JP2000063180A - Hexagonal boron nitride sintered product and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hexagonal boron nitride sintered product having excellent deformation resistance during employment under a high strength and a high temperature by an atmospheric pressure sintering method. SOLUTION: This hexagonal boron nitride sintered product has a three point bending strength of >=20 Mpa at ordinary temperature and a deformation resistance index of <0.75. This method for producing the hexagonal boron nitride sintered product comprises sintering a molded raw material containing boron nitride powder in an inert gas atmosphere having a nitrogen component content of <=10 vol.% (including zero). Therein, the boron nitride powder contains oxygen incapable of being removed with methanol in an amount of 0.4-1.7 wt.%, and 30-60 wt.% of the oxygen is released, when the boron nitride powder is held in a helium gas atmosphere at a temperature of 1,800 deg.C for 180 min.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hexagonal boron nitride sintered body and a method for producing the same. The hexagonal boron nitride sintered body produced according to the present invention has high strength and is excellent in deformation resistance during use. Therefore, a jig for producing or assembling semiconductors, electronic parts, etc., an electrical insulation Used as a heat dissipation material, crucibles and jigs for firing ceramics.

[0002]

2. Description of the Related Art Hexagonal boron nitride has a layered structure similar to graphite and is excellent in properties such as thermal conductivity, electrical insulation, chemical stability, solid lubricity and thermal shock resistance. In addition, the sintered body has great chemical stability, corrosion resistance, machinability,
It is widely used in various fields due to its excellent properties such as heat resistance, low dielectric properties and low dielectric loss properties. In particular,
In recent years, there is an increasing demand for large plate materials such as jigs for firing ceramics, and the maximum length is 400 mm and 1000 mm.
Corresponding to a sintered body having a shape reaching a certain degree is required.

The hexagonal boron nitride sintered body is manufactured by a hot pressing method or an atmospheric pressure sintering method using an oxide type sintering aid. However, in the hot press method, sleeve
Due to the strength limit of dies and the like, or the limitation of soaking, only products with a maximum length of about 400 mm can be manufactured. The maximum length here is a diagonal line if it is quadrangular,
A circular shape means a diameter, and an elliptical shape means a major axis.

Therefore, the atmospheric pressure sintering method, which can cope with a large shape having a maximum length of 400 mm or more, has been increasingly adopted. The hexagonal boron nitride sintered body produced by the atmospheric pressure sintering method has advantages that there is little variation in density after firing and that it has suitable characteristics as a jig for firing ceramics.

However, such a hexagonal boron nitride sintered body produced by the atmospheric pressure sintering method has a problem that it tends to be deformed during use at high temperature due to its increase in size. This is because the sintering aid component remaining inside the hexagonal boron nitride sintered body, especially oxygen, promotes deformation by growing a boron nitride crystal during use at high temperature. It is believed that.

Further, when the jig of the hexagonal boron nitride sintered body is used at a high temperature, a stress load is applied from the material to be fired to cause plastic deformation, so that repeated use is difficult. The amount of plastic deformation increases with applied stress, and decreases as the strength of the hexagonal boron nitride sintered body increases. From the above, as a jig for firing ceramics, the appearance of a hexagonal boron nitride sintered body having excellent deformation resistance and high strength has been awaited.

Conventionally, as the atmospheric pressure sintering method, there has been proposed a method of sintering amorphous boron nitride powder as a raw material in a reducing nitriding atmosphere or a nitriding atmosphere (Japanese Patent Publication No. 3-36781). According to the present invention, a high-strength hexagonal boron nitride sintered body can be obtained, but it does not become a hexagonal boron nitride sintered body excellent in deformation resistance.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to provide a hexagonal boron nitride sintered body having high strength and excellent deformation resistance. In particular, it is to provide a large-sized hexagonal boron nitride sintered body having these characteristics and having a maximum length of 400 mm or more.

[0009]

That is, the present invention has a hexagonal boron nitride sintered body characterized by a room temperature three-point bending strength of 20 MPa or more and a deformation resistance index of less than 0.75 as defined in the present invention. Is. The present invention also provides a method of firing a forming raw material containing boron nitride powder in an atmosphere of an inert gas having a nitrogen content of 10 vol% or less (including 0),
The boron nitride powder contains 0.4 to 1.7% by weight of oxygen that cannot be removed with methanol, and the oxygen content is 30 to 60% by weight when kept at a temperature of 1800 ° C. for 180 minutes in a helium gas atmosphere. % Of the hexagonal boron nitride sintered body is produced.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below.

