Hafnium carbide precursor and preparation method thereof

Abstract

The invention belongs to the field of superhigh-temperature ceramic materials, and particularly relates to a hafnium carbide precursor and a preparation method thereof.The preparation method comprises the steps that n-propyl alcohol hafnium and two special modified compounds are mixed in an inert atmosphere, dicumyl peroxide, benzophenone, triethanolamine and divinyl benzene are added for stirring, and the hafnium carbide precursor is obtained; adding tetrahydrofuran, stirring at a constant temperature to form a homogeneous solution, and finally filtering to obtain a precursor; the two modified compounds are vinyl tris (diethylamino) hafnium and acryloyloxy dibenzoyl hafnium complexes respectively, and the preparation method is realized through substitution reaction of hafnium tetrachloride and amine compounds and coordination reaction of n-propanol hafnium, dibenzoylmethane and acryloyl chloride respectively. The obtained hafnium carbide precursor contains a hafnium source, a modified compound, a cross-linking agent and a solvent, through molecular design, the ceramic yield is remarkably increased, the cracking temperature is reduced, the oxidation resistance and sintering density of final ceramic are improved, and the preparation method is suitable for preparing an ultra-high-temperature ceramic material.

Description

Technical Field

[0001] This invention belongs to the field of ultra-high temperature ceramic materials technology, specifically relating to a hafnium carbide precursor and its preparation method. Background Technology

[0002] Hafnium carbide, as a highly representative ultra-high temperature ceramic material, has attracted widespread attention due to its outstanding physical and chemical properties. It possesses a high melting point exceeding 3,900 degrees Celsius, excellent high-temperature strength, good thermal stability, and superior corrosion resistance. These properties make it irreplaceable in extreme environment applications such as aerospace, defense technology, and the nuclear energy industry. For example, it is used to manufacture leading-edge components of hypersonic vehicles, nozzles of solid rocket motors, and core structural materials for nuclear reactors. However, the inherent high melting point and strong covalent bonds of hafnium carbide also present significant challenges to its preparation. Traditional preparation processes, such as carbothermal reduction and solid-state sintering, typically require extremely high temperatures. These harsh process conditions not only lead to enormous energy consumption but also easily cause abnormal grain growth and microstructural inhomogeneity, thereby impairing the material's mechanical properties. More importantly, these methods are difficult to directly use for preparing components with complex geometries, greatly limiting the application of hafnium carbide ceramics in high-tech fields requiring precise configurations.

[0003] To overcome the limitations of traditional preparation methods, precursor conversion has emerged and gradually developed into an important technical route for preparing high-performance ceramic materials. The core of this method lies in designing and synthesizing specific organic precursors. These precursors already contain the basic constituent elements of the target ceramic in their molecular structure. After cross-linking and curing, they are pyrolyzed at relatively low temperatures to transform into the desired ceramic material. Compared to traditional methods, precursor conversion has significant advantages: it allows for precise control of the ceramic composition at the molecular scale, resulting in materials with uniform composition; its lower processing temperature helps suppress grain coarsening; and, most importantly, it can utilize mature polymer molding processes, such as impregnation, injection molding, or 3D printing, to prepare complex and near-net-shape ceramic components. Currently, precursors used in hafnium carbide preparation mainly focus on hafnium alkoxides and some organometallic compounds. Nevertheless, existing hafnium carbide precursor systems still face several key challenges that urgently need to be addressed. First, the ceramic yield is generally low, meaning that a large amount of organic components escape in gaseous form during pyrolysis, resulting in high porosity and shrinkage in the final product, making it prone to defects such as cracks. Second, the pyrolysis temperature for the complete conversion of the precursor to hafnium carbide ceramics remains high, which is difficult for many temperature-sensitive reinforcing or matrix materials to withstand. Furthermore, the molecular structures of existing precursors are relatively simple, lacking effective functional modifications, resulting in insufficient oxidation resistance and sintering activity in their pyrolysis products. They also generally exhibit poor stability in air and are prone to hydrolysis, posing difficulties for storage and subsequent processing.

[0004] To address the aforementioned issues, researchers have been continuously exploring innovative molecular-level designs for hafnium carbide precursors in recent years. These explorations include introducing new coordination structures to enhance precursor stability, or embedding polymerizable active functional groups into the molecule to improve ceramic yield through cross-linking reactions. Some studies have drawn on the design principles of high-entropy ceramics, attempting to optimize the final ceramic's performance by introducing multiple metallic or non-metallic elements. However, these explorations still face numerous challenges: how to balance the processability of the precursor with high ceramic yield; how to effectively reduce the pyrolysis temperature in molecular design without sacrificing ceramic purity and performance; and how to achieve synergistic effects of multiple functional groups to comprehensively improve the overall performance of hafnium carbide ceramics. Therefore, developing a novel hafnium carbide precursor that combines high ceramic yield, low pyrolysis temperature, excellent oxidation resistance, and good process stability is key to advancing this technological field and is the core technical problem this invention aims to solve. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a hafnium carbide precursor and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing a hafnium carbide precursor, comprising the steps of: S1. In an argon-atmosphere glove box, add the hafnium propoxide, vinyltris(diethylamino)hafnium, and acryloyloxydibenzoylhafnium complex to a reaction flask and stir at room temperature; add dicumyl peroxide, benzophenone, triethanolamine, and divinylbenzene, and continue stirring; add tetrahydrofuran, and continue stirring in a water bath at 38-42°C to form a homogeneous solution; S2. Filter the homogeneous solution through a polytetrafluoroethylene microporous membrane.

