National Aerospace Laboratories

Abstract: The recent developments and trends in combustion science towards the synthesis of nanomaterials are discussed. Different modifications made to conventional combustion approaches for preparation of nanomaterials are critically analyzed. Special attention is paid to various applications of combustion synthesized nanosized products.

Abstract: Lead zirconate titanate (PZT) based piezoelectric materials are well known for their excellent piezoelectric properties. However, considering the toxicity of lead and its compounds, there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union in this effect. Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been made to review the recent developments on lead-free piezo materials emphasizing on their preparation, structure–property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail. From the above study, it is concluded that some lead-free compositions show stable piezoelectric responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large number of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of the others.

Abstract: Combustion synthesis has emerged as a facile and economically viable technique for the preparation of advanced ceramics, catalysts and nanomaterials. Recent innovations in the combustion and processing parameters have resulted in a better understanding of combustion phenomena and control of microstructure and property of the products.

Abstract: Solar energy is the most abundant source of renewable energy. The direct method of harnessing solar energy is the solar thermal conversion method using solar absorbers. The absorbers are coated with solar selective coatings with high absorptance and low thermal emittance. Spectrally selective coatings which are stable up to temperatures ≤300 °C (in air and vacuum) have been developed in the past. These coatings are mainly deposited from wet chemical routes (e.g., electrodeposition) and have been reviewed widely in the literature. Because of the environmental issues as well as low thermal stability of these wet chemical deposited coatings, researchers all over the world started looking for other alternative routes such as physical vapor deposited (PVD) coatings. A great deal of research has been carried out since 1990s to develop PVD coatings for both mid- and high-temperature applications. The mid-temperature coatings are used mainly for solar hot water and industrial process heat applications, whereas, the high-temperature absorber coatings are used in concentrating solar power systems for solar thermal power generation. It is well recognized that in order to increase the efficiency of solar thermal power plants, solar selective coatings with high thermal stability are required. In recent years, significant efforts have been made in the field of solar selective coatings to achieve high solar absorptance and low thermal emittance at higher temperatures ( T ≥400 °C). Transition metal based cermets have emerged as novel high temperature solar selective coatings, which are currently being used for solar thermal power plants for electricity generation. Solar selective coatings based on transition metal nitrides, oxides and oxynitrides also hold great potential for high-temperature applications because of their excellent mechanical and optical properties, which are yet to be commercialized. In this review, we present the state-of-the-art of the physical vapor deposited solar selective coatings used for solar thermal applications with an emphasis on sputter deposited coatings for high-temperature applications. A detailed survey, covering the period 1970-present, has been made for the PVD deposited solar selective coatings with high absorptance and low emittance. This review article also describes in detail about the commercially available PVD coatings for flat-plate/evacuated tube collectors and solar thermal power generation applications.

Abstract: A flux splitting scheme is proposed for the general nonequilibrium flow equations with an aim at removing numerical dissipation of Van-Leer-type flux-vector splittings on a contact discontinuity. The scheme obtained is also recognized as an improved Advection Upwind Splitting Method (AUSM) where a slight numerical overshoot immediately behind the shock is eliminated. The proposed scheme has favorable properties: high-resolution for contact discontinuities; conservation of enthalpy for steady flows; numerical efficiency; applicability to chemically reacting flows. In fact, for a single contact discontinuity, even if it is moving, this scheme gives the numerical flux of the exact solution of the Riemann problem. Various numerical experiments including that of a thermo-chemical nonequilibrium flow were performed, which indicate no oscillation and robustness of the scheme for shock/expansion waves. A cure for carbuncle phenomenon is discussed as well.