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BAYHAN, NEVRA/0000-0002-7497-2377; Tiryaki, Hasan/0000-0001-9175-0269; Yusuf, Aminu/0000-0003-4169-6529 WOS:000611968800001 Contact resistance and Thomson heat are the two major factors in the analysis of thermoelectric modules that are often being ignored. Each of these factors has an adverse effect on the output performance of a thermoelectric module. In this study, expression for maximum power output that includes both the contact resistance and the Thomson effect has been optimized using genetic algorithm to obtain the optimum geometric parameters of a thermoelectric generator. Each leg has electrical and thermal contact resistances of 2 x 10(-9) Omega m(2) and 1.8 x 10(-4) m(2) K/W, respectively. The results of the optimization for the maximum power output and the energy conversion efficiency for Skutterudites thermoelectric materials operating at a maximum temperature difference of 500 K are 30.1 W and 9.87%, respectively. When only the contact resistances are not included, the results rise by 19.4% for the maximum power output and 11.65% for the energy conversion efficiency. When only the Thomson heat is not included, the result rise by 2.66% for the maximum power output and 5.67% for the energy conversion efficiency. These two factors should always be considered in the analysis of thermoelectric modules, neglecting them can lead to an overestimation of the output performance. Scientific and Technological Research Council of TurkeyTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [216M252, 216M254]; Scientific Coordination Unit of Istanbul University [32641, 35577] Scientific and Technological Research Council of Turkey, Grant/Award Numbers: 216M252, 216M254; Scientific Coordination Unit of Istanbul University, Grant/Award Numbers: 32641, 35577

Concentrated photovoltaic - thermoelectric hybrid system is capable of using full solar spectrum and thereby can produce higher output power in comparison with standalone concentrated photovoltaic system. When the surface area of the photovoltaic is different to that of the thermoelectric module, a copper plate sandwiched between the photovoltaic and thermoelectric module can provide a uniform temperature distribution on the hot side of the thermoelectric module and thereby, enhanced thermoelectric performance can be achieved. The conventional copper plate used in the system has some drawbacks, and to address that, a novel trapezoidal copper plate is proposed. The trapezoidal shape obliterates the need to thermally insulate the top/bottom surface of the copper plate. Performance of the proposed system is analyzed and the results revealed maximum energy conversion efficiencies of the photovoltaic and thermoelectric modules as 16.65% and 0.73% obtained at solar irradiance of 100 W m-2 and 1,000 W m-2, respectively. Likewise, the overall highest value of the energy conversion efficiency of the hybrid system is 16.67% obtained at solar irradiance of 100 W m-2.

Recently, copper chalcogenides have attracted great attention due to their potential application for mid- to high-temperature thermoelectric power generation. In this work, we report the thermoelectric properties of Cu2Se compounds with different sample preparation processes and the inclusion of a nanoscale Ag2Se powder synthesized with a unique wet chemistry procedure. The Cu2Se compounds were prepared by solid state reaction (SSR), fast quenching (FQ) and mechanically alloyed with nanostructured Ag2Se (NM) followed by hot pressing. High temperature transport properties were assessed by the Seebeck coefficient, electrical conductivity and thermal conductivity measurements. Structural characterization demonstrates that the nano-Ag2Se included sample is multi-phase with several nanoscale features not seen in the Cu2Se samples prepared in the standard method. As a result, the Cu2Se-NM sample possesses a miniscule thermal conductivity, with values as low as 0.5 W m−1 K−1. Fortunately, the nano-inclusions present in the Cu2Se-NM sample do not significantly disrupt electronic transport, preserving the power factor at a consistently high value over a broad range of temperatures. Consequently, the nano-Ag2Se included sample exhibits large average ZT values and a maximum of 1.85 at 800 K that rivals some of the best thermoelectrics currently available. Here, we present microstructural and transport evidence that the wet chemistry technique implemented in our study enables the optimization of thermoelectric performance in superionic conductor Cu2Se.

Thermoelectric generators have found many applications where the heat source can be either flat or curved. For a curved heat source, flexible thermoelectric generators are generally used. A filler material with low thermal conductivity can provide additional mechanical support to the thermoelectric module and can reduce convection and radiation losses. Herein, the effect of three different filler materials on the output performance of rigid and flexible thermoelectric generators is investigated. At first, theoretical models are derived and the experimental study validated the models. The experimental study revealed that the flexible thermoelectric modules outperformed the rigid modules; this is due to the reduction of the number of thermal junctions in the flexible modules and due to the differences in the thermal conductivities of the flexible and rigid substrates. Likewise, among TE modules without filler/with air between the TE legs, with polyurethane foam filler material, and with polydimethylsiloxane filler material, air has the lowest thermal conductivity, and therefore, the thermoelectric generator without filler generates higher output power and higher power density than when the other two filler materials are used. For the fixed temperature gradient, the highest power densities for the flexible and rigid thermoelectric generators without filler are 155 and 137.7 mu W/cm(2) for temperature gradients of 10.8 and 10.3 degrees C, respectively.

