Preparation And Properties Of Black Zinc Selective Absorbers Formed By Reactive rf Sputtering

Abstract

ABSTRACT Black zinc selective solar absorber coatings with solar absorptance O.94 and thermal emittance O.21 have been prepared by the reactive rf sputtering of Zn targets in Ar-O2 atmospheres. For these films the zinc to oxygen ratio is greater than one and the films are composed of both zinc and zinc oxide. The surface microstructure of the films considerably enhances the short wavelength absorptance properties. The coatings represent a possible low-cost selective absorber for flat plate and evacuated tube solar collector applications. 1. INTRODUCTION The potential for zinc oxide coatings for solar energy conversion as an inexpensive heat mirror coating has been investigated recently by a number of researchers (1-4). ZnO is a II-VI compound with an intrinsic bandgap of 3.4 eV which allows transmission of almost all incident solar radiation. The material can be made n-type, conducting and hence infrared reflective, by heavily doping with a trivalent impurity such as Al (1) or In (2). Undoped, non-stoichiometric and electrically conducting ZnO films have also been prepared (3,4) in which the material behaves as an n-type defect semiconductor. The majority of these films have been prepared by sputtering techniques and reported optical and electrical properties are comparable to those of the better known indium-tin oxide coatings (5).We are currently investigating ZnO based films for heat mirror applications and first results of this work have been reported (6,7). In our work we have used both diode and magnetron rf sputtering of either Zn or ZnO targets in Ar, Ar-02 and Ar-H^ atmospheres. Our most recent studies of Al-doped ZnO films prepared by magnetron sputtering from Zn/Al targets have produced excellent heat mirror coatings with values of solar transmittance up to 0.7 and thermal emittance below 0.3 when the films are prepared on Corning 7O59 quartz substrates. These results will be communicated in detail shortly (8). During the course of these studies we observed that black films were produced when Zn and Zn/Al targets were sputtered in weakly reactive Ar-O2 atmospheres. Our laboratory has a long association with selective solar absorbers research, most recently in the area of black cobalt films (9,1O), and consequently we undertook a separate study of the properties of these black zinc coatings to assess their potential as solar absorber surfaces. The results of this study are reported herein.Black zinc coatings have not been researched as extensively as more common selective absorbers such as black chrome and black nickel (11) but some impressive studies have been conducted (12,13). Black coatings of ZnO may be formed by the anodic oxidation of leaf-zinc. These coatings were found to be zinc rich oxides with a surface microroughness of the order of solar wavelengths. Values of solar absorptance, a, above O.97 with thermal emittance, e, below O.25 were obtained (13). Surface optical properties have been successfully modelled using effective medium theory and used as a basis for the assessment of surface change following durability testing and exposure to elevated temperatures (14). A recent paper (15) reports the fabrication of a spectrally selective zinc oxide multilayer prepared by reactive bias sputtering wherein a sputtered black zinc oxide coating was overlaid with a transparent conducting ZnO heat mirror film. This coating had a solar absorptance of O.9O and thermal emittance of O.26.