Effects of oxide coating on the growth of single grain YBCO bulk superconductors

Surface oxide coating and bottom inserting of oxide plates have been conducted to top seeded melt growth (TSMG) processed YBa2Cu3O7−y (Y123) bulk superconductors with an aim of controlling the Y123 nucleation and growth. The coating medium for surfaces was Yb2O3 solution and the bottom inserts were Yb2O3/Y2O3 powder compact. Many vertical cracks were found to develop at the compact/insert interfaces when an Yb2O3 insert was used, but the crack evolution was greatly reduced when a (Yb2O3 + Y2O3) insert was used. The formation of the vertical cracks is ascribed to the difference in thermal expansion between the YBCO compact and bottom insert. Presence of vertical cracks was found to be crucial to the trapped magnetic field and levitation forces of single grain YBCO bulk superconductors. The Y123 nucleation and growth in TSMG-processed YBCO bulk superconductors were successfully controlled by conducting surface coating and bottom plating using a (Yb2O3 + Y2O3) insert and as a result, the levitation properties were much enhanced.