Conductive Principle And Test Of Carbon Fiber

 

 

Carbon fiber is a conductive material. Although its conductivity is lower than that of traditional metal materials, its conductivity as a non-metallic conductor is still very popular. In recent years, carbon fiber heating pipes, carbon fiber floor heating and other products have gradually become popular, making carbon fiber a widely-watched electric heating material. This article analyzes in detail the conductive principle of carbon fiber, the factors affecting conductivity and the resistivity test method.

 

Conductive Principle Of Carbon Fiber Materials

Carbon fiber is composed of a microcrystalline structure. The structural arrangement of carbon atoms in a graphite single crystal is shown in Figure 1. Each carbon atom is bonded to three other adjacent carbon atoms by a strong covalent bond (σ bond); at the same time, graphite microcrystals have a layered structure, and the layers are connected by van der Waals forces, which belong to secondary bond forces. Therefore, it has good mechanical properties and conductivity in the direction parallel to the basal plane, but has low strength between layers.

 

 

 

Figure 1 Schematic diagram of graphite single crystal structure

 

As a carbon graphite material with a carbon content of more than 95%, the conductive principle of carbon fiber is similar to that of graphite. All C atoms inside the fiber participate in the formation of large π bonds, thereby forming a continuous energy band. Each C can have one electron that can move freely and realize conductivity along the continuous energy band. However, due to the unique chaotic layer structure of carbon fiber, the conductivity of the carbon fiber layer (fiber axial direction) is significantly better than the conductivity between layers (fiber longitudinal direction).

Factors affecting the conductivity of carbon fiber

 

The reason why carbon fiber can conduct electricity is closely related to the graphite microcrystalline structure inside it. Therefore, the key factor affecting the conductivity of carbon fiber lies in its internal microstructure.

 

Generally speaking, the higher the degree of graphitization of carbon fiber, the more perfect the graphite microcrystalline structure, and the better the conductivity of carbon fiber. For high-strength medium-modulus carbon fibers such as T300 and T800, the fiber has a typical two-dimensional chaotic layer graphite structure inside, as shown in Figure 2. Due to the defective structure in the graphite layer, its conductivity is poor.

 

 

Figure 2 Typical Turbostratic Graphite Structure Inside High-strength Medium-Modulus Carbon Fiber

 

After graphitization, the two-dimensional turbostratic graphite structure inside the fiber gradually transforms into a three-dimensional graphite microcrystalline structure, and the higher the degree of graphitization, the closer the graphite microcrystalline is to the ideal graphite single crystal structure. For the degree of graphitization, the most objective reflection of the degree is the tensile modulus of the carbon fiber, because the higher the degree of graphitization, the higher the tensile modulus of the carbon fiber.

 

The quantitative relationship curve between the tensile modulus and resistivity of PAN-based carbon fiber is shown in Figure 3. As the tensile modulus of the carbon fiber increases, the resistivity of the fiber gradually decreases. By increasing the heat treatment temperature, high tensile modulus carbon fiber can be prepared, and during the heat treatment process, the graphite flakes will gradually grow, the interlayer spacing of the graphite microcrystalline will gradually decrease, and the orientation will increase, providing a good space for π electron conduction, so the conductivity of the fiber is significantly improved.

 

 

Figure 3 Relationship between tensile modulus and resistivity of PAN-based carbon fiber

 

Resistivity Test Of Carbon Fiber

 

In order to quickly and conveniently test the conductive properties of carbon fiber, researchers from the Special Fiber Division of Ningbo Institute of Materials, Chinese Academy of Sciences, based on many years of experience in carbon fiber research and development, invented a carbon fiber resistivity test device, as shown in Figure 4. When using this device to test the resistivity of carbon fiber, by reading the voltage and current meter values ​​and calculating according to a specific formula, the resistivity test of carbon fiber can be quickly realized.

 

 

Figure 4 Carbon Fiber Resistivity Test Device

 

The specific test process is as follows: cut a certain length of carbon fiber sample from the winding frame, fix it on the bottom plate under tension, adjust the two electrode adjustment blocks to a specific distance according to the scale line, then make the electrode contact the carbon fiber sample to be tested, adjust the power supply to achieve the current value through the sample to be tested to stabilize at a value, then read the voltage value at this time, and finally calculate the resistivity according to the resistivity formula.

 

In order to verify the reliability of the device and test data, the resistivity of domestically produced M50J and M55J grade high-strength and high-modulus carbon fibers independently developed by Ningbo Institute of Materials was tested using the device. The test results showed that the resistivity of the two samples was 0.94×10-3Ω•cm and 0.86×10-3Ω•cm, respectively, which is close to the nominal data of the resistivity of the same model of Toray products in Japan.

 

The test device is simple in design, the test process is very convenient, and the test data is true and reliable. Domestic units with carbon fiber material resistivity testing needs are welcome to negotiate cooperation.