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• The influence of cross-plane strain on the thermal conductivity and phonon characteristics of the GaN lattice is studied. • The thermal conductivity of GaN at room temperature is 257 and 275 W/(m·K) for in-plane and cross-plane directions. • The high-frequency optical acoustic branch was changed by cross-plane strain significantly. With the development of power devices towards high integration and power, the electrical and thermal stresses per unit area become more concentrated and intensified. The impact of cross-plane strain resulting from inverse piezoelectric effect and thermal expansion on power devices cannot be ignored. Cross-plane strain has a substantial influence on the thermal properties of GaN. However, the research on the influence of cross-plane strain on the thermal conductivity of GaN has not been reported in the literature. Based on the first-principles calculation method and the phonon Boltzmann transport equation, the influence of cross-plane strain on the thermal conductivity and phonon characteristics of the GaN lattice is systematically studied in this study. The thermal conductivity of GaN has anisotropy and increases significantly with the decrease in temperature. The thermal conductivity of GaN at room temperature under the free state is calculated to be 257 and 275 W/(m·K) for in-plane (k ⊥) and cross-plane (k ∥) directions, respectively. Compared with previous theoretical reports, our calculation results are more consistent with the existing experiment values. Under the state of cross-plane strain, the lattice thermal conductivity changes remarkably. In detail, the average thermal conductivity at room temperature decreased by 35 % under a 5 % cross-plane tensile strain state, while it increased by 11.5 % under a 5 % cross-plane compressive strain state. According to the calculation results, the influence of cross-plane on lattice thermal conductivity is mainly due to the change in phonon lifetime. The analysis of two mechanisms of phonon lifetime suppression indicates that the cross-plane strain will significantly change the frequency of the high-frequency optical acoustic branch. This result leads to a change in the phonon scattering process and thus affects the phonon lifetime. Besides, the anisotropy of thermal conductivity changes under different strain values, which may be due to the weakened piezoelectric polarization effect induced by strain. [ABSTRACT FROM AUTHOR]

• Performance of ten density functionals on graphite are benchmarked. • Non-local density functionals are needed to predict thermal properties of graphite. • A-axis thermal expansion is negatively correlated with c-axis thermal conductivity. Graphite is a typical layered material, exhibiting many exceptional thermal properties, such as large ratio of thermal conductivity anisotropy and orientation dependent thermal expansion. These features are largely attributed to the weak interlayer van der Waals interaction and strong intralayer covalent bonds. To accurately predict the thermal properties of graphite and other layered materials, it is essential to correctly describe the interlayer van der Waals interaction. Here, we evaluate the performance of different treatments of van der Waals interaction in first-principles calculations by examining the thermal expansion behavior and thermal conductivity of graphite. We show that non-local van der Waals density functional is essential to correctly predict the thermal expansion coefficients and phonon dispersion, while the local-density approximation substantially overestimates the interlayer anharmonicity. Furthermore, the correlation between basal-plane thermal expansion and through-plane thermal conductivity is uncovered, which originates from the anharmonicity of interlayer bonds. The data presented here could serve as a benchmark for van der Waals density functionals in first-principles calculations and this work paves the road to fully understand the thermal transport in van der Waals materials with highly anisotropic vibrational properties. [ABSTRACT FROM AUTHOR]

• An H-type measurement device was used for the thermophysical characterization of single microparticles. • The interfacial thermal resistance between single microparticles was analyzed by analytical thermal resistance model. • The effects of temperature and size on the heat transfer performance of microparticles were studied. • The uncertainty of the whole experimental system is about 4.4 %. With the incorporation of microparticles in composite materials becoming increasingly widespread, it is important to study the effect of the thermophysical properties and thermal contact between individual microparticles on heat transfer in composite materials. However, studies on the heat transfer performance of single microparticles are scarce, mainly because the micrometre scale size makes the measurement difficult. Furthermore, the area of contact between microparticles is smaller than the micrometre scale, further enhancing the difficulty of heat measurements. In this study, an H-type measurement device was developed for the thermophysical characterization of single microparticles. Through experimental and theoretical analyses, we obtained the thermal conductivity and thermal resistance of single microparticles of silicon dioxide, polyethylene (PE), poly ether ketone (PEEK) and polymethyl methacrylate (PMMA), apart from the interfacial thermal resistance between two PMMA particles. Furthermore, effective experimental methods were also developed for the study of the heat transfer mechanism of composite materials at the microscopic level. [ABSTRACT FROM AUTHOR]

• Influence of natural convection on the experimental characterization of liquid thermal conductivity of octadecane. • Inverse method based on heat flux measurements to characterize the thermal conductivity and specific heat capacity of the PCM at liquid state. • A comparison of the Nusselt numbers estimated from combining the numerical and experimental results with the known correlations for the different experimental conditions was carried out. This paper numerically makes use of the inverse method coupled with experimenal results obtained through the fluxmetric method in the Laboratoire de Génie Civil et géo-Environnement (LGCgE) to determine the specific heat, thermal conductivity and thermal expansion coefficient of liquid n -octadecane in an enclosure accounting for natural convection present in the experimental system. The impact of ommiting the presence of natural convection for the thermophysical propertie determination is studied and shows very little effect on the specific heat value but a large error (by a factor of two in this study) on the thermal conductivity value. The correct properties identified when accounting for natural convection (ρ = 2150 J/kg‧K; k = 0.16 W/m‧K; β = 9.38 × 10−4 K−1) are consistent with the literature. Additional direct numerical work was done to determine the impact of using the different thermal conductivity values, comparing numerical to additional expermental restuls where natural convection was stronger. The thermal conductivity value played a minial role when simulating the behavior of the systems rightly accounting for natural convection. Combining the numerical and experimental resutls allowed the calculation of encolsure specific Nusselt numbers for the various experimental conditions used and a comparison to known correlations. It was determine that the Nusselt number obtained in the liquid n -octadecane were larger by 10 to 48%. [ABSTRACT FROM AUTHOR]

In this study, the effective thermal conductivity of materials based on a porous TPMS structure is investigated. The resulting structure is similar to a matrix and consists of identical pore cells that are strictly periodic in all directions. These pores have several characteristic sizes, and by altering them, materials with predictable effective thermal conductivity can be created. This work investigated materials based on Triply Periodic Minimal Surface (TPMS) of Schwarz P, Schoen IWP and Neovius. The calculation of thermophysical properties was performed using the numerical finite element method in the Ansys software package. Heat transfer in the structures was studied both with and without considering air. The empirical coefficient "n" for the Aivazov–Domashnev formula is expressed analytically. This relationship allows for the creation of materials with predictable thermophysical properties, taking into account the thermal conductivity of non-porous materials. The results of numerical modeling are validated by analytical Austin's models and experimental data. The samples for the experimental study were made using additive technologies from PETG, ABS, PLA plastic and photopolymer resin. The study has scientific and applied significance, since a method is proposed for predicting the thermal conductivity of materials with an ordered structure. Several solid-state samples of elementary cells were made publicly available for designing materials with predictable thermal conductivity. The novelty of the work lies in obtaining the dependence of effective thermal conductivity on the geometric parameters of the ordered structure for materials with thermal conductivity in a wide range from 0.12 to 400 W / (m ° C). • The methodology for determining thermal conductivity of TPMS structure is presented. • A study on the thermal conductivity of TPMS structures with air was conducted. • Porous structures of the types Schwarz' P, Schoen IWP, and Neovius were considered. [ABSTRACT FROM AUTHOR]

• Calculation of cross-scale anisotropic thermal transport in carbon fiber composites. • Use of the dual coordinate system to describe thermal conductivity anisotropy. • The mechanism of interface rotation affecting interfacial thermal resistance. • The heat transport properties of fiber tortuosity and orientation within the resin substrate were investigated. The thermal conductivity parameter of materials is a fundamental factor for thermal management and the study of heat transfer processes. In the present work, we constructed the microscopic topology and interfacial structure of the fiber structure based on the homogenization of finite elements and molecular dynamics theory. For the anisotropy of fiber thermal conductivity, we proposed an anisotropic cross-scale heat transport model for carbon fiber/epoxy resin (CF/ER) composites, with consideration of interfacial thermal resistance (ITR). Based on the numerical validation of the reliability of the method, we established a dual coordinate system to analyze the mechanism of the geometric topological parameters on the structural anisotropic heat transport. The fiber volume fraction significantly affected the effective thermal conductivity (ETC) within a range of 10% to 15%, with ETC growth rates of up to 50%. The fiber orientation angle influenced the main heat transfer direction, and when the fiber volume fraction was 40%, increasing the fiber orientation angle to 90° could enhance the ETC in the axial direction up to 130.28%. Meanwhile, the larger orientation angle could reduce the interfacial phonon heat dissipation and achieve the better phonon transmission effect. The change of fiber structure could effectively modulate the ETC, while the fiber length and cross-sectional shape would have less effect on the ETC. With the introduction of fiber tortuosity, the heat transport could be regulated by changing the tortuosity. Within the variation range of tortuosity from 1 to 3, the ETC regulation of the composite could be achieved from 12% to 42% in the axial direction and 14% to 34% in the radial direction. With increasing curvature, the ETC variation trend became gradually nonlinear. The study serves as a theoretical basis for the preparation and thermal properties evaluation of fiber-reinforced composites. [ABSTRACT FROM AUTHOR]

