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Known as a weak topological insulator (TI), BiSe structurally exhibits alternating stacks of quantum spin Hall bilayer ("Bi2") and three-dimensional TI layer ("Bi2Se3"). The low lattice thermal conductivity of BiSe due to the presence of Bi2 bilayers promises potentially good thermoelectric performance. Herein, the thermoelectric properties of nominal Bi1−xCuxSe samples were studied as the functions of the content of Cu additive and temperature. It is found that Cu additives in BiSe (1) profoundly affect the texture of densified polycrystalline samples by inclining the crystallographic c-axis parallel toward the pressure direction in the densification process, (2) increase considerably the effective mass and thus the Seebeck coefficient, and (3) yield point defects and Cu–Se secondary phases that effectively scatter heat-carrying phonons. As a result, the optimized electrical and thermal properties yield a thermoelectric figure of merit of zT ~ 0.29 in Bi1−xCuxSe (x = 0.03) sample at 467 K in parallel to the pressure direction and a zT ~ 0.20 at 468 K in the perpendicular direction. [ABSTRACT FROM AUTHOR]

The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity, reaching values larger than two, consistently across materials and temperatures. Central to this ZT improvement is the drastic reduction in the material thermal conductivity due to the scattering of phonons on the numerous interfaces, boundaries, dislocations, point defects, phases, etc., which are purposely included. In these new generation of nanostructured materials, phonon scattering centers of different sizes and geometrical configurations (atomic, nano- and macro-scale) are formed, which are able to scatter phonons of mean-free-paths across the spectrum. Beyond thermal conductivity reductions, ideas are beginning to emerge on how to use similar hierarchical nanostructuring to achieve power factor improvements. Ways that relax the adverse interdependence of the electrical conductivity and Seebeck coefficient are targeted, which allows power factor improvements. For this, elegant designs are required, that utilize for instance non-uniformities in the underlying nanostructured geometry, non-uniformities in the dopant distribution, or potential barriers that form at boundaries between materials. A few recent reports, both theoretical and experimental, indicate that extremely high power factor values can be achieved, even for the same geometries that also provide ultra-low thermal conductivities. Despite the experimental complications that can arise in having the required control in nanostructure realization, in this colloquium, we aim to demonstrate, mostly theoretically, that it is a very promising path worth exploring. We review the most promising recent developments for nanostructures that target power factor improvements and present a series of design 'ingredients' necessary to reach high power factors. Finally, we emphasize the importance of theory and transport simulations for materialoptimization, and elaborate on the insight one can obtain from computational tools routinely used in the electronic device communities. [ABSTRACT FROM AUTHOR]

For the present investigation, AgPb16SbTe18 material was prepared by the inclusion of AgSbTe2 nanopowder into PbTe matrix, employing mechanical alloying route. This material was characterized for phase formation, microstructure, and transport properties. The microstructure showed densely populated grains with a grain size ~ 80 nm which plays an important role in controlling the transport properties. A remarkable enhancement in Seebeck coefficient (~ − 500 µV at 450 K) value of AgPb16SbTe18 sample was observed which is attributed to the phonon scattering and the potential barrier, encountered by the charge carriers at the nanostructured grain boundaries. The other important findings are enhancement in electrical conductivity and reduction in thermal conductivity which directly enhance the figure of merit of the material and make it promising for thermoelectric power generator application in the mid-temperature zone. [ABSTRACT FROM AUTHOR]

Monolayers of atomically thin semiconductors show novel excitonic physics with tunable optical and electronic properties. Excitons along with its charged complexes have significant role in future optoelectronic devices with respect to carrier transport and recombination. Monolayer of MoSe2 and WSe2 have been investigated for their charge transport and photo response at room and cryogenic temperatures in order to optimize the device for enhanced sensitivity and detectivity. Low temperature PL measurements depicts the decoupling of phonons from excitons due to reduction of non-radiative channels. At cryogenic temperature the maximum photo response of the WSe2 and MoSe2 based detector is 7.2 AW−1 and 5.0 AW−1, respectively, whereas the maximum attainable detectivity as observed for device fabricated from WSe2 and MoSe2 is 2.0 × 1011 Jones and 3.5 × 108 Jones, respectively. This indicates that phonon decoupling and scattering needs to be optimized for better device performance. [ABSTRACT FROM AUTHOR]

