Michal Skarba

Rare-earth barium copper oxide (REBCO) coated conductors are the suitable candidate materials for superconducting tapes on round tubes (TORTs) cables due to their excellent mechanical strength, high current-carrying capacity, and magnetic strength. It is commonly known that AC losses can be decreased when TORT cables are constructed using REBCO multi-filamentary tapes. However, the mechanical strength of these geometrically adjusted tapes is just as important when winding them into cables. The objective of this study was to understand the effects of the mechanical loading on the I _c (critical current) of the striated tapes with different filament geometries and, therefore, determine the safe strain limits. This work involved the preparation of various types of REBCO striated tapes through chemo-mechanical striating, as well as the deposition of stabilization multilayers. Subsequently, the striated tapes were bent at a lay angle of 45° over bender diameter from 13 mm to 1 mm, and current-carrying performance under self-field conditions at 77 K was measured. We observed that striated tapes with narrower gaps remained without I _c reduction to a bender diameter of 2 mm, while for non-striated tape there was a more significant decrease at 3 mm. Finally, the effect of filament geometrical irregularities on the mechanical performance of striated REBCO tapes were supported by finite element analysis and correlated with experimental results.

Alternating current (AC) losses in REBCO (RE – rare earth metal, B – barium, C – copper, O – oxygen) high-temperature superconductors, can be reduced by several orders of magnitudes by the tape striation. Various methods were investigated so far for the decrease of the superconductor's width-to-thickness ratio, and new methods (or novel combination of previously reported methods) are still developed. A reason for the latter efforts is to reduce the superconductor's degradation, which cannot be completely avoided, and it is inherent to each method. Unfortunately, the degradation also seems to depend on the architecture (i.e., on the thicknesses of layers, deposition techniques used, chemical composition, etc.) We thus compared the ac losses of REBCO tapes striated by a chemo-mechanical method, by photolithography, and by laser ablation approach. The ac loss measurements and transport properties of Ag-stabilized superconducting tapes from SuperPower are complemented by the analysis of their structure. All methods resulted in a substantial decrease in normalized ac loss (more than 9 times at 36 Hz for tape with 15 striations in compare with non-striated tape. The photolithography method exhibited the highest decrease of ac loss, but also led to a notable decline in the critical current.

This study aimed to enhance the performance of Ag-stabilized high-temperature superconducting (HTS) tapes with a focus on reducing magnetization losses. Two approaches were employed: dividing the tapes into narrower widths and introducing striation at the level of the superconducting layer. The process of laser ablation proved to be an effective method for implementing these modifications. The quality of the cut edges and grooves was assessed using scanning electron microscopy. To evaluate the electrical properties, measurements were conducted on the critical current and magnetization loss in samples at different stages: in their initial state, after cutting, and after the striation process. Of the two modifications, the striation process more effectively reduced the AC losses in the HTS tapes, approximately by one order of magnitude. The retention of critical current remained high after cutting, but varied with the number of created filaments after the striation process. Subsequently, a short cable was wound from the cut and striated HTS tape. This cable demonstrated a remarkable sixfold reduction in AC losses compared to the initial HTS tape.

Alternating current (AC) losses in superconducting tapes (SCTs) are one of the main concerns for their use in applications where costs of heat dissipation need to be reduced, such as pulse magnets. Modification of the tapes by striation has been shown to significantly reduce AC losses, and several filamentization methods have been developed. Up to two orders of magnitude decrease in AC losses was reported in previous works with laser ablation, mechanical grooving, or chemical etching. However, a different approach may have to be used to achieve similar results for those SCTs (or their applications after striation), which are more prone to degradation by the side effects of the mentioned methods. We proposed a procedure based on chemo-mechanical etching, which avoids problems arising from thermal degradation and mechanical pressure. Our method intended for the REBCO (RE = rare earth metal, B = Ba, C = Cu, O = oxygen) based high-temperature SCTs yielded low and reproducible AC losses, low cost, retention of the relative critical current up to 93%, and preservation of the mechanical properties of the tape substrate.

TORT (Tapes on Round Tube) cables, which implement high-temperature superconducting (HTS) tapes, offer additional benefit over other HTS cables, which is the internal cooling in the former itself. Up to now, the superconducting tapes of existing TORT cables have been wound on a copper tube of various diameters. However, in the alternating current (AC) mode, the high conductivity of the former causes undesirable eddy currents. Therefore, it is necessary to replace the copper with a less electrically conductive material. We designed a former from polyethylene terephthalate glycol-modified and reinforced with carbon fibers (PETG CF), polycarbonate (PC) and polycarbonate reinforced with carbon fibers (PC CF) materials in order to reduce the AC losses in TORT cables, using 3D printing technology. Because of the low thermal conductivity of polymers, the former must be specially designed to improve the cooling of the HTS tapes inside the former. Moreover, thermal deformation of the former during cooling from room temperature to liquid nitrogen temperature and resilience against low-diameter bending is another important issue that was taken into account. Detailed structural finite-element (FE) analysis was carried out using the FE modeling software ANSYS, in prototyping of the former. Based on the numerical models, formers differing in the materials used were fabricated, and subsequently a short TORT cable with wound HTS tape was electromagnetically characterized.

Resistive superconducting fault current limiters (R-SCFCL) are novel devices which implement high-temperature superconducting (HTS) tapes. Unfortunately, the tapes in as-delivery state cannot withstand higher electric fields and thus high temperatures during a quench event, so they must be modified by a thermal stabilization layer. Detailed thermal finite-element (FE) analysis was carried out using the finite element modeling software (FEM SW) ANSYS, to investigate the heat transfer in (1) high-heat capacity materials applied on the HTS tapes, as well as in (2) the HTS tapes themselves. The individual models differ in materials of thermal stabilization, thicknesses and geometry configuration. Their thermal behavior was investigated under different quenching temperatures. Numerical simulations proved that epoxy composite filled with 20 vol.% of SiC particles as a filler in form of 100 m thick overlayer coating is a promising high-heat capacity material, suitable for the thermal stabilization of HTS tapes in R-SCFCL application.

The strain behavior of SiC/Stycast 2850 FT composites under thermomechanical loading using a finite element analysis (FEA) was studied. These composites can serve as thermal stabilizers of high-temperature superconducting (HTS) tapes during limitation event in resistive superconducting fault current limiter (R-SCFCL) applications. For this purpose, the thermomechanical properties of four composite systems with different filler content were studied experimentally. The FEA was calculated using an ANSYS software and it delivered useful information about the strain distribution in the composite coating, as well as in particular layers of the modified HTS tapes. The tapes were subjected to bending over a 25 cm core, cooled in a liquid nitrogen (LN2) bath, and finally, quenched from this temperature to various temperatures up to 150 °C for a very short time, simulating real limitation conditions. The outputs from simulations were also correlated with the experiments. The most promising of all investigated systems was SB11-SiC20 composite in form of 100 µm thick coating, withstanding a temperature change from LN2 up to 120 °C.

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 °C of not modified superconducting tape to 55 °C during the quench with electric field up to 130 V m⁻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.