Search

Fabrication of a V3Si multifilamentary superconductor comprising a high overall Jc is possible through a process involving modified bronze. Overall Jc has 10 times the value of conventional V3Si wire values. The values are 1 times 10 to the 18th power A/square meter at 12.5 T and 1.3 times 10 to the 9th power A/square meter at 5T. Orthonucleic constants may also influence the Tm3+ ion, which is already influenced by a tetragonal crystal field.

Deals with a study which investigated improvements in high-field superconducting performance of V[sub3]Ga by a two-stage reaction process. Preparation of the V[sub3]Ga specimens; Findings.

Abstract: Jelly Roll (JR) processed Nb3Sn wires have been fabricated from laminated composites of Sn-based alloy and Nb sheets. The wires show an offset Tc of 18.1K with a transition width of less than 0.1K, and an offset Bc2 (4.2K) of 26.5T. The wire keeps Bc2 of ∼13T at 12K. Large non-Cu Jc values have been obtained at 4.2K and above 21T. Relative sheet thickness of the Sn-based alloy and Nb yields significant effect on the structure and Jc of the JR wire. [Copyright &y& Elsevier]

The development of a large-aperture (120–180 mm) dipole magnet is proposed in the framework of the CERN-KEK cooperation program. The application target is the D1 magnet (separation dipoles) replacement for the HL-LHC (High Luminosity—Large Hadron Collider) upgrade. The Cos-theta type coil cross section and the shell-based structure are adopted in the conceptual design of this magnet. The nominal field is estimated to be 6–10 T at 1.9 K with a 30-mm-width coil arranged in two layers. The candidates of superconductor are \Nb3\Al, \Nb3\Sn and Nb-Ti. We present the analytical estimation of the key parameters of this magnet and the simulation of the actual design which includes the field quality, the stray field, and the stress distribution in the coil. [ABSTRACT FROM PUBLISHER]

The critical current of a Nb3Al wire (RHQT process) with a rectangular cross section of 1.81\times 0.80\ mm^2 has been measured at 4.2 K under transverse compressive loads at fields between 15 T and 19 T. The same wire (HE2432) was previously used for winding an insert coil at NIMS (coil #B) that generated 4.5 T in a background field of 15 T at 4.2 K. In this work the load is applied perpendicularly to the narrow side of the conductor and the corresponding stresses reach 300 MPa. The critical current stays almost constant up to 150 MPa and shows a flat maximum around 75 MPa. At 300 MPa the critical current is reduced to 93%, 90% and 88% of its stress free value at 15 T, 17 T and 19 T, respectively. This indicates a rather small field dependence and significant degradation being seen only above 200 MPa. Depending on the field, unloading from 300 MPa yields to an almost complete recovery of Ic between 94% and 97% of its initial value, respectively. These observations are compared to the behavior of a Nb3Sn bronze wire. [ABSTRACT FROM AUTHOR]

Research and development of Nb3Al strands and cables for a high field accelerator magnet is ongoing under the framework of the CERN-KEK collaboration. In this program, new Ta-matrix Nb3Al strands were developed and their mechanical properties and superconducting properties were studied. The non-Cu Jc values of these strands were 750\sim 800\ \A/mm^2 at 15 T and 4.2 K. Using these strands, test fabrication of 27-strand Rutherford cable was carried out in collaboration with NIMS and Fermilab. The properties of the strands extracted from the cable were examined and it was found that there was no degradation of the superconducting properties of the strands. In this paper, we report the fabrication of the strands and the cable in brief and present some of the results obtained by studying their properties. [ABSTRACT FROM AUTHOR]

Sn-based sheets with Ta, B or Nb particle dispersion have been prepared from consolidated alloys. A small amount of Ti addition strengthened the bonding between dispersed particles and the Sn matrix, which improved the workability of the sheet. The sheet was laminated with a Nb sheet, and wound into a Jelly Roll (JR) composite. The composite was easily fabricated to a wire without intermediate annealing. Thick and uniform \ Nb3{{\ {Sn}}} layers with nearly stoichiometric Sn concentration were synthesized by the mutual diffusion between Nb and Sn-based alloy layers. An offset Tc of \sim18.1 K and an offset Bc2 (4.2 K) of \sim26.5 T have been obtained after the heat treatment. The non-Cu Jc of the wires increased with decreasing the wire diameter. A non-Cu Jc of \sim \ 150 A/mm^2 has been obtained at 4.2 K and 22 T. The Sn–Ta, Sn–B and Sn–Nb based sheet JR wires exhibited similar microstructures and superconducting performances. [ABSTRACT FROM AUTHOR]

