WO2015008924A1

WO2015008924A1 - Method of preparing aluminum nitride using wet-mixed boehmite slurry

Info

Publication number: WO2015008924A1
Application number: PCT/KR2014/003205
Authority: WO
Inventors: Gwang Hee Shin; Pyoung Seop Seo; Chang Yel Yang; Jong Oh Baek; Sang Chun Park; Dae Woong Kim
Original assignee: KC Corp
Current assignee: KC Corp
Priority date: 2013-07-17
Filing date: 2014-04-14
Publication date: 2015-01-22
Anticipated expiration: 2016-01-17
Also published as: KR20150009803A; KR101728517B1

Abstract

According to a method comprising obtaining a boehmite slurry by performing a hydrothermal synthesis of an aqueous solution of an aluminum hydroxide, wet-mixing the boehmite slurry and carbon, sintering the resulting mixture under a nitrogen atmosphere, and heat treating the sintered product in an air atmosphere for decarbonization, homogenized nitridation reaction may be performed through a wet process, and a homogeneous and stable aluminum nitride powder having high purity may be economically obtained in high yield.

Description

DESCRIPTION

METHOD OF PREPARING ALUMINUM NITRIDE USING WET-MIXED BOEHMITE SLURRY

FIELD OF THE INVENTION

The present invention relates to a method of preparing aluminum nitride used for the manufacture of an equipment for manufacturing a semiconductor, a heat-radiating substrate, etc., and more particularly, to a method of manufacturing aluminum nitride using a wet-mixed boehmite slurry by carbothermal reduction.

BACKGROUND OF THE INVENTION

Aluminum nitride (A1N) is a material having low thermal expansion coefficient (4.5xl0^-6) and high thermal conductivity (at least 150 W/mK), and thus, withstands strong thermal impact and has high corrosion resistance with respect to a fluorine gas. Therefore, the use of aluminum nitride is growing as a part used in a dry etching process and a chemical vapor deposition (CVD) process during conducting a semiconductor process, in which a fluorine gas is positively necessary and strong thermal impact is generated. In addition, aluminum nitride is used as a material of a semiconductor equipment. Particularly, aluminum nitride may be applied in various fields such as an electrostatic chuck (ESC), a member for manufacturing a semiconductor, a heat sink for a light-emitting diode (LED), a substrate for controlling power source of a hybrid car, a part of a ceramic chamber, a heater, a direct bonded copper (DBC) substrate, an additive of a resin, a heat radiating sheet, a submerged nozzle, a hot plate, and the like.

Methods for preparing aluminum nitride known until now include a carbothermal reduction, a direct nitridation, a flotation nitridation, a CVD, a vapor phase method, an organometallic precursor method, and the like.

Among the methods, a nitridation process by the carbothermal reduction is a method of easily obtaining aluminum nitride having relatively high purity, and a method developed by Tokuyama Co. is disclosed at present (Korean Registration Patent Nos. 10-0029699 and 10-0029764). However, according to the method, the nitridation process is performed by using a sintered material of alumina, and the homogeneous mixing of alumina and carbon is difficult.

In addition, a high temperature of about 1,700°C or above is necessary, and a production yield is not high.

In addition, Korean Registration Patent No. 10-0788196 discloses a method of preparing aluminum nitride by dry mixing a dry boehmite powder with a carbon-containing powder and performing a nitridation process.

However, in this method, carbon is dry mixed with an alumina sintered material or the dried boehmite powder during the mixing process of raw materials for the nitridation process, and homogeneous mixing is difficult, and the purity of finally obtained aluminum nitride is not high.

Accordingly, a novel and efficient method of preparing homogeneous aluminum nitride having high purity in high yield is required.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a novel method of efficiently preparing homogeneous aluminum nitride having high purity with high yield.

