WO2025232210A1 - Method for preparing silicon nitride by hot-press molding and nitridation of silicon powder, and silicon nitride product and use thereof - Google Patents

Abstract

Disclosed are a method for preparing silicon nitride by hot-press molding and nitridation of silicon powder, and a silicon nitride product and a use thereof. The silicon nitride product is formed by hot-press molding and nitridation of silicon powder and an additive. Dry mixing is carried out without a solvent, and the viscosity difference before and after melting of a binder is used for hot-press molding. The process is simple, stable, and efficient, and the charging amount and working efficiency are improved. In addition, during nitridation, the binder and a pore-forming agent undergo high-temperature decomposition and volatilization, so that the porosity of a formed silicon blank is greatly improved, thereby facilitating efficient mass transfer between gas and solid phases, and improving the nitriding rate. More importantly, activated carbon formed from the binder at high temperature reacts with a surface oxide layer of the silicon powder, so that the oxygen content of the silicon nitride can also be reduced in the absence of H₂, and the prepared silicon nitride product has the characteristics of high nitriding rate and α phase content and low oxygen content. The silicon nitride prepared by the method can be used for preparing a ceramic material or an electronic material.

Description

Methods for preparing silicon nitride by hot pressing and nitriding of silicon powder, its products and applications

Cross-reference to related applications

This application claims priority to Chinese Patent Application No. 202410559192.3, filed on May 8, 2024, entitled "Method for preparing silicon nitride by hot pressing and nitriding of silicon powder, and its products and applications", the entire contents of which are incorporated herein by reference.

Technical Field

This application relates to the field of inorganic non-metallic materials technology, specifically to a method for preparing silicon nitride by hot pressing and nitriding of silicon powder, and its products and applications.

Background Technology

Compared with other methods such as carbothermal reduction, gas-phase synthesis, thermal decomposition, and combustion synthesis, the direct nitridation method for preparing silicon _nitride has the advantages of superior performance, simple preparation method, low cost, and ease of industrial production. It is one of the main methods for preparing high-performance _Si3N4 powder.

Currently, most silicon powder nitriding methods, both domestically and internationally, employ saggers or crucibles for nitriding, supplemented with hydrogen to reduce the oxygen content of the system. This approach suffers from problems such as cumbersome silicon powder loading and unloading, low batch yield, and poor batch-to-batch stability. Therefore, nitriding silicon powder after it has been shaped can not only improve efficiency but also increase the furnace loading to boost batch yield. There are many methods for dry powder shaping, such as dry pressing, isostatic pressing, injection molding, and gel casting, but these methods all present challenges for silicon powder nitriding, including complex processes and high forming densities. Because silicon powder undergoes significant volume expansion during nitriding to form silicon nitride, a high-density silicon blank leads to a gradual reduction in porosity between silicon powder particles during nitriding, slowing down gas-solid mass transfer and significantly reducing nitriding efficiency.

Summary of the Invention

In view of this, this application provides a method for preparing silicon nitride by hot pressing and nitriding of silicon powder, as well as its products and applications. The method for preparing silicon nitride using this application not only has a large furnace loading capacity and high batch yield, but also a high nitriding rate, low oxygen content, and does not require the participation of hydrogen; it solves the problems of low furnace loading capacity, low batch yield, low nitriding rate, poor batch-to-batch stability, and low working efficiency in the production of silicon nitride in the prior art.

In one aspect, this application provides a silicon nitride product.

The silicon nitride product is formed by hot pressing silicon powder and additives and then nitriding it.

The additives include diluents, binders, and pore-forming agents;

The weight ratio of silicon powder, diluent, binder and pore-forming agent is (49-63): (7-21): (5-10): (20-25).

The adhesive includes sugar-based adhesives or ceramic powder adhesives;

The pore-forming agent is an inorganic ammonium salt.

The pore-forming agent includes ammonium chloride, ammonium carbonate, and/or ammonium bicarbonate;

The sugar binder includes glucose, fructose, and/or maltose;

The ceramic powder binder includes polyvinyl alcohol, polyvinyl ketone, polyacrylic acid and/or polyethylene glycol;

The diluent is _{Si3N4 .}

The silicon nitride product has a nitridation rate (the proportion of silicon powder converted to silicon nitride in the total silicon powder before the reaction) of more than 99.9% by mass percentage, and a relative content of α- _Si3N4 (the proportion of α _phase in the total silicon nitride) of more than 93.4%.

The silicon nitride product also includes, by weight, no more than 0.69% oxygen, less than or equal to 0.1% carbon, and/or less than 500 ppm of metallic impurities.

Secondly, this application provides a method for preparing silicon nitride by hot pressing and nitriding silicon powder.

