Low-expansion ceramics and method of producing the same

The invention relates to low-expansion ceramics, the chemical composition of which consists of 1.2 to 20% by weight of magnesia (MgO), 6.5 to 68% by weight of alumina (Al.sub.2 O.sub.3), 19 to 80% by weight of titanium in terms of titanium oxide (TiO.sub.2), 1 to 20% by weight of silica (SiO.sub.2), and 0.5 to 20% by weight of iron in terms of ferric oxide (Fe.sub.2 O.sub.3); the major component of crystalline phase thereof is a solid solution of magnesium oxide-aluminum oxide-titanium dioxide-silicon oxide-iron oxide; the ceramics have a coefficient of thermal expansion of not more than 20.times.10.sup.-7 (1/.degree.C.) in a temperature range of 25.degree. C. to 800.degree. C., a four-point flexural strength of not smaller than 50 kg/cm.sup.2 at room temperature, and a melting point of not lower than 1,500.degree. C.; and a method of producing low-expansion ceramics comprising the steps of preparing a batch, plasticizing the batch, if necessary, and shaping the batch; drying the body thus shaped; and firing the shaped body at 1,300.degree. C. to 1,700.degree. C., thereby producing ceramics having a coefficient of thermal expansion of not more than 20.times.10.sup.-7 (1/.degree.C.) in a temperature range of 25.degree. C. to 800.degree. C., a four-point flexural strength of not smaller than 50 kg/cm.sup.2 at room temperature, and a melting point of not lower than 1,500.degree. C., and wherein the shape of ceramics is a honeycomb structure.

BACKGROUND OF THE INVENTION 
This invention relates to low-expansion ceramics having a small coefficient 
of thermal expansion, a high melting point, and a high mechanical 
strength. The invention also relates to a method of producing the 
low-expansion ceramics. 
With the progress of technology in recent years, demand for material having 
an excellent heat-resistance and an excellent thermal shock-resistance is 
increasing. The thermal shock-resistance of ceramics depends on 
characteristics of the materials, such as the coefficient of thermal 
expansion, the heat conductivity, the mechanical strength, the Young's 
modulus, and the Poisson's ratio. The thermal shock-resistance is also 
affected by the size and shape of the goods concerned and the conditions 
of heating and cooling or the rate of heat propagation. Among those 
factors affecting the thermal shock-resistance, the contribution of the 
coefficient of thermal expansion is especially large, and when the rate of 
heat propagation is high, the thermal shock-resistance is ruled almost 
solely by the coefficient of thermal expansion, as well known to those 
skilled in the art. Accordingly, there is a strong demand for development 
of low-expansion material with excellent resistance against thermal shock. 
As ceramics with a comparatively low thermal expansion, which has a 
coefficient of thermal expansion in the order of 5 to 20.times.10.sup.-7 
(1/.degree.C.) in a temperature range of 25.degree. C. to 800.degree. C., 
cordierite (MAS) and lithium-aluminum-silicate (LAS) are known. However, 
such known ceramics have a low melting point, e.g., the melting point of 
cordierite is 1,450.degree. C. and that of lithium-aluminum-silicate is 
1,423.degree. C. For instance, when the ceramics honeycomb is used as a 
catalyst substrate for catalytic purifying apparatus of automobiles, even 
the honeycomb substrate using cordierite with a high melting point have 
been found vulnerable to plugging due to melting if the temperature of the 
catalyst bed is increased by 100.degree. C. to 200.degree. C. over that of 
conventional catalyst beds. The increase of the temperature of the 
catalyst bed is caused by modification of the mounting position of the 
catalytic converter from the conventional location of under bed to engine 
proximity for improving the purifying efficiency of the catalyst and by 
modification of design involving the mounting of a turbo-charger for 
improving the fuel economy and enging output, which modifications cause an 
increase in the exhaust gas temperature as compared with that of 
conventional apparatus. Accordingly, the development of low-expansion 
material having an excellent heat-resistance, which also has an excellent 
thermal shock-resistance equivalent to or better than that of cordierite, 
has been strongly demanded. 
