I. Manna

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Laser processing of materials

Light amplification by stimulated emission of radiation (laser) is a coherent and monochromatic beam of electromagnetic radiation that can propagate in a straight line with negligible divergence and occur in a wide range of wave-length, energy/power and beam-modes/configurations. As a result, lasers find wide applications in the mundane to the most sophisticated devices, in commercial to purely scientific purposes, and in life-saving as well as life-threatening causes. In the present contribution, we provide an overview of the application of lasers for material processing. The processes covered are broadly divided into four major categories; namely, laser-assisted forming, joining, machining and surface engineering. Apart from briefly introducing the fundamentals of these operations, we present an updated review of the relevant literature to highlight the recent advances and open questions. We begin our discussion with the general applications of lasers, fundamentals of laser-matter interaction and classification of laser material processing. A major part of the discussion focuses on laser surface engineering that has attracted a good deal of attention from the scientific community for its technological significance and scientific challenges. In this regard, a special mention is made about laser surface vitrification or amorphization that remains a very attractive but unaccomplished proposition.

Discontinuous reactions in solids

Abstract Discontinuous reactions are solid state moving boundary phase transitions characterised by a discontinuous or abrupt change in orientation and composition between the matrix phases in the reactant and product aggregate across the migrating boundary or reaction front that provides a short circuit path of diffusion. The reactions include discontinuous precipitation, discontinuous coarsening, discontinuous dissolution, and diffusion induced grain boundary migration. All these reactions may account for a substantial change in microstructure, composition, and material properties, and hence, deserve adequate scientific attention for a better understanding. The present review provides a comprehensive discussion on the current status of understanding about nucleation and growth mechanisms, genesis and driving force, product morphology and distribution, kinetic growth models, and related experimental techniques, and above all, the unresolved questions concerning these discontinuous reactions. In addition, exhaustive lists have been provided to document the important literature on the concerned subjects, whenever possible. Finally, a particular emphasis has been placed on analysing the recent findings about dynamic behaviour of grain boundaries, scope of determination of Arrhenius parameters by kinetic analysis, and orientation/structural dependence of boundary migration and diffusion.

Laser material processing

Abstract Light amplification by stimulated emission of radiation (laser) is a coherent and monochromatic source of electromagnetic radiation that can propagate in a straight line with negligible divergence. As a result, laser finds diverse applications ranging from mere mundane to most sophisticated uses either for totally commercial or purely scientific purposes, and from life saving to life threatening causes. High power lasers can produce intense heating and perform various manufacturing operations or material processing. The present contribution provides an overview of the application of high power laser only for material processing in engineering applications, and intentionally excludes the scope of application of laser in metrology, biomedical technology, spectroscopy, etc. The manufacturing processes covered have been broadly divided into four major categories, namely, laser assisted forming, joining, machining and surface engineering. Besides discussing the scope and principle of these processes, each section enumerates a detailed update of literature, scientific issues and technological innovations. At the beginning, a brief introduction to different types of lasers and their general applications, fundamentals of laser–matter interaction and classification of laser material processing has been provided. The entire discussion primarily focuses on correlating the properties with processing parameters and microstructure and composition of the material.

Formation of face-centered-cubic titanium by mechanical attrition

This study reports a metastable hcp→fcc polymorphic transformation in elemental titanium induced by high-energy mechanical attrition in a planetary ball mill. The transformation is monitored and verified by x-ray and electron diffraction and high-resolution transmission electron microscopy. The grain size decreases and lattice parameter increases with continued milling. The phase change is gradual and accompanied by about 16% increase in volume per atom. The milling intensity and deformation mode seem crucial for the completion of the change in crystal structure. The extent and influence of both substitutional and interstitial impurities in this transformation have been assessed. It is suggested that structural instability due to negative (from core to boundary) hydrostatic pressure arising out of nanocrystallization or grain refinement, increasing lattice expansion, and plastic strain/strain rate is responsible for this hcp→fcc polymorphic transformation in titanium. Thus, the present transformation is similar in nature and genesis to those in elemental niobium and zirconium earlier reported by us.

