With the rapid development of science and technology, electronic information products tend to be compact in structure and efficient in operation. They generally face problems such as high heat generation, poor high temperature resistance of chips, and insufficient heat dissipation. The large amount of accumulated heat will seriously affect the normal operation of electronic devices. And the stability of the system. In order to solve such problems, people have developed heat-conducting materials based on carbon-based materials with high heat dissipation coefficient and light weight. Among them, graphite film has obvious advantages in the application of microelectronic packaging and integration due to its excellent electrical conductivity, thermal conductivity, and lightness.

As a special engineering material, polyimide (PI) has been widely used in aviation, aerospace, microelectronics and other fields, and is called "problem-solving expert". As early as the 1970s, A BÜRGER et al. processed the PI film at a high temperature of 2 800 ~ 3 200 ℃ to obtain a highly oriented graphite film. After that, many scholars conducted in-depth studies on the carbonization-graphitization behavior and mechanism of the PI film. . Although the performance of the graphite film prepared by the PI film is better than most thermal conductive materials, it still has problems such as the thermal conductivity to be improved and the bending resistance. On this basis, scholars have explored the factors that affect the performance of graphite film and made in-depth research on the improvement of its unilateral performance (thermal conductivity, electrical conductivity). Although my country's development in PI film preparation of graphite film is relatively late, there have been great breakthroughs in academic research and patent layout in recent years. This article mainly summarizes the research on the preparation of high thermal conductivity graphite film with PI base film.

Study on the preparation of graphite film

At present, there are four main technical routes for preparing high thermal conductivity graphite films: expanded graphite calendering method, graphene oxide (GOx) reduction method (solution chemical method), vapor deposition (CVD) method, PI thin film carbonization-graphitization method. The expanded graphite calendering method is mainly formed by the expansion and calendering of natural flake graphite particles. The GOx reduction method uses chemical reagents to reduce graphene by gaining and losing electrons. The CVD method uses a gaseous carbon source to grow graphene on copper and nickel substrates. The PI film carbonization-graphitization method mainly uses high polymers (PI, polyacrylonitrile) as raw materials, and prepares high-performance graphene thermal conductive sheets and fibers through pre-formed carbonization and high-temperature graphitization of precursors. Table 1 is a comprehensive comparison of the four technical routes.

Compared with the other three methods, the PI thin film carbonization-graphitization method has more advantages in the preparation of highly crystalline and highly oriented graphite films with high thermal conductivity. PI thin film carbonization-graphitization method to prepare high-performance graphene thermal conductive sheet and fiber includes two processes: carbonization and graphitization. Carbonization is to pre-heat the PI film under reduced pressure or in a nitrogen (N2) atmosphere, and the carbonization temperature is 800 to 1500°C. Appropriate pressure can be applied to the PI film when the temperature is raised to avoid wrinkling of the film material. Graphitization is carried out under reduced pressure or under the protection of inert gas (argon (Ar), helium (He), etc.), and the temperature of graphitization is 1800-3000°C.

Early research on the preparation of graphite films from PI-type films used commercial PI films as the base film to explore the carbonization-graphitization transition process.

M INAGAKI and others carbonized a Kapton®PI film with a thickness of 25μm, and then graphitized it at different temperatures, and then observed the changes in the cross-section of the film. The results show that at 550～1000℃, the C-N and C=O bonds are broken and separated from the membrane material in the form of CO, CO2, and N2. The quality of the membrane material first declines rapidly and then becomes stable. At 1000-2000°C, the membrane material aggregates to form a chaotic layer structure, the C, H, O, and N in the chaotic layer structure are gradually discharged, the void left by the non-carbon atom separation becomes smaller, and the boundary of the microcrystalline structure gradually disappears. At 2 000-2 500°C, the crystallites aggregate to form graphite crystals, and the film material is partially graphitized.

After the temperature exceeds 2500°C, the crystal lattice is gradually perfected, the disordered layer structure gradually becomes an orderly parallel graphite hexagonal network layer structure, and the membrane material exhibits a high degree of graphitization. They also did a comparative experiment with Upilex® PI film and found that the more oxygen content in the PI structure, the smaller the diameter of the crystallites initially formed, and the lower the graphitization ability. Y HISHIYAMA et al. studied the graphitization changes of carbon film prepared by PI base film at 1800～3200℃, and found that as the temperature increases, the graphite structure gradually tends to be ordered.

Subsequently, domestic scholars carried out detailed research on the carbonization process of PI film. Zhao Genxiang et al. studied the structural transformation of three domestic PI membranes during pyrolysis of carbon from room temperature to 1000°C in a high-purity N2 atmosphere. The experimental results show that as the pyrolysis temperature increases, the carbon content in the sample increases. And the increase is most intense at 550-700°C, which may be the thermal polycondensation reaction of the molecules, which leads to the breaking of the CO and CN bonds to form new bonds, resulting in the growth of heterocyclic rings. The oxygen content in the sample has been decreasing until 800°C. This is due to the breaking of the C-O bond in the sample molecule, which may cause the oxygen to escape in the form of CO. They also studied the thermal decomposition behavior of Kap-ton® PI film heated to 1,000°C in N2. Experiments show that the mass loss and size shrinkage of samples mainly occur at 500-800°C. When the temperature exceeds 800°C, this phenomenon tends to alleviate.