In this paper，the microstructures of ordinary-electrode graphite and three-high graphite were studied by high resolution transmission electron microscopy(JEM － 3010)． The existing forms of carbon and the relationship between different organizations were observed，and constituency electron diffraction (SAED) and high resolution images (HREM) were analyzed． The results show that the graphite material organizational structure is relatively complex，containing crystalline and amorphous and microcrystalline． The graphite material presents as scaly graphite materials embedded in grey matrix organization by macroscopic examination and fibrous-tissue coated rose-organization by high-multiples microscope． The physicochemical properties of ordinary graphite electrode and three high graphite difference mainly comes from the different preparation methods which lead to different grain size and porosity． The roses organization in graphite materials is closely related to graphitized carbon the preparation of raw material，and contain amorphous part．
Graphite has excellent electrical and thermal conductivity, good thermal stability, excellent lubricity and abrasion resistance, good chemical stability and corrosion resistance, and excellent machinability. Graphite materials and their derivatives are found in various fields from civilian to military to aerospace. Since the end of the 20th century, the world’s industrialized countries have consumed 190,000 tons of graphite per year. China consumes about 50,000 to 60,000 tons of graphite per year, mainly used in metallurgy, electric power, foundry and chemical industries. With the development and application of science and technology and equipment, new graphite products and deep processing of graphite material have also emerged. For example, expanded graphite retains the characteristics of heat resistance, corrosion resistance and self-lubricating of graphite, and also has the characteristics of lightness, elasticity, low temperature resistance and organic solvent that graphite does not have. It has a tendency to replace traditional sealing materials such as asbestos and rubber. . Micronized graphite and colloidal graphite are the base materials of modern high-conductivity and high-lubrication technology, can be used as solid lubricants in graphite-oil systems to save fuel consumption. In addition, graphite high-energy batteries, graphite plastics and anti-static graphite rubber are new products and deep processing technologies to be developed. Scientists Andre Geim and Kostya Novoselov stripped graphene from graphite crystals. As a single substance, they are not only very strong and hard, but also transfer electrons faster than known conductors at room temperature, which lays a solid foundation for the application of low-dimensional graphite materials.
The characteristics and preparation of graphite materials have been valued by many scholars. However, due to the particularity of graphite, its microstructure and fine microstructure have become the bottleneck of its development. For this reason, the microstructure of ordinary graphite and special graphite is intensively detailed. The analysis aims to provide a basis for understanding the graphite materials.
1 Experimental materials and methods
The graphite material is selected from ordinary electrode graphite and special (high-strength, high-density, high-purity) graphite. The performance parameters are shown in Table 1. The graphite material is soft and must be handled with care during the sample preparation process, so that no artificial defects or few artificial defects are observed in the observation part to ensure the smooth progress of the observation stage. The metallographic samples were cut into small pieces, and the two materials were smoothed, then thickly polished and thin polished throwed until the surface without obvious scratches. The samples were without corrosion, and a representative part was selected under the XJL-03 metallographic microscope. Observe the analysis. Transmission electron microscopy (TEM) samples were taken from small samples and ground to 200 nm or less by FISCHIONE 1010 low temperature ion thinner. The microstructure was observed under a transmission electron microscope of JEM-3010. Selected area electron diffraction (SAED) images and high resolution transmission electron microscopy (HRTEM) images were analyzed.
2 Experimental results and analysis
2. 1 Metallographic microscopic analysis
Figure 1 is a photograph of the metallographic structure of a common electrode graphite and a special graphite material. It can be seen from Figures 1a and 1b that they are composed of a gray matrix carbon and a uniformly distributed black void. However, there are differences in the pore sizes of the two materials. The voids of graphite materials in common electrodes are larger than those in the special graphites. The grain size of the two samples is uniform, and the special graphite grains are finer than the ordinary graphite graphite grains. The phase structure consists of white scaly tissue and dark irregular tissue at a ratio of about 1:1. Since the preparation of the graphite material is carried out by a powder metallurgy process, the product must have pores. The graphite preparation process is characterized in that the green body is often densified before graphitization, and the impregnation is a common method for densifying the graphite body matrix, including liquid phase impregnation and gas phase impregnation. The matrix is ??densified mainly by reducing pores. Graphitization is the most critical and complicated process in the preparation of graphite. The calcined green body is placed in a high temperature furnace, and a protective medium is added to keep the hexagonal carbon atom plane from two dimensions at 2 300 to 2 500 °C. The spatial disordered arrangement is transformed into an orderly arrangement in three dimensions. Both sets of graphite materials showed polycrystalline characteristics, and the grain and pore differences were observed in the metallographic photographs. For the special graphite, the industry needs to perform multiple impregnations, which is the main reason why the pores are smaller than the ordinary electrode graphite and the density is larger than that of the ordinary electrode graphite. Polycrystalline graphite materials are isotropic on a macroscopic scale, and the organization includes scaly and dark irregularities which are embedded in scaly. It is believed that this is related to the difficulty of graphitization and the rate of graphitization during the preparation of the material. From the metallographic photograph, it can be found that the special graphite grains are fine, and the fine grain strengthening is beneficial to increase the strength of the matrix, while the ordinary electrode graphite grains are relatively coarse and slightly inferior in strength. At the same time, due to the high melting point of graphite, many grain boundaries and microcrystalline boundaries are beneficial to interdiffusion between elements when combined with dissimilar materials.
