Magnetron-sputtered metal carbide solar selective absorbing surfaces

A range of magnetron reactively sputtered metal carbide interference filters has been deposited onto sputtered copper on glass substrates and aged in vacuum at elevated temperature. These films show considerable promise as selective surfaces for incorporation in all glass vacuum insulated solar energy collectors. Surfaces produced by depositing homogeneous metal carbide films have solar absorptances α∠80% and emittances ε∠2.0% at 300 K. Solar absorptances above 90% and emittances 3.0%–4.0% at 300 K are obtained for multilayer metal carbide films fabricated by varying the reactive gas flow rate in steps during the deposition. Ageing experiments show that the reflectances of the multilayer films alter due to interdiffusion between the layers, resulting in a slow degradation of solar absorptance. Some diffusion coefficients for carbon in stainless steel and titanium films have been evaluated. Surfaces fabricated using chromium, molybdenum, and stainless steel carbides are superior to titanium carbide surfaces. Magnetron sputtering technology offers the possibility of more rapid and efficient deposition of these selective surfaces compared with the high pressure diode deposition technique utilized in previous work.