Growth of Nb3Sn coating in tin vapor-diffusion process

Abstract

The potential of Nb3Sn for superconducting radio frequency cavities is widely recognized, and renewed R&D efforts continue to bring new insights about the material's structure and properties. We have systematically coated niobium samples with Nb3Sn for different durations. Several samples were coated over (“overcoat”) multiple times to elucidate the growth kinetics of Nb3Sn during the tin vapor-diffusion process. Analysis of coated samples is consistent with the model in which tin diffuses via grain boundaries to the Nb3Sn-Nb interface, where the growth of Nb3Sn into the niobium bulk takes place. Similar scaling laws are found for Nb3Sn grain growth and layer thickness. Nonparabolic layer growth is consistent with significant grain growth, which reduces the number of Sn transport channels. An examination of patchy regions in Nb3Sn coating, revealing large single-crystalline grains, points to impeded Nb3Sn layer growth due to low grain boundary density, resulting in a significantly thinner coating in those areas.