Rainer Nast

Cube-textured nickel and Ni alloy substrates for YBCO coated conductors

Abstract This paper deals with the texturing of nickel and NiCr, NiW and NiCrW alloys. Concurrently with measurements of the primary recrystallization kinetics, the recrystallization texture of these materials was investigated in dependence on the annealing temperature and time. An increase of the recrystallization temperature or time sharpens the texture due to grain growth, limited however by the occurrence of secondary recrystallization when a sample specific temperature is exceeded. The most favorable recrystallization temperature was found to correlate with the recrystallization kinetics, varying with the sample composition. The reduced stacking fault energy (SFE) in binary Ni alloys in comparison with pure Ni favors the formation of recrystallization twins {1 2 2}〈2 1 2〉 . The effect of a ternary alloy on the SFE is not easy to predict. So we tried to find, experimentally, a ternary alloy with medium to high SFE to produce the cube texture.

Low AC loss cable produced from transposed striated CC tapes

In this work we demonstrate that the use of striated tapes from coated conductors (CCs) significantly reduces the dissipation of a cable made of tapes wound helically on a round core when it is exposed to AC magnetic field. The coupling loss can vanish provided that the striations ensure electrical insulation between filaments and the cable length corresponds to an entire number of lay pitches. In our study we compare the magnetization loss in two cable models exposed to magnetic field perpendicular to their longitudinal axis. The overall geometry of the models was identical: each consisted of three tapes 4?mm wide that were placed with a pitch of 50?mm in a single layer on the 8?mm diameter round core. The cable length was designed to reach two complete tape pitches. In the first cable (the reference cable) tapes without striation were used; the second cable was prepared using similar tapes but striated to five filaments by laser processing. The AC loss was measured for cables without terminations as well as with low resistance terminations; this latter configuration simulates the conditions in a magnet winding. Our experiments have clearly shown the loss behavior expected in the regime of uncoupled filaments. In particular, at AC fields of 0.1?T amplitude the loss in the cable from striated tapes is five times lower than in the reference cable. Numerical models have explained the experimentally observed cable behavior in the whole range of AC fields.

Critical current density enhancement in strongly reactive ex?situ? MgB2 bulk and tapes prepared by high energy milling

The ex?situ technique is promising for its ease of preparation, low material shrinkage during annealing and the fact that it is already superconducting without annealing. However the critical current density (Jc) needs to be enhanced. To improve the Jc values of commercial powder high energy milling was used?a method commonly use in industry and easy to apply. It was found that high energy milling is successful in improving Jc in bulk and tapes. The reason for the improvement is reduced crystallite size and the large fraction of fresh grain surfaces. In this paper we also experimentally investigate the problem of decomposition of MgB2?ex?situ during heat treatment.

Reduction of coupling current losses by internal resistive barrier structures in multifilamentary Bi(2223) tapes in external magnetic fields

The AC losses in multifilamentary Bi(2223) tapes in external magnetic fields are dominated by the hysteresis losses in the superconducting filaments and by coupling current losses in the normal conducting matrix. In the last years great effort has been made in the development of low loss barrier tapes with internal interfilamentary resistive barriers, which reduce the coupling currents. The recent improvements of these conductors are presented. 37-filamentary tapes with a reduced barrier thickness, enhanced superconductor content as well as overall critical current densities were developed. Different barrier materials were used to study the effect of different barrier hardnesses on the deformation, barrier homogeneity, matrix resistivity and critical current density. The tapes were twisted with a twist length down to lp ¼ 5 mm. The parameters of filament twist and the AC losses of the tapes were investigated and are discussed in comparison to former conductors to find an optimum conductor design. 2002 Elsevier Science B.V. All rights reserved.

Study of thermal oxidation of NiO buffers on Ni-based tapes for superconductor substrates

Abstract NiO on Ni-based textured substrates is a potential buffer layer system for growth of superconductor films for conductor applications. In this work, the high temperature oxidation of NiO has been studied under different oxygen partial pressures. Pure Ni, Ni–10%Cr and Cr-coated Ni have been studied. Pure Ni when oxidised at 1250°C for 3 h in P O 2 ∼1 atm has shown excellent pure cube texture. Ni–10%Cr oxidised under the same condition has shown the same texture, however the tape becomes very brittle. The conditions for reproducible growth of highly cube textured NiO are determined.

