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Curvature induced by mechanical deformation in nanostructures has been found to significantly affect their stability and reliability during applications. In this work, we investigated the effects of curvature induced by mechanical bending on the thermal properties of silicon nanowire (SiNW) by using molecular dynamics simulations. By examining the relationship between the curved geometry and local temperature/heat flux distribution, we found that there is no temperature gradient/heat flux along the radial direction of the bent SiNW, and the local heat current density along the circumferential direction varies with the radius of curvature. Interestingly, a ∼10% reduction in the thermal conductivity is found in the bent SiNW due to the depression of long-wavelength phonons caused by its inhomogeneous deformation. The present work demonstrates that the curvature induced by mechanical bending can be used to modulate the thermal conductivity of SiNWs. [ABSTRACT FROM AUTHOR]

Nowotny Chimney-Ladder (NCL) compounds are attracting increased attention as good thermoelectric (TE) materials. Although the TE properties of Si-based NCL compounds such as higher manganese silicides are well investigated, those of Ge-based ones are scarcely studied. Here, we demonstrate that a series of the Ge-based NCL compounds in the Mo13Ge23-Ru2Ge3 pseudobinary system, i.e., Mo1-xRuxGe1.769, shows quite low lattice thermal conductivity as well as good electrical properties like a material called phonon-glass electron-crystal and thus it shows good TE properties. By tuning the valence electron concentration, the TE properties are optimized and the maximum zT, called materials dimensionless figure of merit, reaches 0.23 for x = 0.6 in Mo1-xRuxGe1.769, which is approximately 15 times higher than that of x = 0 in Mo1-xRuxGe1.769, i.e., Mo13Ge23. [ABSTRACT FROM AUTHOR]

As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between "high-entropy" and "entropy-stabilized" phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10−6 K−1. Mechanical softening is also observed across the transition. [ABSTRACT FROM AUTHOR]

As the pseudo-binary alloys between GeTe and Sb2Te3, GeSbTe-based compounds are promising thermoelectric materials. Although Ge2Sb2Te5 and GeSb2Te4 have widely been studied, the thermoelectric properties of GeSb4Te7 have not been well understood yet. In this work, we design a series of GeSb4-xBixTe7 solid solutions and systematically study the variation in the crystal structure, electrical, and thermal transport properties. Alloying Bi effectively reduces the carrier concentration and, thus, enhances the Seebeck coefficient. Meanwhile, the thermal conductivity is greatly reduced via large mass fluctuations. A maximum zT of 0.69 at 550 K and an average zT value of 0.52 between 300 and 700 K have been achieved for GeSb1.5Bi2.5Te7. These findings will promote the understanding and development of GeSbTe-based thermoelectric materials. [ABSTRACT FROM AUTHOR]

Particle-matrix interfaces can play an important role in heat propagation through particulate composites. We investigated the possibility of using frequency domain thermoreflectance combined with a numerical thermal model to measure in situ the thermal resistance of the particle-matrix interface in particulate composite materials. We found that the sensitivity of the technique depended on the matrix thermal conductivity, the particle size, and the value of the interface thermal resistance. In general, high thermal conductivity matrix materials and small particles will present higher sensitivity to the interface thermal resistance. We tested the technique with diamond particles embedded in tin and showed that the interface thermal resistance could be measured for these samples. [ABSTRACT FROM AUTHOR]

We analyze, through molecular dynamics simulations, the temperature dependence of the thermal conductivity (k) of chain-oriented amorphous polyethylene (PE). We find that at increasing levels of orientation, the temperature corresponding to a peak k progressively decreases. Un-oriented PE exhibits the peak k at 350 K, while aligned PE under an applied strain of 400% shows a maximum at 100 K. This transition of peak k to lower temperatures with increasing alignment is explained in terms of a crossover from disorder to anharmonicity dominated phonon transport in aligned polymers. Evidence for this crossover is achieved by manipulating the disorder in the polymer structure and studying the resulting change in temperature corresponding to peak k. Disorder is modified through a change in the dihedral parameters of the potential function, allowing a change in the relative fraction of trans and gauche transformations. The results shed light on the underlying thermal transport processes in aligned polymers and hold importance for low temperature applications of polymer materials in thermal management technologies. [ABSTRACT FROM AUTHOR]

Embedded nanocrystals are capable of dramatically reducing the thermal conductivity of alloy semiconductors through increased phonon scattering, but many aspects of thermal wavelength phonon interactions with embedded nanoparticles remain understudied. Here, the wavevector- and polarization-resolved capabilities of the Frequency Domain Perfectly Matched Layer (FDPML) computational technique are exploited to study several fundamentally important phonon scattering problems across the entire Brillouin zone. We compare the atomistic predictions of FDPML against continuum mechanics approaches for spherically embedded particles. For long to mid-wavelength phonons, reasonable agreement is found with continuum theories that consider the Mie regime accurately, while commonly used “patching” theories which empirically connect the Rayleigh scattering to the geometric limit are shown to have poor agreement with more rigorous approaches. Next, the scattering cross section of optical phonons from nanoparticles is explored for the first time. We show that the scattering behavior of optical phonons is fundamentally different than their acoustic counterparts in that long wavelength optical phonons exhibit a scattering cross section nearly independent of wavelength, which we interpret as being due to zone folding of short wavelength acoustic modes from the geometric scattering regime into long wavelength optical modes. Finally, we study the scattering cross section of nanoparticles exhibiting atomic interdiffusion at the matrix-particle interface, where we find that interdiffusion both suppresses Mie oscillations and substantially increases the scattering cross section at short wavelength, compared to a solid nanoparticle with the same number of impurity atoms. [ABSTRACT FROM AUTHOR]

