The use of high temperature superconductors in current leads to reduce the refrigerator power consumption has been investigated in the last few years by many groups. Potential candidates are Y-123 and Bi-2212 bulk material as well as Bi-2223 tapes sheathed with an Ag/Au alloy. In the frame of the European Fusion Technology Programme, FZK Karlsruhe and CRPP Villigen have started a development program for design and construction of a 60 kA current lead for the ITER Toroidal Field Coils. The task composes of three parts: material selection based on test results of 1 kA current leads, construction and test of 10 and 20 kA leads using the selected HTSC material giving proof of the modularity and scale-ability of the design, and finally construction and test of a 60 kA current lead which will replace an existing conventional lead in the superconducting coil test facility TOSKA at FZK. The paper describes the status of the development program. Experimental results of the first step and a first approach of the realisation of the 10 kA HTSC lead are given.

The transport critical current of Bi(2223)/Ag tapes depends on the mechanical stress applied during or before j(c) determination. We previously reported on our reinforced tapes with a yield strength of 300 - 350 MPa[1], which is more than 5 times the value for conventional Ag-sheathed tapes. The maximum tolerable strain is increased from about 0.3% to 0.7 - 0.8% for the modified tapes. In this contribution we extend our investigations on the variation of filament number and filament/matrix ratio and other sheath materials, e.g. AgAu. We discuss the situations for axial tensile-, hoop- and bending stress. For the special case of bending strain a carbon fibre reinforcement enhances the mechanical performance of the tapes.

BSCCO(2223) tapes for the application in current leads for superconducting coils need replacement of the pure Ag sheath through a low thermal conduction material as AgAu alloy to achieve reduced thermal losses compared to conventional current leads. 7-filamentary BSCCO tapes with Ag92Au8 alloy sheaths were prepared and critical current densities J(c) up to 11 kAcm(-1) at 77 K were achieved. The J(c)(B,T) behavior indicates a nearly pure Bi(2223) phase, but compared to Ag sheathed tapes a stronger J(c) degradation with fields presumably due to a less good texture of the BSCCO phase. The mechanical data characterized by I-c vs. axial tensile stress investigation and a special distortion experiment demonstrate that AgAu sheaths are mechanically weaker than Ag sheaths with an unsufficient performance for technical applications. These tape properties will be discussed with respect to the application aspects in a 1 kA current lead.

The cubic recrystallization of the deformation texture in tapes of fee metals and alloys like Cu, Ni, Ag, Al is applied to prepare biaxially textured substrate tapes for YBCO films. In Cu tapes of suitable purity a grain alignment within < 5 degrees (FWHM) in each direction was realized. In Ni tapes different purifies and rolling techniques led to significantly different cubic textures. A strong influence of side limited rolling was found. Grain misorientation of < 5 degrees and > 10 degrees in different sample directions were observed. For small sample sections (approximate to 1 mm(2)) the out of plane grain misalignment sharpens to < 3 degrees. Cubic texture was also achieved in a mechanically tough commercial Ni65 Cu33Fe2 ahoy, but still with broader grain alignment, presumably due to the non optimized treatments. In Ag tapes we found an indication for a possible cubic recrystallisation.

For reduced AC losses of multifilamentary BSCC0(2223) tapes, it is necessary to apply a twist to the filaments along the tape direction reducing the filament coupling losses. Twists with different twist pitches (10-80 mm) were realized in either Ag sheathed and mechanically reinforced AgMg sheathed 37 filament tapes. Possible reasons for the observed degradations of the transport critical current densities of 22000 Acm/sup -2/ (Ag sheath) and 19000 Acm/sup -2/ (AgMg sheath) down to 50% in the worst case (10 mm twist pitch) for twisted tapes, were investigated analyzing the influence of twisting on the conductor geometry and deformation via quantitative image processing of micrographs. The critical current I/sub C/ of differently twisted tapes was measured in a magnetic field for various tape orientations. From the J/sub C/(B) anisotropy the misalignment angle of the Bi(2223) phase texture was analyzed, and a comparison with rocking angles from X-ray experiments was performed.

