Search

The transport and magnetic properties of the undoped and yttrium doped NdFeAs(O,F) superconductor are investigated in detail for the first time. The transition temperatures of the yttrium doped samples are found to increase monotonically and attain a maximum of 53.3 K for 30% yttrium doping, ∼4 K higher as compared to the yttrium free sample. The T C values observed from the temperature dependent ac susceptibility measurements are also in agreement with the resistivity measurements. Yttrium doping is found to decrease the lattice parameters of the system; causing an inner chemical pressure and thereby accomplishing a T C enhancement. Moreover, the substitution of relatively smaller yttrium ion at Nd site creates lattice defects which act as effective pinning centers thereby enhancing the J C ( H ) performance of the system.

Ni sheathed multifilamentary MgB2 wires with Fe barrier and Cu stabilizer were prepared by the in situ Powder-In-Tube (PIT) method. After rolling, the ends of the wires were sealed by a simple capping technique and the wires were directly heat treated in air, without vacuum or any inert atmosphere. The quality of the wires was assessed by analysing the phase assemblage and measurement of superconducting properties such as R–T, JC–T and JC–H. Phase analysis revealed that only traces of MgO was formed in the superconductor core. Typical multifilamentary wires prepared by this method showed a TC ≈ 38.5 K and ΔTC ≈ 1 K and JC of the order of 105 A/cm2 at 6 K (0 T) and 104 A/cm2 at 4.2 K (6 T) respectively. These values are quite comparable with the values obtained for wires heat treated in inert atmosphere.

Bulk MgB 2 samples added with 0, 2, 5, 10 and 15 wt% of SiC nanoparticles were prepared by a powder-in-sealed-tube (PIST) method. XRD analysis shows the presence of secondary phase Mg 2 Si and some unreacted SiC as impurities in MgB 2 matrix in SiC added samples. The a axis length decreases with increasing SiC doping level which is mainly due to carbon substitution at boron site which causes local strains in MgB 2 lattice. From SEM analysis, it is found that as the addition level of SiC increases the sharpness of MgB 2 grain boundaries loses and more grains coalesce to form bigger molten-like grains. DC magnetization measurements show systematic variations in T C and Δ T C as SiC doping increases. The in-field critical current density [ J C ( H )] is highly enhanced for all doped samples and it is more for 10 and 15 wt% SiC added samples at higher fields. The uniformly dispersed nanosize intragrain impurities as seen in TEM and the lattice distortions caused by C substitution are the reasons for enhanced J C ( H ) and F P ( H ) at higher fields in doped samples.

A systematic study on the superconducting properties of polycrystalline MgB2 synthesized by in situ Powder-In-Sealed-Tube technique is carried out at different temperatures (750–900 °C). Both XRD and SEM results show well-crystallized MgB2 grains in all the samples and grain size is found to be increasing with the sintering temperature. Sharp superconducting transitions are observed for all samples, irrespective of sintering temperatures, which implies the high degree phase purity and homogeneity of MgB2 formed, while JC(H) plot gives sample dependent critical current density. The samples heat treated at relatively low temperatures show enhanced flux pinning and hence improved JC(H) performance. The reduced grain size and hence increased density of grain boundary pinning centers of MgB2 bulks synthesized at low temperature are mainly responsible for the enhanced flux pinning and JC.

The effects of starting Mg particle size on the reactivity of Mg with B, and on remnant Mg in in situ MgB 2 , and their influence on the superconducting system are studied. Samples were prepared by a powder-in-sealed tube (PIST) method, heat treated at 850 °C for 2 h in air and were characterized using XRD, SEM and magnetization measurements. As the particle size of Mg powder increases, residual Mg increases significantly. The MgB 2 prepared using smaller sized Mg powder does not have any residual Mg and show the best infield critical current density ( J C ( H )).

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.

A systematic study on the reactivity of commonly used sheath materials (Ag, Cu, Fe, SS, Ta and Ni) with Mg/B during in situ powder-in-sealed-tube processing of MgB2 superconductor was carried out at different temperatures. Characterization of the samples using XRD, SEM and EDS showed that the heat treatment temperature and type of metal sheath have profound influences on the reactivity. The quantitative data reported here are highly useful for choosing the right sheath material and processing temperature for the fabrication of high quality MgB2 based conductors.

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.

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.

Bulk MgB2 samples with 0, 1, 5 and 10 wt% of Tb4O7 nanoparticles added were prepared by a powder-in-sealed-tube (PIST) method. X-ray diffraction (XRD) analysis showed small quantities of TbB4, Tb2O3, Mg and MgO as impurities in the MgB2 matrix. No Tb4O7 is observed even for the 10 wt% addition. Scanning electron microscopy analysis showed a distinct microstructural change with the Tb4O7 addition. DC magnetization measurements showed a sharp superconducting transition in all the samples, without significant change in critical temperature (TC). The in-field critical current density (JC(H)) is improved for all the doped samples, and the JC(H) improvement in high fields is the highest for 10 wt% of Tb4O7 addition. The irreversibility field (Hirr) of the samples also improved with Tb4O7 addition. The improvement in JC(H) and Hirr of the added samples is due to the enhanced flux pinning consequent to the Tb4O7 addition.