Theoretical insights into thermal transport and structural stability mechanisms of triaxial compressed methane hydrate

The heat transfer and stability of methane hydrate in reservoirs have a direct impact on the drilling and production efficiency of hydrate resources,especially in complex stress environments caused by formation subsidence.In this study,we investigated the thermal transport and structural stability of methane hydrate under triaxial compression using molecular dynamics simulations.The results suggest that the thermal conductivity of methane hydrate increases with increasing compression strain.Two phonon transport mechanisms were identified as factors enhancing thermal conductivity.At low compressive strains,a low-frequency phonon transport channel was established due to the overlap of phonon vibration peaks between methane and water molecules.At high compressive strains,the filling of larger phonon bandgaps facilitated the opening of more phonon transport channels.Additionally,we found that a strain of0.04 is a watershed point,where methane hydrate transitions from stable to unstable.Furthermore,a strain of0.06 marks the threshold at which the diffusion capacities of methane and water molecules are at their peaks.At a higher strain of0.08,the increased volume compression reduces the available space,limiting the diffusion ability of water and methane molecules within the hydrate.The synergistic effect of the strong diffusion ability and high probability of collision between atoms increases the thermal conductivity of hydrates during the unstable period compared to the stable period.Our findings offer valuable theoretical insights into the thermal conductivity and stability of methane hydrates in reservoir stress environments.

Thermal transport in composition graded silicene/germanene heterostructures

Through equilibrium and non-equilibrium molecular dynamics simulations,we have demonstrated the inhibitory effect of composition graded interface on thermal transport behavior in lateral heterostructures.Specifically,we investigated the influence of composition gradient length and heterogeneous particles at the silicene/germanene(SIL/GER)heterostructure interface on heat conduction.Our results indicate that composition graded interface at the interface diminishes the thermal conductivity of the heterostructure,with a further reduction observed as the length increases,while the effect of the heterogeneous particles can be considered negligible.To unveil the influence of composition graded interface on thermal transport,we conducted phonon analysis and identified the presence of phonon localization within the interface composition graded region.Through these analyses,we have determined that the decrease in thermal conductivity is correlated with phonon localization within the heterostructure,where a stronger degree of phonon localization signifies poorer thermal conductivity in the material.Our research findings not only contribute to understanding the impact of interface gradient-induced phonon localization on thermal transport but also offer insights into the modulation of thermal conductivity in heterostructures.

Effects of Rattling Behavior of K and Cd Atoms along Different Directions in Anisotropic KCdAs on Lattice Thermal Transport and Thermoelectric Properties

We employ advanced first principles methodology,merging self-consistent phonon theory and the Boltzmann transport equation,to comprehensively explore the thermal transport and thermoelectric properties of KCdAs.Notably,the study accounts for the impact of quartic anharmonicity on phonon group velocities in the pursuit of lattice thermal conductivity and investigates 3ph and 4ph scattering processes on phonon lifetimes.Through various methodologies,including examining atomic vibrational modes and analyzing 3ph and 4ph scattering processes,the article unveils microphysical mechanisms contributing to the lowκL within KCdAs.Key features include significant anisotropy in Cd atoms,pronounced anharmonicity in K atoms,and relative vibrations in non-equivalent As atomic layers.Cd atoms,situated between As layers,exhibit rattling modes and strong lattice anharmonicity,contributing to the observed lowκL.Remarkably flat bands near the valence band maximum translate into high PF,aligning with ultralowκL for exceptional thermoelectric performance.Under optimal temperature and carrier concentration doping,outstanding ZT values are achieved:4.25(a(b)-axis,p-type,3×10^(19)cm^(−3),500 K),0.90(c-axis,p-type,5×10^(20)cm^(−3),700 K),1.61(a(b)-axis,n-type,2×10^(18)cm^(−3),700 K),and 3.06(c-axis,n-type,9×10^(17)cm^(−3),700 K).

