JP7027196B2 - Manufacturing method of aluminum nitride powder - Google Patents

Description

The present invention relates to a method for producing aluminum nitride powder, which is suitably used as a material for an insulating high thermal conductive member.

Aluminum nitride has excellent properties such as high electrical insulation, high plasma resistance, and high thermal conductivity, and is therefore widely used as an insulating heat dissipation substrate, a material for semiconductor manufacturing equipment, and the like. These are produced by adding a sintering aid to the aluminum nitride powder, if necessary, and sintering under normal pressure or pressure. When yttrium oxide, which is a typical sintering aid, is used, high thermal conductivity is achieved by trapping the impurity oxygen in the aluminum nitride.

By the way, as a general method for producing aluminum nitride powder, a reduction nitriding method in which a mixture of aluminum oxide powder and carbon powder is heated in nitrogen, and a direct nitriding method in which metallic aluminum and nitrogen are reacted at a high temperature are known.

Aluminum nitride powder obtained by the direct nitriding method tends to cause an increase in metal impurities and surface oxygen because the manufacturing process includes a pulverization process, and when it is made into a sintered body, lattice defects due to these impurities are generated. It causes a decrease in thermal conductivity. Further, since the shape of the particles is distorted, there is a drawback that it is difficult to form and bake the molded product.

On the other hand, the aluminum nitride powder obtained by the reduction sintering method has fewer coarse particles having a particle size of more than 10 μm and has a particle shape close to a sphere and high purity as compared with the aluminum nitride powder obtained by the direct sintering method. It has excellent formability and sinterability, and it is easy to obtain high thermal conductivity when it is made into a sintered body. Therefore, aluminum nitride powder produced by the reduction nitriding method is suitable as a raw material for a highly thermally conductive ceramic member, particularly a member for a semiconductor manufacturing apparatus that requires high purity.

However, the aluminum nitride powder produced by the reduction nitride method has a problem that unreacted raw aluminum oxide and carbon used as a reducing agent locally remain inside the aluminum nitride particles. The oxygen component resulting from such residual aluminum oxide is not sufficiently trapped by the sintering aid during firing, and causes local lattice defects to hinder uniform heat conduction. Further, if the sintered body to be produced is a member having a metal interior such as a heater plate for heating a semiconductor wafer, the carbon component caused by the remaining carbon locally reacts with the interior metal to heat the heater. Create a temperature distribution on the plate. (See Patent Document 1)

In aluminum nitride powder produced by the reduction nitriding method, aluminum oxide, which is a raw material, tends to remain because the rate-determining process in the reduction nitriding reaction is the diffusion of nitrogen near the aluminum oxide particles, so all aluminum oxide particles. It is considered that the time until the nitriding reaction is completed is not uniform for each particle unless nitrogen is uniformly supplied to the aluminum particles. Further, it is considered that the reason why carbon tends to remain is that aluminum oxide, which is a raw material, aggregates in the nitriding reaction process, and carbon is incorporated into the particles in the aggregation process.

Conventionally, as a method having a possibility of improving the uniformity of the oxygen concentration and the carbon concentration of the aluminum nitride powder obtained by the reduction nitriding method, for example, a raw material aluminum oxide having a specific surface area of 30 m ² / g or more is used. By improving the reaction rate and further slowing the temperature rise rate to suppress the progress of sintering while nitriding the surface of the particles, the primary particles complete the nitriding in a porous state and remain in the particles. Methods for reducing unreacted aluminum oxide are known.

However, since a raw material aluminum oxide powder having a specific surface area of 30 m ² / g or more is used, it is necessary to maintain the state at the time of the reduction nitriding reaction for a long time until the sphericalization of the primary particles is sufficiently promoted. .. Therefore, a large amount of reducing gas and electric power are required, and it cannot be said that the method has good productivity. Further, when the state at the time of the reduction nitriding reaction is maintained for a long time, sintering between the particles proceeds in parallel with the spheroidization of the primary particles, and the obtained aluminum nitride tends to have many coarse particles ( See Patent Document 2).

Further, for example, aluminum nitride powder obtained by reducing and nitriding an alkaline earth metal compound or a rare earth metal compound and a sodium compound that can co-melt with aluminum oxide at a specific temperature together with aluminum oxide powder and carbon powder. There are known methods for reducing the carbon concentration of aluminum.