The study of heat transfer properties in mesoporous silicon, fabricated from highly doped p-type and n-type silicon wafers, is presented. Measurements were performed by a laser induced transient thermal grating technique, which allowed us to determine the effective (in-plane) thermal conductivity. It is shown that the thermal conductivity undergoes a significant decrease with respect to bulk values mainly due to a reduction of the phonon mean free path of the solid matrix. This reduction can be ascribed to the formation of nano-crystalline domains, which are a consequence of the wet etching fabrication method. Additionally, the in-plane thermal conductivity was analyzed by employing a modified effective medium approach, which includes the phonon mean free path reduction due to the presence of both the nanometric pores and the nano-crystalline domains. The theoretical analysis shows good agreement with our measurements, indicating that the inclusion of phonon mean free path reduction to an effective medium approach is a well-suited method for studying the thermal conductivity of porous silicon. [ABSTRACT FROM AUTHOR]

Simulations are performed using the reverse non-equilibrium molecular dynamics (rNEMD) method and the Stillinger-Weber (SW) potential to determine the input parameters for achieving ±1% convergence of the calculated thermal conductivity of silicon. These parameters are then used to investigate the effects of the interatomic potentials of SW, Tersoff II, Environment Dependent Interatomic Potential (EDIP), Second Nearest Neighbor, Modified Embedded-Atom Method (MEAM), and Highly Optimized Empirical Potential MEAM on determining the bulk thermal conductivity as a function of temperature (400–1000 K). At temperatures > 400 K, data collection and swap periods of 15 ns and 150 fs, system size ≥6 × 6 UC2 and system lengths ≥192 UC are adequate for ±1% convergence with all potentials, regardless of the time step size (0.1–0.5 fs). This is also true at 400 K, except for the SW potential, which requires a data collection period ≥30 ns. The calculated bulk thermal conductivities using the rNEMD method and the EDIP potential are close to, but lower than experimental values. The 10% difference at 400 K increases gradually to 20% at 1000 K. [ABSTRACT FROM AUTHOR]

Nature employs lipids to construct nanostructured membranes that self-assemble in an aqueous environment to separate the cell interior from the exterior environment. Membrane composition changes among species and according to environmental conditions, which allows organisms to occupy a wide variety of different habitats. Lipid bilayers are phase-change materials that exhibit strong thermotropic and lyotropic phase behavior in an aqueous environment, which may also cause thermal rectification. Among different types of lipids, archaeal lipids are of great interest due to their ability to withstand extreme conditions. In this paper, nonequilibrium molecular dynamics simulations were employed to study the nanostructures and thermal properties of different archaeols and to investigate thermal rectification effects in asymmetric archaeal membranes. In particular, we are interested in understanding the role of bridged phytanyl chains and cyclopentane groups in controlling the phase transition temperature and heat flow across the membrane. Our results indicate that the bridged phytanyl chains decrease the molecular packing of lipids, whereas the existence of cyclopentane rings on the tail groups increases the molecular packing by enhancing the interactions between isoprenoid chains. We found that macrocyclic archaeols have the highest thermal conductivity, whereas macrocyclic archaeols with two cyclopentane rings have the lowest. The effect of the temperature on the variation of thermal conductivity was found to be progressive. Our results further indicate that small thermal rectification effects occur in asymmetric archaeol bilayer membranes at around 25 K temperature gradient. The calculated thermal rectification factor was around 0.09 which is in the range of rectification factor obtained experimentally for nanostructures such as carbon nanotubes (0.07). Such phenomena may be of biological significance and could also be optimized for use in various engineering applications. [ABSTRACT FROM AUTHOR]

The layered solid electrolyte Li2ZrCl6 and Li metal electrodes have a very good contact stability, but the thermal transport properties of Li2ZrCl6 are still unclear. Here, we systematically study the intrinsic lattice thermal conductivity (κp) of Li2ZrCl6 using the machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory. The results show that the κp of Li2ZrCl6 at room temperature is 3.94 W/mK along the in-plane (IP) direction and 1.05 W/mK along the out-plane (OP) direction, which means that the κp is significantly anisotropic. In addition, under the compressive stress in the OP direction, the κp evolution along the IP and OP directions exhibits completely different trends, because the stress has a significant regulatory effect on the contribution of optical phonons to κp. With the increase in stress, the κp in the IP direction monotonically decreases, while the κp in the OP direction increases by a factor of 2.2 under a compressive strain of 13%. This is because the contribution of low-frequency optical phonons to κp in the IP direction is as high as 58% when no stress is applied, and this contribution is significantly suppressed with increasing compressive strain. However, the contribution of optical phonons in the OP direction to the κp increases with the increase in stress. Our results reveal the thermal transport properties of Li2ZrCl6 and the effect of the compressive strain on the κp of Li2ZrCl6, thereby providing a reference for the use of Li2ZrCl6 in Li-metal batteries. [ABSTRACT FROM AUTHOR]

