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We prepared overlap soldered joints of high-temperature superconducting tapes, using various materials and preparation conditions. In order to select the joints with optimal performance, we correlated their electrical properties (derived from current–voltage curves) with the microstructure of the respective joint cross-section by scanning electron microscopy. With the first group of joints, we focused on the effect of used materials on joint resistivity and critical current, and we found that the dominant role was played by the quality of the internal interfaces of the superconducting tape. Initial joint resistivities ranged in the first group from 41 to 341 nΩ·cm2. The second group of joints underwent a series of thermal cyclings, upon which the initial resistivity range of 35‒49 nΩ·cm2 broadened to 25‒128 nΩ·cm2. After the total of 135 thermal cycles, three out of four joints showed no signs of significant degradation. Within the limit of 100 thermal cycles, reliable soldered joints can be thus prepared, with normalized resistivity not exceeding 1.4 and with normalized critical current above the value of 0.85.

We enhanced the performance of superconducting tapes during quenching by coating the tapes with various composites, with regards to the application of such coated systems in superconducting fault current limiters. In composition of the coating, we varied the type of epoxy matrix, the content of ceramic filler particles and the use of reinforcement in order to optimize the thermal and the mechanical stability of the coated tapes. By this way modified superconducting tapes were able to reduce the maximum temperature 170 &deg, C of not modified superconducting tape to 55 &deg, C during the quench with electric field up to 130 V m&minus, 1.

The activities of the young STU MTF research team focused on the material properties of superconductors were reviewed for the first time. We performed structural analyses on multiple types of superconducting tapes in order to get deeper insight into the correlation between their structure and electromagnetic properties, both experimentally and by modelling. We also addressed the joining of tapes by lead-free solders.

The influences of stresses/strains on critical current, I c in REBCO high-temperature superconducting (HTS) tapes wound onto a round cable were investigated. In the first step, the HTS tapes were helically wound on copper tubes with various diameters in the range of 2.95-8 mm. Considerably better results were achieved in case the superconducting layer was positioned during the winding at the inner side of the helix. The cables with tape wound on the tube with a diameter of 6.35 mm were subjected to bending tests with various bending radii in range from 21 to 60.5 mm. The I c degradation as a function of both the tube diameter and bending radius has been assessed from a current- voltage curve, from which the critical current was determined. The I, degradation in all samples was confirmed by microstructural investigation of an HTS layer by scanning electron microscopy.

High-temperature superconductor devices often need conductors with transport capacity obtained by assembling more REBCO (RE = rare earth, B = barium, C = copper, O = oxygen) coated conductor (CC) tapes in parallel, thus forming a cable. One method of cabling flat REBCO CC tapes is the tape-on-round-tube (TORT) concept. In TORT cables, multiple CCs are wound in a helical fashion on a copper tube, through which the coolant (liquid nitrogen or cold helium vapor) may flow. A detailed structural finite-element (FE) analysis using the FE code ANSYS was carried out to investigate the stress-strain behavior of CC tapes under tension and bending loading during the winding process. In addition to the finite-element method (FEM) investigations, the measurements of critical current dependence on the axial and bending load of REBCO CC tapes at room temperature were performed. Finally, the possibility of the application of FEM simulation to predict the critical loads of REBCO CC tapes subjected to mechanical loading during the winding process is assessed.

The present paper aims to investigate topographical, microstructural, mechanical and tribological behaviour of precipitation hardened Al alloy subjected to massive laser shock processing (LSP) without protective coating at 2500 pulses/cm2, using three beam diameters. Wear tests under dry sliding conditions resulted in severe wear, whereas the main wear mechanisms were adhesion accompanied by abrasive wear. Nevertheless, LSP with optimal processing parameters reduce the friction coefficient and wear rate with lower degrees of adhesion and abrasion inside the wear track in comparison to the untreated sample. The enhanced tribological performance is attributed to the positive influence of LSP induced surface topography, surface compressive residual stresses (RS) and dense dislocation arrangements, as the result of high-pressure shock waves. Nonetheless, due to the narrow window of optimal parameters reduced wear resistance as a consequence of undesired thermal/softening effect due to laser ablation and melting was detected with non-optimal processing parameters.

