CN111423232B

CN111423232B - Method for preparing compact polycrystalline diamond and boron-doped polycrystalline diamond

Info

Publication number: CN111423232B
Application number: CN202010483868.7A
Authority: CN
Inventors: 王明智; 唐虎; 袁小红
Original assignee: Yanshan University
Current assignee: Yanshan University
Priority date: 2020-06-01
Filing date: 2020-06-01
Publication date: 2021-06-29
Anticipated expiration: 2040-06-01
Also published as: CN111423232A

Abstract

The invention provides a method for preparing compact polycrystalline diamond and boron-doped polycrystalline diamond, and relates to the technical field of high-performance materials. Mixing carbon nano-onions with diamond cores with a boron source to obtain a mixture; the boron source is elemental boron or boron oxide; then, sequentially carrying out primary prepressing molding and secondary high-pressure prepressing molding on the mixture to obtain a preformed material; sintering the preformed material at high temperature and high pressure to obtain a compact boron-doped polycrystalline diamond block; the conditions of the high-temperature high-pressure sintering comprise: the sintering temperature is 1200-2000 ℃, the sintering pressure is 5-10 GPa, and the heat preservation and pressure maintaining time is 1-200 min. According to the invention, the carbon nano-onion with the diamond core is taken as a carbon raw material, and the boron element is doped, so that the synthesis condition of the polycrystalline diamond can be reduced to a great extent, and the method is suitable for industrial production; and the performance of the polycrystalline diamond is improved.

Description

Method for preparing compact polycrystalline diamond and boron-doped polycrystalline diamond

Technical Field

The invention relates to the technical field of high-performance materials, in particular to a method for preparing compact polycrystalline diamond and boron-doped polycrystalline diamond.

Background

Diamond is currently the hardest material known in the world. Since the 50 s of the last century, synthetic diamonds and their products have been widely used in the machining field as cutting and polishing tools. However, synthetic diamond cannot be directly used as a cutting tool because its particles are fine, and isotropic polycrystalline diamond is generally obtained by sintering fine diamond powder together by means of powder sintering. However, due to thermodynamic considerations, diamond is a metastable phase at atmospheric pressure and a certain high temperature may promote its transformation to graphite. At the same time, diamond has a very high decomposition temperature, meaning that polycrystalline diamond is not feasible at atmospheric pressure directly by self-diffusion sintering of diamond powder. In order to ensure that the sintering conditions during sintering are in the phase stable region of diamond (above the equilibrium line of the temperature and pressure of diamond and graphite), an appropriate sintering condition is important for sintering diamond. However, to satisfy both of these two factors, sintering of nano polycrystalline diamond requires very harsh sintering conditions, which are difficult to achieve at high temperature and high pressure in the industrial level.

Based on the above facts, for decades, polycrystalline diamond produced industrially is generally sintered by adding an additive. Sintering aids are generally classified as metals and ceramics, and their properties are much lower than those of diamond, thereby greatly impairing the properties of polycrystalline diamond. Thus, the ability to industrially synthesize additive-free polycrystalline diamond has long been a desire of those skilled in the art. In 2003, Irifune, university of love of Japan teaches and collaborators to successfully prepare nano polycrystalline diamond under high temperature and high pressure conditions by taking polycrystalline graphite as a precursor (Irifune T, et al. Nature,2003,421(6923): 599-. Because the crystal grains are fine (10-30 nm), the synthesized nano polycrystalline diamond has very high hardness, the Knoop hardness of the nano polycrystalline diamond is as high as 110-140 GPa, and the hardness of the nano polycrystalline diamond exceeds that of a natural diamond single crystal. Subsequent studies demonstrated that nano polycrystalline Diamond performs well as a cutting tool than single crystal Diamond and traditional polycrystalline Diamond (SUMIYA H, et al. Diamond and Related Materials,2012,24: 44-48; Harano K, et al. Diamond and Related Materials,2012,24: 78-82.). Nano polycrystalline diamond is therefore considered to be the most promising new generation of cutting tools.

