Glass sintering with concurrent crystallisation. Part 2. Nonisothermal sintering of jagged polydispersed particles

Abstract

Relevance, literature review and objectives The preparation of porous and dense glass articles by sintering is a well established practice in both the laboratory and the industry. However, the development of advanced materials such as sintered glass ceramics, solgel derived glasses, amorphous and crystalline thin films and novel metallic alloys, have rekindled the need for a deeper understanding of viscous flow sintering parameters to control spontaneous crystallisation (which hinders sintering) and residual porosity in such materials. (1) A few models and a variety of experiments have been proposed and conducted on viscous flow sintering. The classical models of Frenkel and MackenzieShuttleworth for the isothermal densification of a porous body, composed of monodispersed glass particles or porous compacts having identical pores, successfully describe parts of the sintering process, e.g. Refs 2-5. In Part 1, In some situations however, depending on the relative rates of heating, sintering, and crystallisation, there may be an onset of these phenomena (or even completion) before the designed annealing temperature is reached. This is particularly true in the case of industrial processes in which large pieces and low heating rates are normally used. Thus it is important to understand and control the sintering and crystallisation process along the heating (and cooling) paths. To our knowledge very few attempts have been made to describe nonisothermal sintering kinetics of glass powders. In one of the earliest publications on the subject, Cutler (7) described the initial linear shrinkage (DL/L 0 <15%) of 25×25×4 mm compacts of 15-25 µm diameter soda-lime-silica glass spheres at heating rates of q=0·5-2·9 K/min, up to temperatures varying from 580 to 680°C. Although no corrections were made for particle size distribution and surface crystallisation his experimental data were described by the Frenkel equation. Fortuitously, perhaps, the complications caused by these factors and other experimental errors cancelled out. Nonisothermal sintering is further complicated by concurrent crystallisation as demonstrated by a fe