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We demonstrate thermoelectric transport properties of Zintl arsenide EuCuAs. The crystal structure of EuCuAs (hexagonal P63/mmc space group) consists of a covalently-bonded honeycomb-type [CuAs] network sandwiched by nearly divalent Eu ions. Undoped EuCuAs exhibit the thermoelectric power factor of 1.2 and 0.4 mW/mK2 at 673 K along directions perpendicular and parallel to the uniaxial hot-pressing direction, respectively. Despite the relatively high power factor, the dimensionless figure-of-merit is limited to 0.1 at 673 K due to the high lattice thermal conductivity, > 2.0 W/mK. In addition to the relatively high power factor, first-principles calculations predicted that, owing to a peculiar shape of the Fermi surface, heavy hole-doped EuCuAs with a hole concentration of > 1.5 × 1021 cm–3 will exhibit axis-dependent conduction polarity, which enables us to construct transverse thermoelectric devices. We investigated the synthesis and thermoelectric transport properties of Eu1–xNaxCuAs using sodium (Na) as a hole dopant, resulting in a hole concentration of 1.0 × 1021 cm–3. The absolute value of the Seebeck coefficient was decreased by Na-doping, as predicted by first-principles calculations, but no conduction polarity switching was observed. This may have resulted from an insufficient hole concentration and/or preferred orientation of the samples. [ABSTRACT FROM AUTHOR]

A concentrated photovoltaic–thermoelectric (CPV–TE) hybrid system, if operated properly, can generate higher output power than a stand-alone concentrated photovoltaic system, due to the contribution of a thermoelectric generator in the system. Herein, to understand the effect of the transport properties of thermoelectric (TE) materials on the hybrid system, a figure of merit-based analysis is conducted. The results reveal that the output performance of the TE module can be enhanced by almost 100% when the value of the dimensionless figure of merit increases from 1 to 2. Moreover, at higher concentration ratios, the output power of the TE module with an average figure of merit of 2 can exceed that of the photovoltaic cell. Under the same circumstances, the energy efficiency of the hybrid system increases with an increase in the concentration ratio. Furthermore, the output performance of the CPV–TE hybrid system can be improved if TE materials with very low thermal conductivity are used. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) in conjugation with non-equilibrium Green's function (NEGF) is used to investigate the structural, electronic, and transport properties of the pristine and fluorinated zigzag ZnO nanoribbons (ZnONRs). For the fluorinated nanoribbons, the bond lengths at the edges are increased due to the excessive atomic radius of fluorine (F) relative to hydrogen (H). The stability increases with fluorination at the Zn-rich edge and decreases when F is passivated at the O-rich edge. The structure with fluorination at the Zn-rich edge (F-Zn-ZnONR) is the most stable with a binding energy ( E b ) of about −5.052 eV, and the structure with fluorination at the O-rich edge (F-O-ZnONR) is the least stable with a E b of −4.607 eV. All the structures are semiconducting and the pristine structure has the highest energy band gap ( E g ) of about 0.531 eV followed by the lowest E g of 0.011 eV for the structure with fluorination at both edges (F-ZnO-ZnONR). Partial density of states (PDOS) study depicts that the new energy states introduced closer to the Fermi energy level are due to the F atoms. All the devices except the pristine device exhibit negative differential resistance (NDR) characteristics. A significant NDR mechanism is observed in all the fluorinated ZnONRs. The highest peak-to-valley current ratio (PVCR) of about 1.79 × 10 2 is reported for the F-ZnO-ZnONR device. ZnONRs with the reported NDR behavior can be used for designing the future nanoelectronic devices, oscillators, frequency converters, switches, rectifiers, resonators etc. [ABSTRACT FROM AUTHOR]

In this work, gel polymer electrolyte systems based on a polymethyl methacrylate (PMMA) and polylactic acid (PLA) blend that was doped with various compositions of lithium bis(oxalato) borate (LiBOB) were successfully prepared. Several characterizations, which included Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and electrical impedance spectroscopy, were carried out to determine their structural and ionic conduction properties. FTIR analysis revealed that molecular interactions had occurred via Li+ ions in several regions, representing the functional groups of C-O, C=O and C-H stretching of the PMMA–PLA. Moreover, an increase in the amorphous phase upon the incorporation of LiBOB was revealed through XRD analysis. A sample containing 20 wt.% of LiBOB (PPLi20) was found to be the most amorphous sample in this study. This result is in alignment with the ionic conduction properties, showing an increase of ionic conductivity up to the PPLi20 sample, which exhibited the highest ionic conductivity with a value of 1.37 × 10−3 S cm−1. The contribution of Li+ ions towards the enhancement of ionic conductivity was determined through the transport parameter analysis. It was proven that upon the addition of LiBOB, the value of η, μ , and D increased, which signified the high dissociation of ions. Beyond 20 wt.%, the transport parameters decreased due to the overcrowding of ions. [ABSTRACT FROM AUTHOR]

With the rapid evolution of microelectronics, the field of integrated circuits is facing unprecedented challenges. Traditional silicon-based transistors cannot maintain the advantages of high performance during the process of further ultra-scaling due to severe short-channel effects. Two-dimensional (2D) materials are potential channel materials that can replace silicon. Herein, 2D SiP is predicted to have a band gap of 1.49 eV with anisotropic electronic properties by means of first-principles calculations, which is suitable as a channel candidate of transistors. Hence, we investigate the ballistic transport properties of 2D SiP double-gate metal oxide semiconductor field-effect transistors (MOSFETs) by using ab initio quantum transport simulations. Despite anisotropic electronic properties of 2D SiP, the performances of monolayer SiP MOSFETs have weak directional dependence due to high valley degeneracy. The n-MOSFETs with 10-nm gate length can fulfill the high-performance requirements of the International Roadmap for Devices and Systems 2020 Edition (IRDS 2020). However, the p-MOSFETs cannot fulfill the demands of IRDS 2020 because of heavy hole effective masses. Considering the appropriate on-current of 1292 μA/μm for SiP n-MOSFETs, 2D SiP could be utilized as a potential channel material in the next-generation FETs. [ABSTRACT FROM AUTHOR]

