1. Polyacrylonitrile-based carbon fiber
Polyacrylonitrile (PAN) carbon fiber and graphite fiber are prepared from PAN raw materials through pre-oxidation, carbonization and graphitization processes. Among them, the carbon content of more than 99% is graphite fiber (wave absorbing material). The structural morphology of PAN is that the cross-section of the fiber is circular, and the fiber diameter is 25-100nm. Each fiber is composed of a large number of graphite crystallites of about 5nm, and the basal surface of the crystallites shows a very regular arrangement along the central axis of the fiber.
(1) Preparation method High-strength, high-modulus fiber is prepared by pre-oxidation, carbonization, graphitization and other stages of polyacrylonitrile precursor. The temperature of CN graphitization is not high, and the one made at 10001500℃ is called carbon fiber, and the one made at 300℃ is called graphite fiber. However, the two are often collectively referred to as carbon fiber, and the production process is as follows:
Acrylonitrile polymerization —→ spinning —→ polyacrylonitrile precursor —→ pre-oxidation —→ carbonization —→ carbon fiber —→ graphitization —→ graphite fiber
(200-300℃) (1000-1500℃) (above 2800℃)
(2) Performance Carbon fiber is a fibrous carbon material, so it has the characteristics of carbon material, such as low density, thermal conductivity, electrical conductivity, self-lubricating and so on. In addition, carbon fiber is the fiber with the highest specific modulus among fiber materials. Its tensile strength is similar to that of glass C√ glass fiber, and its elastic modulus is 4 to 5 times that of glass fiber.
①Electrical performance. Carbon fiber and graphite fiber have good electrical conductivity, with a resistivity of 1010-4g.
② Thermal performance. The linear expansion coefficient of carbon fiber and graphite fiber is small. In the fiber axis direction, the high model is 1×10-6K1, the high-strength model is -0.5×10-6K-1; in the transverse direction, it is about 16.8×10-6K1. The fiber does not melt or soften at a high temperature of 3000°C in a non-oxidizing atmosphere. In addition, the thermal conductivity of carbon fiber is high, the thermal conductivity in the fiber axis direction is 83.7~125.6W/(m·K), and the transverse thermal conductivity of the fiber is as low as 0.84W (m·K), and it increases with the temperature. And decrease, the thermal conductivity at 1500℃ is 15%~30% of normal temperature
③Chemical properties. Carbon fiber and graphite fiber can be oxidized by strong oxidizing agent, but acid and alkali resistance are very good. For example, after placing the fiber in an acid solution for 200 d, when measuring its elastic modulus, fiber diameter and tensile strength, it is found that the mass fraction is 50% hydrochloric acid. There is no obvious change in sulfuric acid and phosphoric acid; it swells slightly in 50% nitric acid in mass fraction; only the diameter decreases in hypochlorous acid solution. In addition, it can withstand oil, benzene, acetone and other media. In addition, it also has the characteristics of preventing atomic radiation (magnetic barrier), absorbing gas and decelerating neutrons.
2. Viscose-based (or rayon-based) carbon fiber
The matrix chain of viscose-based carbon fiber is cellobiose residues, and each glucose residue contains three hydroxyl groups, which must be dehydrated before carbonization and activation, so that the hydroxyl groups are dehydrated and transformed into a heat-resistant ladder structure. The carbon content of viscose-based carbon fiber is above 90% (mass fraction), and the carbon content of graphite fiber is close to 100% (mass fraction), and its cross-section is irregular or sawtooth.
(1) Preparation method The basic process of preparing carbon fiber and graphite fiber from rayon.
(2) Performance Viscose-based carbon fiber has low density, poor thermal conductivity, and low alkali metal content (usually only 28-60mg/kg), which makes it very advantageous as an ablation material. The density of viscose-based carbon fiber is 1.3~1.9g/cm3, and the tensile strength is 0.69~3.4. The modulus of elasticity is 690-760GPa. Its electrical conductivity has a very direct relationship with the modulus of elasticity, that is, with the modulus of elasticity. Increase, almost linear increase. When the modulus of elasticity is 70GP, the conductivity is 4005cmc and the modulus of elasticity is 760GPa, the conductivity is about 1900S/cm. Other properties are similar to polyacrylonitrile-based fibers.
3. Pitch-based carbon fiber and graphite fiber
Pitch-based carbon fiber is a carbon fiber made from PVC pyrolysis pitch, lignin pitch and kerosene pitch as base materials. This kind of carbon fiber has a high carbon content. Even if the heat treatment temperature reaches 1000 ℃, its carbon content is more than 99%. The cross-sectional structure of the fiber is shown in Figure 218. There are radial structure, onion skin structure and no Regulatory structure. Under normal circumstances, the fiber is a mixed structure, in which the radial and random structures are the main ones, and the onion skin structure is less.
Radial structure Onion skin structure Random structure
(1) Preparation method The preparation methods of pitch-based carbon fiber include: ① general (low performance) isotropic method; ② mesophase pitch to prepare carbon fiber; ③ pre-mesophase method to prepare carbon fiber; ④ potential mesophase pitch to prepare carbon fiber.
(2) Performance The tensile strength and elastic modulus of pitch-based carbon fibers are greatly affected by the heat treatment temperature. For example, when the heat treatment temperature is 1700 ℃, the strength is 1.38 GPa and the elastic modulus is 1 GPa; when the heat treatment temperature is 300 ℃, the strength is 2.20 GPa and the elastic modulus is 700 GPa.