Microstructural limits of TSMG REBCO bulk superconductors

The diameter of TSMG (top-seeded melt-growth) REBCO bulks is directly proportional to the trapped magnetic field. Unfortunately, the microstructural changes, which negatively influence superconducting and mechanical properties of the bulk, are associated with increasing sample diameter. Subgrains increase their misalignment with the growth length. The other microstructural phenomena limiting properties is a macroscopic inhomogeneity in 211 particle concentration. The refinement of 211 particles to increase critical current density (Jc) leads to their pushing mainly by the c-growth front. High 211 particle concentration inhomogeneity induces high thermal dilatation stresses into the bulk. Therefore, looking for new particles by additional alloying is fully approved. The shortening of lattice parameters with the oxygen concentration in the REBa2Cu3Ox phase is responsible for formation of cracks in the a/b- and a/c-planes during oxygenation. The role of these cracks is controversial. They significantly enhance oxygen penetration into the sample and shorten oxygenation time. High density of the a/c-cracks can allow for relaxation of macroscopic stresses and prevent fatal cracking of the sample. On the other hand they reduce the overall Jc of the sample.