We study the effects of interfaces on 2-demensional electron gas (2DEG) sandwiched between two semi-infinite leads with different width. The tight-bind Hamiltonian and the recursive Green's function formalism are used to determine the electronic structure and transport properties. The results show that the quantized conductance can be obtained only when the leads are wider than the center device. Each conductance plateau possesses many oscillations which are associated with the aspect ratio of the center device. If the leads become narrower than center device, the conductance is suppressed dramatically and the conductance plateaus disappear. In the limit width of leads, we can observe some discrete conductance spikes with a maximum of 1. These spikes are determined by the extended eigenstates of our system. These results show that the lead-device interface plays an important role in two-terminal transport. [ABSTRACT FROM AUTHOR]

We report the magnetization and electrical resistivity measurements of LaFeAsO and oxygen deficiency LaFeAsOx prepared by solid-state reaction. Superconductivity at around 35 K and a giant diamagnetic contribution at a higher temperature have been observed in LaFeAsOx, while the parent LaFeAsO is non-superconducting. However, after subtracting the paramagnetic background magnetic contribution, the parent compound exhibits a very small superconductivity at low temperatures below Tc ~ 35. The ZFC/FC (zero-field and field cooled magnetization) curves for both samples exhibit apparent irreversibility. The susceptibility increases significantly at low fields. All these behaviors seem to be explained by considering the system consisting of micro-scale weak poly-crystallite magnets owing to the dipolar magnetic interactions among them and individual localized nanoscale magnetic regions within crystallites. Furthermore, we have obtained the critical current density and the pinning force as a function of the applied field using the conventional Beans model. We conclude that the observed superconductivity has a filamentary character based on the implication of the superconductivity properties analyses. [ABSTRACT FROM AUTHOR]

A first-principles investigation is carried out on palladium (Pd)-doped armchair graphene nanoribbons (AGNRs) to investigate their structural, electronic and transport properties. The structural analysis of the considered Pd-doped nanoribbons reveals that single Pd doping at the edges of AGNRs results in the most stable configuration. The present findings reveal that the electronic transport properties are strongly dependent on the number of dopant atoms and their positions. Furthermore, it is noted that the proposed two-probe devices exhibit peculiar nonlinear I–V characteristics indicating potential for rectification behavior. Excellent high rectification ratio (RR) and reverse rectification ratio (RRR) are on the order of 1.8 × 10 5 and 9.7 × 10 4 , respectively, are found for center-Pd-doped 7-AGNRs. The interesting rectifying I–V behavior can be explained by the localization/delocalization effect of frontier orbitals along with the variation of the transmission spectra with the applied bias voltage. These findings indicate that Pd-doped armchair GNRs are a potential candidate for use in next-generation ultralow-power nanoscale switching devices. [ABSTRACT FROM AUTHOR]

The electronic and transport properties of Sn-doped Sb2Te3 have been investigated by using the Heyd–Scuseria–Ernzerhof hybrid functional with spin orbit coupling. Results show that Sn preferentially substitute Sb to induce the p-type characteristics in Sb2Te3. Detailed thermodynamic examinations have been conducted to elaborate the favorability for Sn to act as a shallow acceptor in Sb2Te3. Extensive calculations of transport properties reveal that Sn doping gives rise to remarkable enhancements in hole mobility, electrical conductivity, Seebeck coefficient and power factor of Sb2Te3, thus significantly improving the thermoelectric performance. The present work offers a valuable insight on how Sn doping strongly influences the electronic and transport properties of Sb2Te3. [ABSTRACT FROM AUTHOR]

CuInTe2 is a ternary chalcopyrite of the type I-III-VI2 (where I = Cu, Ag; III = Al, Ga, In; and VI = S, Se, Te). This group of materials represents a broad class of semiconductors with potential applications in optoelectronics and photovoltaics and could be interesting thermoelectric materials in the middle temperature range. CuInTe2 is able to show relatively large stoichiometric deviations due to the low energies of formation of native defects. In this work, the effects of non-stoichiometric indium on the transport and thermoelectric properties of the CuIn1−xTe2 system are systematically studied in the x range of − 0.01 to 0.06. The results x-ray diffraction, differential thermal analysis and scanning electron microscopy measurements suggest that eutectoid of Cu-Te is present as an extraneous phase in most of the prepared samples. Moreover, the chemical composition of the extraneous phases is temperature-dependent, and thus the chemical composition of the main phase is equally so. The observed changes in all the studied transport and thermoelectric parameters upon varying the indium stoichiometry support the idea that unlike the Cu sublattice, the indium sublattice in CuInTe2 can be doped to improve the thermoelectric properties of the material. [ABSTRACT FROM AUTHOR]

