Reactions Between Silicon and Graphite Substrates at High Temperature: In Situ Observations
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TRODUCTION
THE unique thermophysical and chemical properties of graphite make it an extremely versatile material that is used extensively in the production, melting, refining, and solidification of photovoltaic silicon. Graphite as a refractory material has the advantages of high chemical purity, compatible thermal expansion with Si, good thermal shock resistance, high thermal conductivity, and excellent machinability. As such, it is a very practical material for silicon melting crucibles and dies.[1] A number of studies have investigated the wettability and reaction of liquid silicon with different substrates, including graphite and vitreous carbon.[2–7] The open porosity of the graphite, surface roughness, and not the least, contact time have all been documented to greatly influence the ‘‘dynamic wetting’’ of liquid silicon on graphite. By dynamic wetting, it is implied that the system is not in a state of chemical equilibrium—liquid silicon rapidly reacts with carbon to form silicon carbide according to the reaction[8]: SiðlÞ þ CðgrÞ ¼ SiCðbÞ
½1
DG1687 K ð1414 CÞ ¼ 60; 124 J/mol;
½2
and in doing so, the apparent contact angle between a droplet of molten silicon and a carbon substrate rapidly decreases with time until either the melt is completely absorbed by the porous graphite substrate, or an intermediate layer of SiC is formed, and the wetting JESSE F. WHITE, Research Engineer, and KARL FORWALD, Senior Metallurgical Engineer, are with the Elkem AS, Technology, Kristiansand. Norway. Contact e-mail: [email protected] LUYAO MA, Graduate Student, and DU SICHEN, Professor, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted November 19, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
reaches equilibrium. In all of these studies, the experimental technique was essentially the same: A hightemperature sessile drop apparatus was used to visually document the change in apparent contact angle over time of a molten silicon droplet on a substrate under high-purity argon atmosphere or vacuum, followed by scanning electron microscopy (SEM) of the specimens to analyze the condition at the reaction interface. In some studies, oxygen concentration in the furnace inlet gas was maintained at low levels by scrubbing the argon with heated Mg chips.[3,4] Liquid silicon is known to wet silicon carbide; the equilibrium apparent contact angle of liquid silicon on SiC is reported to be approximately 8 deg at 1693 K (1420 C) and an oxygen partial pressure of 1021 MPa.[5] Li and Hausner[3] documented the effect of surface roughness on the equilibrium apparent contact angle of liquid silicon on graphite (after reaction to a SiC layer). The study of Ciftja et al.[4] confirms that the surface roughness affects the equilibrium contact angle, but not the initial rate of spreading. A few studies have taken a more fundamental approach and focused on the kinetics and mechanisms of the Si-C reaction at the liquid–solid inter
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