Preparation process of superfine aluminum nitride powder based on high and low temperature carbon thermal reduction method

Abstract

This invention discloses a process for preparing ultrafine aluminum nitride powder based on a high-low temperature carbothermal reduction method. Nano-Al₂O₃ and carbon black are used as raw materials, and a nonionic surfactant containing carbon, hydrogen, and oxygen elements is added as a dispersant. The mixture is stirred to form a viscous slurry, which is then ball-milled for a certain period to ensure uniform mixing. The slurry is then dried in a drying oven, and ultrafine aluminum nitride powder is obtained through a high-low temperature composite carbothermal reduction synthesis reaction. This invention employs a high-low temperature composite carbothermal reduction synthesis process. First, an aluminum nitride layer is formed on the surface of alumina particles at a high temperature of 1580–1630℃ for a short time. This aluminum nitride layer inhibits the sintering and agglomeration of alumina particles. Then, the temperature is lowered to a lower temperature of 1450–1550℃ for a long time, ultimately resulting in the in-situ nitridation of nano-alumina to generate ultrafine aluminum nitride.

Description

Technical Field

[0001] This invention belongs to the field of ceramic powder preparation and relates to a process for preparing ultrafine aluminum nitride powder based on high and low temperature carbothermal reduction method. Background Technology

[0002] Aluminum nitride (AlN) possesses high thermal conductivity, high resistivity, low coefficient of thermal expansion, non-toxicity, and excellent chemical stability. It is used as a heat dissipation material, semiconductor substrate, compound semiconductor substrate, and reinforcing phase in metal composites. AlN is also an excellent advanced ceramic material. A crucial prerequisite for preparing high-performance ceramics is the synthesis of high-quality powders. The particle size and purity of AlN powder raw materials play a decisive role in the performance of AlN ceramics, especially the oxygen content, which has a significant impact on thermal conductivity. Therefore, to obtain AlN ceramics with high thermal conductivity, high density, and excellent performance, it is essential to first prepare high-purity, fine-grained, narrow-size-distribution, and stable powders. Currently, the most commonly used method for preparing aluminum nitride powder is the carbothermic reduction method.

[0003] The carbothermic reduction method uses alumina powder and carbon powder as raw materials, and reduces and nitrids them to produce aluminum nitride under high temperature conditions and a flowing nitrogen atmosphere. The main reaction formulas are as follows:

[0004] Al2O3(s)+3C(s)+N2(g)→2AlN(s)+3CO(g)

[0005] The carbothermal reduction method for preparing AlN has the advantages of abundant raw material sources and suitability for large-scale production. The powder synthesized by carbothermal reduction has high purity, stable performance, and fine, uniform particle size, making it easy to shape and sinter, thus becoming the main method for preparing aluminum nitride powder. Currently, about 70% of commercially available aluminum nitride powder is produced using this method. However, this method also has shortcomings. Since the raw materials used are usually extremely fine alumina powder and carbon black powder, the particles are prone to agglomeration, making it difficult to thoroughly mix them uniformly. Therefore, to obtain carbothermal reduced aluminum nitride powder with a high degree of nitride, high temperatures and times are required. Furthermore, high synthesis temperatures and long synthesis times can cause severe agglomeration of the aluminum nitride powder product. To reduce this agglomeration, the aluminum nitride powder product needs to be ground for a long time during application. This grinding not only introduces more impurities but also reduces production efficiency and increases costs. For example, patent CN111825066A reports a method for preparing nano-aluminum nitride powder using aluminum hydroxide powder, carbon powder, and aluminum nitride powder as raw materials, and nitrogen and propane as nitrogen sources. This method can prepare high-quality aluminum nitride powder, but it cannot guarantee good dispersibility and small particle size, which is not conducive to the production of high-performance aluminum nitride ceramics. Furthermore, the inventors previously proposed a Chinese patent CN113292053A for a carbothermal reduction process to prepare highly dispersible aluminum nitride powder based on polymer dispersants. The dispersants used are polyvinyl alcohol (PVA), polyacrylic acid (PAA), and polyethylene glycol (PEG), etc. The main advantage is that during the drying process after mixing, as water evaporates, the polymer chains form a cross-linked network. This cross-linked network prevents the alumina and carbon powder from agglomerating again during prolonged static drying due to large differences in density and size. However, because nano-alumina and carbon powder have small size differences and high surface activity, it is difficult to achieve a dispersion effect similar to micron-sized alumina using only polymer dispersants. Without ball milling, the product obtained is still in the micron range, not reaching the submicron range. Although in-situ synthesis of nano-alumina can further reduce the particle size of the product, the nano-alumina powder that is not fully separated by carbon powder is prone to sintering and agglomeration before nitriding, which leads to an increase in the particle size of the product.

