SUPERCONDUCTIVITY IN THE NIOBIUM-TIN SYSTEM,

The critical current, temperature, and field of compositeNb-Sn wires and sintered Nb-Sn powder compacts of varying composition were found to depend on the properties and distribution of the phases present after heat treating at temperatures up to 1500 C. A voltage drop was used to judge the onset of normal behavior in specimens carrying up to 100,000 ampssq. cm which were tested in fields up to 90,000 gauss and at temperatures ranging from 4.2 to 1.95 K. The temperature ranges of stability for Nb3Sn, Nb3Sn2, Nb2Sn3 and a terminal solid solution of Sn in Nb were determined and a tentative phase diagram proposed. The transition temperature for Nb2Sn3 varies from 3.4 to 2.4 K. The Nb solid solution has a critical field of 10,500 gauss and a critical current density of 4,400 ampssq. cm at 2.2 K in a 10,000 gauss field. A systematic decrease in critical current density occurs with increasing heat treating temperature for composite wires heat treated at temperatures above 1000 C for 6 minutes. This decrease appears to be associated with a decrease in interstitial content in the Nb3Sn, or the annealing out of chemical inhomogeneities that may be responsible for high-field, high-current superconductivity. Author