Microstructural Evolution And Properties Of Bi-2223/Ag/Ni High Temperature Superconductor Tapes

Using Nickel (Ni) as outer sheath of the Bi-2223/Ag tapes can lower the costs andenhance the mechanical properties of the Bi-2223/Ag tapes. However, machining andheat-treatment parameters which decided the critical current of tapes have not beenstudied in detail yet. In this work, systematical investigations on machining process,phase transformation, microstructural evolution and properties of the Bi-2223/Ag/Nitapes are presented.The first part of this dissertation studies some key factors during the farbricatingprocess of the Bi-2223/Ag/Ni tapes.10-meter-long Bi-2223/Ag/Ni tapes were fabricatedby optimizing annealing and mechanical process. To the begining, the best suit materialis choden as the outer sheath. Besides,“sausaging effect” is found in the Bi-2223/Ag/Nitapes with183filaments by observing the microstructures of the tapes.Growth kinetics of the Bi-2223grains in Ag/Ni sheathed tapes have been studiedbased on observing microstructural evolution by using the means of DSC, XRD andSEM in the second part of this work. Samples were sintered at833℃for various timeunder low oxygen partial pressure. Bi-2223formation rate presents a relatively slowtrend after20h sintering. An analysis of the Bi-2223phase evolution using Avramirelation reveals a three-stage process with obvious changes of the n-exponent during theBi-2223formation after about3and20h sintering. The n values are2.18,1.41and0.62,respectively.Effects of some conditions on the Bi-2223content, microstructural evolution andcritical current of the Bi-2223/Ag/Ni tapes have been discussed in the third part of thisdissertation. Firstly, the best fit thickness of the Bi-2223/Ag/Ni green tapes is chosen. Itis found that tapes whose thickness is0.3mm have the highest properties for its bestalignment of the Bi-2223grains. Secondly, influences of sintering time in the firstsintering process on the formation of the Bi-2223phase, microstructural evolution andJchave been investigated. The results show that the mono-filamentary Bi-2223/Ag/Nitape whose dwelling time in the first sintering process is30h has the highest Jc, thevalue is6656A cm-2, however, multi-filamentary sample whose dwelling time in thefirst sintering process is40h has the highest Jcand the value is11420A cm-2. Besides,effects of heat-treatment temperature on the Bi-2223/Ag/Ni tapes have been researched.Bi-2223content increases with the rise of the temperature and reaches the maximum at834℃. Critical currents of the Bi-2223/Ag/Ni tapes are well improved by using the optimal paramaters.