JPS5938165B2 - Manufacturing method of cubic boron nitride - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for producing cubic boron nitride.

It is known that cubic boron nitride is a highly hard material with a diamond-like crystal structure.

Although it is inferior to diamond in terms of hardness, its thermal, chemical, and properties are superior to those of diamond, so its use as a new polishing and cutting material for hardened steel, special steel, ceramics, etc. is being developed. .

Furthermore, by doping a suitable element into the crystal lattice, it is possible to manufacture a P-type and N-type amphoteric semiconductor, and there are great expectations for its application as an electronic material by taking advantage of its excellent thermal conductivity.

From this point of view, there is a demand for a high-purity, high-quality single crystal of cubic boron nitride having a desired grain size and crystal morphology.

Conventionally, a manufacturing method is known in which cubic boron nitride is converted into hexagonal boron nitride using a catalyst substance.

The catalyst materials include Ia and I in the periodic table of elements.
[a, Ha group elements and their nitrides or alloys were mainly used.

In addition, tin, lead, antimony alone or alloys, urea, ammonium salts, etc. are also known.

However, when the above-mentioned metal catalyst is used, unstable borides and free boron are produced as by-products, and boron is mixed into the resulting cubic boron nitride crystals, making the crystals black and opaque. However, the particle strength is also significantly reduced.

When nitrides are used, unreacted nitrides remain, and the resulting cubic nitrides contain nitrides in the boron crystals.
I couldn't get anything of high quality.

On the other hand, when urea or ammonium salts are used, the particle size of cubic nitride nitride is extremely small, 0.1 to 0.5 microns, and in addition, with these catalysts, hexagonal nitride nitride cannot be obtained. Cubic nitridation has the drawback that the mechanism of conversion from elemental to boron has not yet been elucidated, and it is difficult to set conditions for obtaining desired crystals.

It is also known to use Ba3N2.5r3N2 as a catalyst, but since the reaction between HBN and nitride is not uniform, unreacted nitride remains and is trapped in the resulting CBN crystal. Excess nitrogen is contained as an impurity.

As a result, it is difficult to obtain high-quality crystals and the particle strength is low.

First, the present inventors developed a method for converting hexagonal boron nitride into cubic boron nitride by using calcium nitride, which is a solvent for hexagonal boron nitride, in its molten state. We have invented a method in which cubic nitriding is carried out at temperatures above the thermodynamically stable pressure range of boron.

By this method, high quality, high purity single crystals were obtained.

. However, when trying to obtain a cubic nitrided boron crystal with a good crystalline state and well-aligned surfaces using this method,
This reaction system can be maintained for several tens of minutes at a pressure and temperature very close to the phase equilibrium line of cubic boron nitride and hexagonal boron nitride (within 30°C). is necessary.

However, since cubic boron nitride is generally produced under high pressures of 50,000 atmospheres or more, it is difficult to maintain the temperature within the vicinity of the phase equilibrium line while applying such ultra-high pressures. Poor reproducibility.

In addition, in order to obtain a single crystal of cubic nitrided boron with larger grains, it is necessary to hold the material under specified temperature and pressure conditions for several tens of hours, which requires special care. 14 However, in reality, when producing cubic boron nitride, it is difficult to directly measure the temperature by inserting a thermocouple into the high-pressure reaction vessel, so the heating temperature is controlled by an electric power method that supplies the heating temperature. Therefore, temperature control becomes inaccurate.

Therefore, this manufacturing method has technical limitations unless precise temperature and pressure control is achieved.

The present invention aims to solve these problems, and its purpose is to provide a method for easily producing cubic boron nitride having large crystal grains.

The present invention is based on strontium nitride (Sr3B2N4) or barium nitride (5a3).
B2N4 ) and hexagonal boron nitride are heated at a temperature of 1250°C or higher under the thermodynamically stable pressure of cubic boron nitride and in the presence of cubic boron nitride seed crystals. I was able to solve the problem by doing this.