The emerging aerogel fibers, which combine the high porosity of aerogel materials with the softness and weavability of fibers, demonstrate significant potential for the next-generation material for wearable thermal protection. However, there is currently a lack of fundamental understanding regarding their thermal insulation performance. In this work, aramid nanofibers-derived aerogel fibers (ANAFs), which exhibit high strength and excellent thermal insulation properties, are adopted as model aerogel materials for the numerical calculation of thermal conductivity. ANAFs are woven into aerogel fabrics with plain, twill, and satin structures, and their three-dimensional structural models are established based on fabric structure parameters. Building upon this foundation, a relationship model between thermal conductivity of ANAFs and thermal conductivity of aramid aerogel fabrics is proposed by series-parallel hybrid model, which is derived from thermal resistance theory. The model comprehensively considers the effects of thermal radiation and conduction. By an iterative approximation strategy, the thermal conductivity of ANAFs is calculated as 0.0345 W / mK. During the calculation process, the fabric structure plays a crucial role in determining the volume fraction of different components in the model, thereby affecting the calculation of ANAFs thermal conductivity. Furthermore, prediction models for the thermal conductivity of twill and satin weave fabrics are constructed to verify the accuracy of the calculated thermal conductivity of ANAFs. The results derived from the prediction models proposed in this work demonstrate a deviation of <5 % when compared to experimental measurements. This work provides a convenient and feasible method for calculating the thermal conductivity of aerogel fibers, while also offering a viable approach for optimizing fabric structural parameters to enhance their thermal insulation performance. [ABSTRACT FROM AUTHOR]

• A neuro-evolution potential model of water filling CNT is developed. • The thermal conductivity of CNT is greatly reduced after filling water molecules. • Anomalous temperature dependence is observed after quantum correction. • Size effect of thermal conductivity is thoroughly investigated. Understanding the behaviors of filling water in narrow carbon nanotubes (CNTs) is critical for a wide range of emerging technologies, including energy conversion and water desalination. Further probing the vibrational coupling between CNT and water contributes to investigating the corresponding thermal properties. However, classical molecular dynamics (MD) fails to exactly characterize the complex CNT-Water interactions due to the inaccuracy of empirical potentials. To cope with this, we develop an accurate and highly efficient neuro-evolution potential (NEP) model for CNT-Water systems and perform extensive MD simulations to investigate the effect of filling water on CNT thermal conductivity. The trained NEP obtains a force accuracy of 69.43 meV/Å and an atomic energy precision of 0.785 meV/atom. Anticipatedly, the structural and vibrational characteristics of the NEP-driven MD simulations show an excellent agreement with ab initio MD simulations. We computationally prove that the thermal conductivities of CNT filled with water are reduced by 23 % and 28 % for different filling ratios, which lies in the transport blockage of the phonon at low frequency. We further find that water filling has less effect on the ultra-long mean free path (MFP). The maximum MFP of empty CNT is 42.94 μm, and the CNT filled with water can be as large as 39.29 μm. Consequently, the thermal conductivity of CNT filled with water is firmly size dependent, accompanied by a rise in thermal conductivity from 88.37 to 436.96 W/m∙K as the size goes from 107.0 Å to 2148.0 Å. Besides, the thermal conductivity presents a different temperature dependence before and after the quantum correction due to the overestimation of the heat capacity. This work sheds light on precise assessment and detailed insights into the thermal properties of CNT filling water, which would considerably affect the following areas including electronic devices cooling, wastewater treatment and adsorption-driven steam power. An atomic-level view of water molecules filling in a small-diameter carbon nanotube (CNT). Water filling could influence phonons transport in CNT, which enables the thermal management for relevant CNT-based devices. [Display omitted] [ABSTRACT FROM AUTHOR]

• Thermal conductivity of a MOF (HKUST-1) was investigated by EMD. • A new method to truncate heat current autocorrelation function of the Green-Kubo method was devised. • The influence of Ar molecules on decomposed thermal conductivity of the MOF was investigated. • The contribution of the interaction of the atoms was dominant to the thermal conductivity. Metal-organic frameworks (MOFs) are promising porous materials for various applications, which have attracted considerable attention recently. Optimizing their thermal properties, such as their heat accumulation characteristics during the adsorption of fluid molecules, is crucial for enhancing their performance. Molecular dynamics studies are vital for meticulously understanding the thermal transport of MOFs. To determine the thermal conductivity of MOFs using the Green–Kubo formula through equilibrium molecular dynamics (EMD), the heat current autocorrelation function (HCACF) must be truncated at a specific time. However, few studies have been conducted on establishing a method for truncating HCACFs for MOFs, rendering it challenging to assess the reliability of MOF thermal conductivities calculated through EMD. In addition, previous studies focused on the overall thermal conductivity, and the contribution of the components have not been identified. In this study, we devised a method for determining the truncation time of an HCACF to calculate the thermal conductivity of MOFs. After confirming the validity of the method, we decomposed the instantaneous heat flux into individual components and obtained the decomposed thermal conductivities. The results revealed that the contribution of the interaction of atoms was more pronounced than that of the microscopic convection of atoms. In addition, the method introduces an ambiguity while calculating the thermal conductivity of MOFs through EMD. The proposed method is useful for truncating of HCACFs and determining the thermal conductivity of complex materials such as MOFs. [ABSTRACT FROM AUTHOR]

The thermal transport properties are very significant for the performance, reliability, and service life of nano-electronic devices. However, effectively measuring the thermal transport properties of nano-metal films is a challenge job, especially for the interfacial thermal conductance between nano-metal films and substrate with low thermal diffusivity. In this study, we improve the traditional time-domain thermoreflectance (TDTR) technique to simultaneously measure cross-plane thermal conductivity and interface thermal conductance of film/substrate assembly with an improved measurement sensitivity. By capturing the reflected beams from the film side and substrate side, and fitting the captured signals with thermal transport models, we simultaneously measure the cross-plane thermal conductivity of Al film, Cr film and Si O 2 substrate, as well as the interfacial thermal conductance of Al/Cr and Cr/ Si O 2 interfaces. The experimental results show that the interfacial thermal conductance of Al/Cr interface is much larger than that of Cr/ Si O 2 interface, and the cross-plane thermal conductivity of Al film and Cr film is much smaller than their corresponding bulk value. Furthermore, we use the Monte Carlo method to analyze the uncertainty of measured parameters and obtain the corresponding error range. • Propose a both-side incidence configuration to improve the traditional TDTR. • Measure the thermal conductivity and interface conductance of films thoroughly. • Enhance accuracy by simultaneously fitting a theoretical model to both side signals. • Use Monte Carlo simulation to demonstrate the low uncertainty of the proposed method. • Verify the ability of the proposed method on a low thermal diffusivity substrate. [ABSTRACT FROM AUTHOR]

• Analyze the variation of heat transfer as a function of diameter and temperature. • Investigate heat transfer mechanism by crucial direct parameters. • Explore internal mechanism by phonon density of state and phonon overlap energy. Carbyne is a sp1 linear carbon chain (LCC) with characteristics of strong chemical reactivity and extreme instability under environmental conditions, which has become an obstacle to the application of LCC with high thermal and electrical conductivity. Utilizing CNT as a container to store LCC is one of the most feasible ways to realize LCC applications. However, current research of LCC encapsulated in CNT (LCC@CNT) focuses on its preparation and lacks of in-depth exploration of the intrinsic heat transfer mechanism. This paper reports the high axis thermal conductivity of LCC@CNT, explore the thermal transport enhancement degree of LCC@CNT under a wide temperature range and multiple diameters, and analyzed the underling mechanism through crucial thermal parameters and phonon theory. When LCC is loaded in CNT, the axial thermal conductivity of CNT is further increased by 15 %. Moreover, LCC@CNT shows the lower sensitivity diameter than pristine CNT. This work is expected to provide data support for the application of LCC, the lower production standard of CNT, and also attempt the higher efficient one-dimensional thermal energy transfer micro devices. [Display omitted] [ABSTRACT FROM AUTHOR]

