M. D. Sumption

Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor

Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length (∼ 2–4 nm for YBCO at temperatures ≤77 K) and when the areal number density of defects is of the order of (H/2) × 1011 cm-2, where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of ∼8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.

High-transport critical current density above 30 K in pure Fe-clad MgB2 tape

Abstract Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2 . The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2 , it confers on the finished wire the additional benefit of magnetic screening.

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 conductors 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.

Large upper critical field and irreversibility field in MgB2 wires with SiC additions

Resistive transition measurements are reported for MgB2 strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB2 [(MgB2)0.9(SiC)0.1]. Three different SiC powders were used; the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ0Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ0Hirr of 29 T and a Bc2 higher than 33 T.Resistive transition measurements are reported for MgB2 strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB2 [(MgB2)0.9(SiC)0.1]. Three different SiC powders were used; the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ0Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ0Hirr of 29 T and a Bc2 higher than 33 T.

A Round Table Discussion on

This paper summarizes a round table discussion on MgB2 held in the evening of September 20, 2005, during the 19th International Conference on Magnet Technology. The discussion, by the panelists and the audience, includes both material features and applications of MgB 2 as well as views on the commercial prospects for MgB2 magnets

rm MgB_2

Abstract Magnetization vs. applied field measurements ( M–H loops) were taken on short samples of YBa 2 Cu 3 O 7− δ (YBCO) thin films which were divided into narrow filaments. The YBCO was deposited using pulsed laser deposition onto single-crystal LaAlO 3 substrates, with a range of film thicknesses from 0.25 to 0.33 μm. Using a YAG laser, the thin films were patterned into linear striations by removing strips of the superconductor by laser ablation. The resulting striated filamentary structure serves to reduce the effective width of the YBCO films and hence the hysteresis loss in the superconducting samples. The magnetization measurements were taken over the temperature range of 4.2–77 K in applied fields of 0–17 kOe using a vibrating sample magnetometer. The measured hysteresis losses show a highly linear relationship between superconductor filament width and hysteresis loss as anticipated. However, the laser ablation process did result in the redeposition of YBCO along the edges of individual filaments. Degradation of T c and J c due to the ablation process is discussed.

Toward a Wide Market or a Niche Production?—A Summary

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

Hysteretic loss reduction in striated YBCO

A controlled introduction of second-phase Y 2 BaCuO 5 (211) nanoparticles into YBa 2 Cu 3 O 7 - δ (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >10 1 1 cm - 2 of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211 y nanoparticles/123 z )xN were deposited by pulsed laser deposition on LaAlO 3 substrates, with N bilayers = 24 to 100, y thickness = 1 to 2 nm, and z thickness = 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼1 μm).

High transport critical current density and large Hc2 and Hirr in nanoscale SiC doped MgB2 wires sintered at low temperature

In this paper, we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB2. It was found that iron is compatible with MgB2 and can be used as sheath material for fabrication of Fe-clad MgB2 wires. We show that at low temperatures the size of the pellet and its critical current density, Jc(H) – i.e. its M(H) – ensure low-field flux jumping, which of course ceases when M(H) drops below a critical value. With further increase of H and T the individual grains decouple and the M(H) loops drop to lower lying branches, unresolved in the usual full M(H) representation. After taking into account the sample size and grain size, respectively, the bulk sample and the grains were deduced to exhibit the same magnetically determined Jcs (e.g. 105 A/cm2, 20 K, 0 T) and hence that for each temperature of measurement Jc(H) decreased monotonically with H over the entire field range, except for a gap within the grain-decoupling zone.

Island growth of Y 2 BaCuO 5 nanoparticles in (211 ∼1.5 nm /123 ∼10 nm )× N composite multilayer structures to enhance flux pinning of YBa 2 Cu 3 O 7−δ films

YBa2Cu3O7−x (YBCO) films with nanoparticles of BaSnO3 (BSO) were processed using pulsed laser ablation of a special target made with dual phase sectors of YBCO and BSO. Transport critical current density (Jct) of these YBCO+BSO films in applied magnetic fields and angular dependence of Jct on the applied field orientation was measured. It was observed that in the YBCO+BSO films, the Jct (H‖c orientation) increased considerably as compared to regular YBCO films and was 1.3 times higher than Jct in H‖ab orientation. Cross-sectional transmission electron microscopy images on YBCO+BSO films showed the presence of high density (3.5×1011cm−2) of nanoparticles (∼10nm size) and nanocolumns that extended throughout the thickness of the films with high density of dislocations and stacking faults (1000μm−2). The observed results of enhancements in Jct in H‖c and Jct in H‖ab orientations were discussed in the light of the observed microstructural details.