Process for preparing cubic boron nitride

A process for preparing cubic boron nitride comprises heating hexagonal boron nitride and magnesium boron nitride at a temperature of at least 1350.degree. C. under a pressure at which the cubic boron nitride is thermodynamically stable, whereby the cubic boron having high strength and high purity can readily be obtainable. Also disclosed is a process for the preparation of magnesium boron useful as a starting material for the above process. This process comprises mixing hexagonal boron nitride and magnesium nitride or metal magnesium in a molar ratio of hexagonal boron nitride/magnesium being at least 0.6, and heating the mixture thus obtained, at a temperature of from 950.degree. to 1250.degree. C. under atmospheric pressure in a non-oxidizing atmosphere, e.g. a nitrogen atmosphere.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for preparing cubic boron 
nitride. It relates further to a process for preparing magnesium boron 
nitride which is useful as a starting material for the production of the 
cubic boron nitride. 
2. Description of the Prior Art 
Cubic boron nitride, i.e. boron nitride having a cubic crystal structure, 
is superior in its chemical stability to diamond, although it is inferior 
in its hardness to diamond. For instance, it is durable in an oxidizing 
atmosphere at a high temperature, and its reactivity with an iron group 
element is extremely small. Accordingly, it exhibits substantially 
superior mechanical properties to those of diamond when used for grinding 
a heat resistant high strength material containing nickel or cobalt as the 
basic component, or a high speed steel. Thus, the cubic boron nitride is a 
quite useful substance. 
It has been common to prepare the cubic boron nitride by treating hexagonal 
boron nitride at a high temperature under high pressure in the presence of 
a catalyst. As the catalyst, there have been known (1) elements belonging 
to Groups Ia and IIa, (2) rare earth elements, actinide elements, tin, 
antimony, lead, etc. (in practice, these elements are used in the form of 
their nitrides or alloys), and (3) urea and ammonium salts. 
However, when the above-mentioned metals or alloys are used as the 
catalyst, unstable boron compounds and free boron are likely to form as 
by-products and they tend to be included in the cubic boron nitride 
crystals thereby obtained. The crystals thereby obtained have drawbacks 
such that they are blackish and opaque, and the strength of the particles 
is rather low. 
In the case where the above-mentioned nitrides are used as the catalyst, 
unreacted nitrides will remain in the system, and they are likely to be 
trapped in the cubic boron nitride. Accordingly, it is thereby difficult 
to obtain cubic boron nitride crystals having high quality. Besides, the 
chemical reaction system is rather complicated, and it is difficult to 
properly set the conditions to obtain the desired crystals. 
In the case where the above-mentioned urea or ammonium salts are used as 
the catalyst, the cubic boron nitride thereby obtained tends to have an 
extremely small particle size at a level of from 0.1 to 0.5 micron. Its 
usefulness as grinding particles will thereby be limited. Further, the 
mechanisms of the chemical reaction and the product formation are not yet 
been clearly known, and accordingly, it is not possible to set the 
reaction conditions to increase the particle size. 
Other than the methods using such catalysts, it is known to prepare the 
cubic boron nitride with use of calcium boron nitride as a solvent. The 
cubic boron nitride thereby obtained has a high quality with a minimal 
content of impurities, and it is also superior in its mechanical strength. 
Namely, during the process of conversion of the hexagonal boron nitride to 
the cubic boron nitride, the calcium boron nitride serves as a solvent for 
the hexagonal boron nitride, and accordingly, it is possible to 
crystallize the cubic boron nitride from the liquid phase by the treatment 
at a temperature sufficiently high to solubilize both reactants, i.e. to 
establish a co-solubilization condition, and under a pressure required to 
establish a thermodynamically stable phase of the cubic boron nitride at 
the temperature. 
However, calcium boron nitride (Ca.sub.3 B.sub.2 N.sub.4) has a drawback 
that it is relatively unstable against moisture. For instance, when 
exposed in the air, it will be decomposed into a hydroxide by the moisture 
in the air. Accordingly, in order to maintain its desired function as the 
solvent, it is necessary to take special cares for its storage or at the 
time of filling it to the high pressure cell. 
Now, with respect to a process for the preparation of magnesium boron 
nitride useful as a starting material for the production of the cubic 
boron nitride, it has been reported to form magnesium boron nitride by 
heating boron nitride and metal magnesium at a temperature of 1150.degree. 
