The process to enhance anti-oxidation property of the pure graphite materials by impregnation with phosphates and its influence factor eree investigated. the results indicates that the initial oxidiating temperature of the materials is enhanced 300℃, and it is hardly oxidated below 950℃.
Graphite material has the characteristics of high temperature strength, good conductivity and heat transfer, thermal shock resistance, corrosion resistance and self-lubrication. Graphite has become an important conductive material and structural material in modern industry. It has shown excellent application characteristics in mechanical, electronic, chemical, metallurgical, nuclear and aerospace industries. For example, it can be used as electrode, electrolytic anode, casting mold and high temperature bearing, as surface coating of neutron deceleration material and nuclear fuel in nuclear reactor, graphite material can be used in satellite antenna, space shuttle hull, rocket engine nozzle, throat lining and other components in space field.
However, the decomposition and condensation of organic matter in the manufacturing process of graphite make the artificial graphite material have porous, most of which are through-holes. In addition, in the process of high temperature graphitization, there is a certain amount of small ash in the gap of graphite. These properties make graphite materials easy to be invaded by oxidizing gases in high temperature oxidizing medium. In the case of ash being the active center, the oxidation corrosion rate of graphite materials is accelerated, the structure of graphite materials is destroyed and the size of graphite materials is changed, so that it can not be used. At the same time, the usage of graphite materials is limited.
So in recent years, scholars at home and abroad have done a lot of research on the oxidation resistance technology of graphite materials, and successfully developed new technologies to reduce the oxidation consumption of graphite materials. For example: surface coating method, gas phase deposition method and immersion salt solution method. Among them, solution impregnation method is simple, inorganic salts and graphite materials are closely combined after impregnation to form antioxidant film, low cost and high temperature oxidation resistance, which has become a more effective method widely studied.
Different scholars have different experimental results on the formulation of impregnating agent and impregnating process conditions. Borate is used as impregnating agent in foreign countries. In China, the emphasis is on the use of phosphate. Based on the best antioxidant property of phosphate compound as impregnating agent, the impregnating process and formula are improved.
Oxidation resistance principle by impregnation
The difference between high-purity graphite and ordinary graphite is that after twice impregnating pitch and high-temperature graphitization, its static physical properties such as flexural and compressive properties and initial oxidation properties are superior to those of ordinary artificial graphite materials. For example, the initial oxidation temperature of high-purity graphite is higher than that of ordinary graphite materials by 300 ℃ (400 ~ 750 ℃). The surface porosity decreased significantly. However, the static physical properties of graphite materials do not change linearly with the increase of asphalt impregnation times, and the increase of impregnation times also greatly increases the cost of materials. Therefore, it is of great significance to treat graphite materials with low cost and convenient treatment technology for its industrial application.
The oxidation resistance principle of impregnation method is that the air in the pores of graphite material is extracted under vacuum condition, and then the impregnation solution is inhaled by the self-absorption of graphite micro-pores, and the impregnation effect is enhanced by pressure at the same time. After removing the ash from the pores of the graphite material with activator, the inorganic salt thermal decomposition material fills the pores of the graphite material and covers the surface of the matrix to form the anti-oxidation film, which has good thermal stability at high temperature, thus achieving the purpose of anti-oxidation.
Test part
1. Materials for testing
BEG-2 high purity graphite, phosphate, borate, phosphoric acid, glycerin and so on.
2. Impregnation process
After the graphite sample is processed, wash the surface with water and dry until the constant weight is dried. After the impregnating agent is proportionally prepared, the sample is placed in a storage tank, and the sample is vacuum-impregnated and heated to 100~120℃. When the vacuum degree reaches the requirement, close the vacuum valve, impregnate the agent and press 2~3h. After soaking, the graphite material is taken out, dried in oven at 150℃, and then heat-treated. During this process, the activator takes out the ash in the micropore, and the phosphate phase changes to form an oxidation-resistant layer. The heating curve is: room temperature – > 110℃ (> 10 min) – > 200℃ (> 10 min) – > 310℃ (> 20 min) – > 420℃ (> 20 min) – > 510℃ (> 10 min). After the sample is taken out, put it in a vacuum container and cool it to room temperature.
The results show that the surface treatment of graphite material by vacuum impregnation method can reduce the porosity of the material surface, enhance its static physical properties such as flexural and compressive resistance, have a significant impact on the oxidation resistance of graphite material, and can effectively improve the oxidation resistance of graphite material and prolong its service life. Moreover, through the study of the impregnation process, it is found that the impregnation temperature, vacuum, pressure, impregnation times and volume density have a certain impact on the impregnation effect. At the same time, the control of the heating curve has a direct impact on the post-treatment process and oxidation resistance. Therefore, in order to get the best results, the test process must be further improved in order to put into production more quickly.
Author: CHEN Han-yu
