H.C. Freyhardt

Large area YBCO-coated stainless steel tapes with high critical currents

Recent progress in developing large area HTS-coated stainless steel tapes is reported. YBCO films have been grown on IBAD-YSZ buffered 0.1 mm-thick steel tapes using a high-rate pulsed-laser-deposition technique which allows a deposition rate up to 70 nm /spl middot/ m/sup 2//h. The coated conductors (CC) are provided with an optimized shunt layer and current contacts which reduce the generation of Joules heat. An advanced technique for critical current I/sub c/ measurements was developed. The effects of self-field and time relaxation of current in helically wound tapes are analyzed. In long tapes of 10 m /spl times/ 4 mm, the highest I/sub c/ of 78 A at 77 K was observed. Critical currents of 67-75 A were reproducibly achieved in (8-10) m /spl times/ 4 mm coated tapes. For shorter tapes with a length of 0.2-1 m, critical currents of 317-391 A/cm width of the tape were observed. Fifteen wider CC tapes of 0.6 m /spl times/ 43 mm with I/sub c/ = 700 A have been manufactured and employed in newly developed fault current limiters with a nominal current of 3 kA. Critical current densities J/sub c/ = (1.3-2.5) MA/cm/sup 2/ at 77 K were observed in YBCO films. The CC tapes exhibit favorable behavior under axial stress and sufficiently small ac losses.

YBaCuO thick films on planar and curved technical substrates

For high-current applications, homogeneous well-textured high-temperature-superconducting Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBaCuO) films on technical ceramic or metallic substrates are required. The ion-beam-assisted deposition (IBAD) of yttria-stabilized zirconia (YSZ) buffer layers, which serve as templates for the YBaCuO, was extended to large planar areas (20 cm/spl times/20 cm) as well as curved or cylindrical (diameter /spl les/15 mm) substrates. For both types of substrates a pronounced in-plane alignment is observed. A deposition equipment is developed to grow high-quality YBaCuO films also on tubes by pulsed laser deposition (PLD), which is characterized by a high long-term stability both of the deposition rate and of the surface temperature of the growing film. Current densities up to 0.9/spl times/10/sup 6/ A cm/sup -2/ at 77 K in self fields are observed in films on planar Ni foils with biaxially aligned YSZ buffers (bi-YSZ). For the coating of long lengths of tubes and tapes a translation and a rotation of the samples are incorporated both in the buffer as well as in the YBaCuO-deposition process. Furthermore, the dependence of the critical transport current densities, J/sub c/, of the HTS layer on magnetic fields and on mechanical stresses are investigated.

Simultaneous inductive determination of grain and intergrain critical current densities of YBa2Cu3O7−x coated conductors

An inductive methodology simultaneously enabling the determination of grain- and intergrain critical current densities of YBa2Cu3O7−x coated conductors is developed. This noninvasive method is based on the identification of a clear peak in the reverse branch of the magnetization loop at a positive magnetic field, as a signature of the electromagnetic granularity inherent to these materials. A quantitative evaluation of the return magnetic field at the grain boundaries allows us to understand the existence of this magnetization peak and quantify the grain critical current density. This methodology is envisaged to sort out granularity effects from vortex pinning effects on coated conductors.

Critical currents in long-length YBCO-coated conductors

Recent progress in developing large-area HTS-coated stainless-steel (SS) tapes is reported. YBCO films have been grown on IBAD-YSZ buffered 0.1 mm-thick SS foils using a high-rate pulsed laser deposition technique which allows an increase of the deposition rate to 40 nm m 2 h -1 , YBCO-coated SS tapes containing a shunt layer, with a length up to 5.5 m, have been manufactured and tested. A model treatment was performed to describe an interchange of current between the shunt layer and the YBCO film. The optimal parameters of the shunt layer have been employed to reduce the resistive loss in the coated tapes. The critical dc current of more than 105 A was observed in a 2 m-long and 10 mm-wide coated tape.

Batch production of high-quality-customized-shaped-monolithic HTSC

Abstract Customized-shaped-monolithic YBaCuO (YBCO) were produced applying the top-seeded-melt-growth (TSMG) method. These high-temperature superconductors (HTSCs) are single-domain monoliths according to optical microscopy, X-ray diffraction, neutron diffraction, as well as Hall probe measurements. The trapped magnetic induction of such a single-domain monolith at 77 K usually well exceeds 600 mT. An external scan of the remanent induction of a 17 mm×17 mm×8 mm large-single-domain sample at 77 K yields a record value of 865 mT. The zero-field-cooled (zfc) levitation forces of standard samples (38 mm×38 mm×12 mm) at 77 K typically amount to 60–70 N. Actually, 16 units with these standard sizes of the quality described can be produced in one batch. Furthermore, YBCO monoliths with diameters up to 130 mm were successfully melt-textured.

