Pyrolytic rhombohedral boron nitride and method thereof

 

(57) Abstract:

The invention relates to refractory compounds, namely pyrolitic the rhombohedral boron nitride and technology of its receipt by the method of chemical deposition from the gas phase. Pyrolytic rhombohedral boron nitride is used for the manufacture of various parts (rods-sinks, output energy) for microwave devices. The technical result is the production of high-oriented pyrolytic boron nitride containing up to 100% of the rhombohedral phase, a set of not of the lamellae thickness of 50 - 70 nm, the reflection coefficient of 0.58 - 0,62, wavelength = 457 nm and a density of 2.21 - of 2.27 g/cm3. The boron nitride contains from 0 to 25% of the hexagonal phase. To obtain pyrolytic rhombohedral boron nitride with the specified characteristics and to increase the rate of deposition in the reactor serves separate streams of gaseous boron TRIFLUORIDE and ammonia, and boron TRIFLUORIDE before mixing with a stream of ammonia enriched boron before the formation of the lower fluorides of boron by passing it through a boron-containing mixture at a temperature of 1600 - 2000oC, and the deposition of boron nitride is carried out at vzaimodeistviyakh BF3and NH3from 1:1 to 1:6. As the boron-containing mixture using amorphous boron, boron carbide, waste of boron nitride or a mixture thereof. 2 S. and 3 C.p. f-crystals, 3 ill., table 1.

The invention relates to the field of refractory compounds, namely pyrolytic rhombohedral boron nitride and technology of its receipt by the method of chemical deposition from the gas phase. Pyrolytic rhombohedral boron nitride is used for the manufacture of various parts (rods-sinks, output energy) for appliances microwave applications.

Known pyrolytic rhombohedral boron nitride, having a form whiskers white (application Japan N 58-74511, class C 01 B 21/064, 1983 ). The boron nitride is obtained by chemical deposition from the gas phase in the interaction of gaseous streams of boron oxide and compounds containing the group CN-(HCN).

Known pyrolytic rhombohedral boron nitride is not compact material, and its production method is not practically feasible due to the use of highly toxic gaseous compounds of CYANOGEN.

Known pyrolytic rhombohedral boron nitride, having a polycrystalline structure, high purity (above 99.9% for cations) and PL is CLASS="ptx2">

Known pyrolytic rhombohedral boron nitride obtained by the way, the closest to the technical nature of the offer, which is fed through the reactor separate streams of gaseous trichloride boron (in the application materials listed on the possibility of obtaining boron nitride, using boron TRIFLUORIDE, but specific examples get no) and ammonia in gas-carrier (hydrogen), mixing and chemical deposition from the gas phase boron nitride by the reaction of

BCl3+ NH3= BN + 3HCl (1)

on the substrate at a temperature of up to 1700oC and pressure up to 2 mm Hg.

A disadvantage of the known pyrolytic rhombohedral boron nitride is the presence of an amorphous phase and, as a consequence, the imperfection of the crystal structure, which determines its opacity in the visible wavelength range and a milky-white color.

The disadvantages of the method of its production can be attributed to the low deposition rate, which ranges from 0.02 to 0.05 mm/h, which, in turn, reduces the efficiency of the process and increases the cost of the final product. This method also does not allow to obtain high-oriented pyrolytic boron nitride content ry high-oriented polycrystalline structure of the rhombohedral phase, consisting of crystalline lamellae fragments with identically oriented gratings and different stacking layers (N. Dist. Sarapin in. A. Lopatin, V. S. Dedkov and other properties of pyrolytic rhombohedral boron nitride. "Inorganic materials", 1996, T. 32, N 6, S. 690).

Known pyrolytic rhombohedral boron nitride has an opacity in thin layers and metallic Shine in the volume of material has a density of 2.20 - 2.24 g/cm3and contains from 15 to 75% of the rhombohedral phase and from 25 to 85% of the hexagonal phase. The material obtained by the method of chemical deposition from the gas phase at high temperatures and reduced pressure on the graphite substrate.

The disadvantages of the known pyrolytic boron nitride can be attributed to insufficiently high content of rhombohedral phase and goveroment layers of crystal fragments, and, as a consequence, a relatively high reflection coefficient, which characterizes the transparency of the material in thin layers, as well as the impossibility to achieve a density corresponding theoretical (2,275 g/cm3).

The objective of the present invention to provide a pyrolytic boron nitride having a high (up to 100%) content wysokosci density, as well as developing ways to obtain the boron nitride content of high-oriented rhombohedral phase to 100% and with high efficiency by increasing the speed of deposition of boron nitride.

