Increase In Seebeck Coefficient Research Articles

Significant enhancement of the thermoelectric performance in Ca3Co4O9 thermoelectric materials through combined strontium substitution and hot-pressing process

This work explores the possibilities for a further enhancement of the thermoelectric properties of Ca3Co4O9 by Sr-doping combined with hot-pressing. Modified hot-pressing process resulted in highly-textured and dense ceramics. Sr-doping significantly improves electrical properties, resulting in extremely large power factor (1.2 mW/K2m at 800 °C) due to simultaneous electrical resistivity decrease and Seebeck coefficient increase. The main effect on cumulative electrical performance is provided by the Seebeck coefficient, reaching 270μV/K at 800 °C. XPS revealed relatively high average cobalt oxidation state at room temperature (+3.3), compared to materials produced by conventional sintering. The results of combined XPS and Auger electron spectroscopy emphasize the importance of high densification in Ca3Co4O9-based ceramics for preventing phase decomposition and interaction with CO2 and moisture. Still, despite the exceptional electrical performance, the calculated figure-of-merit (estimated as 0.29 at 800 °C) is around the best reported in the literature due to a high thermal conductivity (4.4 W/K m at room temperature).

Melt spinning: A rapid and cost effective approach over ball milling for the production of nanostructured p-type Si80Ge20 with enhanced thermoelectric properties

In this work, we have adopted melt-spinning technique followed by spark plasma sintering (SPS) to synthesize a p-type nanostructured Si80Ge20 alloy. To illustrate the impact of the technique, the results are compared with that of prepared by ball milling and subsequent SPS. The room temperature XRD of the alloys prepared by both the techniques confirms that they crystallize with diamond-like cubic structure of space group Fd3¯m. The remarkably decreased thermal conductivity and increased Seebeck coefficient results in a higher ZT of melt spun sample (nearly ∼46% higher at room temperature) as compared to the ball milled sample. The short preparation time of melt spun p-type Si80Ge20 alloy coupled with its enhanced thermoelectric performance indicate that this technique provides a novel strategy to improve the thermoelectric properties of SiGe alloys thus making this synthesis process of interest for commercial purposes.

Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide

This work aims to explore zirconium as a possible dopant to promote thermoelectric performance in bulk ZnO-based materials, both within the single-doping concept and on simultaneous co-doping with aluminum. At 1100–1223 K mixed-doped samples demonstrated around ∼2.3 times increase in ZT as compared to single-doped materials, reaching ∼0.12. The simultaneous presence of aluminum and zirconium imposes a synergistic effect on electrical properties provided by their mutual effects on the solubility in ZnO crystal lattice, while also allowing a moderate decrease of the thermal conductivity due to phonon scattering effects. At 1173 K the power factor of mixed-doped Zn0.994Al0.003Zr0.003O was 2.2–2.5 times higher than for single-doped materials. Stability tests of the prepared materials under prospective operation conditions indicated that the gradual increase in both resistivity and Seebeck coefficient in mixed-doped compositions with time may partially compensate each other to maintain a relatively high power factor.

Uniform dispersion of SiC in Yb-filled skutterudite nanocomposites with high thermoelectric and mechanical performance

Although filled skutterudites are very promising thermoelectric (TE) materials at intermediate temperature, most of the high-performance skutterudites reported are multiple-filled materials that tend to have poor stability and machinability. Here, we successfully fabricate single-filled skutterudite based composites with uniformly dispersed β-SiC nanoparticles via cryogenic grinding and spark plasma sintering. The nanocomposites show not only increased Seebeck coefficient owing to a possible energy filtering effect, but also reduced lattice thermal conductivity through enhanced phonon scattering. A maximum ZT value of 1.42 at 850 K with greatly improved mechanical properties is achieved, which is very attractive for the application in TE devices.

