Tantalum carbide based spectrally selective coatings for solar thermal absorber applications

Abstract

Tantalum carbide (TaC) based tandem coatings for selective absorption of solar radiation were fabricated by magnetron sputtering technique. Tandem structure of Ti/TaC/Al2O3 coating was deposited on a stainless steel substrate in which Ti acts as back reflection layer, TaC acts as main absorber layer and Al2O3 acts as an antireflection layer. The composition and thicknesses of the individual component layers have been optimized by adjusting the sputtering process parameters. Reactively sputtered TaC coating deposited at 85 W of sputtering power and 8 sccm of C2H2 flow rate showed an absorptance of 0.845 and emissivity of 0.14. After depositing approximately 50 nm thick Al2O3 on Ti/TaC, the absorptance increased to 0.957 and emissivity increased to 0.15. The refractive indices and extinction coefficients of each layer were used to simulate the reflectance spectra of the deposited tandem stack using SCOUT simulation software. For individual layer thicknesses of Ti, TaC and Al2O3 of 15, 55 and 50 nm, respectively a good match was obtained with the experimental and simulated reflectance spectra. The layer thicknesses were consistent with the thickness data of the tandem stack obtained from cross-sectional field emission scanning electron microscope image. The thermal stability of the tandem stack was examined in vacuum at different temperatures for a short duration. The results showed that the tandem stack is stable up to 500 °C for 2 h.