Nanometer thick tunable AlHfN coating for solar thermal applications: Transition from absorber to antireflection coating

A novel AlHfN(H)/AlHfN(L) coating with tunable optical properties was designed and developed for high temperature solar thermal applications. The transition from absorber to antireflection behavior was achieved by varying the chemical composition of AlHfN coatings. The aluminum and hafnium target power densities and nitrogen flow rates were varied in order to attain the different chemical compositions of AlHfN layers, which resulted in different optical constant values. The ellipsometric measurements indicated that bottom AlHfN(H)) is the main absorber layer and the top AlHfN(L) acts as an antireflection coating. The AlHfN(H)/AlHfN(L) coating deposited on stainless steel (SS) substrates exhibited absorptance of 0.93 with an emittance of 0.13. In order to decrease the emittance, approximately 1 μm thick tungsten coating was deposited on SS substrates. The tungsten coating acts as an infrared reflector due to which the emittance decreased drastically from 0.13 to 0.06, without affecting the absorptance. The optimized W/AlHfN(H)/AlHfN(L) coating deposited on SS substrates was thermally stable in air at 425 °C for 2 h. These coatings also exhibited high thermal stability in vacuum at 580 °C for 260 h