CN107663092B

CN107663092B - Preparation method of AlN powder

Info

Publication number: CN107663092B
Application number: CN201710883274.3A
Authority: CN
Inventors: 陈卫武; 詹国彬
Original assignee: Shanghai Dongyang Carbon Co Ltd
Current assignee: Shanghai Dongyang Carbon Co Ltd
Priority date: 2017-09-26
Filing date: 2017-09-26
Publication date: 2019-12-31
Anticipated expiration: 2037-09-26
Also published as: CN107663092A

Abstract

The invention relates to the technical field of powder preparation, in particular to a preparation method of homogeneous high-purity AlN powder. In the process of synthesizing AlN powder by nitriding aluminum powder, a small amount of high-purity argon is introduced, and the synthesis temperature is reduced under the condition of ensuring complete nitriding, so that the synthesized AlN powder is homogeneous and easy to crush. The AlN powder prepared by the method can reduce the crushing time of the subsequent process and reduce the impurities introduced by a crushing medium, thereby ensuring the high purity of the AlN powder after fine grinding, and compared with the common process, the crushing time of the AlN powder is reduced by about 30 percent under the condition of the same grain diameter, and the oxygen content is reduced by about 0.2 percent.

Description

Preparation method of AlN powder

Technical Field

The invention relates to the technical field of powder preparation, in particular to a preparation method of homogeneous high-purity AlN powder.

Background

With the development of high power semiconductor devices for LED lighting, electric locomotives, hybrid cars, and the like, high thermal conductivity aluminum nitride ceramic substrates are gaining market acceptance, and are beginning to be used in large scale as heat dissipation materials replacing glass fibers and alumina ceramic substrates. The theoretical thermal conductivity of aluminum nitride is 320W/mK, but the highest reported value of the thermal conductivity of the aluminum nitride ceramic substrate is about 270W/mK, while the thermal conductivity of the aluminum nitride ceramic substrate in practical use is generally between 120-190W/mK.

For the production of the aluminum nitride ceramic substrate, the aluminum nitride powder is usually mixed with a corresponding sintering aid, and then the process steps of dry pressing or wet forming (casting, grouting, etc.), binder removal, sintering, etc. are performed. Each process step affects the thermal conductivity of the final product, which causes the thermal conductivity of the aluminum nitride ceramic substrate to be far lower than the theoretical value in actual use.

Among the links affecting the thermal conductivity, as the most basic aluminum nitride powder, the performance of the aluminum nitride powder has a decisive influence on the subsequent production links and the thermal conductivity of the final product. The properties of AlN powder include the particle size, specific surface, grain shape, metal impurity content, surface oxygen content and the like. Many studies have shown that metal impurities in AlN powder and oxygen adsorbed on the surface cause defects due to solid solution into aluminum nitride crystal lattices during sintering, thereby affecting phonon propagation, resulting in a decrease in thermal conductivity. Therefore, attempts to reduce the metal impurities and oxygen content in the AlN powder have become important approaches to improving the characteristics of the AlN powder.

The preparation process of the aluminum nitride powder mainly comprises two technical routes, one is a carbon thermal reduction nitridation method, the method uses alumina powder and carbon powder as raw materials, nitrogen is introduced at high temperature, and the aluminum nitride powder is obtained by a reaction formula 1, but the process needs higher reaction temperature and subsequent treatment of unreacted carbon powder; the other method is an aluminum powder nitriding method, namely, metal aluminum powder (aluminum liquid) and nitrogen are used to obtain an aluminum nitride primary product through a reaction formula 2 at a certain temperature, and then the aluminum nitride primary product is crushed and ground to obtain powder.

2Al₂O₃+3C+2N₂＝4AlN+3CO₂ (1)

2Al+N₂＝2AlN (2)

In specific implementation, the aluminum powder nitriding method can be divided into a combustion synthesis method (external heating), a self-propagating combustion synthesis method (external non-heating), an aluminum liquid nitriding method and the like. When the aluminum powder/liquid reacts with nitrogen, a large amount of heat is evolved, resulting in a sharp temperature rise. This creates two problems: (1) the over-high temperature causes the synthesized aluminum nitride to be locally decomposed into yellow/brown aluminum; (2) the excessive temperature causes partial sintering of the aluminum nitride locally, and the aluminum nitride becomes a high-hardness block.

