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Abstract A good understanding of the piezoresistive behavior of carbon fiber is highly valuable in studying the processing-structure-property relationship of this very important engineering material. Up-to-date, there is still a lack of agreement on the physical origin of the unique modulus-dependent piezoresistivity for carbon fibers. In the present work, an experimental and theoretical modeling combined approach was taken to attack this long outstanding issue. A cantilever testing method was developed which allows for the evaluation of the piezoresistive behavior of a single carbon fiber filament under axial tension and compression mode. It was found that the tensile and compressive piezoresistive behavior showed antisymmetric characteristics. This supports the concept that the orientation of the basic structural units (BSUs) is responsible for the piezoresistivity of carbon fibers. Such an argument was further augmented by theoretical modeling of the carbon fiber piezoresistivity based on a Maxwell Garnett theory approach. The new piezoresistivity model identifies the critical role of the compound effect of the BSU orientation and its volume fraction in dictating the piezoresistive behavior of carbon fibers. With consideration of this compound effect, the modulus-dependent piezoresistivity data of carbon fibers reported in the past few decades can now be well explained. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

A filter assembly in the upper stream of the spinneret was used to disturb the mesophase pitch (MP) melt flow during the spinning of precursor fibers. We studied the microstructure evolution mechanism of the MP-based carbon fibers during this process. Results showed that the use of filter assembly had a significant effect on the size of the split in the cross-section of carbon fibers. The split has been completely depressed in the carbon fibers spun with 50 layers of plain-weave. Selected area electron diffraction results showed that carbon fibers spun with 50 layers of plain-weave screens exhibited an apparent decrease of the orientational order of the (002) diffraction plane in both center and transition regions. The results of Raman spectroscopy indicated that, the size of graphitic microcrystalline domain in carbon fibers decreased with increasing the number of plain-weave screens in the filter assembly. Our studies confirmed the roles of the filter assembly in precursor fiber spinning to disrupt the mesophase domain formation in the MP melt and thus induce a reduced domain size. [ABSTRACT FROM AUTHOR]