CN114538946A

CN114538946A - High-strength and high-toughness graphite and preparation method thereof

Info

Publication number: CN114538946A
Application number: CN202210224359.1A
Authority: CN
Inventors: 申克; 曹欣磊; 徐昆
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2022-03-09
Filing date: 2022-03-09
Publication date: 2022-05-27

Abstract

The invention provides high-strength and high-toughness graphite and a preparation method thereof. The preparation method comprises the following steps: step 1, heating and kneading coke aggregate, chopped carbon fiber and a binder to obtain a paste; step 2, cooling the paste, crushing, and performing isostatic pressing to obtain a green body; step 3, carbonizing the green body to obtain a carbonized product; and 4, graphitizing the carbonized product to obtain the high-strength and high-toughness graphite. According to the preparation method of the high-strength high-toughness graphite, the chopped carbon fibers are added into the fine-particle coke aggregate, and the high-strength high-toughness graphite is obtained after isostatic pressing, carbonization and graphitization.

Description

High-strength and high-toughness graphite and preparation method thereof

Technical Field

The invention relates to the technical field of chemical material preparation, in particular to high-strength and high-toughness graphite and a preparation method thereof.

Background

Graphite has the following excellent properties: (1) a high neutron scattering cross section and a low neutron absorption cross section, which are excellent neutron moderators; (2) good high temperature resistance and stable chemical properties; (3) the heat conductivity is high, and the anti-irradiation performance is good; (4) the raw materials of the artificial graphite such as coke, natural graphite and the like are rich in sources and low in price, and various nuclear grade graphite with high purity, high strength and different particle size requirements can be easily prepared, so that the artificial graphite is widely used on nuclear reactors. Particularly, in high temperature gas cooled reactors and molten salt reactors which belong to fourth generation nuclear power technologies, graphite is used as a moderator material and a reflective layer material.

In the pebble bed high temperature gas cooled reactor, the graphite material of the reflecting layer is not replaceable, so the service life of the nuclear graphite directly influences the service life of the reactor. However, since graphite is a brittle material; and the existence of pores and defect features inside the graphite material, which cause stress concentration, therefore, the graphite material has the characteristic of low fracture toughness. When the nuclear graphite is in service in a reactor, the nuclear graphite is influenced by conditions such as mechanical load, irradiation damage (which causes the volume of the nuclear graphite to change), high-temperature oxidation and the like. The characteristic of low fracture toughness can lead to cracking and failure of the nuclear graphite material, ultimately affecting the proper operation of the reactor. Thus, both (fracture) toughness and strength are important performance indicators for nuclear graphite materials.

The HTR-10 and HTR-PM projects developed in China at present all use imported nuclear grade graphite, the service life of the nuclear grade graphite is about 30-40 years, and the design service life of a nuclear reactor in the future is 50-60 years. In order to prolong the service life of the high-temperature gas cooled reactor and accelerate the development of the nuclear power industry in China, the development of the high-strength and high-toughness nuclear grade graphite is necessary.

Graphite is used in large quantities not only in the nuclear power field, but also in the photovoltaic field and the semiconductor field, for example, for manufacturing structural members such as heaters, crucibles, sleeves, and the like in devices for pulling up single crystal silicon or polycrystalline silicon. Large-size single crystal silicon is a future development trend in the photovoltaic and semiconductor fields, and accordingly, large-size graphite heaters and graphite crucibles are also required. However, graphite is not easy to produce and process large-sized graphite members due to its low strength and low toughness, and the transportation risk is also large. Graphite materials are subjected to various loads in use in the metallurgical and mechanical fields. The high toughness and strength can increase the deformation resistance and crack propagation resistance of the graphite material, and comprehensively improve the damage resistance of the graphite.

Therefore, the development of high-strength and high-toughness graphite is also necessary in these fields.

Disclosure of Invention

In view of this, the present invention provides a method for preparing high-strength and high-toughness graphite with fine particles, high strength and high toughness.

The invention also provides high-strength and high-toughness graphite with fine particles, high strength and high toughness.

In order to solve the technical problems, the invention adopts the following technical scheme:

according to the first aspect of the invention, the preparation method of the high-strength high-toughness graphite comprises the following steps:

step 1, heating and kneading coke aggregate, chopped carbon fiber and a binder to obtain a paste;

step 2, cooling the paste, crushing, and carrying out isostatic pressing to obtain a green body;

step 3, carbonizing the green body to obtain a carbonized product;

and 4, graphitizing the carbonized product to obtain the high-strength and high-toughness graphite.