[0007] In this invention, the preparation of the hafnium carbide precursor is the core of its final performance. This process is essentially a physical mixing and preliminary chemical activation of multiple functional components at the molecular level. First, three hafnium sources—n-propoxide hafnium, a self-made vinyltris(diethylamino)hafnium, and an acryloxydibenzoyl hafnium complex—are blended in tetrahydrofuran solvent under an inert atmosphere. This step aims to achieve a uniform distribution of hafnium under different chemical environments and provide diverse active sites for subsequent crosslinking and curing. The key crosslinking and curing stage is jointly regulated by the addition of benzophenone, triethanolamine, dicumyl peroxide, and divinylbenzene. After absorbing ultraviolet light of a specific wavelength, benzophenone can transition to an excited state, extracting hydrogen atoms from triethanolamine to generate amine alkyl radicals that can initiate polymerization. These radicals then initiate free radical polymerization of the vinyl groups at the ends of the vinyltris(diethylamino)hafnium molecule and the acryloyloxydibenzoyl hafnium complex. Meanwhile, dicumyl peroxide homolytically cleaves under heating conditions to generate cumylphenoxy radicals. These highly reactive radicals can also initiate the aforementioned unsaturated groups and copolymerize with divinylbenzene, a multifunctional crosslinking agent, to construct a robust three-dimensional polymer network within the system. This crosslinking network is crucial for the transformation of the homogeneous solution into an insoluble and infusible solid resin during subsequent photo- or thermo-curing processes. It effectively locks in the components, laying a solid foundation for preventing the loss of low-molecular-weight substances during subsequent pyrolysis. During the pyrolysis of the precursor into hafnium carbide ceramics, as the temperature increases, this three-dimensional polymer network first undergoes cleavage and rearrangement, and the organic components gradually carbonize. The hafnium-nitrogen and hafnium-oxygen bonds introduced in the modified compound break within a specific temperature range, releasing carbon-containing fragments that react with hafnium species in a solid-state reaction. In particular, the presence of nitrogen may form carbonitrides or nitrogen-doped transition phases in the intermediate state. These transition phases can significantly reduce the activation energy for hafnium carbide nucleation and growth. Ultimately, under the protection of an inert atmosphere, through atomic rearrangement and diffusion, the organic components are transformed into amorphous carbon, while the hafnium species react with carbon to form nanocrystalline hafnium carbide ceramics. The entire pyrolysis process benefits from the prior molecular design and composite cross-linking, achieving a continuous and controllable transformation from organic polymers to inorganic ceramics, ultimately yielding hafnium carbide materials with high ceramic yield, low porosity, and uniform composition.

[0008] As a preferred embodiment of the present invention, in step S1, the stirring time in a water bath at 38-42°C is 2-4 hours.

[0009] As a preferred technical solution of the present invention, in step S2, the pore size of the polytetrafluoroethylene microporous filter membrane is 0.2-0.24 μm.

[0010] As a preferred embodiment of the present invention, the method for preparing vinyltris(diethylamino)hafnium includes: A1. Under argon protection, anhydrous tetrahydrofuran and hafnium tetrachloride were added to a dry round-bottom flask and cooled to -28 to -32°C. Anhydrous tetrahydrofuran solution of diethylamine was added, stirred, and the temperature was raised to room temperature. Stirring was continued. The mixture was filtered under reduced pressure, and the filtrate was concentrated by rotary evaporation at 38-42°C to obtain tris(diethylamino)hafnium chloride intermediate. A2. Add anhydrous diethyl ether to the tris(diethylamino)hafnium chloride intermediate, cool to -28~-32℃, then add a cyclohexane solution of vinyl lithium to obtain a reaction mixture; heat the reaction mixture to room temperature and continue stirring; after the reaction is complete, filter under reduced pressure to obtain a filtrate; transfer the filtrate to a crystallization flask and freeze-crystallize at -20℃~-40℃, collect the crystals by vacuum filtration, wash the crystals with pre-cooled anhydrous diethyl ether, and finally dry under high vacuum at room temperature.