The minimum quench energy (MQE) and normal zone propagation velocity (NZPV) of three kinds of Nb3AI superconductors fabricated by the rapid heating, quenching and transformation (RHQT) process were measured under various conditions of applied magnetic field (10-14 T), temperature (7-11 K), and transport current (80-95% of the critical current), while cooled by a cryocooler for developing the over 20-T class cryogen-free magnet. As a result, MQE values were related to the critical current density (Jo); high MQE was obtained for low Jc. It is assumed that Jc has a stronger influence on the MQE than specific heat, thermal conductivity, resistivity, and other parameters of the composite superconductor including the matrix and the stabilizer. NZPV was mainly proportional to the transport current density varying with applied field and temperature. The second contribution to NZPV is assumed to be heat capacity depending on the wire configuration. [ABSTRACT FROM AUTHOR]

We have reconsidered V3Ga compound wire for future fusion reactor application. 14 MeV fast neutrons will be formed during the Deuterium-Tritium fusion reaction, consequently future fusion reactors will be constructed using low activation materials wherever possible. V3Ga compound has a short radioactive decay time and it will be one of the candidate superconductors with both low activation and high field proper- ties so called "Low activation superconducting materials". The present critical current density, Jc, is insufficient for large fusion magnet. We investigated a new PIT process, using high Ga content compound, in order to improve superconducting properties. The upper critical field, Hc2, of the samples using high Ga content Cu-Ga compounds, was increased with increasing Ga content, and measured 22.5 T when was used 50 at % Ga compound powder. Moreover, the effect of Mg addition into the powder filament was also investigated. The small amount of Mg addition within 1 at %, was improved Hc2 to about 23.0 T. The Ga content dependence and the Mg addition effect on the superconducting properties in the new PIT process are reported in this paper. [ABSTRACT FROM AUTHOR]

We attempted to reduce the matrix ratio down to 0.5 and 0.25 for RHQT (rapid-heating, quenching and transformation) Nb3 Al wires, which is directly related to improvement of the non-Cu Jc. The Ta matrix was chosen to improve the wire strength. It was found that the matrix ratio of 0.25 is too small to carry out RHQ treatment although drawing was possible by using cassette-roller dies (CRDs) as well as conical dies. The reduction of the matrix ratio down to 0.5 can also be achieved. However, the scale-up would make the drawing very difficult. It would be necessary for the scale-up to improve adhesion among the jelly-roll (JR) cores and also the outer Ta sheath in the multi-billet. The RHQ treatment was possible for the wire with a matrix ratio of 0.5. However, it was successful only when the wire diameter was less than 1.13 mm. It is necessary to suppress the experienced bend strain in the operation. The non-Cu Jc was found to be improved by about 20% by the reduction of the matrix ratio, compared to conventional Nb3Al wires. [ABSTRACT FROM AUTHOR]

Two Cu stabilized Nb3Al strands, F1 (Nb matrix) and F3 (Ta matrix), fabricated at NIMS were wound into Rutherford cables at Fermilab in collaboration with NIMS. A Small Race-track magnet using F! Rutherford cable, the first Nb3Al dipole magnet in the world, was constructed and tested to full current at Fermilab. This magnet was tested extensively to full short sample data and its quench characteristics were studied and reported. The 3-D magnetic field calculation was done with ANSYS to find the peak field. The quench characteristics of the magnet are explained with the characteristics of the Nb3 Al strand and Rutherford cable. The Small Racetrack magnet built using Ta matrix F3 strand was constructed and will be tested in the near future. The advantages and disadvantages of these Nb3Al cables are briefly discussed. [ABSTRACT FROM AUTHOR]

In the development of RHQT-processed Nb3Al wires for high-field accelerator magnet applications, one of the major objectives is to increase the non-copper critical current density in the field range of 12-15 T. To pursue this goal, a low-matrixratio wire was fabricated, and the effects of the RHQ conditions and the area reduction after the RHQ treatment on the properties of the wire were investigated. This paper describes the details and results of this study. [ABSTRACT FROM AUTHOR]

