Investigation Of TiN/TiSiN/SiN Solar Selective Absorbing Coatings

Nowadays human beings begin to make full use of clean energy to replace fossil fuels in order to meet the requirements of sustainable development. As a representative of clean energy, solar energy has drawn more and more attentions. Solar water heaters is the most widely used and fastest growing field of the utilization of solar energy. Though flat solar water heaters has a small market share in China now, it is predicted that it would have a very good potential due to its good integration of buildings.As one of the most important parts, solar selective absorbing coatings (SSAC) plays an important role in the solar-thermal conversion of solar water heaters and has direct relationships with the quality of solar water heaters. In order to convert the low energy density solar energy into high energy density thermal energy, a good SSAC should have a high absorptance and low emittance. To prepare a new kind of SSAC without oxygen is very important under the current production condition of China. The key point of this thesis is to prepare a new flat oxygen-free SSAC with high absorptance, low emittance and good thermal stability.In this thesis, TiN/TiSiN/SiN tandem absorbers with and without a copper underlayer were deposited on stainless steel (SS) substrates via a direct-current (dc) reactive magnetron sputtering technique. In this solar selective absorbing coating (SSAC), SS/Cu or SS, TiN, TiSiN and SiN act as the metal reflector layer, high metal volume fraction (HMVF) layer, low metal volume fraction (LMVF) layer and the anti-reflection layer, respectively. An average absorptance (a) of 0.95 and a total hemispherical emittance (ε) of 0.04 were achieved in coatings with a copper underlayer, resulting in a solar selectivity of 23.8. On the other hand, TiN/TiSiN/SiN tandem absorber deposited directly on SS showed a similar absorptance value (-0.95) but poorer emittance (s=0.10) and solar selectivity performance (～9.5). X-ray diffraction (XRD) analysis showed that the Cu layer is polycrystalline, while TiN, TiSiN and SiN are amorphous. The chemical-bonding states of each layer of the TiN/TiSiN/SiN tandem absorber were analyzed by using X-ray photoelectron spectroscopy (XPS). The solar selective absorbing performance of these coatings did not show significant changes after they were heat-treated in vacuum up to 700℃. Raman spectra and atomic force microscopy (AFM) images were collected to verify thermal stability of SS/TiN/TiSiN/SiN. Lastly, heat treatments in air for 150 and 300 hours based on the PC nomenclature were carried out to demonstrate long term thermal stability of the TiN/TiSiN/SiN tandem absorber system.