Thermal conductivity of metal-organic framework 5 (MOF-5): Part I. Molecular dynamics simulations

The phonon thermal conductivity of MOF-5, a metal-organic framework crystal with a phenylene bridge, is predicted between temperatures of 200 K and 400 K using molecular dynamics simulations and the Green–Kubo method. The simulations are performed using interatomic potentials obtained using ab initio calculations and experimental results. The predicted thermal conductivity of MOF-5 is low for a crystal, 0.31 W/m K at a temperature of 300 K, and its temperature dependence is very weak. By decomposing the thermal conductivity into components associated with short- and long-range acoustic phonons, and optical phonons, the weak temperature dependence is found to be related to the mean free path of the majority of phonons, which is of the order of lattice parameter (and is essentially temperature independent). To interpret the results, an analytical thermal conductivity relation is derived, which reduces to the Cahill–Pohl and Slack models under appropriate assumptions. The relation contains a critical frequency, which determines the relative contributions of the short- and long-range acoustic phonons. The small long-range acoustic phonon contribution is found to be related to the long and flexible phenylene bridge, and to the mass mismatch between the cages and the bridges.     

Transport properties of Ar–Kr binary mixture in nanochannel Poiseuille flow

Transport properties, including thermal conductivity and shear viscosity, of the Ar–Kr binary mixture confined in a nanochannel under Poiseuille flow are calculated by equilibrium molecular dynamics (EMD) simulation through Green–Kubo formula. An external force is applied in the x-direction to drive the Poiseuille flow. Thermal conductivity of the confined mixture in the x- and y-direction is obviously higher than that in macroscale, as a result of the strong interacting potential between the fluid atoms and the wall atoms. Thermal conductivity of the flowing binary mixture is obviously anisotropic. With increasing the external driving force, in the x-direction the thermal conductivity increases, whereas in the y-direction it keeps constant. The xz- and yz-component of the shear viscosity of the confined mixture are enhanced comparing with the xy-component owing to the collisions between the fluid atoms and the wall atoms in the z-direction. They are higher than the results in macroscale and decrease with the external driving force increasing. For the binary mixture, thermal conductivity and shear viscosity vary with the mole fraction of the Kr atoms. The interactions between the fluid atoms and the wall atoms play a key role in the transport properties of the binary mixture confined in the nanochannel.     

An atomistic Green’s function method hybrid with a substructure technique for coherent phonon analysis

ABSTRACT

By virtue of substructure technique, the atomistic Green’s function method can be applied to efficiently evaluate the coherent phonon transport in a large device. Comprised of several substructures without inner atoms, the device can be described by a small system of algebraic equations, which can give a small but crucial submatrix of the retarded Green’s function of the open device without loss in accuracy. While avoiding solving the entire device problem, the method can basically give all the information to describe the quantum transport within the ballistic limit, such as the transmission function and local density of states. The validity and efficiency of the proposed method is demonstrated by the one- and three-dimensional cases. The multilevel substructure technique is also feasible.     

Thermal Wave in Phonon Hydrodynamic Regime by Phonon Monte Carlo Simulations

ABSTRACT

Thermal wave, namely wavelike behavior of heat propagation in transient heat conduction, enjoys much attention due to the recent investigations into phonon hydrodynamics in low-dimensional materials. In this paper, an improved phonon Monte Carlo (MC) simulation algorithm is developed based on the Callaway’s dual relaxation time approximation model, which can deal with the coupling of normal and resistance scattering processes. Via the method, more thermal wave evidences are observed from the microscopic view of phonons, including overshooting and diffraction. Furthermore, the ballistic and hydrodynamic thermal waves are deeply studied. Two kinds of dissipation are found to exist in thermal waves, namely spatial dissipation and resistance dissipation. The former keeps the conservation of phonon momentum, but it lengthens the wavelength and decreases the peak temperature. The latter destroys the phonon momentum and keeps the original profile, lowering the peak temperature. Finally, phonon transport phenomena in Ziman hydrodynamic regime and diffusive regime are investigated, by introducing the scattering probability. The propagation tendency of thermal energy is found to decrease with the increasing scattering probability. The investigations into phonon hydrodynamics help to understand the heat transport characteristics and improve thermal management in low-dimensional materials.     

An investigation of the phonon properties of silicon nanowires

The phonon dispersion relations and density of states under the size confinement effect are crucial in order either to obtain accurate solutions of the phonon Boltzmann transport equation or to obtain properly quantum-corrected temperatures for low-dimensional materials. This work draws the confined phonon properties of silicon nanowires from the equilibrium molecular dynamics simulations. The simulation results show discrete acoustic phonon modes with smaller group velocities and many additional modes in the region of large wave numbers and small frequencies, compared to the continuous bulk counterparts. The latter shifts the distribution of phonon density of states toward the lower frequency. The lattice thermal conductivities of infinitely long silicon wires of diameter 4.1 nm, 7.6 nm, and 10.6 nm are next calculated using the non-equilibrium molecular dynamics simulations, with temperatures properly quantum corrected based on the confined phonon density of states. The lattice thermal conductivities are found to be significantly smaller than the bulk value and depend only weakly on the temperature, implying that the surface scattering strongly dominates over the phonon–phonon interaction.     