Characteristics of recently developed Nb3Al strand though the RHQT (rapid heating/quenching and transformation) process are described. One kilometer-long copper stabilized Nb3 Al round strands were economically fabricated with ion-plating and electroplating. The copper electroplating was successfully done with the high velocity of 5 m/h. A strong bonding between the copper and the precursor was achieved, and the mechanical rolling test did not show the separation of the copper stabilizer from the precursor. The rolled Nb3 Al strands showed no degradation both in critical current density and RRR value. Magnetic instability at 4.2 K at low fields was apparently improved on the recent F3 strand, relative to the previously F1 stand because a tantalum was used for the interfilament matrix of precursor. The large magnetic flux jumps, which were observed with the F! strand, were suppressed at 4.2 K, although appeared again below 3.0 K. The twisted strands are shown to be effective to reduce the time-dependent eddy current coupling. It was obtained 48 μm of Deff on F3 strand with twist pitch of 45 mm. In addition, the transverse pressure test for the F1 Nb3 Al strand was performed at 4.2 K and 12 T. The critical, current did not degrade with the transverse pressure up to 210 MPa. [ABSTRACT FROM AUTHOR]

In conventional superconductors, the superconducting gap in the electronic excitation spectrum prevents scattering of low-energy electrons. In high-temperature superconductors (HTSs), an extra gap, the pseudogap, develops well above the superconducting transition temperature TC. Here, we present a new avenue of investigating the pseudogap state, using scanning tunnelling microscopy (STM) of resonances generated by single-atom scatterers. Previous studies on the superconducting state of HTSs have led to a fairly consistent picture in which potential scatterers, such as Zn, strongly suppress superconductivity in an atomic-scale region, while generating low-energy excitations with a spatial distribution—as imaged by STM—indicative of the d-wave nature of the superconducting gap. Surprisingly, we find that similar native impurity resonances coexist spatially with the superconducting gap at low temperatures and survive virtually unchanged on warming through TC. These findings demonstrate that properties of impurity resonances in HTSs are not determined by the nature of the superconducting state, as previously suggested, but instead provide new insights into the pseudogap state. [ABSTRACT FROM AUTHOR]

The nature and behaviour of electronic states in high-temperature superconductors are the centre of much debate. The pseudogap state, observed above the superconducting transition temperature, Tc, is seen by some as a precursor to the superconducting state. Others view it as a competing phase. Recently, this discussion has focused on the number of energy gaps in the system. Some experiments indicate a single energy gap, implying that the pseudogap is a precursor state. Others indicate two, suggesting that it is a competing or coexisting phase. Here, we use temperature-dependent scanning tunnelling spectroscopy of (Bi1−yPby)2Sr2CuO6+x to clarify the situation. We find a previously unobserved narrow and homogeneous gap that vanishes near Tc, superimposed on the typically observed inhomogeneous and broad gap, which is only weakly temperature dependent. These results not only support the two-gap picture, but also explain previously troubling differences between scanning tunnelling microscopy and other experimental measurements. [ABSTRACT FROM AUTHOR]

The advantages of MgB2 compound for a future fusion reactor application are not only high Tc (39 K) and low cost but also low activation material. MgB2 compound is non Nb-based superconductor and its decay time of induced radioactivity may be about 10 years and significantly shorter than Nb-based superconductors. In this paper, we fabricated the Mg2Cu doped MgB2 wires using several metal sheath materials to improve Jc property. The selection of metal sheath materials is also important to apply for fusion reactor. Pure Fe, Ta and Cu have faster decay time of induced radioactivity compared the other metals. Tc values of all Mg2Cu doped samples were higher than those of non-doping samples, and Tc values were improved with increasing amount of Cu doping. And the maximum core-Jc value was estimated to be 100 A/mm2 over at 4.2 K under 8 T for 3 at% Cu doping sample. The comparisons of superconducting property between different metal sheath materials are discussed. [ABSTRACT FROM AUTHOR]

Long Nb3Al strands with copper stabilizer are promising for future high field accelerator magnets. A 1.2 kilometer Nb3A1 strand with Cu stabilizer was fabricated at the National Institute for Materials Science in Japan. Using this strand a 30 meter Cu stabilized Nb3Al Rutherford cable was made for the first time by a collaboration of NIMS and Fermilab. The Nb3Al strands extracted from cable with a relatively low packing factor showed almost no Jc degradation. But the extracted strands from the highly compacted cable showed some degradation in both Jc and n value, which may be caused by local separation of the copper stabilizer. Still, its Jc degradation is lower than that of typical Nb3Sn strands. The current limit due to magnetic instability in low field is about 500 A at 4.2 K. The magnetization of the strands, which was measured with balanced coils at 4.2 K, showed large flux jumps, usually around 1.5 T. This value is much larger than the Bc2 (4.2 K) of the Nb matrix, which is around 0.4 Tesla. The magnetic instability of the Nb3Al strand at low field is not completely understood, but it might be explained by the superconducting coupling current through the Nb matrix. [ABSTRACT FROM AUTHOR]

