CN113493191B - Method for preparing high-purity alpha-silicon nitride powder and high-purity alpha-silicon nitride powder - Google Patents
Method for preparing high-purity alpha-silicon nitride powder and high-purity alpha-silicon nitride powder Download PDFInfo
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Abstract
The invention provides a method for preparing high-purity alpha-silicon nitride powder and the high-purity alpha-silicon nitride powder, wherein the method comprises the following steps: adding a catalyst into fine silicon powder with the particle size of below 100 microns, wherein the catalyst can reversibly react to generate a metal compound under the reaction condition of preparing silicon nitride powder, and a metal halide exists; and (3) placing the fine silicon powder added with the catalyst into a nitriding furnace, and preserving the heat for 50-180 hours at 1050-1400 ℃ in the mixed atmosphere of nitrogen and argon to perform nitriding reaction to obtain the silicon nitride powder. According to the method, the catalyst capable of volatilizing metal halide formed by reversible reaction is transported back and forth between the surfaces of the powder particles and repeatedly plays a catalytic role at a plurality of sites, so that the special effect of fully catalyzing surface reaction by less metal impurities is achieved, the purity of the product is ensured, the alpha phase content of the product is improved, and the requirement of high-purity silicon nitride powder is met.
Description
Technical Field
The invention belongs to the technical field of inorganic non-metallic materials, and particularly relates to a method for preparing high-purity alpha-silicon nitride powder and the high-purity alpha-silicon nitride powder.
Background
Silicon nitride ceramics are important engineering ceramic materials and electronic ceramic materials, have excellent performances such as high hardness, high strength, high toughness, low thermal expansion coefficient, excellent thermal shock resistance, electric insulation and the like, and are widely applied to the fields of light metal smelting, semiconductor manufacturing, high-precision bearings, industrial cutting tools, power electronic devices and the like.
However, the presence of transition metal impurities can reduce the hardness, strength, and especially high temperature mechanical properties of silicon nitride ceramic materials. Therefore, high-purity high-quality silicon nitride powder is very important for the silicon nitride ceramic material. Industrially, the mainstream synthesis process of silicon nitride powder comprises a silicon powder direct nitriding method, a silicon dioxide reduction nitriding method, a silicon imine decomposition method, an auto-ignition method and the like. Compared with the defects that the silicon dioxide reduction nitridation method inevitably leaves residual free carbon and other insufficient reactant impurities, the silamine method has high cost, the self-combustion method has an irregular product fluffy shape and is not suitable for high-grade ceramic products, and the like, the direct nitridation method provides a process route for synthesizing silicon nitride powder with high cost performance and good mass production quality.
However, the direct nitridation reaction of high-purity silicon powder is an exothermic reaction with a high energy barrier, which is difficult to occur at low temperature, but at a sufficiently high temperature, once the reaction is initiated, the reaction is easily overheated to run away, which affects the quality of the silicon nitride powder. Therefore, direct nitridation of high purity silicon feedstock is costly and inefficient, and silicon powder is often required to be extremely fine to increase reactivity, which results in a series of side effects, including high pulverization costs, impurity incorporation due to vigorous pulverization, and very low apparent density of ultra-fine powder feedstock that is excessively pulverized, low charge, which affects production efficiency, etc.
In order to improve the production efficiency, the industry generally relies on the direct nitridation reaction of silicon powder catalyzed by metal impurities. The main principle is that a nano-scale oxide layer always exists on the surface of silicon powder to block the occurrence of nitridation reaction; by introducing metal impurities and locally forming a low-melting-point melt at high temperature, on one hand, the obstruction of an oxide layer is overcome to assist mass transfer, and the nitriding reaction of a solid-phase raw material is promoted, and on the other hand, the volatilization of silicon oxide is promoted and the gas-phase nitriding reaction is generated. However, the introduction of metal impurities requires the incorporation of sufficient impurity powder to contaminate all the silicon powder surfaces, reducing the product purity, and on the other hand, the introduction of more molten liquid phase leads to beta-Si 3 N 4 The content is correspondingly increased, and the content of alpha phase in the silicon nitride is reduced.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a method for preparing high-purity alpha-silicon nitride powder and high-purity alpha-silicon nitride powder aiming at the defects in the prior art, wherein the prepared silicon nitride powder has high purity, high content of alpha phase, stable reaction and high cost performance.
The technical scheme adopted for solving the technical problem of the invention is as follows:
the invention provides a method for preparing high-purity alpha-silicon nitride powder, which comprises the following steps:
adding a catalyst into fine silicon powder with the particle size of below 100 micrometers, wherein the catalyst can generate a metal compound by a reversible reaction under the reaction condition of preparing the silicon nitride powder and enables metal halide to exist;
and (3) placing the fine silicon powder added with the catalyst into a nitriding furnace, and preserving the heat for 50-180 hours at 1050-1400 ℃ in the mixed atmosphere of nitrogen and argon to perform nitriding reaction to obtain the silicon nitride powder.
Further, under the reaction condition of preparing the silicon nitride powder, the catalyst comprises one or more of metal, metal compound and ammonium halide;
the reaction conditions for preparing the silicon nitride powder refer to that: the volatile metal halide exists in the reaction or generates a volatile metal halide intermediate, and for this reason, the method specifically comprises the following steps:
when the catalyst contains metal, the reaction condition for preparing the silicon nitride powder comprises charging gas containing halogen and hydrogen; or
When the catalyst comprises a metal compound and no halogen component exists in the metal compound, the reaction condition for preparing the silicon nitride powder comprises charging gas containing halogen and hydrogen; or
When the catalyst comprises ammonium halide, the catalyst also comprises metal impurities or metal compounds which can react to generate metal halide under the catalytic condition.
The metal impurities refer to: fe. Al, ca, cr, ni, cu, zn, etc., wherein Fe is the most predominant metal impurity.