Transport properties of Indium Arsenide (InAs) films deposited at different working pressures are investigated in the temperature interval of T ∼ 2-250 K. Electrical transport of InAs films is related to the degree of disorder, which increases as working pressure increases. Activated conductivity models satisfactorily describe conduction in the localized band-tails at high temperatures, while conductivity shows metallic characteristic at low temperatures for μc-InAs samples. The critical conductivity exponent of μc-InAs is close to 1, which indicates that μc-InAs samples exhibit a close proximity to the metal-insulator transition. The product of aB and n1/3 for samples S1 and S2 is 0.236 and 0.207, respectively, which is roughly consistent with the Mott criterion. But for α-InAs samples in the high temperature region, electron transport is described by conduction in extended states and in localized states near the Fermi level. Furthermore, a crossover from Mott variable range hopping (VRH) to the Efros-Shklovskii VRH mechanism is observed in α-InAs thin films at low temperatures. [ABSTRACT FROM AUTHOR]

The electronic, mechanical and thermoelectric properties of Zr2CoZ (Z = Si, Ge) Heusler alloys are investigated by the full-potential linearized augmented plane wave method. Using the Voigt-Reuss approximation, we calculated the various elastic constants, the shear and Young's moduli, and Poisson's ratio which predict the ductile nature of the alloys. Thermoelectric coefficients viz., Seebeck, electrical conductivity and figure of merit show Zr2CoZ alloys as ntype thermoelectric materials showing a linearly increasing Seebeck coefficient with temperature mainly because of the existence of almost flat conduction bands along L to U directions of a high symmetry Brillouin zone. The efficiency of conversion was measured as the figure of merit by taking into effect the lattice thermal part that achieves an upper-limit of 0.14 at 1200 K which may favour their use for waste heat recovery at higher temperatures. [ABSTRACT FROM AUTHOR]

Polycrystalline Cu3BiS3 was synthesized and densified using hot pressing in order to investigate the physical properties of this material. Both the thermal conductivity and the Seebeck coefficient of Cu3BiS3 are reported for the first time in order to investigate the thermoelectric properties of this material. The ultralow thermal conductivity coupled with the relatively high Seebeck coefficient, 0.17 W/m-K and 540 μ/K at room temperature, respectively, suggest Cu3BiS3 may show promise for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Ceramic samples with the composition BiGeOSe ( x = 0, 0.05, 0.075, and 0.1) were synthesized by solid-state reaction and compacted using a hot-pressing technique. The prepared materials were characterized by x-ray diffraction analysis, electron microscopy, and measurements of electrical conductivity σ, Seebeck coefficient S, and thermal conductivity in the temperature range 300-780 K. Ge in the BiOSe host structure led to an increase of the free electron concentration compared to pristine BiOSe. The donor effect is attributed to point substitutional defects in the Bi sublattice- $$ {\hbox{Ge}}_{\rm{Bi}}^{ + }$$ , and oxygen vacancies $$ {\hbox{V}}_{\rm{O}}^{ + 2}$$ producing free electrons. As a result, we observe an increase in the electrical conductivity and decrease in Seebeck coefficient while thermal conductivity κ changes slightly. The highest value of the dimensionless figure of merit ZT = σS T/ κ reaches 0.25 for the composition BiGeOSe at T = 723 K, which is, to date, the highest ZT value reported for BiOSe ceramics. Our results suggest that BiOSe is still worth exploring. [ABSTRACT FROM AUTHOR]

Ab initio calculations of Ag-based fluoroperovskite AgMF (M = Co and Ni) compounds are investigated using the full-potential linearized augmented plane wave method. Wien2k and BoltzTrap codes are used to calculate the different physical properties. The structural parameters of the present compounds are within reasonable agreement with previous calculations. This study shows that AgCoF and AgNiF are anisotropic, ductile, mechanically and thermodynamically stable␣compounds, where AgCoF is found to be stiffer and less compressible than AgNiF. The spin-polarized electronic band structure illustrates that AgCoF is metallic, while AgNiF is a semiconductor with indirect (M-Г) band gap energy of 0.43 eV. The bonding force between atoms is found to be mainly ionic with some covalent nature. The total magnetic moment of AgCoF (3.04 μ ) is found to be higher than that calculated for AgNiF (2.00 μ ). Using the magnetic susceptibility calculations, AgCoF is classified as antiferromagnetic, whereas AgNiF is a ferromagnetic compound. The calculated static refractive index of AgCoF (3.85) and AgNiF (3.60) is inversely proportional with the energy band gap. Suitable␣applications are predicted for AgCoF and AgNiF based on their absorption and reflection properties. Furthermore, beneficial thermoelectric applications are expected for the present compounds due to their large Seebeck coefficient ( $$ S_{{{\rm{AgCoF}}_{ 3} }} = 2.92 \times 10^{3} \,\, \mu {\hbox{V/K}}\,\,{\hbox{and}}\,\,\,S_{{{\rm{AgNiF}}_{3} }} = 2.84 \times 10^{3} \,\,\mu {\hbox{V/K}} $$ ) and their thermoelectric power factor with respect to relaxation time ( $$ S^{2} \sigma /t_{{AgNiF_{3} }} = 1.11 \times 10^{9} \,\,{\hbox{W/K}}^{ 2} \,\,{\hbox{and}}\,\,S^{2} \sigma /t_{{AgNiF_{3} }} = 1.28 \times 10^{11} \,\,{\hbox{W/K}}^{ 2} $$ ). [ABSTRACT FROM AUTHOR]