On the other hand, ceramics with low-expansion characteristics generally 
have different values of the coefficient of thermal expansion for 
different directions of crystalline axes of the crystals forming the 
ceramics, which different values tend to cause thermal stress in the 
ceramics, and as the thermal stress exceeds critical strengths of the 
constituent crystals and grain boundaries, micro cracks are formed in 
grains and grain boundaries to reduce the mechanical strength thereof. For 
instance, in the case of ceramic honeycombs for catalytic substrate of 
automobile catalytic purifying apparatus, breakage may be caused in the 
ceramics as the ceramic honeycomb is pushed into a catalytic converter, or 
cracks and breakages are easily caused in the ceramics during automobile 
running due to vibration and other mechanical shocks. To overcome such 
difficulties, there is a strong demand for developing low-expansion 
materials having a high strength available for catalytic substrate. 
BRIEF SUMMARY OF THE INVENTION 
The low-expansion ceramics according to the present invention obviates the 
aforesaid shortcomings and difficulties of the prior art. The inventors 
have succeeded in achieving a low-expansion ceramics having a low 
coefficient of thermal expansion, a high melting point, and high strength, 
by producing the ceramics with magnesia, alumina, silica, and iron oxide. 
The low-expansion ceramics of the invention have a chemical composition 
consisting of 1.2 to 20% by weight of magnesia (MgO), 6.5 to 68% by weight 
of alumina (Al.sub.2 O.sub.3), 19 to 80% by weight of titanium in terms of 
titanium oxide (TiO.sub.2), 1 to 20% by weight of silica (SiO.sub.2), and 
0.5 to 20% by weight of iron in terms of ferric oxide (Fe.sub.2 O.sub.3), 
preferably 2 to 17% by weight of magnesia (MgO), 11 to 62% by weight of 
alumina (Al.sub.2 O.sub.3), 25 to 75% by weight of titanium in terms of 
titanium oxide (TiO.sub.2), 2 to 15% by weight of silica (SiO.sub.2), and 
2 to 10% by weight of iron in terms of ferric oxide (Fe.sub.2 O.sub.3). 
The low-expansion ceramics of the invention further have a major component 
of the crystalline phase thereof consisting of a solid solution of 
magnesium oxide-aluminum oxide-titanium dioxide-silicon oxide-iron oxide, 
a coefficient of thermal expansion of not more than 20.times.10.sup.-7 
(1/.degree.C.) in a temperature range of 25.degree. C. to 800.degree. C., 
a four-point flexural strength of not smaller than 50 kg/cm.sup.2 at room 
temperature, and a melting point of 1,500.degree. C. or higher. 
In this specification, titanium is assumed to be four valency, even though 
it is able to form non-stoichiometric composition with oxide. 
An object of the present invention is to provide low-expansion ceramics 
essentially consisting of 1.2 to 20% by weight of magnesia (MgO), 6.5 to 
68% by weight of alumina (Al.sub.2 O.sub.3), 19 to 80% by weight of 
titanium in terms of titanium oxide (TiO.sub.2), 1 to 20% by weight of 
silica (SiO.sub.2), and 0.5 to 20% by weight of iron in terms of ferric 
oxide (Fe.sub.2 O.sub.3); wherein the major component of crystalline phase 
thereof is a solid solution of magnesium oxide-aluminum oxide-titanium 
dioxide-silicon oxide-iron oxide; and the ceramics has a coefficient of 
thermal expansion of not more than 20.times.10.sup.-7 (1/.degree.C.) in a 
temperature range of 25.degree. C. to 800.degree. C., a four-point 
flexural strength of not smaller than 50 kg/cm.sup.2 at room temperature, 
and a melting point of not lower than 1,500.degree. C. 
Another object of the present invention is to provide low-expansion 
ceramics, wherein said chemical composition consists of 2 to 17% by weight 
of magnesia (MgO), 11 to 62% by weight of alumina (Al.sub.2 O.sub.3), 25 
to 75% by weight of titanium in terms of titanium oxide (TiO.sub.2), 2 to 
15% by weight of silica (SiO.sub.2), and 2 to 10% by weight of iron in 
terms of ferric oxide (Fe.sub.2 O.sub.3). 
A further object of the present invention is to provide low-expansion 
ceramics, wherein said ceramics contains as a second crystalline phase not 
more than 20% by weight of at least one crystal selected from the group 
consisting of rutile, spinel, mullite, corundum, and cordierite. 