Unfortunately, the harsh synthesis conditions (15 GPa or more and 2300 ℃) limit the industrialized production of the nano polycrystalline diamond, and the small-amount production causes the market to be unacceptable due to the over-high selling price caused by the cost.

Disclosure of Invention

In view of the above, the present invention aims to provide a method for preparing a dense polycrystalline diamond and a boron-doped polycrystalline diamond. The method provided by the invention can greatly reduce the synthesis conditions of the polycrystalline diamond and improve the performance of the polycrystalline diamond.

In order to achieve the above object, the present invention provides the following technical solutions:

the invention provides a method for preparing compact polycrystalline diamond, which comprises the following steps:

(1) mixing the carbon nano-onions with the diamond cores with a boron source to obtain a mixture; the boron source is elemental boron or boron oxide;

(2) sequentially carrying out primary prepressing forming and secondary high-pressure prepressing forming on the mixture to obtain a preformed material; the pressure of the preliminary pre-pressing molding is 300-500 MPa; the pressure of the secondary high-pressure pre-pressing forming is 3-5 GPa, and the temperature is 600-800 ℃;

(3) sintering the preformed material at high temperature and high pressure to obtain a compact boron-doped polycrystalline diamond block; the conditions of the high-temperature high-pressure sintering comprise: the sintering temperature is 1200-2000 ℃, the sintering pressure is 5-10 GPa, and the heat preservation and pressure maintaining time is 1-200 min.

Preferably, the boron source in step (1) is chemically pure, and the particle size of the boron source is less than or equal to 5 microns.

Preferably, the mass of the boron source in the step (1) is 0.1-10% of the total mass of the mixture.

Preferably, the mass of the boron source in the step (1) is 1-3% of the total mass of the mixture.

Preferably, the carbon nano-shallot with the diamond core in the step (1) is obtained by taking detonation nano-diamond as a raw material and performing vacuum annealing treatment, wherein the vacuum degree of the vacuum annealing treatment is 1-5 Pa, the annealing temperature is 1000-1350 ℃, and the heat preservation time is 0-60 min; the particle size of the carbon nano-onion with the diamond core is 2-20 nm.

Preferably, the mixing in step (1) is ball milling mixing.

Preferably, the sintering temperature in the step (3) is 1200-1800 ℃, the sintering pressure is 5-8 GPa, and the heat preservation and pressure maintaining time is 20-50 min.

Preferably, the high-temperature high-pressure sintering in the step (3) is specifically as follows: increasing the pressure to the sintering pressure within 1-200 min; and then maintaining the pressure, raising the temperature to the sintering temperature at a temperature rise rate of 10-100 ℃/min, and carrying out heat preservation and pressure maintaining at the sintering pressure and the sintering temperature.

The invention provides the compact boron-doped polycrystalline diamond prepared by the method in the scheme.

