Method for producing silicon nitride

Abstract

This application provides a method for preparing silicon nitride, comprising the following steps: Intermediate generation: Silane gas and ammonia gas are introduced into a reactor, and the reactor temperature is controlled to increase. The silane gas and ammonia gas react to obtain a silicon-nitrogen intermediate; Silicon nitride generation: The reactor temperature is controlled to increase, and silicon powder is introduced into the reactor. The silicon-nitrogen intermediate decomposes to obtain amorphous silicon nitride, while the silicon powder reacts with ammonia gas to obtain amorphous silicon nitride. This application reacts silane gas and ammonia gas at a lower temperature to obtain a silicon-nitrogen intermediate, and then heats it to decompose the silicon-nitrogen intermediate to obtain silicon nitride. Simultaneously, the silicon powder reacts with ammonia gas to obtain silicon nitride. The reaction at a lower temperature greatly reduces production energy consumption, and the simultaneous reaction to obtain silicon nitride can increase the yield of silicon nitride.

Description

Technical Field

[0001] This application relates to the field of silicon nitride generation technology, and more particularly to a method for preparing silicon nitride. Background Technology

[0002] Silicon nitride (Si3N4), as an important high-temperature structural ceramic material, has a series of advantages such as high hardness, high strength, wear resistance, high temperature resistance, low coefficient of thermal expansion, high thermal conductivity, good thermal shock resistance, and low density. It has extremely broad application prospects in fields such as ceramic engines, machining, microelectronics, space science, and nuclear power engineering.

[0003] Currently, the main methods for preparing silicon nitride include direct silicon powder nitridation, carbothermal reduction, silicon halide ammonolysis, and precursor methods. Among these, direct silicon powder nitridation is the most widely used method in China. This method is relatively simple, inexpensive, and uses polycrystalline or monocrystalline silicon as raw materials. However, the silicon powder nitridation process has some problems. For example, silicon powder nitridation is an exothermic reaction, and the nitridation rate must be carefully controlled; otherwise, a "silicon flow" phenomenon can easily occur in local areas, affecting the powder quality. Furthermore, the silicon nitride powder produced by this process is in block form and must be ball-milled to obtain fine powder, which is inefficient, easily introduces impurities, and is time-consuming.

[0004] Carbothermic reduction offers lower raw material costs, produces small-particle-size powders, and boasts a fast reaction rate with high α-Si3N4 content, making it suitable for large-scale production. However, the resulting silicon nitride powders often contain residual carbon or silicon carbide, resulting in low purity and impacting product quality and applications. Ammonolysis of silicon halides can yield high-purity α-silicon nitride or amorphous silicon nitride powders, but some modified methods require sophisticated equipment and have relatively high manufacturing costs. Precursor methods can obtain high-purity or doped silicon nitride, with some methods offering high yields, but process control is complex. Summary of the Invention

[0005] The purpose of this application is to provide a method for preparing silicon nitride, which involves reacting silane gas and ammonia gas to obtain a silicon-nitrogen intermediate, then decomposing the silicon-nitrogen intermediate while simultaneously performing a silicon powder nitriding reaction, and finally preparing silicon nitride together. This method solves the problems of low purity of silicon nitride products, long processing time, and high equipment requirements in the prior art.

[0006] To achieve one of the above-mentioned objectives, one embodiment of this application provides a method for preparing silicon nitride, comprising the following steps:

[0007] Intermediate formation: Silane gas and ammonia gas are introduced into the reactor, and the reactor temperature is controlled to increase. The silane gas and ammonia gas react to obtain a silicon-nitrogen intermediate. Silicon nitride generation: The reactor temperature is controlled and silicon powder is introduced into the reactor. The silicon-nitrogen intermediate decomposes to obtain amorphous silicon nitride. At the same time, the silicon powder reacts with ammonia to obtain amorphous silicon nitride.

[0008] In one embodiment of this application, in the intermediate generation step, the reactor temperature is controlled to rise to 300-380°C.

[0009] In one embodiment of this application, in the intermediate generation step, the molar ratio of silicon atoms in silane gas to nitrogen atoms in ammonia gas is 1:(3-6).

[0010] In one embodiment of this application, in the silicon nitride generation step, the mass of the introduced silicon powder satisfies the following: the molar ratio of silicon atoms in silane gas, nitrogen atoms in ammonia gas, and silicon powder is 1:(3-6):(1.5-4), and the silicon powder is introduced into the reactor via nitrogen gas as a carrier gas.

