Investigation of tungsten cathodes activated with Ba 2CaWO 6

Abstract

The investigation of chemical and topographic structure of activated tungsten cathodes has great importance in the optimisation of high pressure discharge lamps. In spite of the numerous investigations, there are many open questions in connection to the transport mechanism of the activator substances from the reservoir to the electron emitting tip. Investigations of tungsten cathodes activated by Ba 2CaWO 6 were performed by means of simultaneous surface analytical methods: scanning electron microscopy (SEM), electron microprobe (EMP), cathodoluminescence (CL) and secondary ion mass spectrometry (SIMS). The topography of the surface was studied by SEM and CL, giving the result that the activator substances are inhomogeneously distributed on the surface of the tip. According to the EMP analyses the chemical composition of the observed precipitates on the surface of the samples corresponds to the original activator compound with O excess. According to the SIMS spectra, parallel migration of atomic and oxide species is possible. Lateral and in-depth investigations show that the substances migrate mostly on the surface of the tungsten rod, but their minor quantity may migrate also along the grain boundaries. Ratio of fluxes of migration channels could be estimated from the crystal sizes determined and are compared to the heat of formation of the oxides.