Transparent mullite ceramics from diphasic aerogels by microwave and conventional processings

Transparent mullite ceramics were developed by both microwave and conventional sintering of compacts starting with a diphasic aerogel near 1300 degrees C. Both sintering processes were carried out in air at ambient pressure. The conventionally sintered sample was essentially non-crystalline, whereas the microwave sintered sample was highly crystalline mullite. Using a xerogel of the same composition, no transparency was achieved under the same conditions. The results indicate that the agglomeration-free microstructure of the starting aerogel was the key for achieving the transparency. The achievement of transparent mullite ceramics by microwave processing is attributed to the rapid-heating, accelerated-mullitization, enhanced densification, and limited grain-growth of the diphasic mullite gel in the microwave field.