Fig. 5 indicates that drying time decreased with increasing
microwave power density. When the power density was low,
drying process could be divided into three stages: slow rate
drying I, fast rate drying and slow rate drying II. On the other
hand when the power density was high, the drying process
only underwent only the last two stages. It is because energy
offered by microwave field per unit time was much higher than
that for evaporation under low material and high microwave
power condition. So the total water in materials was heated
quickly causing the elimination of the slow rate drying step
(first drying stage). However, the utilization rate for microwave
energy was lower under the latter condition. The utilization
rate for microwave energy was the highest when microwave
power density was 2 W/g by comparing the drying time and
the paste mixture weight.