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Flexible electronic devices with skin-like properties are hailed as revolutionary for the development of next-generation electronic devices, such as electric-skin and humanoid robotics. Graphene is intrinsically flexible due to its structural thinness in nature and are considered next-generation materials for wearable electronics. These devices usually experience a large mechanical deformation in use so as to achieve intimate conformal contact with human skin and to coordinate complex human motions, while heat dissipation has been a major limitation when the device is under a large mechanical strain. Unlike the small deformation (<1%) induced by intrinsic material factors such as lattice mismatch between material components in devices, a large mechanical deformation (>1%) by an external loading condition could lead to apparent changes to global geometric shapes and significantly impact thermal transport. In this study, we investigated the thermal conductivities of graphene under several large mechanical strains: 2.9%, 4.3%, and 6.1%. We used a refined opto-thermal Raman technique to characterize the thermal transport properties and discovered the thermal conductivities to be 2092 ± 502, 972 ± 87, 348 ± 52, and 97 ± 13 W/(m K) for the relaxed state, 2.9%, 4.3%, and 6.1% tensile strain, respectively. Our results showed a significant decreasing trend in thermal conductivities with an increasing mechanical strain. The findings in this study reveal new thermal transport mechanisms in 2D materials and shed light on building novel flexible nanoelectronic devices with enhanced thermal management. [ABSTRACT FROM AUTHOR]

Bond line thickness (BLT) is an essential parameter of thermal conductive composite gels as the thermal interface material (TIM). Extensive research has been performed on designing next-generation TIMs with high thermal conductivity; however, it remains elusive how to link laboratory measurements and the theoretically predicted BLT. Here, we propose a new model to estimate BLT based on a rheological property, in which the TIM is assumed to be a simple power law fluid. To avoid the unrealistic situation of the BLT tending to zero, we introduce a decaying exponential function to describe the influence from fillers during the lid attach process and rebuild the force balance equation. Compared with previously reported models, the theoretical prediction BLT based on the proposed model is in good agreement with the experimental data. Our model has a guiding significance in predicting the BLT, which may help to optimize the thermal performance of TIMs. [ABSTRACT FROM AUTHOR]

The two-dimensional laser-based Ångstrom method measures the in-plane thermal properties for anisotropic film-like materials. It involves periodic laser heating at the center of a suspended film sample and records its transient thermal response by infrared imaging. These spatiotemporal temperature data must be analyzed to extract the unknown thermal conductivity values in the orthotropic directions, an inverse parameter fitting problem. Previous demonstration of the metrology technique used a least-squares fitting method that relies on numerical differentiation to evaluate the second-order partial derivatives in the differential equation describing transient conduction in the physical system. This fitting approach is susceptible to measurement noise, introducing high uncertainty in the extracted properties when working with noisy data. For example, when noise of a signal-to-noise ratio of 10 is added to simulated amplitude and phase data, the error in the extracted thermal conductivity can exceed 80%. In this work, we introduce a new alternative inverse parameter fitting approach using physics-informed neural networks (PINNs) to increase the robustness of the measurement technique for noisy temperature data. We demonstrate the effectiveness of this approach even for scenarios with extreme levels of noise in the data. Specifically, the PINN-approach accurately extracts the properties to within 5% of the true values even for high noise levels (a signal-to-noise ratio of 1). This offers a promising avenue for improving the robustness and accuracy of advanced thermal metrology tools that rely on inverse parameter fitting of temperature data to extract thermal properties. [ABSTRACT FROM AUTHOR]

The binary high-pressure phase diagram of the Ta–Sb system was constructed for the first time in this study, utilizing the evolutionary algorithm USPEX and density functional theory (DFT). Ten pressurized dynamically and mechanically stable or metastable novel phases of Ta–Sb were discovered, including I4/mmm-TaSb2, P4/nmm-TaSb, P-3-Ta2Sb7, I4/mmm-Ta2Sb3, P-4m2-Ta7Sb, Pm-3-Ta7Sb, Pmm2-Ta15Sb, P4/nmm-TaSb3, I4/mmm-Ta3Sb4, and I4/mmm-Ta2Sb5. The compounds P-4m2-Ta7Sb and Pmm2-Ta15Sb exhibit promising characteristics as non-centrosymmetric superconductors (NCSs), with their superconducting critical temperature (TC) being 3.831 and 3.221 K, respectively. The application of pressure tuning is predicted to transform the topological characteristics of P4/nmm-TaSb, causing it to transition from a topological insulator state to a Dirac semimetal state and ultimately reverting back to a topological insulator state. Therefore, the P4/nmm-TaSb compound is considered a promising candidate to investigate topological and superconducting excitations. Moreover, the mechanical and thermal properties of Ta–Sb binary phases were also investigated. The thermal conductivity of I4/mmm-TaSb2, P4/nmm-TaSb, and P4/nmm-TaSb3 all surpasses 20 W m−1 K−1 at 1000 K, showcasing their excellent thermal conductivity properties. The present study addresses the research gap concerning high-pressure structures in the Ta–Sb binary system, thereby offering valuable insights for the design and development of intermetallic compounds within this binary system. [ABSTRACT FROM AUTHOR]

Owing to the unique asymmetric geometry, Janus monolayer compounds exhibit various exotic thermal properties and have promising applications in thermal management. In this study, we combine machine learning potentials and the phonon Boltzmann transport equation to perform a comparative study of the thermal transport properties in Janus XBAlY (X = Se, S, Te; Y = S, Se, O; X ≠ Y) monolayers. Our findings unveil a thermal conductivity (κ p) ranking as SeBAlS > TeBAlO > SBAlSe, contradicting the conventional expectation that a higher κ p is typically observed when the average atomic mass is smaller. At room temperature, the κ p of SeBAlS is 174 Wm−1 K−1, which is 4.8 times that of SBAlSe when considering three-phonon scattering processes. Moreover, the consideration of four-phonon scatterings does not alter such ranking. The anomalous κ p phenomenon was explained through a detailed analysis of the phonon–phonon scattering mechanism, phonon bandgap, phonon anharmonicity, and chemical bond strength. This study highlights the intricate relationship between atomic mass, bonding characteristics, and thermal properties, offering insights for designing Janus materials with tailored thermal conductivity. [ABSTRACT FROM AUTHOR]

