JPH09301772A - Low heat conductivity and high strength ceramic material and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problems of the conventional technique and to produce a low heat conductivity and high strength ceramic material having higher strength than the conventional material. SOLUTION: A mixture of 100 pts.wt. powders of Si3 N4 , Y2 O3 and Al2 O3 in a molar ratio of 90:5:5 with 50 pts.wt. cordierite powder is compacted and fired at 1,350 deg.C in an atmosphere of nitrogen to obtain the objective low heat conductivity and high strength ceramic material having <=4.0W/m.K heat conductivity and >=394MPa four-point bending strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low thermal conductivity and high strength ceramic material having low thermal conductivity and sufficient strength, and a method for producing the same.

[0002]

2. Description of the Related Art Silicon nitride ceramics are ceramics superior in strength, hardness, toughness and chemical stability as compared with other ceramics, and are widely applied as structural ceramics. However, since silicon nitride ceramics have a high thermal conductivity of 20 to 70 W / mK in the case of a dense body, they are not suitable for a heat insulating material, which is one of the general application examples of ceramic materials. Is.

In order to apply the silicon nitride ceramics to a heat insulating material and a heat resistant material, conventionally, a reaction sintered silicon nitride ceramics has been used in which metallic silicon powder is molded, and is nitrided and fired in a nitriding furnace.

In addition, Japanese Laid-Open Patent Publication No. H5-
The high-strength and low-heat-conductivity ceramics described in Japanese Patent No. 139823 and the low-heat-conductivity ceramics described in JP-A-5-330907 are known.

[0005]

However, since the reaction-sintered silicon nitride ceramics are weak in strength, they can be used only for applications such as a kiln tool which does not require relatively high strength.

In recent years, aluminum titanate (Al ₂ O ₃ .TiO ₂ ) has been added to reaction-sintered silicon nitride ceramics as described in Japanese Patent Application Laid-Open No. 5-139823 to reduce thermal conductivity and increase strength. Some of them are known. However, 35 wt% aluminum titanate
When added, the thermal conductivity has been reduced to about 3 W / m · K, but the three-point bending strength is still low at 250 MPa or less.

Further, in Japanese Unexamined Patent Publication (Kokai) No. 5-330907, similarly, silicon nitride whiskers or the like are added to reaction-sintered silicon nitride so that 2 W / perpendicular to the silicon nitride whiskers.
It is described that it has low heat conductivity of m · K and high strength of 320 MPa. However, although the thermal conductivity is as low as 3 W / m · K in the direction parallel to the silicon nitride whiskers, the strength is reduced to 208 MPa.

The present invention has been made in view of the above circumstances, and a first object of the present invention is to provide a novel low thermal conductive and high strength ceramic material different from the prior art and a method for producing the same. is there.

A second object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a ceramic material having a small thermal conductivity and a higher strength than conventional materials, and a manufacturing method thereof. Is.

[0010]

According to the present invention, at least one of the above objects can be achieved by at least one of the following low thermal conductive high strength ceramic material and its manufacturing method.

A thermal decomposition product of cordierite powder and a reaction product obtained by reacting silicon nitride powder in the presence of yttria powder and alumina powder at a temperature of 1325 ° C. or higher have a thermal conductivity of 6 W / m · K. Below, bending strength is 2
A low heat conductive high strength ceramic material having a pressure of 50 MPa or more (claim 1).

Preferably, the total amount of the silicon nitride powder, the yttria powder and the alumina powder is 100 parts by weight, and the cordierite powder before thermal decomposition is 10 to 50.
Parts by weight (claim 2).

The molar ratio of the silicon nitride powder, the yttria powder and the alumina powder is preferably 90-9.
2: 3 to 5: 5 to 8 (claim 3). The reaction product is preferably one or more of silicon oxynitride microcrystals and sialon microcrystals (claim 4).

The low thermal conductivity high strength ceramic material preferably contains at least one of cordierite and its pyrolyzate (claim 5). The pyrolyzate is preferably spinel crystallites (claim 6). The low thermal conductivity high strength ceramic material preferably contains α-silicon nitride crystallites (claim 7).

