Antioxidant treatment methods for graphite materials

As we all know, during the manufacturing process of graphite products, the pyrolysis and condensation of organic carbonaceous materials lead to the formation of porous artificial graphite materials, with a general porosity of around 20-30%. Most of these pores are open pores, which increases the diffusion speed and depth of oxidizing gases, deteriorating the oxidation resistance. Generally, oxidation begins at around 400℃ in air. One of the main measures to prevent the oxidation of graphite materials is to reduce the contact area between the graphite materials and oxygen. Essentially, it involves covering the pores or active sites of the graphite materials with antioxidant substances, preventing their surfaces from being directly exposed to air. Currently, there are roughly three methods for preventing the oxidation of graphite materials: surface coating, vapor deposition, and impregnation. Among them, the impregnation method is gaining increasing attention due to its advantages such as simple equipment, easy operation, significant effect, and good economy.

Vapor deposition method:

Pyrolytic carbon and pyrolytic graphite exhibit excellent high-temperature resistance and corrosion resistance. Therefore, depositing a certain thickness of pyrolytic carbon or pyrolytic graphite on the surface of graphite materials through chemical vapor deposition can enhance the oxidation resistance of the graphite materials. However, a significant drawback of pure pyrolytic carbon coatings is their high anisotropy, which makes them prone to spalling in practical applications. If needle-like silicon carbide crystals are embedded in the pyrolytic carbon deposition process, with the axial direction of the crystals perpendicular to the basal plane of the pyrolytic carbon, it can reduce the anisotropy of the pyrolytic carbon, increase the strength of the c-axis direction of the pyrolytic carbon, and improve other properties such as expansibility. Additionally, the vertically embedded needle-like silicon carbide disrupts the layered structure of the pyrolytic carbon, thereby reducing the delamination of the pyrolytic carbon layers. Since silicon forms SO during oxidation and melts to form a protective layer, this enhances the oxidation resistance and corrosion resistance of the product. However, due to its high cost and applicability only to small-sized products, this technology is currently mainly used in aerospace and aviation materials and other fields.

Impregnation method:

The impregnation method involves impregnating graphite materials with oxidation-resistant materials to reduce the porosity of graphite products, thereby achieving the objective of reducing the surface area of carbon in contact with oxygen and enhancing the oxidation resistance of graphite materials. To achieve satisfactory results, different impregnation methods should be adopted based on the specific conditions of the impregnant. (a) Impregnation with Phosphoric Acid Solution - This process is simple and feasible, and the resulting graphite products exhibit an oxidation resistance temperature of 760°C or higher. The specific method involves immersing graphite products in a phosphoric acid solution containing phosphate for about 10 minutes. Then, the impregnated products are heated to at least 500°C for about 5-10 minutes. Oxidation tests at 680°C and 800°C indicate that the oxidation resistance of the impregnated graphite products is significantly enhanced compared to non-impregnated graphite products. (b) Impregnation with Borosilicate Glass - Borosilicate glass is placed in a stainless steel crucible in the impregnation device, vacuumed, heated, melted, and graphite products are added. Ammonia gas is introduced for a certain period of time to allow the melted glass to penetrate into the graphite products. The graphite products after impregnation exhibit significantly improved performance compared to those before impregnation. Specifically, the compressive strength is doubled, the flexural strength is increased by more than double, the porosity is significantly reduced, and the resistivity remains almost unchanged. The oxidation resistance is enhanced, and the oxidation loss is significantly reduced. (c) Impregnation with Anti-oxidant - The impregnant is an anti-oxidant, which is a mixture of one or more alkaline earth metal inorganic salts and one or two types of phosphoric acid or phosphate salts. A small amount of metal salts such as Si and Ca, which act as conductivity aids and arc stabilizers, are added to the anti-oxidant mixture. Products with good arc resistance and conductivity can be obtained. Graphite products impregnated with anti-oxidant show no oxidation on the surface after a 6-hour test at 700°C. (d) Impregnation with Metal Salts - A saturated solution of refractory metal salts is used as the impregnant. Before heating at high temperatures, the graphite products after impregnation are soaked in an aqueous solution of ammonium hydroxide or ammonium bisulfate for 10 minutes to convert the refractory metal salts deposited in the pores into oxides. Then, the graphite products are heated at high temperatures in vacuum or protective gas to convert the metal oxides into carbides. This method can be applied to the tip of graphite, especially under melting conditions that control carbon concentration. In addition to the above three methods, there are also mixing methods and coating methods for the oxidation resistance treatment of graphite materials. The mixing method involves mixing refractory metals or metal powders during the production process of graphite products. The coating method combines the advantages of impregnation and coating, reducing the contact surface area between the product and oxygen. Of course, this treatment method depends on the application of the product and the treatment agent used. Artificial graphite is usually treated by impregnation, and the cost of the impregnation solution and related treatment costs are not very high, making it an effective method to improve quality and enhance the oxidation resistance of graphite in smelting.