JPH05117022A - Mullite ceramic raw powder and production of mullite ceramic sintered compact - Google Patents

Abstract

PURPOSE:To industrially, easily obtain the ceramic sintered compact little in mechanical strength drop at an elevated temperature of >1400 deg.C, hardly developing deflection due to load application and good in moldability. CONSTITUTION:The objective mullite ceramic material powder, having a specific surface area of 3-10m<2>/g and mullite crystallite diameter of <=200nm and comprising (1) 35-75 pts.wt. of spherical powder >=5mum in particle diameter and (2) 65-25 pts.wt. of powder <=2mum in particle diameter. The objective sintered compact can be obtained by molding the above powder followed by sintering at >=1600 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a raw material powder for a mullite ceramics sintered body and a method for producing the mullite ceramics sintered body, which shows a small decrease in strength at high temperatures exceeding 1400 ° C.

[0002]

2. Description of the Related Art A mullite ceramics sintered body is known as a material that does not easily undergo strength reduction at high temperatures. The following three methods are known as industrial methods for producing such a mullite ceramics sintered body. (1) A method in which a viscosity containing a large amount of kaolin and / or silinite group minerals is used as a starting material.

(2) A method in which so-called electro-melting mullite powder is used as a starting material, which is obtained by pulverizing a high-purity silica or alumina raw material melted in an electro-melting furnace. (3) A method in which mullite powder obtained by a so-called sol-gel method, which is obtained by gelling a mixed sol of silica sol and alumina sol and then firing, is used as a starting material.

[0004]

By the way, the above (1)
The mullite ceramics sintered body obtained by the method of 1) has many impurities as a raw material, so that there is a problem that the strength is remarkably reduced at a high temperature of 1400 ° C. or higher. Further, in the above method (2), the fused mullite powder is extremely hard to sinter by itself,
Even at 1700 ° C., it is difficult to manufacture a dense body having sufficient strength.

On the other hand, since the mullite powder of the sol-gel method in the above method (3) is a fine powder of high purity, it is easy to sinter, and the decrease in strength at 1400 ° C. or higher is moderate. However, there is a problem that sufficient strength cannot be ensured at a high temperature of 1400 ° C. or higher where various ceramics are fired, and bending due to load tends to occur. Further, since it is generally a fine powder, there is a problem that moldability is poor and a sintered body cannot be easily manufactured.

Therefore, the object of the present invention is to reduce the decrease in strength at high temperatures exceeding 1400 ° C., to prevent bending under load, to have good formability, and to manufacture mullite ceramic sintered bodies. It is to provide a technique that can be easily manufactured.

[0007]

The purpose of the present invention means to solve the above-mentioned object, a specific surface area of 3m ² / g~10m ² / g, a less mullite powder mullite crystallite diameter 200 nm, of which the particle diameter is more than 5μm The mullite ceramics raw material powder is characterized in that the spherical powder is 35 to 75 parts by weight and the powder having a particle diameter of 2 μm or less is 65 to 25 parts by weight.

[0008] In addition, a specific surface area of 3m ² / g~10m ² / g
And the mullite powder having a mullite crystallite diameter of 200 nm or less, 3 of which is a spherical powder having a particle diameter of 5 μm or more.
5 to 75 parts by weight, powder having a particle diameter of 2 μm or less is 65 to 2
This is achieved by a method for producing a mullite ceramics sintered body, which comprises firing 5 parts by weight of mullite ceramics raw material powder at a temperature of 1600 ° C. or higher after molding.

In the above invention, SiO ₂ and A
Impurities of components other than l ₂ O ₃ are preferably 1% or less, and the SiO ₂ content is preferably 30% or less. That is, if the amount of impurities exceeds 1% and increases, the problems of strength and bending at high temperature become large. However, non-oxide components such as SiC and Si ₃ N ₄ are not regarded as impurities. On the other hand, if the SiO ₂ content exceeds 30%, the problems of strength and bending at high temperatures become large.

