Method for in-situ synthesis of self-sorted aluminum nitride powder

Abstract

A preparation method for in-situ synthesizing self-graded aluminum nitride powder comprises the following steps: step S1: ball milling a predetermined mass of D 50 =200~700nm aluminum oxide powder with a ball-to-material ratio (4~20):1 for 2~12 hours, so that the aluminum oxide powder is further ground and the grain micro-defects are accumulated under the action of mechanical ball milling, thereby preparing high-activity aluminum oxide powder, i.e., activated aluminum oxide powder; step S2: wet mixing the activated aluminum oxide powder with the same non-activated aluminum oxide powder and a carbon source according to a predetermined proportion, and spray drying the mixed slurry to form a reaction precursor; step S3: loading the reaction precursor into a boat and performing high-temperature carbon thermal reduction nitriding reaction in a flowing nitrogen atmosphere, with a synthesis temperature of 1400℃~1700℃ and a synthesis time of 10~30 hours; and step S4: obtaining the in-situ self-graded aluminum nitride powder by keeping the product of step S3 in a flowing air or oxygen atmosphere at 500~700℃ for 3~12 hours.

Description

Technical Field

[0001] This invention relates to the field of aluminum nitride synthesis technology, and in particular to a method for preparing self-graded aluminum nitride powder by in-situ synthesis. Background Technology

[0002] High-quality aluminum nitride powder is not only essential for obtaining high-quality aluminum nitride ceramic products, but it is also used as a thermally conductive filler in thermally conductive interface materials (TIMs). In these applications, heat needs to be conducted through the thermally conductive channels formed by AlN particles. Therefore, it is often necessary to grade AlN particles of different sizes. Larger particles can significantly improve the continuity of the thermal conductive pathways, while smaller particles can fill the gaps between larger particles to form more thermally conductive pathways. For thermally conductive filling applications in ultra-thin interfaces of microelectronic devices, the maximum particle size of the powder is limited to less than the designed total interface thickness. Generally, specially formulated aluminum nitride particles with larger grains are dispersed separately from conventional 1-2 micrometer aluminum nitride particles and then compounded before use.

[0003] High-end aluminum nitride micro powders are generally synthesized and prepared by carbothermic reduction nitridation. The powder produced by this process has the characteristics of narrow particle size distribution, low oxygen content, high purity and regular micro morphology. However, it does not have the regular gradation characteristics of large and small AlN particles, and it is difficult to meet the requirements of high thermal conductivity aluminum nitride filler applications on its own. Summary of the Invention

[0004] In view of this, it is necessary to provide an energy-saving in-situ synthesis method for preparing self-graded aluminum nitride powder with a large particle size threshold and good thermal conductivity.

[0005] A method for preparing in-situ synthesized self-graded aluminum nitride powder includes the following steps:

[0006] Step S1: Transfer the predetermined mass of D 50 =200~700nm alumina powder is ball-milled at a ball-to-material ratio of (4~20):1 for 2~12 hours. This process further refines the alumina powder and accumulates microscopic defects in the grains under the action of mechanical ball milling, thus producing highly active alumina powder, i.e. activated alumina powder.

[0007] Step S2: The activated alumina powder is wet-mixed with the homogeneous unactivated alumina powder and carbon source in a predetermined ratio, and the mixed slurry is spray-dried to form a reaction precursor.

[0008] Step S3: The reaction precursor is loaded into a boat and subjected to a high-temperature carbothermic reduction nitridation reaction in a flowing nitrogen atmosphere. The synthesis temperature is 1400℃~1700℃ and the synthesis time is 10~30 hours.

[0009] Step S4: The S3 composite is kept at 500-700℃ for 3-12 hours in a flowing air or oxygen atmosphere to obtain in-situ self-graded aluminum nitride powder.

[0010] Preferably, in step S1, ethanol is used as a solvent in the ball mill for the alumina powder, and the solid-liquid ratio is 1:(0.2~1.0).

[0011] Preferably, in step S2, the reaction precursor also includes aluminum nitride powder, which accounts for 3% to 20% of the total mass of the mixed raw materials.

[0012] Furthermore, the particle size of the aluminum nitride powder is 1–2 μm.

