Abstract
Cathode-ray tubes (CRT) have been used for the last 70 years and are expected to be incorporated in television sets for a further 15 years. The oxide cathode, located within the CRT, generates electrons which are focused and scanned across a phosphorescent screen to produce an image. The oxide cathode consists of a porous BaO/SrO spray layer deposited on a Ni alloy base. During operation, the Ni cap is heated. Activators (alloying additions, such as A1 and Mg) reduce BaO to 'free' Ba, which migrates to the emissive oxide surface, reducing the work function. It has been postulated for sometime that an interfacial layer forms between the nickel and emissive layer, leading to a decrease in the performance during life. Early work suggested that the interfacial layer inhibits diffusion of activating elements to the reaction area and reduces the electron flux drawn from the cathode surface. This work examines lifetime tested commercial cathodes using modern surface and interfacial analysis techniques. The results obtained are used to provide information on interfacial reactions, the role of activators, interface layer formation and the reasons for degradation in cathode performance. The nickel-emissive oxide interface has been studied after stripping off the emissive oxide. Auger electron spectroscopy (AES) and scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis show the presence of an interfacial layer (composed of Ba, Sr, A1 and O) on cathodes (activated and aged) with no operational lifetime and those operated for 1000 and 2000 hours. Cross-sections of cathodes after varying operation times have also been examined. Cracks can be observed in the nickel cap and these cracks grow with increasing operational lifetime. There is a significant enrichment of Mg in the cracks. Focused ion beam (FIB) sections of samples exposed for 0, 1000 and 2000 hours have been prepared. Results from transmission electron microscopy (TEM) studies on these samples will be presented and clarify that the reaction products that form in the oxide cathode are composed of MgO and (Ba,Sr)Al204. Time of flight-secondary ion mass spectroscopy (ToF-SIMS) has also been used to acquire depth profiles from the underlying Ni cap and crack regions. These results are used as the basis of a model of the interactions occurring at the nickel/emissive oxide interface as a function of operational lifetime.