Interactions of thin oxide films with a low-pressure mercury discharge

Abstract

Mercury vapour is essential for the efficient conversion of electrical power in fluorescent lamps. However, the mercury dose, which is necessary to guarantee lifetimes in the order of 15 000–20 000 h, is several times larger than the amount of mercury needed to sustain the discharge. Significant quantities of mercury are lost during the operation of the lamp, because discharge species are reactive with respect to lamp components. The reactions of mercury can proceed via different mechanisms, depending on the material. Layers of aluminium oxide and cerium oxide on soda-lime glass have been tested in a mercury discharge in order to characterise possible reaction paths leading to different kinds of bound mercury. Surface-sensitive techniques like Rutherford backscattering and X-ray photoelectron spectroscopy (XPS) have been used to characterise the mercury distribution in the samples. The results show that most of the mercury is bound at the surface and in the oxide layer. Only minor amounts were detected in the glass. Redox reactions between cerium oxide and mercury species from the discharge lead to the formation of mercury oxide, whereas mercury is bound in a metallic state in aluminium oxide.