A physical model of the optical behaviour of aluminium nitride cermet solar coatings has been used to optimize the metal volume fraction and layer thickness of the coatings. A modified photo-thermal conversion efficiency for solar collector tubes is presented and used. The cermet layers are generally deposited by reactive sputtering in a gas mixture of argon and nitrogen. Sputtered aluminium Al-sp is used as a metallic component in the cermet and its refractive index, evaluated in this study, is employed. Due to oxygen contamination, aluminium oxynitride (AlON) is used as a ceramic component in the cermet. Bruggeman approximations are used to calculate the dielectric function for composite materials. An initial ten-layer grade film optimized to one nearly identical to a double cermet film structure when maximizing photo-thermal conversion efficiency at 80 degrees C under a concentration of 1. The optimized films, F10Lm (for initial ten-layer graded film) and F3Lm (for three-layer film) have an identical solar absorptance of 0.957 and an identical hemispherical emittance of 0.048 at 80 degrees C. The optimized film consists of one anti-reflection layer and two cermet layers with metal volume fractions of 0.093 and 0.255, and thicknesses of 30 nm and 93 nm, respectively, going from the anti-reflection coating to the infrared reflector layer. The solar performance can be further improved using a lower refractive index anti-reflection layer and a lower emittance infrared reflector. For example, using an Al2O3 anti-reflection layer, the solar absorptance increases to 0.974, and using a Cu infrared reflector, hemispherical emittance decreases to 0.033 at 80 degrees C. For these different anti-reflection and infrared reflector materials, optimized calculations have also predicted that the double cermet layer film structures have the highest photo-thermal conversion efficiency.