The transport critical current, j/sub c/, depends on the mechanical stress applied during or before the j/sub c/-determination. Two experiments with different aims were performed : a) bending of the tape tests the resistance against winding (coils, power cables); b) axial strain tests give information on the effect of forces acting on the conductor during operation. Bending of Bi(2223)/Ag tapes from a radius r=/spl infin/ to r=1 cm as well as an axial strain of about 0.2% decreases j/sub c/ by some 80%. For bent as well as for elongated tapes, we find an almost unaffected field dependence of j/sub c/, which is only on a lower j/sub c/ level due to crack formation, i.e. a reduction of the percolation path. The strong j/sub c/ reduction can be avoided by reinforcement of the sheath material by dispersion hardening with 2% Mg. The j/sub c/ decrease after bending in these improved tapes is only some 20%, the maximum axial strain tolerance about 0.8%. The yield of the reinforced tapes is 300-350 MPa, which is more than 5 times the value for Ag sheathed tapes.

For the technical application of Bi(2223) tapes the use of a dispersion hardened AgMg sheath, with improved mechanical properties is of most importance. For the characterization of the strain sensitivity of the superconducting properties, the transport currents of the tapes were investigated with applied axial tensile strain, transverse compressive stress and as a function of bending strain applied at T=77 K. The application of dispersion hardened AgMg sheaths led to significantly improved mechanical properties of the tape, especially at temperatures below 77 K, compared to Bi(2223) tapes with pure Ag sheath. Bi(2223)/AgMg tapes are therefore promising for practical application since long lengths up to 102 m and high transport currents up to J/sub c/=18 000 A/cm/sup 2/ (77 K, OT) were obtained.<>

Bulk YBa2Cu4O8 (Y-124) is prepared from YBa2Cu3O7−σ (Y-123) and CuO by a powder-metallurgical method. The superconducting features of the Y-124, in particular critical current densities and activation energies, are measured resistively using a four-probe technique and magnetically using a Faraday magnetometer. In a second step the Y-124 is decomposed at high temperatures. The intragranular critical current density is measured at different annealing times, tA, in order to determine and discuss the characteristics of the jc(tA) curves.

Bi2Sr2Ca2Cu3O10 (Bi-2223) tapes are prepared by the powder in tube (PIT) method. The mechanical properties of the tapes are significantly enhanced by Mg alloying the Ag sheath, i.e. by MgO dispersion hardening. The tolerable axial strain of the tapes is increased by a factor of 2 as compared with pure Ag sheathed tapes. The formation of the Bi-2223 phase is strongly affected by the reinforced sheath material, thus the kinetics of the MgO formation in the sheath and the hardness of the sheath depending on the heat treatment of the AgMg material is studied. An optimization of the heat treatment gives reinforced tapes with j c ’s in the order of 10000 – 20000 A/cm2[4].

The field and angular dependent critical current densities, jc(B), in granular YBa2Cu3O7-δ (YBCO) thin films, deposited on Zr substrates, are investigated. The critical current density is limited by the weak links caused by the boundaries between the grains and is strongly history dependent, jc depends only weakly on the angle φ between the magnetic field B→ and the crystallographic C→, which is almost normal to the film surface. The jc(B) curves exhibit a maximum at low field values which weakly depend on φ. The results are discussed in terms of the model of trapped flux in the grains as proposed by Evetts and Glowacki.

The field and angular dependent critical current densities, j(c)(B), in granular YBa2Cu3O7-delta (YBCO) thin films, deposited on Zr substrates, are investigated. The critical current density is limited by the weak links caused by the boundaries between the grains and is strongly history dependent. j(c) depends only weakly on the angle phi between the magnetic field B over arrow pointing right and the crystallographic c over arrow pointing right-orientation, which is almost normal to the film surface. The j(c)(B) curves exhibit a maximum at low field values which weakly depend on phi. The results are discussed in terms of the model of trapped flux in the grains as proposed by Evetts and Glowacki.