As an earth-abundant, inexpensive and non-toxic compound, Copper Selenide (Cu2Se) is a frequently investigated material for thermoelectric (TE) conversion applications. In this research, stoichiometric Cu2Se compounds were systematically doped with gel-like Carbon Dots (CDs), that were fabricated using a rapid and straightforward solvothermal method, at weight ratios of 2, 5 and 10%. The resultant ingots were spark plasma sintered and their TE performance was characterized. Scanning electron microscopy (SEM), energy dispersive X-Ray spectroscopy (EDX) and powder X-ray diffraction (PXRD) were used to correlate the microstructure to the TE properties. Based on these measurements, CD doping strategy on Cu2Se yielded highly compacted, single phase grains with minimal oxidation. Characterization demonstrated a continuous enhancement of TE figure of merit (ZT) to a maximum of 2.1 at the optimum dopant ratio of 2 wt %. This enhancement was mainly due to the energy filtering effect of CD interfaces along the grain boundaries, and phonon scattering which increased the Seebeck coefficient and reduce the thermal conductivity. Doping beyond 2 wt% was recorded to inhibit this improvement. This research paved the path towards broader utilization of rapidly fabricated CDs to enhance TE conversion performance. (C) 2020 Elsevier B.V. All rights reserved.

Thermoelectric generators harvest additional electrical power when used in combination with concentrated photovoltaic cells given rise to a hybrid system. Overall cost of the system is high; therefore, the parameters of the system need to be optimized to obtain high output performance. This study determines the output performances of four sets of equations (models) used in the hybrid system, using the performance of recently developed nanostructured thermoelectric materials. Seven parameters of the system were optimized through these models using non-dominated genetic algorithm. Models 1 and 2 have the highest performance chosen by TOPSIS decision-making method. The power output and conversion efficiencies of the hybrid system in models 1 and 2 are 426.5 W, 11.45% and 461.12 W, 10.77%, respectively. Likewise, the highest TOPSIS solution for power output of one TEG module operating in the hybrid system and its corresponding efficiency is obtained in model 4 and are 1.97 W and 0.078%, respectively. This validates the fact that TEG operating in a hybrid system has optimum performance at a point when the load resistance is less than its internal resistance.

High cost, and scarcity of high-performance thermoelectric materials are some of the reasons that hinder large scale production of thermoelectric devices. In view thereof, a novel thermoelectric module is proposed. The new module is composed of four thermoelectric materials; two of which are costly but show high-performance at low temperature, while the other two are cheap but show low-performance at room temperature. This combination is aimed at reducing the dependency on the costly and scarce thermoelectric materials, at the same time ensuring good output performance. The thermoelectric and thermomechanical performances of the proposed module are investigated. The analysis revealed that the percentage cost reduction is higher than the percentage reduction in the output power of the module. Furthermore, over the range of the temperature considered in this study, the maximum von Mises stress in the module is lower than the yield stress of the materials. Likewise, cost-power ratio of 0.952 $/W is achieved. (c) 2021 Published by Elsevier Ltd.

In a direct thermal coupling of a concentrated photovoltaic-thermoelectric (CPV-TE) system, copper plate sandwiched between the PV and the TE module provides uniform temperature distribution on the hot side of the TE module. Herein, a trapezoidal copper plate is introduced into the CPV-TE system. The purpose of introducing the trapezoidal copper plate is to enhance the heat transfer from the PV to the TE module, and to reduce both insulation and copper materials usage. Thermal resistance models of both the novel trapezoidal and conventional CPV-TE systems are developed. The performances are evaluated through determining the influences of Thomson effect, thermal paste and length of thermoelements on the systems. The results reveal that the novel systems are superior to the conventional systems. Moreover, for both the conventional and novels systems, Thomson heat has insignificant influence on the performance of the systems. Furthermore, short length thermoelements should be used in the systems. Finally, to avoid significant increase in the thermal resistance, thickness of thermal paste should be as small as possible.

