In the production of carbon-graphite material, the amount of binder, always being a focus of research, has an important influence on the properties of the final product. The microstructure of green product with different amount of coal-tar pitch binder in the production of carbon-graphite material, and the key parameters of the carbon-graphite material were studied in this paper, which may provide a reference for future production of carbon-graphite material.
Since the 20th century, coal tar pitch has been widely used as a binder in the preparation of carbon-graphite materials, because it is a thermoplastic material. The asphalt coke formed after carbonization has good cohesiveness and similar physical and chemical properties as carbon aggregate. Many studies have shown that in the production of carbon graphite materials. The amount of binder asphalt has a very important influence on the preparation of materials and the quality of final products. Generally, people pay attention to the macroscopic appearance of raw and calcined carbon-graphite materials, because in the production process, green and calcined products are more likely to show visible and perceptible defects such as cracks, voids, abnormal sound, and so on, while ignoring the deep-seated quality problems in the production process, such as paste mixing process. And paste forming process. By means of microstructural analysis and evaluation, it is helpful to find hidden problems in the process of material preparation. In order to produce certain carbon and graphite materials with certain physical and chemical properties.
Effect of asphalt content on Green Bodies Quality
The production of carbon-graphite material has to go through the mixing and molding processes, which are closely related and have a very important impact on the quality of products. Mixing and kneading is a process of mixing various carbon aggregates and binder pitches into a paste with good plasticity at a certain temperature. It is a process of combining several aggregate units through a bonding matrix. The paste determines the microstructure and composition of the product with irreversibility. Molding is a product that makes use of external force to make paste deformation and get certain shape and size.
If the binder amount is too low. One is that the asphalt can not wet the carbon aggregate enough, and some coke particles in the paste can not wet. The other is that the coating of the binder matrix is insufficient or even discontinuous, resulting in the uneven distribution of carbon particles. After forming such paste, there are large pores in the billet, resulting in defects in the structure.
Too much binder asphalt will adsorb more asphalt on the surface of coke. such a paste requires lower pressure, larger spacing between grains in green body, lower bulk density, and more pores in binder matrix. There are many pores in carbon-graphite material. The pores mainly come from the interface of coke, binder pitch matrix, coke and matrix. The pores are often the inducement of cracks. Even at lower levels of loading in the use of materials, some microcracks begin to form at and near the tip of the inducer. Therefore, in the preparation of materials, if the amount of binder asphalt is too small, the bonding matrix is discontinuous, and the final prepared materials have more void defects. If the amount of binder is too high, the porosity of the material will be more. Too much or too little asphalt binder will aggravate the production of micro cracks and reduce the service life of materials.
Effect of binder amount of asphalt on properties of products
The bulk density, strength, hardness and resistivity of fine carbon-graphite materials with different asphalt dosages were measured in the preparation of fine grain carbon-graphite materials (150 mesh of the largest particles). With the increase of binder content, the volume density, strength and hardness of the products gradually increase. Resistivity shows a downward trend. The maximum volume density of the product corresponds to the asphalt content of 42%. When the asphalt content is 48%, the product ruptures during sintering. It was also found that the apparent porosity decreased with the increase of binder asphalt content. In summary, the amount of binder be increased can increase the density of the material, decrease the porosity, and increase the strength of the combination bridge between carbon particles and asphalt coke. However, when the amount of binder exceeds a certain limit, the thickness of asphalt film in green body is too large, and the asphalt film will undergo intense decomposition reaction and condensation reaction during carbonization, resulting in great thermal stress. The structure of the material is destroyed.
Conclusion：
(1) The amount of binder asphalt has great influence on the kneading effect of carbon paste and the quality of green bodies molding. When the amount of binder is too small, the coating of the binder matrix is imperfect, and the binder matrix appears to be fractured. The amount of binder used is too much.
The asphalt film between the grains in the billet is thicker and the distance between the grains is enlarged.
(2) The density, strength, hardness and conductivity of fine grain carbon-graphite materials are positively correlated with the amount of binder asphalt. However, when the addition of asphalt exceeds a certain value, cracks will easily occur when the gry is processed in the later stage.
(3) The amount of binder asphalt can be determined quantitatively. The quantitative method can be used to measure the thickness of asphalt film and the method of asphalt soaked coke. Reasonable determination of binder asphalt content can avoid low bulk density of the formed green body or poor local structure of the product, and produce green body with good quality. It is also conducive to roasting and graphitizing high temperature heat treatment of the product in the later process.
Author：Lv Hong-bing，LiuYun-ping，YuanQiang，YangWei-feng
