Tubular cobalt nanocomposites for selective solar absorber applications

Abstract

This thesis reports on the structural and optical properties of laser surface structured Co nanocylinders-Al2O3 cermets on flexible Aluminium substrate for enhanced solar selective absorbers applications. More accurately, the laser surface nano-structured cermet solar absorbers developed consists of oriented metallic nano-cylinders embedded in porous refractory oxide host matrix. Relatively to the 6 existing primary configurations of selective solar absorbers, the investigated nano-composite can be considered as an additional one. This later, which could be classified between textured surfaces and cermet composites consists of non-percolated aligned tubular metallic regions (nanowires) embedded in an oxide host matrix. The methods that are commonly used to produce such nanowires in porous host matrices include electrodeposition into nanometer wide porous material/template. Molecular sieves, track-etched polymer membranes and porous anodic alumina are examples of some of this family of porous materials. The electrodeposition method has attracted much attention because the pore density of the nanowires is high, with controllable diameters in addition to the pores uniformity. In this particular study, the solar selective absorber coating designed consists of Co nanocylinders embedded into nanoporous alumina template produced by standard electrodeposition and thereafter submitted to femtosecond laser surface structuring. While their structural and chemical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and atomic force microscopy, their optical characteristics were investigated by specular & diffuse reflectance. The optimized samples exhibit an elevated optical absorptance α(λ) above 98% and an emittance ε(λ) ~0.03 in the spectral range of 200-1100nm. This set of values was suggested to be related to several surface and volume phenomena such as light trapping, plasmon surface effect as well as angular dependence of light reflection induced by the ultrafast laser multi-scale structuring. The thermal stability of laser surface structured Co nanocylinders-Al2O3 cermets on flexible Aluminium substrate was investigated at temperatures of 200–600 °C.