Mobility and Size-Distribution Analyses of Hetero-Coagulated Structures of Colloidally Mixed Mullite Precursor Suspensions and Their Sinterability

Abstract

To control the hetero-coagulation structure of ultra-fine γ-Al₂O₃ and SiO₂ powders for mullite ceramics, hetero-coagulation structures under different pH conditions were investigated by focusing on electrophoretic mobility and aggregate size distribution in suspensions. A bimodal electrophoretic mobility distribution was observed when precursor powders were prepared by colloidal mixing processes at pH=9.5. Since each peak almost corresponded to the electrophoretic mobility of γ-Al₂O₃ and SiO₂ particles, heterocoagulates were not formed in the suspension. In the case of pH=6.0, weak hetero-coagulates were generated in suspension. However, uniform mixing could not be attained and large coagulates were formed. On the other hand, narrow and single mode electrophoretic mobility distribution was observed after colloidal mixing at pH=4.4. It is concluded that the uniform hetero-coagulates for mullite precursor with the stoiciometric composition can be obtained. The change in structures of hetero-coagulates with pH conditions was analyzed by DLVO theory. Uniform and fine grained mullite ceramics were prepared from uniformly hetero-coagulated precursor powders mixed at pH=4.4.