Materials with complex inner structure can be challenging to characterise in an effective way. The effective material parameters significantly depend on the structure, and thus on the interface properties. In the present paper, we focus on metal matrix syntactic foams, and we attempt to determine their effective thermal parameters. We supplement the results with their mechanical properties as well. For thermal characterisation, we use the heat pulse experiment to study their transient thermal response. We observed deviation from Fourier's law in multiple situations, and thus we propose an evaluation procedure for such experimental data using the Guyer–Krumhansl heat equation. We provide an iterative procedure to efficiently find the effective thermal diffusivity values provided by the Guyer–Krumhansl and Fourier equations based on the measured transient response. Furthermore, we studied the effective parameters, the specific heat, mass density, thermal conductivity and thermal diffusivity. We found that the standard estimations significantly overestimate the thermal conductivity. Furthermore, we propose a method for deducing a more reliable thermal diffusivity and thermal conductivity based on the transient temperature history, i.e., we propose to estimate Fourier's thermal diffusivity based on averaging the parameters of the Guyer-Krumhansl equation according to the observed diffusive time scales. We also found that for such metal matrix syntactic foams, even with knowing the constituents, the contact thermal resistance makes more challenging to provide a prior estimate for the effective thermal conductivity, but this difficulty can be resolved using proper effective thermal diffusivity values. • Proposing an iterative evaluation procedure for the Guyer-Krumhansl equation. • Determining the effective thermal parameters of metal matrix synthetic foams. • Correlating the Fourier and Guyer-Krumhansl parameters. • Comparing the effective thermal conductivity estimations to the measured one. [ABSTRACT FROM AUTHOR]

• Internal structure of the composite material is adjusted by AC electric field. • The high in-plane thermal conductivity of h-BN is fully exploited. • New thermal conductivity model on h-BN orientation was established. The excellent electrical insulation properties and mechanical properties of epoxy resin (EP) have led to its rapid development in the field of microelectronic packaging. However, the poor thermal conductivity of epoxy resins (thermal conductivity of only 0.14 W/(m·K)) limits the development of components for high power. Adding h-BN to epoxy resin to prepare composites can effectively improve the thermal conductivity of epoxy resin. In this paper, h-BN/EP high thermal conductivity composites were prepared by means of AC electric field induction with the goal of making the composites low loading and high thermal conductivity (loading of 10 wt%). It was shown that the h-BN/EP composite had the highest thermal conductivity of 0.310 W/(m·K) when applied to an AC electric field of 120 V/mm and 50 Hz for 1 h. It was 2.214 times higher than that of EP and 1.325 times higher than that of the same-loaded h-BN/EP composite (0.234 W/(m·K)). A thermal conductivity model that can predict the thermal conductivity of composites prepared using the same type of method is also developed, which extends the applicability of the thermal conductivity model. The prepared composites have good thermal management capability. The method is easy to be engineered and has great potential for use in electronic packaging and even other high thermal conductivity applications. [ABSTRACT FROM AUTHOR]

Carbon phosphide is a newly discovered two-dimensional semiconductor material which wrinkles and has a significant carrier mobility. Due to lack an accurate force field, the use of molecular dynamics to study its phonon-dominated thermal conductivity which lead to inaccurate results. At present, the use of machine learning to construct a high-precision force field has become the mainstream research method to solve this problem. The main work of this study is to construct a comprehensive training sets for Phosphorus-Doped Graphene (PC n) (n = 3, 5, 6) and to use the fitted potential to calculate the related thermal properties. The research found that (PC 5) exhibited anisotropic behavior, with a thermal conductivity of 106.6 Wm−1 K−1 in the y-direction and 63.6 Wm−1 K−1 in the x-direction. In comparison, (PC 6) and (PC 3) showed isotropic behavior, with thermal conductivity of approximately 104 Wm−1 K−1 and 76.83 Wm−1 K−1, respectively. Compared to monolayer graphene, the lower thermal conductivity of PC n is mainly attributed to phonon-phonon scattering effects, which are limited by the regular wrinkled structure. Additionally, low-frequency phonon have been found to have a significant impact on the thermal performance of PC n. Furthermore, we investigated the influence of uniaxial strain on the PC 6 and observed an increase in the thermal conductivity with increasing strain. This study used key computational and analytical techniques, including phonon dispersion relations, homogeneous nonequilibrium molecular dynamics method, spectral thermal conductivity analysis. These findings provide a theoretical basis for understanding the thermal transport properties of PC n and will guide its potential applications value. [ABSTRACT FROM AUTHOR]

• Thermal transport through SEI of Li-ion batteries was studied using MD simulations. • Three representative inorganic components of SEI were considered. • Impact of structural properties and temperatures were investigated. • Thermal conductivity of amorphous compounds is closely related to the components. • Results provide valuable insights for the design of electrolytes and SEI. The thermal behavior during operation of Lithium-ion batteries (LIBs) is widely concerned with respect to their electrochemical performance and safety. The solid electrolyte interphase (SEI) is a critical layer formed during electrochemical reactions in LIBs. A thorough understanding of SEI's thermal transport properties is essential to identify limitations within the internal heat transfer of LIBs. In this work, a computational study of the thermal transport through SEI was performed based on classical molecular dynamics (MD) simulations. Three representative inorganic components of SEI, namely Li 2 CO 3 , Li 2 O and LiF, were explored. First, the force fields were evaluated for accurate MD simulations. Subsequently, the impact of structural properties and temperatures on the thermal conductivities of SEI were investigated, followed by discussion on radial distribution functions and vibrational density of states. It was found that the comparison of thermal conductivity of the ordered crystals is Li 2 O > LiF > Li 2 CO 3. As temperature increases, the thermal conductivity of inorganic components decreases significantly. Additionally, it was discovered that the thermal conductivity of amorphous compounds is notably lower than that of ideal crystals and is closely related to the molar ratio of inorganic components. The results of this work can help understand the thermal transport properties of SEI and offer valuable insights for the design of electrolytes and SEI toward improving the thermal safety performance of LIBs. [ABSTRACT FROM AUTHOR]

• Despite relatively weak hydrogen bonds, tetracarboxylic-based frameworks (TCFs), a three-dimensional (3D) HOF, show relatively high thermal conductivity. • Electrostatic interactions in TCFs dominate thermal conductivity and anisotropy but central atoms in the monomers show negligible impact. • Electrostatic interactions sustain phonon modes with frequencies of 5–25 THz to carry heat and are key to maintaining high crystallinity in TCFs. Hydrogen-bonded organic frameworks (HOFs), an emerging class of porous molecular materials assembled through hydrogen bonds (H-bonds), have shown intriguing latent for multifunctional materials or devices. Understanding thermal transport in HOFs is crucial to guiding the functional material design with desired thermal properties. Here, we report thermal transport properties in HOFs and reveal the significance of electrostatic interactions (H-bonds and π-π interactions) to heat conduction with molecular dynamics simulations. It is found that despite relatively weak H-bonds, tetracarboxylic-based frameworks (TCFs), a three-dimensional (3D) HOF, show relatively high thermal conductivity compared with known 3D porous covalent organic frameworks and metal-organic frameworks. Electrostatic interactions in TCFs dominate thermal conductivity and anisotropy but central atoms in the monomers show negligible impact. The structural analysis and x-ray diffraction patterns highlight the importance of electrostatic interactions in maintaining the high crystallinity of TCFs. The vibrational density of states and spectral thermal conductivity further demonstrate that phonon modes with frequencies below 25 THz contribute about 90 % of total thermal conductivity in TCFs, and electrostatic interactions sustain phonon modes with frequencies of 5–25 THz to carry heat. These findings provide a fundamental understanding of structure-thermal conductivity relation in HOFs and advance their applications for multifunctional materials and devices. [ABSTRACT FROM AUTHOR]

• The study employs topological data analysis (TDA) to identify and analyze atomic networks in amorphous materials, establishing meaningful connections to their thermal properties. • The observed enhancement in thermal conductivity is attributed to the presence of larger atomic rings. This enhancement is explained by the longer mean free paths (MFPs) and lower frequencies associated with these larger atomic rings. • The study emphasizes that changes in atomic networks, influenced by deposition temperature, play a pivotal role in determining the thermal conductivity of a-Ge. • This structural characterization technique is not limited to the specific material analyzed here but has broader applicability to various amorphous materials. Compared to crystalline materials, amorphous materials lack periodicity and exhibit distinct thermal and lattice vibration properties. Hence, analyzing atomic networks in the transmission electron microscopy (TEM) images of amorphous materials is challenging. In this study, we employ topological data analysis (TDA) and principal component analysis (PCA) to extract structural features from the TEM images of amorphous germanium (a-Ge) and analyze its atomic networks. Our findings demonstrate that the thermal conductivity of a-Ge is influenced by larger atomic rings with more vertices, which facilitates the heat transfer through longer atomic chains and results in a higher thermal conductivity. A comparison of the experimental and simulation data confirms the non-random nature of the atomic arrangements in a-Ge. We propose herein an approach that employs the TDA to identify and analyze the atomic networks in amorphous materials, establishing connections to their thermal properties. This study enhances our understanding of amorphous materials and paves a way for tailored material design and engineering to achieve the desired thermal properties. [ABSTRACT FROM AUTHOR]