We report thermoelectric transport properties of dual-substituted CaMnNbMO (M = Mo, W; 0.02 ≤ x ≤ 0.06) synthesized by conventional solid-state methodology. The electrical resistivity decreases with the increase of the doping level, which indicates an increase in charge-carrier concentration in the system. The increase in carrier concentration leads to the formation of Mn ions with e electrons in the Mn matrix of CaMnNbMO (M = Mo, W). The electrical resistivity shows non-metal-like temperature dependence. In contrast, the Seebeck coefficient of CaMnNbMO (M = Mo, W) initially decreases with temperature up to 550 K and then increases. The dual substitution by pentavalent Nb and hexavalent Mo or W at the Mn-site causes partial reduction of Mn → Mn and produces a defect center in the Mn-sublattice of CaMnNbMO (M = Mo, W). This defect center acts as a static point-like defect center to cause an additional contribution to the entropy of the system which results in an increase of the Seebeck coefficient with temperature. The highest power factor obtained is of 192 μW m K at 950 K for CaMnNbMoO. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) nanowire structures with and without hydrogen (H) and ammonia (NH) molecules have been constructed and optimized using density functional theory to study their electronic and transport properties. The adsorption energies calculated for the SiC nanowire structures reveal that the adsorption process of H and NH molecules is endothermic in nature. Nonequilibrium Green's function transport theory is employed to study the electronic transport properties of the SiC nanowire devices with and without H and NH molecules. The voltage-current ( V- I) characteristic shows negative differential resistance (NDR) behavior for all the SiC nanowire devices when bias voltage is applied. It is inferred that the NDR behavior is due to shift of quasibound states near the Fermi level because of the applied bias voltage. This observed NDR behavior may be useful for fabrication of nanoelectronic devices. [ABSTRACT FROM AUTHOR]

BiTe compound and BiTe-based solid solutions have attracted much attention as promising thermoelectric materials for refrigerating devices. The possibility of enhancing the thermoelectric efficiency in low-dimensional structures has stimulated studies of BiTe thin films. Now, interest in studying the transport properties of BiTe has grown sharply due to the observation of special properties characteristic of three-dimensional (3D) topological insulators in BiTe. One of the possible manifestations of quantum size effects in two-dimensional structures is an oscillatory behavior of the dependences of transport properties on film thickness, d. The goal of this work is to summarize our earlier experimental results on the d-dependences of transport properties of BiTe thin films obtained by thermal evaporation in a vacuum on glass substrates, and to present our new results of theoretical calculations of the oscillations periods within the framework of the model of an infinitely deep potential well, which takes into account the dependence of the Fermi energy on d and the contribution of all energy subbands below the Fermi level to the conductivity. On the basis of the data obtained, some general regularities and specificity of the quantum size effects manifestation in 3D topological insulators are established. [ABSTRACT FROM AUTHOR]

Bi doped tetrahedrites with nominal compositions of CuSbBiS ( x = 0, 0.2, 0.4, 0.6, 0.8) were synthesized by the solid state reaction method. Powder x-ray diffraction patterns confirmed that CuSbS (tetrahedrite structure) was the main phase, along with CuSbS and CuSbS as the secondary phases. Electron probe microanalysis provided the elemental composition of all the samples. It was confirmed that the main phase is the tetrahedrite phase with slight deviations in the stoichiometry. All the transport properties were measured between 423 K and 673 K. The electrical resistivity increased with an increase in Bi content for all the samples, possibly induced by the variation in the carrier concentration, which may be due to the influence of impurity phases. The increase in electrical resistivity with an increase in temperature indicates the degenerate semiconducting nature of the samples. The absolute Seebeck coefficient is positive throughout the temperature range indicating the p-type nature of the samples. The Seebeck coefficient for all the samples increased with an increase in Bi content as electrical resistivity. The variation of electrical resistivity and the Seebeck coefficient with doping can be attributed to the changes in the carrier concentration of the samples. The total thermal conductivity increases with an increase in temperature and decreases with an increase in the Bi content that could be due to the reduction in carrier thermal conductivity. The highest thermoelectric figure of merit ( zT) ~0.84 at 673 K was obtained for the sample with x = 0.2 due to lower thermal conductivity (1.17 W/m K). [ABSTRACT FROM AUTHOR]

The phonon and carrier thermal conductivities of thermoelectric materials were calculated using the Wiedemann-Franz law, Boltzmann equation, and a method we propose in this study called the Debye specific heat method. We prepared polycrystalline n-type doped bismuth telluride (BiTe) and bismuth antimony (BiSb) bulk alloy samples and measured six parameters (Seebeck coefficient, resistivity, thermal conductivity, thermal diffusivity, magneto-resistivity, and Hall coefficient). The carrier density and mobility were estimated for calculating the carrier thermal conductivity by using the Boltzmann equation. In the Debye specific heat method, the phonon thermal diffusivity, and thermal conductivity were calculated from the temperature dependence of the effective specific heat by using not only the measured thermal conductivity and Debye model, but also the measured thermal diffusivity. The carrier thermal conductivity was also evaluated from the phonon thermal conductivity by using the specific heat. The ratio of carrier thermal conductivity to thermal conductivity was evaluated for the BiTe and BiSb samples, and the values obtained using the Debye specific heat method at 300 K were 52% for BiTe and <5.5% for BiSb. These values are either considerably larger or smaller than those obtained using other methods. The Dulong-Petit law was applied to validate the Debye specific heat method at 300 K, which is significantly greater than the Debye temperature of the BiTe and BiSb samples, and it was confirmed that the phonon specific heat at 300 K has been accurately reproduced using our proposed method. [ABSTRACT FROM AUTHOR]

The aim of this work is to investigate the electronic, mechanical, and transport properties of the fluoroperovskite compound LiBeF by first-principles calculations using the full-potential linear muffin-tin orbital method based on density functional theory within the local density approximation. The independent elastic constants and related mechanical properties including the bulk modulus ( B), shear modulus ( G), Young's modulus ( E), and Poisson's ratio ( ν) have been studied, yielding the elastic moduli, shear wave velocities, and Debye temperature. According to the electronic properties, this compound is an indirect-bandgap material, in good agreement with available theoretical data. The electron effective mass, hole effective mass, and energy bandgaps with their volume and pressure dependence are investigated for the first time. [ABSTRACT FROM AUTHOR]

