Microwave processing has been emerging as an innovative sintering method for many traditional ceramics, advanced ceramics, specialty ceramics and ceramic composites as well as polymer and polymer composites. Development of functionally gradient materials: joining; melting; fibre drawing; reaction synthesis of ceramics; synthesis of ceramic powder, phosphor materials, whiskers, microtubes and nanotubes; sintering of zinc oxide varistors; glazing of coating surface and coating development have been performed using microwave heating. In addition, microwave energy is being explored for the sintering of metal powders also. Ceramic and metal nanopowders have been sintered in microwave. Furthermore, initiatives have been taken to process the amorphous materials (e.g. glass) by microwave heating. Besides this, attempt has been made to study the heating behaviour of materials in the electric and magnetic fields at microwave frequencies. The research is now focused on the use of microwave processing for industrial applications.

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Prospects of microwave processing: An overview

Spark plasma sintering (SPS) or pulsed electric current sintering (PECS) is a sintering techni‐ que utilizing uniaxial force and a pulsed (on-off) direct electrical current (DC) under low at‐ mospheric pressure to perform high speed consolidation of the powder. This direct way of heating allows the application of very high heating and cooling rates, enhancing densifica‐ tion over grain growth promoting diffusion mechanisms (see Fig. 1), allowing maintaining the intrinsic properties of nanopowders in their fully dense products.

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Challenges and Opportunities for Spark Plasma Sintering: A Key Technology for a New Generation of Materials

Si-29, Al-27, H-1 and Na-23 solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The Si-29 chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast H-1 MAS and Si-29 T (SiH)/T-1 rho relaxation measurements demonstrated that occluded pore H2O mobility within the disordered cavities is slow in comparison with H2O mobility characteristics observed within the ordered channel structures of zeolites. The Al-27 MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the Si-29 MAS data, the corresponding Al-27 MAS line shapes are relatively narrow, suggesting that the AlO4 tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si-O bond angles and bond lengths. Corresponding Na-23 MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved Na-23 resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed.

29Si, 27Al, 1H and 23Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers

Influences of pH value on the microstructure and phase transformation of aluminum hydroxide

Effect of sintering temperatures on corrosion and wear properties of sol–gel alumina coatings on surface pre-treated mild steel

Physicochemical characteristics of alumina gel in hydroxyhydrogel and normal form

Synthesis of low expansion ceramics in lithia–alumina–silica system with zirconia additive using the powder precursor in the form of hydroxyhydrogel

Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering

The present work deals with the sintering of SiC with a low melting additive by microwave technique. The mechanical characteristics of the products were compared with that of conventionally sintered products. The failure stress of the microwave sintered products, in biaxial flexure, was superior to that of the products made by conventional sintering route in ambient condition. In firing of products by conventionally sintered process, SiC grain gets oxidized producing SiO2 (∼ 32 wt%) and deteriorates the quality of the product substantially. Partially sintered silicon carbide by such a method is a useful material for a varieties of applications ranging from kiln furniture to membrane material.

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Mechanical characteristics of microwave sintered silicon carbide

Oxidation of SiC is a major constraint during development of metal oxide–silicon carbide composites when processed in oxygen containing environment such as in air. In the present investigation, Mg+2, Al+3 and Zr+4 hydrogels were used as a source of respective oxide and oxidation of SiC in each system was studied. A three-stage mechanism was found to be operative in Al+3 and Zr+4 systems where oxidation at the initial stages was found to be controlled by the nature of the polynuclear complexes formed on the surfaces of SiC particles. At the intermediate stage a transition from polynuclear complex to metal silicate protective layer formation changes the oxidation characteristics. Finally the metal silicates provided the ultimate protection. Mg+2 was found to be ineffective. The extent of retention of SiC in the final composites could be premonitored by controlling the amount and the nature of complexing cations.

S. Ghatak

Papers

Physicochemical characteristics of alumina gel in hydroxyhydrogel and normal form

Synthesis of low expansion ceramics in lithia–alumina–silica system with zirconia additive using the powder precursor in the form of hydroxyhydrogel

Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering

Mechanical characteristics of microwave sintered silicon carbide

Retention of SiC during development of SiC-MxSiyOz composites [M = Al, Zr, Mg] by reaction bonding in air