The particle size of each of the silicon oxynitride microcrystal, the sialon microcrystal, the spinel microcrystal and the α-silicon nitride microcrystal is preferably 1 μm or less (claim 8).
Preferably Si-Al-Mg-O system and Si-Al-M
Either or both of the g-O-N glassy grain boundary phases are present between the crystallites (claim 9).

A total of 100 parts by weight of silicon nitride powder, yttria powder and alumina powder and cordierite powder 1
A molded body of the mixture of 0 to 50 parts by weight is fired at 1325 ° C. or higher to pyrolyze the cordierite powder to obtain a pyrolyzed product of cordierite. A method for producing a low-thermal-conductivity, high-strength ceramic material, which comprises the step of reacting silicon powder (claim 10).

The molar ratio of the silicon nitride powder, yttria powder and alumina powder is preferably 90 to 92: 3.
5: 5-8 (claim 11). As the silicon nitride powder, preferably, a silicon nitride powder having an average particle diameter of 1.5 μm or less and manufactured by an imide decomposition method or a direct nitriding method is used (claim 12).

As the cordierite powder, chemically synthesized cordierite having an average particle size of less than 10 μm is used (claim 13). A precursor of cordierite can be used as a part or all of the cordierite powder (claim 14).

First, the concept of the present invention will be described. There are three possible ways to reduce the thermal conductivity. (1) Reduce the heat capacity. (2) The transmission speed of the heat medium (mainly phonons in ceramic materials) is slowed down. (3) Decrease the density.

Of these, the most popular method is the method (1) of reducing the heat capacity. The heat capacity of ceramics can be easily reduced by forming a composite with ceramics having a low heat capacity, but materials having a low heat capacity such as zirconia and zircon generally have a high density. Therefore, even if it is made into a composite with a material having a low heat capacity, the heat capacity is decreased, and at the same time, the density is increased, and a large decrease in the thermal conductivity cannot be expected. Further, introduction of pores is a very effective method for reducing the heat capacity, but introduction of pores into the material causes a marked decrease in strength of the material, which is not desirable in the present invention.

Therefore, in the present invention, the thermal conductivity is reduced by slowing the transfer speed of the heat medium in (2). The heat medium of ceramics is mainly phonons, and the scattering of phonons is promoted by increasing the grain boundary phase in the ceramics.
The transmission speed can be reduced. Furthermore, in order to increase the grain boundary phase in the material, different particles (including pores) may be added or the crystal grains in the material may be made smaller. It is generally known that the crystal grains obtained by synthesis or decomposition during firing are smaller than the crystal grains introduced in the raw material preparation. The different particles were thermally decomposed by firing at a temperature higher than the temperature at which the particles were thermally decomposed, and as a result, low thermal conductivity was achieved by introducing different particles having small crystal grains.

In the present invention, the description of the numerical range includes not only the both end values but also any arbitrary intermediate value included therein.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION

(Low Heat Conduction High Strength Ceramic Material) The low heat conduction high strength ceramic material of the present invention is cordierite (2MgO 2
Al ₂ O ₃ 5SiO ₂ ) powder thermal decomposition product and silicon nitride (Si ₃
N ₄ ) powder contains a reaction product obtained by reacting at 1325 ° C. or higher (preferably 1325 to 1500 ° C.) in the presence of yttria (Y ₂ O ₃ ) powder and alumina (Al ₂ O ₃ ) powder.

The low thermal conductivity and high strength ceramic material of the present invention has a thermal conductivity of 6 W / m · K or less (preferably 6 to
2 W / m · K, more preferably 4 to 2 W / m · K) and a bending strength of 250 MPa or more (preferably 370 M).
Pa or more).

The molar ratio of the silicon nitride powder, the yttria powder and the alumina powder is preferably 90-9.
2: 3-5: 5-8.

The reaction product is preferably silicon oxynitride (Si ₂ ON ₂ ) crystallites and sialon (Si-Al-).
One or two of the (O-N) crystallites.

The low heat conductive high strength ceramic material of the present invention may contain one or more of cordierite and its pyrolyzate. The thermal decomposition product includes spinel microcrystals. Further, the low thermal conductive high strength ceramic material of the present invention contains α-silicon nitride microcrystals. Between at least one of silicon oxynitride microcrystals, sialon microcrystals, and spinel microcrystals and α-silicon nitride microcrystals, Si-
It is possible to have a glassy grain boundary phase of either one or both of Al-Mg-O system and Si-Al-Mg-O-N system.

