Reactive silicon infiltration is accomplished putting silicon lumps in contact with a porous carbonaceous preform. Often the two materials are separated by a wick. Then the set up is heated (with or without vacuum) till reaching silicon melting. Over the years I have seen people putting silicon on top of the preform (using gravity and capillarity forces) or at its bottom (only capillarity). I am curious to read opinions on this point
There is no universal advice. It depends on what the purposes and conditons of infiltration processes are. I can suggest to apply the mixture of powdered silicon and refractory oxide compositions as a "bitstone" in the case if you prefer to carry on the process mainly in the mode of gas transportation (not wettability!).
Dear Dr. Nobre, Sorry, but commonly porous bodies in the Si-SiC composite production is absolutely different to those in ceramic foam technologies...
Dear Alberto Ortona,
I know you mentioned asked about. The best way put piece of Si on the top of the sample before thermal treatment. Better if you can empirically (with previouse calculation) determinate exact necessary quantity of Si - that quantity, lets it 100% keept in pores structure. Exactly that do in mass production, for Cu infiltration in porouse metal parts. But remember, as you change type of wood (or other material for obtaining carbon scleleton) you need determinate it again.
And one more also necessary necessary to control obtained material for Si which not able react with C and kept in porous structure. By this method made SiC-Si composites (SiC made from carbon skeleton in situ impregnated with excess of Si)
Hope it help you!
Dear Dr.Ortona,
The capillary forces are dominated in the infiltration process. Traditionally, silicon is supplied on the top surfaces of the preforms using the fibrous lumps. Preparation of the graphite preforms and detailed design of technology are the commercial “know how” for any producer. I have some experience in silicon infiltration in fibrous carbon materials and pressed exfoliated graphite. Microstructure of siliconized graphite cloth after burning off carbon fibers are enclosed.
A view of individual SiC fiber after chemical removal of silicon matrix.
This is exactly the discussion I wanted to trigger.
Industrially, I would then argue, silicon is supplied from the bottom. Check this video on you tube.
https://www.youtube.com/watch?v=MAzbbID6BZ0
Dear Alberto,
as mentioned above, this is part of the know how of the companies. When green bodies are made by slip casting and firing, the (brown) ceramic pieces are placed in a crucible filled with a defined amount of pices of silicon metal. Additional silicon pieces are put at the ceramic bodies on horizontal surfaces. I guess it depends on capillary forces to which height the bodies can be infiltrated from the bottom side.
Due to the anomaly of silicon solidification there are some free silicon drops on the surface after the infiltration process what could be removed by abrasive blasting
There are 2 alternative criteria in the Si-SiC composite production: "a cheaper way" and "a better structure"... You have to choose it for yourself!
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