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Fabrication of High Temperature Bi-based Superconducting Film on Ag Surface

Author: SongYang
Tutor: HuangYong
School: Tsinghua University
Course: Materials Science and Engineering
Keywords: dip-coating method partial-melting temperature solid sintering tem-perature oxygen content critical temperature
Type: PhD thesis
Year: 2006
Downloads: 157
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The preparation of Bi-system superconducting films by non-vacuum method issystematically presented in this paper. The formation mechanism of superconductingphase is also studied thoroughly, including the sintering temperature, atmosphere, post-annealing parameters, etc. The main results are as following:A weighing method was used to estimate the thickness of the Bi-2212 film. Dif-ferent thick films were obtained by different dip-coating technologies on various sil-ver tapes and wires, which had favorable superconducting and current carrying prop-erties. The 0.5mm diameter wire coated with 3μm thick Bi-2212 film could carry10000A/cm2 critical current at liquid nitrogen temperature, under zero magnetic field.The dip-coated thin wire could be applied in high frequency circuits since it can avoidAC losses caused by skin-effect.The effect of sintering parameters on the Bi-2212 phase content was systematicallystudied. 883 ?C was the best value for Tmax. At this temperature, Bi-2212 phase contentreached its climax, while second phase content was minimized. When Ts was at 860 ?C,850?C, 840?C and 830?C, there was no difference between the formation of Bi-2212phases. The peritectic reaction could be easily carried on and the characteristics of thesamples were almost the same. At last, the sintering parameters were optimized andconfirmed, the Tmax and Ts were respectively 880 ?C and 840 ?C.The relationship between transition temperature Tc and oxygen content was stud-ied as well. The experiments proved that the samples after pure oxygen sintering wouldcarry extra oxygen, the aim of post-annealing was to diffuse oxygen to the outside of thesample. Higher post annealing temperature, longer dwelling time and larger differencein oxygen content between sample and atmosphere contribute to increased diffusion ca-pacity, the corresponding Tc was obtained. The quenching applied in the post-annealingprocess could enhance the Tc value, but after a long idle time, Tc of different samplestend to be the same.During the ball-mill and sintering process to synthesize Bi-2212 precursor powder,wet-mill reduced the grain size, but led to a content deviation of Bi-2212 phase. Ad-ditional pressure on the precursor powder or extension of sintering time could increasethe Bi-2212 phase content and decrease the content of second phases simultaneously.”Ethanol+SPAN(20)+PVB(SD-3)”is the best organic carrier of the superconduct-ing powder to prepare suspension. In this kind of suspension, under the same dip-coating technology, the film thickness increases as idle time of the suspension is pro-longed.Bi-2223 thin films were prepared on silver substrates by non-vacuum method. Theimportant role of Pb during the phase formation of Bi-2223 was to increase the liquidphase, reduce the sintering temperature, stabilize Bi-2223 phase. At the same time, weput forward three ways to restrain the volatilization of Pb.

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