Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Including plural heating steps
Reexamination Certificate
1999-08-06
2001-03-06
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Including plural heating steps
C423S598000, C501S127000
Reexamination Certificate
active
06197248
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for preparing a powder of aluminum titanate and to a process for preparing a sintered body of aluminum titanate.
PRIOR ART
Aluminum titanate (TiAl
2
O
5
) ceramics were heretofore known as having high cutting capability and machinability. Aluminum titanate is a compound having a high melting point of 1860° C. However, aluminum titanate materials conventionally used have low purity and contain oxides such as Al
2
O
3
, TiO
2
or the like. Consequently molded products of these materials can not exhibit the inherent heat resistance of aluminum titanate due to a large amount of impurities present therein. At high temperatures, such molded products tend to form small cracks in the interior and show poor physical strength. Thus said molded products are unsatisfactory as heat-resistant products for use at high temperatures.
Various ceramics usable at high temperatures are known and include, for example, those made of oxides (such as alumina, mullite, magnesia, zirconia and the like), carbides, nitrides, borides and the like. Such ceramics have limited applications because they are susceptible to thermochemical erosion and can not be machined because of low cutting capability.
DISCLOSURE OF THE INVENTION
A main object of the present invention is to provide a process for preparing ceramics which are excellent in cutting capability and thus capable of being machined, further the ceramics being outstanding in heat resistance and stably usable for a long term in high temperature environments.
The present inventors carried out extensive research to overcome the foregoing problems of the prior art. Focusing on aluminum titanate ceramics excellent in cutting capability, the inventors made investigations to increase the heat resistance of such ceramics. For this purpose, the inventors pressed a mixture of Al
2
O
3
, TiO
2
, specific additives and a small amount of an organic substance powder; sintered the obtained compact molded product in a closed container at a high temperature; and pulverized the obtained sintered product of aluminum titanate to give a powder of aluminum titanate. Then, it was found that in the obtained powder, the aluminum titanate was stably crystalized and the amount of impurities was very small. Thus the powder is suitable as a material for aluminum titanate ceramics. A further finding is as follows: when the obtained powder is molded, the molded product gives a heat-resistant ceramic molded product by being sintered at a relatively low temperature lower than the temperature conventionally applied for sintering the molded product of aluminum titanate because stable aluminum titanate crystals have already been formed in preparing the aluminum titanate powder. The present invention was completed based on these novel findings.
According to the present invention, there is provided a process for preparing a powder of aluminum titanate, comprising the steps of:
subjecting to pressure molding a mixture of 100 parts by weight of a mixture of Al
2
O
3
and TiO
2
at a molar ratio of the former:the latter of 1:0.95-1.05, 2 to 5 parts by weight of SiO
2
, 2 to 5 parts by weight of iron oxide calculated as Fe
2
O
3
and 1 to 3 parts by weight of a powder of an organic substance to give a molded product;
sintering the molded product at a temperature of 1600 to 1700° C. in a closed container; and
pulverizing the molded product.
According to the present invention, there is also provided a process for preparing a sintered body of aluminum titanate, comprising the steps of molding the powder of aluminum titanate prepared by the above-mentioned process and sintering the molded product at 1450 to 1550° C.
In the present invention, Al
2
O
3
, TiO
2
, iron oxide, SiO
2
, and a powder of an organic substance are used as raw materials.
Among the raw materials, Al
2
O
3
and TiO
2
are the components which form aluminum titanate when sintered. These components are used at a molar ratio of Al
2
O
3
:TiO
2
of 1:0.95-1.05, preferably in an equimolar amount. Useful materials as Al
2
O
3
and TiO
2
are not limited insofar as they are capable of forming aluminum titanate crystals on sintering. Usually they are suitably selected for use from the materials conventionally used as Al
2
O
3
and TiO
2
. Especially when activated alumina is used as Al
2
O
3
and anatase-type oxide is used as TiO
2
, aluminum titanate can be formed in a short time in a high yield due to a high reactivity of the two components.
Iron oxide serves to promote the sinterability of Al
2
O
3
and TiO
2
. Various iron oxides such as Fe
2
O
3
, FeO, mixtures thereof and the like can be used for this purpose. The amount of iron oxide used is about 2 to about 5 parts by weight, preferably about 3 to about 4 parts by weight, calculated as Fe
2
O
3
per 100 parts by weight of the combined amount of Al
2
O
3
and TiO
2
.
SiO
2
prevents the diffusion of TiO
2
due to an increase in the viscosity of a melt of Al
2
O
3
and TiO
2
on sintering thereof for forming aluminum titanate, thereby making it easy to obtain a melt of Al
2
O
3
and TiO
2
in an equimolar amount. The amount of SiO
2
used is about 2 to about 5 parts by weight, preferably about 3 to about 4 parts by weight, per 100 parts by weight of the combined amount of Al
2
O
3
and TiO
2
.
The powder of an organic substance decomposes and dissipates on sintering of molded product formed from a mixture of raw materials, giving a porous structure to the molded product. There is no restriction on the powders of organic substances insofar as the powders decompose and dissipate at the sintering temperature without leaving any ash. Specific examples of the organic substance powder are powders of resins, powders of charcoal, etc. Useful organic substance powders are those having a particle size of preferably about 50 to about 5000 &mgr;m, more preferably about 100 to about 2000 &mgr;m. The amount of the organic substance powder used is about 1 to about 3 parts by weight, preferably about 2 to about 3 parts by weight, per 100 parts by weight of the combined amount of Al
2
O
3
and TiO
2
.
In the present invention, Al
2
O
3
, TiO
2
, iron oxide, SiO
2
, and the powder of an organic substance are uniformly mixed and subjected to pressure molding, giving a molded product having a high powder density. In the pressure molding, a pressure of about 300 to about 600 kg/cm
2
is preferably applied. A binder may be used as selected from binders conventionally used in preparing a sintered body. Specific examples of such binders are polyvinyl alcohol (PVA), microwax, carboxymethyl cellulose (CMC), ester of acrylic acid, stearic acid, etc. The amount of the binder used may be the same as the amount thereof conventionally used and is about 1 to about 5 parts by weight, per 100 parts by weight of the raw materials.
The shapes of molded products are not limited insofar as the molded products have a high powder density. A block shape is preferred from the viewpoints of easiness in molding and an efficiency in filling the molded product into a furnace for sintering.
Then the molded product is sintered in a closed container at a temperature of about 1600 to about 1700° C., preferably about 1600 to about 1650° C.
The molded product used in the sintering process is far smaller in surface area than in the form of a powder, because it was molded under a high pressure and made compact. Therefore in the sintering process, a co-melt of Al
2
O
3
and TiO
2
in an equimolar amount is likely to form without diffusion of semi-molten TiO
2
to the surface of the molded product, so that stable aluminum titanate crystals are formed, also due to the effect of iron oxide and SiO
2
used as additives. During sintering, the oxygen in the closed container is consumed for the decomposition of powdery organic substance, whereby the oxidation of titanium or aluminum is hindered, and aluminum titanate (TiAl
2
O
5
) crystals are formed in a high yield. Further, the powdery organic substance used as the raw material decomposes and dissipates during sintering, so
Fukuda Masahiro
Fukuda Tsutomu
Armstrong, Westerman Hattori, McLeland & Naughton
Fiorilla Christopher A.
Fukuda Tsutomo
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