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

Abstract

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 (Ø 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.