Chemistry of inorganic compounds – Boron or compound thereof – Binary compound
Reexamination Certificate
2001-02-07
2003-01-21
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Boron or compound thereof
Binary compound
Reexamination Certificate
active
06508996
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved method of synthesizing cubic boron nitride from hexagonal boron nitride.
2. Description of the Related Art
Cubic boron nitride is second only to diamond in hardness but has a greater chemical stability and, therefore it is becoming increasingly more important as a grinding, polishing and cutting material. A variety of methods have been proposed for producing cubic boron nitride, but the most well-known of these, which is widely used industrially, is a method of converting hexagonal boron nitride to cubic boron nitride under high-temperature, high-pressure conditions of about 4.5-6.0 GPa and about 1400-1600° C. in the presence of a solvent (catalyst). The well-known solvents (catalysts) for this method have conventionally been nitrides and boronitrides of alkali metals and alkaline earth metals. (see, for example, U.S. Pat. No. 3,772,428).
Nevertheless, the cubic boron nitrides obtained by use of said solvents (catalysts) have problems of low toughness and heat resistance, resulting in breakage of abrasives or of significant decrease in the strength of abrasives when exposed to high temperature. Also, the cubic boron nitrides obtained by use of said solvents (catalysts) have irregular shapes or nearly spherical shapes and are poor in the development of euhedral planes.
SUMMARY OF THE INVENTION
The above problems can be solved in accordance with the present invention by providing a method for producing cubic boron nitride which is characterized by keeping hexagonal boron nitride under temperature and pressure conditions within the range of stability of cubic boron nitride, (1) in the presence of (i) at least one compound selected from the group consisting of amides, imides and carbides of alkali metals and alkaline earth metals and (ii) a silicon source; or (2) in the presence of (i) at least one compound selected from the group consisting of amides and imides of alkali metals and alkaline earth metals, (ii) at least one selected from the group consisting of carbides of alkali metals and alkaline earth metals and (iii) a boron source; or (3) in the presence of (i) at least one compound selected from the group consisting of amides, imides and carbides of alkali metals and alkaline earth metals, (ii) a silicon source and (iii) a boron source, to convert the hexagonal boron nitride to cubic nitride.
(1) In the presence of (i) at least one compound selected from the group consisting of amides, imides and carbides of alkali metals and alkaline earth metals and (ii) a silicon source, the obtained cubic boron nitride has an improved toughness and heat resistance. (2) In the presence of (i) at least one compound selected from the group consisting of amides and imides of alkali metals and alkaline earth metals, (ii) at least one selected from the group consisting of carbides of alkali metals and alkaline earth metals and (iii) a boron source, the obtained cubic boron nitride has sharp edges with developed euhedral planes and is excellent in its cutting ability. (3) In the presence of at least one compound selected from the group consisting of amides, imides and carbides of alkali metals and alkaline earth metals, (ii) a silicon source and (iii) a boron source, the obtained cubic boron nitrides has both an improved toughness and heat resistance as well as an improved cutting ability.
The hexagonal boron nitride used as the starting material may be commercially available hexagonal boron nitride powders. Since oxygen impurities contaminated in the form of boron oxide or the like may retard conversion of the hexagonal boron nitride to cubic boron nitride, the starting materials with a less oxygen content are desired. The particle size of the hexagonal boron nitride is not particularly limited, but 150 mesh or less is generally preferred. If the particle size is too large, the reactivity with the solvent (catalyst) may be lowered.
The carbides, amides and imides of alkalimetals, alkaline earth metals are also preferably those having a low content of oxygen, which is similar to the hexagonal boron nitride of the starting material. The particle size is not particularly limited but 1 mm or less is generally preferred. If the particle size of these compounds is too large, the reactivity with the hexagonal boron nitride is lowered.
The amides and imides of alkali metals and alkaline earth metals used in the present invention are basically
LiNH
2
, NaNH
2
, KNH
2
, RbNH
2
, CsNH
2
,
Li
2
NH, Na
2
NH, K
2
NH, Rb
2
NH, Cs
2
NH,
Be(NH
2
)
2
, Mg(NH
2
)
2
, Ca(NH
2
)
2
, Sr(NH
2
)
2
, Ba(NH
2
)
2
,
BeNH, MgNH, CaNH, SrNH, BaNH, etc.,
and the carbides of alkali metals and alkaline earth metals used in the present invention are basically
Li
2
C
2
, Na
2
C
2
, K
2
C
2
, Rb
2
C
2
, Cs
2
C
2
,
Be
2
C, BeC
2
, MgC
2
, Mg
2
C
3
, CaC
2
, SrC
2
, BaC
2
, etc.,
but solid solutions, complex compounds, non-stoichiometric compounds, etc. of the above compounds may be also used to obtain similar effects.
The silicon source and the boron source used may be Si, B
4
Si, Si
3
N
4
, B, B
4
C, SiC, metal silicides, metal silicofluorides, metal silicohydrides, metal siliconitrides, organic silicon compounds, metal borosilicides, metal borofluorides, metal borohydrides, ammonium borofluorides, ammonium silfluorides, etc.
Preferable compounds as the amides, imides and carbides of alkali metals and alkaline earth metals are amides, imides and carbides of Li, Mg and Ca. The amides, imides and carbides of alkali metals and alkaline earth metals other than Li, Mg and Ca allow conversion of hexagonal boron nitride to cubic boron nitride only at a temperature and pressure higher than those in the case of the amides, imides and carbides of Li, Mg or Ca. The cubic boron nitride obtainable by using amides, imides or carbides of Li, Mg or Ca is more excellent in grinding ratio and required power for grinding than that obtainable by the other amides, imides or carbides.
Preferable combinations of at least one first compound of amides and imides of alkali metals and alkaline earth metals with at least one second compound of carbides of alkali metals and alkaline earth metals are combinations of at least one amide and/or imide of Li, Mg and/or Ca with at least one carbide of Li, Mg and/or Ca. The amides, imides and carbides of alkali metals and alkaline earth metals other than Li, Mg and Ca allow conversion of hBN to cBN only at a temperature and pressure higher than those in the case of amide or imide or carbide of Li, Mg or Ca. The cBN obtainable by using an amide, imide or carbide of alkali and alkaline earth metals other than Li, Mg and Ca has a grinding ratio and a required power for grinding interior to those obtainable by an amide, imide or carbide of Li, Mg or Ca to some extent.
A particularly preferable combination is LiNH
2
with CaC
2
. This combination allows a cubic boron nitride with less defects and excellent in its transparency to be obtained, by which a cubic boron nitride with particularly excellent properties due to added silicon source and boron source can be obtained under relatively low temperature and pressure conditions.
Preferred silicon and boron sources are Si and B. These are easy to handle and are available and have a high reactivity, providing the desired effects in a relatively short reaction time.
The present invention is characterized by conducting conversion of hBN to cBN (1) in the presence of (i) at least one compound selected from the group consisting of amides, imides and carbides of alkali metals and alkaline earth metals and (ii) a silicon source, or (2) in the presence of (i) at least one compound selected from the group consisting of amides and imides of alkali metals and alkaline earth metals, (ii) at least one selected from the group consisting of carbides of alkali metals and alkaline earth metals and (iii) a boron source, or (3) in the presence of (i) at least one compound selected from the group consisting of amides, imides and carbides of alkali metals and alkaline earth metals, (ii) a silic
Ihara Eiji
Shioi Kousuke
Langel Wayne A.
Showa Denko K.K.
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