METHOD FOR THE PREPARATION OF BORON NITRIDE POWDER

Abstract

This invention is directed to a process for the preparation of boron nitride powder, particularly a fine powder with a low degree of contamination, which demonstrates good caking, heat conductivity and dielectric properties. Specifically, a process for the preparation of amorphous boron nitride (a-BN) is provided wherein the process comprises: mixing powders of boric acid and a carbamide at a temperature in the range of about 250-300° C., thereby forming: ammonium polyborates; boron imide or a mixture thereof and ammonia; and heating of the materials formed in step (a) to a temperature in the range of about 500-600° C., thereby forming a powder of a-BN.

Description

FIELD OF THE INVENTION

This invention is directed to a process for the preparation of boron nitride powder, particularly a fine powder with a low degree of contamination, which demonstrates good caking, heat conductivity and dielectric properties.

BACKGROUND OF THE INVENTION

Ceramic materials, such as boron nitride (BN), have useful properties including high melting temperature, low density, high strength, stiffness, hardness, wear resistance, and corrosion resistance. Many ceramics are good electrical and thermal insulators.

For most applications using ceramics, a fine powder with small particle sizes, as small as nano-sized particles, is required. Small particle-size powders are not easily obtained by current methodology and usually require additional grinding and cleaning operations.

Boron nitride (BN) is a white powder with high chemical and thermal stability and high electrical resistance. Boron nitride possesses three polymorphic forms; one analogous to diamond, one analogous to graphite and one analogous to fullerenes. Boron nitride can be used to make crystals that are extremely hard, second in hardness only to diamond, and the similarity of this compound to diamond extends to other applications. Like diamond, boron nitride acts as an electrical insulator and is an excellent conductor of heat.

Boron nitride, like graphite, has the ability to lubricate, in both extreme cold and hot conditions, is suited for extreme pressure applications, is environmentally friendly and is inert to most chemicals powders.

Due to its excellent dielectric and insulating properties, BN is used in electronics, e.g. as a substrate for semiconductors, microwave-transparent windows, structural material for seals, electrodes as well as catalyst carriers in fuel cells and batteries.

BN can be prepared as amorphous BN (a-BN), hexagonal BN (h-BN), turbostratic BN (t-BN) and cubic BN (c-BN). Generally, a-BN is prepared at relatively low temperatures, while both h-BN and t-BN are prepared at higher temperatures. c-BN may be prepared by high pressure and high temperature treatment of h-BN.

There are several known processes in the art for preparing BN powders, such as those presented in U.S. Pat. No. 6,306,358. However, the methods known in the art are generally inefficient, and tend to produce powders that need to be cleaned and/or ground before used. U.S. Pat. No. 6,306,358, for example, discloses a method for preparing a-BN powder at temperature below 1000° C., mostly in the range of 850-950° C. However, since boric anhydride (B₂O₃), which is one of the reactants in the process, evaporates at such high temperatures, the yield of the process is relatively low.

Therefore, there is a need in the art for a process for preparing various forms of BN, which would be efficient, and would provide pure powders.

SUMMARY OF THE INVENTION

This invention is directed to a process for the preparation of amorphous boron nitride (a-BN) comprising:

• • a. Mixing powders of boric acid and a carbamide at a temperature in the range of about 250-300° C., thereby forming: ammonium polyborates; boron imide or a mixture thereof and ammonia; and
  • b. heating of the materials formed in step (a) to a temperature in the range of about 500-600° C., thereby forming a powder of a-BN.

According to some embodiments of the invention, the process further comprises heating the a-BN to a temperature between about 1200-1800° C. under an atmosphere of nitrogen, ammonia, or a mixture thereof, thereby providing h-BN, t-BN, or a combination of h-BN and t-BN. According to some embodiments, the amount of crystallinic BN prepared, including the h-BN and the t-BN is at least 98.35 w/w. According to some embodiments, the amount of crystallinic BN prepared is at least 99.0% w/w. According to some embodiments, the amount of crystallinic BN prepared is at least 99% w/w.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a-BN powder;

FIG. 1 b shows an example of an X-ray powder diffraction diagram of a-BN according to an embodiment of the invention;

FIG. 2: shows an example of an X-ray powder diffraction diagram of h-BN/t-BN XRD diagram;

FIGS. 3 a-b: represents EM photomicrographs showing h-BN/t-BN powder, showing the high degree of purity thereof;

FIGS. 4 a-b shows tables describing physical and chemical properties of the h-BN/t-BN powder.

