Thermoelectric analysis of Pyrograf® III carbon nanofiber/polypropylene composites with considerable high positive Seebeck coefficients

Abstract

This study presents, for the first time, the thermoelectric (TE) properties of as provided Pyrograf® III PR 24 HHT XT carbon nanofibers (CNFs) and their effect on the TE properties of polypropylene (PP) processed by melt mixing with 6% by weight of these CNFs. At 30 °C, the CNFs present a modest σ of ∼37 S m−1 and a considerable high S of ∼40 μVK−1, equivalent to a power factor (PF) of 6 × 10−2 μW m−1 K−2, which makes them the CNFs with the greatest TE properties among the others Pyrograf® III grades. As for the PP/CNF composites, they show lower σ and S at 30 °C, with values of ∼7 × 10−2 S m−1, and ∼ 38 μVK−1, respectively, equivalent to a PF of 1 × 10−4 μW m−1 K−2. From the TE results of the CNFs and their derived PP/CNF composite, a simplified quantum chemical model is applied to investigate the transfer of electrons (n-type doping) from PP to CNFs, concluding that this transfer depends on the adsorption mode of PP molecules and CNFs at their interface. Moreover, the σ of the CNFs and PP/CNF composite show negative temperature effect (NTC) in the interval from 30 to 100 °C, explained as thermally activated hopping mechanism by the 3D variable range hopping (VRH) model, while their S shows nonlinear character, also observed in others Pyrograf® III grades and derived PP/CNF composites, in that interval of temperature. These results are presented with the aim of determining the role played by this type of CNFs, on the thermoelectric properties of the polymer composites melt mixed using them, with a view to their use as p-type materials in thermoelectric generators (TEGs).