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Ion implantation is the most useful technology for doping of 4H silicon carbide to fabricate power devices. Semiconductor properties depend on implanted doping activation and ionization, and these properties are critical for the proper design and modeling of SiC power devices. In 4H-SiC, during activation annealing, the atomic diffusion rate is low, and thus full dopant activation is not readily achieved. Also, dopants in 4H-SiC are observed to have very deep energy levels. The temperature behavior of the electrical characteristics depends on the ionization energy level, and these values are not adequately characterized in 4H-SiC over all doping levels. To improve device modeling accuracy and device design predictive capabilities as a function of temperature and doping levels, further study of the behavior of dopants in SiC is crucial. Due to its demonstrated usefulness in epitaxial n-type SiC growth, nitrogen is the element most frequently employed in device manufacturing for n-type doping. However, phosphorus can serve as an alternative, with ionization energies expected to be close to nitrogen. In this work, electrical characterization of phosphorus-implanted 4H-SiC is examined as a function of doping and temperature. For this purpose, four-point van der Pauw probes are prepared, and resistivity and Hall measurements are carried out. Resistivity values are observed to decrease as temperature and doping concentration increase. Concerning Hall mobility, at low temperatures, the Hall mobility decreases due to increased impurity scattering, while at very high temperatures, it decreases because of increased phonon scattering. The increase in carrier concentration with the increase in temperature indicates incomplete ionization and relatively deep ionization energies. From the obtained data, a two-level charge neutrality equation is used to extract and compare the activation percentage and ionization energies for dopants in hexagonal and cubic sites. For a doping level of ~ 3.2 × 1018 cm−3, dopant ionization energy levels of 82 meV and 40 meV are found for the cubic and hexagonal sites, respectively, and these are observed to be a function of doping concentration. The extracted activation rate and ionization energies are useful for improving the predictive capability of SiC device modeling. [ABSTRACT FROM AUTHOR]

Gallium antimonide (GaSb) is a III-V semiconductor of technological interest for low-power, high-mobility field-effect transistors, as well as for mid-wave infrared detectors. In such devices, GaSb interfaces with other III–V semiconductors with different lattice constants that can induce strain in the GaSb layers. Two dominant limiting factors in hot carrier relaxation are the intra-valley and the inter-valley electron–phonon (e-ph) scattering. In GaSb, these are sensitive to the Γ –L energy ordering, which depend intimately on the strain. Here, we report ab initio calculations of electronic structure, phonon dispersion, e-ph scattering and relaxation times for GaSb as a function of strain. As observed previously for other group IV and III-V semiconductors, our results show strong anisotropy, a strong contribution from LO phonons, and the need to go beyond the deformation potential scattering. For GaSb, the main finding is that a compressive strain between 0.4% and 0.6% converts GaSb from a direct-bandgap semiconductor to an indirect-bandgap semiconductor, with dramatic changes in the competing scattering rates and carrier relaxation times. [ABSTRACT FROM AUTHOR]

For two-dimensional transition metal dichalcogenides, the thermoelectric properties of the material are affected by layer thickness and lattice strain. In this paper, we investigate the thermoelectric properties of a HfS2 bilayer under different biaxial tensile strains by first-principles calculations combined with Boltzmann equations. The presence of degenerate bands in the HfS2 bilayer and the absence of its monolayer results in the better thermoelectric performance of the HfS2 bilayer than its monolayer. Moreover, this strain increases the band degeneracy of the HfS2 bilayer even more, and the degenerate bands and stepped 2D density of states lead to a high power factor. In addition, the lattice strain increases the phonon scattering rate and reduces the phonon lifetime of the HfS2 bilayer, resulting in a decrease in the lattice thermal conductivity. Ultimately, we obtained a maximum ZT value of 1.76 for the unstrained HfS2 bilayer at the optimal doping concentration. At this time, its power factor and thermal conductivity are 53.01 mW/mK2 and 9.06 W/mK, respectively. When the strain reaches 3%, for the n-type doped HfS2 bilayer, the power factor and thermal conductivity are 69.87 mW/mK2 and 6.36 W/mK, respectively, and the maximum ZT value is 3.29. For the p-type doped HfS2 bilayer, the maximum ZT value appears at 6% strain, which is 1.83, at which the power factor and thermal conductivity are 13.81 mW/mK2 and 2.27 W/mK, respectively. [ABSTRACT FROM AUTHOR]

Thermoelectric conversion devices based on group IV semiconductor elements can improve conversion efficiency by reducing the thermal conductivity of the material. In particular, it is known that introducing Sn into the system can dramatically reduce the conductivity. It has been experimentally shown that the thermal conductivity of polycrystalline Ge and polycrystalline Si1−xGex can be reduced by introduction of Sn. However, there is no experimental report on the effect of Sn atoms on the phonons responsible for thermal conduction. In this study, we investigated the mechanism of thermal conductivity reduction due to the introduction of Sn by inelastic x-ray scattering measurements on a Ge1−xSnx single-crystalline thin film with 9% Sn composition. The phonon dispersion of Ge1−xSnx was obtained as a result of the measurements, and the slope of the acoustic mode in the phonon dispersion curve of Ge1−xSnx was smaller than that of Ge. The phonon group velocity is expressed as the slope of the dispersion curves of the acoustic mode. Therefore, it was suggested that the reduction of the phonon group velocity by the introduction of Sn partly contributes to the reduction of the thermal conductivity of Ge1−xSnx. Local vibration mode (LVM), which was independent of wavenumber, was also observed in the low-energy region, and we attribute the origin to the local structure of Sn–Sn pairs formed in Ge1−xSnx. Such LVM has also been reported in bulk single- and polycrystalline Si1−xGex and polycrystalline Si1−x−yGexSny and is considered to influence the thermal conductivity reduction. [ABSTRACT FROM AUTHOR]

