Microstructures of the melt-powder-melt-growth processed YBaCuO
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Microstructures of two MPMG processed YBaCuO materials with and without Y 2 BaCuO 5 (211) inclusions were investigated by transmission electron microscopy. Using the MPMG process, it is possible to change the quantity of the 211 inclusions in the YBa 2 Cu 3 0 7 (123) matrix. We prepared two YBaCuO samples with 0 and 30 vol. % 211 and with respective critical current density values of 2000 and 30 000 A/cm 2 at 77 K and 1 T (magnetic field parallel to the c-axis). As possible pinning centers, we found stacking faults in the 123 matrix. However, we observed no appreciable change in their number and structure by introducing the 211 inclusions. Therefore, the difference in Jc values can be attributed to the 211 inclusion itself.
I. INTRODUCTION To make practical use of high-Tc oxide superconductors, fabrication processes to obtain high critical current density (Jc) material must be developed. Recently, several groups reported that high-J c materials were obtained by melt processes.1"5 We have also developed a process called the Melt-Powder-Melt-Growth (MPMG) process, which was reported in detail elsewhere.6 By the MPMG process, superconducting YBa 2 Cu 3 0 7 (123) with finely dispersed nonsuperconducting Y 2 BaCu0s (211) inclusions is obtained. The number of 211 inclusions can be controlled easily by changing the composition of the starting mixture. Jc of the MPMG processed materials with various 211 contents have been measured and found to become larger with increasing number of 211 inclusions.7 Therefore, it is interesting to make clear the role of the 211 inclusions as flux pinning centers. In conventional superconductors, crystallographic defects and nonsuperconducting inclusions are known to act as flux pinning centers.8 In the MPMG processed material, we can consider two candidates for the pinning centers. One is the 211 inclusion itself and another is the crystallographic defects generated by introducing the 211 inclusions. To develop higher Jc material, we need to clarify what the dominant pinning center is in this material. Thus, the microstructures in the MPMG processed material were investigated by means of transmission electron microscopy (TEM).
II. EXPERIMENTAL The material investigated here contains 30 vol. % of 211 inclusions and exhibits a Jc of 30000 A/cm 2 at 77 K with a magnetic field of 1 T parallel to the c-axis.6'7 A material without 211 inclusions with a Jc of 2000 A/cm 2 1404
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J. Mater. Res., Vol. 6, No. 7, Jul 1991
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was also used for comparison. The samples for TEM observations were prepared as follows. Two pieces about 2 mm square and 0.5 mm thick were sliced from the bulk material. The surface normal of one fragment was parallel to the c-axis of the 123 and that of another was perpendicular to the c-axis. After mechanically thinning to about 50 fim thick, they were dimpled and polished mechanically to about 20 /zm at the center of the fragments. Final polishing for electron transparency was performed by Ar ion milling. The observations
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