Electrophysical and thermal properties of NaxCu2-xS (x = 0.05, 0.075, 0.10) and Na0.125Cu1.75S semiconductor alloys

The results of measurements of electronic conductivity and the Seebeck coefficient, as well as X-ray phase analysis, electron microscopy, differential scanning calorimetry (DSC) and studies of the heat capacity of nanocrystalline NaxCu2-xS (x = 0.05, 0.075, 0.10) and Na0.125Cu1.75S semiconducting alloys in the temperature interval of 300 - 600 K are presented. At ambient temperature, the alloys under study consist of three phases: monoclinic Na2Cu4S3, hexagonal Chalcocite Cu2S and rhombohedral digenite Cu9S5. Measurements show that the Seebeck coefficient increases, and the conductivity decreases with increasing sodium concentration in the alloys. Thermal effects near the temperatures of 360 K, 450 K and 640 K have been observed by the DSC method.

Thermoelectric Transport Properties of Cd xBi yGe1- x- yTe Alloys.

Band convergence has been proven as an effective approach for enhancing thermoelectric performance, particularly in p-type IV-VI semiconductors, where the superior electronic performance originates from the contributions of both L and Σ band valleys when they converge to have a small energy offset. When alloying with cubic IV-VI semiconductors, CdTe has been found as an effective agent for achieving such a band convergence. This work focuses on the effect of CdTe-alloying on the thermoelectric transport properties of GeTe, where the carrier concentration can be tuned in a broad range through Bi-doping on Ge site. It is found that CdTe-alloying indeed helps to converge the valence bands of GeTe in both low- T rhombohedral and high- T cubic phases for an increase in Seebeck coefficient with a decrease in mobility. In addition, the strong phonon scattering due to the existence of high-concentration Cd/Ge and Bi/Ge substitutions leads the lattice thermal conductivity to be reduced to as low as 0.6 W/(m-K). These lead to an effectively increased average thermoelectric figure of merit ( ZTave ∼ 1.2) at 300-800 K, which is higher than that of many IV-VI materials with CdTe-alloying or alternatively with MnTe-, MgTe-, SrTe-, EuTe-, or YbTe-alloying for a similar band convergence effect.

Enhanced Cross-Plane Thermoelectric Transport of Rotationally Disordered SnSe2 via Se-Vapor Annealing.

We report cross-plane thermoelectric measurements of SnSe and SnSe2 films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (-38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe2 as the Se source in a sealed tube), a transition from SnSe to SnSe2 is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from -38.6 to -631 μV/K) after Se annealing due to both the SnSe-to-SnSe2 transition and the mitigation of unintentional doping by Se vacancies. We also observe a corresponding 6-fold drop in the electrical conductivity (from 3 to 0.5 S/m) after Se annealing, which is consistent with both a drop in the carrier concentration and an increase in band gap. The power factor S2σ increased by 44× (from 4.5 nW/m·K2 to 0.2 μW/m·K2) after Se annealing. We believe that these results demonstrate a robust method for mitigating unintentional doping in a promising class of materials for thermoelectric applications.

Increased Seebeck Coefficient and Decreased Lattice Thermal Conductivity in Grain-Size-Controlled p-Type PbTe–MgTe System

PbTe systems are known as good thermoelectric materials for midtemperature range applications. Here, we demonstrate that a method of melt-spinning followed by pulsed electric current sintering is e...

Rapid preparation of Ge0.9Sb0.1Te1+x via unique melt spinning: Hierarchical microstructure and improved thermoelectric performance

Thermoelectric semiconductors based on GeTe has been proved to be an important materials for converting waste heat into electricity. In this study, we demonstrate an enhancement of thermoelectric performance for p-type Ge0.9Sb0.1Te1+x by a unique melt spinning method combined with hot pressing (MS-HP), through which increased Seebeck coefficients and reduced thermal conductivities are achieved simultaneously. Moreover, the MS technique greatly gives rise to a variety of microstructures, such as the fine Ge precipitates, twins and nano-domains, all of which contribute to the significantly reduced thermal conductivities. Finally, the Ge0.9Sb0.1Te1.03 reaches a maximum zT value of ∼1.9 at 760 K and a giant average zT of ∼1.2 between 300 K and 820 K, which is comparable to the reported results for GeTe based materials. Combined with the rapid fabrication, Ge0.9Sb0.1Te1+x prepared by melt spinning process is an attractive p-type material for thermoelectric power generation application.

