Heating Principle of Graphite Rods

Graphite rods are commonly used as electric heaters in high-temperature vacuum furnaces. They are prone to oxidation at high temperatures and should only be used in neutral or reducing atmospheres, except in a vacuum. Graphite rods have a low thermal expansion coefficient, high thermal conductivity, high-temperature resistance, resistance to extreme cold and heat, and are cost-effective. The oxidation and volatilization rates of graphite affect the lifespan of the heating element. When the vacuum pressure is 10^-3 to 10^-4 mmHg, the operating temperature should be below 2300°C. In a protective atmosphere (such as H2, N2, AR), the operating temperature can reach up to 3000°C. Graphite should not be used in the air, as it will oxidize and degrade.

Key Characteristics and Behavior of Graphite Rods:

Thermal and Electrical Conductivity:

1.Graphite has a thermal conductivity greater than that of metals like steel, iron, and lead.

2.Its thermal conductivity decreases with increasing temperature, unlike typical metals.

3. It has an electrical conductivity four times that of stainless steel and twice that of carbon steel, and much higher than other non-metallic materials.

High-Temperature Performance:

At temperatures above 1400°C, graphite reacts strongly with tungsten to form carbides.

4. 1. Graphite tends to become adiabatic at very high temperatures, making its insulation properties very reliable under high-temperature conditions.
   2. It has a very high melting point and begins to soften and melt at 3000°C in a vacuum.
   3. Graphite starts to evaporate and sublimate at 3600°C.

Strength and Oxidation:

1.Unlike most materials, whose strength decreases at high temperatures, graphite’s strength doubles at 2000°C compared to room temperature.

2.Graphite’s oxidation resistance is poor, and its oxidation rate increases with temperature.

3.Above 1400°C, the oxidation rate of graphite rods accelerates, shortening their lifespan. Therefore, it’s essential to control the temperature to prevent the surface temperature from becoming too high.

Protective Measures:

1.When heated in the air, a dense silicon oxide film forms on the surface of graphite rods, acting as an anti-oxidation protective layer and extending their lifespan.

2.However, fluctuations in furnace temperature can cause the protective film to crack, reducing its effectiveness and accelerating the increase in the rod’s resistance.

By understanding these principles and characteristics, one can effectively use graphite rods in high-temperature applications while maximizing their performance and lifespan.