P. Pötschke, B. Krause, J. Luo
Leibniz Institute of Polymer Research Dresden (IPF),
Keywords: Seebeck effect, singlewalled carbon nanotubes, polypropylene
Summary:Thermoelectric generators (TEG), which can convert waste heat directly into electricity, are one type of promising energy harvesting devices. The thermoelectric (TE) effect (also called Seebeck effect) describes an electrical potential (voltage ∆U) induced by a temperature difference (∆T) between the two sides of a material. High Seebeck coefficient (S), high electrical conductivity (σ) and low thermal conductivity (k) are favourable for high TE efficiency with high power factor (PF= σ∙S2). The intrinsic low thermal conductivity of polymers (0.1-0.7 W/(mK)) promotes their usage as potential TE materials, in particular for applications near room temperature. Compared to high cost and rigid semiconductors, polymers are cheap, flexible and can be brought easily into different shapes. Polymers or polymer composites based TE materials can be fabricated by solution or melt processing, both representing low cost production techniques. Compared to solution processing, melt processing avoids solvents and can be easily scaled up to currently available industrial level technology. An efficient TEG requires high performance p-type (positive S) and n-type (negative S) TE materials. To avoid problems coming from different thermal expansion coefficients or corrosion effects of two different materials, it is desired to combine similar p- and n-type materials for device fabrication. An ideal scenario is to apply the same base material, which can be doped both into p- and n-type. In our study, polymer composites with an industrially widely used polymer, namely polypropylene (PP) as the matrix, were prepared by small-scale melt processing. Singlewalled carbon nanotubes (SWCNTs) were applied to construct an electrical conducting network in this insulating thermoplastic matrix. In addition, high Seebeck coefficient copper oxide (CuO) microparticles were incorporated into these composites. For the selected CNTs, electrical percolation for charge transport occurs already at 0.1 wt% CNTs. The measured conductivities are high enough to measure the Seebeck effect starting at 0.75 wt% SWCNTs. The effect of SWCNT content on σ, S, and PF was studied. It was shown that the charge carriers injected by SWCNT addition are detrimental to the Seebeck coefficient, leading to an optimized PF at 4 wt% SWCNTs. The addition of copper oxide microparticles having a high S value can enhance the thermoelectric properties. In addition, the use of an ionic liquid during melt mixing leads to higher TE values, whereas the SWCNT surface modification by plasma treatment and the variation of melt-mixing processing conditions did not show much effect. By using special processing additives during the melt mixing, like polyethylene glycol (PEG), doping of the nanotubes can be achieved resulting in negative Seebeck coefficients. To construct demonstrators, finally two composites were selected: p-type PP/ 2wt% SWCNT composite with 5 wt% CuO (with S up to 45 μV/K), and n-type composite (with S up to -56 μV/K) using the same composition and 10 wt% PEG. The two prototypes with 4 and 49 thermocouples of these p- and n-type composites delivered output voltages of 21 mV and 110 mV, respectively, at a temperature gradient of 70 K.