Method for directly synthesizing cubic boron nitride crystal by hot-pressing hexagonal boron nitride nanoparticles

Abstract

The invention relates to a method for directly synthesizing cubic boron nitride crystals by hot-pressing hexagonal boron nitride nano-particles, which comprises the following steps: by taking the hexagonal boron nitride nano-particles as a raw material, sequentially carrying out two times of hot pressing on a domestic cubic hydraulic press and a 1000T Kawai type high-pressure generating device to directly obtain the cubic boron nitride crystals. The invention provides a method for directly synthesizing cubic boron nitride crystals from hexagonal boron nitride under the condition of not adding a metal catalyst, and an obtained cubic boron nitride sample is high in Vickers hardness and fracture toughness.

Description

Technical Field

[0001] The invention belongs to the technical field of superhard material synthesis, and particularly relates to a method for directly synthesizing cubic boron nitride crystals from hexagonal boron nitride nanoparticles. Background Art

[0002] Cubic boron nitride has high hardness and high chemical stability, can withstand cutting temperatures up to 1200°C and does not react with ferrous metals at high temperatures. This high thermal stability and chemical stability make it perform well in high-temperature cutting, and it is particularly suitable for processing difficult-to-process materials such as high-temperature metals, hardened steel, and ferrous metals.

[0003] The methods for synthesizing cubic boron nitride include high temperature and high pressure method, vapor deposition method, hydrothermal method, benzene thermal method, and laser induced reduction method. Among them, the high temperature and high pressure method includes static high pressure catalyst method and impact compression method, which is the most important industrial production method for synthesizing cubic boron nitride crystals so far. However, there are still problems in the preparation of large-sized, high-quality cubic boron nitride single crystals by high temperature and high pressure method, such as purification of raw materials, selection of catalysts and purity control, precise control of reaction chamber temperature, and long reaction time. At present, the research on converting hexagonal boron nitride into cubic boron nitride by high temperature and high pressure method and exploring its mechanical properties is to add metal catalysts to reduce the temperature and pressure of the conversion of hexagonal boron nitride to cubic boron nitride, but the purity of the synthesized cubic boron nitride material will be affected by the catalyst, the impurity content and defect concentration are high, and its mechanical properties such as hardness and fracture toughness will also be greatly affected. In the absence of catalysts, hexagonal boron nitride is directly converted to cubic boron nitride crystals by high temperature and high pressure methods. It is not affected by metal catalysts and can significantly reduce the impurity content and defect density of the synthesized cubic boron nitride, improve the crystal quality and corresponding mechanical properties. However, the temperature and pressure conditions are often very high, and the capacity of the high pressure generating device is limited, resulting in relatively few studies on the mechanical properties of this type of cubic boron nitride crystals. In addition, hexagonal boron nitride nano-precursors with special morphology can also be directly converted into superhard phase cubic boron nitride crystals through high temperature and high pressure. This type of nano-precursor with special morphology often needs to undergo a complex processing process to meet the requirements for synthesizing superhard cubic boron nitride crystals.

[0004] Therefore, using simple hexagonal boron nitride precursors without adding catalysts and adopting high temperature and high pressure methods to directly convert them into cubic boron nitride crystals so that they have excellent mechanical properties is a difficulty and hot spot in current cutting-edge science and industrial applications. Summary of the invention

[0005] In order to solve the problems in the prior art, the purpose of the present invention is to provide a method for directly synthesizing cubic boron nitride crystals from hexagonal boron nitride nanoparticles. The dense hexagonal boron nitride block obtained by the first hot pressing treatment of hexagonal boron nitride nanoparticles is used as a new precursor, and then the hexagonal boron nitride block is subjected to a second hot pressing synthesis, and a cubic boron nitride single crystal block with excellent mechanical properties can be obtained under catalyst-free conditions.

