Preparation method of boron carbide nanopowder and ceramic thereof

Abstract

The present invention discloses a method for preparing a boron carbide nanopowder and a ceramic thereof, comprising the following steps: after hexagonal boron nitride is subjected to carbon coating treatment using a carbon source, the carbon source is placed in a 1500-2000°C vacuum reactor for solid-phase reaction, wherein the carbon source includes a saccharide compound and an oxygen-containing compound; the carbon coating treatment includes the following steps: (1) mixing the saccharide compound and the hexagonal boron nitride and ball milling the obtained mixed powder, adding water to uniformly disperse the obtained mixed powder, and then hydrothermally carbonizing to obtain a first carbon-coated boron nitride; (2) spraying the oxygen-containing compound on the surface of the first carbon-coated boron nitride obtained in step (1), cooling and drying, and then placing the mixture at 500-1000°C for sealed carbonization for 3-6 hours, and naturally cooling to obtain a second carbon-coated boron nitride. The present invention directly uses a carbon source to coat the surface of nano-scale hexagonal boron nitride with amorphous carbon, which can prevent the agglomeration of the boron nitride powder while promoting the reaction between the carbon source and the boron nitride, thereby improving the quality of the boron carbide nanopowder.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic materials, and particularly relates to a preparation method of boron carbide nanopowder and ceramics thereof. Background Art

[0002] Boron carbide (B4C) ceramics have a low density (2.52 g / cm 3 Boron carbide has a range of advantages, including a high melting point (2450°C), high hardness (second only to diamond and cubic boron nitride), and good thermal neutron absorption, making it a promising material for applications in protection, wear resistance, and nuclear energy. However, due to its strong covalent bonds, densification of boron carbide is very difficult.

[0003] Some current research has promoted densification by introducing sintering aids, but the addition of sintering aids can adversely affect material properties, such as an increase in theoretical density and a decrease in hardness, which to some extent limits the application of boron carbide ceramics. To achieve densification of single-phase boron carbide without the addition of sintering aids, the inventor's research team previously reported using a solid-phase reaction method to synthesize boron carbide nanopowders by reacting hexagonal boron nitride with carbon black or graphite under high temperature and vacuum (Zeng Hong, Kan Yanmei, Xu Changming et al., Synthesis of Boron Carbide Nanopowders by Solid-Phase Reaction Method, Journal of Inorganic Materials, Vol. 26, No. 10, October 2011). This method produces boron carbide nanopowders with an average particle size of 100 nm and good sintering activity. However, in order to obtain fine and uniform boron nitride nanopowders in this method, the hexagonal boron nitride, carbon black, etc. used are all nano-level powders, which have some agglomeration phenomenon, which affects both the activity of the solid-phase reaction and the particle size distribution of the final product boron nitride. In addition, there are great restrictions on the particle size of the reaction raw materials hexagonal boron nitride, carbon black, etc., and raw materials with smaller particle sizes cannot be used.

[0004] In the existing technology, the agglomeration of nanopowders is generally solved by adding dispersants or surface grafting modification of the powders. However, when this method is applied to the solid-phase reaction method for synthesizing boron carbide nanopowders, it is easy to form an organic group barrier layer on the surface of boron nitride, carbon black, etc., affecting the reaction activity between the powders. Even if these organic groups can be decomposed during the subsequent high-temperature reaction process, the small molecular impurities produced by the decomposition are likely to affect the purity of the boron carbide nanopowder (such as oxygen content), thereby affecting the sintering performance.

[0005] CN102502535A provides a method for preparing carbon-coated hexagonal boron nitride, which uses maleic anhydride ethylene octene graft copolymer as a carbon source to carbon-coat hexagonal boron nitride, with the purpose of obtaining carbon-coated hexagonal boron nitride with good dispersibility and wear resistance. CN112662449A provides a highly dispersed amorphous carbon-coated hexagonal boron nitride nanosheet and a preparation method thereof, which uses sugar as a carbon source to carbon-coat hexagonal boron nitride nanosheet, giving it dispersibility and lubrication and friction-reducing effects. Although both of the above patents can improve the dispersibility of boron nitride, they do not take into account the reaction between the carbon coating layer and the hexagonal boron nitride, and simply perform surface carbon coating on the hexagonal boron nitride.

[0006] CN108249443A provides a method for preparing carbon-encapsulated boron carbide nanopowders. Boric acid is used as a boron source and sucrose is used as a carbon source. Carbon-encapsulated boron carbide nanopowders are prepared under high vacuum reaction conditions. However, this method is a coating of the boron carbide nanopowders, and this method causes excessive carbon to act as a sintering aid, which is likely to affect the sintering performance of the boron carbide.

[0007] The existing document, "Study on the Synthesis of Boron Carbide Powder by Low-Temperature Precursor Pyrolysis, Lin Shuang," uses hydroxyl-containing organic carbons such as polyvinyl alcohol, glucose, soluble starch, and sucrose as carbon sources, and boric acid as a boron source, using an esterification method to synthesize a polymer precursor. The precursor is then pyrolyzed and vacuum sintered to produce B4C powder. This method, which synthesizes the boron carbide powder after synthesizing the polymer precursor through esterification between carbon and boron sources, is suitable for specific carbon and boron sources capable of esterification. Furthermore, neither this document nor patent CN108249443A considers the carbon coating of hexagonal boron nitride during the preparation of boron carbide nanopowders.

[0008] In summary, how to provide a method for preparing boron carbide nanopowder and ceramics thereof, which can prevent the agglomeration of the reaction raw material powders while promoting the reaction of the carbon source and the boron source, is an urgent problem to be solved. Summary of the Invention

[0009] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for preparing boron carbide nanopowder and ceramics thereof. The method directly uses a carbon source to coat the surface of nano-scale hexagonal boron nitride with amorphous carbon. While preventing the agglomeration of boron nitride powder, it can also promote the reaction between the carbon source and boron nitride, thereby improving the quality of the boron carbide nanopowder.

