Apparatus and method of densifying porous articles
A method and apparatus are disclosed for improving densification of porous substrate using a film boiling process. In particular, the disclosed method and apparatus permit more complete densification of a substrate (i.e., densification closer to the surface of the substrate) by selectively providing a sort of barrier that reduces cooling of the surface of the substrate being densified caused by contact with the relatively cool boiling liquid precursor of the densifying material, such as carbon. In particular, contact between the substrate and the liquid precursor is reduced using one or both of physical barriers (such as a mesh material) or structures that promote the formation of an insulating gaseous layer between the substrate and the liquid precursor (such as a plate closely spaced apart from the surface of the porous substrate). The barrier is moved into operational position before the applied power level increases sharply (as is known) near the end of the film boiling densification process.
1. A method of densifying a porous substrate comprising:
in a reaction chamber, submerging the porous substrate in a liquid precursor so that the liquid precursor infiltrates pores in the porous substrate; and
inductively heating the submerged porous substrate to a temperature sufficient to cause liquid precursor to pyrolyze and deposit a decomposition product within the pores of the substrate so as to densify the porous substrate, using a first power level;
increasing the power level above the first power level as a densified region within the porous substrate approaches an exterior surface of the porous substrate;
moving a barrier towards a surface of the porous substrate when the power level is increased above the first power level, the barrier being constructed and arranged to permit some contact between the liquid precursor and the porous substrate.
2. The method according to claim 1 , wherein the barrier comprises a porous mesh material layer.
3. The method according to claim 2 , wherein the porous mesh material layer is supported by a rigid frame at its periphery.
4. The method according to claim 2 , wherein the porous mesh material layer has an open porosity of between about 30% and about 60%.
5. The method according to claim 4 , wherein the porous mesh material layer is supported by a rigid frame at its periphery.
6. The method according to claim 4 , wherein the porous mesh material layer is a polytetrafluroethylene (“PTFE”) mesh material.
7. The method according to claim 6 , wherein the porous mesh material layer is supported by a rigid frame at its periphery.
8. The method according to claim 1 , wherein the barrier comprises at least one plate member disposed so as to be generally parallel to a surface of the porous substrate.
9. The method according to claim 8 , wherein the barrier comprises two substantially parallel plate members located on opposing sides of the porous substrate.
10. The method according to claim 9 , wherein a peripheral region between the two substantially parallel plate members is open to the liquid precursor.
11. The method according to claim 1 , wherein the liquid precursor comprises a hydrocarbon.
12. The method according to claim 11 , wherein the hydrocarbon is selected from the group consisting of cyclopentane, cyclohexane, 1-hexene, gasoline, toluene, methylcyclohexane, n-hexane, kerosene, hydrodesulfurized kerosene, benzene, and combinations thereof.
13. The method of claim 1 , wherein the decomposition product comprises carbon.
14. The method according to claim 1 , wherein the liquid precursor contains an organosilane.
15. The method according to claim 14 , wherein the organosilane is selected from the group consisting of methyltrichlorosilane, dimethyldichlorosilane, methyldichlorosilane, and tris-n-methyl amino silane.
16. The method according to claim 14 , wherein the decomposition product comprises silicon carbide and silicon nitride.
17. The method according to claim 16 , wherein the decomposition product is one of carbon/silicon carbide or carbon/silicon nitride.
18. The method according to claim 1 , wherein the liquid precursor is a mixture of an organosilane and a hydrocarbon.
19. The method according to claim 1 , comprising selectively moving the barrier away or towards the surface of the porous substrate.
20. The method according to claim 1 , wherein a distance between the barrier and the surface of the porous substrate along a direction perpendicular to a longitudinal axis of the porous substrate when the power level is below the first power level is greater than a distance between the barrier and the surface of the porous substrate along said direction when the power level is above the first power level.
21. The method according to claim 20 , wherein said distance between the barrier and the surface of the porous substrate along said direction when the power level is above the first power level is lower than about 5 mm.