Hafnia (HfO2) is a potential candidate for the high-k gate dielectrics in next-generation high-power electronics. Its thermal transport properties, which determine the performance of these related high-power electronics, are critical while rarely investigated. Here, the thermal transport properties of HfO2 in a wide temperature range of 300–2000 K with a phase transition between monoclinic and tetragonal phases at ∼1765 K, are systematically studied based on the temperature-dependent effective potential landscapes with both propagating and coherence thermal transport considered. It is found that the cage-like structure of monoclinic HfO2 results in the avoid crossing in the phonon band structures, which increases the three-phonon scattering largely. Some phonon modes with significant scattering matrix can have relatively larger 3ph and 4ph scattering rates in tetragonal HfO2. Consequently, the thermal conductivity of HfO2 is only 11.95–1.72 W/mK at 300–2000 K. Our results further show that propagating phonon channels dominate the thermal transport in HfO2 and contribute at least 70% to the total thermal conductivity. The rest of the thermal conductivity of HfO2 results from the coherence thermal transport channels, which is caused by the overlap of phonons. Four-phonon scatterings are found to be significant for the thermal transport in tetragonal HfO2, which can result in a thermal conductivity reduction of ∼50%. Our results here advance the understanding of the thermal transport in HfO2, which may benefit the performance optimization of HfO2-related electronics. [ABSTRACT FROM AUTHOR]

The lattice thermal conductivity (LTC) of Ga 2 O 3 is an important property due to the challenge in the thermal management of high-power devices. In this work, we develop machine-learned neuroevolution potentials (NEPs) for single-crystalline β - Ga 2 O 3 and κ - Ga 2 O 3 and demonstrate their accuracy in modeling thermal transport properties. Combining NEP-driven homogeneous non-equilibrium molecular dynamics simulations with tensor analysis, we determine the spatial distributions of LTCs for two Ga 2 O 3 crystals, showing dissimilar thermal behaviors. Specifically, β - Ga 2 O 3 shows isotropic thermal transport properties, with the LTCs along [100], [010], and [001] directions being predicted to be 10.3 ± 0.2 , 19.9 ± 0.2 , and 12.6 ± 0.2 W/(m K), respectively, consistent with previous experimental measurements. For κ - Ga 2 O 3 , our predictions suggest nearly isotropic thermal transport properties, with the LTCs along [100], [010], and [001] being estimated to be 4.5 ± 0.1 , 3.9 ± 0.1 , and 4.0 ± 0.1 W/(m K). The reduced LTC of κ -Ga 2 O 3 vs β -Ga 2 O 3 stems from its restricted low-frequency phonons up to 5 THz. Furthermore, we find that the β phase exhibits a typical temperature dependence slightly stronger than ∼ T − 1 , whereas the κ phase shows a weaker temperature dependence, ranging from ∼ T − 0.5 to ∼ T − 0.7 . [ABSTRACT FROM AUTHOR]

The carbon nano-peapod is a representative structure with interlayer van der Waals (vdW) interactions, in which encapsulated fullerene molecules play a critical role in modulating the transport properties of the carbon nanotubes (CNTs). In particular, their influence on the thermal transport characteristics has been the focal point of considerable attention. In this study, we trained an accurate machine learning potential for fullerene-encapsulated CNTs based on the efficient NEP model to investigate their thermal properties. Using equilibrium molecular dynamics simulation along with the spectral decomposition method for thermal conductivity, we find that the thermal conductivity of fullerene-encapsulated CNTs is roughly 55 % lower than that of empty CNTs, aligning with experimental observations for CNT bundles with fullerene encapsulation [Kodama et al., Nat. Mater. 16, 892 (2017)]. The research suggests that weak vdW interactions between both the fullerene and CNTs, as well as between fullerene molecules themselves, hinder phonon propagation. The encapsulated fullerene contributes to an increase in phonon scattering within the CNTs, ultimately leading to a reduction in thermal conductivity. We utilized machine learning potential to investigate the structure of fullerene-encapsulated CNTs and their heat transport property. This approach provides valuable insights for performance research of complex systems featuring interlayer vdW interactions. [ABSTRACT FROM AUTHOR]

When an impurity with a light mass is inserted into a crystal, it can undergo a high-frequency oscillation referred to as a local vibrational mode (LVM). A Fermi resonance may occur between the LVM and lower-frequency modes of the defect. The LVM may also interact with phonons or the electromagnetic field. Understanding these interactions can help model and control diffusion, defect reactions, and thermal conductivity. LVMs have been probed in semiconductors using pressure and alloying as experimental parameters, resulting in anticrossing between localized and extended vibrational modes. These types of vibrational interactions could play an important role in the stability and thermal properties of organic–inorganic hybrid semiconductors. The coupling between an LVM and electromagnetic wave yields an "LVM polariton," an excitation that has significant vibrational and electric-field amplitudes. [ABSTRACT FROM AUTHOR]

Time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance techniques have been widely used to measure thermal properties. However, the existence of the metal sensor brings some limitations to the experimental measurement, such as temperature limits, disability to measure low in-plane thermal conductivity, in situ measurement cannot be achieved, etc. This paper proposes a transducerless time-domain thermoreflectance method to measure in- and cross-plane thermal conductivity of nanofilms, in which the optical absorption depth and thermal conductivity tensor are considered to establish a new differential equation that can describe the heat conduction process in multilayer structures. This thermal model can also calculate the effects of spot ellipticity and spot offset distance. Then, the analytical solution and relative deviation of this new model and the surface heat flow boundary model used in conventional TDTR are compared by calculating the phase signals. In terms of experimental measurement, this model is successfully used to derive cross- and in-plane thermal conductivity of PdSi and IrNiTa amorphous alloy nanofilms without a metal sensor. [ABSTRACT FROM AUTHOR]

