JP2849702B2 - Method for producing highly conductive carbon-based composite material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a lightweight, heat-resistant, highly conductive carbon-based composite material from graphite and boride ceramics having electrical conductivity.

[0002]

2. Description of the Related Art Composite materials composed of carbon and ceramics are used as materials having the characteristics of carbon such as light weight, heat resistance, chemical resistance and the like of ceramics such as heat resistance, oxidation resistance and high strength. .

[0003] In this method for producing a composite material, ceramic powder is mixed with raw coke, which is a carbonaceous material having caking properties,
After forming, firing (Japanese Patent Publication No. 61-27352)
No. 2-7907) and a method of preparing a ceramic powder having a carbonaceous mesoface around it by heat-treating the ceramic powder in a pitch, and molding and firing the same (JP-A-6-192660). Furthermore, in order to use this composite as a functional material, a spherical carbon-boride ceramic composite and a method for producing the same have been proposed by heat-treating a mixture of carbon and boride ceramic powder at 2400 ° C. or more. (Japanese Patent Application No. 6-7139
No. 6). However, these carbon-ceramic composite materials are not particularly used as conductive materials.

[0004] On the other hand, carbon composite materials containing titanium boride and zirconium boride have been developed as conductive materials.
JP-A-64-43264 discloses that a compact having a basic composition of raw coke, silicon carbide, and boron carbide as a high-conductivity high-strength carbon material and blended with titanium boride and zirconium boride is fired at 2000 ° C. A method is disclosed. The electrical resistivity of this composite is 480 μΩ · cm or more. Furthermore, it has been reported that a carbon composite material containing titanium boride and zirconium boride was used as a cathode during aluminum refining (Sekine et al., Electrochemical and Industrial Physical Chemistry, Vol. 62 (No. 3), p. 232 (1994).
Year)). According to this report, 0-100% titanium diboride, 0-55% zirconium diboride in carbon powder, and a mixture of pitch and phenol resin
23% is molded and fired at 1000 ° C. to form an electrode. The electric resistance of this electrode is 5 × 10 ^{−3 to} 9 × 10 ^−3.
Ωcm (conductivity 2 × 10 ^{2 to} 1.1 × 10 ² (S / c
m)), and both methods have very low conductivity.

[0005]

The carbon material is used as one of the conductive materials because it is lightweight, has excellent heat resistance and corrosion resistance, and has conductivity. The conductivity of the carbon material depends on the degree of development of the graphite crystal, and the carbon material having the highest conductivity is highly oriented pyrolytic graphite (HOPG) produced by annealing pyrolytic carbon at a temperature of 3000 ° C. or more under pressure. And 2.5 × 10 ⁴ (S / cm) in the a-axis direction (for example, Shirakawa et al., Edited by Synthetic Metals, Kagaku Dojin, Chemical Special Edition 87, 1980). This value is as small as about 1/20 of that of silver or copper metal.

[0006] Graphite is a two-dimensional layered crystal, and easily forms various compounds and interlayer compounds. The conductivity of the intercalation compound generally increases, and the second stage intercalation compound of arsenic pentafluoride (AsF ₅ ) is 6.3 × 10 ⁵ (S / cm) and the third stage of antimony pentafluoride (SbF ₅ ) So 1
× 10 ⁶ (S / cm), which is equal to or higher than silver (for example, edited by Shinjun Watanabe, graphite intercalation compound,
Modern editorial company, 1986). However, the graphite intercalation compound is easily decomposed in the air and the electric conductivity is reduced. for that reason,
Attempts have been made to produce stable intercalation compounds by searching for conditions such as the type and structure of the host and intercalant, but none of them have been produced for practical use.

On the other hand, there are many ceramics having electrical conductivity except for oxide ceramics. Among them, Group II titanium, zirconium and hafnium diborides have an electric conductivity of about 5 to 10 × 10 ⁴ (S / cm).
And have higher values than that of graphite (for example, P.Sc
Hwarzkopf et al., translated by Hariba et al., Raw materials for cemented carbide and cermet, Corona, 1960). Nevertheless, it is considered that these ceramics are not used as highly conductive materials. Although the reason is not clear, it is considered that there is probably a problem in purity and moldability. That is, although boride ceramics are produced in a very high purity as a powder, they are easily oxidized when left in the air to form an oxide film. In addition, this powder has poor moldability, and a sintering aid is used to produce a molded body, but this is considered to decrease the electrical conductivity.

