Interaction of Sc and O on W

Abstract The surface chemistry on top of scandate cathodes plays a fundamental role in the performance and reliability of the cathodes. The surface chemical reaction is a complex dynamical process that involves Sc, Ba, O, W and noble metals at elevated temperatures. An initial approach is to investigate the interaction among the elements in subsets separately. In this study we investigate the interaction of Sc and O on W substrates, combining temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). There are three distinct states of Sc adsorption on W that originate from the first-layer, second-layer and multiple-layer states in a decreasing order of binding energy. The first layer grows to about 2/3 completion when the second layer begins to form, but the second layer is unstable. Agglomeration into multiple layers occurs when the second layer exceeds 1/3 layer. Sc bonds to W more strongly than Ba to W. The desorption maximum of the first-layer Sc occurs at 1575±22 K, in comparison with the desorption maximum of Ba from W at 1250±4 K. The addition of O strengthens the Sc-to-W bonds. There are also three distinct binding states that emerge simultaneously during the growth of oxidized Sc. The weakest binding state has a desorption maximum at 1732±29 K, which is about 300 K higher than the desorption maximum of the oxygen-enhanced Ba adsorption on W. Evaporation rates as a function of temperature are determined from TPD spectra of thick films. The results suggest that during activation of a scandate cathode the loss of metallic Sc can be substantial while the loss of oxidized Sc is minimal. It implies also that thermal evaporation from oxidized Sc may not be an effective method to replenish the Sc, the loss of which could occur from ion sputtering during operation in a vacuum electron device.