Electret behavior of unpoled carbon fiber with and without nickel coating

Unpoled continuous polyacrylonitrile-based carbon fiber is an electret, as tested along the fiber direction of a 12,000-fiber tow. The open-circuit DC electric field before (E₁) and after (E₂) polarity reversal are opposite in sign, with │E₂│ slightly less than E₁, which is contributed by voltmeter-current-induced polarization. Both E₁ and │E₂│increase linearly with inter-electrode distance l, with E₁ varying with l more significantly. At l = 140 mm,│E₂│ is 5.5 × 10⁻⁵ V/m. The short-circuit current density is 3.5 A/m². Both E₁ and │E₂│ decrease to zero upon discharge (short-circuited) and recover upon charge (open-circuited), though E₁ is partly contributed by polarization. For l = 140 mm, discharge-charge testing gives participating charge density 1.8 × 10³ C/m³, participating carrier density 1.2 × 10²² m⁻³, fraction of carriers that participate 1.7 × 10⁻³ and volumetric power density 1.9 × 10⁻⁴ W/m³. Discharge and charge in multiple cycles behave similarly. During discharge/charge, the electric field changes faster than the charge amount. Nickel coating increases the participating charge density, volumetric/gravimetric power density and current density, but decreases the electric field and the time for discharge completion divided by the participating charge. These results and previously reported inverse proportionality of capacitance with l support modeling an electret as electric dipoles of length much smaller than l in series electrically.