Probing the Carrier Dynamics of Polymer Composites with Single and Hybrid Carbon Nanotube Fillers for Improved Thermoelectric Performance

Abstract

The incorporation of carbon nanotubes (CNTs) within polymer hosts offers a great platform for the development of advanced thermoelectric (TE) composite materials. Over the years, several CNT/polymer composite formulations have been investigated on an effort to maximize the TE performance. Meanwhile, several studies focused on the decay dynamics of the charged excitons within CNTs itself and therefrom derived structures, aiming to investigate the lifetimes and the corresponding recombination processes of free charge carriers. The latter physical phenomena play a crucial role in the performance of various types of energy converting and scavenging materials. Nevertheless, up to this date, there is no systematic study on the combination of TE parameters and the critical charge carrier dynamics within CNT containing TE polymer composites. Herein, a variety of composites with single and hybrid CNT fillers based on polycarbonate (PC) and polyether ether ketone (PEEK) polymer matrices were prepared by melt-mixing in small scale. At the same loading, the addition of single fillers in PC results in higher Seebeck coefficients and similar conductivities when compared to the use of hybrid filler systems. In contrast, with hybrid filler systems in PEEK composites, higher power factors could be reached than in single filler composites. Moreover, the PC-based composites are studied using ultrafast laser time-resolved transient absorption spectroscopy (TAS), for the investigation of the exciton lifetimes and the physical origins of free charge carrier transport within the TE films. The findings of this study reveal interesting links between the TE parameters and the obtained charge carrier dynamics. Namely it is found that the Seebeck coefficient of the composites relates directly to the exciton lifetimes, whereas the volume conductivity is independent of the exciton lifetimes and is determined mainly by the average number and the mobility of the free charged electrons at the higher energy states.

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

“…The thermoelectric properties of melt-mixed composites based on PC, PEEK, and PBT filled with SWCNT Tuball TM were reported before in [ 11 , 27 ]. The Seebeck coefficients of the three polymer composite series are shown in Figure 4 .…”

“…In [ 11 ], the S-values of the MWCNT powders were determined to be 6.3 µV/K for MWCNTs of the type NC7000 and 10.1 µV/K for MWCNTs of the type CNSPEG. For PC composites, only about 9 µV/K (1–2 wt% NC7000) or about 16 µV/K (0.5–2 wt% CNSPEG) could be achieved [ 27 ]. In the case of PEEK composites, only about 7 µV/K (3–5 wt% NC7000) or about 7–16 µV/K (0.5–5 wt% CNSPEG) could be measured [ 27 ].…”

“…For PC composites, only about 9 µV/K (1–2 wt% NC7000) or about 16 µV/K (0.5–2 wt% CNSPEG) could be achieved [ 27 ]. In the case of PEEK composites, only about 7 µV/K (3–5 wt% NC7000) or about 7–16 µV/K (0.5–5 wt% CNSPEG) could be measured [ 27 ]. Even in the case of PBT, the S-values of the composites were only 6.8 µV/K (2 wt% NC7000) or 15.7 µV/K (2 wt% CNSPEG) when MWCNTs were incorporated [ 11 ].…”

“…The carbon nanotubes selected for this study are SWCNT Tuball TM (OCSIAl, Luxembourg) with a carbon content of 75% [ 25 , 26 ]. RAMAN spectra of this CNT type are published in the supporting information of [ 27 ]. Thermoelectric parameters of the SWCNTs powder were described in [ 11 ].…”

“… Seebeck coefficients (at 40 °C) of PC, PEEK, and PBT-based composites filled with SWCNT Tuball ™ (values based on Refs. [ 11 , 27 ], newly compiled. Adapted with permission from reference [ 27 ].…”