Whether the presence of adsorbates increases or decreases thermal conductivity in metal-organic frameworks (MOFs) has been an open question. Here we report observations of thermal transport in the metal-organic framework HKUST-1 in the presence of various liquid adsorbates: water, methanol, and ethanol. Experimental thermoreflectance measurements were performed on single crystals and thin films, and theoretical predictions were made using molecular dynamics simulations. We find that the thermal conductivity of HKUST-1 decreases by 40 – 80% depending on the adsorbate, a result that cannot be explained by effective medium approximations. Our findings demonstrate that adsorbates introduce additional phonon scattering in HKUST-1, which particularly shortens the lifetimes of low-frequency phonon modes. As a result, the system thermal conductivity is lowered to a greater extent than the increase expected by the creation of additional heat transfer channels. Finally, we show that thermal diffusivity is even more greatly reduced than thermal conductivity by adsorption. Metal-organic frameworks are attractive adsorbents for gas storage and separations, but appropriate heat dissipation is important for practical application. Here the authors use experiment and theory to show the impact of liquid adsorbates on thermal transport in metal-organic frameworks. [ABSTRACT FROM AUTHOR]

In the present study, the systematic evaluation of thermal properties was performed for CeO2–Gd2O3 (Gd2O3 = 0–15 wt%) samples. Gd2O3 doping into CeO2 reduced the grain size of CeO2–Gd2O3 solid solution and caused the expansion of the lattice parameter. The thermal conductivity of CeO2–Gd2O3 solid solutions decreased with increasing Gd2O3 content and decreased with temperature up to nearly 900 K, though the decreasing rates became smaller at a higher temperature. The values of the thermal conductivity were mostly attributed to phonon mean free path which decreased due to the Umklapp processes at high temperatures and reduced by phonon scattering due to Gd dopants and oxygen vacancies at low temperatures. The phonon scattering caused by grain boundaries, and chemical impurities can be neglected, and the isotope effect is considered to be small. [ABSTRACT FROM AUTHOR]

The phonon modes of quantum-cascade heterostructures based on binary and ternary semiconductor compounds are simulated. The dependences of the frequencies of the structure interface phonon modes on the wave vector in the layer plane and on the phase shift in the superlattice period are calculated. It is found that the range of variation in the quantum energy of phonon modes of the GaAs/Al0.25Ga0.75As structure does not exceed 2 meV. The calculated resulting interband scattering rate in the structure, taking into account the interface and confined modes, barely differs from that calculated in the bulk phonon approximation. [ABSTRACT FROM AUTHOR]

Fermi liquids (FLs) display a quadratic temperature (T) dependent resistivity. This can be caused by electron-electron (e-e) scattering in presence of inter-band or Umklapp scattering. However, dilute metallic SrTiO3 was found to display T2 resistivity in absence of either of the two mechanisms. The presence of soft phonons as possible scattering centers raised the suspicion that T2 resistivity is not due to e-e scattering. Here, we present the case of Bi2O2Se, a layered semiconductor with hard phonons, which becomes a dilute metal with a small single-component Fermi surface upon doping. It displays T2 resistivity well below the degeneracy temperature in absence of Umklapp and inter-band scattering. We observe a universal scaling between the T2 resistivity prefactor (A) and the Fermi energy (EF), an extension of the Kadowaki-Woods plot to dilute metals. Our results imply the absence of a satisfactory understanding of the ubiquity of e-e T2 resistivity in FLs. The electrical resistivity of Fermi liquids usually shows square power law with respect to temperature (T2) due to either inter-bandscattering or Umklapp process. Here, authors report T2 resitivity below a temperature where neither Umkapp nor interband scattering is responsible in a dilute metal Bi2O2Se. [ABSTRACT FROM AUTHOR]

Designing ordered fillers arrangement and superior interfacial adhesion between fillers and matrix can improve the thermal conductivity (TC) of composites. Here, bioinspired dopamine chemistry was firstly used to reduce graphene oxide (GO) and introduce polydopamine nanoparticles on the surface of GO. Then, a well-aligned epoxy/reduced GO films (EP/RGFs) nanocomposites were prepared via the simple vacuum impregnation. Compared with the random distribution of fillers in a traditional blending composite, fillers were selectively distributed in matrix and continuous thermal conductive network structures were constructed in this strategy. As a result, the nanocomposite exhibited a high TC of 0.913 W m−1 K−1 which is 4.8 times higher than pure EP. In addition, curing kinetics showed that RGFs were similar to an amine-type curing agent that reacted with EP and bonded them tightly, and its nanocomposites reaction activation energy is lower than that of pure EP. These results indicated RGFs possessed excellent interface compatibility with EP and suppressing effectively the phonon scattering at the EP–RGFs interface. Cooling experiments showed that nanocomposites can reduce by about 10 °C for a hot source (80 °C), demonstrating it can transfer efficiently heat energy from the heat source. This study provides an effective method for the preparation of high-performance thermal management composites. [ABSTRACT FROM AUTHOR]