C. under pressure of 2.5 GP. The magnesium boron nitride obtained by the 
reported method, has the following X-ray diffraction peaks. 
TABLE 1 
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Lattice spacings and intensities obtained from the major 
X-ray diffraction lines of the magnesium boron nitride 
prepared by the conventional method. 
Lattice Lattice 
spacings Intensities 
spacings Intensities 
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7.76 weak 1.68 strong 
3.88 weak 1.65 moderate 
3.02 moderate 1.57 moderate 
2.69 weak 1.55 moderate 
2.59 strong 1.49 moderate 
2.44 strong 1.46 moderate 
2.38 weak 1.32 weak 
2.21 strongest 1.31 weak 
2.12 strong 1.27 weak 
1.94 moderate 1.22 weak 
1.89 weak 1.19 weak 
1.82 moderate 1.10 weak 
1.75 weak 
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In the above relative intensities, strongest = 100, strong = 70 to 30, 
moderate = 30 to 10 and weak = 10 to 3. 
This method has drawbacks such that high pressure is required and the 
productivity is poor. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a process 
for preparing cubic boron nitride, which is capable of readily producing 
high strength cubic boron nitride containing little impurities without the 
production of chemically complexed reaction products. 
As a result of an extensive research, the present inventors have found that 
the above object can be achieved by subjecting hexagonal boron nitride and 
magnesium boron nitride to heat treatment at a temperature of at least 
1350.degree. C., i.e. a temperature sufficiently high to dissolve both 
reactants, under pressure at which the cubic boron nitride is 
thermodynamically stable. 
Thus, the present invention provides a process for preparing cubic boron 
nitride, which comprises heating hexagonal boron nitride and magnesium 
boron nitride at a temperature of at least 1350.degree. C. under pressure 
at which the cubic boron nitride is thermodynamically stable. 
Another object of the present invention is to provide a process for 
preparing magnesium, boron nitride which is useful for the production of 
the cubic boron nitride. 
As a result of a research with an aim to obtain magnesium boron nitride 
suitable as a catalyst for the synthesis of the cubic boron nitride, the 
present inventors have found that magnesium boron nitride having X-ray 
diffraction peaks are different from those of the magnesium boron nitride 
obtained by the above-mentioned conventional method. 
Thus, the present invention also provides a process for preparing magnesium 
boron nitride, which comprises mixing hexagonal boron nitride and 
magnesium nitride or metal magnesium in a molar ratio of hexagonal boron 
nitride/magnesium being at least 0.6, and heating the mixture thus 
obtained, at a temperature of from 950.degree. to 1250.degree. C. under 
atmospheric pressure in a non-oxidizing atmosphere.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The magnesium boron nitride to be used for the preparation of the cubic 
boron nitride according to the present invention, is extremely stable 
against moisture, and can be synthesized under atmospheric pressure. 
Besides, as compared with the above-mentioned calcium boron nitride, the 
magnesium boron nitride is superior in its function as a solvent for the 
hexagonal boron nitride. The conversion reaction can thereby be completed 
in a short period of time, and it is possible to obtain cubic boron 
nitride grown to have a predetermined particle size. 
The magnesium boron nitride can be prepared by mixing hexagonal boron 
nitride and magnesium nitride or metal magnesium in a molar ratio of 
boron/magnesium of e.g. 2:3, and heating the mixture at a temperature of 
950.degree. to 1250.degree. C. for 2 hours in a nitrogen stream. 
For the production of cubic boron nitride according to the present 
invention, the magnesium boron nitride thus obtained by the 
above-mentioned method may be mixed with hexagonal boron nitride, or an 
excess amount of hexagonal boron nitride may be used for the production of 
the magnesium boron nitride and the product containing the excess amount 
of hexagonal boron nitride, thereby obtained, may be used as the starting 
material. 
The ratio of hexagonal boron nitride to magnesium boron nitride is 
preferably in a range of 15:85 to 75:25 by weight. The magnesium boron 
nitride synthesized under atmospheric pressure undergoes a phase 
transformation in the pressure zone where the cubic boron nitride is 
produced, and forms a finer phase, i.e. a high pressure phase. However, as 
a result of the chemical analysis, it has been confirmed that the chemical 
composition in the high pressure phase is the same as that in the low 
pressure zone, i.e. the composition is Mg.sub.3 B.sub.2 N.sub.4. The 
precipitation or crystallization of the cubic boron nitride is facilitated 
by the solubilization of the hexagonal boron nitride in the molten state 
of the high pressure phase. 
