Nb3Al has been considered as an alternative to Nb3Sn for high-field and large-scale applications, since an extremely high critical current density and excellent strain tolerance were demonstrated in Nb3Al by laboratory tests. Thus, there have been a great number of efforts in the exploration of new fabrication processes for Nb3Al. The processes explored can be classified into three groups: low-temperature processes (jelly-roll, powder metallurgy, clad chip extrusion, rod-in-tube), high-temperature processes (laser- or electron-beam irradiation), and transformation processes. This review describes Nb3Al conductors for each processing group, focusing on current topics relating to the rapid-heating, quenching and transformation method in which an excellent Nb3Al conductor can be obtained with a multifilamentary structure over several hundred metres in length.

We have measured the Hall resistivity for the intermetallic compound Fe 3Ϫx V x Al y ͑xϭ0.5-1.05; y ϭ0.95,1.05͒ at room temperature. The Hall coefficient changed its sign from positive to negative around the Heusler composition ͑i.e., xϭyϭ1͒ with increasing x value or decreasing y value. We have measured the temperature dependence of the Hall coefficient and the electrical resistivity for Fe 1.98 V 1.02 Al and quenched Fe 1.95 V 1.05 Al in the temperature range of 5-300 K. These two compounds showed very different behavior in the electrical resistivity but the behavior of the Hall coefficient was quite similar. At higher temperatures, the Hall coefficient showed a strong temperature dependence but it approached a constant value at low temperatures suggesting that these compounds are semimetals with a pseudogap. The charge carrier density was found to be less than a few tenths per unit cell. The rise of electrical resistivity at low temperatures is not owing to an energy gap but due to magnetic scattering while the negative temperature coefficient at high temperatures is attributable to pseudogap.

Abstruct-The influence of mechanical strain on the critical current (IE) is investigated for (Bi,Pb)zSrzCa2Cu30,(Bi-2223)/ Ag-0.2wt%Mg-O3wt%Sb superconducting tapes a t 77 K. The tensile axial strain along tape length is successfully induced to the sample by using a U-shape holder. Continuous change of the axial strain can be obtained by changing the distance between both ends of the holder. T h e U-shape holders made of Ti, SUS304, or brass are used to examine the effect of thermal strain due to the contraction caused by the cooling. F o r example, the results for the samples glued to Ti or brass holder are as follows.Strain dependence of normalized I, (I,/I,(at as-cooled state)) is affected by holder material. A steep decrease of I, occurs when we apply 0.1% and 0.3% strain for Ti and brass holder, respectively. T h e different thermal expansion (-0.15% for Ti and -0.37% for brass from 300 K to 77 K) explains it. All the results for normalized I, vs. strain relation fall on a master curve with taking into account of the effect of the thermal strain.Thermal expansion of the tape from 300 K to 77 K is measured to be -0.35% by using a strain gage. The critical strain ( , ) where a steep decrease of I, occurs is evaluated to be 0.27% from self-contracted state of the tape. Index Terms-CriticalCurrent, High-Temperature Superconductors, Strain Effect, Thermal Expansion.

Strain effects on critical current densities have been examined for conductors containing nearly stoichiometric Nb3Al filaments with fine grains. The Nb3Al phase in these multifilamentary conductors are prepared by phase transformation from supersaturated Nb(Al) bcc solid solution and show high-field critical current densities much larger than those for conventionally prepared Nb3Al conductors, where the Nb3Al phase is known to be off-stoichiometric. The degradation of critical current densities with −0.7% intrinsic strain is ca. 20% at 12 T, comparable with those for conventional Nb3Al conductors of high strain tolerance.

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The bcc supersaturated solid solution Nb(Al)ss obtained by rapid heating and quenching of a multifilamentary Nb/Al composite wire has shown a crystal structure change from a disordered to an ordered structure before transforming to the A15 Nb3Al phase. Such ordering of the bcc phase seems to be responsible for the A15 phase stacking faults that depress the critical temperature (Tc), the upper critical magnetic field (Bc2) and, hence, the critical current density (Jc) of Nb3Al in high fields. A heat treatment around 1000 °C, higher than conventional transformation temperatures by about 200 °C, suppresses the ordering and yields a new phenomenon termed the `transformation-heat-based up-quenching' (TRUQ). TRUQ is characterized by the self-heating of the bcc phase by the transformation heat, which propagates through the whole length of a composite wire and transforms it to Nb3Al. A subsequent annealing at 800 °C enhances the long-range ordering of the Nb3Al phase and drastically improves the high-field critical current densities of the Nb3Al conductors.