K.S. Kumar

Barium rare-earth zirconates : synthesis, characterisation and their possible application as substrates for YBa2Cu3O7−δ superconductors

A new class of complex perovskite oxides Ba{sub 2}REZrO{sub 6{minus}x} (RE = Pr, Nd, Sm and Gd) have been synthesized and sintered as single phase materials with high sintered density and stability by solid state reaction method. The structure of Ba{sub 2}REZrO{sub 6{minus}x} was studied by X-ray diffraction technique and all of them were found to be isostructural and have a cubic perovskite structure. No chemical reaction was observed between Ba{sub 2}REZrO{sub 6{minus}x} and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} even after annealing a 1:1 volume mixture at 950 C for 20 h. The dielectric constant and loss factor of these materials are in a range suitable for their use as substrates for YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} superconductors. Ba{sub 2}REZrO{sub 6{minus}x} melts congruently and could be grown as single crystals from melt.

YBa2SnO5·5, a novel ceramic substrate for YBCO and BiSCCO superconductors

YBa2SnO5·5 has been synthesized and sintered as single phase material for its use as substrate for both YBCO and BiSCCO superconductors. YBa2SnO5·5 has a complex cubic perovskite (A2BB’O6) structure with the lattice constanta = 8·430 Å. The dielectric constant and loss factor of YBa2SnO5·5 are in a range suitable for its use as substrate for microwave applications. YBa2SnO5·5 is found to be chemically compatible with both YBCO and BiSCCO superconductors. The thick film of YBCO screen printed on polycrystalline YBa2SnO5·5 substrate gave aTc(0) of 92 K and a critical current density (Jc) of 4 × 104 A/cm2 at 77 K. A screen printed BiSCCO thick film on YBa2SnO5·5 substrate gaveTc(0) = 110 K and current density 3 × 103 A/cm2 at 77 K.

Development and characterisation of dysprosium barium niobate: a new substrate for YBa2Cu3O7−δ and (Bi,Pb)2Sr2Ca2Cu3Ox superconductor films

Abstract Dysprosium barium niobate has been developed as a new substrate suitable for both YBa 2 Cu 3 O 7− δ (YBCO) and (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x [Bi(2223)] superconductor films. DyBa 2 NbO 6 (DBNO) has a cubic perovskite (A 2 BB′O 6 ) structure with a lattice constant a =8.456 A. DBNO was found to have a thermal expansion coefficient of 7.806×10 −6 °C −1 and a thermal conductivity of 67.9 W m −1 K −1 . The dielectric constant and loss factor values of DBNO are also in a range suitable for its use as substrate for microwave applications. Both YBCO and Bi(2223) superconductors did not show any detectable chemical reaction with DBNO even under extreme processing conditions. Dip-coated YBCO thick film on polycrystalline DBNO substrate gave a T c (0) of 92 K and J c of ∼1.1×10 4 A cm −2 . Bi(2223) thick film dip-coated on DBNO gave T c (0) of 110 K and J c of ∼4×10 3 A cm −2 .

Superconducting Bi‐cuprate thick film Tc(0)=110 K on DyBa2SnO5.5: A newly developed perovskite substrate

A new ceramic perovskite DyBa2SnO5.5 has been synthesized and developed for its use as a substrate for Bi(Pb)–Sr–Ca–CuO superconductors. The dielectric constant and loss factor of this material are in a range suitable for its use as a substrate for microwave applications. DyBa2SnO5.5 is found to be chemically compatible with Bi(Pb)–Sr–Ca–CuO superconductors. Bi(2223) superconducting thick film fabricated on DyBa2SnO5.5 substrate gave a Tc(0)=110 K with a transition width of 2 K.

