Effects of cold work, stress and temperature on the dielectric behavior of copper

This work reports for the first time the effect of cold work (mechanical rolling) on the dielectric behavior of a conductive dielectric, namely copper (not poled). The positive end of the electret's voltage is where the rolling-induced plastic flow originates. Cold work (37%) increases the relative permittivity κ (2 kHz) from 2380 to 2660, and increases the fraction of carriers that participate in the electret through interaction with the atoms from 4.8 × 10⁻⁶ to 4.3 × 10⁻⁴, in addition to increasing the electret's DC electric field E, power density and energy density by 1730%, 30,500% and 135,000%, respectively. The cold work slows down the electret discharge/charge, but decreases the discharge/charge time per unit participating charge. Annealing after cold work greatly decreases E and the power density. The microstructure, which is obviously affected by cold work and annealing, greatly affects the behavior. During short-circuited discharge and subsequent open-circuited charge, E changes reversibly. The E increases linearly with increasing inter-electrode distance. The electret is also supported by the directional asymmetry in the polarization-induced apparent resistance upon polarity reversal. The effects of tensile stress (elastic regime) and temperature on the dielectric behavior are also reported. The cold work weakens the positive piezopermittivity (the fractional change in κ per unit strain decreasing from 4.0 × 10³ to 9.7 × 10²), negative piezoelectret (the piezoelectret coefficient changing from −1.64 × 10⁻⁹ pC/N to −2.82 × 10⁻¹⁰ pC/N) and negative piezoresisitivty (the gage factor changing from −342 to −167). This work also reports the pyropermittivity (change of κ with temperature) and pyroelectret (change of E with temperature), which are useful for temperature sensing and thermal energy harvesting. The dielectric/electret behavior is strengthened by mild heating. Upon heating from 20 °C to 70 °C, κ (2 kHz) increases reversibly from 2660 to 3200 and E (DC) increases reversibly by 800%, corresponding to pyroelectret coefficient 9.2 × 10⁻¹⁴ C/(m². K).