To obtain single crystals of the cubic boron nitride from the magnesium 
boron nitride and the hexagonal boron nitride, it is necessary to heat the 
magnesium boron nitride and the hexagonal boron nitride at a temperature 
of at least 1350.degree. C., preferably from 1400.degree. to 1600.degree. 
C., i.e. a temperature at which both of them are solubilized, within the 
thermodynamically stable pressure zone for the cubic boron nitride, for 
instance, at a pressure of 50,000 atm. preferably in a range of 52,000 to 
65,000 atm. 
FIG. 1 shows the temperature-pressure conditions in the zone within which 
the cubic boron nitride can be formed. In the Figure, A represents the 
zone within with cubic boron nitride can be formed, A+B represents a zone 
within which the cubic boron nitride is stable, C represents an 
equilibrium curve for the hexagonal boron nitride and the cubic boron 
nitride, and D represents a co-fusion curve for the magnesium boron 
nitride and the boron nitride. The lower limit of the temperature for 
obtaining the cubic boron nitride is 1350.degree. C. 
The starting material hexagonal boron nitride preferably has high purity 
and contains not more than 2.0% by weight of oxygen. Accordingly, when the 
commercially available hexagonal boron nitride is to be used, it is 
preferred to heat it, for instance, at a temperature of about 
2,000.degree. C. for several hours in a nitrogen gas atmosphere under 
about 1 atm. to reduce the oxygen content to a level not more than 1.0% by 
weight. 
EXAMPLE 1 
Referring to FIGS. 2, 3 and 4, reference numeral 1 designates a graphite 
resistance heater, numeral 2 designates a graphite disc, numeral 3 
designates a sodium chloride disc, and numeral 4 designates a sodium 
chloride cylinder. The sodium chloride disc 3 and the sodium chloride 
cylinder 4 serve to keep the pressure within the reaction chamber to be 
constant as the sodium chloride fuses at a high temperature. Reference 
numeral 5 designates a molybdenum cylinder and numeral 6 designates a 
molybdenum disc. The molybdenum cylinder 5 and the molybdenum discs 6 
constitute a container for the reactants. 
About 150 mg. of magnesium boron nitride powder prepared under the 
atmospheric pressure and about 420 mg. of hexagonal boron nitride were 
adequately mixed, and the mixture thereby obtained was pressed by a tablet 
forming device under a pressure of about 2.0 tons/cm.sup.2 to obtain a 
compact 7, which was then packed in the container. Reference numeral 8 is 
a disc made of hexagonal boron nitride, talc, pyrophyllite, silica glass 
or sodium chloride, and it serves to minimize the pressure and temperature 
variations created in the container. 
The reactor thus assembled was then heated at a temperature of 1450.degree. 
C. under 54,000 atm. for about 15 minutes by e.g. a belt type high 
temperature and pressure apparatus as shown in FIG. 5. Thereafter, the 
heating electric power source was switched off, and the temperature within 
the reaction container was rapidly lowered and the pressure released, 
whereupon the reaction product was withdrawn. In FIG. 5, reference numeral 
12 designates a paper gasket, numeral 13 designates a pyrophyllite gasket, 
numeral 14 designates a conductor ring, numeral 15 is a graphite 
resistance heater, numeral 16 is a sodium chloride cylinder, numeral 17 
designates a molybdenum plate, and numeral 18 designates a zirconia disc. 
However, the high temperature and pressure apparatus is not limited to 
such a type and may be any other type so long as it is capable of 
maintaining the required temperature and pressure during the period of 
time required for the reaction. 
The reaction product thus obtained was treated with a strong acid such as 
hydrochloric acid to dissolve magnesium boron nitride contained therein. 
After drying, it was subjected to heavy liquid separation, whereupon 
crystals of cubic boron nitride were obtained. 
The yield of the cubic boron nitride was at least 85%. 
The shape and size distributions of the crystals thereby obtained are 
rather wide. However, the maximum particle size was 120 microns, and in 
any case, the crystals were yellow and transparent. The pressure in this 
Example was applied on the basis of a load-pressure generation curve 
obtained by taking the phase transformation pressures of bismuth, thallium 
and barium to be 25.5, 37 and 55.times.10.sup.3 atm, respectively. The 
temperature in the reaction chamber was reduced from the temperature 
determination curve based on the electric power applied to the graphite 
resistance heater prepared with use of a platinum-platinum 13% rhodium 
thermocouple, by a power control method. 
