Michael Tomsic

Transport Critical Current Densities and

A series of high-performing continuous-tube-filling-forming-fabricated (CTFF) powder-in-tube-type (PIT) MgB₂ strands have been prepared, with critical current densities, J_cs, higher than 1.30×10⁵ A/cm² and n-values above 30 at 4.2 K, 5 T. The transport J_cs and corresponding n-values of a selected sample C1 were reported at temperatures, T, of 4.2-30 K and magnetic fields, B, of 0-12 T. The J_c - B data were fitted by Eisterers percolation model and the temperature dependences of the fitting parameters were analyzed. The n - J_c curves showed a power law relationship n∞J_c^m, also previously observed in state-of-the-art monocore MgB₂ wires. Further analysis revealed that, like monofilamentary wires, the n(B, T) of this multifilamentary strand can also be estimated for all B and T based on the results of transport voltage-current (V- I) measurements made at one arbitrarily chosen temperature.

n

Wires with 6 filaments of cores surrounded by a Nb barrier, and Cu, Monel and Glidcop sheath, drawn by Hyper Tech Inc. to diameters of 0.83 mm, have been treated in a Hot Isostatic Pressure process under high Ar pressures. The pressure of the HIP varied in different processes in order to find the optimal sintering conditions to obtain the highest at high magnetic fields. Also a stainless steel sheathed MgB2 wire has been used for comparison. The effect of sintering under high pressures on transport properties of those wires was measured by means of critical current and pinning forces Fp. The samples of 70 and 20 mm length were measured in parallel and perpendicular magnetic fields, respectively. High magnetic fields, of up to 14 T, were obtained in a Bitter magnet. The microstructure of annealed samples was investigated by SEM analysis.

-Values of Multifilamentary

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 (ρ), superconducting transition temperature (T_c), irreversibility and upper critical fields (H_irr and H_c2), and critical current density (J_c), as well as the pinning force (F_p) were evaluated. We found that the graphene doping has a positive impact on the above mentioned properties. In the case of the optimally doped (x = 3.0%) sample, the critical current density at 5 K corresponds to 1.4 × 10⁵ A/cm² for 2 T field, whereas the undoped sample showed 9.6 × 10⁴ A/cm² for the same field, i.e., 1.5 times improvement. Furthermore, the optimally doped sample showed a J_c of nearly 1 × 10⁴ A/cm² at 5 K, 8 T, which is a significantly high value. The upper critical field has been enhanced to 13 T at 20 K for the optimal doping level. The flux pinning behavior has been evaluated from the curve of flux pinning force against applied magnetic field, and it reveals that the maximum pinning has been improved by nearly 1.2 times at 20 K, due to the graphene doping.

\hbox{MgB}_{2}

Among key design and operation issues for MgB2 relevant to MRI magnets are: uniformity of current-carrying capacity over long lengths (>;2 km) of wire; and reliability of a splicing technique. This paper presents experimental results of current-carrying capacities of a small test coil and joints, both made from MgB2 round wires, multifilament and monofilament (mono), manufactured by Hyper Tech Research, Inc. The test coils were wound with 95-m long unreacted, C (carbon)-doped MgB2 multifilament wire, sintered at 700°C for 90 min. The critical currents were measured in the 4.2 K-15 K and 0 T-5 T ranges. We have modified our original splicing technique, proven successful with unreacted, un-doped multifilament wire sintered at 570°, and applied it to splice both un-doped and C-doped mono wires sintered at 700°C. Most consistently good results were obtained using the un-doped mono wires. Also presented are results of a small joint-coil-PCS assembly of mono wire, operated in persistent mode at 50 A at >;10 K.

Wires at Various Temperatures and Magnetic Fields

To reduce the usage of liquid helium in MRI magnets, magnesium diboride (MgB2), a high temperature superconductor, has been considered for use in a design of conduction cooled MRI magnets. Compared to NbTi wires the normal zone propagation velocity (NZPV) in MgB2 is much slower leading to a higher temperature rise and the necessity of active quench protection. The temperature rise, resistive voltage, and NZPV during a quench in a 1.5 T main magnet design with MgB2 superconducting wire was calculated for a variety of wire compositions. The quench development was modeled using the Douglas–Gunn method to solve the 3D heat equation. It was determined that wires with higher bulk thermal conductivity and lower electrical resistivity reduced the hot-spot temperature rise near the beginning of a quench. These improvements can be accomplished by increasing the copper fraction inside the wire, using a sheath material (such as Glidcop) with a higher thermal conductivity and lower electrical resistivity, and by increasing the thermal conductivity of the wires insulation. The focus of this paper is on the initial stages of quench development, and does not consider the later stages of the quench or magnet protection.

