Aluminum nitride

Abstract

To provide aluminum nitride with a slow oxygen adsorption rate. [Solution] The average particle diameter is 250 μm or more and 1000 μm or less, and the BET specific surface area is 1 m². 2 / g or more 4m 2 Aluminum nitride, which is less than / g.

Description

[Technical Field]

[0001] This invention relates to aluminum nitride. [Background technology]

[0002] Aluminum nitride is attracting attention as a material for next-generation power semiconductors because it possesses superior electrical properties compared to silicon carbide (SiC) and gallium nitride (GaN), such as high resistance to high voltages and low power conversion losses. For aluminum nitride to be used as a material for next-generation power semiconductors, high purity is necessary. For example, in the sublimation method, one of the manufacturing methods for aluminum nitride single crystals, aluminum nitride with an oxygen content reduced to approximately 2000 ppm by mass is required. However, conventional aluminum nitride has an oxygen content of approximately 6000 ppm by mass, making a purification process unavoidable.

[0003] Conventionally, aluminum nitride with reduced oxygen content has an average particle size of D 50 The particle size is 15-200 μm, the content of particles with a particle size of 5 μm or less is 60% or less by number, the content of alkaline earth metal elements and rare earth elements is 0.1% by weight or less, the oxygen content is 0.2% by weight or less, the silicon content is 1000 ppm by weight or less, the iron content is 1000 ppm by weight or less, and the BET specific surface area is 0.05-0.1 m². 2 An aluminum nitride-based powder with a density of / g has been reported (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-65458

[0005] However, because aluminum nitride is highly reactive with oxygen, even if the oxygen content in aluminum nitride is reduced, oxidation can still occur during handling, such as during raw material filling. Therefore, aluminum nitride with a slow oxygen adsorption rate is desired. Therefore, the object of the present invention is to provide aluminum nitride with a slow oxygen adsorption rate. [Overview of the Initiative] [Means for solving the problem]

[0006] As a result of their investigations to solve the above problems, the inventors of the present invention have discovered for the first time that aluminum nitride, in which the average particle size and BET specific surface area are controlled within a specific range, has the attribute of having a slow oxygen adsorption rate and being resistant to oxidation even when exposed to air.

[0007] In other words, the present invention provides the following [1] to [5]. [1] The average particle diameter is 250 μm or more and 1000 μm or less, and the BET specific surface area is 1 m². 2 / g or more 4m 2 Aluminum nitride, which is less than / g. [2] The aluminum nitride described in [1] above, wherein the oxygen content is 2600 ppm by mass or less. [3] The aluminum nitride according to [1] or [2], wherein the oxygen adsorption rate when exposed to air is 160 ppm / h or less by mass. [4] Aluminum nitride according to any one of [1] to [3] above, wherein the circularity is 0.10 or more and 0.55 or less. [5] Aluminum nitride according to any one of [1] to [4] above, for the production of aluminum nitride single crystals by sublimation. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide aluminum nitride that has a slow oxygen adsorption rate and is resistant to oxidation even when exposed to air. [Brief explanation of the drawing]

[0009] [Figure 1] SEM image of aluminum nitride obtained in Example 1. [Figure 2] SEM image of aluminum nitride obtained in Example 2. [Figure 3] SEM image of aluminum nitride obtained in Example 4. [Figure 4] SEM image of aluminum nitride obtained in Example 5.

Mode for Carrying Out the Invention

[0010] <Aluminum Nitride> The aluminum nitride of the present invention has an average particle size of 250 μm or more and 1000 μm or less. The present inventors have first found that when the average particle size of aluminum nitride is within the above range, it has the property of having a slow oxygen adsorption rate and being difficult to be oxidized. Here, in this specification, the "average particle size" means the particle size (D 50 ) corresponding to 50% of the cumulative distribution curve when the particle size distribution of the sample is created on a volume basis in accordance with JIS R 1629. For the measurement of the particle size distribution, for example, a laser diffraction / scattering type particle size distribution measuring device can be used.

[0011] From the viewpoint of further reducing the oxygen adsorption rate, the average particle size of aluminum nitride is preferably 300 μm or more, more preferably 350 μm or more, still more preferably 400 μm or more, even more preferably 450 μm or more, particularly preferably 470 μm or more, and preferably 900 μm or less, more preferably 800 μm or less, still more preferably 700 μm or less, and even more preferably 600 μm or less.

