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.

The present work is devoted to investigating the effect of CeO 2 , PtO 2 and BaCeO 3 additives on the Y 2 BaCuO 5 morphology and the melt processing of YBCO. The samples were synthesized by the melt texture growth (MTG) process in order to observe the Y 2 BaCuO 5 morphology in the liquid phase at high temperature, in the YBa 2 Cu 3 O 7−x matrix resulting from the peritectic reaction, and the resulting microstructure of YBa 2 Cu 3 O 7−x . It is concluded that these investigated dopants exhibit similar effects concerning the limitation of Y 2 BaCuO 5 coarsening in the melt. The evolution of the aspect ratio (l /w ) versus the additive content in the melt demonstrates the change of the morphology of the Y 2 BaCuO 5 grains. The magnetization measurements were carried out on cleaved samples using a SQUID magnetometer at 77 K with H c axis. The best compromise between an homogeneous and submicrometre Y 2 BaCuO 5 distribution through the textured area and a high critical current density corresponds to either 'YBa 2 Cu 3 O 7−x + 20 mol% Y 2 BaCuO 5 + 2 wt% CeO 2 ' or 'YBa 2 Cu 3 O 7−x + 20 mol% Y 2 BaCuO 5 + 0.5 wt% PtO 2 '. The distinct effects of cerium and platinum dopings on the superconducting properties are revealed by the critical current density J c measurements. Indeed, the cerium doped sample exhibits a J c = 5.5 × 10 4 A cm −2 in zero field with a relatively important field dependence behaviour: J c (1 T)/J c (0) = 40%. In contrast the platinum doped sample strongly reduces this field degradation of J c with J c (1 T)/J c (0) = 80%.

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