Thermal transport in individual thermoelectric nanowires: a review

Abstract

Extensive research efforts have been devoted to nanowires because of their novel electronic, optical and thermoelectric properties due to spatial confinement in two dimensions. Among various fields, nanowires have been of interest in the thermoelectric community not only for their novel thermoelectric properties but also for their ease of use in fundamental scientific studies as the physics learned using nanowires can be applied in bulk thermoelectric nanocomposites. In this paper, we limit our discussion to experimental thermal transport in thermoelectric nanowires such as Bi–Te, Pb–Te and Si–Ge nanowires. After reviewing the reasons why nanowires are of interest in the thermoelectric community, we discuss various synthesis methods and thermal transport measurements. Next, we evaluated how thermal transport in nanowires is affected by various scattering mechanisms such as phonon boundary scattering, alloy scattering, etc. We also discuss a recent study concerning how the surface roughness affects phonon transport. This article is useful to gain insight into how to manage thermal transport in various applications.     

Materials Informatics for Heat Transfer: Recent Progresses and Perspectives

ABSTRACT

With the advances in materials and integration of electronics and thermoelectrics, the demand for novel crystalline materials with ultimate high/low thermal conductivity is increasing. However, search for optimal thermal materials is a challenge due to the tremendous degrees of freedom in the composition and structure of crystal compounds and nanostructures, and thus empirical search would be exhausting. Materials informatics, which combines the simulation/experiment with machine learning, is now gaining great attention as a tool to accelerate the search of novel thermal materials. In this review, we discuss recent progress in developing materials informatics (MI) for heat transport: the exploration of crystals with high/low-thermal conductivity via high-throughput screening, and nanostructure design for high/low-thermal conductance using the Bayesian optimization and Monte Carlo tree search. The progresses show that the MI methods are useful for designing thermal functional materials. We end by addressing the remaining issues and challenges for further development.     

Thin Film Non-Noble Transition Metal Thermophysical Properties

The transient thermoreflectance (TTR) technique coupled with a pump-probe experimental setup enables the observation of thermal transport phenomena on a sub-picosecond time scale. The reflectance from non-noble transition metals (at least one unoccupied d-orbital in the conduction band) can be shown to have a linear dependence when compared to small changes in the electron and lattice temperatures. This thermal dependence can be combined with the parabolic two step (PTS) model to enable measurement of the electron-phonon coupling factor and thermal conductivity of thin film materials. Experimental results are presented for thin film samples of the non-noble transition metals platinum and nickel. Results are presented using laser wavelengths ranging from 740 nm to 805 nm and using a range of laser fluences (ranging from ～0.35 to 2 J/m²). Over this range of wavelengths and fluences the material properties are shown to be independent of the measurement conditions.     

On thermo-electro-viscoelastic relaxation functions in a Green–Naghdi type theory

Abstract

We find restrictions on the relaxation functions of thermo-electro-viscoelastic materials. This is achieved within an extension of the Green–Naghdi theory for thermoelasticity, which uses the energy equation to exploit constitutive equations. These restrictions extend the results previously found for thermo-viscoelastic materials and for the classical infinitesimal theory of viscoelasticity.     

Li2MnO3 based Li-rich cathode materials: towards a better tomorrow of high energy lithium ion batteries

Abstract

Li₂MnO₃ based layered Li-rich materials as promising cathode candidates of Li ion batteries (LIBs) have attracted much recent attention mainly owing to their superior high specific capacity and high working voltage. To date, although researchers have put much effort to this family of materials, there are still a number of issues under debates in the fundamental understanding of the crystal structures and the electrochemical reaction mechanisms, before the materials can be ready for practical applications. In this review article, we address the recent progress of this group of Li-rich cathode materials with a good hope to better understanding of the relationships among composition, crystal structure and electrochemical reaction mechanisms. In addition, the use of advanced microscopic characterisation and the strategies of novel material designs will also be discussed for better cathode design for LIBs.     