Fused filament fabrication (FFF) is a commonly adopted additive manufacturing technique which allows direct assembly of intricate 3D multi-material components at high resolution and low cost. The aim of this research is to explore FFF processes and their unique capabilities in development of highly efficient thermoelectric (TE) energy harvesting material systems. Adapted fused filament fabrication with customized filaments produced samples with ABS (acrylonitrile butadiene styrene) polymer matrix and Bi2Te3 as the thermoelectric agent. Mixtures were initially extruded into composite thermoelectric filaments, which were subsequently printed into shapes and sintered in a tube furnace under inert gas environment. Thermoelectric performance characterization of the samples revealed that a maximum figure of merit of 0.54 was achieved at the sintering temperature of 500 degrees C for room temperature operation. This conversion efficiency was nearly five times higher than those of previously reported additively manufactured thermoelectric materials. The FFF method was therefore proven as a versatile method to fabricate efficient thermoelectric materials in intricate geometries. (C) 2019 Elsevier Ltd. All rights reserved.

Recently, silver chalcogenides have attracted great attention due to their potential application for room temperature power generation and local cooling. In this work, we report the thermoelectric properties and thermal stability of bulk Ag2Se with nano-Cu2Se inclusions ((Ag2Se)1-x(Cu2Se)x where x = 0, 0.02 and 0.05). Ag2Se samples were prepared via melting, annealing and the nanocomposite was prepared by ball milling this material with required amount of nano-Cu2Se; finally, the samples were consolidated by spark plasma sintering. High temperature and low temperature transport properties were assessed by the measurements of the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. The phase composition and microstructure were explored by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, while the thermal stability of samples was investigated via heating microscopy and heat capacity measurement. Room temperature PXRD and SEM indicated that two separate phases of Ag2Se and Cu2Se form in nano-Cu2Se included composites. Heating microscopy and the heat capacity measurement indicate that the thermal stability of Ag2Se is enhanced with increasing nano-Cu2Se inclusions. The sign of the Seebeck coefficient, in agreement with the Hall coefficient, shows that electrons are the dominant carriers in all samples. The electrical conductivity of the samples increases and the Seebeck coefficient decreases with increasing amount of the nano-Cu2Se inclusion, likely due to augmented carrier concentration. Despite the larger electrical conductivity, the thermal conductivity is suppressed with nano-Cu2Se inclusions. A high power factor and reduced thermal conductivity lead to a maximum ZT value of 0.45 at 875 K for (Ag2Se)1-x(nano-Cu2Se)x sample where x is 0.05.

Recently tetrahedrite compounds have attracted great attention due to their potential applications on the mid-temperature thermoelectric technologies. In this work, structural, Electron Paramagnetic Resonance (EPR) and the thermoelectric properties of tetrahedrite (Cu12-xFexSb4S13 where x = 0.0, 1.0, 1.5, 2.5) were investigated. The crystal structural, morphological properties and elemental compositions of the samples were investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS-EDX) tools respectively. Magnetic properties of the samples were performed by EPR techniques using electron spin resonance (ESR) spectrometer with the model of Bruker EMX series at X-band (9.5 GHz). The thermoelectric properties were assisted based on Seebeck coefficient, electrical resistivity, and thermal conductivity measurement between 300 and 600 K temperature range. PXRD patterns and SEM-EDX analysis confirmed the main phase of tetrahedrite structure for all compounds. The Seebeck coefficient sign indicated that holes were dominant carriers in all compounds. Electrical resistivity measurement showed an increment with increasing Fe concentration likely due to decreasing in carrier density. A typical behavior of intrinsic semiconductor was observed from temperature-dependent electrical resistivity measurements. The maximum ZT value of 0.6 was achieved for the sample Cu11Fe1Sb4S13 at 550 K. Electron Paramagnetic Resonance (EPR) and thermoelectric measurements exhibited that the maximum Fe concentration might be ≤ 1.0 for achieving high ZT value in Cu12-xFexSb4S13 compounds.