FIG. 5 shows calorimetric analysis of the process according to an embodiment of the invention.

FIG. 6 a shows the XRD diagram of a-BN prepared according to an embodiment of the invention.

FIG. 6 b shows a diagram describing the course of the reaction for preparing a-BN, according to an embodiment of the invention.

FIG. 7A is an XRD diagram of turbostratic h-BN (Grade A), and FIG. 7B is an electron microscope picture of the crystals, prepared according to an embodiment of the invention.

FIG. 8A is an XRD diagram of quasi turbostratic h-BN (Grade B), and FIG. 8B is an electron microscope picture of the crystals, prepared according to an embodiment of the invention.

FIG. 9A is an XRD diagram of quasi graphitic h-BN (Grade C), and FIG. 9B is an electron microscope picture of the crystals, prepared according to an embodiment of the invention.

FIG. 10A is an XRD diagram of graphitic h-BN (Grade D), and FIG. 10B is an electron microscope picture of the crystals, prepared according to an embodiment of the invention.

FIG. 11A is an XRD diagram of cosmetic graphitic h-BN (Grade D), and FIG. 11B is an electron microscope picture of the crystals, prepared according to an embodiment of the invention.

FIG. 12 shows the overlap of the XRD data for Grades A, B, C and D of h-BN, prepared according to embodiments of the invention.

FIG. 13 provides the typical properties of Grades A, B, C, D and E of h-BN, prepared according to embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

This invention provides a process for the preparation of ceramic powders of BN. In one embodiment of this invention, the prepared BN is amorphous BN, i.e., a-BN.

The a-BN is prepared according to this invention by the following steps:

mixing powders of boric acid and a nitrogen comprising compound at a temperature in the range of about 250-300° C., thereby forming: ammonium polyborates ((NH₄)_xB_yO_z); boron imide, or a mixture thereof and ammonia; and heating of the ammonium polyborates and the boron imide formed to a temperature in the range of about 500-600° C., thereby forming a powder of a-BN. By “about” it is meant plus or minus 30%, 20%, 10% or 5%.

A compound containing nitrogen may be for example, ammonia, ammonium and carbamides, including urea.

The a-BN is prepared according to an embodiment of this invention by the following steps:

• • mixing powders of boric acid and a carbamide, such as urea ((NH₂)₂CO, at a temperature in the range of about 250-300° C., thereby forming: ammonium polyborates ((NH₄)_xB_yO_z); boron imide, or a mixture thereof and ammonia; and
  • subsequent heating of the ammonium polyborates and the boron imide formed to a temperature in the range of about 500-600° C., thereby forming a powder of a-BN.

FIG. 1 a shows the a-BN provided by the process of this invention and FIG. 1b shows the XRD diagram of the prepared a-BN.

According to this invention, the ammonium polyborates react with the ammonia when heated to about 500-600° C. thereby forming a-BN. Further, according to this invention heating the boron imide to about 500-600° C. provides a-BN.

According to this invention, the second step of the above process is performed when about less than 50% of the initial weight of the boric acid reactant remains in the reaction vessel. According to another embodiment the second step of the above process is performed when about 55-75% of the initial weight of the boric acid reactant remains in the reaction vessel. According to a further embodiment of the invention, the second step is performed when about 60-65% of the initial weight of the boric acid reactant remains. According to a further embodiment of the invention, the second step is performed when about 70% of the initial weight of the boric acid reactant remains in the reaction vessel. According to a further embodiment of the invention, the second step is performed when about 40-50% of the initial weight of the boric acid reactant remains in the reaction vessel. According to a further embodiment of the invention, the second step is performed when about 30-40% of the initial weight of the boric acid reactant remains in the reaction vessel. According to a further embodiment of the invention, the second step is performed when about 20-30% of the initial weight of the boric acid reactant remains in the reaction vessel. According to a further embodiment of the invention, the second step is performed when about 10-20% of the initial weight of the boric acid reactant remains in the reaction vessel. The term “about” is used herein to mean ±10%.

