2 Optimization of the Filament Size in Multifilamentary - Nb 3 Sn Superconducting Composite

Effects of the filament diameter on the microstructure and the critical current density (J ) were investigated in the external diffusion processed multififamentary Nb Sn-Cu superconducting composites. J was measured in transvere magnetic fields(H) of 10-17 Tesla. The mi~rostructures were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) after each step of the process. Superconducting characteristics were qualitatively rationalized with reference to the microstructures of composites. The ultrafine filaments (diameter<1pm) had severe shape instability problems (spheroidization and agglomeration) in the thermally activated reaction condition. Filaments of large diameter (>5prn) usually contained unreacted Nb cores with heat treaments typical of commercial materials, so that those composites could not exhibit their full potential. Thus the optimum range of filament diameter was decided to be lym-5pm. Nb Sn filaments, when fabricated by one of the conventional diffusional processes, consisted of three concentric shells : the core region with radially directed columnar grains, the outer shell with coarse equiaxed grains, and fine equiaxed grains in the intemediate zone. The columnar structure showed low flux pinning force, due to the orientation of the grain boundary. The coarse grains had low upper critical fields due to the high Sn concentration. The optimum microstructure with the largest volume fraction of equiaxed fine grains might be found in the filament of optimum diameter for a given heat treatment. In such filament the innenmost columnar grains barely disappear. Since most of above phenomena came from the long range diffusional processes of the reaction system, a "diffusionless" powder metallurgical process has been proposed for future work. Thesis Supervisor Dr. Robert M. Rose Title Professor of Metallurgy