The grain size of each of the silicon oxynitride microcrystal, the sialon microcrystal, the spinel microcrystal and the α-silicon nitride microcrystal is preferably 1 μm or less.

(Production Method of Low Heat Conduction High Strength Ceramic Material) In the production method of the low heat conduction high strength ceramic material of the present invention, a total of 100 parts by weight of silicon nitride powder, yttria powder and alumina powder and 10 to 50 cordierite powders are used.
A molded body of a mixture of parts by weight (more preferably 20 to 50 parts by weight, further preferably 40 to 50 parts by weight) is preferably 1325 ° C or higher (preferably 1325 to 1500 ° C, more preferably 1325 to 1525) in a nitrogen atmosphere. Thirteen
Baking at 50 ° C. By firing in this way,
The cordierite powder can be thermally decomposed to obtain a thermally decomposed product of cordierite, and the thermally decomposed product of cordierite can be reacted with the silicon nitride powder.

The molar ratio of the silicon nitride powder, yttria powder and alumina powder is preferably 90 to 92: 3.
5: 5-8.

As the matrix material in the low thermal conductive high strength ceramic material of the present invention, silicon nitride ceramics having high strength among ceramics was selected. The silicon nitride ceramics used for the heat insulating material is generally a reaction sintered silicon nitride ceramic obtained by nitriding and firing metallic silicon. However, since the reaction sintered silicon nitride ceramic has a weak strength, the low thermal conductivity high strength ceramic of the present invention is used. The silicon nitride ceramic in the material is produced from silicon nitride raw material powder. The silicon nitride raw material powder preferably has an α conversion rate (α-S
i ₃ N ₄ production rate) 96% or more α-silicon nitride (α-S
i ₃ N ₄ ) powder is used.

The dense silicon nitride ceramics produced by using the silicon nitride raw material powder has a thermal conductivity of 20 to 70 W / m.
・ K is a ceramic with a relatively high thermal conductivity, but such a dense silicon nitride ceramic material has almost 1
Since 00% is β-phase silicon nitride, and β-silicon nitride has a higher thermal conductivity than α-silicon nitride, the dense silicon nitride ceramics has a high thermal conductivity. Β silicon nitride raw material powder
By firing in a temperature range (1500 ° C. or less) that does not cause a phase transition to a phase to obtain α-phase silicon nitride ceramics, the thermal conductivity can be made equal to that of reaction-sintered silicon nitride ceramics.

The average particle size of the silicon nitride raw material powder used in the manufacturing method of the present invention is preferably 1.5 μm or less, more preferably 1.0 μm or less. The silicon nitride raw material powder used in the production method of the present invention can be preferably obtained by an imide decomposition method or a direct nitriding method.

As the silicon nitride raw material powder used in the manufacturing method of the present invention, in order to increase the strength, use a silicon nitride raw material powder having an average particle diameter of 1.0 μm or less and produced by a fine and active imide decomposition method. Is more desirable, but the average particle size is 1.5μ
A silicon nitride raw material powder produced by a direct nitriding method can also be used if it is fine and active and has a fineness of m or less.

A sintering aid is added to the silicon nitride raw material powder in order to obtain a dense sintered body. Although yttria, alumina, magnesia, tungsten oxide and the like are known as sintering aids for silicon nitride ceramics, in the present invention, one or more of yttria and alumina are preferable for low temperature sintering and high strength. Is used as a sintering aid, and more preferably two types of yttria and alumina are used in combination. The addition amount of the sintering aid is such that the silicon nitride raw material powder can be sintered.

The different particles added to the silicon nitride raw material powder are
As described above, it is a ceramic that thermally decomposes in a high temperature atmosphere, and cordierite is used in the method of the present invention. Cordierite and mullite (3Al ₂ O ₃ .2Si) are used as ceramics that thermally decompose in a high temperature atmosphere.
O ₂ ), aluminum titanate, zircon (ZrO ₂ · S
iO ₂ ) and the like are known.

