Process for producing wurtzitic or cubic boron nitride

Disclosed is a process for producing wurtzitic or cubic boron nitride comprising the steps of: PA0 [A] preparing an intimate mixture of powdered boron oxide, a powdered metal selected from the group consisting of magnesium or aluminum, and a powdered metal azide; PA0 [B] igniting the mixture and bringing it to a temperature at which self-sustaining combustion occurs; PA0 [C] shocking the mixture at the end of the combustion thereof with a high pressure wave, thereby forming as a reaction product, wurtzitic or cubic boron nitride and occluded metal oxide; and, optionally PA0 [D] removing the occluded metal oxide from the reaction product. Also disclosed are reaction products made by the process described.

FIELD OF THE INVENTION 
This invention relates to a process for producing cubic boron nitride. More 
specifically, it relates to a process for making wurtzitic or cubic boron 
nitride utilizing inexpensive starting compounds to produce hexagonal 
boron nitride which in turn is converted to wurtzitic or cubic boron 
nitride by the use of a high pressure shock wave. 
BACKGROUND OF THE INVENTION 
Wurtzitic and cubic boron nitrides are extremely hard refractory materials 
which possess superior properties as compared to diamond when used for 
grinding or cutting of materials containing nickel, cobalt or iron. These 
metals, i.e., nickel, cobalt and iron, and their alloys, are known to 
chemically attack diamond at the hot cutting edge. Thus, cutting tools 
with diamond cutting edges, deteriorate quickly when used to cut these 
metals or their alloys and are, therefore, expensive to use. 
Though expensive, diamonds are used for cutting tools because boron 
nitrides, such as cubic born nitride, or wurtzitic boron nitride, are even 
more expensive overall. 
Wurtzitic and cubic boron nitrides are expensive because they are not 
naturally occurring, and the process used to make them involves the use of 
expensive high temperature-high pressure equipment and reactants that are 
relatively expensive. 
Boron nitride can be found in at least five different states, i.e., 
hexagonal boron nitride, rhombohedral boron nitride, graphitic boron 
nitride, wurtzitic boron nitride, and cubic boron nitride. Of the five 
states, cubic boron nitride is most desirable because it is the hardest. 
It is suitable for use not only as a cutting tool, but also as a crucible 
in the melting of metals, as a polishing material and the like. 
THE PRIOR ART 
U.S. Pat. No. 4,443,420, discloses a cubic system boron nitride which is 
produced by a shock wave compression method in which a thermodynamically 
stable shock wave having a compressing pressure of from about 100 to 1500 
kbar is applied to a rhombohedral system boron nitride to convert the 
rhombohedral system boron nitride to cubic system boron nitride. The 
thermodynamically stable pressure is applied by propelling a flyer plate 
or projectile plate by an explosion wave generated by detonation of an 
explosive. The flyer plate collides with a sample vessel to produce a 
strong shock wave. When the shock wave pressure is imparted to the 
starting material, the rhombohedral system boron nitride is converted to 
cubic system boron nitride. 
U.S. Pat. No. 4,014,979 relates to a method of producing highly imperfect 
wurtzitic boron nitride with enhanced activity, which consists of 
preparing a mixture of a powder of graphitic boron nitride and a 
sufficient amount of a water or aqueous alkaline additive to fill the 
pores between the particles of the graphitic boron nitride, and subjecting 
the mixture to the action of a shock wave with a pressure of not less than 
100 kbar. The shock wave is applied by the use of an explosive charge. 
U.S. Pat. No. 4,446,242 describes a process of synthesizing refractory 
metal nitrides using a solid source of nitrogen. In the process, a metal 
azide is mixed with an amount of a transition metal of the III B, IV B 
groups, a rare earth metal, or a mixture thereof, igniting the resulting 
mixture and forming a refractory nitride composition. 
Sodium azide is the preferred azide for use in the process, and preferred 
metals include: Sc, Y, La, Ti, Zr, Hf, Yb, Er, and the like. 
U.S. Pat. No. 4,016,244 describes a method of synthesizing cubic boron 
nitride from hexagonal boron nitride. In this patent, it is disclosed that 
if water is incorporated into the raw material (graphitic hexagonal boron 
nitride) in an amount of at least 3 percent by weight, cubic boron nitride 
can be obtained under lower temperature and lower pressure conditions than 
in conventional methods. Even so, unacceptably high pressures and 
temperatures are required. 
