Abstract: In order to absorb electromagnetic waves in a measurement chamber, the sidewalls and the ceiling of the chamber are lined with contiguous pyramids (20). The pyramid vertices point into the chamber. The structure element (20) has a frame (22) formed by bars made of an electrically insulating glass fiber material and an outer skin (24). The outer skin is cut out of a surface resistance material web. The surface resistance material web is produced by continuously or almost continuously coating a mechanically flexible support web with an electroconductive layer made of a metallic material.

Abstract: The invention relates to a process and an apparatus for converting metal nitrate or mixed metal nitrate solutions into the corresponding metal oxides in a microwave field. The invention consists in heating, while the feed stream is being metered in, in such a manner that the solvent always evaporates within seconds and the decomposition product is obtained as a porous, sponge-like, purely oxidic material, which can be ground easily to give a powder having particle sizes of 0.5 to about 10 .mu.m. The controlled metering in of the nitrate solution simultaneously makes it possible to control the reaction temperature below the sintering temperature of the powder. The reaction of the nitrate starting solution can take place continuously/batchwise in a reaction flask or continuously in a rotating reaction pipe, which is preferably charged with grinding balls, which possibly exert a reaction-activating effect similar to that exerted very particularly by the metal oxide sponge formed during the reaction.