[0012] The aluminum nitride of the present invention has a BET specific surface area of 1 m 2 / g or more and 4 m 2It is below / g. The inventors have first found that when the BET specific surface area of aluminum nitride is within the above range, it has the property that the oxygen adsorption rate is slow and it is difficult to be oxidized. Here, in this specification, the "BET specific surface area" means the surface area measured by the BET method (a method for measuring the surface area by utilizing the adsorption of gas molecules) in accordance with JIS R 1626:1996. For example, as a flow-type specific surface area automatic measuring device, FrowSorb III 2305 (manufactured by Shimadzu Corporation) can be used.

[0013] From the viewpoint of further reducing the oxygen adsorption rate, the BET specific surface area of aluminum nitride is preferably 1.1 m 2 / g or more, more preferably 1.2 m 2 / g or more, still more preferably 1.3 m 2 / g or more, even more preferably 1.4 m 2 / g or more, and most preferably 3.5 m 2 / g or less, more preferably 3.2 m 2 / g or less, still more preferably 3 m 2 / g or less, even more preferably 2.4 m 2 / g or less is even more preferable.

[0014] From the viewpoint of applying the aluminum nitride of the present invention to the production of aluminum nitride single crystals by the sublimation method, the oxygen content is preferably 2600 mass ppm or less, more preferably 2000 mass ppm or less, still more preferably 1800 mass ppm or less, even more preferably 1500 mass ppm or less, and most preferably 1200 mass ppm or less. The lower limit value of the oxygen content in aluminum nitride is not particularly limited and may be 0 mass ppm. Here, in this specification, the "oxygen content" can be measured by the melting infrared absorption method in an inert gas using an oxygen, nitrogen, hydrogen analyzer. As the oxygen, nitrogen, hydrogen analyzer, for example, TCH-600 manufactured by LECO Corporation can be used. The measurement sample is packed in a Ni capsule in a glove box and sealed, and the measurement is performed with a device installed in the atmosphere.

[0015] Furthermore, from the viewpoint of applying the aluminum nitride of the present invention to the production of aluminum nitride single crystals by sublimation, it is preferable that the oxygen adsorption rate when the aluminum nitride is exposed to air is 160 ppm / h or less, more preferably 140 ppm / h or less, even more preferably 120 ppm / h or less, and even more preferably 100 ppm / h or less. The lower limit of the oxygen adsorption rate when the aluminum nitride is exposed to air is not particularly limited and may be 0 ppm / h. In this specification, "oxygen adsorption rate" is calculated by the following formula based on the change in oxygen content in the aluminum nitride before and after exposure of 1 g of aluminum nitride to air. Exposure of aluminum nitride to air shall be carried out in air at room temperature (20 ± 15 °C) and relative humidity of 65 ± 20% RH, preferably in air at room temperature and relative humidity of 65 ± 5% RH. From the viewpoint of measurement accuracy, the exposure time is preferably 10 hours or more, more preferably 15 hours or more, and from the viewpoint of work efficiency, it is preferably 24 hours or less, and more preferably 20 hours or less.

[0016] Oxygen adsorption rate (mass ppm / h) = (Wa - Wb) / Exposure time (h)

[0017] [In the formula, Wa represents the oxygen content (mass ppm) in aluminum nitride after exposure to air, and Wb represents the oxygen content (mass ppm) in aluminum nitride before exposure to air.]

[0018] Furthermore, from the viewpoint of further reducing the oxygen adsorption rate, the aluminum nitride of the present invention preferably has a circularity of 0.10 or higher, more preferably 0.15 or higher, even more preferably 0.20 or higher, preferably 0.55 or lower, more preferably 0.52 or lower, and even more preferably 0.50 or lower. It should be noted that the closer the circularity is to 1, the closer the particle is to a perfect sphere. The inventors have for the first time discovered that when the circularity of aluminum nitride is within the above range, it has the attribute of having a slow oxygen adsorption rate and being less susceptible to oxidation.

