Method for mixing powdered metal and nanocarbon material, and method for manufacturing nanocarbon/metal composite material

Abstract

A manufacturing method is provided to be used in place of a conventional mechanical alloying method. A powdered metal and a nanocarbon material are placed in an empty metal mill vessel containing no balls, and a mixture in which the powdered metal is coated with this nanocarbon material is obtained by shaking in three dimensions.

Description

FIELD OF THE INVENTION

The present invention relates to a method for mixing a powdered metal and a nanocarbon material, and to a method for manufacturing a nanocarbon/metal composite material.

BACKGROUND OF THE INVENTION

In recent years, special carbon fibers known as “carbon nanofibers” have attracted attention. Carbon nanofibers have a configuration in which sheets of carbon atoms arranged in the form of a hexagonal network are rolled up into a tubular form; such nanofibers have a diameter of 1.0 nm to 150 nm, and a length of a few micrometers to 100 μm. Since such fibers have a nano-size diameter, they are referred to as “carbon nanofibers,” “carbon nanotubes,” or the like (such materials will be called “nanocarbon materials” below).

These nanocarbon materials are reinforcing materials, and are also materials with a good thermal conductivity. Accordingly, strength and thermal conductivity can be improved by mixing these materials with metal materials.

In order to obtain the expected strength and thermal conductivity, it is essential that such nanocarbon materials be uniformly mixed with the metal materials.

One technique for uniformly mixing a nanocarbon material with a metal material is mechanical alloying. This mechanical alloying method has been proposed previously in Japanese Unexamined Patent Application No. 2003-246613. In the mechanical alloying method, balls, a metal material, and a nanocarbon material are placed in a vessel, and the vessel is shaken or rotated. Consequently, the balls strike the nanocarbon material whereby the nanocarbon material is broken up. As a result of the shaking or vibration, this broken-up nanocarbon material is brought into contact with the metal material and is strongly bonded to the metal material.

However, since the carbon nanotubes are mechanically broken up and converted into short fibers, no great improvement in thermal conduction can be expected. Specifically, in the case of long fibers, these fibers constitute passages for heat, so that a high thermal conductivity is obtained. However, in the case of short fibers, the thermal conductivity is small.

Thus, in conventional mechanical alloying methods, it has been ascertained that the desired thermal conductivity performance cannot be sufficiently obtained.

Accordingly, there is a need for a manufacturing method to be used in place of conventional mechanical alloying.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for mixing a powdered metal and a nanocarbon material is provided which comprises the steps of preparing a ball mill, a specified amount of a powdered metal, and a specified amount of a nanocarbon material; placing the aforementioned powdered metal and nanocarbon material in the empty metal mill vessel containing no balls; and obtaining a mixture in which the aforementioned powdered metal is coated with the aforementioned nanocarbon material by shaking the mill vessel in three dimensions using the aforementioned ball mill.

In the aforementioned mixing method, since no balls are placed in the vessel, there is no danger that the nanocarbon material will be excessively broken up. Furthermore, the nanocarbon material can be bonded to the powdered metal by shaking the mill vessel in three dimensions. Accordingly, the nanocarbon material in the form of long fibers can be mixed with the powdered metal in a desirable manner.

The nanocarbon material that is prepared in the aforementioned preparatory step is preferably a nanocarbon material that has been dispersed in advance by ultrasound. If the nanocarbon material is thus dispersed by ultrasound, and the dispersed nanocarbon material is placed in the mill vessel, the metal particles can be more uniformly coated with the nanocarbon material.

According to another aspect of the present invention, a method for manufacturing a nanocarbon/metal composite material is provided which comprises the steps of preparing a ball mill, a specified amount of a powdered metal, and a specified amount of a nanocarbon material; placing the aforementioned powdered metal and nanocarbon material in the empty metal mill vessel containing no balls; obtaining a mixture in which the aforementioned powdered metal is coated with the aforementioned nanocarbon material by shaking the mill vessel in three dimensions using the aforementioned ball mill; and molding and sintering the aforementioned mixture to obtain a sintered body.

