JPH05148038A - Production of sintered boron nitride - Google Patents

Abstract

PURPOSE:To produce a sintered material having good thermal stability and electrical insulation from a commercially available boron nitride powder using a normal-pressure sintering process without using an oxide-based sintering assistant. CONSTITUTION:A powdery mixture produced by adding 0.9-10wt.% of boron carbide to boron nitride is formed, heated in an inert atmosphere (e.g. argon) or in vacuum up to 1600 deg.C and baked in a nitriding atmosphere at 1600-2100 deg.C. The oxide layer on the surface of the baron nitride powder is removed by reduction to improve the activity of the powder surface and the sinterability of the boron nitride powder. The bond between baron nitride particles is reinforced by the conversion of a part of the added B4C into boron nitride.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a boron nitride sintered body by an atmospheric pressure sintering method.

[0002]

2. Description of the Related Art Since boron nitride powder has a very poor sinterability, the sintered body is made of Al ₂ O ₃ , SiO ₂ , B ₂ as a sintering aid.
It is manufactured by a hot pressing method by adding an oxide such as O ₃ . However, the manufacturing by the hot pressing method has the drawbacks of low productivity and high cost.

Therefore, Japanese Patent Application Laid-Open No. 63-45177 discloses a method for producing a boron nitride sintered body by using an amorphous boron nitride powder or an atmospheric pressure sintering method using boron powder as a starting material. Japanese Patent Laid-Open No. 2-111665 discloses a boron nitride sintered body in which a mixed raw material of a boron nitride powder containing water-insoluble oxygen and a reducing component such as B ₄ C for reducing the water-insoluble oxygen is pressure-sintered. A manufacturing method is disclosed.

However, in the method using the amorphous boron nitride powder as a starting material disclosed in Japanese Patent Laid-Open No. 63-45177, the amount of oxygen in the starting material is high, so that the weight loss in the firing process is large, and Since a part of oxygen remains as B ₂ O _{3 in the} fired structure, it deteriorates the properties such as thermal stability and high corrosion resistance of the boron nitride sintered body. Further, in the method of using boron powder, it is necessary to pay particular attention to the storage of the powder because the boron powder reacts with the moisture in the air and changes into B ₂ O _{3 in addition} to the economical problem that the boron powder is expensive. There is also a problem that there is.

The method disclosed in Japanese Patent Laid-Open No. 2-111665 has been proposed in order to avoid the problem caused by oxygen contained in the boron nitride raw material powder, but the obtained fired body is obtained. Is a composite composition of boron nitride and the reaction product of oxygen contained in the raw material powder of boron nitride and the added reducing component, so that the characteristics as a composite material are manifested, but it is more thermally effective than boron nitride alone. The drawbacks are that the characteristics such as stability and electrical insulation deteriorate.

[0006]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems in the production of a boron nitride sintered body by the atmospheric pressure sintering method and the defects in the characteristics of the obtained sintered body. It is intended to provide a method for producing a sintered body having the characteristics inherent in boron nitride by using a boron nitride powder by an atmospheric pressure sintering method.

[0007]

According to the present invention, there is provided a step of molding a mixed powder in which 0.9 to 10% by weight of boron carbide is added to boron nitride, and the obtained molded body is subjected to an inert atmosphere such as argon or the like. The step of heating up to 1600 ° C. in vacuum, and the obtained heat-treated body in a nitriding atmosphere at 160
And a step of firing at 0 to 2100 ° C.

[0008]

The present invention is as follows: (1) Boron carbide heated in an inert atmosphere or vacuum has a reducing action. For this reason, when the boron nitride powder added with boron carbide is heat-treated in an inert atmosphere or in vacuum, the oxide layer on the surface of the boron nitride powder is reduced and removed, so that the surface of the boron nitride powder is cleaned and the powder surface is cleaned. The sinterability should be improved by increasing the activation degree of.

(2) Boron carbide reacts with nitrogen in a high-temperature nitriding atmosphere to form boron nitride, so that this boron nitride forming reaction is added to the atmospheric pressure sintering of the boron nitride powder itself and becomes strong. A sintered body having a bond is obtained.

[0010] It was made by the knowledge that.

The oxygen contained in the boron nitride powder is B ₂ O.
_It is said that it does not exist as a clear oxide of _3, but exists as an amorphous oxide layer on the surface of the boron nitride powder. Such an oxide layer is decomposed by heating in a nitriding atmosphere and removed to the outside of the system, but since sintering proceeds while the oxide layer on the powder surface is decomposed, the original boron nitride powder In addition to the poor sinterability, the sinterability as a molded product becomes extremely poor. When boron carbide powder is added to the boron nitride powder having an amorphous oxide layer on such a surface in an amount of 0.9 to 10% by weight and heated to 1600 ° C. in an inert atmosphere such as argon or in vacuum, the reduction effect of boron carbide is obtained. As a result, the oxide layer on the surface of the boron nitride powder is removed, the surface of the boron nitride powder is cleaned, and the sinterability can be improved by improving the activation degree of the powder surface. Further, the powder having improved sinterability due to the reducing action of boron carbide starts self-sintering at a heating temperature of 1600 ° C. or higher to form a bond between the boron nitride powders. Of the boron carbide added at this time, the remaining boron carbide that was not used for reducing and removing the oxide layer on the surface of the boron nitride powder was formed by reacting with nitrogen in the atmosphere to form boron nitride. It will reinforce the bond between them.