[0019] Here, "circularity" is calculated by processing and measuring the image of particles in SEM images taken using a field emission scanning microscope (SEM) with general-purpose image processing software, and determining the circularity of the particles based on the following formula. Then, the average value of "circularity" is calculated for 20 randomly selected particles. That is, "circularity" in this specification means the average value for 20 particles. For example, the JSM-7001F (manufactured by JEOL Ltd.) can be used as the field emission scanning microscope. For example, NanoHunter NS2K-Pro / Lt (manufactured by NanoSystems Inc.) can be used as the general-purpose image processing software.

[0020] Circularity = 4πS / L 2 [In the formula, S represents the two-dimensional projected area of ​​the particle, and L represents the two-dimensional projected perimeter of the particle.]

[0021] In the formula, the "two-dimensional projected perimeter" is calculated from the diameter of a perfect circle having the same projected area as the captured SEM image.

[0022] The aluminum nitride of the present invention possesses the above-described properties, and even when exposed to air, it has a slow oxygen adsorption rate and is resistant to oxidation, making it useful, for example, as a raw material for manufacturing aluminum nitride single crystals by sublimation. Furthermore, using the aluminum nitride of the present invention can avoid the risk of oxidation during handling, such as during raw material filling.

[0023] <Method for manufacturing aluminum nitride> The present invention provides a method for producing aluminum nitride, which includes a firing step of heating metallic aluminum. While there are no particular limitations on the type of metallic aluminum that is generally available, high-purity aluminum is preferable from the viewpoint of reducing impurities. For example, metallic aluminum with a purity of 4N or higher can be suitably used. The shape of metallic aluminum is not particularly limited because it liquefies when heated. For example, it can be used in any suitable form, such as powder, granules, lumps, plates, or rods, and commercially available products can also be used as is.

[0024] The weighing of metallic aluminum is preferably carried out in an oxygen-free atmosphere. An example of an oxygen-free atmosphere is an inert gas atmosphere. Examples of inert gases include nitrogen, helium, and argon, with argon being preferred. For example, metallic aluminum can be weighed in a glove box under an inert gas atmosphere.

[0025] The reactor is not particularly limited as long as it can accommodate metallic aluminum, allow gas to flow through it, and has heat resistance, but examples include furnace tubes, core tubes, tubular furnaces, electric furnaces, batch kilns, and rotary kilns. The reaction apparatus can be horizontal or vertical, and can be selected as appropriate. Furthermore, the material of the reaction apparatus is not particularly limited as long as it is heat-resistant, but examples include alumina, mullite, boron nitride, and graphite. The size of the reactor can be selected appropriately considering the manufacturing scale, etc. For example, when using a cylindrical furnace core tube, it is usually φ30~300mm × length 500~3000mm.

[0026] The firing process can be carried out under gas flow or in a sealed space with gas replaced. However, from the viewpoint of promoting the nitriding reaction, it is preferable to carry it out under gas flow. Furthermore, firing can be carried out at atmospheric pressure; pressurization or vacuum is not required. Preferably, ammonia gas alone or a mixed gas containing ammonia gas and nitrogen gas is used as the gas. For ammonia gas, a standard high-purity gas (e.g., purity of 5N or higher) can usually be used. For nitrogen gas, for example, gas obtained by vaporizing liquid nitrogen can be used, or high-purity gas filled in nitrogen cylinders can be used.

[0027] The volume ratio of ammonia gas to nitrogen gas in the mixed gas (nitrogen gas / ammonia gas) is preferably 6.5 or higher, more preferably 7 or higher, even more preferably 7.5 or higher, even more preferably 8 or higher, especially preferably 8.5 or higher, and preferably 15 or lower, more preferably 13 or lower, and even more preferably 11 or lower.

[0028] The gas can be supplied, for example, through an openable and closable gas inlet located at the end of the reactor. When using a mixed gas, ammonia gas and nitrogen gas can be mixed in a predetermined ratio and supplied to the reactor, or they can be supplied to the reactor from separate gas inlets in the predetermined ratio. The gas supply rate can be set as appropriate, but is typically 0.01 to 100 L / min, preferably 0.1 to 10 L / min. When using a mixed gas, the supply rate of each gas should be controlled within the above range so that the volume ratio of ammonia gas to nitrogen gas is within the above range.

[0029] The firing temperature is preferably 800 to 1500°C, more preferably 900 to 1300°C, and even more preferably 1000 to 1200°C. The firing time is not uniform depending on the reaction scale, but for example, 0.5 to 30 hours is preferred, 1 to 20 hours is more preferred, and 2 to 10 hours is even more preferred.