[0010] In order to achieve the above object, the technical solution adopted by the present invention is:

[0011] A method for preparing boron carbide nanopowder comprises the following steps: subjecting hexagonal boron nitride to carbon coating treatment using a carbon source, and then placing the carbon source in a vacuum reactor for solid phase reaction, wherein the carbon source comprises a saccharide compound and an oxygen-containing compound.

[0012] Preferably, the molar ratio of the carbon source, calculated as C, to hexagonal boron nitride is (1.1-1.6):4, and the diameter of the hexagonal boron nitride is 0.001-50 μm, more preferably 10-500 nm, and even more preferably 50-100 nm.

[0013] Preferably, the mass ratio of the carbohydrate compound to the oxygen-containing compound is (0.5-2):1, and the carbohydrate compound includes hexose and pentose in a mass ratio of 1:(0.3-0.6), the hexose is one or more of glucose, mannose, fructose and galactose, and the pentose is one or two of xylose and deoxyribose; the number of oxygen atoms in the oxygen-containing compound accounts for 10-24%, and the oxygen-containing compound is one or more of polyethylene glycol (degree of polymerization of 5-50, the number of oxygen atoms accounts for 14.4-15.8%), poly (ethylene glycol) tetrahydrofurfuryl ether (degree of polymerization of 1-50, the number of oxygen atoms accounts for 12.5-14.2%), mannitol, and sorbitol.

[0014] Preferably, the temperature of the solid phase reaction in the vacuum reactor is 1500-2000°C, the heating rate is 30-50°C, the holding time is 3-6h, and the vacuum degree in the vacuum reactor is (0.1-10)×10 -4 Pa.

[0015] The carbon coating process comprises the following steps:

[0016] (1) The sugar compound and hexagonal boron nitride are mixed and ball-milled, the obtained mixed powder is evenly dispersed in water, and then hydrothermal carbonization is performed to obtain the first carbon-coated boron nitride;

[0017] (2) Spraying an oxygen-containing compound (which can be dissolved into a solution by adding a solvent, the mass concentration of the solution is 40-80%, and the solvent is water, ethanol, glycerol or propylene glycol) onto the surface of the first carbon-coated boron nitride obtained in step (1), cooling and drying, placing it in a closed state for carbonization at 500-1000°C for 3-6 hours, and cooling it naturally to obtain the second carbon-coated boron nitride.

[0018] Preferably, in step (1), the ball milling rate is 300-500 rpm, and the ball milling time is 3-8 h; the hydrothermal carbonization conditions are: 160-250°C, a heating rate of 1-10°C / min, and insulation for 2-4 h.

[0019] In step (2), the carbonization heating rate is 5-30°C / min.

[0020] Preferably, in order to further promote the solid-phase reaction effect, sodium chloride is also added during the second carbon coating in the present invention. At this time, the oxygen-containing compound is preferably sorbitol, polyethylene glycol (degree of polymerization of 5-50) and poly (ethylene glycol) tetrahydrofurfuryl ether (degree of polymerization of 1-50), and the mass ratio of the three is (0.001-0.004): (0.2-0.8): 1.

[0021] Preferably, in the case of adding sodium chloride, the carbon coating process comprises the following steps:

[0022] (1) The sugar compound and hexagonal boron nitride are mixed and ball-milled, the obtained mixed powder is evenly dispersed in water, and then hydrothermal carbonization is performed to obtain the first carbon-coated boron nitride;

[0023] (2) Spraying a solution formed by a mixture of saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), and then placing it at a constant temperature of 70-80°C for evaporation for 1-3 hours, and then continuing to spray polyethylene glycol and poly (ethylene glycol) tetrahydrofurfuryl ether. After spraying, drying, first placing it at 500-750°C for vacuum carbonization for 2-5 hours, and then heating it to 850-1000°C and keeping it warm for 0.5-1 hour, and then cooling it naturally, repeatedly rinsing it with clean water to remove sodium chloride, and drying it to obtain a second carbon-coated boron nitride.

[0024] Preferably, the amount of sodium chloride added is 1-6% of the mass of the oxygen-containing compound, the heating rate to 500-750°C is 5-20°C / min, and the heating rate to 850-1000°C is 15-30°C / min.

[0025] Preferably, the change process of the vacuum degree in the vacuum reactor during the solid phase reaction is as follows:

[0026] Before heating to 1500-2000℃, the vacuum degree is (6-10)×10 -4 Pa;

[0027] During the 0-1h period of heating to 1500-2000℃, the vacuum degree is (3-5)×10 -4 Pa;

[0028] During the first 2 hours of heating to 1500-2000℃, the vacuum degree is (0.1-1)×10 -4 Pa;

[0029] During the 2-6h period of heating to 1500-2000℃, the vacuum degree is (2-4)×10 -4 Pa.

[0030] The present invention also provides a method for sintering boron carbide nanopowder, comprising the following steps: sintering the boron carbide nanopowder prepared above at 1900-2100° C. and 28-32 MPa in an inert gas atmosphere (argon or helium) for 40-70 minutes to obtain the boron carbide nanopowder.

[0031] The present invention also provides a boron carbide ceramic, which is manufactured by the above sintering method.

[0032] Technical effects of the present invention:

[0033] 1. The present invention directly uses a carbon source to coat the surface of nano-scale hexagonal boron nitride with amorphous carbon. This prevents boron nitride powder from agglomerating while promoting the solid-phase reaction between the carbon source and boron nitride. Existing carbon-coating of boron nitride does not consider the reaction between the carbon coating layer and boron nitride. To balance carbon coating with reactivity (a small amount of conventional coating makes it difficult to fully react, while a large amount of carbon coating affects the purity and sintering properties of the boron carbide product), the present invention uses a double-coating method to ensure sufficient reaction between boron nitride and carbon. This method allows for better control of the carbon coating and reaction effect, and enables the use of hexagonal boron nitride with a smaller particle size (the smaller the particle size, the higher the reactivity), resulting in a boron carbide nanopowder with a smaller particle size.