Superconducting YBCO-Ag (Tc(0) = 92 K) and Bi(2223)-Ag (Tc(0) = 110 K) composite thick films by dip coating on DyBa2SnO5.5, a new perovskite ceramic substrate

Abstract Superconducting YBCO-Ag and Bi(2223)-Ag composite thick films have been fabricated on polycrystalline DyBa 2 SnO 5.5 , a new cubic perovskite substrate, by dip coating. The thick films of YBCO-Ag dip coated on polycrystalline DyBa 2 SnO 5.5 substrate gave to a T c(0) = 92 K and a critical current density ( J c ) ≃ 7 × 10 4 A/cm 2 at 77 K for 6 vol.% of Ag in the composite. A dip-coated Bi(2223)-Ag composite thick film on DyBa 2 SnO 5.5 gave a T c(0) = 110 K and critical current density ∼1 × 10 4 A/cm 2 at 77 K for an addition of 6 vol.% of Ag in the composite. The effect of Ag volume fraction on the critical current density and orientation in YBCO-Ag and Bi(2223)-Ag composite thick films have also been discussed.

Chemical reactivity and percolation in a rapidly quenched superconducting YBa2Cu3O7- delta -YBa2SbO6 composite system

The chemical reactivity and percolation in a rapidly quenched superconducting YBa2Cu3O7- delta -YBa2SbO6 composite system have been studied by X-ray diffraction and temperature-resistivity measurements. No chemical reaction between Y1Ba2Cu3O7- delta and YBa2SbO6 was observed, even under severe heat treatment, and the system behaved as an ideal conductor-insulator composite with a normal percolation threshold of 22 vol% of YBCO in the system. The values obtained for critical exponents describing the normal transport behaviour of the system match with the theoretically expected values of an ideal conductor-insulator percolation system. The superconducting percolation threshold value is found to be around 40 vol% of YBCO. Superconductivity with a transition temperature of 90 K was obtained in this composite system by rapidly quenching the samples in air directly from a sintering temperature, for a wide range of compositions.

High temperature superconductivity in YBa2Cu3O7−δ−YBa2SbO6 system by rapid quenching in air

Abstract The chemical inter-reactivity and high temperature superconductivity in YBa 2 Cu 3 O 7− δ −YBa 2 SbO 6 system have been studied by XRD and resistivity measurements. It is found that the common procedure of slow cooling at a rate of 2°C/min or annealing for several hours at temperatures between 700–500°C after high temperature sintering is not essential to obtain superconductivity in YBa 2 Cu 3 O 7− δ −YBa 2 SbO 6 system. A superconducting transition of 92 K was obtained in this system, for a wide composition range by directly quenching these samples in air from a sintering temperature of 950°C.

Transport properties of the superconducting YBa2Cu3O7-δ-DyBa2SnO5.5 percolation system

The electrical transport properties and percolation behavior of superconducting YBa2 Cu3O7-δ-DyBa2SnO5.5, a superconductor-insulator composite system, have been studied by X-ray diffraction and temperature-resistivity measurements. The normal-state percolation threshold is ∼22 vol.% of YBa2Cu3O7-δ and the superconducting percolation threshold is ∼30 vol.% of YBa2Cu3O7-δ in the composite. The values obtained for the critical exponents describing the normal-state transport behavior of the system agree with the theoretically expected values for an ideal conductor-insulator percolation system. No detectable chemical reactivity was observed between YBa2Cu3O7-δ and a ceramic insulator DyBa2SnO5.5, even when the two materials were mixed thoroughly and sintered at 1020°C. The implications are discussed.

Superconductivity and electrical transport in YBa2Cu3O7−δ-PrBa2SnO5.5 percolation system

Percolation behaviour of the normal state resistivity and superconductivity of YBa2Cu3O7-δ-PrBa2SnO5.5 composites have been studied by X-ray diffraction and temperature-resistivity measurements. No detectable chemical reactivity was observed between YBa2Cu3O7-δ and a ceramic insulator PrBa2SnO5.5 even when the two materials were mixed thoroughly and sintered at 1020° C. The normal state percolation threshold is 21 vol% of YBa2Cu3O7-δ and the superconducting percolation threshold is 30 vol% of YBa2Cu3O7-δ in the composite. The values obtained for the critical exponents describing the normal state transport behaviour of the system matched with theoretically expected values for an ideal conductor-insulator system. The implications are discussed.