[0011] In one embodiment of this application, in the silicon nitride generation step, the temperature inside the reactor is controlled at 850-900°C and the pressure is 0.1-0.5 MPa.

[0012] In one embodiment of this application, the heating rate of the process from the reaction temperature of the intermediate generation step to the reaction temperature of the silicon nitride generation step is 5-8°C / min.

[0013] In one embodiment of this application, after the silicon nitride generation step, a phase transformation step is further included: controlling the reactor temperature to 1350-1400℃ and holding it for 2-4 hours, amorphous silicon nitride is transformed into α-phase silicon nitride; or, controlling the reactor temperature to 1600-1650℃ and holding it for 1-2 hours, amorphous silicon nitride is transformed into β-phase silicon nitride.

[0014] In one embodiment of this application, the pressure inside the reactor is controlled to be 1-2 MPa during the phase change conversion step.

[0015] In one embodiment of this application, in the intermediate generation step, silane gas and ammonia gas are preheated at 180-200°C before being introduced into the reactor.

[0016] In one embodiment of this application, the purity of silane gas and ammonia gas is ≥99.9%, the purity of silicon powder is ≥99.9%, and the particle size is 1-10 μm.

[0017] One or more technical solutions provided in this application have at least the following technical effects or advantages: In the method for preparing silicon nitride provided in this application, a silicon-nitrogen intermediate is first obtained by reacting silane gas and ammonia gas at a lower temperature. This step greatly reduces the energy consumption in the reaction process. Then, the temperature is raised to decompose the silicon-nitrogen intermediate to obtain silicon nitride. Silicon powder is introduced simultaneously, and silicon nitride is generated by nitriding the silicon-nitrogen intermediate with ammonia gas obtained from the decomposition of the silicon-nitrogen intermediate. The two reactions occur simultaneously, which can increase the yield of silicon nitride and save production time and costs. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This application provides a method for preparing silicon nitride, comprising the following steps: Intermediate formation: Silane gas and ammonia gas are introduced into the reactor, and the reactor temperature is controlled to increase. The silane gas and ammonia gas react to obtain a silicon-nitrogen intermediate. Silicon nitride generation: The reactor temperature is controlled and silicon powder is introduced into the reactor. The silicon-nitrogen intermediate decomposes to obtain amorphous silicon nitride. At the same time, the silicon powder reacts with ammonia to obtain amorphous silicon nitride.

[0020] In this application, silane gas and ammonia are used as reaction raw materials to obtain a silicon-nitrogen intermediate. After heating in the silicon nitride generation step, the silicon-nitrogen intermediate can decompose at high temperature to obtain amorphous silicon nitride, and at the same time, ammonia gas can be released again. The introduced silicon powder can react with ammonia gas to obtain amorphous silicon nitride.

[0021] This application utilizes a low temperature in the intermediate formation step, significantly reducing energy consumption during silicon nitride production and improving safety. In the silicon nitride formation step, the silicon-nitrogen intermediate decomposes while the silicon powder reacts simultaneously; these two reactions occur concurrently, increasing silicon nitride yield and saving production time and costs.

[0022] In some embodiments of this application, the reactor temperature is controlled at 300-380°C during the intermediate generation step to avoid side reactions.

[0023] In some embodiments of this application, in the intermediate generation step, the molar ratio of silicon atoms in silane gas to nitrogen atoms in ammonia gas is 1:(3-6).

[0024] Silane gas, which can be silane, silane, etc., is used as a silicon source, while ammonia is used as a nitrogen source. When controlling the amount of raw materials, the ratio of silicon atoms to nitrogen atoms must be controlled. By controlling the excess of nitrogen source, the reaction of silane gas can be ensured to be complete, and the waste of silane gas can be avoided.

[0025] Furthermore, in some embodiments of this application, in the silicon nitride generation step, the mass of the introduced silicon powder satisfies the following: the molar ratio of silicon atoms in silane gas, nitrogen atoms in ammonia gas, and silicon powder is 1:(3-6):(1.5-4), and the silicon powder is introduced into the reactor via nitrogen gas as a carrier gas.

[0026] The excess ammonia gas serves two purposes: firstly, it ensures that the silane gas is completely consumed during the intermediate formation step; secondly, it also serves as the nitrogen source for silicon powder nitriding.

[0027] When the proportion of ammonia is small (silicon atoms: nitrogen atoms = 1:3), the ammonia remaining after the reaction of silane gas and ammonia gas and the ammonia produced by the decomposition of silicon-nitrogen intermediates are insufficient to completely nitrid the silicon powder. However, the nitrogen gas, which is used as a carrier gas, can also serve as a nitrogen source to nitrid the silicon powder.

