A high-entropy boride ceramic precursor, ceramic powder, and preparation method and application thereof

Abstract

The present invention belongs to the field of high-entropy materials technology and specifically relates to a high-entropy boride ceramic precursor, ceramic powder, preparation methods, and applications thereof. The high-entropy boride ceramic precursor is a polymer composed of a metal source and a boron-carbon source connected by a bridging oxygen bond, and includes a metal element M and a boron element. The metal element M is selected from at least four of Ti, Zr, Hf, V, Nb, Ta, Mo, and W. The ceramic powder of the present invention is a high-purity, completely chemically uniform solid solution in which the elements are uniformly distributed at the molecular level.

Description

Technical Field

[0001] The present invention belongs to the technical field of high entropy materials, and specifically relates to a high entropy boride ceramic precursor, ceramic powder, and a preparation method and application thereof. Background Art

[0002] Ultrahigh-temperature ceramics are a class of compounds with melting points exceeding 3000°C. They possess excellent thermal protection properties, including high modulus, high hardness, and high strength. These ceramics are suitable for extreme environments such as hypersonic long-duration flight, atmospheric reentry, transatmospheric flight, and rocket propulsion systems. They are used in a variety of key components, including aircraft nose cones, wing leading edges, and engine hot ends. Transition metal boride ceramics, with their high melting point, high hardness, high electrical and thermal conductivity, low thermal expansion coefficient, and good oxidation and thermal shock resistance, are promising candidates for ultrahigh-temperature ceramics and hold broad application prospects in the aerospace sector.

[0003] A high-entropy alloy is an alloy formed by alloying five or more elemental components in an equiatomic ratio or a near-equiatomic ratio. High-entropy alloys have superior properties that traditional alloys cannot match, such as high strength, high hardness, high wear resistance, and corrosion resistance. Since ceramic powders can also form a high-mixing-entropy stable solid solution after sintering, and high-entropy ceramics have the advantages of high melting point and high hardness, they have become a major research hotspot.

[0004] High-entropy boride ceramics are an emerging class of single-phase solid solution ceramic materials composed of five or more metal borides in nearly equal molar ratios. They exhibit a hexagonal crystal structure composed of alternating two-dimensional boron and metal layers. High-entropy boride ceramics enrich the field of ceramics by further enhancing their performance through the "high entropy effect" and precise compositional control.

[0005] In 2016, Gild et al. (Scientific Reports 2016, 6, 1-10) successfully prepared (HfB2) using transition metal diborides such as ZrB2, HfB2, TaB2, TiB2, MoB2, NbB2 and CrB2 as raw materials by spark plasma sintering (SPS) technology. 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2、(Hf 0.2 Zr 0.2 Ta 0.2 Mo 0.2 Ti 0.2 )B2、(Hf 0.2 Zr 0.2 Mo 0.2 Nb 0.2 Ti 0.2)B2 and other series of high entropy ceramics, the distribution of transition metal elements and boron elements is relatively uniform, compared with the single-phase boride ceramics prepared by the same method, they are optimized in terms of hardness and oxidation resistance. However, due to the grinding step involved in the preparation of commercial boride raw materials, it is easy to introduce impurities such as oxides, which makes sintering densification difficult, and the highest density is only 92.4%. In order to solve the density problem, Gild et al. (Scripta Materialia, 2019, 170, 106-110) improved the above method by adding a small amount of C to the original boride powder, first pre-sintering it at 1600 ° C and 30 MPa for 5 minutes, and then heat-treating the powder by lightning spark plasma sintering under different power conditions without using a mold, and obtained a density of 99.3% (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2 high entropy ceramics, but still accompanied by a small amount of oxide and carbide impurities in the system.