However, since the alkaline earth metal or the rare earth metal remains in the above method, it is not always desirable as a method for producing an aluminum nitride powder that requires high purity (see Patent Document 2).

Japanese Unexamined Patent Publication No. 2012-09648 Japanese Unexamined Patent Publication No. 4-265208 Japanese Unexamined Patent Publication No. 2013-107805

An object of the present invention is to provide a method for producing an aluminum nitride powder having good sinterability and less localization of aluminum oxide as a raw material and carbon used as a reducing agent.

In order to solve the above problems, the present inventors conducted a detailed analysis on a method for producing aluminum nitride powder by reduction nitride, particularly temperature conditions. As a result, they have found that it is possible to reduce the residual amount of unreacted raw material aluminum oxide and raw material carbon by controlling the heating in a nitrogen atmosphere in two stages, and have completed the present invention.

That is, the present invention is a method for producing aluminum nitride powder by heating aluminum oxide powder in a nitrogen atmosphere and nitriding it.
Use a raw material alumnium oxide with a specific surface area of 2 to 12 m ² / g.
Heating in a nitrogen atmosphere consists of a first heating step in which the reaction proceeds until a conversion of at least 60% or more is achieved, and a second heating step in which the reaction is heated under conditions higher than the first step. The method for producing aluminum nitride powder is characterized in that the temperature of the first heating step is 1450 ° C. or higher and 1600 ° C. or lower, and the temperature of the second heating step is more than 1600 ° C. and 1800 ° C. or lower. ..

According to the present invention, it is possible to obtain an aluminum nitride powder having a small amount of residual aluminum oxide or carbon as a raw material, and excellent electrical characteristics in an insulated heat dissipation substrate and a semiconductor manufacturing equipment member manufactured using this aluminum nitride powder. It is possible to achieve uniform heat dissipation.

An example of an electron micrograph of particles having a maximum length of 2 μm or more of the aluminum nitride powder of Example 1. An example of an electron micrograph of particles having a maximum length of 2 μm or more of the aluminum nitride powder of Comparative Example 1. An example of an electron micrograph of unreacted raw material carbon in aluminum nitride. An example of an electron micrograph of unreacted raw material aluminum oxide in aluminum nitride. The result of differential thermal analysis of the residue obtained by dissolving the aluminum nitride powder of Example 1 in the aqueous sodium hydroxide solution.

[Aluminum oxide powder]
In the production method of the present invention, aluminum oxide having a specific surface area of 2 to 12 m ² / g is used as a raw material. If the specific surface area is large (that is, if the particle size is small), the specific surface area becomes large when aluminum nitride is used, and the amount of oxygen derived from the surface oxygen increases. In addition, aggregation tends to proceed in the nitriding reaction process, and coarse particles increase in the obtained aluminum nitride. If the specific surface area is too small, it becomes difficult to nitrid to the center of the raw material. Further, by using the raw material aluminum oxide having this specific surface area, the volume-based 50% particle size (D50) of the aluminum nitride produced is in the range of 0.5 to 1.5 μm, and the volume-based 90% particle size. (D90) can be 4 μm or less. If D50 is too small, the specific surface area per unit weight will be large, and the amount of oxygen derived from surface oxygen will be large. On the other hand, if it is too large, it adversely affects moldability and sinterability. The preferred range for D50 is 0.8-1.3 μm.

Further, since D90 is small, the content ratio of coarse particles is small. Therefore, good sinterability can be obtained. D90 is preferably 3 μm or less. It is also possible that the amount of particles having a particle size four times or more that of D50 is 5% or less of the total amount.

These particle sizes are measured by a laser diffraction method.

Alumina or a hydrate thereof is used as the aluminum oxide powder without particular limitation. Alumina has a crystal structure such as α, γ, θ, δ, η, κ, and χ, and is dehydrated and transferred by heating such as boehmite, diaspore, gibbsite, bayarite, and todite, and finally all or part of it. All alumina hydrates that transfer to α-alumina are available.

These may be used alone or in a mixed state of different types, but α-alumina, γ-alumina, and boehmite, which have particularly high reaction activity and are easy to control, are preferably used.

Further, it is preferable to use aluminum oxide as a raw material having the highest possible purity, and specifically, the total content of Fe, Ca, Si, Ti, V, Cr and Ni is 500 mass ppm or less, particularly 400 mass. It is preferable to use one having a ppm or less. This makes it possible to reduce the amount of contained metal elements other than aluminum contained in the produced aluminum nitride to 400 mass ppm or less.