EXAMPLE 2 
With use of the arrangement shown in FIG. 3, a compact 9 prepared by 
pressing magnesium boron nitride by a tablet forming device under a load 
of about 2.0 tons/cm.sup.2 and a hexagonal boron nitride disc 10 were 
alternately laminated and packed in the molybdenum container. The 
proportions of the magnesium boron nitride and the hexagonal boron nitride 
were about 1:3 in the weight ratio. They were reacted for about 20 minutes 
at a temperature of 1450.degree. C. under 56,000 atm. The after-treatment 
was carried out in a manner similar to Example 1. The cubic boron nitride 
crystals thereby obtained were yellow and transparent, and their particle 
sizes were from about 80 to 120 microns. The majority of them had a shape 
of twin or octahedron. Although the yield was as low as 45%, crystals 
having irregular crystal forms were substantially reduced as compared with 
Example 1. 
EXAMPLE 3 
Instead of the sodium chloride cylinder 4 shown in FIGS. 2 and 3, a 
hexagonal boron nitride cylinder 11 was used to form an arrangement as 
shown in FIG. 4. Magnesium boron nitride 9 compacted under pressure of 2.0 
tons/cm.sup.2 and a hexagonal boron nitride disc 10 were alternately 
packed therein. They were reacted for about 20 minutes at a temperature of 
1450.degree. C. under 56,000 atm. The after-treatment was carried out in a 
manner similar to Example 1. The cubic boron nitride crystals were yellow 
and transparent, and their particle sizes were from 65 to 120 microns. The 
majority of them had a shape of octahedron having a smooth surface. In 
this method, the cubic boron nitride crystals were obtained in an amount 
of about 1.9 times as much because the contacting surface between hBN and 
magnesium boron nitride increase. 
COMATIVE EXAMPLE 
Instead of the magnesium boron nitride, calcium boron nitride was mixed 
with the hexagonal boron nitride and the mixture was packed in the same 
container as used in Example 1. 
Under the same reaction conditions as in Example 1, i.e. at 1450.degree. 
C., and under 54,000 atm., they were reacted for about 15 minutes. 
The cubic boron nitride thereby obtained was yellow or light yellow, and it 
was fine transparent crystals (at most 60 microns, and the majority of 
them was from 30 to 40 microns). 
The yield was not more than 45% and the majority remained to be hexagonal 
boron nitride crystals. 
Now, the process for preparinng magnesium boron nitride will be described 
in detail. 
The hexagonal boron nitride to be used in this process may be a 
commercially available product. However, it is desired to heat it in a 
nitrogen atmosphere to remove oxides and water, before use. 
The magesium nitride or metal magnesium is preferably the one which 
contains a minimal amount of impurities such as oxides or hydroxides. In 
order to readily and uniformly produce the magnesium boron nitride, the 
magnesium nitride or metal magnesium is preferably in the form of powder 
or fine particles. 
The mixing ratio of the hexagonal boron nitride and the magnesium nitride 
or metal magnesium must be at least 0.6 in a molar ratio of BN/Mg. If the 
molar ratio is less than 0.6, a substantial amount of unreacted Mg.sub.3 
N.sub.2 tends to remain in the formed product, and the product will be 
unstable because Mg.sub.3 N.sub.2 is susceptible to the effects of 
moisture in the air. The preferred range of the molar ratio is from 0.6 to 
1.0. If the hexagonal boron nitride is excessive, unreacted hexagonal 
boron nitride will remain in the formed product. However, the hexagonal 
boron nitride is stable in the air, and it does not bring about 
unstability of the product. If the product is to be used for the 
above-mentioned process for the production of the cubic boron nitride, the 
hexagonal boron nitride present in the product may be useful as a part of 
the starting material for that process. 
The heating temperature must be within a range of from 950.degree. to 
1250.degree. C., preferably from 1100.degree. to 1160.degree. C. If the 
temperature is less than 950.degree. C., the reaction does not proceed. On 
the other hand, if the temperature exceeds 1250.degree. C., the 
decomposition and dispersion of Mg.sub.3 N.sub.2 become vigorous, and the 
formed magnesium boron nitride will be decomposed and transformed into the 
initial hexagonal boron nitride. 