The thermal properties of graphene oxide containing hydroxyl and epoxy functional groups were studied using non-equilibrium molecular dynamics to understand the thermal transport phenomena involved and the structure factors limiting heat conduction. Estimates were given in terms of phonon mean free paths for the reduction in thermal conductivity by interior defects due to scattering. The mechanism of phonon transport in the graphene oxide was discussed. The results indicated that the degree of oxidation can significantly affect the thermal performance of graphene oxide. A low degree of oxidation is necessary to enhance the phonon transport properties of graphene oxide and reduce the probability of phonon-defect scattering. Phonon transport in graphene oxide with a high degree of oxidation is governed by the mean free path of phonons associated with scattering from interior defects. Oxygen-containing functional groups can adversely affect performance and reduce the efficiency of phonon transport in graphene oxide due to phonon mean free paths limited mainly by interior defects. The calculated intrinsic thermal conductivity of graphene oxide at room temperature is about 72 W/(m K) with an oxidation degree of 0.35 and about 670 W/(m K) with an oxidation degree of 0.05. The phonon mean free path decreases with increasing the degree of oxidation due to enhanced phonon-defect scattering, making the thermal conductivity very sensitive to the concentration of oxygen-containing functional groups. [ABSTRACT FROM AUTHOR]

The temperature dependence of the conductivity of the topological Kondo-insulator (TKI) Sm1 –xYbxB6 is investigated in the temperature range 2 < T < 300 K for compositions with x ≤ 0.024. It is found that the ytterbium impurity most strongly affects the low-temperature (T < 20 K) electrical conductivity: when changing from x = 0 to x = 0.024, the activation energy of bulk conductivity decreases by 1.8 times from 4 to 2.2 meV, and the 2D surface conductivity increases by six times. The separation made for the contributions to the conductivity enables us to establish that the one-parameter scaling model can be used for describing the conductivity of 2D surface states in the Sm1 –xYbxB6 TKI in which both the interaction with phonons and the electron–electron scattering effects are taken into account. [ABSTRACT FROM AUTHOR]

This paper reports that a novel polyimide dielectric composite with three-dimensional (3D) thermally conductive networks and enhanced breakdown strength was firstly fabricated by filling with core-double-shell structured F-BA fillers. The F-BA particles were composed of nano-sized boron nitride (nBN) and polydopamine-coated spherical alumina (PDA@Al2O3). Moreover, to ameliorate interfacial compatibility between F-BA fillers and PI matrix as well as restrain phonons scattering during propagation, 1,6-Diisocyanatohexane (HDI) was innovatively used as "bridge agent" to connect and functionalize nBN and PDA@Al2O3 particles to generate core-double-shell structure. The results revealed that breakdown strength of PI dielectric composite with 25 wt % F-BA fillers was increased to 146.3 MV·m−1, showing an increment of 68.5% in comparison with that of pure PI. Furthermore, the thermal conductivity of F-BA/PI composite with 25 wt % F-BA fillers increases to 6.41 W/m·K of in-plane direction and 1.01 W/m·K of through-plane direction, respectively, which shows 36 and 6 times higher than polyimide of 0.18 W/m·K. For dielectric properties of F-BA/PI composite, the dielectric constant and loss are less than 3.5 and 0.02, respectively. Considering these properties, the prepared PI dielectric composite shows potential application in high-performance electronic devices. [ABSTRACT FROM AUTHOR]

Modulation doping has recently emerged as a method for improving the thermoelectric figure of merit using composite materials. By spatially decoupling charge carriers from their dopant impurities, the average carrier mobility can be improved by reducing the influence of ionized impurity scattering. However, as we show in the present work using a simple parabolic band model and effective medium approach, enhancement in such composites is effective only if ionized impurities dominate the scattering of the charge carriers, which is often not the case in many materials. For example, the enhancement is more significant at lower temperatures (T < 300 K for our model material) where acoustic phonon scattering begins to freeze out. Furthermore, for large dielectric constant, which is common for good thermoelectric materials, the ionized impurities are largely screened, and formation of the modulation-doped composite is likely to have little to no benefit, or possibly even degrade the thermoelectric performance of the material. The effective mass of the charge carriers also plays an important role in the relative strength of the scattering mechanism and the possibility for thermoelectric enhancement through modulation doping. [ABSTRACT FROM AUTHOR]