The present invention will be described in more detail below. The mullite powder obtained by the sol-gel method is generally a fine powder and therefore has high purity,
It is easy to sinter, and it is possible to easily manufacture a dense and high-strength mullite ceramics sintered body. Then, the strength decrease at 1300 ° C. or higher is gentler than that of the mullite ceramics sintered body produced by using the viscosity (1) as a raw material. However, it is hard to say that it can withstand long-term use at a high temperature of 1400 ° C. or higher.

The present inventors have conducted extensive studies using various mullite powders obtained by the sol-gel method, and as a result, have revealed the following. (1) The finer the particle size of the sol-gel method mullite powder used for sintering, the lower the temperature and the shorter the sintering time. However, the one sintered at a low temperature tends to be inferior in so-called creep resistance because of strength reduction at a high temperature and bending under load.

(2) On the other hand, irrespective of the particle size distribution of the mullite powder used for sintering, if the mullite crystals in the sintered body have a large particle size of more than 5 μm after firing for a long time at a high temperature of 1700 ° C. or higher. However, there is a tendency for strength to rapidly decrease at high temperatures and creep resistance to improve. The reason why the strength at high temperature and the creep resistance are improved as the grain size of mullite in the sintered body is increased is not clear, but the present inventors consider as follows. That is,
What is sintered at a low temperature consists of many fine crystals,
It has a large amount of grain boundary energy. Thus, exposure to high temperatures results in grain growth due to grain boundary reduction, energy release and stabilization. Since grain growth occurs due to active mass transfer at the atomic level, it is thought that when a load is applied, the bonds between the atoms are broken, slippage is likely to occur, and strength reduction and bending are likely to occur.

In general, it is difficult to make mullite powder having a grain size of 5 μm or more by sintering without adding a sintering aid component to obtain a dense body having sufficiently high strength. Then, the sintering aid component lowers the high temperature strength and promotes bending under load. Therefore, the present inventors mixed various mullite fine powder having high sinterability and coarse powder of 5 μm or more, molded and sintered. However, nothing satisfactory in terms of formability and sinterability was obtained.

Further, the inventors of the present invention have made efforts to make the raw material powder spherical, and have conducted research on various raw material powders such as pulverized powder and spherical powder. As a result, if the powder particle size, the mullite crystallite size, the mixing ratio of the spherical powder, and the specific surface area are predetermined, the moldability and sinterability are good,
It was found that a sintered body having excellent strength at high temperature can be obtained. That is, a specific surface area of 3m ² / g~10m ² / g
And the mullite powder having a mullite crystallite diameter of 200 nm or less, 3 of which is a spherical powder having a particle diameter of 5 μm or more.
5 to 75 parts by weight, powder having a particle diameter of 2 μm or less is 65 to 2
By using 5 parts by weight of the mullite ceramic raw material powder, a sintered body having good formability and sinterability and excellent strength at high temperature was obtained.

That is, if the amount of spherical powder having a particle diameter of 5 μm or more is less than 35% by weight, the moldability is poor and the strength tends to decrease at high temperature. vice versa,
If it exceeds 75% by weight and becomes too large, it becomes difficult to sinter. The spherical powder having a particle size of 5 μm or more is 40 to 70.
It is preferably in the weight%. If the amount of powder having a particle size of 2 μm or less is less than 25% by weight, it becomes difficult to sinter. On the other hand, if the amount exceeds 65% by weight and becomes too large, the moldability is not good. The powder having a particle diameter of 2 μm or less is preferably 60 to 30% by weight.

[0016] Also, it is difficult to sinter the intended value the specific surface area of the powder is too small than 3m ² / g, conversely, 10 m ² /
If the value exceeds g and is too large, the moldability becomes poor.
Further, if the crystallite diameter of mullite exceeds 200 nm, it becomes difficult to sinter, so that the mullite crystallite diameter is 2
It is important that the powder is 00 nm or less (preferably 5 nm or more).

