Microstructural Development in Hot Pressed CaO: Density Decrease and Pore Growth During Post Sintering

Abstract

Calcium oxide derived from calcination of calcium acetate was vacuum hot pressed at 1543–1618 K under 17–34 MPa in a graphite die. Grain-growth and pore-growth studies were performed on low-density (85%) and high-density (98%) materials. Grain growth of CaO followed the cubic law with time, and in both cases the grain growth rate increased with increasing temperature. This was attributed to the increase in surface energy and diffusion coefficient. Microstructural studies demonstrated that grain growth at intermediate temperatures was accomplished by a swift grain boundary movement, and pores lay mainly on the grain boundaries. During post sintering of hot pressed samples, a decrease in bulk density was accompanied by an increase in pore size and a decrease in the number of pores. It was concluded that transfer of gas occurs to the boundary accompanying the pore growth, which causes expansion of specimens with increase in size of pores resulting in the decrease of density.