JPH0478335B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing cubic boron nitride (cBN), and more specifically, to produce cubic boron nitride (rBN) by applying high pressure to it. The present invention relates to a method for producing crystalline boron nitride.

(Prior technology) Cubic boron nitride (cBN), which is the high-pressure phase of boron nitride (BN), has the second highest hardness and thermal conductivity after diamond, and is an extremely stable material both thermally and chemically. be. In particular, its corrosion resistance against high-temperature iron group elements is superior to that of diamond, so it has been used for high-precision, high-efficiency polishing of iron-based materials and as a material for cutting tools.
There are growing expectations for its practical use as a high-performance heat dissipation substrate for electronic devices with significantly increased output.

By the way, cubic boron nitride (cBN) is usually obtained by treating hexagonal BN (hBN), which is the low-pressure phase of BN, at high temperature and high pressure. Industrially, catalysts are used to ease the conditions. Cubic boron nitride (cBN), which can be obtained industrially using such catalysts, has relatively large crystal grains, so it can be used as abrasive grains or sintered to make cutting tools. In this case, cBN is difficult to sinter by itself, so it is necessary to use metal, ceramics, etc. as a binder.

Conventionally, cubic boron nitride (cBN) produced in this way has a catalyst incorporated therein, and a binder is present in the sintered body, so cBN's original hardness, strength, thermal conductivity, etc. There was a problem in that the excellent performance of the system could not be fully demonstrated.

Therefore, the present inventors first used a reaction sintering method in which hexagonal BN (hBN) impregnated with a very small amount of boronitride catalyst was treated at high temperature and high pressure, resulting in almost pure cBN sintering through direct bonding between cBN grains. We developed a method to obtain aggregates (Patent Nos. 1294809 and 1303678) and solved the problem of the presence of binders. However, there was still the problem of the influence of a small amount of catalyst remaining.

Under these circumstances, attempts have been made to directly convert hexagonal BN (hBN) without a catalyst, and in particular, to perform reactive sintering to obtain a cBN sintered body simultaneously with the conversion. but,
In this case, as mentioned above, converting commercially available hBN as it is to cBN without a catalyst requires extremely high temperature and pressure, and there are difficulties such as difficulty in obtaining a strong sintered body. Therefore, the raw material hBN is activated by pre-treatment such as reduced pressure firing (Japanese Patent Application Laid-open No.
55-167110), attempts have been made to ease the temperature and pressure conditions for conversion by synthesizing and using a special hBN under special conditions using a gas phase method.

(Problems to be Solved by the Invention) On the other hand, the present inventor has previously discovered that boron nitride (BN)
We focused on the fact that rhombohedral boron nitride (rBN), another low-pressure phase, has a crystal structure correspondence with cubic boron nitride (cBN). Successfully converted to cBN (Patent No. 1290649). However, this method mixes a large amount of pressure medium with rBN, so
There is metal contamination, and furthermore, the contact time of impact pressure is extremely short, on the order of microseconds, so fine particle cBN
There were some problems, such as not being able to get much.

The present invention relates to the above-mentioned rhombohedral boron nitride (rBN).
The purpose of the present invention is to solve the problems in the production method of cubic boron nitride (cBN) by converting it under high pressure, and to provide a method for obtaining high-purity cBN powder or high-density sintered body without a catalyst. It is something.

(Means for Solving the Problems) In view of the above-mentioned problems, the present inventor conducted research on a solution to the problem by applying pressure at high temperatures for a long period of time, and as a result, established a method for synthesizing rBN necessary for experiments. Based on this, as a result of repeated conversion experiments to cBN, we found conditions under which rBN could be converted to cBN without a catalyst by processing it under static high temperature and high pressure conditions, thus completing the present invention. .

That is, in the present invention, when converting rhombohedral boron nitride into cubic boron nitride by applying high pressure,
The gist of the present invention is a method for producing cubic boron nitride, which is characterized by processing under static high pressure conditions of 5 GPa or higher and high temperature conditions of 1500° C. or higher during pressurization.

The present invention will be explained in more detail below.

(Operation) The method of the present invention uses the fact that rhombohedral boron nitride (rBN) and cubic boron nitride (cBN) have a corresponding relationship in their crystal structures, and statically applies high temperature and high pressure to rBN. This is a cBN synthesis method that easily converts to cBN by adding

In particular, rBN can be obtained by a simple method such as heating a solid raw material under normal pressure, without requiring complicated processes such as special pre-treatment of the raw material.
is used as a raw material, and is processed without a catalyst under the high-temperature, high-pressure conditions used for conventional cBN synthesis, such as 1800℃ and 6Gpa.
cBN can be synthesized, and high-purity cBN powder and high-density sintered bodies can be obtained.

The raw material rhombohedral boron nitride (rBN) is
Preferably, it is as pure as possible and has good crystallinity.
For example, one obtained by the following method is used.

A method of heating a borohydride such as sodium borohydride and an ammonium salt such as ammonium chloride, or adding cyanide during the reaction and heating (Patent Application No. 1989-190967, "Inorganic Materials Research Institute Research Report") (See No. 46, Chapter 11).

A method of heating and vaporizing boron oxide and reducing it with a gas containing cyanide (Patent No. 1290648).

A method of reducing boron oxide, boric acid or borate with cyanide.

