IE80824B1

IE80824B1 - Method for producing cubic boron nitride

Info

Publication number: IE80824B1
Application number: IE940533A
Authority: IE
Inventors: Kousuke Shioi; Hidefumi Nakano
Original assignee: Showa Denko Kk
Priority date: 1993-07-09
Filing date: 1994-06-30
Publication date: 1999-03-10
Also published as: KR960013987A; DE4423987A1; DE4423987C2; IE940533A1; JP3400842B2; KR100351712B1; JPH0768153A

Abstract

Hexagonal boron nitride (hBN) is converted into cubic boron nitride (cBN) by treating it at temperature and pressure conditions in the stability range of cubic boron nitride in the presence of at least one compound selected from among amides and imides of metal elements of the groups Ia and IIa, or in the simultaneous presence of at least one metal selected from among elements of the groups Ia, IIa, IIIa, VIa, VIIa, VIII, IIb and IIIb.

Description

METHOD FOR PRODUCING CUBIC BORON NITRIDE 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 while having 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 5.5 GPa, 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. Of these, lithium-type solvents (catalysts) have been thoroughly studied, and lithium nitride and lithium boronitride are considered to be particularly effective solvents (catalysts) (see, for example, U.S. Patent No. 3,772,428).

Nevertheless, use of the above mentioned solvents (catalysts) does not provide sufficient yields, and consequently the above methods have not been satisfactory from an industrial point of view.

It is an object of the present invention, in light of these circumstances, to provide a method of converting hexagonal boron nitride to cubic boron nitride at a high conversion rate.

DISCLOSURE OF THE INVENTION In order to achieve the above mentioned object, the present invention provides a method for producing cubic - 2 boron nitride which is characterized by keeping hexagonal boron nitride under temperature and pressure conditions within the range of stability of cubic boron nitride, in the presence of one or more compounds selected from amides and imides of elements from Groups la and Ila of the Periodic Table, or in the presence both of one or more compounds selected from amides and imides of elements from Groups la and Ila of the Periodic Table and of one or more metals selected from elements of Groups la, Ila, Ilia, Via, Vila, VIII, lib and Illb of the Periodic Table, to convert the same to cubic boron nitride.

The hexagonal boron nitride to be used as the starting material may be commercially available hexagonal boron nitride (hBN) powder. Contaminant oxygen impurities in the form of boron oxide, etc. slow the conversion from hBN to cubic boron nitride (cBN), and thus materials with low oxygen contents are preferred.

The granularity thereof is not particularly restricted, but generally 150 mesh or lower is suitable. This is because too great a granularity may result in a lower reactivity with the solvent (catalyst).

The present invention is characterized in that the conversion from hBN to cBN is performed in the presence of at least one compound selected from amides and imides of Group la and Ila elements, or in the presence of both at least one compound selected from amides and imides of Group la and Ila elements and one or more metals selected from elements of Group la, Ila, Ilia, Via, Vila, VIII, lib and Illb elements. It has been found that performing the conversion in the presence of these compounds alone or together with the metals results in a dramatic improvement in the conversion rate over the method of the prior art. It is generally believed that the hBN reacts with alkali metals, alkaline earth metals or their compounds, functioning as a solvent, or catalyst, to promote the reaction to cBN, and it is likewise believed that the amide or imide compound or its combination with the above metals according to the present invention also functions in the same manner as a solvent or catalyst.

According to the present invention, the Periodic 5 Table is the long period type, and the elements in each Group are considered to be the following. la: Li, Na, K, Rb, Cs, Fr Ila: Be, Mg, Ca, Sr, Ba, Ra Ilia: Sc, Y, lanthanoids (atomic numbers 57 10 actinoids Via: Vila: VIII: lib: (atomic numbers 89-103) Cr, Mo, W Mn, Tc, Re Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt Zn, Cd, Hg 15 Illb: B, Al, Ga, In, Tl Both the amide or imide of a Group la or Ila element (hereunder referred to as simply amide or imide) and the metal selected from Group la, Ila, Ilia, Via, Vila, VIII, lib and lib elements preferably contain few oxygen impurities, as in the case of the hBN starting material, and usually powder of 150 mesh or lower is used.

