Thermoelectric properties of polypropylene carbon nanofiber melt-mixed composites: exploring the role of polymer on their Seebeck coefficient

Abstract

The effect of polypropylene (PP) on the Seebeck coefficient (S) of carbon nanofibers (CNFs) in melt-extruded PP composites filled with up to 5 wt. % of CNFs was analyzed in this study. The as-received CNFs present an electrical conductivity of ~320 S m−1 and an interesting phenomenon of showing negative S-values of −5.5 μVK−1, with 10−2 µW/mK2 as the power factor (PF). In contrast, the PP/CNF composites with 5 wt. % of CNFs showed lower conductivities of ~50 S m−1, less negative S-values of −3.8 μVK−1, and a PF of 7 × 10−4 µW/mK2. In particular, the change in the Seebeck coefficient of the PP/CNF composites is explained by a slight electron donation from the outer layers of the CNFs to the PP molecules, which could reduce the S-values of the as-received CNFs. Our study indicates that even insulating polymers such as PP may have a quantifiable effect on the intrinsic Seebeck coefficient of carbon-based nanostructures, and this fact should also be taken into consideration to tailor conductive polymer composites with the desired thermoelectric (TE) properties. The higher negative Seebeck coefficients (S) at 30 °C of as-received carbon nanofibers (CNFs) with respect to their polypropylene carbon nanofiber (PP/CNF) melt-mixed composites are explained by a slight electron donation from the outer graphitic shells of the CNFs to the PP molecules. Our study denotes that, contrary to expectations, insulating polymers may play a non-negligible role on the final S-values of conductive polymer composites composed of carbon-based nanostructures.