T. Fukuzaki

Stabilization and coil performance of RHQT-processed Nb/sub 3/Al conductors

A Nb/sub 3/Al superconductor based on jelly-rolled (JR) Nb/Al composites of high performance can be fabricated by rapid-heating and quenching of a wire of such composites with subsequent transformation-annealing to form the Nb/sub 3/Al phase (RHQT process). Since the conductor thereby is heated up to 2000/spl deg/C, far above the melting point of Cu, the Cu stabilizer needs to be externally incorporated by a mechanical-cladding method between quenching and annealing. Such an appropriate deformation promotes the transformation from a disordered bcc phase, and thereby enhances the critical current density J/sub c/. A solenoid coil was fabricated by a wind-and-react method: the Cu-clad conductor (1.61/sup w//spl times/0.71/sup t/ mm/sup 2/) with Al/sub 2/O/sub 3/ yarn insulation was wound into a coil (19.7/40.8/sup /spl phi///spl times/49.7/sup H/ mm), transformed at 800/spl deg/C, and impregnated with beeswax. The resultant coil, while carrying a current of 179 A at 2.1 K in a superconducting back-up field of 21.2 T, generated an additional 1.3 T. The total magnetic field of 22.5 T is the highest record ever for a metallic superconducting coil.

Microchemical and microstructural comparison of high performance Nb/sub 3/Al composites

We have performed a comparison of the microstructures of state of the art Nb/sub 3/Al composites processed using both ordinary RHQT (rapid-heating, quenching and transformation) and TRUQ (transformation-heat-based up-quenching) routes. Cross-sections were examined in the as-quenched, untransformed, and final size strands including Cu-clad strand. Both grain size and microchemistry were examined using a high resolution FESEM in BEI mode, using low accelerating voltage for grain orientation contrast and high voltage for atomic number contrast. The grain size is relatively large in these composites with a Feret diameter of 1300 nm for the TRUQ processed strand (compared with 70 to 160 nm for Nb/sub 3/Sn composites). In the untransformed strand electron backscatter indicated residual chemical inhomogeneity associated with the jelly-roll precursor. In the final strands the variations were much less but longitudinal cross-sections revealed the residual chemical inhomogeneity extending along the strand length. In both the ordinary and TRUQ (Cu and Cu-clad) processed strands a 1 /spl mu/m thick 2-phase reaction layer was revealed on the outside surface of the outer filaments that had an average composition of 10% Al and 90% Nb. D.C. Magnetization measurements at 12 K indicated a 1 T improvement in irreversibility field, H/sup */, for the TRUQ strand compared with ordinary RHQT strands.

Effect of rapid heating, quenching and transformation conditions on the superconducting properties and microstructure of Jelly-Roll processed Nb3Al superconductors

A Jelly-Roll processed Nb3Al superconductor with excellent superconducting properties has been fabricated by the process of rapid heating, quenching and transformation (RHQT). In order to fabricate a long length of wire with homogeneous superconducting properties, the optimization of the RHQT conditions is particularly important because RHQT conditions determine the final microstructure and superconducting properties. In this paper, we have studied the variation of microstructure and superconducting properties with RHQ and transformation annealing conditions, to determine the optimum processing condition. In the study of the RHQ treatment, we found that the RHQ conditions are divided into four regions according to the joule heating current (IRHQ). In the optimum region, the wire is quenched from the temperature in which a bcc solid solution extends to more than 25 at%Al, and the scatter of critical current density (Jc) after transformation annealing is almost negligible. In the study of the deformation and transformation heat treatment, we have found that the deformation of the supersaturated solid solution improves the resultant superconducting properties, and prevents degradation of superconducting properties that occurs when the temperature ramp-up rate is slow.

Microstructure and n-value of RHQT processed Nb/sub 3/Al superconductors for NMR uses

We have examined the critical current I/sub c/ and n-values of RHQT processed Nb/sub 3/Al conductors at 4.2 K in a wide range of fields from 14.5 to 25 T in relation to the microstructures obtained. For a point and a tail of Cu-clad rectangular conductor used for a winding, the n-values were determined by fitting the voltage over a taps spacing of 110 or 220 mm to a relationship of V = CI/sup n/, in a range from 10/sup -5/ to 10/sup -4/ V/m. Such n-values were almost the same with each other, indicating the uniformity of a 370 m length of RHQT Nb/sub 3/Al conductor as the I/sub c/ consistency does. The n-value increased monotonously with decreasing a field from 25 T to 18 T, but got saturated to be around 35 at 4.2 K in fields less than 18 T. The n-value at 21 T and 4.2 K, corresponding to 23.5 T and 1.8 K, was about 25, and confirmed to be large enough for NMR uses.

Optimization of the TRUQ (Transformation-heat-based up-quenching) method for Nb3Al superconductors

The newly developed transformation-heat-based up-quenching (TRUQ) process causes an improvement of the stoichiometry of the A15 Nb3Al phase and an enhancement of the high-field Jc performance in Nb3Al superconductors. Jc properties have been optimized in the fabrication steps for TRUQ Nb3Al superconductors. The TRUQ process in the vicinity of 1000 °C gives the best high-field and low-field Jcs as well as Tc and Bc2. Jcs are not so sensitive to the holding time of the TRUQ process, which makes rapid cooling unnecessary. The TRUQ process can be well applied to conventional Cu-clad stabilized conductors. The Jc of 550 A mm−2 for the superconducting area is obtained at 23 T and 2 K; this is the highest measured value for A15-type superconductors. Thus, TRUQ Nb3Al conductors are very promising for high-field applications, e.g. GHz-class NMR magnets. Temperature profiles of the samples have been measured during the TRUQ process and it has been shown that TRUQ can be induced, even if the heating rate is less than 20 °C s−1. The TRUQ process may be suitable for practical coil fabrications.

