The effective diffusion distance for the sintering of nanocrystalline ceramic powders

Traditional geometrical models of intermediate stage sintering assume that pores are present at every grain edge. In consequence, the grain size is used as a measure of microstructural scale for the prediction of densification kinetics for ceramic materials. In this study, scanning electron micrographs of ZrO2-3mol%Y2O3 and hydroxyapatite that had been pressed to a green density of 46% and isothermally sintered at 1275°C and 1100°C respectively, showed multiple grains between pores. Measurements of the pore size, grain size and pore size distribution indicated that contrary to the assumption of the traditional models of sintering, pore section elimination was involved in densification and coarsening during intermediate stage sintering. Thus the pore separation rather than the grain size should correlate with diffusion length in this material. Similar results were also found for previously studied alpha-Al2O3. However, the expected power law relationship between densification rate and microstructural scale does not fit the data well for pore separation. A new experimentally measured effective diffusion distance based on the arrangement of pores is proposed to predict the effect of microstructure on densification behavior. This effective diffusion distance will allow simple one dimensional flux models like the combined stage sintering model to be adapted for analyzing the sintering of spatially heterogeneous ceramic materials by both pore shrinkage and pore elimination.