The effect of temperature, heating method and state of dispersion on the vacuum filter casting of alumina suspensions

It is well established that with advanced ceramic powders the use of dispersed slips can result in superior microstructures compared to those obtained from partially or fully flocculated slips but at the cost of a significantly slower casting rate. Recent published research has also indicated that bodies produced from coagulated slips have similar properties to those from dispersed slips due to the lubricating nature of the hydrated layer surrounding the powder particles. However, little attention has been given to the possibility of accelerating the slow casting rate obtained with dispersed or coagulated advanced ceramic slips by the use of heating the slip. This paper investigates the effect of elevated temperature on the rheological properties and casting rates of four slips; one dispersed electrostatically, one dispersed electrosterically and two which are based on electrostatic dispersion followed by the addition of different salt levels to achieve coagulated slips. It is observed that the use of elevated temperatures results in increases in the casting rates in all cases, however the coagulated slips behave differently to dispersed slips. The latter display casting rates which increase linearly with increasing temperature whilst coagulated slips display an initial rapid increase after which the casting rate decreases significantly. This is believed to be due to the interaction between the permeability of the green body and the state of dispersion of the slip. A further observation arising from the work is that there is generally an enhanced casting rate when microwave energy is used to heat the slips compared to that achieved with a convection oven. The mechanism of acceleration is believed to be due to the rate of water movement through the cast being increased more than the rate of water uptake by the cast.