The invention provides a method for preparing compact polycrystalline diamond, which comprises the following steps: (1) mixing the carbon nano-onions with the diamond cores with a boron source to obtain a mixture; the boron source is simple substance boron or boron oxide, and the mass of the boron source is 0.1-10% of the total mass of the mixture; (2) sequentially carrying out primary prepressing forming and secondary high-pressure prepressing forming on the mixture to obtain a preformed material; the pressure of the preliminary pre-pressing molding is 300-500 MPa; the pressure of the secondary high-pressure pre-pressing forming is 3-5 GPa, and the temperature is 600-800 ℃; (3) sintering the preformed material at high temperature and high pressure to obtain a compact boron-doped polycrystalline diamond block; the conditions of the high-temperature high-pressure sintering comprise: the sintering temperature is 1200-2000 ℃, the sintering pressure is 5-10 GPa, and the heat preservation and pressure maintaining time is 1-200 min. The invention takes carbon nano-onion with diamond cores as a carbon raw material, boron elements (simple substance boron or boron oxide) are doped, boron and carbon of a shell layer of the carbon nano-onion form boron carbide in the heating and temperature rising process, the temperature is continuously raised, the boron carbide is decomposed into active carbon atoms and boron atoms, the carbon atoms rapidly migrate, most of the carbon atoms migrate to the surface of the original diamond core, the existing diamond cores grow up, a small part of the existing diamond cores are converted into diamond structures in situ, and the diamonds are rapidly connected to form polycrystalline diamond, so that rapid densification is achieved, and the boron atoms enter diamond lattices in the process of converting the carbon into the polycrystalline diamond structures. In the invention, boron is used as an inducer for carbon atom diffusion at high temperature and high pressure, the carbon atoms are induced to diffuse rapidly when the polycrystalline diamond is synthesized at high temperature and high pressure, and grow on the surface of the original diamond core, so that the energy required by nucleation is reduced, the aim of further reducing the synthesis condition of the polycrystalline diamond is fulfilled, the density is improved, and the hardness of the polycrystalline diamond is enhanced; when polycrystalline diamond is in an oxidizing atmosphere, boron remaining on the surface is readily oxidized to boron oxide, which prevents further oxidation of oxygen atoms, thereby improving the oxidation resistance of the polycrystalline diamond.

The method provided by the invention can greatly reduce the polycrystalline diamondThe synthesis conditions of (1) preparing compact polycrystalline diamond under relatively low temperature (1200-2000 ℃) and pressure (5-10 GPa), greatly reducing the synthesis time, improving the synthesis efficiency and being suitable for industrial production, wherein the synthesis conditions are far lower than the conditions reported at present internationally and domestically; and the performance of the polycrystalline diamond is improved. The results of the embodiment of the invention show that the obtained polycrystalline diamond has the grain size of 10-2000 nm, the Vickers hardness of 61-158 GPa, and the fracture toughness of 4.7-16.8 MPa-m^0.5(ii) a The initial oxidation temperature of the polycrystalline diamond in the air is 716-1276 ℃ when the heating rate is 5-10 ℃/min. In addition, the size of polycrystalline diamond is increased due to the reduction of synthesis pressure, thereby facilitating practical use.

Drawings

FIG. 1 is a schematic structural view of a carbon nano-onion with a diamond core according to the present invention;

fig. 2 is an X-ray diffraction pattern of polycrystalline diamond prepared in example 1;

fig. 3 is a transmission electron micrograph of polycrystalline diamond prepared according to example 1;

fig. 4 is an optical photograph of the polycrystalline diamond mass prepared in example 1;

fig. 5 is a graph of vickers hardness indentation of polycrystalline diamond prepared in example 1.

Detailed Description

(2) sequentially carrying out primary prepressing forming and secondary high-pressure prepressing forming on the mixture to obtain a preformed material; the pressure of the preliminary pre-pressing molding is 300-500 MPa, and the temperature is room temperature; the pressure of the secondary high-pressure pre-pressing forming is 3-5 GPa, and the temperature is 600-800 ℃;

Mixing carbon nano-onions with diamond cores with a boron source to obtain a mixture; the boron source is elemental boron or boron oxide, and the elemental boron is amorphous boron or crystalline boron. In the invention, the particle size of the carbon nano-onion with the diamond core is preferably 2-20 nm. In the invention, the carbon nano-onion with the diamond core is preferably obtained by taking detonation nano-diamond as a raw material and carrying out vacuum annealing treatment. In the invention, the vacuum degree of the vacuum annealing treatment is preferably 1-5 Pa, the annealing temperature is preferably 1000-1350 ℃, and the heat preservation time is preferably 0-60 min. The detonation nanodiamond has no special requirement, and the detonation nanodiamond known by the technical personnel in the field can be adopted; the present invention does not require any particular embodiment of the annealing treatment, and the annealing method for preparing carbon nano-onions having diamond cores, which is well known to those skilled in the art, may be used.

