Characterization and properties of YBCO samples containing metastannate inclusion

Abstract

As is well known, it is possible to increase the critical current density of type II superconductors by creating defects in the matrix [1, 2]. Many researchers have tried to increase the defect density by irradiation and cation substitution [3]. Others have obtained highly dense crystal defects such as dislocations and stacking faults around 211 particles by precipitation of the 211 phase from the melt [4]. Because the strain generated by precipitates or inclusions in the neighbouring matrix will interact more strongly with the magnetic flux line as the magnitude of the strains increases [5]. This suggests that stronger pinning might be obtained through the presence of inclusions due to the existence of strains resulting from the mismatch of lattice parameters or phase transformations of inclusions. Following from the above, we attempted to increase the critical current density, and not to decrease superconducting transition temperature, by the addition of perovskite structure particles, which would produce some kind of defects in 123 matrix to enhance the interaction with a magnetic flux. The samples were prepared from the decomposed powders of citric gel [6] with the nominal composition of Y:Ba:Cu as 2:1:1 and 0:1:1, CuO and the powders of BaSnO3, CaSnO3 and SrSnO3, respectively, which were prepared from the decomposition of BaSn(OH)6, SrSn(OH)6 and CaSn(OH)6 coprecipitation powders. The 211 and 011 powders were calcined at 900 °C for 10 h, mixing 211,011 and CuO with nominal composition Y:Ba:Ca = 1:2:3:, adding 5 wt % metastannate, ground and pressed at 40 kgmm -2 into pellets 13 mm in diameter. The pellets were sintered at 920 °C for 20 h in air and cooled in a furnace. X-ray powder diffraction was measured using CuK~ radiation at 40 kV and 100 mA to identify the various phases. The a.c. magnetic susceptibility of samples was measured employing a mutual inductance bridge with precision of 0.1/xV. A Hitach X-650 scanning electron microscope (SEM) equipped with a wavelength-dispersive meter (WDM) was used to observe fracture sections and elemental analysis. An H-800 transmission electron microscope (TEM) was used to determine the structure of phases. Fig. 1 shows the X-ray diffraction patterns of a Y B a C u O sample with BaSnO3 addition. It can be seen that some characteristic diffraction peaks of 211 (20=29 .3 , 30.5 and 31.6 ° ) and BaSnO3