Determination of the changes on the thermal and optical properties of selective solar absorber coatings induced by prolonged thermal treatment

Abstract

High temperature ageing is frequently used in many technical application areas for the assessment of the thermal stability of materials; a high temperature treatment accelerates all kinds of processes, frequently performed to induce stabilization of the material. For solar absorber surfaces composed of small metal particles a high temperature treatment accelerates oxidation of the metal which in turn decreases mainly the optical properties of the coatings. Likewise changes of the surface due to machining, heat treatment, roughness and porosity can induce changes in effective thermal properties of the surface, which would affect cooling or heating process of the surface. In photothermal deflection spectroscopy (PDS) technique the sample absorbs optical radiation (pump laser) and heats the gas or liquid above the surface, the heat changes the refraction index of the fluid. A second low power laser beam is sent to probe the changes of the refraction index, in such a way that the photoinduced deviation of the probe beam are monitored using a position sensor detector. It has been shown that the beam deflection is strongly correlated to the optical and thermal properties of the materials, and it can provide important parameters as the optical absorption coefficient as well as the thermal diffusivity of the material. This non-contact technique can be particularly useful when analyzing materials in extreme conditions or when a methodology in which attachment of the sensors to the sample is not desirable or even possible. In this work, the changes of the thermal and optical properties of selective coatings exposed to a thermal treatment consisting in heating at 573 K for 200 hours, were studied using a combination of photothermal deflection spectroscopy (PDS) technique, UV/VIS/Near spectroscopy and FTIR. Samples consist of steel substrates coated with solar absorber coatings based in black cobalt and black nickel, deposited using electrochemical methods. The layer structure of black cobalt solar absorbers consisted in a bright nickel infrared reflective layer (thickness: 3.12 µm) covered with a black cobalt high solar absorbance thin film (thickness: 0.28 µm). In the same way, layer structure of black nickel solar absorbers consisted in a bright nickel infrared reflective layer (thickness: 3.12 µm) covered with a black nickel high solar absorbance thin film (thickness: 0.33 µm).