Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in-situ processed MgB2 strands

Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C and 700C for times of up to 71 hours and structure-property relationships examined. Drawing-induced elongation of the Mg particles led, after HT, to a textured macrostructure consisting of elongated polycrystalline MgB2 fibers separated by elongated pores. The superconducting Tc, Jc and Fp were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700C, the MgB2 reaction was more or less complete after as little as 30 min; at 600C, full reaction completion did not occur until 71 h. into the HT. Transport, Jct(B) was measured in a perpendicular applied field, and the magnetic critical current densities, Jcm\bot(B) and Jcm{\phi}(B), were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature dropoff of Jcm\bot(B) at fields well below the irreversibility field of Jct(B). This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, Jcm\bot(B), for current flowing transversely to the strand axis that was less than and dropped off more rapidly than Jct(B). In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of Jct(B) and Jcm{\phi} (B). In the present case the latter, leading to the premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing field. This result is supported by Kramer plots of the Jcm{\phi} (B) and Jct(B) data which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, Birr,t.Comment: 41 pages, 14 figure

Prospects for Improving the Intrinsic and Extrinsic Properties of Magnesium Diboride Superconducting Strands

The magnetic and transport properties of magnesium diboride films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Such performance limits are still out of the reach of even the best magnesium diboride magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsic (intragrain) superconducting properties of magnesium diboride, Hc2 and flux pinning, (2) factors that control the efficiency with which current is transported from grain-to-grain in the conductor, an extrinsic (intergrain) property. With regard to Item-(1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux-pinning additives is presented. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. These and other such results indicate that in many cases less than 15% of the conductor's cross sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield five-to ten-fold increases in Jc in relevant regimes, enabling the performance of magnesium diboride in selected applications to compete with that of Nb-Sn

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB2 coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB2/Cu tapes were fabricated over a wide temperature range of 460-520 {\deg}C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (Tc) ranging between 36 and 38 K with superconducting transition width (\DeltaTc) of about 0.3-0.6 K. The highest critical current density (Jc) of 1.34 \times 105 A/cm2 at 5 K under 3 T is obtained for the MgB2/Cu tape grown at 460^{\circ}C. To further improve the flux pinning property of MgB2 tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB2/Cu tapes, the MgB2 on SiC-coated Cu tapes exhibited opposite trend in the dependence of Jc with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB2 grain boundaries lead to strong improvement in Jc for the MgB2/SiC/Cu tapes. The MgB2/Hastelloy superconducting tapes fabricated at a temperature of 520 {\deg}C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher Jc values over the wide field range for MgB2/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The Jc values of Jc(20 K, 0 T) \sim5.8 \times 106 A/cm2 and Jc(20 K, 1.5 T) ~2.4 \times 105 A/cm2 is obtained for the 2-\mum-thick MgB2/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB2 conductors with high Tc and Jc values which are useful for large scale applications.Comment: 17 pages, 12 figures, accepted for publication in Current Applied Physics as a review articl

Growth and Characterization of Ce- Substituted Nd2Fe14B Single Crystals

Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux. Chemical and structural analysis of the crystals indicates that (Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements show that for x = 0.38 the saturation magnetization at 400 K, a temperature important to applications, falls from 29.8 for the parent Nd2Fe14B to 27.6 (mu)B/f.u., the anisotropy field decreases from 5.5 T to 4.7 T, and the Curie temperature decreases from 586 to 543 K. First principles calculations carried out within density functional theory are used to explain the decrease in magnetic properties due to Ce substitution. Though the presence of the lower-cost and more abundant Ce slightly affects these important magnetic characteristics, this decrease is not large enough to affect a multitude of applications. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content

2Flux growth and characterization of Ce-substituted Nd2Fe14B single crystals

Single crystals of (Nd1−xCex)2Fe14B, some reaching ∼6×8×8mm3 in volume, are grown out of Fe-(Nd, Ce) flux. This crystal growth method allows for large (Nd1−xCex)2Fe14B single crystals to be synthesized using a simple flux growth procedure. Chemical and structural analyses of the crystals indicate that (Nd1−xCex)2Fe14B forms a solid solution until at least x=0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements at 300 K show only small decreases with increasing Ce content in saturation magnetization (Ms) and anisotropy field (HA), and Curie temperature (TC). First principles calculations are carried out to understand the effect of Ce substitution on the electronic and magnetic properties. For a multitude of applications, it is expected that the advantage of incorporating lower-cost and more abundant Ce will outweigh the small adverse effects on magnetic properties. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a (LaFeO3)(8)/(SrFeO3)(1) superlattice film capped with four layers of LaFeO3. Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from Fe3+ in the bulk to Fe2+ at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen ©2016 American Physical Society1331sciescopu