[0006] In order to avoid the introduction of oxide impurities from the raw materials, boron thermal / boron carbothermal reduction has become one of the research hotspots for the preparation of high entropy ceramics. Zhang et al. (Journal of the European Ceramic Society, 2019, 39, 3920-3924) used boron carbothermal reduction reaction to convert metal oxides, boron carbide and carbon powder into corresponding boride powders at 1600 ° C, and successfully prepared (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2、(Hf 0.2 Zr 0.2 Mo 0.2 Nb 0.2 Ti 0.2 )B2、(Hf 0.2 Mo 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2 high entropy ceramics, the hardness and density of the high entropy ceramics prepared by this method have been greatly improved, especially (Hf 0.2 Zr 0.2 Mo 0.2 Nb 0.2 Ti 0.2)B2 system high entropy ceramics, its density reached 98.5% and the hardness also reached 27GPa. However, the obtained high entropy ceramics still contain oxide impurities, and the uniformity of the distribution of Nb and Hf elements is poor. Liu et al. (Scripta Materialia, 2019, 167, 110-114) used metal oxides and boron powder as raw materials and prepared (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2 high entropy powder, the particle size is about 310nm, and XRD analysis shows that there is no obvious oxide impurity peak. The Fu Zhengyi team of Wuhan University of Technology (Gu, Science China Materials, 2019, 62, 1898-1909) first used boron carbide to reduce metal oxides and obtained fine, high-purity boride powders at 1700 ° C. The oxygen content and carbon content were only 0.64% and 0.04%, respectively. Then, through SPS, the high-purity and dense (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2 high entropy boride ceramics.

[0007] In addition to the above methods, Tallarita et al. (ScriptaMaterialia, 2019, 158, 100-104) reported that Hf, Mo, Ta, Nb, Ti, and B powders were uniformly reacted by self-propagating reaction (SHS), and then the SHS powder was prepared by spark plasma sintering (SPS) at 1950 ° C to obtain (Hf 0.2 Mo 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B2 high entropy ceramics, but the obtained high entropy ceramics still contain oxide impurities, which affect the sintering densification and hardness improvement of the ceramics.

[0008] At present, most of the reported high-entropy boride ceramics are prepared by inorganic methods, which have the advantage of easy availability of raw materials, but also have problems such as impure products and large particle size. Therefore, innovation in the preparation methods of high-entropy borides is imminent. Summary of the Invention

[0009] To improve the above technical problems, the present invention provides a high entropy boride ceramic precursor, a ceramic powder, and a preparation method and application of the same.

[0010] The high entropy boride ceramic precursor is a polymer composed of a metal source and a boron carbon source connected by a bridging oxygen bond, including a metal element M and a boron element, and the metal element M is selected from at least four of Ti, Zr, Hf, V, Nb, Ta, Mo, and W.

[0011] According to an embodiment of the present invention, the amount of each metal element accounts for 5 to 35% of the total metal amount of the high entropy boride ceramic precursor.

[0012] According to an embodiment of the present invention, the amount of substance of each metal element is the same or different, preferably the same.

[0013] According to an embodiment of the present invention, the ratio of the amount of the boron element to the total amount of the metal elements is 2.2 to 5:1, preferably 3 to 4:1, for example 3:1.

[0014] According to an embodiment of the present invention, the metal-carbon-boron elements in the high entropy boride ceramic precursor are uniformly distributed at the molecular level, and the metal elements, boron and carbon elements are uniformly distributed at a short range.

[0015] The present invention also provides a method for preparing the high entropy boride ceramic precursor, comprising the following steps:

[0016] (1) preparing a metal alkoxide copolymer solution;

[0017] (2) Preparation of boron-carbon source solution: Mix the boron source and carbon source in a C1-C4 organic acid, heat to 50-100°C and react for 0.5-3h;

[0018] (3) mixing the metal alkoxide copolymer solution of step (1) with the boron-carbon source solution of step (2), heating the mixture to 80-100° C. for a first heating reaction, reacting until the system gels, heating the mixture to 110-150° C. for a second heating reaction, drying, and cooling the mixture to obtain a high-entropy boride ceramic precursor.

[0019] According to an embodiment of the present invention, in step (3), the C1-C4 organic acid includes at least one of formic acid, acetic acid, propionic acid or butyric acid, for example, acetic acid.

[0020] According to an embodiment of the present invention, in step (3), the ratio of the amount of boron element in the boron-carbon source to the total amount of metal elements is 2.2 to 5:1, preferably 3 to 4:1, for example 3:1.

[0021] According to an embodiment of the present invention, in step (3), the temperature of the first heating reaction is 85-95° C., and the reaction time is 0.5-3 h, preferably 1-2 h.

[0022] According to an embodiment of the present invention, in step (3), the temperature of the second heating reaction is 120-130° C., and the reaction time is 3-10 h, preferably 4-6 h.

[0023] According to an embodiment of the present invention, the mixing in step (3) is carried out at a temperature of 40 to 80°C, preferably 50 to 70°C, for example, the temperature of the boron carbon source is kept at 45°C, and the metal alkoxide copolymer is added and mixed.

