Electret, piezoelectret and piezoresistivity discovered in steels, with application to structural self-sensing and structural self-powering

Electret, piezoelectret, piezoresistivity and stress-dependent electric permittivity are reported in unmodified steels. Structural stress/strain self-sensing based on piezoelectret/piezoresistivity is demonstrated under tension. Structural self-powering is shown by the electret's inherent electric field, (2.224 × 10^-5) V m^-1 and (1.051 × 10^-5) V m^-1, and power density, 29.2 and 41.8 W m^-3, for low carbon steel and stainless steel, respectively, being enabled by the electret's electrical conductivity. The free-electron-movement-enabled electrets are supported by the asymmetry in the polarization-induced apparent resistance relative to the true resistance upon polarity reversal. The electric field increases linearly with the inter-electrode distance l. An l increase causes the amount of participating free electrons to increase and the fraction of free electrons that participate to decrease; when l is tripled, the amount is increased by a factor of 1.011 and 1.021 for low carbon steel and stainless steel, respectively, while the fraction of free electrons that participate is decreased by a corresponding factor of 0.337 and 0.340. The higher values for stainless steel are consistent with the higher relative permittivity (2 kHz), 1.23 × 10⁶ and 2.89 × 10⁶ for low carbon steel and stainless steel, respectively. The capacitance (2 kHz) and electric field (DC) of the piezoelectret decrease nonlinearly with increasing stress, due to electret weakening; the decrease is reversible at stress ≤210 MPa, but is irreversible at stress ≤340 MPa (elastic regime). This effect is stronger for low carbon steel than stainless steel. The piezoelectret coupling coefficient d ₃₃ is -(6.6 ± 0.1) × 10^-7 and -(3.6 ± 0.2) × 10^-7 pC N^-1 for low carbon steel and stainless steel, respectively. The relative permittivity (2 kHz) decreases nonlinearly by ≤14% with stress ≤340 MPa. The piezoresistivity involves the DC resistivity decreasing nonlinearly and reversibly by ≤10% with stress ≤340 MPa; the gage factor is -1030 and -800 for low carbon steel and stainless steel, respectively. The reversibility upon unloading is superior for piezoresistivity than piezoelectret.