Synthesis and research of carbon fiber

1. What is carbon fiber?

Carbon fiber is a new fiber material with high strength and high modulus fiber containing more than 95% carbon. It is a microcrystalline graphite material obtained by stacking organic fibers such as flake graphite microcrystals along the axial direction of the fiber and undergoing carbonization and graphitization treatments. Carbon fiber is "flexible on the outside and rigid on the inside". It is lighter than metal aluminum, but stronger than steel. It is corrosion-resistant and has high modulus. It is an important material in national defense and civilian applications. It not only has the inherent characteristics of carbon materials, but also has the soft processability of textile fibers. It is a new generation of reinforcing fiber.

2. Physical and chemical properties

Carbon fiber is a new material with excellent mechanical properties. The tensile strength is about 2 to 7GPa, and the tensile modulus is about 200 to 700GPa. The density is about 1.5 to 2.0 grams per cubic centimeter. In addition to being related to the structure of the original fiber, it mainly depends on the temperature of the carbonization treatment. Generally, after graphitization treatment at high temperature of 3000℃, the density can reach 2.0 grams per cubic centimeter. In addition, its weight is very light, its specific gravity is lighter than aluminum, less than 1/4 of steel, and its specific strength is 20 times that of iron.

The thermal expansion coefficient of carbon fiber is different from other fibers in that it has anisotropic characteristics. The specific heat capacity of carbon fiber is generally 7.12. Thermal conductivity decreases with increasing temperature with negative values parallel to the fiber direction (0.72 to 0.90) and positive values perpendicular to the fiber direction (32 to 22).

The specific resistance of carbon fiber is related to the type of fiber. At 25°C, the high modulus is 775 and the high strength carbon fiber is 1500 per centimeter. This gives carbon fiber the highest specific strength and specific modulus of all high-performance fibers.

Compared with metal materials such as titanium, steel, and aluminum, carbon fiber has the characteristics of high strength, high modulus, low density, and small linear expansion coefficient in terms of physical properties. It can be called the king of new materials.

The tensile strength of carbon fiber resin composites is generally above 3500 MPa, which is 7 to 9 times that of steel. The tensile elastic modulus is 230 to 430 GPa, which is also higher than steel; therefore, the specific strength of CFRP is the ratio between the strength of the material and its density. The specific strength can reach more than 2000 MPa, while the specific strength of A3 steel is only about 59 MPa, and its specific modulus is also higher than that of steel.

Compared with traditional glass fiber, Young's modulus (referring to the physical quantity that characterizes the tensile or compressive resistance of a material within the elastic limit) is more than three times that of glass fiber; compared with Kevlar fiber, not only Young's modulus It is about 2 times that. Tests on carbon fiber epoxy laminates have shown that both strength and modulus decrease as porosity increases. Porosity has a great influence on interlayer shear strength, flexural strength, and flexural modulus; tensile strength decreases relatively slowly with the increase of porosity; tensile modulus is less affected by porosity.

Carbon fiber also has excellent fineness (one of the expressions of fineness is the number of grams of 9000-meter-long fiber), which is generally only about 19 grams, and the tensile force is as high as 300 kilograms per micron. Almost no other material has as many excellent properties as carbon fiber, so it has strict requirements in areas such as strength, stiffness, strength, fatigue properties, etc.

When not in contact with air and oxidants, carbon fiber can withstand high temperatures of more than 3,000 degrees and has outstanding heat resistance. Compared with other materials, the strength of carbon fiber does not begin to decrease until the temperature is higher than 1,500 degrees Celsius, and the higher the temperature, the lower the fiber strength. The greater the decrease in intensity. The radial strength of carbon fiber is not as good as the axial strength, so carbon fiber avoids radial strength (that is, it cannot be knotted) and the whisker performance of other materials has also been greatly reduced. In addition, carbon fiber also has good low-temperature resistance, such as not embrittlement at liquid nitrogen temperature.

