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Nanofilms (NFs) have been widely used in many emerging applications, such as microelectronic devices, spintronics, and optical instruments. Characterizing the thermal conductivity (k) of NFs, kNFs, is nontrivial for both fundamental science and industrial applications. Time-domain thermoreflectance (TDTR) is a powerful technique for thermal characterizations under nano-to-micro-scales. However, both the kNFs and the interface thermal resistance between layers are generally unknown parameters when analyzing TDTR signals. So that an effective thermal resistance model (ETRM) is often utilized, where the impact of heat capacity of NFs, cNFs, has been always ignored. Previous studies have proposed viewpoints on how to validate this assumption, however, which still needs to be verified further by considering the parameters' sensitivities for TDTR signals. In this work, we have highlighted the significance of sensitivity analysis for the investigation of the impact of cNFs with the example experiments on Ni NFs. The error of ETRM has been quantitatively and systematically studied. We found that the application requirements of ETRM in TDTR data analysis rely on the sensitivity relations between kNFs and interface thermal resistance. Finally, suggestions for future characterization of the kNFs have been discussed for reference. Our results and conclusions deepen the understanding of both the transient thermal transport process of multilayer NFs and the ability of TDTR on characterizing kNFs. [ABSTRACT FROM AUTHOR]

Recently, cubic boron arsenide (c-BAs) has attracted global attention due to its higher lattice thermal conductivity (κ L), which is comparable to diamond, and excellent thermal properties. Can c-BAs achieve the leap in κ L after transforming its structure from three-dimensional (3D) to two-dimensional (2D) like diamond to graphene? Previous studies have only investigated the κ L considering three-phonon scattering and isotope scattering, and the calculated results are diverse. In this study, we first calculate second-order interatomic force constants (IFCs) and third-order IFCs to iteratively solve the Boltzmann transport equation (BTE) and to obtain the κ L 3 of monolayer hexagonal BX (X = P, As, Sb), h-BX (X = P, As, Sb), considering only three-phonon and isotope scattering. The corresponding κ L 3 of h-BX are 278.2, 205.7, and 20.2 W/mK at room temperature, and we explain the monotonous change that κ L 3 decreases with the increase of average atomic mass (mavg) different from previous studies. Subsequently we use regular residual analysis (RRA) to determine the necessity of including four-phonon scattering when calculating the κ L of monolayer h-BX. By calculating the fourth-order IFCs, we obtain the κ L 3 + 4 of monolayer h-BX including four-phonon scattering. The values of κ L 3 + 4 at room temperature are 61.12, 37.99, and 5.73 W/mK, which are highly consistent with the κ L ∞ of monolayer h-BX as predicted by the phonon spectral energy density (SED) method. The phonon SED method considers all-order scattering and gives values of 54.05 ± 21.48 W/mK (h-BP), 18.20 ± 4.47 W/mK (h-BAs), and 2.46 ± 0.34 W/mK (h-BSb), respectively. Our results show that the influence of four-phonon scattering on the κ L of monolayer h-BX is significant, and the κ L 3 + 4 and κ L ∞ still undergo monotonic changes after including four-phonon scattering. The main factors that determine the low (ultra-low) κ L of monolayer h-BAs (h-BSb) are large mavg and weaker bonding strength, the existence of intermediate frequency ZO and scattered acoustic branches, the strong anharmonicity caused by the in-plane vibrations of As (Sb) atoms, and four-phonon scattering. This study aims to end the variance within monolayer h-BAs κ L numerical simulation and demonstrate the potential of monolayer h-BSb in thermoelectric field applications. [ABSTRACT FROM AUTHOR]

