KR950005298B1 - Method of preparing a nitriding aluminium powder - Google Patents

Abstract

mixing Al powder and carbon powder as a thinner, charging a plate-shape Al compact of constant thickness; which is produced by light pressing the mixture, in reactor; maintaining nitrogen pressure of the reactor to 3-10 air pressure by drawing air out the reactor; creating a nitriding reaction in the reactor by using an electric arc; crushing the mixture of the nitrided Al powder and the carbon powder to use a ball mill; and maintaining the crushed mixture at 650-750 deg.C in an air or in an oxygen atmosphere in order to remove the carbon powder.

Description

Method for manufacturing aluminum nitride powder using high temperature autosynthesis

The present invention manufactures high purity aluminum nitride powders used for packaging materials of high density semiconductor chips and high temperature materials such as heat exchangers, which require excellent electrical insulation and thermal conductivity properties. In particular, aluminum nitride powders using high temperature magnetic synthesis method It relates to a manufacturing method of.

Aluminum nitride not only has excellent electrical insulation, which is a general characteristic of ceramics, but also has a specific physical property that is higher than most metal materials such as the thermal conductivity of 200 W / mK (320 W / mk).

At present, research and practical use of packaging materials for silicon semiconductor chips requiring heat insulation and heat dissipation and heat exchangers requiring heat conductivity and corrosion resistance and other high temperature structural materials have been actively conducted using the characteristics of aluminum nitride.

In general, methods for producing aluminum nitride powder include carbon reduction method, self-propagating high temperature synthesis method, direct nitridation method, fluidized furnace method, and plasma. Rotating electrode methods and vapor phase synthesis methods are known.

Among the known methods, the most commonly used method is the carbon reduction method, which is to produce aluminum nitride by reducing the alumina powder to carbon in a high temperature nitrogen atmosphere.

Al ₂ O ₃ + 3C + N ₂ = 2AlN + 3CO (1)

In order to proceed at a commercially possible rate, the reaction temperature must be maintained at about 1700 ° C. Also, the reaction between alumina and carbon powder is a reaction between a solid and a solid, so that a direct contact between these powders is maintained. There is a disadvantage that can be continued.

That is, when the size of the reaction powder is too large or the voids between the reaction powder are too large, the reaction is not completed and unreacted alumina remains in the material.

In fact, 0.5-2.0 wt% oxygen is present in the commercially available aluminum nitride, which is due to the remaining of unreacted alumina.

Therefore, since oxygen in aluminum nitride has a function of lowering the thermal conductivity of the material, it is essential to reduce the oxygen concentration in order to fully utilize the performance of the material.

The direct nitriding method is to maintain the aluminum powder in a nitrogen atmosphere so that the nitriding reaction occurs. The reaction rate is slow because the reaction temperature must be kept below the melting point of aluminum.

The fluidized reactor method is a method in which aluminum powder is suspended in a high temperature furnace at about 1200 ° C. using nitrogen gas, and the reaction rate is very fast. However, it is difficult to maintain working conditions.

In the plasma rotating electrode method, when a high-temperature plasma is applied to an aluminum consumption electrode in a nitrogen atmosphere, aluminum is vaporized, and vaporized aluminum reacts with nitrogen to form fine aluminum nitride powder.

The plasma rotating electrode method has the advantage of producing an aluminum nitride powder having a fine and uniform size, but has the disadvantage of low productivity and low yield.

In addition, the gas phase synthesis method is a method suitable for the production of thin films, rather than being used for the production of powder, because the gas used is expensive and the production rate is slow.

The reaction reaction formula of the aluminum nitride powder by the gas phase synthesis method is shown in Equation (2) below.

AlCl ₃ + NH ₃ = AlN + 3HCl Formula (2)

On the other hand, high temperature autosynthesis has also been used for the production of aluminum nitride powders and sintered bodies.

Prior art of high temperature autosynthesis has been known in US Pat. No. 3,726,643 and US Pat. No. 4,877,759.

The former prior art is a comprehensive method that can be prepared by high temperature autosynthesis by reacting a high melting point inorganic compound with a metal and its gaseous phase. The latter technique is about 95% relative density when aluminum powder is reacted with nitrogen. Aluminum nitride sintered body was obtained, but the pressure of nitrogen used was about 1,000 atm, which was not suitable for a wide range of applications in commercial production.