The method for preparing silicon nitride by hot pressing and nitriding of silicon powder includes the following steps:

First, the mixture of silicon powder, diluent, binder and pore-forming agent is hot-pressed to form a blank;

The blank is then nitrided at a temperature of 1380°C or less to obtain silicon nitride.

The nitriding temperature is less than or equal to 1380°C;

The hot pressing temperature is 75℃-170℃;

Optionally, the density of the blank is 1.15 g/ ^cm³ - 1.33 g/ ^cm³ ; optionally, the blank is provided with a material hole.

The diameter of the feed hole is 0-1.0 cm.

The nitriding temperature is 1180℃-1380℃;

Optionally, the nitriding process involves first heating and then cooling; the heating is divided into three stages:

The heating rate in the first heating stage is 1℃/min-3℃/min, heating to 1180℃-1220℃, and holding time is 20h-30h.

The heating rate in the second heating stage is 0.5℃/min-1℃/min, heating to 1260℃-1300℃, and holding time is 20h-30h.

The heating rate in the third heating stage is 0.5℃/min-1℃/min, heating to 1340℃-1380℃, and holding time is 5h-8h.

The cooling process includes two stages:

The cooling rate in the first cooling stage is 1℃/min-3℃/min, cooling down to 1200℃±20℃;

The second cooling stage has a rate of 3℃/min-5℃/min, cooling down to room temperature;

Optionally, nitrogen gas is continuously introduced during the nitriding process until the pressure is 0.5 bar to 1.0 bar.

The method also satisfies at least one of the following conditions (1)-(3):

(1) The silicon powder is industrial silicon powder with a particle size ≤3μm, purity ≥99.5%, oxygen content ≤0.5%, and total metal impurity content ≤100ppm;

(2) The D50 of _Si3N4 in the diluent is 0.8μm-3μm, _and the total metal impurity content is ≤500ppm;

(3) The silicon powder, diluent, binder and pore-forming agent are mixed by grinding, preferably by ball milling; the ball milling speed is 300rpm-700rpm, the ball-to-material ratio is (3-6):1, and the ball milling time is 4h-8h; "%" and "ppm" in (1)-(3) are both mass content.

The method also includes decarbonization treatment after nitriding;

Optionally, the decarbonization temperature is 600℃-700℃, the decarbonization time is 2h-4h, and the decarbonization atmosphere is air and/or carbon dioxide.

Thirdly, this application also provides an application of the aforementioned silicon nitride.

An application of a silicon nitride product in the preparation of ceramic and electronic materials, wherein the silicon nitride product is the silicon nitride product provided in the first aspect above or the silicon nitride product prepared by the method of hot pressing and nitriding silicon powder. It can be used to manufacture semiconductor devices, high-performance structural ceramic materials, and high-performance thermally conductive materials, and has wide applications in electronics, ceramics, materials science, and other fields.

Compared with the prior art, the beneficial effects of this application are as follows:

This application provides a silicon nitride product, which is formed by hot pressing silicon powder and additives, followed by nitriding. The additives include a diluent, a binder, and a pore-forming agent. By adding a binder and a pore-forming agent to the silicon powder and mixing them dry without the participation of a solvent, the product is hot-pressed using the viscosity difference before and after the binder melts or before and after glassization. This improves the furnace loading capacity and working efficiency, and the process is simple, stable, and efficient. During the nitriding process, the binder and pore-forming agent decompose and volatilize at high temperatures, causing the silicon blank to present as a porous block, increasing the porosity between silicon powder particles, increasing the gas-solid mass transfer efficiency, and improving the nitriding rate, batch yield, and α-phase nitriding content. The activated carbon formed by the binder at high temperatures reacts with the oxide layer on the surface of the silicon powder, further reducing the oxygen content of the silicon nitride. The prepared silicon nitride product not only has a relatively high nitriding rate and α-phase content but also a relatively low oxygen content. The silicon nitride prepared by this method can be used to prepare ceramic materials or electronic materials.

This application also provides a method for preparing silicon nitride by hot pressing and nitriding silicon powder. First, a mixture of silicon powder, diluent, binder, and pore-forming agent is hot-pressed into shape. Then, the blank is nitrided and decarburized to obtain silicon nitride. By adding a binder to the raw material and pressing it into blocks, not only is loading and unloading facilitated, and the furnace loading capacity and working efficiency increased, but also, by adding solid-phase binders and pore-forming agents, the gas diffusion channel size is increased, the gas-solid contact area is enlarged, and the mass transfer efficiency is greatly improved. This results in improved nitriding rate, batch yield, and batch-to-batch stability, making the process more efficient.

Attached Figure Description

To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

Figure 1 is an XRD pattern of silicon nitride in Example 1 of this application.

Detailed Implementation

The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

In this application, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features.

