A low-oxygen-content silicon nitride powder and its preparation method

Abstract

This invention provides a low-oxygen-content silicon nitride powder and its preparation method, belonging to the field of silicon nitride preparation technology. The invention uses amines with sterically hindered groups as raw materials to synthesize a "nitrophilic and oxygen-repellent" polysilazane precursor that resists water decomposition. The precursor and catalyst precursor are co-encapsulated using PEG-PPG-PEG, which not only ensures excellent dispersibility of the final powder but also embeds a catalyst for subsequent decarbonization steps. Mild low-temperature solidification is achieved through supercritical ammonia. During stepwise pyrolysis, the template is first removed to physically isolate the particles, and then the embedded catalyst is activated to actively convert solid residual carbon into gas for removal. In the high-temperature crystallization stage, the natural segregation of impurities to the particle surface is utilized, supplemented by highly active vapor-phase chemical etching for surface treatment, converting solid oxygen impurities into volatile gaseous products for removal, ensuring the low oxygen content of the final product.

Description

Technical Field

[0001] This invention belongs to the field of silicon nitride preparation technology, and relates to a low oxygen content silicon nitride powder and its preparation method. Background Technology

[0002] Silicon nitride, as an advanced high-performance structural ceramic, possesses comprehensive properties such as high strength, high hardness, excellent wear resistance, outstanding thermal shock stability, and chemical inertness, demonstrating enormous application potential in cutting-edge fields such as aerospace, automotive engines, precision bearings, cutting tools, and high-performance electronic substrates. The final properties of silicon nitride ceramics, especially their high-temperature mechanical and dielectric properties, largely depend on the purity, particle size distribution, phase composition, and dispersibility of the raw material powder. Among these, oxygen content is one of the most critical indicators for evaluating the quality of silicon nitride powder; excessively high oxygen content severely degrades the performance of silicon nitride materials at high temperatures. However, current traditional technologies still face many insurmountable bottlenecks in preparing low-oxygen-content silicon nitride powder.

[0003] In the direct nitriding of silicon powder, a natural oxide film inevitably exists on the surface of the raw silicon powder. This oxide layer transforms into impurities such as silicon oxynitrides during the high-temperature nitriding process, becoming the main source of oxygen in the final product. While the carbothermic reduction nitriding method can handle oxygen-containing raw materials, the reaction itself is difficult to complete, and the high-temperature environment places extremely stringent requirements on equipment and atmosphere purity. Even trace amounts of oxygen or moisture leakage can lead to excessive oxygen content in the product.

[0004] Secondly, powders are prone to hard agglomeration, making their morphology and dispersibility difficult to control. Traditional methods all involve high-temperature solid-state reactions, during which newly formed silicon nitride grains are highly susceptible to sintering, leading to the formation of strong necks between particles and resulting in hard agglomerates that are difficult to disperse effectively using conventional physical methods. These hard agglomerates not only affect the subsequent processing performance of the powder but also become a source of defects in the final ceramic products, reducing their reliability. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a low-oxygen-content silicon nitride powder and its preparation method. The invention first synthesizes a "nitrophilic and oxygen-repellent" polysilazane precursor, resistant to water decomposition, using amines with sterically hindered groups as raw materials. Subsequently, the precursor and catalyst precursor are co-encapsulated using PEG-PPG-PEG (polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer). This not only ensures excellent dispersibility of the final powder but also embeds a catalyst for subsequent decarbonization steps. Mild low-temperature curing is achieved through supercritical ammonia. During stepwise pyrolysis, the template is first removed to physically isolate the particles, and then the embedded catalyst is activated to actively convert solid residual carbon into gaseous substances for removal. In the high-temperature crystallization stage, the natural segregation of impurities to the particle surface is utilized, supplemented by highly active vapor-phase chemical etching for surface treatment. This converts solid oxygen impurities into volatile gaseous products for removal, ensuring the low oxygen content of the final product.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing low-oxygen-content silicon nitride powder, the method comprising:

[0008] S1: Under a nitrogen atmosphere, methyldichlorosilane was added to toluene, and a toluene solution of di-tert-butylamine was added dropwise to obtain reaction mixture A. Reaction mixture A was stirred and matured at room temperature, and then filtered and distilled under reduced pressure to obtain a liquid precursor of polysilazane.

[0009] S2: Add PEG-PPG-PEG to tetrahydrofuran, add polysilazane liquid precursor and acetylacetone iron, and sonicate to obtain composite precursor solution B; place it in a high-pressure reactor, remove oxygen by nitrogen replacement, and pump in high-purity liquid ammonia to obtain reaction solution C. After heating and stirring, after the reaction is completed, cool down, depressurize and dry to obtain solid composite powder.