Tightly shrunk Sn-Ta ingots with good workability were prepared by the reaction at 750°C–800°C from the mixed powder of Sn and Ta containing 25–30at%Ta. Then Jelly Roll processed wires were fabricated starting from Sn-Ta/Nb composites in which thick (Nb, Ta)3Sn layers were produced after the heat treatment. A few wt% Cu addition to the Sn-Ta mixed powder reduces the heat treatment temperature. Moreover, a few at% Ti substitution for Sn + Ta enhances the formation of (Nb, Ta)3Sn layer, and improves the non-Cu Jc of the wire. In the present study, effect of the Sn-Ta-Ti-Cu sheet composition on the structure and high-field performance of the wire has been studied. It has been found that a small amount of Ti and Cu addition decreases the amount of Ta in the sheet from 25–30at% to 10at%, which is favorable for the material cost reduction. Moreover, it has been revealed that thick (Nb, Ta)3Sn layers are synthesized even if the Cu concentration in the sheet is decreased less than lwt% when a few at% Ti is substituted. A non-Cu Jc of ∼ 150 A/mm2 has been obtained at 22 T and 4.2 K in the wires with different sheet composition after the heat treatment at 725°–750°C. [ABSTRACT FROM AUTHOR]

The effects of α-Ti depositions, caused with the heat treatments at 300–5000°C, were studied for various V-Ti alloys through measurements of Tc, Ic, and X-ray diffraction patterns. Ic increased with the α-Ti depositions for the V-Ti alloys with various Ti contents. The α-Ti depositions are very effective pinning centers in V-Ti alloys, being similar to those in Nb-Ti alloys. The maximum Tc of 8.2 K was obtained for the V-40at%Ti alloy annealed at 500°C for 2 hr, while the maximum Birr (4.2 K) of 8.5 T was obtained for the V-(50–60) at%Ti alloys. Jc (4.2 K, and 6.5 T) of about 170 A/mm2 was obtained for the V-45at%Ti alloy annealed at 500°C for 4 hr. We also fabricated the V-Ti alloy multifilamentary wires in order to stabilize the alloy superconductors. The final cold-drawing after heat treatment at 400–450°C is effective on the improvements of Tc, Birr, and Jc of the multifilamentary V-(40–45)at%Ti alloy wires. Jc(4.2 K, 6.5 T) of 200 A/mm2 was obtained for the multifilamentary wire. [ABSTRACT FROM AUTHOR]

An attempt was made to incorporate the Cu stabilizer internally to the Ta-matrix jelly-roll RHQT Nb3Al roundwire, based on less-reactivity of Cu with Ta at elevated temperatures during the RHQ operation of wire. Compared to the conventional internal stabilization technique using Ag filaments jacketed with Nb, the present round wire has several advantages; (1) less induced-radioactivity when irradiated with fast neutrons, (2) higher mechanical strength at both elevated and cryogenic temperatures, and (3) less reactivity with Ga-coolant. The overall residual resistance ratio RRRov of the internally Cu stabilized Ta matrix RHQT Nb3Al round-wire is as much as 75 in spite of a smaller stabilizer-ratio, and magnetoresistance is comparable to that of the conventional Cu clad stabilized Nb matrix RHQT Nb3Al flatwire. Core Jc is also comparable to that of the Nb matrix wire. [ABSTRACT FROM AUTHOR]

The replacement of Nb matrix with Ta for rapid-heating, quenching and transformation annealing (RHQT) processed Nb3Al conductor seems to be advantageous for facilitating the RHQ operation due to a higher mechanical strength at elevated temperatures and less reactivity with the molten Ga in the quench bath and suppressing flux jumps in low magnetic fields. We have fabricated three grades of the Ta matrix precursor, in which the volume fraction of Ta matrix is different, so as to examine the drawability of such Ta matrix precursors. Also examined are the effects of using the Ta matrix on the RHQ condition, Tc values after quenching and subsequent annealing, respectively, Jc (core), overall residual resistivity, and magnetization curves. We have succeeded in drawing all of Ta matrix precursors and confirmed the favorable existence of a plateau region in the IRHQ - Tc relationship even for the Ta matrix RHQT Nb3Al conductors. The resulting superconducting properties were comparative to those of Nb matrix conductors. The replacement of Nb matrix with Ta was very effective in suppressing flux jumps even at 4.2 K. [ABSTRACT FROM AUTHOR]