[0008] The problem of moldability also exists in carbon materials,
Those in which graphite crystals other than HOPG have developed include natural graphite and quiche graphite. However, these are all powders and have poor moldability, so that only molded articles having many pores can be obtained. Therefore, the electric conductivity is much lower than the electric conductivity considered from the degree of crystal development.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to produce a carbon-based compact having high conductivity by compounding graphite and diboride ceramics. As described above, although graphite and diboride ceramics have considerably high conductivity, they cannot exhibit sufficient conductivity because both graphite and diboride ceramics have poor moldability alone. That is, it was considered that the normal state of diboride was due to the formation of an oxide film. Then, it was thought that high conductivity could be exhibited by compounding graphite and diboride ceramics, and as a result of earnest studies, the present invention was accomplished. That is, the present invention relates to 1 part by weight of flaky graphite, and 1 to 2.5 parts by weight of at least one kind of boride ceramics selected from titanium diboride, zirconium diboride and hafnium diboride. An object of the present invention is to provide a method for producing a highly conductive carbon-based composite material, which comprises subjecting a mixture to heat treatment at 2500 to 3000 ° C. in an inert gas atmosphere after molding.

Hereinafter, the method of the present invention will be described. The raw materials used are natural graphite and diboride ceramics. Natural graphite is roughly classified into scaly and muddy graphite,
In the present invention, it is flaky graphite. Moreover, its size is
A large powder having a size of not less than 00 μm is preferable, and a powder obtained by grinding the powder to several microns is not preferable because the electric conductivity is reduced.

The diboride ceramics are titanium diboride (TiB ₂ ), zirconium diboride (ZrB ₂ ) and hafnium diboride (HfB ₂ ), and one or a mixture of two or more thereof is used. These are 99% pure
The above particles having a particle size of about 10 μm or less are manufactured,
Use this as it is.

The natural graphite and the diboride ceramic are collected in a weight ratio of 50:50 to 20:80. If the amount of the boride ceramics is 50% by weight or less, the conductivity of the obtained molded body will be low, and if it is 80% by weight or more, the moldability will be poor.

The raw materials collected are thoroughly mixed in advance.
If the mixing is performed using a grinder or a vibrating ball mill, the particle size of the raw material becomes small due to the action of the grinding, so that the mixing needs to be performed in a short time.

[0014] The mixed raw material is formed. This molding is performed so that the two-dimensional layered graphite is oriented in one direction as much as possible. For example, in plate-shaped molding by uniaxial compression, the thickness is made as thin as possible. If this is too thick, the graphite will be oriented near the surface, but the inside will be random and will have low conductivity. Therefore, forming into a thin plate or ribbon shape with a roller or the like is a preferable method.

The molded product is heat-treated at 2500 to 3000 ° C. in an inert gas such as argon or helium. It is not preferable to use nitrogen gas as the inert gas. The reason is that at high temperatures, boride ceramics may react with nitrogen and change to nitride ceramics.

The heat treatment temperature is in the range of 2500 to 3000 ° C., and when the heat treatment temperature is high, the conductivity of the formed body is increased. However, when the temperature is less than 2500 ° C., a formed body having a desired conductivity can be obtained. Absent. When the temperature exceeds 3000 ° C., the molded body is melted and deformed. The melting and deforming temperature depends on the blending amounts of graphite and diboride, and the lower the amount of graphite, the lower the temperature. Therefore, it is necessary to determine the highest heat treatment temperature depending on the amount of the compound.

The molded body of graphite and diboride ceramic obtained in this manner has the highest conductivity of graphite, H
It is a high conductor having a value close to 2.5 × 10 ⁴ (S / cm) of OPG, and has sufficient strength as a molded body.

This compact is not merely a mixture of natural graphite and diboride ceramics. This is apparent from the fact that, as described above, the electrical conductivity needs to be heat-treated and varies depending on the temperature. That is, those obtained by molding a mixed powder of natural graphite and diboride ceramics is electric conductivity of ~10 ² (S / cm), the strength is low,
When this is heat-treated at 2000 ° C., it is 10 ³ (S / cm)
Conductivity and strength are increased. When the temperature further rises to 2500 ° C. or higher, the conductivity reaches −10 ⁴ (S / cm), and the strength increases. Such a change is due to a change in the structure due to heat treatment, and the change produces graphite other than the natural graphite used as a raw material, and the orientation of the graphite is parallel to the surface of the molded body. Is present as if fused to the ends of the flat natural graphite and the generated graphite. This is considered to be the reason for increasing the conductivity and strength.

