Boron nitride and process for preparing the same

Details Boron nitride and process for preparing the same Boron nitride and process for preparing the same

1999-02-05

2001-11-20

Le, Hoa T. (Department: 1773)

Stock material or miscellaneous articles

Coated or structually defined flake, particle, cell, strand,...

Rod, strand, filament or fiber

C423S290000, C428S372000

Reexamination Certificate

active

06319602

ABSTRACT:

TECHNICAL FIELD
The present invention relates to hexagonal boron nitride (hereinafter referred to as “h-BN”) polycrystalline compounds comprising fine crystals and processes for producing the same.
The invention relates also to h-BN and a process for producing the same.
The invention further relates to h-BN comprising fine particles of novel anisotropic shape and a process for producing the same. h-BN has found a wide variety of industrial uses in lubricating materials, high-temperature structural materials, molten metal containers, insulating heat sink plates, tool materials of high hardness, etc.
BACKGROUND ART
h-BN has a structure resembling that of graphite and comprising planar reticular hexagonal B-N arrangements as stacked, and is known as a compound which is outstanding in characteristics such as thermal conductivity, electrical insulating properties, heat resistance, corrosion resistance, chemical stability and lubricity. Active efforts are directed to the development of uses of this compound. To utilize these characteristics, h-BN is used in various fields in the form of a powder as a solid lubricant, heat-resistant parting agent and material for cubic BN (c-BN), or in the form of sintered bodies prepared from a powder thereof and serving as melting crucibles, electric insulating materials and electronic materials.
Attention has been directed in recent years to the heat resistance and heat dissipating property of the compound in the field of electric and electronic components, and further applications are expected to heat sink plates, for example, for use in computers.
The industrial processes already known for producing boron nitride includes: 1) process for nitriding a boron oxide by reduction, 2) process for nitriding elemental boron, and 3) process for nitriding a boron halide by reduction. The h-BN obtained by these processes has a scalelike form attributable to the particular process and is therefore subject to limitations when to be made into sintered bodies or into composite materials in combination with other ceramics or when to be used in the form of other composite materials. More specifically, the use of the scalelike h-BN as a material for sintered bodies inevitably involves formation of voids and consequently encounters difficulty in giving a compacted sintered body. The nitride has the drawback of giving rise to a similar problem also when to be made into composite materials along with other ceramics or resins.
On the other hand, JP-A-151202/1985 discloses a process for producing a boron nitride. According to the publication, a boron nitride in a scalelike, columnar or acicular form can be obtained from at least one boron compound selected from among boric acids and metal salts of boric acids, and at least one nitrogen-containing compound capable of reacting with the boron compound, by forming a compound first wherein boron and nitrogen atoms are conjointly present, and subsequently heating the resulting compound in an inert gas or reducing gas atmosphere at a temperature of at least 600° C. However the fibrous boron nitride available by this process is limited to not greater than 20 &mgr;m in fiber length if longest and still remains to be fully improved in size for use as a material for giving improved thermal conductivity.
The boron nitride obtained by this process is not in the form of fine particles of high quality on the order of nanometers, and can not be made into fine crystals of the order of nanometers even if pulverized.
A first object of the present invention is to provide a polycrystalline h-BN compound comprising fine crystals of the order of nanometers or a polycrystalline h-BN compound having a fibrous form, and a process for producing such a compound.
DISCLOSURE OF THE INVENTION
The present invention provides as a first feature thereof a polycrystalline h-BN compound comprising fine crystals of the order of nanometers and a polycrystalline h-BN compound having a fibrous form.
The invention further provides a process for producing a polycrystalline h-BN compound comprising fine crystals of the order of nanometers or a polycrystalline h-BN compound having a fibrous form which process is characterized by heat-treating a fibrous compound represented by the formula C
3
N
6
H
12
B
2
O
6
in a nonoxidizing atmosphere at a temperature of 1300 to 1800° C.
The polycrystalline h-BN compound of the present invention comprises fine crystals of the order of nanometers. These fine crystals are in the range of 20 to 200 nm (nanometers), preferably 50 to 200 nm, in particle size, and 5 to 50 nm in average thickness. The polycrytalline h-BN compound of the invention is preferably at least 30 &mgr;m to not greater than 5 mm in length.
The first feature of the invention will be described below.
In developing uses of functional fibrous compounds so far proposed by us, we conducted intensive research from the viewpoint that an increase in apparent volume fraction of a filler as incorporated in composite materials requires an increase in the size of the filler fibers, and consequently found that crystalline fibrous C
3
N
6
H
12
B
2
O
6
(compound) of high quality can be prepared by a simple process as a precursor of boron nitride (compound).
The fibrous compound having the composition of the formula C
3
N
6
H
12
B
2
O
6
is obtained by reacting a melamine compound with a boric acid or boron oxide in a suitable solvent with heating, cooling the reaction mixture to effect growth of crystals, filtering off a fibrous compound separating out and drying the compound preferably rapidly. The compound has a single-crystal structure which belongs to the monoclinic system and wherein the lattice constants are substantially a=3.600 Å, b=20.143 Å, c=14.121 Åand &bgr;=92.11°.
A single-crystal automatic X-ray structural analysis has revealed that this starting material has the following crystal structure.
The compound is useful as an intermediate of boron nitrides and various boron nitride compounds. The compound may contain small amounts of compounds having a structure other than the above crystal structure.
The fibrous compound having the composition of the formula C
3
N
6
H
12
B
2
O
6
can be about 30 &mgr;m to about 5 mm in average fiber length depending on the reaction conditions. The compound is about 10 to about 50 in average aspect ratio.
The present invention provides a process for producing a polycrystalline h-BN compound comprising fine crystals of the order of nanometers by heat-treating the compound having the composition of the formula C
3
N
6
H
12
B
2
O
6
in a nonoxidizing atmosphere.
Usable as the melamine compound in preparing the starting material of the invention is a compound having an NH
2
group such as melamine, ammeline, ammelide, melam or mellon. Usable as the boric acid is orthoboric acid, metaboric acid or tetraboric acid, and as the boron oxide is boron trioxide, diboron dioxide, tetraboron trioxide or tetraboron pentoxide. The boric acid or boron oxide, and the melamine compound are heated individually or together in a solvent and dissolved completely, following by cooling to cause molecular crystals to separate out, with two moles of boric acid combined with one mole of melamine compound by a hydrogen bond. The growth of these crystals affords the starting compound (precursor compound) for use in the invention. When the boric acid and the melamine compound are dissolved each singly, it is necessary to mix the boric acid solution and the melamine compound solution together to obtain the starting compound of the invention. It is desired that the two solutions be mixed together in such predetermined amounts that the mixture has a boric acid/melamine compound molar ratio of 2/1. The starting compound of the invention can be obtained even if an excess of boric acid or melamine compound is present, whereas it is then likely that the reaction mixture will have dissolved therein an amount of boric acid or melamine compound in excess of the solubility at the cooling temperature. In this case, the excessi

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