JPH0383851A - Mullite-based sintered compact and production thereof - Google Patents

Abstract

PURPOSE:To obtain an inexpensive mullite-based sintered compact excellent in high-temperature strength by dispersing BN in a specific proportion in mullite crystals or grain boundary surfaces and providing the sintered compact having a limited crystal grain diameter of the mullite. CONSTITUTION:The aforementioned mullite-based sintered compact is composed of BN and mullite with 5-30wt.% BN content based on the mullite and the grain diameter of the mullite is 10-100mum. The above-mentioned mullite-based sintered compact is obtained by the following method. That is at least two kinds selected from the group composed of purified clay mineral, alumina produced by the Bayer process and quartzite are used as main raw materials and prepared so as to afford the composition ratio of Al2O3/SiO2 within a range producing the mullite. The prepared raw materials are wet pulverized so as to provide >=90% thereof having <=5mum particle diameter and the BN having <=30mum particle diameter in an amount of 5-30wt.% based on the aforementioned prepared raw materials is than added and mixed therewith. The resultant mixture is subsequently dried, disintegrated and then formed using an organic binder. The obtained formed compact is subsequently sintered at >=1600 deg.C temperature for >=1hr.

Description

[Detailed description of the invention] [Field of application in fIi industry] The present invention relates to a mullite-based sintered body and a method for manufacturing the same, and in particular, a mullite-based sintered body that has excellent properties such as high-temperature strength and is provided at a low cost. and its manufacturing method.

[Prior Art] Mullite consists of Ax=on and 5i02, the chemical composition is theoretically 3Aj2203-2S i 02 , and its properties include excellent heat resistance and particularly good creep properties. Also, although the thermal shock properties are good, the electrical properties are not so good.

Mullite Ceramics belongs to old ceramics.
Its research has a long history, and raw materials include kaolin, Bayer alumina, aluminum hydroxide, and
Silica stone is mainly used as a silica source. recently,
By modifying natural mullite, it has become possible to achieve physical properties comparable to synthetic mullite, but the main focus of this research is to prevent the precipitation and vitrification of the S i 02 phase in the mullite composition. The preparation and sintering conditions were studied.

On the other hand, there is also a material with a theoretical composition of high-purity mullite using Fine Ceramics technology, which is produced by a coprecipitation method from metal alkoxide to a theoretical composition.

The mixture of these raw materials according to the purpose and sintering is called mullite-based ceramic material.Mullite-based ceramic material has relatively high high-temperature strength like alkaline and nacella materials, and is a natural raw material. Because it is an inexpensive material, it has been used mainly as a fireproof material, such as furnace materials, sheaths, setter materials, heat-resistant materials, and structural materials.

[Problems to be solved by the invention] Among conventional mullite ceramics, in those modified from natural mullite, the An20s-3iOa bonding originally contained decomposes during long-term use or high-temperature use, and 5iOa becomes mullite grains. It precipitates as a glass phase in the field. For this reason, the strength was significantly reduced, making it difficult to use continuously or repeatedly.

High-purity mullite produced by the alkoxyquito method was developed to solve the above-mentioned drawbacks, but although high-purity mullite has the effect of greatly improving high-temperature strength and durability, it has not been used conventionally due to its high price. It was disadvantageous in terms of cost for use in the field of industrial materials such as heat-resistant materials.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a mullite-based sintered body that has excellent properties such as high-temperature strength and can be provided at low cost, and a method for producing the same.

[Means for solving the problem] The mullite-based sintered body of claim (1) is made of BN (boron nitride).
and mullite, the BN content is 5 to 30% by weight based on mullite, and the mullite particle size is 10 to 100% by weight.
The method for producing a mullite-based sintered body according to claim (2), characterized in that the mullite-based sintered body has a particle diameter of 20
The composition ratio of s/S i O2 is blended to be in the mullite production range, and 90% or more of the blended raw materials have a particle size of 5 μm.
After wet pulverization as follows, BN with a particle size of 30 μm or less is added and mixed in an amount of 5 to 30% by weight based on the raw material, the resulting mixture is dried and crushed, and an organic binder is added. The molded body is then baked at a temperature of 1600° C. or higher for 1 hour or more.

