Dielectric behavior of graphite, with assimilation of the AC permittivity, DC polarization and DC electret

The dielectric behavior of polycrystalline graphite is addressed by assimilating the DC electric polarization, AC permittivity, DC electret behavior, and their grain-size dependence. The AC-permittivity-related polarization is weaker than the DC-electret-related polarization, involving fewer carriers, because the depolarization is slower than polarization. The DC-current-induced polarization is even weaker than the AC-permittivity-related polarization, involving even fewer carriers. A decrease in the grain size increases the fraction of carriers that participate in the DC-polarization/AC-permittivity/DC-electret through carrier-atom interaction at the grain boundaries, thereby increasing the permittivity and the electret's electric field. The electric field increases with increasing resistivity and increasing inter-electrode distance l. DC-polarization/DC-depolarization and electret discharge/charge are faster for a smaller grain size. Both electric field and the reciprocal of the capacitance increase linearly with l, suggesting a model of dipoles in series. The amount of charge at each end of the overall dipole is greater for a smaller grain size, while the discharge time per unit electret-participating charge is shorter. In spite of the decrease in the fraction of carriers that participate with increasing l, the electret energy density is much increased by increasing l, because it depends on the power density and discharge time, both of which increase with l. © 2021 Elsevier Ltd