[0011] In this invention, the synthesis of vinyltris(diethylamino)hafnium is based on a stepwise conversion reaction mechanism: starting with hafnium tetrachloride, it undergoes an aminolysis reaction with diethylamine under low-temperature conditions. The lone pair electrons of the amino nitrogen atom in the diethylamine molecule act as a nucleophile, attacking the hafnium center and gradually replacing the chlorine atom. Simultaneously, diethylamine acts as an acid-binding agent, combining with the hydrogen chloride generated in the reaction to form an amine salt precipitate. Selective substitution is achieved by precisely controlling the material ratio to generate a tris(diethylamino)hafnium chloride intermediate. Subsequently, in an inert solvent, this intermediate undergoes a metal substitution reaction with a vinyl lithium reagent. The chlorine atom coordinated in the intermediate molecule exchanges with the vinyl group, and the vinyl anion coordinates with the hafnium center to form the target product, while a lithium salt precipitate is formed. After the reaction mixture has fully reacted, it is purified by a low-temperature crystallization process to finally obtain vinyltris(diethylamino)hafnium with a complete structure and the required purity. This synthetic route effectively maintains the chemical integrity of the thermosensitive vinyl functional groups through a continuous process of stepwise aminolysis and metal substitution.

[0012] As a preferred embodiment of the present invention, in step A1, the mass ratio of hafnium tetrachloride to diethylamine is 1:(1.08-1.10).

[0013] As a preferred embodiment of the present invention, in step A2, the stirring time is 9-11 hours.

[0014] As a preferred embodiment of the present invention, the method for preparing the acryloyloxydibenzoyl hafnium complex includes: B1. Add anhydrous tetrahydrofuran, hafnium propoxide and dibenzoylmethane to a round-bottom flask and stir at room temperature; add anhydrous tetrahydrofuran solution containing acryloyl chloride dropwise at 0-5℃, raise the temperature to 58-62℃, and reflux with stirring; B2. After the reaction is complete, cool to room temperature and distill under reduced pressure to obtain an oily substance. Add n-hexane to the oily substance, stir, collect the precipitate by vacuum filtration, wash the precipitate with n-hexane, and finally dry it in a vacuum drying oven at 38-42℃.

[0015] In this invention, the preparation of acryloyloxydibenzoyl hafnium complexes is based on a continuous process of "ligand exchange" and "propoxy nucleophilic substitution." The reaction uses n-propoxide hafnium as the metal source. Benzoylmethane, as the key β-diketone ligand, first undergoes a proton exchange reaction with one propoxy group in the n-propoxide hafnium complex, generating one molecule of n-propanol. Simultaneously, the deprotonated benzoylmethane anion, with its two oxygen atoms as donors, undergoes a strong O,O'-chelate coordination with the hafnium center, forming a stable, charge-delocalized six-membered cyclic hafnium-β-diketone chelate. Subsequently, acryloyl chloride is slowly added under low-temperature conditions. The key nucleophilic center is the remaining propoxy group in the hafnium-β-diketone chelate, not the enol oxonium. These propoxy groups act as nucleophiles, directly attacking the acyl carbon of acryloyl chloride, undergoing a nucleophilic substitution reaction. This introduces the acryloyl group as an acrylate ligand into the hafnium center, generating chloropropane as a byproduct. This process does not directly release hydrogen chloride. Subsequently, the hafnium species in solution undergo coordination exchange and adjustment, ultimately forming a stable target complex with an intramolecularly integrated hafnium-β-diketone chelate ring and acrylate ligand. The entire reaction process must be carried out under light-protected and anhydrous conditions to prevent prepolymerization of the acryloyl group. Finally, through precipitation, washing, and drying, a well-defined powdered target complex is obtained.

[0016] As a preferred embodiment of the present invention, in step B1, the mass ratio of hafnium propoxide, dibenzoylmethane and acryloyl chloride is (3.5-4.0):(2.5-3.0):(1.0-1.5).

[0017] As a preferred embodiment of the present invention, in step B2, the drying time in a vacuum drying oven at 38-42°C is 12-14 hours.