European Patent Application 0,240,913 discloses a method of manufacturing a 
sintered compact of cubic boron nitride which is accomplished by mixing 
alkaline earth metal boron nitride powder with hexagonal boron nitride 
powder, forming the mixture into a compact, causing it to adsorb from 
0.005 to 1.000 percent by weight water, then sintering it at a temperature 
of 1200.degree. C. or more, and under high pressure. Hexagonal boron 
nitride is converted to cubic boron nitride by the process described. 
As can be seen from the processes described in the prior art, an essential 
requirement, in most cases, is that the process be conducted at high 
temperatures and under high pressure. This is expensive and inefficient. 
The quantity of product which can be made is limited. 
It would be desirable in the art to develop a process for making cubic 
boron nitride using inexpensive starting materials. It would also be 
desirable to develop a process which eliminates the need for high 
pressures and other expensive process conditions. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an economical process of producing 
wurtzitic and cubic boron nitride using inexpensive starting materials. 
Another object of the invention is to provide a process of producing 
wurtzitic and cubic boron nitride which does not require continuous high 
temperatures or high nitrogen pressures. 
Still another object of the invention is to provide a quick and efficient 
process for producing wurtzitic and cubic boron nitride. 
Additional objects, advantages and novel features of the invention will be 
set forth in the description which follows. The objects and advantages of 
the invention may be realized and obtained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
To achieve the foregoing and other objects, and in accordance with the 
purpose of the present invention, as embodied and broadly described 
herein, the process for producing wurtzitic and cubic boron nitride 
comprises the steps of: 
[A] preparing an intimate mixture of powdered boron oxide, a powdered metal 
selected from the group consisting of magnesium and aluminum, and a 
powdered metal azide; 
[B] igniting the mixture and bringing it to a temperature at which 
self-sustaining combustion occurs, whereby nitrogen is liberated from the 
metal azide and reacts with the boron formed during the combustion process 
to form hexagonal boron nitride; [C] shocking the mixture at the end of 
the combustion thereof with a high pressure wave, thereby converting said 
hexagonal boron nitride to wurtzitic or cubic boron nitride and forming as 
a reaction product, a material containing wurtzitic or cubic boron nitride 
and an occluded metal oxide; and, optionally, [D] leaching the occluded 
metal oxide from the reaction product material. 
The above described process employs economical starting materials and an 
in-situ source of nitrogen, eliminating the necessity of using high 
temperature equipment. 
Additionally, conversion of the starting boron oxide to the refractory 
wurtzitic or cubic boron nitride is maximized. The process of the present 
invention is energy efficient, and requires much less time than processes 
which are currently used. 
In another aspect, the invention encompasses the reaction products of the 
method of the invention, i.e., a composite of wurtzitic or cubic boron 
nitride containing occluded metal oxides, and wurtzitic or cubic boron 
nitride obtained after removal of the occluded metal oxides.

DETAILED DESCRIPTION OF THE INVENTION 
In carrying out the process of the invention, the first step is to prepare 
a mixture of powdered boron oxide, powdered magnesium or aluminum and a 
powdered metal azide. The individual compounds are obtained, and 
intimately mixed with each other until a uniform mixture is obtained. 
Stoichiometric ratios are preferred. 
The individual components of the mixture are all commercially available in 
powder form. Boron oxide can be obtained from the Borax Company, aluminum 
or magnesium from Cerac, Inc., and metal azide from the Alfa 
Products*company. 
Magnesium is the preferred metal for use in the process. 
Nitrogen gas is required in the process to react with boron. The nitrogen 
gas is preferably generated in-situ during the reaction. A solid nitrogen 
compound can be used to generate nitrogen gas in-situ. 
A source of solid nitrogen is a metal azide. Suitable metal azides are 
formed from the alkaline earth metals and the alkali metals, as listed in 
Table I below. The preferred azide is NaN.sub.3. 
TABLE I 
______________________________________ 
NaN.sub.3 Be(N.sub.3).sub.2 
KN.sub.3 Mg(N.sub.3).sub.2 
LiN.sub.3 Ba(N.sub.3).sub.2 
CaN.sub.3 Sr(N.sub.3).sub.2 
RbN.sub.3 Br(N.sub.3).sub.2 
CoN.sub.3 
______________________________________ 
The azides useful in the process of the present invention are readily 
prepared from hydrazoic acid and the oxide or carbonate of the metal, or 
by metathesis of the metal sulfate with barium azide. 
