# Self-Cleaning Solar Mirror Coatings: From the Laboratory Scale to Prototype Field Tests
## 3 Results
### Scaling-Up  
The scaling-up from the laboratory scale to pre-industrial scale requires the optimization of magnetron sputtering processes to obtain large-scale auxetic AlN coatings with the desired performance in terms of optical and wettability properties by means of robust and repeatable processes. In a previous work, we described a family of AlN politypoids fabricated by means of magnetron sputtering on 3 \(\times\) 7 cm mirror substrates, using the tubular geometry of ENEA2 proprietary apparatus, and demonstrated that the auxetic phases simulated by [PERSON] and colleagues [9] can be experimentally obtained using both a saturation and a transition regimen with the aluminum target, which includes doping the hexagonal AlN lattice with metals (Al or Ag) or oxygen. Among such an interesting family of materials, the procedure for fabricating a prototype was selecting the material that was the most transparent in UV-VIS-NIR with a higher WCA (\({}^{\circ}\)) and then scaling-up the fabrication process to obtain the maximum dimensions possible with proprietary apparatus in an applied research laboratory. In particular, the reactive sputtering of aluminum with nitrogen in the transition was preferred to the saturation regime because the deposition rate is higher and occurs at a lower energy and with lower target consumption while, at the same time, preserving the stability and repeatability required for the production process. So, the starting experimental conditions to scale up were selected as a power supply to the aluminum target of 1500 W, p = 0.1 Pa and a gas flow rate of 200 sccm Ar + 25 sccm N\({}_{2}\); last but not least, the process at the laboratory scale was performed with a rotating holder. Here, the main goal of the work was to reproduce, on a planar arrangement, the same material properties. In general, the sputtering deposition process involves many parameters, which introduce different complexities. The pre-industrial ENEA 2 plant allows the processing of a substrate with a maximum size of 50 \(\times\) 40 cm in planar configuration, but to do this, the sample housing has to be replaced, removing the tubular sample holder (designed to fabricate the receiver tubes of a solar CSP plant [10] and utilized for the previous experiments on auxetics) and redefining process conditions such that the deposition on a flat plate reproduces the optical properties of the samples obtained on the small rotating flat substrates typically housed in tubular sample holders. Since Fresnel plants have mirrors with a slightly concave geometry and are equipped with a secondary concentrator, it is still possible to widen the range of the angle of acceptance of the reflected radiation, which translates into less stringent requirements on coating roughness, thickness uniformity and the specularity component. Figure 2 shows a photo of the ENEA2 planar sample holder with the mirror mounted. This plant modification carried out on a support (the carrier) that must interact with the plasma is not trivial at all. The modification, if not carefully studied, can destabilize the plasma that is generated during the process, triggering discharges that affect uniformity and, in some cases, cause it to shut down. To avoid arcs by keeping the power density and process pressure constant, it was necessary to modulate the process gas flows through a throttle valve. Moreover, the cathode voltage was controlled to ensure process stability instead of the nitrogen content, as is the case in plasma emission monitoring. So, after investigating different processes for reproducing the desired material, the experimental conditions utilized in such a planar configuration were decided as a power supply to the aluminum target of 1800 W (I = 4.40 A, V = 410 V), a process pressure = 3.0 Pa and gas flows of 200 sccm Ar + 90 sccm N\({}_{2}\). As the sputtering cathode target length is only 20 cm, it is conceivable that the coverture of the entire panel will not be uniform.