Further, the step 1 comprises:

step 11, dry-mixing the coke aggregate and the chopped carbon fiber to obtain mixed powder;

and 12, kneading the mixed powder and the binder under a heating condition to obtain the paste, wherein the binder is first asphalt.

Still further, the aggregate: the chopped carbon fibers are as follows: the mass ratio of the first asphalt is (50-70): (2-16): (30-40).

Further, the average particle diameter of the coke aggregate is 1 to 50 μm, and the diameter of the chopped carbon fiber is 5 to 20 μm and the aspect ratio is 5 to 20.

Wherein, the coke as the aggregate is a granule material, and the coke can be classified into pitch coke, petroleum coke, needle coke, etc., and the application does not specially limit what kind of coke is specifically adopted.

Further, the step 2 comprises:

step 21, cooling, solidifying and crushing the paste, and then sieving the paste;

and step 22, carrying out cold isostatic pressing on the sieved powder under the condition of the pressure of 100-200MPa to obtain the green compact.

Further, the step 3 comprises:

step 31, carbonizing the green body at the temperature of 800-;

step 32, immersing the carbonized body into second asphalt to impregnate the carbonized body with the second asphalt;

and 33, repeating the step 31 and the step 32 for multiple times, and obtaining the carbide after the final carbonization.

Still further, the step 32 includes:

and immersing the carbonized blank in the second asphalt, heating to 150-220 ℃, vacuumizing and maintaining the pressure in a vacuum state for a first preset time, and then pressurizing to 5-10MPa and maintaining the pressure for a second preset time so as to fully impregnate the carbonized blank with the second asphalt.

Furthermore, in the step S31, nitrogen is introduced at a flow rate of 80-150ml/min, the green body is heated to 800-.

Further, in the step S4, the carbonized product is heated to 2500 ℃ and 3200 ℃, and high-purity argon is introduced into the carbonized product for graphitization in the whole process, so that the high-strength and high-toughness graphite is obtained.

Further, halogen and/or halogenated hydrocarbon gas may be selectively added throughout the graphitization treatment to improve the purity of graphite.

The high-strength high-toughness fine-particle isostatic pressing graphite according to the embodiment of the second aspect of the invention contains 2-16 wt% of chopped fibers having a diameter of 5-20 μm and an aspect ratio of 5-20.

The technical scheme of the invention at least has one of the following beneficial effects:

according to the preparation method of the high-strength high-toughness graphite, the short carbon fibers are added into the fine-particle coke aggregate, and the high-strength high-toughness graphite is obtained after isostatic pressing, carbonization and graphitization treatment;

according to the preparation method provided by the embodiment of the invention, carbonization and impregnation are repeatedly carried out for multiple times, so that the full filling of the gap between the chopped carbon fibers and the coke aggregate is facilitated, the density is further improved, and the strength and the toughness are improved;

the fine-particle high-strength high-toughness graphite provided by the embodiment of the invention has the bending strength value of more than 45MPa and the fracture toughness value of 1 MPa-m^1/2As described above.

Drawings

FIG. 1 is a microscopic image and a particle size distribution diagram of the pitch coke used in example 1, in which: (a) is SEM image of asphalt coke, (b) is particle size distribution diagram thereof;

FIG. 2 is an SEM image of carbon chopped fibers used in example 1;

fig. 3 is an SEM image of high strength and high toughness graphite obtained in example 2.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the described embodiments of the invention, are within the scope of the invention.

First, a method for preparing high-strength and high-toughness graphite according to an embodiment of the present invention is described in detail, which includes the following steps:

step 1, heating and kneading the coke aggregate, the chopped carbon fiber and the binder to obtain the paste.

That is, in the first step, coke as an aggregate, chopped carbon fibers as a toughening component, and a binder are kneaded to obtain a paste. In other words, according to the preparation method of the embodiment of the invention, the appropriate amount of the chopped carbon fibers is added into the coke aggregate, so that the fracture toughness of the graphite powder of the final product can be improved to a great extent.

The chopped carbon fibers have a larger aspect ratio than the coke aggregate. When the crack propagates to the chopped fibers, it either propagates around the fibers or is pulled apart/pulled out of the fibers. When the crack is expanded around the fiber, the tortuosity of the crack is increased, the area of a fracture surface is increased, and the work required by the fracture of the test sample is correspondingly increased; when the crack propagates, the fiber is snapped/pulled off, forming a crack bridge, requiring additional work to be done, again resulting in an increase in the total work required to break the specimen. That is, the graphite with the addition of chopped carbon fibers requires a higher load at break than graphite prepared with a single coke aggregate. In other words, the addition of an appropriate amount of chopped carbon fibers to the aggregate will increase the flexural strength and fracture toughness of the resulting graphite material.

