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 ...

Resistive transition measurements are reported for MgB 2 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 MgB 2 [(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 minutes at temperatures from 675°C to 1000°C. Strands with 200 nm size SiC additions had µ 0 H irr and B c2 which maximized at 25.4 T and 29.7 T after heating at 800°C for 30 minutes. The highest values were seen for a strand with 15 nm SiC heated at 725°C for 30 minutes which had a µ 0 H irr of 29 T and a B c2 higher than 33 T.

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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.

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.

Transport critical current densities and n-values were measured at 4.2 K in fields up to 15 T on 7, 19, and 37-stack multifilamentary MgB 2 strands made using an in-situ route. Some strands included SiC additions (particle size ? 30 nm), while in others Mgrich compositions were used. Two basic multifilamentary variants were measured, the first had Nb filamentary barriers, the second had Fe filamentary barriers. All samples incorporated stabilizer in the form of Cu 101. Simple, one-step heat treatments were used, with temperatures ranging from 700-800?C, and times from 10-30 minutes. Transport critical current densities of 1.75 x 10 5 A/cm 2 were seen at 4.2 K and 5 T in 37 stack strands.

In a study of the influence of ZrB 2 additions on the irreversibility field, µ ο H irr and the upper critical field B c2 , bulk samples with 7.5 at. % ZrB 2 additions were made by a powder milling and compaction technique. These samples were then heated to 700-900 o C for 0.5 hours. Resistive transitions were measured at 4.2 K and µ ο H irr and B c2 values were determined. An increase in B c2 from 20.5 T to 28.6 T and enhancement of µ ο H irr from 16 T to 24 T were observed in the ZrB 2 doped sample as compared to the binary sample at 4.2 K. Critical field increases similar to those found with SiC doping were seen at 4.2 K.At higher temperatures, increases in µ ο H irr were also determined by M-H loop extrapolation and closure. Values of µ ο H irr which were enhanced with ZrB 2 doping (as compared to the binary) were seen at temperatures up to 34 K, with µ ο H irr values larger than those for SiC doped samples at higher temperatures. The transition temperature, T c , was then measured using DC susceptibility and a 2.5 K drop of the midpoint of T c was observed. The critical current density was determined using magnetic measurements and was found to increase at all temperatures between 4.2 K and 35 K with ZrB 2 doping. by an in-situ reaction of a stoichiometric mixture of 99.9 % pure Mg and amorphous B powders with a typical size of 1-2 µm. Powders were mixed in SPEX mill for 48 mins. KeywordsDoping was achieved by adding 7.5 mol% of ZrB 2 (from Alfa Aesar) prior to the mixing.Powders with SiC were doping were also fabricated for comparison. The milled powder was then compacted in the form of a cylindrical pellet in a steel die. These pellets were heat-treated in a steel holder, encapsulated in a quartz tube under 200 torr of Ar. Details of the sample composition, and heat-treatment are given in Table I. After sintering at temperatures of from 700-900 deg C for 0.5 hours, the cylindrical pellets were reshaped into cuboids for property measurements.Magnetization measurements were performed from 4.2 K to 40 K on these samples using a vibrating sample magnetometer (VSM) with field sweep amplitude of 1.7 T and a sweep rate of 0.07 T/s. Susceptibility, χ dc, vs. temperature, T measurements were performed using a 50 mT field sweep amplitude after initial zero field cooling.Magnetically determined critical current density, J c, at various temperatures was extracted from the magnetization loop using the Bean model 15 .At 4.2 K, µ o H irr , and B c2 were determined by resistive transitions with applied field, with the measurements being performed at the National High Magnetic Field Laboratory (NHMFL) Tallahassee. A four point resistance measurement was performed, using silver paste to connect the leads, and the distance between the voltage taps was 5mm. The applied current was 10 mA, and current reversal was used. All measurements were made at 4.2 K in applied fields ranging from 0 to 33 T. The samples were placed perpendicular to the applied field, values of µ 0 H irr and B c2 being obtained taking the 10%and 90%...

Critical current density was measured at 4.2 K for MgB 2 strands with and without SiC additions. In some cases measurements were performed on longer (1 m) samples wound on barrels, and these were compared to magnetic measurements. Most measurements were performed on short samples at higher fields (up to 18 T). It was found that in-situ processed strands with 10% SiC additions HT at 700-800°C show improved H r and F p values as compared to control samples, with H r increasing by 1.5 T. At 900°C even larger improvements are seen, with H r reaching 18 T and F p values maximizing at 20 GN/m 3 .