CN1673070A - Method for synthesizing alpha-phase silicon nitride powder by temperature-controlled activation and self-propagating combustion - Google Patents

Abstract

The invention belongs to the field of inorganic non-metallic materials, and relates to a method for synthesizing α-phase silicon nitride powder through temperature-controlled activation and self-propagating combustion. In parts by weight, 40-94 parts of silicon powder, 6-60 parts of silicon nitride diluent and 0.5-30 parts of catalyst are mixed, then ground and activated, and then loaded into a reactor, in a nitrogen atmosphere of 3-9MPa , inducing the reactant system to undergo a self-propagating combustion synthesis reaction to obtain α-phase silicon nitride powder. The method of the present invention has the advantages of no need for raw material pretreatment, simple process, short production process time, simple induced reaction, low power consumption, low cost, high content of α phase in the product, uniform particle size of the powder product, and easy post-processing.

Description

Method for synthesizing α-phase silicon nitride powder by self-propagating combustion with temperature control activation

Technical field

The invention belongs to the field of inorganic non-metallic materials, and relates to a method for synthesizing α-phase silicon nitride powder through temperature-controlled activation and self-propagating combustion.

Background technique

Silicon nitride ceramics have unique physical and mechanical properties, such as high hardness, high strength, wear resistance, excellent thermal stability (especially low thermal expansion coefficient), good chemical stability, low friction coefficient, low density and High thermal conductivity, etc., so silicon nitride ceramic materials and devices are widely used in many fields such as engine manufacturing, chemical industry, and semiconductor industry. However, the high cost of raw materials has restricted the large-scale application of silicon nitride ceramic parts in various fields. Therefore, the international material community has been committed to developing new low-cost methods for preparing silicon nitride powder.

However, silicon nitride powders are very different in terms of phase composition. There are β-Si ₃ N ₄ powders whose main phase is high temperature stable, and there are α phase-Si ₃ N ₄ powders whose main phase is low temperature stable. In terms of sintering performance, the sintering performance of α phase-Si ₃ N ₄ powder stable at low temperature is much better than that of β phase-Si ₃ N ₄ powder, and the transformation of α→β phase generally occurs during the sintering process. The phase transition proceeds through a dissolution-elution mechanism, and densification occurs simultaneously with the phase transition. Therefore, how to prepare high α phase-Si ₃ N ₄ powder has always been a research hotspot.

The known method of directly synthesizing silicon nitride powder by nitriding reaction can be divided into the following types, the first is the furnace body synthesis method widely used in industry; the second is SiO ₂ carbothermal reduction method, the third It is a relatively new plasma chemical synthesis method; the fourth is the high-temperature self-propagating synthesis method (SHS) in the field of combustion synthesis.

The method for synthesizing silicon nitride in a furnace body is: in a nitrogen atmosphere, heat silicon powder in an electric furnace to cause a nitriding reaction to occur. Silica high-temperature carbothermal reduction method also belongs to one of such preparation methods. Technically, this method is the most effective and relatively simple method for synthesizing high-content α-phase silicon nitride. This method mainly lies in the nitriding process of silicon powder in a flowing nitrogen atmosphere. However, the method for synthesizing silicon nitride in a furnace requires two nitriding steps. The first step is to nitride the silicon powder at a temperature lower than its own melting point, that is, 100-250°C. This step takes 10-20 hours; The step nitriding is completed between 1200-1400°C, it takes 4-5 hours, and the whole production cycle is longer. Moreover, this method of synthesizing silicon nitride in a furnace requires strict control of reaction temperature, nitrogen partial pressure and gas flow during the second nitriding process. These conditions are all to ensure the thermal environment required for the formation of α phase. In addition, this method requires high energy consumption for high temperature and long time processing.