That is, the present invention uses inexpensive raw materials that have been conventionally used as raw materials, and as a means of improving physical properties, high strength is achieved by dispersing specific ceramic particles as a second phase within mullite crystals or at grain boundaries. The aim is to provide a material with properties comparable to that of high-purity synthetic mullite.

The present invention will be explained in detail below.

The mullite-based sintered body according to claim (1) contains BN in an amount of 5 to 30% by weight based on mullite. If the BH content is less than 5% by weight based on mullite, the strength improvement effect of the present invention cannot be obtained, and if it exceeds 301% by weight, BH
A large number of N tatami mats pass through, and the characteristics as a mullite-based sintered body are impaired. Therefore, in the present invention, the BN content is 5 to 30! Weight%. especially,
When the BN content is 5 to 10% by weight based on mullite, a particularly high-strength mullite-based sintered body can be obtained.

The mullite crystal in the mullite-based sintered body of claim (1) is
The particle size is in the range of 100 μm.

When the grain size of the mullite crystal is larger than 100 turns, the bending strength of the obtained mullite-based sintered body decreases;
If it is smaller than μm, it becomes difficult to incorporate BN particles into mullite crystals or at grain interfaces. Therefore, the grain size of the mullite crystals is 10 to 100 μm, preferably 1ON50 μm.

On the other hand, if the particle size of the BN particles is too fine, it is difficult to mix them uniformly with mullite. For this reason, it is no longer incorporated uniformly into the mullite crystal grains. On the other hand, if the grain size of the BN particles is too large, BN will exist only at the mullite crystal grain boundaries, causing grain boundary cracks to occur. Therefore, in the present invention, the particle size of the BN particles is 30 μm or less, particularly 110 μm or less.
It is preferably A1 or less, particularly 3 to 10 μm.

The composition of mullite in the mullite-based sintered body is the theoretical composition (D A 1120 s / S i O2-3 /
2 (molar ratio), that is, 71.8728.2 (wt%). Mullite composition Al2O3/SiO
If Z203 is too much than the theoretical composition, Al2O3/Si
Mullite crystals become dispersed in O2203 and sufficient strength cannot be obtained. On the other hand, if SiO2 in the mullite composition is too much than the theoretical composition, the free silica phase becomes a glass phase in the mullite and grows, making it impossible to obtain sufficient high-temperature strength. Therefore, the mullite in the mullite-based sintered body preferably has a composition as close as possible to the theoretical composition Aj1203/5iO2=3/2 (molar ratio).

The mullite-based sintered body of claim (1) is not covered by claim (1).
By the method 2), it can be manufactured easily and efficiently at low cost.

The method for producing a mullite-based sintered body according to claim (2) will be explained below.

In the method of claim (2), first, refined clay minerals, Bayer alumina, aluminum hydroxide, or silica are used as raw materials, and A fL 203/S i
O2 composition ratio is in the mullite production range, preferably A 122
Blend so that 0 s/S i O2 = 3/2 (molar ratio). In this case, the raw materials include purified kaolin and Bayer alumina or aluminum hydroxide.
Alternatively, it is preferable to use Bayer alumina or aluminum hydroxide and silica stone. The required amount of these raw materials is wet-milled using alcohol or the like using a ball mill or attritor so that 90% or more of the powder has a particle size of 5 μm or less. Next, 5 to 30% by weight, preferably 5 to 10% by weight of BN having a particle size of 3 μm or less, preferably 10 μm or less, particularly 3 to 10 μm is added to the obtained pulverized product, Further, mix using a ball mill or the like.

The resulting mixture is dried, crushed, and then molded using an organic binder such as polyvinyl alcohol (PVA). It is preferable that the molding is carried out in two steps, such as pressure molding at 300 kgf/cm' or higher, followed by isostatic press molding at 100,100 O/cm' or higher.

