Selective absorbing surface for evacuated solar collector tubes

The absorptance of AlN-Al(absorbing coating)/Al solar spectrum(0.34 ≈ 2.5 μ m) selective absorbing surface, containing 8-layer AlN-Al composite materials and with distinctive or non-boundaries between the layers, deposited by sputtering technology and heat-treated at 400 °C (60min) is about 0.95 and its thermal emittance is close to 0.07 ≈ 0.08(80 °C). But on the condition that the absorptance is unchanged, the thermal emittance dropped to 0.04 for the AlN-Al coating composed of matel volum fractionic 0.42, 0.25 and 0.00 films.