As the demand for computing applications capable of processing large datasets increases, there is a growing need for new in-memory computing technologies. Oxide-based resistive random-access memory (RRAM) devices are promising candidates for such applications because of their industry readiness, endurance, and switching ratio. These analog devices, however, suffer from poor linearity and asymmetry in their analog resistance change. Various reports have found that the temperature in RRAM devices increases locally by more than 1000 K during operation. Therefore, temperature control is of paramount importance for controlling their resistance. In this study, scanning thermal microscopy is used to map the temperature of Au/Ti/HfOx/Au devices at a steady power state and to measure temperature dynamics of the top electrode above the filament location during both resistive switching loops and voltage pulsing. These measurements are used to verify the thermal parameters of a multiphysics finite elements model. The model is then used to understand the impact of thermal conductivities and boundary conductances of constituent materials on resistance change during the first reset pulse in RRAM devices. It is found that the resistance change can be reduced significantly when the temperature in the titanium capping layer is reduced. We find that the greatest temperature reduction and, therefore, the lowest resistance change in the device are afforded by capping layers with increased thermal conductivities. This work links thermal properties to the resistance change in RRAM devices, providing critical insights into engineering devices with improved switching dynamics. [ABSTRACT FROM AUTHOR]

Clathrates have been reported to form in a variety of different structure types; however, inorganic clathrate-I materials with a low-cation concentration have yet to be investigated. Furthermore, tin-based compositions have been much less investigated as compared to silicon or germanium analogs. We report the temperature-dependent structural and thermal properties of single-crystal Eu2Ga11Sn35 revealing the effect of structure and composition on the thermal properties of this low-cation clathrate-I material. Specifically, low-temperature heat capacity, thermal conductivity, and synchrotron single-crystal x-ray diffraction reveal a departure from Debye-like behavior, a glass-like phonon mean-free path for this crystalline material, and a relatively large Grüneisen parameter due to the dominance of low-frequency Einstein modes. Our analyses indicate thermal properties that are a direct result of the structure and composition of this clathrate-I material. [ABSTRACT FROM AUTHOR]

Clathrates have been reported to form in a variety of different structure types; however, inorganic clathrate-I materials with a low-cation concentration have yet to be investigated. Furthermore, tin-based compositions have been much less investigated as compared to silicon or germanium analogs. We report the temperature-dependent structural and thermal properties of single-crystal Eu2Ga11Sn35 revealing the effect of structure and composition on the thermal properties of this low-cation clathrate-I material. Specifically, low-temperature heat capacity, thermal conductivity, and synchrotron single-crystal x-ray diffraction reveal a departure from Debye-like behavior, a glass-like phonon mean-free path for this crystalline material, and a relatively large Grüneisen parameter due to the dominance of low-frequency Einstein modes. Our analyses indicate thermal properties that are a direct result of the structure and composition of this clathrate-I material. [ABSTRACT FROM AUTHOR]

At the atomic level, heat is viewed as energy for lattice vibrational waves, i.e., a mechanical wave. Correspondingly, the strain as atomic displacement can have a profound impact on the thermal transport. Despite numerous atomistic simulations, fewer experimental efforts can be found for strain-dependent thermal properties of individual nanostructures and thin films. In this work, suspended 2 μm-thick Si films were stretched to reveal the influence of the uniaxial tensile strain on in-plane thermal conductivity along the stretching direction. In a high vacuum, the room-temperature thermal conductivity of a 2 μm-thick Si film decreased from 135.5 ± 6.9 to 127.2 ± 6.5 W/m K under a ∼0.44% tensile strain. This thermal conductivity decrease followed the predicted trend for Si films. In addition, the heat transfer coefficient of representative thin films in the air was also measured to reveal the impact of the heat loss along the sample sidewall on previous in-air thermal measurements. [ABSTRACT FROM AUTHOR]

Complex intermetallics usually exist as second phases in metal alloys. How these second phases can affect the thermal conductivity of alloys is generally unknown because the intrinsic thermal transport properties of these complex intermetallic compounds are quite less explored. In this work, we propose a computational framework based on first-principles calculations to study the electron and phonon thermal transport in complex intermetallics. Two typical intermetallics, i.e., MgZn2 and Mg4Zn7, are studied as prototypes. The rigorous mode-level first-principles calculations are first carried out to study the thermal transport of MgZn2. The calculations not only provide accurate thermal conductivity results, but also allow to prove that the constant relaxation time approximation and the Slack model work quite well in complex intermetallics. Then these two models are combined with first-principles calculations to predict the thermal transport properties for Mg4Zn7. Our results show that the directional average thermal conductivities for MgZn2 and Mg4Zn7 are 53.9 and 21.9 W/mK, significantly smaller than those of their elemental counterparts. Electrons are found to be the main heat carriers in these compounds, leading to a nearly temperature-independent thermal conductivity. Phonon thermal conductivity is negligible due to large unit cells and weak metallic bondings. Our work provides reliable thermal conductivity values for MgZn2 and Mg4Zn7. The computational framework developed in this work can also be further extended to study the electrical and thermal transport of other complex intermetallics. [ABSTRACT FROM AUTHOR]

Complex intermetallics usually exist as second phases in metal alloys. How these second phases can affect the thermal conductivity of alloys is generally unknown because the intrinsic thermal transport properties of these complex intermetallic compounds are quite less explored. In this work, we propose a computational framework based on first-principles calculations to study the electron and phonon thermal transport in complex intermetallics. Two typical intermetallics, i.e., MgZn2 and Mg4Zn7, are studied as prototypes. The rigorous mode-level first-principles calculations are first carried out to study the thermal transport of MgZn2. The calculations not only provide accurate thermal conductivity results, but also allow to prove that the constant relaxation time approximation and the Slack model work quite well in complex intermetallics. Then these two models are combined with first-principles calculations to predict the thermal transport properties for Mg4Zn7. Our results show that the directional average thermal conductivities for MgZn2 and Mg4Zn7 are 53.9 and 21.9 W/mK, significantly smaller than those of their elemental counterparts. Electrons are found to be the main heat carriers in these compounds, leading to a nearly temperature-independent thermal conductivity. Phonon thermal conductivity is negligible due to large unit cells and weak metallic bondings. Our work provides reliable thermal conductivity values for MgZn2 and Mg4Zn7. The computational framework developed in this work can also be further extended to study the electrical and thermal transport of other complex intermetallics. [ABSTRACT FROM AUTHOR]