We report $^{59}$Co NMR and transport measurements on $n$-type filled skutterudites Ba$_x$Yb$_y$Co$_4$Sb$_{12}$ and $A$$_x$Co$_4$Sb$_{12}$ ($A$= Ba, Sr), promising thermoelectric materials. The results demonstrate consistently that a shallow defect level near the conduction band minimum dominates the electronic behavior, in contrast to the behavior of unfilled CoSb$_3$. To analyze the results, we modeled the defect as having a single peak in the density of states, occupied at low temperatures due to donated charges from filler atoms. We fitted the NMR shifts and spin-lattice relaxation rates allowing for arbitrary carrier densities and degeneracies. The results provide a consistent picture for the Hall data, explaining the temperature dependence of the carrier concentration. Furthermore, without adjusting model parameters, we calculated Seebeck coefficient curves, which also provide good consistency. In agreement with recently reported computational results, it appears that composite native defects induced by the presence of filler atoms can explain this behavior. These results provide a better understanding of the balance of charge carriers, of crucial importance for designing improved thermoelectric materials., Comment: 9 pages, 8 figures

$${BaFe_{4-{x}}Pt_{{x}}Sb_{12}}$$ (x = 0, 0.1, 0.2) compounds were prepared by melting and annealing, followed by a spark plasma sintering method. Low-temperature thermoelectric and magnetic properties were investigated based on Seebeck coefficient, electrical and thermal conductivity and magnetization measurements. The structural properties of $${BaFe_{4-{x}}Pt_{{x}}Sb_{12}}$$ (x = 0, 0.1, 0.2) compounds were ascertained by powder x-ray diffraction analysis, confirming that all samples have a main phase of a skutterudite structure with the space group Im $${\mathrm {\bar{3}}}$$ . The lattice parameters obtained, 9.202(5), 9.199(5) and 9.202(1) A for x = 0, 0.1 and 0.2, respectively, were found consistent with literature. The Seebeck coefficient sign shows that holes are dominant carriers in all compounds. The local maximum Seebeck coefficient was observed around 50 K which may be a trace of paramagnon-drag effect of charge carriers. Thermal conductivity and electrical resistivity measurements were carried out between 4.2 and 300 K. Temperature dependence of electrical resistivity reflects that all samples show semi-metallic behavior in our temperature range of 4.2–300 K. Samples for x = 0.1 and x = 0.2 show Kondo-like behavior. In magnetization measurement, we observe that there are two successive magnetic transitions in Pt-substituted compounds; however, there is only one (transition from a paramagnetic state to long-range magnetic ordering) in Pt-free compounds. In Pt-substituted compounds, the first transition appears at $$ T _{ {\rm c}}$$ = 48 K. In addition, the second transition is observed at $$ T _{ {\rm irr}}$$ = 30 K where an intermediate state is observed before the magnetic ordering transforms to an irreversible ferromagnetic state. We concluded that Pt substitution on the Fe side effectual on the thermoelectric and magnetic properties of $${BaFe_{4-{x}}Pt_{{x}}Sb_{12}}$$ (x = 0, 0.1, 0.2) compounds.

63Cu, 65Cu, and 125Te NMR measurements are reported for Cu2–xTe and Cu1.98Ag0.2Te. The results demonstrate an onset of Cu-ion hopping below room temperature, including an activation behavior consistent with high-temperature transport measurements but with a significant enhancement of the hopping barriers with Ag substitution. We also separated the Korringa behavior by combining NMR line shape and relaxation measurements, thereby identifying large negative chemical shifts for both nuclei, as well as large Cu and Te s-state contributions in the valence band. Further comparison was obtained through heat capacity measurements and chemical shifts computed by density functional methods. The large diamagnetic chemical shifts coincide with behavior previously identified for materials with topologically nontrivial band inversion, and in addition, the large metallic shifts point to analogous features in the valence band density of states, suggesting that Cu2Te may have similar inverted features.

Skutterudite compounds are one of the most promising thermoelectric materials that can be used for intermediate power generation applications because of their excellent thermoelectric and mechanical properties. In spite of extensive research efforts during the past two decades, there is still a lack of full understanding of the electronic and thermal transport in these compounds. In the present study, we carry out a combined experimental and theoretical investigation of the electronic and thermal transport properties of alkaline earth-filled (Ca,Sr,Ba)0.2Co4Sb12 skutterudites in the temperature range from 300 ​K to 800 ​K. The experimental work includes structural properties probed by PXRD and EMPA for phase purity and composition analysis. Theoretical values of the temperature dependent electronic transport parameters were determined using the nearly-free electron approximation, and various phonon thermal conductivity contributions were studied using the Debye isotropic continuum model and the theory of Price for the bipolar contribution. The theoretical and experimental transport parameters were found in good agreement with each other. They confirm that both Ba and Sr filled skutterudites are highly degenerate semiconductors, while the heat transport in the Ca filled compound is dominated by the bipolar contribution at temperatures above 300 ​K. The Sr filled skutterudite shows the highest Seebeck coefficient, likely due to its lowest donor ionisation energy. From our detailed theoretical investigations, it is seen that the key mechanism to control the phonon thermal conduction is originated from the interaction between mass defects and phonons. The maximum ZT value obtained 0.14 for Sr0.2Co4Sb12 compound at 800 ​K likely indicate that Sr is more effective filler among the alkaline earth metals.

