CN101229916B - Method for synthesizing silicon nitride powder by burning polytetrafluoroethylene as additive - Google Patents

Abstract

The invention belongs to the technical field of preparation of non-oxide superfine silicon nitride powder, and in particular relates to a method for burning and synthesizing silicon nitride powder by using polytetrafluoroethylene as an additive when there is no silicon nitride diluent. Using silicon powder, ammonium salt or nitrogen-containing compound and polytetrafluoroethylene as raw materials, the reaction mixture is prepared according to different component ratios. After a short period of grinding and mixing, the mixture is placed in a closed high-temperature-resistant container, loose The packing density is 0.8～2.0g/cm ³ ; the reaction device is vacuumed and filled with nitrogen, ammonia, etc., so that the gas pressure of the combustion synthesis reaction device is controlled within 5MPa, and the product can be obtained after fine grinding. The SiC powder has an α-phase content of 60-80%. The invention realizes the combustion synthesis of silicon nitride by silicon powder under the condition of no silicon nitride diluent, and breaks through the limitation that a large amount of silicon nitride diluent must be added in the traditional combustion synthesis silicon nitride process. The method of the invention improves the net yield of products and reduces costs.

Description

Combustion method for synthesizing silicon nitride powder with polytetrafluoroethylene as additive

technical field

The invention belongs to the technical field of preparation of non-oxide ultrafine silicon nitride powder, and relates to a method for combustion synthesis (also known as self-propagating high-temperature synthesis) of silicon nitride powder, in particular to the use of polytetrafluoroethylene when there is no silicon nitride diluent. A method for synthesizing silicon nitride (Si ₃ N ₄ ) powder by burning vinyl fluoride (PTFE) as an additive.

Background technique

As an important high-temperature structural ceramic material, silicon nitride ceramics have low density, low specific gravity, high hardness, corrosion resistance, wear resistance, high temperature resistance, high elastic modulus, oxidation resistance, thermal shock resistance and electrical insulation properties Good and other advantages, it has been widely used in bearings, rolls, cutting tools, engine parts, etc., and has great development potential. Silicon nitride powder is used as the raw material for the production of silicon nitride ceramic products, and its commercial demand is increasing year by year.

There are many preparation methods of silicon nitride powder that have been researched and developed in the world, the representative ones are silicon powder direct nitriding method, carbothermal reduction method, silicon imine decomposition method, plasma method and combustion synthesis method. Some large-scale manufacturers in the world, such as Stark in Germany and Denka in Japan, generally use the direct nitriding method to prepare silicon nitride powder. The process is: place the silicon powder in a nitrogen (or nitrogen/hydrogen) flowing atmosphere, hold it for a long time for different periods of time, complete nitriding, and then perform grinding and other treatments to obtain silicon nitride powder. This method has a long production cycle (up to 72 hours), requires secondary nitriding, and therefore consumes a lot of energy, and various process parameters must be strictly controlled. The carbothermal reduction process also has disadvantages such as long production cycle, complicated process, and high cost. In addition, the content of impurities such as carbon and oxygen in the product is relatively high. The commercialized silicon nitride powder produced by Japan UBE Company adopts the silicon imine decomposition method. In addition to the disadvantages of complex process, high production cost, and corrosion problems in the production process, this method also has relatively high chlorine content in the product. In recent years, some domestic units have developed a plasma method to prepare nano-silicon nitride. The process is _as follows: inject plasma working gas (such as N ₂ -Ar-H _{2 ,} etc.) into the plasma generator, then use plasma arc to heat the working gas, and finally inject nitrogen-containing raw materials (such as NH ₃ etc.) and silicon-containing raw materials (such as SiCl ₄ ) by chemical vapor phase reaction to prepare silicon nitride nanopowder. Most of the silicon nitride prepared by plasma method is amorphous silicon nitride powder and contains ammonium chloride impurities. The product needs to be removed from ammonium chloride impurities at a temperature above 500°C and treated at high temperature to obtain high α-phase silicon nitride powder. body. The plasma method also has disadvantages such as complex equipment, high cost, and high energy consumption. In addition, it does not have the possibility of forming a large-scale industry in terms of product price and output scale.

