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The isotherms of the lattice thermal conductivity λL for the PbSe1-xTex semiconductor solid solutions (х = 0–0.045) were obtained in the temperature range T = (200–600) K. The λL(x) dependences exhibit a general trend to a decrease in λL with increasing x, but the peaks are observed near x = 0.007, 0.015 and 0.0025 indicating the presence of phase transformations. We associated the first peak with a percolation-type phase transition to the impurity continuum and estimated the critical index for λL within the framework of dynamic scaling theory. The second and third peaks were attributed to the self-organization processes in the PbSe1-xTex after the percolation threshold reaching. The effective cross-sections σs for phonon scattering by impurity (Te) atoms were determined experimentally and calculated theoretically using the Klemens theory. The data obtained are another confirmation of our assumption about the presence of such percolation-type phase transition in any solid solution. [ABSTRACT FROM AUTHOR]

AbstractNanocomposites have already been widely used to enhance thermoelectric performance since they exhibit exceptionally low lattice thermal conductivities. Further improvement is expected to arise from boosting the power factor and charge carrier relevant figure of merit ZT with nanoinclusions. Here we present that Ag nanoscale inclusions embedding in the n-PbTe can meet this requirement. The quantitative relationships between such nanostructures in PbTe and thermoelectric properties, using a versatile model including the details of geometry, electron–phonon interactions, quantization, and tunneling, showed that the Seebeck coefficient can be significantly enhanced due to interface potential barrier induced by Ag nanoinclusions. Additionally, it was found that high-concentration Ag nanoinclusions with 1–2 nm radius can effectively enhance the thermoelectric performance of PbTe through energy-selective carrier scattering, especially around room temperature. The power factor and charge carrier relevant figure of merit ZT can be elevated by at least 80% and 260%, respectively. These findings highlight effective strategies to nanostructuring for overcoming the high threshold to commercial application for cooling and power generation. [ABSTRACT FROM AUTHOR]

Depending on the material properties of the system at hand, in some circumstances the application of a short pulse laser may initially bring a system into a highly non-equilibrium state wherein either the electron temperature may rise up while the lattice still remains cold, or conversely, because the heat capacity of electrons is usually different from that of lattice. In order to correctly describe the heat transport in those situations, therefore, one should properly split the non-equilibrium temperature into two different contributions: the electron temperature and the phonon temperature. By means of a recent two-temperature model (which fully agrees with the Maxwell-Cattaneo theory), in this paper we show that the values of those two temperatures may be always predictable during the thermal-shock propagation in nanowires, after having illustrated both that the employed theoretical model agrees with the basic tenets of continuum mechanics, and that it is mathematically well posed in the case of particular initial and boundary conditions. The main results of this paper could be used to deepen the concept of non-equilibrium temperature. [ABSTRACT FROM AUTHOR]

The miniaturization and higher power density of modern electronics pose a significant challenge in thermal management. A key focus in addressing this challenge revolves around the advancement of thermal interfaces within microchip packaging, aiming to enhance thermal energy dissipation and optimization of performance. Copper nanowires are extensively employed in the chip industry as interconnects for signal transmission and thermal management purposes. Investigating the impact of reduced cross-section on the thermal transport properties of nanowires is crucial. In this study, the thermal conductivity of copper and silicon nanowires is studied with variations in the length of the nanowires. The simulation is conducted with the Equilibrium Molecular Dynamics (EMD) process. The cross-section of the nanowire is kept fixed (10 × 10 nm) and with the increase in length, its thermal conductivity is studied. At room temperature for a 50 nm length, the lattice thermal conductivity value is 1.68 and 0.037 W m − 1 K − 1 for silicon and copper nanowires, respectively. We further studied the phonon scattering, mean free path, and group velocity of silicon and copper lattices. Our study may help to design more thermally efficient microchips and innovate new cooling methods of microelectronics. [ABSTRACT FROM AUTHOR]

Aluminum scandium nitride (AlScN) has been receiving increasing interest for radio frequency microelectromechanical systems because of their higher achievable bandwidths owing to the larger piezoelectric response of AlScN compared to AlN. However, alloying scandium (Sc) with aluminum nitride (AlN) significantly lowers the thermal conductivity of AlScN due to phonon alloy scattering. Self-heating in AlScN devices potentially limits power handling, constrains the maximum transmission rate, and ultimately leads to thermal failure. We grew plasma-assisted molecular beam epitaxy (PAMBE) AlScN on AlN-Al2O3 and GaN-Al2O3 substrates, and compared the cross-plane thermal conductivity to current work on AlScN grown on Si substrates. AlScN grown on AlN-Al2O3 and GaN-Al2O3 substrates achieve a better lattice match and a comparable thermal conductivity to AlScN grown on Si substrates, but with significantly thinner films. [ABSTRACT FROM AUTHOR]

Accurate representation of thermal neutron scattering in Monte Carlo transport simulations requires that the molecular vibrations of the target material be accounted for. Historically, this has been achieved by precomputing large multidimensional tables that are a function of temperature and the cosine of the scattering angle, as well as incoming and outgoing neutron energy. Most commonly used sampling techniques for thermal neutron scattering rely on large multidimensional tables, where higher resolution results in an increase in required memory and attempts to reduce memory can result in grid coarseness errors. An alternative sampling method is introduced here that is a significant departure from precomputed tables and instead relies on a more physical model of the scattering behavior. The phonon sampling method classifies neutron scattering events by the number of phonons excited/de-excited during the scattering collision. In doing so, energy exchange may be obtained via rejection sampling, and an analytical representation of the momentum exchange is obtained. This sampling method has been tested on graphite, yttrium hydride, and uranium nitride, and preliminary implementation of the phonon sampling method shows accurate results for angular and energy distributions, though resulting in up to a 40% slowdown in overall calculation time. This notable slowdown is countered, however, by a large reduction in storage (over 99% reduction compared to standard multidimensional tables). [ABSTRACT FROM AUTHOR]

