JPS61266398A - Method for manufacturing graphite whiskers - Google Patents

Abstract

PURPOSE:To obtain graphite whisker having superior characteristics by treating active carbon previously, then heat-treating. CONSTITUTION:Active carbon having >=500m<2>/g specific surface area is treated with mineral acid, then heat-treated at >=2,000 deg.C. Effective mineral acid is HCl, H2SO4, HNO3 or their mixture. Necessary concn. of the acid is >=ca.0.5 N. If mixed acid is to be used, the proportion of the component acid may be optional. Suitable treating temp. is room temp. (ca.20 deg.C)-100 deg.C, and suitable treating time is >=ca.5min. The treatment is executed by immersing the active carbon in the acid just enough to dip the active carbon, allowing to stand or stirring slightly. Then, the acid is removed by filtration or decantation, and the active carbon is washed throughly with water and dried.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is characterized in that activated carbon, after being previously treated with mineral acid, has a
The present invention relates to a method for producing graphite whiskers with excellent physical properties by heat treatment at 00°C or higher.

Conventional technology and problems Whiskers are fibrous single crystals1. A variety of whiskers are manufactured because they have mechanical properties close to theoretical properties. Several methods have also been proposed for graphite whiskers. A typical method is a method in which a graphite electrode is deposited by arc discharge under high temperature and pressure [R. Bacon (R.

Bacon). Journal of Applied Physics (J, Appli, Phys) No. 31
Volume, 283 pages, 1960]. The physical properties of the whisker obtained by this method are as follows: tensile strength of 2000 kg
/m'' and tensile modulus of 72 tons/am''. In addition, there is a method of depositing COz gas by thermal decomposition on stacking misalignment and rotation twins of the (111) plane of β-3iC crystal [H, B, ■aanstra et al., Journal
Of Crystal Growth (J, Crystal, G
rowwth), Volume 16, Page 71, 1972]
A method of producing amorphous carbon obtained by electrolytic deposition of martensite and martensite by heat treatment at 2800°C [Shae et al.
Gio-(J, G11lotl et al., Kapo-N(
Carbon), Volume 6, Page 381, 1968].

However, since all of these methods require complicated production operations, they have many drawbacks as industrial production methods.

On the other hand, as an industrial production method, there is a method of producing graphite whiskers by heat-treating carbon black having a particle size of about 13I or less at 2000° C. or higher (Japanese Patent Publication No. 8762/1987). This method is very good in that graphite whiskers can be produced with simple operations, but the raw material carbon black must have a very small particle size of about 13n@ or less, and this The drawback is that carbon black with such a small particle size is not necessarily inexpensive.

In view of the above points, the method described in Japanese Patent Publication No. 58-8762 is considered to be the best industrial manufacturing method for producing graphite whiskers. As a result of searching for a carbon material to replace black, it was found that graphite whiskers can also be produced from a carbonaceous material (activated carbon) having fine pores (Japanese Patent Application No. 6O-47752). However, since most of these whiskers have a crystal structure belonging to A below, the physical properties that can be considered from this structure are not necessarily superior to those belonging to B.

The structures of graphite whiskers known so far can be roughly divided into: (A) the C-axis of the graphite crystal is in the same direction as the long axis of the whisker; (B) the C-axis of the graphite crystal is in the direction perpendicular to the long axis of the whisker. There are two. Furthermore, (A) is (A-1) one in which the C axis of the graphite crystal is stacked at an angle of about 20°C with respect to the central axis of the whisker (cone-helix), (
A-2) The graphite crystal C-axis is arranged parallel to the long axis direction of the whisker, and the graphite crystals are stacked (hezlgonal 1ay
er), and (B) is (B-1) where band-shaped graphite crystals are wound in a spiral shape and continuously laminated (scr
oll 1ayer), (B-2) A band-shaped graphite crystal wound concentrically around the center of the whisker (e
oneentric 5ireles)@Therefore, in terms of physical properties, it belongs to 已, that is, R, Ba
A crystal structure similar to that obtained by con is considered to have practical value.

