Pyropermittivity and pyroelectret behavior of graphite

Abstract

This work reports the pyropermittivity (effect of temperature on the relative electric permittivity κ) and pyroelectret behavior (effect of temperature on the electret’s inherent DC electric field E, with an electret meaning a permanent electric dipole) of graphite, namely isotropic (polycrystalline) graphite with DC resistivity ρ (1.16 ± 0.08) × 10−5 Ω.m, κ 530 ± 18 at 2 kHz, and E 1.30 × 10−5 V/m at room temperature. These effects are to be distinguished from the pyroelectric effect. They pertain to the dielectric behavior, which stems from the carrier-atom interaction. They are relevant to temperature sensing and the conversion of thermal energy to electrical energy. Pyropermittivity allows temperature sensing by capacitance measurement. Pyroelectret allows temperature sensing by electric field measurement. The material is a true electret, i.e., an electret that does not require poling. Upon heating (20–70 °C), κ increases by 32% (positive pyropermittivity), ρ increases by 70% (positive pyroresistivity, reflecting the metallic character), and E increases by 1900% (positive pyroelectret). As a result, the electric power volumetric/gravimetric density due to the electret (important for energy conversion) increases by 24000%. The increase in ρ upon heating promotes the dielectric behavior. All effects are reversible upon cooling. The pyroelectret coefficient is (1.53 ± 0.04) × 10−14 C/(m2.K) – first report for materials in general.