Neson Varghese

Significant enhancement of the in-field critical current density of the MgB2 superconductor through codoping of nano-TiC with nano-SiC

Nanoparticles of two carbides, SiC and TiC, having different reactivities with the Mg/B are doped into the MgB2 system. These carbides are added in mono-and codoping conditions through the in situ solid state reaction, and the structural and superconducting properties are studied. The results show distinct modifications in the structural and superconducting properties by the two dopants. SiC causes considerable C substitution at the B site and the formation of intragrain inclusions, whereas TiC remains unreacted and does not cause any substitution. The codoped samples show significantly enhanced JC(H) performance compared to the undoped and monodoped samples. Compared to the undoped sample, the enhancement of JC is more than two orders of magnitude at 15 K for fields >7 T for the best sample. The results of JC(H) performance of the samples are discussed.

Superconductivity of MgB2 in the BCS framework with emphasis on extrinsic effects on critical temperature

The discovery of superconductivity in MgB2 with an unusually high TC of 39?K interested material scientists all over the world. Many of its superconducting properties, including an experimentally observed double energy gap, differ significantly with those of conventional LTS and HTS. Many scientists proposed exotic and unconventional mechanisms to explain the superconductivity in MgB2, but after more than 5 years it is now accepted that MgB2 is a BCS type superconductor with exceptionally high TC. In this review we discuss qualitatively the origin of superconductivity in MgB2 based on both experimental and theoretical results. The variation of TC of MgB2 with various processes, such as irradiation of high energy ions, doping and external pressure, etc., is discussed and reviewed. TC of MgB2 decreases upon irradiation, by applying external pressure and by most of the doping. The reduction of density of states at the Fermi level, increased band scattering and changes in the phonon frequencies are the main factors which reduce the TC under these conditions.

Effect of combined addition of nano-SiC and nano-Ho2O3 on the in-field critical current density of MgB2 superconductor

MgB2 superconducting samples added with nano-Ho2O3 (n-Ho2O3) and/or nano-SiC (n-SiC) have been prepared by an in situ solid state reaction method to investigate and compare the combined and individual effects of n-SiC and n-Ho2O3 on a crystal structure, critical temperature (TC), and critical current density (JC) of MgB2. All the doped samples exhibit significantly enhanced in-field JC and the codoped sample with 2.5 wt % n-Ho2O3 and 5 wt % n-SiC gives the best performance in in-field JC, and the enhancement is around 100 times and 2 times greater than the undoped and monodoped n-SiC samples, respectively, at 5 K and 8 T. For the n-SiC added sample, lattice distortions due to C substitution on the B site and the formation of reacted phase Mg2Si as flux pinners cause enhanced JC up to the maximum field studied (8 T). While in the n-Ho2O3 added sample, a reacted phase HoB4 having a strong magnetic moment forms, without any substitution at the Mg or B site, which acts as a flux pinner in order to enhance the...

Influence of nano-Cu additive on MgB2 phase formation, processing temperature, and transport properties

Pure and nano-Cu doped MgB2/Fe superconducting wires were prepared by in situ powder-in-tube method at different temperatures (550–675 °C). The phase formation, microstructure, and transport critical current density of the wires were investigated as a function of the heat-treatment temperature. A small amount of nano-Cu addition (2.5 wt %) was found to dramatically decrease the reaction temperature of magnesium and boron, forming MgB2 without any degradation in the transport critical current. From x-ray diffraction and scanning electron microscopy analyses, it was found that the added Cu form a reacted phase Mg2Cu with Mg which melts at around 550 °C. This liquid phase helps the formation of MgB2 at a significant lower temperature with improved grain connectivity, grain size, and density. All Cu doped samples heat treated in the range of 550–650 °C exhibited a transport JC quite comparable to that of the pure sample processed at 650 °C, which shows that high quality MgB2 conductors can be produced even at...

Influence of reactivity of sheath materials with Mg/B on superconducting properties of MgB2

We investigated the influence of reactivity of commonly used sheath materials (Fe, SS, Cu, and Ni) with Mg/B, processed by in situ powder in sealed tube method on superconducting properties of MgB2. X-ray diffraction characterization of samples yields quantitative data of the volume fraction of MgB2 core, whereas magnetic measurements show the influence of added sheath materials on TC , JC(H), Hirr, and flux pinning behavior of MgB2. Our comparative study gives better phase purity, JC(H) characteristics, and flux pinning behavior for Fe and SS added samples, which shows the suitability of Fe and SS as sheath materials in MgB2 wire/tape fabrication.We investigated the influence of reactivity of commonly used sheath materials (Fe, SS, Cu, and Ni) with Mg/B, processed by in situ powder in sealed tube method on superconducting properties of MgB2. X-ray diffraction characterization of samples yields quantitative data of the volume fraction of MgB2 core, whereas magnetic measurements show the influence of added sheath materials on TC , JC(H), Hirr, and flux pinning behavior of MgB2. Our comparative study gives better phase purity, JC(H) characteristics, and flux pinning behavior for Fe and SS added samples, which shows the suitability of Fe and SS as sheath materials in MgB2 wire/tape fabrication.

