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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]

Strain provides an additional mechanism in tuning the thermal/electrical properties of nanostructures and therefore has triggered lots of interest in recent years. However, experimental research about the strain effect on the thermal conductivity of nanowires is still limited, especially in the low-temperature range, which is important in understanding the physics of strain-induced regulation in thermal conductivity. Here, we present thermal transport measurements of bent silicon carbide nanowires at temperatures ranging from 20 to 300 K. Reduction in thermal conductivity compared to their straight counterparts is observed. More specifically, the relative change is up to 55% at 20 K and descends with temperature, which is due to the inhomogeneous strain-induced phonon scattering. This study will deepen the understanding of thermal properties in nanostructures with strain. [ABSTRACT FROM AUTHOR]

This study investigates spatially resolved phonon-mediated thermal transport across nano-sized grains and grain boundaries (GBs) in 3C-SiC using molecular dynamics (MD) simulations. This investigation involves controlling the complete range of inter-grain misorientation tilt angles (θ = 0°–90°) and nanoscale grain sizes (d = 2.18–130.77 nm). The grain boundary energy and interfacial thermal transport are found to be highly θ -sensitive and asymmetric with respect to θ = 45° due to the low symmetry associated with two interpenetrating diatomic SiC fcc lattices. When adjacent grains are tilted at θ = 14.25°, the interfacial heat conduction is highly suppressed compared to other θ values, especially for larger grains. The most stable atomic configuration of the GB region associated with the minimal GB energy results in the highest suppression of heat conduction across the GB interface. Spatially resolved thermal anisotropy reveals a strong GB-mediated nanoscale hydrodynamic phonon Poiseuille effect when heat flows parallel to the GB planes, as shown by our perturbed MD study. With the reduction of d , the intra-grain and inter-GB thermal conductivities decrease due to the enhanced phonon scattering from interfaces, but the difference between these conductivities becomes negligible for the heat flow normal to the GB planes. It is envisioned that nanoscale spatially resolved control of thermal energy will provide useful guidance to engineer nanocrystalline ceramics with tunable interfacial thermal properties. [ABSTRACT FROM AUTHOR]

Thermal transport properties of metallic nanowires are crucial to the near-junction heat dissipation of transistors. However, the understanding of the size-dependent thermal conductivity of these advanced interconnect metals is still limited. In this work, we select nine candidate metals and investigate the size effect on thermal transport properties by the mode-level first-principles method combining with the Boltzmann transport equation. Their thermal conductivity, the phonon contribution, and the Lorenz ratio in nanowires with characteristic size from 3 to 30 nm are analyzed. While all these metals have lower bulk thermal conductivity than Cu, we find some of these metals have larger thermal conductivity with characteristic size smaller than 10 nm. We identified that their smaller electron mean free path is the key factor. Moreover, the contribution of phonon thermal conductivity is smaller than 25% to total thermal conductivity. The Lorenz ratio is found to be slightly larger than the Sommerfeld value, mainly due to the phonon contribution. This work can provide important guidance for selecting advanced interconnects in the development of next-generation integrated circuits. [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]

Thermal properties are expected to be sensitive to the network topology, and however, no clearcut information is available on how the thermal conductivity of amorphous systems is affected by details of the atomic structure. To address this issue, we use as a target system a phase-change amorphous material (i.e., Ge2Sb2Te5) simulated by first-principles molecular dynamics combined with the approach-to-equilibrium molecular dynamics technique to access the thermal conductivity. Within the density-functional theory, we employed two models sharing the same exchange-correlation functional but differing in the pseudopotential (PP) implementation [namely, Trouiller–Martins (TM) and Goedecker, Teter, and Hutter (GTH)]. They are both compatible with experimental data, and however, the TM PP construction results in a Ge tetrahedral environment largely predominant over the octahedral one, although the proportion of tetrahedra is considerably smaller when the GTH PP is used. We show that the difference in the local structure between TM and GTH models impacts the vibrational density of states while the thermal conductivity does not feature any appreciable sensitivity to such details. This behavior is rationalized in terms of extended vibrational modes. [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]

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]

The importance of higher-order anharmonic effects on thermal transport has recently been demonstrated in highly symmetrical 2D materials with large acoustic–phonon (A–O) gap. However, the phonon scattering and the thermal transport properties in low-symmetry structures remain ambiguous. In this work, we employed moment tensor potential and Boltzmann transport equation to investigate phonon thermal transport properties of 1T′-ReS2 and 2H-WS2. We show that the mechanism of four-phonon scattering in 1T′-ReS2 is quite different from that in 2H-WS2. In 1T′-ReS2, the four-phonon scattering can reduce the thermal conductivity by up to 40.29%, even in the absence of an acoustic-optical phonon bandgap. The strong four-phonon scattering in 1T′-ReS2 is attributed to A–O phonon scattering mediated by abundant flattening optical phonon modes. However, in 2H-WS2, the strong four-phonon scattering is attributed to the presence of a large A–O phonon bandgap. Our work suggests that considering the four-phonon scattering is essential for calculating the thermal conductivity of 2D materials, even in the absence of an A–O phonon gap. [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]