In one embodiment of this invention, the boric acid is selected from H₃BO₃, H₂B₄O₇ or HBO₂. In another embodiment of the invention, salts of boric acid may be used instead of the boric acid.

According to an embodiment of the invention, the chemical formula of the ammonium polyborates is (NH₄)_xB_yO_z, wherein x is between 1-4, y is between 1-10 and z is between 2-17. The ammonium polyborates may, for example, without being limited, (NH₄)₂B₄O₂, NH₄B₅O₈ or (NH₄)₄B₁₀O₁₇. According to an embodiment of the invention, any of the polyborates may be hydrated. According to an embodiment of this invention, when the carbamide reactant is urea, the ammonium polyborate formed may be ammonium tetraborate. According to this embodiment, the chemical reactions that may take place in the reaction vessel in the first step of the above process are:

4H₃BO₃+(NH₂)₂CO(NH₄)₂B₄O₇+CO₂+4H₂O

4H₃BO₃+3(NH₂)₂CO2B₂(NH)₃+3CO₂+9H₂O

2H₃BO₃B₂O₃+3H₂O

Additionally, part of the urea in the reaction vessel reacts with the water produced in the above reactions thereby forming ammonia according to the following reaction:

(NH₂)₂CO+H₂O2NH₃+CO₂

Then, in the second step, upon heating to about 500-600° C., the ammonium tetraborate reacts with ammonia, thereby forming a-BN, according to the following reaction:

(NH₄)₂B₄O₇+NH₃→4a-BN+7H₂O

Further, the boron imide produced in the first step breaks down, upon heating to 500-600° C., to a-BN and ammonia according to the following reaction:

B₂(NH)₃→2a-BN+NH₃

thus providing a-BN and additional ammonia that may react with ammonium tetraborate for the formation of further a-BN.

According to an embodiment of the invention, the w/w ratio of the carbamide and the boric acid reactants is from about 3:4 to 2:1. According to a further embodiment of the invention, the w/w ratio of the carbamide and the boric acid is about between 1.0-1.5:1.0. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 3.75:4. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 3.5:4. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 3.25:4. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 2.75:4. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 2.5:4. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 2.25:4. According to a further embodiment of the invention, the ratio of the carbamide and the boric acid is about 1:2.

According to an embodiment of this invention, the process of this invention may further comprises heating the a-BN to a temperature between about 1200-1800° C. under an atmosphere of nitrogen, ammonia, or both a mixture thereof, so as to provide h-BN and/or t-BN. According to one embodiment of this invention, the heating of the a-BN is performed when about 40-45% of the initial weight of the boric acid reactant remains. According to one embodiment of this invention, the heating of the a-BN is performed when about 35-40% of the initial weight of the boric acid reactant remains. According to one embodiment of this invention, the heating of the a-BN is performed when about 30-35% of the initial weight of the boric acid reactant remains. According to one embodiment of this invention, the heating of the a-BN is performed when about 25-30% of the initial weight of the boric acid reactant remains. According to one embodiment of this invention, the heating of the a-BN is performed when about 20-25% of the initial weight of the boric acid reactant remains. According to one embodiment of this invention, the heating of the a-BN is performed when about 15-20% of the initial weight of the boric acid reactant remains. According to one embodiment of this invention, the heating of the a-BN is performed when about 10-15% of the initial weight of the boric acid reactant remains. According to some embodiments, additional amounts of any one of the reactants, or a combination thereof, may be added to or removed from the reaction vessel during the preparation of the h-BN/t-BN. According to further embodiments, any amount of the products may be removed from the reaction vessel during the reaction.

According to this invention, when lower range temperatures are used, i.e., about 1200-1400° C. the percentage of t-BN rises, while higher temperatures, i.e., about 1400-1800° C. result in lower amounts of t-BN and higher amounts of h-BN. FIG. 2 shows the XRD pattern obtained from the h-BN/t-BN powder prepared according to this invention at 1500° C.