A density functional theory + Hubbard U (DFT + U) method is employed to investigate the effect of strain on the electronic and transport properties of the correlated metal LaNiO. LaNiO without strain is characterized by a low temperature Fermi liquid behaviour of resistivity, a negative Seebeck coefficient and a positive Hall coefficient. Density of states, resistivity, thermopower and Hall coefficient obtained within the DFT + U approach reveal that LaNiO under both compressive and tensile strain is more metallic compared to the unstrained system. However, LaNiO under tensile strain is found to be more strongly correlated than that under compressive strain. Electron localization function calculation shows that there is a substantial increase in the covalent part of the chemical bonding, which corroborates an increase in the resistivity for LaNiO under tensile strain. Our first-principle-based calculation clearly demonstrates that the transport properties of LaNiO can be tuned by applying suitable strain. [ABSTRACT FROM AUTHOR]

A theoretical calculation of the structural, thermal and transport properties of anatase titanium dioxide (TiO) was investigated with the help of density functional theory and Boltzmann theory. The fully optimized structure was obtained by minimizing the total energy. The variations of the volume ( V), bulk modulus ( B), Debye temperature (Θ), heat capacities at constant volume ( C ) and constant pressure ( C ), entropy ( S), Grüneisen parameter ( γ) and thermal expansion coefficient ( α) as a function of the pressure ( P) and temperature ( T) were all obtained and analyzed in detail. Boltzmann theory calculations have been used to evaluate important transport properties such as Seebeck coefficient ( S), electrical conductivity ( σ), electronic thermal conductivity ( K ) and power factor ( S σ) with respect to scattering time ( τ) as a function of chemical potential ( μ). [ABSTRACT FROM AUTHOR]

II-IV-nitride compounds are tetrahedrally bonded, heterovalent ternary semiconductors that have recently garnered attention for their potential technological applications. These materials are derived from the parent III-nitride compounds; ZnGeN is the II-IV-nitride analogue to the III-nitride GaN. Very little is known about the transport properties of ZnGeN. In this work, we present Seebeck coefficient ( S) data on 3-micron-diameter, 70-micron-long, single-crystal ZnGeN rods, employing a novel measurement approach. The measurements of S show that the majority free carriers are electrons, and imply that the carrier gas is degenerate. Within a single-band model for the conduction band, a carrier concentration of order 10 cm was estimated for a measured S = −90 μV/K. Together with electrical transport measurements, a lower limit for the electron mobility is estimated to be ∼20 cm/V-s. A discussion of this material as a thermoelectric is presented. The background level of free electrons in this unintentionally doped ZnGeN is very near the predicted optimum value for maximum thermoelectric performance. [ABSTRACT FROM AUTHOR]

Thallium-doped TlBiTe ( x = 0.0, 0.05, 0.1, and 0.2) nanopowders were synthesized by the hydrothermal method. The doping effect of thallium on the morphologies of the synthesized nanopowders was investigated. It was found that the doping of thallium can significantly change the morphologies of the synthesized nanopowders. The synthesized nanopowders were hot-pressed into bulk pellets and the doping effects of thallium on the transport properties of these pellets were investigated. The results show that the doping of thallium can enhance the Seebeck coefficient but increase the electrical resistivity. Moreover, the power factors of the thallium-doped samples decrease with the increasing of the thallium doping level as compared with the un-doped sample. This is attributed to the increase of the electrical resistivity and the disappearing of the flower-like morphologies of the doped nanopowders. [ABSTRACT FROM AUTHOR]

Polycrystalline samples with composition SnTlSe (for x = 0 to 0.04) have been synthesized from elements of 5 N purity at elevated temperatures. The phase purity of the products was verified by x-ray diffraction analysis. Samples for measurement of transport properties were prepared by hot pressing. The samples were then characterized by measurement of electrical conductivity, Hall coefficient, Seebeck coefficient, and thermal conductivity over the temperature range from 300 K to 725 K. All samples demonstrated p-type conductivity. Tl markedly enhanced the carrier concentration. We discuss the influence of Tl substitution on the free carrier concentration and thermoelectric performance. Investigation of the thermoelectric properties showed an order-of-magnitude improvement, with ZT reaching 0.6 at 725 K. We discuss the distinctive nature of the thermal conductivity of SnSe. [ABSTRACT FROM AUTHOR]

As part of development of a new assembly technology to achieve bonding for an innovative silicon carbide (SiC) power device used in harsh environments, the aim of this study is to compare two silver sintering profiles and then to define the best candidate for die attach material for this new component. To achieve this goal, the solder joints have been characterized in terms of porosity by determination of the morphological characteristics of the material heterogeneities and estimating their thermal and electrical transport properties. The three dimensional (3D) microstructure of sintered silver samples has been reconstructed using a focused ion beam scanning electron microscope (FIB-SEM) tomography technique. The sample preparation and the experimental milling and imaging parameters have been optimized in order to obtain a high quality of 3D reconstruction. Volume fractions and volumetric connectivity of the individual phases (silver and voids) have been determined. Effective thermal and electrical conductivities of the samples and the tortuosity of the silver phase have been also evaluated by solving the diffusive transport equation. [ABSTRACT FROM AUTHOR]