The experimental data of the temperature dependence of the electrical conductivity σ(T) and the Hall coefficient RH(T) on single crystal samples of n-type CdS reported in literature are reviewed and have been critically analysed including hopping conduction in an impurity band. It is shown that the experimental data of σ(T) and RH(T) reported in almost previous studies can be simultaneously fitted using a common set of the values of material parameters for calculating free-electron conduction, i.e. the conduction and the density-of-state effective mass of electrons, the acoustic-phonon deformation potential, the Fröhlich coupling constant and the piezoelectric coupling coefficient, whereas wide ranges of values for these material parameters had been adopted in different studies. The correct assignment of impurity conduction mechanisms in n-type CdS single crystals is presented. In addition, reasonable explanations are given for unresolved enigmas regarding impurity pairing, unexpected large Hall mobility at low temperatures, and the effect of electric field on hopping Hall mobility. [ABSTRACT FROM AUTHOR]

Half-Heusler compounds have excellent power generation performance at high temperatures. The scattering mechanism is an essential factor affecting the electrical transport properties of thermoelectric materials. In computational simulations, only the role of acoustic phonon scattering on the thermoelectric properties of materials is considered, whereas other scattering mechanisms are neglected. In this work, we investigate the thermoelectric properties of NbCoGe compounds with different combinations of acoustic deformation potential, polar optical phonon, and ionised impurity scattering mechanisms. The calculated results show that the ZT values of n-type and p-type NbCoGe compounds reach 8 and 2.8, respectively, when only acoustic deformation potential scattering is considered, indicating that this sole scattering is insufficient. The calculated ZT values of p-type NbCoGe compounds reach 1.8 at 1200 K and more than 1 at 800 K for p- and n-type NbCoGe compounds under the combined effect of the three scattering mechanisms due to their high–power factor. This provides solid theoretical guidance for the search for potentially high–temperature half-Heusler thermoelectric materials. [ABSTRACT FROM AUTHOR]

Thermal conductivity of hydrogenated hexagonal boron nitride (h-BN) nanosheets was investigated using molecular dynamics simulation method. A newly parameterized reactive force field (ReaxFF) for hydrogen and h-BN interactions was used. ReaxFF was used due to its higher accuracy compared to other simpler interatomic potentials. Accurate thickness selection of a monolayer h-BN nanosheet has been shown to produce high thermal conductivity values for pristine armchair and zigzag nanosheets. It was further found that hydrogenation diminishes thermal conductivity of hydrogenated h-BN nanosheets. This reduction in thermal conductivity was due to the occurrence of sp2 to sp3 bonding transition when hydrogen atoms were placed on top of B and N atoms. The increase in temperature was also found to diminish thermal conductivity due to the occurrence of phonon–phonon scattering at higher temperatures. N-vacancy defect has then been shown to exhibit lower thermal conductivity compared to B-vacancy defect. Furthermore, the removal of more atoms contributes to higher decline in thermal conductivity. However, vacancy defect constructed along vertical direction provides the highest reduction in thermal conductivity. It is expected that this work provides useful insights for the design of an effective hydrogen storage system using these novel h-BN nanosheets. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the lattice thermal conductivity of Janus In2Ge2S6 and In2Ge2S3Se3 bilayers by solving the phonon Boltzmann transport equation using first-principles calculations. We found that this is mainly due to the fact that the frequencies at which larger gaps appear in the intermediate and high frequency optical branches of In2Ge2S3Se3 are smaller than those of In2Ge2S6, which shifts the phonon dispersion curve of In2Ge2S3Se3 downward, which makes the overall phonon group velocity of In2Ge2S3Se3 material smaller than that of In2Ge2Se6 material, and also due to the fact that In2Ge2S3Se3 soft bending in the finite layer thickness coupling and the tight connection of the in-plane acoustic modes, resulting in increased phonon-phonon scattering processes, shorter phonon relaxation times, and larger Grüneisen parameters indicating a stronger anharmonic In2Ge2S3Se3 structure, all these factors combined lead to a lattice thermal conductivity of In2Ge2S3Se3 smaller than the lattice thermal conductivity of In2Ge2S6. At a temperature of 1000 K, the In2Ge2S3Se3 structure has a minimum lattice thermal conductivity of about 0.22 W/mK, and In2Ge2S6 has a minimum lattice thermal conductivity of about 0.4 W/mK. Our results suggest that Janus In2Ge2S6 and In2Ge2S3Se3 bilayers are potential for future thermal management of nanoelectronic devices and thermoelectric devices. two-dimensional materials. [ABSTRACT FROM AUTHOR]