The induced activity and compositional change of several superconducting materials such as Nb-based alloys, V-based alloys and MgB2 have been evaluated with a numerical calculation. We assume that the materials are irradiated for 10 MW a/m2 at the magnet of the inboard position of a tokamak-type fusion reactor with a neutron wall loading of 1 MW/m2. The materials are Nb3Sn, Nb3Al, NbTi, NbZr, V2Zr, V2(Zr,Hf), V3Ga, V3Si and MgB2. Most of the induced activity of V-based alloys and MgB2 after the shutdown of the reactor is controlled by the 60Co formed from Cu. After the irradiation, the dose rate decreases to a safe level of 10 μSv/h within 30 years. However, Nb-based alloys are predicted to emit gamma rays for tens of thousands years. The compositional changes and irradiation damage of V-based alloys and MgB2 are minimal. [Copyright &y& Elsevier]

Energy–momentum (E–k) dispersion and the shape of the Fermi surface (FS) in the (Bi,Pb)2(Sr,La)2CuO6+δ (Bi2201) superconductors of various hole-concentrations were investigated by the angle-resolved photoemission spectroscopy (ARPES) with high energy and momentum resolutions (ΔE=10 meV and Δk=0.005 Å-1). The E–k dispersion is kept rigid and the chemical potential moves towards lower or higher energies with increasing or decreasing hole-concentration, respectively, over a wide hole-concentration range from the nearly optimal-doped condition with a transition temperature of 32 K to a heavily over-doped condition where the superconducting transition disappears. By comparing the shape of the FS determined for the present Bi2201 with that reported for the Bi2Sr2CaCu2O8+δ (Bi2212), we found that the area surrounded by the FS in the Bi2201 is fairly larger than that in the Bi2212. Estimated hole-concentration in the Bi2201 was found to vary from 0.25 to 0.43 holes/Cu, that is more than twice as large as that in the optimally doped La2-xSrxCuO4 (LSCO) and Bi2212 superconductors of 0.17 holes/Cu. [Copyright &y& Elsevier]

Structure and high-field performance of(Nb, Ta)3 Sn conductors fabricated by using ductile Sn-Ta matrix are reported. A mixed powder of Sn and Ta with or without a small amount of Cu addition was melted at 800° C in vacuum. The resulting Sn-Ta alloy was rolled into a sheet 100 μm in thickness. The Sn-Ta sheet was laminated with a Nb foil of the same thickness, and wound around a Nb-4at%Ta rod forming a Jelly Roll composite. The composite was encased in a Nb-4at%Ta tube, and then fabricated into a wire without intermediate annealing. The wire with no Cu addition to the Sn-Ta sheet and reacted at 900°C showed a non-Cu Jc of 100 A/mm2 at 23 T and 4.2 K. The non-Cu Jc of the wire at 2.1 K and 25 T reached 100 A/mm<2, the upper critical field of the wire being 28.4 T. A small amount of Cu addition to the Sn-Ta sheet appreciably reduced the reaction temperature, and produced a non-Cu Jc of 100 A/mm2 at 22 T and 4.2 K after the reaction at 800° C. Another process for fabricating multifilamentary-type (Nb, Ta)3 Sn conductors using ductile Sn-Ta/Nb composite is also being studied. [ABSTRACT FROM AUTHOR]

Nb3Ga wires fabricated by using RHQ (rapidly heating and quenching) treatment show Tc = 29.7 K and Bc2 (4.2 K) = 32 T. In addition Jc of the Nb3Ga phase is very high, comparable to those of practical (Nb, Ti) 3 Sn wire and RHQT-processed Nb3Al wire. However we cannot obtain the large overall Jc for the wire, because of small volume fraction of Nb3Ga in the wire. By using Ta-matrix Nb3Ga precursor wire, we performed the RHQ treatment at higher temperatures, resulting in two-times improvement of overall Jc and a little improvement in Jc. We measured temperature dependence of Jc vs. B curve for the Nb3Ga wire. The pinning force vs. B curves did not show any agreement with Kramer's scaling law. TEM observations clarified that the composition of Nb3Ga varies from near-stoichiometry (24at%Ga) nearby the Nb5Ga3 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 Nb3Ga wire. The strain effect of Nb3Ga wire is as large as that of Nb3Sn wire. [ABSTRACT FROM AUTHOR]

