New concept of electret-based capacitance, as shown for solder and other conductors

An electret refers to a permanent electric dipole. Nonconductive electrets (polymers and ceramics) have long been studied and utilized, but conductive electrets (metals and carbons) are emerging. This paper introduces the concept of the electret-based capacitance C′ to elucidate the discharge behavior of conductive electrets in the form of unpoled solder and other conductors (metals). The C′ is defined as the electret charge Q′ (determined by integrating the discharge curve) divided by the DC electret voltage V′. The conventional permittivity-based capacitance C (measured) is lower than C′ by orders of magnitude. For Sn–4Ag lead-free solder, C′ (given by Q′/V′) is 620 F, whereas C is 175 pF. The electret stems from the interaction of a small fraction of the carriers (free electrons) with the atoms, as supported by DC polarization asymmetry. For Sn–4Ag, this fraction is 3.5 × 10^–9. Electret discharge (V′ decreasing to zero) occurs upon short circuiting. Self-charge (V′ restored) occurs upon subsequent open circuiting, being slightly more sluggish than discharge. The electret discharge/charge amounts to discharge/charge of C′. The discharge/charge time constant is found to approximately equal RC′ (R = resistance) and the discharge/charge energy is found to approximately equal ½ C′V′², as expected for the discharge/charge of C′ and shown for Sn–4Ag, copper (0 and 37% cold work) and low-carbon steel. This two-fold agreement between theory and experiment for each of the metals studied strongly supports the C′ concept. Strong correlation occurs among high fraction of carriers that participate, high participating charge density and high discharge time constant.