Preparation and enhanced thermoelectric performance of Pb-doped tetrahedrite Cu12-xPbxSb4S13

Pb-doped tetrahedrite Cu12-xPbxSb4S13 (x = 0, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) samples were prepared and their thermoelectric properties were investigated in the temperature range from 300 K to 723 K. The results show that Pb is successfully doped in Cu(1) site, which brings about both 10–118% increase in Seebeck coefficient owing to the decrease of hole concentration and 87–168% decrease in thermal conductivity due to the enhanced phonon scattering by the dopant atoms. As a result, a maximum ZT = 0.8 at 723 K is achieved for Cu11PbSb4S13, which is ∼43% larger than that of pristine Cu12Sb4S13. Present results demonstrate that Pb is an effective dopant to elevate thermoelectric performance of Cu12Sb4S13.

(Invited) Development of Thermoelectric Materials and Thermal Interface Materials By Pulsed Electroplating

In this talk, I will share our recent research effort on development of thermoelectric materials and devices as well as thermal interface materials by using the pulsed electroplating method. We systematically studied the effects of deposition parameters on the composition, microstructure, and thermoelectric properties of the electroplated Bi2Te3 thin films. Our results show that the thermoelectric figure of merit of the electroplated Bi2Te3 films can be enhanced by increasing the pulse off-to-on ratio, which is mainly due to the reduced thermal conductivity and the increased Seebeck coefficient. Based on the fundamental materials research, we further developed cross-plane micro-thermoelectric generators (TEG) based on the electroplated Bi2Te3 and Sb2Te3 micropillars. A power density of 9.2 mW/cm2 is achieved at a temperature difference of 52.5 K for a TEG with 127 thermoelectric pairs. Recently, we are developing thin thermal interface materials by combining pulsed electroplating with microfabrication processes. I will introduce the fabrication process of thermal interface materials and our preliminary results on thermal resistance measurements.

Fabrication of Porous Polyvinylidene Fluoride/Multi-Walled Carbon Nanotube Nanocomposites and Their Enhanced Thermoelectric Performance.

In this paper, a solvent vapor-induced phase separation (SVIPS) technique was used to create a porous structure in polyvinylidene fluoride/Multi-walled carbon nanotube (PVDF/MWNTs) composites with the aim of increasing the electrical conductivity through the incorporation of MWNTs while retaining a low thermal conductivity. By using the dimethylformamide/acetone mixture, porous networks could be generated in the PVDF/MWNTs composites upon the rapid volatilization of acetone. The electrical conductivity was gradually enhanced by the addition of MWNTs. At the same time, the thermal conductivity of the PVDF film could be retained at 0.1546 W·m−1·K−1 due to the porous structure being even by loaded with a high content of MWNTs (i.e., 15 wt.%). Thus, the Seebeck coefficient, power factor and figure of merit (ZT) were subsequently improved with maximum values of 324.45 μV/K, 1.679 μW·m−1·K−2, and 3.3 × 10−3, respectively. The microstructures, thermal properties, and thermoelectric properties of the porous PVDF/MWNTs composites were studied. It was found that the enhancement of thermoelectric properties would be attributed to the oxidation of MWNTs and the porous structure of the composites. The decrease of thermal conductivity and the increase of Seebeck coefficient were induced by the phonon scattering and energy-filtering effect. The proposed method was found to be facile and effective in creating a positive effect on the thermoelectric properties of composites.