According to some embodiments, the different grades of t-BN and h-BN may be prepared according to the process, including graphitic-BN, quasi-graphitic-BN, quasi-turbostratic-BN and turbostratic-BN. According to further embodiments, the temperature needed to prepare each grade of BN is dependent on the degree of crytallinity of the prepared grade. The higher the crystallinity, the higher the temperature used to prepare that specific grade.

According to some embodiments of the invention, the prepared crystallinic BN is at least 97% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 98% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 98.3% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 98.4% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 98.5% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 98.7% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 99.0% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 99.5% w/w pure. According to further embodiments, the prepared crystallinic BN is at least 99.7% w/w pure.

According to an embodiment of this invention, the a-BN is ground to particles smaller than about 2-3 micron, before heating to about 1200° C.-1800° C. to prepare the h-BN/t-BN.

Once the h-BN/t-BN is prepared, according to an embodiment of the invention, the t-BN/h-BN powder is cleaned from remaining boric acid, boric anhydride, or any other contaminants, by washing with hot water in temperature that is higher than about 70° C. and/or alcohol. Since the alcohol is capable of providing cleaner material, when highly pure material is desired, according to this invention, the t-BN/h-BN is washed first with water and then with alcohol. According to a further embodiment, the washing with hot water is performed until the remaining amount of boric anhydride in the reaction vessel is less than about 0.5% w/w. According to a further embodiment, the washing with hot water is performed until the remaining amount of boric anhydride in the reaction vessel is less than about 1-2% w/w. According to a further embodiment, the washing with hot water is performed until the remaining amount of boric anhydride in the reaction vessel is less than about 2-3% w/w. According to a further embodiment, the washing with hot water is performed until the remaining amount of boric anhydride in the reaction vessel is less than about 3-4% w/w. According to a further embodiment, the washing with hot water is performed until the remaining amount of boric anhydride in the reaction vessel is less than about 4-5% w/w. According to a further embodiment, the washing with alcohol is performed until the remaining amount of boric anhydride is less than about 0.1% w/w.

According to an embodiment of the invention, the water used to wash the product materials is distilled or demineralized water, wherein the concentration of the h-BN/t-BN powder in the water is less than about 2-5%. According to another embodiment, the powder is separated from the water by centrifuge.

Once the h-BN/t-BN materials are washed there may still be up to 1% residual oxygen (not from boric anhydride) that probably results from free orbitals on the surface of the h-BN/tBN material that react with the oxygen in the air. Thus, according to a further embodiment of this invention, after the h-BN/t-BN material is washed with water and/or alcohol, it is heated to about 300° C. under a light gas, such as hydrogen or helium, thereby causing the oxygen to leave the surface. Then, under hermitic conditions, a heavier gas, such as argon or nitrogen, is streamed over the h-BN/t-BN material.

According to this invention, the h-BN/t-BN products contain up to about 2% impurities. According to another embodiment of this invention, the h-BN/t-BN product contains up to about 1% impurities. According to yet another embodiment of this invention, the h-BN/t-BN product contains up to about 0.5% impurities. According to yet another embodiment of this invention, the amount of impurities found in the h-BN/t-BN product is less than 0.5%.

FIG. 3 a shows an electron microscope picture of the h-BN/t-BN powder prepared according of this invention, demonstrating the high degree of purity of the product. FIG. 3b shows additional electron microscope pictures of the h-BN/t-BN powder prepared according to this invention. An analysis of the h-BN/t-BN powder prepared according to this invention indicates the following composition: carbon 0.053%, oxygen 0.608%, nitrogen 55.8%, calcium 280 ppm, silicon 100 ppm and sol. Borates 0.133% mean particle size of 5.5 μm.

The physical and chemical properties of two different batches of the h-BN/t-BN prepared according to this invention are provided in FIGS. 4a and 4b. The time of endurance for preparing the t-BN is 1.5-3 hours at a temperature of 1200-1500° C. The time of endurance for preparing the h-BN is 3 hours at a temperature of 1500-1800° C.

Various aspects of the invention are described in greater detail in the following Examples, which represent embodiments of this invention, and are by no means to be interpreted as limiting the scope of this invention.

