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Unreacted MgB2 wires fabricated from SiC-doped precursor material by Hyper Tech Research, Inc. have been used to make small-diameter (14 mm) superconducting coils. All coils made of 500 mm length wires were subjected to hot isostatic pressure (HIP) treatment. The critical current density (Jc) parameters of coils were compared to straight samples characteristics. Both types of samples have been measured in perpendicular magnetic field configuration for Jc and pinning force density (Fp) evaluation. No significant Jc difference between the long wires on coils and straight wires was found. These results suggest that the critical current (Ic) for coils can be determined for straight samples (25 mm). SEM analysis indicated that a small diameter of the coil does not influence the structure of in situ MgB2 wire. [ABSTRACT FROM AUTHOR]

A prototype resistive superconducting fault current limiter (SFCL) was developed using single-strand round magnesium diboride (MgB2) wire. The MgB2 wire was wound with an interleaved arrangement to minimize coil inductance and provide adequate inter-turn voltage withstand capability. The temperature profile from 30 to 40 K and frequency profile from 10 to 100 Hz at 25 K were tested and reported. The quench properties of the prototype coil were tested using a high current test circuit. The fault current was limited by the prototype coil within the first quarter-cycle. The prototype coil demonstrated reliable and repeatable current limiting properties and was able to withstand a potential peak current of 372 A for one second without any degradation of performance. A three-strand SFCL coil was investigated and demonstrated scaled-up current capacity. An analytical model to predict the behaviour of the prototype single-strand SFCL coil was developed using an adiabatic boundary condition on the outer surface of the wire. The predicted fault current using the analytical model showed very good correlation with the experimental test results. The analytical model and a finite element thermal model were used to predict the temperature rise of the wire during a fault. [ABSTRACT FROM AUTHOR]

Raw MgB2 wires have been manufactured by Hyper Tech Inc. Each of their 18 cores contained undoped in situ powder surrounded by an Nb barrier and placed in a Cu sheath, and cores were coated in a common Monel sheath. Wires of diameters 0.63 and 0.83 mm were annealed under high argon pressure (up to 1 GPa). Parameters of such HIP process (temperature, time, and pressure) were varied in order to determine their optimal values. Superconducting properties of such samples were investigated by means of four-probe critical current j c measurement. Bitter magnet producing magnetic field up to 14 T was used. It provided place for samples 70 mm long in parallel field and 20 mm long in perpendicular field. Results include critical current j c and pinning force density F p dependencies on magnetic field as well as Kramer plots. Critical current of 104 A/cm 2 was achieved at 12 T magnetic field. SEM pictures of wire cross sections were also taken to determine quality of Nb barrier and microstructure of superconducting material. [ABSTRACT FROM PUBLISHER]

Monocore powder-in-tube MgB2 precursor strands were cold-drawn and heat-treated at 600 and 700 C for times of up to 71 h, and structure-property relationships examined. Drawing induced elongation of the Mg particles led, after heat treatment (HT), to a textured macrostructure consisting of elongated fine polycrystalline MgB2 fibers (or veins) separated by elongated pores. The superconducting transition temperature (Tc), critical current density (Jc) and bulk pinning force density (Fp) were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600 and 700 C. Critical current density and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700 C the MgB2 reaction was more or less complete after as little as 30 min; at 600 C full reaction completion did not occur until 72 h into the HT. Transport, Jct.B/, was measured in a perpendicular applied field, and the magnetic critical current densities, J? cm.B/ and J cm.B/, were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature drop-off of J? cm.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, J? cm.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 J cm.B/: in the present case the latter, leading to the premature drop-off of the measured J? cm.B/ compared to Jct.B/ with increasing field. This result is supported by Kramer plots of the J cm.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. [ABSTRACT FROM AUTHOR]

we report the synthesis and characterization of \MgB2 made from nano-boron and doped with graphene in the following mole percentages, x=0, 3.0 and 12.0. The effect of graphene doping on the normal state resistivity (\rho), superconducting transition temperature (Tc), irreversibility and upper critical fields (Hirr and Hc2), and critical current density (Jc), as well as the pinning force (Fp) were evaluated. [ABSTRACT FROM AUTHOR]

The synthesis of MgB-based materials under high pressure gave the possibility to suppress the evaporation of magnesium and to obtain near theoretically dense nanograined structures with high superconducting, thermal conducting, and mechanical characteristics: critical current densities of 1.8-1.0×10 A/cm in the self-field and 10 A/cm in a magnetic field of 8 T at 20 K, 5-3×10 A/cm in self-field at 30 K, the corresponding critical fields being H=15 T at 22 K and irreversible fields H=13 T at 20 K, and H=3.5 T at 30 K, thermal conduction of 53±2 W/(m⋅K), the Vickers hardness H=10.12±0.2 GPa under a load of 148.8 N and the fracture toughness K=7.6±2.0 MPa⋅m under the same load, the Young modulus E=213 GPa. Estimation of quenching current and AC losses allowed the conclusion that high-pressure-prepared materials are promising for application in transformer-type fault current limiters working at 20-30 K. [ABSTRACT FROM AUTHOR]

