According to the source of raw materials, carbon fibers can be divided into polyacrylonitrile (PAN) -based carbon fibers, pitch-based carbon fibers, and viscose-based carbon fibers. Among them, the production of PAN-based carbon fibers accounts for more than 90% of carbon fibers, followed by pitch-based carbon fibers of about 6-8% Gum-based carbon fiber yields the lowest.
Asphalt-based carbon fibers can be divided into general-grade asphalt-based carbon fibers and high-performance asphalt-based carbon fibers. The former is generally short filaments, which are mostly used for thermal insulation materials, while the latter is composed of mesophase pitch that is spun, unmelted, pre-oxidized, and carbonized. And graphitization treatment to obtain a continuous tow-like carbon fiber, so it is also called mesophase pitch-based carbon fiber (MPCF).
MP-based carbon fiber has the advantages of high tensile modulus, small thermal expansion coefficient, high thermal conductivity, and good impact resistance. The modulus of high-modular pitch-based carbon fiber is up to 930 GPa, which is 1.5 times that of PAN-based high-modulus carbon fiber. The carbonization yield of carbon fibers made from asphalt is as high as 80% to 90%. In view of the excellent properties of mesophase pitch-based carbon fibers, it has broad application prospects in the fields of aerospace, defense industry, and advanced technology.
Thermal conductivity of carbon fiber materials
Carbon fiber relies on unique performance advantages and has been widely used in the aerospace field. In the aerospace field, the current top speed of modern aircraft has reached 2.7 times the speed of sound. Such a fast supersonic flight causes a large amount of friction between the aircraft and the air. Heat requires materials to have high thermal conductivity.
In the field of electronic products, the necessity of material heat conduction is also very prominent. Since it is impossible to make an ideal flat surface by mechanical processing, there are many gaps or gaps between the CPU, chips, and the heat sink. Air is a poor conductor of heat. The air gap will seriously affect the heat dissipation efficiency, greatly reducing the performance of the radiator, and even fail to play a role.
The main parameter used to characterize the thermal conductivity of carbon fiber is the thermal conductivity (also known as thermal conductivity or thermal conductivity), which refers to a material with a thickness of 1m under steady-state conditions and a surface temperature difference of 1K on both sides. The heat transferred. The legal unit in China is W / (m · K).
The thermal conductivity comparison between PAN-based carbon fibers and mesophase pitch-based carbon fibers is shown in Figure 1 below. It can be seen that the tensile modulus and thermal conductivity of MP-based carbon fibers are much higher than those of PAN-based carbon fibers. Among them, the thermal conductivity of MP-based carbon fibers is up to 1100W / (m · K), while M55J-class PAN-based high modulus Carbon fiber is only 155.7W / (m · K), but MP-based carbon fiber has no advantage in tensile strength.
Structural factors affecting the thermal conductivity of carbon fibers
The previous article of this public account discussed the effect of micro-defect structure on mechanical properties, and the micro-structure affects the conductive properties of carbon fibers. As in the previous introduction, in the field of carbon fibers, the truth that the micro-structure restricts the macro-performance of the fibers is the same truth. PAN-based carbon fibers and MP-based carbon fibers The difference in thermal conductivity is also due to the difference in the internal microstructure of the two fibers.
The structural evolution during the preparation of PAN-based carbon fibers and MP-based carbon fibers is shown in Figure 2. The bottom left is the M65J-grade PAN-based high modulus carbon fiber developed under the laboratory conditions of Ningbo Institute of Materials. Below is Japan's Mitsubishi MP-based carbon fiber with a tensile modulus of 900 GPa. The two figures are obtained by using a scanning electron microscope under the same detection conditions. In addition to the significant difference in cross-sectional structure, the diameter of MP-based carbon fibers is significantly higher than that of PAN-based carbon fibers at the same magnification.
Because PAN is difficult to graphitize, the cross-sectional structure of PAN-based high-modulus carbon fibers prepared by graphitization is relatively disordered, and its structural model is difficult to determine, but its essence is still a lamellar structure composed of graphite microcrystals.
Due to the double-diffusion during spinning and pre-oxidation, there will be some structural differences between the surface layer and the core of the PAN-based carbon fiber. Some scholars have proposed a three-dimensional spatial structure model. The schematic diagram of the structure is shown in Figure 4. In this structure model, Carbon fiber surface layer and core layer have significant differences in microcrystalline density and sorting surface, so it is also called skin-core structure model.
As the mesophase pitch is easy to graphitize under high temperature environment, as the processing temperature increases, the graphite structure inside the pitch fiber is gradually improved, and after high temperature graphitization treatment above 2200 ° C, the pitch-based carbon fiber graphite sheet can be aligned in the axial direction, thereby It guarantees that the fiber has excellent thermal conductivity. For PAN-based carbon fibers, the graphite sheet structure is not perfect, and there are certain microcrystalline defects, which results in poor thermal conductivity.
Although the thermal conductivity of PAN-based carbon fibers is significantly different from that of MP-based carbon fibers, the thermal conductivity of M55J-class PAN-based high-mode carbon fibers has reached 155 W / (m · K), which is comparable to that of some metals and aluminum alloys.
Because MP-based carbon fiber is more difficult and costly to produce than PAN-based carbon fiber, and PAN-based carbon fiber has obvious advantages in terms of tensile strength, etc., under some requirements that emphasize both structural and functional components, try M55J grade Carbon fiber reinforced metal matrix composites.