Nonlinear Thermopower Behaviour of N-Type Carbon Nanofibres and Their Melt Mixed Polypropylene Composites

Abstract

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).

Electronic Features of Cotton Fabric e-Textiles Prepared with Aqueous Carbon Nanofiber Inks

“… 40 Likewise, the W D (absolute value) for the CNF powder (46 meV) is close to the activation energy (60 meV) reported for n-type graphitized carbon fibers in the 250–750 K interval. 41 Notably, the CNF powder used in this study shows W D < 0, which confirms the results found in a precedent work for CNFs Pyrograf III PR 19 LHT XT 26 (it is reminded that the Pyrograf III PR 24 LHT XT grade is used in this study). This negative W D can be explained by the presence of impurities such as the oxygen (∼1.8%) and sulfur (∼0.1%) detected by XPS.…”

“…It is noteworthy that π−π* peaks are also detected in the C 1s spectra recorded for CWF@3.2 CNF and CWF@6.4 CNF (Figure 5g,i), which means that the signal from the CNFs is stronger in those samples. The O 1s spectra in as-received CNFs (Figure 5b) yielded peaks at ∼531.9 and ∼533.5 eV assigned to C�O and C�O, respectively, 26 whereas in CWF, the peak at 532.1 eV can be associated with both C�O and C�O, which it is then shifted to ∼532.2 eV in CWF@6.4 CNF (Figure 6j). 27 In summary, the Raman and XPS analysis seem to match well with the SEM images.…”