In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

In this application, unless otherwise specified, percentage concentrations refer to final concentrations. The final concentration refers to the percentage of the added component in the system after its addition.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

To address the issues of low furnace charge, low batch yield, poor batch-to-batch stability, and low efficiency in silicon nitride production, this application considers increasing the furnace charge. However, silicon powder undergoes approximately 20% volume expansion during nitriding; increasing the charge would inevitably lead to denser silicon powder packing, significantly reducing gas-solid mass transfer efficiency. Therefore, this application utilizes a binder to increase density while adding a volatile or decomposable solid (pore-forming agent) to fill gaps. Then, by increasing the temperature, the pore-forming agent volatilizes and creates pores between silicon powder particles within the billet, thereby increasing production capacity and improving nitriding efficiency.

According to a first aspect of this application, this application provides a silicon nitride product.

The silicon nitride product is formed by hot pressing silicon powder and additives and then nitriding. The additives include diluent, binder and pore-forming agent. The weight ratio of silicon powder: diluent: binder: pore-forming agent is (49-63): (7-21): (5-10): (20-25).

By adding binders and pore-forming agents to silicon powder and dry mixing them without solvents, followed by hot pressing until the binder softens and becomes viscous, the diluent, silicon powder, and pore-forming agent can be bonded together. The process utilizes the viscosity difference before and after the binder melts for hot pressing, resulting in a simple, stable, and efficient process that also increases furnace loading capacity and overall efficiency. Furthermore, during nitriding, the binder and pore-forming agent decompose and volatilize at high temperatures, causing the silicon blank to become porous and blocky, significantly increasing the porosity of the formed silicon blank. This facilitates efficient mass transfer between the gas and solid phases, improving the nitriding rate, batch yield, and α-phase nitriding content. More importantly, the activated carbon formed by the binder at high temperatures reacts with the oxide layer on the silicon powder surface, reducing the oxygen content of silicon nitride even in the absence of _H₂ . The resulting silicon nitride product exhibits high nitriding rate, high α-phase content, and low oxygen content. Silicon nitride prepared using this method can be used to prepare ceramic or electronic materials.

Adjusting the ratio of silicon powder, diluent, binder, and pore-forming agent can further increase the furnace charge while connecting the pore network between the stacked silicon powder, thereby further improving the production capacity and nitriding rate.

In the embodiments, the adhesive includes a sugar-based adhesive or a ceramic powder adhesive;

Ceramic powder adhesives are glues (adhesives) that can bond ceramics. Ceramic adhesives are generally divided into three categories: organic ceramic adhesives, inorganic ceramic adhesives, and metal powder adhesives.

Sugar-based binders or ceramic powder binders have low melting points and high viscosity after melting, allowing for increased furnace loading and higher batch yields. The activated carbon produced after high-temperature carbonization of the binder can also reduce and destroy the _SiO₂ layer on the silicon powder surface, exposing the Si surface, which helps to increase the reaction rate and reduce the oxygen content in the finished product.

Optionally, the sugar binder includes glucose, fructose and/or maltose, preferably glucose, and the ash content of the ignition residue of the sugar binder is ≤0.02%.

Glucose has a relatively low melting point of 147℃, and its viscosity after melting is 4300-8600 centipoise, which is quite high. This allows it to be bonded to silicon powder and pore-forming agents using hot pressing to form blocks, increasing furnace loading and batch yield. Furthermore, glucose provides a porous network through high-temperature volatilization or decomposition, facilitating the construction of large-diameter pore networks within silicon powder. This increases the size of gas diffusion channels, enlarges the gas-solid contact area, and significantly improves mass transfer efficiency. Simultaneously, the anaerobic high-temperature carbonization of glucose produces activated carbon, a strong reducing agent that can reduce the oxide layer on the silicon powder surface, exposing silicon reaction sites and further reducing the oxygen content in the finished product, thus further increasing the reaction rate.

In the embodiments, the ceramic powder binder includes polyvinyl alcohol, polyvinyl ketone, polyacrylic acid and/or polyethylene glycol.

In the embodiments, the pore-forming agent is an inorganic ammonium salt;

Optionally, the pore-forming agent includes ammonium chloride, ammonium carbonate, and/or ammonium bicarbonate. Volatile inorganic ammonium salts (ammonium chloride) are preferred.

Inorganic ammonium salts act as solid site occupiers, are easily decomposed and volatilized. With increasing temperature, after volatilization, they construct a large-diameter porous network of silicon powder within the billet, increasing the gas-solid contact area and significantly improving mass transfer efficiency and nitriding efficiency. Ammonium chloride is the preferred pore-forming agent, as it not only creates pores but also provides ammonia as an active nitrogen source, reducing nitrogen consumption.

The diluent is _{Si3N4 powder} .

In the embodiments, the silicon nitride product has a silicon nitride ...

The silicon nitride product also includes, by weight percentage, no more than 0.69% oxygen, less than or equal to 0.1% carbon, and/or less than 500 ppm of metallic impurities.

