Temperature and porosity gradients developed during nonisothermal microwave processing of zinc oxide

Temperature gradients in sintering ZnO were characterized by in situ differential temperature measurements. The sintering was performed at a constant heating rate (15°C/min) in a 2.45-GHz microwave applicator. Particular attention was given to the measurement of temperature in the microwave environment. When the core temperature reached 600°C, the difference between the core and the surface temperatures attained a maximum of 260°C. As the sample temperature approached 1100°C, this difference diminished to zero. Microstructural analysis using scanning electron microscopy (SEM) showed that the porosity near the surface was higher than at the core for samples sintered to intermediate bulk densities. Actual density versus temperature behavior was obtained by correlating local porosities measured from postsintering SEM micrographs with local temperature measurements made during sintering. From these measurements, the parameters determining the sintering kinetics were calculated. The analysis indicated a significantly lower apparent activation energy for microwave sintering than for conventional sintering.