Abstract: The paper evaluates the effects of phosphorylation series antioxidants through the post-impregnation of graphite materials treated with soluble antioxidative impregnants made from water-soluble Al(H2PO4)3, Mg(H2PO4)2, boric acid and sodium hexametaphosphate. It is shown that the initial oxidation temperature of graphite materials dealt with the water-soluble impregnant rise from 400 ℃ to 500 ℃, and the graphite materials are still effectively protected from oxidation at 1 000 ℃.
Graphite materials have very good properties such as good electrical and thermal conductivity, self-lubricity, low hardness, low friction and high temperature resistance. They are widely used in metallurgy, electrical, electronics, machinery, chemistry, nuclear industry and other fields. However, graphite materials also have obvious disadvantages, that is, insufficient antioxidant properties. The graphite material is slightly oxidized from about 450 °C in an aerobic environment and violently oxidized at around 750 °C. Therefore, when graphite is used as a high-temperature material, it is generally used in a low-oxygen or oxygen-free environment, and can only be used as a consumable material under aerobic condition.
In order to broaden the application range of graphite materials, scientists at home and abroad have done a lot of research on the oxidation resistance of graphite materials, and have obtained many methods to improve the oxidation resistance of graphite materials, such as: solution impregnation method, coating method, self-healing method and carbon ceramic composite method . However, each of the above methods has its own disadvantages. For example, the coating method has a relatively high price, and is generally only suitable for a product with a small amount and a high added value, and the coefficient of thermal expansion between the coating and the graphite material is difficult to match, and it is easy shedding under severe temperature changes. The self-healing method generally oxidizes the surface of the graphite material into a ceramic film by the ceramic particles added to the graphite substrate, so that the surface of the graphite material has some ceramic material properties, which inevitably affects some properties such as hardness and self-lubricity of the graphite material. . Solution impregnation has little effect on the properties of graphite materials, but the antioxidant effect is not as good as coating method and carbon ceramic composite method. Therefore, the above various methods have their advantages and disadvantages, and the specific choices need to look at the environment in which the graphite materials are served.
1.1 Sample preparation
The test graphite material was sampled from the Φ318 mm joint of the ultrahigh power graphite electrode produced by Kaifeng Carbon Co., Ltd., and the sample was made into a cylinder of Φ25 mm × 30 mm. The sample had a bulk density of 1.85 g/cm3, a apparent porosity of 15%, and a resistivity of 3.3 μΩ·m.
1.2 Preparation of impregnating agent
Take a certain amount of deionized water, phosphoric acid, magnesium dihydrogen phosphate, aluminum dihydrogen phosphate, sodium hexametaphosphate in a beaker, heat to about 70 °C, and add the weighed boric acid separately under stirring to prepare a colorless and transparent The solution was then added with a small amount of glycerol as a surfactant.
1.3 Test equipment
Electronic analytical balance: the index value is 0.1 mg; electrothermal constant temperature blast drying oven: model 101-3, working temperature RT ~ 300 °C; box type resistance furnace: working temperature RT ~ 1 200 °C; thermal analyzer: TG / DSC ; Water ring vacuum pump.
2 Tests and results
2.1 Differential thermal-thermogravimetric analysis of antioxidant impregnating agent
A small amount of impregnating agent was placed in a 100 mL beaker and kept at 110 °C for 4 h, at which time the impregnating agent became a paste. Take a small amount of paste-like antioxidant impregnant for differential thermal-thermogravimetric analysis.
It can be seen from the analysis results that there is a wide endothermic peak A between 120 and 150 °C, which may correspond to the removal of solvent water and crystal water remaining in the impregnating agent, and that H3BO3 forms metaboric acid upon heating. Or water molecules lost by pyroboric acid. As the temperature increases, there is an absorption peak B around 220 °C, which may be the endothermic peak of the decarburic acid or pyroboric acid in the impregnating agent which continues to be thermally decomposed to form B2O3 and a small amount of glycerol which is heated and volatilized. When the temperature continues to rise, the remaining components in the impregnating agent undergo a polycondensation reaction with each other, in which water molecules and other small molecules volatilize, and the quality continues to decrease. However, when the temperature is higher than 400 ° C, the rate of mass reduction becomes slow, indicating that the reaction between the components in the impregnant is substantially completed after the temperature is greater than 400 °C.
2.2 Impregnation and heat treatment process
The contaminants and floating ash layer on the surface of the prepared sample were removed, and then placed in an ultrasonic cleaner for ultrasonic cleaning for 20 min, and then dried at 120 ° C in an oven at a constant temperature. In order to study the effect of vacuum degree and immersion time on the impregnation effect, the samples were divided into 5 groups, 3 samples of each group were subjected to the immersion-antioxidation test, and all the samples were subjected to 2 times of immersion-heat treatment. The specific impregnation process is as follows: When the vacuum impregnation is not performed, the impregnation liquid is heated to 65 ° C, and the dried sample is placed. When vacuum impregnation was performed, the degree of vacuum was maintained at -0.09 MPa, after which the vacuum was broken and the sample was taken out.
