Densification of Sol-Gel-Derived Mullite Ceramics after Cold Isostatic Pressing up to 1 GPa

Molecular-designed ultrafine mullite precursor powders with a stoichiometric composition were prepared by copolymerization of alkoxides. The precursor powders were calcined in the range from 800° to 1200°C and consolidated by ultra-high-pressure cold isostatic pressing up to 1 GPa. Ultrahigh isostatic pressure of 1 GPa led to a closed packing structure in the green compacts. Interaggregate pores in the green compacts were collapsed by the ultrahigh cold isostatic pressure to reduce the pore size below 6 nm. As a result, the maximum density of the green compacts reached 70% of theoretical. These closely packed green compacts of precursor powders with a stoichiometric composition and calcined at relatively low temperatures could be sintered to >95% of theoretical at 1500°C. Relatively low-temperature sintering below the liquid formation temperature resulted in fine microstructure of the resultant mullite ceramic with a grain size below 300 nm.