Abstract
The emission material in High Intensity Discharge (HID) lamps plays a significant role in lowering the electrode work function and thus the lamps´ operating temperature. Ba-containing rare earth- and alkali earth tungstate materials are commonly used as cathode thermionic emission materials because of the production of high intensity discharge. The goal of this present work is to model a cathode tip surface evolved during sintering and to compare the physical and chemical properties of emission materials currently used in the industry. In order to achieve this goal, we investigated and compared the layer structure of Ba-Ca- and Ba-Y-containing emission materials evolved on polycrystalline tungsten foils. Simultaneously, the tendency of the work function was also monitored as a function of Ba/BaO layer thickness. The Ba coverage of cathode is one of the most important factors during the lifetime of HID lamps. The initial Ba diffusion was also examined. We also proposed a layer model, valuable for the structures occurring during the operation of lamps. The chemical composition of the flat samples was analysed by X-ray Photoelectron Spectroscopy (XPS) and the electron emission properties by Work Function Spectroscopy (WFS).
Highlights
Barium has a considerable importance in lighting technology as a work function lowering element, which is shown by the Richardson-Dushmann equation [1], where a lower work function and a higher thermionic current are achieved at the same temperature
In the commercial high-pressure discharge lamps the mixture of Ba-containing rare earth- and alkali earth tungstate materials are used as an emission material to lower the electrode work function [2]
The work function was measured at 4,1 eV in case of both emission materials
Summary
Barium has a considerable importance in lighting technology as a work function lowering element, which is shown by the Richardson-Dushmann equation [1], where a lower work function and a higher thermionic current are achieved at the same temperature. The Ba coverage results in an improved lamp performance by lowering the electron work function ( ) of the tungsten electrodes, and by lowering the operating temperature ( gg= 4,5 eV for clean W surface [4], = 2,0-2,30 eV for BaO monolayer on W at 1000 – 1200 K [5,6,7]). In 1957 Rittner et al [4] found the following three facts: (i) the emitting surface of the cathode consists of barium on clean tungsten, (ii) oxygen is an important constituent of the dipole layer throughout the cathode life, and (iii) the oxygen coverage appears to be nearly monoatomic. Rittner observed that the end of lamp life is in
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