• V Prasad

      Articles written in Bulletin of Materials Science

    • Upper critical field of Bi1·5-Pb0·5-Ca2-Sr2-Cu3-Ox superconductor

      V Prasad S V Subramanyam

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      The highTc superconductor of nominal composition Bi1·5-Pb0·5-Ca2-Sr2-Cu3-Ox was prepared by solid-state reaction method. The upper critical fieldHc2 of the material with a zero-resistance transition at 110K was investigated by observing variations in resistance with temperature down to 90 K and with a magnetic field up to7T. The slope ofHc2 with temperature was about −0·41T/K for zero resistance transition and −6·86T/K for onset of superconductivity.Hc2 values at 0K were estimated to be 31·3T and 563T for zero-resistance transition and onset of superconductivity respectively.

    • Thermal conductivity measurements at low temperatures

      P Balaya H S Jayanna Hemant Joshi G Sumana V G Narasimha Murthy V Prasad S V Subramanyam

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      We describe briefly the experimental facilities developed for the measurement of thermal conductivity of solids in the temperature range 10K–300K. Different techniques have been used for the determination of thermal conductivity, depending on the relaxation time of the system under investigation. Measurements on stainless steel 304, using steady state and non-steady state methods are presented. Values of thermal conductivity obtained by both these methods agree to each other and are consistent with those reported earlier.

    • Tuning magnetoresistance and electrical resistivity by enhancing localization length in polyaniline and carbon nanotube composites


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      We report low temperature electrical resistivity and magnetoresistance (MR) measurements of conducting polyaniline (PANI) and multiwalled-carbon nanotube (MWCNT) composites. We have used an in-situ oxidative polymer-ization method to synthesize hydrochloric acid-doped PANI composites with MWCNT weight percentages of 0, 5, 10 and 15. The temperature dependence of resistivity is studied from room temperature to 4.2 K and analysed by a Mott variable range hopping (VRH) model. The resistivity increases from 1.1 × 10⁻³ Ωm at 300 K to 65.75 Ωm at 4.2 K, almost four orders of the magnitude change with temperature for pure PANI. Whereas the PANI composite with 15% MWCNTs shows less variation from 4.6 × 10⁻⁴ to 3.5 × 10⁻² Ωm. The huge change in resistivity is due to the localization of charge carriers in the presence of disorder. At 4.2 K MR shows transition from positive to negative with higher MWCNT loading. Samples with 5 and 10% MWCNTs show positive MR, whereas the 15% MWCNT loaded sample shows negative MR. The positive and negative MR are discussed in terms of the wave function shrinkage effect and quantum interference effect on VRH conduction.

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