In the present invention, the structure of the carbon nano-onion with diamond core is shown in FIG. 1, and is represented by SP²A carbon shell layer and a diamond core. In the present invention, the carbon nano-onions have diamond cores, so that carbon atoms can easily grow on the surface of the existing diamond cores without requiring higher energy, thereby reducing the synthesis pressure.

In the invention, the boron source is preferably chemically pure, and the particle size of the boron source is preferably less than or equal to 5 micrometers; the mass of the boron source is preferably 0.1-10% of the total mass of the mixture, and more preferably 1-3%. The source of the boron source is not particularly critical to the present invention and commercially available products well known to those skilled in the art may be used. In the present invention, the mixing is preferably ball milling mixing, which is preferably performed in an inert atmosphere, which is preferably argon. The invention has no special requirements on the specific conditions of ball milling and mixing, and the carbon nano-onion with the diamond core can be uniformly mixed with the boron source.

After the mixture is obtained, the invention sequentially carries out primary prepressing forming and secondary high-pressure prepressing forming on the mixture to obtain the preformed material. In the invention, the pressure of the preliminary pre-pressing molding is preferably 300-500 MPa, and the temperature is preferably room temperature; the preliminary pre-pressing molding is preferably performed by a hydraulic press. In the invention, the pressure of the secondary high-pressure pre-pressing molding is preferably 3-5 GPa, the temperature is preferably 600-800 ℃, more preferably 650-750 ℃, and the secondary high-pressure pre-pressing molding is preferably carried out by adopting a cubic press; the invention realizes the preliminary densification of the raw materials by secondary high-pressure prepressing molding and eliminates the adsorbed gas on the surfaces of the carbon nano onion particles.

After the preformed material is obtained, the preformed material is sintered at high temperature and high pressure to obtain the compact boron-doped polycrystalline diamond block. In the present invention, the conditions of the high-temperature high-pressure sintering include: the sintering temperature is 1200-2000 ℃, and preferably 1200-1800 ℃; the sintering pressure is preferably 5-10 GPa, and more preferably 5-8 GPa; the heat preservation and pressure maintaining time is 1-200 min, preferably 20-50 min. The invention preferably utilizes a six-face top or a six-face top additionally provided with a 6-8 type multistage supercharging device to generate high temperature and high pressure for high temperature and high pressure sintering; when the sintering pressure is more than or equal to 7GPa, the 6-8 type multistage supercharging device is preferably additionally arranged on a six-surface top. In the present invention, the high-temperature high-pressure sintering is specifically preferably: increasing the pressure to the sintering pressure within 1-200 min; and then maintaining the pressure, raising the temperature to the sintering temperature at a temperature rise rate of 10-100 ℃/min, and carrying out heat preservation and pressure maintaining at the sintering pressure and the sintering temperature. After the heat preservation and pressure maintaining time is reached, the temperature and the pressure are preferably synchronously reduced to the atmospheric environment. In the invention, the time for raising the sintering pressure is preferably 30-180 min, and more preferably 60-150 min; the heating rate is preferably 20-80 ℃/min, and more preferably 30-50 ℃/min. The invention adopts the high-temperature high-pressure sintering procedure to ensure that all reactions are carried out under a high-pressure state.

The invention takes carbon nano shallot with diamond core as carbon raw material, boron element (simple substance boron or boron oxide) is doped, boron element in situ and carbon element generate rapid chemical combination reaction to generate boron carbide in the heating and temperature rising process (high pressure condition), the boron element in situ and the carbon element are rapidly decomposed into boron and carbon when reaching the sintering and heat preservation temperature, so that the carbon atom is rapidly migrated (the carbon atom has high chemical activity and is easy to be converted into diamond crystal structure), most of the carbon atom is migrated to the surface of the original diamond core, and the existing diamond core is grown up; and a small part of the diamond is converted into a diamond structure in situ, and the diamonds are quickly connected to form polycrystalline diamond, so that the rapid densification is achieved, and boron atoms enter the diamond crystal structure in the process of converting carbon into the polycrystalline diamond structure. Boron is used as an inducer for carbon atom diffusion at high temperature and high pressure, and the carbon atoms are induced to diffuse rapidly and grow on the surface of the original diamond core when the polycrystalline diamond is synthesized at high temperature and high pressure, so that the energy required by nucleation is reduced, and the aim of further reducing the synthesis condition of the polycrystalline diamond is fulfilled. The method provided by the invention can reduce the synthesis conditions of the polycrystalline diamond to a greater extent, the compact polycrystalline diamond is prepared under the conditions of relatively low temperature (1200-2000 ℃) and pressure (5-10 GPa), the synthesis conditions are close to the phase boundary of diamond and graphite, and the method is suitable for realizing industrial production.

