Polypropylene-based melt mixed composites with singlewalled carbon nanotubes for thermoelectric applications: Switching from p-type to n-type by the addition of polyethylene glycol Jinji Luo a , Giacomo Cerretti b , Beate Krause a , Long Zhang c , Thomas Otto d , Wolfgang Jenschke a , Mathias Ullrich a , Wolfgang Tremel b , Brigitte Voit a, e , Petra P € otschke a, * a Leibniz-Institut für Polymerforschung Dresden e.V. (IPF), Hohe Str. 6, D-01069, Dresden, Germany b Institut für Anorganische und Analytische Chemie, Johannes-Gutenberg-Universit€ at Mainz, Duesbergweg 10-14, 55128, Mainz, Germany c Leibniz-Institut für Festk€ orper- und Werkstoffforschung Dresden e.V. (IFW), Helmholtzstr. 20, 01069, Dresden, Germany d Fraunhofer-Institut für Elektronische Nanosysteme, Technologie-Campus 3, 09126, Chemnitz, Germany e Technische Universit€ at Dresden, Organic Chemistry of Polymers, 01062, Dresden, Germany article info Article history: Received 22 September 2016 Received in revised form 5 December 2016 Accepted 7 December 2016 Available online 7 December 2016 Keywords: Thermoelectric Composite Polymer Carbon nanotube Copper oxide n-type abstract The thermoelectric properties of melt processed conductive nanocomposites consisting of an insulating polypropylene (PP) matrix filled with singlewalled carbon nanotubes (CNTs) and copper oxide (CuO) were evaluated. An easy and cheap route to switch p-type composites into n-type was developed by adding polyethylene glycol (PEG) during melt mixing. At the investigated CNT concentrations of 0.8 wt% and 2 wt% (each above the electrical percolation threshold of ~0.1 wt%), and a fixed CuO content of 5 wt%, the PEG addition converted p-type composites (positive Seebeck coefficient (S)) into n-type (negative S). PEG was also found to improve the filler dispersion inside the matrix. Two composites were prepared: P- type polymer/CNT composites with high S (up to 45 mV/K), and n-type composites (with S up to 56 mV/ K) through the addition of PEG. Two prototypes with 4 and 49 thermocouples of these p- and n-type composites were fabricated, and delivered an output voltage of 21 mV and 110 mV, respectively, at a temperature gradient of 70 K. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction A thermoelectric generator (TEG) is an energy harvesting device that can convert waste heat directly into electricity. It consists of multiple p-type and n-type thermoelectric (TE) materials that are connected electrically in series and thermally in parallel. The performance of a TE material is evaluated by a dimension- less figure of merit ZT (ZT ¼ sS 2 T/k), where s is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conduc- tivity and the numerator sS 2 is defined as the power factor [1]. Depending on the dominant charge carrier type, the Seebeck co- efficient can be positive (holes, p-type) or negative (electrons, n- type). For room temperature applications, semiconductors (e.g. bulk bismuth telluride (Bi 2 Te 3 ) alloys), are widely used as TE materials due to their high power factors [2]. However, it is difficult to reduce the high thermal conductivity of these materials (e.g. Bi 2 Te 3 has a k of 1.2 W/(m$K)) to a value lower than 1 W/(m$K) so that their ZT is still around 1. The toxicity and scarcity of employed component (e.g. Te) are of concern. Their rigidity and high pro- duction cost limit semiconductor based TE materials to niche ap- plications [3]. On the contrary, organic materials are flexible and contain abundant atoms (mostly C, H, O) [4]. Polymers are widely available, can be easily processed into different shapes and have much lower cost. In addition, pure polymers in general have intrinsic low thermal conductivity ranging from 0.1 to 0.6 W/(m$K) [5], which is one of the desired TE parameters. They can be processed either in the solution or melt state, both of which could be scaled up for mass fabrication. In comparison, melt processing is more environmental friendly as it does not require the use of solvents and enables the production of larger amounts of material. * Corresponding author. E-mail address: poe@ipfdd.de (P. P€ otschke). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2016.12.019 0032-3861/© 2016 Elsevier Ltd. All rights reserved. Polymer 108 (2017) 513e520