• B Viswanathan

      Articles written in Bulletin of Materials Science

    • Positron annihilation studies of rare-earth mixed valence compounds

      K P Gopinathan C S Sundar B Viswanathan A Bharathi

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      Positron lifetime has been measured in SmS under external pressures from 0 to ∼ 16 kbar, and in (Sm0·8Gd0·2)S at temperatures from 10 K to 300 K. An abrupt drop in the lifetime was observed at the valence transition at a pressure of 6·5 kbar in SmS and at a temperature of ∼ 120 K in (Sm0·8Gd0·2)S. Measurement of the Doppler broadening of the annihilation photons was made in (Sm0·8 Gd0·2)S as a function of temperature. The line shape parameter of the Doppler broadened spectrum also showed an abrupt change at the transition. Angular correlation of annihilation photons was measured in the semiconducting phase of SmS and in the metallic phase of (Sm0·8Tb0·2)S. The correlation profile in the mixed valence (metallic) phase was broader than that in the semi-conducting phase. These results have been explained on the basis of the electronic structure of the semiconducting and the metallic phases of these materials.

    • X-ray crystallographic and thermal studies on the hydrides of magnesium and its intermetallics

      P Selvam B Viswanathan C S Swamy V Srinivasan

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      Magnesium nickel hydride, Mg2NiH4, exists in two crystallographic modifications, the low temperature phase crystallizing in monoclinic structure and the high temperature phase having a cubic structure. The phase transition (∼510 K) was accompanied by a small composition change. The enthalpies and entropies of formation of these hydrides were calculated from the DTA data and compared with the values obtained by other methods.

    • Characteristics of materials for microwave devices

      V R K Murthy K C James Raju B Viswanathan

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      The characteristics of materials like ferrites, garnets, ferroelectrics temperaturecompensated dielectrics and copper-coated PTFE-based composites, extensively used in microwave devices are discussed.

    • Effect of substitution at the barium site in YBa2Cu3O7-δ

      N S Raman B Viswanathan

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      The oxygen content in YBa2Cu3O7-δ has been varied by substitution at Ba site with Sr or K and its influence on the structural and superconducting properties has been studied. These properties are compared with that of unsubstituted YBa2Cu3O7-δ prepared by quenching. The increase inδ value of YBa2Cu3O7-δ decreases theTc drastically, whereas in YBa2-χKχCu3O7-δ system increase in theδ value (as high as 0·92) does not affect the superconducting transition temperature, which remains above 77 K. In the YBa2-χSrχCu3O7-δ system, as Sr content increases there is marginal decrease in the oxygen content as well as in the superconducting transition temperature.

    • Cold-worked and hydrogen chargedNbHf as studied by TDPAC and positron lifetime measurements

      R Govindaraj R Rajaraman K P Gopinathan B Viswanathan

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      Time differential perturbed angular correlation (TDPAC) and positron life-time measurements on cold-worked and hydrogen chargedNbHf indicate a strong binding of hydrogen decorated vacancy clusters with Hf impurities. Both TDPAC and positron lifetime results are mutually consistent on this aspect. Transformation of H-vacancy clusters into H-bubbles is indicated by the positron lifetime behaviour at annealing temperatures beyond 900 K. Impurity effects like oxygen pickup and coating of voids cannot be ruled out at elevated temperatures.

    • Mössbauer and XRD studies on the effect of doping iron in KCaY(PO4)2

      T N Sairam G V Narasimha Rao B Viswanathan

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      The effect of doping iron at the yttrium site in hexagonal KCaY(PO4)2 is studied for various concentrations ofx (0≤x≤1), of iron using Mössbauer and X-ray diffraction methods. For low iron concentrations, very little changes in structure are seen but atx≈0·1, onset of new peaks in the XRD pattern is observed. The Mössbauer study of the doped samples reveals that iron has a solubility of up to 2·5% in the parent phase with any excess iron precipitating out to form a new and unknown phase. From a detailed analysis of the X-ray diffraction pattern (corresponding tox=1) of the latter phase, it is found that this phase is rhombohedral with the possible space groupR3.

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