[0010] S3: The solid composite powder is placed in a tube furnace and heated to a first temperature at a first heating rate under a nitrogen atmosphere and held at that temperature; then the temperature is continued to rise to a second temperature while the atmosphere is switched to a hydrogen / nitrogen mixture and held at that atmosphere to obtain high-purity amorphous silicon nitride precursor powder.

[0011] S4: The high-purity amorphous silicon nitride precursor powder is heated to the third temperature at the second heating rate in a pure nitrogen atmosphere and held at the temperature. During the holding period, a hexafluoroethane / nitrogen mixture is pulsedly introduced into the nitrogen gas flow. After the reaction is completed, the powder is cooled to room temperature in a pure nitrogen atmosphere to obtain silicon nitride powder with low oxygen content.

[0012] As a preferred technical solution of the present invention, in step S1, the reaction mixture A is obtained by adding a toluene solution of di-tert-butylamine dropwise at -10 to 0°C. For example, the dropwise addition can be carried out at -10°C, -9°C, -8°C, -7°C, -6°C, -5°C, -4°C, -3°C, -2°C, -1°C or 0°C, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0013] In some optional embodiments, the volume ratio of methyldichlorosilane to toluene is 1:(10-20), for example, it can be 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19 or 1:20, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0014] In some optional embodiments, the mass fraction of the di-tert-butylamine in toluene solution is 20-30 wt.%, for example, it can be 20 wt.%, 21 wt.%, 22 wt.%, 23 wt.%, 24 wt.%, 25 wt.%, 26 wt.%, 27 wt.%, 28 wt.%, 29 wt.%, or 30 wt.%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0015] In some optional embodiments, the molar ratio of di-tert-butylamine to methyldichlorosilane is (2.2-2.5):1, for example, it can be 2.20:1, 2.23:1, 2.26:1, 2.29:1, 2.32:1, 2.35:1, 2.38:1, 2.41:1, 2.44:1, 2.47:1 or 2.50:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0016] In some optional embodiments, the reaction mixture A is stirred and matured at room temperature for 6-12 hours, for example, 6.0 hours, 6.6 hours, 7.2 hours, 7.8 hours, 8.4 hours, 9.0 hours, 9.6 hours, 10.2 hours, 10.8 hours, 11.4 hours, or 12.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0017] As a preferred technical solution of the present invention, in step S2, the mass ratio of PEG-PPG-PEG to polysilazane liquid precursor is (0.8-1.2):1, for example, it can be 0.80:1, 0.84:1, 0.88:1, 0.92:1, 0.96:1, 1.00:1, 1.04:1, 1.08:1, 1.12:1, 1.16:1 or 1.20:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0018] In some optional embodiments, the mass ratio of iron acetylacetonate to the polysilazane liquid precursor is (0.08-0.12):10, for example, it can be 0.080:10, 0.084:10, 0.088:10, 0.092:10, 0.096:10, 0.100:10, 0.104:10, 0.108:10, 0.112:10, 0.116:10 or 0.120:10, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0019] In some optional embodiments, the total solids mass fraction in the composite precursor solution B is 8-15 wt.%, for example, it can be 8.0 wt.%, 8.7 wt.%, 9.4 wt.%, 10.1 wt.%, 10.8 wt.%, 11.5 wt.%, 12.2 wt.%, 12.9 wt.%, 13.6 wt.%, 14.3 wt.%, or 15.0 wt.%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0020] In some optional embodiments, the mass ratio of liquid ammonia to polysilazane liquid precursor is (30-50):1, for example, it can be 30:1, 32:1, 34:1, 36:1, 38:1, 40:1, 42:1, 44:1, 46:1, 48:1 or 50:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0021] In some optional embodiments, the reaction solution C is heated to 180-240°C, for example, 180°C, 186°C, 192°C, 198°C, 204°C, 210°C, 216°C, 222°C, 228°C, 234°C or 240°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0022] In some optional embodiments, the reaction solution C is stirred at 18-25 MPa, for example, 18.0 MPa, 18.7 MPa, 19.4 MPa, 20.1 MPa, 20.8 MPa, 21.5 MPa, 22.2 MPa, 22.9 MPa, 23.6 MPa, 24.3 MPa or 25.0 MPa, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0023] In some optional embodiments, the reaction time of the reaction solution C is 3-5 hours, for example, 3.0 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4.0 hours, 4.2 hours, 4.4 hours, 4.6 hours, 4.8 hours or 5.0 hours, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of the present invention, in step S3, the first heating rate is 1-3℃ / min, for example, it can be 1.0℃ / min, 1.2℃ / min, 1.4℃ / min, 1.6℃ / min, 1.8℃ / min, 2.0℃ / min, 2.2℃ / min, 2.4℃ / min, 2.6℃ / min, 2.8℃ / min or 3.0℃ / min, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] In some alternative embodiments, the first temperature is 500-550°C, for example, it can be 500°C, 505°C, 510°C, 515°C, 520°C, 525°C, 530°C, 535°C, 540°C, 545°C or 550°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0026] In some optional embodiments, the holding time for the first temperature is 1-2 hours, for example, it can be 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2.0h, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0027] In some alternative embodiments, the second temperature is 900-950°C, for example, it can be 900°C, 905°C, 910°C, 915°C, 920°C, 925°C, 930°C, 935°C, 940°C, 945°C or 950°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0028] In some optional embodiments, the holding time for the second temperature is 1.5-2.5h, for example, it can be 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2.0h, 2.1h, 2.2h, 2.3h, 2.4h or 2.5h, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0029] The atmosphere for the second stage of treatment is a hydrogen / nitrogen mixture;

