A novel and potentially economical microwave route has been developed for the synthesis and sintering of mullite from the naturally available minerals kaolinite and alumina. Mullite obtained by this route could be sintered to as high as 98% of theoretical density in under 60 min. A detailed structural investigation has been carried out using XRD, IR, and MAS NMR. It is clearly evident from the structural studies that there is a partial conversion of octahedral Al to tetrahedral Al with time during the formation of mullite. Also, it is evident that the exsolved SiO2 reacts with the added Al2O3 to form secondary mullite over primary mullite which is nucleated from the decomposition of kaolinite. A detailed microstructural evolution of this conversion has been followed. A possible microwave−material interaction has been discussed for the rapid formation of mullite.

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A novel and fast microwave route is described for the synthesis of yttrium aluminum garnet (YAG) and for its sintering to translucent bodies. Precursor was made by microwave decomposition (20 min) of aluminum tri-sec-butoxide and yttrium nitrate dissolved in ethyl acetate. The precursor, conventionally calcined at 1000 °C (1 h), was sintered in microwave using SiC as secondary heater for just 35 min. Resulting translucent YAG has a microhardness (HV) of 18.1 GPa and fracture toughness (KIC) of 4.3 MPa m1/2. A 0.86-mm-thick sintered pellet exhibits approximately 45% transmission for 520-nm radiation. The entire microwave process requires less than 3 h.