Combustion synthesis (CS for short), also known as self-propagating high temperature synthesis (SHS for short), is an effective new technology for synthesizing new special ceramic powder materials such as nitrides and carbides. Its basic process features are: a highly exothermic chemical reaction system undergoes a local chemical exothermic reaction after being induced by external energy (this process is called ignition), forming a combustion reaction front (that is, a combustion wave), and then reacts in the heat released by itself Under the maintenance of the combustion wave, the reaction continues to proceed spontaneously (called self-propagation). During the propagation of the combustion wave, the reactants are transformed into synthetic materials. For the combustion synthesis of silicon nitride technology, its characteristics are in the aspects of rapid reaction, low energy consumption, self-purification, high sintering activity of powder, simple equipment and process, small investment, and strong versatility of equipment.

Since the 1980s, material researchers at home and abroad have shown high attention to the preparation of silicon nitride powder by combustion synthesis technology, and there have been a large number of patents and research papers. These reports have different focuses.

Since the Russian scientist Merzhanov et al. used the SHS technology invented by him to prepare silicon nitride powder, he applied for a patent named "Preparation Method of Silicon Nitride with High α Phase Content" (US Patent No. US5032370). The gas pressure required for the synthesis reaction is too high (up to 30MPa), and the amount of ammonium salt added in the reaction raw material is too large (up to 60wt% of the added silicon powder content); in addition, in the raw materials of this process, not only Si powder needs to pass through Special methods such as hydrofluoric acid are used for pretreatment, and different contents of amorphous Si or silicon imine are used, which increases the cost of raw materials. The "combustion synthesis of fine-grained α-phase silicon nitride" (US Patent No. US4944930) invented by Holt J. Birch in the United States uses a large amount of NaN ₃ (accounting for about 50wt% of the reactants) as a nitrogen source. Since NaN ₃ is highly toxic and explosive, it will not only introduce impurities, but also is not suitable for safe production.

"A Method for Synthesizing High α-phase Silicon Nitride Powder by Low-Pressure Combustion" (Chinese Patent No. 02100183.9) invented by Chen Kexin in China, uses the technology of suspending silicon nitride powder to prepare silicon nitride, which can reduce nitrogen gas The effect of pressure, but a certain amount of silicon imide or amorphous silicon is added to the raw materials used, and at the same time, up to 58wt% of silicon nitride is added as a diluent. The purpose is to synthesize silicon nitride with a high α-phase content. The "Method and Equipment for Synthesizing Silicon Nitride or Ferrosilicon Nitride by Low-Pressure Combustion" (Chinese Patent Application No. 02158760.4) invented by China Sun Jialin et al. improved the combustion synthesis equipment and played a role in reducing the nitrogen pressure, but Up to 95wt% of silicon nitride is also added as a diluent, and the raw material needs to be preheated, and the temperature required for preheating is as high as 1200°C, which will also lead to high energy consumption and production costs. This method synthesizes silicon nitride with high β-phase content. Invented by Wang Shenghong of China, "A production method for pressurized control during silicon nitride combustion synthesis" (Chinese patent number 03149961.9) and "A production method for granulation combustion synthesis of silicon nitride" (Chinese patent number 200410037807.9 ), respectively improved the pretreatment of raw materials and subsequent process control, the purpose of which is to synthesize homogeneous silicon nitride with high β phase content, but also added up to 60wt% silicon nitride as a diluent , and the method used makes the whole process operation complicated and also introduces impurities. Two patents issued by Shanghai Institute of Ceramics "Self-propagating high-temperature synthesis method for preparing β-silicon nitride whiskers" (China Patent No. 01126400.4) and "Preparation of β-nitride whiskers by combustion synthesis using silicon magnesium nitride as a growth aid" "Silicon Rod Crystal" (Chinese Patent No. 200410017913.0) introduces the preparation of β-silicon nitride by combustion synthesis method, and also adds up to 50wt% silicon nitride as a diluent. The former uses oxide as a growth aid. agent, the latter uses magnesium silicon nitride as an auxiliary agent, both of which inevitably introduce oxygen or metal impurities into the product. At the same time, the subsequent treatment of the product of this method makes the process complicated.