The amount of the amide or imide, or the amide or imide and a metal, to be used is such that the total number of metal atoms making up the additive (the amide or imide, or the amide or imide and a metal) is 2 parts or more, and preferably 5-50 parts, to 100 parts by the number of boron atoms making up the hBN. If the amount of the additive is less than 2 parts, a long time will be required to obtain a sufficient conversion rate. On the other hand, since there is no improvement in the conversion rate even at greater than 50 parts, it is not economical, and therefore neither situation is preferred.

If one or more compounds selected from amides or imides of Group la and Ila elements are combined with one or more metals selected from Group la, Ila, Ilia, Via, Vila, VIII, lib and Illb elements, they may be used in any desired proportion, but the atomic ratio of the metal elements is preferably between 95:5 and 5:95. Outside of this range, the effect of the simultaneous addition of the above mentioned compounds and metals will not be sufficiently exhibited, and as a result it will be difficult to achieve a sufficiently high conversion rate.

Furthermore, when an amide or imide is used in combination with a metal, it is possible to obtain cBN at a higher conversion rate than when an amide or imide is used alone, even at lower temperature and pressure.

As a preferred mode of combining the above mentioned additives with the hexagonal boron nitride, their powders may be mixed together, but layers of the hexagonal boron nitride and the additives may also be arranged for alternate lamination in a reaction container.

Actually, the hBN and the amide or imide, or the amide or imide and a metal, are preferably compacted at about 1-2 t/cm2 pressure, either separately or after being filled into the reaction container. This will have an effect of improving the handleability of the crude powders while increasing the productivity by reducing the amount of shrinkage in the reaction container.

The reaction container may be a high-temperature, high-pressure generator capable of maintaining crude powders (hBN and additives) or their compacts, etc. under conditions of temperature and pressure in the range of stability of cBN. This range of stability (temperature and pressure) is reported in P. Bundy, R.H. Wentorf, J. Chem. Phys. 38(5), pp.1144-1149, (1963), and in most cases a minimum temperature and pressure of 1100°C and 3.8 GPa are effective; however, this will vary depending on the types and combination of the additives (solvent, catalyst), and conversion to cBN is also possible at lower than 1100°C, 3.8 GPa. The retention time is not particularly limited and should be enough to allow the desired conversion rate to be attained, but in most cases it is from about one second to 6 hours.

The hBN is converted to cBN by being kept in the above mentioned stability range, and if the temperature and pressure conditions are extremely high a near 100% conversion rate may be attained; however, usually a composite lump consisting of a mixture of hBN and cBN is obtained.

The composite lump is crushed to isolate the cBN.

The method used for the isolation may be the one described in Japanese Examined Patent Publication (Kokoku) No. 49-27757, wherein, for example, the composite lump is crushed to a size of 5 mm or smaller, preferably 1 mm or smaller, after which sodium hydroxide and a small amount of water are added thereto and heating is effected to about 300°C to selectively dissolve the hBN, and thus upon cooling, acid cleaning and filtration the cBN is obtained. Also, in the case of residue of a metal used as an additive, it may be removed using hydrochloric acid, nitric acid or the like.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a sectional view of a reaction container used for converting hBN to cBN in the Examples.

EXAMPLE Example 1 Lithium amide was added to hexagonal boron nitride with a granularity of 150 mesh or lower and containing, as impurities, 0.8 wt% of oxygen and 0.2 wt% of a metal impurity other than an alkali metal or alkaline earth metal, at an atomic ratio of 20 parts lithium to 100 parts of the boron atoms in the compound. This mixture was subjected to 1.5 ton/cm2 pressure to make a 26 πιιηφ x 32 mmh compact, which was kept in the reaction container shown in Fig. 1.

In the reaction container shown in Fig. 1, the outer wall 1 of the container is made of pyrophyllite as a pressure conveyor, into a cylindrical shape, while the inner side thereof is provided with a heater 2 consisting of a graphite cylinder and pyrophyllite 8 as a partitioning material. Also, the top and bottom ends of the container are each provided with a conducting steel ring 3 and a conducting steel plate 4, while the inner sides thereof are provided with a sintered alumina plate and pyrophyllite 6 as a pressure conveyor, and the space surrounded by this pyrophyllite 6 and the pyrophyllite 8 as a partitioning material is used as the · holding compartment 7 for holding the raw materials for the reaction.