According to the method of the present invention, since the permissible value range in the temperature and pressure range is wide, it is easy to control the grain growth rate, and since seed crystals are present, it is possible to produce cubic crystals with large grain size, high purity, and good quality. Nitriding has the excellent effect of making it possible to easily produce single crystals of boron.

The strontium nitride used in the present invention is composed of metallic strontium or strontium nitride and hexagonal boron nitride in a molar ratio of strontium to boron of 3:2.
0 ratio and heated at 950-1200℃ in a nitrogen stream for 2 hours.
Obtained by heating for more than an hour.

Further, barium nitride can also be obtained in the same manner by using barium in place of strontium.

As raw materials for the production of the present invention, hexagonal boron nitride is mixed with strontium boron nitride or barium boron nitride, or strontium boron nitride or barium boron nitride is produced by the method described above. In this case, an excess amount of hexagonal boron nitride is used in advance, and an excess amount of hexagonal boron nitride is used.

The mixing ratio of hexagonal boron nitride is preferably 35% by weight or more based on strontium boron nitride or barium boron nitride.

The relationship between the temperature and pressure conditions under which cubic nitridation of boron can be synthesized in the present invention is as shown in FIG.

In the figure, 13 is the boundary curve between the synthesis possible range and the stable range of cubic boron nitride when strontium boron nitride is used, 14 is the same boundary curve when barium boron nitride is used, and 15 indicates the thermodynamic equilibrium line between cubic boron nitride and hexagonal boron nitride.

Region A is a region where cubic boron nitride can be synthesized, region B is a stable region of cubic boron nitride, and region C is a stable region of hexagonal boron nitride.

As is clear from the drawings, approximately 1350° C. and 1250° C. are the lower limit temperatures for synthesizing cubic boron nitride obtained using strontium boron nitride and barium boron nitride, respectively.

Further, it is preferable that the raw material hexagonal boron nitride has high purity and an oxygen content of 2 to 3 or less by weight.

Therefore, for example, in a nitrogen gas atmosphere of 1 atm, approximately 2000
Upon exposure to temperatures of 0.degree. C., the oxygen content can be reduced to less than 1 wt.

Comparative Example 1 FIGS. 2, 3, and 4 show longitudinal cross-sectional views of the sample structure.

The test was conducted using the sample configuration shown in FIG.

In the figure, 1 is a graphite resistance heating element, 2 is a graphite disk, and AC or DC current is applied to both ends of the graphite disk to supply a predetermined temperature inside the reaction chamber.

3 and 4 are a salt cylinder and a disk, respectively, which measure the uniformity of the pressure inside the reaction chamber under high temperature.

The reaction chamber is surrounded by 5 and 6 molybdenum cylinders and discs, preventing direct contact with the common salt.

7 is a hexagonal boron nitride disk or plug, 8 is a strontium nitride powder made by a tablet molding machine at 2.0 tons/cr.
It is press-formed under a load of 100 liters, and is filled by laminating alternately with hexagonal boron nitride disks.

The ratio of strontium boron nitride to hexagonal boron nitride is 1:3 by weight.

This is heated to 54,000 atmospheres and 1
It was treated at 350°C for about 60 minutes.

Thereafter, the heating power was turned off, the temperature of the container was rapidly lowered, the pressure was removed, and the crystals were taken out.

When the obtained crystal was subjected to X-ray diffraction, a scratch test on a cemented carbide plate, and microscopic observation, it was confirmed that the cubic nitride was a boron crystal.

Since the strontium nitride contained in the reaction product is easily decomposed by a weak acid, it can be easily removed by treatment with a weak acid.

In addition, unreacted hexagonal nitride crystals crystallized at the boundary with boron were dissolved and separated in a solution containing sodium fluoride in hot concentrated sulfuric acid, and then subjected to specific gravity separation using a heavy liquid. Ta.

The obtained cubic boron nitride was a pale yellow transparent single crystal with a diameter of about 170 microns and had a combination of hexahedrons and octahedrons, and the conversion rate from HBN to CBN-\ was around 45%. .

