Computational analysis of the lattice contribution to thermal conductivity of single-walled carbon nanotubes

Molecular dynamics based heat-flux auto-correlation functions are combined with a Green-Kubo relation from the linear response theory to quantify the lattice contribution to thermal conductivity of single-walled carbon nanotubes with three different chiralities (screw symmetries). The interactions between carbon atoms within a nanotube are analyzed using the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. The results obtained show that, due to a long-term exponential-decay character of the heat-flux auto-correlation functions, converging values of the lattice thermal conductivities can be obtained using computational cells considerably smaller than the phonon mean free path. However, to obtain accurate values of the thermal conductivity, a spectral Green-Kubo relation and a phonon-based extrapolation function are found to be instrumental for quantifying the thermal conductivity contribution of the long-wavelength phonons not allowed in the computational cells of a finite size. The results further show that chirality of the carbon nanotubes can affect the lattice contribution to the thermal conductivity by as much as 20%. Also, the simulation results of the effect of temperature on the thermal conductivity clearly show a competition between an increase in the number of phonons and an increased probability for phonon scattering at higher temperatures.     

Microstructure and thermal conductivity of carbon/carbon composites containing zirconium carbide

Carbon/carbon (C/C) composites containing zirconium carbide (ZrC) were prepared by a novel method. Carbon fiber felt with addition of zirconia was prepared by a microwave-hydrothermal reaction, followed by densification and graphitization. The crystalline structure of the pyrolytic carbon and morphology of the composites were investigated by X-ray diffraction, Raman spectrascope, polarized light microscope, and scanning electron microscopy. Results show that the ZrC grains with sub-micron size present a homogeneous distribution in carbon matrix. The degree of order of the pyrolytic carbon matrix is decreased due to adding ZrC into the C/C composites. Graphitization degree of the C/C composites is decreased by the addition of ZrC. ZrC grains uniformly embedded in the pyrolytic carbon matrix act as pinning particles blocking the conversion of disordered to ordered structure during graphitization. Thermal conductivity is higher in the C/C composites containing ZrC, which is attributed to the increased phonon-defect interaction produced by the thermal motion of the CO in the micropores and gaps of the composites.     

Nano-scale compositional oscillation and phase intergrowth in Cu2S0.5Se0.5 and their role in thermal transport

Solid solution alloying is a promising strategy to establish high performance thermoelectrics.By alloying different elements,phase structures and phase compositions may vary accompanied by appearance of variety of interesting microstructures including mass fluctuation,lattice strain,nano-scale defects and spinodal decomposition,all of which may greatly influence the electrical and specifically the thermal transport of the material.In the present study,atomic structures of Cu2S0.5Se0.5 solid solution have been examined by using atom-resolved electron microscopy in order to investigate the structure-correlated physical insights for the abnormal thermal transport in this solid solution.Then the exceptional inter-growth nanostructures were observed.The solid solution consists of two high symmetrical phases,i.e.the hexagonal and cubic phase,which alternately intergrow to form highly oriented ultra-thin lamel-lae of nano or even,unit cell scales.The compositional oscillation in Se/S atomic ratio during alloying is responsible for the phase stability and intergrowth nanostructures.The unique binary phase intergrowth nanostructures make great contribution to the ultra-low lattice thermal conductivity comparable to glass and extremely short phonon mean free path of only 1.04 (A),peculiar continuous hexagonal-to-cubic struc-tural transformation without a critical transition temperature and its corresponding abnormal changes of thermal characters with temperatures.The present study further evokes the unlimited possibilities and potentials for tailoring nanostructures by alloying for improved thermoelectric performance.     

空位缺陷对单晶硅薄膜热导率影响的分子动力学模拟研究

采用非平衡分子动力学(NEMD)方法及Tersoff势能函数模拟了微尺度下空位结构缺陷对单晶硅薄膜热导率的影响。结果表明,硅薄膜的热导率随空位浓度的增加而明显减小。当温度为300~700K时,随着温度的升高,空位浓度对热导率的影响以及同一空位浓度下温度对热导率的影响都逐渐减弱。     

A blend of first-principles and kinetic lattice Monte Carlo computation to optimize samarium-doped ceria