[0030] In some optional embodiments, the volume fraction of hydrogen in the hydrogen / nitrogen mixture is 4-6%, for example, it can be 4.0%, 4.2%, 4.4%, 4.6%, 4.8%, 5.0%, 5.2%, 5.4%, 5.6%, 5.8% or 6.0%, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0031] As a preferred technical solution of the present invention, in step S4, the second heating rate is 5-10℃ / min, for example, it can be 5.0℃ / min, 5.5℃ / min, 6.0℃ / min, 6.5℃ / min, 7.0℃ / min, 7.5℃ / min, 8.0℃ / min, 8.5℃ / min, 9.0℃ / min, 9.5℃ / min or 10.0℃ / min, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] In some optional embodiments, the third temperature is 1500-1550°C, for example, it can be 1500°C, 1505°C, 1510°C, 1515°C, 1520°C, 1525°C, 1530°C, 1535°C, 1540°C, 1545°C or 1550°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0033] In some optional embodiments, the holding time at the third temperature is 1.5-2.5 hours, for example, it can be 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours, 2.0 hours, 2.1 hours, 2.2 hours, 2.3 hours, 2.4 hours or 2.5 hours, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0034] In some optional embodiments, the instantaneous concentration of hexafluoroethane in the total gas stream is 80-150 ppm, for example, it can be 80 ppm, 87 ppm, 94 ppm, 101 ppm, 108 ppm, 115 ppm, 122 ppm, 129 ppm, 136 ppm, 143 ppm or 150 ppm, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0035] In some optional embodiments, the time ratio of the hexafluoroethane / nitrogen mixture to the time of its inlet and outlet is 1:(8-12), for example, it can be 1:8.0, 1:8.4, 1:8.8, 1:9.2, 1:9.6, 1:10.0, 1:10.4, 1:10.8, 1:11.2, 1:11.6 or 1:12.0, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0036] Secondly, the present invention provides a silicon nitride powder with low oxygen content.

[0037] In the initial stage of preparation (S1), this application constructs the first line of defense against oxygen attack. Using di-tert-butylamine, a sterically hindered amine with a tert-butyl group, as a reactant, it is polymerized with methyldichlorosilane under strictly anhydrous, oxygen-free, and low-temperature conditions. The tert-butyl group forms an effective "protective shield" around the silicon atoms of the polymer chain, increasing the steric hindrance of water molecules and thus reducing the risk of accidental hydrolysis of the precursor to form Si-O bonds during storage and subsequent operations. This "nitrophilic and oxygen-repellent" molecular characteristic lays the chemical foundation for the entire low-oxygen preparation route.

[0038] In step S2, the precursor is mixed with PEG-PPG-PEG and the catalyst precursor ferric acetylacetonate. The self-assembly properties of PEG-PPG-PEG are used to form nanomicelles, encapsulating the precursor and constructing an independent reactor. This not only provides a physical template for the subsequent formation of uniformly dispersed nanoparticles to prevent aggregation but also pre-embeds a uniformly dispersed catalyst for in-situ catalytic carbon removal. Subsequently, low-temperature solidification is carried out in supercritical ammonia fluid. Utilizing the high permeability of supercritical ammonia compared to gases and the high density of liquids, efficient and uniform penetration and reaction of ammonia molecules into the precursor are achieved, completing the initial nitridation from liquid to solid state and avoiding oxidation problems caused by residual oxygen in the equipment or atmosphere during the initial high-temperature reaction.

