Superconducting properties and uniaxial strain characteristics of Nb3Sn fiber-reinforced superconductors with tantalum reinforcement fibers

We have been developing fiber-reinforced superconductors (FRS) for high-field and large-scale magnets. Tungsten fibers have hitherto been selected as the reinforcement fiber for FRS because tungsten has the highest elastic modulus, approximately 400 GPa, which can minimize the strain for electromagnetic force. The preparation of FRS consists of sputtering deposition and heat treatment because it may be difficult to apply drawing methods to materials of high elastic modulus such as tungsten. Tantalum has a high elastic modulus of 178 GPa and its thermal expansion coefficient is closer to that of Nb3Sn than that of tungsten, which means that prestrain in Nb3Sn in FRS is reduced by adopting tantalum fibers. Tantalum has been used as a barrier between bronze and copper in conventional Nb3Sn superconductors which are usually prepared by the drawing process despite tantalum's high elastic modulus. This implies that drawing may be used to prepare FRS with tantalum reinforcement fibers. In this paper, FRS using tantalum fibers prepared by sputtering are described and compared with FRS of tungsten to clarify the basic properties of FRS using tantalum fibers. Depth profiles in the Nb3Sn layer in FRS were measured to examine the reaction between superconducting layers and reinforcement fibers. Superconducting properties including strain and stress characteristics are shown. Those data will contribute to the design of FRS using tantalum reinforcement fibers in conjunction with the drawing processes. (C) 1999 Scripta Technica.