XPS analysis of the surface composition of niobium for superconducting RF cavities

• Published in: Applied surface science Vol. 126; no. 3; pp. 219 - 230
• Main Authors: Daccà, A., Gemme, G., Mattera, L., Parodi, R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.04.1998; Elsevier Science
• Subjects: Applied sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Metals, alloys and compounds (a15, 001c15, laves phases, chevrel phases, borocarbides, etc.); Metals. Metallurgy; Metals; alloys and binary compounds (including A15, Laves phases, etc.); Physics; Properties of type I and type II superconductors; Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.); RF superconductivity; Superconducting materials (excluding high-tc compounds); Superconductivity; X-ray photoelectron spectroscopy; 75.25.Nf; SVC; 74.70.Ad; PG; RF superconductivity; NG; 82.80.Pv; X-ray photoelectron spectroscopy; Cavities; Superconductors; Work functions; Transition elements; Surface properties; X radiation; Experimental study; Photoelectron spectroscopy; Niobium
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/S0169-4332(97)00790-3

The surface chemical composition of Nb samples prepared by following procedures commonly employed in the preparation of RF cavities has been studied by XPS and ARXPS. In order to understand the process occurring at surface, the thermal evolution of the surface has been studied in the 30°C÷1000°C range of temperature both in the heating and cooling phases. During the heating phase, an irreversible transition has been observed near T≅200÷300°C; it is characterized by a progressive reduction of Nb oxides from Nb 2O 5 to NbO 2 and finally to NbO. The O signal disappears near T≅1000°C and it reappears below 900°C, during the cooling phase. This can be interpreted as due to O migration in the Nb matrix, raising T, and to O diffusion towards the surface as the temperature decreases. Work function measurements show that the values of Φ are strongly correlated with the chemical composition of the surface.