• A K Singh

      Articles written in Bulletin of Materials Science

    • The kinetics of pressure-induced polymorphic transformations

      A K Singh

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      The various methods of obtaining the kinetics data in pressure-induced transformations have been briefly reviewed. Some of the recent results obtained for the pressure-induced alpha-omega transformation in titanium, and fcc-bcc transformation in ytterbium have been discussed. The general features of the kinetics data have been compared with those for other transformations available in literature.

    • The pressure-volume relation of ytterbium up to 9 GPa

      S Usha Devi A K Singh

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      The pressure-volume relation of ytterbium has been determined up to 9 Gpa using tungsten carbide opposed anvil high pressure x-ray camera. The fcc phase of ytterbium is observed between one atmosphere and 4 GPa and the bcc phase above 3·5 GPa. The bcc phase can be metastably retained down to 1 GPa by gradually decreasing the pressure from a region where only bcc phase alone is observed. The bulk modulus,B0, at zero pressure and the pressure derivative of the bulk modulus,B’0, are determined by fitting Murnaghan equation to the pressure-volume data. The following values were obtained:B0=16·3 GPa andB’0=3·6 for the fcc phase, andB0=14·7 GPa andB’0=1·5 for the bcc phase. Based on the present data it is suggested that the thermodynamic equilibrium pressure for fcc ⇆ bcc transformation in ytterbium is below 3·5 GPa. The valence change under pressure has been discussed.

    • Elastic properties of As-Sb-Se glasses

      A Giridhar Sudha Mahadevan A K Singh

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      Results of measurement of elastic modulii on As-Sb-Se glasses are reported and their composition dependence discussed. The Young’s and the shear modulii lie in the range of 170–210 and 65–80 kb respectively. These values are typical of chalcogenide glasses. For (As, Sb)40Se60 glasses, the modulii increase monotonically with increasing Sb2Se3 content. The observed composition dependence of the modulii for the AsxSb15Se85 −x glasses is examined in terms of the chemically ordered structural units in the glasses.

    • Thermal expansion studies on As-Sb-Se glasses

      A Giridhar Sudha Mahadevan A K Singh

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      The results of measurement of thermal expansion coefficient from 50°C to the softening temperature on eight glass compositions of the As-Sb-Se system are presented. Measurements have been made on (As, Sb)40Se60 and AsxSb15Se85−x glasses at heating rates of 1°C/min and 5°C/min. The composition and temperature dependence of the thermal expansion coefficient is discussed in the light of results of other chalcogenide glasses.

    • Non-isothermal crystallization of As2Se3 glass

      A Giridhar Sudha Mahadevan A K Singh

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      The results of non-isothermal crystallization studies performed at different heating rates on batches of As2Se3 glasses prepared from melts at 400°C, 600°C and 800°C are reported. The peak temperature of crystallizationTp, the enthalpy of crystallization ΔHc and the activation energy for crystallizationEc are independent of the melt temperature used in the preparation. Bulk nucleation with three-dimensional growth of crystals is indicated for As2Se3. The values of ΔHc andEc are found to be respectively 23·3 ± 0·9 cal/g and 36·5 ± 0·9 kcal/mol for As2Se3.

    • Use of energy dispersive method with tungsten carbide opposed anvil high pressure set-up

      A K Singh S Usha Devi

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      The use of energy dispersive method has been discussed to record the X-ray diffraction data from a tungsten carbide opposed anvil high pressure set-up. It is pointed out that a proper collimation of the incident and the diffracted beams can eliminate the scattering from the gasket material. This results in an appreciable improvement in the quality of the recorded diffraction data. The choice of the various parameters has been discussed. The data on sodium chloride are presented in support of this analysis.

    • X-ray analysis of polycrystalline aluminium subjected to fatigue cycling

      Kalyani Vijayan A Mani C Balasingh A K Singh

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      Variations in the halfwidth values of X-ray reflections from fatigue-cycled, polycrystalline aluminium samples have been analysed. An oscillatory variation of the halfwidths with fatigue cycling has been observed. Analysis of the diffraction line profiles indicates that broadening arises mainly because of the build-up of microstrains during fatigue cycling. The present data indicate that (i) broadening due to fatigue cycling increases with glancing angle; (ii) changes in halfwidth and integral widths, due to fatigue cycling, are comparable and (iii) (b/b0) versusN curves for fatigue cycling under constant stress amplitude and flight loading conditions are comparable.

    • Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates

      D Pandey V S Tiwari A K Singh Sangeeta Chaudhry

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      The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 − y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 − y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics.

    • Preparation and characterization of aluminium alloy sheet Aramid fibre-laminated composites

      P Kanakalatha M K Sridhar Chadra Ajay C Balasingh A K Singh

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      Laminated composites consisting of alternate layers of aluminium alloy sheets and unidirectional Kevlar-49 fibre epoxy composites were prepared using two different aluminium alloys DTD 687 and aluminium-lithium alloy. Tensile, compressive and interlaminar shear strengths of the laminates were measured. The residual stresses in the aluminium alloy sheets arising out of thermal mismatch between aluminium alloys and aramid fibres were also measured. It is found that the laminates have lower density, higher tensile strength and marginally lower Young’s modulus as compared with monolithic alloy sheets.

    • Measurement of ultrasonic wave velocity in a solid under gas pressures up to 0·4 GPa

      C Divakar A K Singh

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      A set-up is described for the measurement of longitudinal and transverse wave velocities in a solid under gas pressures up to 0·4 GPa. To check the performance of the set-up, the elastic constants of Se and As2Se3 glasses and their pressure derivatives were obtained from the wave velocity data and compared with the data available in the literature.

    • Thermoelectric power of YBa2Cu3O7−δ under pressure up to 9 GPa

      Sheela K Ramasesha A K Singh

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      Thermoelectric power (TEP) of two YBa2Cu3O7−δ compounds (with δ=0·17 and 0·21) was measured as a function of quasi-hydrostatic pressure up to 9GPa at 300K on samples with low porosity. In both cases TEP decreases with increasing pressure, at a rate ∼ 0·8 μVK−1/GPa. The data obtained under hydrostatic pressure up to 3 GPa are in good agreement with those under quasi-hydrostatic pressure. The TEP of both compositions is found to decrease linearly at a rate 0·8 μVK−1/GPa above 1·5 GPa.

    • The effect of uniaxial stress component on the lattice strains measured by a diffraction method using opposed anvil device: trigonal system

      A K Singh C Balasingh

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      The equations have been derived for the trigonal system to calculate the lattice strains produced by the non-hydrostatic pressure condition which arises when the sample is compressed between the anvils without any pressure transmitting medium.

    • Ferroelectric phase transition in Ba5RTi3Nb7O30 [R=Nd, Eu, Gd] ceramics

      R N P Choudhary S R Shannigrahi A K Singh

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      Polycrystalline samples of Ba5RTi3Nb7O30 [R=Nd, Eu, Gd], were prepared using high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the compounds shows the formation of single phase compounds (orthorhombic crystal system) at room temperature. Detailed studies of dielectric properties (ɛ, tanδ,σ) as a function of frequency (400 Hz to 10 kHz) and temperature (30° to 380°C) show that these compounds exhibit diffuse ferroelectric phase transition.

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