In this study, we investigated the electric field effect on the transport properties of a Bi nanowire. These effects were modeled by a surface potential. The electron states of Bi nanowires were numerically analyzed by effective mass equations at the T-point and L-point taking into account surface potential due to an external electric field. The Seebeck coefficients of Bi nanowires were calculated by using the Boltzmann equation with a constant relaxation time. It was found that the Seebeck coefficients increased when the sign of the surface potential parameter was negative. In particular, when the surface potential parameter was $$-$$ 1 eV and the skin depth was 10 nm in a 20-nm-diameter nanowire, the maximum absolute value of the Seebeck coefficient was larger than 1 mV/K, which was greatly improved compared to that without an external electric field. [ABSTRACT FROM AUTHOR]

ZnSb would be a good thermoelectric material with carrier concentration above 10/cm, but unfortunately this has been shown to be difficult to achieve, particularly with Sn as a dopant. Two series ZnSb samples doped with Sn and ZnSn were prepared using hot-pressing technics, and their thermoelectric properties were investigated in the temperature range from 300 K to 700 K. The tin content of the samples was in the range from 0.1 to 0.5 at.%. Surprisingly, samples with lower tin content achieved higher carrier concentration, which is beneficial for thermoelectric performance. Samples doped with 0.1 at.% Sn achieved Hall carrier concentration above 1 × 10/cm, reaching ZT of 0.9, while for samples doped with 0.5 at.% Sn, the Hall carrier concentration was close to the hole concentration of pure ZnSb. Also, by analyzing hysteresis present in the heating-cooling cycles, we conclude that the role of intrinsic defects in ZnSb is important and that these defects clearly determine the ability of ZnSb to achieve ZT near 1. [ABSTRACT FROM AUTHOR]

We performed Monte Carlo simulations on the electron and phonon transport properties of Si nanowires with constant widths and of nanowires modulated by a constriction. We discuss and compare the transport properties and the thermoelectric efficiency in the nanowires. An overall figure of merit ( ZT) enhancement is predicted compared to the corresponding non-modulated nanowires. The ZT enhancement in thick, modulated nanowires has been found comparable to that in thin, non-modulated nanowires. [ABSTRACT FROM AUTHOR]

Bismuth nanocrystals were synthesized using a polyol process and densified using spark plasma sintering to form polycrystalline bulk materials with uniformly dispersed nanoscale grains. The Bi nanocrystals were consolidated at different pressures in order to investigate the magnitude and temperature dependence of transport properties as a function of porosity. We also compare these transport properties with those of single-crystal bismuth. This investigation expands our understanding of the role of porosity in the thermoelectric properties of nanostructured bismuth. [ABSTRACT FROM AUTHOR]

Ba-Co-Ge intermetallic clathrates were synthesized in both type-I and chiral-type crystal structures to investigate thermoelectric and physical properties. Seebeck coefficients, thermal conductivities, electrical resistivities, and Hall coefficients were measured, as well as specific heat and magnetic susceptibility. Type-I BaCoGe was formed with a large number of spontaneous vacancies, similar to a Zintl condition, but without the vacancy ordered superstructure of BaGe. However, the vacancies for this composition do not moderate the carrier density as expected from Zintl electron balancing. Instead, the physical properties point to a complex Fermi surface property with a large effective carrier density, a behavior consistent with other materials close to BaGe. In the case of chiral samples, Co substitution strongly suppresses the temperature of only the lower of the two structural transformations. Susceptibility and specific heat measurements, coupled with the measured transport properties, demonstrate that the electron densities of states near the Fermi energy change very little in the transformations, a significant reduction in the effect of these transitions compared to the case of unsubstituted BaGe. [ABSTRACT FROM AUTHOR]

Single crystals of Se- and Fe-doped BiTe have been synthesized via the zone melting method. Energy-dispersive x-ray and x-ray powder diffraction analyses have been carried out to identify the constituent elements and determine the lattice parameters of the grown crystals. Surface topological features of the as-grown single crystals have been studied. The transport properties of doped stoichiometric BiTe single crystals have been studied by measuring the thermoelectric power and electrical conductivity in the temperature range from 303 K to 473 K. The thermoelectric power, S, effective mass, scattering parameter, and Fermi energy have been calculated from thermoelectric power measurements. The temperature dependence of the electrical conductivity, σ, shows that the dopants in the crystals are thermally activated. All the crystals exhibit semiconducting behavior as confirmed by the temperature dependence of σ and S. The effective mass of electrons and the effective density of states have been determined and are reported for BiTeSe (0 ≤ x ≤ 0.3) and BiFeTe (0 ≤ y ≤ 0.3). [ABSTRACT FROM AUTHOR]

Mg(Si,Sn) compounds are promising candidate low-cost, lightweight, nontoxic thermoelectric materials made from abundant elements and are suited for power generation applications in the intermediate temperature range of 600 K to 800 K. Knowledge on the transport and mechanical properties of Mg(Si,Sn) compounds is essential to the design of Mg(Si,Sn)-based thermoelectric devices. In this work, such materials were synthesized using the molten-salt sealing method and were powder processed, followed by pulsed electric sintering densification. A set of MgSiSnSb (0 ≤ x ≤ 0.072) compounds were investigated, and a peak ZT of 1.50 was obtained at 716 K in MgSiSnSb. The high ZT is attributed to a high electrical conductivity in these samples, possibly caused by a magnesium deficiency in the final product. The mechanical response of the material to stresses is a function of the elastic moduli. The temperature-dependent Young's modulus, shear modulus, bulk modulus, Poisson's ratio, acoustic wave speeds, and acoustic Debye temperature of the undoped Mg(Si,Sn) compounds were measured using resonant ultrasound spectroscopy from 295 K to 603 K. In addition, the hardness and fracture toughness were measured at room temperature. [ABSTRACT FROM AUTHOR]

Characterization of powder-metallurgically manufactured (BiSb)(TeSe) thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type ( x = 0.2, y = 1) as well as n-type ( x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed. [ABSTRACT FROM AUTHOR]