[0034] 2. When the surface of hexagonal boron nitride is carbon-coated, the present invention first uses a sugar compound for the first hydrothermal carbon coating. The carbon-coated surface contains more oxygen-containing groups, which can have good compatibility with the oxygen-containing compound of the second carbon coating, thereby promoting the effect of the second carbon coating. In addition, the present invention uses a certain proportion of hexose and pentose as carbonization materials in the first carbon coating. The hydrothermal intermediate of hexose is hydroxymethylfurfural, which is hydrophilic and has good compatibility in water, but the carbonized product is easy to stick together. The hydrothermal intermediate of pentose is furfural, which is relatively hydrophobic. The carbonized product has good dispersibility and is not easy to stick together, but has poor compatibility with the oxygen-containing compound of the second carbon coating, which affects the effect of the second carbon coating. Therefore, the hexose and pentose are mixed in a certain proportion for hydrothermal carbonization, which can improve the hydrothermal carbonization effect while taking into account the effect of the second carbon coating.

[0035] 3. When the present invention performs a second carbon coating on hexagonal boron nitride, the carbonized material used is an oxygen-containing compound. During the high-temperature carbonization process of the second carbon coating, the oxygen atoms can be removed by high temperature and generate pores in the carbon coating layer, which increases the specific surface area of ​​the product while promoting the solid-phase reaction between amorphous carbon and boron nitride. The proportion of oxygen atoms in the oxygen-containing compound has a greater influence on the solid-phase reaction. When the proportion of oxygen atoms is too low, there are fewer pores in the carbon coating layer, the solid-phase reaction efficiency is insufficient, and it is easy to cause an excessively high graphitized structure, affecting the solid-phase reaction; when the proportion of oxygen atoms is too high, there are too many pores in the carbon coating layer, resulting in an insufficiently strong structure, and the increase in oxygen atoms easily promotes the formation of carbon oxides, resulting in carbon loss, resulting in insufficient carbon source and insufficient reaction. Therefore, the present invention selects the proportion of oxygen atoms in the oxygen-containing compound to be 10-24%, which can comprehensively improve the solid-phase reaction effect.

[0036] 4. Compared with the prior art in which the coating layer obtained by only using the hydrothermal method during carbon coating has a higher oxygen content and causes many defects, the present invention can not only remove the oxygen atoms in the second carbon coating layer through the second carbon coating high-temperature carbonization, but also remove the oxygen atoms in the coating layer formed by the first carbon coating hydrothermal carbonization, thereby reducing defects.

[0037] 5. During the second carbon coating, the present invention further adds a saturated sodium chloride solution to the oxygen-containing compound (sorbitol, a common additive for forming star-shaped dendrites). The sodium chloride is first heated and evaporated in the presence of sorbitol to form star-shaped dendrites. These highly dispersed crystals allow the subsequently added oxygen-containing compound (polyethylene glycol and poly(ethylene glycol) tetrahydrofurfuryl ether) to form amorphous carbon based on the star-shaped dendrites during the first high-temperature carbonization process at 500-750°C, further enhancing the reactivity of the second carbon coating layer. Then, during the second high-temperature carbonization process at 850-1000°C, the sodium chloride crystals melt to form a highly fluid liquid that can flow within the voids of the carbon coating, reducing the residence and reaction of small molecular compounds and promoting their removal, thereby comprehensively reducing the oxygen content of the carbon coating and improving the sintering performance of boron carbide. Finally, the highly water-soluble sodium chloride is removed by rinsing with clean water, leaving pores of this specific crystal structure in the carbon coating, thereby increasing the reactivity of the carbon coating and preventing any residual sodium chloride from remaining in the nano-boron carbide powder.

[0038] The oxygen-containing compound of the present invention includes a relatively large content of poly(ethylene glycol) tetrahydrofurfuryl ethyl ether, which is a liquid with gel properties at room temperature. Before high-temperature carbonization, it can well cover the surface of sodium chloride crystals and be highly dispersed, thereby improving the carbonization efficiency of the poly(ethylene glycol) tetrahydrofurfuryl ethyl ether and the carbonization rate.

[0039] 6. Small-particle boron carbide powder can improve sintering density. The particle size of the boron carbide nanopowder obtained in the present invention is small, but its sintering performance (density, toughness) is not significantly improved compared with the existing technology. This may be because after the present invention performs secondary carbon coating on hexagonal boron nitride, the discharge of nitrogen generated by boron nitride in the coating layer during the solid-phase reaction is delayed, making the structure of the boron carbide nanopowder loose and rough, and the friction between the powders is enhanced. In addition, since the particle size of the boron carbide nanopowder obtained in this application is small and the adhesion between the powders is large, the fluidity of the boron carbide powder during application is poor, which increases the difficulty of densification during the sintering process of the boron carbide powder, thereby affecting the sintering performance.

[0040] Therefore, in the preparation process of boron carbide nanopowder of the present invention, different vacuum degrees are adopted in different reaction stages. A lower vacuum degree is first applied in the early stage when the reaction has not yet been intense, and a higher vacuum degree is applied during the subsequent intense reduction reaction, so that the nitrogen generated by the reaction can be discharged through the pores of the carbon coating layer in a timely manner, and loose pores will not be formed in the synthesized boron carbide powder. Then, in the process of the subsequent reaction gradually slowing down, the vacuum degree is reduced to adapt to the solid-phase reaction speed. Through such a specific vacuum degree curve, boron carbide with better sintering performance can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a scanning electron microscope image of the boron carbide nanopowder prepared in an embodiment of the present invention;

[0042] Figure 2 The following is the XRD spectrum of the boron carbide nanopowder prepared in the present invention. DETAILED DESCRIPTION

[0043] The above scheme is further described below in conjunction with specific examples; it should be understood that these examples are used to illustrate the basic principles, main features and advantages of the present invention, and the present invention is not limited to the scope of the following examples; the implementation conditions adopted in the examples can be further adjusted according to specific requirements, and the implementation conditions not specified are generally the conditions in routine experiments.

[0044] Unless otherwise specified in the following examples, all raw materials were purchased from commercial sources or prepared by conventional methods in the art. Example 1

[0045] This embodiment provides a method for preparing boron carbide nanopowder, comprising the following steps: hexagonal boron nitride (100 nm) is carbon-coated with a carbon source, and then placed in a vacuum reactor for solid-phase reaction. The solid-phase reaction temperature in the vacuum reactor is 1800°C, the heating rate is 40°C, the holding time is 3 hours, and the vacuum degree in the vacuum reactor is 1×10 -4 Pa.