EXAMPLES

Example 1

300 g H₃BO₃ are mixed with 600 g (NH₂)₂CO at 250° C. for 2 hours and then heated to 500° C. for 0.25 hour for obtaining 120 gr of a-BN. The reaction vessel is then heated to a temperature of 1200° C. for 3 hours in a nitrogen atmosphere for obtaining 84.6 gr t-BN.

Example 2

300 g H₃BO₃ are mixed with 600 g (NH₂)₂CO at 250° C. for 2 hours and then heated to 600° C. for 0.5 hour for obtaining 130 gr of a-BN. The reaction vessel is then heated to a temperature of 1500° C. for 2 hours under an atmosphere of nitrogen for obtaining 104.5 gr t-BN.

Example 3

300 g H₃BO₃ are mixed with 600 g (NH₂)₂CO at 250° C. for 2 hours and then heated to 600° C. for 0.5 hour for obtaining 135 gr of a-BN. The reaction vessel is then heated to a temperature of 1500° C. for 5 hours in a nitrogen atmosphere for obtaining 101.2 gr h-BN.

Example 4

300 g H₃BO₃ are mixed with 600 g (NH₂)₂CO at 250° C. for 2 hours and then heated to 600° C. for 1.0 hour for obtaining 132 gr of a-BN. The reaction vessel is then heated to a temperature of 1800° C. for 3 hours in a nitrogen atmosphere for obtaining 88.6 gr h-BN.

Example 5

To simulate the process, we conducted a thermogravimetric analysis using TG-50 and a calorimetric analysis using DSC-823E. Both Analyzers of the company Mettler Toledo, USA.

For the thermogravimetric analysis 25.5600 mg of a mixture of urea and boric acid taken as a ratio of 2:1 was used. Heating was conducted from 25° C. to 1000° C. at a rate 10° C. per minute in a nitrogen atmosphere (200 ml per minute). The results indicate that heating above 600° C. for production of the amorphous BN is not effective.

For the calorimetric analysis 6.2900 mg of a mixture of urea/boric acid, taken as a ratio of 2:1 was used. Analysis was conducted in an atmosphere of nitrogen (80 ml per minute) in the temperature range from 30° C. to 600° C. at a heating rate 10° C./min. The results are shown in FIG. 5.

Example 6

80 kg of H₃BO₃ were mixed with 160 kg of urea for 250 minutes. The reaction mass was heated according to the profile shown in FIG. 6B. After 250 minutes, 31 kg of a-BN were obtained. The weight profile of the entire reaction mass throughout the reaction is shown in FIG. 6B. The XRD diagram of the prepared a-BN is shown in FIG. 6A. Table I below shows the certificate of analysis of the prepared a-BN:

TABLE I
Certificate of analysis of a-BN
Physical analysis	Chemical composition
|Graphite index{circle around (3)}	n/a	N	52.6%	Al	120	ppm
XRD: confirm		B	42%	Ba	<1	ppm
		B2O3sol.	3.50%	Ca	<10	ppm
		C	0.05%	Na	70	ppm
		O	5.2%	Fe	150	ppm
		H2O	0.3%	Si	30	ppm
		BN	92.2%	K	<5	ppm
		Poly-borates	2.5%	Mn	<1	ppm
				Mg	<1	ppm
{circle around (1)}under detection limit
{circle around (2)}not analyzed
{circle around (3)}variation range

The obtained a-BN was heated to temperatures ranging from 1200-1800° C., to prepare turbostratic h-BN (Grade A), quasi turbostratic h-BN (Grade B), quasi graphitic h-BN (Grade C), graphitic h-BN (Grade D) and cosmetic graphitic h-BN (Grade D), wherein the higher the temperature, the higher the crystallinity of the prepared BN. The XRD data and electron microscope pictures of the turbostratic h-BN (Grade A), quasi turbostratic h-BN (Grade B), quasi graphitic h-BN (Grade C), graphitic h-BN (Grade D) and cosmetic graphitic h-BN (Grade D) are shown in FIGS. 7-11(A-B), respectively. Tables II-VI below provide the certificates of analysis of the turbostratic h-BN (Grade A), quasi turbostratic h-BN (Grade B), quasi graphitic h-BN (Grade C), graphitic h-BN (Grade D) and cosmetic graphitic h-BN (Grade D), respectively. As shown in the certificates of analysis, the turbostratic h-BN has 98.3% w/w purity, the quasi turbostratic h-BN has 98.5% w/w purity, the quasi graphitic h-BN has 98.3% w/w purity, the graphitic h-BN has 98.4% w/w purity, and the cosmetic graphitic h-BN has a purity of more than 99.5% w/w.

