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The purpose of this investigation is to reveal the correlation between microstructures and curitical current density (Jc) in YBCO ceramic superconductors by TEH. Further analyses have shown that these microstructural features are denpendent on Cu-O planes behaviour of two dimensions and key to explain the mechanism of the relation between microstructure and Jc. Three YBCO samples which have the same composion but different processing were measured and compared. The Tc values of three samples were approximately the same (90K) but the Jc values were quite different:30A/cm2 for sample S1 300A/cm2 for sample S2 and 2000A/cm2 for sample S3. Hundreds pictures for three samples were randomly taken in low magnification by TEM. Using the statistical method, the microstructural information were measured from TEM pictures.A number of observations by TEM have found that three kinds of microstructural features, including the grain of c/a≥3 (grain length c, width a),the grain boundary between parallel-straight grains and the grain with twin domains play a major role in YBCO ceramics superconductors as shown in Fig. 1. The results of investigation have been shown that their content increases as Jc value are increased and as a function of Jc as shown in Fig. 2.

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