Sodium azide is readily prepared by reacting NaNH.sub.2 with N.sub.2 O, as 
illustrated in the following equation: 
EQU 2 NaNH.sub.2 +N.sub.2 O.fwdarw.NaN.sub.3 +NaOH+NH.sub.3 
A complete description of this process is found in B.T. Fedoroff, et al., 
Encyclopedia of Explosives and Related Items, pages A601 to A619 
[Picatinny Arsenal, Dover, N.J., USA 1960], incorporated herein by 
reference. 
The metal azide is mixed with at least a stoichiometric amount of boron 
oxide and magnesium or aluminum. Excesses of the metal azide can be used, 
to ensure complete reaction of the nitrogen which is liberated on heating, 
with the boron which is liberated. 
Preferably, the materials in the mixture will have particle sizes ranging 
from about 10 to about 30 microns; however, mixtures of compounds having 
particle sizes falling outside of this range are also suitable for use. 
Once the mixture is obtained, it is ignited so that the top or bottom 
surface of the mixture is brought to the ignition temperature of the 
composition. Once this temperature is reached, the process becomes 
self-sustaining. Suitable methods for heating or igniting the mixture 
include use of heated tungsten coils or carbon strips; pulsed laser beams; 
electric arcs; focused high intensity radiation lamps, and the like. 
Although a sufficient amount of nitrogen for the process is obtained from 
the metal azide, a nitrogen atmosphere is preferably additionally employed 
during the synthesis. A nitrogen atmosphere of about 1 atmosphere is 
preferred. If desired, however, the reaction can be conducted in vacuum. 
Once the mixture is ignited, and combustion begins, the temperature of the 
mixture rises to a point where the metal azide decomposes, and nitrogen is 
liberated therefrom. The liberated nitrogen reacts with the boron which is 
produced by reduction of boron oxide with the active metal, Mg or Al. The 
steps believed to take part in the reaction are shown below, when 
magnesium is used as the reducing element, and sodium Azide the source of 
nitrogen. 
##STR1## 
The temperature of combustion will vary depending to some extent on the 
specific ratios of starting compounds in the mixture, but in general will 
range from about 1800.degree. to about 2100.degree. C. The combustion of 
the mixture occurs very fast and is completed within seconds after 
ignition commences. 
While the mixture can, if desired, be ignited when the mixture is in a 
loose powder stage, preferably the mixture is formed into a compact prior 
to ignition. The compact is made by compressing the loose powder into a 
formed shape conforming to the dimensions of a specific die. Normally, the 
shape will be that of a flat tablet or wafer, having a dimension wider 
than it is thick. 
After completion of combustion, which is determined by a signal from a 
thermocouple located at the bottom surface of the burning compact, the 
completely combusted mixture is subjected to a shock wave which has the 
effect of converting hexagonal boron nitride, produced as a consequence of 
the combustion of the mixture, to a reaction product material which is 
wurtzitic or cubic boron nitride occluded with a metal oxide, i.e., 
magnesium oxide, or aluminum oxide. 
The shock wave parameters applicable to this invention are described in 
U.S. Pat. No. 4,014,979. That patent is hereby incorporated by reference 
to the extent provided by law. It has been found in conjunction with the 
process of this invention, that shock waves ranging between about 100 and 
300 kbar in pressure result in the production of wurtzitic or cubic boron 
nitride. It is essential that the shock waves be applied to the combustion 
mixture as uniformly as possible, and normally this is accomplished by 
means of an explosive device, or a gas gun. 
After the combustion products are subjected to the shock wave, which 
produces wurtzitic or cubic boron nitride and occluded metal oxide, the 
Mg0 is thereafter optionally leached from the reaction product, yielding 
substantially pure wurtzitic or cubic boron nitride. 
The leaching can be done by subjecting the reaction product to a suitable 
acid such as hydrochloric acid or phosphoric acid. 