Yellow/brown aluminum as an impurity, which needs to be avoided with effort; the high hardness block aluminum nitride needs a long time pulverization process in the subsequent process to be ground into required fine powder (usually about 1 μm). During the pulverization, the pulverizing media (metal or ceramic) is worn away, and the worn metal or metal oxide enters the AlN powder to affect the purity. Further, the long-term grinding also increases the contact time between the AlN powder and the air, and since AlN is easily deliquesced, the chance of increasing the surface hydrated oxygen content thereof upon exposure to the air for a long time is also increased (formula 3).

AlN+3H₂O＝Al(OH)₃+NH₃ (3)

In order to obtain a loose AlN block so as to facilitate the subsequent crushing and grinding process, the nitrogen pressure can be reduced in the synthesis process, but the nitrogen pressure can possibly cause incomplete nitridation of aluminum powder at the bottom of the crucible; the proportion of the aluminum nitride powder serving as a diluent in the raw materials can also be increased, but the production efficiency is reduced; or, when an ammonium salt (ammonium chloride, ammonium fluoride, etc.) is added to the raw material, F, Cl ions may remain, and the decomposed acid-base gas may damage the equipment.

Therefore, how to obtain homogeneous and easily-crushed aluminum nitride blocks in the synthesis process, that is, on the premise of ensuring that aluminum powder and nitrogen gas fully react to synthesize aluminum nitride, reducing the reaction temperature becomes an important subject for enterprises and researchers who use the aluminum powder nitriding process to produce AlN powder.

Disclosure of Invention

The invention aims to provide a method for obtaining homogeneous and high-purity aluminum nitride powder by reducing the synthesis temperature of aluminum nitride.

The invention aims to reduce the reaction speed and the reaction temperature by mixing a small amount of inert gas argon into high-pressure nitrogen serving as a reaction raw material in an aluminum powder nitriding synthesis process, thereby avoiding the phenomenon of aluminum nitride sintering or redissolution caused by overhigh local temperature, obtaining an aluminum nitride block with high purity and easy crushing, and realizing the aim of preparing homogeneous high-purity AlN powder.

In order to achieve the purpose, the invention provides a preparation method of AlN powder, which adopts an aluminum powder nitriding method and has the reaction formula as shown in the specification:

2Al+N₂＝2AlN，

in this, argon gas was mixed into high-pressure nitrogen gas as a reaction raw material.

Further, the amount of argon gas mixed is 0.01 to 10%, preferably 1 to 5% of the total volume of nitrogen gas and argon gas.

Further, the aluminum powder nitriding method includes an external heating combustion synthesis method, a self-propagating combustion synthesis method, and an aluminum liquid nitriding synthesis method, and more preferably an external heating combustion synthesis method and a self-propagating combustion synthesis method. The aluminum liquid liquefaction synthesis method is that Al liquid drops are sprayed out to directly react with nitrogen.

Further, the preparation method of the AlN powder comprises the following steps: putting aluminum powder into a high-purity graphite crucible or an aluminum nitride ceramic crucible, putting the crucible into a synthetic furnace, introducing argon, and then introducing high-pressure nitrogen to 0.1-5 MPa. Wherein the high purity graphite crucible can be made of isostatic pressure graphite with a purity of 10 ppm.

Further, in the method for producing the AlN powder, the maximum temperature of the reaction region is maintained at 2400 ℃ or lower.

Generally, in the aluminum powder nitriding process, after the temperature is raised, nitrogen gas and aluminum powder in reactants generate violent exothermic reaction, so that the temperature of the reactants is rapidly raised to be close to 3000 ℃. However, in the present invention, the severity of the reaction is reduced by the presence of argon gas, and the maximum temperature in the reaction zone is maintained at 2400 ℃ or lower.

