N. Selvakumar

Bio: N. Selvakumar is an academic researcher from National Aerospace Laboratories. The author has contributed to research in topics: Coating & Sputter deposition. The author has an hindex of 20, co-authored 35 publications receiving 2043 citations. Previous affiliations of N. Selvakumar include Indian Institute of Science.

A comparative study of reactive direct current magnetron sputtered CrAlN and CrN coatings

Abstract: Approximately 1.5 μm thick CrN and CrAlN coatings were deposited on silicon and mild steel substrates by reactive direct current (DC) magnetron sputtering. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the coatings was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD data showed that the CrN and CrAlN coatings exhibited B1 NaCl structure. Nanoindentation measurements showed that as-deposited CrN and CrAlN coatings exhibited a hardness of 18 and 33 GPa, respectively. Results of the surface analysis of the as-deposited coatings using SEM and AFM showed a more compact and dense microstructure for CrAlN coatings. The thermal stability of the coatings was studied by heating the coatings in air from 400 to 900 °C. The structural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data revealed that CrN coatings got oxidized at 600 °C, whereas in the case of CrAlN coatings, no detectable oxides were formed even at 800 °C. After annealing up to 700 °C, the CrN coatings displayed a hardness of only about 7.5 GPa as compared to CrAlN coatings, which exhibited hardness as high as 22.5 GPa. The potentiodynamic polarization measurements in 3.5% NaCl solution indicated that the CrAlN coatings exhibited superior corrosion resistance as compared to CrN coatings.

TiAlN∕TiAlON∕Si3N4 tandem absorber for high temperature solar selective applications

Abstract: A tandem absorber of TiAlN∕TiAlON∕Si3N4 is prepared using a magnetron sputtering process The graded composition of the individual component layers of the tandem absorber produces a film with a refractive index increasing from the surface to the substrate, which exhibits a high absorptance (095) and a low emittance (007) The tandem absorber is stable in air up to 600°C for 2h, indicating its importance for high temperature solar selective applications The thermal stability of the tandem absorber is attributed to high oxidation resistance and microstructural stability of the component materials at higher temperatures

High Performance Thermoelectric Materials: Progress and Their Applications

Abstract: Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high-performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar-thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

Concentrating Solar Power.

Abstract: United States. Advanced Research Projects Agency-Energy (Awards DE-AR0000471 and DE-AR0000181)

Metallic nanostructures for light trapping in energy-harvesting devices

Abstract: Solar energy is abundant and environmentally friendly. Light trapping in solar-energy-harvesting devices or structures is of critical importance. This article reviews light trapping with metallic nanostructures for thin film solar cells and selective solar absorbers. The metallic nanostructures can either be used in reducing material thickness and device cost or in improving light absorbance and thereby improving conversion efficiency. The metallic nanostructures can contribute to light trapping by scattering and increasing the path length of light, by generating strong electromagnetic field in the active layer, or by multiple reflections/absorptions. We have also discussed the adverse effect of metallic nanostructures and how to solve these problems and take full advantage of the light-trapping effect. In recent years, researchers have demonstrated a number of new schemes for enhancing the absorption of light in solar cells. Chuan Fei Guo and colleagues from the University of Houston in the USA and National Center for Nanoscience and Technology of China in Beijing have now reviewed the use of metallic nanostructures for trapping light in photovoltaic devices. In particular, the presence of metallic nanoparticles in a solar cell or a solar absorber can aid light absorption by inducing strong, local field-enhancement effects or coupling to resonant plasmon modes. Such particles can also promote scattering and thus increase path lengths for light within the device. Solar cells that utilize this approach are either more efficient or substantially thinner than those that do not, thus reducing material costs and creating the opportunity for ultrathin, flexible devices.

A review of cermet-based spectrally selective solar absorbers

Abstract: Spectrally selective solar absorbers harvest solar energy in the form of heat. Solar absorbers using cermet-based coatings demonstrate a high absorptance of the solar spectrum and a low emittance in the infrared (IR) regime. Extensive work has been done to optimize cermet-based solar absorbers to achieve high performance by exploring different cermet (ceramic–metal composite) materials and film configurations through different preparation techniques such as electrodeposition, sputtering, pulsed laser deposition, and solution-based methods. In this article, we review the progress of cermet-based spectrally selective absorbers with high solar absorptance and low thermal emittance, such as Cr2O3, Al2O3, AlN, SiO2, and ZrO2 based cermets as absorption layers. We also present an outlook for cermet-based spectrally selective absorbers with high thermal stability and high conversion efficiency from sunlight to heat.