The task is achieved by the fact that known pyrolytic rhombohedral boron nitride with high-oriented polycrystalline structure of the rhombohedral phase, consisting of crystalline lamellae fragments with identically oriented gratings and different paving grids, characterized by a metallic luster and a high density, contains up to 100% of the rhombohedral phase, which is a set of smooth, not of the lamellae thickness of 50 - 70 nm, has a reflection coefficient of 0.58 - 0,62, wavelength = 457 nm and a density of 2.21 - of 2.27 g/cm3.

In addition pyrolytic rhombohedral boron nitride contains from 0 to 25% of the hexagonal phase.

The task is achieved by the fact that in the known method of obtaining pyrolytic rhombohedral boron nitride, comprising feeding to the reactor separate streams of gaseous boron halide and ammonia, mixing and chemical deposition from the gas phase boron nitride on the substrate p is that before mixing with a stream of ammonia enriched boron before the formation of the lower fluorides of boron, by passing it through a boron-containing mixture at a temperature of 1600 - 2000oC, and the deposition of boron nitride is carried out at the interaction of gaseous lower boron fluoride with ammonia at a temperature 1550-1700oC.

In addition, as the boron-containing mixture using amorphous boron, boron carbide, waste of boron nitride or a mixture thereof, and the deposition of boron nitride is carried out at a pressure of 1-5 mm Hg.

Offer pyrolytic rhombohedral boron nitride contains up to 100% of the rhombohedral phase with high-oriented structure of the lamellae crystal fragments, in the absence of corrugated layers. It is the perfection of the structure, close to ideal, determines the transparency of boron nitride that in thin layers is confirmed by the relatively low reflection coefficient at a wavelength of =457 nm, equals 0.58-of 0.62, and a density value to 2.27 g/cm3close to theoretical (2,275 g/cm3). The proposed method uses instead of trichloride boron - TRIFLUORIDE boron enrichment which boron to lower fluorides of boron occurs according to the reaction:

[B] + BF3= 3BF and [B]+ 2BF3= 3BF2(2)

and their interaction with ammonia reactions

BF + NH3= BN + HF + H2

The method also has a high efficiency because the rate of deposition of boron nitride in amounts of 0.10 - 0.20 mm/h

In Fig. 1 shows a General view of the reactor used to obtain the claimed boron nitride proposed method, Fig. 2 - fractograms plates offer pyrolytic rhombohedral, obtained by the claimed method of boron nitride; Fig. 3 - x-pyrolytic rhombohedral boron nitride, with different content of the rhombohedral phase: a - 100%, b - 85%, 50%, g - 0%.

To obtain boron nitride is used reactor (Fig. 1), which is a two-chamber furnace of stainless steel. The reactor includes a housing 1, a settling chamber 2 within which is placed boron charge 3, the pipe 4 to enter incoming reagents having the outer tube 5 to enter boron TRIFLUORIDE and the inner tube 6 for ammonia, the deposition chamber 7 within which is placed the substrate 8 and the pipe 9 to output exhaust gases.

The outer tube 5 of the socket 4 in the chamber 2 is fed boron TRIFLUORIDE, which is about the Idov boron according to reaction (2), which are then introduced into the deposition chamber 7. At the same time through the pipe 6 pipe 4 into the chamber 7 of deposition served ammonia, where it is mixed with the thread formed in the prechamber 2 lower fluorides of boron. Feed streams BF3NH3is carried out at a ratio of 1:1 to 1:6.

In the chamber 7 of the deposition, at a temperature of 1550-1700oC, interaction of lower boron fluoride with ammonia according to reaction (3) deposition from the gas phase boron nitride on a substrate 8 at a pressure of 1 to 5 mm Hg. Gases formed in the reactor are removed through pipe 9.

As the boron-containing mixture used amorphous boron, boron carbide, waste of boron nitride or a mixture thereof. The substrate may be made of graphite, special ceramics, various refractory materials (carbides, nitrides, borides, silicides of refractory metals Mo, W, and others).

Below are examples illustrating the proposed boron nitride and method of its production.

Example 1

In the prechamber 2 reactor placed amorphous boron, formed on the starch, and then heat it to a temperature of 1800oC. the deposition Chamber 7 is heated to a temperature of 1650oC and vacuum until a pressure of 1 mm Hg. On the outer tube 5 of petracchi (2) with the formation of the lower fluorides of boron, then flows into the chamber 7 of the deposition. The inner tube 6 of the socket 4 in the deposition chamber 7 serves the flow of ammonia with a flow rate of 160 l/h

In the deposition chamber 7 by mixing ammonia with the lower fluorides of boron, coming from the pre-chamber 2, is their interaction according to reaction (3) deposition of boron nitride on a substrate 8 made of graphite. The deposition process is conducted for 10 hours and Then the reactor is cooled and the substrate 8 is removed, a layer of boron nitride whose thickness is 1.8 mm

The resulting boron nitride containing 99.99% of the main product, characterized by the presence of 85% rhombohedral (r-BN) and 15% of the hexagonal phase, which is confirmed by x-ray (position b) Fig. 3) obtained with the device "DRON-3" registered monochromatizing CuK-radiation. The average thickness of the lamellae is 65 nm, which was determined by x-ray structure analysis - modified harmonic analysis of x-ray line shape (Krawczyk A. E., Moshkina T. I., Osmanov. A. C. "laboratory", 1988, I. 54, No. 9, S. 67-71).