Solid oxide fuel cells (SOFCs) have been acknowledged as a possible future source for clean and efficient electric power generation. One of the most important goals in the SOFCs research is to decrease the operating temperature, which in turn will improve the stability and decrease the cost of various components enabling its widespread utilization. For realizing the aforementioned goal, it is imperative to identify suitable electrolyte materials that show enhanced conductivity in the intermediate temperature range (773–1,073 K). Sm-doped ceria (SDC) is considered a promising candidate for use as an electrolyte material for SOFC operation in intermediate temperature range due to the high oxygen ion conductivity. In this article, we present a theoretical investigation using first-principles and kinetic lattice Monte Carlo (KLMC) computations to highlight the trends in oxygen ion conductivity as a function of dopant content and temperature in SDC. Using first-principles calculations, oxygen vacancy formation and migration were examined at first, second, and third nearest neighbor positions to a Sm ion. The activation energies for oxygen vacancy migration along various pathways in SDC computed using first-principles were used as input to the KLMC model to study vacancy mediated diffusion. SDC with 20 % mole fraction of dopant content yields the maximum conductivity, which is in very good agreement with experimentally identified compositions. Rationale for increase in conductivity as a function of increase in dopant content and subsequent decrease in conductivity at higher dopant fractions in SDC is presented. This combined methodology of first-principles and KLMC computations is a useful tool for the design and identification of various ceria-based electrolyte materials used in SOFCs.     

Phonon-mediated thermal transport: Confronting theory and microscopic simulation with experiment

We discuss recent advances in the microscopic simulations of thermal conductivity through the prism of comparisons with experimental measurements. By dissecting the thermal conductivity into its constituent properties, heat capacity, phonon structure and anharmonic phonon properties, we show that the reliable prediction of the thermal transport properties over a range of conditions requires each to be described correctly. However, it is sometimes possible to obtain thermal conductivity values in overall good agreement with experiment through a cancellation of errors in the constituent properties. Major advances in the prediction of thermal transport properties in the last few years have come through increases in computational power and through development of numerical algorithms for the essentially exact solution of the linearized Boltzmann Transport Equation, with interatomic interactions described by first-principles electronic-structure calculations. This approach enables consistent ab initio determination of the thermal conductivity in the pure crystals. We also discuss the effects of various defects on thermal conductivity and compare results from the atomistic simulations, classical theories from the 1950s, and experimental measurements.     

Molecular dynamics simulations of lattice thermal conductivity and spectral phonon mean free path of PbTe: Bulk and nanostructures

In this work, molecular dynamics (MD) simulations are performed to predict the lattice thermal conductivity of PbTe bulk and nanowires. The thermal conductivity of PbTe bulk is first studied in the temperature range 300-800 K. Excellent agreement with experiments is found in the entire temperature range when a small vacancy concentration is taken into consideration. By studying various configurations of vacancies, it is found that the thermal conductivity in PbTe bulk is more sensitive to the concentration rather than the type and distribution of vacancies. Spectral phonon relaxation times and mean free paths in PbTe bulk are obtained using the spectral energy density (SED) approach. It is revealed that the majority of thermal conductivity in PbTe is contributed by acoustic phonon modes with mean free paths below 100 nm. The spectral results at elevated temperatures indicate molecular scale feature sizes (less than 10 nm) are needed to achieve low thermal conductivity for PbTe. Simulations on PbTe nanowires with diameters up to 12 nm show moderate reduction in thermal conductivity as compared to bulk, depending on diameter, surface conditions and temperature.     

Second-moment interatomic potential for aluminum derived from total-energy calculations and molecular dynamics application

We have obtained an interatomic potential for Al within the second-moment approximation of the tight-binding theory by fitting to the volume dependence of the total energy of the metal, computed by first-principles APW calculations. This scheme was applied to calculate the bulk modulus, elastic constants, vacancy formation and surface energies of Al. The predicted values are in good agreement with the measurements. We also have used this potential to perform molecular-dynamic simulations and determine the temperature dependence of the lattice constant and atomic mean-square displacements (MSDs), as well as the phonon spectra and surface related thermodynamic properties. A satisfactory accuracy has been obtained, denoting the success of the method.     

Temperature Dependent Thermal and Elastic Properties of High Entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2: Molecular Dynamics Simulation by Deep Learning Potential

High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now.In this work,variations of thermal and elastic properties of high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 with respect to temperature were predicted by molecular dynamics simulations.Firstly,a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/(A),in comparison with first-principles calculations.Then,temperature dependent lattice constants,anisotropic thermal expansions,anisotropic phonon thermal conductivities,and elastic properties of high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 from 0 ℃ to 2400 ℃ were evaluated,where the predicted room temperature values agree well with experimental measurements.In addition,intrinsic lattice distortions of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 were analyzed by displacements of atoms from their ideal positions,which are in an order of 10-3 (A) and one order of magnitude smaller than those in (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C.It indicates that lattice distortions in (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 is not so severe as expected.With the new paradigm of machine learning potential,deep insight into high entropy materials can be achieved in the future,since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential.     

Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.     

Low-temperature thermal properties of polypropylene

We have studied the low temperature thermal properties of a polypropylene copolymer (PP): thermal conductivity (between 0.1 and 4 K), specific heat (between 0.06 and 1 K) and thermal expansion (from 4.2 K to room temperature).Both the thermal conductivity and the specific heat temperature data were interpreted using the tunnelling model for phonon scattering.The measured thermal properties show that PP is suitable for use as thermal insulating support material in cryogenic devices.     

First-principles study on the formation of a vacancy in Ge under biaxial compressive strain

The effects of the biaxial compressive strain on the atomic relaxation and the formation energy of a neutral vacancy in Ge were investigated using first-principles calculations. Prior to this, the effects of the supercell size and Brillouin zone sampling were tested. The vacancy formation energy and atomic configuration around a vacancy are strongly affected by the inter-vacancy distance determined by the supercell size, due to the periodic boundary condition. The biaxial compressive strain reduced the formation energy of the vacancy nearly linearly by up to 1.34 eV as the magnitude of the biaxial compressive strain increased to the “Ge on Si (GoS)” condition. This was explained in terms of the bond strength characterized by the spatial electron density. The behavior of the vacancy in Ge was also compared with that in Si.     

Thermal conductivity modeling of core-shell and tubular nanowires

The heteroepitaxial growth of crystalline core-shell nanostructures of a variety of materials has become possible in recent years, allowing the realization of various novel nanoscale electronic and optoelectronic devices. The increased surface or interface area will decrease the thermal conductivity of such nanostructures and impose challenges for the thermal management of such devices. In the meantime, the decreased thermal conductivity might benefit the thermoelectric conversion efficiency. In this paper, we present modeling results on the lattice thermal conductivity of core-shell and tubular nanowires along the wire axis direction using the phonon Boltzmann equation. We report the dependence of the thermal conductivity on the surface conditions and the core-shell geometry for silicon core-germanium shell and tubular silicon nanowires at room temperature. The results show that the effective thermal conductivity changes not only with the composition of the constituents but also with the radius of the nanowires and nanopores due to the nature of the ballistic phonon transport. The results in this work have implications for the design and operation of a variety of nanoelectronic devices, optoelectronic devices, and thermoelectric materials and devices.     

Enhancement of thermal and mechanical properties of flexible graphene oxide/carbon nanotube hybrid films though direct covalent bonding

The thermal conductivity and mechanical properties of graphene oxide/multiwalled carbon nanotube (GO/MWCNT) hybrid films with and without covalent bonding were examined. Chlorinated GO and amino-functionalized MWCNT were bonded covalently to fabricate chemically bonded GO/MWCNT hybrid films. Mixtures of surface-modified GO and MWCNT were filtered and then subjected to hot-pressing to fabricate stacked films. Examination of these chemically bonded hybrid films revealed higher thermal conductivity than in physically bonded hybrid films, because of the synergetic interaction of functional groups in GO and MWCNT in the films. However, the addition of excess MWCNT to the films led to an increased phonon scattering density and a decreased thermal conductivity. The hybrid films fabricated by the optimized process endured about 20000 bending cycles without rupturing or losing their thermal conductivity. The mechanical properties showed enhanced performance after increased MWCNT loading at elevated temperature due to the reinforcement effect of the MWCNT between GO layers.     

Nanostructure design for drastic reduction of thermal conductivity while preserving high electrical conductivity

The design and fabrication of nanostructured materials to control both thermal and electrical properties are demonstrated for high-performance thermoelectric conversion. We have focused on silicon (Si) because it is an environmentally friendly and ubiquitous element. High bulk thermal conductivity of Si limits its potential as a thermoelectric material. The thermal conductivity of Si has been reduced by introducing grains, or wires, yet a further reduction is required while retaining a high electrical conductivity. We have designed two different nanostructures for this purpose. One structure is connected Si nanodots (NDs) with the same crystal orientation. The phonons scattering at the interfaces of these NDs occurred and it depended on the ND size. As a result of phonon scattering, the thermal conductivity of this nanostructured material was below/close to the amorphous limit. The other structure is Si films containing epitaxially grown Ge NDs. The Si layer imparted high electrical conductivity, while the Ge NDs served as phonon scattering bodies reducing thermal conductivity drastically. This work gives a methodology for the independent control of electron and phonon transport using nanostructured materials. This can bring the realization of thermoelectric Si-based materials that are compatible with large scale integrated circuit processing technologies.     