Phonon transport properties of two-dimensional materials can play a crucial role in the thermal management of low-dimensional electronic devices and thermoelectric applications. In this study, both the empirical Stillinger–Weber (SW) and machine learning interatomic potentials are employed to investigate the lattice thermal conductivity of monolayer GeS and SnS through solving the phonon Boltzmann transport equation. The accuracy of the two types of interatomic potentials and their performance for the evaluation of thermal conductivity are verified by analyzing phonon harmonic and anharmonic properties. Our results indicate that the thermal conductivity can be predicted more accurately with a machine learning approach, while the SW potential gives rise to an overestimated value for both monolayers. In addition, the in-plane anisotropy of thermal transport properties existing in these monolayers can be confirmed by both potential models. Moreover, the origins of the deviation existing in calculated thermal conductivities, including both the effects of interatomic potential models and monolayer compositions, are elucidated through uncovering the underlying phonon transport mechanisms. This study highlights that in contrast to the machine learning approach, more careful verification is required for the simulation of thermal transport properties when empirical interatomic potential models are employed. [ABSTRACT FROM AUTHOR]

A microscopic theory of dielectric relaxation of the hexagonal ice (Ih) is proposed based on the multiple-trapping model. The theory explains the distinctive peculiarities of the relaxation time temperature behavior and the peak broadening parameter in a wide temperature range from the unified positions. [ABSTRACT FROM AUTHOR]

A microscopic theory of dielectric relaxation of the hexagonal ice (Ih) is proposed based on the multiple-trapping model. The theory explains the distinctive peculiarities of the relaxation time temperature behavior and the peak broadening parameter in a wide temperature range from the unified positions. [ABSTRACT FROM AUTHOR]

One way to reduce the lattice thermal conductivity of solids is to induce additional phonon–surface scattering through nanostructures. However, the way in which phonons interact with surfaces, especially at the atomic level, is not well understood at present. In this work, we perform two-dimensional atomistic wave-packet simulations to investigate angular-resolved phonon reflection at a surface. Different surface morphologies, including smooth surfaces, periodically rough surfaces, and surfaces with amorphous coatings, are considered. For a smooth surface, mode conversion can occur after reflection, with the resulting wave-packet energy distribution depending on the surface condition and the polarization of the incident phonon. At a periodically rough surface, the reflected wave-packet distribution does not follow the well-known Ziman model but shows a nonmonotonic dependence on the depth of the surface roughness. When an amorphous layer is attached to a smooth surface, the incident wave packet is absorbed by the amorphous region and is then reflected diffusively at the surface. Our results show that the commonly adopted specular-diffusive model is insufficient to describe phonon reflection at a periodically rough surface and that an amorphous layer can induce strong diffusive reflection. This work provides a comprehensive analysis of phonon reflection at different types of surfaces, which is important for better understanding of thermal transport in various nanostructures [ABSTRACT FROM AUTHOR]

We use a magnetothermal resistance method to measure lattice thermal conductivity of pure single crystal metals over the intermediate temperature range of 5–60 K. Large transverse magnetic fields are applied to suppress electronic thermal conduction. The total thermal conductivity and the electrical conductivity are measured as functions of applied magnetic field. The lattice thermal conductivity is then extracted by extrapolating the thermal conductivity versus electrical conductivity curve at zero electrical conductivity. We used this method to experimentally measure the lattice thermal conductivity and Lorenz number in single crystal Al (100), Cu (100), and Zn (001) in the intermediate temperature range. Our results show that the measured phonon thermal conductivity versus temperature plot has a peak around ΘD/10, and the Lorenz number is found to deviate from the Sommerfeld value in the intermediate temperature range. [ABSTRACT FROM AUTHOR]

We report on the detailed microstructural features in a low thermal conductivity Sn1-xSbxTe (x = 0.04, 0.08, 0.15) alloy investigated by transmission electron microscopy and density functional theory calculation. A near theoretical minimum thermal conductivity is obtained for Sn0.85Sb0.15Te alloy composition containing distinct microstructures. The crisscross lines along {111} planes forming nano-scale structures have been identified as areas with Sb replacing both regular Sn sites and Te anti-sites. This leads to the modulation in the {111} inter-planar spacing (d111) and results in superstructure spots in the electron diffraction pattern. The formation of such structures is supported by theoretical calculation. In general, two different phases are observed in the system, one with Sb replacing the regular Sn sites and another with crisscross lines where Sb replaces both the Sn and Te sites. Theoretical calculation further reveals that while the areas with Sb at the regular site give rise to large thermo-power, the areas with Sb substituting regular and anti-sites combination forming a superstructure contribute towards low lattice thermal conductivity and the combined effect increases the zT to ~1. [ABSTRACT FROM AUTHOR]

We use equilibrium molecular dynamics simulations to determine the relative role of compositional and structural disorder in a phononic thermal conductivity reduction by studying three 50-50 SiGe alloy structures: ordered alloys, disordered alloys, and amorphous alloys, as well as pure amorphous Si and Ge structures for reference. While both types of disorders significantly reduce thermal conductivity, structural disorder is much more effective to this aim. The examination of phonon lifetimes in disordered alloys shows high values in a low frequency regime governed by Umklapp scattering that are reduced rapidly with increasing frequency following Rayleigh scattering behavior. The local properties analysis reveals that the structural disorder leads to elastic heterogeneities that are significantly larger than density heterogeneities, which is likely the key reason for amorphous semiconductor alloys having lower thermal conductivity than disordered alloys. Temperature dependence of thermal conductivity indicates the importance of propagating phonons and associated Umklapp scattering in SiGe alloy structures. Interestingly, longitudinal modes in amorphous and disordered alloys exhibit similar lifetimes, while transverse modes lifetimes show significant differences and are more temperature dependent. [ABSTRACT FROM AUTHOR]