[0024] According to an embodiment of the present invention, in step (3), the boron source is boric acid, and the carbon source is a polyol, such as sorbitol.

[0025] According to an embodiment of the present invention, the molar ratio of the boron source, carbon source and acetic acid in step (3) is 1:0.2-1.5:0.4-4, preferably 1:0.5-1:1-3, for example 1:0.5:1.

[0026] According to an embodiment of the present invention, the drying in step (3) is vacuum drying, the drying temperature is 80-150° C., and the drying time is 3-8 hours. Preferably, the drying temperature is 100-120° C., and the drying time is 4-6 hours.

[0027] According to an embodiment of the present invention, the metal alkoxide copolymer solution comprises the following steps:

[0028] a. Adding a complexing agent to the metal alkoxide M(OR)n at room temperature to 80°C to react and obtain a metal alkoxide complex;

[0029] b. Select at least four metal alkoxide complexes containing different metal elements prepared in step a, mix them evenly, and slowly add a mixture of water and monohydric alcohol at room temperature to 90° C., reflux for 1 to 5 hours after the addition is completed, to obtain a metal alkoxide copolymer solution.

[0030] According to an embodiment of the present invention, the M in step M(OR)n has the definition as described above.

[0031] As an example, when the M is selected from Hf, V, Nb, Ta, Mo or W, the metal alkoxide is prepared as follows: the metal salt of M is dispersed in an organic solvent, a monohydric alcohol is added dropwise at -10 to 5°C, and then triethylamine is added dropwise. After the addition is completed, the mixture is heated under reflux for 1 to 5 hours and filtered to obtain a metal alkoxide solution.

[0032] According to an embodiment of the present invention, the metal salt of M is selected from MCln or M(NO3)n.

[0033] According to an embodiment of the present invention, n is determined according to the valence state of M. For example, when M is selected from Ti, Zr or Hf, n is 4; when M is selected from V, Nb, Ta or Mo, n is 5; when M is W, n is 6.

[0034] According to an embodiment of the present invention, the ratio of the metal alkoxide, the monohydric alcohol and the triethylamine is 1: (1-2)n: (1-1.5)n.

[0035] According to an embodiment of the present invention, the organic solvent is one or more of n-hexane, n-heptane, toluene, xylene, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and tert-butyl methyl ether.

[0036] According to an embodiment of the present invention, the monohydric alcohol is selected from one or more of methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, ethylene glycol methyl ether, and ethylene glycol ethyl ether.

[0037] According to an embodiment of the present invention, the molar ratio of the metal alkoxide to the complexing agent in step b is 1:(0.12-0.4)n, preferably 1:(0.2-0.3)n.

[0038] According to an embodiment of the present invention, the complexing agent in step b is one or a mixture of acetylacetone and ethyl acetoacetate.

[0039] According to an embodiment of the present invention, the reaction time in step b is 0.1 to 5 hours, preferably 1 to 3 hours.

[0040] According to an embodiment of the present invention, the molar ratio of water to total metals in step c is 0.9-1.5:1, and the mass ratio of the monohydric alcohol to water is 3-8:1. Preferably, the molar ratio of water to total metals is 1-1.2:1, and the mass ratio of the monohydric alcohol to water is 5-7:1.

[0041] According to an embodiment of the present invention, the monohydric alcohol in step c is selected from one or more of methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, ethylene glycol methyl ether, and ethylene glycol ethyl ether.

[0042] The present invention also provides the use of the high-entropy boride ceramic precursor in the preparation of high-entropy boride ceramic powder.

[0043] The present invention also provides a high-entropy boride ceramic powder, which is obtained by debinding and cracking the high-entropy boride ceramic precursor.

[0044] According to an embodiment of the present invention, the chemical formula of the high entropy boride ceramic powder is MB y , for example (Ti1Zr 0.99 Hf 1.02 Nb 1.02Ta 1.00 )B 10.06 .

[0045] According to an embodiment of the present invention, the high entropy boride ceramic powder is a solid solution.

[0046] According to an embodiment of the present invention, the solid solution is a crystal, and the crystal has a hexagonal phase.

[0047] According to an embodiment of the present invention, the size of the high entropy boride ceramic powder particles is 100 nm to 1 μm, preferably 300 nm to 800 nm, for example 300 nm to 500 nm.