Compared with traditional thermoelectric (TE) materials, bismuth oxyselenide (Bi 2 O 2 Se) consists of nontoxic and lower-cost elements and has better chemical and thermal stability. The highest ZT value of Bi 2 O 2 Se ever obtained experimentally was recently reported at 0.69. To further improve its TE performance, the electron and phonon transport properties of intrinsic n-type Bi 2 O 2 Se are evaluated by first-principles calculations. Due to the density of states at the conduction band minimum being too small resulting in a low intrinsic carrier concentration, the reported TE properties of Bi 2 O 2 Se are not ideal. The TE parameters we calculated are in good agreement with the experimental values. The average optimal ZT values at 800 K for the p-type and n-type Bi 2 O 2 Se are 1.83 and 1.81, respectively, indicating that Bi 2 O 2 Se is a promising TE material. It is an innovative approach to improve the TE performance by regulating the vacancy defects in Bi 2 O 2 Se, which could achieve the cooperative optimization of electrons and phonons through the energy filtering effect and enhanced phonon scattering. Our results could provide a theoretical basis for further improving the TE properties of Bi 2 O 2 Se in experiments. [ABSTRACT FROM AUTHOR]

The ultralow thermal conductivity (TC) (0.1–0.3 W m−1 K−1) of amorphous epoxy resins (EPs) limits their applications to high heat flux; thus, designing EPs with high TC is of great significance. In this work, the individual and synergistic effects of three different factors, chain length, tensile strain, and curing agent structures, on the intrinsic TC of single crosslinked EP chains were investigated mainly by equilibrium molecular dynamics simulations. The results show that, compared with the linear polyethylene chain, EP chains in the unstretched state retain a much lower TC below 3 W m−1 K−1 due to the more significant phonon scattering in the complex crosslinked structures. The TC can be significantly improved over a critical tensile strain due to the significant reduction in the folding and kink structures. The critical tensile strain varies with the types of crosslinked EP chains. A high TC of EP chains can be easily achieved due to the synergistic effect of chain length and stretching. By constructing two types of structural evolutions, the molecular structure of the curing agents was found to have a significant effect on the TC of crosslinked EP chains. The additional functional groups on the benzene ring cause a significant reduction in the TC of single EP chains. In addition, the decrease rate generally slows down as the benzene ring number increases. [ABSTRACT FROM AUTHOR]

Recently, derivatives of molybdenum disulfide have attracted considerable attention. Among them, the thermal conductivity of Janus-based MoSSe SeS, SeSe and SS bilayers has not been investigated. Along these lines, in this work, the lattice thermal conductivities of Janus MoSSe-based bilayer structures were examined. More specifically, three different combined modes were used, including SMoSe/SMoSe (SeS stacking), SMoSe/SeMoS (SeSe stacking) and SeMoS/SMoSe (SS stacking), based on first-principles calculations. The extracted results show that the lattice thermal conductivity of all three structures is decreased with increasing temperature, whereas the SeS structure has a maximum lattice thermal conductivity value of about 22 W/mK at 300 K. The SS structure exhibits also the strongest phonon anharmonicity and highest phonon scattering effects, which leads to the smallest lattice thermal conductivity value of about 1.57 W/mK in the x-direction at room temperature, rendering the proposed configuration well suited for thermoelectric applications. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a junctionless carbon nanotube field-effect diode in the presence of electron–phonon scattering is examined. A self-consistent mode-space simulation approach based on non-equilibrium Green's function is used. We have studied the influence of incoherent transport on electrical characteristics such as on-current ( I ON ), off-current ( I OFF ), on/off current ratio (ION/IOFF), intrinsic delay (τ), energy-delay product, and sub-threshold power dissipation (SPD ) at various gate lengths. We have shown that phonon-assisted tunneling dominates the band-to-band tunneling regime under off-state conditions. Therefore, this phenomenon deteriorates I OFF and, as a result, SPD . Furthermore, the scattering results in an increase in the intrinsic delay and energy consumed per switching event. Finally, in the on-state, acoustic phonons interaction is the overriding scattering mechanism and causes about a 35% reduction in I ON . [ABSTRACT FROM AUTHOR]

High-performance thermoelectric material, SnTe, has a good application prospect in thermoelectric power generation, but the low thermoelectric performance limits its application. There are many ways to improve the thermoelectric properties of materials, one of which is the composite method of heterogeneous materials. In this work, SnTe-based composites with different content (0, 0.5, 1.0, and 1.5) wt% single-walled carbon nanotubes (SWCNTs) were prepared by ultrasonics, magnetic stirring, grinding, and hot pressing. The experimental results showed that the electrical conductivity and thermal conductivity of the composites decreased with the increase of SWCNTs content. The thermoelectric properties of all the composites were improved. The thermoelectric properties of the composites might benefit from the high electrical conductivity of SWCNTs, and the increased phonon scattering at interfaces between the matrix and the SWCNTs. The figure-of-merit (ZT) of all the composites increased with the increase of temperature. At 523 K, SnTe + 1.5 wt% SWCNTs composite had the maximum ZT ~ 0.1. [ABSTRACT FROM AUTHOR]

