Preparation and characterization of poly(3-octylthiophene)/carbon fiber thermoelectric composite materials

Poly(3-alkylthiophene) (P3AT) with a high Seebeck coefficient has recently been reported. However, P3AT/inorganic conductive composites exhibit relatively poor thermoelectric performance because of their low electrical conductivity. In this work, carbon fiber sheets with a high electrical conductivity were chosen as the inorganic phase, and poly(3-octylthiophene)(P3OT)/carbon fiber composites were prepared by casting P3OT solution onto the carbon fiber sheets. The carbon fiber sheets incorporated into the composites can provide good electrical conductivity, and P3OT can provide a high Seebeck coefficient. The highest power factor of 7.05 μW m⁻¹ K⁻² was obtained for the composite with 50 wt% P3OT. This work suggests a promising method for preparing large-scale thermoelectric composites with excellent properties.     

Thermoelectric behavior of aerogels based on graphene and multi-walled carbon nanotube nanocomposites

In this work, we presented that the Seebeck coefficient and electrical conductivity can be increased simultaneously in aerogels based on graphene and multi-walled carbon nanotube (graphene-MWCNT) nanocomposites, and at the same time the thermal conductivity is depressed due to 3D porous skeleton structure. As a result, graphene-MWCNT aerogels possess ultra-low thermal conductivities (∼0.056 W m⁻¹ K⁻¹) and apparent density (∼24 kg m⁻³), thereafter the figure of merit (ZT) of ∼0.001 is achieved. Although the ZT value is too low for practical application as a thermoelectric (TE) material, the unique structure in this project provides a potential way to overcome the challenge in bulk semiconductors that increasing electrical conductivity generally leads to decreased Seebeck coefficient and enhanced thermal conductivity.     

DC resistivity and thermoelectric power in Ni-Cd ferrites

The electrical resistivity and Seebeck coefficient for Ni-Cd ferrites have been studied as a function of temperature. The lattice constant of the phases have been evaluated from X-ray powder data. The thermoelectric power measurements indicate that the samples aren-type semiconductors and the conduction mechanism is interpreted on the basis of localized model of polarons.     

Effect of electron donor/acceptor substituents on the Seebeck coefficient and thermoelectric properties of poly(3-methylthiophene methine)s/graphite composites

Recently, the use of polymers as thermoelectric materials has attracted considerable attention. However, relatively few studies have investigated the effects of polymer structures on the corresponding thermoelectric properties of the polymers. In this work, a series of poly(3-methylthiophene methine)s (PMMs) were synthesized for use as thermoelectric materials, and the effects on the Seebeck coefficient of donor or acceptor side groups at the methine carbon were studied. The PMMs with strongly electron-withdrawing and electron-donating groups exhibited the highest Seebeck coefficients. Motivated by the high Seebeck coefficients of the selected PMMs, PMM/graphite composites were prepared via solution mixing followed by mechanical ball milling and cold pressing. The thermoelectric properties of the composites were investigated as a function of the graphite (G) concentration. The highest ZT (6.23 × 10⁻³) was measured for the poly[(3-methylthiophene-2,5-diyl) (p-(methoxy)benzylidene)]/G composite that contained 90 wt% G. The results of this work suggest that the thermoelectric properties of polymer-inorganic composites can be improved by designing polymers with high Seebeck coefficients.     

Effects of annealing on thermoelectric properties of Sb2Te3 thin films prepared by radio frequency magnetron sputtering

Antimony telluride (Sb₂Te₃) thin films were deposited on silicon substrates at room temperature (300 K) by radio frequency magnetron sputtering method. The effects of annealing in N₂ atmosphere on their thermoelectric properties were investigated. The microstructure and composition of these films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. Both of the Seebeck coefficient and Hall coefficient measurement showed that the prepared Sb₂Te₃ thin films were p-type semiconductor materials. By optimizing the annealing temperature, the power factor achieved a maximum value of 18.02 μW cm^?1 K^?2 when the annealing temperature was increased to 523 K for 6 h with a maximum electrical conductivity (1.17 × 10³ S/cm) and moderate Seebeck coefficient (123.9 μV/K).     