Hexagonal boron nitride sintered body by atmospheric pressure sintering method
The sintering mechanism of is as follows. That is, Hara
The boron nitride powder used as a raw material is
It contains a trace amount of oxygen as an impurity that cannot be removed with a nor.
This trace amount of oxygen is a tri-elemental compound of B, N, and O (generally,
Expression "B_xN_yO_z"Represented by")
It is believed that. This raw material powder is molded and pressureless baked.
When tied, first B_xN _yO_zOxygen is B₂O₃Released in the form of
Will be issued. Then this B₂O₃Form a liquid phase
Hexagonal boron nitride particles were dissolved and re-precipitated in the
By the intergranular bond that grows and coincides with this grain growth
It becomes a hexagonal boron nitride sintered body.

The feature of the present invention is that, in the firing of such a hexagonal boron nitride sintered body, the amount of oxygen which cannot be removed by methanol contained in the boron nitride raw material powder and its release characteristic are optimized, as described later. In addition, by lowering the partial pressure of the nitrogen component in the firing atmosphere and firing, the desired high strength (at room temperature, three-point bending strength of 20 MPa or more) and deformation resistance (deformation resistance index of 0. It was possible to produce a hexagonal boron nitride sintered body having a value of less than 75).

The deformation resistance index defined in the present invention is the amount of warpage that occurs when a plate-shaped specimen of a hexagonal boron nitride sintered body is held at a temperature of 1900 ° C. for 10 hours under an atmosphere of nitrogen gas at atmospheric pressure. Measured and calculated using the formula of deformation resistance index = [warpage amount (mm) × test piece width (mm) × {test piece thickness (mm)} ² ] / [test piece length (mm)] It is a thing.

The larger the deformation resistance index, the more easily deformation occurs during use at high temperature, and in the hexagonal boron nitride sintered body by the hot pressing method, the deformation resistance index reaches 5 or more.

The hexagonal boron nitride raw material powder used in the manufacturing method of the present invention contains 0.4 to 1.7% by weight of oxygen which cannot be removed by methanol.

Here, "oxygen which cannot be removed by methanol" is mainly oxygen contained in the hexagonal boron nitride powder as an impurity as described above, and is B _x N _y.
It exists in the form of O _z and is considered to release oxygen in the form of B ₂ O ₃ during pressureless sintering to form a liquid phase.

On the other hand, regarding oxygen which can be removed by methanol, B ₂ O ₃ and H ₃ are added to the hexagonal boron nitride powder from the beginning.
It is considered to be oxygen existing in the form of BO _{3 and the} like.
These oxygens are released at a low temperature stage during normal pressure sintering, and thus do not contribute much to the improvement of the physical properties of the sintered body.

Regarding the removal of oxygen with methanol,
Hexagonal boron nitride dried in air at 120 ° C for 2 hours
Add 3 ml of methanol per 1 g of powder, and add B₂O₃
And H ₃BO₃Oxygen present in the form of
After extraction, add methanol at a temperature of 70-80 ° C.
It is carried out by volatilizing and drying for 1.5 hours.
It

For the measurement of the amount of oxygen that could not be removed with methanol, B ₂ O ₃ or H ₃ was measured by the above method.
After removing oxygen existing in the form of BO _{3 or} the like, it can be performed using, for example, an O / N simultaneous analyzer manufactured by Horiba Ltd.

Further, regarding the release characteristics of oxygen which cannot be removed by methanol, when the boron nitride powder containing this oxygen is held at a temperature of 1800 ° C. for 180 minutes in a helium gas atmosphere, the oxygen which cannot be removed by this methanol is removed. Three
It is preferred that 0-60% by weight be released. This time,
It is considered that oxygen is volatilized and released in the form of B ₂ O ₃ . Such oxygen release characteristics can be obtained by quantifying the amount of carbon monoxide produced when a uniform mixture of hexagonal boron nitride powder and carbon powder is held at the above temperature under a helium gas flow. .