[0048] According to an embodiment of the present invention, the metal element in the solid solution is the same as the metal element in the high entropy boride ceramic precursor.

[0049] According to an embodiment of the present invention, the metal elements and boron elements in the solid solution are uniformly distributed at the molecular level.

[0050] The present invention also provides a method for preparing the high-entropy boride ceramic powder, comprising the following steps: subjecting the high-entropy boride ceramic precursor to debinding and cracking to obtain the high-entropy boride ceramic powder.

[0051] According to an embodiment of the present invention, the debinding temperature is not higher than 500°C, and the debinding time is 0.5-4h; preferably, the debinding temperature is 400-500°C, and the debinding time is 1-3h, for example, debinding at 400°C for 2h.

[0052] According to an embodiment of the present invention, the cracking temperature is not less than 1500°C, and the cracking time is 0.5-5h; preferably, the cracking temperature is 1700-2000°C, and the cracking time is 1-4h, for example, cracking at 1800°C for 4h.

[0053] According to an embodiment of the present invention, the debinding is performed under an inert atmosphere, and the cracking is performed under vacuum or an inert atmosphere, wherein the inert atmosphere is selected from argon, helium or a mixture of the two.

[0054] The present invention also provides a high-entropy boride ceramic, which is made from the high-entropy boride ceramic powder.

[0055] The present invention also provides a use of the high-entropy boride ceramic in heat protection, such as use in aircraft and rockets.

[0056] The present invention also provides a heat-resistant component, the surface of which is provided with the above-mentioned high-entropy boride ceramic or is made of the above-mentioned high-entropy boride ceramic.

[0057] In the above scheme, researchers of the present invention discovered that different types of metal elements have different reactivity during the complex formation process. If the complexing agent is added in a ratio outside the scope of the present invention, although the complex can be formed, the reaction equilibrium will be tilted due to the difference in the amount of complexing agent added during the subsequent mixed hydrolysis of the multiple metal element alkoxide complex, resulting in the inability to form a precursor with uniform molecular distribution. In addition, the amount of complexing agent added affects the amount of residual active groups in the resulting metal alkoxide copolymer, which in turn affects the reactivity of the metal alkoxide copolymer and the boron-carbon source. Using the metal alkoxide to complexing agent ratio provided by the present invention can overcome this problem, making the reactivity difference of each metal relatively small during the subsequent hydrolysis, while retaining the reactivity of the metal alkoxide copolymer and the boron-carbon source.

[0058] In the above scheme, the ratio of alkoxide to water provided by the present invention is obtained based on the consideration of mixing metal alkoxides with different reactivity, so that the reactivity of multiple metal alkoxides during co-hydrolysis tends to be similar, thereby obtaining an alkoxide copolymer with a uniform distribution of metal elements at the molecular level, while retaining the reactivity of the metal alkoxide copolymer with the boron-carbon source.

[0059] In order to further improve the reactivity of the metal alkoxide copolymer and the boron-carbon source, the present invention, on the basis of controlling the amount of ligand and water, mixes the metal alkoxide copolymer in the form of a solution (rather than the metal alkoxide copolymer from which the solvent has been removed by distillation) with the boron-carbon source for reaction. Adding the metal alkoxide copolymer in the form of a solution can prevent the active groups in the metal alkoxide copolymer from being reduced due to continued reaction during the distillation process, thereby avoiding the problem of mismatch in the reactivity of the metal alkoxide copolymer and the boron-carbon source.

[0060] In the above preparation method, the boron source and the carbon source need to first undergo a polymerization reaction to prepare a boron-carbon source. This is mainly to solve the problem of mismatched reaction rates between the boron source boric acid and the carbon source polyol and the metal polymer. Specifically, the boron source boric acid reacts slowly with the metal copolymer due to poor solubility and low reactivity, while the carbon source polyol reacts too quickly with the metal polymer. If both react with the metal copolymer at the same time, it is very easy for the polyol and the metal copolymer to react rapidly, resulting in precipitation. At the same time, the reactivity of the metal copolymer is reduced, and boric acid cannot participate in the reaction, resulting in the inability to obtain a precursor with the target element content and the elements uniformly distributed at the molecular level.