Thermal Transport in Methane Hydrate by Molecular Dynamics and Phonon Inelastic Scattering

The heat conduction and thermal conductivity for methane hydrate are simulated from equilibrium molecular dynamics. The thermal conductivity and temperature dependence trend agree well with the experimental results. The nonmonotonic temperature dependence is attributed to the phonon inelastic scattering at higher temperature and to the confinement of the optic phonon modes and low frequency phonons at low temperature. The thermal conductivity scales proportionally with the van der Waals interaction strength, The conversion of a crystal-like nature into an amorphous one oecurs at higher strength. Both the temperature dependence and interaction strength dependence are explained by phonon inelastic scattering.

Review of thermal transport and electronic properties of borophene

In recent years, two-dimensional boron sheets （borophene） have been experimentally synthesized and theoretically proposed as a promising conductor or transistor with novel thermal and electronic properties. We first give a general survey of some notable electronic properties of borophene, including the superconductivity and topological characters. We then mainly review the basic approaches, thermal transport, as well as the mechanical properties of borophene with different configurations. This review gives a general understanding of some of the crucial thermal transport and electronic properties of borophene, and also calls for further experimental investigations and applications on certain scientific community.

A Genetic Algorithm for Simultaneous Determination of Thin Films Thermal Transport Properties and Contact Resistance

A genetic algorithm （GA） was studied to simultaneously determine the thermal transport properties and the contact resistance of thin films deposited on a thick substrate. A pulsed photothermal reflectance （PPR） system was employed for the measurements. The GA was used to extract the thermal properties. Measurements were performed on SiO2 thin films of different thicknesses on silicon substrate. The results show that the GA accompanied with the PPR system is useful for the simultaneous determination of thermal properties of thin films on a substrate.

Nanoscale thermal transport: Theoretical method and application

With the size reduction of nanoscale electronic devices, the heat generated by the unit area in integrated circuits will be increasing exponentially, and consequently the thermal management in these devices is a very important issue. In addition, the heat generated by the electronic devices mostly diffuses to the air in the form of waste heat, which makes the thermoelectric energy conversion also an important issue for nowadays. In recent years, the thermal transport properties in nanoscale systems have attracted increasing attention in both experiments and theoretical calculations. In this review, we will discuss various theoretical simulation methods for investigating thermal transport properties and take a glance at several interesting thermal transport phenomena in nanoscale systems. Our emphasizes will lie on the advantage and limitation of calculational method, and the application of nanoscale thermal transport and thermoelectric property.

Study on Thermal Transport Coefficient of Electron in the Siliconized HT-7 Tokamak

Siliconization is a normal method for the first-wall conditioning on the HT-7 toka-mak. After siliconization the total radiation loss is reduced significantly. Heat-diffusion coefficient the electron of is reduced obviously at the outer half radius (r/a > 0.5) after siliconization. And the plasma confinement is improved effectively. At the core of the plasma, electromagnetic drift-wave mode driven by the temperature gradient of electron gives a good representation of the experimental data not only before siliconization but also after siliconization. But at the outer half radius, the Parail's electromagnetic drift-wave even mode gives a good description of the experimental data before siliconization, and the experimental data of Xe is close to the collisionless electrostatic drift-wave mode turbulence after siliconization.

First-principles analysis of phonon thermal transport properties of two-dimensional WS2/WSe2 heterostructures

The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.

Thermal transport in twisted few-layer graphene

Twisted graphene possesses unique electronic properties and applications, which have been studied extensively. Recently, the phonon properties of twisted graphene have received a great deal of attention. To the best of our knowledge,thermal transports in twisted graphene have been investigated little to date. Here, we study perpendicular and parallel transports in twisted few-layer graphene（T-FLG）. It is found that perpendicular and parallel transports are both sensitive to the rotation angle θ between layers. When θ increases from 0° to 60°, perpendicular thermal conductivity κ（｜｜） first decreases and then increases, and the transition angle is θ = 30°. For the parallel transport, the relation between thermal conductivity κand θ is complicated, because intra-layer thermal transport is more sensitive to the edge of layer than their stacking forms. However, the dependence of interlayer scattering on θ is similar to that of κ⊥. In addition, the effect of layer number on the thermal transport is discussed. Our results may provide references for designing the devices of thermal insulation and thermal management based on graphene.