The raw material rBN has a higher rBN content and purity as BN, and the better the crystallinity, the higher the conversion rate to cBN, and the higher the quality of the cBN sintered body. In this regard, according to the above method, BN with a purity of 99% or more, an rBN content of 90% or more, and an average crystal grain size of several hundred nanometers can be easily obtained.
Also, while normal hBN consists of thin plate-like crystallites with a width to thickness ratio of 10:1 or more, this rBN
The crystallites have a small ratio of 3:1, for example, and are therefore excellent in that they are easy to fill and have no orientation. Although the above method provides a raw material with a high content of rBN as whiskers, the yield is low. In this respect, what can be obtained by the above method is a large amount of hBN.
Although it is a mixture of

Synthesized rhombohedral boron nitride (rBN) may have residual starting materials or hydration during washing, so it is purified by high purification treatment such as heating to 2100℃ in nitrogen gas. It is preferable to use it, and it is especially necessary when trying to obtain a high quality cBN sintered body.

The conditions for high-temperature, high-pressure treatment of rhombohedral boron nitride (rBN) are cBN of 1500°C or higher and 5GPa or higher.
The temperature and pressure are required to be in the thermodynamic stability range, preferably 5.5GPa or higher and 1600℃ or higher, more preferably 6.0GPa or higher and 1800℃ or higher.The higher the pressure and temperature, the higher the conversion rate to cBN. A high-density sintered body can be obtained.

Note that the high-pressure device may be any device that can generate the above-mentioned temperature and pressure, and for example, a belt-type device equipped with a graphite heating element can be used. The rBN raw material can be filled into a high-pressure device as a powder, but it is better to cold-form or sinter it, since there is less crushing allowance and there is contamination from the heating element and pressure medium. It is preferable to cover and fill with foil or the like.

In the high-temperature and high-pressure treatment, according to the usual procedure, the pressure and temperature are raised and held, and then the temperature and pressure are lowered in order and the sample is taken out.

The pressurized sample thus obtained is a white or light brown powder or sintered body, or a translucent high-density sintered body, and according to X-ray diffraction, it is 100% cBN or partially amorphous. It is a mixture of cBN and cBN.

(Example) Next, examples of the present invention will be shown.

Example 1 Sodium boronitride and ammonium chloride were mixed, heated to 600°C in a nitrogen stream, cooled once, potassium cyanide was added, and heated again to 1030°C for 24 hours.
By heating for a period of time, cooling, and washing with water, a powder mainly composed of rBN with an average crystal grain size of 250 nm was obtained.
This was then heated to 2100°C for 3 hours using a molybdenum heating element in a nitrogen stream, resulting in an rBN content of 95%.
In this way, a starting material consisting of 100% BN was obtained within the analytical precision (see Figures 1 and 2).

Next, this was cold pressed into a disk shape with a diameter of 6 mm and a thickness of 2 mm, surrounded by tantalf oil and filled into a high-pressure device, held at a temperature and pressure of 1800°C and 6 GPa for 30 minutes, and then cooled and decompressed. The sample was taken out.

The obtained sample was a milky-white translucent sintered body, and as a result of X-ray diffraction, it was confirmed that it was 100% cBN (see Figure 3), and as a result of measurement using the Archimedes method, it was confirmed that it was almost the same as the theory. It has a density (3.48), and hardness measurements confirmed that it has a high value of 6000Kg/mm ² or more.

Example 2 6.0GPa, 1800℃ using the same system as Example 1
When treated with , a 100% cBN sintered body with a light brown color was obtained.

Example 3 5.5GPa, 1600℃ using the same system as Example 1
When treated with
A BN mass containing cBN was obtained.

(Effects of the Invention) As detailed above, according to the present invention, since the correspondence relationship in the crystal structure of rhombohedral boron nitride (rBN) and cubic boron nitride (cBN) is utilized, hexagonal boron nitride It can be converted to cBN without a catalyst without any special pre-treatment like when using BN (hBN), and not only can high-purity cBN be obtained.
A dense cBN sintered body is obtained. Therefore, it is suitable for use as a heat dissipation substrate for semiconductors, for example.

Furthermore, since the starting material rBN can be synthesized by reaction under normal pressure, the synthesis and control of the starting material is easier than with conventional methods.

[Brief explanation of drawings]

Figure 1 is an X-ray diffraction diagram of the starting material rBN used in the examples, and Figure 2 is the starting material rBN used in the examples.
Figure 3 is an X-ray diffraction diagram of the cBN sintered body obtained in the example.

Claims (1)

[Claims] 1. When high pressure is applied to rhombohedral boron nitride to convert it into cubic boron nitride, the pressure is a static high pressure of 5GPa or more, and the temperature at the time of pressurization is a high temperature of 1500°C or more. A method for producing cubic boron nitride, characterized by processing under certain conditions. 2. The method according to claim 1, wherein the cubic boron nitride obtained is in the form of a dense sintered body. 3 The rhombohedral boron nitride raw material contains 90% or more of rhombohedral boron nitride, and the average crystal grain size is 10n.
The method according to claim 1 or 2, wherein the method is at least m. 4. According to claim 3, the rhombohedral boron nitride raw material is either one obtained by subjecting a boron hydride to a heating reaction with ammonium halide, or one obtained by reacting this with cyanide. the method of.