Electret behavior discovered in solder, specifically tin–silver

An electret is a permanent electric dipole exhibited by a material. This work provides the first report of the electret behavior of a solder, which is Sn–4Ag in the form of a fluxless unpoled wire of diameter 2.79 mm. The electret voltage increases linearly with the inter-electrode distance l, due to Ohm’s Law. The electret’s electric field also increases with l, increasing by 2.8% upon increasing l from 100 to 1000 mm. The electret discharges upon short-circuiting (with the electric field decreasing to zero) and charges back upon subsequent open-circuiting (with the electric field fully restored). For l = 1000 mm, the voltage is 0.77 µV, the electric field is 0.77 µV/m, the current is 32 µA, the discharge/charge time is 45 s, and the fraction of valence electrons that participate is 3.5 × 10⁻⁹, which is much greater than the fraction of valence electrons that participate in the AC permittivity (up to 3.6 × 10⁻¹⁴, decreasing with increasing l), which is in turn much greater than the fraction of valence electrons that participate in DC-current-induced polarization (up to 5.4 × 10⁻²¹). The electret’s electric field is lower for solder than previously reported copper or steel. The fraction of carriers that participate in the discharge is much smaller for solder than copper. The discharge time is shorter for solder than copper, but the discharge time per unit participating charge is longer for solder than copper. The microstructure is believed to primarily govern the electret behavior, though the resistivity affects the permittivity and discharge time.