Crystallite Orientation in Ybco Ceramic Layers Sintered on Silver Substrates
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CRYSTALLITE ORIENTATION IN YBCO CERAMIC SINTERED ON SILVER SUBSTRATES
LAYERS
CLAUS CHR. SCHOLER, FELIX GREUTER AND PETRA KLUGE-WEISS ASEA BROWN BOVERI, Corporate Research, CH-5405 Baden-D~ttwil, Switzerland ABSTRACT Strongly c-axis textured layers of YiBa 2 Cu 3 07_x were prepared by sintering a precursor powder on Ag-foils. Critical current densities of layers with a few gm thickness are presently Jc - 5000-8000 A/cm2 (77 K, H = 0) over a few centimeters length. INTRODUCTION In epitaxially grown YBCO films, critical current densities of Jc > 106 A/cm2 (77 K, H = 0) are now routinely achieved. Sintered bulk ceramics hardly exceed Jc - 103 A/cm2 in 1 x 1 mm cross sections with the corresponding self-field limitation [1]. The generally accepted causes for low Jc values in sintered YBCO are weak-links through the grain boundaries and strong crystal anisotropy of the physical properties. With the latter argument in mind, many attempts were made to introduce favourable crystallite orientation into polycrystalline YBCO by wire drawing2 or tape-rolling. Considerably enhanced Jc values up to 3330 A/cm have been reported for thin tapes [2] and seem to support this idea. We have tried to achieve crystallite orientation by controlling the stages of reaction, nucleation and growth during reactive sintering, rather than using thermomechanical treatments of YBCO powder. Here, we briefly describe our process and report on the first Jc measurements. PREPARATION OF POWDER LAYERS We have chosen a reactive sintering process with the idea, that on a suitable substrate growth of the (123)-phase would be controlled by its well-known anisotropy, with the surface providing orientation at the nucleation stage. The precursor powder of homogeneously mixed Y2 0 3 , BaCO 3 and CuO, with particle sizes below 1 gm was derived from a stoichiometric coprecipitation of the oxalates followed by decomposition at 580*C in air. Details have been reported in [3]. Thin powder layers are best deposited on a substrate from a well dispersed suspension. As a medium we have used isopropanol with a small amount of a dispersant added. The decomposed powders were first roll-milled in this medium with ZrO2 (Y2 0 3 stabilized) balls to break up soft agglomerates. The resulting suspension was then classified by gravitational settling to a narrow particle size Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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with d - 0.5 jim. Powder layers on Ag-foils, typically 50 gm x 5-20 mm x 50 mm in dimensions, were dip-coated to a thickness (after sintering) of - 2-4 gm. In Fig. la the rather dense packing after drying is shown. The success of the subsequent sintering process depends critically on two qualities achieved in the green state, i.e. narrow-sized, sub-micron particles and good packing. They promote a fast, homogeneous reaction process from the precursor constituents to the (123)-phase, reduce shrinkage on densification and in particular assist early formation of sinter-necks between particles. The layer is thus strengthened against cr
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