Polyethylene Glycol as Additive to Achieve N-Conductive Melt-Mixed Polymer/Carbon Nanotube Composites for Thermoelectric Application

Abstract

The development of thermoelectric (TE) materials based on thermoplastic polymers and carbon nanotubes is a focus of current TE research activities. For a TE module, both p- and n-conductive composites are required, whereby the production of n-conductive materials is a particular challenge. The present study investigates whether adding polyethylene glycol (PEG) as n-dopant during the melt-mixing of the conductive composites based on polycarbonate, poly(ether ether ketone), or poly(butylene terephthalate) with singlewalled carbon nanotubes (0.5 to 2 wt%) is a possible solution. It was shown that for all three polymer types, a change in the sign of the Seebeck coefficient from positive to negative could be achieved when at least 1.5 wt% PEG was added. The most negative Seebeck coefficients were determined to be −30.1 µV/K (PC), −44.1 µV/K (PEEK), and −14.5 µV/K (PBT). The maximal power factors ranged between 0.0078 µW/m·K2 (PC), 0.035 µW/m·K2 (PEEK), and 0.0051 µW/m·K2 (PBT).

Influence of Polyvinylpyrrolidone on Thermoelectric Properties of Melt-Mixed Polymer/Carbon Nanotube Composites

“…To overcome the problems with PVP, polyethylene glycol (PEG) is a suitable alternative additive for n-doping in polymer/SWCNT composites. It was shown that for PP [ 5 ] as well as PC, PBT, and PEEK [ 18 ] based composites with SWCNTs the addition of PEG leads to negative Seebeck coefficients. Unfortunately, only Luo et al [ 5 ] investigated the long-term stability after 8 months of storage in air.…”

“…After cooling down, the obtained extruded strands were compression molded into plates (60 mm diameter, 0.5 mm thickness) using the hot press PW40EH. The preparation conditions for melt compounding and compression molding are described in Krause et al [ 18 ].…”

“…The thermoelectric (TE) characterization was carried out in a measuring device developed and constructed at IPF Dresden [ 30 ]. The measuring temperature was set to 40 °C with four or eight temperature variations up to ±8 K. More details are given in Krause et al [ 18 ]. The measurements were performed within two weeks after sample preparation.…”

“… SEM image of PBT composites with 2 wt% SWCNT ( a ) [ 18 ] and PBT with 2 wt% SWCNT + 10 wt% PVP ( b , c ). …”

“…Recently, it could be shown that the incorporation of p-type SWCNTs can lead to n-type composites when using specific polymer matrices, such as acrylonitrile butadiene styrene (ABS) [ 9 ] or different kinds of polyamides [ 9 , 13 ]. As further alternatives to produce n-type composites, n-type CNTs can be incorporated, such as nitrogen-doped CNTs [ 14 ], or p-type CNTs can be combined with additives such as ionic liquids [ 15 ], polyethylenimine (PEI) [ 16 ], or polyethylene glycol (PEG) [ 5 , 17 , 18 ] before being added to the polymer matrix. In this context, the stability of the n-type character and the negative Seebeck coefficients over the storage time of the composite is a crucial point, which unfortunately has not been studied sufficiently and reported in the literature.…”