Contemporary thermoelectric literature is rife with material structure-related terminologies like interfaces and grain boundaries, signaling the significance of these structures. Interfaces decide the characteristics of electronic and thermal transport and mechanical properties of polycrystalline and nano thermoelectric (TE) materials. Understanding the relationship between grain boundaries/interphase boundaries and property connections in materials is a key component of material design with desired characteristics and performance. It is now widely recognized that the microstructure of materials is intimately connected to their bulk properties. Accordingly, microstructure control and interface manipulation have emerged as critical topics in the field of materials science and engineering, particularly in thermoelectrics. This paper narrates recent breakthroughs in high-performance TE material design from the standpoints of interface structure and grain boundary manipulation. First, it provides a glimpse of strategies for thermal conductivity reduction through nano and microstructure control, embedded nanoinclusions, grain size reduction, and all-scale hierarchical architectures. It then deliberates on electron and phonon transport decoupling via coherent interfaces, matrix/precipitate electronic band alignment, and charge carrier filtering effects. It proceeds to review the recent results on TE properties of materials prepared with aforementioned strategies emphasizing Bi2(Te,Se)3 and (Bi,Sb)2Te3, SnSe, SnTe, Cu2Se, skutterudides, PbTe-based compounds, GeTe, polymer TE composites, and other materials. At the end, possible strategies for further enhancing zT are addressed. [ABSTRACT FROM AUTHOR]

In this study a 2D cubic chamber model filled with paraffin is analyzed with and without the inclusion of magnetic Fe3O4 nanoparticles at concentrations of 0.5, 1, 1.5 and 2 wt%, and an external magnetic field of intensities 0.005, 0.01, 0.015 and 0.02 T. It is ascertained that adding magnetic nanoparticles leads the horizontal temperature gradient to be reduced owing to increments in thermal conductivity. Additionally, this feature is found to be accelerated by applying an external magnetic field, which shapes highly conductive cluster formations of nanoparticles. However, since the increase in nanoparticle concentration and magnetic intensity increases the composite viscosity, there is an optimum configuration while applying both schemes. As such, the addition of 1 wt% nanoparticles provides the best results, as the melting time is reduced up to 25% compared to pure paraffin. Meanwhile, the melting time of a 1 wt% nanoparticle-containing phase change material (PCM) in the presence of an external magnetic field is improved up to 24% compared to the case with no external magnetic field. Also, the heat transfer coefficient of a 1 wt% nanoparticle-containing PCM both with and without an external magnetic field is also staggeringly enhanced compared to pure paraffin. Good correspondence with experimental data was achieved. [ABSTRACT FROM AUTHOR]

We presented a comprehensive study of electronic properties and phonon thermal transport properties in layered WX2 (X = Se, Te) using first-principles calculations combined with density functional perturbation theory (DFPT). Our results indicated that the spin–orbit coupling (SOC) and biaxial strains have significant effects on the band gaps and band alignments of the bilayer WX2 (X = Se, Te), and the d-orbital of the W atom played a dominant role at the valence and conduction band edge positions. We found that the band gaps decreased with the increase of strain and WTe2 have the transition from indirect band gap to direct band gap at 2% strain. We also calculated their lattice thermal conductivities k l , group velocities, cumulative thermal conductivities k c along the x and y directions, and scattering rates at room temperature. The calculated k l along x and y directions of WSe2 are 70.81 and 78.38 W/m K, respectively, whereas those of WTe2 were relatively low (55.05 W/m K along x-direction and 59.97 W/m K along y-direction), which mainly originated from their relatively smaller phonon group velocities and atomic weights. Two materials exhibited that k l , k c and group velocities in the y-direction were higher than those in the x-direction. We also investigated the size dependence of the k c and contributions of different acoustic and optical branches to the total k l . It was found that the nanostructure may be efficient to reduce k l . [ABSTRACT FROM AUTHOR]

High entropy carbides ceramics with randomly-distributed multiple principal cations have shown high temperature stability, low thermal conductivity, and possible radiation tolerance. While chemical disorder has been shown to suppress thermal conductivity in these materials, little investigation has been made on the effects of additional, extrinsically-generated structural defects on thermal transport. Here, (Zr 0.25 Ta 0.25 Nb 0.25 Ti 0.25 )C is exposed to Zr ions to generate a micron-scale, structural-defect-bearing layer. The reduction in lattice thermal transport is measured using laser thermoreflectance. Conductivity changes from different implantation temperatures suggest dislocation loops contribute little to phonon scattering while nanoscale defects serve as effective scatterers, offering a pathway for thermal engineering. [ABSTRACT FROM AUTHOR]

For high-energy single ion events, the relationship between electronic energy loss mechanism and atomic processes is generally described by the inelastic thermal spike model. However, the parameters required for the model are not accurately known and are often estimated from a free electron gas model. To ensure this model is more reliable and predictive, a more accurate calculation for the parameters should be taken. In this paper, the temperature dependence of the electronic heat capacity and electron–phonon coupling factor (e-ph) has been calculated, using density functional theory (DFT). The calculation results demonstrate that the effect of electron thermal excitation on the thermodynamic parameters is sensitive to the bandgap, and these parameters vary immensely with the electron temperature. Besides, to verify the accuracy of obtained parameters, both the swift heavy ions (SHI) irradiation in crystalline 6H-SiC and amorphous 6H-SiC are simulated, using the inelastic thermal spike model. And the results are more consistent with the experiment conclusion, for the obtained parameters by DFT. Hence, the calculated temperature-dependent parameters are more suitable for the inelastic thermal spike model. [ABSTRACT FROM AUTHOR]

Silicon semiconductors with excellent thermoelectric characteristics have been widely used in integrated electronic systems. Thermal rectification has important potential applications in semiconductor devices such as thermal management improvement of electronics and energy saving. The thermal conductivities and thermal rectifications of silicon devices with sets of triangular holes are investigated by nonequilibrium molecular dynamic (NEMD) simulations. The influences of the dimension and angle of the distributed triangular-shaped holes are investigated. The results indicate that the phonon scattering on the side and base of the right-angled triangular holes are quite different, which causes the thermal rectification. The thermal rectification effect is strengthened with the increase of the triangle dimension. However, the variations of the triangular angle have little effect on the thermal conductivities of the silicon device. Besides, the phonon spectral energy density along the frequency axis of the silicon system is given to explain the mechanism behind the rectification phenomenon. Based on the present simulation results, a possible design strategy is proposed for developing highly efficient thermoelectric devices. [ABSTRACT FROM AUTHOR]

