JP2002274949A - Process for producing aluminum nitride ceramic and aluminum nitride ceramic produced through this process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for producing high-density, inexpensive aluminum nitride ceramics and the aluminum nitride ceramics produced through this process. SOLUTION: In the process, >=97 wt.% aluminum nitride powders are subjected to discharge plasma sintering in a nitrogen atmosphere. The process contains steps wherein aluminum nitride powders are filled in a mold made of an electrically conductive material, the atmosphere is replaced with nitrogen, the aluminum nitride powders filled in the mold are heated to a predetermined temperature while being pressurized in a vertical direction, and the aluminum nitride powders heated to the predetermined temperature is subjected to discharge plasma sintering at an elevated temperature while being pressurized in a vertical direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing aluminum nitride ceramics, and more particularly to a method for producing high-density aluminum nitride ceramics and aluminum nitride ceramics produced by the method.

[0002]

2. Description of the Related Art Aluminum nitride ceramics have high thermal conductivity and insulating properties, have a coefficient of thermal expansion close to that of Si and compound semiconductors, and have excellent characteristics of no toxicity. I have. Also, if a high-quality and inexpensive method of manufacturing aluminum nitride ceramics is established in the future, it will be possible to apply it not only as a heat dissipation substrate material currently used but also to various fields such as optical materials and electronic materials. Expected.

[0003] Such aluminum nitride ceramics are:
Generally, aluminum nitride powder is added with a sintering aid to lower the melting point of the powder surface, molded by pressing or other molding means, and then obtained in an electric furnace to obtain high-density aluminum nitride ceramics. Sintered at a high temperature of about 1800 ° C.

However, the above-mentioned general method for producing aluminum nitride ceramics involves a problem that the obtained aluminum nitride ceramics are expensive due to the need for a sintering aid and sintering at a high temperature. There is.

As a technique for solving such a problem,
There is a method of sintering using a discharge plasma sintering method. For example, a method of producing a green compact by isotropically pressing a powder material and subjecting the green compact to discharge plasma sintering (Japanese Patent Laid-Open No. 2000-2000) -128648
No. 2, pp. 1 to 3), a prior step of isotropically pressing a powder material to produce a green compact is required, and the resulting aluminum nitride ceramic is The problem of being expensive has not been fully solved.

[0006] Further, although not a method for producing aluminum nitride ceramics, at least 40% of the alumina powder is made amorphous or at least 40% of the alumina powder is made to have a particle size of 1%.
A method for producing high-density aluminum ceramics by ultra-miniaturization to ~ 50 nm and then discharge plasma sintering
No. (Method for producing high-density aluminum ceramics). In this conventional technique, a pre-process is required to make the powder ultrafine by a high energy ball mill or the like, and there is a problem that the obtained ceramic becomes expensive. Also,
When this conventional technique is applied to the production of aluminum nitride ceramics, there is a problem that the ratio of an oxidized layer on the powder surface is increased by making the powder ultrafine, and the quality of the produced aluminum nitride ceramics is rather deteriorated. is there.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to provide a method for producing a high-density and inexpensive aluminum nitride ceramic and an aluminum nitride ceramic produced by the method. To the purpose.

[0008]

In order to solve the above-mentioned problems, a method for producing aluminum nitride ceramics according to the present invention is directed to a method for producing an aluminum nitride powder of 97% by weight or more by discharge plasma sintering in a nitrogen atmosphere. It is.

Further, the method for producing aluminum nitride ceramics comprises a step of filling aluminum nitride powder into a mold made of a conductive material, a step of replacing the atmosphere with nitrogen, and a step of filling the mold with aluminum nitride powder. A process of raising the temperature to a predetermined temperature while pressing from above and below, and a step of subjecting the aluminum nitride powder heated to the predetermined temperature to discharge plasma sintering in a high-temperature atmosphere while pressing from above and below It is.

The aluminum nitride powder is not limited to the present invention, but is preferably a powder having an average particle diameter of 0.5 to 2 μm, and the mold for filling the aluminum nitride powder is preferably made of graphite. preferable.

The step of raising the temperature of the aluminum nitride powder to a predetermined temperature while pressurizing the aluminum nitride powder from above and below is performed in order to easily release gas from the aluminum nitride powder as the temperature rises. It is preferable to adopt a step including an intermediate step.

