Do You Know the Heating Principle of Graphite Rods?

Graphite rods are commonly used as electric heaters in high-temperature vacuum furnaces. They oxidize easily at high temperatures and can only be used in neutral or reducing atmospheres, apart from vacuum conditions. Graphite rods have a low thermal expansion coefficient, high thermal conductivity, high-temperature resistance, and are cost-effective. The oxidation and volatilization rates of graphite impact the lifespan of heat generators. In a vacuum environment with a pressure of 3 to 4 mmHg, the operating temperature should be below 2300°C. Under a protective atmosphere (such as H2, N2, AR), the temperature can reach up to 3000°C. Graphite should not be used in the air to avoid oxidation and consumption.

Above 1400°C, graphite reacts strongly with tungsten to form carbides, exhibiting high thermal and electrical conductivity. Its electrical conductivity is four times that of stainless steel, twice that of carbon steel, and many times higher than that of general non-metallic materials. Graphite’s thermal conductivity is higher than that of metals like steel, iron, and lead, and it decreases with increasing temperature, unlike typical metals. At very high temperatures, graphite tends to become adiabatic. Therefore, graphite’s insulation performance under high-temperature conditions is very reliable, and it has a very high melting point. Under vacuum conditions, graphite starts to soften and tends to melt at 3000°C, and it evaporates and sublimates at 3600°C. Unlike ordinary materials, whose strength gradually decreases at high temperatures, graphite’s strength doubles at 2000°C compared to room temperature.

However, graphite has poor oxidation resistance, and its oxidation rate increases with temperature. When the temperature inside the furnace exceeds 1400°C, the oxidation rate of graphite rods accelerates, shortening their lifespan. Therefore, it is essential to control the temperature to prevent the surface temperature from becoming too high. Heating in air forms a dense silicon oxide film on the surface, which acts as an anti-oxidation protective layer, extending the rods’ lifespan. As the furnace temperature rises and falls, the protective film on the rod surface may crack, reducing the protective effect and accelerating the increase in rod resistance.