The obtained molded body is fired by hot pressing or pressureless sintering to obtain a mullite-based sintered body. In this case, the heating rate is 5
0 to b Preferably, the firing temperature is 1600°C or higher, preferably 16
It is preferable that the temperature is 00 to 1650°C and the firing time is 1 hour or more, preferably 1 to 3 hours. In addition, when using a hot press, the pressure is 300 to 600 kg/crr.
It is preferable to set it to about 1.

[Function] In general, even if raw materials such as refined kaolin, Bayer's crystal, or silica stone are used, and the raw materials are finely pulverized using a ball mill or the like and mixed, theoretical iJ! It is impossible to mix it into the soil, and it takes a long time to diffuse it by sintering.

On the other hand, if 5 to 30% by weight of BN particles are added as a second phase to the mullite composition, a mullite-based sintered body with excellent high-temperature strength can be obtained by normal molding and firing, even when pulverized and mixed using a ball mill etc. It will be done.

Although the details of the mechanism of this improvement in high-temperature strength due to the addition of BN are not clear, the BN particles incorporated into the mullite crystals or at the grain interfaces block the movement of SiO2 in the mullite to the glass phase. This is thought to be because the BN particles are dispersed within the mullite crystal grains and at the grain boundaries, suppressing the growth of the mullite crystals.

[Example] The present invention will be described in more detail with reference to Examples and Comparative Examples below.

Example 1, Comparative Example 1 Composition of purified kaolinite is A j22 '03/5
Alumina was added so that iO2 = 3/2 (molar ratio), and the mixture was milled using alcohol in a ball mill (ZrO2 ball) for 48 hours. In this case, if a media agitation type pulverizer (attritor) is used, the treatment can be completed in 1 to 2 hours. After pulverizing the raw material so that 90% or more has a particle size of 5 μm or less, BN powder (
(manufactured by Showa Denko Co., Ltd.: average particle diameter 5 μm) was added in the amount shown in Table 1 (comparative example 1 was not added), and further mixed in a ball mill for 5 hours. After drying and crushing this, an organic binder (
PVA) was added in an amount of 5% by weight and thoroughly kneaded.

The kneaded material was press-molded to a size of 50mmφ x 5rnm.
After molding at 0 kg/ctn', it was further pressed at 1500 kg/cm" using a rubber press to obtain a molded body. This molded body was sintered to obtain a mullite-based sintered body.

Note that sintering uses a hot press, and the temperature increase rate is 150°C.
/ hr, I 60 at 300 kg/crrr
The test was carried out at 0°C for 1 hour.

Table 1 shows the properties of the obtained mullite-based sintered body.

From Table 1, it is clear that the mullite-based sintered body to which a predetermined amount of BN has been added can stably provide extremely high strength from room temperature to high temperatures such as 1300 tons.

[Effects of the Invention] As detailed above, the mullite-based sintered body of the present invention can be provided at low cost using inexpensive raw materials, and has outstanding properties such as high-temperature strength and durability. . Therefore, the mullite-based sintered body of the present invention can be used extremely effectively for a long period of time as an industrial refractory material.

Therefore, such a mullite-based sintered body of the present invention can be easily and efficiently manufactured at low cost by the method of the present invention.

Claims (2)

[Claims] (1) Consisting of BN and mullite, the BN content is 5 to 30% by weight relative to mullite, and the mullite particle size is 1
A mullite-based sintered body having a diameter of 0 to 100 μm. (2) Al
_2O_3/SiO_2 is blended so that the composition ratio falls within the mullite production range, and more than 90% of the blended raw materials have a particle size of 5μ.
After wet pulverization to a particle size of 30 μm or less, 5 to 30% by weight of BN with a particle size of 30 μm or less is added and mixed to the raw material, and the resulting mixture is dried and crushed.
Molded using an organic binder and heated to 1600℃
A method for producing a mullite-based sintered body, which comprises firing at a temperature above 1 hour or more.