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

Vacuum. 2018. Vol. 154. P. 333–339.

Gaidar A. I., Bondarenko G.G., Petrov V. S., Kashirina N. V.

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 emissionactive
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.

Keywords: палладий-бариевый катод, electron emission, электронная эмиссия, palladium-barium cathode, work function, работа выхода