Acoustic phonon scattering is mostly regarded as the deciding scattering in half-Heusler studies. This oversimplification needs scrutiny because there are other scatterings which might be more prominent. If other scattering mechanisms are ignored, the calculation results may be affected. In this paper, we systematically study the electronic structure, dynamic stability, and thermoelectric properties of the p-type VFeBi compound based on three scattering mechanisms: acoustic phonon, ionized impurity, and polar optical phonon. We analyze the change of electronic transport parameters under different combinations of the three scattering mechanisms to explore the effects of different scattering on thermoelectric performance. The results show that, in addition to an acoustic phonon, ionized impurity and polar optical phonon also have a significant impact on thermoelectric performance. With the increase in scattering mechanism, the mobility, electronic conductivity, power factor, and electronic thermal conductivity gradually decrease. It can be shown that the predicted ZT of p-type VFeBi under the acoustic phonon and polar optical phonon scattering can reach up to 2.431 at 1.07 × 1020 cm−3 and 875 K, while it is 2.47 under the three scatterings at 1.14 × 1020 cm−3 and 910 K. However, the maximum value of ZT is 3.424 under acoustic phonon scattering at 5.48 × 1019 cm−3 and 784 K. Therefore, it is inaccurate to only consider acoustic phonon scattering. Comparing the ZT values under different conditions, we predict that the p-type VFeBi compound has a good thermoelectric performance. [ABSTRACT FROM AUTHOR]

Filled-skutterudite materials are promising candidates for thermoelectric applications in the intermediate-temperature range to recover waste heat. In this work, mechanical alloying and spark plasma sintering (SPS) techniques were used to consolidate double-filling InxLa0.25Co4Sb12 (x = 0, 0.1, 0.3, 0.5) samples. X-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy were used to analyze the microstructure of the InxLa0.25Co4Sb12 SPS-ed samples. The microstructure results revealed the main phase of the CoSb3 skutterudite structure with a small amount of InSb and CoSb2 attributed to secondary phases. Electrical resistivity remarkably decreased to 9.7 μΩ m at room temperature for the In0.5La0.25Co4Sb12 sample, mainly due to the effective impurity of the InSb nanoinclusions. Moreover, the InSb nanoinclusions yielded a substantial depression in lattice thermal conductivity due to phonon scattering. The reduction in lattice thermal conductivity was approximately 62% for the In0.5La0.25Co4Sb12 sample compared with the La0.25Co4Sb12 sample, resulting in a remarkable enhancement in the dimensionless figure-of-merit (ZT) of 1.25. [ABSTRACT FROM AUTHOR]

A transversely applied magnetic field can alter the energy bandgap and other corresponding electrical parameters of a PN-device at the nanoscale. The present theoretical analysis demonstrates that the applied magnetic field controls the bandgap and resistivity of a phosphorene nanoribbon (PNR)-based PN-device. This allows the device to function as a magnetic field sensor as well. For this purpose, the electron–acoustic phonon scattering mechanism is used to estimate phonon-mediated electronic resistivity, which is a parameter directly linked to the bandgap. The armchair configuration of nanoribbons is subjected to energy bandgap and resistivity calculations owing to their semiconducting nature. It is found that in the absence of a magnetic field, the device exhibits a semiconducting nature, but when a magnetic field is supplied, the device acquires a conducting (metallic) state. This semiconductor-to-metal transition is attributable to the change in resistivity of the PN-device under a transversely applied magnetic field, which ultimately results in a change in bandgap. The outcome of the work is pivotal for (i) various nanoelectronic applications that demand semiconductor–metal transition and (ii) designing a magnetic field sensor via bandgap regulation of PNR-based PN-devices. [ABSTRACT FROM AUTHOR]

Ternary Cu3SbSe4 thermoelectric material with a diamond-like structure exhibits good thermoelectric performance in the middle-temperature region. In this study, Cu2.98Co0.02SbSe4 material with intrinsically low thermal conductivity was prepared by a melting–ball milling–hot pressing process. The maximum zT value for the Cu2.98Co0.02SbSe4 sample was 0.7 at 650 K, which was about 190% higher than that of the pristine sample. The improvement in thermoelectric performance was ascribed to the realization of multi-scale features induced by ball milling. It was found that multiple scattering centers of phonons were formed via interfacial engineering, including dislocations, nano-holes and grain boundaries, which offers an applicable pathway for the reduction of lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

Half-Heusler MNiSn (M = Ti, Zr, Hf) shows a good thermoelectric performance in a medium temperature range of 500–1100 K. In this study, we investigate the effect of annealing on the microstructure and thermoelectric properties of half-Heusler Ti0.26Zr0.37Hf0.37NiSn. The Ti0.26Zr0.37Hf0.37NiSn composition which was designed to improve phonon scattering via mass difference and strain fluctuation. The samples were fabricated through arc melting and then annealed at 1173 K for 24–216 h. The as-cast sample showed a microstructure in which the Ti-rich and Ti-poor half-Heuslers were mixed with some secondary phases like Hf, Sn, and Ti6Sn5. The secondary phases disappeared, and the difference in the Ti fraction between the Ti-rich and Ti-poor half-Heuslers was reduced with increasing annealing time. In other words, the samples become homogeneous with annealing, leading to the reduction of the anti-site defects commonly observed in MNiSn alloys. This was confirmed by the increase of the band gap calculated from the Seebeck coefficient. The electrical conductivity decreased, and the Seebeck coefficient increased as the annealing time increased. This is attributed to the decrease of the carrier concentration caused by the reduction of the anti-site defects. The figure-of-merit value for the sample annealed for 216 h reached 0.74 at 773 K, showing an increase of approximately 20% compared with that of the as-cast sample. [ABSTRACT FROM AUTHOR]

