KR100252590B1 - Process for manufacturing ultra-fine powders by using thermal plasma - Google Patents

Abstract

PURPOSE: Provided is a preparation method of ultrafine aluminum nitride(AlN) through a reaction evaporated Al with N2 or NH4OH gas by using thermal plasma. CONSTITUTION: The method is characterized in that Al bulk is evaporated by plasma jet using the mixed gas of Ar and N2, plasma-generating gas, and flowing a reaction gas such as N2 or NH4OH gas, preferably NH4OH, in a rate of 1.5L of/min. to a place 50mm higher than a nozzle of plasma jet from a low temperature region. The resultant AlN powder has 50-100m¬2/g of specific surface area.

Description

Method of manufacturing aluminum nitride ultrafine particles using thermal plasma

The present invention relates to a method for producing aluminum nitride ultrafine particles using thermal plasma, and more particularly, to a method of producing aluminum nitride ultrafine particles by evaporating an aluminum mass using thermal plasma and reacting the vaporized aluminum with nitrogen or ammonia gas. will be.

Aluminum nitride (AIN) has attracted attention as a new semiconductor substrate material due to its high thermal conductivity (about 200 W / mK), high insulation, and low coefficient of thermal expansion (4.1x10 ^-6 / ° C). The production process of aluminum nitride which is currently applied generally includes direct nitriding (2Al + N ₂ = 2AIN) and carbon reduction nitriding (Al ₂ O ₃ + 3C + N ₂ = 2AlN + 3CO).

The direct nitriding method is an economical process for producing aluminum nitride using cheap raw materials and low energy. Once the reaction starts, the nitriding reaction proceeds automatically by exothermic reaction, but the melting point of aluminum (660 ℃) starts the nitriding reaction. Lower than the temperature (800 ° C.), the aluminum powder agglomerates with each other before reacting, so the rate of conversion to AIN is low.

In addition, the carbon reduction nitriding method can produce a powder having a relatively small particle size and high purity, but the raw material powder is expensive, a lot of energy is required due to the endothermic reaction, and the prepared powder requires a decarburization process to remove excess carbon. .

In order to apply AIN to electronic materials, a compact sintered compact is required. Therefore, a high-purity, submicron-sized uniform powder is required as a raw material powder. The characteristics of the powder are closely related to the product manufacturing process and the sintered compact. As a result, high-purity, ultra-fine AIN powder for application to semiconductor substrate materials has been studied. Floating nitriding method, which complements the direct nitriding method, reacts aluminum halide and ammonia, and plasma gas phase reaction method. come.

The gas phase reaction method using plasma is a process that is attracting attention in various fields recently because of its excellent characteristics such as ultra high temperature, high activity and rapid temperature gradient (cooling rate: ~ 10 ⁶ K / s). Method of synthesizing ultrafine particles by reaction with ammonia gas by injecting and evaporating an alkali compound (Itatani, K., Sano, K., Howell, FS, Kishioka A. and Kinoshita, M .: J. Mat. , 28, 1631 (1993)) and aluminum bulk by evaporation by nitrogen plasma followed by reaction with ammonia or nitrogen gas (Ageorges, H., Megy, S., Chang, K., Baronnet, J.-M., Williams, JK and Chapman, C .: Plasma Chemistry & Plasma Processing, 13, 613 (1993)., Kasra Etema:: oama Chemistry & Plasma Processing, 11, 41 (1991) .Uda, M. , Ohno, S. and Okuyama, H.,: Yogyo-Kyokai-Shi, 95, 86 (1987).).

The present inventors have completed the present invention by studying a method for producing aluminum nitride ultra-fine particles by evaporating the aluminum mass (bulk) using a thermal plasma and then reacted with nitrogen or ammonia gas.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for efficiently producing ultra-fine aluminum nitride particles having a small particle diameter by producing aluminum nitride from an inexpensive aluminum mass using thermal plasma.

1 shows an apparatus used in the manufacturing method of the present invention.

2 shows the formation mechanism of aluminum and aluminum nitride ultra-fine particles.

3 is a photograph observing the growth of dendrite and the reaction of aluminum evaporation on the surface of the aluminum mass,

3a is a photograph observed when the aluminum mass is melted,

3b is a photograph observed when 30 seconds have elapsed after the melting of aluminum,

3c is a photograph observed when 3 minutes have elapsed after the aluminum is melted.

