Comparative thermoelectric properties of polypropylene composites melt-processed using Pyrograf® III carbon nanofibers

• Published in: Journal of composites science 2023-04, Vol.7 (4), p.1-16
• Main Authors: Paleo, Antonio J., Krause, Beate, Mendes, Ana R., Tavares, C. J., Cerqueira, M. F., Muñoz, Enrique, Pötschke, Petra
• Format: Article
• Language: English
• Subjects: Analysis; Carbon fibers; Carbon nanofibers; Chemical vapor deposition; Composite materials; Conductivity; Electrical modeling; Electrical resistivity; Figure of merit; Graphitization; Heat conductivity; Materials; Morphology; n-type polymer composites; Nanofibers; Polymers; Polypropylene; Power factor; Properties; Raman spectroscopy; Scanning electron microscopy; Seebeck effect; Spectrum analysis; Thermoelectric properties; Thermoelectricity; Thermoplastics
• ISSN: 2504-477X; 2504-477X
• DOI: 10.3390/jcs7040173

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. A.J.P. gratefully acknowledges support from FCT-Foundation for Science and Technology by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) co-financed by FEDER through the PT2020 program and “plurianual” 2020–2023 Project UIDB/00264/2020. C.J.T. acknowledges the funding from FCT/PIDDAC through the Strategic Funds project reference UIDB/04650/2020-2023. E.M. acknowledges support from Project ANID PIA Anillo ACT/192023.