High temperature thermoelectric properties and figure-of-merit of sintered Nd2−xCexCuO4

Thermoelectric properties such as the Seebeck coefficient, electrical resistivity, and thermal conductivity are measured in the temperature range of 300– 673 K on Nd2−xCexCuO4 (x = 0–0.1) sintered bodies in order to estimate the figure-of-merit for thermoelectric energy conversion. The temperature dependences of both the Seebeck coefficient and electrical resistivity indicated n-type semiconducting behaviour. The thermal conductivities whose value decreased with increasing temperature were in the range of 3.7–7.5 Wm-1K-1. The maxima of the power factor and the figure-of-merit estimated from data measured at 320 K on a sample of a composition of x = 0.05 were 9.2×10-5 Wm-1K-2 and 1.7×10-5 K-1, respectively. The limitation of the power factor is discussed based on the measured Seebeck coefficient and electrical conductivity data. The thermal conductivity could be separated into a small electron component and a large phonon component by applying the Wiedemann–Franz law. This suggests the possibility of improving the figure-of-merit to some extent by a reduction of the phonon thermal conductivity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Improved thermoelectric properties of La-doped Bi2Sr2Co2O9-layered misfit oxides

Bi₂Sr_2−x La _x Co₂O₉ (x = 0, 0.02, 0.04, and 0.08) polycrystalline-layered misfit oxides have been prepared by solid-state reactions. Electrical property measurements indicated that all the samples are p-type semiconductors. The electrical conductivity decreased and the Seebeck coefficient increased with increasing temperature. The thermal conductivities were very low, only 0.6–0.7 W m⁻¹ K⁻¹ at room temperature. La doping was effective in increasing the Seebeck coefficient, reducing the thermal conductivity, and hence improving the thermoelectric performance. A highest dimensionless figure of merit ZT of 0.147 was obtained for Bi₂Sr_1.96La_0.04Co₂O₉ sample at 737 K, about two times higher than that of the sample without La doping.     

Layer-by-layer deposition of MnSi1.7 film with high Seebeck coefficient and low electrical resistivity

A good thermoelectric material should have a high Seebeck coefficient, a low electrical resistivity, and a low thermal conductivity. For conventional thermoelectric materials, however, increasing the Seebeck coefficient also leads to a simultaneous increase in the electrical resistivity. In this paper, a method of layer-by-layer deposition of MnSi_1.7 film with high Seebeck coefficient and low electrical resistivity is developed. After deposition of the first MnSi_1.7 sub-layer, the deposition process is interrupted for several minutes, and then continues for another MnSi_1.7 sub-layer. Therefore, the MnSi_1.7 film contains two sub-layers for one interruption, three sub-layers for two interruptions, and so on. It is found that the n-type MnSi_1.7 film with two sub-layers has a higher Seebeck coefficient, −0.451 mV K⁻¹, and a lower electrical resistivity, 19.4 mOhm-cm, at 483 K as compared to that of without deposition interruption, −0.152 mV K⁻¹ and 44.3 mOhm-cm. The p-type MnSi_1.7 film with three sub-layers also has a higher Seebeck coefficient, 0.238 mV K⁻¹, and a lower electrical resistivity, 5.5 mOhm-cm, at 733 K in comparison with that of without deposition interruption, 0.212 mV K⁻¹ and 10.4 mOhm-cm.     

Influence of annealing on thermoelectric properties of bismuth telluride films grown via radio frequency magnetron sputtering

Bismuth telluride films were prepared via radio frequency magnetron sputtering. Mixed powders with different composition were used as sputtering targets. Influence of the annealing temperature on surface topography, crystal structure and thermoelectric properties of the films has been investigated. It was found that the grain size increased and the surface roughness decreased with a rising annealing temperature. X-ray diffraction analysis revealed an improved crystallization after the annealing, and that crystal planes perpendicular to c-axis became prominent. High temperature treatments resulted in a decrease of Seebeck coefficient and an increase of electrical conductivity. The highest power factor was obtained after being annealed at 300 °C.     