Thick textured films of Bi2Sr2Ca1Cu2O x are obtained by partially melting and slow cooling doctor-blade-processed tapes. Heat treatment between 890 and 870°C produces aligned microstructures on substrates of either Ag or Ag coated MgO. During heat treatment dissolution of Ag into the liquid oxide occurs. Because silver lowers the melting point of the oxide, the processing temperatures must be increased by about 10°C when an MgO substrate is used. The resulting films show that the oxide surface plays an important role in grain alignment. The Bi2Sr2Cu1O x phase, stable for temperatures greater than about 870°C, always transforms to the desired Bi2Sr2Ca1Cu2O x phase regardless of the heat treatment history (quenched from 890°C and annealed at 870°C or slowly cooled from 890°C to 870°C). The texture of the microstructure, however, is dependent upon the heat-treatment path, with a higher texture obtained with slower cooling.

The physical properties of RBa 2 (Cu 1- x T x ) 3 O 7- δ , R=Y/Gd, T=Ni/Fe are represented. Bulk samples of different concentrations of the magnetic ions up to 100% are prepared and the oxygen contents, structure, microstructure, magnetic and critical properties such as T c and B c2 are determined by resistive and magnetic measurements, X-ray diffraction, TEM and TGA. The transition temperature, T c , is strongly suppressed as a function of Fe, Ni or Gd concentration. The experimentally determined data for the Pauli susceptibility, the temperature dependence of the upper critical field near T c and T c are used to derive the Curie-Weiss temperature, θ, the density of states at the Fermi level, N (ϵ F ), γ, ǂ; GL and ξ GL . An interpretation of the T c -dependence on the concentration of the magnetic ions in terms of the classical Abrikosov-Gor'kov pair-breaking theory is given. A good agreement between the experimental data and the theory is obtained.

Amorphous and crystalline thin NbAl films were prepared in an UHV-sputter system from single metal targets. Films of different concentrations were deposited on sapphire substrates at room temperature. The structure of the films was detected by X-ray and scanning electron microscopy (SEM), including energy dispersive X-ray analysis (EDX). Measurements of the transition temperature Tc and critical transport current measurements in magnetic fields up to 15.5 T characterize the superconducting properties. The films were heat treated (220°C; 1 h) and the resulting effect on structure and superconducting properties was recorded. For the heat treated films a reduction of Tc and Bc2 as well as the volume pinning force Fp were observed. The field dependence of Fp, however, was similar for all films: Fp ∼ b(1−b)2.

The superconducting and magnetic properties of TBa2(Cu1−xMx)O7−δ with M=Fe,Ni and T=Y,Gd were studied. Ni and Fe cause a linear decrease of Tc with x, but for small concentrations of Fe, x<0.023, Tc is almost constant. If Fe and Gd is substituted simultaneously for Cu and Y, the decrease of Tc is larger (−dTc/dx=700K instead of 591K). For Ni plus Gd substitution no additional decrease of the transition temperature is observed. These results and resistive measurements of Bc2 indicate, that the decrease of Tc is at least partly caused by scattering of superconducting quasi particles at the magnetic ions substituted at different positions in the perovskite structure of Yba2Cu3O7−δ.

The superconducting, structural and magnetic properties of YiBa 2 ( Cu 100− z − Fe z ) 0.03 O 7−δ (O ⩽ x ⩽ 18) were determined by resistive and magnetic measurements as well as by X-ray diffraction experiments and thermogravimetrical analysis. Four Fe-concentration regimes can be distinguished: Below x = 2 T c remains constant, above 2.3 % Fe contents T c decreases linearly with x, first with 5.4 K jat% Fe and above x 10 with 9.0 Kiat% Fe. Above 15 at% Fe no superconductivity could be detected. This behaviour of T coincides with changes in the structure and the oxygen contents of the Y 1 Ba 2 ( Cu 100− x − Fe x ) 0.03 O 7−δ . Below 2.3 at% the orthorhombic perovskite structure is retained. At larger Fe concentration a tetragonal structure is attained with a = b /3 below x = 10 and with a = b = c /3 above x = 15. Between 10 and 15 at% Fe c /3 approaches the value of a = b . Magnetic measurements show an anomaly below T c , which indicates antiferromagnetic ordering. The magnetic ordering temperature increases with Fe concentration.