Thermal transport within nanostructures is highly confined by interfaces, and non-trivial physics can emerge at boundaries. Theoretical studies have shown that different phonon modes can exhibit varying thermal resistances at interfaces. Experimental observation of these variations, however, is lacking. Using the steady-state Raman thermometry, the E 2 g 1 and A1g vibrational modes of MoS2 were utilized to characterize the thermal transport properties across the MoS2/Si interface. Our results revealed distinct temperature rises associated with different modes, indicating various mode contributions in the interfacial thermal conductance. Combining experimental and numerical simulations, the out-of-plane mode in MoS2 was found to contribute less to the interfacial transport, by 21.5%, attributed to the less variational mode mismatch of the in-plane phonon, compared to the in-plane mode. Furthermore, our results confirmed a 26.9% higher thermal conductivity from the out-of-plane mode than the in-plane one. [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]

The 3ω method is a well-established thermal technique used to measure the thermal conductivity of materials and the thermal resistance of interfaces. It has significant advantages over other steady state and transient thermal techniques in its ability to provide spatially resolved thermal property measurements over a wide range of thermal conductivity. Despite its advantages, it has been restricted to lab-scale use because of the difficulty involved in sample preparation and sensor fabrication and is limited to non-metallic substrates. High-throughput 3ω measurements with reusable sensors have not been realized yet. In this work, we demonstrate a method of applying reusable 3ω sensors fabricated on flexible polyimide films to measure bulk and spatially resolved thermal properties. We establish the limits of thermal conductivity measurement with the method to be 1 to 200 W/mK, and within the measurement limit, we verify the method by comparing the measured thermal conductivities of standard samples with established values. From the 3ω measurements, we also determine the thermal resistance of an interlayer of thermal grease as a function of pressure and compare it against the resistance calculated from direct thickness measurements to demonstrate the ability of this method to provide spatially resolved subsurface information. The technique presented is general and applicable to both metallic and non-metallic substrates, providing a method for high-throughput 3ω measurements with reusable sensors and without considerable sample preparation. [ABSTRACT FROM AUTHOR]

An improved method to simultaneously determine the heat capacity, thermal diffusivity, and thermal conductivity of a small-sized material is described. In this method, the heat of a square wave with a superimposed constant component is applied to one side of a plate-shaped sample using a thin-film heater, which is thermally linked to a heat reservoir. The response temperature is measured by a thermometer attached to the heater. In contrast to a previously reported method, the amplitude of the temperature oscillation detected by the thermometer is enhanced by the internal thermal relaxation in the improved method. This feature is advantageous for determining thermal properties with low-heat modulation. We theoretically analyzed the proposed method using a one-dimensional model and demonstrated the method on synthetic quartz (SiO2) and poly(methyl methacrylate) plates in the temperature range of 80–300 K. The thermal properties obtained for both samples using the proposed method were consistent with values reported in the literature. The deviations from the data for the specific heat capacity, thermal diffusivity, and thermal conductivity were estimated to be ∼1%, 2%, and 2%, respectively. [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]

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]

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]

We show that the centroid molecular dynamics (CMD) method provides a realistic way to calculate the thermal diffusivity a = λ/ρcV of a quantum mechanical liquid such as para-hydrogen. Once a has been calculated, the thermal conductivity can be obtained from λ = ρcVa, where ρ is the density of the liquid and cV is the constant-volume heat capacity. The use of this formula requires an accurate quantum mechanical heat capacity cV, which can be obtained from a path integral molecular dynamics simulation. The thermal diffusivity can be calculated either from the decay of the equilibrium density fluctuations in the liquid or by using the Green–Kubo relation to calculate the CMD approximation to λ and then dividing this by the corresponding approximation to ρcV. We show that both approaches give the same results for liquid para-hydrogen and that these results are in good agreement with the experimental measurements of the thermal conductivity over a wide temperature range. In particular, they correctly predict a decrease in the thermal conductivity at low temperatures—an effect that stems from the decrease in the quantum mechanical heat capacity and has eluded previous para-hydrogen simulations. We also show that the method gives equally good agreement with the experimental measurements for the thermal conductivity of normal liquid helium. [ABSTRACT FROM AUTHOR]