超导射频腔用铌材的表面化学抛光技术

Chemical Polishing of Niobium for Superconducting Radio Frequency Cavity

摘要:

高能粒子加速器的性能很大程度上依赖于超导射频铌腔的内表面状态。本文通过分析化学抛光过程中铌材的抛光速率、表面形貌和氧化状态等参数,确定了超导腔用铌材的表面化学抛光工艺：混合酸(HNO₃: HF: H₃PO₄= 1: 1: 2,体积比)为适合的抛光试剂,抛光速率方程为h = 29.359 × t _0.9247,抛光6 ~ 15 min可移除厚度154 ~ 360 μm,表面粗糙度Ra小于0.65 μm,表面Nb₂O₅层厚度小于10 nm。同时,利用电子背散射电子衍射技术(EBSD)结合表面形貌讨论了化学抛光机制,发现机加纹路消除后继续延长抛光时间,晶粒内高指数晶面(原子疏排面)会被优先抛光,从而在晶界处形成较深的沟槽,在晶粒内出现尖锐突起。

Abstract:

The performance of niobium superconducting radiofrequency (SRF) accelerator cavities strongly depends on its interior surface state. This paper describes the experimental characteristics of chemical polishing technology for niobium metal, namely polishing rate, surface topography, chemical composition and grain orientation, which have been conducted to optimize process parameters. The mixed solution of hydrofluoric acid, Nitric acid and prthoposphoric acid, 1: 1: 2 in parts by volume is proposed as appropriate polishing reagents with rate equation h = 29.359 × t _0.9247. Polishing for 6 ~ 15 min can remove 154 ~ 360 μm of material using this solution, corresponding roughness Ra and Nb2O5 layer thickness of polished surface is less than 0.65 μm and 10 nm respectively. Meanwhile, we discussed the polishing mechanism using electron back scatter diffration (EBSD) and laser scanning confocal microscope (LSCM), found that the outmost surface of over-polishing sample consists of many fine high-index planes which intersects each other to form sharp egdes in grains or at boundaries.