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    Applied Surface Analysis Thursday Sessions
       Session AS-ThM

Paper AS-ThM5
Oxidation of Scandium Layers on Tungsten

Thursday, November 1, 2001, 9:40 am, Room 134

Session: Oxides and Oxidation
Presenter: A. Shih, Naval Research Laboratory
Authors: A. Shih, Naval Research Laboratory
J.E. Yater, Naval Research Laboratory
C. Hor, Naval Research Laboratory
R. Abrams, Naval Research Laboratory
Correspondent: Click to Email
Scandate cathodes have demonstrated an emission density as high as 400A/cm@super 2@ at 1300 K. In order to develop a reliable fabrication technique for scandate cathodes, a thorough understanding of the surface chemistry involving Sc, Ba, O and W is essential. An initial approach is to investigate the interaction among the elements in subsets separately. In this study, Sc on W is studied by AES and TPD. The study reveals three states of Sc adsorption on W that originate from the first-, second- and multi ple-layer. It is desirable to incorporate metallic Sc in the fabrication because its high mobility is conducive to coating uniformity and to Sc replenishment. However, can metallic Sc survive the cathode activation process, which takes a few hours at 1500 K? The desorption-rate equation for the multiple-layer state has been determined as a function of the substrate temperature. At 1500 K, the evaporation rate is 40 µm/hour. Oxidation increases the binding energy of Sc. At 1500 K the evaporation rate of oxidized Sc is less than 0.04 layers/hour. Consequently Sc in the scandate coating should be in an oxidized form. We studied the oxidation process of Sc films and found that at room temperature oxygen has a very limited diffusion length in Sc films. In a 13-layer film after an exposure of 2000 L, 7 layers of Sc remains unoxidized. However, an elevated substrate temperature accelerates the oxidation process. For a 13-layer film, 3 layers and 1.5 layers of Sc remain un-oxidized after an oxygen exposure 48 L at substrate temperatures of 520K and 720K, respectively. This underlies the need of a thermal treatment of a scandate cathode in an oxidizing environment before activation.