Laser surface engineering of a magnesium alloy with Al+Al2O3

The present study concerns an attempt to improve the wear resistance of a commercial magnesium alloy, MEZ (rare earth 2 wt.%, Zn 0.5 wt.%, Mn 0.1 wt.%, Zr 0.1 wt.%) by dispersion of Al2O3 particles and alloying with aluminium on the surface by laser surface engineering. Laser processing was carried out with a 10-kW continuous wave CO2 laser by melting and subsequent feeding of Al+Al2O3 particles (in the ratio of 3:1) on the surface of MEZ. Following laser processing, a detailed microstructural and phase analysis of the surface modified layer were carried out. The microhardness of the surface layer was measured as a function of laser parameters and wear resistance property was evaluated in details. Microhardness of the surface layer was significantly improved to as high as 350 VHN as compared to 35 VHN of the substrate. The optimum processing region for formation of a homogeneous microstructure and composition for laser surface modification of MEZ with Al+Al2O3 was established. The wear resistance of the laser surface modified samples was considerably improved as compared to the as-received specimen.

CdS-Decorated ZnO Nanorod Heterostructures for Improved Hybrid Photovoltaic Devices

Cadmium sulfide (CdS)-decorated zinc oxide (ZnO) nanorod heterostructures have been grown by a combination of hydrothermal and pulsed laser deposition techniques. Hybrid photovoltaic devices have been fabricated with CdS modified and unmodified ZnO nanorods blended separately with regioregular poly(3-hexylthiophene) (P3HT) polymer as the active layer. The solar cell performance has been studied as a function of ZnO concentration and the casting solvent (chlorobenzene, chloroform, and toluene) in the unmodified ZnO:P3HT devices. The power conversion efficiency is found to be enhanced with the increase of ZnO concentration up to a certain limit, and decreases at a very high concentration. The surface modification of ZnO nanorods with CdS leads to an increase in the open circuit voltage and short-circuit current, with enhanced efficiency by 300% over the unmodified ZnO:P3HT device, because of the cascaded band structure favoring charge transfer to the external circuit.

Friction and wear behavior of Ti following laser surface alloying with Si, Al and Si+Al

Abstract This study concerns the friction and wear behavior of Ti following laser surface alloying (LSA) with Si, Al or Si+Al. The said tribological characteristics of the laser-alloyed samples, subjected to the earlier determined optimum conditions of LSA, were investigated in terms of the variation of wear depth as a function of load and time using a computer-controlled reciprocating ball-on-disc wear testing machine fitted with an oscillating hardened steel ball. A detailed post wear microstructural analysis was conducted to determine the mechanism of wear and role of alloying elements in improving the resistance to wear. It appears that LSA with Si is more effective in improving the wear resistance of Ti than that by Si+Al or Al alone. The enhanced wear resistance in Si surface alloyed samples has been attributed to the presence of uniformly distributed Ti 5 Si 3 in the alloyed zone (AZ).

Laser composite surfacing of a magnesium alloy with silicon carbide

The present study concerns an attempt to improve the wear resistance of a recently developed magnesium alloy (MEZ) by formation of a SiC reinforced composite layer on the surface by laser surface engineering. Laser processing was carried out with a 10 kW continuous wave CO 2 laser by melting and subsequent feeding of SiC particles on the surface of MEZ. Following laser processing, detailed microstructural and phase analysis of the composite surfaced layer were carried out and correlated with the laser parameters. The microhardness of the surface layer was measured as a function of laser parameters and wear resistance was evaluated in details. Microstructure of the composite surfaced layer mainly consists of uniform dispersion of SiC particles in grain refined MEZ matrix. The volume fraction of SiC particles was found to vary with laser/process parameters. Microhardness of the surface layer was significantly improved to as high as 270 VHN as compared to 35 VHN of the substrate. The wear resistance of the composite surfaced samples was considerably improved as compared to the as-received specimen.

Formation of face-centered-cubic zirconium by mechanical attrition

This study reports a hcp→fcc polymorphic transformation in elemental zirconium induced by mechanical attrition in a planetary ball mill. The transformation is monitored and verified by x-ray and electron diffraction and high-resolution transmission electron microscopy. Grain size decreases and lattice parameter increases with continued milling. The phase change is gradual and is accompanied by about 9% increase in volume per atom. It is suggested that structural instability due to plastic strain, increasing lattice expansion, and negative (from core to boundary) hydrostatic pressure is responsible for this hcp→fcc polymorphic transformation in zirconium.