4 is a graph showing an X-ray diffraction pattern of powder and dendrite formed by thermal plasma.

5 is a photograph of dendrite grown on the surface of an aluminum mass,

5a is a photograph of an initial state,

5b is a photograph of the maximum growth state.

6 is a graph showing an X-ray diffraction pattern of powders prepared at various ammonia flow rates.

6a is 0 1 / min, 6b is 0.5 1 / min, 6c is 1.0 1 / min and 6d is 2.0 1 / min, respectively.

7 is a graph showing the content of aluminum nitride in the resulting powder according to the ammonia flow rate.

8 is a graph showing the formation cast free energy according to temperature.

9 is an SEM image of a powder prepared under different conditions,

9a is a photograph of the powder produced under the conditions of FIG. 6a, and FIG. 9b is a condition of FIG. 6d, respectively.

10 is a graph showing particle size distribution measured by a particle size analyzer.

11 is a STEM picture of the powder and dendrite produced,

Figure 11a is a powder collected on the chamber wall, Figure 11b is another form of Figure 11a, Figure 11c is a partially sintered powder, and Figure 11d is a photograph of a dendrite.

The method for producing aluminum nitride ultrafine particles using the thermal plasma of the present invention comprises generating a plasma jet to evaporate an aluminum mass and reacting the evaporated aluminum with nitrogen or ammonia gas.

As the plasma generating gas used in the present invention, a mixed gas of nitrogen and argon is used. In a nitrogen-arcon mixed plasma, nitrogen molecules dissociate into atomic nitrogen and dissolve in the molten aluminum mass. The dissolved nitrogen atoms recombine to form a molecular state and are released from the molten aluminum into the outer region of the plasma, and the heat of reaction causes the aluminum to evaporate.

The evaporated aluminum is converted into aluminum nitride by reacting with nitrogen or ammonia as the reaction gas. At this time, it is better to use ammonia than nitrogen in order to increase the conversion rate, and in the case of ammonia, the conversion rate varies depending on the injection position. The conversion rate is improved when the injection position of ammonia is a low temperature region.

Hereinafter, with reference to the drawings will be described the present invention in more detail.

[Manufacturer]

1 shows an apparatus used in the manufacturing method of the present invention, wherein the manufacturing apparatus of the present invention comprises a plasma torch part, a reaction part, and an exhaust part. The torch is composed of a water-cooled anode nozzle and a tungsten cathode rod, and generates a plasma jet by flowing argon and nitrogen gas between the anode nozzle and the cathode rod. The reactor consists of a stainless double tube that is water cooled, and a holder is installed in the center of the reactor to control the distance between the plasma jet and the specimen. In addition, a gas injector is installed between the torch and the reactor to inject the reactant gas, and a coil for injecting the reactant gas and the quenching gas is installed in the reactor.

[Evaporation Process of Aluminum]

Figure 2 shows the evaporation and reaction of aluminum under a direct-current plasma jet, as modeled by Uda, M., Ohno, S. and Okuyama, H., Yogyo-Kyokai-Shi, 95, 86 (1987). It shows the process. First, nitrogen atoms are easily dissociated in the argon and nitrogen mixed plasma and rapidly dissolved in molten aluminum. The dissolved nitrogen atoms are released from molten aluminum in the form of molecules because the temperature of the aluminum surface is not high enough to maintain the atomic state of nitrogen. That is, the supersaturated nitrogen atoms are recombined and released by the gasification reaction (2N-> N ₂ ), and the aluminum surface rapidly evaporates due to the reaction heat. In addition, a large number of dendrites are formed on the surface by the partial pressure of nitrogen released by rapidly nitriding and recombining the aluminum surface. The capillary action generated between them causes the molten aluminum to rise, from which the protrusions gather and grow to the top of the plasma jet. The grown protrusions are immediately evaporated and extinguished by the high temperature at the top of the plasma, and another protrusion is formed on the aluminum surface. At this time, a large amount of aluminum vapor is discharged to the outside of the plasma is converted to ultra-aluminum ultrafine particles by a sudden temperature gradient or combined with the surrounding nitrogen source is converted to ultra-nitride ultrafine particles.