[0018] In a second aspect, the present invention provides a hafnium carbide precursor prepared by the method described above, comprising the following raw materials in parts by weight: 25-45 parts of n-propanol hafnium; 10-20 parts of vinyltris(diethylamino)hafnium; 8-15 parts of acryloyloxydibenzoylhafnium complex; 5-12 parts of benzophenone; 5-10 parts of divinylbenzene; 0.5-3 parts of triethanolamine; 15-30 parts of tetrahydrofuran; and 0.05-0.5 parts of dicumyl peroxide.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) The most prominent technical effect of this invention is that it significantly improves the ceramic yield of the precursor and effectively reduces its pyrolysis temperature. By introducing two newly designed modified compounds: vinyltris(diethylamino)hafnium and acryloyloxydibenzoylhafnium complex, the active functional groups are effectively integrated at the molecular level. The unsaturated groups such as vinyl and acryloyloxy contained in the molecules of these two compounds can participate in the polymerization and crosslinking reaction through thermal or photoinitiation during the curing stage of the precursor, forming a dense and stable three-dimensional network polymer structure. This highly crosslinked network can minimize the escape of small molecule volatiles during subsequent pyrolysis. At the same time, the hafnium-nitrogen bond and hafnium-oxygen bond introduced in the modified compound have lower bond energy and higher reactivity than the hafnium-oxygen bond in traditional alkoxides. They can break and recombine earlier during pyrolysis, effectively reducing the energy barrier for the formation of hafnium carbide crystal phase; (2) This invention effectively improves the microstructure, density, and high-temperature performance of the final hafnium carbide ceramic material. The synergistic effect of the two modified compounds is key. During the pyrolysis of vinyltris(diethylamino)hafnium, the decomposition of its hafnium-nitrogen bond may promote the formation of a transition state nitrogen-doped carbon structure. This not only helps to reduce hafnium oxide at lower temperatures but also inhibits the excessive precipitation of free carbon during the cracking process, thereby promoting the formation of hafnium carbide grains with finer grain size and more uniform distribution. The benzoylmethyl group in the acryloyloxydibenzoyl hafnium complex acts as a chelating ligand, enhancing the thermal stability of the precursor molecule. The carbon species remaining after its decomposition can participate in the carbonization reaction in a more orderly manner, reducing the formation of pores. In addition, the introduction of nitrogen brings a solid solution strengthening effect to the final ceramic material, which may form a hafnium carbide-hafnium nitride solid solution. This significantly improves the oxidation resistance and creep resistance of the ceramic under high-temperature extreme environments. (3) This invention greatly enhances the process adaptability and application potential of hafnium carbide precursors. The constructed composite crosslinking system simultaneously contains benzophenone photoinitiator and dicumyl peroxide thermal initiator, and is combined with divinylbenzene crosslinking agent and various polymerizable functional groups, making the precursor suitable for both traditional thermosetting processes and emerging ultraviolet curing technologies. This provides the possibility of directly molding complex-shaped hafnium carbide ceramic components using advanced additive manufacturing technologies such as stereolithography and digital light processing, realizing the integration of material preparation and molding technologies. At the same time, the precursor solution has good storage stability and suitable rheological properties, and is not easily gelled by trace amounts of moisture in the air, making it exhibit excellent operability and repeatability in various processes such as impregnation, coating, and molding. In summary, this invention, through systematic material design, successfully overcomes several technical bottlenecks in existing hafnium carbide precursor ceramics, such as low yield, high pyrolysis temperature, single performance, and poor process adaptability, providing a new and highly competitive material solution for the preparation of high-performance, complex-structure, ultra-high temperature ceramic products. Detailed Implementation

[0020] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0021] The sources of some components in the examples and comparative examples are as follows: The n-propanol hafnium was purchased from Jiangxi Jinhe New Materials Co., Ltd.

[0022] The dicumyl peroxide was purchased from Shandong Aifu Technology Co., Ltd.

[0023] The benzophenone was purchased from Jiangsu Xinhan New Materials Co., Ltd.

[0024] The divinylbenzene was purchased from Jiangsu Evergreen New Material Technology Co., Ltd.

[0025] The hafnium tetrachloride was purchased from Hunan Haoweite Technology Development Co., Ltd.

[0026] The dibenzoylmethane was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0027] The acryloyl chloride was purchased from Tianjin Xiens Biochemical Technology Co., Ltd.

[0028] Example 1: This example provides a method for preparing a hafnium carbide precursor, the steps of which include: Preparation of vinyltris(diethylamino)hafnium: 200 mL of anhydrous tetrahydrofuran and 25.3 g of hafnium tetrachloride were added to a 1000 mL round-bottom flask, and the system temperature was lowered to -30 °C and maintained. A solution consisting of 27.5 g of diethylamine and 100 mL of anhydrous tetrahydrofuran was slowly added dropwise at a rate of 1 drop per second using a constant-pressure dropping funnel while stirring at 300 rpm. After the addition was complete, stirring was continued at -30 °C for 2 hours. Subsequently, the reaction system was gradually heated to 25 °C and stirred continuously for 12 hours. The diethylamine hydrochloride precipitate was removed by vacuum filtration through a Buchner funnel, yielding a clear filtrate. The filtrate was transferred to a rotary evaporator and evaporated at 40 °C water bath temperature and 50 rpm for 1.5 hours to remove the solvent, yielding tris(diethylamino)hafnium chloride intermediate. Add 200 mL of anhydrous diethyl ether to the intermediate, cool to -30 °C, and then slowly add 110 mL of a 1.0 M vinyllithium cyclohexane solution. After the addition is complete, gradually heat the reaction system to 25 °C and stir continuously for 10 hours. Remove the lithium salt precipitate by vacuum filtration, transfer the filtrate to a crystallization flask, and freeze-crystallize at -30 °C. Collect the white crystals by vacuum filtration, wash three times with pre-cooled anhydrous diethyl ether, and finally dry under high vacuum at room temperature to obtain the product.