In accordance with one aspect of the present invention, there is provided a method of preparing aluminum nitride comprising:

(a) preparing an aluminum hydroxide,

(b) obtaining a boehmite slurry by performing a hydrothermal synthesis of an aqueous solution of the aluminum hydroxide,

(c) wet-mixing the boehmite slurry and carbon, and sintering the resulting mixture under a nitrogen atmosphere, and

(d) heat treating the sintered product in an air atmosphere for decarbonization.

In accordance with another aspect of the present invention, there is provided aluminum nitride prepared by the above-described method.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, which respectively show:

Fig. 1 : a flowchart of a process of preparing aluminum nitride step by step according to an embodiment of the present invention;

Fig. 2: mapping images of energy dispersive X-ray spectroscopy (ED AX) of (a) a wet mixture of boehmite and carbon, and (b) a dry mixture thereof; and

Fig. 3: images of X-ray diffraction (XRD) analysis of (a) aluminum nitride prepared by sintering a wet mixture of boehmite and carbon at 1,700°C, and (b) aluminum nitride prepared by sintering a dry mixture thereof at 1,700°C.

DETAILED DESCRIPTION OF THE INVENTION

Method of preparing aluminum nitride

Hereinafter, a method of preparing aluminum nitride according to the present invention will be particularly described step by step referring to Fig. 1.

Preparation of aluminum hydroxide

This step is a step of preparing an aluminum hydroxide (Steps SO and S 1 in Fig. 1).

Aluminum hydroxide used as a raw material may be prepared through a common Bayer process. Aluminum hydroxide prepared by the Bayer process usually includes a trace amount of impurities.

The aluminum hydroxide may have a powder state, and may have a particle size of 0.1 μηι to 50 μπι, and may preferably have the particle size of 0.2 μηι to 30 μπι.

Preparation of boehmite slurry

This step is a step of preparing a slurry state boehmite, which is a transition type alumina, by putting aluminum hydroxide with water in a reaction bath for a hydrothermal synthesis and performing a pyrolysis process under steam pressure of high temperature (Steps S2 and S3 in Fig. 1).

The water used in the hydrothermal synthesis may be distilled water or industrial water.

The amount of the water with respect to aluminum hydroxide is controlled so that the concentration of an aqueous aluminum hydroxide solution is in the range of 1 to 10 M, and more preferably, in the range of 1.5 to 5 M. In the above-described range, stirring may be easily performed, and the preparation of the slurry may be completed in a short time.

Appropriate stirring conditions in the reaction bath for the hydrothermal synthesis may be 150 times per minute or less. In this range, the generation of precipitation of the slurry may be restrained, and disturbance with respect to the hydrothermal synthesis may be reduced.

During the transformation of the aluminum hydroxide to the transition type alumina in the reaction bath for the hydrothermal synthesis in the pressurized conditions at high temperature, a trihydrated compound is transformed into a monohydrated compound as illustrated in the following Reaction 1. In this case, Na⁺ ions present in aluminum nitride crystal react with activated water molecules to form NaOH or Na₂CO₃ so as to be deionized from aluminum hydroxide.

[Reaction 1]

Al₂O₃-3H₂O → Al₂O₃ H₂O + 2H₂O

The aluminum hydroxide contains about 34.5% of crystallized water, and the crystallized water may be eliminated through the pyrolysis process in high temperature and high pressure conditions of the hydrothermal synthesis. The boehmite thus obtained in a crystallized water state contains about 16 to 18 wt% of water, and during performing the reaction, the Na⁺ ions may be eliminated from the aluminum hydroxide crystal and increase the activity of contacting an aqueous solution, thereby improving the eliminating efficiency of the Na⁺ ions.

Preferable hydrothermal synthesis conditions are as follows. A reaction temperature may be controlled in the range of 120°C to 250°C, more preferably in the range of 190°C to 230°C. A reaction pressure may be controlled in the range of 4 to 25 bars, more preferably in the range of 15 to 20 bars. A reaction time may be controlled in the range of 30 to 150 minutes, more preferably in the range of 60 to 120 minutes. A pH range at the initial stage of the reaction may be controlled in the range of 3.0 to 8.0, more preferably in the range of 5.5 to 7.5.