Secondly, this application provides a method for preparing silicon nitride by hot pressing and nitriding silicon powder.

In this embodiment, the method for preparing silicon nitride by hot pressing and nitriding of silicon powder includes the following steps:

First, the mixture of silicon powder, diluent, binder and pore-forming agent is hot-pressed into shape;

The blank is then nitrided at a temperature of 1380°C or less to obtain silicon nitride.

In the embodiments, the hot pressing temperature is 75℃-170℃; the hot pressing temperature depends on the melting point or glass transition temperature of the sugar binder and the ceramic powder binder (organic polymer binder).

Optionally, the density of the blank is 1.15 g/ ^cm³ - 1.33 g/ ^cm³ .

Optionally, the mixture is pressed into a 20cm×10cm×4cm blank using a dry powder hot press, with a density of 1.12g/ ^cm³ , and 2×4 holes are evenly punched (two horizontal holes and four vertical holes), with a cross-sectional diameter of 0.6cm for each hole.

By hot pressing the raw materials, the furnace loading capacity is further increased, and the pressing temperature is limited. The binder can be melted during the pressing process, making it easy to bond with silicon powder into blocks, which is easy to handle and reduces labor costs. At the same time, hot pressing is less labor-intensive than room temperature pressing, thus reducing costs.

Optionally, the blank is provided with a material hole;

The diameter of the feed hole is 0-1.0 cm. The cross-sectional diameter of the hole is 0.5 cm-0.8 cm, preferably 0.6 cm.

The pressed billet has a high density. During nitriding, the pore-forming agent inside forms pores. However, if the amount of gas generated in the early stage is small, it is not easy to overflow to the surface of the billet. The billet has material holes, which solves the above problem. After the gas is generated, it can quickly form a pore network channel, accelerate the reaction rate, increase the gas-solid contact area, greatly improve the mass transfer efficiency, and improve the nitriding efficiency.

In the examples, the nitriding temperature was 1180℃-1380℃;

The nitriding process involves heating followed by cooling; the heating process is divided into three stages:

The heating rate in the first heating stage is 1℃/min-3℃/min, heating to 1180℃-1220℃, and holding time is 20h-30h.

The heating rate in the second heating stage is 0.5℃/min-1℃/min, heating to 1260℃-1300℃, and holding time is 20h-30h.

The heating rate in the third heating stage is 0.3℃/min-0.5℃/min, heating to 1340℃-1380℃, and holding time is 5h-8h.

Gradual heating is used to prevent the silicon powder nitriding process from generating violent exothermic reactions that could cause the temperature to skyrocket and reduce the α phase content.

The cooling process includes two stages:

The cooling rate in the first cooling stage is 1℃/min-3℃/min, cooling down to 1200℃±20℃;

The second cooling stage has a rate of 3℃/min-5℃/min, cooling down to room temperature;

Optionally, nitrogen gas is continuously introduced during the nitriding process until the pressure is 0.5 bar to 1.0 bar.

In the embodiments, the method also satisfies at least one of the following conditions (1)-(3):

(1) The silicon powder is industrial silicon powder with a particle size ≤3μm, purity ≥99.5%, oxygen content ≤0.5%, and total metal impurity content ≤100ppm;

(2) The D50 of _Si3N4 in the diluent is 0.8μm-3μm, _and the total metal impurity content is ≤500ppm;

(3) The silicon powder, diluent, binder and pore-forming agent are mixed by grinding, preferably by ball milling; the ball milling speed is 300rpm-700rpm, the ball-to-material ratio is (3-6):1, and the ball milling time is 4h-8h.

This application uses a solid-phase binder, which is non-sticky in its solid state and is easy to mix with silicon powder after grinding and stirring.

In the embodiments, the method further includes decarbonization treatment after nitriding;

Optionally, the decarbonization temperature is 600℃-700℃, the decarbonization time is 2h-4h, and the decarbonization atmosphere is air, oxygen, and/or carbon dioxide.

Thirdly, this application also provides an application of the silicon nitride product.

In the embodiments, the silicon nitride product or the silicon nitride product prepared by the hot pressing and nitriding method of silicon powder can be used to prepare ceramic materials and electronic materials, such as semiconductor devices, high-performance structural ceramic materials and high-performance thermal conductive materials, and has wide applications in the fields of electronics, ceramics, and materials science.

The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

Example 1

This embodiment provides a method for batch production of silicon nitride and the resulting product. The specific steps are as follows:

1. Material Selection: The silicon powder is industrial-grade silicon powder with a silicon content >99.5%, oxygen content ≤0.4%, and total metal impurities ₍ iron, aluminum _, calcium) content ≤100ppm. The diluent ( _Si₃N₄ ) contains ≥93% α- _Si₃N₄ . The selected binder is glucose, with an ash content of ≤0.02% in the ignition residue.