Transport and magnetic properties of yttrium doped NdFeAs(O,F) superconductor

A.L. Solovjov, V. N. Svetlov, V. B. Stepanov, S. L. Sidorov, V.Yu. Tarenkov, A. I. D’yachenko, and A.B.Agafonov B.Verkin Institute for Low Temperature Physics & Engineering National Academy of Sciences of Ukraine, Lenin Ave. 47, 61103 Kharkov, Ukraine A.Galkin Institute for Physics & Engineering National Academy of Sciences of Ukraine, R. Luxemburg 72, 83114 Donetzk, Ukraine and Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, D-30167 Hannover, Germany (Dated: December 7, 2010)

On the current transfer length and current sharing in short length MgB2 wires

Current transfer length (CTL) and current sharing with the normal paths for a metal sheathed MgB2 superconductor are studied by analytical and numerical methods. The study was aimed at wire geometries with short sample lengths. The computations show low CTL values for Fe sheathed MgB2 and significantly higher CTL for Cu sheathed MgB2. For short length samples a considerable part of the applied current is not transferred into the superconducting layer and flows through the sheath in Cu sheathed conductors. Close to the current contacts the heat generated due to current sharing with normal paths is significantly higher for the Fe sheath than for the Cu sheath.

Rare Earth (RE - Ce, Gd) Modified Nd1-xRExFeAsO0.7F0.3 Superconductor with Enhanced Magneto-transport Properties

The influence of rare earth (RE – Ce, Gd) doping at the Nd site in the NdFeAsO0.7F0.3 superconductor wherein Ce and Gd have ionic radii in the order Ce > Nd > Gd is investigated. The structural and superconductivity characterization of the pure and doped samples show that Ce doping enhances the TC of Nd1−xRExFeAsO0.7F0.3 to a maximum of 53.6 K at x = 0.1 while Gd doping attains a TC of 55.1 K at x = 0.15. Interestingly, both Ce and Gd doping create neither secondary phases nor precipitates within the detection limit of XRD. However, the lattice defects due to Ce and Gd doping modifies NdFeAsO0.7F0.3 and assists in pinning the flux lines on these defects thereby exhibiting an enhanced JC(H) performance especially at high fields. It is also observed that the relatively small ionic size of Gd is more effective in TC enhancement, while the lattice defects due to larger ionic size of Ce favor the remarkable enhancement of JC(H).

An Effort Toward Development of

In this paper, we report an effort toward the development of general-purpose conduction-cooled current leads based on MgB2, which can carry 2000 A at 25 K. The leads have an overall length of 40 cm and a diameter of 5.8 mm, with Cu end caps with a length of 5 cm each for anchoring them to the superconducting system and the current source. Fe, Cu, and Ni are used as the barrier, the stabilizer, and the outer sheath materials, respectively. The critical parameters of the current leads, such as rated current Ic, thermal cycling stability, and heat leak, were tested and evaluated using a cryocooler-integrated cryostat test facility.

\hbox{MgB}_{2}

The effects of rare earth site doping on the structural, superconducting and magnetic properties of SmFeAsO0.7F0.3 iron pnictide are investigated. Gd3+ and Ce3+ ions are chosen by virtue of their position being on either side of Sm3+. Doping of both smaller (Gd3+) and larger (Ce3+) ions at the Sm3+ site increases the TC up to 55 K. Doping with the smaller Gd3+ results in lattice contraction and thereby enhances TC. It is interesting to observe that though Ce3+ doping in SmFeAsO0.7F0.3 exhibits an increase in lattice parameters, a substantial enhancement of TC occurs. The enhancement in TC due to increased charge carrier concentration is also confirmed using Hall Effect measurement. Apart from TC enhancement, the simultaneous doping of both at oxygen and rare earth sites prominently increases the superconducting properties such as JC, HC1 and HC2. The co-doped samples also exhibit better magnetic field dependence of JC over the entire field of study. It is also observed that the Ce3+ doped sample shows higher JC in the high field region due to its enhanced flux pinning properties.