n-type Mg3Sb2-based material is a promising thermoelectric material with less toxicity and more abundance. Herein, we successfully synthesized n-type polycrystalline Mg3.5-xInxSb1.7Bi0.26Te0.04(x = 0, 0.01, 0.02, 0.03, 0.04) samples by rapid induction melting and hot pressing. The effect of indium doping on the thermoelectric properties of Mg3Sb2-based materials was investigated. Indium doping manipulates the grain boundary scattering, resulting in increased electrical conductivity and carrier mobility. Additionally, the lattice disorder induced by In strengthens the phonon scattering and suppresses lattice thermal conductivity. A peak ZT value of 1.27 is obtained at 773 K in the Mg3.47In0.03Sb1.7Bi0.26Te0.04 sample, which is 26% higher than that of the In-free sample. Our work indicates that indium is an alternative metal addition to improve mobility and thermoelectric performance. [ABSTRACT FROM AUTHOR]

n-type Bi2(Te0.90Se0.10)3/(SiC)y (0 ≤ y ≤ 0.01) composites with nano-SiC were fabricated by mechanical alloying combined with microwave sintering. The microstructure and thermoelectric properties of Bi2(Te0.90Se0.10)3/(SiC)y were studied systematically. The nano-SiC particles as the phonon scattering center uniformly distributed in the matrix, with little effect on the electric transport performance and significantly reducing the thermal conductivity of n-type Bi2(Te0.90Se0.10)3/(SiC)y (0 ≤ y ≤ 0.01). The introduction of nano-SiC optimizes the thermoelectric performance, and the sample with SiC y = 0.0075 obtains the highest ZT~0.94 at 323 K. [ABSTRACT FROM AUTHOR]

In this work, we introduced two-dimensional (2D) WSe2 with the lowest heat conductivity to enhance thermoelectric properties of the non-toxic and cost-effective SnTe. On one hand, the improved power factor can be attributed to the enhancement of Seebeck coefficient due to the decrease of low-energy carriers and minority carriers by the addition of WSe2-SnTe p-p-type heterojunction interface barriers. On the other hand, the thermal conductivity also decreased sharply by the heightened phonon scattering from the high-density stacking faults and multiscale hierarchical architectures. Ultimately, benefiting from the dual regulation effect of the introduction of 2D WSe2 on the electro-acoustic decoupling, an improved figure of merit (ZT) of 1.0 is achieved for the composition of SnTe + 6 wt% WSe2 at 873 K, which is enhanced by 134% in comparison with the pure SnTe sample. Moreover, the meaningful attempt represents an effective and feasible strategy for mixing SnTe with other compounds by the method of ball milling. [ABSTRACT FROM AUTHOR]

Thermoelectricity has been proven as a potential technology for the conversion of waste heat into usable electricity. It involves primarily three parameters, namely the Seebeck coefficient, electrical conductivity, and thermal conductivity. However, there are many other interrelated parameters, such as carrier concentration, mobility, effective mass, multi-valley bands, relaxation time, reduced Fermi energy, phonon modes, scattering parameters, and the number of neighbouring atoms in a given structure. The understanding of these parameters is equally important in order to optimize a high figure of merit (ZT). This article addresses the basics of electronic and thermal transport with the help of Boltzmann transport equation, fundamental concepts for the design of thermoelectric (TE) materials, and implementation of several strategies such as alloying, the phonon-glass electron-crystal (PGEC) approach, band engineering and nanostructuring to optimize the ZT of materials, and finally ends with a discussion of the future prospects of heat extraction through different heat sources. [ABSTRACT FROM AUTHOR]

Incorporating a nanophase into a matrix is an effective approach to improve thermoelectric (TE) properties of bulk TE materials. In this study, graphene nanosheets were introduced into a Cu3SbSe4 matrix and graphene/Cu3SbSe4 composites were prepared by ball milling and rapid hot-press sintering. Microstructure and thermoelectric transport properties of graphene/Cu3SbSe4 composites were analyzed. It was found that the graphene distributed uniformly in the form of layers at the grain boundaries of Cu3SbSe4 matrix. The electrical conductivity of graphene/Cu3SbSe4 composites increased significantly while the Seebeck coefficient decreased with the increasing content of graphene. The thermal conductivity was effectively depressed due to the enhancement of phonon scattering. The maximum thermoelectric dimensionless figure of merit was 0.75 for 2.00% graphene/Cu3SbSe4 composite at 650 K, which was 3.85 times higher than that of pure Cu3SbSe4. [ABSTRACT FROM AUTHOR]

Developing reliable synthetic methods for metal telluride micro-nanostructures is essential to achieve a breakthrough in the enhancement of their thermoelectric properties. In this article, a simple and reproducible cation-exchange method to fabricate mesoporous Cu7Te4 flowerlike structures by using pre-synthesized β-ZnTe(ethylenediamine)0.5 (β-ZnTe(en)0.5) flowerlike templates for thermoelectric applications is proposed. The cation-exchange temperature plays a key role in the formation of products, and the optimal reaction temperature for mesoporous Cu7Te4 flowerlike structures is about 135°C. During this process, the Cu ions rapidly occupy the vacancies created by the loss of en-molecules and Zn ions in β-ZnTe(en)0.5 templates, and the crystal grains grow to mesoporous Cu7Te4 flowerlike structures, which are proved by the characterization results of x-ray diffraction, electron microscopy, x-ray photoelectron spectroscopy, and nitrogen adsorption-desorption isotherms. Finally, in thermoelectric measurements, mesoporous Cu7Te4 flowerlike structures exhibit higher figure of merit (ZT) than Cu7Te4 flowerlike structures without pores due to the improvement of phonon scattering and electrical conductivity. [ABSTRACT FROM AUTHOR]

