Sintering: An Overview

Abstract

This chapter explains different aspects of liquid-phase and solid-state sintering. Owing to the generally high melting points of the raw materials involved, the fabrication of ceramic materials commonly includes a heat treatment in which a powder, already formed into a required shape, is converted into a dense solid. In liquid phase sintering, the composition and firing temperature are selected in a way that some liquid is formed, but in a quantity insufficient to fill the residual gas phase simply by viscous flow. Under these conditions, the attainment of complete densification requires some change in the shape of the solid grains so that the residual gas volume is eventually reduced to the point where the liquid can fill it. In solid-state sintering, the composition and firing temperature are such that no liquid is formed, all densification being achieved by reshaping of the powder. This reshaping, which is most commonly achieved by solid-state diffusion of atoms, is driven by the energy reduction achieved by elimination of the solid-gas interface and its replacement by a solid–solid interface. It is found that this process is employed for high-quality technical ceramics such as alumina lamp envelopes and ferrite magnetic ceramics.