Investigation of the processing conditions for the growth of large cross section meet-powder-meet-growth YBCO samples in
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A bidirectional gradient furnace with sample displacement was used for the growth of large meet-powder-meet-growth (MPMG) YBCO samples. The optimization of the macrogranular structure with consequent extension of the central part of the sample, characterized by large aligned domains, was studied. Considering the large cross section of our samples, we studied the combined effects of different ratios between the longitudinal and transversal temperature gradients, different pulling rates (R = 0, 0.5, 1, and 2 mm/h), and one or more stages at the beginning of the sample growth on grain nucleation, dimension, and orientation. Different mechanical pretreatments of sample surfaces were also tested in order to control domain nucleation. We obtained 20 X 25 X 45 mm3 samples with low misoriented extended domains. Moreover, we were able to vary their c-axis directions from perpendicular to parallel with respect to the sample length.
I. INTRODUCTION In processing of bulk high temperature superconductors, the alignment of large grains has turned out to be a crucial achievement for technological applications. As a matter of fact, due to the small coherence length and the strong anisotropy of these materials, the altered structure and chemistry of high angle grain boundaries drastically reduce the current carrying capacity and favor a rapid decay of critical current density (Jc) values in the presence of small external magnetic fields.1 To limit the number of grain boundaries, different methods, based on melt-texturing, have been developed for bulk YBCO.2"4 Centimeter-size domains have been obtained, and intergranular critical current densities on the order of 105 A/cm 2 at 77 K have been measured.3-5 In the fields of large current applications, such as power transmission cables, magnetic bearings for motors, flywheel, etc., it is, however, essential to avoid a random grain orientation over extended dimensions. Using conventional directional solidification methods, such as magnetic field techniques,6 seeding,7 and temperature gradient application,8"11 preferred domain orientation along a few centimeter lengths has been achieved in samples with cross-sectional areas smaller than 1 cm2. In most cases sample diameters just reach a few millimeters. Still, for what concerns magnetic levitation applications, these results are not completely satisfactory.
a) On
leave from CISE S.p.A., 12081 1-20134 Milano, Italy. address: CRISMAT-ISMRA Bd. du Marechal Juin, 14050 Caen cedex, France.
b) Present
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J. Mater. Res., Vol. 10, No. 11, Nov 1995
http://journals.cambridge.org
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As a matter of fact, it would be advantageous to use the sample in the most favorable orientation, depending on the material anisotropy, in order to induce an enhancement of the levitation force. But for this purpose the cross-section diameter of grain-aligned bulk YBCO superconductors, and not only their length, should be extended. The optimization of the production time, from a commercial point of view, would thus also be reached. For th
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