[0039] In step S3, this application employs stepwise pyrolysis. The various substances are decoupled within their respective optimal temperature windows. The first step involves pyrolysis at a lower temperature range (500-550℃) for the complete vaporization and removal of PEG-PPG-PEG, thereby releasing the spatially isolated amorphous ceramic particles. The second step switches to a slightly higher temperature range (900-950℃) with a hydrogen / nitrogen mixed atmosphere to activate the nano-iron catalyst generated from the in-situ decomposition of pre-embedded iron acetylacetone. The catalytic mechanism lies in the fact that the highly active iron nanoparticles can effectively crack hydrogen molecules, and the activated hydrogen atoms catalyze a hydrogenation reaction with the solid amorphous carbon produced by the precursor pyrolysis, generating gaseous methane that escapes. This in-situ chemical carbon removal solves the problem of residual carbon, which is difficult to avoid in traditional precursor conversion methods.

[0040] In step S4, this application utilizes a solid-state transformation supplemented by surface chemical modification. When high-purity amorphous powder crystallizes into low-oxygen-content silicon nitride powder at a high temperature of 1500-1550℃, the "displacement" of impurity atoms during lattice reconstruction—that is, thermodynamically unstable oxygen atoms spontaneously migrate from the interior of the lattice and accumulate on the higher-energy grain surface—forms an extremely thin Si-ON amorphous layer. Subsequently, pulsed hexafluoroethane decomposes at high temperature to generate highly reactive fluorine radicals. The strong oxidizing properties of fluorine are used to selectively chemically etch the oxygen-rich particle surface, converting solid oxygen impurities into volatile gaseous products such as silicon fluoride, thus completely removing them. This "accumulation first, removal later" strategy is akin to a "chemical polishing" of the material, ensuring that the final product achieves an extremely low oxygen content.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] This application uses a sterically hindered amine with a large tert-butyl group as a raw material to synthesize a polysilazane precursor with inherent hydrolysis resistance: its large tert-butyl group forms a "protective shield" that hinders water molecule erosion through a physical barrier effect, thereby inhibiting the formation of silicon-oxygen bonds and laying the chemical foundation for the entire low-oxygen preparation route.

[0043] This application achieves a dual purpose by constructing a "reactor": First, by co-encapsulating the precursor and catalyst precursor with PEG-PPG-PEG, a physical template is pre-set for the subsequent formation of uniformly dispersed particles to prevent agglomeration, and a catalyst is pre-embedded for the in-situ decarbonization step; Second, low-temperature solidification is carried out in supercritical ammonia, and its high permeability and reactivity are used to complete efficient and uniform preliminary nitriding, thereby avoiding the oxidation problem commonly found in traditional high-temperature processes.

[0044] This application employs a stepwise pyrolysis method to decouple the template removal and decarbonization processes. First, the PEG-PPG-PEG template is removed by vaporization in a lower temperature region, yielding spatially isolated amorphous particles. Then, in a higher temperature region, a hydrogen atmosphere is switched to activate a pre-embedded nano-iron catalyst, catalytically hydrogenating the solid residual carbon into gaseous methane, which then escapes. This in-situ chemical decarbonization method solves the common problem of residual carbon in precursor methods.

[0045] This application adopts a "gather first, remove later" strategy: firstly, during the high-temperature crystallization process, oxygen atoms are spontaneously enriched on the particle surface by utilizing the "impurity displacement effect"; then, highly active fluorine radicals generated by hexafluoroethane are introduced in a pulsed manner to selectively chemically etch the oxygen-rich layer, converting the solid oxygen impurities into easily volatile gaseous products and removing them, thus ensuring the low oxygen content of the final product. Detailed Implementation

[0046] The technical solution of the present invention will be described in detail below with reference to specific embodiments. The embodiments described herein are specific implementations of the present invention and are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be construed as limiting the implementation of the present invention or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the content disclosed in the claims and the specification of this application. These technical solutions include technical solutions that employ any obvious substitutions and modifications made to the embodiments described herein.

[0047] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone further purification or processing.