A still further object of the present invention is to provide low-expansion 
ceramics, wherein the shape of ceramics is a honeycomb structure. 
Another object of the present invention is to provide a method of producing 
low-expansion ceramics comprising the steps of 
preparing a batch of compounds so as to provide a chemical composition of 
1.2 to 20% by weight of magnesia (MgO), 6.5 to 68% by weight of alumina 
(Al.sub.2 O.sub.3), 19 to 80% by weight of titanium in terms of titanium 
oxide (TiO.sub.2), 1 to 20% by weight of silica (SiO.sub.2), and 0.5 to 
20% by weight of iron in terms of ferric oxide (Fe.sub.2 O.sub.3); 
plasticizing the batch if necessary and shaping the batch; 
drying the body thus shaped; and 
firing the shaped body at 1,300.degree. C. to 1,700.degree. C., thereby 
having a coefficient of thermal expansion of not more than 
20.times.10.sup.-7 (1/.degree.C.) in a temperature range of 25.degree. C. 
to 800.degree. C., a four-point flexural strength of not smaller than 50 
kg/cm.sup.2 at room temperature, and a melting point of not lower than 
1,500.degree. C.

DETAILED DESCRIPTION OF THE INVENTION 
A method of producing the low-expansion ceramics according to the present 
invention will be now explained. 
A mixture of starting materials selected from the group consisting of 
magnesia, magnesium carbonate, magnesium hydroxide, talc, alumina, 
aluminum hydroxide, bauxite, anatase type titanium dioxide, rutile type 
titanium dioxide, metallic iron, .alpha.-type ferric oxide, .gamma.-type 
ferric oxide, hydrous iron oxide, ilmenite, clay, calcined clay, chamotte, 
agalmatolite, mullite, sillimanite, and kyanite, so as to provide a 
chemical composition of 1.2 to 20% by weight of magnesia (MgO), 6.5 to 68% 
by weight of alumina (Al.sub.2 O.sub.3), 19 to 80% by weight of titanium 
in terms of titanium oxide (TiO.sub.2), 1 to 20% by weight of silica 
(SiO.sub.2), and 0.5 to 20% by weight of iron in terms of ferric oxide 
(Fe.sub.2 O.sub.3). If necessary, a plasticizer is added in the mixture 
thus formed for obtaining a batch which is formable in a plastic manner, 
and the batch is formed by a ceramic forming process selected from the 
processes of extrusion, pressing, slip casting, and injection molding. 
Thus formed body is dried. 
The dried body is then heated by raising its temperature at a rate of 
5.degree. C./hr to 300.degree. C./hr and fired at 1,300.degree. C. to 
1,700.degree. C., preferably for 0.5 to 48 hours. Whereby, the 
low-expansion ceramics of the invention is produced. 
The starting materials for the low-expansion ceramics of the invention are 
not restricted to the aforesaid substances, but various natural materials 
which essentially consist of the aforesaid chemical composition can be 
also used for producing the low-expansion ceramics. 
As pointed out above, the low-expansion ceramics of the present invention 
can be formed by any of the conventional forming processes available for 
ceramics. The shape of the final product is not restricted at all: for 
instance, the final product can be a honeycomb body having a thin walled 
matrix with a plurality of cells extending from one end to the opposite 
end thereof and the cross section of the cells of the honeycomb body can 
be of any geometrical form such as triangular, rectangular, hexagonal, any 
polygonal, circular, or a combination thereof; a complicatedly shaped 
three-dimensional body, a thick body, a block of various shapes, or a body 
of almost any shape and any structure. 
The reasons for the various limitations in the present invention are as 
follows. 
(1) 1.2 to 20% by weight of magnesia (MgO), 6.5 to 68% by weight of alumina 
(Al.sub.2 O.sub.3), and 19 to 80% by weight of titanium in terms of 
titanium oxide (TiO.sub.2): 
Binary ceramics of magnesia-alumina system produces spinel crystals and 
gives a melting point of 2,000.degree. C. or higher, so that magnesia and 
alumina are very useful ingredients for improving the heat-resistance. 