Further, the predetermined temperature at which the temperature of the aluminum nitride powder is increased while being pressed from above and below is 1100 ° C. to 1300 ° C.
° C, and the pressurizing pressure is 0.6 to 1 tf
/ Cm ² is preferable.

The above-mentioned manufacturing method provides a high-density aluminum nitride ceramic having a sintered body density of 90% or more of the theoretical density.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention has been made based on the research results of a semiconductor thermoelectric energy conversion system that the inventors have been working on for many years. In particular, the present invention is used as a heat radiation substrate material for a semiconductor thermoelectric energy conversion system. Focusing on the excellent properties of the used aluminum nitride ceramics, we provide a method of manufacturing high-quality and inexpensive aluminum nitride ceramics, and furthermore, aluminum nitride that can be applied to various fields such as optical materials and electronic materials It is intended to provide ceramics.

According to the present invention, a discharge plasma method, which has excellent characteristics as a sintering method, is adopted for the production of aluminum nitride ceramics, and furthermore, sintering conditions suitable for the production are found and completed. .

In the spark plasma sintering method, pulsed electric energy is directly applied to the gaps between particles of a powder material, and the high energy of high-temperature plasma generated instantaneously by spark discharge is effectively applied to thermal diffusion and electrolytic diffusion. By applying this method, a local neck junction is first efficiently formed between particles before thermal equilibrium is reached, and it is grown into a stable bond.
This technology enables sintering or sintering in about 5 to 60 minutes including the holding time. According to such a spark plasma sintering method, sintering at a low temperature proceeds rapidly without a sintering aid usually used for lowering the melting point of the powder surface, and the structure of the particles is excessively large. A high-density sintered body can be formed without growth.

The spark plasma sintering apparatus generally includes a mold (die) made of a conductive material filled with a powder material, and a pair of punches made of a conductive material fitted into the die. The powder material is filled in the die, and the filled powder material is pressed from above and below by a punch, and a DC pulse current is started to flow through the punch to a predetermined temperature (sintering temperature). The temperature is raised to a sintering temperature, and then maintained at that temperature for a predetermined time to form a sintered body.

The method for producing the aluminum nitride ceramics of the present invention is a production method using such a discharge plasma sintering method, wherein 97% by weight or more of aluminum nitride powder is subjected to discharge plasma sintering in a nitrogen atmosphere. This is a manufacturing method in which an oxide layer on the powder surface can be evaporated and removed by high-energy plasma generated by discharge of particle gaps, so that a higher quality sintered body can be obtained.

In addition, since sintering is performed in a nitrogen atmosphere, nitrogen vacancies are hardly generated, and high quality is obtained. That is, nitrogen vacancies, in addition to lowering thermal conductivity due to scattering of phonons, are likely to become a trapping center for electrons and holes, causing coloring and deterioration due to light irradiation, and the position of the nitrogen vacancies This is one of the important factors affecting the quality of aluminum nitride ceramics, such as the displacement of adsorbed oxygen and the diffusion of oxygen as impurities into the interior. It is possible to obtain a high-quality aluminum nitride ceramic which is less likely to generate vacancies and is applicable not only as a currently used heat radiation substrate material but also in various fields such as optical materials and electronic materials.

In a preferred embodiment of the production process of the present invention, a process of filling an aluminum nitride powder into a die, for example, a die made of graphite, a process of replacing the atmosphere with nitrogen, and a process of filling the die are performed. The process of raising the temperature of the aluminum nitride powder to the sintering temperature while pressing it from above and below with a punch, and the discharge plasma sintering of the aluminum nitride powder heated to the sintering temperature in a high-temperature atmosphere while pressing from above and below And the step of causing

The aluminum nitride powder is 97% by weight or more,
Desirably, it has a high purity of 99% by weight or more and an average particle size of 0.5 to 2
A powder of μm is preferred, and an average particle diameter of about 0.6 to 1 μm is more preferred. Such powders are commercially available and readily available. If the average particle size is smaller than this, the proportion of the oxide layer on the particle surface increases, and if the average particle size is larger, it is difficult to obtain a high-density sintered body, and in any case, quality problems occur.