[0046] The carbon source, calculated as C, has a molar ratio of 1.3:4 to hexagonal boron nitride. The carbon source includes a carbohydrate compound and an oxygen-containing compound in a mass ratio of 1.2:1, wherein the carbohydrate compound includes a hexose (glucose) and a pentose (xylose) in a mass ratio of 1:0.45, and the oxygen-containing compound is polyethylene glycol 1000.

[0047] The carbon coating process includes the following steps:

[0048] (1) The sugar compound and hexagonal boron nitride were mixed and ball-milled at a ball-milling rate of 400 rpm for 5 h. The obtained mixed powder was evenly dispersed by adding water and then hydrothermally carbonized at the conditions of 200 °C, a heating rate of 5 °C / min, and a holding time of 3 h to obtain the first carbon-coated boron nitride.

[0049] (2) Spray an oxygen-containing compound (polyethylene glycol 1000 dissolved in water to form a solution with a mass concentration of 50%) onto the surface of the first carbon-coated boron nitride obtained in step (1), cool and dry it, and then place it in a closed state for carbonization at 650°C for 4 hours at a carbonization heating rate of 20°C / min. After the carbonization is completed, cool it naturally to obtain the second carbon-coated boron nitride.

[0050] The present invention also provides a method for sintering boron carbide nanopowder, comprising the following steps: sintering the boron carbide nanopowder prepared above at 2000° C. and 30 MPa in an inert gas atmosphere (argon) for 55 minutes to obtain the boron carbide nanopowder.

[0051] The present invention also provides a boron carbide ceramic, which is manufactured by the above sintering method. Example 2

[0052] The difference between this embodiment and embodiment 1 is that the hexose in the carbohydrate compound is fructose and the pentose is deoxyribose. Example 3

[0053] The difference between this embodiment and embodiment 1 is that the oxygen-containing compound is poly(ethylene glycol) tetrahydrofurfuryl ethyl ether (polymerization degree of 1-2 components is 95-100%). Example 4

[0054] The difference between this embodiment and embodiment 1 is that the oxygen-containing compound is sorbitol (the amount of oxygen atoms accounts for 23.1%). Example 5

[0055] The difference between this embodiment and embodiment 1 is that the oxygen-containing compound includes polyethylene glycol 1000 and poly(ethylene glycol) tetrahydrofurfuryl ether in a mass ratio of 0.5:1 (the component with a polymerization degree of 1-2 is 95-100%). Example 6

[0056] The difference between this embodiment and embodiment 1 is that the oxygen-containing compound includes sorbitol, polyethylene glycol 1000 and poly(ethylene glycol) tetrahydrofurfuryl ether in a mass ratio of 0.003:0.5:1 (the component with a polymerization degree of 1-2 is 95-100%). Example 7

[0057] This embodiment provides a method for preparing boron carbide nanopowder. The method differs from Example 1 in that the oxygen-containing compound comprises sorbitol, polyethylene glycol 1000, and poly(ethylene glycol) tetrahydrofurfuryl ether (with a degree of polymerization of 95-100% for the 1-2 component) in a mass ratio of 0.003:0.5:1. Sodium chloride is also added during the carbon coating process, with the amount of sodium chloride added being 3% of the mass of the oxygen-containing compound. The carbon coating process specifically comprises the following steps:

[0058] (1) The sugar compound and hexagonal boron nitride were mixed and ball-milled at a ball-milling rate of 400 rpm for 5 h. The obtained mixed powder was evenly dispersed by adding water and then hydrothermally carbonized at the conditions of 200 °C, a heating rate of 5 °C / min, and a holding time of 3 h to obtain the first carbon-coated boron nitride.

[0059] (2) Spray a solution formed by mixing a saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), then place it at a constant temperature of 75°C for evaporation for 2 hours, and then continue to spray polyethylene glycol 1000 and poly (ethylene glycol) tetrahydrofurfuryl ether. After spraying, dry it and place it at 600°C (heating rate of 10°C / min) for vacuum carbonization for 3.5 hours, then heat it to 900°C (heating rate of 20°C / min) and keep it warm for 40 minutes, then cool it naturally, rinse it repeatedly with clean water to remove sodium chloride, and dry it to obtain the second carbon-coated boron nitride.

[0060] The rest is the same as Example 1. Example 8

[0061] This embodiment provides a method for preparing boron carbide nanopowder. The difference from Example 7 is that the change process of the vacuum degree in the vacuum reactor during the solid-phase reaction is as follows:

[0062] Before heating to 1800℃, the vacuum degree is 8×10 -4 Pa;

[0063] During the 0-1h period of heating to 1800℃, the vacuum degree is 4×10 -4 Pa;

[0064] During the first 2 hours of heating to 1800℃, the vacuum degree was 0.5×10 -4 Pa;

[0065] During the 2nd to 3rd hour of heating to 1800℃, the vacuum degree is 3×10 -4 Pa.

[0066] The rest is the same as Example 7. Example 9

[0067] This embodiment provides a method for preparing boron carbide nanopowder, comprising the following steps: hexagonal boron nitride (50 nm) is carbon-coated with a carbon source, and then placed in a vacuum reactor for solid-phase reaction. The solid-phase reaction temperature in the vacuum reactor is 1500°C, the heating rate is 30°C, and the holding time is 4 hours.

[0068] The carbon source, calculated as C, has a molar ratio of 1.1:4 to hexagonal boron nitride. The carbon source includes a saccharide compound and an oxygen-containing compound in a mass ratio of 0.5:1. The saccharide compound includes a hexose (glucose) and a pentose (xylose) in a mass ratio of 1:0.3. The oxygen-containing compound includes sorbitol, polyethylene glycol 1000, and poly(ethylene glycol) tetrahydrofurfuryl ether in a mass ratio of 0.001:0.2:1 (the component with a degree of polymerization of 1-2 is 95-100%).