[Carbon powder]
When nitriding aluminum oxide in a nitrogen atmosphere, carbon powder is usually coexisted for reduction.

As the raw material carbon powder, carbon black or graphite powder can be used. As the carbon black, a furnace method, a channel method carbon black, and an acetylene black are preferably used.

The specific surface area of these carbon powders is arbitrary, but preferably 0.01 m ² / g to 500 m ² / g.

Further, it is preferable to use a raw material carbon powder having as high a purity as possible. Specifically, for example, the total content of Na, Fe, Ca, Si, Ti, V, Cr and Ni is 100 mass ppm or less, particularly. It is preferable to use one having a mass of 70 mass ppm or less. This makes it possible to reduce the amount of contained metal elements other than aluminum contained in the produced aluminum nitride to 400 mass ppm or less.

Further, as long as the effect of the present invention is not impaired, synthetic resin condensates such as phenol resin, melamine resin, epoxy resin and furanphenol resin, hydrocarbon compounds such as pitch and tar, cellulose, sucrose, polyvinylidene chloride, etc. A carbon source such as an organic compound such as polyphenylene, and a reducing gas such as hydrogen, carbon monoxide, and ammonia can also be used as raw materials.

[Mixed raw materials]
In the method for producing aluminum nitride powder of the present invention, the method for mixing the raw materials may be any method as long as the aluminum oxide powder and the carbon powder are present in a uniform composition, regardless of whether they are wet or dry. Mixing with a blender, mixer or ball mill is suitable.

The amount of carbon powder used may be 40 to 60 parts by mass with respect to 100 parts by mass of aluminum oxide powder.

〔nitrogen〕
In the present invention, as the nitrogen gas used for nitriding, a nitrogen gas equivalent to the known nitrogen gas used for the reduction nitriding reaction can be used without particular limitation.

[Reduction nitriding]
In the method for producing aluminum nitride powder of the present invention, the reduction nitriding step is performed by heating the raw material in the presence of nitrogen gas. The heating in the nitrogen atmosphere consists of a first heating step in which the reaction proceeds until the conversion rate reaches at least 60% or more, and a second heating step in which the reaction is heated under conditions higher than the first step. Become.

Further, the temperature of the first heating step is 1450 ° C. or higher and 1600 ° C. or lower, and the temperature of the second heating step is 1600 ° C. or higher and 1800 ° C. or lower.

[First heating step]
In the present invention, the first heating step is carried out until the conversion rate from aluminum oxide to aluminum nitride becomes 60% or more. When the conversion rate is less than 60%, aluminum nitride produced by the reaction between aluminum oxide and aluminum nitride, and the sulfur component and metal component in the raw material react with aluminum oxide at the end of the first heating step. A large amount of aluminum sulfide and the like produced in the above will remain. Since these components generate a liquid phase in the second step and promote liquid phase sintering between the particles, the raw aluminum oxide and the raw carbon are taken into the closed pores in the particles and tend to remain unreacted.

Preferably, in the first heating step, the conversion from aluminum oxide to aluminum nitride is carried out until the conversion rate becomes 80% or more. It may be carried out until the conversion rate reaches 100%.

In the first heating step, the heating temperature is set to 1400 to 1600 ° C., preferably 1450 to 1580 ° C. so that the nitriding reaction proceeds slowly. If the temperature is lower than 1400 ° C., the reduction nitriding reaction hardly proceeds. At a temperature exceeding 1600 ° C., liquid phase sintering of the particles progresses in the nitriding process, and the particles are burnt while the raw material is taken into the agglomerates of the primary particles. It becomes aluminum powder.

In the first heating step, nitrogen gas is sufficiently supplied so as not to be the rate-determining factor of the reduction nitriding reaction.

By carrying out the first heating step in this way, the obtained reaction product is further heated under the same temperature conditions until the conversion rate reaches 100% to be nitrided, and then decarburized. The degree of cohesion of the produced aluminum nitride is 2 or more. When the conversion rate is nitrided to 100% in the first heating step, the degree of cohesion is 2 or more when the conversion is directly decarburized. Whether or not the conversion rate from aluminum oxide to aluminum nitride has reached 100% can be easily determined by analyzing the exhaust gas.

