JPH03187972A - Alumina-silica-based sintered material and production thereof - Google Patents

Abstract

PURPOSE:To obtain the subject alumina.silica-based sintered material having a mullite composition, excellent in properties such as high temperature strength and capable of provision at a low cost by specifying the respective contents of BN and CaO based on mullite. CONSTITUTION:The above mentioned alumina.silica-based sintered material is composed of BN having <=30mum particle size, CaO having <=0.1mum particle size and mullite having 10-100mum particle size. BN content and CaO content are respectively 5-30wt.% and 0.1-1wt.% based on mullite. In the above- mentioned sintered material, more economical raw materials than the conventional ones are used and specified ceramic particles as the second phase are dispersed in the mullite crystalline or on the grain boundary surface as a method for improvement of its physical properties to contrive improvement of the strength thereof. In addition, free glassy silica is occluded as a solid solution by addition of CaO, thus providing the objective material having characteristics equal to those of a high purity synthetic mullite.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an alumina-silica-based sintered body and a method for producing the same, and particularly relates to an alumina-silica-based sintered body that has excellent properties such as high-temperature strength and is provided for cheap sake. The present invention relates to a sintered body and a method for producing the same.

[Prior Art] Mullite is composed of AIL203 and SiO2, and its chemical composition is theoretically 3Aj22032SiO2, 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 belong to old ceramics.
This research has a long history, and the main raw materials used are kaolin and Bayer alumina as an alumina source, and silica as a silica source. Recently, it has become possible to achieve the physical properties of synthetic mullite groups by modifying natural mullite, but the focus of this research is on preventing the precipitation and vitrification of the silica phase in the mullite composition, and The preparation and sintering conditions were investigated.

On the other hand, there is also a material with a theoretical composition called high-purity synthetic mullite using fine ceramics technology, and this is mainly produced by a coprecipitation method using methods such as metal alkoxides to achieve the theoretical composition.

Mullite ceramics are made by mixing and sintering these raw materials according to the purpose, and like alumina ceramics, mullite ceramics have relatively high high-temperature strength, and those made from natural raw materials are inexpensive. Because it is a durable material, it has been mainly used as a refractory material, such as furnace materials, sheaths, setter materials, heat-resistant materials, and structural materials.

[Problem to be solved by the invention] Among conventional mullite-based ceramics, those modified from natural mullite do not exhibit AJ during long-term use or high-temperature use.
! The 20*-5i02 bonding decomposes and silica precipitates as a glass phase at the grain boundaries of mullite. For this reason, the strength was significantly reduced, making it difficult to use continuously or repeatedly.

High-purity synthetic mullite using the alkoxyquito method was developed to solve the above-mentioned drawbacks. Although high-purity mullite has the effect of greatly improving high-temperature strength and durability, it is expensive and has not been used in the past. It was disadvantageous in terms of cost for use in the field of industrial materials such as heat-resistant materials.

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

[i! [Means for Solving the Problem] The alumina-silica-based sintered body of claim (1) is made of BN (boron nitride), Cab (calcium oxide), and mullite, and the BN and CaO contents are respectively lower than that of mullite. 5～
30% by weight, 0.1 to 1% by weight, and the mullite particle size is 10 to 100 μm. , Bayer alumina, aluminum hydroxide, and silica stone as main raw materials, Aj! After blending so that the composition ratio of 20*/5iOa falls within the mullite production range, and wet-pulverizing the blended raw material so that 90% or more has a particle size of 5 μm or less, BN with a particle size of 30 μm or less is mixed into the above blend. 5-30 for raw material
% by weight, CaCO5 (calcium carbonate) with a particle size of 0.1 μm or less, calculated as CaO, is 0.1 to 1% to the above-mentioned blended raw material.
It is characterized by adding and mixing in weight percent, then drying and crushing the obtained mixture, then molding using an organic binder, and baking the molded product 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. Furthermore, the glassy silica liberated by the addition of CaO is fixed as a solid solution, thereby providing a material having the characteristics of a high-purity synthetic mullite group.

The present invention will be explained in detail below.

The alumina-silica sintered body of claim (1) contains 5 to 30% by weight of BN and 0.1 to 1% by weight of C based on mullite.
It contains aO. 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 30% by weight, the amount of BN becomes too large and the alumina-silica sintered body is properties are impaired. Therefore, in the present invention, the BN content is 5 to 30% by weight based on mullite. In particular, when the BN content is 5 to 10% by weight based on mullite, an alumina-silica sintered body with particularly high strength can be obtained.

