Stress and temperature sensing in self-sensing flexible felt composite based on carbon fibers

Self-sensing composites based on carbon fibers (CFs) offer a wide range of remarkable properties, making them suitable for various significant industrial and engineering applications. In this study, the wet-laid method was employed to prepare flexible short carbon fiber felts (CFFs). The microstructure analysis was performed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. To exclude contact capacitance, the relative permittivity was measured using an LCR meter. The resistivity was measured through the four-probe method and a multimeter. We investigated the relationship between stress, temperature, and the dielectric and conduction behavior while also analyzing the underlying mechanisms behind the self-sensing phenomenon. The results indicate that for an areal density of 52.6 g/m2, the CFF exhibits a relative permittivity of 400, which is significantly lower than that of pitch-based carbon fiber. This disparity suggests that smaller crystallite sizes promote AC polarization. Additionally, both permittivity and conductivity increase with rising compressive stress and temperature, implying that compression and heating strengthen the dielectric and conduction behavior. The increase in polarization during compression is attributed to the enhanced polarization continuity, whereas the rise in conduction is attributed to the improved conduction connectivity. The rise in charged carriers induced by thermal energy leads to positive pyropermittivity and negative pyroresistivity. Notably, self-sensing in terms of permittivity is more pronounced compared to resistivity, suggesting that polarization is more sensitive to stress and temperature changes than conduction.