[0069] The carbon coating process includes the following steps:

[0070] (1) The sugar compound and hexagonal boron nitride were mixed and ball-milled at a ball-milling rate of 300 rpm for 8 h. The obtained mixed powder was evenly dispersed by adding water and then hydrothermally carbonized at the conditions of 160 °C, a heating rate of 1 °C / min, and a holding time of 4 h to obtain the first carbon-coated boron nitride.

[0071] (2) Spray a solution of a mixture of saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), then place it at 70°C for constant temperature evaporation for 3 hours, then continue to spray polyethylene glycol 1000 and poly (ethylene glycol) tetrahydrofurfuryl ether, dry it after spraying, first place it at 500°C (heating rate of 5°C / min) for vacuum carbonization for 5 hours, then heat it to 850°C (heating rate of 15°C / min) and keep it at that temperature for 1 hour, then cool it naturally, rinse it repeatedly with clean water to remove sodium chloride, and dry it to obtain the second carbon-coated boron nitride. The amount of sodium chloride added is 1% of the mass of the oxygen-containing compound.

[0072] The change process of vacuum degree in the vacuum reactor during solid phase reaction is as follows:

[0073] Before heating to 1500℃, the vacuum degree is 6×10 -4 Pa;

[0074] During the 0-1h period of heating to 1500℃, the vacuum degree is 3×10 -4 Pa;

[0075] During the first 2 hours of heating to 1500℃, the vacuum degree was 0.1×10 -4 Pa;

[0076] During the 2nd to 4th hour of heating to 1500℃, the vacuum degree is 2×10 -4 Pa.

[0077] The present invention also provides a method for sintering boron carbide nanopowder, comprising the following steps: sintering the boron carbide nanopowder prepared above at 1900° C. and 28 MPa inert gas atmosphere (argon) for 70 minutes to obtain the boron carbide nanopowder.

[0078] The present invention also provides a boron carbide ceramic, which is manufactured by the above sintering method. Example 10

[0079] This embodiment provides a method for preparing boron carbide nanopowder, comprising the following steps: hexagonal boron nitride (200 nm) is carbon-coated with a carbon source, and then placed in a vacuum reactor for solid-phase reaction. The solid-phase reaction temperature in the vacuum reactor is 2000°C, the heating rate is 50°C, and the holding time is 6 hours.

[0080] The carbon source, calculated as C, has a molar ratio of 1.6:4 to hexagonal boron nitride. The carbon source includes a saccharide compound and an oxygen-containing compound in a mass ratio of 2:1. The saccharide compound includes a hexose (glucose) and a pentose (xylose) in a mass ratio of 1:0.6. The oxygen-containing compound includes sorbitol, polyethylene glycol 1000, and poly(ethylene glycol) tetrahydrofurfuryl ether in a mass ratio of 0.004:0.8:1 (the component with a degree of polymerization of 1-2 is 95-100%).

[0081] The carbon coating process includes the following steps:

[0082] (1) The sugar compound and hexagonal boron nitride were mixed and ball-milled at a ball-milling rate of 500 rpm for 3 h. The obtained mixed powder was evenly dispersed by adding water and then hydrothermally carbonized at the following conditions: 250 °C, a heating rate of 10 °C / min, and a holding time of 2 h to obtain the first carbon-coated boron nitride.

[0083] (2) Spray a solution of a mixture of saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), then place it at 80°C for constant temperature evaporation for 1 hour, then continue to spray polyethylene glycol and poly (ethylene glycol) tetrahydrofurfuryl ether, dry it after spraying, first place it at 750°C (heating rate of 20°C / min) for vacuum carbonization for 2 hours, then heat it to 1000°C (heating rate of 30°C / min) and keep it at that temperature for 0.5 hours, then cool it naturally, rinse it repeatedly with clean water to remove sodium chloride, and dry it to obtain the second carbon-coated boron nitride. The amount of sodium chloride added is 6% of the mass of the oxygen-containing compound.

[0084] The change process of vacuum degree in the vacuum reactor during solid phase reaction is as follows:

[0085] Before heating to 2000℃, the vacuum degree is 10×10 -4 Pa;

[0086] During the 0-1h period of heating to 2000℃, the vacuum degree is 5×10 -4 Pa;

[0087] During the first 2 hours of heating to 2000℃, the vacuum degree is 1×10 -4 Pa;

[0088] During the 2nd to 6th hour of heating to 2000℃, the vacuum degree is 4×10 -4 Pa.

[0089] The present invention also provides a method for sintering boron carbide nanopowder, comprising the following steps: sintering the boron carbide nanopowder prepared above at 2100° C. and 32 MPa inert gas atmosphere (argon) for 40 minutes to obtain the boron carbide nanopowder.

[0090] The present invention also provides a boron carbide ceramic, which is manufactured by the above sintering method.

[0091] Comparative Example 1

[0092] The difference between this comparative example and Example 6 is that the carbon source is calculated as C, and the molar ratio of the carbon source to the hexagonal boron nitride is 0.9:4.

[0093] Comparative Example 2

[0094] The difference between this comparative example and Example 6 is that the carbon source is calculated as C, and the molar ratio of the carbon source to the hexagonal boron nitride is 2.0:4.

[0095] Comparative Example 3

[0096] The difference between this comparative example and Example 6 is that the carbon source does not contain a carbohydrate compound. In this case, an oxygen-containing compound is used instead of the hexagonal boron nitride for the first carbon coating. Step (1) is: the oxygen-containing compound (sorbitol, polyethylene glycol 1000, poly (ethylene glycol) tetrahydrofurfuryl ether, dissolved in water to form a solution with a mass concentration of 50%) and the hexagonal boron nitride are mixed and ball milled at a ball milling rate of 400 rpm and a ball milling time of 5 h. The obtained mixed powder is evenly dispersed by adding water and then hydrothermally carbonized. The hydrothermal carbonization conditions are: 200 ° C, a heating rate of 5 ° C / min, and heat preservation for 3 h to obtain the first carbon-coated boron nitride.

