Grain Boundary Modelling and Correlation with Critical Current Densities in High-T c Superconductors
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Grain Boundary Modelling and Correlation with Critical Current Densities in High-Tc Superconductors K. Jagannadham and J. Narayan Department of Materials Science and Engineering North Carolina State University Raleigh, North Carolina 27695 Abstract Geometrical modelling of grain boundaries in 123-YBaCuO and 2223-TlBaCaCuO systems is carried out for several misorientations. The a-b and the a-c type coincidence boundaries are analyzed to determine the fraction of Cu-O planes that are continuous and the excess charge present at the boundary voids. The intergcurrent density is determined as a function of the rain critical misorientation and the width of the boundary. The tunneling of superconductor pairs through the regions of distortions,giving rise to depression of the order parameter at the boundaries, is current density in the weak coupused to determine the critical ling limit. Introduction The high critical current densities observed [1-3] in epitaxial thin films(>6.0XJ0 amps/5m at 77 K and zero field) and single crystals(l.OXl0 amps cm [4,51 c?mpared to smaller values in bulk sintered products(10 -10 amps/cm ) [6-9] emphasize that grain boundaries are responsible for these differences. The intergrain (between grains) critical current density, Jc(gb) measured for the a-b tilt boundaries is found [10-13] to decrease sharply with the misorientation angle. In addition, the temperature dependence of Jc(gb) is found to follow from the tunneling behavior of superconductor pairs across interfaces. These experimental observations support the proposition that grain boundaries form weak links between two superconducting grains. High resolution electron microscopy observations of high Tc superconductors illustrate the presence of triangular shaped voids at small angle grain boundaries [14,15]. In this paper, we describe the structure of tilt boundaries in highTc oxides using geometrical modelling (16] and derive the critical current density in terms of misorientation angle, width and depression of the order parameter associated with the boundaries [17]. Primary Coincidence Boundaries The layered nature of superconducting oxides necessitates consideration of different possible configurations. Figures la and lb show the four different atomic planes in the 123-YBaCuO and 2223-TlBaCaCuO systems, respectively. The four different atomic planes give rise to four independent grain boundary configurations. Among these, the bonding will be weak when atomic planes containing dissimilar atoms such as Cu-O-Cu-O and Ba-0-Ba-0 adjoin at the boundary. Such configurations are expected to give rise to low strength and brittle behavior [16]. The primary coincidence boundaries can be classified as either Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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