Sn Tapes Research Articles

水素化物を用いた(Nb, Ta)<SUB>3</SUB>Sn超伝導線材の組織と特性

水素化物を用いた(Nb, Ta)<SUB>3</SUB>Sn超伝導線材の組織と特性

Critical state in disk-shaped superconductors.

We have calculated the magnetic fields and currents occurring in a disk-shaped superconductor (radius \ensuremath{\gg}thickness) in the critical state in a self-consistent way using finite-element analysis. We find that the field shielded (or trapped) in the center of the disk is roughly equal to ${J}_{c}$d, where d is the thickness of the disk. The shielding currents also create radial fields which are or order ${J}_{c}$d/2 on the disk surface. For low applied fields ${H}_{\mathrm{appl}<{J}_{c}\mathrm{d}}$ these self-field effects dominate, leading to a deviation of the local field direction from the applied field, which can exceed 90 deg towards the outside perimeter of the disk. If ${J}_{c}$d is large, as is the case for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals at 4.2 K, self-field effects persist up to several telsa applied field. The field dependence of the calculated magnetic moment in the self-field dominated regime is independent of whether ${J}_{c}$ is weakly or strongly (\ensuremath{\propto}1/H) dependent on field. The calculations were validated by comparison to both magnetic and resistive measurements on a disk-shaped section in ${\mathrm{Nb}}_{3}$Sn tape.

Shape anisotropy in critical current densities of Nb<inf>3</inf>Sn tapes produced by the bronze process

A systematic study of the shape anisotropy in critical current densities of bronze processed Nb 3 Sn tapes was made. [Critical current densities of the Nb 3 Sn tapes are greater when applied magnetic field H is perpendicular, J c ( \perp ), to the flat face of the tape than when it is parallel, J c (∥), to the face while current is perpendicular to H in both cases.] In these tapes, the anisotropy is not only due to the shape (columnar or equiaxed) of the Nb 3 Sn grains, but also in many cases, it is related to the similar anisotropy in the critical magnetic fields H c2 . However, the origin of this shape anisotropy in H c2 remains unknown.

Critical current of Nb<inf>3</inf>Sn practical superconductors in high magnetic field

The critical currents of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn practical superconductors were measured in high magnetic fields up to 22 T at 4.2 K. The conductors investigated were MF Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wire, CVD-Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape and diffusion processed Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape. The measurements show that the conductors have excellent superconducting properties. The multifilamentary Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wire possesses a higher overall current density Jc <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ov</sup> (Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn + bronze), especially in high fields above 12T. Its Jc <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ov</sup> , for example, is 2.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 15T. The CVD-Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape displays a higher Jc in the fields up to 12T, while the diffusion processed Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape has an excellent Jc(Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn) in high field. Their Jc(Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn) ,for instance, are 4.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (at 11T) for CVD-Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn and 1.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (at 15T) for diffusion processed Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape, respectively. The upper critical field Bc2 are determined extrapolately by using Kramer's scaling law for flux pinning. A discussion is presented respect of the results of measurement and the upper critical field.

Internal friction and elastic softening in polycrystallineNb3Sn

The vibrating-reed technique was used to measure internal friction and Young's modulus of polycrystalline ${\mathrm{Nb}}_{3}$Sn in the form of composite Nb-${\mathrm{Nb}}_{3}$Sn tapes from 6 to 300 K. In tapes with only small residual strain in the $A15$ layers, a dramatic increase in internal friction with decreasing temperature is observed with an abrupt onset at \ensuremath{\sim}48 K. The internal friction ${Q}^{\ensuremath{-}1}$ between 6 and 48 K is believed to be associated with stress-induced motion of martensitic-domain walls. In this temperature range, ${Q}^{\ensuremath{-}1}$ is approximately proportional to the square of the tetragonal strain of the martensitic phase; ${Q}^{\ensuremath{-}1}\ensuremath{\propto}{(\frac{c}{a}\ensuremath{-}1)}^{2}$. With residual compressive strains of \ensuremath{\sim}0.2%, the internal friction associated with domain-wall motion is considerably reduced. This is attributed to a biasing of domain-wall orientation with residual stress, which reduces wall motion induced by the (much smaller) applied stress. The transformation temperature, however, is unchanged (within \ifmmode\pm\else\textpm\fi{} 1 K) by residual strains of up to 0.2%. Young's modulus exhibits substantial softening on cooling from 300 to 6 K. This softening is substantially reduced in the presence of small residual compressive strains, indicating a highly nonlinear stress-strain relationship as previously reported for V $_{3}\mathrm{Si}$.

Design and testing of a dual 8-T 380-mm/12-T 220-mm split superconducting solenoid for ORNL

A superconducting high field magnet facility has recently been prepared for operation at the Oak Ridge National Laboratory (ORNL). The facility consists of a background NbTi coil and an insert coil made of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape. The background coil produces an 8-T central field, with a peak field of 8.8 T, in a bore of 380 mm and contains radial access ports of 67- mm diam. The insert in conjunction with the background coil produces a central field of 12 T, with a peak field of 13.5 T, and provides radial access ports of 67-mm diam. Details of magnet design both for the background coil and insert coil will be presented. The protection scheme will be discussed and test results will be given.