In the aforementioned method for manufacturing a nanocarbon/metal composite material, since no balls are placed in the vessel, there is no danger that the nanocarbon material will be excessively broken up. Furthermore, the nanocarbon material can be bonded to the powdered metal by shaking the mill vessel in three dimensions. Accordingly, the nanocarbon material in the form of long fibers can be mixed with the powdered metal in a desirable manner. In addition, in the method of the present invention, a sintered body, i.e., a nanocarbon/metal composite material can be obtained by sintering the uniformly mixed mixture of a powdered metal and nanocarbon material. Since the nanocarbon material is uniformly mixed with the powdered metal, a nanocarbon/metal composite material which has a large strength and a large thermal conductivity can be manufactured.

The nanocarbon material that is prepared in the aforementioned preparatory step is preferably a nanocarbon material that has been dispersed by ultrasound in advance. If a nanocarbon material that has thus been dispersed by ultrasound is used, a nanocarbon/metal composite material in which the nanocarbon material is more favorably dispersed can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

Several preferred embodiments of the present invention will be described in detail below with reference to the attached figures, wherein:

FIGS. 1A through 1C show the method for mixing a powdered metal and nanocarbon material in accordance with the present invention;

FIG. 2 shows the mixture obtained by the mixing method shown in FIG. 1;

FIG. 3 shows the principle of a plasma sintering apparatus that is used to perform a sintering treatment on the mixture shown in FIG. 2;

FIG. 4 is a graph showing the relationship between the sintering time and sintering temperature in the sintering treatment;

FIG. 5 is a graph showing the relationship between the amount of carbon nanofibers added and the maximum tensile stress in samples 1 through 5; and

FIG. 6 shows a graph comparing the maximum tensile stress between sample 6 and sample 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for mixing a powdered metal and a nanocarbon material in accordance with the present invention will be described with reference to FIGS. 1A through 1C.

As is shown in FIG. 1A, which shows the dispersion treatment step, a nanocarbon material 13 is placed in a vessel 12 filled with an acetone solution 11, and the vessel is placed on an ultrasonic vibrating apparatus 14. Next, the acetone solution 11 is shaken by the action of the ultrasonic vibrating apparatus 14. As a result, the nanocarbon material 13 is dispersed. The nanocarbon material 13 is then removed from the vessel 12 and dried.

As is shown in FIG. 1B, no balls are placed in the metal mill vessel 15. Then, a specified amount of a powdered metal (e.g., powdered aluminum) 16 and the nanocarbon material 17 dispersed in FIG. 1A are placed in the empty metal mill vessel 15.

As is shown in FIG. 1C, a cover 19 is placed on the mill vessel 15. Then, this mill vessel 15 is shaken in three dimensions. As a result, a mixture of the powdered metal 16 and nanocarbon material 17 can be obtained.

The nanocarbon material 13 that has been dispersed by ultrasonic vibration in FIG. 1A may be dried while the powdered metal 16 is mixed. In this case, a mixed powder of the nanocarbon material 13 and powdered metal 16 is placed in the metal mill vessel 15 shown in FIG. 1B.

Furthermore, the apparatus that shakes the mill vessel 15 in three dimensions in FIG. 1C is preferably a ball mill that has an agitating action in three dimensions, such as a tri-axial shaking ball mill, planetary ball mill, or the like.

As is shown in FIG. 2, the mixture of a powdered metal and nanocarbon material obtained in FIG. 1C is a mixture 18 having a configuration in which the surfaces of a simple powdered metal 16 are coated with a fine nanocarbon material 17.

The nanocarbon material 17 consisted of long fibers, and no signs of cutting were observed.

Specifically, since only the powdered metal 16 and nanocarbon material 17 were placed in the mill vessel, without any balls being placed in this vessel, and the mill vessel was then shaken, no large cutting force was applied to the nanocarbon material 17. Accordingly, it appears that it was possible to coat the powdered metal 16 with the long-fiber nanocarbon material 17 in a substantially uniform manner.

Furthermore, it is desirable to perform a sintering treatment on the mixture 18 shown in FIG. 2. The principle of a plasma sintering apparatus that is suitable for this sintering treatment will be described next.