The sintering temperature of the aluminum nitride powder is 1100
The temperature is preferably from 1 to 1300 ° C, more preferably about 1200 ° C. 90% of theoretical density when sintering temperature is lowered
It is difficult to obtain a high-density sintered body as described above. For example,
At 1000 ° C, the sintered density becomes 50-60%, and at 1100 ° C,
Although a high density sintered body can be obtained, the density may be low, and the yield is about 70%. If the sintering temperature is increased, the sintering density is increased, but at the same time, a large amount of power is required, which causes an economic problem.

In the step of raising the temperature of the aluminum nitride powder to the sintering temperature, a step of maintaining a constant temperature in the middle of the temperature increase is provided in order to facilitate gas release from the aluminum nitride powder accompanying the temperature rise. For example, at 200 ° C and 900 ° C each
It is preferable to suspend the heating for about 5 minutes.

The pressure for pressing the aluminum nitride powder is as follows:
The pressure is preferably 0.6 to 1 tf / cm ^2, and when the pressure is reduced, the sintering density deteriorates. If the pressurizing pressure is increased, the sintering time can be shortened, but the apparatus becomes large and expensive.

The holding time is not limited to the present invention, but is preferably 30 to 40 minutes.

According to the embodiment of the method for manufacturing an aluminum nitride ceramic of the present invention described in detail above, it is possible to obtain a high-density aluminum nitride ceramic having a sintered body density of 90% or more of the theoretical density. Can be.

According to the production method of the present invention, green compact production,
Alternatively, high-purity aluminum nitride powder is spark-plasma-sintered at a low sintering temperature and a short sintering time without the need for a pre-process such as ultra-fine powdering, the addition of a sintering aid, etc. Therefore, a high-density and inexpensive aluminum nitride ceramic can be obtained.

The aluminum nitride ceramics of the present invention also uses high-purity aluminum nitride powder as a raw material, does not contain a sintering aid, etc., and an oxide layer on the surface of the raw material powder is evaporated and removed by high-energy plasma during sintering. In addition, nitrogen sintering hardly occurs due to sintering in a nitrogen atmosphere, resulting in high purity and high quality.

By changing the shape of the die and the punch according to the shape of the final product to be manufactured, the subsequent processing steps can be omitted or simplified, and the present invention can be more effectively implemented. it can.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

First, a spark plasma sintering apparatus will be described with reference to FIGS. 1 and 2. However, the present invention is not limited to this apparatus but is a known apparatus.

FIG. 1 is a view showing the schematic arrangement of a powder sealing die and a punch used in the confirmation test of the present invention, and FIG. 2 is a schematic view of the spark plasma sintering apparatus used in the confirmation test of the present invention. It is a lineblock diagram (inside a chamber) showing a schematic structure.

In the discharge plasma sintering, a sintered body is manufactured by applying a pulse current while pressing a powder filled in a die with a pair of punches, and the die has a powder charging portion. A cylindrical punch having a cylindrical shape and slidably fitted at both ends is arranged. As a structural material of the die and the punch, for example, a conductive material such as a carbon-based material (graphite, graphite, etc.) or a cemented carbide is used, and the material used in this confirmation test is made of graphite. When filling the powder, the pressing surface of the punch is covered with a carbon sheet to prevent adhesion between the punch and the powder.

As shown in FIG. 2, the discharge plasma sintering apparatus comprises a vacuum chamber, a pair of upper and lower punch electrodes for pressurizing and supplying electricity to the punch, a sintering power supply (not shown) for generating a pulse voltage, and a punch. A hydraulic pressure driving mechanism (not shown) for raising and lowering the electrodes, a vacuum pump (not shown), a control unit (not shown) for controlling these, and the like.
The die filled with the powder and the pair of punches are set in a vacuum chamber.

The inside of the vacuum chamber is generally controlled by a vacuum pump in order to prevent oxygen, nitrogen, water and the like in the air from reacting with the metal powder contained in the powder and adversely affecting the sintered body. The discharge plasma sintering is performed in a vacuum state (in a reduced pressure state) or in an inert gas atmosphere by vacuum replacement.

The control unit controls the output of the sintering power supply so that the powder temperature detected by the temperature sensor (thermocouple in FIG. 2) installed on the die matches a preset temperature rising pattern. . Further, the control unit controls driving of the pressurizing drive mechanism and the vacuum pump.