[0048] Example 1

[0049] This embodiment provides a low-oxygen-content silicon nitride powder and its preparation method. The preparation method of the low-oxygen-content silicon nitride powder specifically includes the following steps:

[0050] S1: Under a nitrogen atmosphere, methyldichlorosilane was added to toluene, and a toluene solution of 25 wt.% di-tert-butylamine was added dropwise at -5°C to obtain reaction mixture A, wherein the volume ratio of methyldichlorosilane to toluene was 1:15, and the molar ratio of di-tert-butylamine to methyldichlorosilane was 2.4:1; reaction mixture A was stirred and matured at room temperature for 8 h, and then filtered and distilled under reduced pressure to obtain a polysilazane liquid precursor;

[0051] S2: PEG-PPG-PEG (model Pluronic® P-123) was added to tetrahydrofuran, along with a polysilazane liquid precursor and iron acetylacetone. The mixture was ultrasonically treated to obtain a composite precursor solution B. The mass ratio of PEG-PPG-PEG to the polysilazane liquid precursor was 1:1, and the mass ratio of iron acetylacetone to the polysilazane liquid precursor was 0.10:10. The total solids content in composite precursor solution B was 10 wt.%. The solution was placed in a high-pressure reactor, and after nitrogen purging and deoxygenation, high-purity liquid ammonia was pumped in to obtain reaction solution C. The mass ratio of liquid ammonia to the polysilazane liquid precursor was 40:1. The solution was heated to 200°C and stirred at 20 MPa for 4 hours. After the reaction, the solution was cooled, depressurized, and dried to obtain a solid composite powder.

[0052] S3: The solid composite powder is placed in a tube furnace and heated to a first temperature of 520°C at a first heating rate of 2°C / min under a nitrogen atmosphere and held for 1.5h; then the temperature is continued to rise to a second temperature of 920°C, while the atmosphere is switched to a hydrogen / nitrogen mixture, wherein the volume fraction of hydrogen is 5%, and the mixture is held for 2.0h under this atmosphere to obtain high-purity amorphous silicon nitride precursor powder.

[0053] S4: The high-purity amorphous silicon nitride precursor powder is heated to a third temperature of 1520℃ in a pure nitrogen atmosphere at a second heating rate of 8℃ / min and held for 2.0h. During the holding period, a hexafluoroethane / nitrogen mixture is pulsedly introduced into the nitrogen gas flow, wherein the instantaneous concentration of hexafluoroethane in the total gas flow is 100ppm and the ratio of gas flow to gas stop time is 1:10. After the reaction is completed, the powder is cooled to room temperature in a pure nitrogen atmosphere to obtain silicon nitride powder with low oxygen content.

[0054] Example 2

[0055] This embodiment provides a low-oxygen-content silicon nitride powder and its preparation method. The preparation method of the low-oxygen-content silicon nitride powder specifically includes the following steps:

[0056] S1: Under a nitrogen atmosphere, methyldichlorosilane was added to toluene, and a toluene solution of 30 wt.% di-tert-butylamine was added dropwise at 0 °C to obtain reaction mixture A, wherein the volume ratio of methyldichlorosilane to toluene was 1:20, and the molar ratio of di-tert-butylamine to methyldichlorosilane was 2.2:1; reaction mixture A was stirred and matured at room temperature for 12 h, and then filtered and distilled under reduced pressure to obtain a polysilazane liquid precursor;

[0057] S2: PEG-PPG-PEG (model Pluronic® P-123) was added to tetrahydrofuran, along with a polysilazane liquid precursor and iron acetylacetone. The mixture was ultrasonically treated to obtain a composite precursor solution B. The mass ratio of PEG-PPG-PEG to the polysilazane liquid precursor was 1.2:1, and the mass ratio of iron acetylacetone to the polysilazane liquid precursor was 0.08:10. The total solids content in composite precursor solution B was 15 wt.%. The solution was placed in a high-pressure reactor, and after nitrogen purging and deoxygenation, high-purity liquid ammonia was pumped in to obtain reaction solution C. The mass ratio of liquid ammonia to the polysilazane liquid precursor was 30:1. The solution was heated to 240°C and stirred at 25 MPa for 3 hours. After the reaction, the solution was cooled, depressurized, and dried to obtain a solid composite powder.

[0058] S3: The solid composite powder is placed in a tube furnace and heated to a first temperature of 550°C at a first heating rate of 3°C / min under a nitrogen atmosphere and held for 1 hour; then the temperature is continued to rise to a second temperature of 950°C, while the atmosphere is switched to a hydrogen / nitrogen mixture with a hydrogen volume fraction of 6% and held for 2.5 hours under this atmosphere to obtain high-purity amorphous silicon nitride precursor powder.