Preparation of high performance Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tape by a high tape rate CVD process

Preparation of high performance Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tape by a high tape rate CVD process

Low AC loss Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tape for transmission line applications

A process for modifying the surface topography of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tapes so as to achieve low ac losses has been further developed by experimental operation at pilot plant scale. Diffusion process Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tapes are treated by etching followed by hot dip coating with tin. Problems involved in control of the etch process and the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn input have been studied for the purpose of optimizing the product superconductor with respect to minimum losses in combination with high values of critical current. Performance capability has been demonstrated by processing a group of four tapes totaling about 3,000 feet. The average ac loss for the group, as determined from measurements at each end of each tape, is at the target specification: 10 μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (500 rms A/cm, 60 Hz, 4.2 K, unclad). The target specification for critical current was also met.

Design and first stage test of 50-meter flexible superconducting cable

A 50-meter flexible superconducting cable model is described, including a copper-stabilized Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn-tape current-carrying core and cable-paper electric insulation of 13 mm total thickness which is submerged in cold helium while under working conditions. The helium-cooled inner channel is 36 mm in diameter and the outer diameter of the current-carrying core is 75 mm. Helium also flows in an annular channel between the outer surface of the core and a corrugated enclosure. The core is incorporated in a flexible cryostating envelope consisting of 4 corrugated enclosures; the two middle pipes form an annular channel for liquid nitrogen which reduces heat leaks to helium. The design of a 110 kV, 12 kA lead was developed to test the flexible superconducting cable model. For d.c. and a.c. tests of the model, a technique of inductive current introduction through a superconducting transformer is offered. A multisection transformer with a superconducting primary of insulated Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape was manufactured, the current-carrying core being the secondary. Before assemblying, the sections were tested in a liquid helium cryostat to check the design characteristics and to determine the critical current in the winding. A.C. losses (50 Hz) in the transformer's primary were measured. The cable model was cooled by two improved helium refrigerator plants, HGU-150/4,5. Current tests were performed at a working temperature.

Fabrication and properties of Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tapes by the "Bronze process" application to superconducting power transmission lines

Long lengths of Nb 3 Sn tapes have been prepared by extrusion and rolling of bronze clad niobium billets. Fine grained layers up to 5 μm thick, were formed by heat treating at 750°C for several hours. By combining the bronze-process with an external Sn diffusion technique layers up to 15 μm thick were produced. Loss data at 60 Hz are given for various configurations with and without copper stabilizing layers. DC critical current densities are also given.

AC loss and DC critical current densities of Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tapes by the solid state diffusion process

The effects of metallurgical processing on 60 Hz ac losses and dc critical currents in Nb 3 Sn tapes fabricated by the solid state diffusion technique were investigated. An addition of Al to the Cu-Sn alloy for the matrix resulted in large reduction in the ac losses of Nb 3 Sn tapes, but the highest linear critical current densities were observed in Nb 3 Sn tapes produced with a Nb-1 wt% Zr core in a Cu-13 wt% Sn matrix. Values of the losses and the critical currents in these tapes can meet the present requirements for the ac superconducting power cables.

The effect of ternary additions to Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn on the upper critical field at 4.2K

The effect of ternary additions of Group III <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> -IV <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> metallic elements (Al,Ga,In, Tl,Pb) to Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Sn on the upper critical field H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (4.2K) has been studied. Samples were prepared by arc melting in an argon atmosphere. The measurements of H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (4.2K) were performed with a pulsed magnet, and the value of H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> was determined from measurements of the electrical resistivity. H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (4.2K) of the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn compounds was observed to increase by about 70 KG to a maximum of 300 KG through the replacement of several percent of the Sn by Group III <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> -IV <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> metallic elements. Of these systems, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Sn-In) and Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Sn-Pb) were also prepared by the diffusion method: Cold-rolled tapes of Nb doped with the third element were immersed and heat treated in a Sn bath or a Cu-30wt%Sn bath. H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> as high as 270 KG was achieved for the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Sn-In) tape prepared in the Cu-30wt% Sn bath at a reaction temperature of 850°C. The critical current of this tape was substantially higher than that of the standard Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape in a high field region.

Development of Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tapes having low AC losses

A process has been developed for modifying the surface topography of Nb 3 Sn tapes produced by the diffusion process such that losses at 60 Hz and 4.2 K are reduced by a factor of 4 or more. The process comprises an etching sequence followed by reapplication of tin; the latter operation restores solderability for the subsequent fabrication of laminated conductor and, under some conditions, contributes to the reduction of ac losses. The treated superconductor tapes apparently fulfill the essential requirements established for superconducting ac power transmission cables.