[0028] When the proportion of ammonia is relatively large (silicon atoms: nitrogen atoms = 1:6), and the amount of silicon powder added is small (1.5 parts), the ammonia remaining after the reaction of silane gas and ammonia gas and the ammonia gas produced by the decomposition of silicon-nitrogen intermediates can completely nitrid the silicon powder and discharge it from the reactor as tail gas. When more silicon powder is added (4 parts), there is a surplus of silicon powder, and nitrogen gas can be used as a nitrogen source to nitrid the remaining silicon powder.

[0029] In some embodiments of this application, during the silicon nitride generation step, the temperature inside the reactor is controlled at 850-900°C, and the pressure is 0.1-0.5 MPa. During the intermediate generation step, the pressure can also be 0.1-0.5 MPa.

[0030] Before introducing silane gas and ammonia gas into the reactor, the reactor is evacuated to prevent the silicon-nitrogen intermediate, as well as the silicon nitride produced and the silicon powder added in the silicon nitride generation step, from being oxidized, thus avoiding the generation of unnecessary impurities.

[0031] After evacuation, silane and ammonia gases are introduced into the reactor, raising the pressure inside the reactor to 0.1-0.5 MPa. The silicon-nitrogen intermediate undergoes a decomposition reaction at 850-900℃, which is much lower than the 1300℃ or higher temperature required for the direct nitridation of silicon powder. At the same time, the silicon powder can also undergo a slow reaction under this condition.

[0032] A pressure higher than atmospheric pressure can promote the forward reaction and improve reaction efficiency.

[0033] In some embodiments of this application, the heating rate of the process from the reaction temperature of the intermediate generation step to the reaction temperature of the silicon nitride generation step is 5-8°C / min.

[0034] After the intermediate reaction is completed, the reactor needs to be heated to start the silicon nitride generation step. This stage requires rapid heating while avoiding damage to the reactor. The heating rate between the two steps should be controlled at 5-8℃ / min.

[0035] In some embodiments of this application, after the silicon nitride generation step, a phase transformation step is further included: controlling the reactor temperature to 1350-1400°C and holding it for 2-4 hours to convert amorphous silicon nitride into α-phase silicon nitride, or controlling the reactor temperature to 1600-1650°C and holding it for 1-2 hours to convert amorphous silicon nitride into β-phase silicon nitride.

[0036] The silicon nitride obtained from the silicon nitride formation process is amorphous silicon nitride. Amorphous silicon nitride has a loose structure, low density, low hardness, poor strength, and poor thermal stability, resulting in low application value. Depending on the desired crystal form, it is subjected to high-temperature holding to promote the formation and growth of α-phase nuclei, or to induce the transformation of the α-phase to the β-phase, to obtain α-phase or β-phase silicon nitride with a dense structure, high density, high hardness, high strength, and good thermal stability. This makes the final silicon nitride have extremely high application value.

[0037] In the phase transformation step, in addition to converting amorphous silicon nitride into α-phase silicon nitride or β-phase silicon nitride, it can also promote the reaction between silicon powder and ammonia / nitrogen, so that the silicon powder is completely nitrided and avoids the formation of impurities in the silicon nitride.

[0038] In some embodiments of this application, the pressure inside the reactor is controlled at 1-2 MPa during the phase transformation step. Under high pressure, the conversion of amorphous silicon nitride to α-phase or β-phase silicon nitride can be promoted, shortening the holding time of amorphous silicon nitride at high temperatures and saving energy and production costs.

[0039] After holding at high temperature, α-phase silicon nitride powder or β-phase silicon nitride powder is obtained. The particle size distribution and morphology of the powder can be further adjusted through steps such as ball milling and sieving to improve product quality and performance. The ball milling process is preferably carried out under the protection of an inert gas to prevent the silicon nitride powder from being oxidized.

[0040] In some embodiments of this application, the purity of silane gas and ammonia gas is ≥99.9%, the purity of silicon powder is ≥99.9%, and the particle size is 1-10 μm. Using high-purity silane gas, ammonia gas, and silicon powder as raw materials avoids introducing impurities. Smaller silicon powder particle size results in a larger specific surface area, increasing the contact area between silicon powder and ammonia / nitrogen gas, thereby enhancing the reactivity of silicon powder with ammonia / nitrogen gas and increasing the reaction rate.

[0041] In some embodiments of this application, in the intermediate generation step, silane gas and ammonia gas are preheated at 180-200°C before being introduced into the reactor.