Further, the step 1 may include, for example:

and 11, dry-mixing the coke aggregate and the chopped carbon fiber to obtain mixed powder.

That is, after dry-blending the powdered coke aggregate and the chopped carbon fiber, a binder (e.g., pitch) is added and kneaded under heating to sufficiently mix the powdered and chopped carbon fiber and to provide a certain moldability.

Among them, asphalt is preferably used as the binder. The asphalt is a dark brown complex mixture composed of hydrocarbons with different molecular weights and nonmetal derivatives thereof, is one of high-viscosity organic liquids, mostly exists in a liquid or semisolid petroleum form, has a black surface, and can be dissolved in carbon disulfide and carbon tetrachloride. The asphalt can be mainly divided into coal asphalt, petroleum asphalt and natural asphalt: among them, coal pitch is a by-product of coking. Petroleum pitch is the residue of crude oil distillation. Natural bitumen is stored underground, and some forms a mineral layer or is accumulated on the surface of the crust. The asphalt is mainly used in the industries of paint, plastics, rubber and the like and pavement and the like.

In the present application, pitch is used as the binder, and kneading is performed by a kneader under heating.

For example, the coke aggregate and the chopped carbon fiber are weighed according to a certain mass ratio, poured into a conical blender, fully mixed for 1 hour and then taken out.

Thereafter, the uniformly mixed powder raw material was poured into a double-shaft kneading mixer having a zigzag reamer, and the temperature was raised while stirring. After the temperature is increased to 180 ℃, adding coal pitch (namely first pitch) into the kneading machine, stirring at the constant temperature of 180 ℃ for about 2 hours, stopping heating, and taking out to obtain the mixed paste.

Wherein the coke aggregate: the chopped carbon fibers are: the mass ratio of the first asphalt is (50-70): (2-16): (30-40). The proportion can ensure that the framework material with enough fine particles can improve the strength, and the short carbon fibers can improve the toughness, and can be fully mixed without agglomeration and other phenomena.

Further, the average particle diameter of the coke aggregate is 1 to 50 μm, the diameter of the carbon chopped fiber is 5 to 20 μm, and the aspect ratio is 5 to 20. That is, the strength of the target product can be improved by controlling the particle size of the aggregate, and the toughness and strength of the target product can be further improved by controlling the diameter and aspect ratio of the chopped carbon fibers.

Among these, as the aggregate, for example, pitch coke, petroleum coke, and the like in the coke category can be used.

And 2, cooling the paste, crushing, and carrying out isostatic pressing to obtain a green body.

That is, after obtaining the paste, it is first allowed to cool to room temperature, so that the first pitch as a binder is solidified, and thereafter, pulverization and cold isostatic pressing are performed to prepare a green compact.

Further, the step 2 comprises:

After pulverization, the powder may be passed through a 120-mesh or 200-mesh sieve, for example, so that the undersize powder is sufficiently fine.

And 3, carbonizing the green body to obtain a carbonized product.

That is, after a green compact is obtained, a carbonization treatment is first performed to convert pitch coke, pitch, and the like into carbon.

Further, the step 3 comprises:

step 31, carbonizing the green body at the temperature of 800-;

step 32, dipping the carbonized body in second asphalt to make the second asphalt dip the carbonized body;

That is, the combination of impregnation and multiple charring can further increase the density of the charred material, which is beneficial to further increase the strength of the target product.

Wherein, in the carbonization step, the green body can be heated to 800-1200 ℃ at the heating rate of 10-25 ℃/h and is kept at the temperature for 2-6 h. Of course, the temperature rise rate and the holding time can be appropriately adjusted according to the size, shape, and the like of the green body.

In addition, in the step S31, nitrogen is introduced at a flow rate of 80-150ml/min for carbonization, and the furnace is cooled to room temperature after the carbonization is finished. Under the condition of inert gas, the oxidation in the raw material can be avoided. Is favorable for improving the comprehensive performance of the target product.

Still further, the step 32 includes:

More specifically, the carbonized green body is immersed in an impregnation tank containing second asphalt, and the impregnation tank is heated and vacuumized simultaneously so as to remove gas in the carbonized body and enable the second asphalt to be better impregnated; after a certain period of evacuation, the gas is substantially removed, at which point pressurization is carried out, which facilitates further pressing of the second pitch into the char and, at the same time, densification thereof.