The invention provides the compact boron-doped polycrystalline diamond prepared by the method in the scheme. All phases of the polycrystalline diamond provided by the invention are in a diamond structure; the diameter of the polycrystalline diamond is 4-30 mm, and the height of the polycrystalline diamond is 1-30 mm; the grain size is 10 to 2000nm, the Vickers hardness is 61 to 158GPa, and the fracture toughness is 4.7 to 16.8 MPa.m^0.5(ii) a The initial oxidation temperature of the polycrystalline diamond in the air is 716-1276 ℃ when the heating rate is 5-10 ℃/min. The invention reduces the synthesis pressure of the polycrystalline diamond, and increases the size of the polycrystalline diamond, thereby facilitating practical application. In the present invention, the grain size of the polycrystalline diamond can be controlled by the temperature and pressure conditions of synthesis and the amount of boron doping element, for example, increasing the temperature can increase the grain size, and decreasing the pressure by the same temperature can increase the grain size. The compact boron-doped polycrystalline diamond provided by the invention has excellent performance.

The method for preparing a dense polycrystalline diamond and a boron-doped polycrystalline diamond according to the present invention will be described in detail with reference to the following examples, which should not be construed as limiting the scope of the present invention.

Example 1

(1) Firstly, carrying out vacuum annealing treatment (the vacuum degree is 1Pa, the annealing temperature is 1300 ℃, and no heat preservation) on industrial detonation nano-diamond to obtain carbon nano-onions (the particle size is 2-20 nm) with diamond cores, doping 3 mass percent of elemental boron (accounting for the mass sum of the carbon nano-onions and the elemental boron), and uniformly mixing in an inert environment by using a ball milling mode;

(2) preliminarily pre-pressing and molding the uniform mixture by using a hydraulic press, wherein the pressure is 500MPa and the room temperature is high;

(3) performing secondary high-pressure pre-pressing forming on the preliminary pre-pressed forming sample prepared in the step (2) by using a domestic cubic press, wherein the forming pressure is 5GPa, and the temperature is 800 ℃, and obtaining a pre-forming material;

(4) a 6-8 type multistage supercharging device is additionally arranged on a domestic six-sided top to generate a pre-forming material which is obtained by high-temperature and high-pressure (8GPa and 1800 ℃) treatment; the high-temperature high-pressure process mainly comprises the following steps: firstly, increasing the pressure to 8GPa within 60min, increasing the temperature to 1800 ℃ at the temperature increase rate of 30 ℃/min under the pressure maintaining condition, preserving the heat and the pressure for 50min under the conditions of 8GPa and 1800 ℃, and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the polycrystalline diamond doped with the boron element with a compact structure.

An X-ray diffraction pattern and a transmission electron micrograph of the polycrystalline diamond prepared in example 1 are shown in fig. 2 and 3, respectively. As can be seen from fig. 2, the product phase of example 1 was entirely diamond-structured, and polycrystalline diamond was obtained; fig. 3 is a transmission electron micrograph of the polycrystalline diamond block prepared in example 1, and the left side and the right side of fig. 3 are a transmission electron micrograph of the polycrystalline diamond block and an in-situ diffraction pattern, respectively, of the transmission electron micrograph, and it can be seen from fig. 3 that the grain size of the product of example 1 is fine, and the in-situ diffraction proves that the product has a diamond structure and nanocrystalline properties. Fig. 4 is an optical photograph of the polycrystalline diamond mass prepared in example 1, and it can be seen from fig. 4 that the obtained columnar product has a diameter of more than 10mm, no pores, and a dense structure.

