Browsing by Author "S.K. Malik"

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Effect of la doping on microstructure and critical current density of MgB2
(2005) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; D.S. Rana; S.K. Malik; O.N. Srivastava
In the present study, La-doped MgB2 superconductors with different doping levels (Mg1-xLaxB2; x ≤ 0.00, 0.01, 0.03 and 0.05) have been synthesized by the solid-state reaction route at ambient pressure. Effects of La doping have been investigated in relation to microstructural characteristics and superconducting properties, particularly intragrain critical current density (Jc). The microstructural characteristics of the as-synthesized Mg(La)B2 compounds were studied employing the transmission electron microscopic (TEM) technique. The TEM investigations reveal inclusion of LaB6 nanoparticles within the MgB2 grains, which provide effective flux pinning centres. The evaluation of intragrain Jc through magnetic measurements on the fine powdered version of the as-synthesized samples reveal that Jc values of the samples change significantly with the doping level. The optimum result on Jc is obtained for Mg 0.97La0.03B2 at 5 K; Jc reaches ∼1.4 × 107 A cm-2 in self-field, ∼2.1 × 106 A cm-2 at 1 T, ∼2.5 × 10 5 A cm-2 at 2.5 T and ∼1.8 × 104 A cm-2 at 4.5 T. The highest value of intragrain Jc in the Mg0.97La0.03B2 superconductor has been attributed to the inclusion of LaB6 nanoparticles, which are capable of providing effective flux pinning centres. © 2005 IOP Publishing Ltd.
Enhancement of flux pinning and high critical current density in graphite doped MgB2 superconductor
(2007) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; O.N. Srivastava; S.K. Malik
We report the synthesis and characterization of graphite (C) doped MgB2-x Cx (x=0.0, 0.1, 0.2, and 0.3) samples. The crystal structure and microstructural characterization have been investigated by x-ray diffractometer and transmission electron microscopic (TEM) analysis. The superconducting properties especially Jc and Hc2 have been measured by employing physical property measurement system. We found that the graphite doping affects the lattice parameters as well as the microstructure of MgB2 superconductor. In case of optimally doped (x=0.1) sample, the critical current density at 5 K corresponds to 1.1× 106 and 5.3× 104 A cm2 for 3 and 5 T fields, respectively. The upper critical field has been enhanced nearly two times after doping. The flux pinning behavior has been investigated by flux pinning force density curve and it reveals that the flux pinning behavior has improved significantly by doping. TEM micrographs show the graphite nanoparticles of size ∼5-10 nm which are invariably present in MgB2 grains. These nanoparticles act as flux pinning center and are responsible for enhancement of superconducting properties of MgB2. © 2007 American Institute of Physics.
High critical current density and improved flux pinning in bulk MgB 2 synthesized by Ag addition
(2007) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; O.N. Srivastava; S.K. Malik
In the present investigation, we report a systematic study of Ag admixing in Mg B2 prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural microstructural characterization employing x-ray diffraction and transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by physical property measurement system. The TEM investigations reveal the formation of MgAg nanoparticles in Ag admixed samples. These nanoparticles may enhance critical current density due to their size (∼5-20 nm) which is compatible with the coherence length of Mg B2 (∼5-6 nm). In order to study the flux pinning effect of Ag admixing in Mg B2, the evaluation of intragrain critical current density (Jc) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on intragrain Jc is obtained for 10 at. % Ag admixed sample at 5 K. This corresponds to ∼9.23× 107 A cm2 in self-field, ∼5.82× 107 A cm2 at 1 T, ∼4.24× 106 A cm2 at 3.6 T, and ∼1.52× 105 A cm2 at 5 T. However, intragrain Jc values for Mg B2 sample without Ag admixing are ∼2.59× 106, ∼1.09× 106, ∼4.53× 104, and 2.91× 103 A cm2 at 5 K in self-field, 1 T, 3.6 T, and 5 T, respectively. The high value of intragrain Jc for Ag admixed Mg B2 superconductor has been attributed to the inclusion of MgAg nanoparticles into the crystal matrix of Mg B2, which are capable of providing effective flux pinning centers. A feasible correlation between microstructural features and superconducting properties has been put forward. © 2007 American Institute of Physics.