Among these ceramics, zircon and mullite are thermally decomposed at a temperature of 1500 ° C. or higher,
The β phase of the silicon nitride matrix occurs, which is not suitable for low thermal conductivity.

Aluminum titanate and cordierite are thermally decomposed at 1500 ° C. or lower, but cordierite is thermally decomposed by a ternary compound to form a more complicated crystal phase than aluminum titanate which is a binary compound, and phonon Most desirable because it facilitates scattering.

The average particle diameter of cordierite is preferably 10 μm or less. The cordierite used in the method of the present invention is preferably a chemically synthesized cordierite, although cordierite precursors can also be used.
Examples of cordierite precursors include spinel and cristobalite.

The cordierite is more preferably a fine cordierite powder which is chemically synthesized and has a high purity and an average particle size of 10 μm or less.

The cordierite powder is silicon nitride powder and its sintering aid (yttria and alumina, the same applies hereinafter).
10 to 50 parts by weight are added to 100 parts by weight of the powder. The effect of lowering the thermal conductivity cannot be confirmed even if a smaller amount of cordierite than 10 parts by weight is added. If cordierite is added in an amount of more than 50 parts by weight, the cordierite will be dissolved in the sintering process, or cracks will be generated in the cooling process of the sintering to lower the strength.

Part or all of the cordierite powder can be replaced with the cordierite precursor powder. The cordierite precursor powder is obtained by converting all cordierite contained in the conversion into cordierite powder (the total weight of the cordierite powder and the cordierite obtained from the cordierite precursor powder) to silicon nitride. The powder and the sintering aid powder are contained in an amount of 10 to 50 parts by weight based on 100 parts by weight.

The silicon nitride raw material powder, the sintering aid, and the cordierite raw material powder are blended and mixed by wet mixing, and then molded into a predetermined shape, at a temperature not lower than the temperature at which cordierite is thermally decomposed (usually Baking at a temperature of 1325 ° C. or higher). Cordierite is not decomposed at a temperature lower than 1325 ° C., so that the low thermal conductivity is insufficient.

In FIG. 1, the addition amount (value of B / A × 100% in the case where the total weight of the silicon nitride powder and its sintering aid powder is A and the weight of cordierite is B) is 50 wt%.
Silicon nitride with cordierite added at 1300-1
The X-ray-diffraction pattern at the time of sintering at 400 degreeC (Examples 9, 10 and reference example 2) is shown. 1350 ℃ according to Fig.
The cordierite peak disappeared, and silicon oxynitride,
The peaks of sialon and spinel are confirmed, and it can be confirmed that the crystal phase becomes complicated. This is because cordierite is thermally decomposed and reacts with silicon nitride as in the following reaction formula,
This is because silicon oxynitride and sialon were generated.

(1) Thermal decomposition of cordierite: 2MgO2Al ₂ O ₃ 5SiO ₂ → 2MgOAl ₂ O ₃ +5
SiO ₂

(2) Reaction with silicon nitride Si ₃ N ₄ + SiO ₂ → 2Si ₂ ON ₂

In FIG. 2, the addition amount after firing at 1300 to 1400 ° C. (B / in the case where the total weight of the silicon nitride powder and its sintering aid powder is A and the weight of cordierite is B /
A value of (A × 100%) shows a change in thermal conductivity with respect to the sintering temperature of silicon nitride ceramics added with cordierite at 50 wt% (Examples 9 and 10 and Reference Example 2).
When fired at 1350 ° C, the thermal conductivity is 5.3 W / m
The lowest is K. However, when firing at a temperature higher than 1400 ° C., the cordierite addition amount is 50 wt.
If it is more than%, it will be dissolved in the sintering process. If the cordierite is thermally decomposed, the holding time at the maximum temperature is preferably about 1.5 hours to 4.5 hours. If the holding time is long, grain growth is promoted, grain boundaries are reduced, and thermal conductivity increases.

During firing, the air should be evacuated to 2 × 10 ^-3 torr or less, then nitrogen gas should be introduced, and firing should be performed in a nitrogen atmosphere. This is to prevent the silicon nitride ceramics from being oxidized. Nitrogen gas is 9 kgf / cm
^{Although a} gas pressure of about ² (0.882 MPa) may be applied, since silicon nitride ceramics is not gasified in the sintering process when sintering is performed in the temperature range of 1325 to 1500 ° C.,
An atmosphere in which nitrogen gas is flowed at about 1 to 5 liters / minute may be used.