Purpose of the Invention From the above points, the present invention provides that when activated carbon is treated at high temperature,
The generated whiskers are not of a structure belonging to A, but are of a structure belonging to B.
There is a need to generate whiskers belonging to the above group, and it has been found that activated carbon can be treated with a mineral acid in advance.

Structure of the Invention The present invention consists of the following steps. That is, ■ about 5001
Selection of wood-based, vegetable-based, or coal-based activated carbon with a specific surface area of 11″/g or more.

■ Treatment of these activated carbons with mineral acids.

■ Heat treatment at approximately 2000℃ or higher.

Among these steps, step (2) is the main point in the present invention. These steps will be described in detail below.

Approximately 500m of activated carbon is the raw material for producing whiskers.
” / It is necessary to have a specific surface area of 8 or more. If it is less than this, the amount of whiskers produced is small, so it is not suitable as a raw material. Also, the activation method may be either chemical activation or gas activation, and the shape can be changed. It may be in powder or granular form.The specific surface area here refers to liquid nitrogen temperature (77K).
This is the value determined by the BET equation from the adsorption equidistant line due to nitrogen gas adsorption at .

These activated carbons are then treated with mineral acids. Mineral acids useful for the purposes of this invention are hydrochloric acid, sulfuric acid, nitric acid, or mixed acids thereof. Phosphoric acid has no particular effect, and whiskers belonging to A are produced even when treated with this acid. Note that treatment with an aqueous solution of aqueous sodium hydroxide or aqueous ammonia, which are alkaline, has no effect, and whiskers belonging to A are generated.

The conditions for treatment with these acids are as follows. The concentration of acid may be about 0.5N breath. Further, in the case of a mixed acid, each acid may be used in any proportion.

The effect does not particularly change depending on the concentration ratio. Temperatures range from room temperature (approximately 20°C) to 100°C. Processing time is about 5 minutes or more. In the treatment, these acids are added to the extent that the activated carbon is fully immersed, and the mixture is left to stand or is slightly stirred.

The acid is then removed by filtration and decantation, followed by thorough washing and drying.

Even if activated carbon containing acid is heat-treated, the effect remains the same, but it is not preferable because it may corrode the equipment. What is more important is the PR when removing the acid after treatment with the acid. The acid removal operations usually include (1) washing with water and (2) neutralization with an aqueous alkali solution. In the case of (2) washing with water, the pH is 7 or lower and there is no problem; however, in the case of (2) using an aqueous alkaline solution, the structure of the whisker produced differs depending on the pH. That is, PI(
When PR is 7 or less, the whisker belongs to the same category as shown in Figure 1, but when PR is 7 or more, it belongs to the fourth whisker.
This results in a whisker belonging to A similar to that shown in the figure. Therefore, the use of excess alkali will lose the effectiveness of the present invention. Note that if the activated carbon with a PI of 7 or more is treated again with mineral acid to make the PH 7 or less, whiskers belonging to B, similar to those shown in FIG. 1, are generated.

The activated carbon treated with a mineral acid is heat treated at about 2000° C. or higher in an inert gas such as nitrogen or argon. Hold times at these temperatures are about 30 minutes or more. Even after holding for a long period of time, such as several hours, no significant increase in the amount of whiskers produced was observed. At temperatures below 2000°C, the amount of whiskers produced decreases.

By this heat treatment, whiskers are generated in the form of fluff on the surface of the activated carbon particles. The situation was observed using a scanning electron microscope (SEM).
The results are shown in Figure 1. This sample was obtained by treating activated carbon treated according to Example 1 at 2800°C. As seen in this figure, most of the whiskers formed on the particle surface have a diameter of 0.2 .mu.m and a length of several tens to hundreds of degrees.