Fig． 1 Microstructure of graphite． a:Ordinary-electrode graphite; b: special graphite．
2. 2 TEM analysis
2. 2. 1 Transmission electron microscopic observation of common electrode graphite
Fig. 2 is a microscopic TEM image of a common electrode graphite and its corresponding SAED diagram. A large number of rose tissues and fibrous structures can be clearly observed from the figure, and the grain boundaries and microcrystalline boundaries are complicated, and the electron diffraction pattern is amorphous or polycrystalline crystal features, the upper right part of Figure 2a is the SAED diagram of the rose tissue, with a more pronounced amorphous diffraction ring, the diffraction ring is divergent and should be related to the degree of graphitization of the material.
Figure 2b shows the TEM image and SEAD of the fibrous structure in the graphite material. The microstructure is fibrous and has a clear stratification. Compared with the electron diffraction pattern of Fig. 2a, the diffraction ring is clear and regular, the degree of crystallization is high, and the degree of graphitization is good. The measurement of the diffraction ring in the figure shows that it is in good agreement with the close-packed hexagonal crystal graphite (where a = 0. 247, c = 0. 679 nm).
2. 2. 2 TEM observation of special graphite
Figure 3 shows the TEM image of rose tissue and irregular tissue in special graphite materials. From Fig. 3a, we can see the obvious river-like fibrous tissue and rose tissue, and the fibrous tissue coated rose tissue grows to the outer layer. The rose tissue is mosaic and the two tissues are well transitioned, while the corresponding SAED diffraction circle has a slight divergence, and the intergranular distance is basically the same as that of the common electrode graphite. The special graphite grains are fine, and the graphitization degree is better than that of the ordinary electrode graphite, and the amorphous component is small. However, compared to Figure 2b, the amorphous diffraction ring in the SAED diagram is heavier, more divergent, and contains amorphous components. It can be observed from Fig. 3b that irregular regions (Z zone) exist in a part of the microstructure, which may be related to impurities in the graphite preparation raw material and hardly graphitized carbon structure. Comparing the TEM image and SAED chart of the two graphite materials, the microstructure is basically similar, the carbon exists in various forms, mainly in the form of fibrous and rose tissue coating, and the rose tissue contains amorphous components and different carbons. The existence form will directly affect the activity of carbon elements at high temperatures, and has a certain effect on the deep processing of graphite materials and the connection with dissimilar materials.
Fig． 2 TEM and SAED images of ordinary-electrode graphite． a:TEM and SAED images of roses microstructure;
b:TEM and SEAD images of fiber microstructure．
Fig． 3 TEM and SAED images of the special graphite roses and irregular microstructure．
a:TEM and SAED images of roses microstructure;b:TEM of irregular microstructure．
2. 2. 3 HRTEM image of graphite
Theoretically, the graphitization mechanism can be divided into three stages: the first stage is carried out at about 1 000 to 1 800 °C, the bond of C-H, C=O is broken, and the impurity atoms such as hydrogen, oxygen, nitrogen and sulfur are excluded. There is no obvious change in the accumulation, mainly in the order of two-dimensional orientation, and the two-dimensional ordered size is small, which shows that some microcrystalline boundaries disappear, the process is accompanied by complex endothermic exothermic reaction; the second stage temperature range is at 1 600 ~ 2 200 °C, the thermal vibration frequency of carbon atoms increases, transition to the three-dimensional grid structure, the interlayer distance decreases, the dislocation lines and grain boundaries decrease the exotherm; the third stage is above 2 200 °C, the grain grows further along the a-axis and the c-axis, and the c-axis direction is about 60 layers. The crystal grains shrink and the grain boundary gap tends to expand. The recrystallization process is completed by lattice recombination and three-dimensional arrangement of carbon plane molecules and intermolecular carbon movement. Each stage is intertwined with each other, accompanied by an endothermic exothermic reaction. In essence, more is an exothermic process and the system is stabilized.