Most of the above patents have been improved in the process, the purpose is to prepare silicon nitride with high β phase or high α phase content, but whether it is experimental research or industrialization, it usually involves adding different proportions of silicon nitride to the raw material composition. Silicon (up to 95wt%) is used as a diluent to reduce the reaction temperature, thereby avoiding the melting and agglomeration of Si during the reaction process, thereby ensuring that the combustion synthesis of Si powder in nitrogen can be carried out. The main problem with this approach is that it makes the net yield of the product low, resulting in high cost. Obviously, these processes will increase the cost for the preparation of silicon nitride powder with an α-phase content of 60-80%, and are not suitable for industrial production. Therefore, it is extremely necessary to develop a new process for synthesizing silicon nitride by combustion without using silicon nitride as a diluent under lower nitrogen pressure.

Contents of the invention

The object of the present invention is to provide a new method for burning and synthesizing silicon nitride powder with polytetrafluoroethylene (PTFE) as a novel additive.

The method of the present invention uses silicon powder, ammonium salt or nitrogen-containing compound and PTFE as raw materials, and prepares a reaction mixture according to different component ratios. After grinding and mixing for a short time, the mixture is packed in a closed high-temperature-resistant container. , the bulk density is 0.8-2.0g/cm ³ ; then put the airtight high-temperature-resistant container containing the reaction mixture into the high-pressure reaction device with a cooling water jacket, and the reaction device is vacuumed and filled with nitrogen, ammonia Gas or nitrogen mixed with NH ₃ , Ar or H ₂ to control the gas pressure of the combustion synthesis reaction device within 5MPa, and then through local heating to initiate the combustion synthesis reaction, and the synthesized silicon nitride product is cooled with the furnace . After the product is finely ground, homogeneous silicon nitride powder can be obtained, and the alpha phase content is 60-80%.

The method for burning and synthesizing silicon nitride powder with polytetrafluoroethylene as an additive of the present invention comprises the following steps:

(1). Preparation of reactants

According to silicon powder: ammonium salt, nitrogen-containing compound or their mixture: the weight ratio of polytetrafluoroethylene is 95 ~ 60: 4.5 ~ 30: 0.5 ~ 10 Weigh raw materials and fully mix, prepare reactant; Preferred silicon powder: The weight ratio of ammonium salt, nitrogen-containing compound or their mixture:polytetrafluoroethylene is 95-63:4.5-27:0.5-10.

(2). Pretreatment of reactants

The mixed material in step (1) is placed on a grinding device for a short period of grinding and mixing treatment, or placed in an ultrasonic device for mixing treatment;

(3). Combustion synthesis reaction

Put the pretreated mixture in step (2) in a closed high-temperature-resistant container with a bulk density of 0.8 to 2.0 g/cm ³ ; then put the container into a high-pressure reaction device with a cooling water jacket, After vacuuming, fill it with nitrogen, ammonia or the mixed gas of nitrogen and NH ₃ , Ar or H ₂ (the proportion of Ar or H ₂ in the mixed gas of nitrogen and Ar or H ₂ is 0-30vol%), The gas pressure of the combustion synthesis reaction device is controlled within 5MPa, preferably the gas pressure is controlled between 1.0-5MPa; and then the combustion synthesis reaction is triggered by local heating (electric ignition), and the synthesized silicon nitride product is cooled with the furnace . The homogeneous silicon nitride powder can be obtained after post-processing such as pulverization, fine grinding and grading of the product, and the alpha phase content thereof is 60-80%.

After the materials are ground and mixed, the powder is sieved, and then put into a porous graphite crucible with a bulk density of 0.8-2.0g/cm ³ , and then put the powdered porous graphite crucible into a closed resistant The combustion synthesis reaction is carried out in a high-temperature container.

The purity of the raw silicon powder is greater than 99%, and the particle size ranges from 0.1 to 10 μm.

When selected from a mixture of ammonium salts and nitrogen-containing compounds, they are in an equal weight ratio.

