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

Antonio J Paleo,Enrique Muñoz,Petra Pötschke,Ana M Rocha,Maria F Cerqueira,Beate Krause

doi:10.3390/polym14020269

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).

Highlights

Thermoelectric (TE) materials are able to transform waste heat into electrical energy. Their efficiency is characterized by the dimensionless figure of merit zT = Skσ T where the thermopower (Seebeck coefficient) (S), calculated as S = ∆V/∆T, reflects the creation of a potential difference (∆V) when the ends of a TE material are exposed to a temperature difference, T is the absolute temperature, and k is the thermal conductivity [1,2]
Of carbon nanofibre (CNF) in the temperature range between 30 ◦ C and 100 ◦ C have been analysed in this study
The PP/CNF composite, for its part, shows lower conductivities of ~15 S m−1, and less negative S-values of −3.4 μV K−1, which correspond to a power factor (PF) of 1.8 × 10−4 μW m−1 K−2 at 30 ◦ C

Summary

Introduction

Thermoelectric (TE) materials are able to transform waste heat into electrical energy. Their efficiency is characterized by the dimensionless figure of merit zT = Skσ T where the thermopower (Seebeck coefficient) (S), calculated as S = ∆V/∆T, reflects the creation of a potential difference (∆V) when the ends of a TE material are exposed to a temperature difference, T is the absolute temperature, and k is the thermal conductivity [1,2]. The combination of their difficult processibility together with their high cost, partial toxicity, geopolitical risks and poor flexibility have increased interest in searching for different TE materials [6]. In this context, organic materials mainly including conducting polymers (CPs) such as poly(3,4-ethylene dioxythiophene)

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Conclusion