René Flükiger

Large transport critical currents in dense Fe- and Ni-clad MgB2 superconducting tapes

We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3×105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ∼1 MA/cm2, corresponding to a critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2.

Growth of thick YBa2Cu3O7−x single crystals from Al2O3 crucibles

Abstract The growth of large, isometric and free-standing single crystals of YBa 2 Cu 3 O 7− x using a CuOBaO self-flux is reported. Within an optimum concentration range in the phase diagram centered around 4 at% Y, 30 at% Ba and 66 at% Cu, isometric crystals with aspect ratios between 1–10 and sizes of 5×5×2 mm 3 were grown. Outside this range of favourable flux compositions thin platelets up to 8×8 mm 2 with aspect ratios ≫ 10 were obtained. The thickness of the isometric crystals is essentially determined by the cooling rate: 500μm, 1 mm and 2 mm thick crystals were obtained for cooling rates of 0.5, 0.3 and 0.1° C/h, respectively. For cooling rates 30° C/h a dendritic non equilibrium morphology with a zigzag-like habit of the edge of the (001) face can be observed. Aluminum impurities arising from the Al 2 O 3 crucible have a strong influence on the crystal growth: they considerably reduce the soak time and the viscosity of the melt. The presence of Al also favours the formation of yellow-greenish needles of a new quaternary phase, YBa 3 Al 2 O 7.5 .

Transport and inductive critical current densities in superconducting MgB2

Abstract The critical current density of four MgB 2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples yield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-granular current density measured in a dispersed powder. Also the correspondence between the inductive and transport data indicates that the grain boundaries in the bulk samples are totally transparent for the current. The current–voltage relation becomes rapidly shallow in the vicinity of a depinning line, well below the second critical field. Between the depinning line and the second critical field the material is ohmic and shows a marked magneto-resistivity, indicative of a flux-flow regime.

Current status of high-Tc wire

This paper is the result of the work of a SCENET (The European Network for Superconductivity) material working groups efforts on giving values for present and future expected performance of high-temperature superconducting (HTS) wires and tapes. The purpose of the work is to give input to the design of HTS applications like power cables, motors, current leads, magnets, transformers and generators. The current status performance values are supposed to be used in the design of todays prototypes and the future values for the design of fully commercial HTS applications of the future. We focus on what is expected to be the relevant parameters for HTS application design. The most successful technique by far for making HTS tapes has been on the (Bi, Pb)2Sr2Ca2Cu3Ox (Bi-2223) material by the powder-in-tube (PIT) technique and this paper therefore focuses on giving the current status and expected future performance for Bi-2223 tapes.

The influence of thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)2223 tapes with different matrices

Abstract The behaviour of the critical current in longitudinally strained Bi,Pb(2223) tapes shows a strain-insensitive plateau up to an irreversible strain limit e irr . For higher strains, the formation of cracks induces an abrupt decrease of the critical current. We investigate the relationship between precompression and irreversible strain limit with a set of tapes made with different filling factors as well as an in situ Inconel 600-reinforced tape. I c ( e ) curves were measured in a longitudinal strain apparatus at 77 K. The precompression at the measurement temperature is numerically estimated for each sample as well as the evolution of precompression during the cool-down. These calculated values are compared to the fracture susceptibility of extracted filaments, which gives an empirical estimate of the precompression. The main hypothesis of the “Irreversible I c Reduction Model” is confirmed, i.e. the irreversible strain limit essentially depends on precompression. However, we also found that the regime where I c remains constant contains a tensile component: the plateau extends beyond the external tensile strain needed to relieve the thermal precompression and includes a regime where the ceramic is further elongated non-destructively. This non-destructive deformation can be understood as a “connected-grains” behaviour, and extends the strain-insensitive plateau ∼0.1% beyond the precompression strain. This value is confirmed with a three points bending experiment performed on single filaments which gives a similar value for the bending failure strain. A comparable regime was found to exist also under compressive strain. These non-destructive regimes are of great importance for practical applications since up to a certain level the precompression can develop without any I c degradation.

Electronically ‘atypical’ a 15-type compounds based on chromium and molybdenum

Abstract The superconducting transition temperature has been investigated as a function of composition and degree of long-range order in a number of binary and ternary A 15-type phases on Cr and Mo. It is concluded that these compounds are unlike the typical Nb- or V- based phases with the same structure. The maximum T c does usually not occur at the ideal composition. Nevertheless, a correlation has been found between T c and the degree of order.

On the formation and stability of the A15 phase in the Niobium-Gallium system

Abstract The Nb-rich portion of the Nb-Ga equilibrium phase diagram has been investigated up to 50 at.% Ga by means of X-ray diffractometry, metallography, hardness measurements and microprobe analysis. The direct observation of the phase relations at temperatures up to 2200 °C has been carried out independently by differential thermal analysis and a complementary technique, that of “simultaneous stepwise heating”. Particular attention was given to the high temperature part of the A15 phase. The Ga-rich limit of this phase was found to depend strongly on the temperature: it extends from 21 at.% Ga at 1000 °C to 25 at.% Ga at 1740 °C, the eutectic temperature.

Preparation and characterization of (100) cube textured Ag substrates for in-plane oriented HTS tapes

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

Improvements in crystal growth and crystal homogeneity and its impact on physics

With the development of the novel crucible material BaZrO3 in Geneva a new class of 123 — single crystals has become available, the purity of which reaches now semiconductor grade (> 99.995%). Due to the exceptional purity the experiments are not longer obscured by residual impurities and the influence of different oxygen content and distributions could be studied in detail. This has given way to a set of new findings and basic understanding of the physics of high Tc superconductivity. Examples for the impact of this new crystal generation on physics are the reversible suppression of the so called fishtail effect, which is accompanied with the absence of the Schottky anomaly in low temperature specific heat and the calorimetric observation of the vortex lattice melting. On such samples it is possible to observe the vortex lattice by STM without any surface conditioning and to study the influence of pinning centers such as twin boundaries, micro-structural inhomogeneities of the metallic sub-lattice and oxygen vacancies on pinning of vortices.

Mechanical properties of Bi,Pb(2223) single filaments and Ic(ε) behaviour in longitudinally strained tapes

The Youngs modulus and fracture stress of isolated Bi,Pb(2223) filaments were deduced from three-point bending tests performed at different stages of the tapes preparation. These results were introduced in the model describing the evolution of critical current of tapes submitted to a longitudinal strain in view to predict their irreversible strain limit eirr. These calculated irreversible strain limits were compared to measured values, taken from a set of tapes made with different filling factors and composite matrices. This experiment shows that the predicted irreversible strain limits correspond to the measured ones. Presenting the Ic behaviour of highly stressed tapes in a magnetic field, we discuss the evolution of the ratio Istrongc0/Ic0 versus strain. This value, representative of the fraction of the critical current attributed to strongly connected grains, increases significantly during the crack formation regime at e > eirr. This indicates that mechanically weak links correspond to electromagnetically weak ones. This result is further confirmed by comparing the modulus of rupture obtained in single filaments extracted from tapes with different Ic values.