The temperature dependence of the conductivity of the topological Kondo-insulator (TKI) Sm1 –xYbxB6 is investigated in the temperature range 2 < T < 300 K for compositions with x ≤ 0.024. It is found that the ytterbium impurity most strongly affects the low-temperature (T < 20 K) electrical conductivity: when changing from x = 0 to x = 0.024, the activation energy of bulk conductivity decreases by 1.8 times from 4 to 2.2 meV, and the 2D surface conductivity increases by six times. The separation made for the contributions to the conductivity enables us to establish that the one-parameter scaling model can be used for describing the conductivity of 2D surface states in the Sm1 –xYbxB6 TKI in which both the interaction with phonons and the electron–electron scattering effects are taken into account. [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]

Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance. Chemical doping plays an important role in tuning carrier concentration of materials, but its influence on other aspects of electrical properties is less known. Here, the authors find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering. [ABSTRACT FROM AUTHOR]

Engineering of thermoelectric materials requires an understanding of thermal conduction by lattice and electronic degrees of freedom. Filled skutterudites denote a large family of materials suitable for thermoelectric applications where reduced lattice thermal conduction attributed to localized low-frequency vibrations (rattling) of filler cations inside large cages of the structure. In this work, a multi-wavelength method of exploiting x-ray dynamical diffraction in single crystals of CeFe4P12 is presented and applied to resolve the atomic amplitudes of vibrations. The results suggest that the vibrational dynamics of the whole filler-cage system is the actual active mechanism behind the optimization of thermoelectric properties. [ABSTRACT FROM AUTHOR]

High thermal conductivity polymer composites have increased application in modern electronics. To achieve high thermal conductivity at relatively low filler content, two critical approach are used: surface modification of the filler and construction of thermal conductive network in polymer composites. This article provided a new simple and feasible method to modify h-BN consisting of physical absorption of polyvinylpyrrolidone (PVP) and hydrolysis of tetraethyl orthosilicate (TEOS). Then, SiO2@BN/PMMA composites were fabricated via solution-mixing and hot compression. The surface modification of BN enhanced the interface interaction between adjacent BN, which led to reduced thermal resistance and phonon scattering. The solution-mixing and hot compression process ensured the formation of thermal conductive pathway at the same time. The highest thermal conductivity of 40 vol% SiO2@BN/PMMA composite reached 5.583W /m K, which exhibited 3292% and 200% enhancement compared with the pure PMMA and melt-mixing BN/PMMA composites, respectively. [ABSTRACT FROM AUTHOR]

A nanoscale phononic crystal filter (reflector) is designed for a kinetic inductance detector where the reflection band is matched to the quasiparticle recombination phonons with the aim to increase quasiparticle lifetime in the superconducting resonator. The inductor is enclosed by a 1-μm-wide phononic crystal membrane section with two simple hole patterns that each contain a partial spectral gap for various high-frequency phonon modes. The phononic crystal is narrow enough for low-frequency thermal phonons to propagate unimpeded. With 3D phonon scattering simulation over a 40 dB attenuation in transmitted power is found for the crystal, which is estimated to give a lifetime enhancement of nearly two orders of magnitude. [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]

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]

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]

Yttria-stabilized zirconia (YSZ), 15 wt.%Al2O3-doped YSZ (YSZ-15 wt.%Al2O3), and 35 wt.%Al2O3-doped YSZ (YSZ-35 wt.%Al2O3) coatings were prepared by atmospheric plasma spraying. Composition and microstructure of the coatings were characterized. Moreover, the effect of composition and microstructure on the thermal conductivities and thermal expansion coefficients (TECs) of the coatings was investigated. The YSZ-15 wt.%Al2O3 coatings possess much lower thermal conductivity compared to YSZ coating which is attributed to the smaller grains, more oxygen vacancies and the large atomic weight difference between Al and Zr. The mentioned factors improve the phonon scattering thereby reducing the thermal conductivity. The lower TECs of YSZ-15 wt.%Al2O3 coatings are attributed to the smaller lattice parameters due to the substitutional Al3+ doping at Zr4+ sites. The decrease in lattice parameter values increases the crystal lattice energy. And the TEC is inversely proportional to the lattice energy. Moreover, the TEC of the YSZ-35 wt.%Al2O3 coating is much lower than the YSZ coating, which is due to the precipitation of Al2O3 and the formation of amorphous phases. Compared to ZrO2 phase, the Al2O3 phase possesses lower TECs values. The amorphous phase can account for larger expansion tolerance compared to the corresponding crystalline phase due to its loose atomic arrangement. [ABSTRACT FROM AUTHOR]