[0030] After firing, the fired product may be cooled, for example, to room temperature (20±15℃).

[0031] Aluminum nitride produced by this method can typically possess the average particle size and BET specific surface area described above. Therefore, since the oxygen content of aluminum nitride produced by the above method is reduced, a purification process is unnecessary, and the energy and time required for production can be reduced. In addition, the cooled aluminum nitride may be subjected to a grinding process if necessary to adjust the average particle size and BET specific surface area. Furthermore, the cooled aluminum nitride or its pulverized material may be sieved to adjust the average particle size and BET specific surface area.

[0032] Grinding can be carried out using a grinding device. The grinding device is not particularly limited as long as it is capable of grinding aluminum nitride and can be sealed, but examples include a media grinder. The media grinder may be batch type or continuous type, and may be rolling type, vibrating type or planetary type. Examples of grinding media include rod-shaped media (rods), spherical media (balls), and cylindrical media (rings), and these may be used one or more in combination. Among these, planetary ball mills, ball mills, and disc mills are preferred.

[0033] The grinding conditions can be set appropriately depending on the type of grinding equipment and the manufacturing scale. For example, the typical rotation speed is 50-400 rpm and the grinding time is 1-1440 minutes. The temperature during grinding is, for example, room temperature (20°C ± 15°C).

[0034] From the viewpoint of preventing oxidation, an oxygen-free atmosphere is preferred during grinding. An example of an oxygen-free atmosphere is an inert gas atmosphere. Specific examples of inert gases are as described above, and for example, grinding can be performed in a glove box with an oxygen-free atmosphere.

[0035] For sieving, a sieving machine can be used. The sieving machine is not particularly limited, and industrial equipment can be used, for example, vibrating, in-plane motion, rotary, or stationary types. The sieve size can be appropriately selected to achieve the desired average particle size. From the viewpoint of preventing oxidation, an oxygen-free atmosphere is preferred during sieving. Specific examples of an oxygen-free atmosphere are as described above; for example, sieving can be performed in a glove box with an oxygen-free atmosphere. [Examples]

[0036] The embodiments of the present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following embodiments.

[0037] 1. Average particle diameter (D 50 ) The particle size distribution of aluminum nitride powder was prepared on a volume basis in accordance with JIS R 1629 "Method for Measuring Particle Size Distribution of Fine Ceramic Raw Materials by Laser Diffraction and Scattering Method". Then, the particle size (D) corresponding to 50% of the integrated distribution curve was calculated. 50 The particle size distribution was determined using a Microtrac MT3300EX II (manufactured by Microtrac-Bell) laser diffraction / scattering method, and measurements were performed at 25°C.

[0038] 2.BET specific surface area The specific surface area (BET) was measured using a flow-type automatic surface area analyzer (FlowSorb III 2305, manufactured by Shimadzu Corporation). The measurement was performed at 25°C using a nitrogen-helium mixture containing 30% nitrogen.

[0039] 3.Oxygen content Aluminum nitride powder was packed into Ni capsules inside a glove box and sealed to prepare the samples for measurement. Measurements were performed using the fusion infrared absorption method in an inert gas environment with an oxygen, nitrogen, and hydrogen analyzer (TCH-600, LECO).

[0040] 4. Oxygen adsorption rate One g of aluminum nitride powder was weighed in a glove box and removed from the glove box. The sample was left in the air at 25°C and 65% RH for 18 hours, and then recovered in the glove box. The oxygen content of the aluminum nitride after exposure to air was measured at 25°C, and the change in oxygen content before and after exposure to air was calculated using the following formula.

[0041] Oxygen adsorption rate (mass ppm / h) = (Wa - Wb) / 18 (h)

[0042] [In the formula, Wa represents the oxygen content (mass ppm) in aluminum nitride after exposure to air, and Wb represents the oxygen content (mass ppm) in aluminum nitride before exposure to air.]

[0043] 5. Circularity Particles in SEM images taken using a field emission scanning microscope (JSM-7001F, JEOL Ltd.) were processed and measured at 25°C using general-purpose image processing software (NanoHunter NS2K-Pro / Lt, NanoSystems Inc.), and the circularity of each particle was calculated based on the following formula. Then, the average value of the circularity was calculated for 20 randomly selected particles.