[0013] Preferably, in step S3, the reaction precursor is subjected to gradient sintering, specifically, held at 1400-1500℃ for 4-10 hours; held at 1500-1600℃ for 4-10 hours; and held at 1600-1700℃ for 4-10 hours.

[0014] Beneficial Effects: The in-situ synthesis method for preparing self-graded aluminum nitride powder of this invention lowers the reaction temperature when aluminum oxide reacts with carbon powder and nitrogen to form aluminum nitride by mechanically activating a portion of the alumina. At a lower temperature, the mechanically activated alumina reacts first to generate aluminum nitride seed crystals. These seed crystals continuously grow during the subsequent reaction, and new seed crystals are also formed. After a period of reaction, as the temperature inside the sintering furnace increases, ordinary alumina also begins to react. Due to the presence of aluminum nitride seed crystals, some of the subsequently generated aluminum nitride forms its own seed crystals, while the rest grows on the basis of the existing aluminum nitride seed crystals, ultimately resulting in aluminum nitride particles with a large particle size threshold and a relatively uniform distribution of different particle sizes. Simultaneously, due to the larger proportion of aluminum nitride grains, the powder particles have better dispersibility, a smaller increase in oxygen content after carbon removal, and superior particle thermal conductivity. Attached Figure Description

[0015] Figure 1 The image shows an electron microscope (EM) image of the self-graded aluminum nitride powder prepared according to the preparation method of Example 1.

[0016] Figure 2 The image shows an electron microscope (EM) image of the self-graded aluminum nitride powder prepared according to the preparation method of Example 2.

[0017] Figure 3 The image shows an electron microscope (EM) image of conventional aluminum nitride powder prepared according to the preparation method of control group 1.

[0018] Figure 4 The image shows an electron microscope (EM) image of conventional aluminum nitride powder prepared according to the preparation method of control group 2.

[0019] In the image: the left image is an electron microscope image with higher magnification, and the right image is an electron microscope image with lower magnification. Detailed Implementation

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] A method for preparing in-situ synthesized self-graded aluminum nitride powder includes the following steps:

[0022] Step S1: Transfer the predetermined mass of D 50 =200~700nm alumina powder is ball-milled at a ball-to-material ratio of (4~20):1 for 2~12 hours. This process further refines the alumina powder and accumulates microscopic defects in the grains under the action of mechanical ball milling, thus producing highly active alumina powder, i.e. activated alumina powder.

[0023] Step S2: The activated alumina powder is wet-mixed with the homogeneous unactivated alumina powder and carbon source in a predetermined ratio, and the mixed slurry is spray-dried to form a reaction precursor.

[0024] In a preferred embodiment, the total molar ratio of alumina powder to carbon is 1:(3.3-4.2), and the mass ratio of activated alumina to unactivated alumina is arbitrary. The total alumina powder refers to the combined amount of activated and unactivated alumina.

[0025] Step S3: The reaction precursor is loaded into a boat and subjected to a high-temperature carbothermic reduction nitridation reaction in a flowing nitrogen atmosphere. The synthesis temperature is 1400℃~1700℃ and the synthesis time is 10~30 hours.

[0026] Step S4: The S3 composite is kept at 500-700℃ for 3-12 hours in a flowing air or oxygen atmosphere to obtain in-situ self-graded aluminum nitride powder.

[0027] In a preferred embodiment, in step S1, the alumina powder is processed in a ball mill using ethanol as a solvent, with a solid-liquid ratio of 1:(0.2 to 1.0).

[0028] In a preferred embodiment, in step S2, the reaction precursor further includes aluminum nitride powder, which accounts for 3% to 20% of the total mass of the mixed raw materials.

[0029] Furthermore, the particle size of the aluminum nitride powder is 1–2 μm.

[0030] In a preferred embodiment, in step S3, the reaction precursor is subjected to gradient sintering, specifically, held at 1400-1500°C for 4-10 hours; held at 1500-1600°C for 4-10 hours; and held at 1600-1700°C for 4-10 hours.