2. Mixing ratio: Mix Si: diluent: glucose: ammonium bicarbonate (pore-forming agent) in the ratio of 49:21:5:25, and ball mill for 8 hours at a speed of 500 rpm. The ball-to-material ratio is 6:1.

3. Pressing: The mixture is pressed into a 20cm×10cm×4cm blank using a dry powder hot press, with a density of 1.33g/ ^cm3 , and 2×4 holes are evenly punched, with a hole diameter of 1cm. The pressing temperature is 150-170℃.

4. Nitriding: First, evacuate the nitriding furnace to 10 Pa at room temperature, and then slowly introduce nitrogen gas to atmospheric pressure, repeating this process 3 times. At the same time, raise the temperature to 800℃ at a rate of 5℃/min, and continuously replace moisture and air with nitrogen gas during the process.

After the displacement process was stopped, _N₂ gas was introduced for nitriding. The temperature was increased to 1200℃ at a rate of 3℃/min and held for 20 hours. After the holding period, the temperature was increased to 1290℃ at a rate of 1℃/min and held for 20 hours. Then, the temperature was increased to 1360℃ at a rate of 0.5℃/min and held for 8 hours. After nitriding, the temperature was decreased to 1200℃ at a rate of 1℃/min and then to room temperature at a rate of 5℃/min. Nitrogen gas was continuously introduced during the nitriding process until the pressure reached 0.5 bar–1.0 bar.

5. Carbon removal: The temperature is 600-700℃, the carbon removal time is 4 hours, and carbon dioxide is introduced for carbon removal.

6. The nitrided powder is grayish-white, as shown in Figure 1, which is the XRD pattern of the nitrided silicon nitride. The obtained silicon nitride has formed a phase. Phase composition analysis showed that the nitriding rate of the silicon nitride powder was 99.9%, and the relative content _of α- _Si3N4 was 93.6%. The oxygen content was measured to be 0.6% by a nitrogen-oxygen analyzer, and the carbon content was measured to be 0.10% by a carbon-sulfur analyzer. ICP elemental analysis showed that the metal impurity content was <500ppm.

Example 2

A method for preparing silicon nitride by hot pressing and nitriding of silicon powder, and silicon nitride raw powder, the specific steps of which are as follows:

1. Raw Material Processing: The selected silicon powder is industrial silicon powder with a silicon content >99.5%, oxygen content ≤0.4%, and total metal impurities (iron, aluminum _, calcium) ≤100ppm. The industrial silicon powder is graded and ground to a D50 below 3μm. The selected _Si₃N₄ diluent contains ≥93% α- _Si₃N₄ with a D50 of 0.8-1.2μm. The selected polyethylene glycol (binder) is ground through a 300-mesh sieve, _and the ash content of the ignition residue is ≤0.02%.

2. Mixing ratio: Mix Si: diluent: polyethylene glycol: ammonium chloride (pore-forming agent) in a ratio of 56:14:5:25 by ball milling for 8 hours.

3. Pressing: The mixture is pressed into a 20cm×10cm×4cm blank using a dry powder hydraulic press, with a density of 1.18g/ ^cm³ , and 2×4 holes are evenly punched in each blank with a diameter of 0.6cm. The pressing temperature is 70-90℃.

4. Nitriding: As described in step five, the nitriding furnace is first evacuated to 20 Pa at room temperature, and nitrogen gas is slowly introduced to atmospheric pressure, repeated twice. At the same time, the temperature is increased to 800°C at a rate of 5°C/min, and nitrogen gas is continuously introduced to displace and remove moisture and air.

After the displacement process was stopped, _N₂ gas was introduced for nitriding. The temperature was increased to 1200℃ at a rate of 3℃/min and held for 20 hours. After the holding period, the temperature was increased to 1290℃ at a rate of 1℃/min and held for 20 hours. Then, the temperature was increased to 1360℃ at a rate of 0.5℃/min and held for 8 hours. After nitriding, the temperature was decreased to 1200℃ at a rate of 1℃/min and then to room temperature at a rate of 5℃/min. Nitrogen gas was continuously introduced during the nitriding process until the pressure reached 0.5 bar–1.0 bar.

5. Decarbonization: The temperature is 600-700℃, the decarbonization time is 2 hours, and carbon dioxide gas is introduced during the decarbonization process.

6. The powder after nitriding is grayish-white. The nitriding rate of the product was measured to be 99.9%, the relative content of α _{- Si3N4} was 93.9%, the oxygen content was 0.63%, the carbon content was 0.09%, and the metal impurity content was <500ppm.

