Investigation of tungsten cathodes activated with Ba2CaWO6

The investigation of chemical and topographic structure of activated tungst en cathodes has great importance in the optimisation of high pressure disch arge lamps. in spite of the numerous investigations, there are many open qu estions in connection to the transport mechanism of the activator substance s from the reservoir to the electron emitting tip. Investigations of tungst en cathodes activated by Ba2CaWO6 were performed by means of simultaneous s urface analytical methods: scanning electron microscopy (SEM), electron mic roprobe (EMP), cathodoluminescence (CL) and secondary ion mass spectrometry (SIMS). The topography of the surface was studied by SEM and CL, giving th e result that the activator substances are inhomogeneously distributed on t he surface of the tip. According to the EMP analyses the chemical compositi on of the observed precipitates on the surface of the samples corresponds t o the original activator compound with O excess. According to the SIMS spec tra, parallel migration of atomic and oxide species is possible, Lateral an d in-depth investigations show that the substances migrate mostly on the su rface of the tungsten rod, but their minor quantity may migrate also along the grain boundaries. Ratio of fluxes of migration channels could be estima ted from the crystal sizes determined and are compared to the heat of forma tion of the oxides. (C) 2001 Elsevier Science Ltd. All rights reserved.