Three keynote thermophysical and transport properties of ³He and 4He, namely, the second virial coefficient, the shear viscosity, and the thermal conductivity, are reported for the "extended Dirac bubble potential" (EDbp), a novel model for He--He [M. Chrysos, J. Chem. Phys. 146, 024106 (2017)]. Comparisons with the experiment as well as with potentials with a proven track record and with the oversimplified Dbp are being made in the range 0.1-500 K to analyze the performance of the EDbp, which is shown here to emerge as a promising analytic model for He--He. A flowchart of how to treat the "buffer" in scattering cross section measurements is designed and conducted, offering a route to EDbp optimization. An impressive consistency with state-of-the-art calculations (which is just striking for such a simple analytic model) is found, essentially thanks to the performance of the phase-shift expression cot ... A Multimedia view of δl(k, rc) versus k and rc is part of the material presented in this article. Data for the "best" rc(k) is given as a supplementary material. [ABSTRACT FROM AUTHOR]

This paper reports on the measurement of the thermal conductivity of ytterbium doped CaF2 transparent ceramics using the 3ω method. Samples with different doping levels were studied in a temperature range of 10K to 310 K. The results for doped samples are very similar to those obtained for Yb:CaF2 single crystals with the same doping levels, which demonstrates an important improvement in the thermal properties of transparent ceramics. However, the results obtained for non-doped ceramics and single crystals differ for the maximum conductivity due to the presence of grain boundaries inside the ceramic. Theoretical calculations were also conducted using Callaway's model and are in a very good agreement with experimental results. The fitting parameters found tend to indicate that the introduction of a doping element in the lattice has more impact on thermal conductivity than the presence of grain boundaries in the material. [ABSTRACT FROM AUTHOR]

In this work, we aim to study the thermal properties of materials using classical molecular dynamics simulations and specialized numerical methods. We focus primarily on the thermal conductivity κ using non-equilibrium molecular dynamics (NEMD) to study the response of a crystalline solid, namely hematite (α−Fe2O3), to an imposed heat flux as is the case in real life applications. We present a methodology for the calculation of κ as well as an adapted potential for hematite. Taking into account the size of the simulation box, we show that not only the longitudinal size (in the direction of the heat flux) but also the transverse size plays a role in the determination of κ and should be converged properly in order to have reliable results. Moreover we propose a comparison of thermal conductivity calculations in two different crystallographic directions to highlight the spatial anisotropy and we investigate the non-linear temperature behavior typically observed in NEMD methods. [ABSTRACT FROM AUTHOR]

In this work, we aim to study the thermal properties of materials using classical molecular dynamics simulations and specialized numerical methods. We focus primarily on the thermal conductivity κ using non-equilibrium molecular dynamics (NEMD) to study the response of a crystalline solid, namely hematite (α−Fe2O3), to an imposed heat flux as is the case in real life applications. We present a methodology for the calculation of κ as well as an adapted potential for hematite. Taking into account the size of the simulation box, we show that not only the longitudinal size (in the direction of the heat flux) but also the transverse size plays a role in the determination of κ and should be converged properly in order to have reliable results. Moreover we propose a comparison of thermal conductivity calculations in two different crystallographic directions to highlight the spatial anisotropy and we investigate the non-linear temperature behavior typically observed in NEMD methods. [ABSTRACT FROM AUTHOR]

We use molecular dynamics simulations to investigate how the energy input and distribution in contacts affect the thermal transport in silicon as described by the Stillinger-Webber potential. We create a temperature difference across a Si specimen by maintaining the temperature of two contacts (also made of Si) using widely used thermostats: the deterministic Nosé-Hoover approach and a stochastic Langevin bath. Quite surprisingly, the phonon thermal conductivity of the channel obtained using the two thermostats but under otherwise identical conditions can differ by a factor of up to three. The discrepancy between the two methods vanishes as the coupling strength between the thermostat and material is reduced and for long channels. A spectral analysis of the contacts and channel shows that increasing the coupling of the stochastic Langevin thermostat affects the spectral energy distribution in the contacts away from that based on the vibrational density of states, broadening peaks and smoothening the distribution. This results in contacts injecting phonons preferentially in low frequency modes and in transport through the channel away from local equilibrium. A comparison of the MD results with Boltzmann transport equation simulations provides an additional insight into the role of contacts on thermal transport in nanoscale specimens. These results stress the importance of contacts in nanoscale thermal transport in simulations and in the interpretation of experimental data. [ABSTRACT FROM AUTHOR]