The stacking of 2D layered materials can be an effective tool to modulate low-dimensional electronic structures and transport properties. In this work, using first-principles calculations, the thermal transport properties of a GeS bilayer are systematically investigated by solving the phonon Boltzmann transport equation. Various stacking configurations for bilayer GeS are introduced, and two dynamically stable structures are confirmed. The results indicate that the thermal transport property of the GeS bilayer can be dramatically suppressed due to a decreased phonon relaxation time, which is dependent on the stacking patterns and interlayer distances. The underlying phonon transport mechanisms and the stacking effects on the lattice thermal conductivity for bilayer GeS are further revealed through a comparative study among monolayer, bilayer, and bulk GeS. In addition, the in-plane anisotropy of the thermal transport properties is also enhanced for the GeS bilayer, which is also found to be dependent on the stacking pattern. The significantly suppressed thermal conductivity for the GeS bilayer evaluated in this work implies great potential for 2D multilayer-based thermoelectric devices and applications. [ABSTRACT FROM AUTHOR]

To explore the underlying mechanism of chemical disorder in high-entropy pyrochlores, ten rare earth zirconates (nRE1/n)2Zr2O7 (n = 1, 2, and 4, RE = La, Nb, Sm, Eu, and Gd) are studied by using first-principles calculations. The mechanical and thermal properties are carefully analyzed with a special focus on local structural evolution and interatomic interaction. It is found that all three kinds of bond lengths increase linearly with lattice parameters whether the pyrochlore involves chemical disorder or not. Compared with the single-component counterparts, the multi-component pyrochlores are recognized to exhibit higher elastic constants and moduli but lower elastic anisotropy. Meanwhile, (LaSmEuGd)2Zr2O7 shows the lowest thermal conductivity, which can be attributed to the larger La atoms and the weaker La–O bonding. Furthermore, the abnormal strengthening of phonon anharmonicity in (SmEu)2Zr2O7 emphasizes the significance of fluctuation in local distortion rather than enhancement in chemical disorder on decreasing thermal conductivity for high-entropy ceramics. This work uncovers the physical origins of the chemical disorder effect on mechanical and thermal properties for pyrochlores and further shed some lights on the design of high-performance high-entropy ceramics with great potential applications including thermal barrier coatings. [ABSTRACT FROM AUTHOR]

To explore the underlying mechanism of chemical disorder in high-entropy pyrochlores, ten rare earth zirconates (nRE1/n)2Zr2O7 (n = 1, 2, and 4, RE = La, Nb, Sm, Eu, and Gd) are studied by using first-principles calculations. The mechanical and thermal properties are carefully analyzed with a special focus on local structural evolution and interatomic interaction. It is found that all three kinds of bond lengths increase linearly with lattice parameters whether the pyrochlore involves chemical disorder or not. Compared with the single-component counterparts, the multi-component pyrochlores are recognized to exhibit higher elastic constants and moduli but lower elastic anisotropy. Meanwhile, (LaSmEuGd)2Zr2O7 shows the lowest thermal conductivity, which can be attributed to the larger La atoms and the weaker La–O bonding. Furthermore, the abnormal strengthening of phonon anharmonicity in (SmEu)2Zr2O7 emphasizes the significance of fluctuation in local distortion rather than enhancement in chemical disorder on decreasing thermal conductivity for high-entropy ceramics. This work uncovers the physical origins of the chemical disorder effect on mechanical and thermal properties for pyrochlores and further shed some lights on the design of high-performance high-entropy ceramics with great potential applications including thermal barrier coatings. [ABSTRACT FROM AUTHOR]

Electrical percolation in two-phase materials involves a very singular behavior, manifested as a huge change in the electrical conductivity, for a given volume or mass fraction of the phase with higher conductivity. In contrast, in the case of heat transfer, in two-phase composite systems, analogous percolative phenomena are far more elusive and have been rather difficult to observe in various physical systems. In this Perspective, we present a critical analysis of experimental results and the application of theoretical models aimed to study the effects of percolation phenomena on the thermal and electrical properties of two-phase materials. Our attention will be focused on composites made of high conductivity particles in a polymeric matrix. The effect of several factors, such as the geometrical and physical characteristics of fillers and their connectivity with the matrix, the proportion between the conductivity of filler and the matrix, as well as the crucial role of interfacial thermal resistance, is considered. In particular, the differences between the thermal and electrical thresholds and the physical and geometrical conditions that should be fulfilled to observe thermal percolation are discussed. Future trends, to be followed in the development of new materials, in order to enhance the thermal conductivity as well as in making the thermal percolative effects notable, based on including additional phases and 2D fillers, are also discussed. [ABSTRACT FROM AUTHOR]

The thermal properties of two-dimensional (2D) materials, such as MoS2, are known to be affected by interactions with their environment, but this has primarily been studied only with SiO2 substrates. Here, we compare the thermal conductivity (TC) and thermal boundary conductance (TBC) of monolayer MoS2 on amorphous (a-) and crystalline (c-) SiO2, AlN, Al2O3, and h-BN monolayers using molecular dynamics. The room temperature, in-plane TC of MoS2 is ∼38 Wm−1 K−1 on amorphous substrates and up to ∼68 Wm−1 K−1 on crystalline substrates, with most of the difference due to substrate interactions with long-wavelength MoS2 phonons (<2 THz). An h-BN monolayer used as a buffer between MoS2 and the substrate causes the MoS2 TC to increase by up to 50%. Length-dependent calculations reveal TC size effects below ∼2 μm and show that the MoS2 TC is not substrate- but size-limited below ∼100 nm. We also find that the TBC of MoS2 with c-Al2O3 is over twice that with c-AlN despite a similar MoS2 TC on both, indicating that the TC and TBC could be tuned independently. Finally, we compare the thermal resistance of MoS2 transistors on all substrates and find that MoS2 TBC is the most important parameter for heat removal for long-channel (>150 nm) devices, while TBC and TC are equally important for short channels. This work provides important insights for electro-thermal applications of 2D materials on various substrates. [ABSTRACT FROM AUTHOR]

