Faculty Publications
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Item Measurement of high temperature emissivity and photothermal conversion efficiency of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO spectrally selective coating(Elsevier B.V., 2017) Jyothi, J.; Soum-Glaude, A.; Nagaraja, H.S.; Barshilia, H.C.A spectrally selective TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO coating was deposited on stainless steel substrate by unbalanced magnetron sputtering system. Each individual layer of the tandem absorber was optimized by varying the reactive gas flow rates (C2H2, N2 and O2) and target power densities (Ti, Al and Si). The optimized tandem absorber shows a solar absorptance of 0.960 and an emittance of 0.15 at 82 °C, measured using solar spectrum reflectometer and emissometer, respectively. In order to study the optical properties of the deposited tandem absorber at high operating temperatures the reflectance spectra of the tandem absorber were measured at temperatures ranging from 80 °C to 500 °C by UV–Vis–NIR spectrophotometer and FTIR spectrometers. The reflectance spectra of the as-deposited sample and after high temperature reflectance measurements did not show any significant changes. The thermal emittance of the tandem absorber at high temperatures (80–500 °C) was studied in detail. At the temperature of 200 °C, 300 °C, 400 °C and 500 °C the tandem absorber shows the emittance of 0.152–0.157, 0.181–0.19, 0.214–0.246 and 0.251–0.275, respectively with an absorptance of ~0.930. These results show the good selectivity of the tandem absorber even at high operating temperatures (e.g., 500 °C) with a photothermal conversion efficiency of 88%, thus demonstrating that the tandem absorber is suitable for solar thermal power generation applications. Reflectance and roughness data of the absorber coating post annealing in air up to 600 °C for 2 h, carried out independently, corroborated the present results. © 2017 Elsevier B.V.Item Enhanced bremsstrahlung X-ray emission from Ag nanoparticles irradiated by ultrashort laser pulses(Elsevier B.V., 2019) Sankar, P.; Thomas, J.; Shashikala, H.D.; Philip, R.In this work, an Ag nanoparticle colloidal suspension flowing in the form of a thin jet (250 ?m) is irradiated by 150 femtosecond, 800 nm laser pulses to form a plasma which emits bremsstrahlung X-rays of up to 100 keV energy. The flowing jet ensures long-term durability of the plasma source during continuous laser irradiation. The laser pulse is p-polarized and the angle of incidence is normal to the jet surface, to optimize resonance absorption of laser radiation by the plasma electron density gradient. A 30-fold enhancement is observed in the X-ray yield in the nanoparticle suspension, compared to the precursor salt solution. This is because of the local field enhancement (LFE) associated with the localized surface plasmon resonance (LSPR) in Ag nanoparticles. Multiphoton ionization will be greatly enhanced in the presence of LFE, resulting in the generation of a relatively larger number of free electrons, which become “hot” electrons of high kinetic energy by resonance absorption. Bremsstrahlung in the X-ray regime occurs due to the deceleration of these hot electrons. Under identical excitation conditions the corresponding X-ray enhancement measured in Au nanoparticles is relatively lower at 18-fold. This decrease is due to the higher ionization potential of Au (9.22 eV) as compared to Ag (7.58 eV). On the other hand, absorption spectra and SEM images measured after continuous irradiation reveal that Au nanoparticles are more photostable compared to Ag nanoparticles. These studies show that Ag nanoparticles are better suited for X-ray generation compared to Au nanoparticles under the experimental conditions employed. Applications include dynamics studies, microscopy, and lithography. © 2019