Thin-wall geometr yi mplies a regular network of holes in a bulk, centimetric sample so that its effective thickness can be considered to be smaller than 1.5 mm. Thin-wall geometry offers a significant potential for helping the oxygenation process, mechanical reinforcement and thermal stabilization. Thin-wall geometry was obtained by drilling holes in slightly sintered pellets or by pressing pellets with embedded needles. The growth of a single domain, up to 50 mm in diameter, on such thin-wall geometry pellets was confirmed, despite the presence of many-hole patterns, by in situ high-temperature video monitoring. A significant decrease of porosity and cracks is observed, associated with the reduction of diffusion paths produced by thin-wall geometry. The improvement of the material quality is established by a significant increase (about 40%) of the magnetic trapped field in thin-wall samples. (Some figures in this article are in colour only in the electronic version) M This article features online multimedia enhancements

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Fabrication and characterization of thin-wall YBCO single-domain samples

Isothermal growth of large YBaCuO single domains through an artificial array of holes

The oxygenation process of the top seeded melt-grown (TSMG) single-grain YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 (123/211) bulk superconductors was studied by microstructure observations It is shown that the c -cracks in these bulks are formed in the oxygenated orthorhombic layer along the a / b -macrocracks The lattice contraction of the 123 phase in a / b -plane induces tensile stresses in the oxygenated layer, which cannot be relaxed by twin boundary motion The critical spacing of the c -macrocracks, λ cr =14 μm, at oxygenation temperature 380 °C, estimated using Thoules’s model, is in a good agreement with the c -macrocrack spacing measures on standard TSMG 123/211 single-grain samples

Formation of c-macrocracks during oxygenation of TSMG YBa2Cu3O7/Y2BaCuO5 single-grain superconductors

Considering resistive YBCO fault current limiter, a more compact design can be achieved with the current flowing along the c-axis instead of the (a, b) planes. The higher resistivity along the c-axis allows reducing the required length of material. The oxygen annealing step of the samples is the key factor for developing the necessary superconducting properties. Usually an intense cracking along the (a, b) plane is observed due to the mechanical constraints introduced during the oxygen uptake. An annealing treatment has been designed to reduce the cracking drastically. However, this treatment can only be applied to samples with thickness lower than 1.5 mm. To simplify manufacturing and increase reproducibility, single domains (O 20 and 40 mm) were grown by the TSMG method through a triangular array of equidistant holes. Holes as small as 0.8 mm in diameter and distant of 2 mm were drilled parallel to the c-axis in YBaCuO sintered pellets in order to produce a geometry with walls having a thickness less than 1.5 mm. The growth proceeds similarly to a plain pellet. The holes remain open. The growth front is slightly distorted by the holes, but reaches the edges. The growth of a single domain is confirmed by flux mapping experiment as well as by microscopic observation under polarized light. An indirect advantage of this geometry is the disappearance of the porosity usually trapped within a plain single domain.

Growth of YBCO single domains through an array of holes for FCL c-axis superconducting elements

A strategy has been envisaged leading to extremely high current-limitation capability of melt-textured, single-domain YBa2Cu3O7−x. Underdoped, single-domain, Bridgman-grown bars having an enhanced normal state resistivity allowed to generate extremely high electric fields during a current pulse (E≈4850 V/m), and maximum total energy density dissipated during a quench transition (e≈4×109 J/m3). We suggest that the enhanced performance for current limitation applications results from the combination of a strongly anisotropic electrical resistivity in the normal state due to the opening of a pseudogap and to the creation of impurity local density of states distributed at a nanometric scale in the superconducting chains, which then generate a dense array of hot spots.

Extremely high current-limitation capability of underdoped YBa2Cu3O7−x superconductor

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.

E. Floch

Papers

c-Axis YBCO domains for current limiting applications