In the present invention, the amount of oxygen contained in the hexagonal boron nitride powder that cannot be removed by methanol is 0.
If it is less than 4% by weight, sintering will be insufficient and the strength will decrease. On the other hand, if it exceeds 1.7% by weight, the amount of residual oxygen in the hexagonal boron nitride sintered body increases, which causes plastic deformation during use. Moreover, since the amount of liquid phase during sintering becomes too large, the grain growth of hexagonal boron nitride particles increases,
As a result, the sintered body is likely to be deformed, the purity and density are lowered, and further, cracks are generated due to rapid volatilization of a large amount of B ₂ O ₃ .

The content of oxygen which cannot be removed with methanol in the hexagonal boron nitride powder raw material is preferably adjusted based on the thickness of the hexagonal boron nitride sintered body produced, and for example, the thickness is about 35 mm. In the case of 0.5
The content is preferably 0.9% by weight.

Further, in the present invention, when the oxygen release characteristic which cannot be removed with methanol is less than 30% by weight, the amount of oxygen in the hexagonal boron nitride sintered body increases, and 60% by weight.
If it exceeds, cracks will occur. Optimal release characteristics are 40
~ 50% by weight.

The boron nitride powder raw material having the above-mentioned release characteristics and proper amount of oxygen which cannot be removed by methanol is composed of hexagonal boron nitride powder whose release characteristics and content are known and the primary particle size. It can be produced by appropriately mixing with a high crystal that has been sufficiently developed to an average value of about 10 to 15 μm and a hexagonal boron nitride powder that contains almost no oxygen that cannot be removed with methanol.

The former hexagonal boron nitride powder has a temperature range of 1 when a boric acid source such as orthoboric acid or borax is reacted with a nitrogen source such as nitrogen urea, melamine or ammonia.
The release characteristics and appropriate amount of oxygen that is produced at 200 to 1400 ° C. and cannot be removed with methanol can be adjusted by the calcination temperature, the type and amount of catalyst, and the like. The latter high-crystal hexagonal boron nitride powder has a reaction temperature range of 1
Manufactured at 800-2100 ° C. To mix both powders,
A mixer such as a ribbon blender, a V type mixer, a double cone blender, a Henschel mixer, etc. is used.

As a method for molding the hexagonal boron nitride powder raw material, a general method such as a die pressing method, a hydrostatic pressing method, a slurry casting molding method, an extrusion molding method, and an injection molding method can be adopted. . Moreover, these can also be combined. In these methods, when a molding organic binder or the like is used, it is necessary to remove it before firing. In particular, in order to obtain a hexagonal boron nitride sintered body having high strength and excellent resistance to deformation, it is desirable to apply a hydrostatic press of 0.5 ton / cm ² or more.

The firing of the hexagonal boron nitride compact is performed in an inert gas atmosphere having a nitrogen content of 10 vol% or less (including 0). As the inert gas, a simple substance or a mixture of helium, neon, argon and the like can be used. As the nitrogen component, in addition to nitrogen gas, ammonia and other gases containing nitrogen in its composition are used. When the nitrogen component exceeds 10 vol%, it is not possible to achieve both reduction of residual oxygen content and strength development. The reason for this is not clear, but the nitrogen component in the atmosphere should be 10
By controlling the content to be not more than vol% (including 0), as described below, in the temperature range in which sintering proceeds,
This is to promote the volatilization and release of oxygen that cannot be removed by methanol in the form of B ₂ O ₃ .

[0028] That is, the temperature range for sintering proceeds in the liquid phase by dissolution and reprecipitation of the hexagonal boron nitride particles to create a B ₂ O ₃ is is a 1,200-1,900 ° C., is more than 1900 ° C. B ₂ The saturated vapor pressure of O ₃ rises sharply. The B ₂ O ₃ liquid phase formed in the temperature range where the saturated vapor pressure is high volatilizes rapidly and cannot contribute to the sintering of hexagonal boron nitride. Therefore, it is convenient for the sintering of hexagonal boron nitride to change B _x N _y O _z to B ₂ O ₃ in a low temperature range where the saturated vapor pressure is low. Under the inert atmosphere of the present invention, the formation of B ₂ O ₃ is promoted in a temperature range where the saturated vapor pressure is low, so that oxygen existing in the hexagonal boron nitride powder raw material that cannot be removed by methanol is sufficiently sintered. It is thought that it can contribute to.