On the other hand, the content of CaO is 0.1% by weight relative to mullite.
If it is less than 1% by weight, the glass phase liberated during the formation of mullite, which will be described later, cannot be sufficiently fixed, and the strength improving effect will not be sufficient, and if it exceeds 1% by weight, the CaO phase will become large, which is not preferable. Therefore, in the present invention, the CaO content is 0.1 to 1% by weight based on mullite. In particular, when the CaO content is 0.5 to 1% by weight based on mullite, an alumina-silica sintered body with particularly high strength can be obtained. Conventionally, there have been reports of using CaO as an additive during sintering of mullite, and in this case, 5 to 15% by weight is added. This is because the particles of CaC0a and Ca(OH)2 (calcium hydroxide), which are common CaO raw materials, are several μm in size, and it is necessary to add a large amount in order to uniformly disperse them.

On the other hand, in the present invention, by using CaCCl3 in the form of ultrafine particles of submicron size or less, a sufficient effect could be obtained with addition of a small amount of 0.1 to 1% by weight.

The mullite crystals in the alumina-silica sintered body according to claim (1) have a grain size in the range of 100 μm. If the grain size of the mullite crystal is larger than 100 μm, the bending strength of the alumina-silica sintered body obtained will decrease;
If it is smaller than m, it becomes difficult to incorporate 88 particles and CaO particles into mullite crystals or grain boundaries. Therefore, the grain size of mullite crystals is 10 to 100 μm, preferably 10 to 50 μm.
Let it be m.

On the other hand, if the particle size of the 88 particles is too fine, it is difficult to mix them uniformly into mullite. On the other hand, if the grain size of the 88 grains is too large, BN will exist only at the mullite grain boundaries, causing grain boundary cracks. Therefore, in the present invention, the particle size of the 88 particles is preferably 30 μm or less, particularly 10 μm or less, particularly 3 to 10 μm.

In addition, if the particle size of CaCoa particles is large, the effect cannot be obtained unless a large amount is added, and the reactivity is also poor.
．． The thickness is preferably 1 μm or less, preferably 0.05 μm or less.

The composition of mullite in the alumina-silica sintered body is the theoretical composition Aj! 203/s i 02 = 3/2
(molar ratio), that is, 71.8/28.2 (wt%). If Al12'3 in the mullite composition is too much than the theoretical composition, mullite crystals will be dispersed in Al2O3 and sufficient strength will not be obtained. On the other hand, if SiO2 in the mullite composition is too much than the theoretical composition, a free silica phase will form in the mullite as a glass phase, making it impossible to obtain sufficient high-temperature strength. Therefore, the mullite in the alumina-silica sintered body has the theoretical composition A1202/S
It is preferable that the composition be as close as possible to iO2=3/2 (molar ratio).

As described above, even if the silica glass phase is prevented from precipitating as much as possible, some precipitation still occurs, and therefore the strength cannot be sufficiently increased. When CaC0a is added here, some silica glass liberated during mullite formation reacts with CaO generated by the decomposition of CaCO5 and is fixed, so it no longer precipitates as a glass phase at the mullite grain boundaries, resulting in high strength. Become something. The amount of CaCO5 added is 0.1 to 1% by weight in terms of CaO, more preferably 0.5%.
When the content was 1% by weight, high strength was obtained. As mentioned above, if the amount of CaC0:+ added is too large, the CaO phase will become large, which is not preferable. On the other hand, if the amount is too small, the free glass phase cannot be sufficiently fixed, resulting in no effect.

The alumina-silica-based sintered body of claim (1) can be easily and efficiently produced at low cost by the method of claim (2).

The method for producing an alumina-silica-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 11203/S i O
2 composition ratio is within the mullite production range, preferably A It20
Blend so that s/S i O2 = 3/2 (molar ratio). In this case, it is particularly preferable to use purified kaolin and Bayer alumina or aluminum hydroxide, or Bayer alumina or aluminum hydroxide and silica stone as raw materials. The required amount of these raw materials is wet-pulverized using alcohol or the like using a ball mill or an attritor so that 90% or more has a particle size of 5 μm or less. Next, the obtained pulverized material was added with a particle size of 30 μm.
m or less, preferably 10 μm or less, particularly 3 to 10 μm, 5 to 30% by weight, preferably 5 to 30% by weight of the pulverized material.
Added ~10% by weight and further added CaCO3 of 0.1 μm or less
0.1 to 1% by weight in terms of CaO, preferably 0.5 to 1% by weight
Add 1% by weight and 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). After pressure forming at 300 kgf/Crd or more, 1
00100O/crr? It is preferable to carry out the two-stage molding by isostatic press molding as described above.