[0061] First, a boron source is reacted with a carbon source to prepare a boron-carbon source, and then a metal source is polymerized with the boron-carbon source. The metal source and the boron-carbon source are connected by a bridging oxygen bond to form a polymer in which metal-carbon-boron elements are uniformly distributed at the molecular level. The metal elements and B and C elements in the polymer are uniformly distributed at a short range, which is conducive to the occurrence of boron-carbon thermal reduction reaction during the cracking of the precursor, thereby obtaining high-entropy boride ceramics with uniform element distribution at a relatively low temperature.

[0062] In the present invention, the reactivity of the metal alkoxide is reduced after copolymerization to form a metal alkoxide copolymer, and the activity of the carbon source and the boron source is also reduced after the reaction. This appropriate activity reduction treatment is beneficial to the control of the reaction process to obtain a more uniform precursor.

[0063] It is worth noting that when the carbon source reacts with the boron source to prepare the boron-carbon source, due to the excess of boric acid, it cannot be guaranteed that the boron source and the boric acid will react completely with the carbon source. In order to prevent the unreacted boric acid in step (3) from precipitating after cooling, thereby affecting its reaction activity and further affecting the elemental uniformity of the product, the mixing temperature of the metal copolymer and the boron-carbon source in the present invention is 40-80°C.

[0064] The entire process of precursor cracking involves boron-carbon thermal reduction reaction, so there is an appropriate ratio range for the feeding of metal source, boron source and carbon source. Otherwise, metal carbide impurity phase will appear due to insufficient boron or excessive carbon, or boron carbide impurity phase will appear due to excessive boron.

[0065] The high entropy boride ceramic precursor in the present invention is a polymer in which a metal source and a boron-carbon source are connected by a metal bridge oxygen bond. The metal source is prepared by co-hydrolysis of a metal alkoxide, and the boron-carbon source is prepared by reacting a boron-containing compound and a polyol. Since the reactivity of the metal alkoxide is high and the activity of different metal alkoxides is quite different, if the metal alkoxide is directly used to react with the boron-carbon source, the metal alkoxide with high reactivity will first occupy the reactive sites of the boron-carbon source, and at the same time, insoluble matter will be precipitated due to the excessively fast reaction speed, while the metal alkoxide with low reactivity will hardly react with the boron-carbon source, thereby causing the ratio between the metal elements to change and the metal elements to be unevenly distributed. Therefore, the metal alkoxide is first co-hydrolyzed to prepare the metal source, which, on the one hand, avoids the change in the ratio of metal elements caused by the difference in reactivity between the metal alkoxides, and on the other hand, also allows multiple metals to be uniformly distributed at the molecular level in the precursor. At the same time, in order to avoid element loss or uneven element distribution caused by the mismatch of reaction activity between the boron source and the carbon source and the metal copolymer, the present invention pre-reacts the boron source and the carbon source to prepare a boron-carbon source, which is then reacted with the metal polymer, thereby ensuring that the elements in the precursor are evenly distributed at the molecular level.

[0066] Beneficial effects

[0067] In the prior art, boride high-entropy ceramics are mostly prepared by inorganic powder reaction sintering methods, which often result in low-purity solid solutions (impurity peaks on XRD) and uneven element distribution. The present invention adopts a polymer precursor method to prepare boride high-entropy boride ceramic precursors. Because the elements in the polymer precursor are uniformly dispersed at the molecular level, the uniform distribution of elements is maintained during the solidification and cracking processes, which is conducive to achieving uniform element distribution in the boride solid solution. Therefore, a high-purity, completely chemically uniform solid solution can be obtained at a relatively low temperature (1700-2000°C, for example, 1800°C), and the elements in the solid solution are uniformly distributed at the molecular level. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 is the XRD pattern of the high entropy boride ceramic powder obtained in Example 2.

[0069] Figure 2 These are the SEM and EDS images of the high-entropy boride ceramic powder obtained in Example 2.

[0070] Figure 3 This is the XRD pattern of the ceramic powder obtained in Comparative Example 1.

[0071] Figure 4 This is the XRD pattern of the ceramic powder obtained in Comparative Example 2.

[0072] Figure 5 This is the XRD pattern of the ceramic powder obtained in Comparative Example 3.

[0073] Figure 6 This is the XRD pattern of the ceramic powder obtained in Comparative Example 5. DETAILED DESCRIPTION

[0074] The following will further describe the high-entropy ceramic precursor of the present invention, its preparation method, and application in conjunction with specific examples. It should be understood that the following examples are merely exemplary illustrations and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope of protection intended by the present invention.