In addition, primary carbonization, primary impregnation, secondary carbonization, secondary impregnation and finally tertiary carbonization can be carried out under the same conditions to obtain a carbonized product after the tertiary carbonization.

Here, the second pitch used as the impregnation liquid may be the same as or different from the first pitch. From the viewpoint of facilitating impregnation, it is preferable to use a pitch having a lower molecular weight, a low viscosity value, and a low softening temperature than the first pitch.

That is, after the carbonization treatment, the carbonized product is graphitized at a high temperature to obtain high-strength and high-toughness graphite.

Specifically, the carbonized product is heated to 2500 ℃ plus 3200 ℃, high-purity argon is introduced in the whole process for graphitization treatment, and halogen and/or halogenated hydrocarbon gas can be selectively added to further purify the product, so that the high-strength high-toughness graphite is obtained. The heating rate of the graphitization treatment can be 5 ℃/min, the graphitization temperature is 3000 ℃, the temperature is kept for 20min, then the product is naturally cooled, and high-purity argon is introduced in the cooling process.

The high-strength and high-toughness graphite obtained in the above way has a bending strength of 45MPa or more and a fracture toughness of 1MPa m^1/2As described above.

The method for producing the high-strength and high-toughness graphite according to the present invention will be described in further detail with reference to specific examples.

Raw materials:

coke aggregate: selecting pitch coke (average particle diameter 3.5um) produced by Mitsubishi corporation, wherein SEM image is shown as (a) in figure 1, and particle size distribution is shown as (b);

short carbon fiber: short cut fibers (diameter 10um, length 50 um; length-diameter ratio 10) from Japan graphite fiber company were selected as raw materials, and SEM images thereof are shown in FIG. 2.

Adhesive: coal pitch provided by Sichuan carbon is selected.

The specific physicochemical properties of the coal pitch are shown in Table 1.

Table 1: physical and chemical properties of coal tar pitch

Name (R)	Softening point	Coking value	QI content	Beta resin	Ash content
						Coal tar pitch	114.3℃	60.57％	8.94％	19.33％	0.1％

Impregnating material: the impregnated coal pitch provided by Changyu science and technology development Limited in Hunan province is selected.

Example 1

Preparing pitch coke and carbon chopped fibers according to the mass ratio of 92:8, pouring the pitch coke and the carbon chopped fibers into a conical mixer, fully mixing for 1 hour, and taking out.

And pouring the uniformly mixed powder raw materials into a double-shaft stirring kneader with a Z-shaped reamer, and heating while stirring. When the temperature rises to 180 ℃, the molten coal pitch is added to the kneader. The mass ratio of the powder raw material (asphalt coke and chopped fiber) to the coal pitch is 65: 35. stirring at 180 deg.C for 2 hr, stopping heating, and taking out the mixed paste.

After the paste was cooled, it was pulverized by a small mechanical pulverizer, and the pulverized powder was passed through a 150-mesh screen. The sieved powder was charged into a rubber mold having an inner diameter of 28mm and a height of 300mm and compacted. And sealing the charged mold, placing the mold into a cold isostatic press for molding, and keeping the pressure at 150MPa for 3 min.

The molded green body is heated to 1000 ℃ at the speed of 20 ℃/h for carbonization, and the temperature is kept for 3 h; and the nitrogen atmosphere is introduced in the whole process, and the nitrogen flow is 100 ml/min. And stopping heating after the heat preservation is finished, and taking out the product after the carbonization furnace is naturally cooled to room temperature to finish the carbonization process.

The carbonized product is heated to 3000 deg.c, high purity argon gas is introduced for graphitization, and halogen and/or halohydrocarbon gas is added for purification to obtain the high strength and high toughness graphite. The temperature rising rate of the graphitization treatment is 5 ℃/min, the heat preservation is carried out for 20min, then the natural cooling is carried out, and high-purity argon is still introduced in the cooling process.

Example 2

The same as in example 1, except that the carbonization process was different. The following is a description of only the carbonization process, and the other steps refer to example 1.

Primary carbonization:

heating the molded green body to 1000 ℃ at a rate of 20 ℃/h for primary carbonization, and preserving heat for 3 h; and the nitrogen atmosphere is introduced in the whole process, and the nitrogen flow is 100 ml/min. And stopping heating after the heat preservation is finished, and taking out the product after the carbonization furnace is naturally cooled to room temperature to finish the carbonization process.

