Effects of diameter and hollow structure on the microwave absorption properties of short carbon fibers

A series of polyacrylonitrile (PAN)-based solid or hollow carbon fibers were prepared with outer diameters ranging from sub-micrometer to two hundred micrometers. They have similar compositions and intrinsic conductivities. Their electromagnetic parameters and microwave reflections were measured and compared. It was found that both the real and imaginary parts of the complex permittivity increased with decreasing diameter, especially at around 1.0 μm. The microwave absorption peaks moved to lower frequencies as well. However, some different properties were observed among the hollow carbon fibers with similar outer diameters. For example, the imaginary part of the complex permittivity increased much more gently and the microwave absorption peak located at the lower frequency had a stronger absorption. A connected network was finally proposed to understand these diameter-dependent properties. That is, a finer diameter means more fibers and a denser network, which produces a higher increase of the complex permittivity, especially for its imaginary part. Hollow structures could accelerate the increasing rate of the real part while lowering that of the imaginary part. It can be concluded that decreasing diameter and introducing a hollow or porous structure are both beneficial to the strong absorption at lower frequencies.