Preparation of an amorphous optically transparent and hydrophobic Al2O3 top-protective layer for first-surface CSP reflectors

In Concentrated Solar Power (CSP) application, top coatings are of tremendous importance as a way to protect the reflective layers form degradation and conserve the efficiency and durability of the mirrors. The choice of Al
2
O
3
as a top-protective coating for CSP reflectors is based on its high stability, hardness and transparency. In this study, high quality and stable CSP first-surface silvered thick glass mirrors were prepared with a transparent and hydrophobic amorphous Al
2
O
3
top-protective layer with different thicknesses (1µm − 4µm) in order to investigate the effect of the film-thickness on the optical properties and surface morphology of the samples. The spectrophotometric measurements were conducted using a Perkin Elmer Lambda 950 UV/VIS/NIR Spectrophotometer and showed no significant change in the optical properties of the amorphous Al
2
O
3
layers with different thicknesses. The surface morphology was characterized using scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results show that increasing the thickness of the Al
2
O
3
layer up to 3 µm increased the surface hydrophobicity of the mirrors whereas a decrease in the water contact angle was noticed with 4 µm thickness. The measured water contact angles (WCA) were 94°, 98°, 102° and 95° for 1 µm, 2 µm, 3 µm and 4 µm, respectively. A decrease in the water contact angle (WCA=33°) was noticed by achieving a phase transformation from amorphous to crystalline (γ-Al2O3) using annealing at 800°C for 2 hours, exhibiting a hydrophilic behavior.