An in-depth study of thermal and electron transport properties including thermal conductivity κ(T), thermoelectric power S(T), and electrical resistivity ρ(T) of the heavy fermion Kondo lattice Ce6Pd12In5 and its nonmagnetic reference compound La6Pd12In5 is presented. The absolute #954;(T) value of Ce6Pd12In5 is smaller that than of La6Pd12In5, which indicates that conduction electron--4f electron scattering has a large impact on the reduction of thermal conductivity. The isolated 4f electron contributions to the electrical resistivity ρ4f(T), electronic thermal resistivity displayed in the form Wel,4f(T)⋅T, and thermoelectric power S4f(T) reveal a low- and high-temperature --lnT behaviour characteristic of Kondo systems with strong crystal-electric field (CEF) interactions. The analysis of phonon scattering processes of lattice thermal conductivity #954;ph(T) in (Ce, La)6Pd12In5 was performed over the whole accessible temperature range according to the Callaway model. In the scope of a theoretical approach based on the perturbation type calculation, we were able to describe our experimental data of ρ4f(T) and Wel,4f(T)⋅T by using the model incorporating simultaneously the Kondo effect in the presence of the CEF splitting, as it is foreseen in the framework of the Cornut-Coqblin and Bhattacharjee-Coqblin theory. Considering the fact that there are not many cases of similar studies at all, we also show the numerical calculations of temperature-dependent behaviour of spin-disorder resistivity ρs(T), magnetic resistivity ρ4f(T), and occupation number 〈Ni〉 due to the various types of degeneracy of the ground state multiplet of Ce3+ (J = 5/2). [ABSTRACT FROM AUTHOR]

An in-depth study of thermal and electron transport properties including thermal conductivity κ(T), thermoelectric power S(T), and electrical resistivity ρ(T) of the heavy fermion Kondo lattice Ce6Pd12In5 and its nonmagnetic reference compound La6Pd12In5 is presented. The absolute #954;(T) value of Ce6Pd12In5 is smaller that than of La6Pd12In5, which indicates that conduction electron--4f electron scattering has a large impact on the reduction of thermal conductivity. The isolated 4f electron contributions to the electrical resistivity ρ4f(T), electronic thermal resistivity displayed in the form Wel,4f(T)⋅T, and thermoelectric power S4f(T) reveal a low- and high-temperature --lnT behaviour characteristic of Kondo systems with strong crystal-electric field (CEF) interactions. The analysis of phonon scattering processes of lattice thermal conductivity #954;ph(T) in (Ce, La)6Pd12In5 was performed over the whole accessible temperature range according to the Callaway model. In the scope of a theoretical approach based on the perturbation type calculation, we were able to describe our experimental data of ρ4f(T) and Wel,4f(T)⋅T by using the model incorporating simultaneously the Kondo effect in the presence of the CEF splitting, as it is foreseen in the framework of the Cornut-Coqblin and Bhattacharjee-Coqblin theory. Considering the fact that there are not many cases of similar studies at all, we also show the numerical calculations of temperature-dependent behaviour of spin-disorder resistivity ρs(T), magnetic resistivity ρ4f(T), and occupation number 〈Ni〉 due to the various types of degeneracy of the ground state multiplet of Ce3+ (J = 5/2). [ABSTRACT FROM AUTHOR]

One of the key challenges of phase change memory (PCM) is its high power consumption during the reset operation, when the phase change material (typically Ge2Sb2Te5, i.e., GST) heats up to ∼900 K or more in order to melt. Here, we study the temperature-dependent behavior of PCM devices by probing the reset power at ambient temperatures from 80 to 400 K. We find that different device structures exhibit contrasting temperature-dependent behavior. The reset power in our confined-type PCM is nearly unchanged with ambient temperature, corresponding to a temperature-dependent thermal resistance, whereas results for mushroom-type PCM from the literature show a linear relation between power and temperature, suggesting a more constant thermal resistance. This discrepancy is ascribed to different temperature distributions and thermal properties of the dominant components of the PCM cell thermal resistance, as shown by electro-thermal modeling. In the confined cell, the thermal boundary resistance of the GST and the thermal conductivity of the bottom electrode dominate the thermal resistance, while for the mushroom cell, the GST thermal conductivity plays a greater role. These findings can help to design more power- and energy-efficient PCM devices by better focusing thermal management efforts on the key components of the device. [ABSTRACT FROM AUTHOR]

We blend n-type Bi2Te3 with an inexpensive abundant conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), to gain a bulk-structured composite, in which energy filtering and phonon scattering effects should occur at the interface of two components. The composite records somewhat higher electrical resistivity than pristine Bi2Te3, because the interface possibly interrupts carrier transport. However, the composite completely compensates for the resistivity increment with a significant increase in the Seebeck coefficient, which is caused by energy filtering effects at the interface; thus, it exhibits the improved power factor. The composite also records a much lower thermal conductivity than the pristine Bi2Te3 because of phonon scattering effects at the interface. The composite induces significant decoupling of electrical and thermal properties, thus affording the remarkably enhanced figure of merits (ZTmax ∼ 1.19 at 132 °C, ZTave ∼ 1.14 at 50–150 °C), which are approximately double those of the pristine Bi2Te3. The ZT values are not only predominant among the performance of n-type binary Bi2Te3, but they are also as competent as the excellent performance of n-type ternary Bi2(Te,Se)3 previously reported. [ABSTRACT FROM AUTHOR]

Compared to extensive studies of thermal transport in two-dimensional materials, very limited attention has been paid to the corresponding phenomenon in quasi-one-dimensional van der Waals crystals. Here, we show that Ta2Se3 can be easily exfoliated into thin nanowires, indicating strong anisotropy in the bonding strength within the basal plane. Systematic thermal property measurements disclose signatures of one-dimensional phonons as the nanowire hydraulic diameter reduces below 19.2 nm with linearly escalating thermal conductivity as temperature increases and size dependence inconsistent with the classical size effect. We further show that these unusual transport properties are induced by elastic stiffening occurring for wires of <30 nm diameter. [ABSTRACT FROM AUTHOR]