[0006] This invention addresses the difficulty in preparing low-agglomeration ultrafine (submicron size) aluminum nitride powder through carbothermal reduction synthesis. It uses nano-alumina as raw material, precisely controls the type and amount of dispersant, and employs a high-low temperature composite synthesis process to obtain submicron nano-aluminum nitride powder with fine size and good dispersibility. Summary of the Invention

[0007] One objective of this invention is to address the shortcomings of existing processes by providing a process for preparing ultrafine aluminum nitride powder based on high and low temperature carbothermal reduction.

[0008] The present invention adopts the following technical solution:

[0009] Nano-Al2O3 (20-100nm) and carbon black (10-100nm) are used as raw materials. A certain proportion of dispersant aqueous solution is added and mixed to form a viscous slurry. Then, the slurry is ball-milled for a certain time to make it uniformly mixed. After that, the mixed slurry is placed in a drying oven to dry. Then, aluminum nitride and carbon mixed powder is obtained by high and low temperature composite carbothermal reduction synthesis. Finally, aluminum nitride powder is obtained by removing excess carbon.

[0010] The specific steps are as follows:

[0011] Step (1): Add nano alumina and carbon black to a ball mill jar and mix them evenly to obtain a mixture; add the dispersant to deionized water, mix it after heating in a water bath, and dissolve it into a dispersant solution; then add the dispersant solution to the above mixture and stir to form a viscous slurry A.

[0012] Preferably, the dispersant is a nonionic surfactant containing carbon, hydrogen, and oxygen elements, specifically at least one of polyoxyethylene sorbitan monolaurate (Tween), glyceryl monostearate (GMS), and alkylphenol polyoxyethylene ether (OP).

[0013] Preferably, the median diameter of the nano-alumina is 20–100 nm; the carbon black is an ultrafine powder with a median diameter of 10–100 nm.

[0014] Preferably, the mass ratio of nano-alumina, carbon black and dispersant in the viscous slurry A is 100:(60-80):(6.4-12.8).

[0015] Step (2): Place the above viscous slurry A into a planetary ball mill for ball milling, and after mixing, obtain a viscous slurry B with a certain viscosity and uniform mixing.

[0016] Preferably, the ball mill rotation speed is 100-160 r / min and the ball milling time is 2-8 h.

[0017] Step (3): Place the viscous slurry B into a drying oven and dry it to obtain powder C.

[0018] Preferably, the drying temperature is 70–100°C.

[0019] Step (4): Powder C is placed in a graphite sintering furnace and synthesized by high and low temperature carbothermic reduction under flowing nitrogen to obtain nitrided powder D.

[0020] Preferably, the high-low temperature composite carbothermal reduction synthesis process involves first synthesizing at a high temperature of 1580–1630℃ for 10–40 minutes, and then cooling to 1450–1550℃ for 2–8 hours.

[0021] Step (5): Place the nitrided powder D into a high-temperature box furnace and keep it at 600-750℃ for 4-8 hours to remove excess carbon and obtain the raw material powder for firing aluminum nitride ceramics.

[0022] Another objective of this invention is to provide nano-aluminum nitride powder prepared by the above process.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] (1) The raw material is nano alumina powder. Since in-situ carbothermic reduction synthesis occurs, it provides a reaction basis for the direct generation of ultrafine aluminum nitride powder.

[0025] (2) Nano-alumina powder is prone to agglomeration. Compared with existing methods for dispersing submicron or micron alumina, this invention uses non-ionic surfactants containing carbon, hydrogen and oxygen elements and precisely controls their type and amount, which has a better dispersion effect on nano-alumina.

[0026] (3) Since carbothermal reduction synthesis requires high temperature, and high temperature can easily cause nano-alumina raw materials to sinter and agglomerate before synthesizing aluminum nitride, the present invention adopts a high and low temperature composite carbothermal reduction synthesis process. First, an aluminum nitride layer is formed on the surface of alumina particles at a high temperature of 1580-1630℃ for a short time (10-40 min). The aluminum nitride layer is used to prevent the sintering and agglomeration of alumina particles. Then, the temperature is reduced to a lower temperature of 1450-1550℃ and nitrided for a long time (2-8 h), and finally the nano-alumina is nitrided in situ to generate ultrafine aluminum nitride. Attached Figure Description

[0027] Figure 1 This is a morphology diagram of the synthesized powder in Example 4.