The said ammonium salt is chemically pure NH ₄ F, NH ₄ Cl, (NH ₄ ) ₂ CO ₃ or mixtures thereof; when selected from two or more mixtures, they are equal in weight portion ratio.

The nitrogen-containing compound is chemically pure CO(NH ₂ ) ₂ .

The purity of the polytetrafluoroethylene additive is chemically pure.

The invention breaks through the limitation that a large amount of silicon nitride diluent must be added when the silicon nitride powder is prepared by the combustion synthesis method, especially under industrial scale production conditions. The synthesized silicon nitride powder has low impurity content and uniform and fine particles. Moreover, the production cost is low, the production efficiency is high, the operation is easy, the equipment investment is small, and it is suitable for large-scale production.

Advantage of the present invention compared with prior art:

1) The present invention provides a novel polytetrafluoroethylene (PTFE) additive, which realizes Si powder without

Combustion synthesis of Si ₃ N ₄ with the addition of silicon nitride diluent breaks through the traditional combustion synthesis of silicon nitride process, especially the limitation of adding a large amount of silicon nitride diluent under industrial scale production conditions, and improves the net product yield. The cost is reduced, and it is helpful to give full play to the low-cost advantage of the combustion synthesis silicon nitride process.

2) The technological process of the present invention is simple, and operability is strong, has avoided tedious and complicated technological process. The raw materials only need to be mixed and pretreated for a short time, which reduces the energy consumption and pollution caused by the long grinding time; the powder blank does not need special treatment, which reduces the difficulty of operation.

Description of drawings

Fig. 1. X-ray diffraction diagram of the silicon nitride powder prepared in Example 1 of the present invention.

Fig. 2. SEM microscopic analysis diagram of the silicon nitride powder prepared in Example 1 of the present invention.

Detailed ways

Example 1

Weigh the raw materials according to the weight ratio of silicon powder (purity > 99%, particle size range 0.1-10 μm): ammonium chloride: PTFE 90:9:1, put the prepared raw materials on a vibrating grinder, mix and grind for 1 hour , After grinding, the powder is sieved, and then loaded into a porous graphite crucible, with a bulk density of 0.8-2.0g/cm ³ . Put the powdered porous graphite crucible into a 40L combustion synthesis reaction device, and place a spiral tungsten wire and titanium powder at one end of the powder blank as an ignition source. After vacuuming, fill the combustion synthesis reaction device with nitrogen until the pressure reaches 1.5 MPa. The tungsten wire is energized and heated to ignite the titanium powder, inducing the combustion synthesis reaction of the powder system. Combustion synthesized products with furnace cooling. The obtained product is subjected to post-treatment processes such as pulverization and fine grinding to obtain a homogeneous silicon nitride powder with an α phase content of 77.7% and an average particle size of 2.6 μm.

The X-ray diffraction pattern and scanning electron microscope microanalysis diagram of the combustion synthesis product in this embodiment are shown in accompanying drawings 1 and 2. Through phase analysis, it can be known that the product is silicon nitride, and no diffraction peak of residual Si appears, which shows that Si powder has been completely nitrided. Quantitatively calculated by X-ray diffraction analysis method, the content of α-phase silicon nitride in the product is 77.7%. It can be seen from the scanning electron microscope microscopic analysis of the product in Figure 2 that the particle size of the obtained product is uniform, consistent in size, and has the characteristics of a homogeneous silicon nitride powder.

Example 2

Take the raw material according to the weight ratio of silicon powder (purity > 99%, particle size range is 0.1 ~ 10 μ m): ammonium salt: PTFE is 73: 22: 5, wherein the ammonium salt is the mixture of ammonium chloride and ammonium fluoride, The ratio of parts by weight is 1:1. The prepared raw materials are placed on a vibrating grinder for mixing and grinding for 1 hour. After grinding, the powder is sieved and then loaded into a porous graphite crucible with a bulk density of 0.8-2.0 g/cm ³ . Put the powdered porous graphite crucible into a 40L combustion synthesis reaction device, and place a spiral tungsten wire and titanium powder at one end of the powder blank as an ignition source. After vacuuming, a mixture of nitrogen and argon is charged into the combustion synthesis reaction device, wherein the proportion of argon is 20 vol%, until the pressure reaches 3 MPa. The tungsten wire is energized and heated to ignite the titanium powder, and (by local heating) induces the combustion synthesis reaction of the powder system. Combustion synthesized products with furnace cooling. The obtained product was treated to obtain silicon nitride powder with an α-phase content of 78.4% and an average particle size of 8.5 μm.