Some are partially nodular. In addition, although in small quantities, it has a length of about 10 to 20 μm with a maximum diameter of about 5 μm.
There are also "cigar-shaped" intestines. Structural analysis using high-resolution electron microscopy and microlawe Xs diffraction reveals that the former is a 5-croll 1-ay structure consisting of band-shaped graphite crystals wound in a helical shape.
The structure belongs to B of er or eoneantrtc s+reles, the latter belongs to A, and the eo of A-1
It is of the ne-helix type.

The whiskers produced when activated carbon is heat-treated as it is at about 2800°C have a structure belonging to A, but by treating this activated carbon with a mineral acid in advance, whiskers belonging to B are produced. At present, it is not clear why Iskar with this different structure is generated.

When heat-treated activated carbon is observed with an SEM, in the present invention, the particle surface cannot be observed because the particles are covered with flocculent whiskers, but it is possible to observe the activated carbon that has been heat-treated as it is. From this result, it can be seen that several cigar-shaped voice cars are generated from specific locations on the particle surface of the present whisker, and there is a point where whiskers are generated. In the case of the present invention, assuming that whiskers are similarly generated from the generation point on the particle surface, it is assumed that the treatment with mineral acid changes the state of this generation point, and as a result, the crystal structure of the generated whisker changes. Presumed.

It is a well-known fact that when activated carbon is produced, it is generally activated with gas or chemicals and then subjected to acid treatment. Therefore, commercially available activated carbon is considered to have already been acid-treated. However, as mentioned above, acid treatment is necessary to generate whiskers belonging to B.

The reason for this is presumed to be that, as described on page 7, an alkali is used to neutralize the acid after the acid treatment.

Example 1 The specific surface area determined by the BH3 formula from nitrogen girth adsorption at 77K is 986//g-The average pore diameter determined from the pore distribution by the Cranston-Inclay (CI) method is 1
．． Crushed coal-based activated carbon with a diameter of 2 to 3 nm, 20
Put 100 g of 38% hydrochloric acid into a 300-mL Erlenmeyer flask
After adding -, the mixture was kept in a constant temperature water bath at 30°C for 5 minutes.

Then, it was immediately put into water in Step 11, and after being allowed to stand still, most of the water was removed using a decanting soda, fresh water was added to the remainder, and the water that had been left to stand was removed by heating and boiling. Perform this operation 3
After repeated rotations, the mixture was filtered through a glass filter. At this time, the P of the 5p solution was 6.3. Approximately 1 liter of filtered material
It was dried overnight at 0° C. to obtain hydrochloric acid-treated activated carbon.

5 g of hydrochloric acid-treated active material was placed in a lidded graphite crucible with an inner diameter of 25 m and a depth of 25 m, and heated in a Tammann furnace in an argon stream at an average heating rate of 450°C/hr to 2800°C. Hold for minutes.

It was observed that whiskers were formed in the form of fluff on the surface of the activated carbon heat-treated in this manner.

The situation was observed using a scanning electron microscope (SEM), and the first
As shown in the figure. Figure 1a) shows the overall appearance observed under low magnification, and Figure 1b) shows the appearance of the flocculent whiskers observed under high magnification. As seen in this figure, it can be seen that whiskers with a diameter of about 0.2 μm are generated.

Because these whiskers are intertwined in countless numbers, it is impossible to know the length of a single whisker, but it appears that there are several whiskers that are long. xi this whisker
As analyzed by a microanalyzer, h is composed of carbon, and crystal structure analysis by transmission electron microscopy and electron beam diffraction reveals that it has a structure belonging to B among the graphite whisker structures mentioned above. It turned out to be. However, since the structures B-1 and B-2 give the same structural pattern, it is not known which one it is.

In addition, in FIG. 1, a cigar-like shape with a large diameter and a short length can be seen. The crystal structure of this substance is A-
Therefore, not all of the generated whiskers have a crystal structure belonging to B.

The same activated carbon as above was treated with various mineral acids and then heat treated. The results are summarized in Table 1 and Figures 2 and 3.

