High-temperature performance of graphite materials

Characteristics and application scope of graphite materials

Carbon materials are inorganic non-metallic materials primarily composed of carbon. Carbon materials are essentially composed of non-graphitic carbon, while graphite materials are primarily composed of graphitic carbon. Graphite is further divided into two categories: natural graphite and synthetic graphite.

Graphite materials are primarily composed of polycrystalline graphite. Graphite is a layered crystal with a hexagonal planar lattice between carbon atoms. Although graphite belongs to the category of inorganic non-metallic materials, it is referred to as a semi-metal due to its excellent thermoelectric conductivity. Graphite exhibits higher thermoelectric conductivity than some metals, while also having a much lower thermal expansion coefficient and high chemical stability, which makes it of great value in engineering applications. Graphite is chemically inert in non-oxidizing media and has excellent corrosion resistance. Except for strong acids and oxidizing media, graphite is not corroded by other acids, alkalis, or salts, and does not react with any organic compounds.

Graphite material is also a high-temperature resistant material. It does not melt at high temperatures and has a very high gasification temperature. It only sublimates into gas at 3350°C under normal pressure. Generally, the strength of materials decreases gradually at high temperatures, but the strength of graphite increases with temperature up to 2500°C. Above 2000°C, its strength doubles compared to that at room temperature. Graphite material also exhibits excellent thermal shock resistance. Therefore, graphite material has unique advantages as a high-temperature material.

Due to its advantages such as high temperature strength, electrical and thermal conductivity, thermal shock resistance, corrosion resistance, and good lubricity, graphite material has become an indispensable structural material, high-temperature material, conductive material, wear-resistant material, and functional material in the development of the national economy. Currently, graphite materials are widely used in sectors such as metallurgy, chemical industry, electronics, electrical appliances, machinery, as well as nuclear energy and aerospace industries. They can be used for electrolysis, casting molds, and high-temperature bearings; as neutron moderating materials and surface coatings for nuclear fuels in nuclear reactors; and in aerospace fields, graphite materials can be used for artificial satellite antennas, space shuttle casings, and rocket engine nozzle throat liners.

The physicochemical changes that occur in graphite materials at high temperatures, as well as the characteristics of their use at high temperatures:

Graphite material is chemically stable and thus is a corrosion-resistant material. However, under certain conditions, carbon can react with other substances. The main reactions include: oxidation in oxidizing atmospheres or strong oxidizing acids at high temperatures; melting in metals at high temperatures and forming carbides; and forming graphite interlayer compounds.

 At room temperature, carbon does not react chemically with various gases. At around 350℃, amorphous carbon begins to undergo significant oxidation reactions, and graphite also starts to undergo oxidation reactions at around 450℃. The higher the degree of graphitization, the more complete the crystal structure of graphite, the greater its reaction activation energy, and the better its oxidation resistance. Within the temperature range of 800℃, graphite materials reach the same oxidation rate at a temperature about 50~100℃ higher than carbon materials. Within the same material, binder carbons tend to oxidize preferentially. Therefore, when the oxidation reaction progresses to a certain extent, the aggregate particles will fall off. At lower temperatures, if the air supply is sufficient, carbon and graphite materials mainly undergo the following reactions: C + O2 — CO2. At higher temperatures, carbon and graphite materials begin to undergo the following reactions: C + 1/2 O2 — CO. Red-hot carbon and graphite materials react with water vapor at around 700℃: C + H2O — CO + H2. C + 2H2O — CO2 + 2H2. The oxidation reaction of red-hot carbon and graphite with CO only proceeds at higher temperatures: C + CO2 — 2CO. The reaction between carbon and gases belongs to gas-solid reactions, and the oxidation reaction rate is related to factors such as the reaction area size, the porosity of the material, and the gas pressure. The reaction rate depends not only on the chemical reaction rate on the surface but also on the diffusion of gas molecules into the material. If the porosity of the material is high, especially when there are many open pores, gas molecules easily diffuse into the material, and the surface area participating in the reaction is large, leading to a fast oxidation rate. When the use temperature is low, the oxidation reaction rate is not high, and gas molecules have sufficient time to diffuse into the material. At this time, the oxidation reaction rate is related to the pore structure and reactivity of the material. When the temperature is above 800℃, the chemical reaction rate is fast, but the diffusion of gas molecules into the material pores slows down due to thermal motion, and the oxidation reaction only proceeds on the surface. The oxidation rate is dominated by the surface gas flow rate and is less related to the type of material. Impurities contained in graphite materials catalyze the oxidation reaction, so there is a significant difference in the oxidation resistance between high-purity graphite and ordinary graphite. (b) Formation of Carbides. At high temperatures, carbon melts and reacts with metals such as Fe, Al, Mo, Cr, Ni, Ti, and non-metals such as B and Si to form carbides. (c) Formation of Graphite Interlayer Compounds. The carbon atoms in graphite are firmly connected through covalent bonds within their layers, while they are bound by weaker van der Waals forces between layers. Therefore, by inserting various molecules, atoms, and ions into the interlayer spaces of graphite without breaking its two-dimensional crystal lattice, only increasing the interlayer spacing, a unique compound of graphite called graphite interlayer compound can be formed. Natural flake graphite is commonly used as the raw material for the production of graphite interlayer compounds. Flexible graphite, which has been widely used in industry, is one of the graphite interlayer compounds. Flexible graphite not only has self-lubricating properties and high temperature resistance but also has flexibility, malleability, and compressive resilience. It can be used as an insulating material for refining furnaces and high-temperature furnaces and is widely used as a sealing material. To improve the oxidation resistance of flexible graphite, binders such as boric acid, thermosetting resins, and inorganic colloids are added to flexible graphite. It can be seen that carbon materials integrate heat resistance and electrical conductivity in non-oxidizing media, but in oxidizing environments, oxidation reactions begin at temperatures above 627K, and the oxidation rate increases with rising temperature, causing structural corrosion and damage, affecting its use at high temperatures. Therefore, the issue of oxidation resistance protection for graphite materials is receiving widespread attention.