Development of carbon fiber

Compared with traditional metal materials and composite materials made of other fibers, carbon fiber composite materials have the characteristics of light weight, high strength, and high modulus of elasticity. They can reduce the weight by 30% compared with traditional aluminum alloy structures, and make a huge contribution to the improvement of weapon equipment performance. It is widely used in the manufacture of aircraft bodies and engines, missile shells, etc. The proportion of carbon fiber composite materials used in American F-22 and F-35 fighter jets reached 24% and 36%, respectively, and the proportion of carbon fiber composite materials used in new large-scale civil aircraft represented by the A350 and Boeing 787 reached more than 50%. The use of carbon fiber composite materials has become one of the symbols to measure the advanced nature of weapons and equipment. Carbon fiber is a key raw material for composite materials, and it bears about 90% of the load of composite materials. Its tensile strength and elastic modulus are the keys to achieving the structural performance goals of composite materials.

The first generation of carbon fiber

Carbon fiber takes tensile strength and elastic modulus as the main indicators. At present, commercial products have developed to the second generation. Japan and the United States have similar performance on the widely used second-generation carbon fiber products. The first generation is represented by Toray’s T300 and Hexcel’s AS4 low-strength low-modulus carbon fiber in the 1960s. T300 is mainly used for the secondary load-bearing components of Boeing 737 and other models. AS4 is used in the early F-14 fighter jets. Flat tail and other parts.

Second-generation carbon fiber

The second-generation high-strength, medium-modulus carbon fiber is represented by Toray’s T800 and Hexcel’s IM7 series in the 1980s. The same generation products also include Toray’s T700 and T1000, Hexcel’s IM8, IM9 and so on.

The strength of T800 is 68% higher than that of T300, and the modulus is increased by 28%. It is widely used in the main bearing structure of the wing and fuselage of aircraft such as A350 and Boeing 787. Compared with AS4, the strength of IM7 is increased by 37%, and the modulus is increased by 21%. It is widely used in the US "Trident" II submarine-launched missiles and F-22 and F-35 fighters.

At this stage, the second-generation high-strength medium-modulus carbon fiber is the most widely used in aerospace and other fields. Due to the low modulus and the brittleness of carbon fiber materials, it is easy to cause fatigue damage to composite structural components and even catastrophic damage. This limits the improvement of the performance of aviation weapons and equipment, and it is even more difficult to meet the performance requirements of the new generation of aviation weapons and equipment. As the United States launched the development of the sixth-generation fighter, the new-generation long-range bomber, and the first-generation unmanned carrier-based combat aircraft, aviation weapons and equipment have proposed indicators such as cruise speed, range, maneuverability, stealth performance, protection capabilities, and maintainability. In order to meet higher requirements, carbon fibers with higher comprehensive properties such as tensile strength, fracture toughness, and impact performance are required. To obtain carbon fiber with high comprehensive performance, it is necessary to make a breakthrough in the two basic properties of strength and modulus. The main technical feature of the third-generation carbon fiber is to achieve high tensile strength and high elastic modulus at the same time.

At the same time, achieving high tensile strength and elastic modulus is a technical difficulty in the development of carbon fiber. The preparation and carbonization of precursor are the two core processes of carbon fiber preparation: high-quality PAN precursor is the key to achieving high performance and mass production of carbon fibers; the control of the carbonization process is directly related to the tensile strength and elastic modulus of carbon fibers. Years of carbon fiber development experience has shown that when the elastic modulus of carbon fiber is greatly increased, the tensile strength will be significantly reduced; while maintaining the high tensile strength of carbon fiber, it is difficult to greatly increase the elastic modulus of the fiber. The reason is that carbon fiber is an anisotropic material composed of a large number of graphite crystallites. High-strength carbon fiber usually requires a small crystallite size, while high-modulus carbon fiber usually requires a larger crystallite size. How to solve this contradiction is the biggest problem in the development of carbon fiber.

Third-generation carbon fiber

Japan and the United States have obtained high-strength, high-modulus carbon fiber from two different technical approaches.

Judging from the current research results, Toray’s third-generation carbon fiber products have higher strength and are more suitable for structural parts with high tensile strength design values; American products have higher elastic modulus and are more suitable for bending, impact, and Components with high fatigue strength design value. Relevant Japanese and American companies and institutions have clearly stated that the application target of the third-generation carbon fiber is the high-end aerospace market, replacing the current T800 and IM7 second-generation carbon fiber products, and improving the overall performance of military aircraft structural components such as strength and stiffness. Toray is the pioneer of traditional PAN solution spinning technology. The raw silk technology is highly mature and has strong industrialization capabilities. From the perspective of the first and second generation products, its third generation products are expected to achieve industrialized production and integration in the next 5 to 10 years. Fully put on the market. The United States abandons the traditional solution precursor preparation process and adopts the gel spinning technology, which has more room for process optimization, and the carbon fiber performance also has more room for improvement. The sixth-generation fighter jets, the new-generation long-range bombers, and the first-generation unmanned carrier-based combat aircraft planned by the United States to be launched before 2030 are very likely to greatly improve combat performance through the application of third-generation carbon fiber technology.

Japan's Toray Company made a breakthrough in the carbonization process to increase the strength and modulus of carbon fiber by more than 10% at the same time, and was the first to meet the technical requirements of the third-generation carbon fiber. Toray believes that the key to achieving high tensile strength and high elastic modulus of carbon fiber at the same time lies in the heat treatment technology and high temperature equipment during the carbonization process. In terms of heat treatment technology, many factors such as temperature, drafting, catalysis, and magnetic field will affect the performance of the fiber after carbonization. In March 2014, Toray announced the successful development of T1100G carbon fiber. Toray uses traditional PAN solution spinning technology to finely control the carbonization process, improve the microstructure of carbon fibers on the nanometer scale, and control the orientation, crystallite size, defects, etc. of the graphite crystallites in the carbonized fiber, so as to increase the strength and elasticity. The modulus has been greatly improved. The tensile strength of T1100G is 6.6 GPa, which is 12% higher than that of T800; the modulus of elasticity is 324 GPa, which is 10% higher, and it is entering the stage of industrialization.

The Georgia Institute of Technology research team in the United States has broken through the preparation process of the original silk, while maintaining the high strength of carbon fiber, the elastic modulus has increased by more than 28%. Hexcel's carbon fiber products have remained at a medium elastic modulus level for 30 years, and their performance is difficult to break through. The National Defense Preliminary Research Agency (DARPA) launched the Advanced Structural Fiber Project in 2006, with the purpose of convening the nation’s superior scientific research forces to develop next-generation structural fibers based on carbon fiber. Georgia Institute of Technology, as one of the participating institutions, started with the raw silk preparation process to improve the elastic modulus of carbon fiber. In July 2015, the research team used the innovative PAN-based carbon fiber gel spinning technology to increase the tensile strength of carbon fiber to 5.5-5.8 GPa and the tensile modulus of elasticity to 354-375 GPa. Although the tensile strength is equivalent to IM7, the elastic modulus has been greatly improved by 28% to 36%. This is the combination of high strength and highest modulus of carbon fiber reported so far. The mechanism is that the gel connects the polymer chains together to generate strong internal chain force and orientation of the crystallite orientation, ensuring high strength even with the larger crystallite size required for high elastic modulus. This shows that the United States already possesses independent research and development capabilities for third-generation carbon fiber products.

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