As can be seen from these figures in Table 1, the largest amount of thin and long whiskers belonging to B is produced in the case of hydrochloric acid treatment. In the case of other acids, in addition to this whisker, a considerable amount of cigar-like material is also produced. Furthermore, when treated with phosphoric acid or alkali, only cigar-shaped particles are produced, and no effect of the treatment is observed.

Comparative Example Activated carbon similar to Example 1 was treated in 36% hydrochloric acid at 30°C for 5 minutes. A 1N caustic soda aqueous solution was gradually added to the mixture to neutralize it, and finally it was made into Pfull. This was kept for 37 minutes, thoroughly washed with water, and then dried. This activated carbon is heated in a Tammann furnace for 2
It was heated to 800°C and held at this temperature for 30 minutes. When this heat-treated product was observed with a SEM, as shown in FIG. 4, cigar-shaped whiskers were formed on the particle surface, and a few thin and long whiskers with a diameter of 0.2 μm were observed. This situation is almost the same as when untreated activated carbon is heat treated. Therefore, the activated carbon treated to pH 1 was treated again in 36% hydrochloric acid at 30°C for 5 minutes, thoroughly washed with water, filtered through 3p, and dried. 3P after 2
The pH of the liquid was 6. This was heated to 2800℃ in a Tamman furnace.
When the sample was heat-treated for 30 minutes, whiskers similar to those shown in FIG. 1 were generated.

Example 2 Coal-based spherical activated carbon (steam activation, specific surface area 767'/
g s Average pore diameter 1. Onm) and wood-based powder activated carbon (zinc chloride activated, specific surface area 1313 m''/g, average pore diameter 1.9 nm) 20 g each to 300 m
Collect the sample in an Erlenmeyer flask and add 100ml of 36% hydrochloric acid to it.
J added.

Then, the mixture was placed in an oil bath heated to 100° C., maintained for 60 minutes, and then filtered under reduced pressure through a glass filter (No. 4). The 5p filtered product was washed with hot water, etc. until the pH of the furnace solution reached 6, and then dried under reduced pressure at about 80° C. all day and night.

These hydrochloric acid-treated activated carbons were treated in the same manner as in Example 1, and 28
Heat treatment was performed at 00°C for 30 minutes. When we examined the state of whisker formation in this treated material using SEMII inspection, we found that in the case of coal-based activated carbon, many long whiskers with a diameter of about 0.2 μm were formed, similar to that shown in Figure 1, but in the case of wood-based activated carbon, Similar to Figure 3'
Most of them are short, with a diameter of about 0.2 μm, and their quantity is not large. Furthermore, cigar-shaped substances were also produced.

Effect The crystal structure of the whisker obtained by the method of the present invention belongs to B. Since this has the same crystal structure as the whisker obtained by R. Bacon, it is thought that it has similar physical properties. Therefore, this whisker can be said to be useful, for example, as a raw material for composite materials. Moreover,
The raw material is activated carbon, which is relatively inexpensive, and can be easily obtained by treating it with a mineral acid, making it a useful industrial manufacturing method.

[Brief explanation of drawings]

Figure 1 is a scanning electron micrograph of whiskers formed on coal-based activated carbon treated with hydrochloric acid. Figure 1 a) is a low magnification, and Figure 1 b) is a high magnification. Figure 2 is 50%
These are scanning electron micrographs of whiskers formed on coal-based activated carbon treated with sulfuric acid, Figure 3 treated with 62% nitric acid, and Figure 4 treated with hydrochloric acid followed by alkali treatment (PH11).

Claims (3)

[Claims] (1) A method for producing graphite whiskers, which comprises treating activated carbon with a specific surface area of 500 m^2/g with a mineral acid and then heat-treating it at 2000°C or higher. (2) The method according to claim 1, wherein the mineral acid is hydrochloric acid, sulfuric acid, nitric acid, or a mixed acid thereof. (3) The method according to claim 1, wherein the activated carbon is treated with a mineral acid and then the mineral acid is removed at a pH of 7 or less.