[0029] Preparation of the acryloyloxydibenzoyl hafnium complex: In a 1000 mL three-necked round-bottom flask, 350 mL of anhydrous tetrahydrofuran, 48.6 g of n-propoxide hafnium, and 36.0 g of dibenzoylmethane were added sequentially. The mixture was stirred at 25 °C and 200 rpm for 30 minutes until the solid was completely dissolved. The system was cooled to 3 °C in an ice-water bath, and 50 mL of anhydrous tetrahydrofuran solution containing 17.6 g of acryloyl chloride was added dropwise at a rate of 1 drop per second using a constant-pressure dropping funnel. After the addition was complete, the ice-water bath was removed, and the reaction system was heated to 60 °C. A reflux condenser was installed, and the mixture was stirred under reflux at 300 rpm for 8 hours. After the reaction was completed, the system was cooled to 25 °C, and concentrated to one-third of its original volume by rotary evaporation at a water bath temperature of 40 °C, yielding a viscous yellow oil. 300 mL of n-hexane was added to this oil, and the mixture was stirred at 200 rpm for 30 minutes to induce precipitation. The light yellow precipitate was collected by vacuum filtration through a Buchner funnel, washed with 3 × 50 mL of cold n-hexane (0 °C), and finally dried in a vacuum drying oven at 40 °C for 13 hours to obtain the product.

[0030] Preparation of hafnium carbide precursor: In an argon-atmosphere glove box, 35.0 g of n-propoxide hafnium, 15.0 g of vinyltris(diethylamino)hafnium, and 12.0 g of acryloyloxydibenzoylhafnium complex were added to a 250 mL reaction flask and stirred at 25 °C and 200 rpm for 30 minutes. Then, 0.3 g of dicumyl peroxide, 8.0 g of benzophenone, 2.0 g of triethanolamine, and 7.0 g of divinylbenzene were added sequentially, and stirring continued for another 30 minutes. Next, 23.0 g of tetrahydrofuran was added, and the reaction flask was transferred to a 40 °C constant temperature water bath. Stirring continued at 200 rpm for 3 hours to form a homogeneous transparent solution. Finally, the homogeneous solution was pressure filtered through a 0.22 μm polytetrafluoroethylene microporous membrane, and the filtrate was collected to obtain the hafnium carbide precursor.

[0031] Example 2: This example provides a method for preparing a hafnium carbide precursor, the steps of which include: Preparation of vinyltris(diethylamino)hafnium: 200 mL of anhydrous tetrahydrofuran and 22.0 g of hafnium tetrachloride were added to a 1000 mL round-bottom flask, and the system temperature was lowered to -30 °C and maintained. A solution consisting of 24.0 g of diethylamine and 100 mL of anhydrous tetrahydrofuran was slowly added dropwise at a rate of 1 drop per second using a constant-pressure dropping funnel while stirring at 300 rpm. After the addition was complete, stirring was continued at -30 °C for 2 hours. Subsequently, the reaction system was gradually heated to 25 °C and stirred continuously for 12 hours. The diethylamine hydrochloride precipitate was removed by vacuum filtration through a Buchner funnel, yielding a clear filtrate. The filtrate was transferred to a rotary evaporator and evaporated at 40 °C water bath temperature and 50 rpm for 1.5 hours to remove the solvent, yielding tris(diethylamino)hafnium chloride intermediate. Add 180 mL of anhydrous diethyl ether to the intermediate, cool to -30 °C, and then slowly add 95 mL of a 1.0 M cyclohexane solution of vinyl lithium. After the addition is complete, gradually heat the reaction system to 25 °C and stir continuously for 10 hours. Remove the lithium salt precipitate by vacuum filtration, transfer the filtrate to a crystallization flask, and freeze-crystallize at -30 °C. Collect the white crystals by vacuum filtration, wash three times with pre-cooled anhydrous diethyl ether, and finally dry under high vacuum at room temperature to obtain the product.

[0032] Preparation of the acryloyloxydibenzoyl hafnium complex: In a 1000 mL three-necked round-bottom flask, 350 mL of anhydrous tetrahydrofuran, 42.0 g of n-propoxide hafnium, and 31.5 g of dibenzoylmethane were added sequentially. The mixture was stirred at 25 °C and 200 rpm for 30 minutes until the solid was completely dissolved. The system was cooled to 3 °C in an ice-water bath. Using a constant-pressure dropping funnel, 50 mL of anhydrous tetrahydrofuran solution containing 14.0 g of acryloyl chloride was added dropwise at a rate of 1 drop per second. After the addition was complete, the ice-water bath was removed, and the reaction system was heated to 60 °C. A reflux condenser was installed, and the mixture was stirred under reflux at 300 rpm for 8 hours. After the reaction was completed, the system was cooled to 25 °C and concentrated to one-third of its original volume by rotary evaporation at a water bath temperature of 40 °C, yielding a viscous yellow oil. 300 mL of n-hexane was added to this oil, and the mixture was stirred at 200 rpm for 30 minutes to induce precipitation. The light yellow precipitate was collected by vacuum filtration through a Buchner funnel, washed with 3 × 50 mL of cold n-hexane (0 °C), and finally dried in a vacuum drying oven at 40 °C for 13 hours to obtain the product.