The plasma chemical synthesis method is that the raw materials containing silicon (such as SiH ₄ , SiCl ₄ ) and nitrogen (NH ₃ , N ₂ ) are plasmaized and reacted in a high-frequency generator to prepare silicon nitride powder. The silicon nitride prepared by this method is mostly amorphous and contains high oxygen. In addition, compared with silicon nitride powder prepared by other methods, silicon nitride powder prepared by plasma method has higher chemical activity and is easily hydrolyzed in humid air. Take certain protective measures during the process. Although the silicon nitride powder prepared by the plasma method has the advantage of being easy to sinter, the quality of this powder cannot meet the requirements for preparing ceramic materials with high physical and mechanical properties. In addition, plasma chemical synthesis, like furnace synthesis, also requires greater power consumption.

Technically, high-temperature self-propagating synthesis is the most promising method. This method utilizes the exothermic effect of the reaction, and its key lies in the induction of local reactions in the reaction layer of the mixture and the continuous interaction between the components in the combustion synthesis. That is to say, the early stage of self-propagating combustion is mainly due to the self-exothermic reaction between layers of the reaction mixture caused by the sufficient exothermic effect of the reaction system. The huge thermal effect (180Kcal/mol) of the reaction between silicon and nitrogen allows silicon nitride to be prepared by combustion synthesis, that is, high-temperature self-propagating synthesis.

The currently disclosed combustion synthesis method to prepare silicon nitride powder also has some limitations. For example, "a method for preparing high-α-phase silicon nitride" invented by Merzhanov et al. (patent US5032370), the nitrogen pressure required is too high (4-30MPa), the requirements for equipment are harsh, and the production safety factor is low, which is not conducive to large-scale production. . Similarly, the "preparation method for self-propagating high-temperature synthesis of silicon nitride ferrous powder" (CN1275526) invented by Jiang Guojian and others also has the disadvantages of high reaction pressure and strict requirements on equipment, and due to the use of ferrosilicon alloy powder with lower purity as raw material , The iron impurity content in the product is high, basically it is β-phase silicon nitride with low commercial value, which cannot meet the needs of fine structure ceramics. "A method for synthesizing high-α-phase silicon nitride powder by low-pressure combustion" (CN1362358A) invented by Chen Kexin of Tsinghua University and others uses the suspension nitriding technology of silicon, which is mainly completed in the following steps: 1. Metal silicon powder is pickled or ultrasonically pretreated to improve the activity of the original silicon powder; 2. Add active agents, diluents and additives; 3. Mill the mixed raw material powder on a rolling ball mill for 15 to 30 hours; 4. The mixed raw materials are dried at 50-70°C, put into a low-pressure combustion synthesis reaction device, and after vacuuming, nitrogen gas with a pressure of 0.1-1 MPa is blown from the bottom of the powder to induce the combustion of the raw material powder to realize silicon synthesis under low pressure. Suspension nitriding. Although this synthesis technology can greatly reduce the nitrogen pressure used and reduce the requirements for the high-pressure resistance of the equipment, from the perspective of the entire reaction process, there are many procedures, time-consuming, long production cycle, complicated induced reaction methods and easy introduction of impurities. , Energy consumption is big, and production cost is improved.

Contents of the invention

The purpose of the present invention is to overcome the disadvantages of the prior art, such as numerous procedures, long production cycle, complicated induced reaction mode, large energy consumption and high cost, and provide a temperature-controlled activated automatic A method for synthesizing α-phase silicon nitride powder by spreading combustion.

The method for synthesizing α-phase silicon nitride by temperature-controlled activated self-propagating combustion provided by the present invention comprises the following steps:

(1) Ingredients:

In parts by weight, silicon powder: 40-94 parts, silicon nitride diluent: 6-60 parts, catalyst: 0.5-30 parts are mixed to obtain a mixture;

Wherein said catalyst is ammonium halide;

(2) Activation treatment:

The mixture obtained in step (1) is subjected to grinding and activation treatment;

(3) Combustion reaction:

Put the ground mixture in step (2) into the reactor with a bulk density of 1.0 to 2.4 g/cm ³ , and after vacuuming, fill it with nitrogen, nitrogen and ammonia or nitrogen and argon to control the pressure At 3 ~ 9MPa, then induce the mixture to undergo combustion synthesis reaction;

When filling the mixed gas of nitrogen and ammonia or nitrogen and argon, the partial pressure of ammonia or argon is 0.1-0.5 MPa;

(4) The reaction is completed:

When the pressure in the reactor is reduced to 2-7MPa, the combustion synthesis reaction is completed. After cooling to room temperature, the pressure in the reactor is released to obtain a loose block product, which is finely ground to obtain α-phase silicon nitride powder.

