# Self-Cleaning Solar Mirror Coatings: From the Laboratory Scale to Prototype Field Tests
## 2 Materials and Methods  
In this study, AlN layers were deposited by the sputtering technique using a proprietary multicathode sputtering apparatus, named ENEA2 (Figure 1), equipped with a process chamber and load lock chamber. ENEA2 is the apparatus on which they were developed, patented and determined as international industry key technologies in the field of selective thin-film solar coatings for receiver tubes [8].  
In fact, it can process tubular substrates with a maximum length of 600 mm, external diameter between 7 and 10 cm (typically 7 cm) and wall thickness between 2 and 5 mm. In the prevailing tubular configuration, the tube-substrate is placed in a horizontal position on a system with a \"trolley + pallet\" that transports it from the loading chamber to the process chamber.  
The tube is made to translate and rotate repeatedly in front of magnetron cathodes (sputtering sources) arranged vertically on both sides of the apparatus. In the process chamber, six cathodes are mounted and arranged in pairs on two opposite sides, specifically, three standard magnetron and three dual magnetron cathodes. The standard magnetron cathodes can be operated in DC and DC pulsed sputtering mode, whereas the dual magnetron cathodes can be operated in bipolar DC pulsed and MF sputtering mode. In the load lock chamber, plasma etching and heating pre-treatments can be performed. The machine is equipped, in the process chamber, with a high-vacuum pumping system. Load lock pumping is carried out by means of a cryo-pump. The general tubular layout of the ENEA2 sputtering plant was changed in this work to a proprietary planar pallet for making deposits on flat panels (e.g., metallized glasses) with a maximum length of 50 cm and a height of up to 40 cm.  
The mirror substrate was positioned in front of the target in a planar configuration and the fabrication process was a reactive sputtering with an Ar + N\({}_{2}\) pressure of 3 Pa and a power of 1800 W supplied to the Al cathode.  
Silicon substrates for homogeneity tests were purchased from Merk \({}^{\circ}\)KGaA, Darmstadt, (Germany) SpA.  
Solar mirrors with dimensions of 50 \(\times\) 40 \(\times\) 1 cm were purchased from Societa Vetraria Biancaledese SpA., Roncade (PV), Italy.  
UV-VIS-NIR analysis was performed using a double-beam Perkin-Elmer (Waltham, MA, USA), Lambda 900 instrument, equipped with a 15 cm integrating sphere to measure global spectral reflectance and transmittance.  
The thickness of sample layers was measured by means of a surface profiler, TENCOR P-10.  
The static water contact angle (WCA) was measured with the direct optical method of drop-shape analysis, by means of the contact angle meter, KRUSS, Hamburg (Germany) DSA-100.  
Tests on-field were performed on prototypes obtained by gluing coated mirror panels on BSM commercial constitutive mirrors (as better explained in the following paragraphs) outdoors in the demonstrative Fresnel plant ENEASHIP, located in Casaccia (Rome) ENEA Research Center. Reflectance measurements on-site were performed by means of a portable D&S R15-USB, averaging 3 values for each 50 \(\times\) 40 panel and 9 values for the entire solar mirror.  
Measurements were carried out on different days at the same points to correctly assess the change in reflectance due to soiling.  
Water retention was estimated by means of a gravimetric procedure using deionized water with a resistant of 10 M\(\Omega\) and a technical-scale Giorgio Bormac S.r.l., Carpi (Mo) Italy, XS BALANCE BL 2002 suitable for weighing glass sheets with an area of 10 cm\({}^{2}\) operating in the temperature range between 10\({}^{\circ}\) and 40 \({}^{\circ}\)C. Each measurement was performed at a defined temperature and humidity before and 10 s after a washing step, performed by  
Figure 1: ENEA2 sputtering deposition pre-industrial apparatus.  
positioning samples on a holder inclined at 70\({}^{\circ}\) with respect to the laboratory plane and spraying a defined amount of deionized water on them.