It is well known that nanoparticles have the ability to enhance the performance of thermoelectric materials. A comprehensive analytical model exploring the mechanism by which this enhancement takes place remains largely absent from the literature. We address this deficiency by introducing a simple model of a nanoparticle as a circular nanoinhomogeneity and analyze its effect on thermal–electric conversion efficiency and the effective properties of thermoelectric composites. Taking interface phonon scattering into consideration, the effective material parameters around a nanoinhomogeneity are derived explicitly, while the effective properties and optimal conversion efficiency are discussed in detail via numerical analysis. Our results show that the nanoinhomogeneity equally effects both the figure of merit and the conversion efficiency. A carefully selected inhomogeneity not only improves the thermoelectric power factor but also suppresses the effective thermal conductivity, and thus greatly improves the optimal conversion efficiency. Specifically, a nanoinhomogeneity with higher electric and thermal conductivities generates a higher thermoelectric figure of merit regardless of the magnitude of its Seebeck coefficient. Noting that the thermoelectric performance improved by the presence of the inhomogeneity is restricted by the intensity of interface phonon scattering, we further calculate the expressions for the effective figure of merit in certain extreme cases. Furthermore, we see that a smaller inhomogeneity has a stronger ability to improve the conversion efficiency for a given doping proportion, mainly because the interface phonon scattering of the smaller inhomogeneity has a relatively greater effect on thermal conduction. [ABSTRACT FROM AUTHOR]

The Raman spectra of bulk samples of the InSb–MnSb eutectic composite and its thin films produced by flash evaporation are studied. In the Raman spectra of the bulk samples, the TO and LO modes of InSb at the frequencies 179.5 and 192.4 cm–1 and a number of peaks at the frequencies 122, 127, 167, 211, and 245.5 cm–1, close to the available theoretical data on the phonon frequencies in MnSb, are observed. In the Raman spectra of the films, the TO mode is shifted to lower energies and corresponds to 178 cm–1, and the LO mode is shifted to higher energies and corresponds to 196 cm–1. The shift of the LO mode to higher frequencies in the composite with respect to the corresponding mode in InSb is apparently caused by strains at the matrix–inclusion interface and by the contribution of scattering at surface phonons to the Raman spectrum. [ABSTRACT FROM AUTHOR]

The abundant waste heat (below 500 K) from industry and automobile exhaust stimulate the development of thermoelectric materials for power generation. The commercial Bi2Te3-based alloys are generally used near room temperature. Herein, n-type Bi2Te2.7+xSe0.3 was prepared by a method of mechanical alloying combined with hot pressing, which is more efficient, time-saving, and energy-saving than the method of melting and spark plasma sintering. The n-type Bi2Te2.7Se0.3 being suitable for power generation (below 500 K) is obtained by manipulating the intrinsic point defects to enhance carrier concentration and suppress the intrinsic excitation at elevated temperature using a tiny amount of excess Te. Enhanced carrier concentration improves electrical conductivity, while enhanced phonon scattering reduces lattice thermal conductivity, originating from increased anti-site defects Te.Bi in Te-rich condition. Consequently, the enhanced ZT peak gradually shifts to higher temperature while increasing the excess Te content. The maximum ZT peak of 0.9 at 373 K and an average ZT of 0.82 between 300 and 498 K are obtained in Bi2Te2.71Se0.3, indicating potential for thermoelectric power generation (below 500 K). [ABSTRACT FROM AUTHOR]

Si/Bi0.4Sb1.6Te3 bulk composites have been prepared by combining mechanical alloying with spark plasma sintering, and their thermoelectric properties have been investigated in the temperature ranges from 298 K to 498 K. The results indicate that with silicon content increasing, the thermopower (S) of the composite system increases substantially. Simultaneously, silicon nano-inclusions cause significant reduction in thermal conductivity (κ) owing to the decreased electrical conductivity and the enhanced phonon scattering of nanoparticles as well as phase boundaries. For the 0.5 vol% Si/Bi0.4Sb1.6Te3 sample, S increases to 224.6 µV K−1 from 210.6 μV K−1 for the sample without silicon and κ decreases to 0.96 Wm−1 K−1 at 423 K from 1.07 Wm−1 K−1 for the sample without silicon, respectively. As a result, the highest ZT of 1.36 is obtained at 423 K of the 0.5 vol% Si/Bi0.4Sb1.6Te3 sample. [ABSTRACT FROM AUTHOR]