[0029] When the ratio of the nitrogen content exceeds 10 vol%, the inability to sufficiently balance the expression of reduction and strength of the residual amount of oxygen, when the larger part of the ratio of nitrogen content, the B ₂ O ₃ In the temperature range where the saturated vapor pressure is low, the change of oxygen to B ₂ O _{3 that} cannot be removed by methanol becomes insufficient, and the required amount of B ₂ O ₃ liquid phase that effectively contributes to sintering cannot be secured, resulting in a sintered body. It is considered that this is because it is difficult to obtain sufficient strength.

The firing temperature is preferably 1700 ° C. or higher. If it is lower than this temperature, the sintering does not proceed sufficiently. It is also possible to reduce the residual oxygen and raise the purity of the sintered body by further raising the temperature. By raising the temperature, it is possible to vaporize oxygen, which is harmful when the sintered body is used. In this case, the temperature can be industrially raised up to about 2300 ° C.

The temperature rising rate during firing is 1000 ° C.
Although any amount may be used in the temperature range below, industrially, it is advantageous to be fast. In the temperature range of 1000 ° C or higher, it is desirable to set the rate to about 50 to 500 ° C / hour.

According to the method described above, the high strength of three-point bending strength at room temperature is 20 MPa or more, and the deformation resistance index is 0.7.
A hexagonal boron nitride sintered body having an excellent deformation resistance of less than 5 can be produced.

[0033]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples.

Examples 1 to 10 Comparative Examples 1 to 8 Various hexagonal boron nitride powder raw materials having release characteristics as shown in Table 1 and containing oxygen which cannot be removed by methanol in the proportion shown in Table 1 are rubbers. It is filled in a mold and pressed by isostatic pressing at 1.5 ton / cm ^{2 so} that the dimensions are approximately 550 mm × 6.
It was molded into 50 mm × 35 mm. This compact was filled in a hexagonal boron nitride crucible and heated to the firing holding temperature shown in Table 1 at a temperature rising rate of 100 ° C./hour under the atmosphere gas flow shown in Table 1 at that temperature. Hold for 3 hours.
Thereafter, the temperature was raised to 2000 ° C. at a temperature rising rate of 100 ° C./hour, and the temperature was maintained for 3 hours and then allowed to cool to produce a hexagonal boron nitride sintered body.

The density, the three-point bending strength at room temperature, the amount of residual oxygen, and the deformation resistance index of the obtained hexagonal boron nitride sintered body were measured as follows. The results are shown in Table 2.

(1) The density is 3 mm × from each part of the sintered body.
Cut 10 pieces of 4mm x 50mm bending sample evenly,
The density was calculated from the weight and size, and the average value was calculated. (2) The room temperature three-point bending strength was measured according to JIS R1601 on the test piece used for the density measurement. (3) The residual oxygen content was measured in the same manner as the oxygen content of the hexagonal boron nitride powder raw material by crushing a part of the sintered body in a mortar. (4) Deformation resistance index is the warpage that occurs when a plate-shaped sample (50 mm x 100 mm x 4 mm) is cut out from a hexagonal boron nitride sintered body and held in a nitrogen gas atmosphere at a temperature of 1900 ° C for 10 hours. The amount was measured and calculated by the above formula.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

According to the present invention, a hexagonal boron nitride sintered body having a high density, a high strength, a small amount of residual oxygen, and excellent deformation resistance when used at high temperatures is obtained by pressureless sintering. be able to.

Claims (2)

[Claims] 1. A hexagonal boron nitride sintered body having a room temperature three-point bending strength of 20 MPa or more and a deformation resistance index of less than 0.75 defined in the present invention. 2. A method of firing a forming raw material containing boron nitride powder in an atmosphere of an inert gas having a nitrogen content of 10 vol% or less (including 0), wherein the boron nitride powder contains oxygen that cannot be removed with methanol. 0.4 to 1.7% by weight, and the oxygen is contained in a helium gas atmosphere at a temperature of 18
A method for producing a hexagonal boron nitride sintered body, wherein 30 to 60% by weight is released when held at 00 ° C for 180 minutes.