XPS ANALYSIS OF THE SURFACE-COMPOSITION OF NIOBIUM FOR SUPERCONDUCTING RF CAVITIES

The surface chemical composition of Nb samples prepared by following p rocedures commonly employed in the preparation of RF cavities has been studied by XPS and ARXPS. In order to understand the process occurrin g at surface, the thermal evolution of the surface has been studied in the 30 degrees C divided by 1000 degrees C range of temperature both in the heating and cooling phases. During the heating phase, an irreve rsible transition has been observed near T congruent to 200 divided by 300 degrees C; it is characterized by a progressive reduction of Nb o xides from Nb2O5 to NbO2 and finally to NbO. The O signal disappears n ear T congruent to 1000 degrees C and it reappears below 900 degrees C , during the cooling phase. This can be interpreted as due to O migrat ion in the Nb matrix, raising T, and to O diffusion towards the surfac e as the temperature decreases. Work function measurements show that t he values of Phi are strongly con elated with the chemical composition of the surface. (C) 1998 Elsevier Science B.V.