Overlap joints of the SuperPower 2G HTS tape SCS12050 with surrounding copper stabilizer were prepared, in controlled conditions, using a lead-free solder and two types of fluxes in air atmosphere. A clamping device was developed, in order to control the evolution of temperature during the process. The reflow time (during which the solder melting is expected) and the pressure exerted on the joint area were utilized as parameters in the optimization of the soldering procedure. The melting point of the used Sn96.5Ag3Cu0.5 alloy was 217 °C; soldering temperatures slightly exceeded the latter temperature. Quality of the joint was evaluated by measurement of the voltage–current curve, from which the joint resistance, critical current, and $n$ -value were determined. Resilience against temperature cycling was tested on one of the samples. Electrical characterization is complemented by structural investigation with scanning electron microscopy, which was focused on the microstructure of the soldered joint layer. Surprisingly, the best joints were obtained at the lowest pressures used in these experiments. With regard to the reflow times and used fluxes, a slight reduction in the joint resistance was observed at longer duration of melting conditions and using rosin flux.

This paper systematically investigates the effect of laser shock peening without coating parameters on the microstructural evolution, and dislocation configurations induced by ultra-high plastic strains and strain rates. Based on an analysis of optical microscopy, polarized light microscopy, transmission electron microscopy observations and residual stress analysis, the significant influence of laser shock peening parameters due to the effect of plasma generation and shock wave propagation has been confirmed. Although the optical microscopy results revealed no significant microstructural changes after laser shock peening, i.e. no heat effect zone and differences in the distribution of second-phase particles, expressive influence of laser treatment parameters on the laser shock induced craters was confirmed. Moreover, polarized light microscopy results have confirmed the existence of well-defined longish grains up to 455 μm in length in the centre of the plate due to the rolling effect, and randomly oriented smaller grains (20 μm × 50 μm) in the surface due to the static recrystallization effect. Laser shock peening is reflected in an exceptional increase in dislocation density with various configurations, i.e. dislocation lines, dislocation cells, dislocation tangles, and the formation of dense dislocation walls. More importantly, the microstructure is considerably refined due to the effect of strain deformations induced by laser shock peening process. The results have confirmed that dense dislocation structures during ultra-high plastic deformation with the addition of shear bands producing ultra-fine (60–200 nm) and nano-grains (20–50 nm). Furthermore, dislocation density was increased by a factor of 2.5 compared to the untreated material (29 × 1013 m− 2 vs. 12 × 1013 m− 2).

(RE)BCO-based superconducting tapes were studied to clarify the correlation between structure of the (RE)BCO layer and the critical current density. For this purpose, both etched and cross-sectioned samples of (RE)BCO tapes were investigated by means of SEM and EBSD techniques. The outgrowths penetrating deeply into the (RE)BCO layer were found. Their non-uniform distribution density profile across a tape width was closely related to the profile of critical current density. Grains in (RE)BCO outgrowth-free regions, outgrowths themselves and neighboring regions of outgrowths were found to differ from each other in crystal orientation. It was shown that the presence of outgrowths may cause localized changes in crystal orientation of superconducting layer and in this way it may affect the critical current density.

Superconducting layer in coated conductor was investigated to assess the inhomogeneities present in its structure and to get better insight into the effect of inhomogeneities on degradation of electric properties. In the investigation scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscattered diffraction, and laser scanning confocal microscopy were used. The results obtained showed good correlation between the density of inhomogeneities across the tape width and the degradation of current transport properties determined by measurements of the current density

An adsorbed layer of a cationic polyelectrolyte, poly(diallyldimethyl-ammonium) chloride (PDADMAC) on negatively charged colloidal latex particles was investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). SANS gives a layer thickness of 8 +/- 1 A and a polymer volume fraction of 0.31 +/- 0.05 within the film. DLS gives a somewhat larger thickness of 18 +/- 2 A, and the discrepancy is likely due to the inhomogeneous nature of the layer and the existence of polymer tails or loops protruding into solution. These results show that a highly charged polyelectrolyte adsorbs on an oppositely charged colloidal particle in a flat configuration due to the attractive forces acting between the polyelectrolyte and the substrate.

