Laser induced circular texture in Bi-2212 bulk

Abstract

Laser Zone melting (LZM) has been widely used to induce texture in Bi-2212 bulk materials, both in cylindrical geometries and in monolithic planar samples. Texture is induced due to its strong crystal growth anisotropy, obtaining a microstructure characterized by planar superconducting grains with the a-b plane aligned perpendicular to the solidification front. This technology has produced samples with high critical current values when the current flows in the longitudinal direction (i.e. parallel to the grain’s a-b planes). In addition, this geometry induces critical current anisotropy, with values of the order of 0.45 lower when the critical current is measured perpendicular to the longitudinal direction. In this work we present the use of Laser Zone melting techniques to process planar monolithic samples either with a ring geometry or with more complex shapes, such helical, meander, etc, where the current path direction is not constant, and therefore superconducting grains have to change their growth direction. Combining different sample movements during LZM process it is possible to assure that the solidification front is always perpendicular to the current path direction. We have performed a microstructural analysis of these samples in order to analyse the effect of changing growth direction during process for the different analysed geometries. The microstructure depends on the size of the superconducting grains and on the radius needed to perform this change in direction. It is important to have in mind that these grains can reach sizes of the order of millimetres. It has been observed that the superconducting grains form a set of domains, changing smoothly their growth direction. The processing parameters allow the control of the angles between consecutive domains needed to minimize the reduction in the critical current thattakes place when the current has to change its flowing direction. In all the cases, the critical current values in the different regions of the samples have been measured.