Effects of thermal transport properties on temperature distribution within silicon wafer

A combined conduction and radiation heat transfer model was used to simulate the heat transfer within wafer and investigate the effect of thermal transport properties on temperature non-uniformity within wafer surface. It is found that the increased conductivities in both doped and undoped regions help reduce the temperature difference across the wafer surface. However, the doped layer conductivity has little effect on the overall temperature distribution and difference. The temperature level and difference on the top surface drop suddenly when absorption coefficient changes from 104 to 103 m-1. When the absorption coefficient is less or equal to 103 m-1, the temperature level and difference do not change much. The emissivity has the dominant effect on the top surface temperature level and difference. Higher surface emissivity can easily increase the temperature level of the wafer surface. After using the improved property data, the overall temperature level reduces by about 200 K from the basis case. The results will help improve the current understanding of the energy transport in the rapid thermal processing and the wafer temperature monitor and control level.

Material properties dependent on the thermal transport in a cylindrical nanowire

Using the elastic wave continuum model, we investigate the effect of material properties on ballistic thermal transport in a cylindrical nanowire. A comparative analysis for the convexity-shaped and concavity-shaped structure is made. It is found that in the convexity-shaped structure, the material with higher wave velocity in the convexity region can increase the thermal conductance at the lower temperature range; the thermal conductance of the nanowire with higher wave velocity in the convexity region is lower than that of the nanowire with lower wave velocity in the convexity region at the higher temperature range. However, in the concavity-shaped structure, the material properties of the concavity region have less effect on the thermal conductance at the lower temperature range; the material with higher wave velocity in the concavity region can reduce the thermal conductance at the higher temperature range. A brief analysis of these results is given.

Ultra-efficient and parameter-free computation of submicron thermal transport with phonon Boltzmann transport equation

Understanding thermal transport at the submicron scale is crucial for engineering applications,especially in the thermal management of electronics and tailoring the thermal conductivity of thermoelectric materials.At the submicron scale,the macroscopic heat diffusion equation is no longer valid and the phonon Boltzmann transport equation(BTE)becomes the governing equation for thermal transport.However,previous thermal simulations based on the phonon BTE have two main limitations:relying on empirical parameters and prohibitive computational costs.Therefore,the phonon BTE is commonly used for qualitatively studying the non-Fourier thermal transport phenomena of toy problems.In this work,we demonstrate an ultra-efficient and parameter-free computational method of the phonon BTE to achieve quantitatively accurate thermal simulation for realistic materials and devices.By properly integrating the phonon properties from first-principles calculations,our method does not rely on empirical material properties input.It can be generally applicable for different materials and the predicted results can match well with experimental results.Moreover,by developing a suitable ensemble of advanced numerical algorithms,our method exhibits superior numerical efficiency.The full-scale(from ballistic to diffusive)thermal simulation of a 3-dimensional fin field-effect transistor with 13 million degrees of freedom,which is prohibitive for existing phonon BTE solvers even on supercomputers,can now be completed within two hours on a single personal computer.Our method makes it possible to achieve the predictive design of realistic nanostructures for the desired thermal conductivity.It also enables accurately resolving the temperature profiles at the transistor level,which helps in better understanding the self-heating effect of electronics.