Characterization of natural and artificial low-angle boundaries in YBCO TSMG samples

Abstract Since several years, technical applications of bulk HTS YBCO superconductors are of growing interest. However, shapes of HTS tiles needed for complex applications require the joining of two or more single-domain monoliths. The welding technique leads to the formation of a low-angle grain boundary and then requires a high connectivity between two adjacent single-domain materials. This study was devoted to the characterization of the microstructure and the superconducting properties of natural and artificial low-angle grain boundaries. The natural grain boundary formed during the multi-seeded melt growth (MSMG) process exhibits a non-uniform microstructure during the growth. An energy dispersive spectroscopy (EDS) analysis carried out across the grain boundaries always reveals a depletion of copper and an accumulation of yttrium. Mechanical joining of two single-domain monoliths leads to the formation of an artificial grain boundary. This kind of joining was performed either without or with a welding agent, i.e., YbBa 2 Cu 3 O 7− x (Yb-123). The first method leads to a good connectivity between welded Y-123 platelets. In contrast, the control of the mechanical welding process with a welding agent is more difficult. At 940°C, Yb-123 decomposes into Yb 2 BaCuO 5 , BaCuO 2 and CuO. This decomposition deteriorates the superconducting properties between two adjacent domains.

Thermally driven shape instabilities of Nb/Cu multilayer structures: instability of Nb/Cu multilayers

The disintegration of Nb layers in Nb/Cu multilayers during heat treatments was investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray microscopy (XRM). Spherically shaped Nb particles were observed after a heat treatment at 870 K for 3 h. The growth velocity of the Nb particles depends strongly on the initial interface roughness and the Nb layer thickness. The XRM investigations indicate a process of self-assembly, leading to a non-statistical distribution of the particles. Measurements of the superconducting properties of both annealed and non-annealed samples, confirm the microstructural observations.

Growth model for melt-textured Y1Ba2Cu3O7−gd

Abstract A simplified two-dimensional growth model is proposed to explain the growth of highly textured melt-processesed YBa 2 Cu 3 O 7− δ . direction of These considerations take into account the anisotropic growth and distinguish between the growth rate and the processing rate. Due to the anisotropic growth of YBa 2 Cu 3 O 7− δ , differently oriented grains grow with different speeds parallel to the the temperature gradient. During the growth process grains with fast growing crystal directions surround grains with slow growing crystal directions and stop the latter. Therefore, a selection between differently oriented grains occurs during the growth the temperature process. While other models only consider the growth of the fast growing ab -planes parallel to the temperature gradient, this modified growth model could explain the observed growth of YBa 2 Cu 3 O 7− δ textured with the c -axis parallel to gradient. Due to this model it is possible to design highly textured YBa 2 Cu 3 O 7− δ with its c -axis either parallel or perpendicular to the temperature gradient.

Possible origin of the anomalous magnetization behavior in melt‐textured Y1Ba2Cu3O7−δ crystallites

Magnetization measurements of the hysteresis loop, M(B), of melt‐textured Y1Ba2Cu3O7−δ (YBCO) crystallites were performed in external magnetic fields up to 10 T. The M(B) curves exhibit, between the superconducting transition temperature Tc and almost 39 K, a peak at a finite field B* which increases with decreasing temperature. The crystallites possess a high density of dislocations which could have been introduced during the high temperature treatment and/or by stresses created due to differences in the lattice parameter of small inclusions and the YBCO matrix. The large dislocation density together with the high sensitivity of the superconducting order parameter on deviations from the regular crystal structure and the small coherence length are suggested to be responsible for the observed fishtail shape of the magnetization curves.

Flux pinning mechanism in unorientated grains of YBa2Cu3O7−δ

Abstract Magnetization measurements of the intragranular pinning force in polycrystalline YBa2Cu3O7−δ and YBa2(Cu1−xMx)3O7−δ, where M = Fe,Ni, address two major questions of flux pinning: 1, what are the dominant pinning sites; and 2, what is the relevant pinning mechanism in these materials. It is demonstrated that the microstructure, i.e. the twin spacing, is unaltered by Ni, but drastically decreased by addition of Fe. Thus these systems provide an ideal tool with which to examine the role of twin boundaries in flux pinning. The volume pinning force is interpreted in terms of flux line shear, i.e. FP is related to the onset of dissipative flux flow within weakly pinning channels in the (inhomogeneous) superconductor. The theoretically expected field and temperature scaling agrees with the experimental data. A quantitative analysis of the data yields a channel width comparable to the flux line spacing, ao. This indicates that single rows of flux lines move once the Lorentz force exceeds the flow stress of the flux line lattice. The applicability of this pinning mechanism to YBa2Cu3O7−α more homogeneous than the metallurgically prepared powder used here is discussed.