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

Si-doped nanostructured hematite (α-Fe2O3) has attracted significant attention as a low-cost, high-efficiency candidate material for photoelectrochemical water splitting. In this work, we investigated the effect of Si incorporation on the preparation and performance of α-Fe2O3 films produced by atmospheric pressure chemical vapor deposition (APCVD). Structural, optical, electrical, and photoelectrochemical characterization of doped and undoped hematite films was performed using XRD, FIB/SEM, Raman spectroscopy, UV−vis absorption spectroscopy, J-V and electrochemical capacitance measurements. It was concluded from the XPS data that Si is incorporated in the hematite structure as Si4+. The results suggest that Si-free additives as well as the use of fluorinated transparent conducting oxide (FTO) substrates can influence the preferred orientation of hematite films. It was also found that the incorporation of silicon at very low levels led to the formation of disorder in the hematite structure. Moreover, it is shown that the optical bandgap of Si-doped film increased with the increase of TEOS flow rate. It contributed to the reduction in the size of the hematite nanoparticles and the size quantization effect. The observed donor densities for the doped samples seemed to be much higher than the true values, mainly because the total capacitance measured was higher than space charge layer capacitance, which resulted from the surface area enhancement in the doped films. Therefore, it is considered that donor densities of doped films were smaller than that of the undoped hematite films.

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A microwave/conventional hybrid furnace has been used to sinter three ceramics with different microwave absorption characteristics under pure conventional and a range of microwave/conventional hybrid heating regimes. The precursor powder particle size was also varied for each material. In each case it was ensured that every sample within a series had an identical thermal history in terms of its temperature/time profile. An increase in both the onset of densification and the final density achieved was observed with an increasing fraction of microwave energy used during sintering, the effect being greatest for the materials that absorbed microwaves most readily. Twenty‐three percent greater densification was observed for submicron zinc oxide powder, the material with the largest microwave absorption capability, when sintered using hybrid heating involving 1 kW of microwave power compared with pure conventional power under otherwise identical conditions. For the ceramic with the lowest microwave absorption characteristic, alumina, the increase in densification was extremely small; partially stabilized zirconia, a moderate microwave absorber, was intermediate between the two. Temperature gradients within the samples, a potential cause of the effect, were assessed using two different approaches and found to be too small to explain the results. Hence, it is believed that clear evidence has been found to support the existence of a genuine “microwave effect.”

Current generation carbon-carbon (C-C) and carbon-silicon carbide (C-SiC) materials are limited to service temperatures below 1800 • C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre-UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB 2 , ZrB 2 -20 vol% SiC, ZrB 2 -20 vol% SiC-10 vol% LaB 6 , HfB 2 and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 • C and the results are compared with that of a C-C benchmark composite.