[0075] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0076] Example 1 Preparation of high entropy boride ceramic precursor:

[0077] (1) Obtain metal alkoxides: Ti(OPr)4, Zr(OPr)4, Hf(OPr)4, Nb(OCH2CH2OCH2CH3)5 and Ta(OCH2CH3)5

[0078] Ti(OPr)4 and Zr(OPr)4 were purchased directly, and Hf(OPr)4, Nb(OCH2CH2OCH2CH3)5 and Ta(OCH2CH3)5 were prepared by the following method:

[0079] Among them, Hf(OPr)4 and Nb(OCH2CH2OCH2CH3)5 are prepared by dispersing metal salts HfCl4 and NbCl5 in n-heptane respectively, dropping monohydric alcohol n-propanol and ethylene glycol ethyl ether respectively at -10°C, and then dropping triethylamine respectively. After the dropwise addition is completed, the solution is heated under reflux for 1 hour and filtered to obtain metal alkoxide solutions; wherein the molar ratio of HfCl4, n-propanol and triethylamine is 1:4:4, and the molar ratio of NbCl5, ethylene glycol ethyl ether and triethylamine is 1:5:6.

[0080] Ta(OCH2CH3)5 is prepared by dispersing the metal salt TaCl5 in ethylene glycol dimethyl ether. At -5°C, monohydric alcohol ethanol is first added dropwise, followed by triethylamine. After the addition is complete, the solution is heated under reflux for 1 hour and filtered to obtain a metal alkoxide solution. The molar ratio of TaCl5, ethanol, and triethylamine is 1:5:5.

[0081] (2) Preparation of metal alkoxide complexes: At 40°C, acetylacetone, a complexing agent, was added dropwise to metal alkoxides Ti(OPr)4, Zr(OPr)4, Hf(OPr)4, Nb(OCH2CH2OCH2CH3)5 and Ta(OCH2CH3)5, respectively. The mixture was stirred for 0.1 h to obtain metal alkoxide complexes. The molar ratios of metal alkoxides Ti(OPr)4, Zr(OPr)4, Hf(OPr)4, Nb(OCH2CH2OCH2CH3)5 and Ta(OCH2CH3)5 to acetylacetone were 1:0.48, 1:0.8, 1:1, 1:2 and 1:2, respectively.

[0082] (3) Co-hydrolysis: The metal alkoxide complex obtained in step (2) is uniformly mixed in an equal metal molar ratio, and a mixed solution of water and n-propanol is slowly added dropwise to the system at room temperature, wherein the molar ratio of water to total metal is 1.5:1 and the mass ratio of n-propanol to water is 4:1. After the addition is completed, the mixture is refluxed for 5 hours to obtain a metal alkoxide copolymer solution.

[0083] (4) Preparation of a boron-carbon source: Boric acid and sorbitol were mixed uniformly in acetic acid, and then reacted at 60° C. for 2 h to obtain a boron-carbon source solution; the molar ratio of boric acid, sorbitol, and acetic acid was 1:0.5:1.

[0084] (5) Preparing a precursor: maintaining the temperature of the boron-carbon source solution obtained in step (4) at 45°C, adding the metal alkoxide copolymer solution of step (3) thereto, then heating to 80°C for a first heating reaction, reacting for 1 hour to allow the system to gel, then heating to 120°C for a second heating reaction, the reaction time being 5 hours; finally, drying the obtained gel in a vacuum oven at 120°C for 4 hours, and cooling to obtain a high-entropy boride ceramic precursor;

[0085] The ratio of the total amount of metal in the metal alkoxide copolymer to the amount of boron in the boron-carbon source is 1:3.

[0086] Example 2 Preparation of high entropy boride ceramic powder

[0087] The high entropy boride ceramic precursor prepared in Example 1 was debinded at 400°C for 2 h under argon, and the debinded product was then pyrolyzed at 1800°C for 4 h under vacuum conditions to obtain high entropy boride ceramic powder (hereinafter referred to as ceramic).

[0088] like Figure 1 As shown, this is the XRD diagram of the ceramic. There is only one set of diffraction peaks in the diagram, indicating that solid solution has occurred, so that the metal atoms are completely dissolved into a crystal lattice, and the system does not contain impurity peaks of metal oxides, metal carbides or boron carbide.