Note that the pressure values in this comparative example are the phase transition pressure points of bismuth, thallium, and barium at room temperature of 25.5, respectively.
It is based on load-pressure development curves prepared for kilobar, 37 kilobar and 55 kilobar.

Further, for the temperature at the center of the sample section, a temperature-power consumption curve was created using a platinum-platinum 13 rhodium thermocouple, and a predetermined temperature was estimated from the power value applied to the graphite resistance heating element.

Comparative Example 2 In place of the common salt cylinder used in Comparative Example 1, an experiment was conducted using a sample configuration shown in FIG. 3 using a hexagonal boron nitride cylinder.

Barium nitride powder is molded using a tablet molding machine at a load of about 2.0 tons/crA 8 and is alternately laminated with hexagonal boron nitride disks 7 to form a hexagonal boron nitride cylinder 9. filled inside.

This was subjected to high-temperature and high-pressure treatment at 57,000 atmospheres and 1,500° C. for about 70 minutes to obtain a cubic nitride crawler crystal in the same manner as in Example 1.

The crystal grain size is large, about 80 microns (mostly 60 microns or less), colorless and transparent crystals (one pale yellow transparent crystal is also available), and mainly consists of octahedrons, with each (111) plane being completely smooth and beautiful. It was crystal.

The conversion rate from HBN to CBN-\ was around 30%.

Example 1 Sample composition similar to that used in Comparative Example 1, 3 and 4
are a salt cylinder and a disk, respectively, and in order to avoid a decrease in the apparent pressure generated due to the volume reduction that occurs during the conversion from hexagonal boron nitride to cubic boron nitride, these are
It was inserted into the sample constructor shown in the figure.

Since there is a temperature gradient between the center and bottom of the graphite resistance heating element 1, a mixture of fine crystals (about 1 to 3 microns in size) of hexagonal boron nitride and cubic boron nitride is used. placed in the high temperature section 7, and cubic boron nitride (approximately 4 oo
Micron) seed crystals 10 were placed.

Therefore, in this sample configuration, only the lower half forms the effective sample space.

The materials 11 and 12 filled in the lower half may be hexagonal boron nitride, pyrophyllite, talc, silica glass, salt, etc., but do not undergo phase transition or decomposition under high temperature and high pressure conditions, resulting in volume reduction. It is preferable to use

In this example, a salt disk was used as 11, and a hexagonal boron nitride disk was used as 12.

Note that a Fe-AA 10% alloy thin plate was inserted between the strontium boron nitride and the cubic boron nitride seed crystal.

This is preferably used at the beginning of the reaction to avoid dissolving the seed crystals in the strontium boron nitride until the boron nitride is saturated in the strontium boron nitride solution.

This was treated at 55,000 atm and 1,570°C for about 24 hours.

The obtained cubic boron nitride crystal was observed to have grown from the surface of the seed crystal to a diameter of approximately 2.2 mm.

This crystal was an octahedral crystal with (100) planes and was transparent and yellow.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between temperature and pressure in the synthesis possible range of cubic boron nitride, Figures 2, 3 and 4 are cross-sectional views of the sample structure; 4 shows a comparative example, and FIG. 4 shows an embodiment of the present invention. , 1: graphite resistance heating element, 2: graphite disk, 3: salt cylinder,
4: Salt disk, 5: Molybdenum cylinder, 6: Molybdenum disk, 7: Hexagonal boron nitride disk, 8: Solvent part (Sr3B
2N4 or Ba3B2N4), 9: Hexagonal boron nitride cylinder, 10: Cubic nitride boron seed crystal, 11: Salt disk, 12: Hexagonal boron nitride disk, A: Cubic nitride foil Synthesizable range of elements, B: Cubic nitridation is the stable range of boron, C
: Hexagonal nitridation is the stable region of boron.

Claims (1)

[Claims] 1. Strontium nitride or barium boron nitride and hexagonal boron nitride are combined in the presence of a seed crystal of cubic boron nitride to achieve the thermodynamic stability pressure of cubic boron nitride. under,
A method for producing cubic boron nitride, which comprises heating at 1250°C or higher.