Study On Related Physical Properties Of Bi-2223 High Temperature Superconductor In The Mixed State

Bi-2223 is one of the most promising high Tc superconductors that could be commercicalized currently. Power applications require not only high Jc and good mechanical properties of Bi-2223 tapes but also low alternating current losses that could reduce its operation cost and increase the stability of devices in its application. Through the investigation on alternating current losses in traditional low Tc superconductors, large quantity of experimental results and perfect theories have been accumulated. The alternating current losses characters of Bi-2223 high Tc superconductors tapes are different from that of traditional low Tc superconductors. But the characteristics of high Tc superconductors, such as high critical temperature, short coherence length, granularity and strong anisotropy, lead to the alternating current losses and flux pinning mechanisms partially different from those of traditional low Tc superconductors. Recently, many studies have been made on the physical properties of high temperature superconductors at home and abroad, which accelerates the application of high temperature superconductors. In this paper, we investigated two sides of Bi-2223 high temperature superconductors on the flux pinning mechanism and the alternating current losses.One of the works, we investigated the flux pinning mechanism of Bi-2223 high temperature superconductor. The flux pinning force of superconductors was at the bottom of magnetism reciprocity and core reciprocity. The flux pinning force of magnetism reciprocity reduced faster in comparison with the flux pinning force of core reciprocity with aggrandizement of GL's parameterκ. Due to the GL's parameterκwas big, only core pinning mechanism was considered, the magnetism reciprocity mechanism can be ignored for Bi series high temperature superconductor. To analyze the pinning mechanism in Bi-2223 high temperature superconductors, a predigestion formula of pinning has been proposed according to the D. Dew-hughes model. In accordance with the experimental data of the references, the pinning mechanism in Bi-2223 high temperature superconductors was confirmed by curve fitting. At last, the simulation result is consistent with the conclusion of the references.In the other part of this dissertation, the transport current losses of the sample tape under 50mT DC field carrying 7.4A alternating current losses at 77K is presented. The experimental results show that the alternating losses for B//c is much larger than that for B//ab due to the effects of I_c(B) behavior. Under DC magnetic field, the alternating current losses is also a little in verse proportion to the frequencies of current, which could be explained by flux creep model. But the alternating current is mainly dominated by hysteresis in general. Finally, according to the ellipse model of the Norris equation, we present the variational law of the alternating current losses with different orientations of DC magnetic field. The result showed the bigger the angle of DC magnetic field and ab plane of the Bi-2223 tapes is, the bigger the alternating current losses is. The calculational values of the alternating current losses are also consistent with the experimental data.