Within the above-described ranges, the boehmite thus prepared may appropriately exhibit the properties of transition type alumina having high pyrolysis temperature.

According to an aspect of the present invention, the hydrothermal synthesis reaction may be performed with a pH of 3.0 to 8.0, at a temperature of

120°C to 250°C under a pressure of 4 to 25 bars for 30 to 150 minutes at the initial stage of the reaction.

According to another aspect of the present invention, the hydrothermal synthesis reaction may be performed with a pH of 5.5 to 7.5, at a temperature of

190°C to 230°C under a pressure of 15 to 20 bars for 60 to 120 minutes at the initial stage of the reaction.

During performing the hydrothermal synthesis, oxalic acid (C₂H₂O₄) is added as a pH controlling agent to control the acidity of a reaction solution. Na⁺ ions produced during the reaction may form sodium oxalate (Na₂C₂O₄) and may be stably removed. In this case, the amount of the oxalic acid in the reaction solution may be preferably in the range of 0.01 to 0.1 M. In this range, the formation of a complex due to the dissolution of aluminum may be restrained, and the reaction of soda may be further activated.

As a result, a boehmite slurry may be obtained. An average particle size of a solid may be from 5 μιτι to 60 μηι, and the purity of the boehmite may be from 99.9% to 99.999%.

A mixing process, a dispersing process, etc. may be additionally performed with respect to the boehmite slurry thus obtained to control the viscosity thereof appropriately in a subsequent process.

Wet mixing (boehmite slurry and carbon)

In this step, the boehmite slurry obtained in the previous step is wet mixed with carbon (Step S4 in Fig. 1).

The boehmite slurry used in this step may have a solid content ranging from 3 to 70 wt%, and a dispersing solvent may be additionally used to control the concentration. As the dispersing solvent, water, alcohols or a mixture solvent thereof may be used, for example, a mixture solvent of water/isopropyl alcohol (from 90: 10 to 50:50, w/w) may be used. To mix the boehmite slurry with carbon, a Lodiger mixer, a kneader mixer, a non-gravitational mixer, etc. may be used. For example, the boehmite slurry may be dispersed well by using the kneader mixer and then homogeneously mixed by using a bead mill, etc. in liquid conditions.

During conducting a nitridation process, a mixing ratio (weight ratio) of the boehmite slurry and carbon may be in a range of from 30:70 to 70:30, for example, from 40:60 to 60:40.

Nitridation process

In this step, a wet mixture of the boehmite slurry thus obtained and carbon is sintered at high temperature under a nitrogen atmosphere (nitridation reaction, Step S5 in Fig. 1). As a result, aluminum nitride is synthesized by the nitridation process through carbothermal reduction. In this case a COx gas is emitted as a by-product, and unreacted carbon remains.

The flowing rate for injecting a gas containing nitrogen may be from 0.5 to 10 L/min, for example, from 5 to 7 L/min.

In addition, a hydrogen gas, an ammonia gas, or a mixture gas thereof other than the nitrogen gas may be additionally injected and a sintering process may be performed. When the hydrogen gas is additionally injected, the volume ratio on the injecting amounts of the nitrogen gas and the hydrogen gas is preferably from 7:3 to 4:6.

In addition, a sintering time may be from 1 to 7 hours, for example, from 2 to 5 hours. In addition, a sintering temperature may be from 1,200°C to 1,800°C, for example, from 1,400°C to 1,600°C. In addition, a pressure of a reactor during sintering may be from 0.1 to 1 bar, for example, from 0.3 to 0.5 bars.

Decarbonization process

In this step, the product obtained in the previous step is heat treated (sintered at a low temperature) in the air to remove carbon remaining in the product (decarbonization, Step S6 in Fig. 1). As a result, the remaining carbon is eliminated as a COx gas state, thereby producing a pure aluminum nitride powder (Step S7 in Fig. 1). The heat treatment may be performed in the air, more preferably in an oxygen gas atmosphere.