[0049]

EXAMPLES The present invention will be described below with reference to examples.

[Example 1] Silicon nitride powder was produced by the imide decomposition method and manufactured by Ube Industries, Ltd. E-10 (average particle size; 1.07).
μm) and yttria (YF, manufactured by Mitsubishi Chemical) and alumina (AKP-30, manufactured by Sumitomo Chemical Co., Ltd.) are used as Si ₃ N ₄ : Y.
₂ O ₃ : Al ₂ O ₃ = 90: 5: 5 (mol%) was added. Cordierite powder is K manufactured by Kyoritsu Ceramics Co., Ltd.
YORIX (average particle size: 8.22 μm) is used, and the total amount of silicon nitride, yttria, and alumina is 20 to 20.
It was added so as to be 50 wt%.

Silicon nitride, yttria, alumina, and cordierite were put in a polyethylene pot together with alumina boulders and mixed by wet mixing for 16 hours. After mixing, the slurry was dried, and further dry mixed with a ball mill for 48 hours.

The mixed powder was subjected to CIP molding at 147 MPa and then fired. The firing was performed by evacuation to 2 × 10 ⁻³ torr, and then nitrogen gas was introduced at a rate of 2 liter / min to create a nitrogen atmosphere. The firing was carried out at each sintering temperature of 1325 to 1350 ° C. for 1.5 to 6.0 hours at the maximum temperature. Table 1 shows various characteristics of the low thermal conductive ceramic materials produced under the respective conditions.

Further, Table 1 shows various characteristics of silicon nitride to which 50 wt% cordierite added at 1300 ° C. which does not cause thermal decomposition of cordierite is added as a reference example.

[0054]

[Table 1]

Example 2 A low thermal conductive ceramics was prepared by using a silicon nitride powder (S-1 manufactured by Hermann-C-Stark Co., Ltd., average particle size: 1.18 μm) produced by a direct nitriding method as the silicon nitride powder. It was made. Yttria (YF from Mitsubishi Chemical) as a sintering aid, and alumina (A from Sumitomo Kasei)
KP-30) a _{Si 3 N 4: Y 2 O} 3: Al 2 O 3 = 92:
It was added so as to be 3: 5 (mol%). The cordierite powder used is KYORIX manufactured by Kyoritsu Ceramics Co., Ltd., and is 50 wt% with respect to the total amount of silicon nitride, yttria, and alumina.
% Was added.

Silicon nitride, yttria, alumina, and cordierite were put in a polyethylene pot together with alumina boulders and mixed by wet mixing for 16 hours. After mixing, the slurry was dried, and further dry mixed with a ball mill for 48 hours.

The mixed powder was subjected to CIP molding at 147 MPa and then fired. After baking the vacuum, 2
Nitrogen gas was introduced at a rate of 1 / min to create a nitrogen atmosphere. The firing was performed by holding each sintering temperature of 1350-1400 ° C. for 4.5 hours. Table 2 shows various characteristics of the low thermal conductive ceramic materials obtained under the respective conditions.

Table 2 also shows various characteristics of the silicon nitride / cordierite composite material fired at 1300 ° C. as a reference example.

[0059]

[Table 2]

In Tables 1 and 2, the thermal conductivity is a value obtained by the laser flash method at room temperature,
The 4-point bending strength is a value obtained by using a JIS test piece (thickness 3 mm x width 4 mm x total length 40 mm) based on the JIS standard (the number of test pieces is 10 or more).

[0061]

The low heat conductive high strength ceramic material of claims 1 to 9 is obtained by reacting the pyrolyzed product of cordierite powder with silicon nitride powder in the presence of yttria powder and alumina powder at 1325 ° C or higher. Contains a reaction product, has a thermal conductivity of 6 W / mK or less, and a bending strength of 250.
Since it is at least MPa, it has the basic effects of low thermal conductivity and high strength.

Since the low heat conductive high strength ceramic material according to claims 2 to 9 further comprises the specific constituent requirements, the above-mentioned basic effects are remarkable.

