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Mechanical Behavior of Lightweight Ceramics Based on Sintered Hollow Spheres 395rattling in a cage, which has been suggested as the mechanism for the high microwave susceptibility of pure sodium nepheline.8 The role of the size and shape of the sample in relation to the wavelength at 2.45 GHz has yet to be studied. Also, the role of the microstructure of each gel which is dependent upon various process parameters" also needs to be investigated. Such detailed studies are under way. V. ConclusionsIt has been demonstrated that alumina, silica, and mullite gels could be heated rapidly to very high temperatures in a 600-W kitchen oven with proper insulation. In fact, silica gel was melted under the present experimental conditions. Studies are presently under way to unravel the mechanism for the high susceptibility of the gels to microwave radiation. With the emergence of sol-gel technology in the ceramic industry, the high susceptibility of gels to microwave radiation gives an excellent processing option. A simple micromechanical model, describing the Young's modulus, fracture toughness, and density of a lightweight ceramic structure produced by the sintering of hollow spheres and based on shell theory, was developed. The approach combined an analysis of the elastic deformation using shell theory and a simple analysis of the densification in terms of the microstructural parameters. The results were compared with experimental data for materials produced by the sintering of hollow glass spheres and gave reasonable to good agreement, especially in the prediction of the nonlinear variation of Young's modulus and fracture toughness with density. The model should provide a useful basis for rationalizing the relation between mechanical properties and fabrication procedure for such materials.