[0033] Preparation of hafnium carbide precursor: In an argon-atmosphere glove box, 28.0 g of n-propoxide hafnium, 12.0 g of vinyltris(diethylamino)hafnium, and 10.0 g of acryloyloxydibenzoylhafnium complex were added to a 250 mL reaction flask and stirred at 25 °C and 200 rpm for 30 minutes. Then, 0.1 g of dicumyl peroxide, 6.0 g of benzophenone, 1.0 g of triethanolamine, and 5.0 g of divinylbenzene were added sequentially, and stirring continued for another 30 minutes. Next, 18.0 g of tetrahydrofuran was added, and the reaction flask was transferred to a 39 °C constant temperature water bath. Stirring continued at 200 rpm for 2.5 hours to form a homogeneous transparent solution. Finally, the homogeneous solution was pressure filtered through a 0.22 μm polytetrafluoroethylene microporous membrane, and the filtrate was collected to obtain the hafnium carbide precursor.

[0034] Example 3: This example provides a method for preparing a hafnium carbide precursor, the steps of which include: Preparation of vinyltris(diethylamino)hafnium: 200 mL of anhydrous tetrahydrofuran and 28.0 g of hafnium tetrachloride were added to a 1000 mL round-bottom flask, and the system temperature was lowered to -30 °C and maintained. A solution consisting of 30.5 g of diethylamine and 100 mL of anhydrous tetrahydrofuran was slowly added dropwise at a rate of 1 drop per second using a constant-pressure dropping funnel while stirring at 300 rpm. After the addition was complete, stirring was continued at -30 °C for 2 hours. Subsequently, the reaction system was gradually heated to 25 °C and stirred continuously for 12 hours. The diethylamine hydrochloride precipitate was removed by vacuum filtration through a Buchner funnel, yielding a clear filtrate. The filtrate was transferred to a rotary evaporator and evaporated at 40 °C water bath temperature and 50 rpm for 1.5 hours to remove the solvent, yielding tris(diethylamino)hafnium chloride intermediate. Add 220 mL of anhydrous diethyl ether to the intermediate, cool to -30 °C, and then slowly add 125 mL of a 1.0 M cyclohexane solution of vinyl lithium. After the addition is complete, gradually heat the reaction system to 25 °C and stir continuously for 10 hours. Remove the lithium salt precipitate by vacuum filtration, transfer the filtrate to a crystallization flask, and freeze-crystallize at -30 °C. Collect the white crystals by vacuum filtration, wash three times with pre-cooled anhydrous diethyl ether, and finally dry under high vacuum at room temperature to obtain the product.

[0035] Preparation of the acryloyloxydibenzoyl hafnium complex: In a 1000 mL three-necked round-bottom flask, 350 mL of anhydrous tetrahydrofuran, 52.0 g of n-propoxide hafnium, and 39.0 g of dibenzoylmethane were added sequentially. The mixture was stirred at 25 °C and 200 rpm for 30 minutes until the solid was completely dissolved. The system was cooled to 3 °C in an ice-water bath. Using a constant-pressure dropping funnel, 50 mL of anhydrous tetrahydrofuran solution containing 18.0 g of acryloyl chloride was added dropwise at a rate of 1 drop per second. After the addition was complete, the ice-water bath was removed, and the reaction system was heated to 60 °C. A reflux condenser was installed, and the mixture was stirred under reflux at 300 rpm for 8 hours. After the reaction was completed, the system was cooled to 25 °C and concentrated to one-third of its original volume by rotary evaporation at a water bath temperature of 40 °C, yielding a viscous yellow oil. 300 mL of n-hexane was added to this oil, and the mixture was stirred at 200 rpm for 30 minutes to induce precipitation. The light yellow precipitate was collected by vacuum filtration through a Buchner funnel, washed with 3 × 50 mL of cold n-hexane (0 °C), and finally dried in a vacuum drying oven at 40 °C for 13 hours to obtain the product.

[0036] Preparation of hafnium carbide precursor: In an argon-atmosphere glove box, 42.0 g of n-propoxide hafnium, 18.0 g of vinyltris(diethylamino)hafnium, and 14.0 g of acryloyloxydibenzoylhafnium complex were added to a 250 mL reaction flask and stirred at 25 °C and 200 rpm for 30 minutes. Then, 0.4 g of dicumyl peroxide, 10.0 g of benzophenone, 2.5 g of triethanolamine, and 9.0 g of divinylbenzene were added sequentially, and stirring continued for another 30 minutes. Next, 28.0 g of tetrahydrofuran was added, and the reaction flask was transferred to a 41 °C constant temperature water bath. Stirring continued at 200 rpm for 3.5 hours to form a homogeneous transparent solution. Finally, the homogeneous solution was pressure filtered through a 0.22 μm polytetrafluoroethylene microporous membrane, and the filtrate was collected to obtain the hafnium carbide precursor.

[0037] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not contain vinyltris(diethylamino)hafnium and acryloyloxydibenzoylhafnium complexes.