Example 3

A method for preparing silicon nitride by hot pressing and nitriding of silicon powder, and silicon nitride raw powder, the specific steps of which are as follows:

1. Raw Material Processing: The selected silicon powder is industrial silicon powder with a silicon content >99.5%, oxygen content ≤0.4%, _and total iron, aluminum, and calcium metal impurities ≤100ppm. The industrial silicon powder is graded and ground to a D50 below 3μm. The selected _Si₃N₄ diluent ( _Si₃N₄ ) contains ≥93% α- _Si₃N₄ with a D50 of 0.8-1.2μm. The selected binder is polyvinyl ketone ₍ PVC ₎ , with an ash content of ≤0.02% in the ignition residue.

2. Mixing ratio: Mix Si: diluent: polyvinyl ketone: ammonium carbonate (pore-forming agent) in the ratio of 63:7:10:20 and then ball mill the mixture. The ball milling speed is 700 rpm, the ball-to-material ratio is 3:1, and the ball milling time is 6 hours.

3. Pressing: The mixture is pressed into a 20cm×10cm×4cm blank using a dry powder hydraulic press, with a density of 1.26g/ ^cm³ , and 2×4 holes are evenly punched in each blank with a diameter of 0.6cm. The pressing temperature is 100-120℃.

4. Nitriding: As described in step five, firstly, the nitriding furnace is evacuated to 12 Pa at room temperature, then evacuated to 1-20 Pa at room temperature, and nitrogen gas is slowly introduced until atmospheric pressure is reached. This process is repeated three times. Simultaneously, the temperature is increased to 800°C at a rate of 5°C/min, and nitrogen gas is continuously introduced to displace and remove moisture and air.

After the displacement process was stopped, _N₂ gas was introduced for nitriding. The temperature was increased to 1200℃ at a rate of 3℃/min and held for 20 hours. After the holding period, the temperature was increased to 1290℃ at a rate of 1℃/min and held for 20 hours. Then, the temperature was increased to 1360℃ at a rate of 0.5℃/min and held for 8 hours. After nitriding, the temperature was decreased to 1200℃ at a rate of 1℃/min and then to room temperature at a rate of 5℃/min. Nitrogen gas was continuously introduced during the nitriding process until the pressure reached 0.5 bar–1.0 bar.

5. Decarbonization: The temperature is 600-700℃, the decarbonization time is 3 hours, and carbon dioxide gas is introduced during the decarbonization process.

6. The powder after nitriding is grayish-white. The nitriding rate of the product was measured to be 99.9%, the relative content of α _{- Si3N4} was 93.5%, the oxygen content was 0.66%, the carbon content was 0.09%, and the content of metal impurities was <500ppm.

Example 4

This embodiment provides a method for preparing silicon nitride by hot pressing and nitriding of silicon powder, and silicon nitride raw powder. The preparation method is basically the same as in Example 3, except that no pores are drilled. Details are as follows:

1. Raw Material Processing: The selected silicon powder is industrial silicon powder with a silicon content >99.5%, oxygen content ≤0.4%, and total metal impurities (iron, aluminum _, calcium) ≤100ppm. The industrial silicon powder is graded and ground to a D50 below 3μm. The selected _Si₃N₄ diluent contains ≥93% α- _Si₃N₄ with a D50 of 0.8-1.2μm. The selected polyethylene glycol (binder) is ground through a 300-mesh sieve, _and the ash content of the ignition residue is ≤0.02%.

2. Mixing ratio: Mix Si: diluent: polyethylene glycol: ammonium chloride (pore-forming agent) in a ratio of 56:14:5:25 by ball milling for 8 hours. The ball milling speed is 500 rpm and the ball-to-material ratio during ball milling is 5:1.

3. Pressing: The mixture is pressed into 20cm×10cm×4cm blanks by a dry powder hydraulic press, with a density of 1.21g/ ^cm3 and a pressing temperature of 70-90℃.

4. Nitriding: As described in step five, firstly, the nitriding furnace is evacuated to 20 Pa at room temperature, then evacuated to 1-20 Pa at room temperature, and nitrogen gas is slowly introduced until atmospheric pressure is reached. This process is repeated twice. Simultaneously, the temperature is increased to 800°C at a rate of 5°C/min, and nitrogen gas is continuously introduced to displace and remove moisture and air.

After the purging process is stopped, N2 gas is introduced for nitriding. The temperature is increased to 1200℃ at a rate of 3℃/min and held for 20 hours. After the holding period, the temperature is increased to 1290℃ at a rate of 1℃/min and held for 24 hours. Then, the temperature is increased to 1360℃ at a rate of 0.5℃/min and held for 10 hours. After nitriding, the temperature is decreased to 1200℃ at a rate of 1℃/min and then to room temperature at a rate of 5℃/min. Nitrogen gas is continuously introduced during nitriding until the pressure reaches 0.5 bar–1.0 bar.

5. Decarbonization: The temperature is 600-700℃, the decarbonization time is 2 hours, and carbon dioxide gas is introduced during the decarbonization process.