N-type doping and thermoelectric properties of co-sublimed cesium-carbonate-doped fullerene

Organic semiconductors exhibit a large Seebeck coefficient and a poor thermal conductivity allowing them to become strong candidates for thermoelectric applications. These materials have been widely used in organic electronics with the fabrication of organic light-emitting diodes, organic solar cells, and transistors. However, few studies have reported on thermoelectric properties of organic materials even though they offer specific advantages such as cost-effectiveness and flexibility. In this article, we discuss the fabrication and characterization of fullerene C60 doped with cesium carbonate (Cs₂CO₃). The evolution of the morphology, electrical conductivity, and Seebeck coefficient was analyzed as a function of the dopant concentration. An optimal power factor of 28.8 μWm^?1 K^?2 was obtained at room temperature for a molar ratio of 15.2 %. Thus far, this power factor value constitutes the best thermoelectric performance achieved with N-type organic materials.     

Dielectric properties of epoxy/short carbon fiber composites

The dielectric properties of epoxy/short carbon fiber composites at different concentrations 0, 5, 10 and 15% by weight, different thicknesses 2 and 4 mm, and frequency in the range from 20 Hz to 1 MHz were characterized. Scanning electron microscopy and differential scanning calorimetry were utilized. The alternating current (ac) electrical properties (complex impedance, dielectric constant, dielectric loss, real part of electric modulus, imaginary part of electric modulus, electrical conductivity, and relaxation time) were determined. It was found that the applied frequency, filler concentrations, and composite thickness affected the ac electrical properties of the epoxy/carbon fiber composites. The dielectric behaviors of the interfacial polarization between epoxy matrix and carbon fibers could be described by the Maxwell–Wagner–Sillars relaxation. The analysis of the complex electric modulus in the frequency range from 20 Hz to 1 MHz revealed that the interfacial relaxation followed the Cole–Davidson distribution of relaxation times. The universal power-law of ac conductivity was observed in the epoxy/carbon fiber composites. The calculated power exponent (near unity) is physically acceptable within this applied model.     

Switching effects in magnetic semiconductors

The Seebeck coefficient, or thermoelectric power (TEP), couples the fluctuations in electric current with those of heat transport. We derive a new relationship between the thermoelectric power and the thermal conductivity using the generalized theory of Brownian motion. The TEP temperature derivative in ferromagnets has a singular behavior like that of the specific heat. In antiferromagnets, the singularities in the TEP and in the electrical resistivity temperature derivatives are similar.     

Electronic transport properties of 1-(p-R-phenacyl)-4-{[(1′-ethylcarboxylate)-(3′-p-R′-phenacyl)]-7′-indolizinyl}pyridinium bromides in thin films

The temperature dependences of electrical conductivity, σ, and Seebeck coefficient, S, for some new pyridium monoquaternary salts derivatives, 1-(p-R-phenacyl)-4-{[(1′-ethylcarboxylate)-(3′-p-R′-phenacyl)]-7′-indolizinyl}pyridinium bromides have been studied.The film samples (d = 0.08-0.28 μm) have been deposited onto glass by an immersion technique (dimethylformamide was used as a solvent).The investigated compounds behave as typical p-type polycrystalline semiconductors. The activation energy of electrical conduction ranged between 0.65 and 1.72 eV, while the ratio of charge carrier mobilities laid in the range (0.60-0.83).The model based on band gap representation is suitable in explaining the electronic transport in present compounds in the higher temperature range (385-500 K).     

Novel thermoelectric materials based on boron-doped silicon microchannel plates

The thermoelectric properties of boron-doped silicon microchannel plates (MCPs) were investigated. The samples were prepared by photo-assisted electrochemical etching (PAECE). The Seebeck coefficient and electrical resistivity at room temperature (25 °C) were measured to determine the thermoelectric properties of the samples. In order to decrease the very high resistivity, boron doping was introduced and by modulating the doping time, a series of samples with different resistivity as well as Seebeck coefficient were obtained. Boron doping changed the electrical resistivity of the samples from 1.5 × 10⁵ Ω cm to 5.8 × 10⁻³ Ω cm, and the absolute Seebeck coefficient deteriorated relatively slightly from 674 μV/K to 159 μV/K. According to the Seebeck coefficient and electrical conductivity, the power factor was calculated and a peak value of 4.7 × 10⁻¹ mW m⁻¹ K⁻² was obtained. The results indicate that silicon MCPs doped with boron are promising silicon-based thermoelectric materials.     

Extraordinary Thermoelectric Performance Realized in n‐Type PbTe through Multiphase Nanostructure Engineering

Lead telluride has long been realized as an ideal p‐type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n‐type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record‐high ZT value of ≈1.83 can be achieved at 773 K in n‐type PbTe‐4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state‐of‐the‐art thermoelectric systems.     