Here, the degree of cohesion is a value obtained by dividing the specific surface area per unit weight of the object by the specific surface area per unit weight of a true sphere of aluminum nitride having a diameter of 50% based on the volume of the object. Yes, it is expressed by the following formula (1).

Cohesion = SA / ((4 × π × (D50 / 2) ² ) / (ρAlN × 4/3 × π × (D50 / 2) ³ ) (1)

In the above formula (1), D50 is the 50% particle size [m] based on the volume of the object, SA is the specific surface area per unit weight of the object [m ² / g], π is the circumference ratio, and ρAlN is. The true specific surface area of aluminum nitride [g / m ³ ].

Regarding the degree of agglomeration, (1) when the agglomerates of the primary particles have a porous structure, and (2) the sintering of the agglomerates of the primary particles progresses, and they are necked to each other, resulting in a large number of coarse agglomerates. Although it becomes large when formed, in the present invention, by adopting the above conditions, the aggregates of the primary particles have a porous structure, and the reaction gas cannot enter the inside of the aggregates of the primary particles. By preventing the formation of closed pores, the reaction can proceed sufficiently and the amount of unreacted raw materials can be reduced. The lower the reaction temperature, the higher the degree of cohesion tends to be.

The longer the reaction time, the lower the degree of cohesion, which is preferable. However, it is difficult to reduce the conversion rate to 0.1 hours or less in order to increase the conversion rate to 60% or more. On the other hand, it is not necessary to heat for more than the time required for the conversion to reach 100%. Rather, when heated for more than 50 hours even at a low temperature, sintering proceeds not only between the primary particles of the aluminum nitride particles but also between the secondary particles generated by the sintering of the aggregates of the primary particles, and coarse particles are likely to be generated. Become. The coarse particles tend to deteriorate the moldability and hinder the sintering when the sintered body is produced. Generally, the time required for the conversion to reach 100% at the heating temperature is ± 5 hours, particularly ± 2.5 hours. Specifically, although it depends on the temperature, for example, it is 5 to 10 hours at 1500 ° C. and 2 to 7 hours at 1550 ° C.

In the present invention, it is preferable to set the reaction temperature and time so that the degree of cohesion is 2.5 or more.

[Second heating step]
In the present invention, the reaction product obtained by heating in a nitrogen atmosphere as described above is further heated in temperature and heated at a temperature of more than 1600 ° C. and 1800 ° C. or lower under a nitrogen atmosphere. If the conversion rate is less than 100% in the first heating step, this heating is performed until the conversion rate reaches 100%. Whether or not the conversion rate has reached 100% can be easily determined by analyzing the CO concentration in the exhaust gas.

The temperature increase range is preferably 20 to 200 ° C, more preferably 100 to 150 ° C. By raising the temperature, unreacted raw materials are reduced and surface oxygen is reduced. However, if the maximum reaction temperature exceeds 1800 ° C., coarse particles in which aluminum nitride particles are aggregated are likely to be generated. Therefore, the second heating step is performed at 1800 ° C. or lower, preferably 1750 ° C. or lower.

Even if heating is continued at the same temperature as in the first step without raising the reaction temperature, the primary particles remain distorted and formability deteriorates, or the specific surface area remains large and is derived from surface oxygen. There is a risk that the amount of oxygen components will increase. In addition, there is a risk that the reduction of carbon components and metal impurities due to heating after nitriding is completed will be insufficient.

The heating time of the second heating step depends on the conversion rate in the first heating step and the temperature of the second heating step, and the higher the conversion rate of the first step, the higher the temperature of the second step. The higher it is, the shorter it may be. Specifically, for example, when the conversion rate in the first step is 60% and the temperature in the second step is 1680 ° C., it may be carried out for about 3 to 6 hours.

When the first heating step is performed up to a conversion rate of 100%, conversion does not occur in this second heating step, but even in that case, the amount of unreacted raw materials is reduced and the primary particles are formed into spheres. It is necessary to heat for at least 1 hour, preferably 2 hours or more for the purpose of improving the properties and reducing the amount of surface oxygen.

On the other hand, even after the conversion rate reaches 100% in the second heating step, if the aluminum nitride particles are heated for too long, the aluminum nitride particles may be sintered and coarse particles may be generated. Therefore, the heating time is preferably +10 hours or less, which is the time required to achieve a conversion rate of 100%, and more preferably +6 hours or less.

