Chemistry of inorganic compounds – Boron or compound thereof – Binary compound
Reexamination Certificate
1998-06-02
2001-10-23
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Boron or compound thereof
Binary compound
C428S403000
Reexamination Certificate
active
06306358
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a boron nitride powder, especially to high purity turbostratic boron nitride powder having good sinterability and a method for producing same which enables mass scale production of the boron nitride powder in low cost.
GENERAL BACKGROUND OF THE INVENTION
There are several kinds of polymorphs in boron nitride They are hexagonal boron nitride (hereinafter h-BN), rhombohedral boron nitride (hereinafter r-BN), amorphous boron nitride (hereinafter a-BN), turbostratic boron nitride (hereinafter t-BN), zinc blend type cubic boron nitride (hereinafter c-BN), and wurtzite type hexagonal boron nitride (hereinafter w-BN), the latter two being high pressure stable phases. In case where boron nitride is synthesized at relatively low temperatures, for example at 900° C. or below, a-BN is generated. It is known that if boron nitride synthesis is carried out at elevated temperature or if a-BN is heat treated at elevated temperature, h-BN is generated. The primary particles of h-BN powder are usually hexagonal platelets.
FIG. 1
is X-ray powder diffraction diagram of h-BN which exhibits remarkable [
002
], [
100
], [
101
], [
102
] and [
004
] diffraction spectra. Compared with this h-BN diffraction diagram, the diffraction diagram of a-BN generally exhibits a broad diffraction peak having a large half peak height width at the site where the [
002
], [
100
] and [
101
] diffraction peaks of h-BN are present. As shown in
FIG. 8
the broad diffraction peak seems to be a composite of the [
100
] and [
101
] diffraction peaks of h-BN (hereinafter, for convenience of explanation, every diffraction peaks of boron nitride other than h-BN in respective X-ray powder diffraction diagrams are simply named with indexes of h-BN without notices). The crystal of h-BN has a crystal structure having hexagonal B-N net layers stacked up in a pattern of a,a′, a,a′, a,a′, a,a′, . . . With other typed patterns of stacked hexagonal B-N mesh layers, other kinds of crystal phases other than h-BN will result, and a crystal phase having a random manner of stacked hexagonal B-N mesh layers is generally called t-BN.
In a wide sense, as explained in “Shigen-Sozai-Gakkai-Shi (J. Resources and Materials Association) ” Vol. 105 No. 2, p 201, a-BN may be classified as one sort of BN having a turbostratic structure. However, a boron nitride having such broad X-ray diffraction peaks should be amorphous BN, i.e., a-BN. In the present invention, a well crystallized boron nitride, having sharp and remarkable [
004
] and other diffraction peaks in the X-ray powder diffraction diagram, is termed “t-BN”. (Particularly, as explained later, the present invention intends a crystal phase having a specified small half peak height width of the [
004
] spectra 2&thgr; and having no ordering (pattern) in the laminating manner, as the crystalline t-BN).
Boron nitride crystals have crystal structures similar to graphite and exhibit properties similar to graphite crystals except for electrical insulating property. For example, boron nitride crystals have weak bonding strength between each hexagonal B-N mesh layers, and are easily cleaved into flakes to exhibit solid lubricity. Boron nitride is stable up to high temperatures under a nonoxidizing atmosphere, hard to sinter, with its sintered body having good machinability. Yet more, it is not oxidized up to about 1000 ° C., which is by about 500 ° C. higher than the oxidation resistant temperature of graphite. Boron nitride has attractive properties as a material. If boron nitride powder having superior sinterablity or high purity is produced cheaply and sintered bodies can be supplied in low cost, boron nitride will find many new applications which are not economically feasible at present.
RELATED ART
As methods for producing boron nitride, the following methods have been known.
(1) Heating a mixture of borax and urea in an atmosphere of ammonia (Japanese Patent Kokai publication JP-A-38-1610).
(2) Heat treating a mixture of boric acid or ammonium borate with nitrogen source material such as urea, ammonia, melamine, dicyandiamide etc. (Japanese Patent Kokoku publications JP-B-48-14559, JP-B-5-47483, Japanese Patent Kokai publication JP-A-7-172806, J. Am. Chem. Soc. Vol. 84. p 4619-4622, 1963).
(3) Heat treating boron powder in atmosphere of nitrogen and ammonia (Japanese Patent Kokoku publication JP-B-7-53610).
(4) Synthesize boron nitride by vapor reaction of boron chloride and ammonia under reduced pressure (Japanese Patent Kokoku publication JP-B-2-296706).
(5) Thermal decomposition of borazine or borazine derivatives (Japanese Patent Kokoku publication JP-B-4-4996).
(6) Boron nitride powder obtained by heat treating a mixture of urea and boric acid in ammonia is reported to have a turbostratic structure which is similar to turbostratic graphite structure (J. Am. Chem. Soc. Vol. 84. p 4619-4622, 1963).
SUMMARY OF THE DISCLOSURE
In the course of development toward the present invention, we encountered the following problems in the prior art:
It is disclosed that the product powder obtained by method (1) at a low temperature is a-BN, whereas if the synthesis is carried out at elevated temperatures, h-BN powder is obtained. The resultant boron nitride powders contain plenty of sodium ingredient and are not suited for electrical insulator applications. According to method (2), it is disclosed that a-BN results at synthesizing temperature below 1000° C., although the product obtained by synthesizing at higher temperatures is h-BN because boric acid promotes crystallization of h-BN. The product includes almost no impurities other than boric acid, but it is a problem from the view point of present invention that h-BN is apt to be produced because of coexisting boric acid.
As for method (3) or (5), cost of such starting materials as boron powder, borazine and borazine derivatives is expensive, entailing a high cost of produced boron nitride. Method (4) lacks in productivity and generates hydrogen chloride which is corrosive and has stimulus odor. To secure working environment, it necessitates expensive equipment to trap the hydorogen chloride. None of these reports mentions production of crystalline t-BN powder.
On the other hand from the view point of the present invention, method (6) explains only that a-BN powders exhibit two broad (i.e., with a large half peak height width) peaks, namely, a [
002
] diffraction peak at the [
002
] site of h-BN and a [
10
] diffraction peak at a site around the [
100
] and [
101
] sites of h-BN, respectively, while exhibits no [
004
] diffraction peak in the X-ray powder diffraction diagram(refer to FIG.
1
A). It is reported that the transformation from t-BN to h-BN begins when heated up to 1450° C., and is completed at about 1850° C. As the transformation (crystallization) proceeds, partial three dimensional ordering of atoms will occur (refer to FIG.
1
. B,C) and finally get to perfect three dimensional ordering of the hexagonal system. In the case of this method, impurity of boron oxide remaining in the resultant boron nitride powder is inevitable.
As explained above, in the known methods of prior art, there is no known method which is suited for the mass production of the crystalline t-BN powder, particularly crystalline t-BN powder having a higher extent crystallization.
It is a principal object of the present invention to provide a new crystalline t-BN powder.
In detail, it is further objects of the present invention to provide a solution for the above mentioned problems in the prior art and to provide a low cost boron nitride powder easy to sinter, especially a crystalline t-BN powder, and a novel method for mass-producing the crystalline t-BN powder.
In another aspect, it is a still further object of the present invention to provide a high purity crystalline t-BN powder.
Also it is a yet further obj
Langel Wayne
Morrison & Foerster / LLP
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