Wetting Properties of Molten Silicon with Graphite Materials

Abstract

The wetting behavior of refractory materials by molten silicon is important in the refining and casting of silicon with respect to production of low-cost solar cells. Here we studied the wetting properties of several graphite materials by molten silicon. These materials are used in the photovoltaic industry. The sessile drop method is employed to measure the contact angles. Initially, molten silicon does not wet graphite materials. The initial contact angles measured are approximately 120 deg. Molten silicon will react with C to form b-SiC and to infiltrate the refractory. Because losses of Si should be minimized, infiltration of Si into the refractory also is a problem. Surface roughness increases the contact area between Si and refractory and thus the loss of Si. Equilibrium wetting angles of 0 deg to 31 deg are measured. With increasing surface roughness, the equilibrium wetting angles decrease. The results show that the infiltration depth of molten silicon increases with the average pore size of graphite materials.

Wear Mechanisms of Carbon-Based Refractory Materials in Silicomanganese Tap Holes—Part I: Equilibrium Calculations and Slag and Refractory Characterization

“…[7] In these experiments small initially cylindrical slag samples were heated on a flat refractory substrate while observing the sample shape with a digital video camera. To form the cylinders, slag was ground to particles smaller than 106 lm and then compressed into a pellet with volume approximately 0.013 cm 3 .…”

“…After experiments, vertical cross sections through the slag and refractory were prepared, similar to the method described by Ciftja et al [7] : The samples were mounted in resin and subsequently cut in half through the center of the substrate and slag droplet, grinding the section with SiC paper and polishing with diamond paste, cleaning with soap and water, and ethanol, and finally drying with hot air. The polished sections were examined by reflected-light optical microscopy and scanning electron microscopy.…”

Wettability and Infiltration of Liquid Silicon on Graphite Substrates

“…However, the reactivity between Si and graphite can be a determining factor of the lifetime of graphite crucibles. Moreover, the open porosity increases this disadvantage [7].…”

“…Nevertheless, such materials have not been found yet because molten silicon is very reactive. Hence, wetting and infiltration of molten silicon into the refractories should be minimized [7]. In addition to silicon purification, knowledge about wetting and infiltration of silicon in graphite is relevant for techniques such as reaction bonding of silicon carbide (RBSC) and liquid silicon infiltration (LSI) [8].…”

“…It is known that the reaction between silicon and graphite allows the phenomena of wetting and infiltration. Wettability depends on the roughness [10], temperature [11], atmosphere [12], open porosity, and type of substrate [7]. Since the exact influence of each factor is still not known, there is a large scatter of results [6][7][8][9][10][11].…”