JP2614874B2 - Pressureless sintered boron nitride compact - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a normal pressure sintered boron nitride molded article.

<Conventional technology> Boron nitride (BN) has electrical insulation, thermal conductivity, corrosion resistance,
While having excellent properties such as thermal shock resistance and lubricity, it is one of the few ceramics that is easy to machine. For this reason, including various containers for metal melting where the above properties are required,
Widely used for electrical insulation materials, high-temperature heat transfer materials, etc.

The BN sintered body is generally made by a hot press (pressure sintering) method because BN is hardly sintered. that is,
Since the process is carried out at 1500 to 2300 ° C. under a pressure of more than 100 kg / cm ² , there are problems that a large-sized product cannot be obtained and that it is not suitable for a method of producing a complicated-shaped product. In addition, currently available BN compacts are expensive because they employ a method in which a sintered body that has been hot-pressed into a cylindrical shape is machined to finish the final product shape.

Various normal pressure sintering methods have been attempted to solve the above problems, but to date, no BN sintered body that can exhibit the characteristics of BN has been obtained. For example, JP-A-61-
In the publication 132563, sintering under the condition of restricting free expansion by inserting a preform obtained by high-pressure rubber press molding into a graphite mold is attempted, but the density and strength are low and the production efficiency is low. And there is a problem that a large-sized product cannot be obtained. Therefore, it is desired to provide a normal pressure sintered BN molded body having excellent properties inherent to BN, such as electrical insulation, heat transfer, corrosion resistance, and thermal shock resistance, and which can be manufactured at low cost, easily and efficiently. Was.

<Problems to be Solved by the Invention> The present invention improves the shape restriction and productivity of a BN sintered body, and achieves normal pressure sintering with good thermal conductivity and electrical insulation that could not be obtained conventionally.
It is intended to provide a BN molded body.

<Means for Solving the Problems> That is, the present invention provides an alkaline earth metal borate comprising
40% by weight, relative density 65-80%, flexural strength 55
Insulation resistance of 10 ¹⁰ Ω, characterized by being 0 kg / cm ² or more
As described above, the normal pressure sintered boron nitride molded body having a thermal conductivity of 0.04 cal / cm · sec · ° C. or more.

 Hereinafter, the present invention will be described in detail.

The BN powder used in the present invention may be a commercial product, but is preferably a hexagonal BN powder having high crystallinity. Since this powder is excellent in plastic deformation at the time of preforming, a high-density preformed body is easily obtained. As the alkaline earth metal of the alkaline earth metal boron salt, one or more of magnesium, calcium, strontium, and barium are arbitrarily selected.

The compact of the present invention is a method of baking after molding a mixed powder obtained by mixing the above BN powder and alkaline earth metal borate to a final product composition at a pressure of 5 ton / cm ² or more,
And mixing the alkaline earth metal borate with the BN powder obtained by pulverizing the specific surface area to at least twice the obtained specific surface area, or mixing the mixture of BN and the alkaline earth metal borate with the original specific surface area. It can be manufactured by a method of crushing such as breaking, shearing, grinding and the like until it becomes twice or more, molding at a pressure of ² ton / cm ² or more, and firing.

As a molding device, a usual molding machine such as a mold molding machine and a cold isostatic pressing machine (CIP) can be used. The molding pressure when using highly crystalline hexagonal BN powder or amorphous BN powder as it is, 5 ton / cm ² or more, preferably 7 t
on / cm ² or more. At a molding pressure of less than 5 ton / cm ^2, it is difficult to obtain a normal pressure sintered BN molded body having a relative density of 65% or more. On the other hand, when using crushed BN powder or powder obtained by mixing the obtained raw materials, the molding pressure is 2 ton / c
Similar effects can be obtained in m ² or more. As a pulverizer at this time, a normal pulverizer such as a ball mill, a vibrating ball mill, an attritor, and a raikai machine can be used. The pulverization is performed until the specific surface area of the original powder becomes 2 times or more, preferably 10 times or more. If the pulverization is less than twice, it is difficult to obtain a normal pressure sintered BN compact having a relative density of 65% or more.