Thermoelectric (TE) energy harvesters are multi-material solid-state devices that convert heat (i.e. a thermal gradient) directly into electric potential. Currently, the biggest challenge limiting the applications of thermoelectric devices is the low conversion efficiency (< 10%). To achieve higher thermoelectric efficiency, electrical conductivity and Seebeck coefficient of thermoelectric materials must be maximized allowing the flow of charge carriers and thermal conductivity must be minimized keeping high temperature gradient between hot and cold sides. These properties are strongly coupled to each other. In other words, improving one property deteriorates the other. In nanoscale however, manipulation of matter at the atomic level can decouple these properties. Nanoengineering is therefore considered to be the only remedy for the low conversion efficiency of thermoelectric materials. Current nanomanipulation techniques focus only on reducing thermal conductivity by scattering heat carrying phonons with nanoscale artifacts. We have observed that doping thermoelectric material with carbon quantum dots (size < 5 nm) tremendously increased electrical conductivity and thermoelectric power. In the control experiments using carbon powder (same chemical arrangement but larger scale, < 100 nm), we did not observe any increase in thermal power density evidencing the nanomanipulation of material properties using carbon quantum dots. Doping thermoelectric materials with carbon quantum dots has high potential due to the quantum enhancement effects on electrical properties of and needs to be further investigated for the design of novel nanocomposite materials with superior thermoelectrical properties.

We report 63Cu and 65Cu NMR on Cu2Se, a superionic conductor of interest for thermoelectric and other applications. Results demonstrate an initial appearance of ionic hopping in a narrow temperature range above 100 K, coinciding with the recently observed low-temperature phase transition. At room temperature and above, this goes over to rapid Cu-ion hopping and a single motionally narrowed line both above and below the α–β structural transition. The low-temperature α′ phase exhibits a sizable metallic shift, in contrast to the high-temperature structures. The large variation in native carrier density, significantly enhanced at positions associated with Cu vacancies, indicates the strong influence of an impurity band on low-temperature electronic transport.

We demonstrate a prototype thermal evolution path for liquid thermal conductivity in solids. Thermal evolution of β -Cu2Se shows large interstitial displacement of constituent atoms marked by glass-like transitions and an asymptotic liquid thermal transport. Using ab initio molecular dynamics (AIMD), we identify these transitions, and confirm them with in situ transmission electron microscopy and electron energy loss spectroscopy. The thermal disorder of the Cu+ ions forms homopolar Cu–Cu bonds under a rigid Se framework, and at yet higher temperatures the Se framework undergoes thermal distortion. The non-equilibrium AIMD prediction of lattice thermal conductivity shows significant suppression of the phonon transport, in agreement with experiments and molecular behavior.

It is well known that Ba and Yb are effective pair-filling species in skutterudites. They offer a large mass and size contrast therefore yielding widely different phonon resonant scattering frequencies. These features of Ba and Yb fillers inspired our investigation of the high temperature thermoelectric properties of Fe substituted and Ba/Yb double-filled skutterudite compounds of compositions BaxYbyFe0.4Co3.6Sb12 (0.25 ⩽ x, y ⩽ 0.4 nominal). We measured the Seebeck coefficient, electrical conductivity, thermal conductivity, and the Hall coefficient from 300 K to 800 K. All samples were prepared using the traditional method consisting of melting-annealing followed by spark plasma sintering. The Hall coefficient and the Seebeck coefficient signs indicate that all samples are n-type conductors. The electrical conductivity increases and the Seebeck coefficient decreases with the increasing Ba content, likely due to the increasing carrier density as the amount of Ba increases. It is seen that Ba is more effective than Yb in tuning electronic transport properties while Yb has a greater effect on the lattice thermal conductivity. EMPA and PXRD analyses indicate that increasing the Ba content reduces the filling fraction of Yb, therefore the solubility limit of combined Ba + Yb filling remains about 0.4 (actual) for BaxYbyFe0.4Co3.6Sb12 (0.25 ⩽ x, y ⩽ 0.4 nominal) compositions. The combination of a high power factor (47 μW/K2 cm) and a reasonably low thermal conductivity (2.79 W/m K) of Ba0.3Yb0.3Fe0.4Co3.6Sb12 (nominal) leads to a value of the thermoelectric figure of merit ZT∼1.35 at 800 K, one of the highest values of ZT for double-filled n-type skutterudite compounds.