The above mentioned compact was treated for 10 minutes in this reaction container, under conditions of 4.5 GPa and 1400°C.

The specimen was crushed in a mortar, and an X-ray powder diffraction instrument was used to determine the conversion rate to cubic boron nitride from the intensity ratio of the diffracted rays of the cubic boron nitride (111) and the hexagonal boron nitride (002) with respect to CuK a-rays, and the conversion rate was found to be 84%.

The cubic boron nitride may be isolated (purified) by adding sodium hydroxide and a small amount of water to the specimen prepared by crushing to about 1 mm or less in a mortar or the like, heating it to 300°C, following this with cooling, cleaning with distilled water and hydrochloric acid and filtration, and then drying the filtered residue.

Example 2 Magnesium amide was added to hexagonal boron nitride at an atomic ratio of 20 parts magnesium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 88%.

Example 3 Calcium amide was added to hexagonal boron nitride at an atomic ratio of 20 parts calcium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 82%.

Example 4 Lithium imide was added to hexagonal boron nitride at an atomic ratio of 20 parts lithium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 83%.

Example 5 Magnesium imide was added to hexagonal boron nitride at an atomic ratio of 20 parts magnesium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 85%.

Example 6 Calcium imide was added to hexagonal boron nitride at an atomic ratio of 20 parts calcium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 85%.

Example 7 Lithium amide and magnesium amide were added to hexagonal boron nitride at an atomic ratio of 10 parts each of lithium and magnesium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 93%.

Comparison 1 Lithium nitride was added to hexagonal boron nitride at an atomic ratio of 20 parts lithium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 7%.

Comparison 2 Lithium boronitride was added to hexagonal boron nitride at an atomic ratio of 20 parts lithium to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 14%.

Comparison 3 Magnesium was added to hexagonal boron nitride at an atomic ratio of 20 parts to 100 parts of the boron atoms in the compound, to synthesize cubic boron nitride in the same manner as in Example 1, and upon evaluation the conversion rate to cubic boron nitride was found to be 54%.

Example 8 Hexagonal boron nitride containing, as impurities, 0.8 wt% of oxygen and 0.2 wt% of a metal impurity, was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and magnesium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was subjected to 1.5 ton/cm pressure to make a 26 ιηπιφ x 32 mmh compact, which was kept in the reaction container shown in Fig. 1, in the same manner as in Example 1.

The above mentioned compact was treated for 10 minutes in this reaction container, under conditions of 4.0 GPa and 1200°C.

The specimen was crushed in a mortar, and an X-ray powder diffractor was used to determine the conversion rate to cubic boron nitride from the intensity ratio of the diffracted rays of the cubic boron nitride (111) and the hexagonal boron nitride (002) with respect to CuK arays, and the conversion rate was found to be 96%.

Example 9 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and lithium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 73%.

Example 10 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and chromium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 80%.

Example 11 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and manganese at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 81%.

Example 12 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and iron at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 79%.

Example 13 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and cobalt at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 80%.

Example 14 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and nickel at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 88%.

Example 15 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and zinc at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 85%.

Example 16 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and aluminum at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 72%.

Example 17 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and lanthanum at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 76%.

Example 18 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and cerium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 77%.

Example 19 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and praseodymium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 78% .

Example 20 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and neodymium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 77%.

Example 21 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and samarium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 76%.

Example 22 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium and gadolinium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 76%.

Example 23 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium, magnesium at an atomic ratio of 10 parts and nickel at an atomic ratio of 5 parts, to 100 parts of the boron atoms in the compound. This was used to synthesize < cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 95%.

Example 24 Hexagonal boron nitride was mixed with weighed out portions of lithium amide at an atomic ratio of 10 parts lithium, magnesium amide at an atomic ratio of 10 parts magnesium and manganese at an atomic ratio of 5 parts, to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 86%.

Example 25 Hexagonal boron nitride was mixed with weighed out portions of lithium imide at an atomic ratio of 10 parts lithium and magnesium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 74%.