When pulverization is performed in an oxidizing atmosphere, the formation of boron oxide is observed, and if the pulverization is performed as it is, free boric acid is present in the sintered body, and the moisture resistance and the thermal conductivity are reduced. No cracks occur in the sintered body. In this case, it can be used as a raw material for obtaining the product of the present invention by performing a treatment for removing the generated borate oxide. As a method for removing boron oxide, treatment with alcohols such as methanol, ethanol, and glycerin is used.
More specifically, the method includes heating of a slurry containing alcohol or washing and filtration using alcohol. If the pulverization is performed in a non-oxidizing atmosphere such as N ₂ , Ar or the like in which boron oxide is not generated, the above-described device for removing boron oxide is not necessarily required.

The pulverized product obtained by the above-mentioned method can obtain a normal-pressure sintered BN compact having a relative density of 65% or more without performing high-pressure compaction. This is presumably because the lattice irregularity and partial amorphization of the crystal proceeded, and at the same time, a newly formed particle surface appeared and became a powder activated by the so-called mechanochemical effect. In order to obtain a high-strength normal pressure sintered BN compact, it is desirable to increase the compacting pressure to increase the density of the preformed compact as much as possible.

The firing is performed in a non-oxidizing atmosphere at 1400 to 2100 ° C. If the sintering temperature is lower than 1400 ° C, it is difficult for the BN particles to bond directly to each other and the alkaline earth metal borate, so high-strength normal pressure sintering
BN compact cannot be obtained. On the other hand, when the temperature exceeds 2100 ° C., the alkaline earth metal borate undergoes thermal decomposition / evaporation and loses its original function as an additive. Desirably, especially high-pressure atmospheric sintering
This is a non-oxidizing atmosphere at 1600 to 2100 ° C where a BN molded body can be obtained. As the non-oxidizing atmosphere, an inert atmosphere such as He, Ar, and N ₂ is suitable. As a firing device, a Tamman furnace, a resistance heating furnace, a high-frequency furnace, or the like is used.

The atmospheric pressure sintered BN compact of the present invention produced as described above has a BN content of 98 to 96% by weight and an alkaline earth metal borate of 2 to 40%.
% By weight. In particular, when a high degree of electrical insulation and thermal conductivity is required, the BN content is preferably 98 to 80% by weight and the alkaline earth metal borate is preferably 2 to 20% by weight. When the content of the alkaline earth metal borate is less than 2% by weight, a sintered body having high density and high strength is not obtained. On the other hand, if it exceeds 40% by weight, the excellent properties of BN, such as thermal conductivity and electrical insulation, will be significantly reduced. In particular, when the working temperature of the molded body increases, this tendency becomes more severe. Further, the sintered body density of the normal pressure sintered BN compact of the present invention produced as described above is a relative density of 65 to 80.
% Gives a bending strength of 550 kg / cm ² or more. If the relative density is less than 65%, there are many pores and it is not dense, so that the bending strength and the thermal conductivity are not improved, and it is not suitable for applications such as break rings and heat sinks. On the other hand, when the relative density exceeds 80%, machinability decreases.

<Examples> Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1 Commercially available BN powder (hexagonal, purity 99.0%, specific surface area 6 m ² /
g) 15 parts by weight of CaO.B ₂ O ₃ was added to 85 parts by weight, and then mixed by a ball mill to obtain a mixed powder for molding. Next, this mixed powder was subjected to cold isostatic pressing at a pressure of 5 ton / cm ² . The obtained preform was embedded in the graphite container containing the BN powder, and fired at 1900 ° C. for 60 minutes in an Ar atmosphere in a high-frequency furnace. The measurement results of the composition, relative density, insulation resistance, and thermal conductivity of the obtained BN sintered body are shown in the table.

Example 2 Using the mixed powder for molding obtained in Example 1, the molding pressure was 7 ton.
The procedure was performed in the same manner as in Example 1 except that the density was set to / cm ² .

Example 3 MgO.CaO.2B ₂ O ₃ 10 was added to 90 parts by weight of the BN powder used in Example 1.
The parts by weight were added and mixed to obtain a mixed powder for molding. Except that this powder was used and the sintering temperature was 1700 ° C., the same procedure as in Example 1 was carried out.