• The thermal cloak in homogeneous isotropic multilayer materials is analyzed. • The influences of the layer's properties in the thermal cloak on the thermal control ability are investigated. • The temperature field of the arbitrary layout composite thermal cloak can be predicted by the deep learning model with almost no time cost. Thermal cloaks can realize thermal manipulation and have been widely applied in the engineering field. Traditional methods, like the finite element method (FEM) and finite difference method, obtain the temperature of the thermal cloak with time-consuming. A deep learning model is proposed to predict the temperature field in homogeneous isotropic multilayer materials. The FEM is applied to solve the thermal cloak and the training data are obtained. The layer's properties, including different layers, thickness, relative thermal conductivities and distribution modes, of the thermal control or manipulative ability for temperature and heat diffusion are also investigated. The deep learning mode is developed by mapping the parameters layout to the temperature field as an image-to-image regression task and the relationship between the layer's properties and the cloaking performances is intelligently established. The temperature field of the arbitrary layout composite can be acquired almost with no time cost. The temperature field with the largest node numbers of medium temperature by the FEM and the deep learning model, is exhibited and compared. Furthermore, we extend the surrogate model to different cases with different boundary conditions, which can still achieve good prediction results. Owing to the high efficiency and complete independence from the mesh densities, the proposed method offers a novel and robust technique for predicting the temperature field and suggestion for the design of the thermal cloak. [ABSTRACT FROM AUTHOR]

• Twisted MoS 2 exhibits significant thermal conductivity anisotropy (∼400). • Cross-plane thermal conductivity of MoS 2 shows strong twist-angle dependence. • In-plane thermal conductivity of MoS 2 exhibits twist-angle independence. • Registry-dependent potential accurately predicts thermal conductivities of MoS 2. • Spectral heat current analysis provides insights into thermal transport mechanisms. Thermal transport property of homogeneous twisted molybdenum disulfide (MoS 2) is investigated using non-equilibrium molecular dynamics simulations with the state-of-art force fields. The simulation results demonstrate that the cross-plane thermal conductivity strongly depends on the interfacial twist angle, while it has only a minor effect on the in-plane thermal conductivity, exhibiting a highly anisotropic nature. A frequency-decomposed phonon analysis showed that the cross-plane and in-plane thermal conductivity of MoS 2 are dominated by the phonons with frequencies below 12.5 THz and 7.5 THz, respectively. As the interfacial twist angle increases, these low-frequency phonons significantly attenuate the phonon transport across the interface, leading to impeded cross-plane thermal transport. However, the in-plane phonon transport is almost unaffected, which allows for maintaining high in-plane thermal conductivity. Furthermore, our study revealed a strong size dependence for both cross-plane and in-plane thermal conductivities due to the influence of low-frequency phonons in MoS 2. The maximum thermal conductivity anisotropy ratio is estimated as ∼400 for twisted MoS 2 from our simulation, which is in the same order of magnitude as recent experimental results (∼900). Our study highlights the potential of twist engineering as a tool for tailoring the thermal transport properties of layered materials. [ABSTRACT FROM AUTHOR]

• New approach to prediction of thermal properties from EM sounding data is proposed. • Forecasting accuracy beyond boreholes is comparable with that in the borehole scale. • Dry and wet thermal conductivity sections for Soultz geothermal area are built. • Specific heat capacity section for Soultz geothermal area is constructed. • Vertical heat flow density at the surface of Soultz geothermal area is estimated. Estimation of deep temperature, thermo-hydro-dynamic and basin simulation, heat exchange and geothermal resource assessment require knowledge of the rock thermal properties beyond boreholes. At the absence of suitable methods and instruments for such estimates we propose a new approach to prediction of thermal conductivity, specific heat capacity and heat flow density based on results of electromagnetic sounding of the study area. Artificial neural network used to this end is taught by correspondence between laboratory measurements on cores from the drilled boreholes and adjacent electrical conductivity profiles determined by inversion of the electromagnetic sounding data collected in their vicinity. This approach is used to estimate rock thermal properties along NW-SE magnetotelluric profile crossing Soultz-sous-Forêts geothermal area, Alsace, France. We demonstrate that accuracy of such prediction both in depths below the boreholes and in the interwell space is comparable with that usually achievable in the borehole scale. In particular, it is shown that relative errors of thermal conductivity prediction at depths twice as large as the borehole depth are as small as 2%, and the accuracy of cross-well thermal conductivity prediction is 6% and 10% for dry and wet rocks, respectively. On the other hand, the prediction accuracy for matrix thermal conductivity of a rock from the known profile of dry thermal conductivity is about 8%. Based on the results of our studies, we constructed dry and wet thermal conductivity sections from the electrical conductivity model determined earlier by 2-D inversion of magnetotelluric data. The temperature model of the study area built along the same profile by means of electromagnetic geothermometer and the obtained thermal conductivity sections enabled estimating the profiles of the vertical component of heat flow density at the surface from dry and wet thermal conductivity sections. They could be considered as minimum and maximum constraints, respectively, bounding the area of its possible values, which, in turn, depend on in situ thermal conductivity of fluids filling the rock pores. The range of possible vertical heat flow density variations (100–150 ± 10% mW· m − 2) agrees well with heat flow estimates in the Upper Rhine graben and, in particular, in Soultz-sous-Forêts geothermal area roughly estimated earlier from borehole measurements. Using the artificial neural network trained on the correspondence between the specific heat capacity determined in the borehole and adjacent electrical conductivity profile, we built the model of specific heat capacity. Against the background, a zone of enhanced specific heat capacity ranging between 2.05 and 2.15 MJ· m − 3· K − 1 was observed in the domain located at the base of the sedimentary cover and upper part of the granitic basement. [ABSTRACT FROM AUTHOR]

• Effective thermal diffusivity and conductivity of pebble bed are measured up to 1200 °C under vacuum and helium conditions. • Two vacuum tests and two helium tests are conducted to verify experimental repeatability and validity. • An improved method is used to retrieve results by alleviating wall effect. • Effective thermal conductivity is comparable with previous results of SANA and HTTU. Effective thermal diffusivity and conductivity are two crucial parameters to determine inherent safety in porous pebble bed of High Temperature Gas-cooled Reactor. A full-radius-scale heat test facility has been developed to measure these thermal properties at high temperature. Two vacuum tests and two atmospheric-pressure helium tests up to 1200 °C have been conducted to determine the temperature-dependent parameters by the inverse method. Repeatable results are validated by both tests with different heating processes. Furthermore, the primary experimental errors from installation positions of thermocouples as well as random packing effect are reduced by retrieving positions in the inverse method. Sensitivity and uncertainty analyses are carried out in respective tests to give the reasonable descriptions of the results of effective thermal diffusivity and conductivity in these tests. [ABSTRACT FROM AUTHOR]

• Improvement of thermal conductivity of polymer composites with segregated structure. • Use of a mixture of fillers with different shape factor (anthracite and graphene). • Modeling of composites thermal conductivity with a Lichtenecker model. This article presents the study of electrical and thermal properties of segregated polymer composites based on ultra-high-molecular-weight polyethylene (UHMWPE) filled with carbon fillers (nanofiller graphene (Gr), microfiller anthracite (A) and hybrid filler Gr/A). It is shown that the formation of a segregated structure with an ordered distribution of the filler leads to a high local concentration in the intergrain boundaries, which causes a lower percolation threshold. Thus, in the composite UHMWPE + A, the percolation threshold is an order of magnitude lower than for a system with a random distribution of the filler. The segregated composite with nanofiller UHMWPE + Gr provides a 14-fold lower percolation threshold than the composite with microfiller UHMWPE + A. Composite with the hybrid filler Gr/A also exhibits a low percolation threshold close to the UHMWPE + Gr. The plot of the thermal conductivity versus filler content does not show the percolation behavior and obeys the equation of the Lichtenecker. The thermal conductivity parameter λ f in the segregated system is 4.4 times higher than for the uniform distribution of the filler that indicates an increased thermal transport through the filler phase located at the boundaries in the segregated structure. [ABSTRACT FROM AUTHOR]