[0031] In existing technologies, the sintering of alumina powder, carbon powder, and nitrogen to prepare aluminum nitride results in a homogeneous precursor composition, preventing the formation of aluminum nitride powder with a large threshold particle size. Furthermore, large-grained alumina cannot maintain its original particle size and morphology after synthesis, and its excessive size can lead to incomplete reaction. Therefore, in conventional carbothermic reduction nitridation synthesis of aluminum nitride, alumina almost simultaneously begins deoxidation and nitridation to form aluminum nitride particles. Although new aluminum nitride particles are subsequently formed, the earlier synthesized particles, acting as seed crystals, lack an absolute advantage in competing with other seed crystals for raw materials. This means that the size of all aluminum nitride grains remains within a relatively narrow range.

[0032] This invention first mechanically activates a portion of the alumina powder, thereby lowering the reaction temperature between the alumina powder and carbon powder and nitrogen. At a lower temperature, holding the temperature allows for the preferential formation of aluminum nitride crystals with a size advantage. Subsequently, as the temperature increases, unactivated alumina also participates in the reaction. The already formed aluminum nitride crystals continue to grow due to their "first-mover advantage," while newly formed aluminum nitride particles also form new seed crystals that grow independently. Thus, after the reaction is complete, the particle size threshold of the aluminum nitride powder is significantly higher than that of aluminum nitride powder formed by sintering using existing techniques. This allows for the one-time in-situ synthesis of aluminum nitride powder containing two or more distinct particle sizes, exhibiting a high tap density. It can be used to improve the thermal conductivity of aluminum nitride thermal interface materials and the density of ceramic green bodies.

[0033] The invention will be illustrated below through examples.

[0034] Example 1: D 50=400nm alumina powder was ball-milled in a stirred ball mill at a ball-to-powder ratio of 12:1, using ethanol as the medium, a solid-liquid ratio of 2:1, a grinding ball diameter of 4mm, a ball milling speed of 75 rpm, and a milling time of 10 hours to obtain a slurry. The weight of activated alumina powder was calculated as 1 / 3 of the slurry weight. Unactivated alumina was weighed at a ratio of activated alumina to unactivated alumina of 3:7, and nano-carbon black was weighed at 3.6 times the total molar amount of alumina. After conventional ball milling and mixing, a homogeneous slurry was prepared, which was then spray-dried to prepare a reaction precursor. The reaction precursor was loaded into a reaction boat and subjected to a high-temperature carbothermic reduction nitridation reaction in a flowing nitrogen atmosphere. The reactions were held at 1450℃, 1550℃, and 1650℃ for 8 hours, 8 hours, and 8 hours respectively, and then held at 600℃ in a flowing air atmosphere for 8 hours to remove excess carbon, resulting in self-graded aluminum nitride powder. The resulting powder had an oxygen content of 0.61% wt and a tap density of 0.78 g / cm³. 3 The micrographs shown below show that the powder is composed of a gradation of particles with a median particle size of about 7 μm and particles with a median particle size of about 1.5 μm.

[0035] Example 2: D 50 =400nm alumina powder was ball-milled in a stirred ball mill at a ball-to-powder ratio of 12:1, using ethanol as the medium, a solid-liquid ratio of 2:1, a grinding ball diameter of 4mm, a milling speed of 75 rpm, and a milling time of 10 hours to obtain a slurry. The weight of activated alumina powder was calculated as 1 / 3 of the slurry weight. Unactivated alumina was weighed at a ratio of activated alumina to unactivated alumina of 3:7. Nano-carbon black was weighed at 3.6 times the total molar amount of alumina. Then, 5% of the total alumina amount of D was added. 50 =2μm aluminum nitride powder was prepared into a homogeneous slurry through conventional ball milling, and then spray-dried to form a reaction precursor. The precursor was loaded into a reaction boat and subjected to a high-temperature carbothermic reduction nitridation reaction in a flowing nitrogen atmosphere. The reactions were carried out at 1450℃, 1550℃, and 1650℃ for 8 hours, 8 hours, and 8 hours respectively, followed by a 8-hour holding time at 600℃ in a flowing air atmosphere to remove excess carbon, yielding self-graded aluminum nitride powder. The resulting powder had an oxygen content of 0.53%wt and a tap density of 0.81 g / cm³. 3 The micrographs are shown below. It can be seen that the powder is composed of a particle size distribution of a particle with a median particle size of about 10 μm, a particle with a median particle size of about 5 μm, and a particle with a median particle size of about 1.5 μm.