[0097] Comparative Example 4

[0098] The difference between this comparative example and Example 6 is that the carbon source does not contain oxygen-containing compounds. In this case, a sugar compound is used instead of the hexagonal boron nitride for the second carbon coating. Step (2) is: spraying a sugar compound (glucose and xylose are dissolved in water to form a solution with a mass concentration of 50%) on the surface of the first carbon-coated boron nitride obtained in step (1), cooling and drying, and then placing it in a closed state for carbonization at 650°C for 4 hours. The carbonization heating rate is 20°C / min. After the carbonization is completed, it is naturally cooled to obtain the second carbon-coated boron nitride.

[0099] Comparative Example 5

[0100] The difference between this comparative example and Example 6 is that the carbon source does not contain oxygen-containing compounds, and the carbon coating process only includes step (1), that is, only the first carbon coating is performed.

[0101] Comparative Example 6

[0102] The difference between this comparative example and Example 6 is that the carbon source does not contain a sugar compound, and the carbon coating process only includes step (2), which is to directly carbon-coat the surface of the hexagonal boron nitride with an oxygen-containing compound.

[0103] Comparative Example 7

[0104] The difference between this comparative example and Example 6 is that the carbonization temperature in step (1) is the same as that in step (2), which is 650°C.

[0105] Comparative Example 8

[0106] The difference between this comparative example and Example 6 is that the carbonization temperature in step (2) is the same as that in step (1), which is 200°C.

[0107] Comparative Example 9

[0108] The difference between this comparative example and Example 6 is that the carbon source does not contain glucose.

[0109] Comparative Example 10

[0110] The difference between this comparative example and Example 6 is that the carbon source does not contain xylose.

[0111] Comparative Example 11

[0112] The difference between this comparative example and Example 6 is that in the carbohydrate compound, the mass ratio of hexose to pentose is 1:0.1.

[0113] Comparative Example 12

[0114] The difference between this comparative example and Example 6 is that in the carbohydrate compound, the mass ratio of hexose to pentose is 1:0.8.

[0115] Comparative Example 13

[0116] The difference between this comparative example and Example 6 is that the oxygen-containing compound in the carbon source is glucose (the amount of oxygen atoms accounts for 25%).

[0117] Comparative Example 14

[0118] The difference between this comparative example and Example 6 is that the oxygen-containing compound in the carbon source is heptanoic acid (the amount of oxygen atoms accounts for 8.7%).

[0119] Comparative Example 15

[0120] The difference between this comparative example and Example 8 is that sodium chloride is replaced by potassium chloride.

[0121] Comparative Example 16

[0122] The difference between this comparative example and Example 8 is that the oxygen-containing compound in the carbon source does not contain sorbitol.

[0123] Comparative Example 17

[0124] The difference between this comparative example and Example 8 is that the oxygen-containing compound in the carbon source does not contain poly(ethylene glycol) tetrahydrofurfuryl ether.

[0125] Comparative Example 18

[0126] The difference between this comparative example and Example 8 is that the carbon coating treatment step (2) is carbonized at 600°C at one time. At this time, step (2) is as follows: spraying a solution formed by mixing a saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), then placing it at 75°C for constant temperature evaporation for 2 hours, then continuing to spray polyethylene glycol 1000 and poly (ethylene glycol) tetrahydrofurfuryl ether, drying after spraying, placing it at 600°C (heating rate of 10°C / min) for vacuum carbonization for 3.5 hours, then naturally cooling, repeatedly rinsing with clean water to remove sodium chloride, and drying to obtain the second carbon-coated boron nitride.

[0127] Comparative Example 19

[0128] The difference between this comparative example and Example 8 is that the carbon coating treatment step (2) is carbonized at 900°C at one time. At this time, step (2) is as follows: spraying a solution formed by mixing a saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), then placing it at 75°C for constant temperature evaporation for 2 hours, and then continuing to spray polyethylene glycol 1000 and poly (ethylene glycol) tetrahydrofurfuryl ether. After spraying, drying is carried out, heating to 900°C (heating rate of 20°C / min) and keeping the temperature for 40 minutes, then naturally cooling, repeatedly rinsing with clean water to remove sodium chloride, and drying to obtain the second carbon-coated boron nitride.

[0129] Comparative Example 20

[0130] The difference between this comparative example and Example 8 is that the amount of sodium chloride added is 0.5% of the mass of the oxygen-containing compound.

[0131] Comparative Example 21

[0132] The difference between this comparative example and Example 8 is that the amount of sodium chloride added is 8% of the mass of the oxygen-containing compound.

[0133] Comparative Example 22

[0134] The difference between this comparative example and Example 8 is that the change process of the vacuum degree in the vacuum reactor during the solid phase reaction is as follows:

[0135] Before heating to 1800℃ and during the 0-1h holding period, the vacuum degree is 4×10 -4 Pa;

[0136] During the first 2 hours of heating to 1800℃, the vacuum degree was 0.5×10 -4 Pa;

[0137] During the 2nd to 3rd hour of heating to 1800℃, the vacuum degree is 3×10 -4 Pa.

[0138] Comparative Example 23

[0139] The difference between this comparative example and Example 8 is that the change process of the vacuum degree in the vacuum reactor during the solid phase reaction is as follows:

[0140] Before heating to 1800℃, the vacuum degree is 8×10 -4 Pa;

[0141] During the 0-2h period of heating to 1800℃, the vacuum degree was 0.5×10 -4 Pa;

[0142] During the 2nd to 3rd hour of heating to 1800℃, the vacuum degree is 3×10 -4 Pa.

[0143] Comparative Example 24

[0144] The difference between this comparative example and Example 8 is that the change process of the vacuum degree in the vacuum reactor during the solid phase reaction is as follows:

[0145] Before heating to 1800℃, the vacuum degree is 8×10 -4 Pa;

[0146] During the 0-2h period of heating to 1800℃, the vacuum degree is 4×10 -4 Pa;

[0147] During the 2nd to 3rd hour of heating to 1800℃, the vacuum degree is 3×10 -4 Pa.