Highlights • Thermal conductivities of the H 2 O/CO 2 /H 2 and H 2 O/CO 2 mixtures in supercritical regions of water are predicted. • EMD models are established for the calculation of thermal conductivities of the mixtures. • Recommendations for the selection of force field models and simulation strategies are given. • Predicted values by EMD method and theoretical models are compared. Abstract Coal gasification technology is an important means of clean coal utilization. In the process of coal supercritical water gasification, H 2 O/CO 2 /H 2 or H 2 O/CO 2 mixtures can be produced and used as the working medium for thermodynamic cycle power generation systems. The thermal conductivity of H 2 O/CO 2 /H 2 or H 2 O/CO 2 mixtures is one of the most fundamental thermal properties required for the design and optimization of a thermodynamic system based on coal supercritical water gasification. Thus far, the thermal conductivity of H 2 O/CO 2 /H 2 , H 2 O/CO 2 , and H 2 O/H 2 mixtures in supercritical regions of water remains unknown. In this paper, the thermal conductivity of these mixtures in supercritical regions of water is predicted by equilibrium molecular dynamics (EMD) simulation and various theoretical models. The force field models and simulation strategies for the MD model are discussed and recommended. To validate the simulation method, the thermal conductivity of pure H 2 O, CO 2 , H 2 and CO 2 /H 2 mixtures is calculated by MD simulations and compared with available experimental and NIST data. The method and data provided in this article may facilitate the practical applications of coal supercritical water gasification. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Temperature responsive thermal metamaterials (TMs) comprising of tunable cells were developed. • Tunable shifters as unit cells turn on/off anisotropic heat transfer as temperature changes. • Tunable shifters were realized in layered structures of metals and phase change nanocomposites. • Modular 4-by-4 cells as one TM perform dynamic thermal shielding by temperature changes. • The potential use was demonstrated as a tunable interface of thermal dissipation-insulation. Abstract Integrated circuits or miniaturized portable electronics require adaptive thermal control under certain temperatures. Thermal metamaterials (TMs), which artificially manipulate the heat passing through mediums have shown innovative thermal functions at a continuum scale. However, they cannot implement tunable thermal functions at local spots depending on the operating temperatures. Herein, we introduce temperature-responsive TMs enabled by modular design of thermally tunable unit cells. As ambient temperature changes, tunable thermal shifters can dynamically turn on/off their intrinsic functions to guide anisotropic heat transfer through the transition of thermal conductivities from the inner phase change nanocomposites (PCNCs), and their modular design realizes temperature-responsive thermal shields having switchable functions. The layered structures of stainless steel and the PCNC of n -octadecane embedding carbon nanotubes and copper powder are fabricated as tunable thermal shifters. Their 4 × 4 modular structure confirms the feasibility of temperature-responsive TMs, verified by the disappearance and appearance of thermally shielded regimes at low- and high-temperature ranges. The potential use of the developed concept was demonstrated as tunable interfaces between thermal dissipation and insulation for protecting temperature-sensitive components. This work can offer new capabilities for conventional passive TMs, such as local thermal adaptation, active thermal control interface, and thermal disturbance mitigation. [ABSTRACT FROM AUTHOR]

Highlights • Thermal properties of concrete components can be estimated by solving reverse problem. • The thermal conductivity is most preferably measured on water soaked concrete. • The specific heat is preferably measured on concrete dried to stable mass. • The rule of mixtures can predict specific heat of concrete for insufficient data. Abstract Results of measurements of the specific heat and the thermal conductivity of concrete with blended special aggregate for neutron and gamma radiation shielding are presented. Experimental tests were performed on concrete with heavyweight aggregate (magnetite, barite), hydrogen-bearing aggregate (serpentine) and amphibolite aggregate. The thermal properties of concrete were determined using a nonstationary method. The highest specific heat was found for concrete with serpentine aggregate. Simple models for predicting the specific heat and the thermal conductivity on the basis of concrete mix design were evaluated to include the blends of heavyweight and hydrogen-bearing aggregates. The thermal conductivity of concrete was found to be linearly dependent on the concrete density in the range from 2200 to 3500 kg/m3. Its increase due to water saturation of concrete was not dependent on the open porosity of concrete. It was found that the specific heat can be fairly well predicted using the rule of mixtures formula. The thermal conductivity of concrete can be approximately predicted using a parallel model in the case of water-saturated concrete. The thermal conductivity prediction for dry concrete is also discussed. [ABSTRACT FROM AUTHOR]

Highlights • Thermal conductivity model for nanoparticle packed bed is established with nanoparticle deformation considered. • Our model give good prediction at 0.3 ≤ φ ≤ 0.9, while EMA model predicting well at φ ≥ 0.75. • R cd dominates R at 0.75 ≤ φ ≤ 0.9, while R cb preponderates at 0.3 ≤ φ ≤ 0.75. • The overestimation of k e by EMA model at 0.3 ≤ φ ≤ 0.75 arrives from the ignored R cb. Abstract Theoretically understanding the thermal conductivity of the nanoparticle packed beds (NPBs) is critical for designing high-performance thermal insulation materials. Currently, the classical effective medium assumption (EMA) model, Nan model, just show their good prediction at a high porosity of NPBs (≥0.75). Herein, we propose a generalized model of the thermal conductivity that almost covers the whole porosity range by considering the effect of the nanoparticle deformations on the thermal contact resistance (R). It has been demonstrated that our model matches the experimental results great well. It is also found that at high porosity R is dominated by the phonon diffusive scatterings ( R cd ), while it is determined by the phonon ballistic scatterings ( R cb ) at a low porosity. More interestingly, R can determine the porosity at which the lowest thermal conductivity of NPBs appears. This work opens a new way to design the desired thermal insulation materials. [ABSTRACT FROM AUTHOR]

Highlights • Thermal conductivity data collected changing temperature and compression. • Effective medium model developed to capture variation in conductivity. • Model tuned to data collected in an air environment. • Model found to be in good agreement with collected and published data sets. • Model could aid in thermal designs in operational environments. Abstract Ceramic fiber insulation materials are used in numerous applications (e.g. aerospace, fire protection, and military) for their stability and performance in extreme environments. However, the thermal properties of these materials have not been thoroughly characterized for many of the conditions that they will be exposed to, such as high temperatures, pressures, and alternate gaseous atmospheres. The resulting uncertainty in the material properties can complicate the design of systems using these materials. In this study, the thermal conductivity of two ceramic fiber insulations, Fiberfrax T-30LR laminate and 970-H paper, was measured as a function of atmospheric temperature and compression in an air environment using the transient plane source technique. Furthermore, a model is introduced to account for changes in thermal conductivity with temperature, compression, and ambient gas. The model was tuned to the collected experimental data and results are compared. The tuned model is also compared to published data sets taken in argon, helium, and hydrogen environments and agreement is discussed. [ABSTRACT FROM AUTHOR]

Highlights • New method for estimation of thermal properties of charring ablators is proposed. • Sensitivity analysis shows thermal conductivity has stronger effect on temperature. • Estimation of thermal properties in order can shorten the inversion time. • Thermal properties can be estimated by both simulations and test data. Abstract The accurate assessment of thermal properties is crucial for charring materials simulation and design. In this work, a new inversion method for estimating temperature-dependent thermal conductivity and specific heat capacity from temperature data is presented for charring ablators. Firstly, a one-dimensional thermal response model with surface recession is developed to simulate the thermal behavior of charring ablator. Then based on the developed model, sensitivity analysis is conducted to investigate the correlation between thermal parameters and temperature for determining inversion sequence and find that virgin thermal conductivity has the biggest influence on temperature which should be estimated at first. Finally, the temperature-dependent thermal conductivities and specific heat capacities are obtained by the inversion method from temperature data. The inversion values from simulation temperature data coincide with the reference values, which the average inversion error of thermal conductivities is 4.3% and the average inversion error of specific heat capacities is 3.1%. And the calculated thermal response temperature employing the inversion thermal properties shows a good agreement with the arc jet test data, which the average error is 8.5%. This inversion method can accurately determine unknown temperature-dependent thermal properties such as thermal conductivity and specific heat capacity, which provides an insight into the analysis and design of thermal protection materials for spacecraft. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Several applications for diamond nanoparticles were discussed (automotive, electronic cooling and cryosurgery industries). • The methods used for characterizing the thermal conductivity were reviewed and summarized. • The main factors affecting heat transfer behavior and thermal conductivity of the diamond nanofluids are nanoparticle size, concentration, temperature, viscosity, and preparation method. • It was determined that the amount of nanodiamond, which becomes a nanofluid in the host liquid system, has a direct relationship to thermal conductivity improvement. Abstract Nanofluids offer many opportunities to enhance the efficiency of thermal systems by improving the thermal properties of the host fluids. Due to its exceptional thermal, mechanical and electrical properties, diamond has attracted attention for use as a nanoparticle in fluids. Thereby, nanofluids have proven to be useful in applications, especially electronic cooling, where increasing thermal properties and avoiding electrical conductivity are vital. There have been numerous investigations of nanodiamond-nanofluid systems with many suggesting that considerable property enhancements, e.g., increased thermal conductivity, might be possible. However, those results have not been collectively compared or discussed. This paper reviews and summarizes the experimental values of the physical and thermal properties, including an appropriate focus on the thermal conductivity enhancement, of nanodiamond-nanofluid systems found in the literature as well as different techniques for thermal conductivity measurement. The effect of various factors on the thermal-physical properties of diamond nanofluids is also reviewed, and the stability and viscosity of diamond nanofluids are tabulated and compared. The preparation and properties of hybrid nanofluids containing diamond nanoparticles are analyzed and discussed in detail, and a list of potential uses for these enhanced heat transfer fluids in industrial and commercial applications is provided and deliberated. [ABSTRACT FROM AUTHOR]