Independently controlling electronic and thermal transport in solids is a challenge, because these properties are coupled. Here, we show that disordered nanoporosity in Ca3Co4O9 thin films can decrease the thermal conductivity without significantly hampering electronic transport. Scanning thermal microscopy was used to determine the out-of-plane thermal conductivity and estimate the in-plane values. Nanoporous Ca3Co4O9 films exhibit a thermal conductivity of 0.82 W m−1 K−1, which is nearly twofold lower than that obtained from nonporous Ca3Co4O9 films. Nanoporous Ca3Co4O9 exhibit a room-temperature electrical resistivity of 4 mΩ cm, which is comparable to polycrystalline Ca3Co4O9 and twice that reported for single-crystal Ca3Co4O9. Our results suggest that controlling nanoporosity and their degree of disorder can offer a means of decoupling electrical and thermal properties in materials. [ABSTRACT FROM AUTHOR]

The thermal conductivity and thermal diffusivity of both metallic and non-metallic microwires are simultaneously measured by a cross-wire geometry. In this method, the heating wire serves both as a thermometer and a heater. The deflection of the heating wire is in situ modified by using the Ampère force to contact and separate the test wire. By using the quasi-steady-state measurement, the thermal contact resistances under different contact conditions are obtained so that the effect on thermal conductivity can be eliminated. This method is verified by both the metallic wires and carbon fiber to clarify the effect of the surface radiation heat loss of the test wire. The obtained thermal properties are repeatable though the magnitude of the thermal contact resistance under different contact conditions changes significantly. The cross-wire geometry overcomes the obstacle introduced by different thermal interfacial materials, which provides an accurate and convenient way to measure the thermal properties of microwires. [ABSTRACT FROM AUTHOR]

In a frequency-domain thermoreflectance (FDTR) experiment, the phase lag between the surface temperature response and the applied heat flux is fit with an analytical solution to the heat diffusion equation to extract an unknown thermal property (e.g., thermal conductivity) of a test sample. A method is proposed to reduce the impact of uncertainty in the laser spot radius on the resulting uncertainty in the fitted property that is based on fitting to the quotient of the test sample phase and that of a reference sample. The reduction is proven analytically for a semi-infinite solid and was confirmed using numerical and real experiments on realistic samples. When the spot radius and its uncertainty are well known, the reference phase can be generated numerically. In this situation, FDTR experiments performed on Au–SiO2–Si and PbS nanocrystal test samples demonstrate 32% and 82% reductions in the overall uncertainty in thermal conductivity. When the spot radius used in the test sample measurement is not well known, a real reference sample, measured under conditions that lead to the same unknown spot radius, is required. Although the real reference sample introduces its own uncertainties, the total uncertainty in the fitted thermal conductivity can still be reduced. A reference sample can also be used to reduce uncertainty due to other sources, such as the transducer properties. Because frequency-domain solutions to the heat diffusion equation are the basis for time-domain thermoreflectance (TDTR) analysis, the approach can be extended to TDTR experiments. [ABSTRACT FROM AUTHOR]

In the last decade, several works have focused on exploring the material and electrical properties of GeTe/Sb2Te3 superlattices (SLs), in particular because of some first device implementations demonstrating interesting performances such as fast switching speed, low energy consumption, and non-volatility. However, the switching mechanism in such SL-based devices remains under debate. In this work, we investigate the prototype GeTe/Sb2Te3 SLs to analyze fundamentally their electronic and thermal properties by ab initio methods. We find that the resistive contrast is small among the different phases of GeTe/Sb2Te3 because of a small electronic gap (about 0.1 eV) and a consequent semi-metallic-like behavior. At the same time, the out-of-plane lattice thermal conductivity is rather small, while varying up to four times among the different phases, from 0.11 to 0.45 W m−1 K−1, intimately related to the number of Van der Waals (VdW) gaps in a unit block. Such findings confirm the importance of the thermal improvement achievable in GeTe/Sb2Te3 superlattices devices, highlighting the impact of the material stacking and the role of VdW gaps on the thermal engineering of the phase-change memory cell. [ABSTRACT FROM AUTHOR]

Back in 2008, Panofsky gave an empirical formula, T = 1 1.5 W 2 / 3 , for turkey baking time, T, in hours vs turkey weight, W, in pounds, the so-called Panofsky formula or Panofsky constant. Compared to the previously existed recipes that are based on the simple linear relationship between turkey weight and baking time, the Panofsky formula provides a more accurate estimate for the baking time. For instance, a general guideline of 13–20 min/lb was widely recommended in all previous turkey baking recipes. In this work, we conduct a comprehensive study of the turkey baking process that leads to a mathematical derivation of the Panofsky formula under some approximations. We also generalize the Panofsky formula to define a general Panofsky formula, T = 1 P W 2 / 3 , where P is defined as the Panofsky constant. Under spherical approximations, we then apply an accurate physical solution of the heat transfer equation and use the rigorous solution with numerical methods to study the generalized Panofsky formula and the Panofsky constant. We found that the generalized Panofsky formula can be perfectly applied to all turkey baking scenarios for baking time calculations. Furthermore, we did a careful analysis of the Panofsky constant, which equals 1.5 in the original Panofsky formula. The dependency of the new Panofsky constant on thermal properties of the turkeys and other initial parameters of baking, e.g., initial and final center temperature of the turkeys, oven temperature, thermal conductivity, specific heat, and turkey's density, was carefully analyzed and mapped out. The Panofsky constant, P, could vary from 1.1 to 1.9 depending on these thermal parameters. [ABSTRACT FROM AUTHOR]