In order to better realize the combustion synthesis of high-alpha-phase silicon nitride, the present invention expands the range of combustible components of raw materials by designing the reactant system components and changing the nitrogen pressure, and reduces the maximum combustion temperature to adjust it to be suitable for α-phase nitriding Suitable temperature range for silicon formation (<1600°C); at the same time, the activity of the reactants is increased to ensure that the combustion wave is sufficient to spread at a lower combustion temperature. We call this technology of controlling the maximum combustion temperature and increasing the activity of reactants as temperature-controlled activation technology.

Because the heat release of silicon powder and nitrogen reaction is too high, therefore, in step (1), reduce the combustion reaction temperature by adding silicon nitride powder to the raw material silicon powder to dilute; method (such as mixing Ar and other gases into _N2 gas). At the same time, in order to make the combustion reaction self-sustained at low temperature, a catalyst is added to the raw silicon powder and activated to improve the activity of the reactants. The catalyst used plays a dual role in the synthesis. First, it can be used as a diluent for raw silicon powder, that is, to reduce the combustion temperature and provide a suitable temperature for the synthesis of α-phase silicon nitride. Second, it is gasified at the initial stage of combustion and forms Si(NH ₂ ) ₂ intermediate product with silicon, which will promote the formation of α-phase silicon nitride during the combustion process. For this purpose, either a single ammonium halide salt or a mixture of two ammonium halide salts can be used as catalyst.

In the present invention, the Si powder used has a particle size range of 0.1 to 110 μm; the silicon nitride diluent, i.e. silicon nitride powder, has a particle size range of 0.05 to 78 μm, and preferably the α phase content is greater than 85 wt %; the used The catalyst is an ammonium halide salt, preferably NH ₄ Cl, NH ₄ F or a mixture thereof, wherein the weight ratio of NH ₄ Cl to NH ₄ F in the mixture is 0.1-10.

In the compounding process of the step (1) of the present invention, it is preferable to first mix the silicon powder and the Si ₃ N ₄ diluent in proportion and then perform drying treatment, for example, put them in an oven for drying treatment, and then add the catalyst.

For the activation treatment of the mixture obtained in step (1), mechanical activation treatment can be adopted, including high-energy planetary ball milling, high-energy horizontal rotor milling, high-energy vibration milling or stirring ball milling, etc., and the ball milling time is 1 to 10 hours. Grinding, also known as ball milling, can be ground and activated. When it is ground to a certain extent, microcrystalline-induced amorphization will occur, that is, Si powder changes from crystalline to micro-amorphous. The requirements for the degree of activation It is advisable that the amorphous phase content is not less than 5wt%. Then, the ground mixture powder is loosely packed in a high-temperature-resistant container (such as a porous graphite crucible) with a bulk density of 1.0-2.4 g/cm ³ , and put into the reactor together. The reactor used in the present invention is a high-pressure vessel made of stainless steel with a cooling water jacket.

In the present invention, W helical wire or carbon paper is used as the heating element, and a current of 10-30A is generally passed through to induce the combustion synthesis reaction of the powder system by local heating. For example, a φ0.5mm tungsten wire is used to wind a coil, and a pulse current of 10-30A is applied for 5-10s to heat the coil, thereby heating the raw material mixture layer in contact with it, so that the raw material mixture layer reaches the point where silicon and nitrogen react. temperature, and then, the chemical reaction advances the combustion layer by layer in the way of spreading in the raw material mixture. In this system, the spreading rate of the burning wave is 0.01-20mm/s (the spreading rate of the burning wave is related to the nitriding medium and related to the composition of the raw material mixture).

When the pressure in the reactor is reduced to 2-7MPa, which is 80% of the initial pressure, the combustion synthesis reaction is completed, and the whole reaction process lasts for 10-30 minutes; after the combustion synthesis reaction is completed, cool to room temperature, and then release the pressure, a loose block product can be obtained, and after fine grinding, an α-phase silicon nitride powder with a content of 85-97wt% is obtained.