A method for producing a low heat conductive high strength ceramic material according to claims 10 to 14 is a method for producing a molded body of a mixture of a total of 100 parts by weight of silicon nitride powder, yttria powder and alumina powder and 10 to 50 parts by weight of cordierite powder. 1325 ° C
The thermal conductivity includes the step of firing the above, thermally decomposing the cordierite powder to obtain a thermally decomposed product of cordierite, and reacting the thermally decomposed product of cordierite with the silicon nitride powder. It is possible to manufacture the low heat conduction high strength ceramic material of the present invention having a small heat resistance and a high strength.

Since the method for producing a low heat conductive high strength ceramic material according to claims 11 to 14 further includes the specific constituent requirements, the above-mentioned basic effects are remarkable.

[Brief description of drawings]

FIG. 1 is a diagram showing an X-ray diffraction pattern of a composite obtained by sintering a mixture containing silicon nitride and cordierite at 1300 ° C., 1350 ° C. and 1400 ° C., respectively.

FIG. 2 is a diagram showing a change in thermal conductivity with respect to a sintering temperature of a composite body obtained by sintering a mixture containing silicon nitride and cordierite.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hirokazu Matsunaga No. 36 Noritake Shinmachi 3-chome Noritake Nishi-ku, Nagoya-shi, Aichi Noritake Company Limited Limited

Claims (14)

[Claims] 1. A thermal decomposition product of cordierite powder at 1325 ° C. or higher and a reaction product obtained by reacting silicon nitride powder in the presence of yttria powder and alumina powder, and having a thermal conductivity of 6 W / m.・ Below K, bending strength is 25
A low heat conductive high strength ceramic material characterized by having a pressure of 0 MPa or more. 2. The total of the silicon nitride powder, the yttria powder and the alumina powder is 100 parts by weight, and the cordierite powder before thermal decomposition is 10 to 50 parts by weight. The low heat conductive high strength ceramic material described in. 3. The mol ratio of the silicon nitride powder, the yttria powder, and the alumina powder is 90 to 92: 3 to 5:
It is 5-8, The low heat conduction high strength ceramic material in any one of Claims 1-2. 4. The low heat conductive high strength ceramic material according to claim 1, wherein the reaction product is one or more of silicon oxynitride microcrystals and sialon microcrystals. . 5. The low heat conductive high strength ceramic material according to claim 1, which contains one or more of cordierite and a thermal decomposition product thereof. 6. The low heat conductive high strength ceramic material according to claim 5, wherein the thermally decomposed material is spinel crystallites. 7. The low heat conductive high strength ceramic material according to claim 1, which contains α-silicon nitride microcrystals. 8. The grain size of each of the silicon oxynitride microcrystal, the sialon microcrystal, the spinel microcrystal, and the α-silicon nitride microcrystal is 1 μm or less.
7. A low heat conductive high strength ceramic material according to any one of 7. 9. Si-Al-Mg-O system and Si-Al-
The glassy grain boundary phase of any one or both of Mg-O-N system is present between the crystallites.
8. A low heat conductive high strength ceramic material according to any one of 8. 10. A total of 100 parts by weight of silicon nitride powder, yttria powder and alumina powder, and cordierite powder 1
A molded body of the mixture of 0 to 50 parts by weight is fired at 1325 ° C. or higher to pyrolyze the cordierite powder to obtain a pyrolyzed product of cordierite. A method for producing a low heat conductive high strength ceramic material, comprising the step of reacting silicon powder. 11. The low heat conductive high strength ceramic material according to claim 10, wherein the molar ratio of the silicon nitride powder, the yttria powder and the alumina powder is 90 to 92: 3 to 5: 5 to 8. Production method. 12. The silicon nitride powder having a mean particle size of 1.5 μm or less, which is produced by an imide decomposition method or a direct nitriding method, is used as the silicon nitride powder.
11. The method for producing a low heat conductive high strength ceramic material according to any one of 1 to 11. 13. The low heat conductive high strength ceramic material according to claim 10, wherein a chemically synthesized cordierite having an average particle size of less than 10 μm is used as the cordierite powder. Method. 14. The method for producing a low heat conductive high strength ceramic material according to claim 10, wherein a precursor of cordierite is used as a part or all of the cordierite powder.