[0042] Preheating silane and ammonia gases before they enter the reactor serves two purposes. First, it ensures that the gases reach a suitable initial reaction temperature before entering the reactor, reducing the temperature drop within the reactor and thus shortening the required heating time after entry, thereby increasing the reaction rate and efficiency. Second, it reduces the temperature gradient within the reactor, resulting in a more uniform reaction, minimizing side reactions, and ensuring the stability and safety of the reaction process. Third, preheating reduces the reactor's heat load, lowers energy consumption, and extends the equipment's lifespan.

[0043] This application utilizes a fluidized bed reactor for the reaction. The reactor material is selected to be high-temperature resistant and corrosion-resistant, such as stainless steel or quartz. An efficient heating system is installed outside the reactor, enabling precise temperature control. The heating method can be electric heating or induction heating.

[0044] The fluidized bed reactor is connected to a silane gas storage tank and an ammonia gas storage tank via a mixer. Silane gas and ammonia gas are introduced into the mixer through flow controllers, preheated in the mixer, and then introduced into the fluidized bed reactor. During the preheating process in the mixer, the silane gas and ammonia gas are thoroughly mixed to form a uniform reaction atmosphere.

[0045] A spiral guide plate is installed inside the fluidized bed. The spiral guide plate agitates the silicon powder and silicon-nitrogen intermediate, keeping the solid material in a fluidized state and preventing solid material deposition, which would affect the decomposition / reaction efficiency.

[0046] During the reaction, after evacuating the fluidized bed reactor, silane and ammonia gas can be introduced to a preset pressure, then the gas supply can be stopped. After the intermediate formation step is completed, nitrogen gas carrying silicon powder can be introduced again to a preset pressure to continue the reaction. During the phase change step, nitrogen gas can be introduced to the target pressure and then held at the temperature. Alternatively, the required gases can be continuously introduced and controlled at a preset pressure, and the solid material can be returned to the fluidized bed reactor through filtration or other methods.

[0047] The technical solution of this application will be further described below with reference to some specific embodiments.

[0048] Example 1 Intermediate formation: The fluidized bed reactor was evacuated. Silane and ammonia gas in a molar ratio of 1:3, preheated to 180°C, were introduced into the fluidized bed reactor at 320°C until the pressure reached 0.15 MPa. The reaction was carried out for 5 hours to obtain a silicon-nitrogen intermediate. The purity of both the silane gas and the ammonia gas was 99.99%. Silicon nitride generation: The fluidized bed reactor was heated to 850°C at a heating rate of 5°C / min, and nitrogen gas carrying silicon powder was introduced until the pressure reached 0.3 MPa. The reaction was carried out for 3 hours to obtain amorphous silicon nitride. The silicon powder had a purity of 99.99% and a particle size of 3-5 μm. The molar ratio of silicon powder to silane gas was 1.5:1. Phase transformation: The fluidized bed reactor was heated to 1350℃ and pressurized to 1.1MPa, and held at that temperature for 2 hours to obtain α-phase silicon nitride.

[0049] XRD analysis showed that the purity of the α-phase silicon nitride was 5N, and the α-phase purity was 95%.

[0050] Example 2 Intermediate generation: The fluidized bed reactor was evacuated. Silane and ammonia gas in a molar ratio of 1:5.5, preheated to 190°C, were introduced into the fluidized bed reactor at 350°C until the pressure reached 0.2 MPa. The reaction was carried out for 6 hours to obtain a silicon-nitrogen intermediate. The purity of the silane gas and the ammonia gas was 99.999%. Silicon nitride generation: The fluidized bed reactor was heated to 870℃ at a heating rate of 6℃ / min, and nitrogen gas carrying silicon powder was introduced until the pressure reached 0.45MPa. The reaction was carried out for 3.5h to obtain amorphous silicon nitride. The silicon powder had a purity of 99.999% and a particle size of 3-7μm. The molar ratio of silicon powder to silane gas was 4:1. Phase transformation: The fluidized bed reactor was heated to 1600℃ and pressurized to 1.5MPa, and held at that temperature for 1 hour to obtain β-phase silicon nitride.

[0051] XRD analysis showed that the purity of the β-phase silicon nitride was 5N, and the β-phase content was 96%.