[0038] Comparative Example 2 The difference between this comparative example and Example 1 is that only vinyltris(diethylamino)hafnium was added in this comparative example, and no acryloyloxydibenzoylhafnium complex was added; and the acryloyloxydibenzoylhafnium complex was replaced with an equal mass of vinyltris(diethylamino)hafnium.

[0039] Comparative Example 3 The difference between this comparative example and Example 1 is that this comparative example only adds acryloyloxydibenzoyl hafnium complex and does not add vinyltris(diethylamino) hafnium; and vinyltris(diethylamino) hafnium is replaced with an equal mass of acryloyloxydibenzoyl hafnium complex.

[0040] The performance of the hafnium carbide precursors obtained in Examples 1-3 and Comparative Examples 1-3 was tested in accordance with national and industry standard testing specifications.

[0041] The hafnium carbide precursor solutions prepared in each example and comparative example were poured into petri dishes and placed in a vacuum drying oven at 40°C for 24 hours to completely remove the solvent tetrahydrofuran. Subsequently, they were heat-treated in an oven at 200°C for 1 hour to fully crosslink and solidify, obtaining a solid resin. This solid resin was ground into powder, and 5.000 g was accurately weighed and placed in a 50 mL alumina crucible, which was then placed in a GSL-1700X tube furnace. First, the temperature was increased from 25°C to 200°C at a rate of 5°C / min under air atmosphere and held for 60 min to complete the thermal crosslinking and curing process. Then, the temperature was switched to an argon protective atmosphere, with a gas flow rate controlled at 50 mL / min, and the temperature was increased from 200°C to 1400°C at a rate of 3°C / min and held for 120 min to complete the pyrolysis process. Finally, the product was cooled to below 50°C in the furnace and removed. The mass of the ceramic product was weighed using an electronic balance with an accuracy of 0.0001 g.

[0042] Thermogravimetric analysis was performed using a Netzsch STA449F3 thermogravimetric analyzer. The sample amount was approximately 10 mg. The temperature was increased from 25 °C to 1500 °C at a rate of 10 °C / min under an argon atmosphere (flow rate 50 mL / min), and the mass change curve was recorded in real time.

[0043] Phase analysis was performed using a Bruker D8 Advance X-ray diffractometer with CuKα radiation (λ=0.15406nm), operating voltage 40kV, current 40mA, scanning range 10°-80° (2θ), scanning speed 2° / min, step size 0.02°. The collected diffraction data were used to calculate the relative content of hafnium carbide phase using the Rietveld refinement method, which was recorded as the phase purity.

[0044] Microscopic morphology was observed using a Hitachi SU8010 field emission scanning electron microscope with an accelerating voltage of 15kV and a working distance of 8mm. The samples were sputter-coated with gold.

[0045] For the antioxidant performance test, the ceramic sample was processed into a size of 10mm×10mm×2mm, placed in an Al2O3 crucible, and then placed in a KSL-1700X muffle furnace. The sample was kept at 1500℃ in static air for 60 minutes, and the mass change was measured after cooling.

[0046] Bulk density was measured using the Archimedes displacement method with an analytical balance of 0.1 mg accuracy, using deionized water as the immersion medium. Each sample was measured three times and the average value was taken. All tests were conducted under standard experimental conditions of 25°C and 45% relative humidity, and each sample was tested three times to ensure data reliability.

[0047] The performance test data above are shown in Table 1.

[0048] Table 1 Performance Test Results

[0049] The test results in Table 1 above clearly show that, based on the comparative analysis of performance test data, Examples 1-3 effectively solve the core technical problems existing in the current hafnium carbide precursors compared to Comparative Examples 1-3.

[0050] Regarding ceramic yield, the yields of Examples 1-3 remained stable in the range of 56-59%, significantly higher than the 34.2% of Comparative Example 1. This confirms that the two modified compounds successfully locked in the organic components by forming a dense three-dimensional network structure through the vinyl and acryloxy groups in their molecules during the crosslinking and curing stage, thus greatly reducing the mass loss during pyrolysis.

[0051] Regarding the pyrolysis temperature, the hafnium carbide crystal phases of Examples 1-3 were fully formed at temperatures below 1400°C, while Comparative Example 1 was not fully transformed at 1500°C. The completion temperatures of Comparative Examples 2 and 3 were also at least 60°C higher than those of Examples 1-3. This indicates that the hafnium-nitrogen bonds and hafnium-oxygen bonds introduced into the modified compounds effectively reduced the activation energy barrier for the formation of hafnium carbide.

[0052] Regarding ceramic quality, the hafnium carbide crystal phase purity of Examples 1-3 was all higher than 97.8%, far superior to the 85.3% of Comparative Example 1. Moreover, their cross-sectional morphology showed dense and uniform characteristics, with a porosity of less than 4%, while Comparative Example 1 showed a porous structure and obvious cracks. This is due to the fact that the two modified compounds promoted the uniform reaction and diffusion of hafnium carbon elements during pyrolysis and suppressed the formation of impurity phases.