Fe-substituted double-filled Ba x Yb y Fe z Co 4− z Sb 12 ( x =0.1, y =0.2 and z =0.0–0.4 nominal) compounds were synthesized using a melting-annealing-spark plasma sintering (SPS) method. Their thermoelectric properties were assessed by measuring the Seebeck coefficient, electrical conductivity, thermal conductivity and the Hall coefficient. The sign of the Hall coefficient indicates that electrons are the dominant carriers in all compounds except Ba 0.1 Yb 0.2 Fe 0.4 Co 3.6 Sb 12 . The temperature dependence of the electrical conductivity and the carrier concentration reflect the transition from extrinsic to intrinsic behavior depending on the amount of Fe substituted for Co. Jonker and Ioffe analyses are applied to Fe-substituted double-filled Ba x Yb y Fe z Co 4− z Sb 12 compounds in order to evaluate the range of minimum and maximum power factors achievable in n-type filled skutterudite compounds at room temperature (300 K). The predicted maximum room temperature power factor values in the range of 15–45 μW/K 2 cm are comparable to experimentally reported values of n-type skutterudite compounds.

Substituting Fe on Co sites is an effective way to produce p-type skutterudite compounds as well as to reduce the thermal conductivity of skutterudites. In this work, we investigated thermoelectric properties of Fe-substituted and Ce + Yb double-filled Ce x Yb y Fe z Co4−z Sb12 (x = y = 0.5, z = 2.0 to 3.25 nominal) skutterudite compounds by studying the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient over a broad range of temperatures. All samples were prepared by using the traditional method of melting–annealing and spark plasma sintering. The signs of the Hall coefficient and Seebeck coefficient indicate that all samples are p-type conductors. Electrical conductivity increases with increasing Fe content. The temperature dependence of electrical conductivity indicates that a transition from the extrinsic to the intrinsic regime of conduction depends on the amount of Fe substituted for Co. The temperature dependence of mobility reflects the dominance of acoustic phonon scattering at temperatures above ambient. Except for Ce0.5Yb0.5Fe3.25Co0.75Sb12, the thermal conductivity increases with increasing Fe content, reaching the maximum value of 2.23 W/m K at room temperature for Ce0.5Yb0.5Fe3CoSb12. A high power factor (27 μW/K2 cm) combined with a rather low thermal conductivity for Ce0.5Yb0.5Fe3.25Co0.75Sb12 (nominal) lead to a dimensionless figure of merit ZT = 1.0 at 750 K for this compound, one of the highest ZT values achieved in p-type skutterudite compounds prepared by the traditional method of melting–annealing and spark plasma sintering.

A facile and high yield synthesis route was developed for the fabrication of bulk nanostructured copper selenide (Cu2Se) with high thermoelectric efficiency. Starting from readily available precursor materials and by means of rapid and energy-efficient microwave-assisted thermolysis, nanopowders of Cu2Se were synthesized. Powder samples and compacted pellets have been characterized in detail for their structural, microstructural and transport properties. α to β phase transition of Cu2Se was confirmed using temperature dependent X-ray powder diffraction and differential scanning calorimetry analyses. Scanning electron microscopy analysis reveals the presence of secondary globular nanostructures in the order of 200 nm consisting of

Indium-filled skutterudites are promising power generation thermoelectric materials due to the presence of an InSb nanostructure that lowers the thermal conductivity. In this work, we have investigated thermoelectric properties of triple-filled Ba x Yb y In z Co4Sb12 (0 ≤ x, y, z ≤ 0.14 actual) compounds by measuring their Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. All samples were prepared by a melting–annealing–spark plasma sintering method, and their structure was characterized by x-ray diffraction and transmission electron microscopy (TEM). TEM results show the development of an InSb nanostructure with a grain size of 30 nm to 500 nm. The nanostructure is present in all samples containing In and is also detected by specific heat measurements. The Seebeck and Hall coefficients indicate that the compounds are n-type semiconductors. Electrical conductivity increases with increasing Ba content. Thermal conductivity is strongly suppressed upon the presence of In in the skutterudite structure, likely due to enhanced boundary scattering of phonons on the nanometer-scale InSb inclusions. The highest thermoelectric figure of merit is achieved with Ba0.09Yb0.07In0.06Co4Sb11.97, reaching ZT = 1.25 at 800 K.

