Process for manufacturing SiC composite ceramics
A method is described for manufacturing a ceramic composite structure. The method includes wrapping ceramic fibers ( 22 ), such as SiC fibers, about the external surface of at least one form. The method further includes heating the wrapped fibers ( 22 ) to a temperature no greater than a first temperature, infiltrating voids ( 24 ) in the wrapped fibers ( 22 ) with the ceramic composite in a first vessel ( 12 ) at the first temperature, transferring the infiltrated wrapped fibers ( 22 ) from the first vessel ( 12 ) to a second vessel ( 14 ), distinct from the first vessel ( 12 ), and coating the infiltrated wrapped fibers ( 22 ) with the ceramic composite in the second vessel ( 14 ) at a second temperature, higher than the first temperature.
1. A method of manufacturing a ceramic composite structure for use as cladding for a nuclear fuel rod consisting essentially of:
providing a tube of crystalline silicon carbide fibers having an inner portion, an outer portion, and voids defined among the fibers;
optionally applying an interface coating to the tube of silicon carbide fibers in a separate vessel;
heating the tube of crystalline silicon carbide fibers to a temperature no greater than a first temperature between 800° C. and 1300° C. by one of: (i) placing the wrapped fibers in a heated vessel, (ii) flowing gases heated to the first temperature over the wrapped fibers, or (iii) inserting a heating element within the wrapped fiber and heating the inner portion of the wrapped fibers to a temperature higher than the temperature of the outer portion of the wrapped fibers;
in a first vessel, uniformly infiltrating the voids in the fibers with a crystalline silicon carbide deposit at the first temperature and at a first pressure by one or a combination of a chemical vapor infiltration process and an electrophoretic deposition process;
transferring the uniformly infiltrated fibers from the first vessel to a second vessel, separate and distinct from the first vessel, wherein the first and second vessels are separated from each other such that the conditions in each vessel do not affect the conditions in the other vessel; and,
in the separate and distinct second vessel, coating the infiltrated fibers with the silicon carbide composite to form a corrosion resistant dense crystalline outer barrier coating having less than ten percent amorphous material and greater than ninety-five percent theoretical density to form a coated crystalline silicon carbide composite structure;
wherein coating is carried out by a chemical vapor deposition process at a second temperature between 1200° C. and 1800° C. and at a second pressure by flowing a coating step precursor gas and a coating step carrier gas over the infiltrated fibers, wherein the second temperature is higher than the first temperature and the second pressure is higher than the first pressure, and
wherein prior to coating the infiltrated fibers, uniformly heating the infiltrated fibers to a temperature higher than the first temperature up to the second temperature.
2. The method recited in claim 1 wherein prior to the infiltration step, the fibers are heated to within 25° C. of the first temperature.
3. The method recited in claim 1 wherein the tube of crystalline silicon carbide fibers are formed by wrapping crystalline silicon carbide fibers about the external surface of at least one form and,
heating the form about which the fibers are wrapped.
4. The method recited in claim 3 wherein the at least one form is heated to raise the temperature of the inner portion of the wrapped fibers to a temperature higher than the temperature of the outer portion of the wrapped fibers.
5. The method recited in claim 3 wherein the wrapping step is selected from braiding and winding the ceramic fibers around the form.
6. The method recited in claim 5 wherein the form is a removable form removed after the infiltration step.
7. The method recited in claim 5 wherein the form is a SiC tube that hermetically seals the inner portion of the wrapped fibers.
8. The method recited in claim 1 wherein the infiltrating step comprises injecting an infiltrating step precursor gas carried in an infiltrating step carrier gas into the first vessel and flowing the infiltrating step precursor and carrier gases over the heated fibers.
9. The method recited in claim 8 wherein the infiltrating step precursor gas is methyltrichlorosilane and the infiltrating step carrier gas is hydrogen.
10. The method recited in claim 1 wherein the first temperature is between 1000° C. and 1200° C.
11. The method recited in claim 1 wherein the step of applying an interface coating in a separate vessel before the infiltrating step is not optional.
12. The method recited in claim 1 wherein transferring the infiltrated fibers to the second vessel comprises transfer to an intermediate vessel.
13. The method recited in claim 12 wherein the infiltrated fibers are heated in the intermediate vessel to a temperature higher than the first temperature up to the second temperature.
14. The method recited in claim 1 wherein the coating step precursor gas is methyltrichlorosilane and the coating step carrier gas is hydrogen.
15. The method recited in claim 1 wherein the coated composite structures are cooled following the coating process.
16. The method recited in claim 1 wherein the second temperature is between 1300° C. and 1500° C.
17. The method recited in claim 1 wherein the method is a batch process comprising a plurality of silicon carbide fiber tubes.
18. A method of manufacturing a ceramic composite structure for use as cladding for a nuclear fuel rod consisting essentially of:
wrapping crystalline silicon carbide fibers about the external surface of at least one form, the wrapped fibers defining voids among the wrapped fibers;
optionally applying an interface coating to the tube of silicon carbide fibers in a separate vessel;
heating the wrapped fibers to a temperature no less than 25° C. below a first temperature between 800° C. and 1300° C. by one of placing the wrapped fibers in a heated vessel, flowing gases heated to the first temperature over the wrapped fibers, or inserting a heating element within the wrapped fiber and heating the inner portion of the wrapped fibers to a temperature higher than the temperature of the outer portion of the wrapped fibers;
in a first vessel, injecting a first precursor gas carried in a first carrier gas at the first temperature and at a first pressure and flowing the precursor and carrier gases over the heated wrapped fibers to infiltrate voids in the wrapped fibers with a crystalline silicon carbide deposit;
cooling the walls of a second vessel, separate and distinct from the first vessel, wherein the first and second vessels are separated from each other such that the conditions in each of the first and second vessels do not affect the conditions in the other of the first and second vessels;
transferring the infiltrated wrapped fibers from the first vessel to the second vessel;
in the separate and distinct second vessel, uniformly heating the infiltrated wrapped fibers to a temperature higher than the first temperature up to a second temperature between 1200° C. and 1800° C.; and,
flowing a second precursor gas and a second carrier gas over the heated crystalline infiltrated wrapped fibers to deposit a corrosion resistant dense crystalline outer barrier coating thereon, the dense crystalline outer barrier coating having less than ten percent amorphous material and greater than ninety-five percent theoretical density to form a coated crystalline silicon carbide composite structure.