Silicon-germanium (SiGe) alloys are prominent high-temperature thermoelectric (TE) materials used as a powering source for deep space applications. In this work, we employed rapid cooling rates for solidification by melt-spinning and rapid heating rates for bulk consolidation employing spark plasma sintering to synthesize high-performance p-type SiGe nano-alloys. The current methodology exhibited a TE figure-of-merit (ZT) ≈ 0.94 at 1123 K for a higher cooling rate of ∼3.0 × 107 K/s. This corresponds to ≈ 88% enhancement in ZT when compared with currently used radioisotope thermoelectric generators (RTGs) in space flight missions, ≈ 45% higher than pressure-sintered p-type alloys, which results in a higher output power density, and TE conversion efficiency (η) ≈ 8% of synthesized SiGe nano-alloys estimated using a cumulative temperature dependence (CTD) model. The ZT enhancement is driven by selective scattering of phonons rather than of charge carriers by the high density of grain boundaries with random orientations and induced lattice-scale defects, resulting in a substantial reduction of lattice thermal conductivity and high power factor. The TE characteristics of synthesized alloys presented using the constant property model (CPM) and CTD model display their high TE performance in high-temperature regimes along with wide suitability of segmentation with different mid-temperature TE materials. [ABSTRACT FROM AUTHOR]

SnxBi0.5-xSb1.5Te3 materials with high ZT values were prepared by vacuum melting, ball milling, cold pressing and ambient pressure sintering. The effect of Sn doping amount on the thermoelectric performance of Bi0.5Sb1.5Te3 -based materials was investigated. The results showed that Sn doping increased the carrier concentration and DOS effective mass to improve the electrical conductivity and Seebeck coefficient, respectively, resulting in an increase in the power factor. Meanwhile, the reduction in lattice thermal conductivity was attributed to enhanced phonon scattering. The decrease in bipolar thermal conductivity was caused by the suppression of intrinsic excitation. Finally, compared with Bi0.5Sb1.5Te3, the power factor increased 66%, to 2.72 mW·m−1·K−2, lattice thermal conductivity decreased by 28% to 0.334 W·m−1·K−1, and the ZT value for Sn0.01Bi0.49Sb1.5Te3 at 350 K was 1.33. [ABSTRACT FROM AUTHOR]

p-Type FeNbSb-based half-Heusler compounds exhibit promising thermoelectric (TE) performance under high temperatures. Herein, the optimal sintering temperature and pressure of these compounds were determined. Sintering temperature can affect relative density, and consequently, improve TE performance. In addition, a series of Sn-doped FeNbSb compounds was prepared in accordance with the optimal sintering temperature and pressure. Results showed that Sn optimized the power factor and reduced lattice thermal conductivity by enhancing point defect phonon scattering. A high ZT of 0.66 was attained at 923 K for FeNbSb0.84Sn0.16. The findings of this study lay the foundation for reducing the thermal conductivity of FeNbSb-based compounds. [ABSTRACT FROM AUTHOR]

It is commonly believed that reducing the thermal conductivity of Heusler alloys is important for improving their thermoelectric properties. In this study, we focused on Fe2TiSn, which exhibits a relatively low thermal conductivity among Heusler alloys, to investigate the advantages of a powder metallurgy method that can easily form samples of desired shapes and produce dense samples with less segregation. We prepared sintered Fe2TiSn specimens using powders milled in air or Ar for 1 h, 3 h, and 12 h. We found that varying the non-stoichiometric composition led to deviations from the Fe2TiSn content of samples milled in air as a result of the appearance of a second phase, and the temperature at which the Seebeck coefficient changes from p-type to n-type decreased with increasing milling time for samples milled in air. For samples milled in Ar, no change in the Seebeck coefficient with milling time was observed. Although no significant difference was found between the electrical resistivities and thermal conductivities of samples milled in air and Ar, increasing the milling time promoted phonon scattering at the grain boundaries and reduced the lattice thermal conductivity. We determined a maximum dimensionless figure-of-merit of 0.0014 at 285 K for the Fe2TiSn alloy milled in Ar for 3 h. [ABSTRACT FROM AUTHOR]

Incorporation of a foreign element is considered as a promising approach to enhance the performance of thermoelectric materials since this can either improve the power factor by a band structure modification or reduce the thermal conductivity by a phonon scattering strengthening. We fabricated the polycrystalline bulk samples of Cu-incorporated Bi0.5Sb1.5Te3 by melt spinning and spark plasma sintering, and evaluated the electronic and thermal transport properties. From the phase analysis and thermoelectric properties measurement, we found that most of the added excess Cu atoms were substituted at a Sb-site and a small amount of Cu was intercalated at the van der Waals gap between quintuple layers. By the formation of two different point defects (substituted Cu and intercalated Cu), the thermoelectric power factor was enhanced because of the increased density of states effective mass, and simultaneously reduced thermal conductivity originated from the intensified phonon scattering and suppressed bipolar contribution. Maximum thermoelectric figure of merit zT of 1.13 was obtained at 400 K. [ABSTRACT FROM AUTHOR]

In this work, the thermoelectric properties of the p-type higher manganese silicide (HMS) were enhanced by partially substituting Mn with Ta. The ingots of the compound Mn36.4−xTaxSi63.6, where x = 0.00, 0.25, 0.50. 0.75 and 1.00, were synthesized via arc melting which were then cast into a ribbon shape by a melt spinning process. After that, the crushed ribbons were consolidated to form a bulk sample by spark plasma sintering. The x-ray diffraction analysis showed that the single phase of HMS existed for x up to 0.50. Above that, the evidence of secondary phases was found, confirmed by scanning electron microscope imaging with elemental mapping. For thermoelectric properties measurement, the Seebeck coefficient and electrical conductivity were insignificantly different in the pure-phase samples. On the other hand, the samples with secondary phases showed increased electrical conductivities but slightly decreased Seebeck coefficients. The thermal conductivity was suppressed in all Ta-substituted samples. The lowest lattice thermal conductivity was found in the sample with the impurity phase (TaSi2) due to the enhanced phonon scattering. Consequently, the ZT of the Ta-substituted HMS was enhanced with the peak ZT of 0.37 at 813 K, which is about 28% higher than that of the pristine HMS. [ABSTRACT FROM AUTHOR]

