Dalla Lana Semione Guilherme, Vonk Vedran, Pandey Arti Dangwal, Grånäs Elin, Arndt Björn, Wenskat Marc, Hillert Wolfgang, Noei Heshmat, Stierle Andreas
Fachbereich Physik, Universität Hamburg, 22607 Hamburg, Germany.
Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany.
J Phys Condens Matter. 2021 May 24;33(26). doi: 10.1088/1361-648X/abf9b7.
Niobium's superconducting properties are affected by the presence and precipitation of impurities in the near-surface region. A systematic wide-temperature range x-ray diffraction study is presented addressing the effect of low temperatures (108 K-130 K) and annealing treatments (523 K in nitrogen atmosphere, 400 K in UHV) on the near-surface region of a hydrogen-loaded Nb(100) single-crystal. Under these conditions, the response of the natural surface oxides (NbO, NbO, and NbO) and the changes in the subsurface concentration of interstitial species in Nb are explored, thereby including the cryogenic temperature regime relevant for device operation. The formation and suppression of niobium hydrides in such conditions are also investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species (oxygen and nitrogen) occupying the octahedral sites of the Nb bcc lattice at room temperature, both in the near-surface region and in the bulk. (ii) A decrease in the concentration of interstitials within the first 10 nm from the surface at 130 K. (iii) Hydride formation suppression at temperatures as low as 130 K. These results show that mild annealing in nitrogen atmosphere can suppress the formation of superconducting-detrimental niobium hydrides, while subsurface interstitial atoms tend to segregate towards the surface at 130 K, therefore altering the local concentration of impurities within the RF penetration depth of Nb. These processes are discussed in the context of the improvement of niobium superconducting radio-frequency cavities for next-generation particle accelerators.
铌的超导性能受近表面区域杂质的存在和析出影响。本文开展了一项系统的宽温度范围X射线衍射研究，探讨低温（108 K - 130 K）和退火处理（在氮气气氛中523 K，在超高真空中400 K）对氢负载Nb(100)单晶近表面区域的影响。在这些条件下，研究了天然表面氧化物（NbO、NbO和NbO）的响应以及Nb中间隙物种的次表面浓度变化，从而涵盖了与器件运行相关的低温区域。还研究了在此类条件下氢化铌的形成与抑制情况。结果表明，这些处理会导致：（i）室温下，近表面区域和体相中占据Nb体心立方晶格八面体位置的间隙物种（氧和氮）浓度增加。（ii）在130 K时，距表面前10 nm内的间隙原子浓度降低。（iii）在低至130 K的温度下抑制氢化物形成。这些结果表明，在氮气气氛中进行温和退火可抑制对超导有害的氢化铌的形成，而在130 K时次表面间隙原子倾向于向表面偏析，从而改变了Nb射频穿透深度内杂质的局部浓度。在改进用于下一代粒子加速器的铌超导射频腔的背景下对这些过程进行了讨论。