Measuring the thermal properties of anisotropic films of hybrid materials poses a challenge to existing metrology techniques. We have developed a new approach for measuring the volumetric heat capacity and anisotropic thermal conductivity of these systems using the 3ω method. While there exist many avenues for measuring the thermal properties of thin films, most carry with them difficult requirements such as smooth surfaces or advanced lithography. Here, we present measurements of a film's in-plane and cross-plane conductance and its volumetric heat capacity using relatively simple sample configurations, each requiring a single heater. For the measurement of volumetric heat capacity, we present a new model fitting method, relying on a standard film-on-substrate configuration. For the measurement of in-plane thermal conductance by 3ω, we have developed the use of an embedded micro-wire heater in suspended drop cast films, allowing for a 12 μm wide heater without the need for advanced lithography. We also expose the surprisingly significant effect of thermal radiation in the suspended film measurement and its associated error. Our measurements reveal a large anisotropy in the thermal conductivity of our test material, Te-PEDOT:PSS, of kin-plane/kcross-plane = 19, consistent with the nanoscale morphology of the material. [ABSTRACT FROM AUTHOR]

Thermal transport through various nanowires has attracted extensive attention in the past two decades. Nanowires provide an excellent platform to dissect phonon transport physics because one can change the wire size to impose systematically varying boundary conditions that can help to distinguish the contributions of various scattering mechanisms. Moreover, novel confinement phenomena beyond the classical size effect promise opportunities to achieve highly desirable properties. Based on a summary of research progresses in nanowire thermal properties, we discuss more intriguing observations due to the classical size effect, coupling between mechanical and thermal properties, and divergent thermal conductivity as a result of conversion from three-dimensional to one-dimensional phonon transport, showcasing the superdiffusive thermal transport phenomenon. We hope that these discussions could provide a new perspective on further exploring thermal transport in nanowires, which may eventually lead to breakthroughs such as achieving thermal conductivity values higher than that of any known materials. [ABSTRACT FROM AUTHOR]

Localization is a well-known wave phenomenon that significantly impedes transport, as uncovered by a pioneering work of Anderson. The localization of thermal phonons based on the phonon wave nature is widely represented in disordered atomic systems. Compared with electron and photon localization, the observations of phonon localization are much rarer, owning to the broadband nature of phonon thermal transport. In this Perspective, we summarize the experimental and theoretical evidences of phonon Anderson localization in disordered atomic systems from the aspects of vibrational spectroscopy, thermal conductivity measurement, phonon transmission, phonon wave packet, phonon participation ratio, and energy distribution. [ABSTRACT FROM AUTHOR]

Driven by the prospective application of thermoelectric (TE) materials, massive efforts have been devoted to improving its performance. However, strong-coupled thermal and electrical transport is still the largest block for the promotion of TE techniques. Here, we report a peak ZT over 1.2 at 400 K in an n-type Bi2Te2.7Se0.3 sample doped by 5 wt. % Sb under the spark plasma sintering method. This high performance is attributed to the synergistically optimized thermal and electrical transport properties. Sb nano-precipitates, which have been observed directly by spherical aberration-corrected transmission electron microscopy, substantially decrease the lattice thermal conductivity of the sample, leading to a low value of 0.35 W m−1 K−1. Meanwhile, band bending caused by the nano-precipitates significantly enhances the Seebeck coefficient, resulting in a high PF of 35 μW cm−1 K−2. The study about optimizing electrical and thermal properties simultaneously opens the door to the high performance of Bi2Te3-based materials. [ABSTRACT FROM AUTHOR]

High-performance thermoelectric (TE) properties of pristine Sn-based colusites are investigated theoretically. A recent experimental article [Bourgès et al., J. Am. Chem. Soc. 140, 2186 (2018)] showed how structural disordering reduced the lattice thermal conductivity in Cu 26 V 2 Sn 6 S 32 and improved the TE figure of merit (ZT). In this article, it is observed that low energy soft optical phonons and the vacancy defect play a crucial role in reducing thermal conductivity. An ultra-low lattice thermal conductivity 0.35 Wm − 1 K − 1 is observed in Cu 26 V 2 Sn 6 Se 32. It is seen that mass variance perturbation and number of vacancies are important to tune the thermal and electrical properties. Though the electrical conductivity decreases with the number of vacant sites, the Seebeck coefficient enhances due to the reduction in carrier concentration. The detailed study of thermoelectric properties of Cu 26 V 2 Sn 6 Se 32 with Se vacancies points to enhanced ZT values of 0.34 at 300 K, which is ∼ 15 times greater than ZT of Cu 26 V 2 Sn 6 S 32. A maximum ZT of 1.68 at 755 K is predicted for Cu 26 V 2 Sn 6 Se 32 with Se vacancies, which is the highest to date reported for Sn-based colusites. [ABSTRACT FROM AUTHOR]

Nanostructured semiconductors are promising candidates for thermoelectric materials owing to their superior thermal insulating properties over their bulk counterparts. In this study, a one-dimensional, crystalline nanostructure synthesized by sintering Si nanoparticles, called Nano Particle Chain (NPC) structures, is proposed. The structure is systematically analyzed for its thermal transport properties and compared with the nanowire counterparts. Both classical molecular dynamics and lattice dynamics tools were employed to evaluate lattice thermal conductivity (k) and to perform phonon mode level decomposition. A marked reduction in the phonon relaxation time of the NPC structure was observed indicating possible effects of phonon-boundary/constriction scatterings. This has resulted in a two-order reduction in k in NPC structures over bulk Si. Further, one order reduction of k of NPC structures was attained with respect to a nanowire of the same constriction size, indicating the effectiveness of the mismatch of particle and constriction diameters as an efficient thermal suppression mechanism. With the addition of a second material of different mass, the NPC structures can be further diversified to core/shell configurations. It was also identified that a non-monotonic variation of k exists, with a minimum in core/shell NPC structures. This effect is materialized by using a Ge-like fictitious material to coat the original Si nanoparticles, owing to competing effects of two phonon suppression mechanisms. Moreover, these core/shell NPC structures are compared with previously reported diameter modulated core/shell nanowire structures [E. Blandre et al., Phys. Rev. B, 91, 115404 (2015)] to highlight their capability to enhance the thermoelectric performance over conventional one-dimensional nanostructure configurations. [ABSTRACT FROM AUTHOR]