Acceptor related photoluminescence and field emission of ZnO:P nanostructures

ZnO:P nanobelts were self-assembly synthesized by thermal evaporation of Zn power and P₂O₅ mixture. The temperature dependence photoluminescence of ZnO:P nanostructures was studied from 81 to 291 K. As the temperature increased from 81 to 111 K, the PL intensity of DAP emission was obviously enhanced. The abnormal PL intensities were ascribed to the acceptor vibration with local phonon and lattice phonon assistant. The PL of zinc vacancy and its replica were well resolved due to the strenuous vibration of Zn vacancy. The replica of zinc vacancy emission increased while the visible emission gradually decreased with the temperature increase. It suggested that there were intensive deep acceptor vibration. The field emission properties of the ZnO:P nanostructures have been investigated according to the acceptor-related PL spectra. The influence of space charge effect on the field emission behaviors was also discussed.     

Enhanced thermoelectric performance of CuAlS2 by adding multi-walled carbon nanotubes

In this paper, we report the first-ever study on a relatively uniform dispersion of multi-walled carbon nanotubes (MWCNTs) in CuAlS₂ nanoparticles, synthesized by high-energy ball-milling, and study the thermoelectric properties of the bulk materials. A vortex mixer and bath sonicator are used to achieve well dispersion of nanotubes in the matrix, and then the powder is hot-pressed. Carbon nanotubes dispersed in the matrix improve electrical conductivity and Seebeck coefficient. The addition of MWCNT causes an increase in the grain boundary and facilitates phonon scattering, resulting in a reduction in the lattice thermal conductivity and finally total thermal conductivity. The optimum amount of carbon nanotubes is effective for reducing thermal conductivity and increasing electrical conductivity, thereby elevating the figure-of-merit of the nanocomposites. Finally, the figure-of-merit is highly influenced by total thermal conductivity, and the maximum figure-of-merit was obtained for CuAlS₂/0.5 wt% MWCNT composite, which indicated about 20% improvement.     

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high‐efficiency Na‐doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.     

Breaking the Minimum Limit of Thermal Conductivity of Mg3Sb2 Thermoelectric Mediated by Chemical Alloying Induced Lattice Instability

Thermal properties strongly affect the applications of functional materials, such as thermal management, thermal barrier coatings, and thermoelectrics. Thermoelectric (TE) materials must have a low lattice thermal conductivity to maintain a temperature gradient to generate the voltage. Traditional strategies for minimizing the lattice thermal conductivity mainly rely on introduced multiscale defects to suppress the propagation of phonons. Here, the origin of the anomalously low lattice thermal conductivity is uncovered in Cd-alloyed Mg₃Sb₂ Zintl compounds through complementary bonding analysis. First, the weakened chemical bonds and the lattice instability induced by the antibonding states of 5p-4d levels between Sb and Cd triggered giant anharmonicity and consequently increased the phonon scattering. Moreover, the bond heterogeneity also augmented Umklapp phonon scatterings. Second, the weakened bonds and heavy element alloying softened the phonon mode and significantly decreased the group velocity. Thus, an ultralow lattice thermal conductivity of ≈0.33 W m⁻¹ K⁻¹ at 773 K is obtained, which is even lower than the predicated minimum value. Eventually, Na_0.01Mg_1.7Cd_1.25Sb₂ displays a high ZT of ≈0.76 at 773 K, competitive with most of the reported values. Based on the complementary bonding analysis, the work provides new means to control thermal transport properties through balancing the lattice stability and instability.     

Low temperature thermal conductivity of twinned and untwinned indium-thallium alloys

The low-temperature lattice thermal conductivity of twinned and untwinned, martensitic and non-martensitic, indium-thallium alloys has been measured to probe the effect of twin boundaries on phonon thermal transport. The phonon scattering by electrons, sample surfaces, dislocations, and thallium impurities is accounted for adequately by existing theoretical models. The reduced lattice thermal conductivity seen in twinned samples is attributed to additional phonon scattering by twin boundaries and, for the polycrystalline samples, by grain boundaries. Phonon scattering by twin boundaries is much weaker than that generally reported for grain boundaries, and is well represented by an acoustic-mismatch model.