6. The powder after nitriding is grayish-white. The nitriding rate of the product was measured to be 99.9%, the relative content of α _{- Si3N4} was 93.4%, the oxygen content was 0.69%, the carbon content was 0.1%, and the metal impurity content was <500ppm.

Comparative Example 1

This comparative example provides an efficient method for preparing silicon nitride by nitriding and the resulting product. The specific steps are as follows:

1. Material Selection: The selected silicon powder is industrial silicon powder with a silicon content >99.5%, oxygen content ≤0.4%, _and total metal impurities of iron, aluminum _, and calcium ≤100ppm. The diluent ( _Si₃N₄ ) contains ≥93% α- _Si₃N₄ . The selected binder is glucose.

2. Mixing ratio: Mix Si: diluent: glucose = 67:28:5, and ball mill for 8 hours at a speed of 500 rpm. The ball-to-material ratio is 6:1. After separating the balls, set aside for later use.

3. Loading the furnace: Place the evenly mixed powder into a sagger sprinkled with silicon nitride powder, with a bulk density of 0.76 g/ ^cm³ and a bulk thickness of 4 cm.

4. Nitriding: First, place the crucible in the nitriding furnace. Evacuate the furnace to 10 Pa at room temperature and slowly introduce nitrogen gas to atmospheric pressure. Repeat this process three times. Simultaneously, raise the temperature to 800℃ at a rate of 5℃/min, continuously purging with nitrogen to remove moisture and air.

After the displacement process was stopped, _N₂ gas was introduced for nitriding. The temperature was increased to 1200℃ at a rate of 3℃/min and held for 20 hours. After the holding period, the temperature was increased to 1290℃ at a rate of 1℃/min and held for 20 hours. Then, the temperature was increased to 1360℃ at a rate of 0.5℃/min and held for 8 hours. After nitriding, the temperature was decreased to 1200℃ at a rate of 1℃/min and then to room temperature at a rate of 5℃/min. Nitrogen gas was continuously introduced during the nitriding process until the pressure reached 0.5 bar–1.0 bar.

5. Carbon removal: Temperature is 600-700℃, carbon removal time is 4 hours, and air is introduced during carbon removal.

The nitrided powder is grayish- _white . The relative content of α- _Si3N4 in the product was measured to be 93.8%, the oxygen content was 0.60%, and the carbon content was 0.09%. The content of metal impurities was <500ppm.

By comparing Example 1 and Comparative Example 1, it can be seen that Example 1 adopted a hot pressing molding process, which increased the furnace loading by about 38% for the same volume, and the α phase content and other impurity content were not significantly different from those of Comparative Example 1, and the performance was also superior.

Comparative Example 2

This comparative example provides an efficient method for preparing silicon nitride by nitriding and the resulting product. The specific steps are as follows:

1. Raw Material Selection: The silicon powder is industrial-grade silicon powder with a silicon content >99.5%, oxygen content ≤0.4%, _and total metal impurities (iron, aluminum _, calcium) content ≤100ppm. The diluent ( _Si₃N₄ ) contains ≥93% α- _Si₃N₄ .

2. Mixing ratio: Mix Si: diluent = 7:3, and ball mill for 8 hours at a speed of 500 rpm. The ball-to-material ratio is 6:1. After separating the balls, set aside for later use.

3. Loading the furnace: Place the evenly mixed powder into a sagger sprinkled with silicon nitride powder, with a bulk density of 0.84 g/ ^cm³ and a bulk thickness of 4 cm.

4. Nitriding: First, place the crucible in the nitriding furnace. Evacuate the furnace to 10 Pa at room temperature and slowly introduce nitrogen gas to atmospheric pressure. Repeat this process three times. Simultaneously, raise the temperature to 800℃ at a rate of 5℃/min, continuously purging with nitrogen to remove moisture and air.

After the displacement process was stopped, _N₂ gas was introduced for nitriding. The temperature was increased to 1200℃ at a rate of 3℃/min and held for 20 hours. After the holding period, the temperature was increased to 1290℃ at a rate of 1℃/min and held for 20 hours. Then, the temperature was increased to 1360℃ at a rate of 0.5℃/min and held for 8 hours. After nitriding, the temperature was decreased to 1200℃ at a rate of 1℃/min and then to room temperature at a rate of 5℃/min. Nitrogen gas was continuously introduced during the nitriding process until the pressure reached 0.5 bar–1.0 bar.

The nitrided powder is light gray. The nitriding rate of the product was measured to be 85.6%, the relative content of α _{- Si3N4} was 91.0%, the oxygen content was 1.4%, the carbon content was 0.13%, and the metal impurity content was <500ppm.

Comparing Example 1 and Comparative Example 2, it can be seen that in Comparative Example 2, without the addition of binder and pore-forming agent, the system has no residual activated carbon, which makes it impossible to quickly remove the oxide layer on the surface of silicon powder, resulting in a slower nitriding rate, lower nitriding ratio, and _lower α- _Si3N4 content. The oxygen content in the system also cannot be removed, leading to poor performance of the obtained silicon nitride powder.

