EXPERIMENTAL DETERMINATION OF WORK FUNCTION ABOUT SILVER ALLOYS LEFORT André, AKBI Mohamed, PARIZET Marie-José Laboratoire "Arc Electrique et Plasmas Thermiques" CNRS : URA 828 - Université Blaise Pascal 24, Avenue des Landais - F 63177 AUBIERE CEDEX ABSTRACT Models about arc roots need a good knowledge of physical constants characterizing contt material. With pure metal, all the constants are well known ; but for alloys some are not known. In this paper we relate how we have measured work function of silver alloys. Measuring method used is the Fowler one where photoelectronic current produced by different ultraviolet radiations is measured in high vacuum. Experimental set-up have been tested for pure materials and then, have been applied to some silver alloys. Results about pure metals show an evolution of the work function with temperature ; it decreases when temperature rncreases. Results about silver-nickel alloys show a large dependence of obtained results with the preparauon of contact surface. For example, polishing and cleaning induce, because of atom migration, a large modification of the ratio of the two components on surface ; this phenomenon induces a variation of work function comparatively with obtained values with priitary product. By heating alloy contact in high vacuum we have observed large variations of work function, which result from material component vaporisation by sheets. Surface analyses by electron microscope produce proves of the phenomenon. An empirical relation is given to calculate work function of binary alloy when this constant is known for each alloy componenL 1. INTRODUCTION Modelization of phenomena describing arc mot evolution needs accurate data on material that gives rise to iL Several authors1'2 have shown that surface state and its composition from initial ones, have an effect on cathodic spot duration, on their dimensions and cathode erosion ; this is due to electron crossing from electrode metallic state to plasma state of ionizing region. In this case, presence of materials different from basic one or existence of micro tips yields electric fields which extract electrons. To improve understanding of these phenomena, we have measured work function for different cases of basic material evolution. Experimental method developed and used3 is based on measuring, in vacuum, photoelectric currents emitted by electrode exposed to ultraviolet radiations of different wave lengths and same intensity. In the first part of this paper, we give our results for three pure metals (silver, copper and nickel) versus temperature. In second part, we show how varies work function of AgNi alloy versus physical conditions and its composition. 2. RESULTS FOR PURE METALS Electrodes used, made with pure metal (99.99 %), have a cylindrical shape with 8 mm in diameter, 3.5mm in height and are exposed to IJV radiation ; their actived surface is curved with curvature radius of 18 mm. Before using them, contacts are polished and then cleaned and rinsed with distilled water. 486 /SPIE Vol. 2259 O-8194-1581-2/94/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/19/2015 Terms of Use: http://spiedl.org/terms