Kyoji Tachikawa

High‐field superconducting properties of the composite‐processed Nb3Sn with Nb‐Ti alloy cores

The Nb3Sn layer growth rate, the upper critical field and the critical current density in high fields of the composite‐processed Nb3Sn are greatly enhanced by the titanium addition to the niobium core. This addition yields a Jc (Nb3Sn layer) value of over 1×105 A/cm2 at 15 T. After the reaction, large amount of titanium is incorporated into the Nb3Sn layer. The optimal amount of titanium addition to the niobium core is about 2 at. %. The Nb‐Ti/Cu‐Sn composite wire seems to be quite promising for applications in magnetic fields up to at least 15 T.

Anomalous superconductivity in black phosphorus under high pressures

Abstract Pressure induced superconductivity in single crystals of black phosphorus has been studied. Maximum onset Tc was near 13 K. The anomalous superconductivity may be explained in terms of excitonic mechanism.

Composite‐processed Nb3Sn with titanium addition to the matrix

The effects of a titanium addition to the matrix on the superconducting properties and the metallurgical aspects of the composite‐processed Nb3Sn have been studied. The titanium added to the matrix is found to diffuse readily into the Nb3Sn layer. The small amount of the titanium addition to the matrix significantly increases the growth rate and the critical current density Jc in high magnetic fields of the Nb3Sn layer. A Jc(Nb3Sn layer) of over 1×105 A/cm2 has been obtained at 16 T for the Nb/Cu‐7 at. % Sn‐0.35 at. % Ti composite wire reacted at 750 °C for 100 h. The titanium addition to the matrix may be practically more promising than the titanium addition to the core.

High Tc superconducting films of Y‐Ba‐Cu oxide prepared by low‐pressure plasma spraying

A low‐pressure plasma spraying technique for depositing high Tc Y‐Ba‐Cu‐O thick films has been developed. Films with a thickness range of 20–100 μm have been prepared by using Y0.3Ba0.7CuOx powders. After post‐annealing in oxygen for 1 h at 950 °C, the films, which were deposited on a nimonic alloy substrate heated at 650 °C during spraying, exhibited a zero resistance temperature of 90.6 K with a transition width (90%–10%) of 2 K and a critical current density (77 K, 0 T) of 690 A/cm2.

Improvements of current‐carrying capacities of the composite‐processed Nb3Sn in high magnetic fields

The Nb3Sn growth rate Hc2 and Jc in high fields (⩾12 T) of composite‐processed Nb3Sn are greatly increased by the addition of hafnium to the niobium core. Furthermore, the simultaneous addition of hafnium to the core and gallium to the matrix more significantly enhances Hc2, and Jc in high fields. Jc of the Nb‐Hf/Cu‐Sn‐Ga wire specimens exceeds 1×105 A/cm2 at 17 T, suggesting the possibility of fabricating multifilamentary Nb3Sn composite wires that stand use in fields at least up to 17 T.

Effects of metal powder addition on the critical current in MgB2 tapes

Abstract MgB2 commercial powder was encased in a Ni sheath and fabricated into a monocore tape. Different kinds of metal powder, i.e., Ag, Ni, Cu, In and Sn, were mixed with the MgB2 powder. The addition of low melting point metal powder, especially In, has been found to enhance appreciably the Jc in MgB2 tapes. The annealing at a low temperature yields further increase in the Jc of tapes with In addition. The enhancement in Jc by the In addition and the low temperature annealing reaches a factor of five. The In addition may improve the linkage among MgB2 grains, which provides the improvement in Jc and avoids a sintering treatment at high temperatures. The optimum amount of In addition to the MgB2 core seems to be about 10 vol%. The effect of other metal powder addition is also described.

Improvements in current-carrying capacities of Nb 3 Sn composites in high fields through titanium addition to the matrix

Single-core and multifilamentary Nb 3 Sn composites with titanium addition to the matrix have been fabricated. The electron-probe microanalysis indicates that the titanium is more rapidly incorporated into the Nb 3 Sn layer from the matrix than from the core. The titanium addition of less than 1.5 at.% to the matrix does not deteriorate the workability of the Nb 3 Sn composites. The titanium addition to the matrix remarkably increases the growth rate and J c in high fields of the Nb 3 Sn layer. The optimum amount of titanium addition to the matrix to produce the highest overall J c at 16 T was found to be about 0.8 at.% for the 160-core Nb/Cu-7at.%Sn-Ti composite wires. The simultaneous titanium addition to the core and to the matrix produces further improvement in J c in high magnetic fields. An overall J c of about 2.7×104A/cm2at 16 T is obtained for the 370-core Nb-1.5at.%Ti/Cu-8at.%Sn-0.5at.%Ti composite wire reacted at 700°C for 200 h.

Effects of magnesium addition to the CuSn matrix in the composite-processed Nb3Sn superconductor

Abstract The effects of the addition of a small amount of magnesium to the CuSn matrix in the composite-processed (bronze-processed) Nb 3 Sn super-conductor were investigated. It was found that the magnesium addition significantly suppresses the Nb 3 Sn grain coarsening. The fine Nb 3 Sn grain structure produced in the Nb/CuSnMg composite enhances the grain boundary diffusion of tin and consequently makes the Nb 3 Sn growth rate nearly twice as much as that of the Nb/CuSn composite. The significant increase in superconducting critical current attained in the Nb/CuSnMg composite shows that the magnesium addition is promising for the improvement of the current-carrying capacities of multifilamentary Nb 3 Sn superconductors.

Nb 3 Al and its ternary A15 compound conductors prepared by a continuous liquid quenching technique

The Nb 3 Al, Nb 3 (Al,Ge) and Nb 3 (Al,Si) tapes have been successfully fabricated by a continuous liquid quenching using a high-speed moving copper, Cu, substrate tape heated at a proper temperature. A small amount of silicon, Si, and/or germanium, Ge, substituted for alminum, Al, improves the fluidity of molten alloy and facilitates the formation of the quenched alloy tape. The Nb 3 Al alloy with small contents of Si(Ge) showed a supersaturated bcc phase as quenched state. This bcc alloy was then transformed into a metastable stoichiometric A15 phase, which had an extremely small grain size of a few hundred angstroms. The increase in Si(Ge) contents in the alloy led to the direct formation of A15 phase from the molten state, whose grain size was about 1 μm. J c of the sample transformed from the bcc phase is much larger than that of the A15 phase directly formed from the molten state because of the finer grains in the transformed A15 phase. The highest J c at 20 T and 4.2 K of about 105A/cm2was obtained for Nb-25 at % Al-2 at %Si alloy annealed at 820°C for 3 h.

A15 Nb3(AlGe) superconductors prepared by transformation from liquid quenched body‐centered cubic phase

A supersaturated body‐centered cubic (bcc) solid solution of Nb3(Al1−xGex) up to x = 0.3 has been prepared by the liquid quenching on a hot Cu substrate. The quenched materials are then transformed to the A15 structure by annealing above 800 °C. Following the transformation, the critical temperature rises above 18 °K. The A15 phase converted from the bcc phase has an extremely small grain size of a few hundred angstroms, resulting in a high critical current density Jc in high magnetic fields. Jc of 3.5×105 A/cm2 at 4.2 °K and 16 T has been achieved in the Nb3(Al0.8Ge0.2) sample annealed at 850 °C for 7 h.