OPTICAL-PROPERTIES OF DENSE REGULAR CERMETS WITH RELEVANCE TO SELECTIVE SOLAR ABSORBERS

As thin films applied to metal substrates, certain cermets form selective surfaces of high solar absorptance and low thermal emittance. A prediction of the optical constants of cermets is valuable in the choosing of a combination of metal and ceramic and in the optimization of the grading profile. The metallic inclusions are in general densely packed, and only those theories which include the effects of proximity are capable of yielding correct values. In order to complement previous work on the optical and transport properties of regular arrays of spheres, we solved the problem of cylindrical inclusions arranged in a square lattice. The calculations were checked by comparison with measurements for perfectly conducting cylinders. The optical constants of gold cylinders in alumina were calculated and were compared with those for spherical inclusions. It is shown that proximity effects are more pronounced for cylinders than for spheres. For the cases of cylindrical and spherical voids in a highly conducting medium it is shown that the Maxwell-Garnett theory is accurate to within 2% for volume fractions of voids up to 0.50 and 0.40 respectively. © 1979.