The arsenic (As) to selenium (Se) ratio in AsxSe1-x glasses ranging from x=0.2 to 0.5 was varied in order to examine the effect of chemical and topological ordering on the glass' thermal transport behavior. The fundamental thermal properties of glass transition temperature (Tg), thermal conductivity (k), and heat capacity (cp) were experimentally measured using differential scanning calorimetry, transient plane source method, and ultrasonic testing. Based on topological constraint theory, inflections in Tg and k were found at the structural coordination number of 2.4, whereas a slight increase in heat capacity (cp) with increasing was observed. A maximum in total thermal conductivity of 0.232 W/m.K was measured for the composition with x=0.4, which corresponds to the stoichiometric As2Se3. Gas kinetic theory was used to derive an expression for the photon (kp) portion of thermal conductivity, which was calculated by measurements of the glass' absorption coefficient (α) and refractive index (n). Models based on Debye theory were used to derive expressions for specific heat (cv) and the lattice (kl) portion of thermal conductivity. The maximum value for kp was 0.173 W/m.K for the composition with x=0.2, and a minimum value of 0.144W/m.K was measured for the composition with x=0.4. Photonic conduction was found to be the dominant carrier mechanisms in all compositions, comprising 60% to 95% of the measured total thermal conductivity. [ABSTRACT FROM AUTHOR]

The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3-4 µm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015-1018 cm-3), n-type layers exhibit a nearly constant thermal conductivity of 180W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110W/mK with increased doping. These trends--and their overall reduction relative to bulk--are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness. [ABSTRACT FROM AUTHOR]

The phonon spectra, Debye temperatures, Grüneisen parameters, and the intrinsic lattice thermal conductivities of the layered oxychalcogenides BiCuOCh (Ch=S, Se, Te) have been studied with first-principles calculations. We find that the lattice thermal conductivities of them are anisotropic and quite low. The lowest thermal conductivity is only 0.14Wm-1K-1 along c-axis for BiCuOTe. The size-dependent thermal conductivity of them is also discussed. [ABSTRACT FROM AUTHOR]

Electrical and thermoelectric (TE) properties for n-type Ag8SnSe6 and (Ag1-xNbx)8SnSe6 are investigated. Ag8SnSe6 has the thermoelectric figure of merit (ZT) close to 1.1 at 803K due to its intrinsic ultralow thermal conductivity ~0:3Wm-1K-1, relativelyμow resistivity ~0.01 X cm, and high Seebeck coefficient ~-200μV/K. The ZT for pure Ag8SnSe6 drops to 0.02 at room temperature due to itsμarge resistivity. Niobium doping increases the carrier concentration nearly 10 times and thus enhances its ZT to 0.11 at room temperature. Ag8SnSe6 is a promising n-type candidate of TE materials which needs further elaborations. [ABSTRACT FROM AUTHOR]

Electrical and thermoelectric (TE) properties for n-type Ag8SnSe6 and (Ag1-xNbx)8SnSe6 are investigated. Ag8SnSe6 has the thermoelectric figure of merit (ZT) close to 1.1 at 803K due to its intrinsic ultralow thermal conductivity ~0:3Wm-1K-1, relativelyμow resistivity ~0.01 X cm, and high Seebeck coefficient ~-200μV/K. The ZT for pure Ag8SnSe6 drops to 0.02 at room temperature due to itsμarge resistivity. Niobium doping increases the carrier concentration nearly 10 times and thus enhances its ZT to 0.11 at room temperature. Ag8SnSe6 is a promising n-type candidate of TE materials which needs further elaborations. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation was used to study the effects of each grain dimension and of grain boundary characteristics on the inter-grain thermal boundary resistance (TBR) and intragrain thermal conductivity of nanocrystalline diamond. The effect of the grain boundaries perpendicular to the heat flow was studied using a multiple slab configuration, which greatly reduced the artifacts associated with the heat source/sink. The TBR between the slabs was found to be more sensitive to the atomic arrangement at the boundary than to the tilt angle between the slabs. When the atomic arrangement at the interface was altered from the minimum energy configuration, the TBR increased by a factor of three, suggesting that a sub-optimal interface quality between the grains could play a large role in reducing the thermal conductivity of nanocrystalline diamond. The thermal conductivity between the boundaries was found to be similar to the bulk value, even when the boundaries were only 25 nm apart. The effect of grain boundaries parallel to the heat flow was found to have a large dependence on the microstructural details. Parallel boundaries which were 2 nm apart reduced the thermal conductivity of defect-free diamond by between one third and a factor of ten. [ABSTRACT FROM AUTHOR]

A theoretical model for the description of thermal conductivity of molten salt mixtures as a function of composition and temperature is presented. The model is derived by considering the classical kinetic theory and requires, for its parametrization, only information on thermal conductivity of pure compounds. In this sense, the model is predictive. For most molten salt mixtures, no experimental data on thermal conductivity are available in the literature. This is a hindrance for many industrial applications (in particular for thermal energy storage technologies) as well as an obvious barrier for the validation of the theoretical model. To alleviate this lack of data, a series of equilibrium molecular dynamics (EMD) simulations has been performed on several molten chloride systems in order to determine their thermal conductivity in the entire range of composition at two different temperatures: 1200 K and 1300 K. The EMD simulations are first principles type, as the potentials used to describe the interactions have been parametrized on the basis of first principle electronic structure calculations. In addition to the molten chlorides system, the model predictions are also compared to a recent similar EMD study on molten fluorides and with the few reliable experimental data available in the literature. The accuracy of the proposed model is within the reported numerical and/or experimental errors. [ABSTRACT FROM AUTHOR]

