Browsing by Author "H. Kishan"

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Effect of Ca doping for Y on structural/microstructural and superconducting properties of YBa2Cu3O7-δ
(Elsevier, 2005) Rajiv Giri; V.P.S. Awana; H.K. Singh; R.S. Tiwari; O.N. Srivastava; Anurag Gupta; B.V. Kumaraswamy; H. Kishan
We report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O 7-δ bulk samples with 0.20 ≥ x ≥ 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu 3O7-δ leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation. © 2005 Elsevier B.V. All rights reserved.
Impact of Zn substitution on phase formation and superconductivity of Bi1.6Pb0.4Sr2Ca2Cu 3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12
(2005) R.B. Saxena; Rajiv Giri; V.P.S. Awana; H.K. Singh; M.A. Ansari; B.V. Kumaraswamy; Anurag Gupta; Rashmi Nigam; K.P. Singh; H. Kishan; O.N. Srivastava
Samples of series Bi1.6Pb0.4Sr2Ca 2Cu3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi-2223 (Bi2Sr2Ca2Cu3O10) phase (c-lattice parameter ∼ 36 Å). The proportion of Bi-2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase (Bi-2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [ρ(T)] and ac magnetic susceptibility [χ(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (∼ 10 K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi-2223 system are compared with other Zn-doped HTSC (high temperature superconducting) systems, namely the RE-123 (REBa2Cu3O7) and La-214 ((La, Sr)2CuO4). © World Scientific Publishing Company.
Magneto-resistance, magneto-caloric and bolometric properties in Pr2/3Sr1/3MnO3:PdO composites near room temperature
(Institute of Physics Publishing, 2014) Ramesh Chandra Bhatt; P.C. Srivastava; V.P.S. Awana; H. Kishan; S.K. Agarwal
Temperature coefficient of resistance (TCR), magneto-resistance (MR) and magneto-caloric properties of polycrystalline pristine Pr2/3Sr1/3MnO3 (PSMO) and its composites with PdO (10-30 mole %) (PSMO:Pd) have been studied. Rietveld refinement of their x-ray diffraction (XRD) patterns confirms their single phase crystalline structure with orthorhombic Pbnm space group and Pd appears in pure metallic phase. Of the various composites investigated, PSMO: Pd30 composite exhibits significant TCR of 9.7%K-1 (at 281 K) and low field MR value of 33.5% (H= 1 T). Magnetic entropy change (ΔSM) has been found to be 3.5 J kg-1K-1 (1 T), 6.3 J kg-1K-1 (2 T) and 8.7 J kg-1K-1 (3 T) at 283K for PSMO:Pd10 sample which show a decreasing trend with increasing Pd content. Present investigations reveal excellent characteristics for bolometric, magnetic sensing and magnetic refrigerating applications near room temperature by PSMO:Pd composites. © 2014 IOP Publishing Ltd.
Near room temperature magneto-transport (TCR & MR) and magnetocaloric effect in Pr2/3Sr1/3MnO3:Ag2O composite
(Elsevier Ltd, 2015) Ramesh Chandra Bhatt; V.P.S. Awana; H. Kishan; P.C. Srivastava
The magneto-transport and magnetocaloric effect in Pr2/3Sr1/3MnO3:Ag2O (PSMO:Ag) with Ag2O (10-30 mol%) have been studied by resistivity and magnetization measurements. Temperature coefficient of resistance (TCR), magneto-resistance (MR) and magnetocaloric properties of the composite have shown different responses to Ag addition. Rietveld refinement confirms single phase crystalline structure with orthorhombic Pbnm space group and presence of silver metal as well. TCR changes randomly with Ag, increasing initially to 7.8% K-1for PSMO:Ag10 composite which then decreases marginally with increasing Ag content. MR has been found to reach a maximum value of 30.5% at 1 T applied field for PSMO:Ag10 composite. Magnetic entropy change (ΔSM) increases with increasing Ag content and is found to be 3.5 J kg-1K-1(1 T), 6.1 J kg-1K-1(2 T) and 8.4 J kg-1K-1(3 T) for PSMO:Ag30 composite at 285 K. The maximum refrigeration capacity of 39.5 J kg-1(1 T), 91.0 J kg-1(2 T) and 107.7 J kg-1(3 T) has been found for the PSMO:Ag20 composite. PSMO:Ag composite exhibits potential for bolometric, magnetic sensing and magnetic refrigerating applications near room temperature. © 2014 Elsevier B.V. All rights reserved.
Room temperature magneto-resistance and temperature coefficient of resistance in la0.7Ca0.3-xAgxMnO3 thin films
(2007) P.K. Siwach; V.P.S. Awana; H. Kishan; R. Prasad; H.K. Singh; S. Balamurugan; E. Takayama-Muromachi; O.N. Srivastava
We investigate the magnetotransport properties of La0.7Ca 0.3-xAgxMnO3 (0 ≤ x ≤ 0.3) polycrystalline films prepared by spray pyrolysis. The virgin La 0.7Ca0.3MnO3 (LCMO) (x=0.0) film shows paramagnetic-ferromagnetic [insulator-metal (IM)] transition at TC ∼ 260 K (TIM ∼245 K). Ag doping induces appreciable enhancement in TC/TIM and interestingly TIM of Ag doped films is higher than the corresponding TC value. In the 30% Ag doped film IM transition occurs at ∼325 K which is ∼ 10 K higher than its TC ∼315 K. Strong irreversibility is observed in zero field cooled-field cooled magnetization of all the films and the irreversibility temperature Tirr >TC increases with Ag doping. This suggests the formation of magnetic clusters above TC. The pure LCMO film exhibits small magnetoresistance (MR) ∼20% and rather poor temperature coefficient of resistance (TCR) in the vicinity of the room temperature. In contrast, around room temperature the Ag doped films show large MR which is ∼60% for x=0.15 and decreases to ∼30% for the pure Ag doped film. Ag doped films also have good TCR ∼4%/K in the vicinity of room temperature. © 2007 American Institute of Physics.