This study analysed the electronic structures of dislocations in 9Cr steel using a spherical aberration-corrected scanning transmission electron microscope equipped with a monochromated electron energy-loss spectroscope. This is the first study to report that dislocations broaden a zero-loss peak (ZLP) and also induce an absorption phenomenon at approximately 0.5 eV in the ZLP tail, which are interpreted to be related to phonon scatterings by dislocations and interference effect of multiple beams, respectively. This work also experimentally demonstrates that the use of a spectrum-imaging method allows imaging of dislocations through ZLP broadening. The approach proposed herein is a promising technique for detecting dislocations in high Cr steel. [ABSTRACT FROM AUTHOR]

This study proposes BN as a new sintering aid for pressureless solid-state sintering of SiC ceramics. The full densification of SiC ceramics for 0.5–2.7 wt% BN addition showed the composition tolerance of the newly developed ceramics. The electrical resistivity decreased by an order of magnitude (107→106 Ω·cm) as BN content increased from ~0.5 to ~0.9 wt% because of the increased BN-derived B doping in the SiC lattice. A further increase in BN content had no significant effect on the electrical resistivity, which is attributed to the limited solubility of B in the SiC lattice. The thermal conductivity decreased with increasing BN content owing to increased phonon scattering at B-doped sites and the thermally insulating BN phase located at the grain boundaries. The fracture toughness increased with increasing BN content owing to interfacial debonding at the weak SiC-BN interfaces. However, intrinsically weak BN grains with low hardness were responsible for reduced flexural strength and hardness with increasing BN content. [ABSTRACT FROM AUTHOR]

We present a method for predicting thermal conductivity by deterministically solving the Boltzmann transport equation for gray phonons by utilizing arbitrary higher-order continuous finite elements on meshes which may also be unstructured and utilize curved surfaces. The self-adjoint angular flux (SAAF) formulation of the gray, steady-state, single relaxation time, phonon radiative transport (PRT) equation was spatially discretized using the continuous finite element method and angularly discretized using the discrete ordinates method. The solution discretization methodology was verified using a method of manufactured solution (MMS) spatial convergence test case and compared favorably to previous work. The angular phonon radiances, heat flux, and temperatures computed in this work compare favorably to previous literature in silicon thin films. Using local values of the temperature gradient and heat flux, the thermal conductivity as a function of position in a one-dimensional perfect crystal was evaluated using a Fourier's Law representation and compared to kinetic theory. Our results show that in the interior of the simulation domain, our transport-based prediction of thermal conductivity converged on the kinetic theory estimation. We also find that near isothermal boundaries, the transport solution deviated from kinetic theory, implying non-equilibrium behavior in the thin-film limit and agreed with previous studies. [ABSTRACT FROM AUTHOR]

This study investigates rare earth RE3TaO7 ceramics and shows that these materials may be optimal for thermal barrier coatings. ZrO2-Sm3TaO7 ceramics were prepared through a solid-state reaction. X-ray diffusion and structural refinement revealed a phase structure with an ordered orthorhombic phase for the Ccmm space group. The degree of structural disorder increased with increasing ZrO2 content. Gaussian function fitting of the oxygen 1s X-ray photoelectron spectra showed that the Sm3+ and Ta5+ ions were replaced by Zr4+ ions. At high temperatures, 8 mol% ZrO2-Sm3TaO7 has a high thermal expansion coefficient (10.9 × 10−6 K−1). The thermal conductivities of ZrO2-Sm3TaO7 (1.17 − 1.75 W·m−1 K−1) are lower than those of 7–8 wt.% yttria-stabilized zirconia, while those of 2% ZrO2-Sm3TaO7 are the lowest. The oxygen vacancies maintain the charge in equilibrium and enhance phonon scattering while decreasing the thermal conductivity. These results indicate that Sm3TaO7 can be used as a TBC. [ABSTRACT FROM AUTHOR]

A thorough understanding of the microscopic picture of heat conduction in solids is critical to a broad range of applications, from thermal management of microelectronics to more efficient thermoelectric materials. The transport properties of phonons, the major microscopic heat carriers in semiconductors and insulators, particularly their scattering mechanisms, have been a central theme in microscale heat conduction research. In the past two decades, significant advancements have been made in computational and experimental efforts to probe phonon-phonon, phonon-impurity, and phonon-boundary scattering channels in detail. In contrast, electron-phonon scatterings were long thought to have negligible effects on thermal transport in most materials under ambient conditions. This article reviews the recent progress in first-principles computations and experimental methods that show clear evidence for a strong impact of electron-phonon interaction on phonon transport in a wide variety of technologically relevant solid-state materials. Under thermal equilibrium conditions, electron-phonon interactions can modify the total phonon scattering rates and renormalize the phonon frequency, as determined by the imaginary part and the real part of the phonon self-energy, respectively. Under nonequilibrium transport conditions, electron-phonon interactions can affect the coupled transport of electrons and phonons in the bulk through the "phonon/electron drag" mechanism as well as the interfacial thermal transport. Based on these recent results, we evaluate the potential use of electron-phonon interactions to control thermal transport in solids. We also provide an outlook on future directions of computational and experimental developments. [ABSTRACT FROM AUTHOR]