The investigation of materials' behavior under high-temperature and high-pressure conditions, such as the correlation with structural characteristics and thermal properties, holds significant importance. However, the challenges associated with the experimental implementation have, to a certain extent, constrained such research endeavors. We utilized the ultrafast laser based non-contact thermal measurement method combined with an externally heated moissanite-anvil-cell to characterize the thermal conductivity of [10-10] oriented SiO2 crystals under high temperature (300–830 K) and high pressure (0–15 GPa) conditions. We investigated the impact of extreme conditions on the microstructure from both Raman spectroscopy and thermal perspectives. The presence of kinetic hindrances associated with the transformation of α-quartz to coesite and stishovite was identified and confirmed. It expands the comprehension and application of the SiO2 pressure–temperature phase diagram in this range and provides insights into the intricate relationship between extreme environments and material structure formation through the analysis of thermal characteristics. [ABSTRACT FROM AUTHOR]

The thermal conductivities along the basal and hexagonal directions of α -quartz silica, the low-temperature form of crystalline SiO2, are predicted from the solution of the Boltzmann transport equation combined with the van Beest, Kramer, and van Santen potential for the temperature up to 900 K and the pressure as high as 4 GPa. The thermal conductivities at atmospheric pressure, which show a negative and nonlinear dependence on temperature, are in reasonable agreement with the experimental data. The influence of pressure on thermal conductivity is positive and linear. The pressure (P) and temperature (T) dependences of the thermal conductivity (λ) in basal and hexagonal directions are fitted to a function of the form λ = (b + c P) T a. The thermal conductivity, influenced by temperature and pressure, is analyzed based on phonon properties, including spectral thermal conductivity, dispersion relation, phonon density of states, phonon lifetime, and phonon probability density distribution function. [ABSTRACT FROM AUTHOR]

The application of graphene in high-performance thermal management has received a lot of attention in recent years, which still needs further exploration and development. Here, based on first-principles calculations, the thermal transport is found to be efficiently modulated by enhancing interlayer interactions in bilayer graphene (BLG), showing a different trend compared to graphite. The results of our work suggest that, by enhancing the interlayer force, the "in-plane" anharmonic phonon transport of BLG while the "out-of-plane" harmonic phonon transport of graphite can be effectively tuned. By manipulating interlayer interactions, a controllable and directed parameter (6% out-of-plane compressing deformation of BLG can achieve more than 25% decrement of in-plane thermal conductivity; 10% out-of-plane compressing deformation of graphite can increase out-of-plane thermal conductivity by more than 5 times) for tuning the thermal conductivity can be achieved. The difference in the effect of the interlayer force on thermal conductivity for low-dimensional and bulk materials emphasizes the significance of the anharmonic phonon transport properties of low-dimensional materials with interlayer interaction and thereby provides an important insight for promoting the future application of bilayer graphene and graphite. [ABSTRACT FROM AUTHOR]

Pressure is an effective way to regulate physical properties of ABO3 perovskites, such as thermal conductivity κ of SrTiO3, which can enhance fundamental understanding of structure–property relationships. In this Letter, κ of SrTiO3 was investigated up to ∼20 GPa using high-pressure time domain thermoreflectance together with Raman spectroscopy and first-principles calculations. Our theoretical predictions effectively explain the measured results. In both cubic and tetragonal phases, κ increased with compression, and optical phonons are the dominant heat carriers. The phonon group velocity and relaxation time make prominent contributions to κ with compression in the cubic phase, while the reduction in the anharmonicity of phonon modes and the phonon scattering channels dominates the increase in κ in the tetragonal phase. Especially, during the transition from cubic to tetragonal phases, there is a significant drop in κ, which originally results from the TiO6 octahedral distortion induced by the soft-phonon-mode, which markedly reduces the phonon group velocity. Our results not only help reveal the pressure effect on κ of complex oxides but also pave their way for applications on high-temperature superconductors and spin devices. [ABSTRACT FROM AUTHOR]

The thickness of the metal-transducer nano-film is an essential parameter for high-pressure time-domain-thermoreflectance (TDTR) measurements. In this article, an accurate method was proposed to determine the transducer thickness in high-pressure conditions using the pressure–volume equation of state combined with an image processing method. Both the elastic and plastic deformation of the sample squeezed in diamond anvil cells were considered in this method. High-pressure TDTR measurements on thermal conductivity of MgO and mica were further taken for comparing the influence from different thickness-characterization methods up to ∼18 GPa, and the proposed method accurately captured the effect of plastic deformation on thermal conductivity for the first time. This work can not only help achieve more accurate TDTR measurements under high-pressure but also provide valuable guidance for the diamond anvil cell application in nanoscale research. [ABSTRACT FROM AUTHOR]