In addition, according to a Japanese research publication as a related art, it is known that nitrogen pressure must be maintained at 5 atmospheres or more in order for high-temperature rotation to proceed stably, in which case, many unreacted aluminum powders remain in the material. In order for the powder to be completely converted to aluminum nitride powder, a high pressure of 100 atm or more must be maintained.

The high temperature autosynthesis method will be described in more detail as follows.

The high temperature autosynthesis method is a method in which the heat of reaction between the elements constituting the compound is heated so that the formation reaction is spontaneously maintained. This is a phenomenon that is frequently observed in a synthesis reaction of a ceramic material or an intermetallic compound having a high heat of synthesis reaction.

This high temperature autosynthesis method uniformly mixes two or more elements constituting the compound, and then compresses the same by applying a pressure in a suitable form at room temperature.

A portion of the pellets thus prepared is locally heated using a heating method such as tungsten filament, carbon strip, etc. to ignite the reaction.

And once the reaction is ignited, even if the external heat source is removed, the reaction is continued by the heating effect of the self-heating reaction, and the reaction zone proceeds sequentially through the entire portion of the pellet.

The raw materials involved in the reaction when aluminum nitride powder is prepared by high temperature autosynthesis method are solid aluminum powder and nitrogen gas, and nitrogen gas and aluminum powder in the unit volume of the reaction pellet to produce aluminum nitride with a constant chemical equivalence ratio They must be present in the same molar ratio.

Therefore, very high nitrogen pressure of 100-2,000 atmospheres is required for this reaction to proceed.

And one of the important manufacturing parameters in the high temperature autosynthesis method is the adiabatic reaction temperature.

The adiabatic reaction temperature is the highest temperature that can be reached when the reaction heat generated when the compound is synthesized is used to raise the temperature of the synthesized compound without loss. In the case of aluminum nitride, the adiabatic reaction temperature is about 3,200. It is reported to reach -4,000 ° C.

However, since the aluminum nitride has a sublimation temperature of about 2500 ° C., the reaction does not proceed stably. Therefore, it is essential to lower the synthesis temperature of aluminum nitride to prevent sublimation and decomposition of aluminum nitride.

In order to lower the synthesis temperature of aluminum nitride, a method of mixing a stable compound which does not participate in the nitriding reaction of aluminum as a diluent as a diluent has been widely used.

That is, silicon nitride, boron nitride, titanium nitride, hafnium nitride, zirconium nitride, and aluminum nitride powder are mixed with aluminum powder by about 20-50% to proceed the nitriding reaction.

At this time, these diluents do not participate in the nitriding reaction, and are known to play a role of reducing the reaction temperature by absorbing the heat of reaction generated during the synthesis reaction.

In addition, in order to prevent decomposition of aluminum nitride, a partial pressure higher than the nitrogen partial pressure obtained from the following chemical reaction formula should be applied.

2Al + N ₂ = 2AIN ΔG = -76, 100-4.82T (3)

The equilibrium partial pressure of nitrogen calculated by Equation (3) increases as the temperature rises and becomes about 1 atm when the reaction temperature reaches around 2800 ° C, and further increases to about 100 atm when reaching about 3,600 ° C. Increases.

Therefore, when the reaction temperature increases, a pressure higher than the equilibrium partial pressure is applied. The biggest problem when producing aluminum nitride by the conventional high temperature autosynthesis method is that the synthetic partial pressure of nitrogen is too high.

The nitrogen partial pressure currently used mainly is 100-200 atmospheres, which is too high to produce commercial aluminum nitride powders.

In this way, if the pressure is high, the amount of nitrogen gas consumed without being reacted is increased, leading to an increase in manufacturing cost, and a strong structure that can withstand the rupture due to the high reaction pressure is required.

Another problem is the amount of oxygen present in the nitrogen gas.