After the reaction is finished, the obtained product is a soft block, and the color is uniform without yellow/brown impurities. After being crushed, the offwhite powder is obtained. Single phase AlN was analyzed by XRD.

Further, the AlN powder obtained as described above was mixed with 5 wt% of Y based on the AlN powder₂O₃Putting into a nylon ball milling barrel, taking absolute ethyl alcohol as a dispersion medium, and Si₃N₄The ball is a grinding medium ball, ball milling is carried out for 12h, slurry is taken out, dried and sieved by a 50-mesh screen, and sintering is carried out after dry pressing forming.

The mixed powder can be sintered and compacted under no pressure at 1780-1950 ℃ after dry pressing and forming, and the density of a sintered sample at 1850 ℃ is 3.33g/cm³The thermal conductivity was 185W/mK.

Further, in the method for producing an AlN powder according to the present invention, AlN powder may be used in place of a part of Al powder, i.e., AlN powder may be mixed with Al powder as a diluent. Wherein the weight ratio of the Al powder to the AlN powder is (45-65): (35-55), preferably (50-60): (40-50), more preferably 55: 45. The dilution effect of the AlN powder further dilutes the intensity of the reaction, so that the reaction can be more completely carried out, and the AlN powder with higher purity can be obtained.

The technical scheme of the invention has the following advantages:

the method can synthesize the soft aluminum nitride block without introducing redundant impurities or damaging equipment, and can ensure enough nitrogen in the synthesis process and reduce local high temperature of a synthesis area by introducing a small amount of argon in the synthesis process, so that the synthesized aluminum nitride is homogeneous, very soft, high in purity and easy to crush.

The AlN powder prepared by the method can reduce the crushing and grinding time of the subsequent process and reduce the impurities introduced by the grinding medium, thereby ensuring the high purity of the AlN powder after fine grinding. The pulverization time is reduced by about 30% and the oxygen content is reduced by about 0.2% for the same particle size as compared with the conventional process, and the thermal conductivity of the final AlN sintered body can be improved.

The method reduces the direct reaction speed of the aluminum powder and the nitrogen, thereby reducing the temperature of a reaction area and avoiding the following steps: (1) the synthesized aluminum nitride is decomposed again to form yellow brown/black hard block inclusion; (2) the synthesized aluminum nitride is partially sintered and forms hard lumps which are hard to crush. The synthesized aluminum nitride block has uniform overall color (close to white) and no impurities; the block body has no hard block and is easy to break.

In addition, the method of the invention has no residual impurities such as Cl, F and the like, and has no damage to equipment caused by gases such as HCl and the like. Compared with the traditional process, the method has the advantages of no great change, easy operation, large yield, no pollution and suitability for industrial production.

Drawings

FIG. 1a shows the morphology of the aluminum nitride bulk synthesized in comparative example 1, which contains yellow/brown inclusions and hard lumps.

FIG. 1b shows the morphology of the bulk of the aluminum nitride synthesized in example 1, which does not contain yellow/brown inclusions and hard lumps.

FIG. 2 shows XRD of the pulverized powders of the products of example 1 and comparative example 1.

Detailed Description

The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.

Comparative example 1

5kg of metal Al powder and aluminum nitride powder in total are mixed according to the weight ratio of 55:45, placing the mixture into a porous graphite container after uniformly mixing, and then placing the container into a self-propagating reaction synthesis furnace. After the furnace is closed, vacuumizing to 20Pa, introducing high-purity nitrogen to 4MPa, then electrifying to ignite the ignition agent to induce the metal aluminum powder in the raw material powder to react with the introduced nitrogen, and ensuring that the subsequent reaction is continuously carried out by the heat generated by the reaction until all the metal aluminum powder and the nitrogen react. The fire-igniting agent is composed of metal Ti powder. The entire synthesis reaction was continued for about 10min and then cooled to room temperature. The resultant aluminum nitride was observed to appear as a soft mass overall after opening the furnace, but with a local yellow/brown inclusion (fig. 1a) which was hard like stone. The XRD result of the powder, which was ball-milled to a D50 ═ 5 μm size, showed that the powder contained a trace amount of metallic aluminum (fig. 2), required 24 hours.