The obtained molded body is fired by hot pressing or pressureless sintering to obtain an alumina-silica-based sintered body.

In this case, it is preferable that the temperature increase rate is 50 to 50°C, the firing temperature is 1600°C or higher, preferably 1600 to 1650°C, and the firing time is 1 hour or more.
Preferably, the time is 1 to 3 hours. In addition, when using a hot press, the pressure is 300 to 600 k
g/crr? It is preferable to set it as approximately.

[Function] In general, even if raw materials such as refined kaolin, Bayer alumina, aluminum hydroxide, or silica stone are used and mixed by finely pulverizing them with a ball mill, etc., it is impossible to mix them to the theoretical composition at the atomic level. However, it takes a long time to diffuse through sintering.

On the other hand, 5 to 30% by weight of BN particles as the second phase in the mullite composition, and 0.0% of CaCO3 particles in terms of CaO.
When 1 to 1% by weight is added, an alumina-silica-based sintered body with excellent high-temperature strength can be obtained by ordinary molding and firing, even by pulverizing and mixing using a ball mill or the like.

In the present invention, although the details of the mechanism of high-temperature strength improvement by BN addition are not clear, the BN particles incorporated within the mullite crystals or at the grain interfaces block the movement of SiO2 in the mullite to the glass phase. Furthermore, it is thought that this is because the BN particles are dispersed within the mullite crystal grains 1 2 and at the grain boundaries, suppressing the growth of the mullite crystals. Regarding the addition of % CaCO2, it is thought that free silica (glass phase) is reacted with CaO and fixed, so that precipitation of the glass phase is eliminated and the high-temperature strength is increased.

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

Example 1, Comparative Example 1 Purified kaolinite has a composition of Al12os/S i
Alumina was added so that the molar ratio was 02 = 372, and the mixture was milled using alcohol in a ball mill (ZrOz ball) for 48 hours. In this case, if a media agitation type pulverizer (attritor) is used, 1 to 2
It is possible to process in 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 size 5 μm) and CaC0a powder (specially manufactured by Mitsubishi Mining Cement: average particle size O, S μm) is added to this. The amounts shown in Table 1 were added (in Comparative Example 1, no addition was made), and the mixture was further mixed in a ball mill for 5 hours. After drying and crushing this, 5% by weight of an organic binder (PVA) was added and thoroughly kneaded.

The kneaded material was press-molded into a size of 50 mmφ x 5 mm.
After molding at kg/crd, the molded product was further pressurized at 1500 kg/crn' using a rubber press. This compact was sintered to obtain an alumina-silica sintered body having a mullite composition. For sintering, a hot press was used, the heating rate was 150°C/hr, and the weight was 300 kg.
/C at 1600°C for 1 hour.

Table 1 shows various properties of the obtained sintered body.

Table 1 3 4 From Table 1, it is clear that an alumina-silica sintered body with a mullite composition to which a predetermined amount of BN and CaCO3 are added can stably obtain extremely high strength from room temperature to high temperatures such as 1300°C. be.

[Effects of the Invention] As detailed above, the alumina-silica-based sintered body of the present invention is provided at low cost using inexpensive raw materials, and has excellent properties such as high-temperature strength and durability. Remarkably superior. Therefore, the present sintered body can be used extremely effectively over a long period of time as an industrial fireproof material.

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

Claims (2)

[Claims] (1) Consisting of BN, CaO and mullite, the BN content is 5 to 30% by weight relative to mullite, the CaO content is 0.1 to 1% by weight relative to mullite, and the mullite particle size is 10 to 30% by weight. An alumina-silica sintered body characterized by a thickness of 100 μm. (2) At least two selected from the group consisting of purified clay minerals, Bayer alumina, aluminum hydroxide, and silica stone are blended as main raw materials so that the composition ratio of Al_2O_3/SiO_2 falls within the mullite production range, and the blended raw materials are After wet grinding so that 90% or more has a particle size of 5 μm or less, BN with a particle size of 30 μm or less is
~30% by weight of CaCO_3 with a particle size of 0.1 μm or less
Addition and mixing of 0.1 to 1% by weight with respect to the above-mentioned raw materials in terms of aO is carried out, and the resulting mixture is then dried and crushed, and then molded using an organic binder to form a molded product with a temperature of 160%
A method for producing an alumina-silica-based sintered body, characterized by firing at a temperature of 0° C. or higher for 1 hour or more.