Furthermore, the content of metal components in the mixed catalyst and fine silicon powder is controlled to be 100-500ppm. The amount of the catalyst used is determined in this way.
Further, the method for preparing the high-purity alpha-silicon nitride powder further comprises the following steps: in the process of carrying out nitridation reaction to obtain silicon nitride powder, hydrogen is filled into the nitriding furnace, wherein the volume content of the hydrogen is a, and a is more than or equal to 1% and less than or equal to 13%.
Further, the method for preparing the high-purity alpha-silicon nitride powder further comprises the following steps: halogen gas is filled into the nitriding furnace in the process of obtaining the silicon nitride powder through nitriding reaction, the volume content of the halogen gas is b, wherein b is more than 0 and less than or equal to 5 percent.
Further, in the process of obtaining the silicon nitride powder through nitridation reaction, the total volume content of the nitrogen and the argon is maintained within the range of x +/-3%, wherein x is more than or equal to 85% and less than or equal to 99%; and the nitrogen proportion is gradually increased along with the nitridation reaction process, and the argon proportion is reduced.
Furthermore, in the process of obtaining the silicon nitride powder through the nitridation reaction, the pressure in the nitriding furnace is kept between 0.15 and 0.5MPa.
Further, the fine silicon powder with the grain diameter of below 100 microns is obtained by crushing high-purity silicon ingots or crushed silicon materials step by step; alternatively, the first and second electrodes may be,
after high-purity silicon ingot or silicon crushed material is crushed step by step to obtain high-purity superfine silicon powder, the high-purity superfine silicon powder is granulated and coarsened to obtain fine silicon powder with the grain size of below 100 microns;
the contents of iron, aluminum and calcium in the high-purity silicon ingot or silicon crushed material are all less than 100ppm, and the total metal impurity content is not more than 400ppm;
the granularity of the high-purity superfine silicon powder is as follows: d50 is 1-10 μm, D90 is less than 30 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total metal impurity content is not more than 400ppm;
the granularity of the fine silicon powder with the particle size of 100 microns or below is as follows: d50 is 1-50 μm, D90 is less than 80 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total content of metal impurities is not more than 400ppm.
Further, after the nitridation reaction is completed, the method further comprises the following steps: the nitriding furnace is stopped, the pressure is released, and replacement gas is introduced to replace the gas in the nitriding furnace; the displacement gas is typically nitrogen.
Further, after the nitriding reaction is performed to obtain the silicon nitride powder, the method further comprises the following steps:
crushing the obtained silicon nitride powder;
and (3) carrying out acid cleaning on the crushed silicon nitride powder by hydrochloric acid or a mixed solution of hydrofluoric acid and hydrochloric acid, then cleaning by deionized water, filtering for 2-4 times, and drying.
Another aspect of the present invention provides a high purity α -silicon nitride powder prepared by any of the above methods.
Has the advantages that:
according to the method for preparing the high-purity alpha-silicon nitride powder, the catalyst capable of forming volatile metal halide through reversible reaction is added into the fine silicon powder, so that the catalyst can be transported in a gas phase and decomposed and deposited when the silicon powder is subjected to nitridation reaction, the catalyst is transported back and forth between the surfaces of powder particles in the nitriding furnace, the catalytic effect is repeatedly realized at a plurality of sites, the special effect of surface reaction is fully catalyzed by less metal impurities, the pressure is relieved after the furnace is stopped, the gaseous halide catalyst is pumped out through gas replacement, the purity of a product is ensured, meanwhile, the conversion of alpha → beta crystal form of the silicon nitride is difficult to occur due to less metal impurities, and the alpha phase content of the product is improved. The silicon nitride powder, alpha-
The phase content is more than or equal to 92 percent, and the total metal impurity content is less than 300ppm, thereby meeting the use requirement of high-purity silicon nitride powder.
Drawings
Fig. 1 is a flowchart of a method for preparing high-purity α -silicon nitride powder according to a first embodiment of the present invention.
Detailed Description
In order to make the technical solutions of the present invention better understood, the present invention is further described in detail below with reference to the accompanying drawings and examples.
It should be noted that the embodiments and features of the embodiments of the present invention may be arbitrarily combined with each other without conflict.
In order to improve the efficiency of direct nitridation to produce silicon nitride, industry generally relies on metal impurities or metal oxides to catalyze the direct nitridation of silicon powder. The main schemes can be summarized in the following categories:
(1) The metal impurities are mixed in the form of simple substances to pollute the surface of the high-purity silicon powder (the surface is oxidized into SiO) 2 ) The mechanism of catalytic action is as follows, M denotes a metal:
M(s)+SiO 2 (s)→MSiO(l)+SiO(g)
3SiO(g)+2N 2 (g)→Si 3 N 4 (s)+3/2SiO 2 (g)
3Si(s)+2N 2 (g)→Si 3 N 4 (s)
the metal catalyst is added in the form of simple substance powder particles, so that the uniform adhesion of metal powder on the surface of silicon powder cannot be realized geometrically; in addition, the metal powder is attached to and fused on the surface of the contacted silicon powder at high temperature, and cannot migrate for a long distance, so that the metal is always added in a large excess amount for the purpose of destroying the local nano-scale silicon oxide film, and the improvement of the product purity is not facilitated.
(2) Metal impurities are mixed into the high-purity silicon powder in a metal oxide form to pollute the surface of the high-purity silicon powder, and the catalytic action mechanism is as follows:
M 2 O y (s)+SiO 2 (s)→MSiO(l)+SiO(g)
3SiO(g)+2N 2 (g)→Si 3 N 4 (s)+3/2SiO 2 (g)
3Si(s)+2N 2 (g)→Si 3 N 4 (s)
in the above reaction formula, MSiO does not represent the atomic ratio of each substance, and represents a molten mixture of several substances. When the metal catalyst is added in the form of oxide powder particles, uniform adhesion of the catalyst powder on the surface of the silicon powder cannot be realized, and the adhesion of the catalyst on the surface of the silicon powder basically does not migrate/diffuse, so that the problems of large total amount of the required catalyst, excessive local feeding of the catalyst and the like are still inevitable.
