• R G Sharma

      Articles written in Pramana – Journal of Physics

    • Critical current in silver clad Y-Ba-Cu-O superconducting wires

      R G Sharma Y S Reddy S R Jha S S Dubey

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      Silver clad wires of highTc superconductor Y1Ba2Cu3O7−x have been fabricated through the powder metallurgy technique. The reacted wires show a midpointTc of 84K. A critical current density of 26·4 A cm−2 (77K, 0T) is obtained in these wires. The wires, however, turn complete normal only at a current density of 280 A cm−2. The reasons for low critical current density obtained in these wires are discussed.

    • Influence of oxygen deficiency on the thermoelectric power of Y1Ba2Cu3O7-x

      S R Jha Y S Reddy D K Suri K D Kundra R G Sharma Deepak Kumar

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      In this paper we report measurements of the thermoelectric power on a series of Y1Ba2Cu3O7-x specimens with varying amounts of oxygen deficiency obtained by changing the cooling rate of the sintered specimens. The specimens have been characterized by X-ray diffraction measurements, electric resistivity and oxygen contents. The temperature variation of the thermopower reveals a peak just before the onset of superconducting transition. We examine possible theoretical explanations of this anomaly. In particular we argue that this anomaly is associated with the pairing fluctuations in the normal state close toTc. We present some theoretical results in support of this conclusion.

    • Role of processing in improving the critical current of Ag/BPSCCO tapes

      S R Shukla Y S Reddy D K Pandya Y Tanaka R G Sharma

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      Silver-clad tapes of highTc Bi-Pb-Sr-Ca-Cu-O superconductors have been fabricated through the powder-in-tube method. The critical current density, Jc, of a thick tape was 534 A.cm−2 (77 K, 0 T). Subsequent rolling and sintering of the tapes led to a gradual decrease inJc, instead of the expected increase. This was caused by the microcracks developed in the core material by a rather drastic reduction during the rolling of the tapes. A modified and well controlled rolling technique, on the other hand, resulted in much improvedJc values. Repeated rolling and sintering resulted in a good grain alignment and no microcracks were observed. In the present studies, maximumJc of 1846 A.cm−2 (77 K, 0 T) and 2.43 × 104 A.cm−2 (4.2 K, 0 T) have been obtained Optimization of the processing and sintering parameters are expected to lead to still higherJc values.

    • Critical current in Ag/BPSCCO tapes using low purity materials

      S R Shukla Y S Reddy R G Sharma

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      Silver-clad Bi1·7Pb0·4Sr1·8Ca2Cu3·5Ox (BPSCCO) tapes have been fabricated using low purity (98–99%) starting materials and following the powder-in-tube technique. MaximumJc values of 6·14 × 103 A·cm−2 at 77 K and 1·4 × 105 A·cm−2 at 4·2 K have been obtained in tapes subjected to the process of intermediate rolling and sintering. The bulk superconducting material used for the tape-fabrication contains both 2223 and 2212 phases in the ratio 60:40. A pure phase material and the optimization of the sintering parameters are expected to yield much higherJc values at 77 K. It is possible that the copper-rich phase(s) and/or a small amount of iron impurity (60 ppm) present in CuO might be acting as flux pinning sites and could be responsible for highJc values.

    • Further studies on Ag/BPSCCO tapes using low purity materials

      S R Shukla Y S Reddy N Kumar S K Sharma R G Sharma

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      Transmission electron microscopic (TEM) studies are reported on Ag-clad Bi1.7 Pb0.4Sr1.8Ca2Cu3.5Ox tapes prepared by using low purity (98–99%) commercial grade materials. The self-fieldJc values of these tapes viz. 6.14 × 103 A.cm−2 at 77 K and 1.4 × 105 A.cm−2 at 4.2 K, reported in an earlier publication, were significantly higher than the correspondingJc values in tapes prepared with high purity (99.99%) materials. The TEM pictures on the low purity core material of the tapes reveal the presence of stacking faults and the intergrowth of the 2212 and 2223 phases which could be acting as flux pinning sites and responsible for enhancedJc values. These defects can perhaps be traced back to the presence of 60 ppm iron in the low purity CuO as revealed by atomic absorption analysis reported earlier.

    • Role of vanadium in Bi-2223 ceramics

      D R Mishra S V Sharma R G Sharma

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      The effect of substitution of vanadium in Bi1.6Pb0.4Sr2Ca2Cu3 Oδ ceramic at Bi and Cu sites has been investigated for the resistivity, a.c. susceptibility, XRD and a.c. magnetization studies. Enhancement in Tc for the smaller concentrations of V at either of the two sites was observed which followed an expected lattice distortion and decrease in Tc for higher V concentrations. V plays a role of substituting element more than just a sintering agent as reflected in the more rapid decrease in Tc at Cu-site and further in the magnetization values that are higher compared to the values at the Bi-site. It indicates higher magnetization scattering at the Cu site. The results were explained keeping in mind the liquid phase mechanism behind the formation of the 2223 phase and the possibility of magnetic scattering.

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