Polymer films with ultrahigh thermoelectric properties arising from significant seebeck coefficient enhancement by ion accumulation on surface

Organic thermoelectric (TE) materials have drawn great interest because of their advantages including mechanical flexibility, easy availability, non-toxicity and low thermal conductivity. TE materials with high dimensionless figure-of-merit ZT are required for highly efficient TE conversion. But the electrical conductivity and Seebeck coefficient of TE materials are interdependent. The increase in Seebeck coefficient is usually at the cost of the decrease in electrical conductivity. In this work, we report a facile approach to significantly enhance the TE properties of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) films by ion accumulation of an ionic liquid on the polymer surface. The ion accumulation can increase the Seebeck coefficient of the PEDOT:PSS films by 1.2–2 fold while it does not remarkably affect the electrical conductivity. The PEDOT:PSS films can exhibit an ultrahigh power factor of 754 μW m−1 K−2, corresponding to a ZT value of 0.75. This ZT value is comparable to that of inorganic TE materials like bismuth telluride at 300 K.

Simultaneously Increased Seebeck Coefficient and Electrical Conductivity by Photoinduced Excited State Base on the Organic Thin Film Device

Simultaneously Increased Seebeck Coefficient and Electrical Conductivity by Photoinduced Excited State Base on the Organic Thin Film Device

Role of surfactant on thermoelectric behaviors of organic-inorganic composites

Hybrid organic/inorganic composites have recently attracted intensive interests as a promising candidate for flexible thermoelectric (TE) devices using inherently soft polymers as well as for increasing the degree of freedom to control TE properties. Experimentally, however, enhanced TE performance in hybrid composites has not been commonly observed, primarily due to inhomogeneous mixing between the inorganic and organic components which leads to limited electrical conduction in the less conductive component and consequently a low power factor in the composites compared to their single-component counterparts. In this study, we investigated the effects of different surfactants on the uniformity of mixing and the TE behaviors of the hybrid composites consisting of Bi0.5Sb1.5Te3 (BST) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). We found that compared to dimethyl sulfoxide, which is the most widely used surfactant, Triton X-100 (TX-100) can lead to homogenous dispersion of BST in PEDOT:PSS. By systematically studying the effects of the surfactant concentration, we can attribute the better mixing capability of TX-100 to its non-ionic property, which results in homogenous mixing with a lower critical micelle concentration. Consequently, we observed simultaneous increase in electrical conductivity and Seebeck coefficient in the BST/PEDOT:PSS composites with the TX-100 surfactant.

The Effects of Excess Co on the Phase Composition and Thermoelectric Properties of Half-Heusler NbCoSb.

NbCoSb with nominal 19 valence electrons, and is supposed to be metallic, has recently been reported to also exhibit the thermoelectric properties of a heavily doped n-type semiconductor. In this study, we prepared Co-rich NbCo1+xSb samples (x = 0, 0.2, 0.3, 0.4, 0.5), and their phase compositions, microstructures and thermoelectric properties were investigated. The Seebeck coefficient increased a great deal with increasing x, due to decreasing carrier concentration, and the total thermal conductivity reduced mainly because of declining κe. Finally, a peak thermoelectric figure of merit, ZT, was about 0.46 for NbCo1.3Sb at 973 K. This enhancement was mainly attributed to the reduction of electric thermal conductivity and the increase of Seebeck coefficient. The excess Co had effects on the carrier concentration, deformation potential Edef and DOS effective mass m*. Adding an excessive amount of Co leads to a very high Edef, which was detrimental for transport characteristics.

