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 average particle sizes of the bimodal distribution were varied. When the difference between the average particle sizes was small, green compacts had a low packing density and uniform packing structure. In this case, the fine and coarse particles were deposited by filtration of gypsum mold, due to the high viscosity of the slips, an insufficient dispersion of the fine particles and the small size of the coarse particles. On the other hand, when the difference 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 skeletal structure; then, the fine particles percolated through the voids of the skeletal structure by filtration and were deposited in the voids. On approaching the bottom part of the green compacts, the skeletal structure showed increased density. Since the water absorption rate was slow, the fine particles did not fill the voids sufficiently. As a result, the maximum packing density of 84% was obtained in the bottom part of the green compact. The packing density decreased with increasing distance from the bottom part of the specimen.