(3) Metal impurities are mixed into the silicon powder in the form of compounds such as metal salt, metal organic matters and the like to pollute the surface of the silicon powder. The above two cases are converted to by using a metal or a metal oxide which is a decomposition product of a metal salt, a metallorganic compound or the like at a high temperature. For example, in the literature, metal silicon powder is systematically impregnated with nitrates of calcium, yttrium, iron, copper, silver, and chromium, and the effect of different metal catalysts on the direct nitridation reaction of silicon is studied, i.e., the characteristic of decomposition of the nitrates at high temperature is utilized. The impregnation of the silica powder is used to generate very fine metal/metal oxide particles in situ due to the decomposition reaction, which can be more beneficial to the pressing of the metal catalyst dosage than the two cases. However, the amount of catalyst needed is still high because the catalyst material adheres to the surface of the silicon powder and cannot migrate efficiently.
(4) Industrial silicon powder containing more metal impurities is directly used as a raw material, and the produced silicon nitride powder is shown in the following table 1.
The impurity content of common industrial silicon powder is high (the typical Fe content is more than or equal to 1000ppm, see the following table 1, extracted from GB/T2881-2014 Industrial silicon), and most impurities are wrapped in the particles and do not participate in the reaction catalysis of the surfaces, so that the purity requirement of high-purity silicon nitride powder cannot be met when the particles are used as raw materials.
TABLE 1 Industrial silicon Specification
The introduction of metallic impurities on the one hand reduces the product purity and on the other hand it is generally accepted that an increase in the molten liquid phase leads to beta-Si 3 N 4 The content rises accordingly. Therefore, in the direct nitriding process, in order to obtain high purity and high alpha phase silicon nitride powder, it is usually adopted to control the mixing of trace metal or metal oxide impurities into high purity silicon powder, for example, JPH111309A, which is obtained by mixing 0.3-2.0wt% of alpha-Fe with particle size of 0.3-20 μm into high purity silicon powder with particle size of 1-50 μm 2 O 3 And (3) a powder catalysis scheme. Similar to the above, there are a magnesium catalyst having a specific open-constitution, i.e., 51-48800, a calcium catalyst having a specific open-constitution, i.e., 54-120298, and a copper and copper compound having a specific open-constitution, i.e., JP2970400B 2. Such a solution of mixing enough impurity powder at one time to contaminate all silicon powder surfaces still has too high impurity content for high purity silicon nitride powder.
Therefore, in order to overcome the defects in the prior art, the invention provides a method for preparing high-purity alpha-silicon nitride powder. The following are examples of the present invention.
Example 1
As shown in fig. 1, the present invention provides a method for preparing high-purity α -silicon nitride powder, comprising:
step S101: adding a catalyst into fine silicon powder with the particle size of below 100 microns, wherein the catalyst can generate a metal compound by a reversible reaction under the reaction condition of preparing silicon nitride powder, and enables metal halide to exist;
step S102: and (3) placing the fine silicon powder added with the catalyst into a nitriding furnace, and preserving the heat for 50-180 hours at 1050-1400 ℃ in the mixed atmosphere of nitrogen and argon to perform nitriding reaction to obtain the silicon nitride powder.
Main reaction in nitriding furnace
SiO 2 (s)+H 2 (g)←→SiO(g)+H 2 O(g)
Si(s)+H 2 O(g)←→SiO(g)+H 2 (g)
6SiO(g)+4N 2 (g)←→2Si 3 N 4 (s)+3O 2 (g)
3Si(s)+2N 2 (g)←→Si 3 N 4 (s)
The function of a catalyst; in this example, in order to reduce the content of impurities in the produced silicon nitride powder, a catalyst having a metal halide or capable of generating a metal halide under reaction conditions is mixed into the prepared silicon powder raw material, and the silicon powder raw material generally contains a small amount of metal impurities such as Fe, al, ca, cr, ni, cu, zn, etc.; the catalyst is generally metal halide, or metal oxide which can react with halogen gas or ammonium halide to generate metal halide, or halogen-free metal salt, metal organic matter and other precursors which can be decomposed at high temperature to generate metal or metal oxide, and halogen-containing gas or ammonium halide which can react with the precursors; the metal halide can be sublimated and gasified at high temperature, is conveyed back and forth among different reaction sites, is decomposed on the surface of the silicon powder through a reversible reaction, generates a complex metal-silicon-oxygen melting system, overcomes the obstruction of an oxide layer on the surface of the silicon powder, improves the mass transfer efficiency, promotes the volatilization of the silicon monoxide and the generation of a gas phase nitridation reaction on one hand, and promotes the nitridation reaction of a solid phase raw material on the other hand. The mechanism of action is shown in the following reaction, wherein M refers to metal, X refers to halogen:
mechanism of formation of metal halides
2M(s)+yX 2 (g)←→2MX y (s)
2M(s)+yH 2 O(g)←→M 2 O y (s)+yH 2 (g)
M 2 O y (s)+yX 2 (g)+yH 2 (g)←→2MX y (s)+yH 2 O(g)
NH 4 X(s)←→NH 3 (g)+HX(g)
2M(s)+2yHX(g)+ySiO 2 (s)←→2MX y (s)+ySiO(g)+yH 2 O(g)
M 2 O y (s)+2yHX(g)←→2MX y (s)+yH 2 O(g)
Transport and catalytic mechanism for metal halides
MX y (s)←→MX y (g)
2MX y (s)+yH 2 (g)←→2M(s)+2yHX(g)
2MX y (s)+yH 2 O(g)←→M 2 O y (s)+2yHX(g)
M(s)+SiO 2 (s)→MSiO(l)+SiO(g)
M 2 O y (s)+SiO 2 (s)→MSiO(l)+SiO(g)
6SiO(g)+4N 2 (g)←→2Si 3 N 4 (s)+3O 2 (g)
In the above reaction formula, MSiO does not represent the atomic ratio of each substance, and represents a molten mixture of several substances.