3 is a photograph observing a phenomenon that aluminum is evaporated and reacted by a plasma jet. The aluminum bulk is melted by a plasma jet, i.e., FIG. 3a, and after about 30 seconds a large number of protrusions are formed on the surface of the molten aluminum, emitting a strong light (figure 3b). At this time, the entire inside of the reactor is filled with aluminum vapor, and the formed protrusion continues to grow to the top of the plasma flow (FIG. 3C), and rapidly evaporates from the top. When the grown projections are evaporated and extinguished, another projection is formed on the aluminum surface, and the process of forming, growing, and disappearing is repeated. In this process, aluminum evaporates rapidly and a large amount of aluminum vapor is produced.

The protrusions formed on the aluminum surface are porous and brittle, and as a result of X-ray diffraction analysis, they appear as polycrystalline aluminum nitride phase as shown in FIG. 4, and the powder produced from aluminum vapor has only a clear aluminum peak. That is, the protrusions formed on the aluminum surface were completely converted to aluminum nitride, and the aluminum vapor was produced as aluminum powder without reacting with the surrounding nitrogen gas.

The protrusions formed on the aluminum surface grow into white porous crystals and appear in different forms depending on the reaction gas. When only nitrogen is injected into the reaction gas, it grows significantly, and when ammonia is injected, growth is suppressed. In addition, hydrogen is injected into the nitrogen gas to suppress the growth of the projections, so it is estimated that hydrogen plays a role in suppressing the growth.The shape varies depending on the degree of cooling of the specimen. do.

5 is a photograph observing the initial shape (a) of protruding growth of aluminum surface and the shape of aluminum surface (b) after maximum growth when the flow rate of ammonia gas is 1.0 l / min. For example, the maximum growth height is 40V, 150A. It reached about 50 mm on condition of.

[Influence of Ammonia Gas Flow Rate and Injection Location]

As shown in FIG. 4, the low conversion rate of the collected powder is due to the low reactivity of the nitrogen gas, which is an atmospheric gas. see. This is because, as shown in FIG. 2, the activated nitrogen in the plasma region acts as a driving force for evaporation of aluminum, but the vaporized aluminum vapor is released into the region outside the plasma to react with the atmosphere gas. That is, since nitrogen is present in a high activated state in the plasma region but immediately recombined in the plasma outer region to become inert nitrogen molecules, the reactivity with aluminum vapor emitted to the plasma outer region is very small.

When ammonia is injected into the reaction gas to improve the conversion rate of the unreacted aluminum vapor, as shown in FIG. 6, the aluminum nitride peak appears strongly as the ammonia flow rate increases, and as shown in FIG. 6d, the ammonia flow rate is 1.5. At 1 / min it is completely converted to aluminum nitride. In addition, in order to examine the influence of the ammonia gas injection position, the conversion rate when ammonia gas is injected while changing the distance from the plasma jet nozzle is shown in FIG. Ammonia gas is preferably injected at a position of 50 mm or more from the plasma jet nozzle. The reduction in the conversion rate when ammonia gas is injected at a position of 25 mm or more from the plasma jet nozzle (the specimen is 40 to 50 mm from the nozzle) is considered to be because the ammonia gas is easily decomposed into nitrogen molecules having low reactivity at high temperatures. This was confirmed when ammonia gas was completely decomposed when ammonia gas was injected into the plasma region through an injector provided between the torch and the reactor. 8 shows the change in Gibbs free energy for the decomposition reaction of ammonia and the formation reaction of aluminum nitride (extrapolation above 2000 K). 8A and 8B illustrate that the molten aluminum or aluminum vapor reacts with atomic nitrogen in the plasma region, so that the molten aluminum surface is quickly nitrided. In addition, the aluminum vapor discharged to the outside of the plasma reacts with nitrogen or ammonia gas, and it can be seen from FIGS. 8c and 8d that the aluminum vapor has excellent reactivity with ammonia. However, at temperatures above 456 K, the ammonia decomposition reaction proceeds competitively as shown in FIG. 8 g, thereby reducing the reaction rate with aluminum. Therefore, in order to effectively react the aluminum vapor, it is preferable to inject ammonia gas at a position capable of suppressing the decomposition of ammonia.

The present invention will be described by the following examples. The following examples are merely illustrative of the invention and do not limit the scope of the invention.

EXAMPLE

Manufacturing method and analysis method

Experimental conditions are as follows.