In the method for producing aluminum nitride powder of the present invention, as another condition at the time of the reduction nitriding, a known method is not particularly limited as long as it is a method in which nitrogen is sufficiently diffused and comes into contact with the raw material. Will be adopted. For example, there are a method of filling the mixed powder in a carbon container and circulating nitrogen, a method of using a rotary kiln, a method of using a fluidized bed, and a method of using a vertical tube furnace. Of these, a method of filling a carbon container and distributing nitrogen is preferable.

When the conversion proceeds in the second heating step, nitrogen gas is sufficiently supplied so as not to be the rate-determining factor of the reduction nitriding reaction as in the first heating step. On the other hand, when the reaction is advanced to a conversion rate of 100% in the first heating step, a small amount of nitrogen may be flowed.

In the present invention, the first heating step and the second heating step may be performed continuously or separately. Industrially, a method of continuously performing the process, that is, a method of performing the first heating step in the same heating furnace and then raising the temperature as it is is convenient and preferable.

[Decarburization treatment]
In the method for producing aluminum nitride powder of the present invention, the aluminum nitride powder obtained after the heating step contains free excess carbon powder, and therefore it is preferable to perform decarburization treatment. The decarburization treatment is performed at a high temperature, and a method of burning excess carbon powder using an oxidizing gas is common.

As the oxidizing gas for decarburization, any gas that can oxidize carbon, such as air and oxygen, can be used without limitation, but air is used in consideration of economic efficiency and the oxygen concentration of the obtained aluminum nitride. Suitable. Further, when the decarburization treatment is performed in an air atmosphere under normal pressure, the treatment temperature is preferably 500 to 1100 ° C. because the aluminum nitride is rapidly oxidized from around 1200 ° C., and the efficiency of decarbonization and the excess of the aluminum nitride surface are excessive. Considering oxidation, 600 to 900 ° C. is more preferable.

The decarburization treatment time may be appropriately set according to the degree of carbon reduction, but if it is performed at 600 to 900 ° C., it can be 1 to 6 hours, for example.

[Aluminum nitride powder]
The aluminum nitride powder obtained by the production method of the present invention has an average circularity of primary particles of 80% or more. In most cases, it is 90% or more. Since it has a high circularity, it has good moldability when a sintered body is obtained from aluminum nitride powder as a raw material. Such a high circularity can be achieved by producing aluminum nitride powder by the reduction nitriding method, and the direct nitriding method involves steps such as pulverization, so that the circularity is extremely low. ..

The method for obtaining the average circularity is as follows. That is, the powder is observed at 10000 times using a scanning electron microscope, 80 primary particles are randomly selected, and the area equivalent to the circle of the target particles (area of the target object) from the center of gravity of the target particles among the surfaces of the target particles. The area of the surface included in the range of half the length of the perfect circle having the same area as) is divided by the area of the perfect circle whose diameter is the equivalent circle diameter of the target particle, and multiplied by 100. The value.

Further, the aluminum nitride powder obtained by the production method of the present invention often contains 70% or less of the average circularity of the aluminum nitride particles having a maximum length of 2 μm or more (the maximum length is any two of the objects). The longest of the dots).

The aluminum nitride powder obtained by the production method of the present invention has a total oxygen concentration of 1.0% by mass or less. Therefore, it is unlikely that the thermal conductivity of the sintered body will decrease. The total oxygen concentration is preferably 0.2 to 0.9% by mass, more preferably 0.5 to 0.8% by mass.

The total oxygen concentration can be easily measured using a commercially available ceramic oxygen-nitrogen analyzer.

In the aluminum nitride powder obtained by the production method of the present invention, the oxygen component comprises surface oxygen of aluminum nitride particles, solid-dissolved oxygen in the particles, and oxygen in unreacted raw aluminum oxide. In the aluminum nitride powder obtained by the production method of the present invention, the concentration of unreacted raw material aluminum oxide is 0.07% by mass or less. Since the concentration of unreacted raw material aluminum oxide in the present state is as low as 0.07% by mass or less, lattice defects derived from oxygen are locally generated when the sintered body is formed, and uniform heat conduction is hindered. Fear is significantly reduced. In many cases, it can be 0.05% by mass or less. On the other hand, it is difficult to make it less than 0.02% by mass, and improvement of the above effect cannot be expected so much.