Superconducting Properties Comparison of SiC Doped Multifilamentary

High temperature superconductors such as MgB2 focus on conduction cooling of electromagnets that eliminates the use of liquid helium. With the recent advances in the strain sustainability of MgB2, a full body 1.5 T conduction cooled magnetic resonance imaging (MRI) magnet shows promise. In this article, a 36 filament MgB2 superconducting wire is considered for a 1.5 T fullbody MRI system and is analyzed in terms of strain development. In order to facilitate analysis, this composite wire is homogenized and the orthotropic wire material properties are employed to solve for strain development using a 2D-axisymmetric finite element analysis (FEA) model of the entire set of MRI magnet. The entire multiscale multiphysics analysis is considered from the wire to the magnet bundles addressing winding, cooling and electromagnetic excitation. The FEA solution is verified with proven analytical equations and acceptable agreement is reported. The results show a maximum mechanical strain development of 0.06% that is within the failure criteria of −0.6% to 0.4% (−0.3% to 0.2% for design) for the 36 filament MgB2 wire. Therefore, the study indicates the safe operation of the conduction cooled MgB2 based MRI magnet as far as strain development is concerned.

{\rm MgB}_{2}

In this paper, a series of high-performing powder-in-tube MgB₂ strands have been prepared. Transport voltage-current measurements were performed to determine the effects of C doping and strand geometry such as filament numbers. The best <i>J</i>_c for our samples was 1.0 × 10⁵ A/cm² at 4.2 K, 7 T, for a strand using B powder with 3% C addition. The current transfer length was also measured for MgB₂ short wires with Nb chemical barrier and Monel outer sheath. The current transfer length ranged from 2 to 12 mm, and had a correlation with the filament numbers.

Wires of Various Sheaths (Cu, Monel, Glidcop) After High Pressure HIP Treatment

The tubular technique for economical production of Nb3Sn material with large numbers of subelements is being explored further by Supergenics I LLC and Hyper Tech Research Inc. The number of subelements has been raised to 271 (246 + 25) by increasing the size at which the restacking is carried out. The product exhibited no fabrication problems and was drawn down and tested at a wire diameter of 0.42 mm, where the subelements are 18 mum in diameter. Heat treatment of a 271 subelement restack has been varied and a pre heat treatment of 48 h at 575degC followed by long times at 635degC has been shown to be advantageous. Restacks with 217 subelements have been subjected to both compositional and heat treatment changes and the best results, closely approaching those found on PIT materials, have been found on samples given 500 h at 625degC.

Improving Superconducting Properties of MgB

Multifilamentary, tube-type composites consisting of subelements of Nb with a simple Cu/Sn binary metal inserts were studied in an attempt to enhance the performance boundaries of these conductors. Transport J c values for these multifilamentary strands were found to be as high as 2440 A/mm2 at 12 T, 4.2 K for 217 strand conductors. We focused on correlating the composition and morphology of the intermetallic A15 to the transport and magnetic properties for varying heat treatments. In particular, lower temperature HTs were studied, specifically 625degC and 635degC as a function of time. The extent of A15, the ratio of the coarse/fine grain areas, and amount of the untransformed 6:5 were then observed as a function of time-temperature at these lower temperatures where the ternary phase diagram three-phase region boundaries have shifted from their 675degC positions. Compositional analysis was carried out on the A15 phase to study the variation in stoichiometry as a function of temperature. Fractography was also performed to investigate the effect of temperature and alloying on the morphology and grain size of the fine grain A15.

_{2}

MgB2 strands with various dopants and various types of B were characterized using transport Jc. In particular, amorphous, nanocrystalline, and plasma spray processed B powder based samples were compared. In a number of cases, various nanoparticle additions were then made to improve the properties with the given strand and B powder type. SiC, TiC, Ag nano ceramics were purchased from a commercial source and were then mixed into the wire during the milling stage. In one case, Malic acid was the dopant, and added at this milling stage. The doped powders were then dispensed into monofilamentary wires in which a Nb-lined monel sheath construction was used. Transport Jc measurements were performed on all samples under a variety of magnetic fields and in some cases temperatures. These results were then compared to our previous results with and without dopants.