JPH01203264A - Ceramics having low thermal expandability and production thereof - Google Patents

Abstract

PURPOSE:To obtain ceramics maintaining low thermal expandability and having superior mechanical properties by forming a compounded structure consisting of beta-spodumene and monoclinic or tetragonal zirconia in a specified ratio as the structure of ceramics. CONSTITUTION:This ceramics having low thermal expandability has a compounded structure consisting of beta-spodumene and monoclinic or tetragonal zirconia in 88/12-60/40 weight ratio as the crystal phases of a sintered body. The ceramics can be formed as follows: 60-88wt.% powdery starting material consisting of 90-95wt.% petalite and 10-5wt.% kaolin is mixed with 40-12wt.% zirconia powder, molded and calcined at 1,250-1,350 deg.C in the air.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to ceramics for structural members that have low thermal expansion and excellent properties such as strength, hardness, and rigidity, and a method for manufacturing the same.

[Conventional technology]

Ceramics exhibiting low thermal expansion include polycrystalline bodies made of crystals such as β-spodumene, eucryptite, and cordierite.

Ceramics that are widely used as low thermal expansion ceramics include those made of β-spodumene or eucryptite as lithium aluminosilicate minerals, polycrystalline materials such as cordierite, and amorphous materials. is Lithia.

There are those mainly made of alumina and silica.

[Problem to be solved by the invention]

However, these low thermal expansion ceramics have the disadvantage that they are inferior in mechanical properties such as strength, hardness, and elastic modulus, compared to ceramics for mechanical structural members such as alumina, zirconia, mullite, silicon carbide, and silicon nitride. .

An object of the present invention is to provide a ceramic that maintains low thermal expansion and has excellent mechanical properties.

[Means to solve the problem]

The low thermal expansion ceramic of the present invention has a sintered crystal phase of β
- has a composite structure of spodumene and monoclinic or tetragonal zirconia, and the weight fraction of β-subodiumene/zirconia is within the range of 8 g/12 to 60/40;
In addition to denseness, low thermal expansion, and thermal shock resistance, it has excellent mechanical properties such as strength and hardness, and thermal shock resistance.

In this specification, monoclinic or tetragonal zirconia will be simply referred to as zirconia.

The low thermal expansion ceramic according to the present invention can be produced by sintering.
- It can be obtained by firing a mixture of natural raw materials such as phyllodespar and spodium pyroxene, or artificial raw materials such as β-subozinimene and eucryptite, and zirconia raw materials as raw materials that produce sposicomene. Here, as the zirconia raw material to be blended, either high-purity zirconia containing no impurities or one containing a zirconia stabilizing material can be used.

However, when a stabilizing material is contained, it is preferable to use an alkaline earth metal oxide as the type of stabilizing material.
Their content is 3 to 6% by weight in case of IJA.
It is desirable that it falls within the range of .

That is, the low thermal expansion ceramic of the present invention is prepared by mixing 60-88% by weight of starting material powder consisting of 90-95% by weight of petalite and 10-5% by weight of kaolin with 40-12% by weight of zirconia powder, 1 in the air after molding
Obtained by firing at 250 to 1350°C.

Regarding the ratio of petalite to kaolin in the starting material, if the ratio of kaolin is less than 5% by weight,
Silica produced when petalite decomposes at high temperatures remains in the ceramic in a free form, which has an unfavorable effect on heat resistance. Furthermore, if the ratio of kaolin exceeds 10% by weight, silica, which is a decomposition product of excess kaolin, will be present, which is also unfavorable for heat resistance.

Next, regarding the ratio of the starting material to the zirconia powder, if the ratio of the zirconia powder is less than 12% by weight, high toughness and high strength due to stress-induced transformation of zirconia cannot be sufficiently achieved.

On the other hand, if the proportion of zirconia powder exceeds 40% by weight, the high thermal expansion characteristic of zirconia will appear, making it impossible to obtain the desired low thermal expansion.

Regarding the conditions for adjusting the blend, it is very important that the zirconia raw material components are uniformly distributed in the blend. For this reason, it is desirable to employ a wet mixing method when mixing each raw material component. Specifically, there are ball mills, attrition mills, etc. At this time, depending on the molding method, binder.

It is common to add molding aids such as lubricants, wetting agents, plasticizers, etc. to the formulation.

As the forming method, conventionally widely practiced methods such as -axis forming, cold isostatic pressing, extrusion, and casting can be applied as they are.

Furthermore, the firing conditions for the molded body are similar to those for conventional low thermal expansion ceramics made of lithium aluminosilicate minerals, and are fired in the atmosphere at 1250 to 1350°C. If the firing temperature is lower than this range, sintering will be insufficient and the high strength and toughness of the present invention will not be achieved. If the firing temperature is higher than this range, softening and melting phenomena of the lithium aluminosilicate mineral occur, which is not preferable.