[0053] Regarding antioxidant properties, the oxidation weight gain rate of Examples 1-3 in static air at 1500℃ for 1 hour was only 2.3-2.7%, far lower than the 8.9% of Comparative Example 1. This demonstrates that the introduction of nitrogen element forms a solid solution strengthening effect, significantly improving the high-temperature stability of the ceramic material. It is particularly noteworthy that although Comparative Examples 2 and 3 each contain a modified compound, their performance indicators are all between those of Examples 1 and Comparative Example 1. This fully demonstrates that the two modified compounds are complementary and synergistic in molecular design. Their combined effect achieves a comprehensive technical effect of high ceramic yield, low pyrolysis temperature, high product purity, and excellent antioxidant properties, successfully overcoming the technical bottleneck of traditional hafnium carbide precursors.

Claims

1. A method for preparing a hafnium carbide precursor, characterized in that the steps include... include: S1. In an argon-atmosphere glove box, add the hafnium propoxide, vinyltris(diethylamino)hafnium, and acryloyloxydibenzoylhafnium complex to a reaction flask and stir at room temperature; add dicumyl peroxide, benzophenone, triethanolamine, and divinylbenzene, and continue stirring; add tetrahydrofuran, and continue stirring in a water bath at 38-42°C to form a homogeneous solution; S2. Filter the homogeneous solution through a polytetrafluoroethylene microporous membrane.

2. The method for preparing the hafnium carbide precursor according to claim 1, characterized in that, In step S1, stirring is continued in a water bath at 38-42℃ for 2-4 hours.

3. The method for preparing the hafnium carbide precursor according to claim 1, characterized in that, In step S2, the pore size of the polytetrafluoroethylene microporous filter membrane is 0.2-0.24 μm.

4. The method for preparing the hafnium carbide precursor according to claim 1, characterized in that, The method for preparing the vinyltris(diethylamino)hafnium includes: A1. Under argon protection, anhydrous tetrahydrofuran and hafnium tetrachloride were added to a dry round-bottom flask and cooled to -28 to -32°C. Anhydrous tetrahydrofuran solution of diethylamine was added, stirred, and the temperature was raised to room temperature. Stirring was continued. The mixture was filtered under reduced pressure, and the filtrate was concentrated by rotary evaporation at 38-42°C to obtain tris(diethylamino)hafnium chloride intermediate. A2. Add anhydrous diethyl ether to the tris(diethylamino)hafnium chloride intermediate, cool to -28~-32℃, then add a cyclohexane solution of vinyl lithium to obtain a reaction mixture; heat the reaction mixture to room temperature and continue stirring; after the reaction is complete, filter under reduced pressure to obtain a filtrate; transfer the filtrate to a crystallization flask and freeze-crystallize at -20℃~-40℃, collect the crystals by vacuum filtration, wash the crystals with pre-cooled anhydrous diethyl ether, and finally dry under high vacuum at room temperature.

5. The method for preparing the hafnium carbide precursor according to claim 4, characterized in that, In step A1, the mass ratio of hafnium tetrachloride to diethylamine is 1:(1.08 - 1.10).

6. The method for preparing the hafnium carbide precursor according to claim 4, characterized in that, In step A2, the stirring time continues for 9-11 hours.

7. The method for preparing the hafnium carbide precursor according to claim 1, characterized in that, The preparation method of the acryloyloxydibenzoylhafnium complex includes: B1. Add anhydrous tetrahydrofuran, hafnium propoxide and dibenzoylmethane to a round-bottom flask and stir at room temperature; add anhydrous tetrahydrofuran solution containing acryloyl chloride dropwise at 0-5℃, raise the temperature to 58-62℃, and reflux with stirring; B2. After the reaction is complete, cool to room temperature and distill under reduced pressure to obtain an oily substance. Add n-hexane to the oily substance, stir, collect the precipitate by vacuum filtration, wash the precipitate with n-hexane, and finally dry it in a vacuum drying oven at 38-42℃.

8. The method for preparing the hafnium carbide precursor according to claim 7, characterized in that, In step B1, the mass ratio of hafnium propoxide, dibenzoylmethane, and acryloyl chloride is (3.5-4.0):(2.5-3.0):(1.0-1.5).

9. The method for preparing the hafnium carbide precursor according to claim 7, characterized in that, In step B2, the drying time in a vacuum drying oven at 38-42℃ is 12-14 hours.

10. A hafnium carbide precursor prepared by the method for preparing hafnium carbide precursor according to any one of claims 1-9, characterized in that, The raw materials include the following parts by weight: 25-45 parts of n-propanol hafnium; 10-20 parts of vinyltris(diethylamino)hafnium; 8-15 parts of acryloyloxydibenzoylhafnium complex; 5-12 parts of benzophenone; 5-10 parts of divinylbenzene; 0.5-3 parts of triethanolamine; and 15-30 parts of tetrahydrofuran. Dicumyl peroxide: 0.05-0.5 parts.

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