[0052] Example 3 Intermediate formation: The fluidized bed reactor was evacuated. Dichlorosilane and ammonia gas, with a molar ratio of 1:8, were preheated to 185°C and then introduced into the fluidized bed reactor at 330°C until the pressure reached 0.17 MPa. The reaction was carried out for 4.5 h to obtain a silicon-nitrogen intermediate. The purity of the silane gas and the ammonia gas was 99.99%. Silicon nitride generation: The fluidized bed reactor was heated to 870℃ at a heating rate of 6℃ / min, and nitrogen gas carrying silicon powder was introduced until the pressure reached 0.35MPa. The reaction was carried out for 3.5h to obtain amorphous silicon nitride. The silicon powder had a purity of 99.99% and a particle size of 2-5μm. The molar ratio of silicon powder to silane was 3.5:1. Phase transformation: The fluidized bed reactor was heated to 1370℃ and pressurized to 1.3MPa, and held at that temperature for 2.5 hours to obtain α-phase silicon nitride.

[0053] XRD analysis showed that the purity of the α-phase silicon nitride was 5N, and the α-phase purity was 96.2%.

[0054] Example 4 Intermediate formation: The fluidized bed reactor was evacuated. Dichlorosilane and ammonia gas, with a molar ratio of 1:9, were preheated to 195°C and then introduced into the fluidized bed reactor at 360°C until the pressure reached 0.18 MPa. The reaction was carried out for 5.5 h to obtain a silicon-nitrogen intermediate. The purity of the silane gas and the ammonia gas was 99.99%. Silicon nitride generation: The fluidized bed reactor was heated to 880℃ at a heating rate of 7℃ / min, and nitrogen gas carrying silicon powder was introduced until the pressure reached 0.4MPa. The reaction was carried out for 4.5h to obtain amorphous silicon nitride. The silicon powder had a purity of 99.99% and a particle size of 2-6μm. The molar ratio of silicon powder to silane was 4.5:1. Phase transformation: The fluidized bed reactor was heated to 1620℃, pressurized to 1.5MPa, and held at that temperature for 1.5 hours to obtain β-phase silicon nitride.

[0055] XRD analysis showed that the purity of the β-phase silicon nitride was 5N, and the β-phase purity was 97.6%.

[0056] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0057] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing silicon nitride, characterized in that, Includes the following steps: Intermediate formation: Silane gas and ammonia gas are introduced into the reactor, and the reactor temperature is controlled to increase. The silane gas and ammonia gas react to obtain a silicon-nitrogen intermediate. Silicon nitride generation: The reactor temperature is controlled and silicon powder is introduced into the reactor. The silicon-nitrogen intermediate decomposes to obtain amorphous silicon nitride. At the same time, the silicon powder reacts with ammonia to obtain amorphous silicon nitride.

2. The method for preparing silicon nitride according to claim 1, characterized in that, In the intermediate generation step, the temperature inside the reactor is controlled at 300-380℃.

3. The method for preparing silicon nitride according to claim 1, characterized in that, In the intermediate generation step, the molar ratio of silicon atoms in the silane gas to nitrogen atoms in the ammonia gas is 1:(3-6).

4. The method for preparing silicon nitride according to claim 3, characterized in that, In the silicon nitride generation step, the mass of the introduced silicon powder satisfies the following: the molar ratio of silicon atoms in silane gas, nitrogen atoms in ammonia gas, and silicon powder is 1:(3-6):(1.5-4), and the silicon powder is introduced into the reactor via nitrogen gas as a carrier gas.

5. The method for preparing silicon nitride according to claim 1, characterized in that, In the silicon nitride generation step, the temperature inside the reactor is controlled at 850-900℃ and the pressure is 0.1-0.5 MPa.

6. The method for preparing silicon nitride according to claim 5, characterized in that, The heating rate for the process of raising the temperature from the intermediate formation step to the silicon nitride formation step is 5-8 °C / min.

7. The method for preparing silicon nitride according to claim 1, characterized in that, Following the silicon nitride formation step, a phase transformation step is also included: controlling the reactor temperature to 1350-1400℃ and holding it for 2-4 hours to transform amorphous silicon nitride into α-phase silicon nitride, or controlling the reactor temperature to 1600-1650℃ and holding it for 1-2 hours to transform amorphous silicon nitride into β-phase silicon nitride.

8. The method for preparing silicon nitride according to claim 7, characterized in that, In the phase change conversion step, the pressure inside the reactor is controlled at 1-2 MPa.

9. The method for preparing silicon nitride according to claim 1, characterized in that, In the intermediate generation step, silane gas and ammonia gas are preheated at 180-200°C before being introduced into the reactor.

10. The method for preparing silicon nitride according to claim 1, characterized in that, The purity of silane gas and ammonia gas is ≥99.9%, the purity of silicon powder is ≥99.9%, and the particle size is 1-10μm.