Ba enrichment on the surface of oxide cathodes

We investigated the chemical structure of actual oxide cathode emission materials using soft x-ray absorption spectroscopy. High-energy resolution spectra of the Ba $3d$ absorption edges reveal that the Ba content significantly increases on the surface layers of oxide cathodes down to several tens of nanometers in depth, after the cathode activation process. Furthermore, we will demonstrate that the excess Ba on the surface is only slightly driven by thermal energy, but rather it is induced by the voltage difference applied during cathode activation. This result suggests that the rate controlling step of the Ba enrichment on the surface during activation is the electrolytic transport of Ba ion from the bulk powder to the interface. We assume that the Ba enrichment on the surface originates from the depletion of barium in bulk powder by the electrolytic transport.