For example, a nitriding furnace is charged with hydrogen and chlorine, and a catalyst FeCl 3 A series of reactions with hydrogen occur at high temperatures:
reduction of ferric chloride to ferrous chloride
2FeCl 3 (s)+H 2 (g)→2FeCl 2 (s)+2HCl(g)
Ferrous chloride and hydrogen or water vapor generate iron simple substance or ferrous oxide and generate reaction for destroying oxide layer on the surface of silicon powder
FeCl 2 (s)+H 2 (g)←→Fe(s)+2HCl(g)
FeCl 2 (s)+H 2 O(g)←→FeO(s)+2HCl(g)
Fe(s)+SiO 2 (s)→FeSiO(l)+SiO(g)
FeO(s)+SiO 2 (s)+H 2 (g)→FeSiO(l)+SiO(g)+H 2 O(g)
Note: feSiO in the above reaction formula does not represent the atomic ratio of each substance, and represents a molten mixture of several substances.
Main reaction of nitriding
6SiO(g)+4N 2 (g)←→2Si 3 N 4 (s)+3O 2 (g)
3Si(s)+2N 2 (g)←→Si 3 N 4 (s)
Reaction for recovering iron component in iron-silicon-oxygen molten mixture to ferrous chloride
FeSiO(l)+HCl(g)→Si(s)+FeCl 2 (g)+H 2 O(g)
FeSiO(l)+H 2 (g)→Fe(s)+Si(s)+SiO(g)+H 2 O(g)
Fe(s)+2HCl(g)←→FeCl 2 (g)+H 2 (g)
Reversible reaction of ferrous chloride gasification (sublimation) transportation and deposition
FeCl 2 (g)←→FeCl 2 (s)
After being gasified, the metal halide can be deposited and decomposed to generate metal or metal oxide, and then forms a complex metal-silicon-oxygen melting system with the silicon powder surface oxide layer, thereby overcoming the obstruction of the silicon powder surface oxide layer and promoting the occurrence of nitridation reaction.
Because the metal substance in the metal-silicon-oxygen melting system can be recovered into metal halide when the oxygen in the melting mixture is exhausted, and the metal substance can repeatedly play a catalytic role at different sites through gas phase transportation and deposition, and the catalyst dosage enough for catalyzing all the sites does not need to be mixed once when feeding, the special effect of fully catalyzing surface reaction by using the catalyst with less metal impurities can be achieved, and the purity of the product is ensured. Meanwhile, less metal impurities are formed corresponding to less mixed melt liquid phase, so that the alpha → beta crystal form conversion is not easy to occur, and the alpha phase content of the product is improved.
Wherein alpha-Si 3 N 4 Belongs to a low-temperature stable crystal form, beta-Si 3 N 4 Is a high temperature stable crystal form. In general, for ceramic applications, it is desirable that the silicon nitride powder have as high an alpha phase content as possible to provide higher sintering activity and better mechanical properties. Because the phase transformation of alpha → beta provides additional sintering activity during sintering, and the silicon nitride powder and sintering aid are transformed into beta type columnar crystal with high length-diameter ratio by eutectic-precipitation mechanism at high temperature, the ceramic product can be improvedThe toughness of (3).
The fine silicon powder can be superfine silicon powder obtained by gradually crushing high-purity silicon materials, and can also be granulated coarse powder prepared from the superfine silicon powder. Generally, it is considered that, on the premise that impurity contamination is not excessively introduced in the step-by-step crushing process, in order to increase the specific surface area and improve the nitriding reaction activity, the smaller the particle size of the silicon powder serving as a reactant is, the better the particle size is. To increase the filling rate, the particle size can also be increased by granulation. In conclusion, the fine silicon powder of the invention needs to control the D50 to be 1-50 μm, the D90 to be less than 80 μm, the particle size to be less than 100 μm, the contents of iron, aluminum and calcium to be less than 100ppm, and the total content of metal impurities to be not more than 400ppm.
Further, under the reaction condition of preparing the silicon nitride powder, the catalyst comprises one or more of metal, metal compound and ammonium halide;
the reaction conditions for preparing the silicon nitride powder refer to that: the reaction has volatile metal halide or intermediate for generating volatile metal halide, and for this purpose, the method specifically includes:
when the catalyst contains metal, the reaction condition for preparing the silicon nitride powder comprises charging gas containing halogen and hydrogen; or
When the catalyst comprises a metal compound and halogen-free components are contained in the metal compound, the reaction condition for preparing the silicon nitride powder comprises the steps of filling gas containing halogen and hydrogen; or
When the catalyst comprises ammonium halide, the catalyst also comprises metal impurities or metal compounds which can react to generate metal halide under the catalytic condition.
The catalyst capable of reversibly reacting under the reaction conditions to produce a metal compound includes various forms such as a metal halide itself, a metal oxide which easily reacts with a halogen gas at a high temperature, a metal compound which is easily decomposed into a metal or a metal oxide, and a mixture of halogen salts such as ammonium halide. Specifically, the catalyst includes but is not limited to FeCl 3 、Fe 2 O 3 、NH 4 Cl、CaF 2 Etc. halogen-containing gases includeBut is not limited to F 2 、Cl 2 HF, HCl, etc., and one or more thereof are selected according to the actual situation.
Furthermore, the content of metal components in the mixed catalyst and fine silicon powder is controlled to be 100-500ppm.
In the process of adding the catalyst, the addition amount of the metal, the metal compound or the ammonium halide is in the order of 100ppm, the total metal component content is 100-500ppm in terms of the impurity level carried by the silicon powder raw material, and the compound is calculated according to the content of only metal in terms of the total metal component content; preferably, the total metal component content ranges from 100 to 500ppm in terms of the total metal impurity content excluding aluminum.