Advantage of the present invention compared with prior art:

1) The raw material powder is conveniently available, and the raw silicon powder does not need special treatment such as pickling.

2) The activation treatment time, that is, the ball milling time is only 1-10 hours, preferably 1-5 hours; while the combustion synthesis reaction is rapid, and the reaction time is 10-30 minutes, which shortens the entire production cycle.

3) Relying on the exothermic effect of the raw material itself to complete the high-temperature self-propagating reaction, after the reaction is induced, no external energy is required, and the electric energy required to ignite the chemical reaction is negligible. Therefore, save energy and reduce costs.

4) The present invention overcomes in the conventional self-propagating process, because the reaction temperature is too high, the reaction speed is too fast, the reaction is difficult to control, resulting in incomplete reaction, low conversion rate, low alpha phase content in the synthetic product, and the shortcomings of being difficult to pulverize , by adding diluent and catalyst, and adjusting the amount of both additions, the reaction temperature can be effectively controlled, and silicon nitride powder with high purity, high α-phase content and uniform particle size distribution can be obtained.

5) Due to the introduction of diluent and catalyst and the introduction of amorphous phase activation treatment means through grinding, the reaction process is effectively controlled, and the nitrogen pressure required for combustion synthesis is reduced (reaction pressure 3-9 MPa). In turn, the requirements for equipment are lowered, and the production safety factor is improved.

Detailed ways

Example 1

Si powder with an average particle size of 0.1 μm, Si ₃ N ₄ powder with an average particle size of 0.5 μm (alpha phase content is about 90wt%) and NH ₄ F are sampled at a weight ratio of 50:28:22, and they are placed in Mill in the ball mill tank of the vibrating ball mill for 9 hours to make it fully mixed and activated; place the mixed and activated material in a porous graphite crucible so that the bulk density is about 1.0-2.4g/cm ³ , and place a winding on the upper layer of the material Spiral tungsten wire with a diameter of 0.5mm is put into the combustion synthesis reactor together; after vacuuming, high-purity nitrogen is filled from the bottom of the reactor until the nitrogen pressure reaches 8MPa; the spiral tungsten wire is passed through Pulse current to make it generate heat and induce the combustion of the raw material powder. After the combustion reaction lasts for 15 minutes, cool down to room temperature, then discharge high-pressure _N2 , and finely grind the product to obtain the combustion reaction product. The reaction product was quantitatively analyzed with a D/MAX-IIB X-ray diffraction analyzer of Japan Rigaku, and the result was a silicon nitride powder with an α-phase silicon nitride content of 91wt%, and a residual Si content of 5wt% %.

Example 2

Si powder with an average particle size of 5 μm, Si ₃ N ₄ powder with an average particle size of 20 nm (alpha phase content is about 90 wt%), NH ₄ F and NH ₄ Cl are sampled at a weight ratio of 45:36:8:11. Put the weighed Si powder and Si ₃ N ₄ into the oven, dry at 100°C for 1.5 hours, take it out, add the weighed NH ₄ F and NH ₄ Cl, and put them together into the ball mill jar of the planetary ball mill Ball mill for 5 hours to make it fully mixed and activated, and the amorphous phase content is not less than 5wt%; put the mixed material in a porous graphite crucible so that the bulk density is about 1.0-2.4g/cm ³ , and place it on the upper layer of the material Put a spiral tungsten wire together into the combustion synthesis reactor; after vacuuming, fill the ammonia gas from the bottom to 0.1MPa, and then fill it with high-purity nitrogen until the pressure in the reactor reaches 7.5MPa; put the spiral tungsten wire Pass a pulse current of 12A to make it generate heat and induce the combustion of the raw material powder. After the combustion reaction lasts for 30 minutes, cool down to room temperature, release the air, and finely grind the product to obtain the reaction product. IIB type X-ray diffraction analyzer conducts quantitative phase analysis, and the result is silicon nitride powder with α-phase silicon nitride content of 93wt%, and there is no free Si in X-ray diffraction analysis.