A rapid heating and quenching (RHQ) operation along a long-length of jellyroll Nb/Al precursor over 300 m has been established. The J[sub c] varies sensitively with the joule-heating current (I[sub RHQ]) that determines the temperature from which a precursor wire is quenched, excluding the "plateau" region where the bcc supersaturated-solid solution (Nb(Al)[sub ss]) only forms. The RHQ operation using such a plateau condition can suppress a scatter of J[sub c] from point to tail of a long-length of wire. The standard deviation of J[sub c] (4.2 K) at 21 T along a long-length of wire was about 5%, which was less than the variation of J[sub c] with I[sub RHQ] in the plateau region (±7%). The mechanical deformation process needed when incorporating the Cu stabilizer also plays an important role in ensuring uniform heat treatment of the coil that is wound with a long-length of wire and, hence, large heat capacity. Although a temperature ramp-up rate might differ from place to place in the coil, the deformation processing makes such a ramp-up rate insensitive to these critical parameters. The coils using 300.5 m and 254 m lengths of Cu-clad RHQT JR Nb[sub 3]Al conductor generated additional 2.3 T and 3.2 T in bores of 90.2 mmφ and 64.6 mmφ, respectively, indicating that a long-length of RHQT JR Nb[sub 3] Al conductor can be trustworthy for practical coil applications. [ABSTRACT FROM AUTHOR]

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 °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. [ABSTRACT FROM AUTHOR]

The mechanism of thick (Nb,Ta)[sub 3]Sn layer synthesis in the Ta-Sn core/Nb (Nb-Ta) sheath composite is studied. The diffusion of Nb from the sheath to the core seems to play a key roll to enhance that of Sn from the core to the sheath synthesizing (Nb,Ta)[sub 3]Sn layer. A simplified powder core process, in which Ta + Sn mixed powder is used instead of Ta-Sn reacted powder as the core, does not degrade the high-field performance of the (Nb,Ta)[sub 3]Sn conductor. A finer dispersion of Ta in the core is found to improve the J[sub c] of the conductor below 23 T. New Jelly Roll processed (Nb,Ta)[sub 3]Sn wires are fabricated using Sn-Ta sheets richer in Sn. The JR wires exhibit attractive high-field performance, and a small amount of Cu addition to the Sn-Ta sheet effectively decreases the reaction temperature. [ABSTRACT FROM AUTHOR]

As the starting material of Nb[sub 3](Al,Ge) precursor wires, gas-atomized Al-Ge alloy powder is very promising because of the fine Ge particle dispersion in the alloy powder. The hardness of Al-Ge alloy is relatively small and well-balanced with that of the pure Nb. Therefore, no intermediate annealing is technically required for drawing a 7 × 220 × 233-core Nb/Al-Ge multifilamentary wire. Furthermore, excellent J[sub c]-B performance without peak effects was obtained in the Nb[sub 3] (Al,Ge) tape specimens through the RHQ (rapid-heating/quenching) treatment. It may be caused by the synthesizing of A15 phases transformed from Nb-Al-Ge ternary supersaturated bcc phase. The present Nb[sub 3](Al,Ge) tapes show a good agreement with Kramer's scaling law described as F[sub p] = A(B[sub c2])[sup 2.14]b[sup 0.5](1 - b)[sup 1.46], where A is temperature and field independent, and b = B/B[sub c2]. [ABSTRACT FROM AUTHOR]

We have performed a comparison of the microstructures of state of the art Nba 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 NbaSn 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 µm 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. [ABSTRACT FROM AUTHOR]

Examines the uniaxial stress/strain tolerance of niobium aluminum alloy superconductors subjected to different heat treatments. Critical current density; Stoichiometry; Stress-strain curve; Degradation.

Examines the fabrication of niobium aluminum alloy superconductors added with copper. Jelly-roll process; Drawing of the multifilamentary wire; Rapid heating, quenching and transformation.

Examines the superconducting properties of niobium aluminum alloy multifilamentary wires during rapid heating condition. Direct current magnetization; Critical current; Magnetic field dependence of pinning force density.