Natural pyrrhotites have gained significant attention due to their interesting electronic, antimicrobial, and chemical properties. The present work attempts to explore the morphology-induced modifications in the thermal characteristics of natural pyrrhotite due to ageing. The morphological, elemental, structure, optical, and thermal characterisations help in understanding the effect of ageing. The effects of five years of ageing of the sample are (i) Field Emission Scanning Electron Microscopic analysis reveals a morphological transformation from flakes to agglomerated powder, (ii) elemental analyses suggest the ageing induced compositional modification (iii) the Tauc plot analysis shows a bandgap energy modification from 1.46 eV to 1.92 eV, (iv) X-ray Diffraction (XRD), Fourier Transform Infrared, and X-ray photoelectron spectroscopic studies affirm the formation of oxy-hydroxides (v) the XRD data indicates an increase of dislocation density, and (vi) Photoluminescence study shows a deep violet emission evidenced through the CIE plot. The study by the thermal lens technique shows a lowering of thermal diffusivity study by 23%, due to the morphological modifications, adsorbed/chemisorbed hydroxyl groups, and the formation of secondary compounds due to oxidation and weathering. The phonon boundary scattering, weathering induced smaller grain size, reduced phonon mean free path, and point defects also account for the lowering of the thermal diffusivity value and thereby influencing its properties. [ABSTRACT FROM AUTHOR]

The energy conversion efficiency of thermoelectric systems can be greatly improved via the process known as "hole-doping" in which small-scale holes are introduced into constituent thermoelectric materials. However, the associated thermal stress induced by the introduction of the holes is known to be one of the main factors contributing to failure of the thermoelectric system. In this paper, we perform a detailed and rigorous analysis of the thermal stress distribution around a small-scale (micro- and nano-scale) elliptic hole by taking into consideration the contribution of size effects. Numerical solutions describing electric, thermal and elastic fields in the vicinity of the hole are obtained using complex variable techniques and Fourier series expansions. In particular, we obtain closed-form analytical solutions for the special case of a circular hole to verify the results of our numerical solutions. We find that the dominant stress distribution around the hole is that induced by the hoop stress which is significantly influenced by surface phonon scattering. Specifically, the maximum hoop stress decreases sharply with the increase of the Knudsen number and reaches a minimum value which therefore identifies an optimal stress distribution. [ABSTRACT FROM AUTHOR]

The thermal transport across the alkanethiol surfactant layer at the nanoparticle/base fluid interface in nanofluids was investigated by molecular dynamics simulation, with consideration of the conformation of the surfactant layer with different surfactant chain lengths and backbone stiffness. The variation of temperature drop at nanoparticle-surfactant interface reveals that the interfacial thermal conductance was mediated by the chain length, possibly due to the difference in the adsorption density of surfactant on the surface of the nanoparticles, because of the blocking effect from the bending of the long alkyl chains. The intrinsic thermal conductivity of the surfactant layer increased with decreasing chain length and increasing chain stiffness because of the phonon scattering effect from the bending and cross-linking of the alkyl chains. We quantified the modes of heat flow across the surfactant layer and found that the contribution of intramolecular bonded interaction was much higher than that of atomic translation and nonbonded interaction separately. By analysing the variation of bonded interaction contrition with chain length and stiffness, it is demonstrated that the increased thermal conductivities benefited from the enhanced thermal transfer through the covalent bonds of surfactant molecules. The results can provide insights into the design of thermally conductive surfactants. [ABSTRACT FROM AUTHOR]

The anharmonic interaction and scattering of phonons by a moving dislocation, the photon wind, imparts a drag force v B (v , T , ρ) on the dislocation. In early studies, the drag coefficient B was computed and experimentally determined only for dislocation velocities v much less than transverse sound speed, c T . In this paper, we derive analytic expressions for the velocity dependence of B up to c T in terms of the third-order continuum elastic constants of an isotropic crystal, in the continuum Debye approximation. In so doing we point out that the most general form of the third order elastic potential for such a crystal and the dislocation-phonon interaction requires two additional elastic constants involving asymmetric local rotational strains, which have been neglected previously. We compute the velocity dependence of the transverse phonon wind contribution to B in the range 1–90% c T for Al, Cu, Fe, and Nb in the isotropic Debye approximation. The drag coefficient for transverse phonons scattering from screw dislocations is finite as v → c T , whereas B is divergent for transverse phonons scattering from edge dislocations in the same limit. This divergence indicates the breakdown of the Debye approximation and sensitivity of the drag coefficient at very high velocities to the microscopic crystalline lattice cutoff. We compare our results to experimental results wherever possible and identify ways to validate and further improve the theory of dislocation drag at high velocities with realistic phonon dispersion relations, inclusion of lattice cutoff effects, MD simulation data, and more accurate experimental measurements. [ABSTRACT FROM AUTHOR]

We develop a physical model to describe the microscopic Doppler effect of phonon states in energetic material and use it to investigate the phonon–strain scattering behaviour of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. The required elastic constants and force constants are obtained by first-principles calculations. By using the phonon–strain scattering probability, a set of dissipation parameters are calculated, such as the viscosity coefficient, damping rate of elastic wave, and heat dissipation across shock wave front. It is interesting that the Doppler effect could describe the microscopic phonon scattering mechanism reasonably. [ABSTRACT FROM AUTHOR]