Performance tests showed that the polycrystalline diamond prepared in example 1 had a Vickers hardness of 120GPa and a fracture toughness (indentation method) of 14.3MPa m^0.5(the Vickers hardness indentation pattern is shown in FIG. 5); at a temperature rise rate of 1At 0 deg.C/min, the initial oxidation temperature in air is 1045 deg.C.

Comparative example 1

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, and the particle size is 2-20 nm), are homogenized in an inert environment by using a mechanical ball milling mode;

steps (2) and (3) are the same as in example 1;

(4) the high-temperature high-pressure process mainly comprises the following steps: firstly, increasing the pressure to 8GPa within 60min, then increasing the temperature to 1800 ℃ at the temperature increase rate of 30 ℃/min, and keeping the temperature for 300min under the conditions of 8GPa and 1800 ℃ (the heat preservation time is 250min longer than that of example 1); and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the polycrystalline diamond.

The performance test shows that the polycrystalline diamond prepared in the comparative example 1 has the vickers hardness of 114GPa and the fracture toughness of 7.6MPa · m^0.5The initial oxidation temperature in air was 798 ℃ at a temperature rise rate of 10 ℃/min, which was lower than 1045 ℃ for example 1.

As can be seen from example 1 and comparative example 1, the present invention improves the hardness, toughness and oxidation resistance of polycrystalline diamond while significantly reducing the synthesis time by doping boron element into carbon nano-onions having diamond cores, thereby improving the synthesis efficiency.

Example 2

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, the particle size is 2-20 nm), are doped with elemental boron with the mass percentage of 1% (accounting for the mass sum of the carbon nano-onions and the elemental boron), and are uniformly mixed in an inert environment by using a ball milling mode;

(3) performing secondary high-pressure pre-pressing forming on the preliminary pre-pressed forming sample prepared in the step (2) by using a domestic cubic press, wherein the forming pressure is 5GPa, and the temperature is 600 ℃, so as to obtain a pre-forming material;

(4) a 6-8 type multistage supercharging device is additionally arranged on a six-sided top of a product made in China to generate a pre-forming material processed at high temperature and high pressure (10GPa and 2000 ℃); the high-temperature high-pressure process mainly comprises the following steps: firstly increasing the pressure to 10GPa within 200min, increasing the temperature to 2000 ℃ at the temperature increase rate of 10 ℃/min under the pressure maintaining condition, preserving the heat and the pressure for 200min under the conditions of 10GPa and 2000 ℃, and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the boron element doped polycrystalline diamond with a compact structure.

Performance tests show that the polycrystalline diamond prepared in example 2 has the Vickers hardness of 158GPa and the fracture toughness of 16.8 MPa-m^0.5The initial oxidation temperature in air was 1276 ℃ at a temperature rise rate of 10 ℃/min.

Comparative example 2

steps (2) and (3) are the same as in example 2;

(4) the high-temperature high-pressure process mainly comprises the following steps: firstly, increasing the pressure to 10GPa within 200min, then increasing the temperature to 2000 ℃ at the temperature increase rate of 10 ℃/min, and keeping the temperature for 330min under the conditions of 10GPa and 2000 ℃ (the heat preservation time is 130min longer than that of example 2); and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the polycrystalline diamond.

The performance test shows that the polycrystalline diamond prepared in the comparative example 2 has the Vickers hardness of 149GPa and the fracture toughness of 15.1 MPa-m^0.5The initial oxidation temperature in air was 855 ℃ at a heating rate of 10 ℃/min.

