Search

A comparative study on the structural and superconducting properties of MgB2 Powder-In-Tube (PIT) wires doped with various weight percentage of nano carbon (n C), nano SiC (n SiC) and Burned Rice Husk (BRH) has been carried out. X-Ray diffraction patterns of n SiC and BRH doped samples showed peaks of Mg2Si which is a useful intragrain inclusion for flux pinning. The Lattice parameter ‘a’ decreased with increasing doping concentration for all the doped samples, the maximum being for n C, indicating C substitution for B. The Full Width at Half Maximum (FWHM) of (hk0) peaks of all samples increased with doping. Doping caused a reduction in grain size and SEM images showed tightly packed smaller grains containing hexagonal crystallites in all the three types of doped samples. All the dopants reduced the transition temperature of the undoped sample and the effect was severe for n C doping. Unlike the pure sample, the transport critical current density (JC) of the doped samples didn't fall rapidly at high fields; but retained large fractions depending on the dopants. The best results were obtained for n SiC and BRH doped samples with BRH closely matching the performance of n SiC. Improvement in critical current density for these samples was an order higher than the undoped sample at 4.2 K and 8 T.

We have studied the impact of simultaneous Ba2+ doping at Sm3+ site (hole doping) and F- doping at O2- site (electron doping) on the structural and superconducting properties of Sm1111 system using a single source, BaF2. With BaF2 doping the extent of layering of the grains and the grain size have been improved considerably. High transport current of 875 A/cm2 at 12 K has been achieved for the doped sample. The binary doped sample shows higher TC of 51 K than the corresponding F-alone doped sample. In addition, substantially high values of HC2 and better JC-H behavior are observed for the BaF2 doped samples. These results strongly indicate the scope for SmFeAsO based conductors with higher current carrying capacity.

The influence of reactivity of different sheath metals such as Ag, Fe, Cu, Ni and SS on the superconducting properties of NdFeAsO0.6F0.4 wires has been investigated. The observed structural and superconducting properties of the metal sheathed wires conclude that among the five candidates Ag and Fe are technically compatible to be used as sheath materials in terms of reactivity. However, the best superconducting properties such as TC and transport JC were exhibited by Ag sheathed NdFeAsO0.6F0.4 wires.

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.

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).

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.

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.

MgB 2 bulk samples added with nano SiO 2 and/or nano diamond were prepared by powder-in-sealed-tube (PIST) method and the effects of addition on structural and superconducting properties were studied. X-ray diffraction (XRD) analysis revealed that the addition caused systematic reduction in ‘ a ’ lattice parameter due to the substitution of C atoms at B sites and the strain caused by reacted intragrain nano particles of Mg 2 Si as evinced by transmission electron microscope image. Scanning electron microscopy images showed distinct microstructural variations with SiO 2 /diamond addition. It was evident from DC magnetization measurements that the in-field critical current density [ J C ( H )] of doped samples did not fall drastically like the undoped sample. Among the doped samples the J C ( H ) of co-doped samples were significantly higher and the best co-doped sample yielded a J C , an order of magnitude more than the undoped one at 5 K and 8 T.

With the aim of improving flux pinning and in-field critical current density [JC(H)], two physically and chemically different additives – burned rice husk (BRH) and nano Ho2O3 were introduced into in situ MgB2 superconductor. The effects of the above two additives were studied individually and combinedly. Ho2O3 decomposed and reacted with B to form HoB4, without any substitution. BRH caused considerable amount of C substitution at B site and formed Mg2Si and Mg2C3 secondary phases. Addition of Ho2O3 improved the JC(H) only marginally, but BRH improved the JC(H) strongly. Combined addition of Ho2O3 with BRH was found to be much more effective than their solo addition for the enhancement of JC(H) of MgB2.

Iron-sheathed in situ MgB2 superconducting tapes with high densities were prepared by hot pressing of electrically self-heated powder-In-tube wires. The tapes were characterized for phase purity, density, microstructure and superconducting properties. It has been possible to achieve a core density of 2.45 g/cm3 (93.2% of theoretical density) and 2.4-fold increase in critical current for the best sample. The method can be transformed into a hot rolling process for large-scale production of long MgB2 conductors.

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.

The iron-pnictide Sm1−xCaxFeAsO1−2xF2x superconductor was prepared and the combined effect of electron and hole doping was studied in detail. It is observed that the binary doping using CaF2 improves the microstructure tremendously with a preferred orientation of the (00l) planes. Moreover, a maximum TC of 53.8 K and a transport JC of 880 A/cm2 (12 K), which is double to that of the F-doped sample, are achieved. The dopant CaF2 seems to be a potential candidate for solving the grain-connectivity concerns in iron-pnictides paving the way towards conductor development.

The effects of addition of nano SiO2 and/or nano diamond on structural and superconducting properties of MgB2 bulk samples were studied. The addition caused systematic reduction in ‘a’ lattice parameter. The in‐field critical current density [JC(H)] of doped samples showed a significant enhancement compared to the undoped sample. The codoped samples exhibited the best results.

We report the synthesis of Nd1−xCaxFeAsO1−2xF2x superconductors using CaF2 as a binary dopant that provides both holes and electrons by simultaneous substitution of Ca2+ ions and F− ions at Nd3+ and O2− sites, respectively. The sample with x = 0.2 exhibits a maximum critical temperature (TC) of 52.3 K and transport critical current density (JC) of 1240 A/cm2 at 12 K. An interesting feature observed for binary doped samples is the preferential alignment of (00l) planes and the refinement of microstructure both in terms of grain size and grain connectivity. NdFeAsO based superconductors are already known for their very high critical fields and field independent JC(H) performance. However, the development of the bulk superconductors with oriented large grains and good grain connectivity has not been reported so far and hence the present work will be definitely a step towards the progress in NdFeAsO based conductor development.

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.

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...

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.

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.