Speaker
Description
The surface chemistry of superconducting radio-frequency (SRF) niobium cavities during medium-temperature baking (MTB, ~250–400 °C) has been widely investigated. However, atomic-scale insight into how water molecules—inevitably introduced by high-pressure rinsing (HPR)—interact with niobium and its surface oxides remains largely absent. Here, using first-principles calculations, we systematically study water adsorption on clean Nb and on its representative oxide surfaces. Our preliminary results show that water physisorbs weakly on metallic niobium, whereas on oxidized surfaces it spontaneously dissociates with the aid of lattice oxygen, forming two surface hydroxyl groups in a significantly more stable chemisorbed state. We further assess the desorption kinetics of these hydroxyl groups through ab initio molecular dynamics (AIMD) simulations at selected temperatures. These adsorption and desorption characteristics may change the loss tangent of the surface oxides. They may also correlate with the precipitation of dissolved hydrogen during cavity cooldown, which leads to hydride-related Q degradation. These findings offer new insight into the microscopic origin of the MTB window.