Suppression of Marangoni Convection with Oxide Films
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SUPPRESSION OF MARANGONI CONVECTION WITH OXIDE FILMS
J. D. VERHOEVEN, M. A. NOACK AND A. J. BEVOLO Ames Laboratory* and Department of Materials Science and Engineering, University, Ames, IA 50011.
Iowa State
ABSTRACT This research is an effort to determine whether surface tension driven convection in molten tin can be eliminated by the formation of one to several monoInitial work is presented describing layers of oxide on the molten tin surface. Auger and electron loss spectroscopies used to detect SnD and Sn02 on molten tin surfaces in a UHV system; Progress on efforts to produce and monitor controlled oxide layers of 8 to 20 A upon floating zones in a disk geometry is presented.
INTRODUCTION Perhaps, the primary motive for doing crystal growth experiments in space is the possibility of eliminating convection in the liquid because of the nearly The float zone geometry is particzero gravitational fields possible in orbit. ularly attractive because it allows controlled solidification in a containerless fashion and therefore has good potential for producing reactive materials, such However, since the as Si, free from the striation defects caused by convection. theoretical analysis of Wilcox and coworkers [1] and the experimental work of Schwabe [2] and Chun [3] it has become widely recognized that surface tension driven flow (which we will also call Marangoni convection) is likely to produce This work significant convection in float zone experiments performed in space. is concerned with the possibility of eliminating Marangoni convection in molten Those metals and semiconductors by producing thin oxide films on the surface. familiar with experimental studies on even moderately reactive molten metals realize
that
it
is
difficult
to avoid surface oxide
films with inert
gases at
the vacuum levels commonly achieved with oil pumped systems, 10-6 Torr (10-4 Pa). This is consistent with the fact that even at this fairly low pressure level, surfaces are bombarded at a rate of approximately 1 monolayer per second. Hence, it is likely that the molten zone in metals which form stable oxides will probably be covered with an oxide layer unless special precautions are taken, such as use of ultra high vacuum (UHV) levels or reducing atmospheres. If the mechanical strength of oxide films is sufficient to support the shear stress produced by the surface tension gradients encountered in float zone configurations, one expects an essentially no-slip condition at the metal/oxide In addition to possessing interface and, hence, zero Marangoni convection. adequate strength, the oxide films must be continuous across the liquid surface and possess adequate ductility to avoid breaking up under the thermal expansion There is qualitative evidence that stresses which are encountered on the zone. the mechanical strength and the ductility of oxide films on molten metals are For example, Piezteapol and Miley [4] have shown that the oxides appreciable. formed on 0.8 mm wires of Pb, Sn, Bi and Zn when heated in air or oxygen will 0 su
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