# From Self-Cleaning to Self-Aware Solar Mirror Skin
## 3 Results and Discussion  
Starting from the previous results about auxetic aluminum nitrides developed as ideal for substituting alumina as BSM self-cleaning skin, here the possibility of conferring sensing properties to this material was explored by utilizing its piezoelectric properties combined to the distributed sensing of a large area of deposition. When dust accumulates on a solar mirror, it exerts pressure on the surface. This pressure generates a piezoelectric signal in the mirror's aluminum nitride coating, as shown in Figure 2. With a large area, the total pressure from dust accumulation is distributed over a large surface, resulting in a signal which is several orders of magnitude larger than the signal from a smaller area. Clearly, the maximization of piezoelectric behavior is crucial for exploiting the distributed sensitive performance. So, the thin film AlN fabrication by means of a reactive sputtering process was directed to maximize the piezoelectric strain constant (d33) of the material, that can be correlated to dust deposition (considered as an applied force), and hence to the soiling of the surface. From a structural perspective, the \(d_{33}\) piezoelectric coefficient of sputtered AlN films is highest when the material crystallizes in the hexagonal wurtzite phase and when its crystallites grow with a c-axis orientation [24]. The crystallographic phase and orientation are closely tied to the deposition technique used. Aluminum nitride thin films can be deposited using chemical vapor deposition (CVD), molecular beam epitaxy, or sputtering in either DC or AC mode [25]. The advantage of the selected reactive sputtering process is that it enables room-temperature deposition, with the c-axis orientation tunable through  
Figure 2: Working mechanism diagram: the direct piezoelectric effect of an AlN surface, on which the deposited dust exerts a compressive force that can be converted into an electric potential. various process parameters, such as the discharge power/pressure ratio and the \(Ar/N_{2}\) gas flow ratio. Even when sputtered thin films exhibit mixed orientations, it is still possible to maximize piezoelectric response. This is because, within the 10-200 nm thickness range required to ensure optical clarity across the full solar spectrum for self-cleaning purposes, the coatings naturally exhibit an intrinsic anisotropy due to their polycrystalline structure. Extensive experimental work was dedicated to optimizing process conditions to balance optical transmittance, self-cleaning wettability (contact angle and WCA), and the maximization of the piezoelectric coefficient in sputtered AlN thin films. Table 1 reports the defined magnetron sputtering conditions and WCA of auxetic, amorphous, and piezoelectric AlN self-cleaning coatings. The measured contact angles (WCAs) are all greater than 90\({}^{\circ}\), confirming self-cleaning behavior compared to uncoated mirrors. It should be noted that in a BSM, the layer exposed to air is alumina, which typically has contact angles around 40\({}^{\circ}\). Therefore, replacing the alumina layer on the glass with an AlN layer, as previously explained, ensures self-cleaning properties, allowing the mirrors to be washed with a reduced amount of water [26]. Once the self-cleaning effect is preserved, we can focus on the piezoelectric properties. To maximize the piezoelectric behavior, process parameters were adjusted until the film became amorphous, followed by further modifications involving a significant reduction in process pressure and an increase in the power supplied to the aluminum target. Both conditions indicate a more energetic process, which promotes the coalescence of fine columnar wurtzite grains into larger ones. Figure 3 presents the XRD spectra of both amorphous and piezoelectric films. A greater thickness was chosen to enhance the overall piezoelectric response by increasing the generated voltage.