[0148] Comparative Example 25

[0149] The difference between this comparative example and Example 8 is that the change process of the vacuum degree in the vacuum reactor during the solid phase reaction is as follows:

[0150] Before heating to 1800℃, the vacuum degree is 8×10 -4 Pa;

[0151] During the 0-1h period of heating to 1800℃, the vacuum degree is 4×10 -4 Pa;

[0152] During the first 3 hours of heating to 1800℃, the vacuum degree was 0.5×10 -4 Pa.

[0153] 1. Particle size of boron carbide nanopowder prepared by the present invention

[0154] The boron carbide nanopowder product obtained in Example 8 of the present invention was scanned by electron microscope, and the results showed that Figure 1 , we can see that the powder particle size distribution is uniform and the appearance is good. The XRD pattern of the product powder is shown in Figure 2 , it can be seen that the product is boron carbide powder.

[0155] The particle sizes of the boron carbide nanopowders obtained in Examples 1-10 of the present invention are respectively measured and shown in Table 1.

[0156] Table 1

[0157]

[0158] Table 1 shows that the boron carbide nanopowders produced in Examples 1-10 of the present invention range from 30 to 150 nm. Comparisons with Examples 1-6 show that the powder produced in Example 6 has a smaller particle size. Examples 8-10 use hexagonal boron nitride of varying particle sizes as raw materials, and the boron carbide nanopowder produced in Example 9 (using 50 nm hexagonal boron nitride as raw material) has an even smaller particle size.

[0159] 2. Effects of Carbon-Coated Hexagonal Boron Nitride of the Present Invention

[0160] 1. Dispersion and coating strength of boron nitride powder after carbon coating

[0161] The dispersion and coating strength of 50 nm, 100 nm, and 200 nm hexagonal boron nitride powders, as well as the carbon-coated boron nitride powders prepared in Examples 1-10 and Comparative Examples 3-14, were observed respectively. The results are shown in Table 2.

[0162] The test method for coating strength is:

[0163] Weigh a certain amount of the powder to be tested, add it to anhydrous ethanol, start the stirrer and stir at a speed of 1500 rpm for 15 minutes. Use an optical microscope to observe the breakage of the powder surface during stirring in anhydrous ethanol to test the coating strength of the carbon-coated boron nitride powder.

[0164] Table 2

[0165]

[0166] As shown in Table 2, compared with the hexagonal boron nitride raw materials (50 nm, 200 nm), the carbon coating treatment of the nano-scale hexagonal boron nitride in Examples 1-10 of the present invention can significantly improve the powder dispersion and no agglomeration occurs.

[0167] Compared with comparative examples 3-11, the first carbon coating of Examples 1-10 of the present invention uses a specific ratio of hexose and pentose as the hydrothermal carbonization material, and the second carbon coating uses an oxygen-containing compound with a specific oxygen atom ratio as the high-temperature carbonization material. The combination of the two can obtain better modified powder dispersibility and coating strength.

[0168] 2. Carbonization rate of boron nitride powder after carbon coating treatment

[0169] The carbonization rates of hexagonal boron nitride subjected to carbon coating treatment in Examples 1-8 of the present invention and Comparative Examples 15-21 were measured respectively. The results are shown in Table 3.

[0170] Table 3

[0171]

[0172] A higher carbonization rate indicates a better carbonization effect and a more complete reaction between carbon and boron nitride. Table 3 shows that the carbonization rates of hexagonal boron nitride in Examples 1-8 range from 30.57% to 46.85%, with Examples 7 and 8 achieving the highest carbonization rates.

[0173] Compared with Example 8, Comparative Example 15 changed sodium chloride to potassium chloride, Comparative Examples 16-17 changed the composition of oxygen-containing compounds, Comparative Examples 18 and 19 changed the carbonization temperature of different steps, and Comparative Examples 20 and 21 changed the addition amount of sodium chloride. As a result, the carbonization rate decreased.

[0174] 3. Basic properties of the boron carbide nanopowder prepared by the present invention

[0175] 1. Yield and purity of boron carbide nanopowder

[0176] The yield and purity of the boron carbide nanopowders prepared in Examples 1-10 of the present invention and Comparative Examples 1-8 and 15-21 were measured. The results are shown in Table 4.

[0177] Table 4

[0178]

[0179] As shown in Table 4, the yields of the boron carbide nanopowders prepared in Examples 1-10 of the present invention are in the range of 90.0%-98.5%, and the purities are in the range of 91.0-99.5%, among which the yields and purities of Examples 8-10 are higher.

[0180] By comparing Examples 1-6, it can be seen that when the sugar compounds in Example 6 are glucose and xylose, and the oxygen-containing compounds are sorbitol, polyethylene glycol, and poly(ethylene glycol) tetrahydrofurfuryl ether, the yield and purity of the boron carbide nanopowder are higher.

[0181] Based on Example 6, Example 7 adds sodium chloride during the carbon coating process to improve the solid phase reaction effect. Based on Example 7, Example 8 designs the vacuum degree during the solid phase reaction, resulting in further improvement in reaction yield and purity.

[0182] Compared with Example 6, Comparative Examples 1-8 changed the specific steps of carbon coating. Compared with Example 8, Comparative Example 15 changed sodium chloride to potassium chloride, Comparative Examples 16-17 changed the composition of oxygen-containing compounds, Comparative Examples 18 and 19 changed the carbonization temperature of different steps, and Comparative Examples 20 and 21 changed the amount of sodium chloride added. As a result, the yield and purity of Comparative Examples 1-8 and 15-21 decreased to varying degrees.

[0183] 2. Oxygen content of boron carbide nanopowder

[0184] The oxygen content of the boron carbide nanopowders prepared in Examples 6-10 and Comparative Examples 15 and 18-21 was analyzed. The results are shown in Table 5.

[0185] Table 5

[0186]

[0187] The higher the oxygen content of boron carbide nanopowders, the more likely they are to affect the performance of boron carbide products. As shown in Table 5, the oxygen content of boron carbide nanopowders produced in Examples 6-10 of the present invention ranges from 0.05% to 0.47%. Examples 8-10 have relatively low oxygen contents, ranging from 0.05% to 0.08%, which is beneficial for improving the sintering performance of boron carbide.