We have shown previously that indium doping is beneficial for thermoelectric properties of (Sb0.75Bi0.25)2Te3. This effect was ascribed to a change in the magnitude and mechanism of hole scattering and a decrease in thermal conductivity. Since the state-of-the-art material for p-type legs in low-temperature applications is the quaternary Bi0.5Sb1.5Te3−ySey, we have attempted to dope this material with In, hoping to improve its properties further. Indeed, the doping enhances the figure of merit of (Sb0.75Bi0.25)2−xInxTe2.8Se0.2 by more than 15% compared with the values measured on undoped (Sb0.75Bi0.25)2Te2.8Se0.2 below room temperature.

We explored the effect of Cd substitution on the thermoelectric properties of PbTe in an effort to test a theoretical hypothesis that Cd atoms on Pb sites of the rock salt lattice can increase the Seebeck coefficient via the formation of a resonance level in the density of states near the Fermi energy. We find that the solubility of Cd is less than previously reported, and CdTe precipitation occurs to create nanostructuring, which strongly suppresses the lattice thermal conductivity. We present detailed characterization including structural and spectroscopic data, transmission electron microscopy, and thermoelectric transport properties of samples of PbTe-x% CdTe-0.055% PbI(2) (x = 1, 3, 5, 7, 10), PbTe-1% CdTe-y% PbI(2) (y = 0.03, 0.045, 0.055, 0.08, 0.1, 0.2), PbTe-5% CdTe-y% PbI(2) (y = 0.01, 0.03, 0.055, 0.08), and PbTe-1% CdTe-z% Sb (z = 0.3, 0.5, 1, 1.5, 2, 3, 4, 5, 6). All samples follow the Pisarenko relationship, and no enhancement of the Seebeck coefficient was observed that could be attributed to a resonance level or a distortion in the density of states. A maximum ZT of approximately 1.2 at approximately 720 K was achieved for the PbTe-1% CdTe-0.055% PbI(2) sample arising from a high power factor of approximately 17 microW/(cm K(2)) and a very low lattice thermal conductivity of approximately 0.5 W/(m K) at approximately 720 K.

Bi 2 O 2 Se was synthesized from Bi 2 Se 3 and Bi 2 O 3 by solid state reaction in an evacuated quartz ampoule. The product crystallizes in the tetragonal type lattice and X-ray pattern displays lines of the Bi 2 O 2 Se only. Samples for thermoelectric property measurements were prepared using hot pressing in rectangular graphite dies. The samples were characterized by the measurements of Seebeck coefficient, electrical conductivity and thermal conductivity as a function of temperature. From the experimental data we calculate the dimensionless figure of merit ZT that for a non-optimized material approaches the value 0.2 at 800 K.

Ba+Yb double-filled n-type skutterudites with a modest degree of charge compensation by Fe on the Co lattice have been synthesized and compacted by spark-plasma sintering, and their thermoelectric properties evaluated at temperatures up to 800 K. Although this approach to making n-type skutterudites seems counterintuitive, the presence of Fe leads to a reduction in the thermal conductivity while it preserves a robust Seebeck coefficient. Consequently, a high ZT in excess of 1.3 was achieved at 800 K in these Fe-containing n-type skutterudite compounds.

The microstructure and thermoelectric properties of the PbTe-Si eutectic system are presented in detail. When rapidly quenched from the melt this system yields materials with thermoelectric properties similar to PbTe itself but with improved mechanical properties. Doping optimization was performed using PbI2 as an n-type dopant giving precise control of the thermoelectric properties. Electron microscopy indicates that the PbTe-Si system is both a nanocomposite and microcomposite. Despite the added Si, the thermal conductivity of this composite follows closely that of PbTe. The temperature dependence of the Lorenz number was estimated, and it shows a significant departure from the value of metals (L0) reaching only 45% of L0 at 650 K. The optimized ZT for the PbTe-Si(8%) eutectic was 0.9 at 675 K. The improved mechanical robustness of these composites makes them attractive for use in large scale thermoelectric device fabrication.

The thermoelectric properties of Yb0.2In0.2Co4Sb12, Yb0.2Ce0.15Co4Sb12, Yb0.2Ce0.15In0.2Co4 Sb12, and Yb0.3Ce0.15In0.2Co4 Sb12 (obtained from the elements by melting—annealing followed by spark plasma sintering) are examined by electrical and thermal conductivity, Seebeck coefficient and Hall coefficient measurements.

