D. Isfort

Cracking and oxygenation of YBaCuO bulk superconductors: application to c-axis elements for current limitation

Abstract A process to elaborate crack free, oxygenated c -axis samples in YBa 2 Cu 3 O x bulk superconductors is reported. This includes texturing of crack free pellets and development of an adequate oxygen annealing treatment to avoid cracking during oxygen uptake. For current limitation in c -direction such a process is needed as c -axis elements are particularly susceptible to ab -plane cracking during oxygenation. Mechanical properties have been compared for different annealing treatments by flexural strength measurements. With the load applied in ab -direction the mean flexural strength is 58 MPa in as-textured c -axis bars and 48 MPa in c -axis bars which were oxygenated using a customized annealing treatment. The conventional oxygen annealing treatments lead to macroscopic oxygenation cracks which reduced flexural strength to 10–20 MPa. Bars cut in ab -direction do not show significant variation in flexural strength (load applied in c -direction) as a function of the annealing condition with an average value of 145 MPa.

Isothermal growth of large YBa2Cu3O7-x single domains up to 93 mm

Abstract The growth of YBa 2 Cu 3 O 7−x single domains larger than 30–40 mm by a classical Top Seeding Melt Texturing (TSMT) method appears to be difficult. Because of the very narrow solidification range between the onset of the nucleation from the seed and the nucleation of grains in the liquid, reaction with the substrate as well as uncontrolled thermal gradients introduce severe limitations to the growth. After suppressing nucleation from the substrate and improving the temperature homogeneity around the sample, the growth rate measured in situ was used to carefully adjust the process. A YBa 2 Cu 3 O 7−x single domain has been grown up to 93 mm without applied thermal gradient nor large overheating. The texture is confirmed on a large volume by neutron scattering and an homogeneous 211 size distribution and content are found across the sample. This suggests that steady growth conditions have been maintained and that even larger samples could be grown.

Kinetics study of ab-plane crack propagation by a modulus measurement in single domain YBa2Cu3Ox

Abstract Young’s modulus experiments have been carried out to study the kinetics of ab -plane crack propagation in single domain YBa 2 Cu 3 O x (YBCO) during a prolonged oxygen heat treatment at 400 °C up to 188 h. It has been found that the modulus value experiences a rapid fall between the annealing time 48 and 96 h. X-ray diffraction (XRD) experiments have been carried out to investigate the structural phase transition during the oxygenation process. Consistently, a diffraction peak shift has been observed in this region that indicates a massive tetragonal-to-orthorhombic (T-to-O) transition. SQUID magnetization measurements of annealed samples have shown clear oxygen inhomogeneity in this regime suggesting a T-to-O phase boundary in the crystal. A physical model is provided to describe the operating mechanism of crack propagation in single domain YBCO.

Superconducting fault current limiter with bulk materials

Abstract Superconducting materials offer the unique possibility to limit fault currents by their self-triggering quench to a resistive state above a given threshold current. Their use in power networks would improve the power quality, which is a real need today. YBCO or BSCO bulk materials are good candidates for current limitation. Their behaviours are rather different, in particular the electric field versus current characteristics. BSCO compounds are protected by their homogeneous quench due to the numerous defects (grain boundaries) distributed along the material. An assembling of 60 sintered Bi bars was tested under 1 kV. The current limitation is effective (a 5000 A short-circuit has been limited to 1080 A) but it occurs at 24 times the critical current. The increase of the critical current density of Bi compounds by their texturation reduces the current excursion above the critical value. In YBCO bulk elements, the defects are localized and make YBCO very sensitive to hot spots. Those are unavoidable and the operating conditions should be adapted in order to make them non-destructive. That is the self-protection concept. To fulfil it, we chose to operate near the critical temperature (above 90 K) to reduce the critical current density. The small difference with T c is very favourable for the quench. A fault current limiter based on 43 YBCO meanders working at 90.5 K in a pressurized liquid nitrogen bath is presented. The meanders are cut in 45 mm in diameter single domain pellets elaborated using the top-seeding technique. Supplied under a maximum voltage of 1 kV, the current was limited at 740 A instead of the theoretical value of 6500 A without quench.

c-Axis YBCO domains for current limiting applications

Abstract A superconducting fault current limiter using YBCO single domains requires long elements cut along the ab -planes or along the c -axis. These elements have to be assembled in series in order to produce a limited fault current after the quench to the resistive state under the network voltage. The superconductor total length is much smaller with c -axis elements because their high resistivity after transition contributes to a very high electric field during the current limitation. The critical current density has to be decreased in order to be sure that the Joule effect related to the limiting current does not increase too much the material temperature before its interruption by a current breaker. The inhomogeneously oxygenated YBCO elements limit the current even if the only surface layer is superconducting and produce an electric field up to 5000 V/m. A homogeneous oxygenation without macrocrack is obtained when the thickness along the ab -planes is thin. Several elements have been tested by applying a 20 ms duration fault current in a liquid nitrogen bath at T =77 K and above.

Current limitation with bulk YBaCuO meanders

Forty three meanders were mounted and put in series to build a resistive FCL prototype. The nominal current and the temperature were respectively fixed at 100 A and 90.5 K. The voltage and the mean electric field respectively reached 1 kV and 106 V/m during 20 ms. The current was limited to 730 A whereas it would have reached 5.1 kA without limitation. The superconducting parameters were adjusted so as to reduce the temperature (self-protection) reached by the samples. Bulk YBCO proved in our experiment their strong ability to limit the current. However, meanders suffered large electromechanical and thermal stresses during the transition. In this article, we analyse the prototype results.

Transition and transport Jc(H, t, Θ/Φ) measurements in single domain superconductors

Conductors cut from large YBCO and NdBCO single domain crystals, textured by the top seeding technique, are measured in transport current. For YBCO, the magnetic field angular dependence of critical current densities along the ab planes and along the c-axis is measured as a function of temperature. A special accent is put on measurements with a configuration of the field in the ab planes and the current along the c-axis for which vortex pinning is strongly influenced by twin planes.