Further, the method for preparing the high-purity alpha-silicon nitride powder further comprises the following steps: hydrogen is filled into the nitriding furnace in the process of performing nitriding reaction to obtain the silicon nitride powder, wherein the volume content of the hydrogen is a, and a is more than or equal to 1% and less than or equal to 13%.
Nitrogen and silicon powder generate silicon nitride powder at high temperature, nitrogen, argon and hydrogen are filled into a nitriding furnace, wherein the nitrogen is a reactant, the argon is an inert diluent, the hydrogen is used as reducing gas to assist in breaking an oxide layer on the surface of the silicon powder, the content of the filled hydrogen can be adjusted along with the reaction, the volume content of the hydrogen is more than or equal to 1% and less than or equal to 13%, and the preferred range is 5-10%.
Further, the method for preparing the high-purity alpha-silicon nitride powder further comprises the following steps: and (2) charging halogen-containing gas into the nitriding furnace in the process of performing nitriding reaction to obtain the silicon nitride powder, wherein the halogen-containing gas has the volume content of b, and b is more than 0 and less than or equal to 5%.
The halogen-containing gas is a raw material for generating gas-phase metal halide in the reaction, the volume content of the halogen-containing gas is 0-5%, and the preferable range is 0.5-1%, and the halogen-containing gas can be supplemented in times according to the change of the temperature stage of the process. If the catalyst contains sufficient halogen salt, the atmosphere may be such that no additional halogen-containing gas is necessary.
Further, in the process of obtaining the silicon nitride powder through nitridation reaction, the total volume content of the nitrogen and the argon is maintained within the range of x +/-3%, wherein x is more than or equal to 85% and less than or equal to 99%; and the nitrogen proportion is gradually increased along with the nitridation reaction process, and the argon proportion is reduced.
Nitrogen and argon are filled into the nitriding furnace, the content of the nitrogen is 25-95%, and the preferable range is 40-85%, and the nitrogen and the argon vary with the temperature stage of the process; the argon content is between 0 and 70%, preferably between 0 and 55%, as a function of the temperature phase of the process. The rest of the nitriding furnace is hydrogen and halogen-containing gas such as chlorine and hydrogen chloride.
The nitrogen is consumed along with the nitridation process, the proportion of the nitrogen is gradually increased and the proportion of the argon is reduced during gas supplement according to the intensity of the reaction, the proportion of the nitrogen can reach 100 percent at the maximum in the final stage, and the content of the argon can be reduced to zero. In the field production process, along with the reaction, the nitriding furnace is supplemented with gas, the reaction furnace is decompressed to standard atmospheric pressure (the same as external atmospheric pressure) before gas is supplemented every time, and then the gas is pressurized according to the gas proportion set by the process. In this way the furnace volume will provide a buffer and the proportion of the make-up gas is accurately controlled. Preferably, different gases can be introduced according to the proportion and simultaneously enter and exit, so that the atmosphere in the furnace infinitely approaches to the set proportion. Along with the nitridation process, the surface of the silicon powder is gradually coated by a silicon nitride product, the reaction tends to be mild, and the concentration of a reactant needs to be increased to promote the reaction.
Further, in the process of obtaining the silicon nitride powder by the nitridation reaction, the pressure in the nitriding furnace is kept between 0.15 and 0.5MPa.
During the nitridation reaction, the pressure in the nitriding furnace is kept between 0.15 and 0.5MPa, and the preferred range is between 0.2 and 0.4MPa; the nitriding temperature is 1050-1400 ℃, and the heat preservation time is 50-180h, so as to achieve better production efficiency.
Further, the fine silicon powder with the grain diameter of below 100 microns is obtained by crushing high-purity silicon ingots or crushed silicon materials step by step; alternatively, the first and second electrodes may be,
after high-purity silicon ingot or silicon crushed material is crushed step by step to obtain high-purity superfine silicon powder, the high-purity superfine silicon powder is granulated and coarsened to obtain fine silicon powder with the particle size of 50-100 microns;
the contents of iron, aluminum and calcium in the high-purity silicon ingot or silicon crushed material are all less than 100ppm, and the total metal impurity content is not more than 400ppm;
the granularity of the high-purity superfine silicon powder is as follows: d50 is 1-10 μm, D90 is less than 30 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total metal impurity content is not more than 400ppm;
the granularity of the fine silicon powder with the particle size of 100 microns or below is as follows: d50 is 1-50 μm, D90 is less than 80 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total content of metal impurities is not more than 400ppm.
Refining and sieving high-purity coarse silicon powder by means of back-impact crushing, jaw crushing and the like of high-purity silicon ingots or crushed materials to obtain high-purity coarse silicon powder, wherein the granularity of the high-purity coarse silicon powder is 50-100 meshes, and then gradually crushing or grinding the high-purity coarse silicon powder to obtain fine silicon powder with the particle size of below 100 microns;
the fine silicon powder is prepared by finely grinding the high-purity coarse silicon powder in a mode of jet milling, roller ball milling or the like. The granularity of the fine silicon powder is as follows: d50 is 1-50 μm, D90 is less than 80 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total content of metal impurities is not more than 400ppm.
Preferably, in order to increase the loading capacity, the high-purity coarse silicon powder is subjected to step-by-step crushing or grinding to obtain high-purity superfine silicon powder, and then the high-purity superfine silicon powder is subjected to granulation and coarsening to obtain fine silicon powder with the particle size of less than 100 microns.
In order to increase the loading capacity, the granulation process is also used to granulate and coarsen the high-purity superfine silicon powder to obtain fine silicon powder with the particle size of less than 100 microns, and the preferred particle size is 50-100 microns; the granulation method is the same as the granulation method in the production process of the functional ceramic.
Further, after the nitridation reaction is completed, the method further comprises the following steps: and (4) relieving the pressure of the nitriding furnace after the nitriding furnace is stopped, and introducing replacement gas to replace the gas in the nitriding furnace.