MOLECULAR DYNAMICS SIMULATION OF THERMAL CONDUCTIVITY OF NANOSCALE THIN SILICON FILMS

Molecular dynamics simulations are performed to explore the thermal conductivity in the cross-plane direction of single-crystal thin silicon films. The silicon crystal has diamond structure, and the Stillinger-Weber potential is adopted. The inhomogeneous nonequilibrium molecular dynamics (NEMD) scheme is applied to model heat conduction in thin films. At average temperature T = 500 K, which is lower than the Debye temperature Θ_D = 645 K, the results show that in a film thickness range of about 2–32 nm, the calculated thermal conductivity decreases almost linearly as the film thickness is reduced, exhibiting a remarkable reduction as compared with the bulk experimental data. The phonon mean free path is estimated and the size effect on thermal conductivity is attributed to the reduction of phonon mean free path according to the kinetic theory.     

Heat,mass, and crystal growth of GaN in the ammonothermal process: A numerical study

ABSTRACT

This paper presents a computational model for the ammonothermal gallium nitride (GaN) crystal growth process, including fluid flow, heat transfer, dissolution and crystallization rates, GaN metastable phase transport, and crystal interface advancement. The presented article solves the Navier–Stokes equations along with the Brickman–Darcy–Forchheimer extensions for nutrient porous medium and Boussinesq approximation for free convection. Piecewise Linear Interface Calculation (PLIC) method is adopted to construct and advance the crystal interface. Simulations, in particular, were performed for a common research autoclave with a retrograde ammonothermal system. Special attention is given to the regions close to the crystal interface.     

Nanosized Zn–Sn metal composite oxide: a new anode material for Li ion battery

Abstract

The morphological evolution of nanosized Zn–Sn composite oxides, synthesised by the decomposition of ZnSn(OH)₆ precursor at temperature ranged from 300 to 800°C was investigated by using XRD and high resolution TEM. The precursor was also studied by thermal analysis. The electrochemical performance of Zn–Sn composite oxides as anode materials for Li ion batteries was measured in the form of Li/Zn–Sn composite oxides cells. The results reveal that the samples calcined at low temperatures (300 and 500°C) were amorphous Zn₂SnO₄ and SnO₂, and the samples calcined at high temperatures (720 and 800°C) were crystal Zn₂SnO₄ and SnO₂. All the samples have a high reversible specific capacity of over 800 mAh g^?1, and the first charge specific capacity is up to 903 mAh g^?1 for the sample calcined at 500°C. The charge capacity and cyclability were sensitive to the structure and composition of the electrode active materials; the samples calcined at phase transition temperature rage exhibited relatively worse electrochemical properties.     

Structural,electronic, dynamical and thermodynamic properties of Ca10(PO4)6(OH)2 and Sr10(PO4)6(OH)2: First-principles study

Density functional theory (DFT) with optPBE-vdW functional is used to simulate the structural, electronic, dynamical and thermodynamic properties of Ca₁₀(PO₄)₆(OH)₂(Ca-HA) and Sr₁₀(PO₄)₆(OH)₂(Sr-HA). The calculated structural properties within optPBE-vdW functional is found to yield better agreement with the experimental results, which indirectly suggests the important role of weak hydrogen bond in this crystal. The calculated electronic properties indicate that Ca-HA and Sr-HA are insulator materials with indirect band gap of 5.52 eV and 5.10 eV, respectively. The detailed dynamical properties of two apatites are obtained by the linear-response approach. With replacement of Ca by Sr, the librational mode of OH group decreases from 612 cm^?1 to 569 cm^?1, the stretching mode of OH group increases from 3614.5 cm^?1–3649.9 cm^?1, which is consistent with the experimental results. Finally, some phonon related thermodynamic properties, such as Helmholtz free energy F, internal energy E, entropy S and heat capacity C_V of Sr-HA and Ca-HA are studied according to the phonon calculations within the harmonic approximation. The present calculation results of two apatites with optPBE-vdW functional are in good agreement with the existing experimental.     

Modeling heat transfer in Bi2Te3–Sb2Te3 nanostructures

Bi₂Te₃–Sb₂Te₃ nanostructures are gaining importance for use in thermoelectric applications following the finding that the Bi₂Te₃–Sb₂Te₃ superlattice exhibits a figure of merit, ZT = 2.4, which is higher than conventional thermoelectric materials. In this paper, thermal transport in the cross-plane direction for Bi₂Te₃–Sb₂Te₃ nanostructures is simulated using the Boltzmann transport equation (BTE) for phonon intensity. The phonon group velocity, specific heat, and relaxation time are calculated based on phonon dispersion model. The interfaces are modeled using a combination of diffuse mismatch model (DMM), and the elastic acoustic mismatch model (AMM). The thermal conductivity for the Bi₂Te₃–Sb₂Te₃ superlattice is compared with the experimental data, and the best match is obtained for specularity parameter, p, of 0.9. The present model is extended to solve for thermal transport in 2-D nanowire composite in which Sb₂Te₃ wires are embedded in a host material of Bi₂Te₃. Unlike in bulk composites, the results show a strong dependence of thermal conductivity, temperature, and heat flux on the wire size, wire atomic percentage, and interface specularity parameter. The thermal conductivity of the nanowire is found to be in the range of 0.034–0.74 depending on the atomic percentage and the value of p.     