Although the value of the coefficient of thermal expansion of the binary 
ceramics vary somewhat depending on the composition thereof, if is about 
60 to 80.times.10.sup.-7 (1/.degree.C.) and very large. On the other hand, 
the present invention relates to low-expansion ceramics with a coefficient 
of thermal expansion of not more than 20.times.10.sup.-7 (1/.degree.C.). 
As can be seen from FIG. 1, if 19 to 80% by weight of titanium in terms of 
titanium oxide is added to the binary system of magnesia-alumina, the 
ternary system thus produced has a coefficient of thermal expansion of not 
more than 20.times.10.sup.-7 (1/.degree.C.), while keeping the melting 
point thereof above 1,500.degree. C. If the addition of titania exceeds 
80% by weight, the melting point increases with the amount of the titania, 
but the coefficient of thermal expansion rapidly increases into a range of 
20 to 80.times.10.sup.-7 (1/.degree.C.), so that the addition of titania 
should not exceed 80% by weight. On the other hand, if the addition of 
titania is not more than 19% by weight, although the melting point 
increases to 1,700.degree. C. to 2,000.degree. C., the coefficient of 
thermal expansion rapidly increases to an excessively large range of 20 to 
80.times.10.sup.-7 (1/.degree.C.), so that at least 19% by weight of 
titanium in terms of titanium oxide must be added, while considering the 
amounts of other ingredients of ferric oxide (Fe.sub.2 O.sub.3) and silica 
(SiO.sub.2). 
(2) 0.5 to 20% by weight of ferric oxide (Fe.sub.2 O.sub.3): 
The reason for limiting the amount of iron from 0.5 to 20% by weight in 
terms of ferric oxide is in that the addition of iron in this range 
prevents variation of the coefficient of thermal expansion when the 
ceramics is exposed to various heat treatments, especially to heat 
treatment of constant temperature for many hours such as at about 
1,000.degree. C. to 1,200.degree. C. for 2,000 hours or repeatedly heating 
up and cooling down. The addition of iron at the aforesaid rate also gives 
low-expansion ceramics having a small coefficient of thermal expansion of 
less than 20.times.10.sup.-7 (1/.degree.C.) in a temperature range of 
25.degree. C. to 800.degree. C. and a high melting point of 1,500.degree. 
C. or higher. If the amount of iron in the ceramics becomes less than 0.5% 
by weight in terms of ferric oxide, the variation of the coefficient of 
thermal expansion becomes too large when the ceramics is exposed to heat 
treatment, especially that of constant temperature for many hours, e.g., 
at from about 1,000.degree. C. to 1,200.degree. C. for more than 2,000 
hours or that of repeatedly heating up and cooling down. On the other 
hand, if the amount of iron in the ceramics exceeds 20% by weight in terms 
of ferric oxide, the melting point becomes smaller than 1,500.degree. C. 
resulting in an inferior heat-resistance and the coefficient of thermal 
expansion in the temperature range of 25.degree. C. to 800.degree. C. 
exceeds 20.times.10.sup.-7 (1/.degree.C.), so that the thermal 
shock-resistance becomes inferior. 
(3) 1 to 20% by weight of silica (SiO.sub.2): 
Referring to FIG. 2, the reason for limiting the content of silica to 1 to 
20% by weight in the chemical composition of the present invention is in 
that if the content of silica is less than 1% by weight, the four-point 
flexural strength becomes less than 50 kg/cm.sup.2 and the strength of the 
low-expansion ceramics becomes insufficient. On the other hand, if the 
content of silica exceeds 20% by weight, high mechanical strength can be 
achieved but production of different crystalline phases increases so much 
that the coefficient of thermal expansion becomes larger than 
20.times.10.sup.-7 (1/.degree.C.) resulting in an inferior thermal 
shock-resistance. Thus, the content of silica is determined to be 1 to 20% 
by weight. 