The heating atmosphere must be a non-oxidizing atmosphere. In an oxidizing 
atmosphere, the metals will be oxidized. The reaction is carried out under 
atmospheric pressure. 
As the apparatus to be used for the process of the production of magnesium 
boron nitride according to the present invention, there may be used, for 
instance, an apparatus shown in FIG. 6. In the Figure, reference numeral 
21 is a mixture of the starting materials, numeral 22 is a stainless steel 
crucible, numeral 23 is a heater, numeral 24 is a high frequency heating 
coil, numeral 25 is a quartz glass tube, numeral 26 is a nitrogen gas 
inlet, and numeral 27 is a nitrogen gas outlet. 
The relation between the heating temperature and the progress of the 
reaction for the formation of the magnesium boron nitride, is as shown in 
FIG. 7. The hexagonal boron nitride and the magnesium nitride were mixed 
in a molar ratio of 2:1 and the mixture thus obtained was heated for 2 
hours at the respective temperatures, whereupon the amounts of the 
magnesium boron nitride thereby formed and the remaining boron nitride and 
magnesium nitride, were respectively represented by the intensities of the 
X-ray diffraction lines measured with Cu-K.alpha. and related with the 
heating temperatures. 
As shown in the Figure, the magnesium boron nitride starts to form at a 
temperature around 950.degree. C. and its formation reaches the maximum at 
a temperature around 1160.degree. C. 
The product thus obtained is magnesium boron nitride having a yellow 
colour. If unreacted Mg.sub.3 N.sub.2 is present in the product, it may be 
removed by washing the product with water or an aqueous acid solution for 
a short period of time. 
In the case where metal magnesium is to be used as the starting material, 
in order to avoid abrupt evaporation of magnesium, the heating should 
preferably conducted so as to gradually raise the temperature, or the 
temperature should initially be brought to a level of from 1000.degree. to 
1100.degree. C. and then brought to 1150.degree. C. 
Further, it is possible to homogenize the formed magnesium boron nitride by 
pulverizing it and heating it. 
According to the process of the present invention, it is possible to 
readily obtain under atmospheric pressure and in a short period of time 
magnesium boron nitride stable in the air and having a function as a 
catalyst suitable for use in the production of cubic boron nitride having 
a large particle size. Since the reaction is conducted at atmospheric 
pressure, the installation required for the production is simple, and a 
mass production can easily be made. Accordingly, it is possible to reduce 
the costs for the production. 
EXAMPLE 4 
Hexagonal boron nitride purified by heating at 2100.degree. C. in a 
nitrogen stream and Mg.sub.3 N.sub.2 powder were mixed in a molar ratio of 
2:1. The mixture thus obtained was placed in a crucible, and heated for 2 
hours at 1160.degree. C. under the atmospheric pressure in a nitrogen 
stream, whereupon yellow powder of magnesium boron nitride was obtained. 
The lattice spacings obtained from the major X-ray diffraction lines 
thereof were as follows. This product contained a small amount of 
hexagonal boron nitride, but no Mg.sub.3 N.sub.2 was present. 
TABLE 2 
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Lattice Lattice 
spacing Intensities spacing Intensities 
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8.05 moderate 1.84 moderate 
4.01 weak 1.77 strong 
(3.33) weak hexagonal 
1.54 weak 
BN (002) 
3.07 strong 1.53 weak 
3.02 strongest 1.48 moderate 
2.67 strong 1.43 weak 
2.44 strong 1.33 moderate 
2.22 weak 1.28 weak 
2.11 weak 1.23 weak 
2.02 moderate 1.16 weak 
2.00 moderate 
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The relative intensities are the same as used in Table 1. 
EXAMPLE 5 
Hexagonal boron nitride powder and metal magnesium grains having a grain 
size of about 0.5 mm were mixed in a molar ratio of 2:3. The mixture thus 
obtained was placed in a crucible and heated for 2 hours at 1050.degree. 
C. under the atmospheric pressure in a nitrogen stream, whereupon yellow 
powder of magnesium boron nitride was obtained. This powder partially 
contained hexagonal boron nitride and Mg.sub.3 N.sub.2, and therefore it 
was pulverized and again heated for 2 hours at 1160.degree. C. in a 
nitrogen stream, whereupon pure magnesium boron nitride was obtained.