The magnetic and transport properties of MgB2 films present performance goals yet to be attained for powder-processed bulk samples and conductors. Carbon-doped films have exhibited upper critical fields, ?0Hc2, as high as 60?T and a possible upper limit of more than twice this value has been predicted. Very high critical current densities, Jc, have also been measured for films, e.g.?25?MA?cm?2 in the self-field and 7?kA?cm?2 in 15?T. Such performance limits are still out of reach for even the best MgB2 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 MgB2?Hc2 and flux pinning, and (2)?the 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 via direct fluxoid/pinning center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux pinning additives is presented. Dopant selection, chemistry, methods of introduction (inclusion), and homogeneity of distribution (via the rounding of the superconducting electronic specific heat transition) are considered. 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. Overall porosity, including reduced density and intergranular blocking, is quantified in terms of the measured temperature dependence of the normal-state resistivity compared to that of a clean single crystal. Several experimental results are presented in terms of per cent effective cross-sectional area for current transport. 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 fivefold-to-tenfold increases in Jc in relevant regimes, enabling the performance of MgB2 in selected applications to compete with that of Nb3Sn.

https://iopscience.iop.org/article/10.1088/0953-2048/21/10/103001/pdf

Prospects for improving the intrinsic and extrinsic properties of magnesium diboride superconducting strands

TOPICAL REVIEW: MgB 2 thin films

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.

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

Great progress has been made in the deposition of MgB2 thin films since the discovery of superconductivity at 39 K in this two-band superconductor In this review, deposition techniques used for MgB2 are described, with an emphasis on their meeting the critical requirements for growing high-quality MgB2 thin films The availability of these thin films has contributed significantly to the research on the two-gap superconductivity in MgB2 MgB2 thin films have also been investigated for applications in high-field magnets, superconducting electronic devices and circuits, and radio-frequency cavities used in accelerators

https://iopscience.iop.org/article/10.1088/0953-2048/22/4/043001/pdf

MgB2 thin films

We report an in situ heat-treatment technique for the preparation of near-fully-dense un-doped MgB2 bulks that also provides very strong in-field pinning. The high density was achieved without using high-pressure apparatus. The heat-treatment of compacted boron sealed in a Ta tube with Mg pellets employs a short high-temperature sintering at 1100 °C first, followed by a low-temperature annealing below 660 °C. A high density of 2.5 g cm−3 (95% of the theoretical density) was achieved in the bulks treated by the two-step process. The in-field Jc is nearly one order of magnitude higher than for the samples prepared by single-step sintering at high or low temperature. Microstructural analysis suggested a unique feature of well-connected small grains with a high level of disorder in the MgB2 samples created by the two-step process.

https://iopscience.iop.org/article/10.1088/0953-2048/21/3/032004/pdf

Fabrication of highly dense MgB2 bulk at ambient pressure

Significant differences in superconducting and microstructural properties between two types of MgB/sub 2/ films prepared by pulsed laser deposition were determined. A very high H/sub c2/-T slope of 1.1 T/K was achieved in the in situ film. The J/sub c/-H curves of the in situ film also show a much weaker field dependence than that of the ex situ film. The magneto-optical (MO) images show that at 4 K the flux penetrates the in situ MgB/sub 2/ film through random paths, while for the ex situ film, the flux penetration pattern is mostly repeatable, indicating a defect-controlled flux penetration. Microstructural study (transmission electron microscopy and atomic force microscopy) revealed a relatively big grain size in the ex situ film. The correlation between the superconducting properties, microstructure and preparation conditions is discussed with regard to the two types of films.

/pdf/superconducting-and-microstructural-properties-of-two-types-5bf7eoye2q.pdf

Superconducting and microstructural properties of two types of MgB/sub 2/ films prepared by pulsed laser deposition

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Superconducting and microstructural properties of two types of MgB/sub 2/ films prepared by pulsed laser deposition