[0044] Circularity = 4πS / L 2 [In the formula, S represents the two-dimensional projected area of ​​the particle, and L represents the two-dimensional projected perimeter of the particle.]

[0045] Example 1 100 g of powdered metallic aluminum (approximately 150 μm) was weighed in a glove box and placed in a tubular furnace. The tubular furnace was removed from the glove box, ammonia gas was circulated through it, and the temperature was raised to 1150°C over 3 hours, where it was held for 3 hours to calcine the metallic aluminum. After cooling, the aluminum nitride powder was removed from the tubular furnace in the glove box to obtain aluminum nitride. The obtained aluminum nitride was then analyzed for oxygen content, oxygen adsorption rate, average particle size, BET specific surface area, and circularity. The results are shown in Table 1, and the SEM images are shown in Figure 1.

[0046] Example 2 Except for changing the powdered metallic aluminum to granular metallic aluminum (approximately 1 mm), the procedure was carried out in the same manner as in Example 1 to obtain aluminum nitride powder. The obtained aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1, and the SEM images are shown in Figure 2.

[0047] Example 3 The aluminum nitride obtained in Example 1 was sieved in a glove box to obtain an average particle size of approximately 450 μm. The resulting aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1.

[0048] Example 4 Except for changing the flowing gas to a mixed gas of ammonia and nitrogen (N2 / NH3 volume ratio = 9), the procedure was carried out in the same manner as in Example 1 to obtain aluminum nitride powder. The obtained aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1, and the SEM images are shown in Figure 3.

[0049] Example 5 Except for changing the powdered metallic aluminum to solid metallic aluminum (approximately 5 mm in size), the procedure was carried out in the same manner as in Example 4 to obtain aluminum nitride powder. The obtained aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1, and the SEM images are shown in Figure 4.

[0050] Example 6 The aluminum nitride obtained in Example 4 was pulverized using a planetary ball mill, and then sieved in a glove box to obtain an average particle size of approximately 350 μm. The resulting aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1.

[0051] Comparative Example 1 Except for weighing 10 g of powdered metallic aluminum (approximately 150 μm) in air and removing the sample from the tubular furnace in air, the procedure was carried out in the same manner as in Example 1 to obtain aluminum nitride powder. The obtained aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1.

[0052] Comparative Example 2 The aluminum nitride obtained in Example 1 was sieved in a glove box to obtain an average particle size of approximately 1500 μm. The obtained aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1. An attempt was made to measure the circularity by SEM observation, but the particle size was too large to measure.

[0053] Comparative Example 3 The aluminum nitride obtained in Example 1 was sieved in a glove box to obtain an average particle size of approximately 230 μm. The obtained aluminum nitride was then analyzed in the same manner as in Example 1. The results are shown in Table 1.

[0054] [Table 1]

[0055] The aluminum nitride in Comparative Example 1 has a BET specific surface area of ​​4 m². 2 Since the value exceeds / g, it indicates that the oxygen adsorption rate is fast and that it is easily oxidized. In Comparative Example 2, the aluminum nitride has an average particle size exceeding 1000 μm, indicating a fast oxygen adsorption rate and susceptibility to oxidation. The aluminum nitride in Comparative Example 3 has an average particle size of less than 250 μm and a BET specific surface area of ​​4 m². 2 Since the value exceeds / g, it indicates that the oxygen adsorption rate is fast and that it is easily oxidized. In contrast, the aluminum nitrides of Examples 1-6 have an average particle size of 250 μm to 1000 μm and a BET specific surface area of ​​1 m². 2 / g or more 4m 2 Since the value is less than / g, it can be seen that the oxygen adsorption rate is slow and oxidation is difficult. In particular, when the average particle size is 470 μm or larger, the oxygen adsorption rate is even slower and oxidation is even more difficult.

Claims

1. The average particle diameter is between 250 μm and 1000 μm. BET specific surface area is 1 m 2 / g or more 4m 2 It is less than or equal to / g. Aluminum nitride.

2. The aluminum nitride according to claim 1, wherein the oxygen content is 2600 ppm by mass or less.

3. The aluminum nitride according to claim 1 or 2, wherein the oxygen adsorption rate when exposed to air is 160 ppm / h or less by mass.

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