The spherical mullite powder having a particle size of 5 μm or more (preferably 100 μm or less) in the present invention is obtained by spraying and firing an alumina-silica mixed sol. The mullite powder having a particle size of 2 μm or less (preferably 0.1 μm or more) in the present invention can be obtained, for example, by pulverizing a spherical mullite powder having a particle size of 5 μm or more. Then, the mixed mullite ceramic raw material powder of the spherical mullite powder having a particle diameter of 5 μm or more and the mullite powder having a particle diameter of 2 μm or less may be obtained by mixing the respective powders obtained separately, or an alumina-silica mixed sol. Particle size obtained by spraying is 5 μm
The above spherical powder may be pulverized so as to be 35 to 75% by weight and partially pulverized.

It should be noted that 50 parts by weight or less of alumina powder or electrofused mullite powder may be added to 100 parts by weight of the above-mentioned starting material powder. However, if 50 parts by weight or more is added, it tends to be difficult to sinter, or the strength at high temperature tends to decrease, so 50 parts by weight or less is preferable. The specific surface area obtained as described above is 3
M ² / g to 10 m ² / g, mullite crystallite diameter of 200
mullite powder with a particle size of 5 μm or less
35 to 75 parts by weight of spherical powder of m or more, particle diameter of 2μ
After molding the mullite ceramic raw material powder in which the powder of m or less is 65 to 25 parts by weight, 1600 ° C. or higher, preferably 1600 ° C. to 1800 ° C., more preferably 1650 ° C.
When fired at ˜1750 ° C., a mullite ceramics sintered body was easily produced, which had a small decrease in strength under a high temperature of more than 1400 ° C. and was resistant to bending under load.

That is, if the temperature is lower than 1600 ° C., it is difficult to sinter and the strength of the sintered body is apt to decrease at a high temperature. On the contrary, if the temperature is higher than 1800 ° C., the firing furnace is greatly consumed, which is uneconomical. Because. Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited thereto.

[0020]

Examples and Comparative Examples After adjusting the pH of fumed silica and boehmite into silica sol and alumina sol, respectively, the mixture was thoroughly stirred with a high speed mixer, sprayed and fired to obtain spherical mullite powder. Incidentally, spherical mullite powders having different particle sizes (particle size distribution by laser scattering method) and crystallite diameters were prepared by changing the spraying and firing conditions.

Further, the above spherical mullite powder was pulverized to prepare non-spherical mullite powder. The spherical mullite powder and the non-spherical mullite powder obtained as described above are mixed at a predetermined ratio, a predetermined binder is added thereto, and the mixture is kneaded and extruded into a plate shape, and then at a temperature of 1550 ° C to 1750 ° C. Was fired to prepare a sintered body. A 3 mm × 4 mm × 40 mm prism was cut out from the sintered body thus obtained, and 14
A strength test was conducted at 50 ° C. For bending, stress −
The strain curve was checked. Table 1 shows these characteristics.
And shown in Table 2.