[0059] S4: The high-purity amorphous silicon nitride precursor powder is heated to a third temperature of 1550℃ in a pure nitrogen atmosphere at a second heating rate of 10℃ / min and held for 2.5h. During the holding period, a hexafluoroethane / nitrogen mixture is pulsedly introduced into the nitrogen gas flow, wherein the instantaneous concentration of hexafluoroethane in the total gas flow is 150ppm and the ratio of gas flow to gas stop time is 1:8. After the reaction is completed, the powder is cooled to room temperature in a pure nitrogen atmosphere to obtain silicon nitride powder with low oxygen content.

[0060] Example 3

[0061] This embodiment provides a low-oxygen-content silicon nitride powder and its preparation method. The preparation method of the low-oxygen-content silicon nitride powder specifically includes the following steps:

[0062] S1: Under a nitrogen atmosphere, methyldichlorosilane was added to toluene, and a toluene solution of 22 wt.% di-tert-butylamine was added dropwise at -8°C to obtain reaction mixture A, wherein the volume ratio of methyldichlorosilane to toluene was 1:12, and the molar ratio of di-tert-butylamine to methyldichlorosilane was 2.3:1; reaction mixture A was stirred and matured at room temperature for 10 h, and then filtered and distilled under reduced pressure to obtain a liquid precursor of polysilazane;

[0063] S2: PEG-PPG-PEG (model Pluronic® P-123) was added to tetrahydrofuran, along with a polysilazane liquid precursor and iron acetylacetone. The mixture was ultrasonically treated to obtain a composite precursor solution B. The mass ratio of PEG-PPG-PEG to the polysilazane liquid precursor was 0.9:1, and the mass ratio of iron acetylacetone to the polysilazane liquid precursor was 0.11:10. The total solids content in composite precursor solution B was 12 wt.%. The solution was placed in a high-pressure reactor, and after nitrogen purging and deoxygenation, high-purity liquid ammonia was pumped in to obtain reaction solution C. The mass ratio of liquid ammonia to the polysilazane liquid precursor was 45:1. The solution was heated to 220°C and stirred at 22 MPa for 4.5 h. After the reaction, the solution was cooled, depressurized, and dried to obtain a solid composite powder.

[0064] S3: The solid composite powder is placed in a tube furnace and heated to a first temperature of 510°C at a first heating rate of 1.5°C / min under a nitrogen atmosphere and held for 1.8h; then the temperature is further increased to a second temperature of 910°C, while the atmosphere is switched to a hydrogen / nitrogen mixture with a hydrogen volume fraction of 4.5% and held for 1.8h under this atmosphere to obtain high-purity amorphous silicon nitride precursor powder;

[0065] S4: The high-purity amorphous silicon nitride precursor powder is heated to a third temperature of 1510℃ in a pure nitrogen atmosphere at a second heating rate of 6℃ / min and held for 1.8h. During the holding period, a hexafluoroethane / nitrogen mixture is pulsedly introduced into the nitrogen gas flow, wherein the instantaneous concentration of hexafluoroethane in the total gas flow is 120ppm and the ratio of gas flow to gas stop time is 1:11. After the reaction is completed, the powder is cooled to room temperature in a pure nitrogen atmosphere to obtain silicon nitride powder with low oxygen content.

[0066] Example 4

[0067] This embodiment provides a low-oxygen-content silicon nitride powder and its preparation method. The preparation method of the low-oxygen-content silicon nitride powder specifically includes the following steps:

[0068] S1: Under a nitrogen atmosphere, methyldichlorosilane was added to toluene, and a toluene solution of 20 wt.% di-tert-butylamine was added dropwise at -10°C to obtain reaction mixture A, wherein the volume ratio of methyldichlorosilane to toluene was 1:10, and the molar ratio of di-tert-butylamine to methyldichlorosilane was 2.5:1; reaction mixture A was stirred and matured at room temperature for 6 h, and then filtered and distilled under reduced pressure to obtain a polysilazane liquid precursor;

[0069] S2: PEG-PPG-PEG (model Pluronic® P-123) was added to tetrahydrofuran, along with a polysilazane liquid precursor and iron acetylacetone. The mixture was ultrasonically treated to obtain a composite precursor solution B. The mass ratio of PEG-PPG-PEG to the polysilazane liquid precursor was 0.8:1, and the mass ratio of iron acetylacetone to the polysilazane liquid precursor was 0.12:10. The total solids content in composite precursor solution B was 8 wt.%. The solution was placed in a high-pressure reactor, and after nitrogen purging and deoxygenation, high-purity liquid ammonia was pumped in to obtain reaction solution C. The mass ratio of liquid ammonia to the polysilazane liquid precursor was 50:1. The solution was heated to 180°C and stirred at 18 MPa for 5 hours. After the reaction, the solution was cooled, depressurized, and dried to obtain a solid composite powder.

