Researcher Song Shulin, head of the Specialty Fiber Business Department of Ningbo Institute of Materials, told the China Science Journal: 'We have mastered the relationship between structure and properties, and established an application evaluation system of domestic graphite fiber-resin composites for carbon fiber engineering. And the application of composite materials to provide a solid theoretical and practical basis, domestic graphite fiber to achieve continuous and stable production.
What is the difference of graphite fiber
Carbon fiber is a high-strength, high-modulus new fiber material with a carbon content of more than 95%. Carbon fiber 'External soft inside' is lighter than metal aluminum, but its strength is higher than that of steel, and it is corrosion resistant and high. The characteristics of the modulus are important materials in the defense industry and civilian use.
Carbon fiber is divided into general purpose and high performance type according to its mechanical properties. General purpose carbon fiber strength is 1000 MPa and modulus is 100GPa (1GPa=1000 MPa). High performance carbon fiber is divided into high strength type (strength 2000 MPa, Modulus 250GPa) and high model (modulus 300GPa or more).
With the development of the aerospace and aerospace industries, high-strength high-modulus carbon fibers have also emerged, of which the most widely used is polyacrylonitrile-based carbon fibers (PANCF). PANCF can be classified into high-strength medium and high-strength high-modulus types according to its performance. High-strength model of domestic carbon fiber tensile strength up to 6.5GPa, modulus around 300GPa.
Zhang Yonggang, technical director of Ningbo Fiber Materials Specialty Fibers Division, pointed out that the tensile strength of M60J graphite fiber was 5.24GPa and the tensile modulus was 593GPa. 'From the data point of view, the tensile modulus of graphite fiber is much higher than that of high strength. Medium model carbon fiber, slight loss of strength. '
The graphite fibers here have a carbon content of more than 99%, have higher modulus, low thermal expansion coefficient, high electrical conductivity, good thermal stability, dimensional stability, and can be used as key materials in spacecraft, aerospace and other fields.
Breakthrough key preparation technology
Carbon fiber preparation is a systematic project that involves a series of complex physical and chemical reactions. Because the mechanism of partial reactions has not been thoroughly studied, and the equipment required for the relevant reactions has been limited, it has increased the difficulty of carbon fiber preparation.
The international competition in the carbon fiber market is not sufficient, and most of the shares are monopolized by a few companies in countries such as Japan and the United States, which also leads to higher prices of carbon fiber products.
Over the years, the key preparation technology of graphite fiber mastered abroad has been severely blocked in China. This situation has been improved with the gradual improvement of domestic carbon fiber preparation technology. Related scientific research units are improving the maturity of key equipment, providing a theoretical basis for the preparation of graphite fibers. And equipment foundation.
Graphite fibers are obtained after high-temperature carbonization of fibers and further graphitized. The degree of graphitization is very high. Zhang Yonggang told reporters: 'graphitization technology is the key to the preparation of graphite fibers, including key equipment manufacturing technology and carbon fiber full process preparation technology. '
The current known theory is that the higher the graphitization temperature, the higher the tensile modulus of the carbon fiber. Therefore, a graphitization apparatus capable of providing a sufficiently high temperature is first required. Generally, the degree of graphitization of the fiber is gradually increased above 2200° C., and the temperature is further increased. Ascension, the structure of fiber graphitization is further improved. The ideal state is that the graphitization temperature reaches 3000°C.
Zhang Yonggang also pointed out that the difficulty is that, due to the material's temperature resistance and equipment processing level, it is difficult to obtain graphitized equipment that meets the requirements.
In order to break through the high-grade graphite fibers, we must start from the study of the relationship between structure and performance. The Ningbo Ningbo Institute of Special Fibrous Business Unit adopts fine structure control, strictly controls the process conditions, optimizes the graphitization structure of the fibers, and improves the macroscopic properties of the fibers.
After strenuous efforts, the team achieved a breakthrough in the performance of the graphite fibers M55J and M60J at a lower graphitization temperature. In addition, the lowering of the graphitization temperature effectively extends the life of the graphitization equipment and achieves the matching of performance and cost.
Stable technology scale production
In recent years, Ningbo Institute of Materials has successively broken through high-strength medium-mode carbon fiber pilot and engineering technology. In May 2015, special fiber business division obtained M50J graphite fiber, with tensile strength and tensile modulus as high as 5.12GPa and 475GPa, respectively.
In January 2016, Ningbo Materials Co., Ltd. took the lead in China to achieve a major breakthrough in the domestic M55J preparation technology. In September of the same year, it carried out preparation technology verification and obtained graphite fibers with a tensile strength of 4.15 GPa and a tensile modulus of 585 GPa. M55J graphite fiber continuous and stable production.
Afterwards, the Special Fiber Business Department of Ningbo Institute of Materials conducted a detailed study on process analysis, structural research and performance optimization of domestic graphite fiber, especially focusing on the study of tensile strength retention of carbon fiber in a high-temperature graphitization environment, and producing domestic M60J graphite fibers. .
Zhang Yonggang pointed out that compared with Japan Toray M60J graphite fiber (tensile strength 3.92GPa, tensile modulus 588GPa), our domestic M60J (tensile strength 5.24GPa, tensile modulus 593GPa) continue to maintain the tensile strength The advantage.
At present, the domestically-made graphite fiber has developed rapidly. Ningbo Institute of Materials, Beijing University of Chemical Technology and other scientific research units have successively broken through the domestic M55J graphite fiber preparation technology. Some domestic enterprises are carrying out M55J engineering technology research.
The breakthrough in the key preparation technology of M60J graphite fiber made by Ningbo Materials will further promote domestic technology development in the field of graphite fiber. At present, the development of graphite fiber industry in China is still immature, mainly military projects, and civil use has yet to be further explored.
Zhang Yonggang said: 'The potential applications of graphite fibers are very broad, and we will gradually expand on the basis of achieving stable preparation technology and forming a production scale.'