Influence of addition of CuO-BaCuO2 on microstructure of melt-textured YBa2Cu307_ ceramics and relevant transport properties* J.M. Heintz, C. Magro, P. Dordor, D. Chateigner t, P. Germi t, M. Pernet t, D. Machajdik $ and J.P. Bonnet** Laboratoire de Chimie du Solide du CNRS, Universit@ Bordeaux I, 351, Cours de la Lib6ra- tion, 33405 Talence Cedex, France t Laboratoire de Cristallographie-CNRS, 38042 Grenoble Cedex 09, France Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Czechoslovakia **ENSCI, 47 Avenue Albert Thomas, 87065 Limoges, France The present work is devoted to the study of the relationship between microstructure and electrical transport properties in melt-textured YBa2Cu307_ ~ ceramics. The production process described gives rise to a preferential orientation within large domains. SEM micrographs show long platelets stacked in a parallel arrangement. A.c. susceptibility measurements reveal a very low level of weak links. Direct and indirect critical current measurements, performed on long and small samples, respectively, reveal large dif- ferences. This discrepancy is ascribed to pseudo random orientation of the large domains, as confirmed by rocking curve characterizations. Pole figure measurements show that the preferential orientation of the free surfaces of the bulk sample appears to be < 113> or < 103> whichever face of the parallelepiped is considered. The latter observation is in agreement with the existence of non-coherent regions inside the sample. Consequently, a high critical current density (12 000 A cm-2) can only be obtained for samples where long range texturing of the order of centimetres has been achieved. Keywords: microstructure;YBa2Cu307_~; transport properties Sintering of YBa2Cu307_ ~ under normal conditions (900-975°C) leads to the formation of granular and randomly orientated ceramics. Even though much pro- gress has been made concerning the chemistry of the precursors needed to yield fine and high purity powders, the resulting ceramics still present relatively low values of critical current density (Jc ~ 103 A cm -2) although the potential value for the YBa2Cu307_~ phase is much higher. This behaviour is ascribed to the presence of weak link regions between the high Jc grains. Texturing of the microstructure can greatly reduce the weak link problem. Since 1988, many directional solidification techniques have been used to achieve textured ceramics l-m, the latest using a magnetic field to enhance the texturing process ~1,12. The microstructural characterization of such ceramics is of interest for at least two reasons. First, the correlation between the pro- duction technique and the final microstructure could help us understand the mechanism of microstructure for- mation. Second, the study of the influence on the microstructure of chemical parameters such as the addi- * Paper presented at the conference 'Critical Currents in High Tc Superconductors', 22-24 April 1992, Vienna, Austria 0011 2275/93/030270-07 © 1993 Butterworth-Heinemann Ltd 270 Cryogenics 1993 Vol 33, No 3 tion of CuO, BaCuO 2 or Y2BaCuO5 could allow us to control the development of texture and consequently to improve the transport properties 3'6'13'14. This paper deals with the characterization of YBa2Cu307_ ~ textured ceramics obtained using a con- trolled melt process without application of a thermal gradient. Since YBa2CU3OT_~ undergoes peritectic melting, both Y2 BaCuO5 (211) precipitates and a liquid phase are formed above 1040°C under oxygen. Further- more, the addition of CuO-BaCuO2 offers a means by which to vary the quantity of the liquid phase and therefore the level of 211 phase in the system. The influence of this addition on the microstructure is investigated. A.c. susceptibility measurements are used to correlate transport properties to the change in microstructure. Experimental procedure Production process The starting material is a Rh6ne-Poulenc powder. Three types of samples were prepared: type A cor- responds to the stoichiometric composition; type B is a