While the powder is pressed from above and below by the punch by the operation of the pressing drive mechanism, a pulse current is passed through the punch to raise the temperature to the sintering temperature. A sintered body is formed by holding for a time. Most of the current flows in the order of upper punch electrode → upper punch → die → lower punch → lower punch electrode to generate Joule heat, which is used to heat the powder from the outside, and the remaining current is used to heat the powder The spark discharge occurs in the gap between the powder particles, and the high energy of the discharge plasma generated by the spark discharge is effectively used for heat diffusion, electrolytic diffusion, and the like, and the powder particles are strongly bonded. The condition of the pulse current to be applied is, for example, that the pulse ratio (non-energizing time: energizing time) is 1:
The current output can be about 1 to 12: 1, and the current output can be about 1 to 1000 A.

Next, referring to FIGS. 3 and 4, a description will be given of a confirmation test of the method for producing an aluminum nitride ceramic according to the present invention, which was performed using the above-mentioned spark plasma sintering apparatus.

FIG. 3 is a schematic diagram showing an example of the temperature rise pattern used in the confirmation test of the present invention and the measurement of the displacement of the powder sample during the experiment. FIG. It is a scanning electron microscope photograph which shows the Example of the sintered compact structure of aluminum ceramics.

The aluminum nitride powder used was an F type manufactured by Tokuyama, having a purity of 99.0% by weight and an average particle diameter of 0.6.
μm high-purity aluminum nitride powder.

First, the above-mentioned high-purity aluminum nitride powder
0.3 g was packed in a die, and sandwiched between punches on which carbon sheets were laid from above and below, and this was set between electrodes in a vacuum chamber of a discharge plasma sintering apparatus as shown in FIG.

Next, in order to perform sintering in a nitrogen atmosphere, the inside of the vacuum chamber is evacuated to 0.01 Torr or less, and then supply of pure nitrogen gas into the vacuum chamber is started while evacuating with a vacuum pump. And the pressure in the vacuum chamber
The supply nitrogen flow rate was adjusted so as to be 0.01 Torr.

Next, as shown in FIG. 3, the aluminum nitride powder filled in the die was pressed with a punch from above and below at a pressure of 0.8 tf / cm ^{2 at} a heating rate of about 50 ° C./min. Initiate the temperature rise, and on the way, in order to facilitate the release of gas from the powder, interrupt the temperature rise at about 200 ° C and about 900 ° C for about 5 minutes each, until the sintering temperature reaches 1200 ° C. The temperature rose in 34 minutes.

After raising the temperature to 1200 ° C., which is the sintering temperature,
Subsequently, while applying a pressure of 0.8 tf / cm ² from above and below, the temperature was maintained for 40 minutes to perform discharge plasma sintering.

During this time, as shown in FIG. 3, the displacement of the sample decreases with an increase in temperature, that is, the sample expands, but the displacement of the sample rapidly increases from around 1000 ° C., that is, starts to shrink, which is the sintering temperature. Sintering is completed by holding at 1200 ° C. for 30 to 40 minutes.

The aluminum nitride ceramics of the present invention produced as described above has a sintered body density of 9% of the theoretical density.
It is a high-density aluminum nitride ceramic of 5.2%, and its sintered body has a grain size of about 1 μm, which is about the same as that of the aluminum nitride powder as a raw material, as shown in FIG.
It can be seen that almost no grain boundaries grow and a dense sintered body is formed.

As described above, according to the present embodiment, there is no need for a pre-process such as green compact production or powder ultra-fine refining, no addition of a sintering aid or the like, a low sintering temperature, and This is a technique for spark plasma sintering of a high-purity aluminum nitride powder in a short sintering time, and it is possible to obtain an inexpensive aluminum nitride ceramic having a high density (the sintered body density is 90% or more of the theoretical density).

Further, the obtained aluminum nitride ceramic is made of high-purity aluminum nitride powder as a raw material, does not contain a sintering aid or the like, and an oxide layer on the surface of the raw material powder is vaporized and separated by high-energy plasma during sintering. It is removed, and nitrogen vacancies are less likely to be generated by sintering in a nitrogen atmosphere, and high purity and high quality.

While the embodiments of the present invention have been described above, it is to be understood that various changes may be made in form and detail without departing from the spirit and scope of the present invention as defined in the appended claims. it is obvious.