A temperature of the decarbonization may be from 600°C to 900°C, for example, from 700°C to 800°C. A heat treatment time may be from 0.5 to 5 hours, for example, from 2 to 4 hours. In addition, a pressure of a reactor during the heat treatment may be from 0.1 to 1 bar, for example, from 0.3 to 0.5 bars.

Then, an aluminum nitride powder may be further pulverized and sorted.

As a result, the yield of the aluminum nitride thus obtained may be from 80% to 99%, and may be from 90% to 95%. The average particle diameter of the aluminum nitride powder thus obtained may be from 1 μιη to 5 μπι. In addition, the purity of the aluminum nitride powder thus obtained may be from 95% to 99%.

According to the above-described method, aluminum nitride may be synthesized by the nitridation of a boehmite slurry through carbothermal reduction. Therefore, when compared to a conventional process of synthesizing aluminum nitride from sintered a-alumina, more homogenized nitridation reaction may be performed through a wet process, and a homogeneous and stable aluminum nitride powder having high purity may be economically obtained in high yield.

Thus, the aluminum nitride prepared according to the present invention may be usefully used for the manufacture of an equipment for manufacturing a semiconductor, a heat radiating substrate, etc. Hereinafter, the present invention will be explained in more detail through preferred embodiments. However, the following embodiments are only for the illustration of the present invention, and the scope of the present invention is not limited thereto. Example 1: Preparation of aluminum nitride using boehmite slurry

Step (1): Preparation of boehmite slurry

A specimen of an aluminum hydroxide prepared by a Bayer process and having an average particle diameter of 1.5 μηι (composition: Al₂O₃-3H₂O 99.7%, Na₂O 0.25-0.35%, SiO₂ 0.01%, Fe₂O₃ 0.01%, LOI (loss on ignition): 33.4%, water: 0.5%, product name: KH-101LC, manufacturer: KC Co., Ltd.) was used. A stainless steel autoclave of which the temperature was controlled and the pressure was measured was used as a reaction bath for hydrothermal synthesis. 300 g of the specimen was put into a 2 L stainless steel autoclave, and 1L of water was added thereto. Then, oxalic acid (H₂C₂O₄-2H₂O, manufacturer: Duksan pure chemicals Co., Ltd.) was added to the reaction solution by the concentration of 0.06 M to control its pH to 5.5. Then, the reaction temperature was increased to 200°C under the pressure of 16 bar, and a hydrothermal synthesis reaction was performed for 60 minutes. Then, the reaction temperature was gradually decreased. The boehmite slurry thus prepared was filtered by using a cloth filter type vacuum filter having permeability of 0.6 cc/cni . After filtering, industrial water corresponding to the amount of the specimen used in the reaction was divided into two and used for washing. As a result, a boehmite slurry was prepared. An average particle size of a solid was 8.9 μη , and the purity of boehmite was 99.999%.

Step (2): Wet mixing of boehmite slurry

The boehmite slurry obtained in the above Step (1) was mixed by using a kneader mixer and was dispersed using a bead mill. The solid content of the boehmite slurry was controlled to 50 wt% by using a mixture solvent of water/isopropyl alcohol (50:50, w/w). The boehmite slurry and carbon black (manufactured by EVONIK Co.) were mixed with the weight ratio of 50:50, and were dispersed well by using a kneader mixer and homogeneously wet mixed by using a bead mill in liquid conditions.

Step (3): Nitridation and decarbonization

A nitrogen gas was injected to the mixture obtained in the above Step (2) by the flow rate of 5 L/min, while subjecting to carbothermal reduction and nitridation of the mixture at 1,700°C for 5 hours. Then, the decarbonization of the resulting product was performed at 800°C for several hours to produce an aluminum nitride powder.

Comparative Example 1: Preparation of aluminum nitride using dry boehmite powder. Step (1): Preparation of boehmite slurry

A boehmite slurry was prepared by performing the same procedure as described in Step (1) of Example 1.

