Development of Bronze-Processed V₃Si Multifilamentary Superconductors for AC Applications

Abstract

V₃Si multifilamentary superconductors produced by a modified bronze process are promising for AC applications. The combination of adjusting the total proportion of V to Si in the Cu-Si/V composite to -3 and reducing V filaments to -1μm enables the completion of the reaction in a short time; the V₅Si₃ initially formed around V₃Si filaments appropriately decomposes and mostly V₃Si without grain growth is produced to achieve a high overall critical current density J_c. In addition to fine grain structure of V₃Si, an increment in density of V₃Si/normal-metal interface pinning center is also responsible for the high overall J_c of 1.3×10⁹A/cm² at 5T, being comparable to that of Nb₃Sn multifilamentary wires produced by the bronze process for AC use. The resistivity of remaining V₅Si₃ layers (-15μΩcm at 4.2K) is likely to be high enough to separate the filaments from each other for electromagnetic decoupling. The effective filament diameter D_eff in fields ranging from 1.5 to 7T was -3.5μm for 1.2μm (the designed V filament diameter before heat treatment) composites with undecomposed V₅Si₃ layers, being one of the smallest D_eff reported so far for AC Nb₃Sn superconductors. Disappearance of V₅Si₃ layer after long heat treatments increased D_eff, suggesting that the existence of V₅Si₃ layer is effective to reduce D_eff. Thus, with optimal cross-sectional structure (bronze composition, overall V/Si ratio, filament spacing, filament size, etc.), the control of thickness and morphology of V₅Si₃ layer would furthermore reduce D_eff and hence AC losses.