In the present work, the morphology and the electrical and thermal conduction properties of co-continuous poly(vinylidene fluoride) (PVDF), maleated polypropylene (PPgMA) and multiwall carbon nanotubes (CNT) nanostructured blends are investigated. CNT preferentially locates in the PPgMA phase and clearly causes a refinement in the co-continuous structure. Electrical conductivity experiments show that nanocomposites are well above the percolation threshold and evidence for one order of magnitude enhancement in conductivity for the co-continuous nanocomposites compared to the monophasic nanocomposites with the same CNT volume fraction. On the other hand, thermal diffusivity enhancement for the co-continuous blends is found lower than that for the monophasic nanocomposites at the same CNT volume fraction. An explanation is proposed in terms of large interfacial area, causing phonon scattering at the interface between immiscible PVDF and PPgMA domains. Results described in this paper open the way to the preparation of high electrical and low thermal conductivity materials with possible application as thermoelectrics. [ABSTRACT FROM AUTHOR]

Thermoelectric generators, capable of directly converting heat into electricity, hold great promise for tackling the ever-increasing energy sustainability issue. The thermoelectric energy conversion efficiency is heavily dependent upon the materials' performance that is quantified by the dimensionless figure-of-merit (ZT). Therefore, the central issue in the research of thermoelectric materials lies in continuously boosting the ZT value. Although thermoelectric effects were discovered in the nineteenth century, it was only until the 1950s when classic materials like Bi2Te3 and PbTe were developed and basic science of thermoelectrics was established. However, the research of thermoelectrics did not take a smooth path but a rather tortuous one with ups and downs. After hiatus in the 1970s and 1980s, relentless efforts starting from the 1990s were devoted to understanding the transport and coupling of electrons and phonons, identifying strategies for improving the thermoelectric performance of existing materials, and discovering new promising compounds. Rewardingly, substantial improvements in materials' performance have been achieved that broke the ZT limit of unity. Meanwhile, advancements in fundamental understanding related to thermoelectrics have also been made. In this Review, recent advances in the research of thermoelectric materials are overviewed. Herein, strategies for improving and decoupling the individual thermoelectric parameters are first reviewed, together with a discussion on open questions and distinctly different opinions. Recent advancements on a number of good thermoelectric materials are highlighted and several newly discovered promising compounds are discussed. Existing challenges in the research of thermoelectric materials are outlined and an outlook for the future thermoelectrics research is presented. The paper concludes with a discussion of topics in other fields but related to thermoelectricity. [ABSTRACT FROM AUTHOR]

A second-order phonon Boltzmann equation (SOPBE) is proposed based on the phonon Boltzmann equation (PBE) under gray relaxation-time approximation, and the direct collocation meshless (DCM) method is employed to solve the SOPBE. Several numerical tests for phonon transport over a broader range of Knudsen numbers under different boundary conditions are carried out. The results show that the SOPBE solved by the DCM method is applicable for different transport regimes. Moreover, it overcomes the numerical error “ray effects” of other numerical methods under the ballistic limit to a certain extent. When modeling phonon transport in materials with inhomogeneous acoustic property, the superiority of SOPBE will be more obvious compared with the PBE. The results demonstrate the capability of our methodology for ballistic-diffusive phonon heat transport. [ABSTRACT FROM AUTHOR]

By building physically sound interatomic force constants, we offer evidence of the universal presence of a quadratic phonon branch in all unstrained 2D materials, thus contradicting much of the existing literature. Through a reformulation of the interatomic force constants (IFCs) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. We use this approach to predict the thermal conductivity of Pmmn borophene, which is comparable to that of, and displays a remarkable in-plane anisotropy. These qualities may enable the efficient heat management of borophene devices in potential nanoelectronic applications. IMPACT STATEMENTThe newly found universality of quadratic dispersion will change the way 2D-material phonons are calculated. Predicted results for borophene shall become a fundamental reference for future research on this material. [ABSTRACT FROM PUBLISHER]

The authors have proposed a comprehensive analytical model for evaluating the impact ionization rate of electrons in monolayer graphene nanoribbons (GNRs). The impact ionization phenomena have been viewed as a multistage scattering process. All possible combinations of optical phonon scattering and electron–electron collision events prior to the impact ionization have been taken into account in the model. The impact ionization rate of electrons in monolayer GNR has been calculated and results are compared with the numerical data obtained from an analytical model proposed earlier. The effects of temperature and sheet electron concentration on the ionization rate have also been studied. [ABSTRACT FROM PUBLISHER]

The remaining topics are topological nanophononics and chiral phononics, phonon spectroscopy, and phonons in liquid helium. This is followed by three comprehensive chapters on the I ab initio i calculation of phonons in the harmonic approximation, including both direct and linear response methods, anharmonicity, and lattice thermal conductivity, including relaxation-time, variational and linear response methods. This is a welcome revision, about 30 years after the first edition, of one of the most comprehensive monographs on the theory of phonons. [Extracted from the article]

The advent of coupled thermal transport calculations with interatomic forces derived from density functional theory has ushered in a new era of fundamental microscopic insight into lattice thermal conductivity. Subsequently, significant new understanding of phonon transport behavior has been developed with these methods, and because they are parameter free and successfully benchmarked against a variety of systems, they also provide reliable predictions of thermal transport in systems for which little is known. This topical review will describe the foundation from which first principles Peierls-Boltzmann transport equation methods have been developed and briefly describe important necessary ingredients for accurate calculations. Sample highlights of reported work will be presented to illustrate the capabilities and challenges of these techniques and to demonstrate the suite of tools available, with an emphasis on thermal transport in micro- and nanoscale systems. Finally, future challenges and opportunities will be discussed, drawing attention to prospects for methods development and applications. [ABSTRACT FROM PUBLISHER]