Example 3

Si powder with an average particle size of 0.5 μm, Si ₃ N ₄ powder with an average particle size of 78 μm (the α phase content is about 90 wt%) and NH ₄ Cl are sampled at a weight ratio of 89:6:5, and put into the planetary ball mill. In a ball mill tank, ball mill for 4.5 hours to make it fully mixed and activated, and the amorphous phase content is not less than 5wt%; the mixed and activated material is placed in a porous graphite crucible, so that the bulk density is about 1.0-2.4g/ ^cm3 , place a helical tungsten wire on the upper layer of the material, the diameter of the tungsten wire is 0.5mm, and put them together into the combustion synthesis reactor; after vacuuming, first fill with ammonia to 0.5MPa, and then fill with high-purity nitrogen , until the pressure in the reactor reaches 3MPa; pass the spiral tungsten wire with a pulse current of 12A to make it generate heat and induce the combustion of the raw material powder. After the combustion reaction lasts for 28 minutes, cool to room temperature, release the air, and finely grind the product. Obtain the reaction product, and the reaction product carries out XRD analysis with the D/MAX-IIB type X-ray diffraction analyzer of Japan Rigaku, and the result is that the silicon nitride powder body of α-phase silicon nitride content is 92wt%, X-ray diffraction analysis No free Si.

Claims (10)

1. a temperature control activates the method that self-propagating combustion synthesizes the α phase silicon nitride powder, it is characterized in that step is as follows:

(1) batching:

In weight part, with silica flour: 40～94 parts, the silicon nitride thinner: 6～60 parts, catalyzer: 0.5～30 part is mixed, and obtains a mixture;

Wherein said catalyzer is an ammonium halide salt;

(2) activation treatment:

The mixture that step (1) is obtained grinds activation treatment;

(3) combustion reactions:

Mixture after step (2) ground is with 1.0～2.4g/cm ³Loose density pack in the reactor, after vacuumizing, charge into the mixed gas of nitrogen, nitrogen and ammonia or nitrogen and argon gas, make pressure-controlling at 3～9MPa, bring out mixture then and carry out combustion synthesis reaction;

During the described mixed gas that charges into nitrogen and ammonia or nitrogen and argon gas, wherein the dividing potential drop of ammonia or argon gas is 0.1～0.5MPa;

(4) reaction is finished:

When the pressure in the reactor was reduced to 2～7MPa, combustion synthesis reaction was finished, be cooled to room temperature after, the releasing reactor internal pressure, resultant loose block product after fine grinding, obtains α phase silicon nitride powder.

2. method according to claim 1 is characterized in that, in the blending process of described step (1), earlier with silica flour and Si ₃N ₄Carry out drying treatment after thinner is mixed in proportion, add catalyzer again.

3. method according to claim 1 and 2 is characterized in that, described silica flour particle size range is 0.1～110 μ m.

4. method according to claim 1 and 2 is characterized in that, described silicon nitride thinner is a silicon nitride powder, and particle size range is 0.05～78 μ m, and wherein the α phase content is greater than 85wt%.

5. method according to claim 1 is characterized in that, described ammonium halide salt is NH ₄Cl, NH ₄F or its mixture, wherein NH in the mixture ₄Cl and NH ₄The weight ratio of F is 0.1～10.

6. method according to claim 1 is characterized in that, described grinding activation comprises that high-energy planetary formula ball milling, the horizontal rotor of high energy grind, the high energy vibrations are ground or stirring ball-milling.

7. method according to claim 1 is characterized in that, described mixture after step (2) is ground is packed in the reactor, is that the mixture pine after step (2) is ground is loaded in the porous graphite crucible, puts into reactor more together.

8. method according to claim 1 is characterized in that, described activation degree is that amorphous content is not less than 5wt%.

9. method according to claim 1 is characterized in that, the described mixture that brings out carries out combustion synthesis reaction and is, makes heating element by tungsten helix or carbon paper, feeds the electric current of 10～30A, brings out in the local heating mode and realizes.

10. method according to claim 1 is characterized in that, the described combustion synthesis reaction time is 10～30 minutes.