JPH01168328A - Process for synthesizing cubic boron nitride - Google Patents

Abstract

PURPOSE:To synthesize cubic boron nitride at low pressure and temperature, by mixing specific quantities of two kinds of hexagonal boron nitride powders having different degrees of crystallinity, providing them with a synthesizing catalyst composed of cubic boron nitride, pressurizing and heating them under specific conditions. CONSTITUTION:Hexagonal boron nitride powders with a crystallinity of 1000Angstrom or more in X-ray index Lc and hexagonal boron nitride powders with a crystallinity of less than 1000Angstrom in Lc are mixed together at the ratios of 30-90 and 70-10wt.% respectively. Then, synthesizing catalyst composed of cubic boron nitride is added to said mixture, which is then pressurized and heated in a region wherein cubic boron nitride remains stable theremodynamically to synthesize cubic boron nitride(CBN). The embodiments of synthesizing catalysts for CBN are alkali metals, alkakline earth metals and nitrides thereof, wherein the mixing ratio is generally 1-30 pts.wt. catalyst per 100 pts.wt. HBN. Preferably synthesis condition is, for exanple, 1400-1700 deg.C and 40-50kb.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for synthesizing cubic boron nitride (hereinafter referred to as CBN), and in particular to a method for synthesizing CBN of good quality at low pressure and low temperature. be.

Conventional technology Conventionally, the raw material used for CBN synthesis, macrogonal boron nitride (
HBN (hereinafter referred to as HBN) is generally a commercially available product whose crystal size Lc in the C-axis direction is 100
OX or higher is used.

This is because HBN with low crystallinity and an Lc value of 100OX or less usually contains about 5% of impurities such as B2O3, and this B2O3 impurity greatly inhibits the growth of CBN and makes it impossible to obtain good crystals. be.

Problems to be Solved by the Invention When HBN with uniformly developed crystals is used to synthesize CBN, when pressure and heat are applied in the CBN stability region, HBN is simultaneously dissolved in the CBN synthesis catalyst and CBN growth is initiated. As a result, the crystals do not form in the form of skeleton crystals, especially in the early stages of growth, and the subsequent growth is random, resulting in the final crystals having irregularities on the surface and tight crystals with pores inside. There are cases. This provides a method for synthesizing CBN that excludes impurities such as B2O3 in low-crystalline ABN, and furthermore provides a method that can synthesize CBN even if the synthesis temperature and pressure are lower than conventional ones. There is K.

Means to Solve the Problems There is a method of controlling the growth of CBN from the beginning to the end of the synthesis by synthesizing near the equilibrium line of pressure and temperature i CBN - HBN. As a result, it is extremely difficult to maintain the entire sample space at a pressure and temperature that will produce a good product.

The present inventor has determined that the dissolution rate of HBN in the CBN synthesis catalyst is
It was found that this is due to the crystallinity of N. HBN with underdeveloped crystals (Lc value less than 100OX) is crystallographically unstable and dissolves quickly in the catalyst. Also this more than I (
BN generally has a small particle size and a large surface area, so the contact area with the catalyst is large, so the contact area with the catalyst is relatively large, so it dissolves quickly.

Therefore, in the present invention, HBN with a low degree of crystallinity is mixed with HBN with a high degree of crystallinity, thereby dissolving the HBN in stages.

That is, in the present invention, HBN and j9 are synthesized in the presence of a CBN synthesis catalyst.
When synthesizing cBNi, the crystallinity Lc by X-rays is 100
0 to 30-90% by weight of HBN powder and Lc is 100
This method of synthesizing CBN is characterized by using a mixture of 70 to 10% by weight of HBN powder of less than OX as a raw material, and heating and pressurizing the mixture to a thermodynamically stable CBN range.

HBN having an Lc of 100OX or more is commercially available and can be used as is. The particle size of the powder is 30μm
The following are desirable. This commercially available HBN is produced using B2O3 or the like as a raw material and then heat-treated at a high temperature. L
HBN having a c of less than 100OX can be obtained without the above heat treatment or by lowering the heat treatment temperature. Normally, this HBN contains a small amount of impurities such as B2O3, so it is desirable to wash with water, pickle, etc. as follows.

The mixing ratio of HBN with high crystallinity and HBN with low crystallinity is the same as described above, and this is necessary in order to avoid dissolving HBN in the synthesis catalyst all at once.

CBN synthesis catalysts include commonly used catalysts such as alkali metals, alkaline earth metals, nitrides thereof, and specifically Li, Na, Ca, Sr, Li3N, Ca3N2. L
i3BN2°LiBaBN2 or the like is used. The mixing ratio of these to HBN K is generally 1 to 30 parts by weight of catalyst per 100 parts by weight of HBN.

Synthesis of CBN is carried out by preforming these mixed powders and loading them into an ultra-high pressure device.

Synthesis conditions are not limited to normal conditions; low temperature and low pressure are also possible.
For example, 1400℃~1700℃, 40kb~50kb
is appropriate.

In CBN synthesis, by mixing ft of HBN with high crystallinity with HBN with high crystallinity, HBN with low crystallinity is first dissolved in the catalyst and CBN is produced. It is thought that CBN with a high degree of crystallinity is then dissolved in the bath, becomes CBN and precipitates on the surface of the previously generated CBN, and grain growth occurs.

Example Commercially available HBN (Lc value 1ooOX or more, particle size 30μ
HBN with low crystallinity (Lc value zso, j, particle size 5μm or less, purity 99% or more) 50% by weight)
%i by weight, 10 parts by weight of LiCaBN2 as a catalyst was mixed with 100 parts by weight of the mixture, and after cold molding, 50 kb was pressurized and heat treated at 1500° C. for 10 minutes. The obtained CBN was an euhedral and transparent crystal.

Comparative Example 1 When CBN was synthesized under the same conditions as 1) by mixing 10 parts by weight of the same HBN (Lc value of 100OX or more) as above with 10 parts by weight of LiCaBN2f as a catalyst, the obtained CBN showed unevenness on the surface. and had devitrified parts inside.

Comparative Example 2 When 10 parts by weight of HBN (purity < 99%) with Lc of 250X was mixed with 10 parts by weight of LiCaBN2f as a catalyst and CBN 'i was synthesized under the same conditions as in the example, crushed grains with no eutorhesis were obtained. It was mostly black and there was no transparency.

Effects of the Invention According to the present invention, in the synthesis of CBN, the rate of dissolution of HBN into the catalyst can be controlled to a large extent by the degree of crystallinity of the HBN. Therefore, growth can be performed at an almost constant rate from the beginning of growth to the end, and good products can be obtained. can be obtained.

Claims (1)

[Claims] 30 to 90% by weight of hexagonal boron nitride powder with a crystallinity of X-ray index Lc of 1000 Å or more and 70 to 10% by weight of hexagonal boron nitride powder with Lc of less than 1000 Å are mixed, and cubic boron nitride is added to the mixture. A method for synthesizing cubic boron nitride, which is characterized by adding a synthesis catalyst and applying pressure and heating in a thermodynamically stable region of cubic boron nitride.