| The top-seeded infiltration and growth process (TSIG) is one of the most important ways to fabricate YBCO bulk superconductors. In order to furtherly promote the physica properties of these samples, strong magnetic fluxl pinning centers have to be introduced into the high-temperature superconductors. Various means, such as heat treatment, doping, high-energy particle radiation and heavy particle bombardment have been used, and successfully produced effective flux pinning centers in YBCO bulk samples. It is proved that doping is the cheapest and practical way to introduce pinning centers.In this paper, we use doping method. We doped ZrO2 powder and Y2Ba4CuNbOx powder into Y211 by solid-state synthesis. ZrO2 doped and Nb-2411 doped YBCO bulk superconductor have been fabricated by the top-seeded infiltration and growth process, the microstructure and levitation force of the superconductors have also been investigated.For the ZrO2 doped YBCO bulk superconductors, we mainly studied the morphology and the magnetic force. Through analysis of the morphology of the bulk superconductors, it is found that ZrO2 dopant can change the morphology of the YBCO bulk samples, nearly single-domain YBCO bulk can be obtained when ZrO2 dopant is in the range of 0.1 wt% to 1wt%, however, there is a square area around the seed of superconductors, and look like a "pyramidal" shape, and in the area it emerges the rule raised stripes and more stripes when ZrO2 content increases. When the doping ratio reaches 2wt%, the superconductors showes very distinct deformation, "pyramidal" shape nearly coveries the whole sample, the top surface area of the superconducotors is obviously larger than that of the bottom one. When the doping ratio reaches 10wt%, there is no YBCO crystal growth morphology. Though analysis of the magnetic levitation force of the samples, the levitation force reaches the maximum value when ZrO2 dopant is of 0.4wt%. Through the microstructure analysis we can know that the valum of Y211 particles decreases with the ZrO2 doping ratio increases, and so the magnetic force of the samples with higher ZrO2 additions is lower.Before preparation of Nb-2411-doped YBCO bulk superconductors, we need to prepare Y2Ba4CuNbOx powder. It is prepared by solid state reaction, using four milling and three sintering process, the sintering temperature is 1140℃, the sintering time is 30 hours.Nb-2411-doped YBCO bulk superconductors have been fabricated and analysed, the results indicates that Nb-2411 dopant can caused deformation in the bulk superconductor. When the Nb-2411 doping ratio is less than 2wt%, the single domain YBCO bulk superconductor can be prepared, and there is a slightly influence on the morphology of the superconductors, which is happened only near the seed. But when the doping ratio is larger than 2wt%, the doped superconductors show similar deformation like what happened in the samples doped with ZrO2, which formes a square area around the seed of the YBCO superconductors, and the top surface looks like a "pyramid". And when the Nb2411 addition ratio reaches 6wt%, the sample shows no crystal growth characteristics of YBCO superconductors. Judging from the magnetic force, Nb-2411 doping improved the superconducting levitation force. Between 1wt%～5wt% range, superconductors levitation force is greater than the superconductor without doping. The optimum doping ration is 2wt%, the magnetic force reaches the maximum value in this group of the samples. Through the microstructure analysis, we can see that Nb-2411 doping can refine Y211 particles, so as to achieve the purpose of improving superconducting properties. It is also found that the volume of the Y211 particles is reduced with the increasing of Nb-2411 content, and higher Nb-2411 additions will reduce the performance of bulk superconductor. |
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