Figure 4 is an HRTEM image of a graphite graphite fibrous structure of a common electrode. It can be observed from Fig. 4 that the lattice fringe of the A region is clear, while the B region adjacent thereto does not have this feature, which may be closely related to the crystal state and graphitization of the raw material. The measurement is calculated to have a diffraction plane of (002) crystal plane, and the pitch is about 0.34 nm. This is in agreement with the calculated value of the concentric ring of the corresponding selected area electron diffraction pattern of 0.37 nm.
Fig. 5 is a photograph of a rosette tissue and an HRTEM image of special graphite. From Fig. 5a, it can be seen that the outer fibrous region (X region) is closely connected to the striped S region of the flower core, and has the same structure. The dense fibrous tissue extends to the center of the flower and is embedded in the B region in a strip-like free state with a certain flexibility. Figures 5b and 5c are HRTEM images of the center of the flower. It is observed from Fig. 5b that the lattice fringe pattern of the flower core is neatly arranged and clearly visible, while the F area adjacent to it is very blurred, and it is difficult to distinguish the stripe arrangement. The morphology may be an amorphous structure of amorphous carbon that is not completely graphitized. And the lattice streaks of different widths have obvious bending crossover phenomenon (D region), and the magnified image of the D region is measured to have a crystal surface spacing d of 0.33 nm, which is consistent with the (002) plane spacing of the graphite.
Fig． 4 HETEM image of ordinary electrode graphite fibrous tissue．
Fig． 5 Roses images of special graphite and HETEM．
a:TEM image of roses microstructure;b:High resolution TEM image;c:Magnified image of D area．
2. 2. 4 Inference of graphitization mechanism
Franklin studied the graphitization of different types of carbon, and divided the carbon raw materials into easy-graphitizable carbon and non-graphitizable carbon. Such as petroleum coke, coal pitch coke are easy to graphitize raw materials, carbon black, cellulose carbon, phenolic resin carbon, Charcoal and the like are difficult to graphitize carbon. The raw material forms amorphous carbon in the carbonization process, while the amorphous carbon is composed of 2 to 5 layers of parallel carbon network planes and a single carbon network plane and unorganized carbon which do not constitute parallel structures. Figure 6 is a few A typical carbon structure model. Graphitizable carbon (Fig. 6a) The crystallites are arranged neatly during the graphitization growth process, while the disordered carbon microcrystal structure (Fig. 6b) is difficult to graphitize. There are various sayings about the graphitization mechanism, and it is still not possible to obtain a unified understanding, and many of them are completed in the form of post-bonding in each of the easy and difficult graphitization parts.
Figure 6c, 6d are two classic non-graphitizable carbon structural models, and the rose tissue in Figure 5 have much in common, the black-branched bifurcation, bending, overlapping microstructure of rose tissue and typical tissue The models are very similar. In this paper, it is considered that the coating of rose tissue from fibrous tissue and the dense connection can be inferred that the possible mechanism of graphite crystallization is: after carbonization of the raw material, there is a thin layer of microcrystalline graphite appeared, and the thin layer of graphite microcrystalline in the non-graphitizable part as the whole graphitization’s precursor, which drives the atomic arrangement of the entire graphitization process, due to its complex structure, unstable components such as incomplete atomic sequences, provides a structural basis and energy basis for the graphitization process, and grows in the form of a coating, and finally the hardly graphitized part of the carbonized material forms a rose-like structure, and the easily graphitized part forms a fibrous structure, and the micropores in the rose tissue in the graphite are filled with amorphous carbon, which is not easily graphitized due to its structural characteristics, and finally appears to be disordered arranged amorphous components. The entire graphitization process is extremely complex and needs further research to confirm.
Fig． 6 Carbon model of graphitizing difficultly．
a:Franklin graphitic carbon model;b:Franklin hard graphitic carbon model;c:Kawamura hard graphitic carbon model;d:Whitehead hard graphitic．
3 conclusion
(1) The microstructure of graphite is scaly embedded in dark structure at low magnification and rosy structure coated with fibrous structure at high magnification. The difference of physicochemical properties between ordinary electrode graphite and special graphite is mainly due to the difference of grain size and porosity caused by different preparation methods.
(2) the microstructure of graphite materials is rather complicated, including crystals, amorphous and microcrystals. Among them, the rose structure in graphite material is closely related to the refractory graphitized carbon in the raw material of graphite preparation, and contains amorphous components.
(3) It can be concluded from the characteristics of fiber-coated rosette that rose-coated rosette is the precursor of graphitization, which leads to the growth of Graphite-Coated microcrystals and completes the whole graphitization process.
Author: WEN Ya-hui，CHEN Wen-ge ，HOU Lin-tao