The abundance of unreacted raw material aluminum oxide in the aluminum nitride powder can be measured as follows. That is, the aluminum nitride powder was dissolved in a 5 mol / L sodium hydroxide aqueous solution, the residue was collected by filtration, and then the obtained residue was decarburized and then weighed, which was the same as the weight of the aluminum nitride powder before dissolution. It can be calculated from the ratio.

The aluminum nitride powder obtained by the production method of the present invention can have a concentration of metal elements other than aluminum of 400 mass ppm or less by using the above-mentioned high-purity raw materials. Since it can be 400 mass ppm or less, there is almost no possibility of causing a decrease in thermal conductivity in the obtained sintered body. The preferred amount is 350 mass ppm or less, more preferably 300 mass ppm or less.

The concentration of metal elements other than aluminum can be determined by dissolving the sample with an acid and using ICP emission spectroscopy.

In the production method of the present invention, the concentration of the carbon component existing as the unreacted raw material carbon contained in the obtained aluminum nitride powder can be 100 mass ppm or less. Since the carbon concentration in the existing state can be 100 mass ppm or less, when used for a member such as a heater plate for heating a semiconductor wafer with a metal inside, the interior metal reacts locally with carbon to form a heater plate. It is unlikely to cause the problem of causing a temperature distribution. It is preferably 70 mass ppm or less.

The amount of unreacted raw material carbon can be measured by the following method. That is, first, the aluminum nitride powder is dissolved in a 5 mol / L sodium hydroxide solution. As a result, carbon and aluminum oxide are not dissolved and remain as a residue. The residue is thermogravimetrically measured, and the weight loss at 350 ° C. to 600 ° C. is taken as the origin of combustion of the unreacted raw material carbon, and the content in the residue is determined.

From the weight of the melted aluminum nitride powder, the total residual weight, the weight of the residue used for thermogravimetric measurement, and the content of unreacted raw material carbon in the residue, the amount of unreacted raw material carbon in the aluminum nitride powder can be determined by the following formula. Can be calculated.

Amount of unreacted raw material carbon in aluminum nitride powder = (total residual weight x content of unreacted raw material carbon in residue / weight of residue used for thermal weight measurement) / weight of dissolved aluminum nitride powder

Further, the aluminum nitride powder obtained by the production method of the present invention usually has a specific surface area of 1.5 to 4.0 m ² / g by the BET 1-point method, and in most cases 2.0. The range is ~ 4.0 m ² / g.

The aluminum nitride powder obtained by the production method of the present invention has an appropriate particle size, few coarse particles, a spherical shape, and extremely few impurities, and therefore can be suitably used for various applications such as sintered body raw materials and filler applications. When used as a raw material for a sintered body, the aluminum nitride powder of the present invention is molded by a known method such as tape molding or press molding, and is sintered under normal pressure or pressurization. Specific applications include heat dissipation substrates such as LEDs and power modules, heaters for semiconductor manufacturing equipment, and electrostatic chucks.

Hereinafter, the present invention will be described in more detail, but the present invention is not limited to these examples. Various physical properties and related numerical values in Examples and Comparative Examples were measured by the following methods.

(1) 50% particle size (D50) and 90% particle size (D90) on a volume basis
For the average particle size of aluminum oxide powder, aluminum nitride powder, and mixed powder of aluminum nitride and unreacted raw material aluminum oxide, the sample was dispersed in an aqueous solution of sodium pyrophosphate with a homogenizer, and laser diffraction was performed using MICROTRAC HRA manufactured by Nikkiso Co., Ltd. Measured by method.

(2) Specific surface area The specific surface area of aluminum oxide powder, carbon powder, aluminum nitride powder, and mixed powder of aluminum nitride and unreacted raw material aluminum oxide is determined by the BET method using the flow type automatic surface area measuring device Flowsorb 2300 manufactured by Shimadzu Corporation. It was measured.

(3) Content of metal elements other than aluminum The content of metal elements other than aluminum in aluminum nitride powder is obtained by adding 2 mL of nitric acid and 10 mL of phosphoric acid to 0.8 g of a sample and heating and decomposing at 380 ° C for 20 minutes. Measured by ICP emission spectroscopic analysis using -1000-II.

(4) Total oxygen content The total oxygen content in the aluminum nitride powder was measured using an oxygen nitrogen analyzer EMGA-620W in ceramic manufactured by HORIBA, Ltd.