Study of Joint Resistance in

We have developed a novel superconducting joint method for Nb3Al conductors, which can be used for the innermost coil of a 1-GHz NMR magnet. First, we optimized the joint property; then, we prepared a closed circuit with an NbTi coil and a pair of Nb3Al-NbTi joints; finally, we determined the joint resistance by using the field decay of the persistent current. A joint resistance of 1.27 times 10-12 Omega was obtained, which is sufficiently low for high field NMR.

{\hbox{Nb}}_{3}{\hbox{Al-NbTi}}

Heat treatment conditions and procedures of the transformation-heat based up-quenching (TRUQ) process particular to Nb/sub 3/Al coils are studied. It is found that pre-heating at 600/spl deg/C before TRUQ process makes the bcc phase ordered but does not cause transformation. Noticeable is that TRUQ process combined with such a pre-heating does not depress the J/sub c/ performance, compared with TRUQ process without pre-heating. Thus, pre-heating before TRUQ process can be used to help fast-heating of coils. In this work, a vacuum furnace having two heaters or a set of fluidized-bed furnaces are used for TRUQ process combined with pre-heating. Samples used are small coils where the conductor of several tens of centimeters in length is wound on a stainless-steel bobbin. It is found that both ways improve J/sub c/ characteristics, compared with the conventional transformation process. Additionally, untransformed short samples are ohmically heated as TRUQ process, controlling current or voltage of the power supply, to control the application of the ohmic-heating to provide a practical coil heat treatment.

Superconducting Joint for High Field NMR

Nb/sub 3/Ga wires fabricated by using RHQ (rapidly heating and quenching) treatment show T/sub c/ = 29.7 K and B/sub c2/ (4.2 K) = 32 T. In addition J/sub c/ of the Nb/sub 3/Ga phase is very high, comparable to those of practical (Nb, Ti)/sub 3/Sn wire and RHQT-processed Nb/sub 3/Al wire. However we cannot obtain the large overall J/sub c/ for the wire, because of small volume fraction of Nb/sub 3/Ga in the wire. By using Ta-matrix Nb/sub 3/Ga precursor wire, we performed the RHQ treatment at higher temperatures, resulting in two-times improvement of overall J/sub c/ and a little improvement in J/sub c/. We measured temperature dependence of J/sub c/ vs. B curve for the Nb/sub 3/Ga wire. The pinning force vs. B curves did not show any agreement with Kramers scaling law. TEM observations clarified that the composition of Nb/sub 3/Ga varies from near-stoichiometry (24at%Ga) nearby the Nb/sub 5/Ga/sub 3/ phase to Ga-poor composition (21.8at%) nearby the Nb-Ga bcc phase. We also measured the tensile strain effect on the superconducting properties of Nb/sub 3/Ga wire. The strain effect of Nb/sub 3/Ga wire is as large as that of Nb/sub 3/Sn wire.

TRUQ (TRansformation-Heat based up-quenching) for high-performance Nb/sub 3/Al superconductors

Rapid-heating, quenching, and transformation (RHQT)-processed Nb/sub 3/Al was found to provide high critical current density in high field by Cu addition. There seems to be a difference in the reaction path to A15 between binary Nb/Al (non-Cu-addition) and the Cu added ternary. In the binary case, a Nb/Al composite reacted to form a bcc phase through rapid-heating and quenching (RHQ) treatment and then it was transformed into A15 phase including stacking faults by an 800 /spl deg/C heat treatment. On the other hand, when Cu was added, the composite reacted to form A15 phase including stacking faults through RHQ treatment directly. We fabricated Cu added jelly-roll (JR) Nb/Al wire with various Nb/Al layer thicknesses and compositions and investigated their effect on RHQ process. The Cu added JR wire of Al poor composition for Nb/sub 3/Al stoichiometry showed a behavior similar to the binary. Al rich composition showed a behavior similar to the Cu-addition case. In this work we discussed the Cu addition effect by considering these results.

Development of RHQ-processed Nb/sub 3/Ga wire

Recently it has been reported that the critical current density of rapid-heating, quenching, and transformation (RHQT) Nb/sub 3/Al superconductor for high fields was vastly improved by Cu additions. That superconductor was made by rod-in-tube (RIT) process. In this work we tried to fabricate a Cu added Nb/sub 3/Al superconductor by jelly-roll (JR) process, which is one of practical processes of Nb/sub 3/Al superconductor fabrication. We fabricated a miniature sized billet and completed the drawing of multifilamentary wire of 1.10 mm in diameter and 41 m in length with no breakage. A sample subjected to the RHQT showed a core J/sub c/ (4.2 K) of 62 A/mm/sup 2/ at 24 T and exceeded the ordinary RHQT Nb/sub 3/Al JR superconductor (without Cu addition) in core J/sub c/ at high fields.