[0028] Figure 2 This is the volume distribution-differential distribution curve of the synthesized powder in Example 4.

[0029] Figure 3 This is the volume distribution-cumulative distribution curve of the synthesized powder in Example 4.

[0030] Figure 4 This is a morphology diagram of the synthesized powder in Example 5.

[0031] Figure 5 This is a morphology diagram of the synthesized powder in Comparative Example 1.

[0032] Figure 6This is a volume distribution-differential distribution curve of the synthesized powder in Comparative Example 1.

[0033] Figure 7 This is a volume distribution-cumulative distribution curve of the synthesized powder in Comparative Example 1.

[0034] Figure 8 This is a morphology diagram of the synthesized powder in Comparative Example 2.

[0035] Figure 9 This is a morphology diagram of the synthesized powder in Comparative Example 3. Detailed Implementation

[0036] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. In the following embodiments, the median diameter of nano-alumina is 20-100 nm; the carbon black is an ultrafine powder with a median diameter of 10-100 nm.

[0037] Example 1

[0038] First, weigh out a certain amount of nano-alumina, carbon black, and OP in a mass ratio of 100:60:6.4. Add OP to deionized water, mix and heat in a water bath to dissolve into an OP solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir evenly to obtain a mixture, then stir with the OP solution to form a viscous slurry A. Add viscous slurry A to a ball mill jar, and ball mill it at 100 r / min for 8 hours using deionized water as the milling medium to obtain a uniformly mixed viscous slurry B. Then separate slurry B and the milling beads through a sieve. Place the slurry separated from the milling beads in a 70℃ oven to dry the moisture, obtaining a dry mixed powder. Then place the powder in a graphite sintering furnace, calcine at 1580℃ for 40 minutes under a flowing nitrogen atmosphere, then cool to 1550℃ for 2 hours to obtain the nitrided product. The nitriding product was then placed in a box furnace and held at 600℃ for 8 hours to remove carbon, ultimately yielding well-dispersed ultrafine aluminum nitride powder. The aluminum nitride particles exhibited low agglomeration, small agglomerate size, uniform particle distribution, and submicron particle size.

[0039] Example 2

[0040] First, weigh out a certain amount of nano-alumina, carbon black, and GMS in a mass ratio of 100:60:6.4. Add GMS to deionized water, mix and heat in a water bath to dissolve it into a GMS solution. Add the nano-alumina and carbon black to a ball mill jar, mix and stir until homogeneous to obtain a mixture, then stir with the GMS solution to form a viscous slurry A. Add the viscous slurry A to a ball mill jar, and ball mill it on a planetary ball mill at 100 r / min for 8 hours using deionized water as the milling medium to obtain a homogeneous viscous slurry B. Next, separate slurry B and the milling beads through a sieve. Place the slurry separated from the milling beads in a 70℃ oven to dry the moisture, obtaining a dry mixed powder. Then, place the powder in a graphite sintering furnace, calcine it at 1580℃ for 40 minutes under a flowing nitrogen atmosphere, and then cool it to 1550℃ for 2 hours to obtain the nitrided product. The nitriding product was then placed in a box furnace and held at 600℃ for 8 hours to remove carbon, ultimately yielding aluminum nitride powder with small particle size and good dispersibility. The aluminum nitride particles exhibited low agglomeration, small agglomerate size, uniform particle distribution, and a particle size in the submicron range.

[0041] Example 3

[0042] First, weigh out a certain amount of nano-alumina, carbon black, and Tween in a mass ratio of 100:80:12.8. Add Tween to deionized water, mix and heat in a water bath to dissolve it into a Tween solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir evenly to obtain a mixture, then stir with the Tween solution to form a viscous slurry A. Add viscous slurry A to a ball mill jar, and ball mill it on a planetary ball mill at 140 r / min for 6 h using deionized water as the milling medium to obtain a uniformly mixed viscous slurry B. Then separate slurry B and the milling beads through a sieve. Place the slurry separated from the milling beads in an oven at 100℃ to dry the moisture, obtaining a dry mixed powder. Then place the powder in a graphite sintering furnace, calcine it at 1630℃ for 10 min under a flowing nitrogen atmosphere, and then cool it to 1450℃ for 8 h to obtain the nitrided product. The nitriding product was then placed in a box furnace and kept at 750°C for 4 hours to remove carbon, ultimately yielding well-dispersed ultrafine aluminum nitride powder.