Based on the combination of first-principles calculations and Boltzmann transport equation, we investigated the thermal transport properties of K 3 AuO and predicted a figure of merit ZT = 2.01 at 700 K with p-type doping. Such a high thermoelectric efficiency can be attributed to the ultra-low lattice thermal conductivity with a value of 0.48 Wm − 1 K − 1 at 300 K, and detailed research shows that the low lattice thermal conductivity arises from the small phonon group velocity and high scattering rates; moreover, the figure of merit ZT of p-type doped K 3 AuO can be maintained at around 2 in a relatively wide carrier concentration of 5.5 × 10 20 – 1.2 × 10 21 cm − 3 , which demonstrates the stable thermoelectric properties of K 3 AuO. [ABSTRACT FROM AUTHOR]

The effects of length, diameter, temperature, and axial strain on the thermal conductivity of armchair and zigzag WS 2 nanotubes are systematically investigated by nonequilibrium molecular dynamics simulations. It is found that the thermal conductivity gradually increases with the increase in the length, while it is insensitive to nanotube diameter variation. The thermal conductivity of armchair and zigzag WS 2 nanotubes is remarkably reduced as temperature increases due to the increment of phonon–phonon scattering and reduction of the phonon mean free path. In addition, compressive strain can increase thermal conductivity due to increased contributions from low-frequency phonons, while the opposite is true in the case of tensile strain. The chirality has a slight influence on the thermal transport properties of the WS 2 nanotube. [ABSTRACT FROM AUTHOR]

Developing predictive thermal property models for liquids based on microscopic principles has been elusive. The difficulty is that liquids have gas-like and solid-like attributes that are at odds when considering the frameworks of microscopic models: Models for gases are simple due to randomness and low density, whereas models for crystalline solids rely on symmetry and long-range order for easier calculation. The short-range order in liquids does, however, provide structure to neighboring molecules similar to amorphous solids, and there have been recent advances indicating that collective vibrational modes store heat in liquids. Models combining Debye approximations from solid-state physics and Frenkel's theory of liquids can accurately predict the heat capacity of liquids. Phonon-like dispersions in liquids have also been widely observed in neutron scattering experiments. These developments motivate us to propose a model where high-frequency vibrational modes, which travel at the speed of sound and have a mean free path on the order of the average intermolecular distance, conduct heat in liquids. We use this liquid phonon gas model to calculate the thermal conductivity of liquids with varying intermolecular interaction energies from strongest to weakest—Coulomb, hydrogen-bonding, Keesom, and London dispersion energy. Generally, the model is more accurate as the intermolecular interaction energy and density of liquids increase. The calculated thermal conductivity of Coulombic-bound molten sodium nitrate and hydrogen-bonded water is within 1.46% and 2.98% of the experimentally measured values, respectively, across their entire temperature ranges. Further modal analysis of the velocity and the mean free path of collective vibrations could establish the liquid phonon gas model as an accurate model for weakly interacting liquids as well. [ABSTRACT FROM AUTHOR]

This study reports on the elastic properties and the lattice thermal conductivity of amorphous Ge2Sb2Te5 and GeTe thin films using surface Brillouin scattering. It is demonstrated that this method allows for the determination of the isotropic elastic constants from the measured surface acoustic phonon frequencies. We found elastic constants of C 11 = 48.7 / 43.6 and C 44 = 14.3 / 9.4 GPa for GeTe and Ge2Sb2Te5, respectively. These results suggest acoustic hardening in GeTe compared to Ge2Sb2Te5 films, and this was supported by the derived, shear, and Young's moduli. The measured longitudinal and transverse velocities were used to determine the lower limit of the lattice thermal conductivity. In general, both chalcogenides alloys exhibit low lattice thermal conductivities of κ m i n < 0.50 W m − 1 K − 1 . This could be beneficial for thermal management in phase-change memory devices and for thermoelectric application. [ABSTRACT FROM AUTHOR]

In this study, we report on the thermal conductivity of amorphous carbon generated in diamond via nitrogen ion implantation (N 3 + at 16.5 MeV). Transmission electron microscopy techniques demonstrate amorphous band formation about the longitudinal projected range, localized approximately 7 μ m beneath the sample surface. While high-frequency time-domain thermoreflectance measurements provide insight into the thermal properties of the near-surface preceding the longitudinal projected range depth, a complimentary technique, steady-state thermoreflectance, is used to probe the thermal conductivity at depths which could not otherwise be resolved. Through measurements with a series of focusing objective lenses for the laser spot size, we find the thermal conductivity of the amorphous region to be approximately 1.4 W m − 1 K − 1 , which is comparable to that measured for amorphous carbon films fabricated through other techniques. [ABSTRACT FROM AUTHOR]

This article presents three photothermal methods dedicated to the measurement of the thermal properties of chalcogenide alloys, used as a central element in the new generations of non-volatile memory. These materials have two phases, amorphous and crystalline, possessing a sharp contrast in their electrical and thermal properties. In the crystalline phase, the properties also change very significantly with temperature. The control of the temperature of the samples, the choice of transducers, and the time or frequency characteristic values of the photothermal excitation are thoroughly discussed. Each photothermal technique is described from the experimental point of view as well as from the inverse method, performed to identify the parameters of interest. The identified thermal properties mainly concern the thermal conductivity and the thermal resistance at the interfaces between the phase-change materials and the materials in contact as encountered in the production of the microelectronic memory device. Assessing various photothermal techniques, the study suggests that pulsed photothermal radiometry is the most effective method for sensitive high-temperature measurements of thermal properties of the phase-change materials. [ABSTRACT FROM AUTHOR]

Germanium manganese compounds exhibit a variety of stable and metastable phases with different stoichiometries. These materials entail interesting electronic, magnetic, and thermal properties both in their bulk form and as heterostructures. Here, we develop and validate a transferable machine learning potential, based on the high-dimensional neural network formalism, to enable the study of Mn x Ge y materials over a wide range of compositions. We show that a neural network potential fitted on a minimal training set reproduces successfully the structural and vibrational properties and the thermal conductivity of systems with different local chemical environments, and it can be used to predict phononic effects in nanoscale heterostructures. [ABSTRACT FROM AUTHOR]