Oxygen of about 0.2wt% exists in nitrogen gas with industrial humidity, and when high pressure nitrogen like conventional high temperature autosynthesis method is used, the absolute amount of oxygen increases and promotes oxidation reaction of aluminum to The content of is increased, and accordingly, the purity of the aluminum nitride produced by the high temperature autosynthesis method is not very good.

In general, since aluminum nitride has good electrical insulation and higher thermal conductivity than most metals, research and practical use have been actively conducted as high temperature structural materials such as packaging materials and heat exchangers of highly integrated semiconductor chips. Due to the high price resulting from the manufacturing method, there are many restrictions on its use.

Accordingly, an object of the present invention is to improve the conventional high-pressure high-temperature autosynthesis method of 100-2,000 atmospheres, and to manufacture aluminum nitride of high purity and low price by low-pressure high-temperature autosynthesis method of about 3-10 atmospheres.

Hereinafter, the manufacturing method and the effects of the present invention will be described in detail.

The present invention, which is a low-pressure high-temperature autosynthesis method, is prepared using a reactor capable of maintaining nitrogen gas up to 10 atm, which is largely arc ignited between a tungsten electrode and a carbon electrode, which can ignite the autosynthesis reaction between aluminum powder and nitrogen gas. It consists of a device, a temperature measuring device that can measure the synthesis temperature of the powder, and a storage device that can hold the powder.

In synthesizing the nitrogen aluminum powder in the present invention, it was possible to use a low pressure nitrogen pressure as compared with the conventional high-pressure high-temperature autosynthesis method, because a new aluminum powder preparation process was developed.

As a method for preparing an aluminum powder sample for maintaining a stable high temperature autonitrification reaction, a carbon powder having a volume ratio of about 10-30% is quantified in the metal aluminum powder and mixed with a diluent. At this time, the diluent plays a role of securing a passage through which nitrogen gas can be delivered into the aluminum powder molded body. As the content of the diluent increases, the nitrogen gas easily penetrates into the sample to increase the generation of aluminum nitride. However, since the carbon powder does not participate in the reaction in nitrogen gas, when the content thereof is 30% or more, the autothermal synthesis reaction is not performed by absorbing the reaction heat generated in the nitriding reaction of the aluminum powder.

Then, the mixed sample is poured into a cuboid having a depth of 3-15 mm. At this time, if the depth of the sample is too large, the reaction gas is difficult to penetrate into the reaction zone, and thus, it is difficult for complete nitriding reaction to occur. If the depth is too small, the heat loss to the ridge is increased, and the depth of the sample is preferably within 3-15 mm. . Thereafter, the thickness of the sample is uniformed by using a doctor blade, etc., and then lightly patted to uniform the density of the sample. This is to eliminate non-uniformity of sample density that may occur during filling.

At this time, if the sample is too thin, the annual loss to the outside becomes large. On the contrary, when the sample is thick (2 cm or more), infiltration of nitrogen is not easy and an unstable nitriding reaction occurs.

The atmospheric pressure in the high temperature autonitrification reactor is removed to maintain the nitrogen pressure of about 3-10 atm. As such, the nitrogen pressure was lower than that of the conventional manufacturing method, because the powder is used in the present invention in a free-filled state as compared with the conventional method of compressing the powder at a high pressure. However, if the pressure of the nitrogen atmosphere is too low below 3 atmospheres, there is a problem that the penetration of nitrogen into the sample is too slow and aluminum is not completely converted to aluminum nitride. On the other hand, when the nitrogen pressure is higher than 10 atm, there is a problem in that the particle size of the powder is increased due to the rapid increase in calorific value.

This pressure is very low compared to the existing 500-2,000 atmospheres, it can significantly reduce the nitrogen consumption in the production of aluminum nitride powder.

The high temperature autonitrification reaction is ignited at the tip of the sample by using an electric arc whose tungsten electrode is the cathode and the carbon electrode is the anode.

Once the reaction is ignited, the arc is destroyed.

This is because once the reaction is ignited, even if the external heat source is removed, the reaction is continued by the heating effect of the self-reacting heat, and the reaction zone proceeds sequentially through the entire portion of the pellet.

Since the electric arc has a very high heat source density and heats the reactants very locally in a short time, the ignition of the reaction is easy.

