Structure, optical properties and thermal stability of pulsed sputter deposited high temperature HfOx/Mo/HfO2 solar selective absorbers

Solar selective coatings of HfOx/Mo/HfO2 were deposited on copper (Cu) and stainless steel (SS) substrates using a magnetron sputtering system. The HfOx and HfO2 layers were deposited from the sputtering of Hf target in Ar+O2 plasma using an asymmetric bipolar-pulsed direct current generator, whereas the Mo layer was deposited from the sputtering of Mo target in the Ar plasma. The optimized HfOx/Mo/HfO2 multilayer absorber on Cu substrate exhibited high solar absorptance (α=0.905–0.923) and low thermal emittance (ε82 °C=0.07–0.09). Similarly, on SS substrates the optimized coatings exhibited α and ε82 °C in the ranges of 0.902–0.917 and 0.15–0.17, respectively. The X-ray diffraction data showed that the HfOx/Mo/HfO2 coating consists of tetragonal and monoclinic phases of HfO2, which was confirmed by micro-Raman spectroscopy data. The bonding structure of the HfOx and the HfO2 layers were confirmed using X-ray photoelectron spectroscopy data. The optical constants (n and k), measured using spectroscopic ellipsometry, showed that the top HfO2 layer acts as an antireflection coating and the bottom two layers (HfOx and Mo) are the main absorber layers. The analysis of the spectroscopic ellipsometric data indicated that the band gap of HfOx and HfO2 layers were 3.90 and 5.82 eV, respectively, indicating non-stoichiometric nature of HfOx. In order to study the thermal stability of the HfOx/Mo/HfO2 coatings, they were subjected to heat treatment in air and vacuum at different temperatures (TA). The HfOx/Mo/HfO2 coatings deposited on Cu substrates were thermally stable up to 400 °C for 2 h in air. Addition of a thin Mo interlayer (40 nm) in the HfOx/Mo/HfO2 coating (i.e., Mo/HfOx/Mo/HfO2) deposited on Cu substrates exhibited high solar selectivity (α/ε) of 0.872/0.09 even after heat-treatment in air up to 500 °C for 2 h. At TA=525 °C, the solar selectivity decreased drastically (α/ε=0.761/0.35) due to the formation of MoO2, MoO3 and HfMo2O8 phases. The Mo/HfOx/Mo/HfO2 coatings deposited on SS substrates showed no significant changes in α and ε values after annealing at 500 °C in air and at 800 °C in vacuum. These results were confirmed by micro-Raman spectroscopy measurements, which showed the compositional stability of these coatings up to 500 °C in air and 800 °C in vacuum.