(5) Carbon content in aluminum nitride powder The carbon content in aluminum nitride powder was generated by burning the powder in an oxygen stream using the metal medium carbon analyzer "EMIA-110" manufactured by HORIBA, Ltd. Quantified from the amount of CO and CO2 gas.

(6) Content of unreacted raw material aluminum oxide in aluminum nitride powder The content of unreacted raw material aluminum oxide in aluminum nitride powder is as follows: First, 50 g of aluminum nitride powder is added to 300 mL of a 5 mol / L sodium hydroxide aqueous solution at 80 ° C. for 2 hours. It was dissolved under the conditions of. At this time, sodium hydroxide is consumed in the reaction with aluminum nitride, so it was added as necessary. Subsequently, the residue remaining undissolved was centrifuged to remove the supernatant. A 5 mol / L aqueous sodium hydroxide solution was added to suspend the residue, and the residue was centrifuged again. After repeating the same operation three times, the cells were collected by filtration through a 0.1 μm membrane filter. The filtrate on the filter was washed with hot water at 60 ° C. and dried. Then, the obtained residue was decarburized at 600 ° C. in an air atmosphere and then weighed, and calculated from the ratio to the weight of the aluminum nitride powder before melting.

It was confirmed by X-ray diffraction that the obtained residue was aluminum oxide.

(7) Content of unreacted raw material carbon in aluminum nitride powder First, 50 g of aluminum nitride powder was dissolved in 300 mL of a 5 mol / L sodium hydroxide solution at 80 ° C. for 2 hours. Subsequently, the residue remaining undissolved was centrifuged to remove the supernatant. Hot water at 60 ° C. was added to suspend the residue, and the residue was centrifuged again. After repeating the same operation three times, the cells were collected by filtration through a 0.1 μm membrane filter. The filtrate on the filter was washed with hot water at 60 ° C. and dried. A part or all of this residue was thermally weighed under the conditions of an air flow rate of 200 mL / min, a heating rate of 20 ° C./min to 200 ° C., and a heating rate of 5 ° C./min at 200 ° C. to 900 ° C., and 350. The content in the residue was determined by assuming that the weight loss at ° C. to 600 ° C. was derived from the combustion of the unreacted raw material carbon. From the weight of the melted aluminum nitride powder, the total residual weight, the weight of the residue used for thermogravimetric measurement, and the unreacted raw material carbon content in the residue, the amount of unreacted raw material carbon in the aluminum nitride powder is calculated by the following formula. did.

Carbon concentration due to the amount of unreacted raw material carbon in the aluminum nitride powder = (total residual weight x content of unreacted raw material carbon in the residue / weight of the residue used for thermal weight measurement) / of the dissolved aluminum nitride powder weight

(8) Average circularity The average circularity of the aluminum nitride powder is as follows for particles with a maximum length of 2 μm or more from the electron micrographs taken at 10000 times using an electrolytic radiation scanning electron microscope S-5500 manufactured by Hitachi High-Technologies. Twenty particles and 80 primary particles were randomly selected, and each particle was image-analyzed to obtain the average value of the circularity obtained.

(Circular ratio is the range of half the length of the surface of the object from the center of gravity of the object to the equivalent circle diameter of the object (the diameter of a perfect circle having an area equal to the area of the object). It is a value obtained by dividing the area of the included surface by the area of a perfect circle whose diameter is the area equivalent to the circle of the object and multiplying by 100.)

(9) Cohesion degree The cohesion degree of the mixed powder of aluminum nitride and unreacted aluminum oxide was calculated by the following formula (1).
Cohesion = SA / ((4 × π × (D50 / 2) ² ) / (ρAlN × 4/3 × π × (D50 / 2) ³ ) (1)

In the above formula (1), D50 is the 50% particle size [m] based on the volume of the object, SA is the specific surface area per unit weight of the object [m ² / g], π is the circumference ratio, and ρAlN is. The true specific surface area of aluminum nitride [g / m ³ ]. The degree of cohesion is a value obtained by dividing SA by the specific surface area per unit weight of a true sphere of aluminum nitride having D50 as a diameter. That is, the degree of cohesion becomes 1 when the object is a true sphere of aluminum nitride, and increases as the distance from the true sphere increases.

(10) Conversion rate from aluminum oxide to aluminum nitride The conversion rate of aluminum nitride powder and reaction mixed powder (mixed powder of aluminum nitride and unreacted aluminum oxide) was determined by the RIR method using the Rigaku X-ray diffractometer SmartLab. rice field.