The mixture of the produced aluminum nitride powder and carbon powder is pulverized using a ball mill or the like.

After 30 minutes of short grinding, several μm of aluminum nitride powder can be obtained. Here, it is not necessary to limit the grinding time of the powder, but if the grinding time is longer, the productivity is reduced and the mixing of impurities is increased, so that the powder is limited to about 30 minutes to obtain an appropriate powder particle size.

This grinding operation facilitates the removal of carbon and has an effect of making the particle size of the aluminum nitride powder fine.

The mixture of the pulverized aluminum nitride and the carbon powder is maintained at a temperature of 650-750 ° C. in the air or in an oxygen atmosphere to remove the carbon powder. In this case, when the temperature is lower than 650 ° C., the oxidation rate of carbon is too slow, and productivity is low. When the temperature is higher than 750 ° C., oxidation of aluminum nitride occurs to increase the content of oxygen impurities in the material.

The nitrogen pressure used in the present invention is 3-10 atm, which is much lower than the nitrogen pressure (100-1000 atm) used for the production of aluminum nitride powder in the United States or Japan, so that it is possible to produce aluminum nitride at a low price.

In the present invention, the preparation process of the aluminum powder invented to lower the nitrogen pressure to 10 atm or less is the shape of the aluminum powder sample and the type and amount of the diluent added to the sample.

Therefore, the shape of the aluminum powder sample has an important effect because the production of aluminum nitride is a reaction between the solid powder and the gaseous nitrogen.

That is, in order for the nitriding reaction of aluminum to proceed, nitrogen gas must penetrate out of the sample.

If the thickness of the reacting sample becomes thick, the intrusion distance of nitrogen for the nitriding reaction becomes far and unreacted aluminum remains in the aluminum powder.

In addition, when the thickness of the aluminum powder layer becomes thicker, the nitriding reaction does not occur inside, and eventually, the high temperature autonitriding reaction stops.

This phenomenon is observed when the compacting density of aluminum powder is increased.

That is, when the compacting density increases, the volume fraction of the pores through which nitrogen can penetrate falls, thereby stopping the high temperature autonitrification reaction.

Therefore, in the present invention, tapping the thickness of the aluminum powder to about 5mm to facilitate the intrusion of nitrogen gas.

Aluminum powder without diluent does not cause a rotating reaction even in sufficiently high nitrogen.

This is because the melting point of the aluminum powder is low, the aluminum powder is melted by the reaction heat, and the intrusion path of nitrogen gas is blocked, and in particular, a thick reaction zone is formed due to the large thermal conductivity of the aluminum powder.

Therefore, in order to secure the infiltration path of nitrogen gas, it is necessary to add a diluent capable of absorbing the reaction heat and reducing the thermal conductivity of the aluminum powder sample.

The diluent used in the present invention uses carbon powder instead of silicon nitride, titanium nitride, aluminum nitride, or the like, which has been conventionally used.

Because the carbon powder is irregular in shape and does not get wet with molten aluminum, it is not only suitable for instantaneous passage of nitrogen in the reaction zone but also maintains a reducing component at high temperature, so it reacts with oxygen in nitrogen gas first. It acts to prevent oxidation.

Therefore, in order to produce high purity aluminum nitride, industrial nitrogen gas can be used without the need for using high purity nitrogen gas.

In addition, since the carbon powder can remove oxygen by reacting with alumina that is always present on the surface of aluminum, there is an advantage of lowering the oxygen concentration of the produced aluminum nitride powder.

As a result of observing the shape of the powder of the aluminum nitride powder produced by the present invention using an electron microscope, the nitriding reaction is not caused by the reaction of aluminum and nitrogen in a general solid state (or liquid phase). Was found to be due to the reaction of nitrogen gas.

That is, the aluminum nitride powder obtained by the present invention is not a spherical shape generally reported, but is a very asymmetric powder having an orientation. The shape of the powder indicates that the nitriding process of aluminum is not a reaction of a solid phase and a gas phase as in the conventional carbon reduction method, but a vapor phase reaction mechanism in which vaporized aluminum reacts with nitrogen.