Primary impregnation:

and (3) placing the blank subjected to primary carbonization in an impregnation tank filled with impregnated coal pitch, heating the impregnation tank to 200 ℃, vacuumizing the impregnation tank, and maintaining the pressure for 20min under the condition of 0.2 atmospheric pressure.

Thereafter, the pressure was increased to 8MPa, and impregnation was carried out for 2 hours while maintaining the pressure.

And taking out the sample, and naturally cooling to finish primary impregnation.

After the primary impregnation is completed, the secondary carbonization is performed under the same conditions as the primary carbonization. Thereafter, the secondary impregnation was carried out under the same conditions as the primary impregnation, and finally, the carbonization was carried out three times under the same conditions as the primary carbonization.

After the third carbonization, graphitization treatment is performed.

Fig. 3 shows an SEM image of the product obtained after graphitization, from which it can be seen that the chopped fibers are uniformly distributed in the aggregate and the graphite product obtained by the preparation has a high density.

Example 3

The same as in example 2, except that the mass ratio of pitch coke to chopped carbon fiber was 85: 15.

Example 4

The same as example 3, except that the mass ratio of the powder to the coal pitch was 60: 40.

Comparative example 1

The same as example 1 except that the chopped carbon fibers were not added.

Comparative example 2

The same as example 2 except that the chopped carbon fibers were not added.

The samples obtained in the comparative examples of the examples were tested for flexural strength and fracture toughness, respectively, according to the standards ASTM-D7972 and ASTM-D7779, the results of which are given in Table 2.

Table 2: evaluation results of Strength and toughness

	Flexural Strength (MPa)	Fracture toughness (MPa. m)^1/2)
			Example 1	36.25	0.80
Example 2	46.62	1.01
			Example 3	42.35	0.94
Example 4	40.81	0.92
			Comparative example 1	22.73	0.61
Comparative example 2	29.45	0.74

As can be seen from the above table, by adding an appropriate amount of chopped carbon fibers to the matrix, the strength and toughness of the matrix can be improved at the same time, and high-strength, high-toughness, fine-grained isostatic graphite can be obtained. Among them, the effect of example 2 is the best.

Further, as is clear from the above table, by further increasing the density by the multiple carbonization, it is advantageous to further increase the precursor strength and the fracture toughness.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. The preparation method of the high-strength and high-toughness graphite is characterized by comprising the following steps:

step 1, heating and mixing coke aggregate, chopped carbon fiber and a binder to obtain a paste;

step 2, cooling the paste, crushing, and performing isostatic pressing to obtain a green body;

step 3, carbonizing the green body to obtain a carbonized product;

2. The method for preparing according to claim 1, wherein the step 1 comprises:

3. The method of claim 2, wherein the coke aggregate: the chopped carbon fibers are: the mass ratio of the first asphalt is (50-70): (2-16): (30-40).

4. The method according to claim 2, wherein the average particle diameter of the coke aggregate is 1 to 50 μm, the diameter of the chopped carbon fiber is 5 to 20 μm, and the aspect ratio is 5 to 20.

5. The method for preparing according to claim 1, wherein the step 2 comprises:

6. The method for preparing according to claim 2, wherein the step 3 comprises:

step 31, carbonizing the green body at the temperature of 800-;

step 32, immersing the carbonized green body in second asphalt to ensure that the carbonized green body is impregnated by the second asphalt;

and step 33, repeating the step 31 and the step 32 for multiple times, and obtaining the carbide after the final carbonization.

7. The method of claim 6, wherein the step 32 comprises:

and soaking the carbonized green body in the second asphalt, heating to 150-220 ℃, simultaneously vacuumizing and maintaining the pressure in a vacuum state for a first preset time, and then pressurizing to 5-10MPa and maintaining the pressure for a second preset time so as to ensure that the second asphalt fully soaks the carbonized green body.

8. The method as claimed in claim 7, wherein in step S31, nitrogen is introduced at a flow rate of 80-150ml/min, the green body is heated to 800-1200 ℃ at a heating rate of 10-25 ℃/h, the carbonization is performed by keeping the temperature for 2-6h, and the green body is cooled to room temperature along with the furnace after the carbonization.

9. The preparation method according to claim 6, wherein in the step S4, the carbonized product is heated to 2500 ℃ and 3200 ℃, and high-purity argon is introduced into the carbonized product for graphitization treatment to obtain the high-strength and high-toughness graphite.

10. The high-strength high-toughness graphite is characterized by comprising 2-16 wt% of chopped fibers, wherein the diameter of each chopped fiber is 5-20 mu m, and the length-diameter ratio of each chopped fiber is 5-20.