Use of microwaves in the synthesis of materials is gaining importance. Microwave-assisted synthesis is generally much faster, cleaner, and more economical than the conventional methods. A variety of materials such as carbides, nitrides, complex oxides, silicides, zeolites, apatite, etc. have been synthesized using microwaves. Many of these are of industrial and technological importance. An understanding of the microwave interaction with materials has been based on concepts of dielectric heating and of the resonance absorption due to rotational excitation. This review presents a summary of recent reports of microwave synthesis of inorganic materials. Various observations regarding microwave interaction with materials are also briefly discussed. ContentsIntroduction 882 Interaction of Materials with Microwaves and Dielectric Heating 883 Case Studies 885 Epilogue 894 References 894

The characteristics of glasslike transitions in supercooled liquids and plastic crystals are discussed and a nearest neighbor interaction scheme, termed the ``bond lattice'' model, is proposed to account for the thermodynamic aspects of the phenomenon. Although the excitations of the bond lattice have features in common with those of the Ising lattice, the bond lattice has a zeroth order (noncooperative) case in which the thermodynamic properties are meaningful and readily evaluated. The zeroth order expressions appear adequate to account for the configurational heat capacity of the covalent bonded liquid ZnCl2, but are clearly incapable of explaining the transition behavior of most molecular and hydrogenbonded glasses. In these cases it is believed the configurational excitations have considerable cooperative character. While the complex cooperative problem is not properly treated in this paper, we show how an ad hoc introduction of temperature-dependent bond energies (a sort of Bragg-Williams approximation) modifies the bond lattice model properties in the direction demanded by experiment. A second order transition between supercooled liquid and glass is not required by the treatment. An expression for transport properties, based on a postulated exponential relationship between rearrangement probabilities and degree of configurational excitation, is evaluated. The VTF (Fulcher) equation, commonly used to describe transport properties of viscous liquids, results as a good approximation. The VTF T0 parameter is determined mainly by the bond strength parameter of the model, while the B parameter can be related to the change of heat capacity at Tg. It is implied that glasses with little or no thermal manifestation of Tg will have large and almost Arrhenius temperature dependences, as seems characteristic of the strong network glasses, BeF2, GeO2, and perhaps SiO2. The VTF transport parameters for the intermediate case of ZnCl2 are well predicted using the same molecular excitation parameters which account for the configurational heat capacity.

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A simple, novel, and fast method of preparation of metal nitride powders (GaN, TiN, and VN) using microwave-assisted carbothermal reduction and nitridation has been demonstrated. The procedure uses the respective oxides and amorphous carbon powder as the starting materials. Ammonia gas is found to be more effective in nitridation than high-purity N2 gas. Complete nitridation is achieved by the use of a slight excess of amorphous carbon. Metals themselves are not found to be effectively nitrided. The products were characterized using XRD, TEM, and SAED and found to possess good crystallinity and phase purity. The method can be of general applicability for the preparation of metal nitrides.

An overview of the synthesis of materials under microwave irradiation has been presented based on the work performed recently. A variety of reactions such as direct combination, carbothermal reduction, carbidation and nitridation have been described. Examples of microwave preparation of glasses are also presented. Great advantages of fast, clean and reduced reaction temperature of microwave methods are emphasized. The example of ZrOz-CeO 2 ceramics has been used to show the extraordinarily fast and effective sintering which occurs in microwave irradiation.

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