Studies the high-field performance and structure of (Nb,Ta)[sub 3]Sn superconductors produced by Ta-Sn metallic alloy core. X-ray diffraction patterns of (Nb,Ta)[sub 3]Sn superconductors; Relative thickness ratio of quoted layers versus total tape thickness; Critical current versus magnetic field curve measurements.

Studies the superconducting properties and rapid heating condition in transformed jelly-roll Nb[sub 3]Al multifilamentary superconducting wire. Critical temperature and magnetization properties; Temperature rise during rapid heating; Critical current density as a function of annealing time.

Describes the fabrication process of Nb3Al multifilamentary superconductors that enhances the critical current carrying capacity. Increase in the wire diameter; Critical current density value; Effects of stranding and flat-rolling of the composites.

We have developed a nearly-stoichiometric Nb/sub 3/Al multifilamentary superconductor by exploiting the transformation from bcc supersaturated-solid-solution Nb(Al)/sub ss/. This is a candidate conductor for 1 GHz NMR magnets. Flat-roll forming, an effective way to increase the packing factor of the coil and thereby the coil-current-density, has been carried out on the multifilamentary Nb(Al)/sub ss//Nb wire, and its effects are compared with that of the conventional Nb tube processed conductor. A Nb(Al)/sub ss//Nb wire, 0.74 mm in diameter, was successfully deformed into a tape conductor (the thinnest case: 0.15/sup t//spl times/2.2/sup w/ mm) without mechanical fracture. The flat-roll forming creates very little anisotropy in critical parameters and scarcely degrades the J/sub c/ properties. Excess flat-rolling causes a wide distribution of Nb(Al)/sub ss/ filament areas and thus reduces the n-index in the voltage-current characteristic. However, it is probably possible to retain the high value of the n-index of a wire, by appropriately designing the overall-aspect-ratio of a flattened conductor. [ABSTRACT FROM PUBLISHER]

Abstract: Specific heat measurements were performed on a high-quality single crystal of . Peaks at the superconducting transition temperature and the antiferromagnetic transition temperature indicate sharp transitions compared to our previous data. The temperature dependence of specific heat below 0.3K is well described by the equation of with and , evidencing line nodes in the superconducting gap. [Copyright &y& Elsevier]

Abstract: Magnetic and superconducting properties have been investigated on a single crystal of CePt3Si which is the recently discovered heavy-fermion superconductor. The effect of magnetic field on the antiferromagnetic transition temperature was studied by the iso-field thermal expansion coefficient. From these results, together with the results of the electrical resistivity under magnetic fields, magnetostriction and magnetization measurements, we have constructed the magnetic and superconducting phase diagrams for and [001]. [Copyright &y& Elsevier]

We have performed soft X-ray emission (SXE) and high-resolution photoemission (PE) spectroscopy to study the electronic structures of β-HfNCl and Na-intercalated HfNCl. Both the PE and SXE spectra of Na0.37HfNCl show an additional structure within the band gap of β-HfNCl, without an apparent shift in the energy position of valence-band features. These results clearly reveal intercalant-induced changes in the electronic structure of the layered nitride chlorides. From these results, we propose non-rigid band like behavior for this system in contrast to generally expected rigid-band filling upon intercalation. [Copyright &y& Elsevier]

The momentum dependence of the superconducting gap at low temperatures is precisely determined from angle resolved photoemission (ARPES) measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k[sub χ]) — cos(k[sub y]) form. We suggest that this arises due to the increasing importance of long range interactions as one approaches the insulator. Furthermore, a comparison of our ARPES results with available penetration depth data clearly indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent. [ABSTRACT FROM AUTHOR]

Examines the strain effects on critical current densities for conductors with stoichiometric Nb[sub 3]Al filaments. Preparation of the filaments by phase transformation; Comparison of strain sensitivity among different conductors; Degradation of critical current densities.

For Nb3Al multifilamentary superconductors fabricated by a newly developed composite process using continuous ultrafine Al-based alloy cores and pure Nb matrix, Jc properties have been investigated in detail in regard to the size and morphology of the Al core. A significant enhancement in pinning force caused by reducing the Al core size and an anisotropy in Jc observed in a rolled tape have indicated that the Nb3Al-matrix (or core) interface acts as a dominant pinning center of fluxoids. [ABSTRACT FROM AUTHOR]