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Register a new userLarge field generation with Hot Isostatically Pressed Powder-in-Tube MgB2 coil at 25 K
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We present the fabrication and test results of Hot-Isostatic-Pressed (HIPed) Powder-in-Tube (PIT) MgB$_2$ coils. The coils properties were measured by transport and magnetization at different applied fields ($H$) and temperatures ($T$). The engineering critical current ($J_e$) value is the largest reported in PIT MgB$_2$ wires or tapes. At 25 K our champion 6-layer coil was able to generate a field of 1 T at self-field ($I_c >$ 220 A, $J_e sim 2.8 times 10^4$ A/cm$^2$). At 4 K this coil generated 1.6 T under an applied field of 1.25 T ($I_c sim350$ A, $J_e sim 4.5 times 10^4$ A/cm$^2$). These magnetic fields are high enough for a superconducting transformer or magnet applications such as MRI. A SiC doped MgB$_2$ single layer coil shows a promising improvement at high fields and exhibits $J_c > 10^4$ A/cm$^2$ at 7 T.
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The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by d.c. transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr ~ 17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of sub-grains within MgB2 crystallites.
We investigated the effect of nanoscale-C doping on the critical current density Jc and irreversibility field Birr of Fe-sheathed MgB2 tapes prepared by the in-situ powder-in-tube method. The tapes were heat treated at 600-950C for 1 h. Higher values of Jc and Birr were seen for 5 at.%C-doped MgB2 tapes at higher sintering temperatures, where substantial substitution of boron for carbon occurred. The C-doped samples sintered at 950C showed the highest Birr, for example, at 4.2 K, the Birr reached 22.9 T. In particular, at 20 K, Birr for the C-doped tape achieved 9 T, which is comparable to the upper critical field of the commercial NbTi at 4.2 K. This role of nano-sized C particles can be very beneficial in the fabrication of MgB2 tapes for magnetic resonance imaging applications at 20 K.
We demonstrate that Fe sheathed LaO0.9F0.1FeAs wires with Ti as a buffer layer were successfully fabricated by the powder-in-tube (PIT) method. Comparing to the common two-step vacuum quartz tube synthesis method, the PIT method is more convenient and safe for synthesizing the novel iron-based layered superconductors. Structural analysis by mean of x-ray diffraction shows that the main phase of LaO0.9F0.1FeAs was obtained by this synthesis method. The transition temperature of the LaO0.9F0.1FeAs wire is around 25 K. The micrograph shows a homogeneous microstructure with a grain size of 1-3 micrometers. The results suggest that the PIT process is promising in preparing high-quality iron-based layered superconductor wires.
We report dc transport and magnetization measurements of Jc in MgB2 wires fabricated by the powder-in-tube method, using commercial MgB2 powder with 5 %at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. By appropriate heat treatments, we have been able to increase Jc by more than one order of magnitude from that of the as-drawn wire. We show that one beneficial effect of the annealing is the elimination of most of the micro-cracks, and we correlate the increase in Jc with the disappearance of the weak-link-type behavior.
We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully fabricated by the powder-in-tube (PIT) method for the first time. Structural analysis by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs was obtained by this synthesis method. The transition temperature of the SmO0.65F0.35FeAs wires was confirmed to be as high as 52 K. Based on magnetization measurements, it is found that a globe current can flow on macroscopic sample dimensions with Jc of ~3.9x10^3 A/cm^2 at 5 K and self field, while a high Jc about 2x10^5 A/cm^2 is observed within the grains, suggesting that a significant improvement in the globle Jc is possible. It should be noted that the Jc exhibits a very weak field dependence behavior. Furthermore, the upper critical fields (Hc2) determined according to the Werthamer-Helfand-Hohenberg formula are (T= 0 K) = 120 T, indicating a very encouraging application of the new superconductors.
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