Potentiometric titrations and electrophoresis techniques were used to study the charging behavior of aqueous silica particle suspensions in the presence of poly(N, N-diallyldimethylammonium chloride) (DADMAC). The main finding is that the particles undergo charge reversal with increasing pH, which can be directly seen by electrophoresis and as a common crossing point in the titration curves at different salt levels. These features resemble the charging behavior of an amphoteric oxide surface. With increasing polymer dose, the charge reversal point shifts towards higher pH. These trends are well reproduced by a modified Stern model. The main effect of the adsorbed cationic polyelectrolyte is to further deprotonate the surface silanol groups, and thereby to induce additional negative charge on the surface.

Confronting grand canonical titration Monte Carlo simulations (MC) with recently published titration and charge reversal (CR) experiments on silica surfaces by Dove et al. and van der Heyden it et al, we show that ion-ion correlations quantitatively explain why divalent counterions strongly promote surface charge which, in turn, eventually causes a charge reversal (CR). Titration and CR results from simulations and experiments are in excellent agreement without any fitting parameters. This is the first unambiguous evidence that ion-ion correlations are instrumental in the creation of highly charged surfaces and responsible for their CR. Finally, we show that charge correlations result in "anomalous" charge regulation in strongly coupled conditions in qualitative desagreement with its classical treatment., 4 pages, 4 figures, submitted to PRL

Various deposition techniques used in the coated conductor industry result in the formation of different structural inhomogeneities within the superconducting film, having an impact on its electromagnetic properties. In this paper, an attempt for complex characterization of the Y(Gd)Ba 2 Cu 3 O 7-x [(RE)BCO] structure in tapes from two major producers was performed. Scanning and transmission electron microscopy techniques were used for the characterization of the fine structure of (RE)BCO thin films in the cross-sectional and planar views. The examination was focused on structural inhomogeneities, e.g., outgrowths, precipitates, stacking faults, nanocolumns, and their mutual correlations. The structural study was correlated to critical current dependence on magnetic field and temperature, where the effect of particular structural inhomogeneities on functional properties was evaluated.

Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters.

Non-uniformity of superconductor properties, e.g. a critical current reduction close to the edge of a coated conductor (CC) tape could degrade its performance in some power applications. Reliable characterization of such non-uniformity and understanding of its mechanism requires investigation of the character and causes of degradation. In this paper spatial distribution of critical current density across the width of a CC tape is studied. Three different experimental methods allowing estimation of the local current density were utilized for this purpose: (i) magnetic field mapping above the tape through which a DC current is flowing, (ii) measurement of the critical current of separate strips prepared by patterning of the CC tape, and (iii) magnetization measurements of the pieces cut from various positions within the tape width. Very good agreement between the results obtained by these methods was found, showing a reduction of the critical current density at the tape edges with respect to its centre. Moreover, structural investigation by scanning electron microscopy revealed a correlation between the morphology and the critical current density across the tape width. Insertion of such real non-uniform distribution of critical current density into AC loss calculation resulted in a dramatic improvement in the agreement with experimental results. (Some figures may appear in colour only in the online journal)

The design of a superconducting coil from high-performance REBCO coated conductors is often complicated because of complex anisotropy of the critical current density Jc . It is important to understand how much detail of this feature must be taken into consideration in the prediction of maximum achievable current and the expected ac loss. We present the results of investigation performed with a small (ten turns, 60 mm inner diameter) coil made from SuperPower tape of 12 mm width. The knowledge of Jc(B,θ) determined on short sample allowed prediction of the maximum achievable current of the coil and the ac loss behavior. We have also investigated the effect of the tape nonuniformity. Our results confirm that the lateral nonuniformity when Jc at tape edges is lower than in its center leads to significant increase of ac loss. A longitudinal nonuniformity, in particular a reduction of critical current in some portion along the tape length, is hardly observable in the ac loss result. On the other hand, using a piece of tape with lower Jc in the innermost coil turn would significantly reduce the maximum current. We also present calculations showing the change in current-voltage curve and redistribution of ac dissipation in the case of nonuniform tape quality.