[0070] S3: The solid composite powder is placed in a tube furnace and heated to a first temperature of 500°C at a first heating rate of 1°C / min under a nitrogen atmosphere and held for 2 hours; then the temperature is continued to rise to a second temperature of 900°C, while the atmosphere is switched to a hydrogen / nitrogen mixture with a hydrogen volume fraction of 4% and held for 1.5 hours under this atmosphere to obtain high-purity amorphous silicon nitride precursor powder.

[0071] S4: The high-purity amorphous silicon nitride precursor powder is heated to a third temperature of 1500℃ in a pure nitrogen atmosphere at a second heating rate of 5℃ / min and held for 1.5h. During the holding period, a hexafluoroethane / nitrogen mixture is pulsedly introduced into the nitrogen gas flow, wherein the instantaneous concentration of hexafluoroethane in the total gas flow is 80ppm and the ratio of gas flow to gas stop time is 1:12. After the reaction is completed, the powder is cooled to room temperature in a pure nitrogen atmosphere to obtain silicon nitride powder with low oxygen content.

[0072] Comparative Example 1

[0073] This comparative example provides a silicon nitride powder with low oxygen content. The difference from Example 1 is that in S1, methylamine is used instead of di-tert-butylamine. Other operating steps and process parameters are exactly the same as in Example 1.

[0074] Comparative Example 2

[0075] This comparative example provides a silicon nitride powder with low oxygen content. The difference from Example 1 is that PEG-PPG-PEG is not added in S2, while the other operating steps and process parameters are exactly the same as in Example 1.

[0076] Comparative Example 3

[0077] This comparative example provides a silicon nitride powder with low oxygen content. The difference from Example 1 is that in S3, after heating to the second temperature, the gas is not switched to a hydrogen / nitrogen mixture, but a pure nitrogen atmosphere is continued to be used. Other operating steps and process parameters are exactly the same as in Example 1.

[0078] Comparative Example 4

[0079] This comparative example provides a silicon nitride powder with low oxygen content. The difference from Example 1 is that in S4, hexafluoroethane is not pulsed during the heat preservation period, and the reaction is carried out only in a pure nitrogen atmosphere. Other operating steps and process parameters are exactly the same as in Example 1.

[0080] The performance of the low-oxygen-content silicon nitride powders of Examples 1-4 and Comparative Examples 1-4 was tested, and the specific process is as follows:

[0081] The oxygen content of the sample was tested using an oxygen and nitrogen analyzer;

[0082] The particle size distribution of the sample was tested using a laser particle size analyzer.

[0083] The test results are shown in Table 1.

[0084] Table 1. Performance test results of low oxygen content silicon nitride powders prepared in Examples 1-4 and Comparative Examples 1-4

[0085] Oxygen content (%) <![CDATA[Particle size distribution (D 50 ) / nm]]> Example 1 0.45 400 Example 2 0.48 380 Example 3 0.46 450 Example 4 0.41 400 Comparative Example 1 1.65 650 Comparative Example 2 0.55 5500 Comparative Example 3 0.60 500 Comparative Example 4 0.85 410

[0086] As shown in Table 1, the test results of Example 1 and Comparative Example 1 indicate that when methylamine is used to replace di-tert-butylamine, methylamine is a small molecule amine, and its methyl group does not have the huge steric hindrance of the tert-butyl group. The active bonds in the polysilazane precursor synthesized after the replacement are exposed. In subsequent operations, a large number of oxygen atoms will inevitably be generated and further condensed to form chemically stable silicon-oxygen bonds. These oxygen atoms are firmly embedded in the polymer backbone, and subsequent heat treatment or chemical treatment is difficult to remove them effectively, resulting in an increase in the oxygen content of the final product. The higher oxygen content will form a silicon-oxygen-nitrogen glass phase, which acts as a flux at high temperatures, promoting abnormal grain growth and interparticle sintering, resulting in an increase in its particle size.

[0087] As shown in Table 1, the test results of Example 1 and Comparative Example 2 indicate that without the addition of PEG-PPG-PEG, the precursor forms a continuous blocky gel during the S2 reaction. During the temperature treatment, silicon nitride nuclei are generated and grow throughout the block. Due to the lack of physical spacing, adjacent grains directly contact and undergo solid-state sintering, forming strong grain boundaries and sintering necks. The final product is not a dispersed nanoparticle powder, but a micron-sized, large, hard agglomerate formed by the sintering of countless tiny grains, with an increased particle size. The slight increase in oxygen content may be because the large agglomerates may contain some gases or impurities that have not been completely expelled.

