Preparing selective surfaces for solar collectors by dry oxidation and

Abstract

Selective surfaces for photothermal solar collectors consisting of a nickel support, preferably commercial nickel of 99.5% purity, a layer of a porous nickel oxide of about 0.2 .mu.m covering the metallic support, the layer of porous nickel oxide being covered with asperities in the form of a network of nickel oxide discs in which the major portions are oriented at an angle with respect to the vertical. The network of discs has a thickness of about 2 .mu.m. The preparation of these selective surfaces is carried out in the following manner. Thin nickel plates are subjected to a heat treatment by heating and oxidizing in an oxidant gas during a short period of time at a temperature varying between about 1000.degree. C. and 1100.degree. C. The oxidized nickel plates are reduced at a temperature of about 1100.degree. C. in the presence of a reducing gas until the metallic state is obtained. The surrounding temperature is thereafter lowered between 810.degree. and 830.degree. C. and an oxidizing gas is circulated around the plates, at a rate of flow which is sufficient to flush away the reducing gas. The plates are rapidly cooled down to room temperature to give the intended selective surfaces.