Galvanomagnetic properties of epitaxial BiSbTe films grown by the hot-wall technique were investigated in magnetic fields of H = 5 T to 14 T through the temperature interval of 10 K to 300 K. The results were analyzed in terms of a many-valley model of the energy spectrum and anisotropic charge carrier scattering. The degeneracy parameter β, and ratios of components of the reciprocal effective-mass tensor $$ {\mathord{\scriptscriptstyle\leftrightarrow}\over \alpha} $$ and relaxation time tensor $$ {\mathord{\scriptscriptstyle\leftrightarrow}\over \tau } (\varepsilon ) $$ were estimated. Substantial anisotropy of charge carrier scattering was observed in the investigated temperature interval. Anisotropy of charge carrier scattering along bisector axes is enhanced, as compared with that of corresponding bulk samples. High charge carrier mobility and higher angular factor of the temperature dependence of mobility promote enhancement of the figure of merit Z of the films as compared with that of corresponding bulk solid solutions. [ABSTRACT FROM AUTHOR]

In this study, we have numerically analyzed the transport properties of Bi-Sb nanowires, taking into account wire boundary scattering. Wire boundary scattering slightly decreased the Seebeck coefficient of Bi-Sb nanowires. This effect is due to the observation that boundary scattering and the mobility ratio of L-point electrons to T-point holes in the nanowires are smaller than those in bulk Bi-Sb because the wire boundary scattering suppresses the mobilities of L-point electrons and heavy holes. The largest Seebeck coefficient for all wire diameters was obtained when the Sb concentration was 5 at.%. The effective mass approached zero near 5 at.% Sb, and the small effective mass led to a large subband shift in each band. Thus, a small effective mass enhances the quantum effect at a fixed wire diameter, even if wire boundary scattering is taken into account. [ABSTRACT FROM AUTHOR]

β-ZnSb exhibits poor thermal stability, with zinc-rich specimens recently shown to be more stable. In this work, temperature-dependent transport measurements [Seebeck coefficient ( S) and electrical conductivity ( σ)] between room temperature (RT) and 525 K were carried out on a zinc-poor specimen. The sequentially measured S and σ data in the same measurement cycle show sharp changes in their absolute values between 450 K and 500 K during the heating cycle, which is not retraced back during cooling. A repeat measurement carried out on the specimen after ~1 month again shows the sharp changes in the absolute values between 450 K and 500 K, indicating reversibility of the process. Temperature-dependent synchrotron measurements were further carried out between RT and 525 K. Formation of elemental Sb was observed beyond 400 K. Between 450 K and 500 K, movement of zinc from lattice to interstitial position is observed, which is also accompanied by the onset of ZnSb formation. The overall zinc content within the β-ZnSb phase is observed to increase with increasing temperature. These observations indicate that both the overall zinc content and the Zn/Zn ratio are crucial in stabilizing the β-ZnSb phase. [ABSTRACT FROM AUTHOR]

We synthesized a series of samples with composition around 52 at.% zinc (Zn), 44 at.% antimony (Sb), 4 at.% phosphorus (P), and up to 3 at.% copper (Cu) by melting the elements and subsequent annealing. This resulted in dense and almost crack-free samples. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) revealed composites with a majority phase of ZnSb containing varying amounts of ZnP and CuZn, in addition to ZnSb in some of the samples. We measured the Seebeck coefficient, electrical conductivity, and thermal conductivity as a function of temperature. The thermoelectric performance tended to improve with increasing Cu content. At Cu content of 2 at.%, a reduced resistivity allows for the highest dimensionless figure of merit, with a maximum zT value of 0.18 at around 573 K. [ABSTRACT FROM AUTHOR]

We report a simplified sequential evaporation route that can deposit compositionally controllable Bi-Te thermoelectric (TE) thin films without the need for a highly controlled facility. Te and Bi granules were used as starting materials, with their ratio being adjusted to obtain Bi-Te films with different compositions and thicknesses. The as-evaporated and annealed films were subjected to structural and morphological analysis, and their transport properties were measured. X-Ray diffraction data revealed multiple phases for most films. Energy-dispersive x-ray spectroscopy showed that the film composition was Te-enriched due to the large vapor pressure difference of Te and Bi. A BiTe single phase was obtained in the annealed films, having nominal composition of BiTe. The existence of impurity phases, such as BiTe or elemental Te, was found in all the as-evaporated films and in the annealed films with other nominal Te/Bi ratios, which degraded the TE properties of the films by increasing their electrical conductivity and reducing their Seebeck coefficient. A pure BiTe film with nominal Te/Bi ratio of 1.2 exhibited a maximum power factor of 7.9 × 10 W m K after annealing at 200°C. This work demonstrated a simple, undemanding, reliable method to deposit Bi-Te-based TE thin films that can be utilized to fabricate low-cost TE microgenerators. [ABSTRACT FROM AUTHOR]

Polycrystalline p-type CoSb was synthesized by the high-pressure method. The microstructure and temperature-dependent thermoelectric properties of CoSb were investigated. X-ray diffraction and scanning electron microscopy showed that single-phase CoSb with fine grain size could be quickly synthesized under high pressure. The carrier concentrations of CoSb could be tuned by more than a factor of 10 by changing the pressure during synthesis. With the increase of the synthetic pressure, the Seebeck coefficient and resistivity of CoSb increase while the thermal conductivity decreases. [ABSTRACT FROM AUTHOR]

