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The transport properties of commercial carbon nanofibers (CNFs) produced by chemical vapor deposition (CVD) depend on the various conditions used during their growth and post-growth synthesis, which also affect their derivate CNF-based textile fabrics. Here, the production and thermoelectric (TE) properties of cotton woven fabrics (CWFs) functionalized with aqueous inks made from different amounts of pyrolytically stripped (PS) Pyrograf® III PR 25 PS XT CNFs via dip-coating method are presented. At 30 °C and depending on the CNF content used in the dispersions, the modified textiles show electrical conductivities (σ) varying between ~5 and 23 S m−1 with a constant negative Seebeck coefficient (S) of −1.1 μVK−1. Moreover, unlike the as-received CNFs, the functionalized textiles present an increase in their σ from 30 °C to 100 °C (dσ/dT > 0), explained by the 3D variable range hopping (VRH) model as the charge carriers going beyond an aleatory network of potential wells by thermally activated hopping. However, as it happens with the CNFs, the dip-coated textiles show an increment in their S with temperature (dS/dT > 0) successfully fitted with the model proposed for some doped multiwall carbon nanotube (MWCNT) mats. All these results are presented with the aim of discerning the authentic function of this type of pyrolytically stripped Pyrograf® III CNFs on the thermoelectric properties of their derived textiles. [ABSTRACT FROM AUTHOR]

The electrical conductivity (σ) and Seebeck coefficient (S) at temperatures from 40 °C to 100 °C of melt-processed polypropylene (PP) composites filled with 5 wt.% of industrial-grade carbon nanofibers (CNFs) is investigated. Transmission Electron Microscopy (TEM) of the two Pyrograf® III CNFs (PR 19 LHT XT and PR 24 LHT XT), used in the fabrication of the PP/CNF composites (PP/CNF 19 and PP/CNF 24), reveals that CNFs PR 24 LHT XT show smaller diameters than CNFs PR 19 LHT XT. In addition, this grade (PR 24 LHT XT) presents higher levels of graphitization as deduced by Raman spectroscopy. Despite these structural differences, both Pyrograf® III grades present similar σ (T) and S (T) dependencies, whereby the S shows negative values (n-type character). However, the σ (T) and S (T) of their derivative PP/CNF19 and PP/CNF24 composites are not analogous. In particular, the PP/CNF24 composite shows higher σ at the same content of CNFs. Thus, with an additionally slightly more negative S value, the PP/CNF24 composites present a higher power factor (PF) and figure of merit (zT) than PP/CNF19 composites at 40 °C. Moreover, while the σ (T) and S (T) of CNFs PR 19 LHT XT clearly drive the σ (T) and S (T) of its corresponding PP/CNF19 composite, the S (T) of CNFs PR 24 LHT XT does not drive the S (T) observed in their corresponding PP/CNF24 composite. Thus, it is inferred in PP/CNF24 composites an unexpected electron donation (n-type doping) from the PP to the CNFs PR 24 LHT XT, which could be activated when PP/CNF24 composites are subjected to that increase in temperature from 40 °C to 100 °C. All these findings are supported by theoretical modeling of σ (T) and S (T) with the ultimate aim of understanding the role of this particular type of commercial CNFs on the thermoelectrical properties of their PP/CNF composites. [ABSTRACT FROM AUTHOR]

The thermoelectric properties, at temperatures from 30 °C to 100 °C, of melt-processed poly(ether ether ketone) (PEEK) composites prepared with 10 wt.% of carbon nanofibers (CNFs) are discussed in this work. At 30 °C, the PEEK/CNF composites show an electrical conductivity (σ) of ~27 S m−1 and a Seebeck coefficient (S) of −3.4 μV K−1, which means that their majority charge carriers are electrons. The origin of this negative Seebeck is deduced because of the impurities present in the as-received CNFs, which may cause sharply varying and localized states at approximately 0.086 eV above the Fermi energy level (EF) of CNFs. Moreover, the lower S, in absolute value, found in PEEK/CNF composites, when compared with the S of as-received CNFs (−5.3 μV K−1), is attributed to a slight electron withdrawing from the external layers of CNFs by the PEEK matrix. At temperatures from 30 °C to 100 °C, the σ (T) of PEEK/CNF composites, in contrast to the σ (T) of as-received CNFs, shows a negative temperature effect, understood through the 3D variable-range hopping (VRH) model, as a thermally activated hopping mechanism across a random network of potential wells. Moreover, their nonlinear S (T) follows the same behavior reported before for polypropylene composites melt-processed with similar CNFs at the same interval of temperatures. [ABSTRACT FROM AUTHOR]

In a recent paper [R. Soto-Garrido and E. Muñoz, J. Phys. Condens. Matter 30, 195302 (2018)], we studied the electronic transport properties in Weyl semimetals (WSMs) submitted to the combined effects of torsional mechanical strain and magnetic field, showing that this configuration induces a node-polarization effect on the current that can be used to measure the torsion angle from transmission experiments. In this article, we extend our previous work to study thermoelectric transport in WSMs under torsional strain and an external magnetic field. Our analysis involves only the electronic contribution to the transport coefficients, and it is thus valid at low temperatures where the phonon contribution is negligible. We provide exact analytical expressions for the scattering cross section and the transmitted heat current, in order to calculate the thermal conductance and the Seebeck coefficient under this configuration. Our results suggest that thermoelectric transport coefficients in these materials can be engineered by appropriately tuning the magnitude of the magnetic field, torsional strain, and the applied bias or thermal gradient, leading to a potentially very high figure of merit. [ABSTRACT FROM AUTHOR]

The temperature dependent electrical conductivity σ (T) and thermopower (Seebeck coefficient) S (T) from 303.15 K (30 °C) to 373.15 K (100 °C) of an as-received commercial n-type vapour grown carbon nanofibre (CNF) powder and its melt-mixed polypropylene (PP) composite with 5 wt.% of CNFs have been analysed. At 30 °C, the σ and S of the CNF powder are ~136 S m−1 and −5.1 μV K−1, respectively, whereas its PP/CNF composite showed lower conductivities and less negative S-values of ~15 S m−1 and −3.4 μV K−1, respectively. The σ (T) of both samples presents a dσ/dT < 0 character described by the 3D variable range hopping (VRH) model. In contrast, their S (T) shows a dS/dT > 0 character, also observed in some doped multiwall carbon nanotube (MWCNT) mats with nonlinear thermopower behaviour, and explained here from the contribution of impurities in the CNF structure such as oxygen and sulphur, which cause sharply varying and localized states at approximately 0.09 eV above their Fermi energy level (EF). [ABSTRACT FROM AUTHOR]