After the furnace is shut down and the pressure is released, the halide of the gas is pumped away, and the content of impurities such as nitride is reduced.
Further, after the nitriding reaction is performed to obtain the silicon nitride powder, the method further comprises the following steps:
crushing the obtained silicon nitride powder;
and (3) carrying out acid cleaning on the crushed silicon nitride powder by hydrochloric acid or a mixed solution of hydrofluoric acid and hydrochloric acid, then cleaning by deionized water, filtering for 2-4 times, and drying.
The mixed solution of hydrofluoric acid and hydrochloric acid is used for acid cleaning, so that part of redundant metal impurities and metal oxides can be washed away, and the purity of the silicon nitride is improved.
The silicon nitride powder after dry grinding is generally required to have the particle size D50=0.5-5.0um. In the acid cleaning, 5% hydrochloric acid and 4% hydrofluoric acid are generally used in a ratio of 1 to 5:1 by weight, or the acid cleaning is repeated by using 5% hydrochloric acid alone, and HF is added to remove the oxide layer on the surface. Since F has occupational health risks, the addition amount of F is determined by each factory. Washing with deionized water for 2-4 times, and drying.
In the embodiment, the catalyst capable of forming volatile metal halide through reversible reaction is added into the fine silicon powder, so that when the silicon powder is subjected to nitriding reaction, the catalyst can be transported in a gas phase and decomposed and deposited, the powder particles in the nitriding furnace are transported back and forth between the surfaces, the catalytic action is repeatedly performed at a plurality of sites, the special effect of surface reaction is fully catalyzed by less metal impurities, the pressure is relieved after the furnace is shut down, the halide catalyst is pumped away through gas replacement, the purity of a product is ensured, meanwhile, the conversion of an alpha → beta crystal form is difficult to occur due to less metal impurities, and the alpha phase content of the product is improved.
Example 2
This example discloses a method for preparing high purity α -silicon nitride powder, comprising the following steps.
Electronic grade polycrystalline silicon powder with the size of 6-10mm is used as a raw material, and the total content of various metal impurities such as Fe, cr, ni, cu, zn, al, K, na and the like in the raw material is less than or equal to 15ppb; mechanical crushing and airflow crushing are sequentially adopted to crush the silicon powder into superfine silicon powder with the granularity D50=2.5 μm and D90=8 μm, and the superfine silicon powder with the particle size of 50-100 microns is obtained after granulation and coarsening.
Adding ferric ammonium citrate into the fine silicon powder, wherein the specific addition amount is adjusted to be about 100ppm of total metal content according to the metal content, and the weight ratio of the added ferric ammonium citrate to the metal content1% NH 4 And (4) uniformly mixing Cl by using an airflow homogenizing device, and decomposing the ammonium ferric citrate at a high temperature in a reducing atmosphere to generate simple substance iron, wherein the simple substance iron is finer than the particles directly added with the iron powder.
The mixture was stacked in a nitriding furnace for nitriding, the total gas pressure in the nitriding furnace was 0.15MPa, and the initial atmosphere volume was 10% hydrogen, 5% hydrogen chloride, 45% nitrogen, and 40% argon. The initial temperature is 1050 ℃; with the gradual temperature rise and continuous nitridation reaction, the silicon powder raw material is continuously consumed, and the possibility of runaway reaction overtemperature is smaller and smaller, so that the volume ratio of nitrogen as reaction gas is gradually increased, and the proportion of argon as diluent gas is gradually reduced, so as to promote the reaction; in order to reduce impurities in the product, the volume ratio of hydrogen to hydrogen chloride gas in the later reaction stage is gradually reduced by adjustment. When the reaction was completed, the atmosphere in the nitriding furnace was composed of 99% nitrogen, 1% hydrogen, and 0% argon and hydrogen chloride. The maximum nitriding temperature was 1390 ℃ and a total of 50 hours.
The ferric ammonium citrate is heated and decomposed in reducing atmosphere to generate iron simple substance which can be mixed with NH 4 Cl reacts to generate gasifiable ferrous chloride, so that the ferrous chloride is transported and deposited among different sites, and related reactions comprise:
2(NH 4 ) 3 Fe(C 6 H 5 O 7 ) 2 (s)→2Fe(s)+6NH 3 (g)+12CO(g)+6H 2 (g)+2H 2 O(g)
NH 4 Cl(s)←→NH 3 (g)+HCl(g)
Fe(s)+2HCl(g)←→FeCl 2 (g)+H 2 (g)
FeCl 2 (g)←→FeCl 2 (s)
ferrous chloride deposited on the surface of the silicon powder can react with an oxide layer on the surface of the silicon powder under the assistance of hydrogen to form an iron-silicon-oxygen molten mixture, so that the compact oxide layer on the surface of the silicon powder is damaged, the mass transfer efficiency is accelerated, and the nitridation reaction is accelerated:
FeCl 2 (s)+SiO 2 (s)+H 2 (g)→FeSiO(l)+SiO(g)+H 2 O(g)
6SiO(g)+4N 2 (g)←→2Si 3 N 4 (s)+3O 2 (g)
3Si(s)+2N 2 (g)←→Si 3 N 4 (s)
the silicon nitride product was pulverized step by step and ground to D50=0.7um by a sand mill, and then washed with 5% HCl to remove metal impurities used as a catalyst, and then repeatedly washed/filtered 3 times with deionized water, and then dried.
The silicon nitride powder obtained by the method is detected as follows: the total metal impurity content is less than 100ppm, and the alpha phase content is more than or equal to 93 percent.
Example 3
This example discloses a method for preparing high purity α -silicon nitride powder, comprising the following steps.
The high-purity coarse silicon powder with 100 meshes is used as a raw material, and the total content of impurities such as Fe, al, ca, cr, ni, cu, zn and the like in a raw material matrix is less than 50ppm; the fine silicon powder was pulverized by gas flow to D50=12 μm and D90=45 μm.