In this work, a systematic theoretical investigation of thermoelectric properties of n-type dopedsolid solutions withis presented in the temperature range K. Electronic transport properties (,S, and) are calculated using the nearly-free-electron approximation and the Fermi–Dirac statistics. Thermal transport properties including contributions from carriers (), electron–hole pairs () and phonons () computed using the Wiedemann–Franz law, Price’s theory and Srivastava’s scheme, respectively. In a very good agreement with available experimental measurements, among withsamples, the highest value for thermoelectric figure of meritZTis found to be 1.41 at 800 K forsample owing to its highest electrical conductivity and the lowest lattice thermal conductivity values. Additionally, by theoretically considering the doping levels as, we suggest that at 800 KZTgoes up by 30% forsample with the value ofcompared tosample due to increment in the electrical conductivity and additional mass defect effects to the phonon thermal conductivity. [ABSTRACT FROM PUBLISHER]

The thermal resistance of a crystal lattice with a monatomic unit cell due to three-phonon scattering processes is investigated in detail theoretically. A general expression for the lattice thermal conductivity is derived from a combined analysis based on: (i) the Boltzmann equation and (ii) data on the heat current autocorrelation function obtained via molecular dynamics simulations in conjunction with the Green–Kubo formalism. It is argued that the phonon gas in a monatomic lattice conducts heat as if it consisted of two distinct parts (two ‘thermal fluids’), so that the lattice thermal conductivity can be decomposed into contributions from these two parts. The origin of the behaviour of the phonon gas, which is explored in the present work, is due to an intrinsic interplay between Umklapp and normal three-phonon scattering processes. New insight into the nature of the lattice thermal conductivity is demonstrated and the results of the present work are in agreement with previous studies in this area. [ABSTRACT FROM PUBLISHER]

Tersoff-potential - based molecular dynamics method is used to simulate wrinkling deformation of one graphene sheet under shear, and the obtained deformation is compared with analytical solutions of macro-membrane. Furthermore, thermal conductivity of the wrinkled graphene at different temperatures is calculated. It is found that (1) the wrinkling deformation of graphene sheet under shear is close to the analytical solutions of macro-membrane under shear, which implies that the solutions of macro-membrane are applicable to predict the wrinkling deformation of graphene sheets under shear; (2) the more serious the wrinkling of the graphene under shear is, the stronger the phonon scattering is and, therefore, the lower the thermal conductivity of the wrinkled graphene is; (3) within the temperature range of 400–700 K, the thermal conductivity of graphene sheet decreases with increase in temperature. [ABSTRACT FROM AUTHOR]

In this paper, LaGdCe2O7 ceramic was synthesised by solution reaction method. Its phase composition, microstructure and thermophysical properties were investigated respectively. X-ray diffraction result showed that single phase LaGdCe2O7 with fluorite structure was successfully synthesised. Scanning electron microscopy result revealed that the microstructure of the product was dense and no other unreacted oxides or interphases exist in the interfaces between grains. Its thermal expansion coefficient was higher than that of conventional 8YSZ, while its thermal conductivity was much lower than that of 8YSZ. The lower thermal conductivity of this product was mainly attributed to the differences in atomic weight and ionic radius between solute and host atoms. The results imply that LaGdCe2O7 ceramic can be used as candidate material for the ceramic layer in thermal barrier coatings. [ABSTRACT FROM AUTHOR]

Dense and fine grained Zn0·96Al0·04O thermoelectric ceramics were fabricated by spark plasma sintering at 1173-1323 K. Apart from the host ZnO phase with a wurtzite structure, a trace of ZnAl2O4 secondary phase was segregated at the grain boundary and showed an increasing trend with sintering temperature. The largest power factor was achieved at 823 K in the sample sintered at 1223 K, which was 4·01×10−4 W m−1 K−2. The thermal conductivity decreased significantly owing to the phonon scattering via refined grains and dispersed ZnAl2O4 phase and reached a low value of 2·42 W m−1 K−1 for the Zn0·96Al0·04O sample sintered at 1173 K, which has a maximum ZT value of 0·11 at 773 K. [ABSTRACT FROM AUTHOR]

The article reports on several studies that determine the phonon density of states using inelastic neutron scattering measurements. It reviews examples of the results from the mesurements on a couple of negative thermal expansion (NTE) compounds. It points out that the distribution in bond-lengths is capable to produce additional features in the vibrational spectrum.

We explore the physics of thermal impedance matching at the interface between two dissimilar materials by controlling the properties of a single atomic mass or bond. The maximum thermal current is transmitted between the materials when we are able to decompose the entire heterostructure solely in terms of primitive building blocks of the individual materials. Using this approach, we show that the minimum interfacial thermal resistance arises when the interfacial atomic mass is the arithmetic mean, whereas the interfacial spring constant is the harmonic mean of its neighbors. The contact-induced broadening matrix for the local vibronic spectrum, obtained from the self-energy matrices, generalizes the concept of acoustic impedance to the nonlinear phonon dispersion or the short-wavelength (atomic) limit. [ABSTRACT FROM PUBLISHER]

We report on experimental measurements of transient heat transfer characteristics of carbon nanotube aggregates for the temperature range from 120 to 370 K. A very large thermal transport relaxation time (τ) was observed: about 100 s at room temperature and up to 260 s at lower temperatures, one order of magnitude larger than record large values. These enormous τ values may imply a significant thermal transport phase lagging across carbon nanotube contacts. This finding may provide a new way to improve thermoelectric efficiency by minimizing heat leakage in the time domain. Supplementary materials are available online for this article. Go to the publisher's online edition ofNanoscale and Microscale Thermophysical Engineeringto view the supplementary file. [ABSTRACT FROM PUBLISHER]