Thermal transports in the MXenes family:Opportunities and challenges

The carbides and nitrides of transition metals known as“MXenes”refer to a fast-growing family of two-dimensional materials discovered in 2011.Thanks to their unique nanolayer structure,superior electrical,mechanical,and thermal properties,MXenes have shown great potential in addressing the critical overheating issues that jeopardize the performance,stability,and lifetime of high-energy-density components in modern devices such as microprocessors,integrated circuits,and capacitors,etc.The outstanding intrinsic thermal conductivity of MXenes has been proved by experimental and theoretical research.Numerous MXenes-enabled high thermal conductivity composites incorporated with polymer matrix have also been reported and widely used as thermal management materials.Considering the booming heat dissipation demands,MXenes-enabled thermal management material is an extremely valuable and scalable option for modern electronics industries.However,the fundamental thermal transport mechanisms behind the MXenes family remain unclear.The MXene thermal conductivity disparities between the theoretical prediction and experimental results are still significant.To better understand the thermal conduction in MXenes and provide more insights for engineering high-performance MXene thermal management materials,in this article,we summarize recent progress on thermal conductive MXenes.The essential factors that affect MXenes intrinsic thermal conductivities are tackled,selected MXenes-polymer composites are highlighted,and prospects and challenges are also discussed.

Quantifying spectral thermal transport properties in framework of molecular dynamics simulations:a comprehensive review

Over the past few decades,significant progress has been made in micro-and nanoscale heat transfer.Numerous computational methods have been developed to quantitatively characterize the thermal transport in bulk materials and across the interfaces,which benefit the thermal management design in microelectronics and energy conversion in thermoelectrics largely.In this paper,the methods and studies on quantifying thermal transport properties using molecular dynamics simulations are comprehensively reviewed.Two classical methods based on molecular dynamics simulations are first introduced,i.e.,equilibrium molecular dynamics and nonequilibrium molecular dynamics,to calculate the thermal transport properties in bulk materials and across the interfaces.The spectroscopy methods are then reviewed,which are developed in the framework of equilibrium molecular dynamics(i.e.,time domain normal mode analysis,spectral energy density,Green-Kubo modal analysis) and methods proposed based on the nonequilibrium molecular dynamics(i.e.,time domain direct decompose method,frequency domain direct decompose method and spectral heat flux method).In the subsequent section,the calculations of spectral thermal conductivities using these computational methods in various systems are presented,including simple crystals,low-dimensional materials,complex materials and nanostructures.Following that,spectral thermal transport across the interfacial systems is discussed,which includes solid/solid interfaces,solid/solid interfaces with interfacial engineering and solid/liquid interfaces.Some fundamental challenges in molecular dynamics simulations,such as including quantum effects and quantifying the anharmonic contributions,are discussed as well.Finally,some open problems on spectroscopy thermal transport properties in the framework of molecular dynamics simulations are given in the summary.

Mechanical property enhancements and amorphous thermal transports of ordered weberite-type RE_(3)Nb/TaO_(7) high-entropy oxides

A_(3)BO_(7)-type(A=rare earth(RE),B=Nb or Ta)oxides have been studied as protective coating materials because of their low thermal conductivity;however,their hardness,toughness,and stiffness are insufficient,particularly for members with webeirte-type structures.In this work,we have synthesized two high-entropy oxides(HEOs)of weberite-type RE niobates/tantalates(RE_(3)Nb/TaO_(7)),i.e.,(Nd_(1/7)Sm_(1/7)Eu_(1/7)Gd_(1/7)Dy_(1/7)Ho_(1/7)Er_(1/7))_(3)NbO_(7)(7HEOs-Nb)and(Nd_(1/7)Sm_(1/7)Eu_(1/7)Gd_(1/7)Dy_(1/7)Ho_(1/7)Er_(1/7))3(Nb_(1/2)Ta_(1/2))O_(7)(7HEOs-NbTa),to overcome the mechanical deficiencies.The short-and long-range ordered arrangements of RE cations in the A-site and Nb/Ta cations in the B-site were identified by the X-ray diffraction(XRD),scanning electron microscopy equipped with energy-dispersive spectrometry(EDS),and transmission electron microscopy.The enhancements in hardness(H=9.4 GPa)and fracture toughness(KIC=2.0 MPa·m^(1/2))were realized by grain refinement,solid solution strengthening,and high stiffness(K).The exceptional phase stability at 25-1500℃,amorphous thermal conductivity(k=1.5-1.7 W·m^(−1)·K^(−1) at 25-900℃),and high thermal expansion coefficients(TEC>11.0×10^(−6) K^(−1) at 1500℃)further supported their potential application as protective coating materials.