Adding Al with the molar ratio of 1:1 into the fine silicon powder 2 O 3 And Fe 2 O 3 And (3) adjusting the specific addition amount of the powder to about 500ppm of the total metal component content in the mixture according to the metal content, and uniformly mixing the powder by using a V-shaped mixer.
The mixture is stacked in a nitriding furnace for nitriding, the total gas pressure is 0.4MPa, and the volume ratio of the initial atmosphere is 13% of hydrogen, 2% of chlorine, 40% of nitrogen and 45% of argon; the volume ratio of hydrogen to chlorine is kept constant in the whole reaction process, the total volume ratio of nitrogen to argon is kept constant at 85% in the whole reaction process, nitrogen is consumed along with the nitriding process, the nitrogen proportion is gradually increased and reduced according to the intensity of the reaction during air supply, the final volume ratio of nitrogen is 85%, the maximum nitriding temperature is 1400 ℃, and the nitrogen is nitrided for 75 hours in total.
The relevant reactions are:
Fe 2 O 3 (s)+3H 2 (g)+3Cl 2 (g)→2FeCl 3 (s)+3H 2 and O (g), and the rest of the reaction is shown in example one and is not described in detail herein.
The silicon nitride product was pulverized step by step and ground to D50=0.8um by a sand mill, and then metal impurities used as a catalyst were removed by acid washing with 5% HCl, followed by repeated washing/filtration with deionized water 4 times, and further dried.
The silicon nitride powder obtained by the method has the total metal impurity content of less than 300ppm and the alpha phase content of more than or equal to 92 percent.
Example 4
This example discloses a method for preparing high purity α -silicon nitride powder, which includes the following steps.
50-mesh photovoltaic III-grade coarse silicon powder is used as a raw material, and the total amount of impurities of Fe, al, ca, cr, ni, cu and Zn in a matrix is less than 0.2ppm; and mechanically crushing and grinding the silicon powder to obtain silicon powder with D50=3 μm and D90=10 μm sequentially.
Adding FeCl into the silicon powder 3 The total iron content of the mixture is converted to 300ppm. And uniformly mixing by using a V-shaped mixer.
Stacking the mixture in a nitriding furnace for nitriding, wherein the total gas pressure is 0.5MPa, and the initial atmosphere volume ratio is 5% of hydrogen, 40% of nitrogen and 55% of argon; the volume ratio of the hydrogen is kept constant all the time in the whole reaction process, the total volume ratio of the nitrogen and the argon is kept constant at 95% all the time in the whole reaction process, the nitrogen is consumed along with the nitriding process, the nitrogen proportion is gradually increased and reduced according to the reaction intensity during gas supplement, the final volume ratio of the nitrogen is 95%, the highest nitriding temperature is 1400 ℃, and the nitrogen is nitrided for 180 hours in total.
The reaction process is as described in example one, and is not further described here.
The silicon nitride product was pulverized by a dry method until D50=0.8um, washed repeatedly with deionized water and filtered 2 times, and then dried.
The silicon nitride powder obtained by the method has the total metal impurity content of less than 200ppm and the alpha phase content of more than or equal to 93 percent.
Example 5
This example is a comparative example, and compared with the embodiment adopting the technical scheme of the present invention, the production process of using a metal simple substance as a catalyst, and the obtained silicon nitride are as follows:
50-mesh industrial-grade coarse silicon powder is used as a raw material, and the total amount of impurities of Fe, al, ca, cr, ni, cu and Zn of a matrix is 2000ppm; mechanically crushing and air-flow crushing to obtain fine silicon powder with D50=3 μm and D90=10 μm.
Putting the mixed material in a nitriding furnace for nitriding, wherein the total gas pressure is 0.2MPa, and the initial atmosphere volume ratio is 5% of hydrogen, 45% of nitrogen and 50% of argon; the volume ratio of the hydrogen is kept constant all the time in the whole reaction process, the total volume ratio of the nitrogen and the argon is kept constant at 95% all the time in the whole reaction process, the nitrogen is consumed along with the nitriding process, the nitrogen proportion is gradually increased and the argon proportion is reduced according to the reaction intensity during gas supplement, the final volume ratio of the nitrogen is 95%, the highest nitriding temperature is 1400 ℃, and the nitrogen is nitrided for 130 hours in total.
After the silicon nitride product is ground by a dry method, the silicon nitride product is pickled by HF/HCl mixed solution, then repeatedly cleaned/filtered for 3 times by deionized water, and then dried, so that the obtained silicon nitride powder has the total metal impurity content of 1200ppm and the alpha phase content of more than or equal to 90 percent.
In this example, in order to achieve acceptable production efficiency of the nitriding reaction, the content of metal impurities in the raw material was controlled to 2000ppm, i.e., the amount of the catalyst was required to be sufficient enough to allow the catalyst to contaminate all the silicon powder surfaces. Otherwise, the high-purity silicon powder cannot react at all, and the purity and quality of the product are determined by how to initiate the reaction with the lowest possible catalyst.
Example 6
This example is a comparative example, in which the catalyst is Fe compared with the example using the technical solution of the present invention 2 O 3 The conditions without halogen gas, the production process thereof, and the obtained silicon nitride were as follows:
the high-purity coarse silicon powder with 100 meshes is used as a raw material, and the total impurities of Fe, al, ca, cr, ni, cu and Zn of the matrix is less than 50ppm; jet milling was carried out to obtain fine silicon powder with D50=8 μm and D90=25 μm.
Adding Fe into the fine silicon powder 2 O 3 And powder, wherein the Fe impurity content of the mixture is 1500ppm by conversion. And uniformly mixing by using a V-shaped mixer.