TABLE II
Certificate of analysis of turbostratic h-BN (Grade A)
Physical analysis	Chemical composition
BETTriStar	4.62	m2/g	N	55.5%	Al	ppm
Tap densityASTM B527	0.48	g/cm3	B	42.8%	Ba	ppm
Graphite index{circle around (3)}	>100		B2O3 sol.	0.84%	Ca	ppm
Particle size distribution			C	0.05%	Na	ppm
by Malvern-2000			O	0.65%	Fe	ppm
d90	23	μm	H2O	0.308%	Ni	ppm
d50	3.6	μm	BN	98.3	K	ppm
d10	0.48	μm			Mn	ppm
					Mg	ppm
{circle around (1)}under detection limit
{circle around (2)}not analyzed
{circle around (3)}variation range

TABLE III
Certificate of analysis of quasi turbostratic h-BN (Grade B)
Physical analysis	Chemical composition
BETTriStar	7.58	m2/g	N	55.6%	Al	70	ppm
Tap	0.26	g/cm3	B	42.9%	Ba	<1	ppm
densityASTM B527			B2O3 sol.	0.074%	Ca	<10	ppm
Graphite index{circle around (3)}	>50		C	0.03%	Na	110	ppm
Particle size			O	1.01%	Fe	30	ppm
distribution by			H2O	0.21%	Si	40	ppm
Malvern-2000			BN	98.5	K	<5	ppm
d90	9.913	μm			Mn	<1	ppm
d50	4.763	μm			Mg	<1	ppm
d10	0.619	μm
{circle around (1)}under detection limit
{circle around (2)}not analyzed
{circle around (3)}variation range

TABLE IV
Certificate of analysis of quasi graphitic h-BN (Grade C)
Physical analysis	Chemical composition
BETTriStar	9.58	m2/g	N	55.5%	Al	90	ppm
Tap	0.34	g/cm3	B	42.8%	Ba	<1	ppm
densityASTM B527			B2O3 sol.	0.064%	Ca	<10	ppm
Graphite index{circle around (3)}	12		C	0.05%	Na	30	ppm
Particle size			O	1.8%	Fe	30	ppm
distribution by			H2O	0.308%	Ni	<1	ppm
Malvern-2000			BN	98.3	K	<5	ppm
d90	12.154	μm			Mn	<1	ppm
d50	6.075	μm			Mg	<1	ppm
d10	0.753	μm
{circle around (1)}under detection limit
{circle around (2)}not analyzed
{circle around (3)}variation range

TABLE V
Certificate of analysis of graphitic h-BN (Grade D)
Physical analysis	Chemical composition
BETTriStar	5.2	m2/g	N	55.5%	Al	1030	ppm
Tap	0.25	g/cm3	B	43.0%	Ba	<1	ppm
densityASTM B527			B2O3 sol.	0.08%	Ca	<2	ppm
Graphite index{circle around (3)}	2.9		C	0.06%	Na	870	ppm
Particle size			O	0.8%	Fe	600	ppm
distribution by			H2O	0.2%	Si	150	ppm
Malvern-2000			BN	98.4	K	<5	ppm
d90	5.8	μm			Mn	<1	ppm
d50	3.87	μm			Mg	<1	ppm
d10	0.95	μm
{circle around (1)}under detection limit
{circle around (2)}not analyzed
{circle around (3)}variation range