The photoacoustic method with piezoelectric detection for the simultaneous evaluation of the thermophysical properties is proposed. The approach is based on the settling of an additional heat sink for redistribution of heat fluxes deposited on the sample surface. First, the approach was tested on the porous silicon with well-defined morphology and well-studied properties. Then, heat capacity and thermal conductivity of silicon nanowire arrays were investigated by recovering the experimental data through numerical simulations. The decrease in heat capacity and effective thermal conductivity of the samples upon increasing thickness and porosity of the sample was observed. Such a behavior could be caused by the increase of the structure heterogeneity. In particular, this can be related to a larger disorder (increased density of broken nanowires and larger porosity) that appears during the etching process of the thick layers. [ABSTRACT FROM AUTHOR]

The nanoscale properties of materials can have a great influence on their macroscopic behavior; for instance, the generation and accumulation of defects at the nanoscale, such as point defects, porosity, and interfaces, can change their thermal properties. In this work, we study the role of an interface in the thermal conductivity between two nanoparticles without any external load. We consider a system subjected to a temperature gradient perpendicular to the contact surface and study the thermal conductivity, thermal conductance, thermal resistance, and contact resistance vs nanoparticle size. The thermal resistance at the interface increases linearly with nanoparticles' contact radius a c. A model based on the contact area between two nanoparticles allows us to reasonably explain the obtained numerical results for the thermal conductivity, leading to a net decrease in effective conductivity as the nanoparticle size increases, reasonably well described by a (a c / R) dependence. Simulated thermal conductance was found to be proportional to (a c / R). [ABSTRACT FROM AUTHOR]

We use density functional theory and reactive-force-field methods to investigate the electrical and thermal transport properties of long disordered lithiated silicon nanowires. The latter could build the core of future lithium ion batteries with enhanced storage capacity. Due to the amorphous nature of these nanowires, disorder and surface roughness effects inevitably arise, affecting the lithiation process. It is found that the electrical conductivity of the nanowires steadily increases as a function of the lithium concentration, despite the presence of disorder, while the thermal conductivity follows the opposite trend and decreases significantly with reduced heat evacuation capabilities as a consequence. This behavior can be attributed to the influence of Li ions, which on one hand tend to metallize Si nanowires and thus enhance their electron mobility. On the other hand, the random distribution of Li atoms perturbs the phonon propagation through the nanowire, explaining the decrease in thermal conductivity. [ABSTRACT FROM AUTHOR]

We study magnetic, transport, and thermal properties of Co2C and Co3C nanocomposites mixed in a 1:1 ratio. The nanocomposite has clusters with an average diameter of 40 ± 15 nm. We show that the behavior of the nanocomposite is completely different from that of only Co3C or Co2C. We observed that with decreasing temperature, the saturation magnetization MS(T) increases, with a steep rise seen below 100 K. A detailed analysis shows that the increase in MS(T) down to 100 K is explained via a surface spin freezing model. However, below 100 K, the steep increase in MS(T) is explained by a finite size effect related to the confinement of spin waves within nanoparticles. Heat capacity measurements show a broad peak at 100 K along with a low temperature anomaly at 43 K (=Tex). Resistance measurements of the nanocomposite show metallic behavior at high T with an anomalous feature appearing at Tex, which is near the T regime, where MS(T) begins to increase steeply. A measurement of the temperature gradients across the sample thickness indicates an abrupt change in thermal conductivity at Tex. Our results suggest a transformation from a magnetically coupled state with a continuous spectrum of spin waves into a magnetically decoupled state below 100 K with confined spin waves. [ABSTRACT FROM AUTHOR]

The lattice thermal conductivity and thermal transport properties of 2D α-selenene are investigated based on the first-principles calculations. The isotropic in-plane thermal conductivity is as low as 3.04 W m−1 K−1 at room temperature, even abnormally lower than α-tellurene that processes analogous configuration and lower Debye temperature. We find this abnormal phenomenon reasonably stems from the larger anharmonicity of the acoustic phonon branch. Moreover, the phonon spectra, elastic properties, and related thermal properties are also exhibited. Acoustic phonons contribute mainly to the total thermal conductivity. Furthermore, size effect, boundary effect, the total phase space for three-phonon processes, phonon group velocity, and relaxation time are further investigated, and the last one is unveiled to be the key ingredient of thermal transport in 2D selenene. [ABSTRACT FROM AUTHOR]

Strain inevitably exists in practical GaN-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering, and it significantly influences the thermal properties of GaN. In this work, thermal transport properties of GaN considering the effects from biaxial strain and electron-phonon coupling (EPC) are investigated using the first principles calculation and phonon Boltzmann transport equation. The thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which are consistent better with the experimental values in the literature than previous theoretical reports and show a nearly negligible anisotropy. Under the strain state, thermal conductivity changes remarkably. In detail, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63%, while it increases by 53% under the −5% compressive strain, which is mostly attributed to the changes in phonon relaxation time. Besides, the anisotropy of thermal conductivity changes under different strain values, which may result from the weakening effect from strain induced piezoelectric polarization. EPC is also calculated from the first principles method, and it is found to decrease the lattice thermal conductivity significantly. Specifically, the decrease shows significant dependence on the strain state, which is due to the relative changes between phonon-phonon and electron-phonon scattering rates. Under a compressive strain state, the decreases of lattice thermal conductivity are 19% and 23% for in-plane and cross-plane conditions, respectively, comparable with those under a free state. However, the decreases are small under the tensile strain state, because of the decreased electron-phonon scattering rates and increased phonon anharmonicity. [ABSTRACT FROM AUTHOR]