As is shown in FIG. 3, the plasma sintering apparatus 20 consists of a lower base 21; a lower spacer 22 that is mounted on this lower base 21; a lower punch 23 that extends upward from this lower spacer 22; a cylindrical die 24 that is fitted into this lower punch 23; an upper punch 25 that is centered on this die 24 and disposed symmetrically with respect to the aforementioned lower punch 23; an upper spacer 26 which retains this upper punch 25 from above; a lifting member 27 from which the upper spacer 26 is suspended; a vacuum chamber 28 which surrounds the lower spacer 22, lower punch 23, die 24, upper punch 25, and upper spacer 26; a vacuum pump 29 which is attached to this vacuum chamber 28, and which evacuates the interior of the vacuum chamber 28; and a pulse power supply 30 which is electrically connected to the lower spacer 22 and upper spacer 26.

The lower spacer 22, lower punch 23, die 24, upper punch 25, and upper spacer 26 are all parts that are made of graphite and possess electrical conductivity. Accordingly, when a pulse current is supplied to the lower spacer 22 and upper spacer 26 by the pulse power supply 30, plasma is generated between the lower punch 23 and upper punch 25.

The die 24 is filled with the mixture 18 (FIG. 2), and pulse powering is initiated while the mixture is compressed between the lower punch 23 and upper punch 25 by pushing the upper punch 25 downward. Consequently, plasma is generated, and the mixture 18 can be heated by the high heat of the plasma. Graphite will burn in the atmosphere, but there is no oxygen in a vacuum, and hence no danger of burning there.

Since the plasma sintering apparatus 20 is capable of rapid heating, the treatment time is short, which is advantageous from the standpoint of increasing productivity. Various types of sintering apparatuses have been adapted for practical use, and the type of apparatus used is arbitrary.

In the sintering treatment, the relationship between the treatment temperature and treatment time is important. One example of the temperature curve used to determine this relationship will be described next.

As is shown in FIG. 4, the temperature is elevated from room temperature to 530° C. over a period of 6 minutes (360 seconds). The temperature is then elevated to 570° C. over a period of 2 minutes, and is subsequently elevated to 580° C. over a period of 1 minute. The temperature is then maintained at 580° C. for 10 minutes. Following this period of temperature maintenance, the supply of power is stopped, and the plasma sintering apparatus 20 shown in FIG. 3 is allowed to cool naturally in its entirety. This type of cooling is called oven cooling, and it is characterized by an extremely low cooling rate.

This type of temperature curve is merely an example. Specifically, this curve may be appropriately established on the basis of the metal material that is prepared.

The experiments described below were performed in order to confirm the effect of the manufacturing method of the present invention described above.

EXAMPLES

Examples of the present invention will be described below, but the present invention is not limited to these examples.

Preparation:

Ball mill: TKMAC-1200L manufactured by Topologic Systems

Metal mill vessel: internal diameter 55 mm, length 60 mm.

Capacity: approximately 140 mL, material: SUS 304.

Powdered metal: powdered aluminum having a mean particle size of 45 μm. Bulk density: 2.96 g/cm³, melting point: 660° C.

Nanocarbon material: carbon nanofibers having a maximum fiber diameter of 150 nm and a bulk density of 0.04 g/cm³. However, these carbon nanofibers were not dispersed by ultrasound.

Charging into Mill Vessel:

The total mass of powdered aluminum (powdered Al) and carbon nanofibers (CNF) was set at 20.0 g. These materials were placed in the mill vessel so that the amount of carbon nanofibers was 0 mass %, 0.5 mass %, 1.0 mass %, 2.0 mass %, or 5.0 mass %, and the remainder was powdered aluminum. The concrete masses are shown in the following table.

TABLE 1
Sample			Powdered
No.	Total	CNF Ratio	Aluminum	CNF
1	20.0 g	0.0 mass %	20.0 g	0.0 g
2	20.0 g	0.5 mass %	19.9 g	0.1 g
3	20.0 g	1.0 mass %	19.8 g	0.2 g
4	20.0 g	2.0 mass %	19.6 g	0.4 g
5	20.0 g	5.0 mass %	19.0 g	1.0 g

Mixing of Powdered Metal and Nanocarbon Material:

The mill vessel was placed in the aforementioned ball mill and was shaken for 5 hours at 800 rpm.

Sintering:

The mixture thus obtained was set in the plasma sintering apparatus 20 shown in FIG. 3, and was sintered while being molded under the following conditions.