[0043] Example 4

[0044] First, weigh out a certain amount of nano-alumina, carbon black, GMS, and OP in a mass ratio of 100:60:6.4:6.4. Add GMS and OP to deionized water, mix by heating in a water bath, and dissolve to form a dispersant solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir until homogeneous to obtain a mixture, then stir with the dispersant solution to form a viscous slurry A. Add viscous slurry A to a ball mill jar, and ball mill it on a planetary ball mill at 160 r / min for 4 hours using deionized water as the milling medium to obtain a homogeneous viscous slurry B. Next, separate slurry B and the milling beads through a sieve. Place the slurry separated from the milling beads in an oven at 80℃ to dry the moisture, obtaining a dry mixed powder. Then, place the powder in a graphite sintering furnace, calcine at 1610℃ for 15 minutes under a flowing nitrogen atmosphere, and then cool to 1550℃ for 4 hours to obtain the nitrided product. The nitriding product was then placed in a box furnace and held at 650℃ for 4 hours for decarburization, ultimately yielding aluminum nitride powder with small particle size and good dispersibility. The microstructure and particle size distribution of the obtained product were observed using scanning electron microscopy and a particle size analyzer, respectively. (See attached image) Figure 1 , 2 As shown in Figure 3, the aluminum nitride particles exhibit very low agglomeration, with primary particle sizes ranging from 0.2 to 1 μm (submicron). As shown in Appendix Table 1, the particle volume distribution range is narrow, the number of primary particles is large, and particles smaller than 1 μm account for 50.17%.

[0045] Table 1. Cumulative table of size ranges of the synthesized powder in Example 4

[0046] Particle size / μm ≤0.2 ≤0.5 ≤1.0 ≤2.0 ≤5.0 Proportion 1.32 12.96 50.17 94.53 100

[0047] Example 5

[0048] First, weigh out a certain amount of nano-alumina, carbon black, GMS, and Tween in a mass ratio of 100:60:3.2:9.6. Add GMS and Tween to deionized water, mix by water bath heating, and dissolve to form a dispersant solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir until homogeneous to obtain a mixture, then stir with the dispersant solution to form a viscous slurry A. Use deionized water as the milling medium in a planetary ball mill at 160 r / min for 4 hours to obtain a homogeneous viscous slurry B. Next, separate slurry B and the milling beads through a sieve. Place the slurry separated from the milling beads in an 80℃ oven to dry the moisture, obtaining a dry mixed powder. Then, place the powder in a graphite sintering furnace and calcine at 1600℃ for 25 minutes under a flowing nitrogen atmosphere, then cool to 1450℃ and calcine for 8 hours to obtain the nitrided product. The nitrided product was then placed in a box furnace and held at 650℃ for 4 hours for decarburization, ultimately yielding aluminum nitride powder with small particle size and good dispersibility. The microstructure and particle size distribution of the obtained product were observed using scanning electron microscopy, as shown in the attached figure. Figure 4 As shown, aluminum nitride particles exhibit low agglomeration, small agglomerate size, uniform particle distribution, and particle size in the submicron range.

[0049] Comparative Example 1

[0050] First, weigh out a certain amount of nano-alumina, carbon black, GMS, and OP in a mass ratio of 100:60:6.4:6.4. Add GMS and OP to deionized water, mix by water bath heating, and dissolve to form a dispersant solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir until homogeneous to obtain a mixture, then stir with the dispersant solution to form a viscous slurry A. Add viscous slurry A to a ball mill jar, and ball mill at 160 r / min for 4 hours using deionized water as the milling medium on a planetary ball mill to obtain a homogeneous viscous slurry. Next, separate the slurry and milling beads through a sieve. Place the slurry separated from the milling beads in an 80℃ oven to dry the moisture, obtaining a dry mixed powder. Then, place the powder in a graphite sintering furnace and calcine at 1550℃ for 4 hours under a flowing nitrogen atmosphere to obtain a nitrided product. The nitrided product was then placed in a box furnace and held at 650℃ for 4 hours for decarburization. The microstructure and particle size distribution of the resulting powder product were observed using a scanning electron microscope and a particle size analyzer, respectively. (See attached image) Figure 7 As shown, the aluminum nitride particles exhibit both a high degree of agglomeration and relatively large agglomerate size. (See attached image.) Figure 5 , 6 As shown in Tables 7 and 2, the particle volume distribution is wide, the number of primary particles is small, particles smaller than 1 μm account for only 7.92%, and aggregates of 2–5 μm account for 28.92%.

[0051] Table 2. Cumulative table of size ranges for synthetic powders in Comparative Example 1.

