Abstract
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.
Highlights
The transformation of thermally wasted energy into an induced voltage, according to the Seebeckor thermoelectric (TE) effect, is a very attractive approach to contribute to higher energy efficiency [1,2,3]
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
PP nanocomposites filled with p-type CNTs always had a positive S coefficient, while PP nanocomposites filled with n-type MWCNTs had a negative S coefficient
Summary
The transformation of thermally wasted energy into an induced voltage, according to the Seebeckor thermoelectric (TE) effect, is a very attractive approach to contribute to higher energy efficiency [1,2,3]. A very attractive approach to obtain n-type materials by doping carbon structures with nitrogen is, the direct incorporation into the hexagonal carbon lattice of the nanotube walls during the synthesis process, e.g., in chemical vapor deposition [26,27], which is scalable and saves time and costs This is a suitable way to reach long-term stable n-doped nanotubes. By using PP based p- and n-type composites, they were able to prepare a demonstrator of a TE generator, which generated with 49 leg pairs a thermovoltage of 110 mV at 70 K temperature difference Another way to generate n-type PP composites was described recently by Paleo et al [45], who incorporated 1 to 5 wt% carbon nanofibers into a PP matrix and reported Seebeck coefficients of about −8.5 μV/K. TAS measurements on the composites were performed to investigate the exciton dynamics of the polypropylene composites and correlated with the Seebeck coefficient
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