The microstructure of the ceramic obtained in this way is β
- It has a structure in which zirconia crystal grains exist isolated from each other in a matrix of subozinimene. The crystal phase of the zirconia grains may be tetragonal or monoclinic depending on the grain size. When external mechanical or thermal stress is applied to ceramics having such a microstructure, mechanical properties such as strength or toughness are improved due to so-called stress-induced transformation and inherent microcracks. Also, β
- Since the hardness of zirconia is higher than that of sposinimene, the hardness of the low thermal expansion ceramic according to the present invention is higher than the hardness of β-sposinimene itself as a whole.

On the other hand, although the coefficient of thermal expansion of zirconia is very large, in the low thermal expansion ceramic according to the present invention, the zirconia grains exist isolated from each other, so the coefficient of thermal expansion of the sintered body as a whole does not become large. The low thermal expansion properties of β-spodumene remain intact.

It will be maintained.

The above effects are exhibited when the ratio of β-spodumene to zirconia is 88/12 to 60/40 in terms of weight fraction of β-spodumene/zirconia. When the weight fraction is less than 12, the contribution of stress-induced transformation of zirconia is very small, and no improvement in strength, toughness, or hardness is observed. When the number of zirconia particles increases, connections occur between zirconia grains, and the contribution of the high thermal expansion inherent to zirconia cannot be ignored, making it impossible to obtain the desired low thermal expansion.

〔Example〕

A natural raw material powder containing 98% or more of particles with a particle size of 75 mm or less and having a petalite crystal phase, a kaolin powder, and a high-purity zirconia fine powder produced by a hydrolysis method are first prepared.
Wet mixing was performed using a ball mill at the mixing ratio shown in the table. The obtained slip was cast to produce a molded body, and then kept in the atmosphere at 1300° C. for 1 hour using an electric furnace to produce a sintered body. The properties of the sintered body thus obtained are shown in Table 2.

Those marked with O in the table are those according to the present invention.

Table 1 Table 2 *1: Indicates the coefficient of linear expansion between room temperature and 1000°C.

*2: Vickers hardness The ceramic according to the present invention has a very low thermal expansion coefficient of 0.2% or less up to 1000°C, and its mechanical properties such as bending strength and hardness are similar to the stress-induced transformation effects of zirconia. It can be seen that this has improved.

Although not mentioned in Table 1, when the zirconia content exceeds the range of the present invention, low thermal expansion characteristics are no longer exhibited.

If the firing temperature in the electric furnace was less than 1250°C, the ceramic would not be dense and the intended excellent mechanical properties could not be obtained. When fired at a temperature higher than 1350°C, phenomena such as softening and deformation were observed in the fired product.

FIGS. 1 and 2 show photographs (magnification: 280 times) of polished cross sections of sintered bodies with N03 and Nα5 in the table.

Figure 1 shows an example consisting of a composite structure of 88% by weight β-sposinimen and 12% by weight zirconia;
The figure shows an example of a composite structure of 60% by weight β-spodumene and 40% by weight zirconia.

Referring to the same photo, the ceramic of the present invention has a structure in which zirconia crystal grains (white regions) are not unevenly distributed in the β-spodumene matrix (gray regions) and are uniform and isolated from each other. It is shown that. In addition,
In the same photo, the parts that look like irregular grains are voids that are open in the cross section.

From these results, it is easily presumed that when the content ratio of zirconia exceeds the range according to the present invention, the zirconia grains form a continuous phase in which they are interconnected.

〔Effect of the invention〕

The high-strength, low-thermal-expansion ceramic of the present invention can provide the following effects.

(1) Since the crystal phase constituting the sintered body is mainly composed of β-spodumene, the coefficient of thermal expansion is extremely small.

(2) Due to the effect of stress-induced transformation of zirconia crystal grains uniformly dispersed in the sintered body, a sintered body with high strength and high toughness can be obtained.

(3) Compared to a sintered body of β-subozinimen alone,
A sintered body having excellent mechanical properties (hardness, elastic modulus, etc.) other than the above (2) can be obtained.

[Brief explanation of the drawing]

FIGS. 1 and 2 show micrographs of polished surfaces showing the microstructure of the ceramic of the present invention. Patent applicant: Kurosaki Ceramics Co., Ltd. (and 1 other person) Agent: Masu Kobori (and 2 others)1
], Da* 2nd TZ

Claims (2)

[Claims] 1. The crystal phase of the sintered body has a composite structure of β-spodumene and monoclinic or tetragonal zirconia, and the weight fraction of β-spodumene/zirconia is 88/12 to 60/4.
A low thermal expansion ceramic characterized by having a low thermal expansion within a range of 0. 2. After mixing and molding 60-88% by weight of starting raw material powder consisting of 90-95% by weight of petalite and 10-5% by weight of kaolin and 40-12% by weight of zirconia powder,
A method for producing low thermal expansion ceramics, characterized by firing in the atmosphere at a temperature of ~1350°C.