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound's intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials. In order to optimize thermoelectric (TE) materials which are used to convert thermal energy and electrical energy, the underlying physics needs to be understood. Here, the authors show that by exploiting static local structure distortion, transverse acoustic phonons can be suppressed resulting in high performing TE materials. [ABSTRACT FROM AUTHOR]

A Monte Carlo calculation of the secondary electron emission from a SiO2 macro-capillary in the backward direction induced by electron irradiation is presented with the aim to understand transmission guiding of a scanning electron beam through a borosilicate glass macro-capillary. The theoretical modeling of electron transport in SiO2 capillary incorporates the elastic, inelastic and phonon scatterings, resulting, respectively, from the interactions with nucleus, electrons and phonons. The influence on electron inelastic scattering by the insulator bandgap is also considered. In this work a simplified approach has been employed to deal with the charging of the internal wall of the capillary and it is found that at a glancing incident angle, the secondary electrons are mostly originated from the top surface. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid perovskite materials exhibit excellent features in the photovoltaic applications. However, there are still questions unanswered regarding fundamental mechanisms of high-performance perovskite devices. Hall Effect and time-resolved photoluminescence measurements are conducted in our research to reveal the nature of carrier transport in CH3NH3I3 single crystals. We found that the high mobility of the crystals falls with increasing temperature, while the slow recombination rate constant of the crystals increases with increasing temperature. The former supports that coherent transport takes place in the CH3NH3PbI3 single crystals, whereas the latter evidences that photo-induced minority carriers exhibit their heavy effective mass in the transport. The two apparently distinct features are complementary in the description of the nature of polaronic motions in the CH3NH3PbI3 materials. These dynamical considerations depict a large polaron moving slowly while scattering with phonons in the CH3NH3PbI3 single crystals. [ABSTRACT FROM AUTHOR]

We study the heat relaxation in current biased metallic films in the regime of strong electron–phonon coupling. A thermal gradient in the direction normal to the film is predicted, with a spatial temperature profile determined by the temperature-dependent heat conduction. In the case of strong phonon scattering, the heat conduction occurs predominantly via the electronic system and the profile is parabolic. This regime leads to the linear dependence of the noise temperature as a function of bias voltage, in spite of the fact that all the dimensions of the film are large compared to the electron–phonon relaxation length. This is in stark contrast to the conventional scenario of relaxation limited by the electron–phonon scattering rate. A preliminary experimental study of a 200-nm-thick NbN film indicates the relevance of our model for materials used in superconducting nanowire single-photon detectors. [ABSTRACT FROM AUTHOR]

Tin selenide (SnSe), which has high thermoelectric (TE) performance due to its low thermal conductivity, is considered as a promising TE material. It is good that TE properties were reported in single crystal form because polycrystalline SnSe exhibits low electrical conductivity compared to that of single crystal SnSe. To improve the electrical conductivity of polycrystalline SnSe, the effects of the pressure applied during spark plasma sintering (SPS) on the electrical charge transport and the TE properties of the polycrystalline SnSe were investigated. Degree of texture was enhanced with increasing sintering pressure from 30 to 120 MPa during SPS, which lead to the increase in carrier mobility, which resulted in the increase in electrical conductivity. Increase in pressure led to a significant increase in thermal conductivity due to an increase in the lattice thermal conductivity, which can be attributed to the decrease in phonon scattering at the grain boundary. A ZT of ∼ 0.7 was obtained at 823 K from the polycrystalline SnSe sintered with a pressure of 60 MPa, which can result from large increase in electrical conductivity with very small increase in the thermal conductivity. This study shows that the TE properties of the polycrystalline SnSe can be enhanced by controlling the degree of texture which can be accomplished by changing the pressure applied during SPS. [ABSTRACT FROM AUTHOR]