Stacking the mixture in a nitriding furnace for nitriding, wherein the total gas pressure is 0.4MPa, and the volume ratio of the initial atmosphere is 10% of hydrogen, 45% of nitrogen and 45% of argon; the volume ratio of hydrogen is kept constant in the whole reaction process, the total volume ratio of nitrogen and argon is kept constant at 90% in the whole reaction process, nitrogen is consumed along with the nitriding process, the nitrogen proportion is gradually increased and the argon proportion is reduced according to the intensity of the reaction during gas supplementing, the final volume ratio of nitrogen is 90%, the maximum nitriding temperature is 1400 ℃, and the total nitriding time is 150 hours.
The obtained silicon nitride powder is crushed by a dry method, the content of Fe impurities is typically 900ppm, the content of total metal impurities is less than 950ppm, and the content of alpha phase is more than or equal to 90 percent; the method is characterized in that a part of iron impurities are washed away by hydrochloric acid typically by an acid washing technology known in the industry, deionized water is used for washing and filtering for 3 times, silicon nitride powder with the Fe impurity content of more than or equal to 600ppm and the total metal content of not less than 650ppm is obtained after drying, and the alpha phase content is maintained at a level of more than or equal to 90%.
In the practice of this example, fe was added 2 O 3 The powder is more, otherwise, the high-purity silicon powder can not react under the reaction condition.
Example 7
This example discloses a high purity alpha-silicon nitride powder prepared by the method described in examples 1-4. Tests show that the alpha-phase content of the prepared high-purity alpha-silicon nitride powder is more than or equal to 92 percent, and the total metal impurity content is less than 300ppm.
It will be understood that the above embodiments are merely exemplary embodiments taken to illustrate the principles of the present invention, which is not limited thereto. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit and substance of the invention, and these modifications and improvements are also considered to be within the scope of the invention.
Claims (8)
1. A method for preparing high-purity alpha-silicon nitride powder comprises the following steps:
adding a catalyst into fine silicon powder with the particle size of below 100 microns, wherein the catalyst can generate a metal compound through a reversible reaction under the reaction condition of preparing silicon nitride powder and enables metal halide to exist;
wherein the catalyst comprises one or more of a metal, a metal compound, and an ammonium halide;
the reaction conditions for preparing the silicon nitride powder refer to that: the existence of the volatilizable metal halide or the generation of a volatilizable metal halide intermediate in the reaction specifically means that:
when the catalyst contains metal, the reaction condition for preparing the silicon nitride powder comprises charging gas containing halogen and hydrogen; or
When the catalyst contains a metal compound and halogen-free components are contained in the metal compound, the reaction condition for preparing the silicon nitride powder comprises the steps of filling gas containing halogen and hydrogen; or
When the catalyst contains ammonium halide, the catalyst also comprises metal impurities or metal compounds which can react under catalytic conditions to generate metal halide;
putting the fine silicon powder added with the catalyst into a nitriding furnace, and preserving the heat for 50-180 hours at 1050-1400 ℃ in the mixed atmosphere of nitrogen and argon to perform nitriding reaction to obtain silicon nitride powder; and the number of the first and second electrodes,
filling hydrogen and halogen gas into a nitriding furnace in the process of performing nitriding reaction to obtain silicon nitride powder, wherein the volume content of the hydrogen is a, and a is more than or equal to 1% and less than or equal to 13%; the volume content of the halogen gas is b, wherein b is more than 0 and less than or equal to 5 percent.
2. The method for preparing high-purity alpha-silicon nitride powder according to claim 1, wherein the content of metal components in the mixed catalyst and fine silicon powder is controlled to be 100-500ppm.
3. The method for preparing high-purity alpha-silicon nitride powder according to claim 1, wherein the total volume content of nitrogen and argon is maintained as x during the nitridation reaction to obtain silicon nitride powder, wherein x is greater than or equal to 85% and less than or equal to 99%, and the proportion of nitrogen is gradually increased and the proportion of argon is decreased along with the nitridation reaction.
4. The method for preparing high-purity alpha-silicon nitride powder according to claim 1, wherein the pressure in the nitriding furnace is maintained at 0.15-0.5MPa during the nitriding reaction to obtain silicon nitride powder.
5. The method according to claim 1, wherein the silicon powder is a mixture of silicon nitride powder and alpha-silicon nitride powder,
the fine silicon powder with the grain size of below 100 microns is obtained by gradually crushing high-purity silicon ingots or crushed silicon materials; alternatively, the first and second liquid crystal display panels may be,
after high-purity silicon ingot or silicon crushed material is crushed step by step to obtain high-purity superfine silicon powder, the high-purity superfine silicon powder is granulated and coarsened to obtain fine silicon powder with the particle size of less than 100 microns;
the contents of iron, aluminum and calcium in the high-purity silicon ingot or silicon crushed material are all less than 100ppm, and the total metal impurity content is not more than 400ppm;
the granularity of the high-purity superfine silicon powder is as follows: d50 is 1-10 μm, D90 is less than 30 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total metal impurity content is not more than 400ppm;
the granularity of the fine silicon powder with the particle size of 100 microns or below is as follows: d50 is 1-50 μm, D90 is less than 80 μm; the contents of iron, aluminum and calcium are all less than 100ppm, and the total content of metal impurities is not more than 400ppm.
6. The method for preparing high-purity alpha-silicon nitride powder according to claim 1, further comprising, after the nitriding reaction is completed: and (4) relieving the pressure of the nitriding furnace after the nitriding furnace is stopped, and introducing replacement gas to replace the gas in the nitriding furnace.
7. The method for preparing high-purity alpha-silicon nitride powder according to claim 1, wherein after the nitriding reaction is performed to obtain silicon nitride powder, the method further comprises:
crushing the obtained silicon nitride powder;
and (3) carrying out acid cleaning on the crushed silicon nitride powder by hydrochloric acid or a mixed solution of hydrofluoric acid and hydrochloric acid, then cleaning by deionized water, filtering for 2-4 times, and drying.
8. A high-purity α -silicon nitride powder characterized by being prepared by the method according to any one of claims 1 to 7.
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