How to test the electrical conductivity of chopped carbon fibers?

Preparation of test samples before testing

chopped carbon fibers are discrete powdered fibers and cannot be directly tested on the machine. Generally, standard test samples need to be prepared first, and then the electrical conductivity test can be carried out. Before the test, the samples should be properly dried to remove the absorbed moisture and avoid interference from moisture in the test results.

Common sample preparation methods include two types. One is to mix chopped carbon fibers with resin matrix and press them into standard cube or sheet-like composite material samples according to a fixed process, which is suitable for simulating the actual usage conditions downstream; the other is to uniformly fill the fibers in a dedicated insulating mold to form loose fiber samples, which are used to characterize the electrical conductivity properties of the fiber powder itself. During sample preparation, it is necessary to ensure the uniform dispersion of the fibers and avoid agglomeration, otherwise, it will cause uneven local conductive pathways and large data fluctuations.

Main testing methods and determination logic

The volume resistivity is the most commonly used indicator to characterize the electrical conductivity of chopped carbon fibers. The four-probe method is commonly used for testing. The four probes of the four-probe device contact the surface of the sample, and a constant current is introduced through the two ends of the electrodes, and the voltage signal between the two middle points is collected. Then, the volume resistivity is calculated by combining the sample size. This method can reduce the interference caused by the contact resistance of the electrodes and has better data stability, which is suitable for carbon fiber composite material sheets after curing.

For loose fiber samples, a powder resistance testing device is often used. The fibers are compressed under controlled pressure for testing. The pressure changes the contact state between the fibers, and the number of contact points changes with different compression degrees. The test values vary significantly, so the pressure condition needs to be fixed and marked in the report during testing.

Surface resistance testing is mostly used for screening the uniformity of the sample surface's electrical conductivity, which is suitable for rapid comparison of batch differences, but it cannot fully reflect the conductive pathways inside the material. It is generally used as an auxiliary detection method.

During the testing process, environmental temperature and humidity will affect the results. It is necessary to stand still and balance in a standard environment. It is recommended to conduct multiple parallel test samples from the same batch for detection, and take the results to make a comprehensive judgment.

It should be noted that the electrical conductivity performance of chopped carbon fibers is not only determined by the fibers themselves, but also closely related to fiber length, dispersion degree, addition ratio, and base material type. The same carbon fibers, when molded in different resin systems, will have significant differences in the final electrical conductivity effect. When purchasing and accepting, it is recommended to prepare test samples according to the actual downstream formulation for testing, which is more in line with the actual production conditions.

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