Study Of Optimized Control Process Of Microstructure In Bi-2223/Ag/Ni Superconducting Tapes

Bi-2223/Ag/Ni superconducting tapes decreased the cost and enhanced themechanical properties greatly comparing with Bi-2223/Ag tapes. However, machiningand heat-treatment parameters which decided the critical current of tapes have not beenstudied in detail yet. Hence, it is the key for improving the properties of Bi-2223/Ag/Nitapes to study the microstructure evolution of Bi-2223/Ag/Ni tapes and optimize thecontrol process of microstructure further.The first part of this dissertation studies the effects of Ni on the microstructure andproperties of Bi-2223/Ag/Ni tapes. The main conclusions include that: Ni enhanced thehardness of tapes by one time, tensile strength of tapes by about80%; texture was alsoimproved obviously; however, serious bubbling phenomenon appeared during heattreatment owing to the bad gas permeability of Ni, which should be improved by usingslow heating rate.Effects of heating rate on the microstructure evolution of tapes werestudied and an optimized heating rate was determined in the second part. Aheating rate of30oC/h is not only beneficial to obtain higher volume fraction ofBi-2223phase, but also good for decreasing the number and size of thesecondary phases. In addition, the biggest Bi-2223grains were observed in tapesheated at30oC/h.The third part explored the distribution of Bi-2223phase and the evolutionof inhomogeneity in Bi-2223/Ag/Ni tapes. A combined heat treatment wasdesigned to eliminate the inhomogeneous distribution of Bi-2223phasesuccessfully. It was concluded that optimized temperature, long time sinteringand lower oxygen partial pressure were helpful for improving the inhomogeneityin tapes.In the last part, microstructure evolution in different oxygen partialpressures was discussed in detail. The growth process of Bi-2223grain indifferent oxygen partial pressures was also expounded. The main conclusionsincluded that higher oxygen partial pressure could slow down the growth ofBi-2223grain and reduce the number of Bi-2223nuclei which would influencethe eventual size of Bi-2223grain. This result proved that oxygen partial hadgreat effect in the growth process of Bi-2223grain.