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 x 10(-5)) V m(-1) and (1.051 x 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 x 10(6) and 2.89 x 10(6) 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 <= 340MPa (elastic regime). This effect is stronger for low carbon steel than stainless steel. The piezoelectret coupling coefficient d(33) is -(6.6 +/- 0.1) x 10(-7) and -(3.6 +/- 0.2) x 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.