The critical current density in industrial Nb 3 Sn and MgB 2 wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti for Nb 3 Sn wires or SiC or C for MgB 2 wires. In the following, several problems linked to the presence of additives in the two classes of compounds are discussed.A reinvestigation of the site occupancy of Ta and Ti additives in Nb 3 Sn wires shows that the Ta atoms occupy the 6c chain sites, while the Ti atoms are located on the cubic 2a sites. It follows that in perfectly ordered A15 compounds A 1−β B β , the relation ρ o versus β exhibits a 'universal' behavior: the effect of the chemical nature of the constituents on ρ o is negligible. The slopes of ρ 0 versus the Ti, Ga and Ni contents in the A15 layer coincide and are much steeper than for the Ta additive, corresponding to the three times higher number of 6c sites with respect to 2a A15 lattice sites.The presence of two grain morphologies, e.g. equiaxial and columnar, is observed in Nb 3 Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentary Nb 3 Sn bronze route wires is explained by the presence of these two different grain types, which have distinctly different Sn contents and sizes. For these wires, the total pinning force can be represented as the superposition of two contributions with different scaling fields.Simultaneous addition of different additives on 'in situ' Fe/MgB 2 wires is presented as an attempt to combine different possible mechanisms influencing J c . The substitution of boron by carbon is known to enhance the value of ρ o and thus of the critical field. In addition, the pinning behavior is expected to be improved by grain boundary effects or nanosize precipitations, caused by the presence of appropriate additives during the MgB 2 phase formation. Since the two mechanisms are independent, their effect on J c is expected to be cumulative. In the present paper, the results on the additive combination B 4 C + LaB 6 in monofilamentary Fe sheathed MgB 2 wires are reported. The data are compared with the additives B 4 C + SiC and show that simultaneous additives could be promising in view of applications at 20 K.
With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads.
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...
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