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Solar selective characteristics and local electronic bonding states of 3d transition metal oxide and metal nitride based thin film coatings

Rahman, Mohammad Mahbubur (2016) Solar selective characteristics and local electronic bonding states of 3d transition metal oxide and metal nitride based thin film coatings. PhD thesis, Murdoch University.

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The present study focused on the development of transition metal oxide and metal nitride based thin film coatings to be utilized as a cost-effective solar selective surface that constitute a new approach in maximizing the power conversion efficiency. Despite many developments on transition metal oxide and metal nitride based selective solar absorbers, these materials are yet to be commercialized for solar thermal conversion applications. Numerous studies on CuCoO and graphene oxide based thin films are dedicated for their optical applications and light harvesting purposes. However to the best of our knowledge, utilization of mixed metal oxide/graphene oxide thin films as solar selective surface is yet to be explored. Both CuCoO and graphene oxide (GO) have generated significant research interest and have widespread applications in clean energy devices due to the good combinations of many important properties. Therefore, in this work we focus on the introduction of GO to the 3d transition metal-based CuCoO coatings and develop the new types optical thin films via dip-coating sol-gel technology to be used as solar selective surfaces. It is believed that use of graphene oxide in wet chemistry based sol-gel derived thin films will explore the new platform of producing highly efficient selective solar surfaces. Generally, 3d transition metal nitride based thin film coatings are studied for structural, mechanical, and electrical applications. However, a very limited number of investigations are directed in search of their optical and solar selective behaviors. With the increasing demand for clean energy alternatives, and economically viable energy devices our endeavor might bring some fruitful breakthroughs in development of 3d transition metal oxide and metal nitride based thin film coatings.

Due to its flexibility and numerous technical advantages, the soft chemical sol-gel approach has been adopted to synthesize the metal oxides based thin film coatings. Unbalanced magnetron sputtered technique has been used for the development of transition metal nitride based thin film coatings for their spectral selective and local electronic structure studies. Sol-gel derived cobalt-copper oxide based coatings, transition metal nitride based sputtered TiMNx (M = Al or AlSi) and Cr1-xMxN (M = Al, Si and/or Ni, with doping concentration, x varying from 14.3 to 28.5 at.%) coatings were extensively studied in search of their spectrally selective behavior, mechanical properties, thermal stability, surface morphology and surface electronic properties. We discuss the spectral selective features of these coatings with their crystal structure, electronic and chemical bonding states. In order to realize the correlation between crystal structure and surface morphology, bonding states, local bonding structures and structure-property relationships of these nanostructured coatings for their solar selective and local electronic behaviors, characterizations were carried out using XRD, synchrotron radiation X-ray powder diffraction, SEM, EDX, XPS, NEXAFS, UV-Vis and FTIR tests, and nanoindentation measurements.

In the case of CuCoO coatings, a high solar absorptance of 83.40% and a low thermal emittance of 5.70% were recorded which gives a solar selectivity of 14.63 (the ratio of the maximum absorption in visible and the minimum emission in infra-red to far infra-red region; a/e. With the incorporation of 1.5 wt.% of graphene oxide to the copper-cobalt oxide coatings, a high solar selectivity of 29.01 was achieved. Optical studies showed that the solar absorptance, in the visible range, of the TiN coatings improved significantly from 51% to 81% with AlSi-doping. However, an increase of solar absorptance of up to 66% was recorded from coatings doped with Al-content. Meanwhile, the Al doping can reduce the thermal emittance in the infrared range from 6.06% to 5.11%, whereas doping with AlSi reduces the emittance to ca 3.58%. The highest solar selectivity of 22.63 was achieved with TiAlSiN coatings. The high temperature investigations of the sputtered TiAlSiN coatings show the highest solar selectivity of 24.63 at 600 °C. Sputtered Cr1-xMxN coatings were investigated to realize the surface and inner structural properties of the materials through the structural evolution of CrNx matrix with addition of doping (Al, Si or Ni) elements. Investigations on the local bonding states and grain boundaries of these coatings, using NEXAFS technique, provide significant information which facilitates understanding of the local electronic structure of the atoms and shed light on the origins of the high mechanical strength and oxidation resistance of these technologically important coatings. These findings help improve our understanding of local bonding structures, which could potentially lead to improved coating designs for mechanical applications.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor: Jiang, Zhong-Tao, Yin, Chun-Yang, Altarawneh, Mohammednoor, Creagh, Chris and Pryor, Trevor
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