Investigation of the effect of striated strands on the AC losses of 2G Roebel cables

The assembly of meander shaped coated conductor tapes by the Roebel technique is a promising way to manufacture high current cables with low ac losses. The application of longitudinal striations to the single strands can be an option to create a filament structure for further possible reduction of the ac losses. Due to the complex Roebel strand geometry, it was important to identify a reliable technique to produce such structures using a picosecond-infrared (IR) laser for the groove etching process. We analyzed the effects of the filament structure on the magnetization ac loss behavior by comparing the losses of a cable with striated strands with those of a reference one with non-striated strands. The ac loss reduction in the Roebel cable with striated strands was confirmed. The measured magnetization loss of the 125 mm striated single strand is five times lower than that of the non-striated one. In the case of the cable sample the loss reduced by a factor of three, but not in the whole interval of amplitudes of the applied magnetic field. We also compared the results with those for a cable with insulated striated strands: they seem to indicate that the coupling currents occur mostly between the filaments, not between the strands.

Large pinning forces and matching effects in YBa2Cu3O7-δ thin films with Ba2Y(Nb/Ta)O6 nano-precipitates

The addition of mixed double perovskite Ba2Y(Nb/Ta)O6 (BYNTO) to YBa2Cu3O7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m3 at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it.

Influence of laser striations on the properties of coated conductors

Due to their high current carrying capability, coated conductors are regarded as the most promising high-temperature superconductor tapes for power applications. However, their high aspect ratio causes too high magnetization losses. To reduce the ac loss, one way is to striate the wide tapes into filaments. We used a picosecond laser for the structuring of (RE)BCO coated conductors. The laser allows to burn 18 μm to 21 μm wide grooves (Ag-cap) with a depth between 0.5 μm to more than 100 μm into the coated conductors, with negligible heat effects at the edges of the structures. Different numbers of filament were structured in Cu-and Ag-cap coated conductors. Patterns with up to 120 parallel filaments in 12 mm wide conductor were made. The critical current and the total ac-magnetization loss were measured as a function of the filament count. With an increasing number of filaments Ic degradation occurs. This current reduction has two contributions, the removed HTS material and current inhomogeneities within the superconductor for instance defects along the tape causing secondary phases. For 120 filaments Ag-cap tapes the hysteresis loss reduction is about two orders of magnitude, as expected. The observation of some remaining filament coupling was investigated.

Mechanically reinforced MgB2 wires and tapes with high transport currents

Abstract Monofilamentary MgB 2 -wires with a 2- or 3-component sheath containing mechanical reinforcing stainless steel (SS) were prepared and characterized. In direct contact to the superconductor Nb, Ta or Fe was used. For a selection of samples with a Fe and Fe/SS sheath, we investigated the transport critical current behaviour in magnetic fields changing systematically the geometrical shape from a round wire to a flat tape. A strong increase of the current densities in flat tapes was observed and possible reasons for this are discussed. Reinforcing the sheath in the outer layer with different amounts of stainless steel leads to a systematic field dependent decrease of the transport critical current density with increasing steel amount. This is an indication for a pre-stress induced degradation of the critical currents in MgB 2 wires and first I c –stress–strain experiments seem to confirm this observation and interpretation.

Study of thermal oxidation of NiO on pure Ni, Ni-10% Cr and Ni-9% V tapes

In this work, the thermal oxidation of NiO has been studied at different annealing temperatures and oxygen partial pressure. The oxidation properties of Rolling Biaxial Textured Substrate (RABiTS) of pure Ni, Ni-9% V and Ni-10% Cr have been investigated. Oxidation of the substrates either as rolled and recrystallised or as rolled, recrystallised and chemically etched states have been studied. It is found that etched pure Ni forms cube-textured NiO upon oxidation in oxygen flow while Ni-10% Cr and Ni-9% V developed cube textured NiO when oxidised in reduced oxygen partial pressure.