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End date: 2020 × Start date: 2020 × DDC: 620 × Has files: Yes × Version: publishedVersion × Subject: Thermoelectric × WGL Institute: IPF ×

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    Boron doping of SWCNTs as a way to enhance the thermoelectric properties of melt‐mixed polypropylene/SWCNT composites
    (Basel : MDPI, 2020) Krause, Beate; Bezugly, Viktor; Khavrus, Vyacheslav; Ye, Liu; Cuniberti, Gianaurelio; Pötschke, Petra
    Composites based on the matrix polymer polypropylene (PP) filled with single‐walled carbon nanotubes (SWCNTs) and boron‐doped SWCNTs (B‐SWCNTs) were prepared by melt‐mixing to analyze the influence of boron doping of SWCNTs on the thermoelectric properties of these nanocomposites. It was found that besides a significantly higher Seebeck coefficient of B‐SWCNT films and powder packages, the values for B‐SWCNT incorporated in PP were higher than those for SWCNTs. Due to the higher electrical conductivity and the higher Seebeck coefficients of B‐SWCNTs, the power factor (PF) and the figure of merit (ZT) were also higher for the PP/B‐SWCNT composites. The highest value achieved in this study was a Seebeck coefficient of 59.7 μV/K for PP with 0.5 wt% B‐SWCNT compared to 47.9 μV/K for SWCNTs at the same filling level. The highest PF was 0.78 μW/(m∙K2) for PP with 7.5 wt% B‐SWCNT. SWCNT macro‐ and microdispersions were found to be similar in both composite types, as was the very low electrical percolation threshold between 0.075 and 0.1 wt% SWCNT. At loadings between 0.5 and 2.0 wt%, B‐SWCNT‐based composites have one order of magnitude higher electrical conductivity than those based on SWCNT. The crystallization behavior of PP is more strongly influenced by B‐SWCNTs since their composites have higher crystallization temperatures than composites with SWCNTs at a comparable degree of crystallinity. Boron doping of SWCNTs is therefore a suitable way to improve the electrical and thermoelectric properties of composites. © 2020 by the authors.
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    Nitrogen-Doped Carbon Nanotube/Polypropylene Composites with Negative Seebeck Coefficient
    (Basel : MDPI, 2020) Krause, Beate; Konidakis, Ioannis; Arjmand, Mohammad; Sundararaj, Uttandaraman; Fuge, Robert; Liebscher, Marco; Hampel, Silke; Klaus, Maxim; Serpetzoglou, Efthymis; Stratakis, Emmanuel; Pötschke, Petra
    This study describes the application of multi-walled carbon nanotubes that were nitrogen-doped during their synthesis (N-MWCNTs) in melt-mixed polypropylene (PP) composites. Different types of N-MWCNTs, synthesized using different methods, were used and compared. Four of the five MWCNT grades showed negative Seebeck coefficients (S), indicating n-type charge carrier behavior. All prepared composites (with a concentration between 2 and 7.5 wt% N-MWCNTs) also showed negative S values, which in most cases had a higher negative value than the corresponding nanotubes. The S values achieved were between 1.0 µV/K and −13.8 µV/K for the N-MWCNT buckypapers or powders and between −4.7 µV/K and −22.8 µV/K for the corresponding composites. With a higher content of N-MWCNTs, the increase in electrical conductivity led to increasing values of the power factor (PF) despite the unstable behavior of the Seebeck coefficient. The highest power factor was achieved with 4 wt% N-MWCNT, where a suitable combination of high electrical conductivity and acceptable Seebeck coefficient led to a PF value of 6.1 × 10−3 µW/(m·K2). First experiments have shown that transient absorption spectroscopy (TAS) is a useful tool to study the carrier transfer process in CNTs in composites and to correlate it with the Seebeck coefficient.