Property-processing relations in developing thermoelectric ceramics: Na<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript>

Polycrystalline Na_1−x Co₂O₄ is a promising p-type oxide thermoelectric, and it was investigated in a property-processing study to enhance the thermoelectric properties for high temperature applications. The density of the ceramics was improved by a post-milling process, and consequently, we obtained better thermoelectric power factors (PF) due to an associated improvement in electrical conductivity. Through a milling process, and sintering at 1203 K, we obtained an enhanced thermoelectric power factor of ~4 μW/cm K² at 500 K for randomly orientated polycrystalline ceramics. The Seebeck coefficient variation with temperature demonstrates through modeling that the conduction mechanism changed from metallic to a semiconducting behavior between temperatures 300 to 400 K.     

聚合物热电复合材料的研究进展

本文综述了碳纳米材料/聚合物、半导体合金/聚合物、金属纳米粒子/聚合物以及聚合物/聚合物等热电复合材料的研究进展。简要分析了热电复合材料的性能提升机理及现有材料尚存在的问题,并指出了聚合物热电复合材料今后的发展方向。     

Enhancing the thermoelectric performance by defect structures induced in p-type polypyrrole-polyaniline nanocomposite for room-temperature thermoelectric applications

Organic thermoelectric materials mainly conducting polymers are green materials that can convert heat energy into electrical energy and vice versa at room temperature. In the present work, we investigated the thermoelectric properties of polymer nanocomposite of polypyrrole (PPy) and polyaniline (PANI) (PPy/PANI) by varying the pyrrole: aniline monomer ratios (60:40, 50:50, and 40:60). The PPy/PANI composite is prepared by in-situ chemical polymerization of PPy on PANI dispersion. It has been observed that the combination of two conducting polymers has enhanced the electrical and thermal properties in the PPy/PANI composite due to the strong π–π stacking and H-bonding interaction between the conjugated structure of PPy and conjugated structure of PANI. The maximum electrical conductivity of 14.7 S m^?1 was obtained for composite with high pyrrole content, whereas the maximum Seebeck coefficient of 29.5 μV K^?1 was obtained for composite with high aniline content at 366 K. Consequently, the PPy/PANI composite with pyrrole to aniline monomer ratio of 60:40 exhibits the optimal electrical conductivity, Seebeck coefficient, and high power factor. As a result, the maximum power factor of 2.24 nWm^?1 K^?2 was obtained for the PPy/PANI composite at 60:40 pyrrole to aniline monomer ratio, which is 29 times and 65.8 times higher than PPy (0.077 nWm^?1 K^?2) and PANI (0.034 nWm^?1 K^?2), respectively.

     

Thermoelectric measurements of PEDOT:PSS/expanded graphite composites

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/expanded graphite films were cast as thin films with different expanded graphite contents at room temperature. The thermoelectric properties of the composites were investigated as a function of the graphite concentration. The electrical conductivity and Seebeck coefficient were measured as a function of the graphite concentration. The electrical conductivity and power factor show similar trends with a sharp increase at around 55 wt% of expanded graphite content. The Seebeck coefficient does not show a significant dependence with the graphite content. SEM and TEM images indicate a nearly homogenous distribution of the filler in the matrix. The initial thermal stability is not modified with the filler.     

Radial electric field effect on thermoelectric transport properties of Bi2Te3 cylindrical nanowire coaxial structure

Radial electric field effect (REFE) on the thermoelectric figure of merit and Seebeck coefficient S are studied for a coaxial cylindrical capacitor configuration on the basis of bipolar intrinsic semiconductors. Theoretical analysis of REFE nanowire was done based on Poisson's equation in cylindrical geometry with corresponding boundary conditions. Using Newton's method the radial variation of the local Seebeck coefficient, carrier concentration and others transport characteristics are calculated for the bipolar Bi₂Te₃ nanowires neglecting size quantization. The dependence of the thermoelectric parameters on the gate voltage is studied. It is shown that the existence of the transition bipolar–monopolar semiconductor, electric field, differences in carrier masses and mobility essentially affect the thermoelectric properties. The thermoelectric figure of merit can be significantly increased by REFE.