[0089] The metal element content of the ceramic was tested by inductively coupled plasma spectroscopy, and the results were as follows: Ti-6.6%, Zr-2.4%, Hf-25.1%, Nb-13.1%, Ta-25%, B-15%, which is converted into the empirical formula (Ti1Zr 0.99 Hf 1.02 Nb 1.02 Ta 1.00 )B 10.06 , it can be seen that the contents of the five metals and boron are close to the theoretical values.

[0090] The oxygen content of the ceramics tested by an oxygen and nitrogen analyzer was only 0.8%, indicating that the ceramics prepared by this method had a low oxygen content.

[0091] like Figure 2 As shown in the figure, the SEM and EDS images of the ceramics have an average particle size of 300 nm. The EDS image shows that the metal elements are evenly distributed, indicating that no element segregation occurs during the cracking process. The above method forms a fine and uniform high-entropy boride ceramic powder.

[0092] Comparative Example 1 Preparation of high entropy boride ceramic precursor

[0093] In this comparative example, based on Example 1, the ratio of the total amount of metal in step (4) to the amount of boron in the boron-carbon source was adjusted to 1:6; other conditions were the same as in Example 1.

[0094] like Figure 3 As shown, this is the XRD pattern of the ceramic precursor obtained in Comparative Example 1, in which two groups of diffraction peaks appear, one group is high entropy boride, and the other group is boron carbide peaks (impurity peaks), indicating that the high entropy boride ceramic precursor obtained in this comparative example contains boron carbide impurities, that is, when the amount of boron-carbon source exceeds the range specified in the present invention (the ratio of the amount of boron element to the total amount of metal elements is 2.2 to 5:1), a pure phase high entropy boride ceramic precursor cannot be obtained.

[0095] Comparative Example 2 Preparation of High Entropy Boride Ceramic Precursor

[0096] In this comparative example, based on Example 1, the molar ratio of boric acid, sorbitol and acetic acid was adjusted to 1:2:1; other implementation conditions of this comparative example were the same as those of Example 1.

[0097] like Figure 4 As shown, this is the XRD pattern of the ceramic precursor obtained in this comparative example. There are two groups of diffraction peaks in the figure, one group is high entropy boride, and the other group is the impurity peak of high entropy carbide, indicating that when the amount of carbon source exceeds the limited range of the present invention (the molar ratio of boron source, carbon source and acetic acid is 1:0.2~1.5:0.4~4), a pure phase high entropy boride ceramic precursor cannot be obtained.

[0098] Comparative Example 3

[0099] In this comparative example, based on Example 1, the molar ratio of Hf(OPr)4 to acetylacetone in step (2) was adjusted to 1:4. The other conditions of this comparative example were the same as those in Example 1.

[0100] like Figure 5 As shown in FIG, the XRD pattern of the ceramic precursor obtained in this comparative example, there are two groups of diffraction peaks in the figure, one group is the boride obtained by the solid solution of the quaternary metal, and the other group is the peak of HfB2, indicating that the metal Hf fails to participate in the solid solution. This is mainly because when the complexing agent content is high, the hydrolysis process will be slow, so the metal Hf fails to participate in the co-hydrolysis, so that the Hf element and other metal elements in the precursor cannot maintain a close distribution during the cracking process, the diffusion is difficult, and it fails to participate in the solid solution.

[0101] Comparative Example 4

[0102] In this comparative example, based on Example 1, the molar ratio of Hf(OPr)4 to acetylacetone in step (2) was adjusted to 1:0.25. The other conditions of this comparative example were the same as those in Example 1.

[0103] Precipitation occurs during the co-hydrolysis process, which is due to the fact that the amount of complexing agent used is too low, resulting in the hafnium alkoxide complex being hydrolyzed too quickly.

[0104] Comparative Example 5

[0105] In this comparative example, based on Example 1, the system after reflux in step (3) was subjected to atmospheric distillation to remove the solvent, thereby obtaining a metal alkoxide copolymer. This metal alkoxide copolymer was then added to the boron-carbon source solution obtained in step (4). The other conditions of this comparative example were the same as those of Example 1. It was found that the system did not gel during the entire process. This is because the active groups of the metal alkoxide copolymer continued to react during the atmospheric distillation process, affecting its reactivity with the boron-carbon source. The resulting precursor was pyrolyzed as in Example 2.