Received rhombohedral boron nitride is not homerefinance layers. This is reflected in Fig. 2, which presents fractograms (fracture) in the scanning mikros is close to theoretical. The material is transparent in thin layers and has a reflection coefficient of 0.60, wavelength = 457 nm, which was determined on a spectrophotometer reflection FO-1.

The process parameters and the characteristic properties of the obtained boron nitride from this example and all subsequent shows respectively in the table.

Examples 2-4

Getting rhombohedral boron nitride is carried out as described in example 1, by changing the process parameters - temperature pre-chamber and the deposition chamber, the ratio of the flows BF3and NH3and the pressure.

Example 5 (comparative)

The deposition chamber 7 is heated to a temperature of 1650oC and vacuum until a pressure of 1 mm Hg. The inner tube 6 of the socket 4 in the deposition chamber 7 serves the flow of ammonia with a flow rate of 240 l/h Through the pipe 5 pipe 4 serves stream of boron TRIFLUORIDE with a flow rate of 120 l/h of the Specified thread BF3passing through is not filled with boron-containing mixture of the prechamber 2, enters the chamber 7 deposition, where, mingling with the stream of ammonia interacts with him according to reaction (1) with the deposition of the resulting boron nitride on the substrate 8. The deposition process is carried out for 4 hours and Then the reactor is cooled and the substrate 8 is separated image is letestu 2,04 g/cm3with elaborations turbostratic structure (Fig. 3 (d), does not contain high-oriented rhombohedral phase and has a high goveroment layers. The reflection coefficient at wavelength = 257 nm is 0.78.

Examples 6 to 9

Obtaining boron nitride is carried out as in example 1, but in the examples 7 to 9, the change in pressure, and as a boron-containing mixture using boron carbide (example 6), waste boron nitride (example 7), a mixture of amorphous boron is formed on the starch, boron carbide and waste of boron nitride in an equal ratio (example 8), a mixture of boron carbide and waste of boron nitride in an equal ratio (example 9).

Example 10 (comparative)

Obtaining boron nitride is carried out as described in example 1, but instead of boron TRIFLUORIDE through the pipe 5 pipe 4 serves stream trichloride boron in the settling chamber 2. With the passage of trichloride boron through the prechamber 2 does not occur enrichment VSS boron before the formation of the lower chlorides of boron in the deposition chamber 7 is supplied BCl3. In the chamber 7 deposition BCl3mixed with ammonia interacts with him according to reaction (1) with the deposition of boron nitride on the substrate 8.

The resulting material contains 10% of the rhombohedral phase, but not in icient reflection at wavelength = 457 nm, equal to 0.69.

1. Pyrolytic rhombohedral boron nitride with high-oriented polycrystalline structure of the rhombohedral phase, consisting of crystalline lamellae fragments with identically oriented gratings and different paving grids, characterized by a metallic luster and a high density, characterized in that it contains up to 100% of the rhombohedral phase, a set of not of the lamellae thickness of 50 - 70 nm, has a reflection coefficient of 0.58 - 0,62, wavelength = 457 nm and a density of 2.21 - of 2.27 g/cm3.

2. Pyrolytic rhombohedral boron nitride under item 1, characterized in that it contains from 0 to 25% of the hexagonal phase.

3. The way to obtain pyrolytic rhombohedral boron nitride, comprising feeding to the reactor separate streams of gaseous boron halide and ammonia, mixing and chemical deposition from the gas phase boron nitride on a substrate at temperatures up to 1700oC and reduced pressure, characterized in that the boron halide is used boron TRIFLUORIDE, before mixing with a stream of ammonia enriched boron before the formation of the lower fluorides of boron by passing it through Bo is gazoobraznykh lower boron fluoride with ammonia at a temperature of 1550 - 1700oC.

4. The method according to p. 3, characterized in that as the boron-containing mixture using amorphous boron, boron carbide, waste of boron nitride or a mixture thereof.

5. The method according to p. 3 or 4, characterized in that the deposition of boron nitride is carried out at a pressure of 1 to 5 mm Hq.

 

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