Although the major component of the crystalline phase of the low-expansion 
ceramics of the present invention is a solid solution of magnesium 
oxide-aluminum oxide-titanium dioxide-silicon oxide-iron oxide, the 
present invention allows the presence of not more than 20% by weight, 
preferably not more than 10% by weight, of a second crystalline phase 
consisting of at least one crystal selected from the group consisting of 
rutile, spinel, mullite, corundum, and cordierite, because this range of 
the second crystalline phase has effects of improving the heat-resistance 
and strength by increasing the softening temperature and the melting 
temperature and reducing the gradient of softening-shrinkage curve from 
the softening temperature to the melting temperature without deteriorating 
the low-expansion characteristics. 
Examples of the present invention will be now explained. 
Referring to Table 1, compositions of Examples 1 through 5 of the invention 
and References 1 and 2 were prepared by weighing starting materials. Two 
parts by weight of a binder of vinyl acetate system were added to 
100 parts by weight of the composition thus prepared, and the binder was 
mixed thoroughly with the composition, and then rod-shaped test pieces of 
10 mm.times.10 mm.times.80 mm were made by pressing at a pressure of 1,000 
kg/cm.sup.2. 
Separately, honeycomb-shaped bodies with square cell cross section were 
prepared, by adding 4 parts by weight of metnyl cellulose and 30 to 40 
parts by weight of water into 100 parts by weight of each of the aforesaid 
compositions, thoroughly kneading the mixture thus formed by a kneader, 
extruding the kneaded mixture into honeycomb shape by an extrusion 
machine, and drying the extruded goods. The rod-shaped test pieces and the 
honeycomb-shaped bodies thus formed were fired under the firing conditions 
of Table 1, whereby ceramics of the Examples 1 through 5 of the invention 
and References 1 and 2 were made. 
The coefficients of thermal expansion in the temperature range of 
25.degree. C. to 800.degree. C. and the melting points of the rod-shaped 
test pieces of the Examples 1 through 5 of the invention and References 1 
and 2 were measured. A specimen of 4 mm width, 3 mm thickness, and 45 mm 
length was prepared from each one of the rod-shaped test pieces by cutting 
and then grinding. The four-point flexural strength of the specimen was 
measured under the conditions of an inside span 10 mm, an outside span 30 
mm, and a loading rate of 0.5 mm/min. 
Thermal shock tests were applied on the honeycomb-shaped bodies of 100 mm 
diameter and 75 mm length for the Examples 1 through 5 of the invention 
and References 1 and 2 by an electric furnace, and withstanding 
temperature differences for quick heating and quick cooling without cracks 
or breakage were determined. 
The results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Examples of the invention 
References 
1 2 3 4 5 1 2 
__________________________________________________________________________ 
Chemical 
MgO (Wt %) 
4.3 
2.9 
2.1 
3.0 
6.0 
4.6 
0.9 
composition 
Al.sub.2 O.sub.3 
(Wt %) 
37.8 
48.9 
54.8 
48.2 
27.3 
35.0 
65.6 
TiO.sub.2 
(Wt %) 
50.4 
34.4 
25.4 
36.9 
50.1 
54.4 
10.4 
SiO.sub.2 
(Wt %) 
2.0 
10.0 
15.0 
7.8 
12.1 
-- 22.0 
Fe.sub.2 O.sub.3 
(Wt %) 
5.5 
3.8 
2.7 
4.1 
4.5 
6.0 
1.1 
Composition 
Magnesia 
(Wt %) 
4.5 
3.1 
2.2 
-- -- 4.8 
1.0 
ingredients 
Magnesium 
(Wt %) 
-- -- -- 3.0 
11.7 
-- -- 
carbonate 
Talc (Wt %) 
-- -- -- 4.9 
-- -- -- 
Alumina 
(Wt %) 
30.9 
21.1 
15.0 
-- -- 33.3 
6.4 
Bauxite 
(Wt %) 
-- -- -- 46.4 
-- -- -- 
Mullite 
(Wt %) 
7.4 
36.7 
54.1 
6.5 
34.4 
-- 80.8 
Silica 
(Wt %) 
-- -- -- -- 1.6 
-- -- 
Ilmenite 
(Wt %) 
-- -- -- -- 9.9 
-- -- 
Titania 
(Wt %) 
51.8 
35.4 
26.1 
35.7 
42.4 
55.7 
10.7 
Ferric 
(Wt %) 
5.4 
3.7 
2.6 
3.5 
-- 6.2 
1.1 
oxide 
Firing Temperature 
(.degree.C.) 
