Top-seeded Melt-textured Growth Research Articles

Fabrication and characterization of large Nd–Ba–Cu–O grains prepared under low oxygen pressure

A large, single-grain Nd–Ba–Cu–O (NdBCO) composite consisting of superconducting NdBa2Cu3O7-δ containing nonsuperconducting Nd4Ba2Cu2O10 phase inclusions was fabricated up to 2 cm in diameter using a top-seeded melt-textured growth technique. A MgO single-crystal seed was used to provide a heterogeneous nucleation site at the center of a presintered pellet heated above its peritectic temperature and cooled continuously in a conventional tube furnace in reduced oxygen partial pressure. This process produces individual grains with the c axis oriented at ≈10° to the seed surface which, from vibrating-sample magnetization measurements, exhibit a pronounced peak effect in their magnetic moment over a wide temperature range (50–90 K) when the supercurrent flows in the a-b planes. A very high irreversibility field (>9 T at 77 K) is also observed in these grains for field applied both perpendicular and parallel to the crystallographic c axis which is significantly greater than that observed in good-quality melt-processed Y–Ba–Cu–O. These results underline the potential of NdBCO for high-field engineering applications.

Top-seeding melt texture growth of single-domain superconducting pellets

The top-seeding melt texture growth (TSMTG) process is investigated in a small-vertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets. An crystal was used as seed in a Y123 - Y211 composite with additions. It was shown that the role of the seed is to initiate the sympathetic nucleation growth mechanism. Indeed, the additional driving force induced by the seed is clearly demonstrated by a decrease in the undercooling of the system. The seeding crystal introduces a heterogeneous nucleation centre, leading to a very well controlled nucleation rate. Furthermore, we discuss the relation between intrinsic anisotropic growth rate and the processing rate, showing the importance of the thermal conditions of the texturing process to stabilize a sympathetic growth. By careful control of the texturing parameters, we succeeded in producing single-grain pellets as confirmed by a pole figure experiment and neutron diffraction measurements. The levitation force for pellets of 2 cm in diameter reaches 16 N when an NdFeB magnet of the same diameter is used.