Pressure gradient CVI/CVD apparatus and method
An apparatus and method for densifying porous structures inside a furnace using pressure gradient CVI/CVD. The apparatus includes a stack of porous structures where each porous structure has an aperture therethrough. The apparatus also includes at least one ring-like spacer disposed within the stack of porous structures between neighboring porous structures. The ring-like spacer encircles the apertures of the neighboring porous structures. The stack of porous structures and the at least one ring-like spacer define an enclosed cavity including each porous structure aperture. A channel provides fluid communication between the enclosed cavity and an outer volume defined by the interior surface of the furnace.
1. A process for densifying porous structures inside a furnace, the process comprising:
providing a furnace, the furnace defining an outer volume;
assembling a multitude of porous structures and ring-shaped spacers in a stack with a ring-shaped spacer between each adjacent pair of porous structures, wherein at least one of the ring-shaped spacers comprises a plurality of channels;
disposing the stack of porous structures between a bottom plate and a top plate in the furnace, wherein the bottom plate, the stack of porous structures, and the ring-shaped spacers define an enclosed cavity extending from the bottom plate, including each porous structure aperture, and terminating proximate the top plate;
providing fluid communication between the enclosed cavity and the outer volume through the plurality of channels in the at least one ring-shaped spacer;
allowing a gas to flow through the channel while maintaining a pressure gradient between the enclosed cavity and the outer volume; and
densifying the porous structures from an average density of less than 0.60 g/cm 3 to an average density of greater than 1.70 g/cm 3 in a single cycle of pressure gradient CVI/CVD.
2. The process of claim 1 wherein at least one of the ring-shaped spacers comprises a radially oriented channel providing fluid communication between the enclosed cavity and the outer volume.
3. The process of claim 1 wherein the porosity of the porous structures after densification is less than 15%.
4. The process of claim 1 further comprising the step of regulating the pressure differential between the enclosed cavity and the outer volume so that it does not exceed a predetermined value.
5. The process of claim 1 further comprising the steps of
providing a bottom pressure release between the enclosed cavity and the outer volume, and
releasing gas through the bottom pressure release when a pressure differential between the enclosed cavity and the outer volume reaches a predetermined value.
6. The process of claim 5 wherein the predetermined pressure differential value is between 10 and 40 torr.
7. The process of claim 1 further comprising the step of placing a plurality of spacer blocks between the porous structures, wherein spacer blocks are placed near the outer diameter of a porous structure if an inner-diameter type spacer is used, and wherein spacer blocks are placed near the inner diameter of a porous structure if an outer-diameter type spacer is used.
8. The process of claim 1 wherein the pressure in the enclosed cavity is at least about 11 torr.
9. The process of claim 1 wherein the pressure in the enclosed cavity is at least about 16 torr.
10. The process of claim 1 wherein the pressure in the enclosed cavity is between about 11 torr and about 50 torr.
11. The process of claim 1 wherein the gas comprises between 5% and 20% reactant gases.
12. The process of claim 1 wherein the pressure in the outer volume is between 5 and 15 torr.
13. A process for densifying porous structures inside a furnace, the process comprising:
providing a furnace, the furnace defining an outer volume;
assembling a multitude of porous structures and ring-shaped spacers in a stack with a ring-shaped spacer between each adjacent pair of porous structures, wherein a majority of the ring-shaped spacers comprises at least one channel;
disposing the stack of porous structures between a bottom plate and a top plate in the furnace, wherein the bottom plate, the stack of porous structures, and the ring-shaped spacers define an enclosed cavity extending from the bottom plate, including each porous structure aperture, and terminating proximate the top plate;
introducing a reactant gas into the enclosed cavity;
introducing a portion of the reactant gas into the outer volume while maintaining a pressure gradient between the enclosed cavity and the outer volume, wherein the at least one channel in at least one of the ring-shaped spacers provides fluid communication between the enclosed cavity and the outer volume; and
densifying the porous structures from an average density of less than 0.60 g/cm 3 to an average density of greater than 1.70 g/cm 3 in a single cycle of pressure gradient CVI/CVD.
14. The process of claim 13 wherein a concentration of reactant gas in the enclosed cavity is greater than a concentration of reactant gas in the outer volume.
15. The process of claim 13 wherein at least some of the ring-shaped spacers comprise a radially oriented channel providing fluid communication between the enclosed cavity and the outer volume.
16. The process of claim 13 wherein the pressure in the enclosed cavity is at least about 11 torr.
17. The process of claim 13 wherein the pressure in the enclosed cavity is at least about 16 torr.
18. The process of claim 13 wherein the pressure in the enclosed cavity is between about 11 torr and about 50 torr.
19. The process of claim 13 wherein the pressure in the outer volume is between 5 and 15 torr.