We have systematically studied the flux pinning behavior of MgB2 bulks synthesized by direct diffusion of Mg into pressed pellets of high purity crystalline B powder, with and without mixing with C and SiC nanoparticles, at a reaction temperature of 850°C for 10 hrs. All of the samples showed very high purity and high density, but their microstructure and flux pinning behavior showed significant differences. It was found that the pure MgB2 agrees with the δTc pinning model, nano-C doped MgB2 agrees with the 61 pinning model, while the SiC + MgB2 composite agrees with the δϵ pinning model (stress/strain field pinning), since the dominant micro-defects that influence the flux pinning in these three samples are different. [ABSTRACT FROM AUTHOR]

MgB2 samples were prepared using as-supplied commercial 96% boron with strong crystalline phase and the same 96% boron (B) after ball milling. The effects of the properties of the starting B powder on the superconductivity were evaluated. We observed that samples using ball-milled 96% B, in comparison with the one made from the as-supplied 96% B, were characterized by small grain size, broadened full width at half maximum (FWHM), and enhanced magnetic critical current density (Jc). reached 2 × 103 Acm-2 at 5 K and 8 T. The improved pinning of these samples seems to be caused by enhanced grain boundary pinning at high field. [ABSTRACT FROM AUTHOR]

A binary magnesium diboride (MgB2) cable has been assembled by braiding six Nb/Monel sheathed monofilament strands around a central copper stabilizer for improving the operational environment. The total critical current (Ic) of the braided cable is obtained by multiplying the Ic of six single wires, without any dissipation. In this work, various mechanical deformations, i.e., swaging, two-axial rolling, groove rolling, and cold high-pressure densification (CHPD) at 1.8 GPa have been applied to the 6-stranded cable to obtain additional densification. The highest critical current density at both 4.2 and 20 K has been achieved in this work through the CHPD treated cable due to higher filament mass density. The present results are promising in view of the cable, particularly in power applications at industrial lengths that pave the way to seeking an optimal protocol to meet a practical functionality. [ABSTRACT FROM AUTHOR]

Abstract: The critical current density, J c, of in situ MgB2 wires with C4H6O5 (malic acid) has been strongly enhanced by means of Cold High Pressure Densification (CHPD) at pressures up to 2.5GPa prior to reaction. An increase in J c by factors 2 and 8 was observed at 4.2 and 20K, regardless of the applied field. Densified wires were found to exhibit a higher homogeneity due to enhanced connectivity. In addition, a higher C content was found, as shown by the lattice parameter change, the decrease of T c, the shift of the calorimetric T c distribution and a reduced anisotropy in tapes obtained from the same wires. The higher C content in densified wires is responsible for the observed enhancement of B irr by more than 1T, up to 11T at 20K. Almost isotropic J c values were obtained for C4H6O5 added square wires of 1×0.6mm2 cross section, the values of J c(4.2K)=1×104 A/cm2 for fields // and ⊥ to the wider surface being obtained at 13.8 and 13.1T (1μV/cm criterion), the values for 20K being 6.4 and 6.2T, respectively. The method was successfully applied to mono- and multifilamentary wires with lengths up to several meters, suggesting applicability for industrial lengths. [Copyright &y& Elsevier]

Abstract: Resistive joints are found in many systems using superconductors. Joints are used to connect the superconductor to a normal conducting current terminal or to connect two superconductors, for example in pancake type windings. Knowing the resistance between the contacts is important in studying the heat balance of a superconducting system. We performed several experiments with MgB2–MgB2 lap joints to determine the relation between the contact resistance and solder joint length. Also, the effect of the outer sheath material on the contact resistance was studied. To support the experiments, a computational model using Finite Element Method was created. The measured and computed results showed adequate correlation. In the experiments, the soldered joint length was varied from 3 to 20mm resulting in contact areas between 2.5 and 16.6mm2. The results indicated that the outer sheath material has significant effect on the contact resistance. For a Monel sheathed conductor the measured contact resistances varied between 4 and 16μΩ and if a copper sheath was used, the resistances were an order of magnitude smaller. [Copyright &y& Elsevier]

MgB2 samples were prepared using as-supplied commercial 96% boron with strong crystalline phase and the same 96% boron (B) after ball milling. The effects of the properties of the starting B powder on the superconductivity were evaluated. We observed that samples using ball-milled 96% B, in comparison with the one made from the as-supplied 96% B, were characterized by small grain size, broadened full width at half maximum (FWHM), and enhanced magnetic critical current density (Jc). reached 2 × 103 Acm-2 at 5 K and 8 T. The improved pinning of these samples seems to be caused by enhanced grain boundary pinning at high field. [ABSTRACT FROM AUTHOR]