[0036] Control group 1: D that has not been ball-milled and activated was taken. 50Alumina powder with a density of approximately 400 nm was mixed with nano-carbon black at 3.6 times the total molar amount of alumina. The mixture was then ball-milled to form a homogeneous slurry, which was subsequently spray-dried to prepare a reaction precursor. The precursor was loaded into a reaction boat and subjected to a high-temperature carbothermic reduction nitridation reaction in a flowing nitrogen atmosphere. The reactions were held at 1450℃, 1550℃, and 1650℃ for 8 hours, 8 hours, and 8 hours respectively, followed by a 8-hour holding at 600℃ in a flowing air atmosphere to remove excess carbon, yielding aluminum nitride powder. The resulting powder had an oxygen content of 0.74% wt and a tap density of 0.68 g / cm³. 3 The micrographs are shown below. It can be seen that the powder particles have a relatively uniform diameter distribution, with a median particle size of about 2 μm.

[0037] Control group 2: D that has not been ball-milled and activated was taken. 50 Alumina powder with a density of approximately 400 nm was mixed with nano-carbon black at 3.6 times the total molar amount of alumina. The mixture was then ball-milled to form a homogeneous slurry, which was subsequently spray-dried to prepare a reaction precursor. The precursor was loaded into a reaction boat and subjected to a high-temperature carbothermic reduction nitridation reaction in a flowing nitrogen atmosphere. The reactions were held at 1450℃, 1550℃, and 1650℃ for 4 hours, 4 hours, and 6 hours, respectively, and then held at 600℃ in a flowing air atmosphere for 8 hours to remove excess carbon, yielding aluminum nitride powder. The resulting powder had an oxygen content of 0.87% wt and a tap density of 0.63 g / cm³. 3 The micrographs are shown below. It can be seen that the powder particles have a relatively uniform diameter distribution, with a median particle size of about 1.2 μm.

[0038] Compared to Example 1, Example 2 involves the addition of aluminum nitride during the preparation of self-graded aluminum nitride powder. The resulting self-graded aluminum nitride powder in Example 2 shows a significant increase in a type of powder. Based on the aluminum nitride formation process, it is inferred that the 10μm median particle in Example 2 originates from the aluminum nitride powder in the raw materials. The aluminum nitride powder in the raw materials acts as seed crystals. At 1450℃, some of the aluminum nitride generated during the activated alumina reaction adheres to the seed crystals, causing grain growth. Subsequently, at 1550℃, some of the aluminum nitride generated during the unactivated alumina reaction continues to adhere to the grains, ultimately forming 10μm aluminum nitride powder. Simultaneously, at 1450℃, another portion of the aluminum nitride powder generated during the activated alumina reaction acts as new seed crystals and grows. Then, at 1550℃, the unactivated alumina reaction... Some of the aluminum nitride generated during the process continues to adhere to the grains, eventually forming 5μm aluminum nitride powder. Since the grains with aluminum nitride as seed crystals have a first-mover advantage, they will compete with the aluminum nitride seed crystals generated by the reaction of activated alumina for aluminum nitride during the reaction process. Therefore, the grain size of 5μm is smaller than the grain size of 7μm without aluminum nitride in the raw material. At the same time, at 1550℃, another part of the aluminum nitride powder generated during the reaction of unactivated alumina acts as a new seed crystal and grows, eventually forming 1.5μm aluminum nitride powder.

[0039] The experimental results from Examples 1 and 2 show that multi-sized aluminum nitride products can be formed through the self-gradation of activated and unactivated alumina, as well as the self-gradation of activated, unactivated, and aluminum nitride. Electron micrographs reveal that the particle size differences of the multi-sized aluminum nitride products are very large.

