems for which such additives have been successfully found remains quite small [2,3], and with the possible exception of the A1 20 3 /MgO system [4,5], the role of the additives is not really understood. A major reason for the general lack of understanding is the variety of functions which an additive can display [6,7]. An additive can, for example, affect the densification rate through changes in the lattice diffusion coefficient, DL, or the grain boundary diffusion coefficient, DB. It may also influence the coarsening rate through changes in DL, DB, the surface diffusion coefficient, Ds, or evaporation/condensation processes (i.e. the diffusion coefficient for the vapor phase, DG). In addition, the additive may segregate to the grain boundary, resulting in changes in the grain boundary energy, 'y, (thereby influencing the
Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
428
driving force for sintering) and the grain boundary mobility, MB. In the case of the A1 20 3 system, solute segregation of MgO to the grain boundaries has not found definitive support because of the low solubility of A1 20 3 for many additives, which has led to conflicting results for solute concentration by surface analysis techniques such
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