Table 1 (Mullite ceramic raw material powder) 5 μm or more and 2 μm or less Spherical powder Mullite Content of spherical powder powder with specific surface area Crystallite diameter (parts by weight) (parts by weight) (%) (nm) (m ² / g) Example 1 40 55 40 50 50 5.3 Example 2 40 60 40 40 50 8.2 Example 3 55 40 55 55 50 4.4 Example 4 55 40 55 140 140 4.8 Example 5 70 30 70 70 30 3 Comparative Example 1 0 100 0 50 14 Comparative Example 2 30 70 30 30 50 11 Comparative Example 3 30 70 30 50 8.7 Comparative Example 4 40 60 40 350 350 8.1 Comparative Example 5 40 60 40 50 2.2 Comparative Example 6 40 60 40 50 11 Comparative Example 7 55 40 40 55 50 11 Comparative Example 8 55 40 55 55 50 5.2 Comparative Example 9 55 40 55 55 360 4.7 Comparative Example 10 7 0 30 70 50 2.1 Comparative Example 11 70 30 70 360 360 3.7 Comparative Example 12 70 30 70 30 3.8 Comparative Example 13 80 20 80 30 30 3.4 Comparative Example 14 80 20 80 80 350 3.9 Comparative Example 15 80 20 80 30 2.0 2.0 Comparative Example 16 100 0 0 30 2.0 Table 2 (Molding, Drying, Firing Conditions. Strength Property) Formability Dry Drying Strength at 1450 ° C Extrusion Pressure Temperature Time 3 Point Bending Strength Bending Kgf / mm Conditions ℃ Kg / mm ² Existence Example 1 140 ○ ○ 1650 5 31 None Example 2 180 ○ ○ 1650 5 33 None Example 3 100 ○ ○ 1650 5 28 None 100 ○ ○ 1750 5 34 None Implementation Example 4 100 ○ ○ 1750 5 27 None Example 5 60 ○ ○ 1650 5 28 None 60 ○ ○ 1750 5 32 None Comparative Example 1> 200 × × Comparative Example 2> 200 × × Comparative Example 3> 200 × × Comparative Example 4 180 ○ ○ 1650 5 12 Yes 180 ○ ○ 1750 5 23 Yes Comparative Example 5 80 ○ ○ 1650 5 9 None 80 ○ ○ 1750 5 16 None Comparative Example 6> 200 × × Comparative Example 7> 200 × × Comparative Example 8 130 ○ ○ 1550 5 11 None 130 ○ ○ 1750 5 31 Yes Comparative Example 9 110 ○ ○ 1650 5 13 None 110 ○ ○ 1750 5 26 Yes Comparative Example 10 40 ○ ○ 1650 514 Yes 40 ○ ○ 1750 5 22 Yes Comparative Example 11 60 ○ ○ 1650 5 7 No 60 ○ ○ 1750 5 12 No Comparative Example 12 60 ○ ○ 1550 5 6 No Comparative Example 13 50 ○ ○ 1550 5 5 No Comparative Example 14 60 ○ ○ 1650 5 7 No 60 ○ ○ 1750 5 15 Yes Comparative Example 15 50 ○ ○ 1650 5 5 None 50 ○ ○ 1750 5 8 None Comparative Example 16 40 ○ ○ 1750 5 2 None In the column of the condition of formability, The mark indicates that it can be pushed out stably. The X mark indicates that discharge is not possible smoothly. In the dry column, the ○ mark indicates that neither cracking nor warping has occurred. × indicates that cracking or warping has occurred. According to the present invention, there is little decrease in strength at high temperatures exceeding 1400 ° C., bending due to load is unlikely to occur, moldability is good, and a mullite ceramic sintered body is industrially manufactured. It can be easily obtained.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fumio Nemoto 5310 Mikajiri, Kumagaya, Saitama Prefecture Chichibu Cement Corporation Huain Ceramics Division

Claims (4)

[Claims] 1. A specific surface area of 3m ² / g~10m ² / g
And the mullite powder having a mullite crystallite diameter of 200 nm or less, 3 of which is a spherical powder having a particle diameter of 5 μm or more.
5 to 75 parts by weight, powder having a particle diameter of 2 μm or less is 65 to 2
A mullite ceramic raw material powder characterized by being 5 parts by weight. 2. The mullite ceramic raw material powder according to claim 1, wherein impurities of components other than SiO ₂ and Al ₂ O ₃ are 1% or less. 3. The mullite ceramic raw material powder according to claim 1 or _{2, wherein} the content of SiO ₂ is 30% or less. Wherein a specific surface area of 3m ² / g~10m ² / g
And the mullite powder having a mullite crystallite diameter of 200 nm or less, 3 of which is a spherical powder having a particle diameter of 5 μm or more.
5 to 75 parts by weight, powder having a particle diameter of 2 μm or less is 65 to 2
A method for producing a mullite ceramics sintered body, characterized in that 5 parts by weight of mullite ceramics raw material powder is fired at a temperature of 1600 ° C. or higher after molding.