Example 3

According to silicon powder (purity > 99%, particle diameter range is 0.1～10 μ m): (ammonium salt+nitrogen-containing compound): PTFE is the weight ratio of 81: 10: 9 and takes raw material, wherein ammonium salt is ammonium chloride and Ammonium carbonate, the nitrogen-containing compound is urea, and its weight ratio is 1:1:1. The prepared raw materials are placed on a vibrating grinder for mixing and grinding for 1 hour. After grinding, the powder is sieved and then loaded into a porous graphite crucible with a bulk density of 0.8-2.0 g/cm ³ . Put the powdered porous graphite crucible into a 40L combustion synthesis reaction device, and place a spiral tungsten wire and titanium powder at one end of the powder blank as an ignition source. After vacuuming, a mixture of nitrogen and hydrogen is charged into the combustion synthesis reaction device, wherein the proportion of hydrogen is 25 vol%, until the pressure reaches 4.5 MPa. The tungsten wire is energized and heated to ignite the titanium powder, inducing the combustion synthesis reaction of the powder system. Combustion synthesized products with furnace cooling. The obtained product was treated to obtain a silicon nitride powder with an α-phase content of 63.8% and an average particle size of 6.5 μm.

Example 4

Weigh raw materials according to the weight ratio of silicon powder (purity > 99%, particle size range is 0.1 ~ 10 μm): (ammonium salt + nitrogen-containing compound): PTFE is 83: 15: 2, wherein the ammonium salt is ammonium fluoride and Ammonium carbonate, the nitrogen-containing compound is urea, and its weight ratio is 1:1:1. Mix and grind the prepared raw materials on a vibrating grinder for 0.5 hours. After grinding, place the powder in an ultrasonic device for 20 minutes, then sieve and put it into a porous graphite crucible. The bulk density is 0.8-2.0g/cm ³ . Put the powdered porous graphite crucible into a 40L combustion synthesis reaction device, and place a spiral tungsten wire and titanium powder at one end of the powder blank as an ignition source. After vacuuming, a mixture of nitrogen and hydrogen is charged into the combustion synthesis reaction device, wherein the proportion of hydrogen is 10vol%, until the pressure reaches 3.5MPa. The tungsten wire is energized and heated to ignite the titanium powder, inducing the combustion synthesis reaction of the powder system. Combustion synthesized products with furnace cooling. The obtained product was treated to obtain silicon nitride powder with an α-phase content of 67.8% and an average particle diameter of 4.5 μm.

Claims (2)

1. a method for additive combustion synthesis silicon nitride powder with polytetrafluoroethylene, it is characterized in that, the method may further comprise the steps: (1). Preparation of reactants Weigh the raw materials according to the weight ratio of silicon powder: NH ₄ Cl: polytetrafluoroethylene 90:9:1 and mix them thoroughly to prepare the reactant; (2). Pretreatment of reactants The material mixed in step (1) is placed on the grinding equipment and processed through grinding and mixing; (3). Combustion synthesis reaction The mixed material pretreated in step (2) is sieved, and then loaded into a porous graphite crucible with a bulk density of 0.8 to 2.0 g/cm ³ ; In the high-pressure reaction device with cooling water jacket, after vacuuming, nitrogen gas is filled into the high-pressure reaction device until the gas pressure in the high-pressure reaction device reaches 1.5MPa, and then the combustion synthesis reaction is triggered by local electric ignition heating, and the synthesized The silicon nitride product is cooled with the furnace; the silicon nitride product is pulverized, finely ground and classified to obtain a homogeneous silicon nitride powder with a phase a content of 77.7%. 2. The method according to claim 1, characterized in that: the purity of the raw silicon powder is >99%, and the particle size range is 0.1-10 μm.

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