Study Of Preparation,Structure And Properties Of Oxide Based Spectrally Selective Solar Absorber Deposited By Sol-gel Method

The global energy crisis due to growing industries is one of the major problem of 21st century.The energy production is usually accompanied by huge environmental issues which force researcher to focus on cheap,clean and renewable sources to promote the sustainable development of society.Among multi-renewable energy sources,Solar energy,is environmental friendly and abundantly available.Solar energy is potentially advantageous over fissile fuel due to relatively simple processing,low cost and large-scale commercialization.The one of the main advantage of solar-thermal conversion as compared to photovoltaics is the utilization of entire solar spectrum and its conversion to useful form of energy with higher efficiency.Solar selective absorber(SSA)coating is the main component of solar-thermal energy conversion system.The high efficiency solar selective absorber film requires high absorptance in solar spectrum range and low emittance in thermal radiation range and is known as“Ideal Solar Selective Coatings".Researchers have been trying to enhance the materials properties,using different structure modification and composites to develop ideal selective coatings.The most widely used solar selective absorption films are nano-ceramic composites.They exhibit excellent performance in spectral selectivity,film structure and substrate adhesion,however,they were unable to meet the demands for high temperature solar thermal applications.To overcome poor temperature stability of nano-ceramic composites new composites based on carbon-metal—oxide was introduced.This new material showed great improvement in thermal and chemical stability and favors the high-temperature aging properties.The preparation method is also very important factor.Recently,the dominant preparation method for the absorbing films was Magnetron sputtering.However,it has always faced the problems of complicated preparation process,high cost,it is desirable to overcome by developing new methods.In this work we have used simple sol-gel method to prepare different composites.The sol-gel one step facile method was established to fabricate the macro-porous morphology films on the basis of Marangoni effect by using TiO2 as a model system.The Marangoni effect was investigated in term of temperature gradient and found to be a successful strategy to produce porous films.In this proposed mechanism,a suitable evaporation rate in the gel film process leads to the macro-porous film due to Marangoni effect.It was observed that the macro-porous morphology of the film sustains during the annealing process,which suggests the creation of porous surface morphology in gel film stage rather than due to annealing.Furthermore,the optical-thermal properties are studied for the potential applications of such porous surface films as solar selective absorber and the obtained values of absorptance and emissivity are 0.76 and 0.18 respectively.We also prepared the Ni/C/NiO composite thin films by sol-gel method.The prepared samples were annealed at 450 ? and investigated the effect of annealing with different holding times i.e.(10,20,30 and 60 min).Comprehensive analysis techniques have been carried out in order to analyze the structural,optical and solar selective absorber properties of the composite.The main structure variations with various holding times were found to be decreasing film thickness and enhancement of metallic Ni nanoparticles size.The solar selective absorber properties for one-layer Ni/C/NiO composite film annealed at 450 ?with 10 min holding time showed the optimal performance,demonstrating an absorptance of about 0.80 and an emittance of about 0.11,capable as SSA for the low-middle temperature applications.We have prepared nano-ceramic composite i.e.Ag@TiO2 porous thin films,in order to investigate the structural and optical properties,A principle of obtaining macro-porous Ag modified TiO2 thin films by a non-template simple sol-gel method.The macro porous was constructed by photo polymerization induced phase separation(PIPS)method,and the reduction of Ag+ to Ag0 was induced by ultraviolet(UV)irradiation at the same time.The achieved macro-porous Ag@TiO2 films were well interconnected and crack free with an average pores size in the range 350-440 nm.Increasing Ag concentration results in the enhancement of extinction coefficient and reduction of refractive index which was found to be consistent with absorbance and transmittance observations.In addition,Ag concentration caused red shift in absorbance which in turn decreases the band gap energy.These findings may not only help to understand the silver modified macro-porous for visible light activities but also help to develop high performance photocatalyst materials.