In the case of aluminum nitride, the thermal insulation reaction temperature is 3000 ℃ or higher, and the temperature of the sample is raised to 2000 ℃ or higher even if the calorie reduction by the diluent is taken into consideration.

Therefore, at this temperature, aluminum causes vaporization and vaporized aluminum reacts with nitrogen to form gaseous aluminum nitride.

The produced aluminum nitride is supersaturated and condensed according to the cooling of the sample, which causes directional growth.

Such gas phase reaction mechanisms tend to have less residual impurities than solid phase reaction mechanisms, and thus the aluminum nitride to be produced has a lower oxygen concentration.

Therefore, since the thermal conductivity of the sintered compact manufactured using the present aluminum nitride powder falls in proportion to the oxygen concentration, the aluminum nitride powder produced by the high temperature autosynthesis method has a higher thermal conductivity than the aluminum nitride powder produced by other methods. Indicates.

On the other hand, as a result of performing an X-ray examination of the aluminum nitride powder produced by the present invention, it was confirmed that the aluminum nitride powder was pure aluminum nitride powder without any residual aluminum powder.

The carbon reduction method currently commercially produced in the conventional method of manufacturing aluminum nitride powder has to maintain the alumina powder in a nitrogen atmosphere at a high temperature of about 1700 ° C. for a long time, and thus the production cost of the powder is high, and the reaction between the solid phase and the solid phase is unreacted alumina. There is a disadvantage that the residual oxygen to increase the residual oxygen concentration.

Oxygen of the unreacted alumina has a disadvantage in that the thermal conductivity of the aluminum nitride sintered body is lowered, thereby making it impossible to sufficiently utilize the excellent performance of the present material.

In addition, other nitriding methods other than the carbon reduction method, that is, direct nitriding method, plasma rotary electrode method, gas phase synthesis method, flow reactor method, etc. are recognized for their advantages, but all have disadvantages of mass production.

Nevertheless, high temperature autosynthesis is of great commercial value in that it can be manufactured at high reaction rates at room temperature without the need for heating the sample, and also uses a simple manufacturing apparatus and relatively inexpensive raw materials such as aluminum and nitrogen gas.

However, in order to manufacture aluminum nitride powder by the high temperature autosynthesis method until now, about 100-1,000 atmospheres of nitrogen pressure is required, which increases the manufacturing cost of the product and also causes the purity of the reaction product to decrease. .

The present invention solves this problem, and is an excellent method for producing aluminum nitride powder at a relatively low nitrogen pressure of about 3-10 atm, which can be easily obtained commercially. It is possible to drastically reduce the amount of nitrogen gas to be produced and to reduce the manufacturing cost by reducing the gas filling time.

In addition, the carbon powder used as a diluent not only stably promotes high temperature autonitrification, but also played a large role in maintaining a reducing component crisis during the reaction to reduce the amount of residual oxygen in the aluminum nitride powder to obtain a high purity aluminum nitride powder.

Claims (4)

In the method of manufacturing aluminum nitride powder using high temperature autosynthesis method, in order to maintain low pressure high temperature autonitrification reaction, a sample is prepared in which a metal powder is mixed with a carbon powder with a diluent, and the sample is lightly pounded to form a plate-shaped aluminum powder. It is made into a molded body, charged into the reactor, the nitrogen gas is maintained at 3-10 atmospheres by removing the atmospheric gas present in the reactor, and the aluminum nitride powder produced after igniting the high temperature autonitrification reaction using an electric arc. Aluminum nitride using a high temperature autosynthesis method, characterized in that the sample, which is a mixture of carbon powder and carbon powder, is pulverized using a ball mill or the like, and the pulverized sample is maintained at a temperature of 650-750 ° C. in an air or oxygen atmosphere to remove carbon powder. Powder production method. The method of preparing an aluminum powder sample for maintaining a low pressure high temperature autonitrification reaction, the aluminum nitride using a high temperature autosynthesis method characterized in that the carbon powder of 10-30% by volume ratio is mixed with the metal aluminum powder. Powder production method. The method of claim 1, wherein the mixed sample is molded to a thickness of 3-15 mm. The method of claim 1, wherein the sample is ground for 30 minutes using a ball mill or the like to obtain easy carbon removal and fine powder.