[0088] From the test results of Example 1 and Comparative Example 3 in Table 1, it can be seen that in S3, after heating to the second temperature, the gas is not switched to a hydrogen / nitrogen mixture, and a pure nitrogen atmosphere is continued. During pyrolysis in a pure nitrogen atmosphere, the organic groups in the precursor will decompose, but cannot be completely vaporized, leaving a large amount of amorphous carbon residue in the amorphous silicon nitride matrix. The presence of this carbon impurity has two serious consequences: in the high-temperature crystallization stage of S4, some carbon will react with silicon nitride or its surface oxides to generate silicon carbide impurity phases, damaging the phase purity of the product; secondly, the presence of carbon impurities will interfere with the normal nucleation and growth process of silicon nitride, leading to uneven grain development and potentially trapping some oxygen-containing impurities inside, resulting in a poorer refining effect. Therefore, the oxygen content of the product increases, and uneven grain growth leads to an increase in D50.

[0089] As can be seen from the test results of Example 1 and Comparative Example 4 in Table 1, in S4, no pulsed hexafluoroethane was introduced during the heat preservation period, and the reaction was carried out only in a pure nitrogen atmosphere. The oxygen-rich layer that was naturally formed on the surface during the crystallization process could not be etched by the pulsed hexafluoroethane and was completely preserved, resulting in an increase in oxygen content. This had little impact on the core size and agglomeration state of the particles.

[0090] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing silicon nitride powder with low oxygen content, characterized in that, The preparation method includes: S1: Under a nitrogen atmosphere, methyldichlorosilane was added to toluene, and a toluene solution of di-tert-butylamine was added dropwise to obtain reaction mixture A. Reaction mixture A was stirred and matured at room temperature, and then filtered and distilled under reduced pressure to obtain a liquid precursor of polysilazane. S2: Add PEG-PPG-PEG to tetrahydrofuran, add polysilazane liquid precursor and acetylacetone iron, and sonicate to obtain composite precursor solution B; place it in a high-pressure reactor, remove oxygen by nitrogen replacement, and pump in high-purity liquid ammonia to obtain reaction solution C. After heating and stirring, after the reaction is completed, cool down, depressurize and dry to obtain solid composite powder. S3: The solid composite powder is placed in a tube furnace and heated to a first temperature at a first heating rate under a nitrogen atmosphere and held at that temperature; then the temperature is continued to rise to a second temperature while the atmosphere is switched to a hydrogen / nitrogen mixture and held at that atmosphere to obtain high-purity amorphous silicon nitride precursor powder. S4: The high-purity amorphous silicon nitride precursor powder is heated to the third temperature at the second heating rate in a pure nitrogen atmosphere and held at the temperature. During the holding period, a hexafluoroethane / nitrogen mixture is pulsedly introduced into the nitrogen gas flow. After the reaction is completed, the powder is cooled to room temperature in a pure nitrogen atmosphere to obtain silicon nitride powder with low oxygen content.

2. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S1: The volume ratio of methyldichlorosilane to toluene is 1:(10-20).

3. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S1: The mass fraction of the di-tert-butylamine in the toluene solution was 20-30 wt.%. The molar ratio of di-tert-butylamine to methyldichlorosilane is (2.2-2.5):

1.

4. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S2: The mass ratio of PEG-PPG-PEG to polysilazane liquid precursor is (0.8-1.2):1; The mass ratio of acetylacetone iron to polysilazane liquid precursor is (0.08-0.12):10; The total solids mass fraction in the composite precursor solution B is 8-15 wt.%.

5. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S2: The mass ratio of liquid ammonia to polysilazane liquid precursor is (30-50):

1.

6. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S3: The first heating rate is 1-3℃ / min; The first temperature is 500-550℃; The holding time at the first temperature is 1-2 hours.

7. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S3: The second temperature is 900-950℃; The holding time at the second temperature is 1.5-2.5 hours; The hydrogen / nitrogen mixture has a hydrogen volume fraction of 4-6%.

8. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S4: The second heating rate is 5-10℃ / min; The third temperature is 1500-1550℃; The holding time for the third temperature is 1.5-2.5 hours.

9. The method for preparing low-oxygen-content silicon nitride powder according to claim 1, characterized in that, In S4: The instantaneous concentration of hexafluoroethane in the total gas flow is 80-150 ppm; The ratio of the time for the hexafluoroethane / nitrogen mixture to the time for the gas to be supplied and stopped is 1:(8-12).

10. A low-oxygen-content silicon nitride powder prepared by the preparation method according to any one of claims 1-9.