Hereinafter, the method of the present invention will be described in more detail with reference to examples. The symbols used here are SEM for scanning electron microscope and E for energy dispersive X-ray analyzer.
DX.

Example 1 In this example, a molded body was prepared from natural graphite and zirconium diboride, and the electric conductivity was measured.

The natural graphite used as the raw material is flaky graphite produced in China, which was separated by a 32 Tyler mesh (mesh size: 495 μm) sieve and used on the sieve. Zirconium diboride (ZrB ₂ ) having an average particle diameter of 2 μm manufactured by Nippon Shinyaku was used as it was. 5 g of ZrB _{2 in} 5 g of natural graphite
(Weight ratio: 50:50), and mixed for 1 hour with a grinder. Then, 1 g of the mixed powder was put into a cylinder-type molding machine having an inner diameter of 20 mm, and a thickness of about 1 mm was applied under a load of 300 MPa.
Molded. This is placed in a tube furnace in nitrogen gas at 900 ° C for 1 hour.
Firing for 2 hours,
It heated to 000-3000 degreeC, and it heat-processed holding 1 hour. The heat-treated molded body was measured at a conductivity of 25 ° C. using a surface resistance meter Loresta AP manufactured by Mitsubishi Yuka (currently Mitsubishi Chemical).
It was measured by the terminal 4 probe method. Table 1 shows the results.

[0022]

As can be seen from the results, the conductivity increases as the temperature of the heat treatment increases, but is particularly remarkable at 2500 ° C. or higher, and its value is HOPG which is a highly oriented pyrolytic graphite.
Become closer to that of. The bulk density is 2.6 regardless of the processing temperature
It was in the range of 5 to 2.72, and the molded body at 2500 ° C. or higher was hard. Experiment numbers 1-1, 1-2 and 1-3
Is a reference example.

Further, the structure of the compact heat-treated at 2800 ° C. was examined by SEM and EDX attached thereto.
The surface was fairly smooth, and a mixture of irregularly shaped flat plates and triangular, square, and hexagonal flat plates was present, and these flat plates were parallel to the surface of the molded product. These slabs were carbon by EDX analysis. When the molded body heat-treated at 2400 ° C. was examined in the same manner, most of the irregularly shaped flat bodies were present, and slightly polygonal flat bodies were present. From this, it was found that the amorphous plate was natural graphite and the polygon plate was formed by heat treatment. In addition, there were fused parts around these flat bodies (edge parts), and when these parts were examined by EDX, they were found to be zirconium. The molded product was pulverized, and powder X-ray diffraction was measured. The result was a diffraction line of carbon, ZrB _2, and a trace amount of zirconium carbide having extremely excellent crystallinity. From these facts, it can be said that the molded body is composed of natural graphite, the formed polygonal flat graphite and ZrB ₂ , and ZrB ₂ is mainly present at the end of the flat graphite. Polygonal flat graphite is ZrB ₂
Natural graphite in contact with the dissolved in ZrB ₂ by heating at high temperature, believed to be precipitated as graphite during cooling. Therefore, the formation of this polygonal flat graphite and ZrB
_{It can be said that} the fusion of No. 2 is one factor that increases the electrical conductivity and improves the strength of the molded body.

Example 2 In this example, the conductivity of a molded body prepared by changing the blending amounts of natural graphite and ZrB ₂ was examined.

The same natural graphite and ZrB ₂ as in Example 1 were used, and their compounding amounts were 70:30 and 30:70 by weight.
And 20:80, and mixed for 1 hour with a grinder.
When this was molded in the same manner as in Example 1, 30: 7
With a blending amount of 0, a clean molded product was obtained, but 20: 8
In the case of 0, it was broken when it was released from the mold and could not be obtained as a molded product.

The molded body was placed in an argon stream in a Tamman furnace,
Heated to 800 ° C., held for 1 hour and heat treated. As a result of measuring the conductivity at 25 ° C., the compounding amount was 70:30.
Is 4.34 × 10 ³ (S / cm).
It was 1.86 × 10 ⁴ (S / cm) in the molded body of 0:70. The molded product heat-treated at 3000 ° C. with a compounding amount of 70:30 was 6.41 × 10 ³ (S / cm), but when the compounding amount was 30:70, it melted by the heat treatment at 3000 ° C. and was greatly deformed. Therefore, the conductivity could not be measured. From these results, it was found that the electrical conductivity depends not only on the heat treatment temperature but also on the compounding amount. However, in order to exhibit high conductivity, the compounding amount is 50:50 or more together with the result of Example 1.

