First-Principles Study of Carbon and Vacancy Structures in Niobium

Abstract

The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important for niobium metal production and particularly for the optimization of niobium SRF technology. These atoms are present in refined sheets and can be absorbed into niobium during processing treatments, resulting in changes to the residual resistance and the performance of SRF cavities. A first-principles approach is taken to study the properties of carbon in niobium, and the results are compared and contrasted with the properties of the other interstitial impurities. The results indicate that C will likely form precipitates or atmospheres around defects rather than strongly bound complexes with other impurities. On the basis of the analysis of carbon and hydrogen near niobium lattice vacancies and small vacancy chains and clusters, the formation of extended carbon chains and hydrocarbons is not likely to occur. Association of carbon with hydrogen atoms can, however, occur through the strain fields created by interstitial binding of the impurity atoms. Calculated electronic densities of states indicate that interstitial C may have a similar effect as interstitial O on the superconducting transition temperature of Nb.

Interface-Driven Strain in Heavy Ion-Irradiated Zr/Nb Nanoscale Metallic Multilayers: Validation of Distortion Modeling via Local Strain Mapping

“…The vacancy formation energy in bulk Zr and bulk Nb is calculated to be 2.05 and 2.73 eV, respectively. These values are in good agreement with previous studies. − A single vacancy creates a compressive out-of-plane strain of 0.32 and 0.08% in Zr and Nb, respectively. On the other hand, one SIA induces a tensile out-of-plane strain of 0.27% in Nb, while introducing a contraction of 0.19% in the Zr crystal (see Table ).…”

Nitrogen infusion R&D at DESY a case study on cavity cut-outs

“…As a next step, we investigated the impact of thermal feedback. The thermal conductivity of niobium carbides is smaller than that of niobium [36] and in [50] it was argued that an increase of interstitial carbon in niobium can lead to a decrease of T c , hence increasing the surface resistance. The small surface resistance at low accelerating fields is in agreement with this model, since Nb 2 C has a T c of 1.98 K [51] and can be superconducting by proximity effect below a certain breakdown field [52].…”

Insight into reactive oxygen plasma characteristics and reaction mechanism on SRF accelerator plasma cleaning

“…J coil is the applied external current. σ is the electron conductivity, calculated from the cold plasma approximation [32], σ = e 2 n e m e (ν e + jω) (11) where m e and e are the electron mass and charge. ν e is the collision frequency of electrons with neutrals.…”

“…Nageshwar Singh et al demonstrate that techniques such as chemical processing and mechanical polishing can only reduce hydrocarbon contaminant to a certain extent but cannot eliminate them completely [5,10]. The most likely reason for hydrocarbon's appearance on the niobium surface could be the attached contamination in the ambient environment [11].…”