Scanning Thermal Microscopy (SThM) provides efficient thermal property measurement with micro- or nanoscale spatial resolution. However, the sensitivity and accuracy of state-of-the-art thermal probes have been limited by excessive thermal contact resistance between the probe and sample. Introduced herein is a robust thermal microprobe that can increase the probe-sample contact force by more than two orders of magnitude, thereby reducing the probe-sample thermal contact resistance by as much as 96% and increasing measurement sensitivity by more than 240% compared to a commercial thermal probe with the same dimensions and measurement principle. The relationship between the probe-sample thermal contact resistance, thermal exchange radius, and sample thermal conductivity is determined experimentally. Simultaneous measurement of thermal conductivity and Seebeck coefficient with unprecedented sensitivity is demonstrated using the enhanced scanning thermal microprobe on samples of an extended range of thermal conductivity up to 18 W/m K, increasing the range of samples applicable to SThM when compared to the conventional commercial probe with diminished measurement sensitivity above ∼10 W/m K. The probe is further demonstrated by simultaneously mapping thermal conductivity and Seebeck coefficient as a function of depth from the irradiated surface of an ion-irradiated bulk nanostructured thermoelectric material. In addition to enabling microscale thermal conductivity and Seebeck coefficient measurement of materials previously not applicable to SThM, the probe can also facilitate high-throughput characterization of combinatorial materials to aid the rapid discovery of compositions and processing conditions that yield highly desired thermal and thermoelectric properties. [ABSTRACT FROM AUTHOR]

The thermal properties of droplets have a significant effect on the evaporation of sessile droplets. In this study, the influence of nondimensional thermal properties on Marangoni instabilities, especially hydrothermal waves (HTWs), in a sessile droplet evaporating at a constant contact angle mode, is numerically investigated using a nondimensional mathematical model. The model considers the transient deformation of the droplet surface during evaporation in a wide range of Marangoni numbers from 1000 to 40 000, evaporative cooling numbers from 1 to 300, relative heat conductivities from 0.01 to 1000, and Prandtl numbers from 0.01 to 25.0. Included are the different kinds of fluids applied in previous works on Marangoni convection in evaporating sessile droplets. The substrate material varies from a vacuum insulation panel with a heat conductivity of 0.002 W/m·K to silver with 429 W/m·K. The results reveal that a sufficiently large Marangoni number, evaporative cooling number, and relative heat conductivity favor the appearance of HTWs, whereas a large Prandtl number inhibits the appearance of HTWs. The mixture mode of Bénard–Marangoni cells and longitudinal rolls or of longitudinal rolls and HTWs can occur for a small relative heat conductivity. The influence of these thermal properties on the characteristics and dynamic behaviors of HTWs are analyzed and the critical Marangoni numbers for the appearance of HTWs are determined. This work can be helpful for understanding the influence of thermal properties on HTWs in sessile droplets. [ABSTRACT FROM AUTHOR]

Low-dimensional hybrid organic–inorganic metal halide perovskites are rapidly emerging as a fascinating sub-class of the three-dimensional parent structures, thanks to their appealing charge and thermal transport properties, paired to better chemical and thermal stabilities. Extensive investigations of the thermal behavior in these systems are of paramount relevance to understand their optoelectronic and thermoelectric applications. Herein, we present a complete thermophysical characterization of imidazolium lead iodide, (IMI)PbI3, a 1D pseudo-perovskite with chains of face-sharing octahedra, and histammonium lead iodide, (HIST)PbI4, a 2D layered perovskite with corner-sharing octahedra. Upon heating, the two compounds show highly anisotropic thermal expansion effects and high thermal stability until 250–300 °C. The thermal diffusivity of pelletized powders was measured with the laser flash technique from room temperature up to 225 °C. To account for the reduced density of the pelletized powders with respect to the bulk, the diffusivity data in different atmospheres were modeled as a function of the volume fraction and dimensionality of the pores, allowing to extrapolate the thermal conductivity of the bulk materials. The two compounds exhibit an ultralow thermal conductivity of 0.15 W/m K, two to three times lower than that reported on 3D MAPbI3 using the same technique. This finding suggests the primary role of the organic molecules within the hybrid systems, regardless of the octahedra connectivity and dimensionality. [ABSTRACT FROM AUTHOR]

The bulk thermal conductivity of Stillinger-Weber (SW) wurtzite GaN in the [0001] direction at a temperature of 300K is calculated using equilibrium molecular dynamics (EMD), non-equilibrium MD (NEMD), and lattice dynamics (LD) methods. While the NEMD method predicts a thermal conductivity of 166±11 W/m·K, both the EMD and LD methods predict thermal conductivities that are an order of magnitude greater. We attribute the discrepancy to significant contributions to thermal conductivity from long-mean free path phonons. We propose that the Grüneisen parameter for low-frequency phonons is a good predictor of the severity of the size effects in NEMD thermal conductivity prediction. For weakly anharmonic crystals characterized by small Grüneisen parameters, accurate determination of thermal conductivity by NEMD is computationally impractical. The simulation results also indicate the GaN SW potential, which was originally developed for studying the atomic-level structure of dislocations, is not suitable for prediction of its thermal conductivity. [ABSTRACT FROM AUTHOR]