REFERENCE EXAMPLE In this reference example, the effect of the particle size of natural graphite and ZrB ₂ , particularly the effect of the particle size of natural graphite on the electrical conductivity was investigated.

A mixed powder was prepared by using the same natural graphite and ZrB ₂ as in Example 1 and changing the blending amounts thereof to 50:50 by weight and mixing time with a rake machine. This was molded in the same manner as in Example 1 and then molded in a Tamman furnace.
It was heated to 00 ° C. and kept for 1 hour to obtain a molded body. The electrical conductivity of this molded body was measured. Table 2 shows the results.

[0030]

From the results, it was found that when the mixing time by the grinder was prolonged, the electric conductivity was lowered. This is thought to be because natural graphite was pulverized to reduce the particle size, and hard ZrB ₂ coexisted, so that the degree of destruction was considered to be large.

This was confirmed using flaky natural graphite having an average particle size of 6 μm. The conductivity of the compact heat-treated at 2800 ° C. was 5.31 × 10 ³ (S / cm), where the mixing time with a ZrB ₂ triturator was 1 hour, which was compared with the value of Experiment No. 1-6. About one-half.

COMPARATIVE EXAMPLE In this comparative example, natural graphite and ZrB ₂ were individually molded, and the electric conductivity was measured.

Using the same natural graphite and ZrB ₂ as in Example 1, each was molded independently. In the case of natural graphite, it was very weak and brittle, but could be molded as a compact. However, when ZrB ₂ was released from the mold, it was broken very finely, and a molded product could not be obtained. Therefore, the molded body made of only natural graphite was heated to 2800 and 3000 ° C. in a Tamman furnace, and kept for 1 hour to perform heat treatment.

The electric conductivity of this molded product was measured in the same manner as in Example 1.
10 ³ (S / cm) at 3000 ° C is 1.55 × 1
0 ³ (S · cm), and the value was much lower than HOPG. Their bulk densities were 1.84 and 1.86 (g / cm ³ ), indicating low strength.

Example 3 This example uses natural graphite and diboride ceramics
It is a result of examining the electric conductivity of a molded body using titanium diboride and hafnium diboride.

The same natural graphite as in Example 1 was used. Titanium diboride and hafnium diboride are made by Nippon Shinyokuhin, and the average particle size is 4 μm for the former and 4.5 for the latter.
μm.

Natural graphite and titanium diboride or hafnium diboride were mixed at a weight ratio of 50:50, and the mixture was mixed for 1 hour by a triturator. This mixture was molded in the same manner as in Example 1 and heat-treated at 2800 ° C. for 1 hour. As a result of measuring the electric conductivity of this compact in the same manner as in Example 1, it was found that titanium diboride was 9.87 × 10 ³ (S / cm), and that hafnium diboride was 1.38 × 10 ^4. (S / cm)
Met. The structure of the surface of the molded body was composed of natural graphite and polyhedral graphite having borides fused to the ends, as in the case of ZrB ₂ of Example 1.

[0039]

According to the simple operation of molding a mixed powder of flaky natural graphite and titanium diboride, zirconium diboride, hafnium diboride, and heat-treating at 2500 to 3000 ° C. in an inert gas atmosphere, A highly conductive carbon-based composite material that is close to highly oriented pyrolytic black having the highest electrical conductivity as graphite can be produced.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Yasuhiro Yamada 807-1, Nonoshita, Sukumachi, Tosu City, Saga Prefecture Inside the Kyushu Institute of Industrial Technology (56) References Japanese Patent Publication 5-56306 (JP, B2) (58) Survey Field (Int. Cl. ⁶ , DB name) C04B 35/58 105 C04B 35/495 C04B 35/52

Claims (1)

(57) [Claims] 1. A scaly graphite (1 part by weight) and at least one boride ceramic material selected from titanium diboride, zirconium diboride and hafnium diboride
A method for producing a highly conductive carbon-based composite material, comprising: heat-treating a mixture of about 2.5 parts by weight at 2,500 to 3000 ° C. in an inert gas atmosphere after molding.