It should be understood by those skilled in the art that the product 
obtained after the shock wave is Propagated through the combustion 
material can include either wurtzitic boron nitride, or cubic boron 
nitride. The specific product obtained depends upon the combustion 
temperature and pressure of the shock wave. In general, lower combustion 
temperatures and shock wave pressures result in the production of 
wurtzitic boron nitride, rather than cubic boron nitride. Because cubic 
boron nitride is the preferred nitride, the process is preferably carried 
out at sufficiently high combustion temperatures and shock wave pressures 
to insure formation of cubic boron nitride. 
The combustion temperature may be decreased, if desired, by the addition of 
magnesium oxide as a diluent. The shock wave pressure may be controlled by 
the proper selection of the explosive charge. 
The following examples are illustrative of the invention, and are not to be 
regarded as limiting its scope, which is defined in the appended claims. 
EXAMPLE 1 
A powder mixture is prepared by charging 18.5 grams powdered B.sub.2 
O.sub.3, 19 grams powdered Mg, and 12.5 grams powdered NaN.sub.3 into a 
container and mixing thoroughly. The particle size of B.sub.2 O.sub.3 is 
30 micron, Mg is 15 micron, and NaN.sub.3 is 30 micron. Next, the powder 
mixture is cold-pressed into compacts with a L/D ratio of 0.5. One of the 
cold-pressed compacts is placed into a six-inch diameter stainless steel 
die with grafoil lining the sides and bottom of the cavity. Situated at 
the bottom of the die cavity, is a horizontal tungsten coil which acts as 
an igniter for the combustion reaction. Electric leads, which are 
insulated, extend down through the bottom of the die and are connected to 
an 5 appropriate power source. The compact is ignited from the bottom, and 
a combustion wave rapidly propagates to the top surface converting the 
reactants into hexagonal boron nitride, magnesium oxide and sodium, the 
latter which vaporizes off. The completion of the reaction is detected by 
a W-Re thermo couple bead positioned at the top surface of the reactant 
compact. While the combustion products are still at a high temperature 
(approximately 2937.degree. C.) and at the time the combustion wave 
reaches the top surface of the compact, a steel plug is driven into the 
reactant compact with an explosive charge producing a shock wave. The 
shock wave has a pressure of 150 kbar. The shock wave transforms the 
hexagonal boron nitride into the cubic form. The impurity gases adsorbed 
on the powder (water vapor) and the sodium are vented through slots in the 
die wall prior to generation of high pressures. 
The product, which is a multi-phase composite of cubic boron nitride and 
magnesium oxide powder, is leached with hydrochloric acid, which leaches 
out the magnesium oxide. 
EXAMPLE 2 
The procedure of Example 1 is repeated, using a starting mixture of 34.9 g 
B.sub.2 O.sub.3, 36.6g Mg, and 6.8 g MgO, and 21.7g NaN.sub.3. The 
temperature of combustion is 2825.degree. C., and the shock wave pressure 
is 125 kbar. The product produced is 24.9 g wurtzitic boron nitride 
occluded with 67.4 g MgO. The occluded is removed by leaching with HCl. 
EXAMPLE 3 
The procedure of Example 1 is repeated starting with a mixture of 41.7 g 
B.sub.2 O.sub.3, 32.3 g Al, and 26.0 g NaN.sub.3. 
The temperature of combustion is 2970.degree. C., and the shock wave 
pressure is 200 kbar. The product produced is 29.7 g cubic boron nitride 
occluded with 61.1 g Al.sub.2 O.sub.3. The Al.sub.2 O.sub.3 is not 
removed. 
EXAMPLE 4 
The procedure of Example 1 is repeated starting with a mixture of 34.6 g 
B.sub.2 O.sub.3, 26.9 g Al, 16.9 g Al.sub.2 O.sub.3, and 21.6 g NaN.sub.3. 
The temperature of combustion is 2660.degree. C., and the shock wave 
pressure is 100 kbar. The product is 24.7 g wurtzitic boron nitride 
occluded with 67.7 g Al.sub.2 O.sub.3. The Al.sub.2 O.sub.3 is not 
removed. 
The foregoing description of preferred embodiments of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the 5 precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. The embodiments were chosen and described in 
order to best explain the principles of the invention and its practical 
application, to thereby enable others skilled in the art to best utilize 
the invention in various embodiments and with various modifications, as 
are suited to the particular use contemplated. It is intended that the 
scope of the invention be defined by the claims appended hereto.