Isothermal Sintering of Cordierite‐Type Glass Powders

Uniaxially pressed pellets of cordierite-type glass powders were sintered in air at 800°, 820°, 840°, and 860°C. There was pronounced anisotropy of shrinkage for these pellets composed of nonspherical crushed glass particles, and the initial axial shrinkage rate was one-half that of the diameter, as though axial particles had twice the radius of diametral ones. The total fractional shrinkage along axes was only 0.12, whereas that of diameters was 0.17. The temperature dependence of sintering followed that of the solid glass viscosity, which is described by the Fulcher equation using independently measured viscosity data. Viscosity decreased approximately an order of magnitude for every 40°C temperature increase. Surface tension calculated from published coefficients for the molar constituents was 0.36 N/m. Linear shrinkage obeyed the Frenkel viscous flow sintering model well, until complete densification was approached, where particles coalesced and thereby lost their identity. A Mackenzie-Shuttleworth analysis, which is based on pore shrinkage, demonstrates the departure of these real powders from ideality. A simple Avrami-type equation permits an empirical fit of data for the entire sintering range when the shrinkage value for 100% densification is known.