[0106] like Figure 6 As shown in FIG, the XRD pattern of the ceramic precursor obtained in this comparative example, it can be seen that multiple groups of diffraction peaks such as diboride (MB2), monoboride (MB), oxide (MO2) and carbide (MC) appear in the figure. This is because the metal copolymer fails to react with the boron-carbon source, resulting in phase separation between the metal copolymer and the boron-carbon source during the solidification and cracking process, making the metal and carbon-boron atoms remotely distributed, and therefore, it is not easy to react to form a high-entropy boride solid solution.

[0107] The above describes the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing a high entropy boride ceramic precursor, characterized in that: The steps include: (1) preparing a metal alkoxide copolymer solution; (2) Preparation of boron-carbon source solution: Mix the boron source and carbon source in acetic acid, heat to 50-100°C and react for 0.5-3h; The molar ratio of the boron source, carbon source and acetic acid is 1:0.2-1.5:0.4-4; (3) mixing the metal alkoxide copolymer solution of step (1) with the boron-carbon source solution of step (2), heating the mixture to 80-100° C. for a first heating reaction, reacting until the system gels, heating the mixture to 110-150° C. for a second heating reaction, drying, and cooling the mixture to obtain a high-entropy boride ceramic precursor; The high entropy boride ceramic precursor is a polymer composed of a metal source and a boron carbon source connected by a bridging oxygen bond, including a metal element M and a boron element, wherein the metal element M is selected from at least four of Ti, Zr, Hf, V, Nb, Ta, Mo, and W; The ratio of the amount of boron to the total amount of metal elements is 2.2 to 5:1; The amount of each metal element accounts for 5 to 35% of the total metal amount of the high entropy boride ceramic precursor.

2. The preparation method according to claim 1, characterized in that The boron source is boric acid, and the carbon source is polyol.

3. The preparation method according to claim 1, characterized in that The carbon source is sorbitol; And / or, in step (3), the temperature of the first heating reaction is 85-95° C., and the reaction time is 0.5-3 h; And / or, in step (3), the temperature of the second heating reaction is 120-130° C., and the reaction time is 3-10 h; And / or, the mixing in step (3) is carried out at a temperature of 40 to 80°C.

4. The preparation method according to claim 1, characterized in that The metal alkoxide copolymer solution comprises the following steps: a. Adding a complexing agent to the metal alkoxide M(OR)n at room temperature to 80°C to react and obtain a metal alkoxide complex; b. Select at least four metal alkoxide complexes containing different metal elements prepared in step a, mix them evenly, and slowly add a mixture of water and monohydric alcohol at room temperature to 90° C., reflux for 1 to 5 hours after the addition is completed, to obtain a metal alkoxide copolymer solution.

5. The preparation method according to claim 1, characterized in that When the M is selected from Hf, V, Nb, Ta, Mo or W, the metal alkoxide is prepared as follows: the metal salt of M is dispersed in an organic solvent, a monohydric alcohol is added dropwise at -10 to 5° C., and then triethylamine is added dropwise. After the addition is completed, the mixture is heated under reflux for 1 to 5 hours and filtered to obtain a metal alkoxide solution.

6. The preparation method according to claim 4, characterized in that In step b, the molar ratio of the metal alkoxide to the complexing agent is 1:(0.12-0.4)n; And / or, the complexing agent in step b is one or a mixture of acetylacetone and ethyl acetoacetate; And / or, the reaction time in step b is 0.1 to 5 hours.

7. The preparation method according to claim 4, characterized in that The molar ratio of water to total metals is 0.9 to 1.5:1, and the mass ratio of monohydric alcohol to water is 3 to 8:1; And / or, the monohydric alcohol is one or more selected from methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, ethylene glycol methyl ether, and ethylene glycol ethyl ether.

8. A method for preparing high entropy boride ceramic powder, characterized in that: The preparation method comprises the following steps: preparing the high entropy boride ceramic precursor by the preparation method according to any one of claims 1 to 7, and obtaining high entropy boride ceramic powder through debinding and cracking; The high entropy boride ceramic powder is a crystal having a hexagonal phase; The size of the high entropy boride ceramic powder particles is 100 nm to 1 μm.

9. The preparation method according to claim 8, characterized in that The debinding temperature is not higher than 500°C, and the debinding time is 0.5 to 4 hours; The cracking temperature is not less than 1500°C and the cracking time is 0.5 to 5 hours; The debinding is performed under an inert atmosphere, and the cracking is performed under a vacuum or inert atmosphere.

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