Asymmetry of thermal propagation velocity in a long force-cooled superconducting test line

We have observed the phenomenon of thermal destruction of superconductivity by current in a force-cooled test model power transmission line of 20 m length at currents up to 9 kA and temperatures between 8 K and 14 K. This line, fabricated by soldering doubly-stabilized Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tapes to each side of a bar of 0fhc copper (0.3cm × 1.58 cm), was enclosed in a German silver cryogen flow tube. The cryogen (He gas) was controlled at a flow rate of 0.5 to 0.7 gm/sec. Destruction was initiated by application of a short duration heating pulse to a small section of line. The velocity of N-S interface zone propagation was measured over 1.0 m test sections located downstream and upstream from the initial section. The upstream moving zone is always in a nearly-constant-temperature cryogenic environment. However, downstream portions contact an environment preheated by Joule heating in upstream normal N zones and thus transfer less heat and exhibit a much higher velocity. This qualitative explanation of the observed dramatic velocity asymmetry is to be supplemented by a more precise description of the physics of the moving N-S interface which must include a proper model for the heat conductance into the cryogen. Various models for heat conductance can be tested in velocity calculations.

Studies of helical conductor models for superconducting AC power transmission

In the BNL concept of a superconducting ac power transmission cable the conductors are made of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tapes wound helically on cylinders. Recently, it was decided to make each conductor in the form of a double layer winding in which the layers are of opposite helicity. This reduces undesirable consequences of axial flux generation. After reviewing the considerations which lead to the double helix conductor, experiments with short models (0.8 m long) are described. Results are given for ac loss and quench current measurements. Quench currents in excess of 3400 A/cm rms have been obtained. Ac losses are higher than those of short samples of the tapes used but are acceptable. Loss calculations for the double helix configuration are discussed.

Recent progress in superconducting transmission

Expected requirements for high-capacity, modest-cost underground transmission of energy have motivated intense development of superconducting power transmission lines. Recent studies and laboratory developments have generated much reason for optimism. The U.S. now has three ongoing projects to develop superconducting cables - a rigid ac system design employing Nb conductors, a flexible ac system employing Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tapes, and a flexible dc cable system design employing Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape or multifilamentary conductors. In addition there are a number of supporting research projects in the fields of cryogenic dielectrics, helium fluid flow and heat transfer, cryogenic refrigeration, and superconducting materials. This review paper presents an overview of these ongoing activities.

Multifilamantary Niobium tin superconductor tape

An effective method for the fabrication of multifilamentary Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape has been developed. Filamentary superconductive paths are produced in otherwise standard commercial superconductor tape by the chemical milling of separator slots through the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn layer. The present multifilament configuration features a matrix of ten parallel helical superconducting paths along the length of the tape; filament width and interfilament separation are approximately 1.2 mm and 0.4 mm respectively. Thirty-five multifilamentary tapes 0.4 m long were produced. Complete arrays of continuous, fully-defined filaments were reliably formed. Tapes tested as small pancake coils (6-10T) demonstrated the integrity and continuity of the matrix and showed that critical current was sustained in direct proportion to retained superconductor. Interesting behavioral differences resulted from filamentization: critical current anisotropy was reversed, and the slope of the I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> versus H curve was reduced. Other tests found qualitative evidence of substantially improved stability in the multifilamentary tape.

AC critical currents of commercial Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn tapes

Quench currents and critical currents have been measured for a number of commercial Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tapes at 4.2 K. The measurements were made at 60 Hz. The observed quench currents in the as-received tapes are considerably lower than the critical currents, which can be determined by a new technique after stabilizing material is added. The temperature dependence over the entire range above 4.2 K was measured for two of the samples. The critical current falls off approximately as 1-(T/T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

AC losses of Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Sn

60 Hz losses were measured at 4.2 K in a number of Nb 3 Sn rods and tapes produced by the bronze-diffusion process and in commercial Nb 3 Sn tapes. At low fields ( \lsim900 Oe), losses vary by more than two orders of magnitude among the different samples and are related to surface topography. At high fields (≳2 kOe) the variation among samples is much less, and depends mainly on the bulk critical current density J c . The losses are analyzed quantitatively in terms of the critical state model, including a surface current.

Magnetic field microprobe in superconductors carrying a current

A new technique to probe local magnetic fields inside superconductors utilizing the Mössbauer Effect (ME) is reported. The ME nucleus responds to the local magnetic field resulting in a modified hyperfine ME spectrum from which the local field and its direction may be deduced. Concentrations of a few ppm of the radioactive probe nucleus are sufficient; the allowable depth of the probe depends on the attenuation of the ME γ ray. For <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Fe ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co parent) in superconductors the field acts directly on the nuclear moment and the resulting sensitivity is only a few kG. For a thin probe consisting of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co in Pd the response is enhanced by the "giant" localized moment providing a sensitivity at 1 K of better than 100 G. Results of such a probe placed at various locations across a commercial Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn tape carrying a current are reported and compared with calculations of the field profile using several models. For thin tape geometries the usual models prove to be inadequate.