The boron nitride powder is preferably a fine powder having a particle size of 10 μm or less and containing oxygen as low as possible. The boron carbide powder added to the boron nitride is preferably a fine powder having a particle size of 2 μm or less in consideration of the reducing action and the reactivity with nitrogen.

The amount of boron carbide added to the boron nitride powder is defined as 0.9 to 10% by weight, because the addition of less than 0.9% by weight is sufficient to remove the oxide layer on the surface of the boron nitride powder. The reason for this is that the upper limit was defined as 10% by weight, because the remaining boron carbide that was not used to remove the oxide layer on the surface of the boron nitride powder completely reacted with nitrogen and was not converted to boron nitride. This is because boron carbide remains in the sintered body and the absolute value of the amount of residual boron carbide increases, so that the sintered body does not have the original characteristics of boron nitride.

The upper limit of the temperature range of the reduction action by boron carbide is defined as 1600 ° C. because the lower limit of the temperature at which the sintering of boron nitride is recognized is about 1600 ° C., and therefore, the temperature at which the sintering of boron nitride starts. This is because it is necessary to achieve the cleaning of the surface of the boron nitride powder. The time of the reduction treatment varies depending on the type of powder and the state of the surface, but as one guideline, it is necessary to heat until the vacuum degree, which is lowered as the heating temperature rises and recovers when the heating is performed in a vacuum.

The upper limit of the sintering temperature is defined as 2100 ° C. because heating up to 2100 ° C. can provide a sintered body with sufficient characteristics, and sintering exceeding 2100 ° C. Although there is no problem with the characteristics of the body, the amount of weight loss due to evaporation increases as the temperature increases, and it is not economically advantageous.

[0016]

【Example】

Example 1 A mixed powder having a composition shown in Table 1 was prepared by using a boron nitride powder having a particle size of 10 μm or less and a boron carbide powder having a particle size of 2 μm or less.
After molding with a pressure of 0 kg / cm ² , 5 ton / c
CIP treatment was performed at a pressure of m ² to prepare a 50 × 50 × 10 mm molded body.

The obtained molded body was held in vacuum at 1600 ° C. for 1 hour and then heated in a nitriding atmosphere at 2000 ° C. for 2 hours. The characteristics of the obtained sintered body are shown in Table 1.

[0018]

[Table 1] It can be seen that the characteristics of the sintered body are improved as the addition of silicon carbide is increased. However, among the added boron carbide, the boron carbide that has not been used for removing the oxide layer on the surface of the boron nitride powder does not react with nitrogen at all. The reaction rate of boron is less than 40%. For this reason, 6% by weight or more of boron carbide remains in the obtained sintered body and has the characteristics as a composite sintered body, so that the performance as a boron nitride sintered body deteriorates.

Example 2 A molded body of a mixed powder obtained by adding 8% by weight of boron carbide to boron nitride powder was prepared in the same manner as in Example 1, and heated under the conditions shown in Table 2 to prepare a sintered body. did. Table 2 shows the characteristics of the obtained sintered body.

[0020]

[Table 2] When the boron nitride powder is heated in a nitriding atmosphere without reduction treatment or under appropriate conditions without reduction treatment,
The properties of the obtained sintered body are degraded. Further, if the sintering temperature is lower than 1600 ° C, a sintered body having sufficient characteristics cannot be obtained, and if it exceeds 2100 ° C, the weight of the sintered body is largely reduced, the density is lowered, and the strength is also lowered.

[0021]

The present invention has the following effects.

(1) A sintered body having excellent characteristics can be manufactured by the atmospheric pressure sintering method.

(2) Further, the advantages of the atmospheric pressure sintering method can be exhibited, and it becomes possible to manufacture a large-sized and complex-shaped sintered body.

(3) Further, since no oxide type sintering aid is used, the obtained sintered body has the original characteristics of boron nitride.

(4) The sintered body obtained in the present invention is used for a high-voltage insulating jig, an electric insulating material such as a high-temperature electrical insulating material, a jig for a molten metal such as a crucible and a nozzle, and a thermocouple protection tube. It can be used suitably for heat resistant, corrosion resistant material, heat dissipation material, etc.

Claims (1)

[Claims] 1. A step of molding a mixed powder obtained by adding 0.9 to 10% by weight of boron carbide to boron nitride, and heating the obtained molded body to 1600 ° C. in an inert atmosphere such as argon or in a vacuum. A method for manufacturing a boron nitride sintered body, which comprises the steps of: and a step of firing the obtained heat-treated body at 1600 to 2100 ° C. in a nitriding atmosphere.