Theoretical calculations are performed on lattice thermal conductivity (LTC) and related parameters for the zinc blende and wurtzite structure of InAs nanowires (NWs) with diameters of 50, 63, 66, 100 and 148 nm through the Morelli–Callaway model. For the model to be efficiently applicable, the longitudinal and transverse modes are considered. The melting point of the various-sized NWs is considered to estimate the Debye and phonon group velocities. The impacts of Grüneisen parameter, dislocations and surface roughness are also successfully utilized to address the calculated and measured LTC of the semiconductor under investigation. Results show that the Grüneisen parameter increases with decreasing NW diameter and that phonon confinement leads to an observable deviation of the calculated LTC curve from that of the experimental one in the case of bulk InAs. We assume that NW boundaries, dislocations and imperfections are responsible for the scattering of phonons along with electrons and other phonons because of normal and Umklapp processes. Therefore, at a specified temperature, LTC depends on the size and crystal structure of the semiconductor. As such, the thermal and mechanical parameters of InAs can be greatly modified by decreasing the size and dimension of the semiconductor as a result of the quantum-confinement effect. [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]

On the example of synthetic single crystal diamond with negatively charged nitrogen-vacancy center (NV)—center, we have experimentally reviled the changes in the polarized spectrum of the first-order Raman scattering due to the presence of nitrogen impurities. It has been found the appearance of rather intense spectra of the Brillouin zone-center optical phonons in an unpredictable crossed polarization of the incident and scattered light, while in the previously reported Raman data it was observed only in parallel polarization of the incident and scattered light. Such a recovery of Raman intensity is prescribed to the change of the local symmetry from the point group Oh to C3v in accordance with the results of Raman scattering polarization rules based on conclusions of the group theory. The data make obvious also the observation of the strong in-coming resonance with the nitrogen NV 0 center electronic transition. [ABSTRACT FROM AUTHOR]

Magnesium alloys have many excellent properties and possess wide industrial application prospects. Sheets of AZ91D magnesium alloy with different thicknesses were produced by the die-cast process, and the cooling rates lay between 3.77 and 29.27 °C s−1 with the thickness ranging from 1.5 to 6 mm. With the increasing thickness, the grain size increased, and the concentration of Al solute atoms in Mg matrix decreased. The second phases transformed from homogeneous fine particles and short strips at 1.5 mm to a network at 6 mm. These changes can be attributed to the cooling rate. The thermal conductivity was found to increase with the increasing thickness at the same temperature, and to increase with the increasing temperature at the same thickness. The minimum thermal conductivity (45.19 W (m K)−1), and the maximum thermal conductivity (89.32 W (m K)−1) were obtained at a thickness of 1.5 mm at 25 °C and a thickness of 6 mm at 150 °C, respectively. The grain size increased, and the Al solute atoms in Mg matrix decreased with the increasing thickness, which reduced the lattice irregularity and scattering of electrons and phonons, and resulted in an increase in the thermal conductivity. [ABSTRACT FROM AUTHOR]

Phonon-mediated heat transport in two-dimensional superlattices with coherent and incoherent interfaces is simulated by using the concurrent atomistic-continuum method. The energy transmission across superlattices with incoherent interfaces is found to be an order of magnitude lower than that with coherent interfaces. The simulation results provide a direct visualization of the transient processes of phonon propagation and scatterings, which facilitates an improved mechanistic understanding of phonon transport across multiple interfaces. This work finds that heat conduction in superlattices with coherent interfaces is dominated by coherent phonons, while the existence of defects that comprise the incoherent interfaces destroys the wave interference and changes the phonon transport nature from coherent to diffusive. In addition, phonon scattering by interface defects becomes stronger with decreasing phonon wavelength, and the phonon coherence is destroyed for phonons with 5 nm wavelength. [ABSTRACT FROM AUTHOR]

In this work, γ-aminopropyltriethoxysilane treated aluminum nitride nanosheets (TAlN) were blended with cellulose nanofibrils (CNFs), and a novel nano-flexible composite film was prepared by vacuum filtration. The interactions between the CNFs and TAlN were studied, and the effect of the amount of the added TAlN on the properties of the composites was discussed. Due to the function of silane, the dispersibility of the AlN nanosheets in the CNF substrate is improved and resulting in the formation of covalent bonds between them. The silane treatment reduces the phonon scattering between the AlN and CNF substrate interfaces; thus, the composite film exhibits excellent thermal conductivity. The in-plane thermal conductivity of the composite film reaches 5.11 W/mK when the amount of TAlN added is 25 wt%. The high thermal conductivity composite film material can realize thermal management for flexible energy storage electronic products; thus, effectively promoting the development of green flexible electronic devices. [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]