Heavy rare earth element Dy-filled skutterudites (DyFeCoSb) have been synthesized by a melting-quenching-annealing method and sintered by the spark plasma sintering technique. Our results suggest that single-phased DyFeCoSb compounds could be obtained when the Fe content is less than 1.5. The maximum filling fraction of Dy in skutterudites increases with increasing Fe content. We also found significant lattice expansion induced by Fe substitution at Co sites and Dy filling in the voids. The electrical conductivity, Seebeck coefficient, and thermal conductivity have been measured in the temperature range from 300 K to 800 K. The low-temperature Hall coefficient and carrier mobility are reported in the temperature range from 2.5 K to 300 K. The power factor for DyFeCoSb increases with increasing Fe content. A significant reduction in lattice thermal conductivity is observed in heavy rare earth element Dy-filled skutterudites due to the low localized vibrational frequency of Dy that effectively scatters low-frequency lattice phonons. The sample with composition DyFeCoSb has lattice thermal conductivity as low as 1.05 W m K at room temperature. The thermoelectric figure of merit ( ZT) reaches a maximum value of 0.67 at 750 K. [ABSTRACT FROM AUTHOR]

Currently, there is a resurgence of interest in thermoelectric materials with enhanced efficiency. Among investigated classes of bulk thermoelectrics such as partially filled skutterudites, ZnSb-based materials, and clathrates, novel polycrystalline Mo cluster-based chalcogenides were reported recently. Among those, Chevrel phase-derived AgMoSe (with 3.4 ≤ y ≤ 3.9) compounds have shown interesting thermoelectric properties, in particular extremely low thermal conductivity allowing improved thermoelectric efficiency compared with reported Chevrel phases. They also possess a complex crystallographic structure where stacked MoSe units leave channels occupied by Ag atoms. Analysis of the structural determinants of the thermoelectric properties of AgMoSe suggested that performance improvements could result from further Cu insertion. In this paper, we describe the synthesis route we used for preparing quaternary Ag-Cu-Mo-Se compositions by a combination of powder metallurgy and spark plasma sintering techniques. Characterization by x-ray diffraction, scanning electron microscopy, and electrical and thermal measurements has been performed. The results obtained for two compounds (AgCuMoSe and AgCuMoSe) are discussed and compared with those of the parent ternary compound AgMoSe. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effects of scattering on the transport properties of Bi nanowires. The electrical conductivities and Seebeck coefficients of Bi nanowires were calculated using the Boltzmann equation, with an energy-dependent relaxation time corresponding to the scattering process. Decreasing the wire diameter increased the Seebeck coefficient for all of the scattering processes examined, because a semimetal-semiconductor transition occurred. In 80-nm-diameter nanowires, the Seebeck coefficient for ionized impurity scattering was larger than that of the acoustic deformation potential. On the other hand, in 20-nm-diameter nanowires, the dependence of the Seebeck coefficient on the scattering process was negligible, compared with the influence of wire diameter. [ABSTRACT FROM AUTHOR]

We report an investigation of the electronic structures and electrical transport properties of a series of RFeSb skutterudites using density functional calculations and Boltzmann transport theory. The band structure and density of states are calculated and discussed. Based on the results of the band structure, the temperature dependence of the Seebeck coefficient, electrical conductivity, power factor, and carrier concentration are calculated, and the results are in good agreement with experimental data. The results of electrical transport properties indicate that double-filled FeSb with the atomic combinations (Na, Yb), (In, Yb), and (Ca, Yb) may be better compared with the corresponding single-filled FeSb. [ABSTRACT FROM AUTHOR]

The clathrate I BaGe□ [space group $$ Ia\bar{3}d $$, no. 230, a = 21.307(1) Å] has been synthesized as a single phase and characterized by x-ray powder diffraction and metallographic analysis. Electrical and thermal transport measurements have been performed in the temperature range of 5 K to 673 K. BaGe□ displays the electrical resistivity of a poor metal at low temperatures, with semiconducting-like behavior appearing above 300 K. [ABSTRACT FROM AUTHOR]

Thermoelectric and galvanomagnetic properties of p-type solid solutions based on bismuth and antimony chalcogenides (Bi,Sb)(Te,Se) have been studied to analyze the features of the figure of merit Z. The increase of Z and ZT for the p-BiSbTe composition at x = 1.6 in the temperature interval of 370 K to 550 K was shown to be defined by the increase of the density-of-states effective mass, the slope of the temperature dependence of the carrier mobility, and the reduction of the lattice thermal conductivity for optimal charge carrier concentration. High carrier mobility and low lattice thermal conductivity provide the increase of Z and ZT in the p-BiSbTeSe ( x = 1.3, y = 0.06) solid solution in the interval from 300 K to 370 K. The growth of Z in these compositions is determined by the increase of the compression of the constant-energy ellipsoids along binary and bisector directions, and by the change of the tilt angle Θ between the principal axes of the ellipsoids and the crystallographic axes. [ABSTRACT FROM AUTHOR]

tool has been developed at Fraunhofer-IPM to calculate the transport properties of thermoelectric material by using its band structure described in terms of effective masses and the location of the ellipsoids in reciprocal space. The calculated transport properties are compared with experimental data measured on bismuth telluride, antimony telluride, and bismuth antimony telluride. Polycrystalline specimens have been prepared by spark plasma sintering (Fraunhofer-IFAM). Electron backscattering diffraction analysis of sample cross-sections yields the frequency distribution of grain orientations. This texture information permits the generation of appropriate finite-element models of the polycrystalline microstructure (TU Dresden). By means of the commercial code COMSOL, which allows anisotropic thermoelectric properties to be taken into account, the effective electrical and thermal conductivities as well as the Seebeck coefficient both parallel and perpendicular to the pressing direction have been calculated. [ABSTRACT FROM AUTHOR]