The impact of internal polarized monochromatic acoustic phonon emission on heat dissipation at nanoscale

Internal monochromatic acoustic phonon emission has a distinct impact on temperature distribution, as well as transient heat dissipation, depending on the emitted phonon frequency and its polarization branch. A Monte Carlo simulation of phonon transport was used to study the impact of internal acoustic phonon emission on heat dissipation at nanoscale in a silicon thin film. The cause of the emission can be due to other energy carriers or by a direct external source. The simulation utilized parabolic phonon dispersion relations in longitudinal acoustic (LA) and transverse acoustic (TA) polarizations with three phonon–phonon scatterings. The Normal and Umklapp phonon scatterings were included in the simulation. All the scattering events scatter phonons isotropically. Results indicated that monochromatic TA-phonon emission within the film tends to create higher local peak temperature than that of LA-phonon emission with an identical volumetric power generation.     

The direct collocation meshless method based on a second-order phonon Boltzmann equation for ballistic–diffusive phonon heat transport

A second-order phonon Boltzmann equation (SOPBE) is proposed based on the phonon Boltzmann equation (PBE) under gray relaxation-time approximation, and the direct collocation meshless (DCM) method is employed to solve the SOPBE. Several numerical tests for phonon transport over a broader range of Knudsen numbers under different boundary conditions are carried out. The results show that the SOPBE solved by the DCM method is applicable for different transport regimes. Moreover, it overcomes the numerical error “ray effects” of other numerical methods under the ballistic limit to a certain extent. When modeling phonon transport in materials with inhomogeneous acoustic property, the superiority of SOPBE will be more obvious compared with the PBE. The results demonstrate the capability of our methodology for ballistic–diffusive phonon heat transport.     

An investigation on strain and temperature rate-dependent thermoelasticity and its infinite speed behavior

Abstract

The present work is aimed at a mathematical analysis of the newly proposed strain and temperature rate-dependent thermoelasticity theory, also called a modified Green–Lindsay model (MGL) theory, given by Yu et al. (2018). This model is also an attempt to remove the discontinuity in the displacement field observed under temperature rate-dependent thermoelasticity theory proposed by Green and Lindsay. We study thermoelastic interactions in an infinite homogeneous, isotropic elastic medium with a cylindrical cavity based on this model when the surface of the cavity is subjected to thermal shock. The solutions for the distribution of displacement, temperature, and stress components are obtained by using the Laplace transform technique. The inversion of the Laplace transform is carried out by short-time approximation. A detailed comparison of the analytical results predicted by the MGL model with the corresponding predictions by the Lord–Shulman model and the Green–Lindsay model is performed. It is observed that strain rate terms in the constitutive equation avoid the prediction of discontinuity in the displacement field and other significant effects are noted. However, the new theory predicts the infinite speed of disturbance like the classical theory. Variations of field variables at different time are graphically displayed for different models and compared by using a numerical method.     

Superior hydrogen storage capacity of Vanadium decorated biphenylene (Bi+V): A DFT study

Herein, the hydrogen storage competency of vanadium-decorated biphenylene (Bi+V) has been investigated using Density Functional Theory simulations. The metal atom interacts with biphenylene with a binding energy value of −2.49 eV because of charge transfer between V 3d and C 2p orbitals. The structure and electronic properties are studied in terms of adsorption energy values, the spin-polarized partial density of states (PDOS), band structure plots, and charge transfer analysis. The Kubas-type interactions lead to average hydrogen adsorption energy values of −0.51 eV/H₂ which fulfills DOE-US criteria (0.2–0.7 eV/H₂). The diffusion energy barrier value of 1.75 eV lowers the chances of metal clustering. The complex binds 5H₂ on each V-atom resulting in a storage capacity of 7.52 wt% with an average desorption temperature of 595.96 K. The ab-initio molecular dynamics (AIMD) and phonon dispersions validates structural integrity at higher temperatures suggesting the excellent storage properties of this material at room temperature.     

Exploring the Extremes of Heat Conduction in Anisotropic Materials

Anisotropic solids possess thermal conductivities ranging from among the highest found in nature, as in the in-plane thermal conductivity of graphite, to the lowest, as in the cross-plane thermal conductivity of disordered layered crystals. Though these extremes of thermal conductivity make anisotropic materials attractive for diverse applications such as thermal management and thermal insulation, the microscopic physics of heat conduction in these materials remain poorly understood. In this review article, we discuss the recent advances in our understanding of thermal phonon transport in anisotropic solids obtained using new theoretical, computational, and experimental tools.