After all the samples were immersed, they were placed in an oven to be dried. The drying step was maintained at 90 ° C for 3 h, and then the temperature was raised to 110 ° C for 3 h. After the sample is dried, heat treatment is carried out. When heat treatment, N2 is required as a shielding gas. The maximum temperature of the heat treatment is 800 °C, and then the temperature is kept for 30 minutes. After the heat preservation, the temperature is naturally lowered to below 400 °C, and the protective gas can be stopped. After the temperature was lowered to room temperature, the sample was taken out, weighed, and the weight gain rate was calculated.
2.3 Antioxidant experiment
The oxidation test was carried out in a box-type resistance furnace, and the oxidizing atmosphere was static air. The test temperature is 1 000 °C. When the furnace temperature reaches the specified temperature, the sample is placed in the furnace and the temperature is kept for 2 h. The oxidation weight loss rate under various immersion conditions is calculated. The calculation formula of the oxidation weight loss rate is: w=(burning Pre-mass-post-burning mass) / pre-burning mass × 100%, the sample impregnation condition with the lowest oxidative weight loss rate is the optimum impregnation condition of the impregnating agent.
It can be seen from the experimental results that the sample with vacuum impregnation for 40 min has the lowest oxidative weight loss rate at 1 000 °C, and the average weight gain rate of the sample is also the highest under this condition. This indicates that the antioxidant formed by magnesium dihydrogen phosphate, aluminum dihydrogen phosphate, boric acid or the like can be well filled into the pores of the graphite material and covers the active sites therein, and exhibits good oxidation resistance at high temperatures. Compared with the sample impregnated for 40 min, the weight loss rate is very small. It can be seen that the extension of the immersion time has little effect on the oxidation effect, so the subsequent test is selected by vacuum impregnation for 40 min. Experimental conditions.
2.4 Loss of oxidation weight of graphite materials under different temperature conditions
Two sets of samples (three in each group) were taken, one of which was vacuum impregnated for 40 min according to the above procedure, and the other set was not subjected to immersion treatment as a blank sample. After the heat treatment is completed, the sample is subjected to an oxidation test at 400 to 1 000 °C and compared with a blank sample. Every 100 °C as a test temperature point. The sample was thermostated at each test temperature for 1 h, after which the sample was taken out, cooled to room temperature in air, weighed, and the oxidation weight loss rate was calculated. The relationship between the oxidation weight loss rate and the oxidation temperature is obtained.
It can be seen from the results that the graphite sample after impregnation has good oxidation resistance. The unimpregnated sample begins to oxidize at 400 °C, and the oxidative weight loss rate increases sharply when the temperature is greater than 700 °C. The initial oxidation temperature of the sample impregnated with antioxidant is above 500 °C, because the oxidation activity of graphite increases from 194 kJ/mol to 216 kJ/mol when B2O3 is present in the graphite material. Graphite oxidation is inhibited, when the temperature is higher than 700 °C, the oxidation weight loss rate increases more gently. This is because the phosphate (Al(H2PO4)3, Mg(H2PO4)2, sodium hexametaphosphate, etc.) in the antioxidant impregnating agent loses water to polycondensate at high temperature to form polyphosphate, with the wetting and covering effect of boron containing materials on the graphite void surface., these materials are thermally stable, so the oxidation rate of graphite can be lowered at high temperatures. Moreover, due to the solution method impregnation, the antioxidant can well enter the pores inside the graphite material to form an antioxidant protective layer, which is different from the coating type antioxidant, and the coating type antioxidant can only play protective effect on the surface of the graphite material. The oxidation resistance of the entire material will drop sharply once the anti-oxidation layer of the surface is damaged or ablated.
1) The graphite substrate treated by impregnation can make the graphite material have good oxidation resistance at 700~1 000 °C. The impregnation impregnation conditions were optimized by experiments, and a good impregnation effect was achieved under mild conditions.
2) Since the antioxidants used are all water-soluble substances, the most environmentally friendly water can be used as a solvent to avoid the use of organic solvents, making the impregnation operation safer. Moreover, the impregnating agent prepared by using an antioxidant which is completely soluble in water can sufficiently infiltrate into the finer voids of the graphite material to further improve the impregnation effect.
3) Theoretically analyze the reasons why antioxidants make graphite materials resistant to oxidation, and also indicate the role of each substance in the impregnating agent in the anti-oxidation of graphite materials, which provides a theoretical basis for further searching for better antioxidants.
Author: Cheng Xiao-ke, Liang Ran, Feng Jun-jie, Kang Jin-cai, Lu Pei-zhong, Yang Wei-feng