Microstructure, optical properties and thermal stability of ZrB2 and Zr-B-N thin films as high-temperature solar selective absorbers

ZrB2 and Zr B N thin films were deposited by DC magnetron sputtering technique. The microstructure and optical properties of thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and spectrophotometer. Thin films with high selectivity (a = 95"26%IsRT = 5%) were achieved by optimizing the nitrogen content of absorbing layers and thickness of antireflective layer. Thermal stability test illustrated that Zr B N thin film with high nitrogen content showed weak thermal stability due to the loose morphology of thin films. Thus, thin films with low nitrogen content and suitable thickness of Si3N4 layer can exhibit superior solar selectivity (a 91.7%lem- = 4.1%) after annealing at 500 C/60 h in air. The XRD, cross-sectional TEM and EDS results of films annealed at 500 C/100 h in air demonstrated that the solar selectivity degradation of films was due to the surface oxidation of ZrB2 layer. Moreover, the solar selectivity was still kept at high level (a = 89.05%lsRT = 10.5%) after treating in neutral salt spray for 24 h. It can be deduced that ZrB2 thin film by sputtering is a superior candidate as high-temperature solar selective absorbers.