Improved critical current density of MgB 2-carbon nanotubes composite
(2007) Chandra Shekhar; Rajiv Giri; S.K. Malik; O.N. Srivastava
In the present study, we report a systematic study of doping/admixing of carbon nanotubes (CNTs) in different concentrations in MgB 2. The composite material corresponding to MgB 2 - x at.% CNTs (35 at.% ≥ × ≥ 0 at.%) have been prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural/microstructural characterization employing XRD, Scanning electron microscopic (SEM), and Transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by Physical property measurement system (PPMS) and electrical transport measurements have been done by the four-probe technique. The microstructural investigations reveal the formation of MgB 2-carbon nanotube composites. A CNT connecting the MgB 2 grains may enhance critical current density due to its size (∼5-20 nm diameter) compatible with coherence length of MgB 2 (∼5-6 nm) and ballistic transport current carrying capability along the tube axis. The transport critical current density (J ct) of MgB 2 samples with varying CNTs concentration have been found to vary significantly e.g., J ct of the MgB 2 sample with 10 at.% CNT addition is ∼2.3 × 10 3 A/cm 2 and its value for MgB2 sample without CNT addition is ∼7.2 × 102 A/cm2 at 20 K. In order to study the flux pinning effect of CNTs doping/ admixing in MgB2, the evaluation of intragrain critical current density (J c) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on J c is obtained for 10 at.% CNTs admixed MgB 2 sample at 5 K, the J c reaches ∼5.2 × 106 A/cm 2 in self field, ∼1.6 × 106 A/cm 2 at 1 T, ∼2.9 × 105 A/cm 2 at 2.6 T, and ∼3.9 × 104 A/cm 2 at 4 T. The high value of intragrain J c in 10 at.% CNTs admixed MgB 2 superconductor has been attributed to the incorporation of CNTs into the crystal matrix of MgB 2, which are capable of providing effective flux pinning centres. A feasible correlation between microstructural features and superconducting properties has been put forward. Copyright © 2007 American Scientific Publishers All rights reserved.
Investigations on the synthesis and microstructural characteristics of Hg(Bi)-Ba-Ca-Cu-O HTSC in relation to critical current density
(2004) Rajiv Giri; G.D. Verma; S.K. Malik; D. Kundaliya; R.S. Tiwari; O.N. Srivastava
In the present study, we have investigated the effect of Bi doping at the Hg site in the oxygen deficient HgOδ layer of HgBa 2Ca2Cu3O8+δ cuprate superconductor in relation to the occurrence of substructures in the form of long period polytypoid (LPP), defect matrix substructures, stacking faults and their influence on superconducting properties, particularly the critical current density. Bulk polycrystalline samples have been prepared by solid state reaction process through precursor route. It has been observed that the as grown Hg1-xBixBa2Ca2Cu 3O8+δ variants (with x=0.05, 0.10, 0.15, 0.20, 0.25) correspond to the Hg(Bi):1223 phase. Transmission electron microscopic (TEM) observations reveal the presence of long period polytypoid-like structures, associated defect matrix substructure and stacking faults. A correlation between the details of defect substructures associated with LPP and the transport critical current density has been found to exist. It is suggested that this correlation arises due to defect matrix substructure and stacking faults providing effective flux pinning sites. The optimum transport critical current density Jct of ∼1.05×103A/cm 2 at 77K has been observed for the composition Hg 0.85Bi0.15Ba2Ca2Cu3O 8+δ. TEM studies reveal that the phase with this composition has the highest density of ordered LPP structures and associated defect matrix substructures. Such an observation bringing out a correlation between transport critical current density and LPP together with defect matrix substructure appears to be the first of its type. Evaluation of intra-grain critical current density through magnetic measurements for the optimum phase Hg 0.85Bi0.15Ba2Ca2Cu3O 8+δ revealed that this phase also has corresponding highest Jc (intra-grain) ∼4.7×104A/cm2 at 77K. © 2003 Elsevier B.V. All rights reserved.