Highlights • Details thermal properties of electric machine lamination stacks. • Unbiased data for lamination thermal properties facilitates thermal modeling. • Presents method for accurately measuring thermal behavior of lamination stacks. • Presents experimental results for four commonly used stator lamination materials. • Presents mathematical model for predicting thermal properties. Abstract There is lack of information in the open literature on thermal properties of electric machine lamination stacks such as contact resistance and effective thermal conductivity, yet this information is critical for researchers and engineers in electric machine design and development. The thermal conductivity of electromagnetic steel lamination materials was measured, and the thermal contact resistance between laminations in a stack, as well as factors affecting contact resistance between laminations - such as the contact pressure and surface finish – were investigated. A model was also developed to estimate the through-stack thermal conductivity for materials beyond those that were directly tested in this work. Four lamination materials were investigated, including the commonly-used 26-gauge and 29-gauge M19 materials, the HF10, and Arnon 7 materials. Although this paper focuses on electric machines for automotive applications, the information is potentially applicable to any component utilizing electromagnetic steel laminations. [ABSTRACT FROM AUTHOR]

Highlights • An effective thermal conductivity model is derived based on fractal theory. • Predictions of proposed model show good consistent with experimental data. • The proposed theoretical model reveals the physical basis of heat transfer in porous media. Abstract Thermal conduction in porous media has received wide attention in science and engineering in the past decades. Previous models of the effective thermal conductivity of porous media contain empirical parameters typically with ambiguous or even no physical rationales. This study proposes a theoretical model of effective thermal conductivity in porous media with various liquid saturation based on the fractal geometry theory. This theoretical model considers geometrical parameters of porous media, including porosity, liquid saturation, fractal dimensions for both the granular matrix and liquid phases, and tortuosity fractal dimension of the liquid phase. Effects of these geometrical parameters on the effective thermal conductivity of porous media are also evaluated. This proposed fractal model has been validated using published experimental data, compared with previous models, and thus provides a physics-based theoretical model that can provide insight to geoscience and thermophysics studies on thermal conduction in porous media. [ABSTRACT FROM AUTHOR]

Highlights • EMD simulations are carried out to calculate the thermal conductivities and fractal dimensions. • The thermal conductivity increases linearly with decrease of fractal dimension of aggregations. • The liquid atoms in nanolayer are “dynamically balanced”. • The explanation based on Kapitza resistance for heat transfer enhancement by nanolayer is queried. Abstract Nanofluid can enhance heat transfer due to the suspending nanoparticles. The mechanism of heat transportation by nanoparticles remains unclear so far. Aggregation of nanoparticles, one of the important mechanisms to elevate the thermal conductivity of nanofluid, was proved by not a few researches. However, the aggregation morphology of nanoparticles evaluated by fractal dimension will greatly influence the thermal conductivity of nanofluid. In this paper, equilibrium molecular dynamics simulations were carried out to calculate the thermal conductivity via Green-Kubo formula. In contemporary, fractal dimensions of the aggregations with various morphologies were obtained by Schmidt-Ott equation. Comparisons of the fractal dimension and thermal conductivity of the nanofluid with same volume fraction show us that, lower fractal dimension can deduce greater thermal conductivity. In addition, the difference of loose and compact aggregation can be read out of the pair correlation function near nanoparticles. And the solvent atoms in nanolayer are mobilized and dynamically balanced. This is helpful for us to understand the influence of aggregation morphology of nanoparticles on the thermal conductivity of nanofluid. [ABSTRACT FROM AUTHOR]

Abstract Phase change materials (PCMs) possess very high heat storage capacity and are capable of maintaining a constant temperature during phase change, which makes them most prominent candidates for solar energy storage systems, heating, and cooling systems. The low thermal conductivity of PCM results in slow heat transfer and low heat storage and release rate, which is a major drawback for their practical applications. This review focuses on the enhancement of thermal conductivity by the introduction of highly thermally conductive metallic and carbon-based nanoparticles, metallic foams, expanded graphite and encapsulation of PCM. Carbon-based nanoparticles including carbon fiber, carbon nanotubes, and graphene show better performance than metal-based nanoparticles due to lower density and better dispersion. The thermal conductivity of the composite phase change material (CPCM) depends on the shape, size, aspect ratio and concentration of nanoparticles. The thermal conductivity of CPCM increases by increasing concentration and aspect ratio of the additive. Metallic foam and expanded graphite possess high thermal conductivity and good thermo-physical properties and also prevent leakage of PCM during phase change. The porosity of foam has a huge impact on thermal conductivity than pore size. Encapsulated PCM has well-enhanced thermal conductivity and long-lifetime due to the high thermal conductive shell which also protects the PCM from direct contact with the environment. [ABSTRACT FROM AUTHOR]

• A skin calorimeter was built to determine skin thermal properties in vivo. • The device uses a periodic thermal excitation to measure skin thermal properties. • Thermal probing depth increases exponentially with the period of the excitation. • Thermal properties time-variant model has advantages over a time invariant model. • This skin calorimeter allows to perform measurements at different skin depths. In this work we perform an experimental study of the thermal probing depth of a skin calorimeter developed to measure in vivo the heat capacity and the thermal resistance of a 4 cm2 skin region. To determine these properties, a small thermal excitation is applied to the skin and then, the static and dynamic response of the calorimetric signal is studied. This excitation consists of a periodic change of the calorimeter thermostat temperature while the device is applied on the skin. We have found that the thermal penetration depth depends mainly on the period of this periodic thermal excitation. This time dependence is exponential for non-semi-infinite domains. Using a time–variant model, we performed measurements on the dorsal and volar areas of the left wrist of a healthy 64-year-old male subject. For a 4 cm2 region of skin, the results show that the heat capacity in both zones are of the same order of magnitude. Its initial value is 4 J/K and increases exponentially up to 7 J/K with a time constant of 6.5 min. Thermal resistance also increases exponentially with time: from 27 to 36 K/W for the volar zone, and from 32 to 45 K/W for the dorsal zone. [ABSTRACT FROM AUTHOR]

This article presents a simple in-house laboratory experiment method for the simultaneous estimation of principal thermal conductivity and specific heat capacity of an orthotropic composite with non-intrusive temperature measurement. The principle of method involves in symmetrical heating of two identical samples with a planar heater sandwiched in-between them and measuring the temperature of non-heated surface non-intrusively using IR camera. The forward problem that mimics the heat conduction from heater to sample and then to ambient is a 3-D transient conduction equation which is solved using finite volume method whereas inverse problem is solved using Levenberg-Marquardt algorithm. At first numerical estimations are carried out with synthetic temperature data to decide upon the parameters to be identified/fixed as well as to find optimal experimental variables. Then, using measured temperature response of the non-heated surface, the thermal properties of unidirectional fiber reinforced carbon composite are estimated. Induced bias errors in the estimated parameters because of error in the fixed parameters are also approximately quantified. The estimated transverse thermal conductivity is found to be 0.72 W/m K whereas the in-plane conductivities parallel and perpendicular to fiber are 5.43 W/m K, 0.64 W/m K respectively. Also, the estimated specific heat capacity is found to be 1044 J/kg K. A maximum deviation of 10% from the absolute Guarded Hot Plate measurement is found in the estimated thermal conductivity parallel to fiber. [ABSTRACT FROM AUTHOR]

Heat transfer and pressure drop characteristics of R410A condensing flow in horizontal small microfin and smooth tubes with outer diameter of 5.0 mm were studied experimentally at relatively high mass fluxes ranging from 390 to 1583 kg/m 2  s with different saturation temperatures of 309.15 K, 316.15 K, and 323.15 K. The results indicate that heat transfer coefficients and pressure gradients increase with increasing mass flux and with decreasing saturation temperature for both microfin and smooth tubes. The lower vapor density, higher liquid thermal conductivity, and higher surface tension effect all contribute to the higher heat transfer coefficients at a lower saturation temperature. The experimental heat transfer coefficients and frictional pressure gradients fit well with the empirical correlations. The heat transfer coefficient ratios between microfin and smooth tube range from 1.65 to 1.28 which means that microfin tubes can enhance heat transfer coefficients efficiently. Both heat transfer enhancement ratio and pressure drop penalty ration decrease with increasing mass flux and saturation temperature. Compared with other refrigerant properties, the liquid-vapor density contributes more to the heat transfer enhancement ratio at high mass flux. [ABSTRACT FROM AUTHOR]

