Slip casting of alumina powder mixtures with bimodal size distribution - Influence of particle size difference between fine and coarse powders on packing and consolidation process

Slips prepared from bimodal size distributed alumina powder mixtures, which consisted of 30% fine powder and 70% coarse powder, were cast. The two ave rage particle sizes of the bimodal distribution were varied. When the diffe rence between the average particle sizes was small, green compacts had a lo w packing density and uniform packing structure. In this case, the fine and coarse particles were deposited by filtration of gypsum mold, due to the h igh viscosity of the slips, an insufficient dispersion of the fine particle s and the small size of the coarse particles. On the other hand, when the d ifference between the average particle size was large, green compacts had a high packing density and nonuniform packing structure. In this latter case , the coarse particles were first deposited by sedimentation to form the sk eletal structure; then, the fine particles percolated through the voids of the skeletal structure by filtration and were deposited in the voids. On ap proaching the bottom part of the green compacts, the skeletal structure sho wed increased density. Since the water absorption rate was slow, the fine p articles did not fill the voids sufficiently. As a result, the maximum pack ing density of 84% was obtained in the bottom part of the green compact. Th e packing density decreased with increasing distance from the bottom part o f the specimen.