Hachio Shiraki

Fabrication of multifilament Nb3Sn conductors

Multifilament conductors have been fabricated using niobium tubes with a sheathed tin‐copper alloy inside and pure copper matrix outside. These composites have good workability, cold drawn without any intermediate heat treatment. Sample evaluation has included measurements of critical temperature, resistivity, and critical current density. The Nb3Sn formed by this method has much higher critical current density than the previous one with approximately the same tin concentration (12 at.%), but lies on the line extrapolated from the bronze method over the solid solubility limit.

Studies on superconducting Nb3Sn formed from high‐tin‐concentration Cu‐Sn alloy

Multifilament conductors have been fabricated using niobium tubes with a sheathed tin‐copper alloy inside and pure copper matrix outside. These composites have good workability, cold drawn without any intermediate heat treatment. Reactions and their effect upon superconducting properties have been checked for temperatures in the 625–800 °C range and various inner filament sizes ranging from 18 to 1154 μm. The Nb3Sn layer thickness grows as t0.8 in most of the reaction time, which cannot be explained by an ordinary diffusion process. The pinning force is found to be inversely proportional to the square root of the grain size measured through a fractographic technique in the 0.1–0.3‐μm range. Also, the activation energy of the Cu/Sn‐Cu diffusion couple is obtained as 47 kcal/mol from the reaction‐temperature–time relation. These results have been discussed and compared with others.

Strain effects of cable-in-conduit Nb3Sn conductors

Abstract Reduced sized cable-in-conduit Nb 3 Sn conductors were fabricated with various void fractions as a parameter, before fabricating full size conductors. The relation between critical current and bending strain was investigated. The Nb 3 Sn conductors had a 6 × 3 strand cable in a conduit with 0.61 mm diameter strands. The void fractions were changed from 30% to 50%. The conductor with ∼50% void fraction had a critical bending strain ( e bc ) of ∼0.9%, which was nearly equal to that for the strand, while conductors with void fractions below ∼40% behaved like a monolithic conductor with e bc below ∼0.5%.

Stress Effects on W/Cu Reinforced Nb3Sn Composite Conductors

Filamentary Nb3Sn wire has become increasingly important as a high-field conductor because of its high upper critical field and critical temperature, in developing large scale magnets such as those used in a fusion reactor, energy storage and electrical machinery. Compound Nb3Sn, however, has a disadvantage in that superconducting properties of Nb3Sn are rapidly degraded by tensile and bend stresses1,2,3 resulting from reeling and coiling the material and, by electromagnetic force generated when a coil is energized and by thermal contraction stresses due to cooling.

Influences of tensile and bending strains on the critical current in multifilamentary Nb3Sn composites processed by Nb tube method

Abstract The critical current degradation of multifilamentary Nb 3 Sn superconducting composites strained mechanically at room temperature has been investigated. The experimental results show that the effects of the Nb 3 Sn layer thickness and the specimen wire structure, such as monolithic and stranded cable on critical current degradation, are appreciable for the specimens strained by bending stress, but are not for specimens strained by tensile stress. The results of the critical current degradation by tensile strain were discussed, based on the stress-strain characteristics of the composites. It was clarified that the critical strain in the case of applying tensile and bending stresses simultaneously at room temperature lay around the line which was drawn from the point of the critical tensile strain to that of the critical bending strain, when the ordinate was tensile strain and the abscissa was bending strain.