Energy transport in dielectric thin films is mainly governed by the phonon transport across the edges of the film. Depending on the phonon frequencies and the film thickness, some of the phonons do not undergo scattering in the film, which results in ballistic effect on the energy transport in the film. In the present study, the influence of ballistic phonons on the energy transport in silicon thin film is investigated for a spatially varying temperature source at the film edge. The Gaussian temperature distribution at one edge of the film is considered to account for the spatial variation of temperature. The influence of film thickness on energy transport is also incorporated in the analysis. It is found that the Gaussian parameter, defining the spatial distribution of temperature at the film edge, significantly influences equivalent equilibrium temperature variation in the film. Equivalent equilibrium temperature reduces sharply across the film as the film thickness reduces. [ABSTRACT FROM AUTHOR]

Thermal conduction in periodically porous naiwstructures is strongly influenced by phonon boundary scattering, although the precise magnitude of this effect remains open to inves-tigation. This work attempts to clarify the impact of phonon-boundary scattering at room temperature using electrothermal measurements and modeling. Silicon nanobeams, pre-pared using electron beam lithography, were coated with a thin palladium overlayer, which serves as both a heater and thermometer for the measurement. The thermal conductivity along the length of the silicon nanobeams was measured using a steady-state Joule heating technique. The thermal conductivities of the porous nanobeams were reduced to as low as 3% of the value for bulk silicon. A Calla way-I I oil and model for the thermal conductivity was adapted to investigate the relative impact of boundary scattering, pore scattering, and phonon bandgap effects. Both the experimental data and the modeling showed a reduction in thermal conductivity with increasing pore diameter, although the experimentally measured value was up to an order of magnitude lower than that predicted by the model. [ABSTRACT FROM AUTHOR]

A method based on Debye theory is developed to calculate the thermal conductivity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The phonon–phonon interaction model is built up for solid HMX. The phonon lifetime formula is derived by the phonon–phonon scattering mechanism, and the thermal conductivity tensor is derived by the phonon dispersion model. The thermal conductivities of α/β/δ-HMX are calculated in the temperature range 0–700 K and pressure range of 0–10 GPa. The phonon softening process of HMX is investigated. We have proven that the Debye frequency and thermal conductivity tend to 0 at the phonon softening point. A physical picture of the phonon–phonon interaction, phonon lifetime and phonon softening is built up. [ABSTRACT FROM PUBLISHER]

We have adapted R-matrix theory to calculate phonon scattering across systems of molecular to mesoscopic scale. The key novelty of this work is that the only required information about the scattering region is its normal modes, which are evaluated only once for a system. Thus, R-matrix theory is a computationally efficient and simple approach to calculate phonon scattering in larger systems. To validate and to demonstrate the applicability of the theory, we apply it to two systems: a one-dimensional chain of atoms and a graphene nanoribbon. In both cases, we discuss the effect of mass impurities on thermal transport. [ABSTRACT FROM AUTHOR]

The processes of radiation defect formation in Si with 1 MeV electron irradiation in the temperature range 100-633 K have been investigated. It is established that the generation efficiency of vacancies λV increases with temperature, then starts to saturate at temperatures of 250 K and finally stays constant at T>300 K. It is shown that at high temperatures, the λV dependence can be caused by the additional scattering of 'hot' interstitial atoms on acoustical and optical phonons, the numbers of which increase with the temperature. An explanation, based on the creation of quasi-molecule of 'hot' interstitial and lattice atoms, is proposed. [ABSTRACT FROM AUTHOR]

Phonon dispersions in ferroelectric lithium heptagermanate Li2Ge7O15 (LGO) were measured by inelastic neutron scattering in the vicinity of the transition temperature TC = 283.5 K. The (300) reflections show more pronounced temperature dependence than the (500) reflections. The energy of a peak corresponding to the Brillouin zone-center phonon, around 0.5 meV at 297.0 K, decreases as the temperature decreases. Then, this inelastic component disappears into the central component at 288.0 K, just above TC. The present phonon shows the typical soft mode feature and shows good accordance with the B1u mode observed by hyper-Raman and FTIR measurements. It is confirmed that the ferroelectric phase transition in LGO is associated with the soft optical phonon at the zone center. Moreover, elastic diffuse scattering is observed below 0.1 meV. This implies that there exists a critical phenomenon at energies below than 0.1 meV. [ABSTRACT FROM AUTHOR]

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a 'natural superlattice' in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX2 structure which has a general formula of (MX)1+x (TX2)n (M = Pb, Bi, Sn, Sb or a rare earth element; T = Ti, V, Cr, Nb or Ta; X = S or Se and n = 1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)1.2(TiS2)2 has better thermoelectric properties compared with pure TiS2 in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities. [ABSTRACT FROM AUTHOR]

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Gruneisen parameter and Debye temperatures were predicted in wide pressure (0-50 GPa) and temperature ranges (0-500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations. [ABSTRACT FROM AUTHOR]

X-ray thermal diffuse scattering is a powerful probe of lattice dynamics. It is especially effective in the investigation of structural phase transitions. In this article, we review and compare recent experimental studies of the behaviour of phonons in connection with the structural phase transitions in TiSe2, SrTiO3 and Pu-Ga. A complete phonon softening is observed for the second-order transitions in TiSe2 and SrTiO3. By contrast, no phonon softening is detected for the first-order transition in Pu-Ga, which is unusual in comparison to other related systems. [ABSTRACT FROM AUTHOR]