For example, the cross-sectional shape of the powder charging portion of the die and the punch to be fitted into the die can be arbitrarily selected according to the shape of the sintered body. Any shape, such as a square shape, may be used according to the shape of the final product, so that subsequent steps such as cutting can be omitted or simplified, and the present invention can be more effectively implemented.

Further, in the above embodiment, the aluminum nitride powder is of the F type manufactured by Tokuyama Co., Ltd., but this is, of course, an example of aluminum nitride powder and does not limit the present invention in any way.

Further, the sintering temperature, pressurizing pressure, holding time, and the like shown in the above embodiment are also set as preferable conditions for plasma sintering the aluminum nitride powder to discharge. Is a condition that can be changed according to the characteristics of the material to be used, etc., and does not limit the present invention.

[0053]

The production method according to the present invention is a production method in which 97% by weight or more of aluminum nitride powder is subjected to discharge plasma sintering in a nitrogen atmosphere. High-density and inexpensive aluminum nitride ceramics because it sinters high-purity aluminum nitride powder at low sintering temperature and short sintering time without the need for pre-process, addition of sintering aids, etc. The effect is obtained. The aluminum nitride ceramics of the present invention also uses high-purity aluminum nitride powder as a raw material, does not contain a sintering aid, etc., and an oxide layer on the raw material powder surface is evaporated and removed by high-energy plasma during sintering. Nitrogen vacancies are hardly generated by sintering in a nitrogen atmosphere, and there is an effect of high purity and high quality.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a schematic configuration of a powder sealing die and a punch used in a confirmation test of the present invention.

FIG. 2 is a configuration diagram (inside a chamber) showing a schematic configuration of a spark plasma sintering apparatus used for a confirmation test of the present invention.

FIG. 3 is a schematic diagram showing an example of a temperature rise pattern used in a confirmation test of the present invention and measurement of powder sample displacement during an experiment.

FIG. 4 is a scanning electron micrograph showing an example of a structure of a sintered body of the aluminum nitride ceramics manufactured in the confirmation test of the present invention.

Continued on the front page (72) Inventor Naoji Koukko 2-16-1, Tokiwadai, Ube-shi, Yamaguchi F-term (reference) 4G001 BA36 BB36 BC13 BC41 BC42 BC52 BC54 BC62 BD01 BE33

Claims (8)

[Claims] 1. A method for producing aluminum nitride ceramics, comprising subjecting 97% by weight or more of aluminum nitride powder to discharge plasma sintering in a nitrogen atmosphere. 2. The method of manufacturing an aluminum nitride ceramic, comprising: filling the aluminum nitride powder into a mold made of a conductive material; replacing the atmosphere with nitrogen; and filling the mold with the aluminum nitride powder. Raising the temperature to a predetermined temperature while pressing the aluminum nitride powder from above and below, and performing a discharge plasma sintering under a high temperature atmosphere while pressing the aluminum nitride powder heated to the predetermined temperature from above and below. The method for producing an aluminum nitride ceramic according to claim 1, wherein: 3. The method for producing an aluminum nitride ceramic according to claim 1, wherein the aluminum nitride powder is a powder having an average particle size of 0.5 to 2 μm. 4. The method for producing an aluminum nitride ceramic according to claim 2, wherein the mold for filling the aluminum nitride powder is a mold made of graphite. 5. The step of raising the temperature of the aluminum nitride powder to a predetermined temperature while pressing the aluminum nitride powder from above and below, in order to facilitate the release of gas from the aluminum nitride powder as the temperature rises, The method for producing an aluminum nitride ceramic according to any one of claims 2 to 4, wherein the method includes a step of maintaining the temperature at a constant temperature. 6. The production of an aluminum nitride ceramic according to claim 2, wherein the predetermined temperature at which the temperature of the aluminum nitride powder is increased while being pressed from above and below is 1100 ° C. to 1300 ° C. Method. 7. The method for producing an aluminum nitride ceramic according to claim ² , wherein the pressure is 0.6 to 1 tf / cm ² . 8. An aluminum nitride ceramic produced by the method for producing an aluminum nitride ceramic according to claim 1, wherein the density of the sintered body is 90% or more of the theoretical density. Ceramics.