Se-doped half-Heusler compositions, FeVSb1−xSex (0.03 ≤ x ≤ 0.15), were fabricated by mechanical alloying followed by vacuum hot pressing. The goal of this synthesis was to explore the effect of Se doping on the thermoelectric and transport properties of FeVSb system. A near single half-Heusler phase was found to form; however, a second phase of FeSb2 couldn't be avoided in this process. N-type conduction was confirmed and Se acted as a donor for the FeVSb system. Lattice thermal conductivity also considerably decreased after Se doping. The absolute value of Seebeck coefficient is increased to a maximum of 126 μVK−1 at 956 K for x = 0.12, which may help to increase the figure of merit (ZT) of the FeVSb system. The figure of merit is improved by Se doping, and the improvement is possibly owing to the combined effect of fine grain structure, increased effective mass and phonon scattering at the grain boundaries. A maximum ZT of 0.27 was achieved for FeVSb0.88Se0.12 at 847 K. [ABSTRACT FROM AUTHOR]

n-Type polycrystalline SnSe0.95-x%MoCl5 (x = 0.5, 1.0, 1.5, 2) samples have been synthesized by melting and hot-pressing. The effect of MoCl5 doping on thermoelectric properties is investigated. The multipoint defects of Clse and Mosn increased the carrier concentration from 5.3 × 1017 cm−3 (p-type) in undoped SnSe to 1.76 × 1019 cm−3 (n-type) in SnSe0.95-1.5%MoCl5 sample, which leads to increased electrical conductivity. Moreover, the multipoint defects enhanced the phonon scattering and resulted in a suppression of the thermal conductivity. As a result, a peak value ZT of 0.66 was obtained at 773 K for SnSe0.95-1%MoCl5. These results show that MoCl5 could be an effective dopant for improving the thermoelectric performance of n-type SnSe. [ABSTRACT FROM AUTHOR]

BiCuSeO is regarded as one of the most promising oxygenated thermoelectric materials because of its special natural superlattice structure and corresponding ultra-low thermal conductivity. In order to improve the thermoelectric performance, we should pay attention to both composition adjustment and structure adjustment. In this paper, we report a study which combines the two ways to improve the electrical transporting performance and suppress the thermal transporting performance, and we have effectively improved the thermoelectric performance. Firstly, by adjusting the composition with In doping at the Bi site, the band gap widens, the energy offset between heavy band and light band increases, and the mobility and electrical transporting performance improves correspondingly. The maximum ZT value is increased to 0.61. Based on this, the thermoelectric properties are further improved in the whole temperature range by increasing the mechanical alloying strength to refine the powders and adjusting the proportion of fine powder and coarse powder. Grain refinement increases the Cu vacancies and corresponding electrical properties. In addition, Grain refinement contributes to enhancing phonon scattering and effectively reduces the lattice thermal conductivity. The thermal conductivity decreases significantly in the whole temperature range because grain refinement can significantly enhance phonon scattering and reduce the lattice thermal conductivity. The appropriate proportion of coarse powder and fine powder can make full use of the fine grains to enhance phonon scattering while enlarging the frequency range of scattered phonons, thus further reducing the thermal conductivity. Finally, the ZT value increased to 1.08. [ABSTRACT FROM AUTHOR]

Thermoelectric oxide-based multiphase systems gain synergistic properties from different materials. Therefore, multiphase systems based on a thermoelectric oxide, combined with both a polymeric phase (Matrimid) and a highly electrically conducting phase (Ag, carbon black) have been investigated. Compared to single-phase porous Ca3Co4O9, the resulting composite materials showed a decreased electrical conductivity while reaching a high Seebeck coefficient of up to 200 μV/K as well as a 4 times lower thermal conductivity. The strongly enhanced phonon scattering in the multiphase system resulting in low thermal conductivity is an especially interesting concept to design thermoelectric multiphase materials. Additionally, Ioffe plots are revitalized to compare the resulting power factor and thermal properties of the composite materials. The significantly low thermal conductivity due to the heteromaterial interfaces in the composite materials especially underlines the potential of multiphase systems as thermoelectric materials. [ABSTRACT FROM AUTHOR]

An analytical model is developed for calculating the electric field-dependent electron ionization rate in bilayer graphene nanoribbons. All probable arrangements of optical phonon and inter-electron scattering phenomena before the occurrence of an ionizing collision event have been considered in this model, which is viewed as the multistage scattering model of impact ionization. This model considers both lucky ballistic and lucky drift electrons moving under an applied electric field. The numerical results calculated by using this model are compared with the results obtained from an earlier developed analytical model. [ABSTRACT FROM AUTHOR]

In2O3(ZnO)k superlattice ceramics are promising oxide thermoelectric materials, as superlattice interfaces are effective in scattering phonons and filtering low-energy electrons to decrease thermal conductivity and improve the figure of merit (ZT) value. In this work, homologous compounds were prepared by solid-state reaction method using ZnO and In2O3 as raw materials. Phase purity and crystal structure were characterized by x-ray diffraction, revealing that In2O3(ZnO)k crystallizes in an R3 m space group for odd k values and P63/mmc for even k values. The study of thermoelectric (TE) properties showed that as k increased, the thickness of InO(ZnO)3+ also increased, while the electrical conductivity, thermal conductivity, Seebeck coefficient, power factor and ZT value were all decreased. In2O3(ZnO)3 samples obtained a maximum power factor of 651 μW m−1 K−2 at 973 K, and also achieved a maximum ZT value of 0.24 at 973 K. [ABSTRACT FROM AUTHOR]

