Effect of HMVF and LMVF layers thickness and annealing on the optical properties of Ti-Si-O-N films for mid-temperature solar thermal absorbers

Stability upon heat treatment of magnetron sputtered Ti-SiO₂ solar-absorbing cermet thin films was studied via TEM and XPS. The films, produced using Ti and SiO₂ targets, consists of an infrared-reflective metallic base layer (or metallic substrate), two cermet solar-absorbing layers – one with a high metal volume fraction (HMVF) and another with low metal volume fraction (LMVF) – and a SiO₂ anti-reflective layer. The HMVF layer was composed of nanometric Ti crystals embedded on an amorphous Ti-Si-O matrix, whereas the LMVF layer consisted only of an amorphous Ti-Si-O matrix. The influence of the base layer metal and both cermet layer thicknesses on the solar absorbance (α) and thermal emittance (ε) was studied. Films reached a α/ε ratio of 43.0. HMVF layer thickness had a strong influence over thermal emittance, while LMVF layer had more influence over solar absorbance. Thermal stability was tested for selected samples, indicating that films exhibit resistance up to at least 300 °C on air. Reasons for this stability are related either to the formation of a Ti-Si barrier layer for vacuum treated samples or to the further oxidation of the film for the samples treated on air.