Example 3

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, the particle size is 2-20 nm), are doped with elemental boron with the mass percentage of 10% (accounting for the mass sum of the carbon nano-onions and the elemental boron), and are uniformly mixed in an inert environment by using a ball milling mode;

(3) performing secondary high-pressure pre-pressing forming on the preliminary pre-pressed forming sample prepared in the step (2) by using a domestic cubic press, wherein the forming pressure is 3GPa, and the temperature is 600 ℃, so as to obtain a pre-forming material;

(4) the pre-formed material is obtained by utilizing domestic six-face top direct high-temperature high-pressure (5GPa and 1200 ℃) treatment; the high-temperature high-pressure process mainly comprises the following steps: firstly, increasing the pressure to 5GPa within 1min, increasing the temperature to 1200 ℃ at the temperature increase rate of 100 ℃/min under the pressure maintaining condition, preserving the heat and the pressure for 20min under the conditions of 5GPa and 1200 ℃, and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the polycrystalline diamond doped with the boron element with a compact structure.

Performance tests show that the polycrystalline diamond prepared in example 3 has a Vickers hardness of 61GPa and a fracture toughness of 4.7 MPa-m^0.5The initial oxidation temperature in air was 716 ℃ at a temperature rise rate of 10 ℃/min.

Comparative example 3

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, and the particle size is 2-20 nm), are homogenized in a mechanical ball milling mode under an inert environment, and no other substances are added;

steps (2) and (3) are the same as in example 3;

(4) the high-temperature high-pressure process mainly comprises the following steps: firstly, the pressure is increased to 6GPa (higher than that of the embodiment 3) within 1min, then the temperature is increased to 1200 ℃ at the temperature increasing rate of 100 ℃/min and is kept at the temperature of 6GPa and 1200 ℃ for 20min, and then the temperature and the pressure are synchronously reduced to the atmospheric environment, so that the polycrystalline diamond is obtained.

Performance tests show that the polycrystalline diamond prepared in the comparative example 3 has the Vickers hardness of 50.1GPa and the fracture toughness of 4.4 MPa-m^0.5When the temperature rise rate was 10 ℃/min, the initial oxidation temperature in air was 702 ℃.

As can be seen from example 3 and comparative example 3, the present invention improves the hardness, toughness and oxidation resistance of polycrystalline diamond while reducing the synthesis pressure by doping boron element into carbon nano-onions having diamond cores, thereby improving the synthesis efficiency.

Example 4

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, the particle size is 2-20 nm), are doped with boron oxide with the mass percentage of 1% (accounting for the mass sum of the carbon nano-onions and the boron oxide), and the boron oxide are uniformly mixed in a mechanical ball milling manner in an inert environment;

(4) a 6-8 type multistage supercharging device is additionally arranged on a six-sided top of a product made in China to generate a pre-forming material processed at high temperature and high pressure (10GPa and 1900 ℃); the high-temperature high-pressure process mainly comprises the following steps: firstly increasing the pressure to 10GPa within 200min, increasing the temperature to 1900 ℃ at the temperature-increasing rate of 30 ℃/min under the pressure-maintaining condition, preserving the heat and the pressure for 200min under the conditions of 10GPa and 1900 ℃, and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the boron-doped polycrystalline diamond with a compact structure.

The performance test shows that the Vickers hardness of the polycrystalline diamond prepared in example 4 is 118GPa, and the fracture toughness is 12.6 MPa.m^0.5The initial oxidation temperature in air was 1055 ℃ at a temperature rise rate of 10 ℃/min.

Example 5

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, the particle size is 2-20 nm), are doped with elemental boron with the mass percentage of 4% (accounting for the mass sum of the carbon nano-onions and the elemental boron), and are uniformly mixed in an inert environment by using a mechanical ball milling mode;

(3) performing secondary high-pressure pre-pressing forming on the preliminary pre-pressed forming sample prepared in the step (2) by using a domestic cubic press, wherein the forming pressure is 5GPa, and the temperature is 700 ℃, so as to obtain a pre-forming material;

(4) a 6-8 type multistage supercharging device is additionally arranged on a domestic six-sided top to generate a preformed material which is processed at high temperature, high pressure and 10GPa and 1800 ℃; the high-temperature high-pressure process mainly comprises the following steps: firstly increasing the pressure to 10GPa within 180min, increasing the temperature to 1800 ℃ at the temperature increase rate of 30 ℃/min under the pressure maintaining condition, preserving the heat and the pressure at 10GPa and 1800 ℃ for 60min, and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the boron-doped polycrystalline diamond with a compact structure.

