Surface processes at the first stage of magnetron cathode Pd-Pd5Ba activation

Abstract

The electron emission of palladium-barium cathodes is highly effective due to the active BaO substance formation of low work function on the cathode surface (2.3 eV). The work function is a universal parameter characterizing the emission capacity of material. The degree of cathode surface coverage with the emission-active layer and distribution uniformity specificate such important parameters as the emission current and homogeneity of the cathode surface. The active metal – barium – is part of the activation phase – Pd5Ba intermetallic compound. The active substance layer is formed during the activation by vacuum cathode heat treatment. At thermal Pd5Ba dissociation, diffusing Ba atoms flux through the defects in the crystal structure of a palladium matrix to be oxidized by the residual oxygen locked in a device. This article is a first attempt to analyze the correlation of emission-active substance distribution over the cathode surface with the mechanisms of barium diffusion in the emitter. At the first stages of thermal activation, BaO active substance was locatized on the palladium-barium cathode surface by scanning electron microscopy (SEM) and high spatial resolution electron probe microanalysis. The analysis reveals that Ba atoms fluxing through grain boundaries form a line of the emission-active substance along the grain boundaries. The line width is in the region of several microns. In places of triple joints, BaO formations are mostly sizeable. BaO nanoparticles are formed on the surface of individual grains due to Ba diffusion through dislocation tubes. The series of vacuum cathode annealing showed that at T = 750 °C, BaO reproduction from diffusing Ba atoms does not compensate for the BaO evaporation rate. The annealing process, carried out for 30 min at T = 900 °C, has led to the active layer formation along the grain boundaries and to the formation of BaO nanoparticles on the surface of some grains. We introduce an original technique for assessing the surface coverage degree of a cathode covered with an emission-active substance. The technique implies using electronic microimages processed with the ImageMagick.