Method of forming mullite bodies
A method of forming a porous mullite composition of acicular mullite grains having improved properties is described, where the mullite is formed at some time in the presence of a fluorine containing gas. For example, it has been discovered that improved properties may result from heating the mullite to a high temperature in an atmosphere selected from the group consisting of water vapor, oxygen, an inert gas or mixtures thereof or forming the mullite composition from precursors having an AL/Si ratio of at most 2.95.
1. A method for preparing an acicular mullite composition, the method comprising,
a) forming a mixture of one or more precursor compounds having the elements present in mullite wherein one of the precursor compounds is clay,
b) shaping the mixture into a porous green shape
c) heating the porous green shape of step (b) under an atmosphere having a fluorine containing gas and to a temperature sufficient to form an untreated mullite composition comprised substantially of acicular mullite grains that are essentially chemically bound and
d) subsequently heating the untreated mullite composition to a heat treatment temperature of at least 950° C. under a heat treatment atmosphere selected from the group consisting of water vapor, oxygen, air, an inert gas or mixtures thereof for a time sufficient to form the mullite composition.
2. The method of claim 1 wherein the precursor compounds are clay and another compound selected from the group consisting of alumina, silica, fluorotopaz, zeolite, AlF 3 and mixtures thereof.
3. The method of claim 2 wherein the other precursor compounds are selected from the group consisting of alumina, silica, fluorotopaz, zeolite, and mixtures thereof.
4. The method of claim 1 wherein the precursor compounds are alumina, silica and clay.
5. The method of claim 1 wherein the fluorine containing gas is SiF 4 separately provided.
6. The method of claim 1 wherein the heat treatment temperature is at least 1000° C.
7. The method of claim 1 wherein the heat treatment temperature is at least 1050° C.
8. The method of claim 1 wherein the heat treatment temperature is at least 1100° C.
9. The method of claim 1 wherein the heat treatment temperature is at least 1200° C.
10. The method of claim 1 wherein the heat treatment atmosphere is selected from the group consisting of air, an inert gas, water vapor and a mixture thereof.
11. The method of claim 10 wherein the heat treatment atmosphere is air or nitrogen.
12. The method of claim 1 wherein the heating of step (c) is to a first temperature and then to a second higher temperature, wherein fluorotopaz is formed at the first temperature and the mullite is formed at the second higher temperature.
13. The method of claim 8 wherein the fluorotopaz formed at the first temperature is formed in an atmosphere comprised of SiF 4 separately provided.
14. The method of claim 13 wherein the first temperature is from 500° C. to 950° C.
15. The method of claim 14 wherein the first temperature is at least 725° C. to 750° C.
16. The method of claim 13 wherein the second temperature is at least 1000° C. to at most 1300° C.
17. The method of claim 1 wherein the heat treatment time is at least 0.5 hour.
18. The method of claim 1 wherein the heat treatment time is at least 2 hours.
19. The method of claim 1 wherein the heat treatment time is at least 4 hours.
20. The method of claim 1 wherein the heat treatment time is at least 8 hours.
21. The method of claim 1 wherein the precursors have an Al/Si bulk stoichiometric ratio of at most 2.95.
22. The method of claim 21 wherein the Al/Si bulk stoichiometric ratio is at most 2.9.
23. The method of claim 22 wherein the Al/Si bulk stoichiometric ratio is at least 2.