Kyungwho Choi

Improved Thermoelectric Behavior of Nanotube-Filled Polymer Composites with Poly(3,4-ethylenedioxythiophene) Poly(styrenesulfonate)

The thermoelectric properties of carbon nanotube (CNT)-filled polymer composites can be enhanced by modifying junctions between CNTs using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), yielding high electrical conductivities (up to approximately 40000 S/m) without significantly altering thermopower (or Seebeck coefficient). This is because PEDOT:PSS particles are decorated on the surface of CNTs, electrically connecting junctions between CNTs. On the other hand, thermal transport remains comparable to typical polymeric materials due to the dissimilar bonding and vibrational spectra between CNT and PEDOT:PSS. This behavior is very different from that of typical semiconductors whose thermoelectric properties are strongly correlated. The decoupled thermoelectric properties, which is ideal for developing better thermoelectric materials, are believed to be due to thermally disconnected and electrically connected contact junctions between CNTs. Carrier transport at the junction is found to be strongly dependent on the type and concentration of stabilizers. The crucial role of stabilizers was revealed by characterizing transport characteristics of composites synthesized by electrically conducting PEDOT:PSS and insulating gum Arabic (GA) with 1:1-1:4 weight ratios of CNT to stabilizers. The influence of composite synthesis temperature and CNT-type and concentration on thermoelectric properties has also been studied. Single-walled (SW) CNT-filled composites dried at room temperature followed by 80 degrees C exhibited the best thermoelectric performance in this study. The highest thermoelectric figure of merit (ZT) in this study is estimated to be approximately 0.02 at room temperature, which is at least one order of magnitude higher than most polymers and higher than that of bulk Si. Further studies with various polymers and nanoparticles with high thermoelectric performance may result in economical, lightweight, and efficient polymer thermoelectric materials.

Light-Weight Flexible Carbon Nanotube Based Organic Composites with Large Thermoelectric Power Factors

Typical organic materials have low thermal conductivities that are best suited to thermoelectrics, but their poor electrical properties with strong adverse correlations have prevented them from being feasible candidates. Our composites, containing single-wall carbon nanotubes, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and/or polyvinyl acetate, show thermopowers weakly correlated with electrical conductivities, resulting in large thermoelectric power factors in the in-plane direction of the composites, ∼160 μW/m·K(2) at room temperature, which are orders of magnitude larger than those of typical polymer composites. Furthermore, their high electrical conductivities, ∼10(5) S/m at room temperature, make our composites very promising for various electronic applications. The optimum nanotube concentrations for better power factors were identified to be 60 wt % with 40 wt % polymers. It was noticed that high nanotube concentrations above 60 wt % decreased the electrical conductivity of the composites due to less effective nanotube dispersions. The thermal conductivities of our 60 wt % nanotube composites in the out-of-plane direction were measured to be 0.2-0.4 W/m·K at room temperature. The in-plane thermal conductivity and thermal contact conductance between nanotubes were also theoretically estimated.

Air-stable fabric thermoelectric modules made of N- and P-type carbon nanotubes

This report demonstrates an exciting new paradigm for thermoelectric energy conversion with both n- and p-type organic materials that possess mechanical flexibility, simple fabrication processes, and stability in air. In order to synthesize n-type samples with membranes and papers, carbon nanotubes were doped with both polyethyleneimine (PEI) and sodium borohydride (NaBH4), showing excellent n-type characteristics with thermopower values as large as −80 μV K−1. Thermoelectric modules made of both n- and p-type composites were fabricated to demonstrate thermoelectric voltage and power generation with one, two, and three p–n couples connected in series. The testing modules produced ∼6 mV thermoelectric voltage, with ∼25 nW generated power upon application of ∼22 °C temperature gradients. These promising results show that further work with many junctions connected in series would result in scalable organic p–n couple modules, which can generate power from temperature gradients or provide cooling for various electronic devices.

Flexible Power Fabrics Made of Carbon Nanotubes for Harvesting Thermoelectricity

Thermoelectric energy conversion is very effective in capturing low-grade waste heat to supply electricity particularly to small devices such as sensors, wireless communication units, and wearable electronics. Conventional thermoelectric materials, however, are often inadequately brittle, expensive, toxic, and heavy. We developed both p- and n-type fabric-like flexible lightweight materials by functionalizing the large surfaces and junctions in carbon nanotube (CNT) mats. The poor thermopower and only p-type characteristics of typical CNTs have been converted into both p- and n-type with high thermopower. The changes in the electronic band diagrams of the CNTs were experimentally investigated, elucidating the carrier type and relatively large thermopower values. With our optimized device design to maximally utilize temperature gradients, an electrochromic glucose sensor was successfully operated without batteries or external power supplies, demonstrating self-powering capability. While our fundamental study provides a method of tailoring electronic transport properties, our device-level integration shows the feasibility of harvesting electrical energy by attaching the device to even curved surfaces like human bodies.

