Comparative Thermoelectric Properties of Polypropylene Composites Melt-Processed Using Pyrograf® III Carbon Nanofibers

Abstract

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.

Thermoelectric Properties of Cotton Fabrics Dip-Coated in Pyrolytically Stripped Pyrograf® III Carbon Nanofiber Based Aqueous Inks

“…Thus, over this interval of temperatures (30 °C–94 °C), the CNF powder presents a positive temperature effect (PTC) or dσ/dT < 0, defined as the increase in the electrical resistivity during a heating process. Notably, this matches well to the PTC found for Pyrograf ® III CNFs PR 24 LHT XT and PR 19 LHT XT [ 32 ]. From this, it can be deduced that, despite the three Pyrograf ® III CNFs (PR 24 LHT XT, PR 19 LHT XT and PR 25 PS XT) showing different structures, all of them present similar σ (T) behaviors.…”

“…Figure 5 and Table 2 show the experimental Seebeck coefficient of the coated textiles and as-received CNF powder at 30 °C. The CNF powder shows n-type character (−0.60 ± 0.03 μV∙K −1 ), which is in agreement with other Pyrograf ® III CNF grades (PR 24 LHT XT and PR 19 LHT XT [ 32 ]). This is not normally observed with similar carbon allotropes (e. g. CNTs) due to their p-doping with the environment [ 33 ].…”

“…This is not normally observed with similar carbon allotropes (e. g. CNTs) due to their p-doping with the environment [ 33 ]. Notably, the S of this grade is lower (in absolute value) than the S of ~ −5 μV∙K −1 measured in other Pyrograf ® III CNF grades [ 32 ]. This clearly reveals that the Pyrograf ® III grades (PR 24 LHT XT and PR 19 LHT XT) show higher S-values than the Pyrograf ® III PR 25 PS XT used here, which could be related with the lower levels of graphitization of this latter grade.…”

Melamine Network as a Solution for Significant Enhancement of the Mechanical, Adsorptive, and Surface Properties in a Novel Carbon Nanomaterial–Silica Aerogel Composite

“…The large size of the CNFs blocked the mesoporous structure of the silica aerogel and S BET dropped from 1199 to 848 m 2 ·g –1 with the addition of 0.5 wt % CNF. Meador et al observed a similar trend when incorporating up to 5 wt % of CNF (with a diameter of 100 nm and length of >100 μm) in their TMOS silica aerogels cross-linked with di-isocyanate. Lamy-Mendes et al followed two routes to modify the surface of MWCNT to achieve a better affinity between the MWCNT and silica sol: one with HNO 3 and the other with HNO 3 along with silanization.…”

Polypropylene Color Masterbatches Containing Layered Double Hydroxide Modified with Quinacridone and Phthalocyanine Pigments—Rheological, Thermal and Application Properties

“…Polypropylene is one of the most important commodity plastics used in variety of applications, among them fibers [1], composite materials [2,3], and food packaging products [4]. Commonly, additives such as stabilizers and plasticizers [4], fillers [5,6], flame retardants [1] or dyes and pigments [7,8] are incorporated into polypropylene to adjust the mechanical and processing properties and to improve appearance.…”