Phonon thermal transport properties of XB_(2)(X=Mg and Al)compounds:considering quantum confinement and electron-phonon interaction

XB_(2)(X=Mg and Al)compounds have drawn great attention for their superior electronic characteristics and potential applications in semiconductors and superconductors.The study of phonon thermal transport properties of XB_(2)is significant to their application and mechanism behind research.In this work,the phonon thermal transport properties of three-dimensional(3D)and two-dimensional(2D)XB_(2)were studied by first-principles calculations.After considering the electron-phonon interaction(EPI),the thermal conductivities(TCs)of 3D Mg B_(2)and 3D Al B_(2)decrease by 29%and 16%which is consistent with experimental values.Moreover,the underlying mechanisms of reduction on lattice TCs are the decrease in phonon lifetime and heat capacity when considering quantum confinement effect.More importantly,we are surprised to find that there is a correlation between quantum confinement effect and EPI.The quantum confinement will change the phonon and electron characteristics which has an impact on EPI.Overall,our work is expected to provide insights into the phonon thermal transport properties of XB_(2)compounds considering EPI and quantum confinement effect.

Nonequilibrium Green＇s function method for quantum thermal transport

This review deals with the nonequilibrium Green＇s function （NEGF） method applied to the problems of energy transport due to atomic vibrations （phonons）, primarily for small jtmction systems. We present a pedagogical introduction to the subject, deriving some of the well-known results such as the Laudauer-like formula for heat current in ballistic systerms. The main aim of the review is to build the machinery of the method so that it can be applied to other situations, which are not directly treated here. In addition to the above, we consider a nmnber of applications of NEGF, not in routine model system calculations, but in a few new aspects showing the power and usefulness of the formalism. In partkaflar, we discuss the problems of multiple leads, coupled left-right-lead system, and system without a center. We also apply the method to the problem of full counting statisties. In the case of nonlinear svstems, we make general comments on the thermal expansion effect. phonon relaxation timv. and a certain class of mean-field approximations. Lastly, we examine the relationship between NEGF. reduced density matrix, and master equation approaches to thermal transport,

Preparation and heat insulating capacity of Sm2Zr2O7-SiC composites based on photon thermal transport

A series of Sm2Zr2O7-SiC composites doped with different volume fraction and particle size of SiC were prepared by hot pressing at 1300°C.The phase of the composites prepared is P-Sm2Zr2O7 and C-SiC,and no other diffraction peaks exist,which indicates that Sm2Zr2O7 has great chemical compatibility with SiC.The thermal conductivity and phonon thermal conductivity of the Sm2Zr2O7-SiC composites are measured by the laser pulse method.The photon thermal conductivity of the composites is obtained by subtracting the phonon thermal conductivity from the total thermal conductivity.The results show that the photon thermal conductivity of Sm2Zr2O7-SiC composites is lower than that of pure Sm2Zr2O7.The photon thermal conductivity of Sm2Zr2O7-SiC composites decreases first and then increases with the increase of SiC particle size.Sm2Zr2O7-(5 vol%,10µm)SiC composite has the lowest photon thermal conductivity.

SIMULATION OF THERMAL TRANSPORT IN AQUIFER:A GWHP SYSTEM IN CHENGDU,CHINA

With an established convection-dispersion model for the thermal transport in aquifer, the thermal transport processes in an unconfined aquifer of a Groundwater Heat Pump （GWHP） system in Chengdu, China, are simulated with double-well intervals and cooling-load design fluctuations in summer running period under special groundwater flow and heat source conditions, and the stage-characteristics of the thermal transport in the aquifer are investigated in the running cycle （1 year） numerically. The results show that the thermal transport in the aquifer is closely related to the distance between pumping and injecting wells and the cooling-load design fluctuations, especially, to the cycling water volume. The thermal transport in the aquifer sees different characteristics in the two states in the pumping-recharging stages in summer/winter and the storing stages in spring/autumn. With the hydro-geologic theory and the numerical model, the effect of the groundwater-flow on the thermal transport in the unconfined aquifer is discussed.