The silicon nitride was prepared in five batches using the same preparation method as in Example 1, and the results of the five batches were measured.

The results are shown in Table 1 below:

Table 1. Determination results of different batches of silicon nitride

Testing showed that the preparation method of this embodiment has high batch yield, good batch-to-batch stability, low oxygen content in the finished product, and high performance. The powder after nitriding is grayish-white. The nitriding rate of the product was measured to be above 99.9%, the α- _Si3N4 content was ≥93.4%, the oxygen content was ≤0.69%, the carbon content was ≤0.1%, _and the metal impurity content was <500ppm.

Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims (10)

A silicon nitride product, characterized in that the silicon nitride product is formed by hot pressing silicon powder and additives and then nitriding it; The additives include diluents, binders, and pore-forming agents; The weight ratio of silicon powder, diluent, binder and pore-forming agent is (49-63): (7-21): (5-10): (20-25). The silicon nitride product according to claim 1, wherein the binder comprises a sugar binder or a ceramic powder binder; The pore-forming agent is an inorganic ammonium salt. The silicon nitride product according to claim 2 is characterized in that the pore-forming agent comprises ammonium chloride, ammonium carbonate and/or ammonium bicarbonate; The sugar binder includes glucose, fructose, and/or maltose; The ceramic powder binder includes polyvinyl alcohol, polyvinyl ketone, polyacrylic acid and/or polyethylene glycol; The diluent is _{Si3N4 .} According to any one of claims 1-3, the silicon nitride product is characterized in that, by mass percentage, the nitridation rate of the silicon nitride product is above 99.9%, _and the content of α- _Si3N4 is above 93.4%. The silicon nitride product also includes no more than 0.69% oxygen, less than or equal to 0.1% carbon, and/or less than 500 ppm of metallic impurities. A method for preparing silicon nitride by hot pressing and nitriding of silicon powder, characterized by comprising the following steps: First, the mixture of silicon powder, diluent, binder and pore-forming agent is hot-pressed into shape; The blank is then nitrided to obtain silicon nitride. The method for preparing silicon nitride by hot pressing and nitriding of silicon powder according to claim 5 is characterized in that the nitriding temperature is below 1380°C; The hot pressing temperature is 75℃-170℃; The density of the blank is 1.15 g/ ^cm³ - 1.33 g/ ^cm³ ; The blank is provided with a material hole; The diameter of the feed hole is 0-1.0 cm. The method for preparing silicon nitride by hot pressing and nitriding of silicon powder according to claim 5 or 6 is characterized in that the nitriding process involves first heating and then cooling; the heating is divided into three stages: The heating rate in the first heating stage is 1℃/min-3℃/min, heating to 1180℃-1220℃, and holding time is 20h-30h. The heating rate in the second heating stage is 0.5℃/min-1℃/min, heating to 1260℃-1300℃, and holding time is 20h-30h. The heating rate in the third heating stage is 0.5℃/min-1℃/min, heating to 1340℃-1380℃, and holding time is 5h-8h. The cooling process includes two stages: The cooling rate in the first cooling stage is 1℃/min-3℃/min, cooling down to 1200℃±20℃; The second cooling stage has a rate of 3℃/min-5℃/min, cooling down to room temperature; During the nitriding process, nitrogen gas is continuously introduced until the pressure reaches 0.5 bar to 1.0 bar. The method for preparing silicon nitride by hot pressing and nitriding of silicon powder according to claim 5 or 6 is characterized in that the method further satisfies at least one of the following conditions (1)-(3): (1) The silicon powder has a particle size ≤3μm, a purity ≥99.5%, an oxygen content ≤0.5%, and a total metal impurity content ≤100ppm; (2) The D50 of _Si3N4 in the diluent is 0.8μm-3μm, _and the total metal impurity content is ≤500ppm; (3) The silicon powder, diluent, binder and pore-forming agent are mixed by grinding, preferably by ball milling; the ball milling speed is 300rpm-700rpm, the ball-to-material ratio is (3-6):1, and the ball milling time is 4h-8h. The method for preparing silicon nitride by hot pressing and nitriding of silicon powder according to claim 5 or 6 is characterized in that the method further includes a decarbonization treatment after nitriding; The decarbonization temperature is 600℃-700℃, the decarbonization time is 2h-4h, and the decarbonization atmosphere is air and/or carbon dioxide. The application of a silicon nitride product in the preparation of ceramic or electronic materials, characterized in that the silicon nitride product is the silicon nitride product according to any one of claims 1-4 or the silicon nitride product obtained by the method of preparing silicon nitride by hot pressing and nitriding of silicon powder according to any one of claims 5-9.