The resistivity of the undoped In0.53Ga0.47As photoconductive semiconductor switch has been measured over a temperature range of 5–300 K. The experimental temperature characteristic curve of the resistivity exhibits unimodality, and the peak resistivity is more than two orders of magnitude greater than that at 300 K. Based on the two-level model (shallow donor impurities level and deep defect level), we analyzed carrier relaxation rate due to scattering by ionized impurity and polar optical phonons. It is found that the unusually thermal characteristics is dominated by scattering of donor states by ionized impurity scattering and polar optical phonon scattering. The theoretical result is in very good agreement with the experimental results. Our results provide quantitative understanding of charge relaxation in InxGa1−xAs-based devices. Understanding of thermal properties of dark resistance can be used to optimize InxGa1−xAs-based electronic and photonic devices performance in different temperature regimes. [ABSTRACT FROM AUTHOR]

Thermoelectric material is a new functional material that can convert thermal energy into electrical energy, and it has been widely concerned because it can be used as alternative energy in many applications. Bi2Te2.4Se0.6 nanopowders were prepared by electroless plating, and then sintered into bulk by spark plasma sintering. After electroless plating, the thermal conductivity of n-type Bi2Te2.4Se0.6/Cu thermoelectric materials has been significantly reduced due to decreased electronic thermal conductivity and phonon scattering caused by large grain boundaries and defects, of which 0.15 wt%Cu/Bi2Te2.4Se0.6 has the lowest lattice thermal conductivity and reached 0.43 W m−1 k−1. The results show that the average zT value of all components has been enhanced to a certain extent, of which the average zT value of 0.15 wt%Cu/Bi2Te2.4Se0.6 reached 0.89, and the peak zT reached 0.98 at 401 K. [ABSTRACT FROM AUTHOR]

Raman light scattering on phonons and polaritons is measured in a gallium phosphide sample. An unfocused beam of a 532-nm single-mode laser was used for excitation. A scattered radiation was collected using a mobile mirror of a small diameter, which allowed us to measure spectra of scattered light in a 0.6°–8° range of scattering angles with a total angular spread of 0.4°. For different crystallographic directions, intensities of polarized components of the Raman light scattering on longitudinal, transverse phonons and polaritons were measured in the region of a strong dispersion of the polaritonic branch for three fixed axial scattering angles. Components of scattering on longitudinal optical phonons and polaritons have a strong dependence on a crystallographic direction, as predicted by theory, and the component of scattering on transverse optical phonons did not depend on a crystallographic direction. It was found that the intensity of scattering on transverse optical phonons correlates with a width of a spectral line of scattering on polariton. A mechanism explaining this correlation is proposed. [ABSTRACT FROM AUTHOR]

In this work, BiSbTe based graphene composites with different vol% of graphene were processed by the pressure assisted high frequency induction heated sintering. The exfoliated graphene was uniformly dispersed in the BiSbTe powder through magnetic stirring and ball milling. The pristine BiSbTe and composites powders were consolidated by the high frequency induction heated sintering. Thermoelectric properties of the developed bulks were investigated in the temperature range 300–500 K. The results suggest that ball milling as well as incorporation of graphene substantially changes the transport properties of nanostructured BiSbTe composites from pristine bulk. The electrical conductivity of the composites degraded somewhat gradually with the addition of graphene. The effective thermal conductivity reduces by incorporation of graphene, which is attributed to increased phonon scattering by the enormous nanostructured phase boundaries and graphene. The enhanced Seebeck coefficient accompanied by the reduction in thermal conductivity leads to improved figure of merit up to ~ 1.2 at ~ 375 K for 0.5 vol% graphene/BiSbTe composite. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of SiGe–x%SiMo composites are studied from 323 to 1173 K. Electrical conductivity increases with SiMo concentration due to improved carrier mobility and reaches a peak of 1735 S cm−1 when x = 20. The sample SiGe–20%SiMo shows the highest PF of 2.9 mW−1 m−1 K−2 at 1073 K, which is 31.8% higher than the PF value of SiGe. Although total thermal conductivities increase after SiMo addition, the lattice thermal conductivities are reduced due to strengthened phonon scattering by the presence of MoSi2. The ZT of SiGe is 0.64 at 1073 K. It is increased for all the composites in the high-temperature range between 773 and 1073 K, which is beneficial to high temperature application of SiGe alloys. SiGe–20%SiMo shows the highest ZT of 0.79 at 1073 K. [ABSTRACT FROM AUTHOR]