Improving Thermoelectric Performance of Polymer Composites with Carbon Nanotubes

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Aug 9 2022

In an article recently published in the journal ACS Applied Energy Materials, researchers discussed the improved thermoelectric performance by investigating the carrier dynamics of polymer composites with single and hybrid carbon nanotube fillers.

Study: Probing the Carrier Dynamics of Polymer Composites with Single and Hybrid Carbon Nanotube Fillers for Improved Thermoelectric Performance. Image Credit: Forance/Shutterstock.com

Background

Carbon nanotubes (CNTs) have garnered a lot of attention from scientists over the years. A viable method for developing enhanced thermoelectric (TE) nanocomposite materials for converting waste heat to electrical energy involves incorporating CNTs into suitable intrinsically conductive (ICP) or nonconductive (particularly thermoplastic) polymers.

In an effort to improve the performance of these created melt-mixed TE polymer composite films, multi-walled CNTs (MWCNTs), single-walled CNTs (SWCNTs), and boron- or nitrogen-doped CNTs have all been used. A range of thermoplastic polymer matrices was described in the literature to produce such TE materials, depending on the intended application of the composites.

Notably, TE topologies with advanced technology and greater performance have been developed over the years using materials other than CNTs. The transient exciton dynamics of CNTs, meantime, have been the focus of numerous investigations during the past ten years. In particular, time-resolved transient absorption spectroscopy (TAS) using an ultrafast laser is a fantastic tool for examining the processes of exciton generation and free charge carrier transfer occurring at SWCNTs in the picosecond timescale.

According to research using time-resolved spectroscopy, all forms of excitonic reactions occurring inside SWCNTs have revealed recombination times that are many picoseconds long. However, investigations and particularly a systematic correlation of the CNT filler type and the concentration influence on the transient exciton dynamics and consequently on the TE performance of polymer-based composite systems are limited to date, despite the fact that both are thought to be crucial.

About the Study

In this study, the authors discussed the development of a range of composites based on polycarbonate (PC) and polyether ether ketone (PEEK) polymer matrices with single and hybrid CNT fillers by small-scale melt-mixing. The usage of hybrid-filler systems was compared to the inclusion of single fillers in PC, and greater Seebeck coefficients and comparable conductivities were obtained at the same loading. In contrast, PEEK composites with hybrid filler systems could achieve better power factors than composites with a single filler.

The team concentrated examined the PC-based composites using ultrafast laser time-resolved transient absorption spectroscopy to investigate the exciton lifetimes and the physical causes of free charge carrier transit within TE films. The results of this investigation showed intriguing relationships between the measured charge carrier dynamics and the TE parameters. In particular, it was discovered that the volume conductivity of composites was primarily influenced by the average number and mobility of the free-charged electrons at higher energy states and was independent of the exciton lifetimes. In contrast, the Seebeck coefficient of composites was directly related to the exciton lifetimes.

The researchers created a variety of TE composites made from PC and PEEK melt-mixed with single-filler and hybrid-filler CNTs. Microscopy techniques were used to characterize the resulting thin films, and their TE power conversion efficiency was assessed in terms of the Seebeck coefficient and electrical volume conductivity. Additionally, Hall measurements were used to look into the mobility of the charge carriers in both single and hybrid CNT composite systems with PC. The resulting PC/CNT composites' exciton dynamics and related charge carrier lifetimes were fully investigated using ultrafast laser time-resolved TAS.

By showing how the TE conversion efficiency was connected to the excitonic properties of the integrated CNTs, the results of this work provided light on the physical causes of the efficiency. In particular, it was shown that the Seebeck coefficient of both single- and hybrid-filler composites demonstrated a direct association with the CNTs' exciton lifespan while being essentially independent of the concentration of CNTs in the host polymer.

Observations

The amount of S dropped as more MWCNT CNS-PEG filler was added to the composite that contained 0.5 wt.% Tuball, while an improvement on the was observed. The PC/Tuball, PC/CNS-PEG, and PC/NC7000 composite polymers had acquired lifetimes of 2, 1.2, and 0.8, respectively, with S-values of 36.7, 15.8, and 9.2 μV/K. When the filler percentage was increased from 0.5 to 2 wt.% for the NC7000 composites, σ improved from 0.04 to 8.5 S/m, and a similar improvement for the CNS-PEG composites was from 7.8 to 72.8 S/m. Once 0.5 wt.% of the Tuball was replaced by NC7000 or CNS-PEG, respectively, the S of a 1 wt.% single-filler Tuball PC composite decreased from 36.7 to 28.2 and 22.6 μV/K.

The conductivity, Seebeck coefficient S, and power factor PF of both single- and hybrid-filler systems of composites were mostly influenced by the kind of CNT. The size of the S-value was affected by the kind of polymer, although the overall trends that depend on the CNT type were constant. The type of polymer, particularly whether it was amorphous or semicrystalline, affected the PF since it contributed to the development of the electrically conductive network.

Conclusions

In conclusion, this study elucidated the development of a range of single-filler and hybrid-filler polymer composite films by melt-mixing to evaluate the filler effect on TE performance. Polymer matrices were used with PC and PEEK. The PC-based polymers were examined by means of ultrafast laser time-resolved TAS.

The thorough investigation of exciton lifespan, dynamics, and free charge carrier recombination yielded new knowledge regarding the influence of the CNT fillers used on the TE parameters. It was shown that the resulting charged exciton durations strongly correlate with the TE parameter Seebeck coefficient S. The S-values of both PC-based hybrid-filler and single-filler composites exhibited an approximately linear dependency with the exciton lifetime, with enhanced S-values found for longer exciton durations. It was discovered that the electrical conductivity σ, the second parameter of the TE, was principally correlated with the electron mobility inside the composite films.

The authors mentioned that the results of this work open the door to the creation of sophisticated TE polymer composite structures and show that TAS is an effective method for examining carrier dynamics in TE energy harvesting materials.

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