High thermal conductivity and exorbitant cost of Hf has for a long time limited the prospects of half-Heusler (HH) alloys for applicability in thermoelectric (TE) energy conversion devices. This work demonstrates the implication of nanostructuring and efficacy of p-type acceptor dopant in (Ti,Zr)CoSb based HH alloys for enhancing the figure of merit (ZT) while eliminating the use of Hf. A series of (Ti,Zr)CoSb1−x(Si,Sn)x HH composition was synthesized using arc-melting and consolidated employing spark plasma sintering (SPS). The optimal doping of acceptor dopants, namely, Si and Sn significantly improves the power factor and strengthens the phonon scattering resulting in an enhanced TE performance with maximum ZT of 0.26 and 0.5 at 873 K, obtained for TiCoSb0.8Sn0.2 and ZrCoSb0.8Sn0.2, respectively. For further optimization, microstructural modifications by fine-tuning of the Ti to Zr ratio induces strain field effects and mass fluctuation in (Ti,Zr)CoSb0.8Sn0.2 compositions, which remarkably introduces additional phonon scattering resulting in maximum ZT ∼ 0.8 at 873 K for the best performing Zr0.5Ti0.5CoSb0.8Sn0.2 compound. The current study provides a better understanding of p-type dopants in HH materials by which prospective high TE performance can be obtained in low-cost Hf-free p-type (Ti,Zr)CoSb half-Heusler alloys. [ABSTRACT FROM AUTHOR]

We perform first principles calculations to predict the electron-phonon (e-ph) scattering rates in AlAs and their dependence on phonon modes at energies close to the conduction band minima (CBM), as well as high into the conduction band. We then study the effect of hydrostatic pressure on the e-ph scattering in AlAs for pressures up to ∼ 8.77 GPa. The effect of such pressures on the electronic structure and phonon dispersion is well documented. In AlAs, the bandgap becomes smaller, whereas the effect on phonon dispersion is to shift the optical phonon bands to higher frequencies and the acoustic branches to lower frequencies. In light of this, we explore the effect of hydrostatic pressure on the resulting scattering rates with increasing pressure along the high symmetry L→Γ→X path. The results suggest that hydrostatic pressure does not significantly affect electron-phonon scattering rate. [ABSTRACT FROM AUTHOR]

Ideal thermoelectric materials should possess low thermal conductivity κ along with high electrical conductivity σ. Thus, strategies are needed to impede the propagation of phonons mostly responsible for thermal conduction while only marginally affecting charge carrier diffusion. Defect engineering may provide tools to fulfill this aim, provided that one can achieve an adequate understanding of the role played by multiple morphological defects in scattering thermal energy carriers. In this paper, we study how various morphological defects such as grain boundaries and dispersed nanovoids reduce the thermal conductivity of silicon. A blended approach has been adopted, using data from both simulations and experiments in order to cover a wide range of defect densities. We show that the co-presence of morphological defects with different characteristic scattering length scales is effective in reducing the thermal conductivity. We also point out that non-gray models (i.e. models with spectral resolution) are required to improve the accuracy of predictive models explaining the dependence of κ on the density of morphological defects. Finally, the application of spectral models to Matthiessen’s rule is critically addressed with the aim of arriving at a compact model of phonon scattering in highly defective materials showing that non-local descriptors would be needed to account for lattice distortion due to nanometric voids. [ABSTRACT FROM AUTHOR]

To gain deep insight into the mechanism of phonon scattering at grain boundaries, we investigated the boundary thermal resistance by using picosecond pulsed-laser time-domain thermoreflectance for epitaxially grown W/Fe2VAl/W films. By using radio-frequency magnetron sputtering, we prepared a series of the three-layer films whose Fe2VAl thickness ranged from 1 nm to 37 nm. The fine oscillation of reflectivity associated with the top W layer clearly appeared in synchrotron x-ray reflectivity measurements, indicating a less obvious mixture of elements at the boundary. The areal heat diffusion time, obtained from the time-domain thermoreflectance signal in the rear-heating front-detection configuration, reduced rapidly in samples whose Fe2VAl layer was thinner than 15 nm. The ∼ 10% mismatch in lattice constant between Fe2VAl and W naturally produced the randomly distributed lattice stress near the boundary, causing an effective increase of boundary thermal resistance in the thick samples, but the stress became homogeneous in the thinner layers, which reduced the scattering probability of phonons. [ABSTRACT FROM AUTHOR]

Cu-doped BiTe hexagonal nanoplates were fabricated by a simple solvothermal method. Scanning electron microscopy and transmission electron microscopy characterization illustrated that the nanoplates are single-crystal ones with an average width of 300 nm and thickness in the range of 20-30 nm. It was demonstrated that CuBiTe nanoplates possess the lowest thermal conductivity (0.8-1.2 W/m K) at 300 K, which is mainly due to the enhanced phonon scattering by grain boundaries. Furthermore, the doped Cu could evidently enhance the power factor of BiTe through optimizing the carrier transport properties. As a result, CuBiTe nanoplates exhibit the best thermoelectric performance with a figure of merit of 0.33, which is more than two times higher than that of BiTe nanoplates. [ABSTRACT FROM AUTHOR]

We herein report an enhancement of the thermoelectric properties of p-type YbFeCoSb skutterudite by melt spinning combined with spark plasma sintering (SPS). By thermal aging (873 K for 120 h) of the starting YbFe CoSb compound for melt spinning, fabricated by conventional melting and quenching, highly dense single phase bulks with reduced grain sizes of ~300 nm are successfully fabricated after SPS. The power factor value of the sample (~3.6 mW m K at 723 K) is increased, benefiting from an enhancement of the electrical conductivity due to the elimination of the secondary phase CoSb during the thermal aging process. In addition, lattice thermal conductivity is significantly decreased due to the reduced grain size, thus intensifying the grain boundary phonon scattering. Through these synergetic effects, the maximum dimensionless figure of merit ZT increases by 25% (0.70 at 723 K) compared to a pristine sample with microscale grains. [ABSTRACT FROM AUTHOR]

