Cathodes with smooth oxide coatings

Modern electron tubes have the well-known major requirements of high mutual conductance i.e. small separation between grid and cathode, minimum noise level and low cut-off current. In order to provide for these conditions the oxide coating must be dimensioned to very small tolerances. The conventional and new coating techniques of spraying, painting, settling and printing are described. Further means of obtaining smooth oxide coating especially the application of centrifugal forces as well as the shaping of coatings i.e. pressing, shaving, turning etc. are discussed. The properties of cathodes prepared by the above-mentioned techniques are presented. Results obtained about surface roughness, coating thickness, edge contour, coating adherence and shrinkage are compared. Furthermore the influence of the carbonates used, of the milling time and of other processing parameters is outlined. The usefulness of these processes depends upon the geometrical configuration of the cathode sleeve. Painting, for example, is appropriate for cylindrically shaped sleeves while settling, and especially centrifugal spraying, are adapted to planar type cathodes. The latter method provides especially good coating adherence. Sharp edges may be obtained by using the printing technique.