1500 
1500 
1500 
1500 
1450 
1500 
1450 
conditions 
Retention 
time (hr) 5 5 5 3 10 5 5 
Coefficient of thermal 
expansion (.times. 10.sup.-7 for 
5 5 12 0 3 9 25 
25.degree. C. for 800.degree. C.) 
(1/.degree.C.) 
Melting point 
(.degree.C.) 
1700 
1680 
1640 
1690 
1720 
1710 
1550 
Four-point 
flexural (kg/cm.sup.2) 
70 160 
270 
100 
350 
30 390 
strength 
Temperature 
difference for 
(.degree.C.) 
1050 
1150 
850 
1300 
1250 
800 
450 
quick heating and 
quick cooling 
__________________________________________________________________________ 
The Examples 1 through 5 of the invention showed coefficients of thermal 
expansion of not more than 20.times.10.sup.-7 (1/.degree.C.) in the 
temperature range of 25.degree. C. to 800.degree. C., so that they showed 
larger withstanding temperature differences for quick heating and quick 
cooling than those of References 1 and 2, as demonstrated by the thermal 
shock tests by the electric furnace. Thus, the Examples of the invention 
proved excellent thermal shock-resistance. 
Furthermore, the Examples 1 through 5 of the invention showed four-point 
flexural strengths of more than 50 kg/cm.sup.2. i.e., sufficient strength 
for practical applications, and high melting points of not lower than 
1,500.degree. C. 
FIG. 1 is a characteristic diagram of magnesia-alumina-titania ternary 
ceramics, showing the effect of the content of titania on the melting 
point and the coefficient of thermal expansion. In the figure, the curve A 
shows the relationship between the content of titania and the melting 
point of the ceramics, while the curve B shows the relationship between 
the content of titania and the coefficient of thermal expansion of the 
ceramics in the temperature range of 25.degree. C. to 800.degree. C. As 
apparent from the figure, the addition of titania has effects of 
outstandingly reducing the coefficient of thermal expansion. 
FIG. 2 is a characteristic diagram of low-expansion ceramics of the 
Examples 1 through 3 of the invention and the ceramics of References 1 and 
2, showing the effects of the content of silica on the flexural strength 
and the coefficient of thermal expansion. In the figure, the curve A shows 
the relationship between the content of silica and the four-point flexural 
strength of the ceramics concerned, while the curve B shows the 
relationship between the content of silica and the coefficient of thermal 
expansion of the ceramics concerned in the range of 25.degree. C. to 
800.degree. C. Points A.sub.1 and B.sub.1 for the silica content 2% belong 
to the Example 1 of the invention; points A.sub.2 and B.sub.2 for the 
silica content 10% belong to the Example 2 of the invention; points 
A.sub.3 and B.sub.3 for the silica content 15% belong to the Example 3 of 
the invention; points A.sub.4 and B.sub.4 for the silica content 0% belong 
to Reference 1; and points A.sub.5 and B.sub.5 for silica content 22% 
belong to Reference 2. As apparent from FIG. 2, the addition of silica has 
effects of outstanding improving the flexural strength of the ceramics. 
As described in the foregoing, the low-expansion ceramics of the present 
invention has a low coefficient of thermal expansion, a high strength, a 
high melting point, and a high thermal stability even after being exposed 
to heat treatment for a number of hours at any temperature up to 
1,400.degree. C. Whereby, the ceramics of the invention can be widely used 
as ceramic material in the fields where high degrees of heat-resistance, 
thermal shock-resistance, wear-resistance, and corrosion-resistance are 
required; for instance, substrate for catalytsts to purify automobile 
exhaust gas; carriers for catalytic combustion; filters for diesel exhaust 
particulate; industrial or automobile ceramic heat exchangers; engine 
parts such as pistons, cylinder liners, combustion chambers, auxiliary 
combustion chambers, turbo-charger rotors or the like, gas turbine parts 
such as nozzles, rotors, shrouds, scrolls, plenum, combustors, tail 
cylinders, or the like; heat-resistance ceramic materials for receivers of 
solar energy; various refractory materials; and chinawares and porcelains 
for chemical industries. Therefore, the present invention contributes 
greatly to the industry.