• Subhash C Kashyap

      Articles written in Bulletin of Materials Science

    • Dielectric behaviour of ZnO-based ceramic semiconductors

      Subhash C Kashyap K L Chopra Bharat Bhushan

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      The effect of composition, sintering parameters, frequency and temperature on the dielectric parameters of ZnO-based ceramic semiconductors (cersems) having small amounts of Bi2O3, Sb2O3, CoO, MnO2, La2O3 and/or Cr2O3 has been investigated. The unusually high dielectric constant of these composites, arising due to a two-phase microstructure has been explained on the basis of a depletion layer model. The agreement in values of barrier height and donor concentration calculated fromC−2V plot, Hall measurement and I-V characteristics of these cersems supports the validity of barrier and depletion layer models. The depletion and inter-granular layers are estimated to be nearly 102 nm and 1–2 nm respectively. The observed variation of dielectric constant/capacitance with sintering parameters and temperature of measurement has also been explained on the basis of simplified microstructure and depletion layers. The loss peak (fmax) observed at 300 kHz remains practically unaltered with change in composition and sintering parameters. The observed dielectric dispersion in the range 102−106 Hz, exhibiting multiple relaxation times and activation energy of relaxation process as 0·36 eV, has been explained on the basis of Debye-type relaxation process originating due to trapping/detrapping and possibly due to scattering of carriers in the depletion regions.

    • Preparation and degradation behaviour of Bi-cuprate superconductors

      Balvinder Gogia Subhash C Kashyap D K Pandya K L Chopra

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      The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theTc,Jc and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi2O3 after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highTc phase (2223) exhibiting a maximum transition temperatureTc=110 K and ΔTc=2 K. The addition of lead and grain orientation together result in an increase inJc by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inTc,Jc and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withTc=97 K even after 250 days. The highTc phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.

    • Effect of praseodymium substitution on the growth and properties of Y1−xPrxBa2Cu3O7−δ (o <x < 0·4) single crystals

      Veer Singh Subhash C Kashyap

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      In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y1−xPrxBa2Cu3O7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm3 and 4 × 1 × 1 mm3, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y0·8Pr0·2Ba2Cu3O7−δ and Y0·6Pr0·4Ba2Cu3O7−δ materials are ≈ 2 × 3 × 0·1 mm3 and 5 × 1 × 1 mm3 and ≈ 1 × 1·5 × 0·1 mm3 and 7 × 0·2 × 0·1 mm3, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (Tc) of the oxygenated crystals were found to depend on the Pr-content.

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