[0052] Particle size / μm ≤0.2 ≤0.5 ≤1.0 ≤2.0 ≤5.0 ≤10.0 ≤20.0 ≤45 Proportion 0 0.61 7.92 18.46 47.38 76.91 96.17 100

[0053] Comparative Example 2

[0054] First, weigh out a certain amount of nano-alumina, carbon black, GMS, and Tween in a mass ratio of 100:60:6.4:6.4. Add GMS and Tween to deionized water, mix them in a water bath, and dissolve them to form a dispersant solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir evenly to obtain a mixture, and then stir with the dispersant solution to form a viscous slurry A. Add viscous slurry A to a ball mill jar, and ball mill it on a planetary ball mill at 160 r / min for 4 hours using deionized water as the milling medium to obtain a uniformly mixed viscous slurry. Then, separate the slurry and milling beads through a sieve. Place the slurry separated from the milling beads in an oven at 80℃ to dry the moisture, obtaining a dry mixed powder. Then, place the powder in a graphite sintering furnace and calcine it at 1450℃ for 8 hours under a flowing nitrogen atmosphere to obtain a nitrided product. The nitrided product was then placed in a box furnace and held at 650℃ for 4 hours for decarburization. The microstructure of the resulting powder product was observed using a scanning electron microscope, as shown in the attached figure. Figure 8 As shown, the aluminum nitride particles exhibit both a large degree of agglomeration and agglomerate size.

[0055] Comparative Example 3

[0056] First, weigh out a certain amount of nano-alumina, carbon black, GMS, and OP in a mass ratio of 100:60:3:3. Add GMS and OP to deionized water, mix by heating in a water bath, and dissolve to form a dispersant solution. Add nano-alumina and carbon black to a ball mill jar, mix and stir until homogeneous to obtain a mixture, then stir with the dispersant solution to form a viscous slurry A. Add viscous slurry A to a ball mill jar, and ball mill it on a planetary ball mill at 160 r / min for 4 hours using deionized water as the milling medium to obtain a homogeneous viscous slurry. Next, separate the slurry and milling beads through a sieve. Place the slurry separated from the milling beads in an 80℃ oven to dry the moisture, obtaining a dry mixed powder. Then, place the powder in a graphite sintering furnace, calcine at 1610℃ for 15 minutes under a flowing nitrogen atmosphere, and then cool to 1550℃ for 4 hours to obtain the nitrided product. The nitrided product was then placed in a box furnace and held at 650℃ for 4 hours for decarburization. The microstructure of the product was observed using a scanning electron microscope, as shown in the attached figure. Figure 9 As shown, the aluminum nitride particles exhibit both a large degree of agglomeration and agglomerate size.

[0057] The above embodiments are not intended to limit the present invention, and the present invention is not limited to the above embodiments. Any embodiment that meets the requirements of the present invention is within the protection scope of the present invention.

Claims

1. A process for preparing ultrafine aluminum nitride powder based on high and low temperature carbothermal reduction method, characterized in that Includes the following steps: Step (1): Add nano-alumina and carbon black to a ball mill jar, mix and stir evenly to obtain a mixture; dissolve the dispersant in deionized water to obtain a dispersant solution; then add the dispersant solution to the above mixture and stir to form a viscous slurry A; wherein the mass ratio of nano-alumina, carbon black and dispersant in the viscous slurry A is 100:60~80:6.4~12.8; The dispersant is a nonionic surfactant containing carbon, hydrogen and oxygen elements, specifically at least one of polyoxyethylene sorbitan monolaurate, glyceryl monostearate, and alkylphenol polyoxyethylene ether. Step (2): Place the above viscous slurry A into a planetary ball mill and ball mill for 2-8 hours. After mixing, viscous slurry B is obtained. Step (3): Place the viscous slurry B into a drying oven and dry it to obtain powder C; Step (4): Powder C is placed in a graphite sintering furnace and synthesized by high and low temperature composite carbothermal reduction under flowing nitrogen to obtain nitrided powder D; wherein the high and low temperature composite carbothermal reduction synthesis process is to first synthesize at a high temperature of 1580~1630℃ for 10~40min, and then cool down to 1450~1550℃ for 2~8h. Step (5): Place the nitrided powder D into a high-temperature box furnace and keep it at 600-750℃ for 4-8 hours to remove excess carbon and obtain low-agglomeration ultrafine aluminum nitride powder.

2. The process of claim 1 wherein The median diameter of the nano-alumina is 20–100 nm.

3. The process of claim 1 wherein The median diameter of the carbon black is 10–100 nm.

4. The process of claim 1 wherein The drying temperature of the viscous slurry B is 70-100℃.

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