The magnetic and transport properties of MgB 2 films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Carbon-doped films have exhibited upper critical fields, μ 0 H c2 , as high as 60 T and a possible upper limit of more than twice this value has been predicted. Very high critical current densities, J c , have also been measured in films, e.g. 25 MA/cm 2 in self field and 7 kA/cm 2 in 15 T. Such performance limits are still out of the reach of even the best MgB 2 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 MgB 2 --H c2 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 H c2 enhancement is discussed and examples presented. On the other hand their roles in increasing J c , both via H c2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of H c2 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 percent 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 five-to ten-fold increases in J c in relevant regimes, enabling the performance of MgB 2 in selected applications to compete with that of Nb 3 Sn. imaging. But to take the next step, whether we are interested in high field 4 K operation, or in the more energy efficient 20 K temperatures regimes well out of the range of the present NbTi and Nb 3 Sn wires, further improvement of the in-field J c is needed. In addressing this issue we must recognize two classes of properties (what might be termed "intrinsic" and "extrinsic", respectively) that are in urgent need of fundamental research. Below, we summarize some of the key issues that need to be addressed. Keywords Intrinsic PropertiesThe essential intrinsic (i.e.intragranular) properties of polycrystalline MgB 2 are H c2 and flux pinning (or, alternatively, intra-grain J c Necessary ...

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Since 2001, when magnesium diboride (MgB2) was first reported to have a transition temperature of 39 K, conductor development has progressed to where MgB2 superconductor wire in kilometer‐long piece‐lengths has been demonstrated in coil form. Now that the wire is available commercially, work has started on demonstrating a MgB2 wire in superconducting devices. This article discusses the progress on MgB2 conductor and coil development, and the potential for MgB2 superconductors in a variety of commercial applications: magnetic resonance imaging, fault current limiters, transformers, motors, generators, adiabatic demagnetization refrigerators, magnetic separation, magnetic levitation, superconducting magnetic energy storage, and high‐energy physics applications.

Three solenoids have been wound and with MgB 2 strand and tested for transport properties. One of the coils was wound with Cu-sheathed monofilamentary strand and the other two with a seven filament strand with Nb-reaction barriers, Cu stabilization, and an outer monel sheath. The wires were first S-glass insulated, then wound onto an OFHC Cu former. The coils were then heat treated at 675°C/30 min (monofilamentary strand) and 700°C/20 min (multifilamentary strand). Smaller (1 m) segments of representative strand were also wound into barrel-form samples and HT along with the coils. After HT the coils were epoxy impregnated. Transport J c measurements were performed at various taps along the coil lengths. Measurements were made initially in liquid helium, and then as a function of temperature up to 30 K. Homogeneity of response along the coils was investigated and a comparison to the short sample results was made. Each coil contained more than 100 m of 0.84-1.01 mm OD strand. One of the 7 strand coils reached 222 A at 4.2 K, self field, with a J c of 300 kA/cm 2 in the SC and a winding pack J e of 23 kA/cm 2 .At 20 K these values were 175 kA/cm 2 and 13.4 kA/cm 2 . Magnet bore fields of 1.5 T and 0.87 T were achieved at 4.2 K and 20 K, respectively. The other multifilamentary coil gave similar results. Keywords Strand FabricationThe continuous tube forming/filling (CTFF) process was used to produce Table 1. For further details on these multifilamentary strands, see [24]. Coil Winding, Heat Treatment, Epoxy ImpregnationThe former was solenoidal and made from OFHC Cu. The strands for all three coils were insulated with S-glass insulation. The coils had from 364 to 538 turns of strand, see Table 2. Cu-1 was HT for 675°C/30 min, while NbCu-7A and B were HT for 700°C/20 min. The ramp up time was 2.5 h and the ramp-down time was approximately 5-6 h, and all HT were performed under flowing Ar. Coils Cu-1 and NbCu-7A were vacuum impregnated with mixed Stycast 1266 epoxy heated to 40°C. NbCu-7B was merely dipped into degassed epoxy (40°C). After removal from the epoxy bath the coil curing was performed in air (at room temperature). Total curing time was estimated at 6-12 h. Coil Measurement and ResultsTransport properties of the coils were measured in a LHe cryostat (Figure 1 shows Cu-1 mounted and ready for insertion). The 4.2 K measurements were performed in liquid He, while higher temperature measurements were made as the coil warmed up.Two Cernox temperature sensors were mounted on the coil, one on the top and one on the bottom. The temperature difference across the coil was never greater than 0.3 K. Voltage taps were placed an various places along the winding. The typical distance between successive taps was about 14-20 m. The field was measured with a cryogenic hall probe and a Bell gaussmeter calibrated to achieve a 2% or better accuracy. The probe was inserted in the center of the bore during measurement. from the strands with a Nb-chemical barrier (Table 2), as might be expected. Strand and coil J e val...

We have used high-pressure, high-temperature synthesis at 1500–1700 °C and 10 MPa to create homogeneously C-substituted MgB2 from a B4C + Mg mixture. X-ray diffraction analysis showed large peak-shifts consistent with a decrease in the a lattice parameter for the B4C-derived MgB2 as compared to an undoped sample (0.033–0.037 Å, depending on the sample). Microstructural investigation showed a three-phase mixture in the B4C-derived ingots: MgB2−xCx (with 0.178 < x < 0.195), MgB2C2, and Mg. The carbon concentration determined from the lattice parameter shift (5.95 at. %) matched well with the calorimetrically derived concentration of 5.3–5.8 at. % C. Furthermore, the carbon content measured by electron probe micro-analysis was shown to be 6.2 ± 1.3 at. %. Finally, we performed bulk specific heat measurements to determine the homogeneity of C-doping in the MgB2. The width of the Tc distribution for the C-doped MgB2 was only 3–6 K with a full-width half maximum (FWHM) of 1.4 K, compared to a width of 2.5 K and a FWHM of 0.65 for an undoped sample. The consistency of these three measurements on a large-grained homogeneous material is unambiguously supportive of C-substitution.