Electret, piezoelectret, dielectricity and piezoresistivity discovered in exfoliated-graphite-based flexible graphite, with applications in mechanical sensing and electric powering

Abstract

This paper reports the in-plane dielectric, electret, piezoelectret and piezoresistive behavior of exfoliated-graphite-based flexible graphite, with demonstrated feasibility of electret-based electric powering and piezoelectret-based/piezoresistivity-based mechanical stress sensing under elastic tension. The powering is enabled by the inherent DC electric field E and DC electrical conductivity (resistivity ρ = 7.5 × 10−6 Ω m). E is supported by relative permittivity κ = 1170 and directionally asymmetric polarization-induced electrical resistance increase. It increases linearly with increasing inter-electrode distance l, with E = 2.6 × 10−5 V/m at l = 40 mm. With increasing stress, E, ρ, κ and capacitance increase monotonically; E and ρ increase smoothly - totally reversibly for stress ≤1.85 MPa and slightly irreversibly for stress between 1.85 and 3.18 MPa. The fractional increases in E and κ at ≤3 MPa are ≤110% and ≤73%, respectively. The κ-stress curve abruptly increases in slope at 2.1 MPa, which is accompanied by the onset of some κ increase irreversibility. This 2.1-MPa slope increase is attributed to a microstructural change that does not affect the strain reversibility or ρ, but increases κ. The change possibly involves interfacial loosening, which promotes polarization without affecting conduction. The piezoelectret coupling coefficient d33 is 3.7 × 10−8 pC/N. The piezoresistivity gage factor is 50.