The chemical properties of carbon fiber are similar to those of carbon. In addition to being oxidized by strong oxidants, it is inert to general alkali. When the temperature in the air is higher than 400°C, significant oxidation occurs, generating CO and CO2. Carbon fiber has good corrosion resistance to general organic solvents, acids, and alkali, does not dissolve or swell, has outstanding corrosion resistance, and has no rust problem at all. Some scholars soaked PAN-based carbon fiber in a strong alkali sodium hydroxide solution in 1981. More than 30 years later, it still maintains its fiber shape. However, its impact resistance is poor, easy to damage, and oxidized under the action of strong acid. The electromotive force of carbon fiber is positive, while the electromotive force of aluminum alloy is negative. When carbon fiber composite materials are used in combination with aluminum alloys, metal carbonization, carburization and electrochemical corrosion will occur. Therefore, carbon fiber must be surface treated before use. Carbon fiber also has properties such as oil resistance, radiation resistance, anti-radiation, absorption of toxic gases and deceleration of neutrons.

3. Preparation method

Carbon fiber can be made from polyacrylonitrile fiber, pitch fiber, viscose fiber or phenolic fiber by carbonization. The most commonly used carbon fibers are polyacrylonitrile carbon fiber and pitch carbon fiber. The manufacturing of carbon fiber includes four processes: fiber spinning, thermal stabilization (pre-oxidation), carbonization, and graphitization. The accompanying chemical changes include dehydrogenation, cyclization, pre-oxidation, oxidation and deoxygenation, etc.

Preparing carbon fibers with high mechanical properties from viscose fibers must be graphitized by high-temperature stretching. The carbonization yield is low, the technology is difficult, and the equipment is complex. The products are mainly used for ablation-resistant materials and heat insulation materials; carbon fibers are produced from pitch. The sources of raw materials are abundant and the carbonization yield is high. However, due to the complicated preparation of raw materials and low product performance, it has not been developed on a large scale. High-performance carbon fibers can be produced from polyacrylonitrile fiber precursors, and the production process is simpler and mechanically simpler than other methods. It has excellent performance and has developed well in the carbon fiber industry since the 1960s.

To get good quality carbon fiber, you need to pay attention to the following technical points:

(1) Achieving high purification, high reinforcement, densification and flawless surface of raw filaments is the primary task for preparing high-performance carbon fibers. Carbon fiber system engineering begins with the polymerization of monomers. The quality of raw silk not only determines the properties of carbon fiber, but also restricts its production cost. High-quality PAN precursor is the primary prerequisite for manufacturing high-performance carbon fiber.

(2) Minimizing impurities and defects, which is the fundamental measure to improve the tensile strength of carbon fiber, and is also a hot topic for scientific and technological workers. In a sense, the process of increasing strength is essentially the process of reducing and minimizing defects.

(3) During the pre-oxidation process, the pre-oxidation time should be shortened as much as possible while ensuring homogenization. This is a directional issue to reduce production costs.

(4) Research high-temperature technology, high-temperature equipment and related important components. The high-temperature carbonization temperature is generally between 1300 and 1800°C, and the graphitization temperature is generally between 2500 and 3000°C. When operating at such a high temperature, it is necessary to operate continuously and improve the service life of the equipment, so it is particularly important to study a new generation of high-temperature technology and high-temperature equipment. Such as microwave, plasma and induction heating technologies under inert gas protection and oxygen-free conditions.

4. Prospects

The world's carbon fiber production reaches more than 40,000 tons per year. In the world of full carbon fiber production lines, a few countries such as Japan, the United States, Germany and South Korea have mastered the core technology of carbon fiber production and have large-scale production.

Currently, the global core technology of carbon fiber is firmly in the hands of a few developed countries. On the one hand, developed countries led by the United States and Japan have always maintained strict technological blockades on China's carbon fiber industry; on the other hand, leading foreign companies in the carbon fiber industry have begun to enter the Chinese market, which has greatly increased the pressure on China's local carbon fiber companies. Although China has increased its guidance and support for the carbon fiber industry, the road to breakthrough for domestic carbon fiber is still bumpy due to the large technological gap.

(Source: Polymer Physics)

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