Example 4 Boric acid and melamine were mixed at a weight ratio of 1: 1 and subjected to a heat treatment at 1200 ° C. for 4 hours in a flow of ammonia gas to obtain BN purity.
90% of a BN powder having a specific surface area of 50 m ² / g was obtained. X-ray diffraction of this powder revealed that it was amorphous BN. This powder
After adding 10 parts by weight of 2CaO.B ₂ O ₃ to 90 parts by weight, the mixture was mixed with a ball meal to obtain a mixed powder for molding. Using this mixed powder, the same procedure as in Example 1 was carried out except that the molding pressure was 7 ton / cm ² and the firing temperature was 1700 ° C.

Example 5 The specific surface area of the BN powder used in Example 1 was 60
After pulverizing in the atmosphere to m ² / g, the powder was washed with methanol and dried to obtain a fine BN powder. The specific surface area was measured by the BET method. To 95 parts by weight of this powder, 5 parts by weight of CaO.B ₂ O ₃ was added and mixed by a ball mill to obtain a mixed powder for molding. Next, the mixed powder was filled in a mold and uniaxially pressed at a pressure of ² ton / cm ² . Example 1 except that this preform was used.
Was carried out in the same manner as described above.

Example 6 CaO.BaO.2B ₂ O ₃ 2 was added to 80 parts by weight of the BN fine powder obtained in Example 5.
0 parts by weight were added and mixed to obtain a mixed powder for molding. Except using this mixed powder, it carried out by the method similar to Example 1.

Example 7 The specific surface area of the BN powder used in Example 1 was measured with an attritor.
It was pulverized under an N ₂ atmosphere until it reached 70 m ² / g to obtain a fine BN powder. Except using this powder, it implemented by the method similar to Example 5.

Example 8 After adding 15 parts by weight of CaO.B ₂ O ₃ to 85 parts by weight of the BN powder used in Example 1, pulverizing in the atmosphere until the specific surface area becomes 60 m ² / g with a raikai machine, After washing with methanol and drying, a mixed powder for molding was obtained. Except using this mixed powder, it carried out by the same method as Example 5.

Example 9 The same procedure as in Example 1 was carried out except that the mixed powder for molding obtained in Example 8 was used.

Example 10 MgO.CaO.2B ₂ O ₃ 2 was added to 80 parts by weight of the BN powder used in Example 1.
After adding 0 parts by weight, the specific surface area is 70 m ² /
The mixture was pulverized under an N ₂ atmosphere until the mixture became g to obtain a mixed powder for molding. Except that this mixed powder was used and the firing temperature was 1600 ° C., the same procedure as in Example 5 was performed.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the BN powder used in Example 1 was used as it was as a molding powder.

Comparative Example 2 Using the mixed powder for molding obtained in Example 3, the molding pressure was 2t.
The procedure was performed in the same manner as in Example 1 except that on / cm ² was set.

Comparative Example 3 Five parts by weight of B ₂ O ₃ were added to 95 parts by weight of the BN powder used in Example 1, and then mixed by a ball mill to obtain a mixed powder for molding. An example was performed in the same manner as in Example 5, except that this mixed powder was used.

To obtain a molding mixed powder were mixed by a ball mill after the CaO · 2B ₂ O ₃ was added 45 parts by weight BN powder 55 parts by weight used in Comparative Example 4 Example 1. Except using this mixed powder, it carried out by the method similar to Example 1.

The physical properties of the BN sintered bodies described in the table were measured by the following methods.

(1) Relative density: After determining the volume from the dimensions of the sintered body and the density from its weight, the relative density (%) = density (g / c
m ² ) / theoretical density (g / cm ² ) × 100.

(2) Room temperature bending strength; measured in accordance with JIS R-1601.

(3) Insulation resistance: The obtained sintered body has an outer diameter of 20 mm and a thickness of 1 mm
And the insulation resistance (Ω) was measured after processing and in a desiccator containing a (NH ₄ ) ₂ SO ₄ solution for 24 hours after storage and moisture absorption.

(4) Thermal conductivity; by laser flash method.

<Effect of the Invention> The normal pressure sintered BN molded article of the present invention has excellent properties of BN such as electrical insulation, thermal conductivity, corrosion resistance, and thermal shock resistance, and is not subject to shape restrictions.

Claims (1)

(57) [Claims] An insulation resistance of 10 ¹⁰ Ω, comprising 2 to 40% by weight of an alkaline earth metal borate, a relative density of 65 to 80% and a bending strength of 550 kg / cm ² or more. Above, a normal pressure sintered boron nitride molded body having a thermal conductivity of 0.04 cal / cm · sec · ° C or more.