Studies of Liquid Metal Surfaces Using Auger Spectroscopy
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MATERIALS PROCESSING IN THE REDUCED GRAVITY ENVIRONMENT OF SPACE Guy E. Rindone, editor
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STUDIES OF LIQUID METAL SURFACES USING AUGER SPECTROSCOPY
Stephen Hardy and Joseph Fine Metallurgy Division and Surface Science Division, National Bureau of Standards, Washington, DC, USA
ABSTRACT The surface composition of liquid gallium-tin alloys has been studied in an Auger electron spectrometer (AES) as a The sessile function of bulk composition and temperature. drop samples were cleaned by argon ion bombardment sputtering of the liquid. This technique produced surfaces that were entirely free of impurities within the sensitivity of AES Tin was found to be strongly and remained so for many days. Surface concenadsorbed at the liquid-vacuum interface. trations based on Auger measurements are in reasonably good agreement with values calculated from surface tension measurements interpreted in terms of a monolayer depth distribution model for the adsorbed tin. INTRODUCTION Materials processing activities in space often involve free liquid surfaces. These may occur as an inherent feature of a specific procedure such as in containerless melting and solidification or in processes involving bubbles or immiscible liquids. Free liquid surfaces also can occur unintentionally when Whenever liquids detach from container walls due to poor wetting or shrinkage. free surfaces are present, the possibility exists that surface tension gradients due to temperature or composition variations will generate fluid flow. These thermocapillary or Marangoni flows in turn influence the temperature and solute fields in the bulk liquid and ultimately the microstructure of the solid. Thus it is necessary to know what happens at these free liquid surfaces in order to However, the temperature interpret or predict the state of the bulk phases. and chemical concentration dependences of the surface tensions of most materials are poorly known and many basic questions about processes such as segregation and adsorption at liquid surfaces are as yet unanswered. In the last twenty years progress in the physics and chemistry of solid surfaces has been dramatic with the development of a host of surface sensitive These have hardly been applied to liquids, however, measurement techniques. although there would appear to be no fundamental impediment to such studies for Perhaps the oldest and most low vapor pressure and melting point materials. widely used of these surface analysis techniques is Auger electron spectroscopy, Both of these yet we know of only two previous applications to liquids [1,2]. A were attempts to measure the surface segregation in binary alloy systems. related technique, electron energy loss spectroscopy, has recently been used to study the oxidation of liquid Sn surfaces [3]. We will report here our measurements of surface segregation in liquid Ga-Sn This system was chosen for study because of alloys using Auger spectroscopy. the low vapor pressures and melting points of its components and because there are in the literature detailed a
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