Electret behavior of carbon fiber structural composites with carbon and polymer matrices, and its application in self-sensing and self-powering

Continuous carbon fiber carbon-matrix (C/C) and polymer-matrix (CFRP) structural composites without poling are electrets. Their DC electric field E (hence the volumetric power density P_v for structural self-powering) increases linearly with increasing inter-electrode distance l (more significantly for C/C). Short-circuit discharge and open-circuit self-charge occur reversibly. The fraction of carriers that participate is 1.6 × 10⁻³ and 2.8 × 10⁻³ for CFRP and C/C, respectively. At l = 140 mm, E = 1.2 × 10⁻⁴ V/m, and P_v is 5.0 × 10⁻⁵ and 2.5 × 10⁻⁴ W/m³ for CFRP and C/C, respectively. The participating carrier density is 2.4 × 10²¹ and 2.6 × 10²² m⁻³, and the discharge time per unit participating charge is 6.7 × 10⁶ and 8.9 × 10⁴ s/C for CFRP and C/C, respectively. The C/C gives higher power density, whereas CFRP discharges more slowly. The polarization-induced apparent resistance increase upon DC current polarity reversal is asymmetrical. Elastic tension affects E, relative permittivity κ and resistivity ρ essentially reversibly and linearly (more significantly for C/C), enabling piezoelectret/piezoresistive self-sensing and piezoelectret-based energy harvesting. The strain causes CFRP to increase in E, κ and ρ, and C/C to decrease in E and ρ and increase in κ. Increasing the temperature from 20 °C to 70 °C increases E reversibly by 1900% and increases the volumetric power density by 40000%, as shown for C/C.