[0006] To achieve this purpose, the technical solution adopted by the present invention is as follows:

[0007] (1) Select hexagonal boron nitride nanoparticles with a particle size of 200-300 nm, put them into a boron nitride tube with a diameter of 2 mm, put them into a graphite tube heating body, put them into a pyrophyllite assembly block, and put the whole into a six-sided hydraulic press cavity. Keep the temperature and pressure at 5 GPa and 1800°C for 20-40 minutes to form a block hexagonal boron nitride precursor.

[0008] (2) The bulk hexagonal boron nitride precursor is placed in the 10 / 5 assembly of a 1000T Kawai type high pressure generating device, and then placed in a press cavity at a pressure of 10-15GPa and a temperature of 1800-2000°C for 20 minutes to obtain pure cubic boron nitride crystals after hot pressing.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0010] (1) The present invention uses hexagonal boron nitride nanoparticles as the raw material. Under the conditions of the first hot pressing process, the transformation from the hexagonal phase to the cubic phase will not occur, but the hexagonal phase grains will grow and reverse and fill the pores. The hexagonal boron nitride block is obtained by hot pressing using a domestic six-sided hydraulic press, and the density is significantly improved, which provides a good precursor for the subsequent secondary hot pressing process to transform superhard cubic boron nitride crystals.

[0011] (2) The present invention uses a 1000T Kawai type high pressure generating device to perform a second hot pressing, applying a higher pressure to change the atomic arrangement in the boron nitride, and can directly convert hexagonal boron nitride into cubic boron nitride crystals without using a metal catalyst.

[0012] (3) The hexagonal boron nitride nanoparticles used in the present invention as the raw material can be directly converted into cubic boron nitride crystals after two consecutive hot pressings. The raw material does not need special treatment, thus avoiding the use of additional special equipment.

[0013] (4) The cubic boron nitride crystals prepared by the present invention have high transparency and excellent mechanical properties. Their hardness is much better than that of cubic boron nitride crystals synthesized using catalysts, and can be compared with superhard cubic boron nitride nanotwins. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The X-ray diffraction pattern (XRD) of the hexagonal boron nitride block obtained after the first hot pressing process at a pressure of 5 GPa and a temperature of 1800 °C for 40 minutes.

[0015] Figure 2 Scanning electron microscope (SEM) image of the original material of hexagonal boron nitride nanoparticles (a) and the hexagonal boron nitride block (b) obtained after the first hot pressing process at a pressure of 5 GPa and a temperature of 1800 ° C for 40 minutes.

[0016] Figure 3 X-ray diffraction pattern (XRD) of cubic boron nitride crystal synthesized after the second hot pressing process at a pressure of 15 GPa and a temperature of 2000°C for 20 minutes. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below in conjunction with embodiments, but the following description and illustration of the embodiments do not constitute any limitation to the protection scope of the present invention.

[0018] Example 1 Preparation of high-density hexagonal boron nitride bulk precursor

[0019] Hexagonal boron nitride nanoparticles with a particle size of 200-300nm are selected as raw materials, and are pressed into cylinders with a diameter of 2mm using a tablet press. The pressed cylinders are filled into boron nitride tubes, which are successively loaded into graphite tube heating bodies, and then inserted into pyrophyllite assembly blocks. The assembled pyrophyllite assembly blocks are placed as a whole in the cavity of a domestic six-sided hydraulic press. The pressure is 5GPa, the temperature is 1800℃, and the heat and pressure are maintained for 40 minutes to obtain a high-density hexagonal boron nitride block with a density of 95.6% and a hardness of 1244.9MPa. Figure 1 The XRD pattern of the hexagonal boron nitride block treated by the first hot pressing process is given. It can be seen that the hexagonal boron nitride does not undergo phase change after treatment and remains a single hexagonal crystal. Figure 2 This is the SEM image of the hexagonal boron nitride block treated by the first hot pressing process, compared with the original hexagonal boron nitride nanoparticle material. The original hexagonal boron nitride material is layered and has wrinkled edges, which provides space for the hexagonal boron nitride block to grow during the first hot pressing process. It is easy to form a dense block material without phase change. It can be seen that after hot pressing, the grains of the hexagonal boron nitride block are significantly larger, the pores are significantly smaller, and the density is significantly improved.

