• R G Sharma

      Articles written in Bulletin of Materials Science

    • Low percolation concentration for zero thermoelectric power in Y1Ba2Cu3O7−x

      S R Jha Y S Reddy R G Sharma

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      Effect of oxygen deficiency on the thermoelectric power and the resistive transition is studied by changing the cooling rate of the sintered specimens of R1Ba2Cu3O7−x where R=Y, Gd and Sm. Slow-cooled (60°C/h) specimens are superconducting and show positive TEP with a broad peak aboveTc. It is a decreasing function of temperature at higher temperatures. Pair fluctuation aboveTc is thought to be responsible for this peak. Highly O2-deficient sample of Y123 has a finite resistivity atTc and yet shows a zero TEP. It is suggested that the percolation concentration of the superconducting phase for zero TEP should be much smaller than that required for the electrical conduction.

    • Interplay of low and highTc phases in Bi-Sr-Ca-Cu-O superconductor

      P L Upadhyay K C Nagpal R G Sharma

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      In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highTc phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi2Sr2Can−1CunO4 + 2n. X-ray diffraction studies show that the lowerTc material is a relatively pure phase while the higherTc phase only co-exists with the lowerTc phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.

    • Critical current studies on metal clad R1Ba2Cu3O7−x wires

      S R Shukla Y S Reddy R G Sharma

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      Silver clad wires of R1Ba2Cu3O7−x (R=Y, Gd, Sm, Dy and Ho) have been fabricated following the powder-in-tube method and cold-rolling. The critical current densityJc at 77 K and zero magnetic field is restricted to 66 A.cm−2. In a separate experiment we have studied the effect of silver sheath thickness onJc of the Y1Ba2Cu3O7−x wires. Interestingly,Jc is higher for larger sheath thickness indicating that large sheath thickness prevents oxygen loss during sintering. Uniaxial pressing of the wires into flat tapes results in an increase of theJc. MaximumJx is, however, limited to 169 A.cm−2. No grain alignment is found in the core material of our wires which is essential for highJc. Several options are being tried.

    • Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-Tc superconductors

      R G Sharma S Lahiry A Pandey Dipten Bhattacharya

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      A series of high-Tc superconductors have been prepared with the HgO addition/substitution. Significant improvement in theTconset as well asTc0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofTc0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inTc thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyEJ becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJc ∼ exp[−ΔTc/Tc0] and the flux pinning energyU ∼ ΔTc, where ΔTc is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJc, highU yet low ΔTc. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.

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