Understanding mechanism of performance improvement in nitrogen-doped niobium superconducting radio frequency cavity

Figures & data

Figure 1. Surface morphology characterization. The optical (a, c) and back scattered SEM (b, d) images of the non-doped (a-b) and nitrogen-doped (c-d) samples.

Figure 2. EBSD analysis of local misorientation. (a-c) non-doped sample and (d-f) N-doped sample. The blue squares in (a) and (d) indicate the ∼ 250 µm × 250 µm region within a coarse grain for EBSD scanning; IPF map of (b) non-doped and (e) N-doped samples, the domains with different colors in (e) represent regions with different orientations; (c) and (f) the corresponding KAM mapping of (b) and (e).

Figure 3. Microstructural and chemical analysis at Nb surface of the non-doped sample. (a) HRTEM image taken along [001] zone axis (ZA). (b) High-resolution BF-STEM image (top right inset is the corresponding fast Fourier transformation (FFT) image) shows the NbO_x/Nb interface area. The white and red arrows indicate the 100 and 110 facets. The inset at the bottom right shows Nb lattice distortion close to the NbO_x/Nb interface (indicated by light blue arrow). (c) schematic of Nb crystal structure with interstitial oxygen at the octahedral center, viewed along [001] ZA; (d) ABF image near the NbO_x/Nb interface. The oxygen interstitials are indicated by blue arrows. (e-g) EELS analysis of the NbO_x layer: (e) HAADF-STEM image and the first derivative of EELS line scan (color inset) of the Nb-M₃ and O-K edges; (f-g) EELS spectrum of Nb-M_{2, 3} and O-K edges from positions 1–4 in (e).

Figure 4. Microstructural and chemical analysis at Nb surface of N-doped sample. (a) HRTEM image taken along [001] ZA. (b) High-resolution HAADF-STEM image shows the NbO_x/Nb interface area (top right inset is the corresponding FFT), red and white arrows indicate the 110 and 100 facets. (c-e) EELS analysis of the NbO_x layer: (c) HAADF-STEM image and the first derivative of EELS line scan (color inset) of the Nb-M₃ and O-K edges; (d-e) EELS spectrum of Nb-M_{2, 3} and O-K edges from position 1–4 in (c).

Figure 5. Crystallized feature in NbO_x amorphous layer. (a) HRSTEM image shows a crystalized feature in the amorphous layer, amplifying in the inset. (b-c) the EELS data from regions 1 and 2.