Chemical Vapor Deposition of Nb3Ge

Abstract

The discovery by Gavaler [1] that sputtered thin films of the A-15 compound Nb3Ge have a superconducting transition temperature T c in excess of 22 K, as well as attractive high-field properties, prompted an examination of the available methods for the synthesis of high-T c Nb3Ge by a process more adapted to fabrication in useful form, e.g., as a metallic tape. The difficulties in obtaining high-T c Nb3Ge are compounded relative to, for example, Nb3Sn, because of the occurrence of competing tetragonal phases in the Nb-Ge system. Moreover, it is believed that the highest-T c material is only stable at temperatures below 1000°C, so that a synthesis technique that can be used at reasonably low temperatures is required. The choices available were chemical vapor deposition (CVD), physical evaporation, and a diffusion process. Of these the CVD process was chosen as offering the most immediate promise of success. The choice was based on the earlier successes in using CVD for synthesis of Nb3Sn [2] and Nb3Ga [3] and the previous CVD synthesis of Nb3Ge with T c values up to 19 K[4]. Results of initial experiments have been reported in two earlier papers [5,6]. The results of parallel, independent efforts were reported by Newkirk et al. [7], Kawamura and Tachikawa [8], and Vieland and Wicklund[9]. High-T c material was obtained in all cases, thus confirming the potential of CVD. The purpose of this paper is to report on progress of the Nb3Ge synthesis effort and on the superconducting properties obtained.