[0188] 3. Fluidity of boron carbide nanopowder

[0189] The fluidity index, the angle of repose, of the boron carbide nanopowders prepared in Examples 6-10 of the present invention and Comparative Examples 22-25 was measured. The results are shown in Table 6.

[0190] Table 6

[0191]

[0192] As shown in Table 6, compared with Examples 6 and 7, Examples 8-10 of the present invention control the vacuum change process during the solid phase reaction to prepare boron carbide nanopowders, and the repose angles of the obtained powders are all controlled below 40°, and the powder fluidity is significantly improved.

[0193] Compared with Example 8, Comparative Examples 22-25 respectively changed the vacuum degree variation process in the solid phase reaction, resulting in poor powder fluidity.

[0194] 4. Sintering properties of boron carbide nanopowders prepared by the present invention

[0195] The boron carbide nanopowder was sintered using the methods of Examples 6-10 of the present invention and Comparative Examples 15 and 18-25, and its sintering properties were measured. The results are shown in Table 7.

[0196] Table 7

[0197]

[0198] As shown in Table 7, the boron carbide nanopowders prepared in Examples 6-10 of the present invention have good sintering properties, with relative density ranging from 98.2% to 99.6%, hardness ranging from 29.5 GPa to 33.5 GPa, and fracture toughness ranging from 2.81 MPa·m 1 / 2Especially in Examples 8-10, when preparing boron carbide nanopowder by solid phase reaction, the vacuum degree variation process is controlled, and the sintering performance of the obtained boron carbide nanopowder is better.

[0199] Compared with Example 8, Comparative Examples 15 and 18-21 changed the carbon coating treatment step of hexagonal boron nitride, and Comparative Examples 22-25 changed the vacuum degree change process in the solid phase reaction. As a result, the sintering properties of the boron carbide nanopowders were all deteriorated.

Claims

1. A method for preparing boron carbide nanopowder, characterized in that: The following steps are involved: After hexagonal boron nitride is carbon-coated with a carbon source, the carbon source is placed in a vacuum reactor for solid-phase reaction, wherein the carbon source includes a sugar compound and an oxygen-containing compound; The carbon coating process comprises the following steps: (1) The sugar compound and hexagonal boron nitride are mixed and ball-milled, the obtained mixed powder is evenly dispersed in water, and then hydrothermal carbonization is performed to obtain the first carbon-coated boron nitride; (2) spraying an oxygen-containing compound onto the surface of the first carbon-coated boron nitride obtained in step (1), cooling and drying, and then placing it in a sealed state for carbonization at 500-1000°C for 3-6 hours, and cooling naturally to obtain the second carbon-coated boron nitride; The carbohydrate compound includes hexose and pentose in a mass ratio of 1:(0.3-0.6); the oxygen-containing compound is one or more of polyethylene glycol, poly(ethylene glycol) tetrahydrofurfuryl ether, mannitol, and sorbitol.

2. The preparation method according to claim 1, wherein: The carbon source is calculated as C, and the molar ratio of the carbon source to the hexagonal boron nitride is (1.1-1.6):

4. The diameter of the hexagonal boron nitride is 0.001-50 μm.

3. The preparation method according to claim 1, wherein: The mass ratio of the carbohydrate compound to the oxygen-containing compound is (0.5-2):1, the hexose is one or more of glucose, mannose, fructose and galactose, and the pentose is one or two of xylose and deoxyribose; the number of oxygen atoms in the oxygen-containing compound accounts for 10-24%.

4. The preparation method according to claim 1, wherein: The temperature of the solid phase reaction in the vacuum reactor is 1500-2000℃, the heating rate is 30-50℃, the holding time is 3-6h, and the vacuum degree in the vacuum reactor is (0.1-10)×10 -4 Pa.

5. The preparation method according to claim 1, wherein: In step (1), the ball milling rate is 300-500 rpm, and the ball milling time is 3-8 h; the hydrothermal carbonization conditions are: 160-250 ° C, a heating rate of 1-10 ° C / min, and heat preservation for 2-4 h; In step (2), the carbonization heating rate is 5-30°C / min.

6. The preparation method according to claim 3, wherein: The oxygen-containing compound comprises sorbitol, polyethylene glycol and poly(ethylene glycol) tetrahydrofurfuryl ether, and the mass ratio of the three is (0.001-0.004): (0.2-0.8): 1; The carbon coating process comprises the following steps: (1) The sugar compound and hexagonal boron nitride are mixed and ball-milled, the obtained mixed powder is evenly dispersed in water, and then hydrothermal carbonization is performed to obtain the first carbon-coated boron nitride; (2) Spraying a solution formed by a mixture of saturated sodium chloride aqueous solution and sorbitol onto the surface of the first carbon-coated boron nitride obtained in step (1), and then placing it at a constant temperature of 70-80°C for evaporation for 1-3 hours, and then continuing to spray polyethylene glycol and poly (ethylene glycol) tetrahydrofurfuryl ether. After spraying, drying, first placing it at 500-750°C for vacuum carbonization for 2-5 hours, and then heating it to 850-1000°C and keeping it warm for 0.5-1 hour, and then cooling it naturally, repeatedly rinsing it with clean water to remove sodium chloride, and drying it to obtain a second carbon-coated boron nitride.

7. The preparation method according to claim 6, characterized in that: The amount of sodium chloride added is 1-6% of the mass of the oxygen-containing compound, the heating rate to 500-750°C is 5-20°C / min, and the heating rate to 850-1000°C is 15-30°C / min.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The change process of vacuum degree in the vacuum reactor during solid phase reaction is as follows: Before heating to 1500-2000℃, the vacuum degree is (6-10)×10 -4 Pa; During the 0-1h period of heating to 1500-2000℃, the vacuum degree is (3-5)×10 -4 Pa; During the first 2 hours of heating to 1500-2000℃, the vacuum degree is (0.1-1)×10 -4 Pa; During the 2-6h period of heating to 1500-2000℃, the vacuum degree is (2-4)×10 -4 Pa.

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