I wonder what effect should have vacuum on capillary action. When performing silicon infiltration at ambient pressure, large pores remain empty (first pict.) That does not happen when applying vacuum (second pict.)
Capillary infiltration depends on the equilibrium between 3 phases: solid-liquid-gas. If you change one component (vacuum versus ambient atmosphere) you will change the capillary effect.
Inclusion of gas bubbles is less likely when using a vacuum condition. And the capillary force increases with decreasing pore size.
These effects may be responsible for the observed differences in infiltration of larger pores.
May I add a further consideration. Among the reasons which can be invoked to explain the "strange" results, two can have a possibility to be close to reality.
The first one is related to the status of the "wetted" surface and to its condition of surface oxidation, which should be more relevant under argon ( normally at a PO2 of the order of 10-6 atm., largely sufficient to oxidize the Si surface). Vacuum, on the contrary, can help deoxidizing the surface via the production of volatile SiO).
The second one , also interesting, is related to the Laplace capilary pressure gradient across the liquid-vacuum(gas) interface. If we take a pore having a radius R=10 micron (there are not dimensions in the figures...) we have DeltaP= 2S/R. Taking S= 0.750 N/m (roughly), we get DeltaP= 1.5 atm. This is true only if the liquid Si makes a zero contact angle with the solid Si, conditions not true. Thus, with a lower contact angle, the radius of curvature should be larger, and , as a consequence, the DeltaP even lower. This leads to see that the presence of a gas phase at a pressure close to the atmospheric one should hinder the penetration of the liquid phase.
I agree with all comments . To my mind the great difference between of vacuum and gas terms of infiltration is SiO formation. SiO may prevent wetting of graphite surface with a molten silicon. The partial pressure of SiO is 2.3 Torr.
Additional comment. You can use the gas mixture Argon + 3% Hydrogen to prevent the SiO formation.
Gentlemen,
Thank you very much. It is much clear to me why large pores do not fill in presence of a gas (air being the worst case). It is not clear to me though, why a large pore fill in vacuum. The diameter of the capillar strongly infulences the fluid flow. In such a large channel (> 100 microns diameter, to answer to Alberto) the liquid should simply "paint" the pore's wall. May be liquid-vapor-liquid phase change being an additional phenomenon providing pore filling?
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