JPH0815548B2 - Method for synthesizing cubic boron nitride - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for synthesizing cubic boron nitride (hereinafter referred to as CBN), and particularly to a method for synthesizing good-crystal CBN at a lower pressure and a lower temperature. .

Conventional technology Conventionally, a hexagonal boron nitride (hereinafter referred to as HBN), which is a raw material used for CBN synthesis, is generally a commercially available product with a fully developed crystal and a crystal size Lc in the C-axis direction of X-rays of 1000 Å or more. in use. It has low crystallinity and Lc value of 1000Å
The following HBN are impurities such as normal B ₂ O ₃ is contained about 5%, the B ₂ O ₃ impurities is inhibited in many cases the CBN growth, and good because crystals can not be obtained.

Problems to be Solved by the Invention If HBN with uniformly developed crystals is used for the synthesis of CBN, CBN
When pressurized and heated in the stable region, HBN is simultaneously dissolved in the CBN synthesis catalyst, and the growth of CBN is started, so that it becomes a skeletal form especially at the early stage of growth, and the growth thereafter becomes random,
The finally formed crystals may have irregularities on the surface or may have crystals having pores inside. This also applies to the case where impurities such as B ₂ O ₃ in low crystalline HBN are removed.

An object of the present invention is to provide a method for synthesizing CBN having such a good crystal, and further to provide a method for synthesizing CBN even if the temperature and pressure for synthesis are lowered as compared with the conventional method.

Means to solve the problem As a method to control the growth of CBN from the beginning to the end of synthesis, there is a method of synthesizing pressure and temperature near the equilibrium line of CBN-HBN, but in the sample space, pressure and temperature Is inevitable, and for that reason, the pressure for forming good crystals in the entire sample space,
Maintaining temperature is very difficult.

The present inventor has found that the dissolution rate of HBN in the CBN synthesis catalyst is based on the crystallinity of HBN. HBN with undeveloped crystals
(Lc value less than 1000Å) is crystallographically unstable and dissolves quickly in the catalyst. In addition, such HBN generally has a small particle size, a large surface area, and a large contact area with the catalyst, so that the contact area with the catalyst is relatively large and the HBN dissolves quickly.

Therefore, in the present invention, the HBN having a low degree of crystallinity is mixed with the HBN having a high degree of crystallinity to carry out the dissolution stepwise.

That is, according to the present invention, when synthesizing CBN from HBN in the presence of a CBN synthesizing catalyst, HBN having a crystallinity Lc of 1000 Å or more by X-ray is used.
Using a mixture of 30 to 90% by weight of powder and 70 to 10% by weight of HBN powder having an Lc of less than 1000Å as a raw material, thermodynamically CB
It is a method for synthesizing CBN which is characterized by heating and pressurizing to a stable region of N.

HBN with Lc of 1000Å or more is commercially available and can be used as it is. The particle size of the powder is preferably 30 μm or less. This commercially available HBN is manufactured by using B ₂ O ₃ or the like as a raw material and then heat-treated at a high temperature. Lc less than 1000Å
HBN can be obtained without the above heat treatment or at a low heat treatment temperature. Usually, this HBN contains a considerable amount of impurities such as B ₂ O _3, so impurities are removed by washing with water or pickling. This is dried and made into powder. Grind further if necessary. The particle size of the powder is preferably 5 μm or less.

The mixing ratio of HBN having a high degree of crystallinity and HBN having a low degree of crystallinity is as described above, but this is necessary in order to prevent HBN from being dissolved in the synthesis catalyst at once.

CBN synthesis catalysts are commonly used, that is, alkali metals, alkaline earth metals, nitrides of these, specifically, Li, Na, Ca, Sr, Li ₃ N, Ca ₃ N ₂ , Li ₃ BN. ₂ , LiBaBN _{2 and the} like are used. The mixing ratio of these to HBN is generally 1 to 30 parts by weight of catalyst to 100 parts by weight of HBN.

CBN is synthesized by preforming these mixed powders and loading them into an ultrahigh pressure device.

The synthesis conditions may be lower than usual conditions and low pressure, for example, 1400 ° C to 1700 ° C, 40 kb to 50 kb are suitable.

By mixing HBN with high crystallinity with HBN with high crystallinity, in the synthesis of CBN, HBN with low crystallinity is first dissolved in the catalyst to form CBN. It is considered that CBN having a high degree of crystallinity then melts and precipitates as CBN on the surface of the previously produced CBN, causing grain growth.

Example Commercially available HBN (Lc value of 1000Å or more, particle size of 30 μm or less) 50
% By weight and 50% by weight of HBN with low crystallinity (Lc value of 250Å, particle size of 5 μm or less, purity of 99% by weight or more), and the mixture
10 parts by weight of LiCaBN ₂ as a catalyst was mixed with 100 parts by weight, cold-molded, and then subjected to heat treatment at 50 kb and 1500 ° C. for 10 minutes. The obtained CBN was a transparent crystal with an automorphic shape.

Comparative Example 1 For 100 parts by weight of the same HBN (Lc value of 1000Å or more) as above,
When 10 parts by weight of LiCaBN ₂ was mixed as a catalyst and CBN was synthesized under the same conditions as in 1), the obtained CBN had irregularities on the surface and a devitrified portion inside.

Comparative Example 2 100 parts by weight of HBN (purity 90% <) having Lc of 250 Å was mixed with 10 parts by weight of LiCaBN ₂ as a catalyst, and CBN was synthesized under the same conditions as in Example. Most of them were not transparent.

EFFECTS OF THE INVENTION According to the present invention, in the synthesis of CBN, the dissolution rate of HBN in the catalyst can be controlled by the crystallinity of the HBN, so that it can be grown at a substantially constant rate from the initial growth to the end, and a good crystal is formed. You can get it.

Claims (1)

[Claims] 1. A mixture of 30 to 90% by weight of hexagonal boron nitride powder having a crystallinity of Lc of X-ray index of 1000Å or more and 70 to 10% by weight of hexagonal boron nitride powder having an Lc of less than 1000Å. A method for synthesizing cubic boron nitride, comprising adding a catalyst for synthesizing cubic boron nitride to, and thermodynamically pressurizing and heating in a stable region of cubic boron nitride.