[0020] Example 2 Direct synthesis of cubic boron nitride crystals without catalyst

[0021] The high-density hexagonal boron nitride block was placed in the thermocouple assembly of the 1000T Kawai type high-voltage generator and inserted into the LaCrO 3The heated body is then placed in an octahedral press chamber, kept at a pressure of 15 GPa and a temperature of 2000° C. for 20 minutes to obtain a cubic boron nitride crystal block. Figure 3 The XRD diagram of the final product is given. It can be seen that the diffraction peaks of the synthesized sample correspond one to one with the diffraction peaks of cubic boron nitride c-BN of the standard card PDF#15-500, indicating that under this temperature and pressure condition, all the hexagonal boron nitride precursor h-BN can be effectively converted into cubic boron nitride crystal c-BN.

[0022] Comparative Example 1

[0023] In Example 1, when the temperature is lower than 1800°C, other conditions remain unchanged, and the operation steps are the same as in Example 1, the density of the obtained hexagonal boron nitride block will decrease. In the subsequent second hot pressing process, the hardness of the obtained cubic boron nitride crystal will be significantly reduced.

[0024] Comparative Example 2

[0025] In Example 1, the raw material is changed from hexagonal boron nitride nanoparticles to hexagonal boron nitride powder, and other conditions remain unchanged. The operation steps are the same as in Example 1. After the first hot pressing process, the density of the obtained hexagonal boron nitride block will decrease. In the subsequent second hot pressing process, the color transparency of the obtained cubic boron nitride crystals will decrease, and the hardness will also be significantly reduced.

[0026] Comparative Example 3

[0027] In Example 1, the raw material was changed from hexagonal boron nitride nanoparticles to porous hexagonal boron nitride nanomaterials, and other conditions remained unchanged. The operation steps were the same as in Example 1. After the first hot pressing process, no hexagonal boron nitride block could be obtained, and the powder was not formed. The subsequent second hot pressing process could not be performed.

Claims

1. A method for directly synthesizing cubic boron nitride crystals by hot pressing hexagonal boron nitride nanoparticles, characterized in that: For the following steps: (1) Filling hexagonal boron nitride nanoparticles into a boron nitride tube, and then placing them in a graphite tube, a pyrophyllite assembly block, and a domestic six-sided hydraulic press cavity in turn, under a pressure of 5 GPa, a temperature of 1800° C., and keeping the temperature for 20-40 minutes, after the first hot pressing, a block hexagonal boron nitride precursor is formed, with a density greater than 95% and a hardness greater than 1200 MPa; (2) The bulk hexagonal boron nitride precursor is again filled into the 10 / 5 assembly of the 1000T Kawai type high pressure generating device, and the pressure is 10-15GPa, the temperature is 1800-2000℃, and the temperature is kept for 20 minutes. After the second hot pressing, a cubic boron nitride crystal block is obtained, and the density is greater than 99%, the hardness is greater than 72GPa, and the toughness is greater than 9K IC (MPa·m 0.5 ).

2. The method for directly synthesizing cubic boron nitride crystals by hot pressing hexagonal boron nitride nanoparticles according to claim 1, characterized in that: The hexagonal boron nitride nanoparticles are self-developed, are in the form of white powder in macroscopic view, and are in the form of layers and particles in microscopic view. The average particle size is 200-300nm, and the purity is higher than 99.9%.

3. The method for directly synthesizing cubic boron nitride crystals by hot pressing hexagonal boron nitride nanoparticles according to claim 1, characterized in that: The density of the blocky hexagonal boron nitride precursor formed after the first hot pressing by a domestically produced six-sided hydraulic press is increased, which is conducive to obtaining cubic boron nitride with better mechanical properties at a higher pressure and temperature in the second hot pressing.

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