The thermal conductivity of soils is a key factor in calculating soil heat transfer and analyzing temperature fields of geotechnical engineering in cold regions. We measured the thermal conductivity of a silty clay by a QuickLine-30 Thermal Properties Analyzer during a freezing-thawing process, and analyzed the variation of the thermal conductivity. We then calculated the thermal conductivity under the same experimental conditions using the three general models, i.e. weighted arithmetic mean model, weighted harmonic mean model, and weighted geometric mean model. The results show that for the thawed or frozen soils with little variation of unfrozen water content, the thermal conductivity is slightly influenced by temperature; however, for the soils in the major phase transition zone, the variation of the thermal conductivity with temperature is significant. After a freezing-thawing process, the thermal conductivities of the soils with higher initial dry densities become smaller, while those with lower initial dry densities become larger. We also found that the variation of porosity and hysteresis effect of unfrozen water content cause the difference of the thermal conductivity of soils between freezing and thawing processes, however the variation of porosity acts as the primary role. Furthermore, the three general models can all be used to calculate the thermal conductivity of soils; however, the weighted geometric mean model agrees best with the experimental data. [ABSTRACT FROM AUTHOR]

Here we show the performance of a cooling system consisting of one stream that morphs while embedded in a square plate heated uniformly. Several configurations are considered in order to establish the evolutionary direction in which the thermal performance of the cooling system is improving. First, serpentine shapes are considered. The total length of serpentine length is fixed, and the number of meanders is varied. The stream inlet and outlet are free to migrate on the perimeter of the heated plate. The results show that the serpentine with four elbows maintains lower peak temperatures when the inlet and the outlet of the duct are positioned in the middle of the side of the plate. We report the configuration in which the thermal conductance is maximum. Second, we analyzed the effect of various loop cooling configurations such as square, circular and clover leaf. Each loop is centered on the center of the plate, and its length is free to vary. The minimum peak temperature is achieved when the length of the circular loop is L c = 2.5 L, and when the length of the square loop is L c = 2.8 L. The clover leaf designs show even better performance. [ABSTRACT FROM AUTHOR]

• A secondary segmentation method was proposed to build CNT microcolumn arrays. • The mechanism of different structural parameters on the heat transfer performance was investigated in CNT arrays. • The effect of air on heat conduction of CNT arrays was investigated theoretically. • Analysis were conducted on the thermal conductivity of CNT arrays under thermal cyclic loading. Vertically arranged carbon nanotube (CNT) arrays have good flexibility, elasticity, and thermal conductivity. As a new type of thermal interface materials (TIMs), it is increasingly used in heat conduction applications. The thermal conductivity of CNT arrays is an order of magnitude higher than that of traditional silicone oil and solid-state interface composites. However, their thermal properties are susceptible to the ambient temperature and internal structural parameters, and the underlying influence mechanism remains unclear. This lack of understanding hinders the design and application of CNT arrays in practical settings. Therefore, a secondary segmentation method was proposed to establish the structural model of CNT microcolumn arrays. Then, the thermal properties of the array material were simulated, and the influence of different environmental temperatures and structural parameters on thermal properties were analyzed. Simultaneously, the dynamic thermal conductivity of the CNT array was explored by thermal cyclic loading. Simulation results indicate that the increase of structural parameters have a positive effect on the thermal conductivity of the array at different ambient temperatures, but the low temperature environment is more conducive to the improvement. Furthermore, the influence of different array densities predicted by the model on thermal properties is consistent with the corresponding experimental results, which illustrates the feasibility of the segmentation method of the model and provides a new theoretical method for studying the thermal properties of CNT arrays. [ABSTRACT FROM AUTHOR]

• Quasilocalized vibrational modes (QVMs) are found to be efficient heat carriers. • The unexpected high thermal transport performance of QVMs results from the strong mutual coherence effect between QVMs and other modes. • The strong mutual coherence results from the strong mode shape coupling and the high mutual energy gain. While quasilocalized vibrational modes (QVMs) in glassy solids are known to populate the low-frequency spectrum, their thermal transport properties remain largely unexplored. It is known that the energy of localized vibrational modes spatially remains where they reside. QVMs are therefore hypothesized to be inefficient heat carriers. We here show that the modal thermal conductivity of QVMs is at the same magnitude as that of the delocalized vibrational modes, which demonstrates QVMs can be efficient heat carriers as delocalized vibrational modes. We further prove that the mutual coherence between QVMs and other modes explains their high thermal exchange performance. Our finding provides a perspective on the thermal transport behavior of QVMs in glassy solids. [ABSTRACT FROM AUTHOR]

• A new method for the design of thermal metamaterial structure coupling the thermal scattering theory and the structure topology optimization algorithm is proposed, which provides a general, convenient and efficient approach for the design and application of thermally functional materials. • Different from the previous reverse design of scattering cloak, the topology parameter (thickness ratio) is adopted as the design variable instead of the thermal conductivity of materials, so as to avoid the problem caused by the restrictions of the heat conductivity of available natural materials. • By defining the initial structure as two layers according to thermal scattering theory, the dimensionality of the design variables is reduced to one, which reduces the calculation workload. • Under the action of the optimal cloak, the heat flux to the central object is lower than 2% of the background field, the relative difference of temperature to background fields of the outer region is less than 4%. Despite its simple structure, the bilayer scattering cloak exhibits excellent cloaking performance under different thermal boundary conditions. A topology optimization method for the design of thermal cloak is proposed. The thermal scattering theory is introduced to pre-define the initial topology of the thermal invisible cloak as two layers to reduce the number of design variables. In bilayer concentric rings, insulating and high thermal conductivity materials are used to mask heat flow and correct thermal properties. Topology optimization based on the CMA-ES strategy is applied to quickly obtain the best configuration of available materials. The influence of the thermal properties of the material on the optimal configuration and the deflection angle of the thermal flux at the boundary surface has been investigated. Numerical simulations were performed to compare the function of the optimal cloak with that of the ideal metamaterial and the single insulating layer. The results show that despite its simple structure, the bilayer scattering cloak exhibits excellent cloaking performance under different thermal boundary conditions. In this work, the perfect scattering elimination is achieved by adjusting the layer thickness ratio of the cloak with the initial defined structure rather than seeking the optimal thermal conductivity of material, so as to avoid the problem of mismatch between the actual and ideal parameters. Compared to continuous optimization method, the pre-defined bilayer structure is easy to manufacture. Our scheme introduces thermal scattering theory to pre-set the initial structure of the topology optimization method, which provides a general, convenient, and efficient means for the design and application of thermally functional materials. [ABSTRACT FROM AUTHOR]

A comprehensive analysis of the thermal conductivity of graphene under various conditions is presented in this study. Results obtained from different experimental and theoretical methods are analyzed and discussed for numerous conditions such as preparation process, shape, sample size, wavelength, and temperature. Wide discrepancies in the measured thermal conductivity results were found in many studies in the literature. Based on the cited data for the graphene thermal conductivity, the initially measured thermal conductivities appear to be highly overestimated. Majority of the documented results reported lower values of thermal conductivity than the earlier reported results. Furthermore, large differences in the values of the thermal conductivity of graphene were noticed from the cited results using different experimental and numerical methods (0.14 W/m K–20,000 W/m K). This raised an important question on the accuracy of these methods when measuring thermal conductivity of graphene at nanoscale. We have established the existence of a high degree of anomalies in the value of the thermal conductivity of grapheme. Therefore, proper experimental and theoretical studies should be conducted to accurately measure the thermal conductivity of graphene. [ABSTRACT FROM AUTHOR]

• A new Na 2 CO 3 /Graphene based phase change material with sandwich structure was proposed and designed. • The thermal transport properties prediction method of PCM was developed and determined by MD simulations. • The construction of sandwich structure was benefit to the thermal conductivity enhancement. In this paper, a new Na 2 CO 3 /Graphene based phase change material (PCM) with sandwich structure was proposed and designed for the application of thermal energy storage, the microstructure of which was established, and its thermal transport properties were investigated by means of molecular dynamics simulations, and the prediction method for transport properties with desired accuracy was also developed and determined. The results indicate that the construction of sandwich structure leads to the limiting effect of ions contained in the PCM simulation system, and the self-diffusion coefficient was reduced and shear viscosity was increased. Besides, the construction of sandwich structure was benefit to the thermal conductivity enhancement, the thermal conductivity of PCM can be increased by 32.51% on average in the studied temperature range, and the maximum enhancement can reach to 51.37%, which provides an attractive possible strategy for thermal conductivity improvement for molten salt based PCM, which is expected applied in the concentrated solar power generation system. [ABSTRACT FROM AUTHOR]

