Graphite undergoes several changes when heated, primarily involving oxidation, structural transformation, and changes in mechanical properties.
Oxidation of Graphite at Elevated Temperatures: Graphite is sensitive to oxygen and begins to oxidize when exposed to air at temperatures around 500°C (932°F). This oxidation process can lead to a rapid loss of mass, up to 1% per day under certain conditions. Prolonged exposure to air at high temperatures results in the thinning of the graphite material, eventually leading to structural failure. This sensitivity to oxidation limits the practical use of graphite in air at high temperatures and necessitates its use in controlled or inert atmospheres.
Structural Changes Through Heat Treatment: Heat treatment of graphite, particularly in an inert atmosphere, can lead to significant structural changes known as graphitization. This process involves heating the graphite up to 3000°C, which causes the disordered or defective carbon structures to rearrange into a more perfect 3D crystal of pure graphite. The graphite layers, known as graphene, become more aligned and larger, improving the overall quality and properties of the graphite. This transformation enhances the material's performance in high-tech applications.
Changes in Mechanical Properties: Heating graphite from room temperature to high temperatures, such as 2,000 °C, can lead to an increase in its mechanical strength. This counterintuitive behavior is due to a decrease in internal stresses within the material as the temperature rises. This strengthening effect allows for the use of graphite in more demanding applications, where it can support larger loads and endure higher temperatures without failure. Additionally, this property enables the design of smaller and more efficient systems that require less support.
Electrical and Thermal Conductivity: Heating graphite rods also enhances their thermal and electrical conductivity. The thermal conductivity of graphite is higher than that of many common metals and increases with temperature, contrary to most materials where it decreases. Similarly, the electrical conductivity of graphite is notably high, being several times greater than that of stainless steel and carbon steel. These properties make graphite an excellent choice for heating elements and other applications requiring efficient heat transfer and electrical conduction.
In summary, heating graphite leads to oxidation at moderate temperatures in air, structural refinement at very high temperatures in inert atmospheres, and an increase in mechanical strength and conductivity at high temperatures. These changes make graphite a versatile material for various high-temperature and high-tech applications.
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