Example 1

5kg of metal Al powder and aluminum nitride powder in total are mixed according to the weight ratio of 55:45, placing the mixture into a porous graphite container after uniformly mixing, and then placing the container into a self-propagating reaction synthesis furnace. After the furnace is closed, vacuumizing to 20Pa, and then, carrying out vacuum drying according to the following ratio of 99: 1 to 4MPa, then electrifying to ignite the ignition agent to induce the metal aluminum powder in the raw material powder to react with the charged nitrogen, and generating heat to ensure that the subsequent reaction is continuously carried out until all the metal aluminum powder and the nitrogen react. The fire-igniting agent is composed of metal Ti powder. The entire synthesis reaction was continued for about 10min and then cooled to room temperature. The synthesized aluminum nitride was observed to appear as an overall soft mass after opening the furnace, and no hard areas containing yellow/brown inclusions were visible after breaking (fig. 1 b). After the synthesis, 16 hours were required for the powder obtained by pulverizing the powder to D50 ═ 5 μm, and XRD results of the powder showed that all of AlN was single-phase (fig. 2).

Example 2

5kg of metal Al powder and aluminum nitride powder in total are mixed according to the weight ratio of 60: 40, placing the mixture into a porous graphite container after uniformly mixing, and then placing the container into a self-propagating reaction synthesis furnace. After the furnace is closed, vacuumizing to 20Pa, and then performing vacuum treatment according to the following steps of 98: 2, introducing high-purity argon and high-purity nitrogen successively to enable the mixed gas to reach 4MPa, then electrifying to ignite the ignition agent to induce the metal aluminum powder in the raw material powder to react with the introduced nitrogen, and ensuring the subsequent reaction to be continuously carried out by the heat generated by the reaction until all the metal aluminum powder and the nitrogen react. The fire-igniting agent is composed of metal Ti powder. The entire synthesis reaction was continued for about 10min and then cooled to room temperature. After the furnace is opened, the synthesized aluminum nitride is observed to be in a whole soft block shape, and hard areas containing yellow/brown impurities are not seen after the aluminum nitride is knocked to be broken. After the synthesis, 15 hours were required for the powder to be pulverized to D50 ═ 5 μm, and XRD results of the powder showed that all of AlN was single-phase.

Example 3

5kg of metal Al powder and aluminum nitride powder in total are mixed according to the weight ratio of 65: 35, after being uniformly mixed, the mixture is placed in a porous graphite container and then is placed in a self-propagating reaction synthesis furnace. After the furnace is closed, vacuumizing to 20Pa, and then, carrying out vacuum drying according to the following steps of 95: and 5, introducing high-purity argon and high-purity nitrogen successively according to the volume ratio to ensure that the mixed gas is 4MPa, then electrifying to ignite the ignition agent to induce the metal aluminum powder in the raw material powder to react with the introduced nitrogen, and ensuring that the subsequent reaction is continuously carried out by the heat generated by the reaction until all the metal aluminum powder and the nitrogen react. The fire-igniting agent is composed of metal Ti powder. The entire synthesis reaction was continued for about 10min and then cooled to room temperature. After the furnace is opened, the synthesized aluminum nitride is observed to be in a whole soft block shape, and hard areas containing yellow/brown impurities are not seen after the aluminum nitride is knocked to be broken. After the synthesis, it took 13 hours to crush the powder to D50 ═ 5 μm, and the XRD results of the powder showed that all of AlN was a single phase.

