Electret behavior and electret-based self-powering and self-sensing performance of carbon fiber polymer-matrix and carbon-matrix structural composites

Continuous carbon fiber (polyacrylonitrile-based) polymer-matrix (CFRP, polyamide, 2-kHz relative permittivity κ 12500, DC resistivity ρ 2.7 × 10⁻² Ω cm) and carbon-matrix (C/C, κ = 10100, ρ = 5.6 × 10⁻³ Ω cm) composites without poling are electrets and electrically conductive dielectrics, with inherent DC electric field E (hence volumetric power density P_v for structural self-powering) increasing linearly with increasing inter-electrode distance l (more significantly for C/C), because the fraction of carriers that participate decreases with increasing l. Short-circuit discharge and open-circuit self-charge occur reversibly. At l = 140 mm, E = 1.2 × 10⁻⁴ V/m, with the fraction of carriers that participate being 1.6 × 10⁻³ and 2.8 × 10⁻³, and P_v being 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 divided by the 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, κ and ρ essentially reversibly and linearly (more significantly for C/C), enabling piezoelectret/piezoresistive stress/strain structural self-sensing. The strain-induced reversible microstructural change causes CFRP to increase in E, κ and ρ, and C/C to decrease in E and ρ and increase in κ.