Spectrally selective solar coating based on W-AlN cermet fabricated by reactive sputtering processes at high deposition rate

The low cost fabrication of thermally stable solar coatings having high photo-thermal performance through high deposition rate processes represents one of the most demanding challenges in linear focusing Concentrated Solar Power (CSP) technology. Since 2005, ENEA has been developing and patenting solar coatings suitable for medium and high temperature applications based on the technology of double nitride cermet. The absorber layer of these coatings is characterized by a graded multilayer cermet of tungsten nitride and aluminium nitride (WN-AlN), while the ceramics of the antireflection filter are constituted by aluminium nitride (AlN) and silica (SiO2). The technology employed to deposit the ceramic component of all cermet materials, as well as of the antireflection filter, is reactive magnetron sputtering in poisoned mode, which does not allow obtaining the best trade-off between deposition rate and energy consumption. In order to improve the economic feasibility of linear focusing CSP solar plant, a cost-effective process was developed in the present work to produce spectral selective coatings for solar receiver tubes. To achieve this objective, the reactive magnetron sputtering technology in transition mode was applied to deposit all ceramic constituents of solar coatings. In detail, the antireflection ceramics were deposited through "dual magnetron"reactive sputtering in transition mode with Medium Frequency (MF) supply and using a Plasma Emission Monitoring (PEM) control system to maintain unchanged the target conditions during the process. It was found that, when using this technology in place of that in poisoned mode, the deposition rate improves by 1.6 and 2.3 times for AlN and SiO2, respectively. Regarding the graded multilayer cermet, while the ceramic component was deposited by "dual magnetron"reactive sputtering in transition mode with MF supply, the metallic component was deposited by "standard magnetron"sputtering with Direct Current (DC) supply. The process control through PEM system was simplified in this case by introducing N2 only from the gas-ring around the Al targets. Since in this configuration the reactive gas flow was not sufficient to promote the formation of WN as metallic component inside the cermet, the double nitride cermet WN-AlN was replaced by W-AlN. A fast procedure was developed to grow the W-AlN multilayer cermet by employing only one hysteresis curve to control the deposition process and grow the cermet layers in sequence. Specifically, this method presents the advantage that it is not necessary to stop and re-start PEM control, power supply and gas injection between the deposition of consecutive cermet layers. The fast procedure was applied to fabricate the absorber layer of the solar coating allowing to obtain a 44% reduction in the energy consumption by replacing WN with W as a metallic component of the multilayer cermet. The coating was subjected to a stability study at high temperature including the estimation of thermal degradation of the photo-thermal parameters after heat treatments under vacuum (2·10-2 Pa) for a total duration of 33 days at the temperature of 620 °C. The results revealed that no appreciable modification occurred in terms of solar absorptance (αs), while a very small increment of thermal emittance (ϵth) was evaluated at 400°C, thus demonstrating the excellent thermal stability of the produced coating.