Example 4

5kg of metal Al powder and aluminum nitride powder in total are mixed according to the weight ratio of 55:45, placing the mixture into a porous graphite container after uniformly mixing, and then placing the container into a self-propagating reaction synthesis furnace. After the furnace is closed, vacuumizing to 20Pa, and then performing vacuum treatment according to the following steps of: 10 to 4MPa, then electrifying to ignite the ignition agent to induce the metal aluminum powder in the raw material powder to react with the charged nitrogen, and generating heat to ensure that the subsequent reaction is continuously carried out until all the metal aluminum powder and the nitrogen react. The fire-igniting agent is composed of metal Ti powder. The entire synthesis reaction was continued for about 10min and then cooled to room temperature. The synthesized aluminum nitride is observed to be in a whole soft block shape after the furnace is opened, no hard area containing yellow/brown inclusion is seen after the aluminum nitride is knocked to be broken, but brown powder is arranged at the bottom of the crucible, and XRD results of the powder show that the powder contains a small amount of unreacted metal Al powder.

The following table 1 shows the results of observation and test of the products of examples 1 to 4 and comparative example 1.

TABLE 1

As can be seen from Table 1 in combination with the drawings, in comparative example 1, argon gas was not introduced, the product had a large amount of impurities, hard lumps which were difficult to crush and unreacted Al, and the oxygen content in the powder was high. Examples 1-3, in which 1-5% argon was introduced, produced without inclusions/hard lumps and unreacted Al, and the oxygen content of the powder was relatively low. In example 4, 10% argon was introduced, and unreacted Al was produced although there were no inclusions/hard lumps in the product. In summary, the introduction of 1-5% argon into the mixed gas has a better technical effect.

Examples of the experiments

AlN powders having a D50 of 5 μm obtained in examples 1 to 3 were mixed with 5 wt% of Y based on the AlN powder₂O₃Putting into a nylon ball milling barrel, taking absolute ethyl alcohol as a dispersion medium, and Si₃N₄The ball is a grinding medium ball, ball milling is carried out for 12h, slurry is taken out, is dried, is sieved by a 50-mesh screen, is subjected to dry pressing and molding, and is sintered and compacted under no pressure at the temperature of 1850 ℃. The sintered samples obtained have densities close to each other, differing by no more than 0.08%, and a mean density value of 3.33g/cm³(ii) a And the thermal conductivity of the obtained sintered sample is close to that of the sintered sample, the difference is not more than 0.12 percent, and the average value of the thermal conductivity is 185W/mK.

Although the invention has been described in detail above with reference to a general description and specific examples, it will be apparent to one skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.

Claims

1. A preparation method of AlN powder adopts a self-propagating combustion aluminum powder nitriding synthesis method, and the reaction formula is shown as follows:

2Al+N₂＝2AlN，

characterized in that argon gas is mixed into high-pressure nitrogen gas as a reaction raw material; the amount of the mixed argon is 1 to 5 percent of the total volume of the nitrogen and the argon; the preparation method comprises the following steps: putting aluminum powder into a high-purity graphite crucible or an aluminum nitride ceramic crucible, putting the crucible into a synthetic furnace, closing the furnace, vacuumizing to 20Pa, introducing argon, and then introducing high-pressure nitrogen to 4 MPa; the maximum temperature in the reaction zone in the preparation process was maintained at 2400 ℃.

2. The production method according to claim 1, wherein AlN powder is substituted for a part of Al powder to dilute the severity of the reaction.

3. The method according to claim 2, wherein the weight ratio of the Al powder to the AlN powder is (45-65): (35-55).

4. The method of manufacturing according to claim 1, further comprising: mixing the obtained AlN powder with 5 wt% of Y based on the AlN powder₂O₃Putting into a nylon ball milling barrel, taking absolute ethyl alcohol as a dispersion medium, and Si₃N₄The ball is a grinding medium ball, ball milling is carried out for 12h, slurry is taken out, dried and sieved by a 50-mesh screen, and sintering is carried out after dry pressing forming.

5. The production method according to claim 4, wherein the obtained AlN and Y are mixed₂O₃The mixed powder is subjected to dry pressing and molding, and is subjected to pressureless sintering and densification at the temperature of 1780-1950 ℃, wherein the density of a sintered sample at 1850 ℃ is 3.33g/cm³The thermal conductivity was 185W/mK.