Degree of vacuum: 5 Pa

Pressurization: 60 MPa

Heating curve: according to heating curve shown in FIG. 4

Tensile Test:

The sintered body thus obtained was treated by a rolling method at 300° C. A tensile test piece was manufactured from the rolled material thus obtained, this test piece was placed in a tensile tester, and the maximum tensile stress was determined. The results are shown in the following table.

TABLE 2
Sample		Maximum Tensile
No.	CNF Ratio	Stress
1	0.0 mass %	98 N/mm2
2	0.5 mass %	126 N/mm2
3	1.0 mass %	128 N/mm2
4	2.0 mass %	122 N/mm2
5	5.0 mass %	105 N/mm2

FIG. 5 is a graph showing the relationship between the amount of carbon nanofibers added and the maximum tensile stress. The graph shows the numerical values in the above table in the form of a graph. Sample 1 contains no CNF. Samples 2 through 5 contain CNF. It was confirmed that the mechanical strength can be increased by mixing CNF.

Specifically, it can be said that a nanocarbon/metal composite material having a large strength, as confirmed in samples 2 through 5, can be manufactured by performing a preparatory step in which a ball mill, a specified amount of a powdered metal, and a specified amount of a nanocarbon material are prepared; a step in which the aforementioned powdered metal and nanocarbon material are placed in an empty metal mill vessel containing no balls; a step in which a mixture comprising the aforementioned powdered metal coated with the nanocarbon material is obtained by shaking the mill vessel in three dimensions using the aforementioned ball mill; and a step in which the aforementioned mixture is molded and sintered to obtain a sintered body.

Next, a test investigating the effect of a dispersion treatment in improving the effect of the present invention was additionally performed.

Dispersion Treatment:

Solution: acetone solution

Vibration frequency: 28 kHz

Treatment time: approximately 20 minutes

The subsequent preparation, charging into the mill vessel, mixing, sintering, and tensile testing were the same as in the case of samples 1 through 5; accordingly, a description is omitted here.

The maximum tensile stress measured for sample No. 6 was as shown in the following table. Sample 3, which showed the best results among samples 1 through 5, is also shown for comparison.

TABLE 3
Sample No. Maximum Tensile Stress
6          133 N/mm2
3          128 N/mm2

FIG. 6 is a graph showing a comparison of sample 6 and sample 3 according to the present invention. In both sample 6 and sample 3, the amount of CNF added is 1 mass %, and the two samples differ only in terms of the presence or absence of a dispersion treatment, with sample 6 being subjected to a dispersion treatment by ultrasound, and sample 3 not being subjected to any dispersion treatment. The maximum tensile stress of sample 3 is 128 N/mm², and the maximum tensile stress of sample 6 is 133 N/mm²; consequently, according to the calculation of 133÷128=1.04, a 4% increase in strength is seen as an effect of the dispersion treatment.

Furthermore, although acetone is ideal as the solvent used in the dispersion treatment, some other similar solvent may also be used.

Moreover, in the step in which the mixture is molded and sintered to obtain a sintered body, it would also be possible to perform molding and sintering in series by manufacturing a molded body using a powder pressing method, and then transferring this molded body to a sintering apparatus and performing a sintering process, besides performing molding and sintering simultaneously in parallel as in the present example.

Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced-otherwise than as specifically described.

Claims

1. A method for mixing a powdered metal and a nanocarbon material, said method comprising the steps of:

preparing a ball mill, a specified amount of a powdered metal, and a specified amount of a nanocarbon material;

placing said powdered metal and nanocarbon material in the empty metal mill vessel containing no balls; and

obtaining a mixture in which said powdered metal is coated with the nanocarbon material by shaking said mill vessel in three dimensions using said ball mill.

2. The method of claim 1, wherein the nanocarbon material prepared in said preparatory step comprises a nanocarbon material dispersed in advance by ultrasound.

3. A method for manufacturing a nanocarbon/metal composite material, said method comprising the steps of:

preparing a ball mill, a specified amount of a powdered metal, and a specified amount of a nanocarbon material;

placing said powdered metal and nanocarbon material in the empty metal mill vessel containing no balls;

obtaining a mixture in which said powdered metal is coated with the nanocarbon material by shaking said mill vessel in three dimensions using said ball mill; and

molding and sintering said mixture to obtain a sintered body.

4. The method of claim 3, wherein the nanocarbon material prepared in said preparatory step comprises a nanocarbon material dispersed in advance by ultrasound.