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A system has been developed, based on the cavity perturbation technique, for measuring the complex scalar permittivity of materials at frequencies between 50 MHz and 3 CHz, and up to a temperature of 1400°C. Measurements have been made on several classes of materials, including metallic oxides, conventional and advanced ceramics, fabrics, earthenwares, geological and biological materials, foods, glasses, organics and ferrites. Some materials remain unchanged by thermal cycling, but many undergo irreversible changes; the latter are of interest in microwave processing. The system measures the complex dielectric constant of materials during conventional heating, curing or sintering. Correlating these microwave measurements with other diagnostic techniques will help in the understanding of these processes in several classes of materials.

R-6sum-@ -Le comportement des poudres base d'oxyde de thorium produiles par broyage humide et par denitratinn "pot". hydro-thermique ou a l'aide de micro-ondes est compare. La taille des cristaar: contr6le la densite de frittage mais la taiile minimzle des cristaux necessaires pour ia production de pastilles de haute densLt6 dGpend de la methodede preparation. I1 est aussi demontre que des vitesses de frittage trhs rapides peuvent Ptre obtenues en utjlisant les petits cristaux que donne la denitation.

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