• The pores act as a "thermal barrier wall" on the heat transfer path in the MPL. • The fractal characteristics of the MPL inhibit heat transfer. • The structural parameters of the MPL is correlated with the thermal properties. Heat transfer in the fuel cells is generally limited by the effective thermal conductivity of the microporous layer (MPL). Due to the complexity of the microstructure and the difficulty of the test conditions, studies on the key parameters that affect the thermal conductivity of MPL is lacking. In this paper, we develop an analytical fractal model for the effective thermal conductivity of the MPL based on a lumped parametric model of the cube and a fractal particle chain model, and provide the contact ratio of the agglomerates using a 3D FIB-SEM reconstruction. The results indicate that the main factors affecting the thermal conductivity of MPL include the structural parameters, fractal dimension, and the thermal conductivity of the carbon particles and pores. The fractal characteristics of the MPL inhibit heat transfer rate and the pores act as a "thermal barrier wall" on the heat transfer path in the MPL. When the volume fraction of the MPL is 40 percent, the contact area ratio and fractal factor of the agglomerates are 0.351 and 0.84, respectively. In addition, this research will help to develop novel MPL structures with high thermal conductivity and improve simulation results for fuel cell temperatures. [ABSTRACT FROM AUTHOR]

• Nanoparticles are embedded to further reduce the thermal conductivity of the superlattice. • Effect of particle number and size on the thermal conductivity are studied. • Characteristics of the thermal conductivity are deeply revealed by calculating the phonons transport parameters. Utilizing different nanostructures can effectively control phonons transport of their corresponding macrostructures, and this will contribute to the development of thermoelectric energy harvesting and microelectronic thermal management. This work tends to decrease the thermal conductivity (TC) of Si/Ge superlattices (SLs) by embedding germanium nanoparticles (GNPs). The thermal transport property in such structure was investigated by non-equilibrium molecular dynamics (NEMD) method. Results showed that embedding NPs can significantly reduce the TC of SLs, but only when the particle number is less than half of the SL's periods, above which the reduction effect will be greatly decreased. In addition, we also investigated the effects of temperature, system length, particle size and number on the TC. The thermal transport properties of the system are deeply revealed by calculating and analyzing phonons parameters such as density of states, participation ratio, mean free paths (MFPs) and spectral heat flow. [ABSTRACT FROM AUTHOR]

• Uniform and partial heatings are compared when testing isotropic materials. • The plane source-based experimental apparatus is investigated. • A d -optimum criterion ensuring the convergence of the estimation procedure is used. • Experiment times, Δ+ determinant and uncertainty of the estimates are considered. • It is shown that the partial heating is not beneficial for isotropic materials. A d -optimum criterion is applied to the three-layer apparatus used for simultaneously estimating thermal properties (for example, thermal conductivity and effusivity) through the plane source method. The objective is to perform a comparison between uniform heating and piecewise-uniform heating of high-conductivity solid samples. In particular, the latter case is modeled through a two-dimensional heat conduction problem in which a rectangular plate (i.e. the sample) is partially heated at the front boundary through a surface heat flux, while all the other boundaries are kept insulated. The optimal experiment is designed for different set-ups of the experimental apparatus (width of the heated region, number of sensors and their locations). The convergence and the computational efficiency of the estimation iterative procedure are the terms of the comparison, as well as the expected standard deviations of thermal conductivity and effusivity. The results indicate that the use of a piecewise-uniform heating is not completely beneficial for isotropic materials. In fact, if on one hand it may offer standard deviations reduced up to about 40%, on the other hand it would require an experiment about six times longer than that required by a uniform heating to ensure a good convergence of the estimation iterative procedure. Therefore, a major computational effort is required. [ABSTRACT FROM AUTHOR]

• Oscillatory thermal response tests can determine ground thermal properties with high accuracy • Testing duration is reduced compared to traditional in situ thermal response tests • Fitting with analytical model provides accuracy comparable to numerical simulations In situ subsurface thermal conductivity (λ) and thermal diffusivity (α) are critical parameters to design ground-coupled heat pump systems (GCHP). λ is conventionally determined from thermal response tests (TRT), whereas α is commonly selected from literature data or with laboratory analyses. Alternatively, both thermal properties can be assessed independently with an oscillatory thermal response test (O-TRT), which consists of heating the subsurface at an oscillatory rate and monitoring the resulting thermal perturbation. Due to their flexibility and relatively small radius, heating cables can be used to carry out tests in shallow unconsolidated deposits in the scope of horizontal GCHP design. The duration of the tests is short compared to vertical ground heat exchangers (fractions of a day) since the thermal resistance between the heating cable and the soil is negligible. The objective of this work was to develop an apparatus and an analytical approach to assess λ and α from an O-TRT carried out in shallow unconsolidated deposits. Nine different O-TRTs were performed in the lab for a dry standard material and five O-TRTs were performed in situ in a natural sandy deposit. After validation via numerical modelling, results show that the analytical approach using correction factors allowed the estimation of λ and α with an accuracy of < 2%. The study demonstrates that O-TRTs are a useful method to estimate both λ and α. Further testing in other reference materials and with different apparatus configurations is needed to enhance the experimental setup and extend O-TRTs towards new applications. [ABSTRACT FROM AUTHOR]

• This reviews the previous works on phase change materials encapsulation methods. • This compares the properties resulting from different encapsulation methods. • This explores manufacturing potential of synthesis methods for PCM encapsulation. • This summarizes all key main technologies for preparing encapsulation for PCMs. • This explores pros, cons and the challenges relating to the encapsulation of PCMs. Currently, non-renewable resources are heavily consumed, leading to increased global warming resulting from the production of carbon dioxide etc., phase change materials (PCMs) are regarded as a solution to mitigate these global crises attributed to their promising thermal energy storage capability. In this critical review, the thermal properties of different encapsulation methods of PCMs are summarised and compared. Encapsulation ensures that PCMs are used safely and efficiently, therefore the method needs to be thoroughly investigated and improved before their practical implementation. The applicable thermal properties for different encapsulation techniques and encapsulation materials such as particle diameter, enthalpy, encapsulation efficiency and thermal cycling times are reviewed. Future researchers are advised to measure and report thermal conductivities, displaying them in a convenient manner; many studies ignore this parameter, hindering research progression. Evaluation criteria for mechanical properties should be developed to enable comparisons between studies. It is suggested that eutectic and metallic PCMs, sol-gel encapsulation methods, complex coacervation methods, and spray drying are the areas that can be further investigated for better microcapsule performance, higher microcapsule yield, and improved synthesis conditions. In the future, bifunctional microcapsules, copolymer encapsulation, and doped high-performance materials are highly promising developments when compared with current monofunctional capsules with pure polymer shells. [Display omitted] [ABSTRACT FROM AUTHOR]

An experiment was performed to investigate the effects of interface temperature, specimen surface roughness, and contact pressure on thermal contact conductance (TCC). Specimens were prepared using SUS 304 stainless steel, the interface temperature was in the region of 360–640 °C, the contact pressure was between 2.39 and 15.17 MPa, and the surface roughness ranged from 0.25 μm to 2.00 μm. All experiments were conducted in ambient atmosphere. Results indicated that TCC presents a power law relationship with contact pressure and interface temperature. While the contact pressure exponent varied between 0.20 and 0.46 at different surface roughness, the interface temperature exponent showed a much wider range of 0.45–2.36. TCC increased more rapidly with temperature in specimens with higher surface roughness than in those with lower surface roughness. The correlation equations between thermal contact conductance, contact pressure, and interface temperature revealed a relative error of 20%. [ABSTRACT FROM AUTHOR]

This paper presents a new transient analytical model for describing the heat transfer process of pile geothermal heat exchangers with spiral coils (PGHE-SC). A numerical model has been established based on the finite element method to validate this new analytical model, which shows a good agreement between the two models. The temperature responses for different heat transfer conditions were calculated and are discussed. When the thermal conductivities of pile is twice as the one of soil, the dimensionless temperature at middle of the pile is 0.3832 which almost twice as the temperature response of homogeneous case. The results indicate that the thermal difference between pile and soil is an important factor influencing PGHE-SC simulation/design. As this new model not only successfully takes the limited length of an energy pile into consideration but also distinguishes thermal properties in its modeling process, it may provide a desirable and better design tool for the PGHE-CS. [ABSTRACT FROM AUTHOR]