Comparison 4 Hexagonal boron nitride was mixed with a weighed out portion of lithium nitride at an atomic ratio of 10 parts lithium to 100 parts of the boron atoms in the compound.

This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 0%.

Comparison 5 ' Hexagonal boron nitride was mixed with a weighed out portion of lithium boronitride at an atomic ratio of 10 parts lithium to 100 parts of the boron atoms in the > 5 compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 0%.

Comparison 6 Hexagonal boron nitride was mixed with a weighed out portion of magnesium at an atomic ratio of 10 parts to 100 parts of the boron atoms in the compound. This was used to synthesize cubic boron nitride in the same manner as in Example 8, and upon evaluation the conversion rate to cubic boron nitride was found to be 0%.

According to the present invention, it is possible to considerably improve the conversion rate in methods of synthesizing cubic boron nitride using hexagonal boron nitride as the starting material.

Claims

1. A method for producing cubic boron nitride, characterized in that hexagonal boron nitride is treated under temperature and pressure conditions within the range of stability of cubic boron nitride in the presence of one or more compounds selected from amides and imides of elements from Groups la and Ila of the Periodic Table, to convert it to cubic boron nitride.

2. A method according to claim 1, characterized in that said one or more amides and imides are used in such an amount that the total number of metal atoms making up the compounds is 2 parts or more to 100 parts of the total number of boron atoms of the hexagonal boron nitride.

3. A method according to claim 2, characterized in that said one or more amides and imides of metals are used in such an amount that the total number of the metal atoms of the compounds is 5 to 50 parts to 100 parts of the total number of boron atoms of the hexagonal boron nitride.

4. A method according to any of the claims 1 to 3, characterized in that said region of stability of cubic boron nitride comprises a temperature of 1100‘c or higher and a pressure of 3.8 GPa or higher.

5. A method according to any of the claims 1 to 4, characterized in that the converted cubic boron nitride is crushed, then mixed with sodium hydroxide and water and afterwards heated to selectively dissolve the hexagonal boron nitride, followed by cooling, acid cleaning and filtration to isolate the cubic boron nitride.

6. A method for producing cubic boron nitride, characterized in that hexagonal boron nitride is treated under temperature and pressure conditions within the range of stability of cubic boron nitride in the presence both of one or more compounds selected from amides and imides of elements from Groups la and Ila of the Periodic Table and of one or more metals selected from elements of Groups la, Ila, Ilia, Via, Vila, VIII, lib and Illb of the Periodic Table, to convert it to cubic boron nitride.

7. A method according to claim 6, characterized in that said one or more amides and imides of elements from Groups la and Ila and said one or more metals from Groups la, Ila, Ilia, Via, Vila, VIII, lib and Illb are used in such amounts that the total number of the metal atoms of the compounds and the latter metals is 2 parts or more to 100 parts of the number of boron atoms of the hexagonal boron nitride.

8. A method according to claim 7, characterized in that said one or more compounds of amides and imides of elements from Groups la and Ila 5 and said one or more metals from Groups la, Ila, Ilia, Via, Vila, VIII, lib and I'llb are used in such amounts that the total number of the metal atoms of the compounds and the latter metals is 5 to 50 parts or more to 100 parts of the number.of boron atoms of the hexagonal boron nitride.

9. A method according to. any of the claims 6 to 8, characterized in 10. That said region of stability of cubic boron nitride comprises temperature of 1100’C or higher and a pressure of 3.8 GPa or higher.

10. A method according to any of the claims 6 to 9, characterized in that the conversion of the hexagonal boron nitride to the cubic boron nitride provides a composite lump 15 comprising the converted cubic boron nitride and unconverted hexagonal boron nitride and this lump is crushed, then mixed with sodium hydroxide and water and afterwards heated to selectively dissolve the hexagonal boron nitride, followed by carrying out cooling, acid cleaning and filtration to isolate 20 the cubic boron nitride.

11. A method according to claim 1 for producing cubic boron nitride, substantially as hereinbefore described and exemplified.

12. Cubic boron nitride whenever produced by a 25 method claimed in a preceding claim.