[0040] Compared to Control Group 1, Example 1 incorporated activated alumina powder into the raw materials. The aluminum nitride powder prepared in Control Group 1 had a median particle size of 2 μm and exhibited relatively uniform particle size. This indicates that the powder with a median grain size of 7 μm prepared in Example 1 was produced using activated alumina in combination with carbon and nitrogen. Furthermore, the preparation process affects the growth of seed crystals generated from the reaction of unactivated alumina. The results from Control Group 1 also demonstrate that aluminum nitride powder prepared from a single alumina raw material exhibits more uniform particle size and smaller differences in particle size threshold.

[0041] Compared to control group two, control group one showed a shorter reaction time, especially in the low-temperature region. The median particle size of the resulting aluminum nitride powder was also smaller. This indicates that high temperatures promote aluminum nitride crystal growth. However, in the same alumina environment, due to the lack of competition among the powder particles, the final particle size is relatively uniform, and it is difficult to form larger particles, such as 7μm or 10μm particles. Increasing the particle size requires raising the reaction temperature, which significantly increases energy consumption during the preparation process. Furthermore, since seed crystal growth is simultaneous, surrounding seed crystals compete for newly formed aluminum nitride; therefore, even with a significant increase in temperature, the final aluminum nitride particle size is limited, generally not exceeding 3μm. Figures 1 to 4 The electron micrographs of aluminum nitride also show that the self-graded aluminum nitride powder prepared by the in-situ synthesis method of the present invention has a larger particle size threshold and smaller gaps between powder particles.

[0042] Therefore, if aluminum nitride particles of different sizes are prepared first and then mixed using conventional methods, energy consumption will increase significantly, and the particle size threshold for the mixture will also be limited. On the other hand, because aluminum nitride particles are very small, dry mixing is generally not uniform, and wet mixing requires drying the slurry, further increasing energy consumption.

[0043] Therefore, compared with conventional preparation methods, the in-situ synthesis method of self-graded aluminum nitride powder of the present invention has a larger particle size threshold, fewer steps, and is more energy-efficient. Furthermore, due to the larger particle size threshold of the self-graded aluminum nitride powder, the prepared high thermal conductivity aluminum nitride filler also exhibits better thermal conductivity.

[0044] The above-disclosed embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the invention. Those skilled in the art will understand that implementing all or part of the above-described embodiments and making equivalent changes in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A method for preparing self-graded aluminum nitride powder by in-situ synthesis, characterized in that: Includes the following steps: Step S1: Ball mill a predetermined mass of alumina powder with D50=200-700nm, with a ball-to-material ratio of (4-20):1, for 2-12 hours. This allows the alumina powder to be further ground and accumulate microscopic defects in the grains under the action of mechanical ball milling, thus producing highly active alumina powder, i.e. activated alumina powder. Step S2: The activated alumina powder is wet-mixed with homogeneous unactivated alumina powder and a carbon source in a predetermined ratio. The mixed slurry is then spray-dried to form a reaction precursor. The mass ratio of activated alumina to unactivated alumina is 3:

7. The reaction precursor also includes aluminum nitride powder, which accounts for 3%-20% of the total mass of the reaction precursor. Step S3: Load the reaction precursor into a boat and carry out a gradient high-temperature carbothermic reduction nitriding reaction under a flowing nitrogen atmosphere. The gradient sintering parameters are: holding at 1400-1500℃ for 4-10 hours; holding at 1500-1600℃ for 4-10 hours; and holding at 1600-1700℃ for 4-10 hours. Step S4: The S3 composite is kept at 500-700℃ for 3-12 hours in a flowing air or oxygen atmosphere to obtain in-situ self-graded aluminum nitride powder.

2. The method for preparing in-situ synthesized self-graded aluminum nitride powder as described in claim 1, characterized in that: In step S1, the alumina powder is processed in a ball mill using ethanol as a solvent, with a solid-liquid ratio of 1:(0.2-1.0).

3. The method for preparing in-situ synthesized self-graded aluminum nitride powder as described in claim 1, characterized in that: The total molar ratio of alumina powder to carbon is 1:(3.3-4.2).

4. The method for preparing in-situ synthesized self-graded aluminum nitride powder as described in claim 3, characterized in that: The aluminum nitride powder has a particle size of 1-2 μm.

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