Power 120-150A, 6-7kW

Plasma generating gas flow rate Ar: 5 l / min

N ₂ : 0-5 l / min

Reaction gas flow rate N ₂ : 0-5 l / min

NH ₃ : 0-5 l / min

Al specimen position (distance from the anode nozzle) 20-50 mm

Injection position of reaction gas (distance from anode nozzle) 15-75mm

The aluminum specimens were made of a 99.9% aluminum ingot into a 15 g cylindrical shape and placed in a water-cooled copper crucible. Before generating the plasma, the reactor was first placed in a nitrogen atmosphere and an argon-nitrogen mixed plasma was generated. The reaction gas was injected from a coil installed in the reaction gas injector or reactor while raising the specimen into the plasma jet. The synthesized powder was collected from the reactor wall, which was mostly water cooled. The discharged gas was collected by the remaining powder through the filter, the residual ammonia was absorbed by water and discharged.

The conversion of the synthesized powder was analyzed by X-ray diffraction analyzer PW 1710 (Philips). The conversion measurement was corrected by comparing the diffraction intensity according to the AlN content of (Al + AlN) mixed powder, which is a pre-mixed standard sample, with the ratio of Al to AlN diffraction intensity, as confirmed by various researchers. Relative diffraction intensity of the standard sample is a diffraction intensity of the _{d Al (111) = 2.338 (} 2θ = 38.49˚) and _{d AlN (100) = 2.695 (} 2θ = 33.20˚) that represents the maximum diffraction intensity from the Al and AlN I _{Al ( 111)} and I _{AlN (100)} .

The shape and particle size of the synthesized powder were analyzed by SEM (Scanning Electron Microscope, Hitachi X-650) and STEM (Scanning Transmission Electron Microscope, JEOL 2000FX-ASID / EDS). Measurement was performed using a dynamic scattering particle size analyzer (Malvern instruments). The specific surface area was measured by the BET method.

Analysis of the synthesized powder

9 is a SEM observation of the synthesized powder. Figure 9a is mostly unreacted aluminum powder observed in spherical agglomerated form, which appears to agglomerate before aluminum solidifies because of its low melting point. Fine particles converted to aluminum nitride were observed as grayish white crystals with a relatively even particle size distribution as in 9b.

The powder collected from the reactor wall was measured by a particle size analyzer, and the particle size was relatively evenly distributed as shown in FIG. 10, and the average particle diameter was measured to be 150 nm. On the other hand, the powder collected around the reaction gas injection coil was measured at 500 nm, which is considered to be because it is partially sintered by high temperature as shown in FIG. 11C. As a result of STEM measurement of the synthesized powder, it was observed as ultrafine particles having a diameter of about 30 nm in FIG. 11a, and the results measured by the particle size analyzer in FIG. 10 showed that the primary particles observed in the STEM were partially aggregated. It is thought to be the particle size distribution for the secondary particles. Figure 11b was confirmed that the polycrystalline aluminum nitride by the diffraction pattern of the powder. FIG. 11c shows the powder collected around the coil for reactant gas injection, and shows a partially sintered shape. The shapes of the projections formed on the aluminum surface were shown as rod-shaped crystals as shown in FIG.

The specific surface area of the synthesized powder was found to be 50-100 m 2 / g as measured by the BET method. The fine particles generally have a high surface energy due to this large specific surface area, and the large specific surface area of the synthesized fine particles suggests the possibility of easily sintering aluminum sintering which is difficult to sinter.

As described above, the present invention relates to a method for producing aluminum nitride ultra-fine particles by generating a plasma jet to evaporate an aluminum mass, and reacting the evaporated aluminum with nitrogen or ammonia gas. Inexpensive and fine particle size aluminum nitride ultrafine particles can be efficiently produced.

Claims (3)

The aluminum nitride is evaporated using a plasma jet generated by using a mixture gas of argon and nitrogen as an aluminum generating gas, and nitrogen or ammonia gas is injected into the outer region of the plasma, which is a low temperature region, to react the evaporated aluminum to react the vaporized aluminum. Method for producing ultra-fine aluminum nitride, characterized in that for producing a. The method for producing aluminum nitride ultrafine particles according to claim 1, wherein ammonia gas is injected at a position of 50 mm or more from the plasma jet nozzle to react the reaction gas. The method for producing aluminum nitride ultrafine particles according to claim 1, wherein the flow rate of ammonia gas as the reaction gas is 1.5 l / min or more.

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