A DFT study on nitrogen enhancing mechanism of SRF niobium cavity performance

High-temperature nitriding has been shown to improve the quality factor (Q) of the superconducting radio frequency (SRF) cavity recently, while its inherent promoting mechanism is still unclear. Therefore, based on first-principles calculations, the adsorption and dissociation processes of nitrogen molecules on the Nb(1 1 0) surface, and the adsorption, diffusion processes of nitrogen atoms on the Nb(1 1 0) surface, were studied in this paper to explain the principle of its performance improvement. To establish the reaction mechanism, adsorption preference sites were determined. All possible reaction paths were investigated for each step including the energy barrier based on the climbing image nudged elastic band (CI-NEB) method. We establish multiple reaction paths and the only optimal one was found. It is found that the Nb(1 1 0) surface can facilitate the dissociation of nitrogen molecules (with only a 1.14 eV energy barrier), and nitrogen atomic energy can readily reach the sub-surface (only a 1.73 eV energy barrier). Still, the high energy barrier (up to 6.64 eV) prevents it from diffusing inward. This revelation provides the nitriding mechanism of pure niobium and the rationale for enhancing niobium cavity performance after nitrogen mixing.