(11) Thermal Conductivity The thermal conductivity of the aluminum nitride sintered body was measured by a laser flash method using LFA-502 manufactured by Kyoto Denshi Kogyo.

[Example 1]
100 parts by mass of α-alumina having a D50 of 0.7 μm and a specific surface area of 5.0 m ² / g and 50 parts by mass of carbon black having a specific surface area of 125 m ² / g were mixed by a ball mill and filled in a graphite container. .. Then, in a nitrogen atmosphere, the first step was carried out under the condition of 1550 ° C. for 3 hours, and then in the nitrogen atmosphere, the second step was carried out under the condition of 1680 ° C. for 4 hours, and carbon and aluminum nitride were further carried out. Mixed powder was obtained.

It should be noted that the agglomeration of the reaction mixed powder obtained by reducing and nitriding until the conversion rate at the time when the first step is completed under the above conditions and the point at which the conversion rate reaches 100% at the same temperature as the step, and then decarburizing. The degree was confirmed by conducting a separate test.

The above-mentioned reaction mixed powder obtained after the second step was decarburized in an air atmosphere at 630 ° C. for 8 hours to obtain an aluminum nitride powder. By the above method, the average particle size, average circle equivalent diameter, average circularity, total oxygen concentration, unreacted raw material aluminum oxide amount, unreacted raw material carbon amount, and metal element amount of the obtained aluminum nitride powder were measured. did. The results are shown in Table 1.

Yttrium oxide was added to 100 parts by mass of the obtained aluminum nitride powder so as to be 5 parts by mass, and a molded product having a thickness of 3 mm was produced by uniaxial press molding. It was fired at 1800 ° C. for 5 hours in a nitrogen atmosphere to obtain an aluminum nitride sintered body. The thermal conductivity of the sintered body was 185 kW / (m · K).

[Example 2]
Aluminum nitride powder was prepared in the same manner as in Example 1 except that the reaction time in the first step was 6 hours.

[Example 3]
Aluminum nitride powder was prepared in the same manner as in Example 1 except that the reaction time in the first step was set to 2 hours.

[Example 4]
Aluminum nitride powder was prepared in the same manner as in Example 1 except that the reaction time in the second step was 10 hours.

[Example 5]
Aluminum nitride powder was produced in the same manner as in Example 1 except that the reaction temperature in the first step was 1500 ° C. and the reaction time was 10 hours.

[Comparative Example 1]
Aluminum nitride powder was produced in the same manner as in Example 1 except that all the steps were set to 1630 ° C. and 10 hours.

[Comparative Example 2]
Aluminum nitride powder and a sintered body were produced in the same manner as in Example 1 except that all the steps were set to 1680 ° C. and 5 hours. By performing the nitriding reaction at a high temperature, more raw materials are trapped in places where they do not come into contact with the reaction gas, and they tend to remain as unreacted raw materials. The thermal conductivity of the sintered body was 175 kW / (m · K).

[Comparative Example 3]
An aluminum nitride powder was produced in the same manner as in Example 1 except that the first step was a condition of 1550 ° C. and 1 hour. Since the conversion rate at the end of the first step is low, the rate of nitriding reaction at substantially high temperature is high, and as a result, many raw materials are trapped in places that do not come into contact with the reaction gas, and remain as unreacted raw materials. It's getting easier.

[Comparative Example 4]
Aluminum nitride powder was produced in the same manner as in Example 1 except that the first step was set to 1630 ° C. for 5 hours.

Claims (2)

It is a method of producing aluminum nitride powder by heating aluminum oxide powder in a nitrogen atmosphere and nitriding it.
Use a raw material alumnium oxide with a specific surface area of 2 to 12 m ² / g.
Heating in a nitrogen atmosphere consists of a first heating step in which the reaction proceeds until a conversion of at least 60% or more is achieved, and a second heating step in which the reaction is heated under conditions higher than the first heating step. The method for producing aluminum nitride powder, wherein the temperature of the first heating step is 1450 ° C. or higher and 1580 ° C. or lower, and the temperature of the second heating step is more than 1600 ° C. and 1800 ° C. or lower. The manufacturing method according to claim 1, wherein the temperature of the second heating step is 100 to 150 ° C. higher than the temperature of the first heating row step.

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