Effect Of Metal Nanoparticels In Ceremts On Spectral Selectivity

Cermet is a kind of spectrally selective material.It is comprised of ceramic phase and metal phase where ceramics are the matrix and metals are in particle shape.Most researches see cermet as one material using effective medium theories,which ignores the sizes and shapes of metal particles.That is because most of the fabricating method of cermet makes metal particles too small to compare to the wavelength of incident light and the shape of metal particles is hard to control.In the analysis based on effective medium theories,the spectral selectivity of cermet-based coatings is achieved by tandem interference effect.However,recent developments in micro and nanoscale manufacturing technologies make it easier to fabricate a cermet with metal particles of specific shapes and sizes.Since effective media theories ignore the structure inside of coatings,it is necessary to have a clear understanding of the effect of metal particles with a specific size on spectral selectivity based on other theories.First,some important parameters are calculated for a specific working condition with working temperature of 600℃ and concentration ratio of 400.The result shows that the best cutoff wavelength is 12.48μm and the weighing factor that shows how many times solar absorptance should be weighed as thermal emittance is about 12.Then the effect of metal particles in cermet coatings on spectral selectivity is studied by Mie theory and Monte Carlo ray tracing algorithm.Cr,Ni and W particles with radius of 10-200 nm embedded in Al2O3 are considered.2-layer cermet coatings are designed to utilize 2 different metal particles in 2 layers.It is found that a solar absorptance higher than 95% can be achieved in a 2-layer cermet coating,whose thermal emittance in 600℃ is 26%.For this 2-layer coating,its photo-thermal efficiency is higher than 93% in the working condition with a concentrating factor of 400.Finally,the dependent scattering effect was qualitatively considered by the CDA(coupled-dipole approach).With a metal volume fraction of 5%,it was found that the effect of dependent scattering was small and should not change the conclusions made based on independent scattering.