Step (2): Drying and dry mixing of boehmite

The boehmite slurry obtained in the above Step (1) was washed and filtered. The filtered cake thus obtained was used to prepare a slurry, having a high concentration of 40%. This slurry was dried by means of a direct hot air drying method using a spray drier. The temperature of the hot air supplied by the drier was 180 to 250°C, and the remaining conditions were the same as those of a conventional drying method. The dry boehmite powder thus obtained and carbon black (manufactured by EVONIK Co.) were mixed with the weight ratio of 50:50, and were homogeneously dry mixed by using a mixer and a bead mill.

Step (3): Nitridation and decarbonization

An aluminum nitride powder was produced by performing the nitridation and decarbonization of the dry mixture of the above Step (2) under the same conditions as Step (3) of Example 1.

Experimental Example 1: Comparison of carbon distribution according to the mixing method

With respect to the wet mixture of Step (2) of Example 1 and the dry mixture of Step (2) of Comparative Example 1 , mapping images were obtained by an energy dispersive X-ray spectroscopy (ED AX) and are illustrated in Fig. 2.

Fig. 2(a) is an EDAX mapping image of a wet mixture obtained in Step (2) of Example 1 , and it was confirmed that carbon (black dots) is relatively homogeneously distributed. On the contrary, Fig. 2(b) is an EDAX mapping image of a dry mixture obtained in Step (2) of Comparative Example 1, and it was confirmed that carbon (black dots) is unhomogeneously distributed and gathered in several places.

Through the result, the mixing of the boehmite and carbon is found to be performed more homogeneously by the wet mixing than by the dry mixing.

Experimental Example 2: Comparison of aluminum nitride synthesis according to the mixing method

X-ray diffraction analysis (XRD) was conducted on each of the aluminum nitrides finally produced in Example 1 and Comparative Example 1, and the results were illustrated in Fig. 3.

Fig. 3(a) is an XRD image of the aluminum nitride obtained in Example 1, which suggests that specific peaks of aluminum nitride are shown well, and impurity peaks are not found. On the contrary, Fig. 3(b) is an XRD image of the aluminum nitride obtained in Comparative Example 1, and it was revealed that peaks of an intermediate, AION impurity are found other than aluminum nitride peaks.

From the results, it is interpreted that homogeneous nitridation reaction materials can be provided by the wet mixing of the boehmite and carbon, whereas such materials cannot be provided by the dry mixing.

The above results show that better synthetic transition of aluminum nitride could be achieved by using a boehmite slurry than using a dry boehmite powder.

While the embodiments of the subject invention have been described and illustrated, it is obvious that various changes and modifications can be made therein without departing from the spirit of the present invention which should be limited only by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS :

1. A method of preparing aluminum nitride comprising:

(a) preparing an aluminum hydroxide,

2. The method of claim 1, wherein in step (a), the aluminum hydroxide is prepared through a Bayer process.

3. The method of claim 1, wherein in step (b), a concentration of the aqueous solution of the aluminum hydroxide is in the range of from 1.5 to 5 M.

4. The method of claim 1, wherein in step (b), the hydrothermal synthesis is performed at a temperature of from 120°C to 250°C under a pressure of from 4 to 25 bars for 30 to 150 minutes.

5. The method of claim 1, wherein in step (c), the boehmite slurry has a solid content ranging from 3 to 70 wt%.

6. The method of claim 1, wherein in step (c), the wet-mixing is conducted in water, alcohols or a mixture solvent thereof.

7. The method of claim 1, wherein in step (c), a weight ratio of the boehmite slurry and carbon is in a range of from 30:70 to 70:30.

8. The method of claim 1, wherein in the sintering of step (c), a hydrogen gas, an ammonia gas, or a mixture gas thereof other than the nitrogen gas is additionally injected.

9. The method of claim 1, wherein in step (c), the sintering is performed at a temperature of from 1,400°C to 1,600°C for 2 to 5 hours.

10. The method of claim 1, wherein in step (d), the heat treatment is performed at a temperature of from 600°C to 900°C for 2 to 4 hours.