Pure lead-free SnTe has limited thermoelectric potentials because of its low Seebeck coefficients and its relatively large thermal conductivity. Herein, we report on the enhanced electronic transport properties of selenium (Se) doped tin telluride (SnTeSe) nanoparticles (NPs) synthesized by a rapid microwave-assisted solvothermal method and subsequent spark plasma sintering (SPS). Se-doped SnTe NPs, consisting of regular octahedral NPs with sizes from 1.5 μm to 300 nm, are synthesized with sufficient Se doping contents, and detailed structural characterizations reveal a large fraction of grain boundaries in our nanostructures, which is conducive to phonon scattering. Here we demonstrate that it is possible to enlarge the band gap by tuning the doping and composition, ultimately enhancing the power factor. As a result, a power factor value of ∼10.45 μW/cmK is attained at 773 K for the SnTeSe sample, which is 35% higher than that of an undoped SnTe sample. This synthesis and assembly approach provides strategic guidance for maximizing the power factor by nanocrystallization and doping. [ABSTRACT FROM AUTHOR]

We herein report substitutional doping effects on the electronic and thermal transport properties of higher manganese silicides (HMS) MnSi. Polycrystalline bulks of MnASi and MnSiB (A = V, Cr, Mo/B = Al, Ge) were fabricated by a solid-state reaction combined with the spark plasma sintering technique, and their thermoelectric properties were evaluated. We found that thermoelectric performance of MnSi was significantly enhanced due to the simultaneous improvement in electronic transport and phonon scattering via partial substitution of foreign atoms at Mn- and/or Si-sites. Through the small amount of Cr doping at the Mn-site and Al and Ge doping at the Si-site, the power factor was improved due to enhancement in density of the state's effective mass. Thermal transport properties could be also manipulated due to the point defect phonon scattering effect, and reduced lattice thermal conductivity was obtained with Ge-doped HMS. As a consequence, the maximum dimensionless figure␣of merit ZT of 0.64 at 773 K (increased 50% compared to undoped MnSi) was obtained in Ge-doped MnSi. [ABSTRACT FROM AUTHOR]

We present the structure of ZnO nanowires (NWs) embedded in ZnO films for high-performance transparent thermoelectric materials. The design concept is that the ZnO NWs exhibit high power factor and work as phonon scatterers to reduce the thermal conductivity. Here, we form an embedded-ZnO NWs structure on Si(111) substrates using physical vapor transport for ZnO NW formation and pulsed laser deposition for embedding NWs with ZnO. The NWs grew along the c-axis orientation vertically on the ZnO buffer/Si(111) substrates. Nanoscale voids near NWs were also observed in filling ZnO. The electrical measurements of films including NWs exhibited the reduction of electrical conductivity from that of bulk ZnO to a similar extent to the reduction in the case of ZnO films without NWs. This indicates that there was small electron scattering by ZnO NWs and the voids. However, considering that the mean free path of electron becomes lower by increasing carrier concentration, the electron scattering effect by nanostructuring can be found to be even weaker under the high doping condition compared with phonon scattering with large mean free path. Therefore, our study develops embedded-ZnO NWs structures promising for high-performance thermoelectric material with high electrical conductivity and low thermal conductivity. [ABSTRACT FROM AUTHOR]

In this research, the microstructure and transport properties of p-type BiSbTe thermoelectric materials were investigated as a function of milling time. The p-type BiSbTe alloys were fabricated by mechanical alloying of elemental chunks of bismuth, antimony, and tellurium. This was followed by plasma spark sintering at 673 K. The micro-Vickers hardness (98.7 Hv) was considerably improved in the 90-min sample due to the presence of fine grains in the matrix that prevented crack propagation via grain-boundary hardening. The lowest lattice thermal conductivity (0.63 W/mK) was obtained for the 90-min sample, a value slightly lower than the minimum total thermal conductivity (0.872 ± 0.5 W/mK at 300 K) due to strong scattering of phonons and carriers owing to the completely randomness of the distribution of the fine-grain structure in the bulk samples. The maximum figure-of-merit ( ZT = 0.98 ± 0.5 at 300 K) was obtained for the 90-min sample due to its superior power factor values. [ABSTRACT FROM AUTHOR]

Lead-free GeTe compound shows good electrical properties but also high thermal conductivity. GeTeSe alloys with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by conventional melting, quenching, and spark plasma sintering (SPS) techniques to study their phases and thermoelectric properties. Se was chosen as dopant element in GeTe to reduce the thermal conductivity. Experimental results showed that all alloys formed rhombohedral GeTe-based solid solution and minor cubic Ge phase without any phase arising from Se. The alloys retained the p-type conduction of GeTe. Substitution of Se for Te in GeTe increased its electrical resistivity, but increased its Seebeck coefficient and reduced its thermal conductivity significantly. Reduction of the carrier mobility was responsible for the increase of the electrical resistivity and Seebeck coefficient. Enhancement of the phonon scattering on Se solute atoms and minor phase Ge was responsible for the reduction of the thermal conductivity. As a result, the figure of merit ZT of GeTeSe alloys can be enhanced with proper Se substitution. A maximum ZT value of 1.17 at 773 K was eventually achieved in the GeTeSe sample, being 26% higher than that of pure GeTe. [ABSTRACT FROM AUTHOR]