Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Including plural heating steps
Reexamination Certificate
2000-11-06
2002-06-11
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Including plural heating steps
C241S015000, C241S016000, C264S661000, C264S681000, C423S598000, C501S127000
Reexamination Certificate
active
06403019
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for preparing a raw material powder for producing a sintered body of aluminum titanate and to a process for preparing a sintered body of aluminum titanate.
BACKGROUND ART
Sintered bodies of aluminum titanate are known as heat-resistant materials because of their low coefficient of thermal expansion and high corrosion resistance. Such sintered bodies exhibit low wettability with slag, corrosion resistance, spalling resistance and other excellent properties when used as materials of containers, ladles, gutters, etc., for molten metals of aluminum, aluminum alloys, pig irons or the like. However, the sintered bodies of aluminum titanate, whose crystal grains constituting the sintered bodies are anisotropic, tend to suffer the following disadvantages: interfacial displacement between their crystal grains caused by stress due to thermal expansion; and formation of tiny cracks and apertures which may lead to lowered mechanical strength.
Hence, conventional sintered bodies of aluminum titanate can not exert sufficient durability when high temperature and heavy loads are applied thereto.
DISCLOSURE OF INVENTION
A primary purpose of the present invention is to provide a sintered body of aluminum titanate having improved mechanical strength and stability at high temperatures as well as their inherent properties, i.e., low coefficient of thermal expansion, high corrosion resistance and the like.
In view of the foregoing problems of the prior art, the present inventors carried out extensive research. Consequently, the inventors found that a mixture containing a finely pulverized spinel powder dispersed in an aluminum titanate powder is obtained by mixing spinel crystals which have been synthesized by heating a mixture of MgO and Al
2
O
3
at 1250° to 1290° C. with aluminum titanate crystals which have been pulverized until a maximum grain size of 30 &mgr;m to 50 &mgr;m, and then pulverizing the mixture of the spinel crystals and the aluminum titanate crystals by wet grinding process using a ball mill or cylinder mill. The inventors also found the following: by forming the obtained mixture of the spinel crystals and the aluminum titanate crystals into a desired shape and then sintering the formed product, a fine spinel powder is charged into the intergranular spaces of a sintered body of aluminum titanate, and a eutectic reaction of aluminum titanate with spinel occurs during sintering. Hence, a sintered body of aluminum titanate having high strength can be obtained, and the strength of the sintered body hardly decreases even when used at high temperatures because said sintered body has resistance to the formation and growth of tiny cracks.
The present invention provides a process for preparing a raw material powder for producing a sintered body of aluminum titanate, and a process for preparing a sintered body of aluminum titanate as mentioned below.
1. A process for preparing a raw material powder for producing a sintered body of aluminum titanate comprising:
pulverizing a mixture containing:
(1) 100 parts by weight of aluminum titanate crystals pulverized until a maximum grain size of 30 to 50 &mgr;m is attained; and
(2) 2 to 6 parts by weight of spinel crystals obtained by heating a mixture containing MgO and Al
2
O
3
at 1250° C. to 1290°, a molar ratio of MgO to Al
2
O
3
being 1:0.95 to 1:1.05,
said pulverizing being conducted by wet grinding process with a ball mill or cylinder mill.
2. The process for preparing the raw material powder according to item 1, wherein the mixture containing aluminum titanate crystals and spinel crystals is pulverized until the maximum grain size of aluminum titanate crystals becomes 3 &mgr;m or smaller.
3. The process for preparing the raw material powder according to item 1, wherein the aluminum titanate crystals are prepared by heating a mixture of titanium oxide in anatase form with &ggr;-alumina and the spinel crystals are prepared by heating &ggr;-alumina and soft-burned magnesia.
4. A process for preparing a sintered body of aluminum titanate, the process comprising the steps of forming the raw material powder obtained by the process according to item 1 into a desired shape; and sintering the formed product.
5. The process for preparing the sintered body of aluminum titanate according to item 4, wherein sintering is conducted at 1600° C. to 1700° C. and then cooling to 1200° C. is conducted at a cooling rate of 280° C./hour or higher.
The process for preparing a raw material powder for producing the sintered body of aluminum titanate of the present invention employs aluminum titanate crystals and spinel crystals as a starting material.
The aluminum titanate crystals are not particularly limited insofar as they are obtained by heating Al
2
O
3
and TiO
2
. Usable Al
2
O
3
and TiO
2
are any of those which are capable of forming aluminum titanate when heated. Usually, they are suitably selected from the materials for producing alumina ceramics, titania ceramics, aluminum titanate ceramics and like ceramics. Particularly, it is preferable to use &ggr;-Al
2
O
3
as Al
2
O
3
and titanium oxide in anatase form as TiO
2
since the reaction which forms aluminum titanate progresses readily and quickly in a high yield by heating. The &ggr;-Al
2
O
3
preferably has an average grain size of about 0.2 &mgr;m to about 1 &mgr;m and the titanium oxide in anatase form preferably has an average grain size of about 0.4 &mgr;m to about 1 &mgr;m. In this specification, a grain size is the value determined by light scattering method.
Al
2
O
3
and TiO
2
are used at a molar ratio of Al
2
O
3
:TiO
2
of about 1:0.95 to about 1:1.05, preferably in an equimolar amount.
The aluminum titanate crystals can be synthesized by uniformly mixing Al
2
O
3
and TiO
2
and then heating the mixture at about 1500° C. to about 1600° C. The heating can be usually carried out in an oxidizing atmosphere, for example, in air. The heating time is not critical and may be a length of time required to sufficiently form aluminum titanate crystals, usually about 2 hours or longer.
The spinel crystals for use in the invention are prepared by heating a mixture containing MgO and Al
2
O
3
at a molar ratio of MgO : Al
2
O
3
of about 1:0.95 to about 1:1.05, preferably about 1:1 at a temperature of about 1250° C. to about 1290° C. Thus-obtained spinel crystals can be readily pulverized and are so stable that they do not dissociate in water during wet grinding process described below. Heating the crystals at a temperature above the aforementioned range is not favorable since the resulting spinel crystals become difficult to be pulverized. The heating may be carried out in an oxidizing atmosphere, e.g., air, for about 1 to about 5 hours, preferably about 2 to about 4 hours. The starting materials for preparing the spinel may be those which are capable of forming spinel on heating and are suitably selected from conventional materials for producing spinel. For instance, &ggr;-Al
2
O
3
and soft-burned magnesia are useful as Al
2
O
3
and MgO, respectively. The &ggr;-Al
2
O
3
preferably has an average grain size of about 0.2 &mgr;m to about 1 &mgr;m, and the soft-burned magnesia preferably has an average grain size of about 1 &mgr;m to about 3.5 &mgr;m.
As mentioned in the above, it is necessary for the present invention to use aluminium titanate crystals and spinel crystals which have been separately synthesized. When all raw materials for synthesizing aluminium titanate crystals and spinel crystals are mixed and heated together, intermediate products are formed in addition to aluminium titanate and spinel. When the raw material containing such intermediate products is used for producing a sintered body of aluminium titanate, the intermediate products become unstable elements in the sintered body. These elements prevent the formation of the highly strengthened sintered body of the invention.
It is essential for the present invention that aluminum titanate crystals which have been pulverized until a maximum grain size of about 30 &mgr;m to ab
Fukuda Masaaki
Fukuda Masahiro
Fukuda Tsutomu
Armstrong Westerman Hattori McLeland & Naughton LLP
Derrington James
LandOfFree
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