Thermal stability studies of sputter-deposited multilayer selective absorber coatings

Abstract

Multilayer sputtered coatings of the Al 2O 3/M/Al 2O 3/R type, where M is a semitransparent absorbing layer and R is a low emittance reflecting layer, show promise as selective absorber coatings. In this paper we describe an investigation of the thermal stability of Al 2O 3/M/Al 2O 3/R-type coatings with M layers of chromium, nickel, molybdenum, tantalum and a PtAl 2O 3 cermet, and with Al 2O 3 layers deposited by both direct r.f. sputtering of alumina and reactive sputtering of aluminum. In order to simplify the thermal stability studies, the substrates were glass plates and the R layers were made of the same metal as the M layers, except in the PtAl 2O 3 case where the R layers were chromium or platinum. In all cases, proper selection of the coating thicknesses yielded solar absorptances equal to or greater than 0.90 and room temperature emittances of about 0.12 or less. Coatings of the basic M layer, the M layer coated with an Al 2O 3 layer, and the complete multilayer stack were subjectedj to various thermal tests in air and vacuum over the temperature range from 300 to 700°C with test periods of from 8 to about 1000 h. The diffusion barrier properties of the Al 2O 3 layers, and therefore the thermal stability of the coatings, depended primarily on the method of deposition of the Al 2O 3 layers. Thus coatings with reactively sputtered Al 2O 3 layers underwent absorptance changes in the 300–450°C range in air or vacuum. Coatings with directly r.f.-sputtered Al 2O 3 layers were stable in air to 500–600°C and in vacuum to at least 650–700°C.