Thermally Driven Large N‐Type Voltage Responses from Hybrids of Carbon Nanotubes and Poly(3,4‐ethylenedioxythiophene) with Tetrakis(dimethylamino)ethylene

Hybrids of carbon nanotubes (CNTs) and poly(3,4-ethylenedioxythiophene) (PEDOT) treated by tetrakis(dimethylamino)ethylene (TDAE) have large n-type voltages in response to temperature differences. The reduced carrier concentration by TDAE reduction and partially percolated CNT networks embedded in the PEDOT matrix result in high thermopower and low thermal conductivity. The high electron mobility in the CNTs helps to minimally reduce the electrical conductivity of the hybrid, resulting in a large figure-of-merit.

N-Type Thermoelectric Performance of Functionalized Carbon Nanotube-Filled Polymer Composites

Carbon nanotubes (CNTs) were functionalized with polyethyleneimine (PEI) and made into composites with polyvinyl acetate (PVAc). CNTs were dispersed with different amounts of sodium dodecylbenzenesulfonate (SDBS) prior to the PEI functionalization. The resulting samples exhibit air-stable n-type characteristics with electrical conductivities as great as 1500 S/m and thermopowers as large as −100 µV/K. Electrical conductivity and thermopower were strongly affected by CNT dispersion, improving the properties with better dispersion with high concentrations of SDBS. This improvement is believed to be due to the increase in the number of tubes that are evenly coated with PEI in a better-dispersed sample. Increasing the amount of PEI relative to the other constituents positively affects thermopower but not conductivity. Air exposure reduces both thermopower and conductivity presumably due to oxygen doping (which makes CNTs p-type), but stable values were reached within seven days following sample fabrication.

Highly Doped Carbon Nanotubes with Gold Nanoparticles and Their Influence on Electrical Conductivity and Thermopower of Nanocomposites

Carbon nanotubes (CNTs) are often used as conductive fillers in composite materials, but electrical conductivity is limited by the maximum filler concentration that is necessary to maintain composite structures. This paper presents further improvement in electrical conductivity by precipitating gold nanoparticles onto CNTs. In our composites, the concentrations of CNTs and poly (vinyl acetate) were respectively 60 and 10 vol%. Four different gold concentrations, 0, 10, 15, or 20 vol% were used to compare the influence of the gold precipitation on electrical conductivity and thermopower of the composites. The remaining portion was occupied by poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), which de-bundled and stabilized CNTs in water during synthesis processes. The concentrations of gold nanoparticles are below the percolation threshold of similar composites. However, with 15-vol% gold, the electrical conductivity of our composites was as high as ∼6×105 S/m, which is at least ∼500% higher than those of similar composites as well as orders of magnitude higher than those of other polymer composites containing CNTs and gold particles. According to our analysis with a variable range hopping model, the high conductivity can be attributed to gold doping on CNT networks. Additionally, the electrical properties of composites made of different types of CNTs were also compared.

Enhanced Piezoelectric Behavior of PVDF Nanocomposite by AC Dielectrophoresis Alignment of ZnO Nanowires

In contrast to commercial piezoelectric ceramics, lead-free materials such as ZnO and a polymer matrix are proper candidates for use in ecofriendly applications. In this article, the authors represent a technique using ZnO nanowires with a polyvinylidene fluoride (PVDF) matrix in a piezoelectric polymer composite. By aligning the nanowires in the matrix in a desired direction by AC dielectrophoresis, the piezoelectric behavior was enhanced. The dielectric constant of the composite was improved by increasing the concentration of the ZnO nanowires as well. Specifically, the resulting dielectric constant shows an improvement of 400% with aligned ZnO nanowires by increasing the poling effect compared to that of a randomly oriented nanowire composite without a poling process.

Tailoring Thermoelectric Properties of Segregated-Network Polymer Nanocomposites for Thermoelectric Energy Conversion

Carbon nanotube (CNT)-polymer composites were prepared by segregated network approach. CNTs were served as conductive fillers in a polymer matrix to synthesize electrically conducting polymer composites. In the segregated network composites, the thermoelectric properties were further improved by replacing Gum Arabic (GA) with electrically conductive stabilizer PEDOT:PSS doped with dimethyl sulfoxide (DMSO). The electrical and thermal conductivities and Seebeck coefficient were measured to determine the thermoelectric property of the polymer composites. The electrical conductivity of the composites with 9.8wt% of CNT was 3191.8 S/m whereas that of 10wt% CNT composite with GA sample was 400 S/m.Copyright

Inorganic and Organic Hybrid Materials for Improving Thermoelectric Performance of Polymer Nanocomposites

Copper nanoparticles were incorporated with Carbon Nanotube (CNT)/polymer nanocomposites in order to enhance the thermopower by enlarging the energy gap between the Fermi level and the mean of differential electrical conductivity. The thermopower was increased as ∼ 2 times with 1 vol% of copper nanoparticles. The effects of copper concentration on the electrical and thermal properties of the composites were studied.Copyright