Evolution of Thermoelectric Properties of Zn4Sb3 Prepared by Mechanical Alloying and Different Consolidation Routes

In this research, a method combining the mechanical alloying with the vacuum sintering or hot pressing was adopted to obtain the compact of β-Zn4Sb3. Pure zinc and antimony powders were used as the starting material for mechanical alloying. These powders were mixed in the stoichiometry ratio of 4 to 3, or more Zn-rich. Single phase Zn4Sb3 was produced using a nominally 0.6 at. % Zn rich powder. Thermoelectric Zn4Sb3 bulk specimens have been fabricated by vacuum sintering or hot pressing of mechanically alloyed powders at various temperatures from 373 to 673 K. For the bulk specimens sintering at high temperature, phase transformation of β-Zn4Sb3 to ZnSb and Sb was observed due to Zn vaporization. However, single-phase Zn4Sb3 bulk specimens with 97.87% of theoretical density were successfully produced by vacuum hot pressing at 473 K. Electric resistivity, Seebeck coefficient, and thermal conductivity were evaluated for the hot pressed specimens from room temperature to 673 K. The results indicate that the Zn4Sb3 shows an intrinsic p-type behavior. The increase of Zn4Sb3 phase ratio can increase Seebeck coefficient but decrease electric conductivity. The maximum power factor and figure of merit (ZT) value were 1.31 × 10−3 W/mK2 and 0.81 at 600 K, respectively. The ZT value was lower than that reported in the available data for materials prepared by conventional melt growth and hot pressed methods, but higher than the samples fabricated by vacuum melting and heat treatment techniques.

Non-precious transition metal oxide calcium cobaltite: Effect of dopant on oxygen/hydrogen evolution reaction and thermoelectric properties

We report here structural, thermoelectric and electrocatalytic performance of the undoped and doped Calcium Cobaltite: Ca3Co4-xMoxO9 (x = 0, 0.1, 0.2, 0.3 and 0.4) series. The Calcium cobaltite (CCO) crystallizes in a layered structure with alternate rock salt (RS) type arrangement and CoO2 layer stacked along the c axis having monoclinic symmetry with a mis-fit between RS and CoO2 layer along the b axis. Rietveld refinement confirms the formation of the similar crystal structure with no impurity phases for all the doped composites. XPS and FTIR data reveal that at an initial stage, Mo enters RS structure and at higher concentrations of doping Mo may exist in both the layers. Multivalent Mo acts as an electron dopant that reduces hole concentration resulting in an increase in seebeck coefficient (S). In addition to thermoelectric performances, the electrocatalytic behaviour of undoped and doped CCO toward the oxygen/hydrogen evolution reaction (OER/HER) has been demonstrated. We presume that the as-synthesized doped material has a potential to become a future electrocatalyst material.

Enhanced thermoelectric response in the fractional quantum Hall effect

We study the linear thermoelectric response of a quantum dot embedded in a constriction of a quantum Hall bar with fractional filling factors nu=1/m within Laughlin series. We calculate the figure of merit ZT for the maximum efficiency at a fixed temperature difference. We find a significant enhancement of this quantity in the fractional filling in relation to the integer-filling case, which is a direct consequence of the fractionalization of the electron in the fractional quantum Hall state. We present simple theoretical expressions for the Onsager coefficients at low temperatures, which explicitly show that ZT and the Seebeck coefficient increase with m.

Thermoelectric Properties of Co-doped ZnO by Incorporating Organic Nanoparticles

Organic-inorganic nanojunction can result in a selective scattering of charge carrier depending on their energy, which leads to a simultaneous increase in Seebeck coefficient S and power factor PF when the Fermi levels of the constituent materials are aligned appropriately. In this work, the nanojunction was employed at the organic-inorganic semiconductor interface of polyparaphenylene (PPP) and Zn 1- x Co x O nanoparticles through a sol-gel method. The resulting Zn 0.925 Co 0.075 O/9wt% PPP hybrids exhibit a high power factor due to the largest electrical conductivity and higher Seebeck coefficient. Moreover, organic-inorganic nano-interface effectively reduces the thermal conductivity by interface scattering of phonons. All these effects finally lead to a thermoelectric figure of merit, ZT up to 0.22 in these inorganic-organic nanocomposites, which corresponds to a 5-fold enhancement compared to that of the Zn 0.925 Co 0.075 O matrix. This work demonstrates the effectiveness of nanojunctions and provides a rational route to high performance thermoelectrics in a bulk material.