As one of the fundamental problems in fluid mechanics, the flow-induced vibration of a flexible cylinder helps shed light on various complex fluid–structure interaction phenomena, such as the coupling effect of the cross-flow and in-line motions in the relationship between external fluid forces and vortical wake patterns. This paper devised a non-uniform in-flow condition (partially uniform flow and partially still water) for flexible cylinders in experimental and numerical conditions. Consistently, a new phenomenon is observed in different scale experiments and simulations where secondary traveling wave vibration of the flexible cylinder is excited from the still water part to the uniform inflow part due to the positive external fluid energy input in the still water. Furthermore, the detailed flow visualization on the vortical wake patterns around the vibrating flexible cylinders reveals that the external fluid force sources in the still water are due to the existence of the attached vortex pair with an uneven strength, which has been observed before in the still rigid cylinder in the oscillatory flow or the rigid cylinder oscillating in the still water at Keulegan–Carpenter number from 4 to 7. [ABSTRACT FROM AUTHOR]

The detailed knowledge of the low thermal conductivity of host–guest compounds is essential to improve our fundamental understanding of heat conduction in complex solids and develop high-performance thermoelectric materials. In this Letter, the intrinsic ultralow thermal conductivity (0.44 ± 0.06 W m − 1 K − 1 in 140–190 K) of the tetrahydrofuran (THF) clathrate hydrate is characterized by the time-domain thermoreflectance technique. The underlying heat conduction mechanism is further investigated by non-equilibrium molecular dynamics simulations. We find that trapped THF molecules do harmonic motions and behave as parts of a crystalline structure, thus playing negligible roles in thermal conductivity reduction. The large unit cell and complex cage-like host structure dominate the low thermal conductivity of the THF hydrate. [ABSTRACT FROM AUTHOR]

Non-equilibrium molecular dynamics simulations were performed to investigate the thermal rectification effect of a system composed of a 400 nm long horizontally aligned single-walled (10, 10) carbon nanotube (CNT) and Si substrate. By imposing a series of positive and negative heat currents across the interface, a thermal rectification effect was observed. The maximum thermal rectification is about 184% when the interfacial heat flux is 60 W/m, which is very promising for thermal rectifier applications. By phonon-related analysis, we found that for heat flowing from Si to CNT, the increase of the interfacial thermal conductance with heat flux is due to the better match of phonon density of states between CNT and Si substrate at broad moderate frequencies, while for heat flowing from CNT to Si, the low-frequency phonon modes excited at large heat fluxes dominate the interfacial heat transfer and such low-frequency phonon mode mechanism is responsible for the thermal rectification effect. Moreover, we proposed a simple yet very useful method to quantify the directional contributions of lattice vibrations to the total interfacial heat flux and we demonstrated that the out-of-plane lattice vibrations at the interface dominate the heat transfer across the silicon/horizontally aligned carbon nanotube interfaces. [ABSTRACT FROM AUTHOR]

The investigation of materials' behavior under high-temperature and high-pressure conditions, such as the correlation with structural characteristics and thermal properties, holds significant importance. However, the challenges associated with the experimental implementation have, to a certain extent, constrained such research endeavors. We utilized the ultrafast laser based non-contact thermal measurement method combined with an externally heated moissanite-anvil-cell to characterize the thermal conductivity of [10-10] oriented SiO2 crystals under high temperature (300-830 K) and high pressure (0-15 GPa) conditions. We investigated the impact of extreme conditions on the microstructure from both Raman spectroscopy and thermal perspectives. The presence of kinetic hindrances associated with the transformation of α-quartz to coesite and stishovite was identified and confirmed. It expands the comprehension and application of the SiO2 pressure-temperature phase diagram in this range and provides insights into the intricate relationship between extreme environments and material structure formation through the analysis of thermal characteristics., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)