By using state-of-the-art first-principles calculations based on density functional theory (DFT), we conduct a comparative study of the mechanical, electrical, and in-plane thermal transport properties of recently synthesized graphenelike C3B and C3N nanosheets. Our DFT results reveal that the monolayer C3B remarkably possesses a lower elastic modulus and in-plane stiffness as well as ultimate tensile strength compared to C3N, while obviously stronger anisotropy in failure behavior is found in C3B sheets. Both monolayer materials are found as semiconductors with indirect bandgaps of about 1.78 eV and 1.15 eV at the HSE06 level, and their carrier mobilities demonstrate remarkable anisotropy. Additionally, the electron mobility of C3B is found to be much higher than its hole mobility, while for C3N, the reverse is true. For the thermal transport properties, as expected, the intrinsic lattice thermal conductivity of the monolayer C3B (301 W/m K at 300 K) is also lower than that of C3N (380 W/m K at 300 K), while much great anisotropy of in-plane thermal conductivity is found in C3B. The underlying mechanisms governing the phonon thermal transport of these two graphenelike monolayers are thoroughly discussed and compared. Our research will benefit future theoretical research and practical application of these two novel boron-carbide and carbon-nitride materials. [ABSTRACT FROM AUTHOR]

Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. Here, we measure the thermal conductivity of crystalline AlN by the 3ω method, finding that it ranges from 674 ± 56 Wm−1 K−1 at 100 K to 186 ± 7Wm−1 K−1 at 400 K, with a value of 237 ± 6 Wm−1 K−1 at room temperature. We compare these data with analytical models and first-principles calculations, taking into account atomic-scale defects (O, Si, C impurities, and Al vacancies). We find that Al vacancies play the greatest role in reducing thermal conductivity because of the largest mass-difference scattering. Modeling also reveals that 10% of heat conduction is contributed by phonons with long mean free paths (MFPs), over ∼7 μm at room temperature, and 50% by phonons with MFPs over ∼0.3 μm. Consequently, the effective thermal conductivity of AlN is strongly reduced in submicrometer thin films or devices due to phonon-boundary scattering. [ABSTRACT FROM AUTHOR]

An attractive feature of nanomaterials is the possibility of tuning their properties through controlling their size and surface morphology, and understanding the effects of various parameters on thermal transport properties of nanostructures has been an active research topic in the past two decades. Through systematic studies of kinked silicon nanoribbons, we show how the kink morphology, a newly recognized degree of freedom for tuning thermal transport in nanostructures, modulates the thermal conductivity of these nanoribbons. For kinked Si nanoribbons that are 34 nm thick and 141 nm wide, the measured thermal conductivity first decreases as the period length reduces from 2 μm to 0.5 μm, reaching a 21% thermal conductivity reduction as compared to that of a straight counterpart at 300 K. However, as the period length drops to a level at which a straight heat transfer channel opens between the heat source and the sink, the thermal conductivity exhibits a steep increasing trend. Moreover, the comparison of thermal conductivity reduction for kinked ribbons along different crystalline directions indicates that phonon focusing could be exploited to further suppress thermal transport in kinked silicon nanoribbons. These results provide important guidelines on modulating heat transfer in nanostructures using kinks, which could be adopted to tune the thermal properties of nanostructures for different applications, such as thermoelectrics, microelectronic device thermal management, and functional thermal regulators. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to determine simultaneously and in different ways thermal diffusivity and thermal conductivity of thin layers. This is done by using the accurate expression of the temperature distribution derived from the parabolic heat equation when the front surface of the thin layer is excited by a periodic heat flux, while the rear surface is maintained at one of three different types of boundary conditions: modulated periodic heat flux, modulated temperature, or constant temperature. Our approach exploits the modulation frequencies at which the normalized front surface temperature reaches its first maximum and first minimum. It is shown that (i) these characteristic frequencies can be used to obtain the thermal diffusivity of the finite layer under three different types of boundary conditions. (ii) The ratio between the values of the maxima and minima of the temperature can be utilized to determine the thermal conductivity of the finite layer. These two thermal properties are sensitive to the nature of the boundary conditions as well as the modulation frequency of the heat excitation. This paper provides a theoretical basis for the determination of the thermal diffusivity and thermal conductivity of the finite layer using laser-based heating photothermal techniques. [ABSTRACT FROM AUTHOR]

Thermal transport properties of graphene nanoribbons (GNRs) are investigated using phonon transport studies. Ribbons of varying widths are considered to investigate the transition of key thermal properties with width. The lattice structure of the ribbons is fully resolved, and phonon transport is modeled by accounting for all three-phonon scattering processes using a solution of the linearized Boltzmann transport equation. A 3× reduction in intrinsic thermal conductivity is observed compared to bulk graphene arising from increased strength of three-phonon scattering due to the additional nondegenerate phonon modes that appear due to the finite edges of confined nanoribbons. Strong dependence of thermal conductivity on ribbon width is also observed. The underlying mechanisms for thermal conductivity reduction and width dependence are presented by analyzing frequency- and polarization-resolved phonon transport. The additional scattering pathways present in 1D GNRs lead to a significant reduction in the thermal conductivity of otherwise highly conducting flexural phonons in bulk graphene. In contrast, confinement-induced changes to the density of states, specific heat or group velocity, and the subsequent impact on lattice thermal conductivity are found to be relatively small. [ABSTRACT FROM AUTHOR]

We investigate the effects of fullerene functionalization on the thermal transport properties of graphene monolayers via atomistic simulations. Our systematic molecular dynamics simulations reveal that the thermal conductivity of pristine graphene can be lowered by more than an order of magnitude at room temperature (and as much as by ∼ 93% as compared to the thermal conductivity of pristine graphene) via the introduction of covalently bonded fullerenes on the surface of the graphene sheets. We demonstrate large tunability in the thermal conductivity by the inclusion of covalently bonded fullerene molecules at different periodic inclusions, and we attribute the large reduction in thermal conductivities to a combination of resonant phonon localization effects, leading to band anticrossings and vibrational scattering at the sp 3 bonded carbon atoms. The torsional force exerted by the fullerene molecules on the graphene sheets and the number of covalent bonds formed between the two carbon allotropes is shown to significantly affect the heat flow across the hybrid structures, while the size of the fullerene molecules is shown to have a negligible effect on their thermal properties. Moreover, we show that even for a large surface coverage, the mechanical properties of these novel materials are uncompromised. Taken together, our work reveals a unique way to manipulate vibrational thermal transport without the introduction of lattice defects, which could potentially lead to high thermoelectric efficiencies in these materials. [ABSTRACT FROM AUTHOR]

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]

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]