IRREVERSIBILITY FIELD AND CRITICAL CURRENT DENSITY AS A FUNCTION OF HEAT TREATMENT TIME AND TEMPERATURE FOR A PURE NIOBIUM POWDER-IN-TUBE NB_3SN CONDUCTOR

摘要

In order to better understand the relation between microstructure, superconducting properties and flux pinning in the A15 phase, we analyzed the magnetic hysteresis and irreversibility field for various heat treatments of a pure-Nb, Nb_3Sn powder-in-tube PIT wire. The irreversibility field (4.2 K) was ～10% higher after 750℃ heat treatment as compared to 675℃ heat treatment, attaining 25.9 T and 23.5 T respectively. The critical current of the wire increased monotonically with reaction time up to 128 hours at 675℃ but was lower for 750℃ reactions. However, the critical current density of the A15 layer peaked after a heat treatment of 47 hours at 675℃ and was lower yet after 32 hours at 750℃, probably due to A15 grain growth. Our results illustrate a central compromise of Nb_3Sn composites, namely that it is not yet possible to simultaneously optimize the primary properties (T_c, H~* and H_(c2)) and the flux pinning when high critical current density demands grains of order 200 nm in diameter or less.