Microwave Drying of Spontaneous-Coagulation-Cast Wet Alumina Green Body

To solve moisture gradients in the conventional drying with controlled temperature and relative humidity, microwave heating was employed to dry wet alumina green bodies shaped by spontaneous coagulation casting. The weight loss, linear shrinkage, surface temperature, and moisture distribution of the green bodies by conventional drying (temperature: 40 ; humidity: 60%) and microwave drying were investigated. The time for no further weight loss and shrinkage of the body by microwave drying (power: 250 W) were respectively shortened to 1/6.8 and 1/6 of those by conventional drying. Surface temperature of the green body during the microwave drying increased firstly and then decreased with time, which was strongly correlated with the internal moisture, while the temperature in the conventional drying keeping at 40 . Low-field nuclear-magnetic-resonance (NMR) imaging revealed that the moisture distribution in the green bodies dried by microwave drying was more uniform than that by conventional drying, indicating that drying stress in the former was lower than that in the latter. After sintering at 1550 for 6 h, alumina ceramics from microwave drying had a higher flexural strength with a smaller deviation than that from conventional drying.