Piezoresistivity and piezoelectricity discovered in aluminum, with relevance to structural self-sensing

This work provides the first report of piezoresistivity, piezoelectricity (piezoelectret) and dielectricity in aluminum, as observed in the in-plane direction for aluminum foil and Al 2024-O alloy bar, which exhibit similar properties, though the piezoelectricity of the latter is slightly stronger. These phenomena enable structural self-sensing and multifunctionality. The stress (≤66 MPa) and strain (≤0.096%) are in the elastic regime. The piezoresistivity is strong and negative, with gage factor -1630, and provides electrical-resistance-based stress/strain sensing. The resistance (DC) decreases monotonically and reversibly with increasing tensile stress, with the fractional decrease being up to 13%. The in-plane electric field (DC) decreases monotonically and reversibly with increasing tensile stress, with the fractional decrease being up to 29%. The capacitance (2 kHz) increases monotonically and reversibly with increasing tensile stress, with the fractional increase being up to 10%. The capacitance increase (up to 10%) relates to the increase in the relative permittivity (up to 15%). The relative permittivity is high, at 5.48 × 10 ⁴ (2 kHz) in the absence of stress. The piezoelectricity is weak, with the piezoelectric coupling coefficient d ₃₃ being -1.4 × 10 ⁻⁸ pC/N, the electric field output contributing -1.7 × 10 ⁻⁸ pC/N, the relative permittivity increase contributing +3.5 × 10 ^-8 pC/N, and the electric field and permittivity in combination contributing -5.2 × 10 ^-10 pC/N. In spite of its weakness, the direct piezoelectricity enables electric-field-based and capacitance-based stress/strain self-sensing.