Pyropermittivity as an emerging method of thermal analysis, with application to carbon fibers.

Pyropermittivity refers to the effect of temperature on the electric permittivity of a material. It is an emerging thermoanalytical method that is relevant to materials characterization, capacitance-based temperature sensing and thermal energy harvesting. Pyropermittivity is well-known for electrically nonconductive materials, but not conductive materials, which include structural materials (e.g., carbon fibers). This work provides the first determination of the activation energy of permittivity and pyropermittivity-based energy density for any material, and that of the temperature coefficient of permittivity (associated carrier-atom interaction) for conductive materials. Pyropermittivity is discovered in carbon fibers. The permittivity temperature coefficient is positive for uncoated/nickel-coated carbon fibers and polycrystalline graphite of prior work, despite the difference in sign of the temperature coefficient of resistivity between the fibers and graphite. The pyropermittivity is stronger when the fiber is nickel-coated, but is yet stronger for graphite. The coefficient values are all higher than or comparable to those previously reported for nonconductive materials, indicating that nonconductivity does not necessarily enhance pyropermittivity. The activation energy of permittivity is ≤ 40 meV. The pyropermittivity-based volumetric energy density is higher when the carbon fiber is nickel-coated, and is lowest for the graphite. The greater energy density when the carbon fiber is nickel-coated is in line with the greater temperature coefficient of permittivity and the higher permittivity. The low energy density of graphite relates to the low permittivity. The highest energy density obtained is 1.73 × 10–4 J m−3 (nickel-coated carbon fiber for the temperature change of 50 °C); the temperature coefficient of permittivity is 1.37 × 10–3/K for this fiber. [ABSTRACT FROM AUTHOR]