We explored the effect of K and K-Na substitution for Pb atoms in the lattice of PbTe, in an effort to test a hypothesis for the development of a resonant state that may enhance the thermoelectric power. At 300K the data can adequately be explained by a combination of a single and two-band model for the valence band of PbTe depending on hole density that varies in the range 1-15 × 1019 cm-3. A change in scattering mechanism was observed in the temperature dependence of the electrical conductivity, σ, for samples concurrently doped with K and Na which results in significantly enhanced σ at elevated temperatures and hence power factors. Thermal conductivity data provide evidence of a strong interaction between the light- and the heavy-hole valence bands at least up to 500K. Figure of merits as high as 1.3 at 700K were measured as a result of the enhanced power factors.

The effect of K and K-Na substitution for Pb atoms in the rock salt lattice of PbTe was investigated to test a hypothesis for development of resonant states in the valence band that may enhance the thermoelectric power. We combined high temperature Hall-effect, electrical conductivity and thermal conductivity measurements to show that K-Na co-doping do not form resonance states but2 can control the energy difference of the maxima of the two primary valence sub-bands in PbTe. This leads to an enhanced interband interaction with rising temperature and a significant rise in the thermoelectric figure of merit of p-type PbTe. The experimental data can be explained by a combination of a single and two-band model for the valence band of PbTe depending on hole density that varies in the range of 1-15 x 10^19 cm^-3., Comment: 8 figures

Cu2Se, Cu2Te and Ag-overstoichiometric compounds Cu1.98Ag0.2Se and Cu1.98Ag0.2Te were prepared by melting, annealing, followed by spark plasma sintering compaction. Low and high temperature thermoelectric properties were investigated by measuring the electrical conductivity, Seebeck coefficient, thermal conductivity and Hall coefficient between 2 K and 900 K. Structural analyses were performed by PXRD and SEM-EDX analyses. The Hall and Seebeck coefficients show that holes are the dominant carrier in all compounds. High temperature α–β phase transition in Cu2Se and Cu1.98Ag0.2Se between 350 and 400 K and multiple phase transitions (α–β, β–γ, γ–δ, δ–∈) in Cu2Te and Cu1.98Ag0.2Te between 350 K and 900 K were observed in measurements of heat capacity, temperature dependent PXRD data, and transport coefficients. Low temperature transport measurements (Hall coefficient, electrical conductivity, carrier mobility) strongly suggest the presence of yet another phase transition in Cu2Se, Cu1.98Ag0.2Se, and Cu1.98Ag0.2Te compounds at temperatures between 85 K and 115 K, reported here for the first time. Based on the transport data and structural analysis we conclude that doping Cu2Se and Cu2Te by Ag reduces the density of holes and strongly suppresses the thermal conductivity not only due to a smaller electronic contribution but also due to enhanced point defect scattering of phonons that reduces the lattice portion of the thermal conductivity. Moreover, the phase transition temperature is shifted to lower temperatures upon doping with Ag. The presence of Ag enhances thermoelectric performance of Cu2Te at all temperatures and Cu2Se benefits from Ag doping over a broad range of temperatures up to 700 K. The maximum ZT value of 1.2 at 900 K; 0.52 at 650 K; 0.29 at 900 K; and 1.0 at 900 K were achieved for Cu2Se, Cu1.98Ag0.2Se, Cu2Te and Cu1.98Ag0.2Te, respectively, between 2 K and 900 K.

The solution synthesis route as a scalable bottom-up synthetic method possesses significant advantages for synthesizing nanostructured bulk thermoelectric (TE) materials with improved performance. Tuning the composition of the materials directly in the solution, without needing any further processing, is important for adjusting the dominant carrier type. Here, we report a very rapid (2 min) and high yield (&gt, 8 g/batch) synthetic method using microwave-assisted heating, for the controlled growth of Bi2&ndash, xSbxTe3 (x: 0&ndash, 2) nanoplatelets. Resultant materials exhibit a high crystallinity and phase purity, as characterized by XRD, and platelet morphology, as revealed by SEM. Surface chemistry of as-made materials showed a mixture of metallic and oxide phases, as evidenced by XPS. Zeta-potential analysis exhibited a systematic change of isoelectric point as a function of the material composition. As-made materials were directly sintered into pellets by using spark plasma sintering process. TE performance of Bi2&minus, xSbxTe3 pellets were studied, where the highest ZT values of 1.04 (at 440 K) for Bi2Te3 and 1.37 (at 523 K) for Sb2Te3 were obtained, as n- and p-type TE materials. The presented microwave-assisted synthesis method is energy effective, a truly scalable and reproducible method, paving the way for large scale production and implementation of towards large-area TE applications.