Formation of Superconducting Nb3Al and Nb3Al–Ge Films

Abstract

The formation of Nb3Al or Nb3Al–Ge compounds on niobium substrates is described. The film formation was followed using optical microscopy and x-ray diffraction techniques. Electrical resistance measurements at cryogenic temperatures were obtained to determine critical transition temperature for superconductivity. The films wee prepared by three processes: (a) dipping the Nb substrate into an Al–Ge melt contained in a graphite crucible, holding for several seconds followed by a thermal diffusion treatment; (b) vacuum deposition of Al or Al–Ge on Nb followed by heat treatment similar to that in (a). The advantage of vacuum deposition is better control of the film thickness of Al or Al–Ge; (c) and simultaneous deposition of Nb and Al on Nb substrates in vacuum. The deposition rate of Nb and Al was controlled at a 3:1 atomic ratio. Germanium was simultaneously deposited with Al. The Nb3Al or Nb3(Al–Ge) films were formed at temperatures between 1350° and 1650°C for both the (a) and (b) processes with the critical superconductive transition temperature between 15 K and 17.5 K. The films prepared by simultaneous deposition of Al and Ge show that the superconducting behavior depends on the diffusion heat treatment. A heat treatment at 1000°C for 30 min resulted in a Nb3Al film having 3:1 atomic ratio. The critical superconducting temperature for the films produced by these processes was between 15 K and 16 K without an ordering heat treatment. A lowering of the heat treatment (diffusion) temperature for the simultaneously deposited films resulted in a critical superconducting temperature that is in agreement with values reported by Neugebauer for superconducting Nb3Sn. The lower heat treating temperatures of 1000° and 1200°C are expected to result in higher critical current densities.