TABLE VI
Certificate of analysis of cosmetic graphitic h-BN (Grade D)
Physical analysis	Chemical composition
BETTriStar	3.11	m2/g	N	56.3%
Tap densityASTM B527	0.38	g/cm3	B	43.5%
Graphite index{circle around (3)}	2.1		B2O3sol.	0.01%
Particle size distribution by			C	<0.01%
Malvern-2000			O	0.213%
d90		μm	H2O	0.221%
d50		μm	BN	>99.5%
d10		μm	Hg	<0.01 ppm
			Ni	<0.01 ppm
			Cd	<0.01 ppm
			Pb	<0.01 ppm
{circle around (1)}under detection limit
{circle around (2)}not analyzed
{circle around (3)}variation range

FIG. 12 shows the overlap of the XRD data of Grades A, B, C and D, allowing the comparison between the different grades. It is noted that the degree of crystallization of the h-BN phase was evaluated in terms of the “graphitization index” (G.I.) according to:

$G.I.=\frac{\mathrm{Area}\left[\left(100\right)+\left(101\right)\right]}{\mathrm{Area}\left[\left(102\right)\right]},$

wherein areas 100, 101 and 102 are marked in FIG. 12.

FIG. 13 provides the percentage of oxygen, the crystal size, particle size and graphite index of Grades A, B, C, D and E. It is noted that Grade E represents crystallinic BN powder that was sintered into a plate and milled into particles of a pre-defined size.

While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

1. A process for the preparation of amorphous boron nitride (a-BN) comprising: a. mixing powders of boric acid and a carbamide at a temperature in the range of about 250-300° C., thereby forming: ammonium polyborates; boron imide or a mixture thereof and ammonia; andb. heating of the materials formed in step (a) to a temperature in the range of about 500-600° C., thereby forming a powder of a-BN.

2. The process according to claim 1, wherein the ammonium polyborates reacts with the ammonia when heated to about 500-600° C. thereby forming a-BN.

3. The process according to claim 1, wherein heating the boron imide to about 500-600° C. provides a-BN.

4. The process according to claim 1, wherein the carbamide is urea.

5. The process according to claim 1, wherein step (b) is performed when about 55-75% of the initial weight of the boric acid remains in the reaction vessel.

6. The process according to claim 1, wherein step (b) is performed when about 60-65% of the initial weight of the boric acid remains in the reaction vessel.

7. The process according to claim 1, wherein the boric acid is H3BO3, H2B4O7 or HBO2.

8. The process according to claim 1, wherein the ammonium polyborate formed consists essentially from ammonium tetraborate.

9. The process according to claim 1, wherein the w/w ratio of the carbamide and the boric acid reactants is from about 3:4 to 2:1.

10. The process according to claim 1, wherein the w/w ratio of the carbamide and the boric acid reactants is about between 1.0-1.5:1.0.

11. The process according to claim 1, further comprising heating the a-BN to a temperature between about 1200-1800° C. under an atmosphere of nitrogen, ammonia, or a mixture thereof, thereby providing h-BN, t-BN, or a combination of h-BN and t-BN.

12. The process according to claim 11, wherein said a-BN is ground to particles smaller than about 2-3 micron prior to heating the a-BN.

13. The process according to claim 11, wherein the further heating of the a-BN is performed when about 40-45% of the initial weight of the boric acid remains in the reaction vessel.

14. The process according to claim 11, wherein remaining boric acid, boric anhydride and other contaminants are removed from the t-BN and the h-BN by washing with water, alcohol, or a mixture thereof.

15. The process according to claim 14, wherein the washing with water is performed until the remaining amount of boric anhydride is less than about 0.5% w/w.

16. The process according to claim 14, wherein the washing with alcohol is performed until when the remaining amount of boric anhydride is less than about 0.1% w/w.

17. The process according to claim 13, further comprising heating the h-BN and t-BN to about 300° C. under a light gas; andstreaming a heavy gas over the h-BN and t-BN under hermitic conditions.

18. The process according to claim 11, wherein the h-BN, t-BN or the combination thereof includes at least 98.3% w/w h-BN, t-BN or the combination thereof.

19. The process according to claim 11, wherein the h-BN, t-BN or the combination thereof includes at least 99.0% w/w h-BN, t-BN or the combination thereof.

20. The process according to claim 11, wherein the h-BN, t-BN or the combination thereof includes at least 99.5% w/w h-BN, t-BN or the combination thereof.