Performance tests show that the polycrystalline diamond prepared in example 5 has the Vickers hardness of 142GPa and the fracture toughness of 15.7 MPa-m^0.5When the temperature rise rate is 10 ℃/min, the initial oxidation temperature in the air is 990 ℃.

Example 6

(1) Firstly, carbon nano-onions with diamond cores, which are prepared by carrying out vacuum annealing treatment on industrial detonation nano-diamonds (the annealing condition is the same as that in example 1, the particle size is 2-20 nm), are doped with 0.1% of elemental boron by mass percent (accounting for the sum of the mass of the carbon nano-onions and the elemental boron), and are uniformly mixed in an inert environment by using a mechanical ball milling mode;

(4) performing high-temperature and high-pressure (7GPa and 1600 ℃) treatment on a domestic six-side top additionally-installed secondary supercharging device to obtain a preformed material; the high-temperature high-pressure process mainly comprises the following steps: firstly, increasing the pressure to 7GPa within 30min, increasing the temperature to 1600 ℃ at the temperature increase rate of 50 ℃/min under the pressure maintaining condition, preserving the heat and the pressure for 90min under the conditions of 7GPa and 1600 ℃, and then synchronously reducing the temperature and the pressure to the atmospheric environment to obtain the boron-doped polycrystalline diamond with a compact structure.

Performance test meterIt is clear that the polycrystalline diamond prepared in example 6 had a vickers hardness of 86GPa and a fracture toughness of 10.5MPa · m^0.5The initial oxidation temperature in air was 1026 ℃ at a temperature rise rate of 10 ℃/min.

As can be seen from the above examples, the present invention can greatly reduce the synthesis conditions of polycrystalline diamond and improve the performance of polycrystalline diamond by doping boron element into carbon nano-onions having diamond cores.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. A method of making dense polycrystalline diamond comprising the steps of:

(3) sintering the preformed material at high temperature and high pressure to obtain a compact boron-doped polycrystalline diamond block; the conditions of the high-temperature high-pressure sintering comprise: the sintering temperature is 1200-1800 ℃, the sintering pressure is 5-8 GPa, and the heat preservation and pressure maintaining time is 1-200 min.

2. The method according to claim 1, wherein the boron source in step (1) is chemically pure and has a particle size of 5 μm or less.

3. The method according to claim 1 or 2, wherein the mass of the boron source in step (1) is 0.1-10% of the total mass of the mix.

4. The method according to claim 3, wherein the mass of the boron source in step (1) is 1-3% of the total mass of the mix.

5. The method according to claim 1, wherein the carbon nano-onions with diamond cores in the step (1) are obtained by performing vacuum annealing treatment on detonation nano-diamonds, wherein the vacuum degree of the vacuum annealing treatment is 1-5 Pa, the annealing temperature is 1000-1350 ℃, and the heat preservation time is 0-60 min; the particle size of the carbon nano-onion with the diamond core is 2-20 nm.

6. The method of claim 1, wherein the mixing in step (1) is ball milling; the ball milling mixing is carried out in an inert atmosphere.

7. The method according to claim 1, wherein the holding time and pressure in the step (3) is 20-50 min.

8. The method according to claim 1 or 7, wherein the high-temperature high-pressure sintering in step (3) is specifically: increasing the pressure to the sintering pressure within 1-200 min; and then maintaining the pressure, raising the temperature to the sintering temperature at a temperature rise rate of 10-100 ℃/min, and carrying out heat preservation and pressure maintaining at the sintering pressure and the sintering temperature.

9. The dense boron-doped polycrystalline diamond prepared by the method of any one of claims 1 to 8.