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The Kapitza resistance is of fundamental importance for the thermal stability of the interface between the ceramic top coat and the thermal growth oxide layer in the thermal barrier coating structure, which is widely used to protect high-temperature components in current gas turbine engines. The top coat typically consists of the ZrO $$_{\mathrm {2}}$$ partially stabilized by 8% Y $$_{\mathrm {2}}$$ O $$_{\mathrm {3}}$$ (YSZ), and the main component of the thermal growth oxide is $$\alpha $$ -Al $$_{\mathrm {2}}$$ O $$_{\mathrm {3}}$$ . In this work, the Kapitza resistance is found to be a small value of 0.69 $${\hbox {m}^{\mathrm {2}}}\cdot $$ K/GW for the YSZ/ $$\alpha $$ -Al $$_{\mathrm {2}}$$ O $$_{\mathrm {3}}$$ interface based on the heat dissipation simulation method. It indicates that the localization of thermal energy is rather weak, which is beneficial for the thermal stability of the YSZ/ $$\alpha $$ -Al $$_{\mathrm {2}}$$ O $$_{\mathrm {3}}$$ interface. This Kapitza resistance can be further reduced to 0.50 $${\hbox {m}^{\mathrm {2}}}\cdot $$ K/GW by a mechanical or thermal compressive strain of 8%. To explore the underlying mechanism for this strain effect, we analyze the phonon vibration and the microscopic deformation in the interface region. It is revealed that the interface becomes denser through the compression-induced twisting of some Al-O $$_{\mathrm {Zr}}$$ and Al-O $$_{\mathrm {Al}}$$ chemical bonds in the interface region, which is responsible for the reduction in the Kapitza resistance. The temperature effect and crystal size effect on the Kapitza resistance of the YSZ/ $$\alpha $$ -Al $$_{\mathrm {2}}$$ O $$_{\mathrm {3}}$$ interface are also systematically studied. These findings shall provide valuable information for further understanding of the thermal conductivity and thermal stability of the thermal barrier coating structures.

Based on the chemical cross-linking method, this paper uses polydimethylsiloxane with various viscosities of 10 cSt, 20 cSt, 50 cSt, and 100 cSt to synthesize mesoporous and macroporous SiOC ceramics. Their thermal conductivities are measured by using 3ω method with high accuracy. Three typical models for their thermal conductivities, i.e., series model (SM), maxwell-Eucken 1 model (ME1), and effective medium theory (EMT) model, are utilized to derive the empirical formula through the multi-parameter linear optimization algorithm, which agrees well with the experimental results. The effects of pore size and specific surface area on the overall thermal conductivity of the porous structure are explored. Interestingly, it is found that the thermal conductivities of both gas phase and solid phase inside the porous structure increase with the increasing pore size at the nanometer scale, but the overall thermal conductivity of the porous structure decreases with the increasing pore size. Scanning electron microscopy graphs corroborate that the extension of the heat transfer route and the barrier of more pores between the solid phases together cause the reduction of the gas-solid coupling thermal conductivity of SiOC ceramics with larger pore size. On the contrary, the miniaturization of individual particles through modulating the synthesis parameters can increase the number of small pores in the sample itself to meet the pseudo-lattice vibration conditions, which results in the increment of the gas-solid coupling thermal conductivity and the overall thermal conductivity of the porous structure. These findings would provide meaningful guidance for designing SiOC porous ceramic super-insulation materials with extremely low thermal conductivity.

The thermal conductivity of two-dimensional materials like graphene can efficiently be tuned by introducing holes, in which the density and distribution of the holes are the key parameters. Furthermore, the distribution of holes can induce a variation as high as 74% in the thermal conductivity for porous graphene with a given density of holes. Therefore, an existing challenge is to find the optimal distribution of holes that can minimize or maximize the thermal conductivity of porous graphene as the design space expands dramatically with increasing hole density. We therefore apply an inverse design methodology based on machine learning to reveal the relationship between hole distribution and thermal conductivity reduction in monolayer graphene. The methodology reveals that holes that are randomly distributed transverse to the direction of heat flow, but that exhibit some periodicity along the direction of heat flow, represent the optimal distribution to minimizing the thermal conductivity for porous graphene. Lattice dynamics calculations and wave packet simulations reveal that this spatial distribution effectively causes localization of the phonon modes in porous graphene, which reduces the thermal conductivity. Overall, this work demonstrates the power of machine learning-based design approaches to efficiently obtain new physical insights for scientific problems of interest.

A thin film sandwich structure (TFSS) was designed to improve the wearing comfort, sensing performance, immunity to humidity, and miniaturization of the hydrogel sensor synchronously. The TFSS sensor was fabricated using the layer-by-layer spin-coating technology with a thickness of only $12.15\ \mu\mathrm{m}$ for the LiBr-percolated hydrogel layer. Benefiting from the ultrathin device structure, the low thermal capacity of LiBr solution, new capacitance measurement mode and ionic transport manner, the TFSS thermistor displays unprecedented sensitivity of 24.54%/°C, ultrafast response time (0.19 s), recovery time (0.08 s), high resolution (0.8 °C) and a broad detection range (−30∼96 °C). The sensitivity of our temperature sensor is one order of magnitude higher than those of state-of-the-art stretchable temperature sensors. In addition to temperature, the tensile strain can also be selectively detected with distinguishable signals, endowing the bimodal sensor with multifunctional sensing capability.

Resistance change under mechanical stimuli arouses mass operational heat, damaging the performance, lifetime, and reliability of stretchable electronic devices, therefore rapid thermal heat dissipating is necessary. Here we report a stretchable strain sensor with outstanding thermal management. Besides a high stretchability and sensitivity testified by human motion monitoring, as well as long-term durability, an enhanced thermal conductivity from the casted thermoplastic polyurethane-boron nitride nanosheets layer helps rapid heat transmission to the environments, while the porous electrospun fibrous thermoplastic polyurethane membrane leads to thermal insulation. A 32% drop of the real time saturated temperature is achieved. For the first time we in-situ investigated the dynamic operational temperature fluctuation of stretchable electronics under repeating stretching-releasing processes. Finally, cytotoxicity test confirms that the nanofillers are tightly restricted in the nanocomposites, making it harmless to human health. All the results prove it an excellent candidate for the next-generation of wearable devices., Though stretchable strain sensors are attractive for next-generation applications due to their high sensitivity, heat generated in these devices limits their reliability. Here, the authors report boron nitride nanosheet-based stretchable strain sensors with enhanced thermal management.

High thermal conductivity nanofilms play a key role in the heat dissipation of microdevices, and the preparation process greatly influences the thermal conductivity of the nanofilms. In this study, four Al nanofilms with different thicknesses were fabricated via the electron beam evaporation method, and their effective thermal conductivity (ETC) values were measured using a specially designed microsensor-based third harmonic 3ω method. Due to the internal pore channels and nanometer-level feature size, the ETC of nanofilms exhibited a downward trend when the thickness increases. Significantly, the pores produced by the preparation induce the electron-phonon nonequilibrium effect inside the film, resulting in an anomalous change of the film thermal conductivity versus the thickness.

Using non-equilibrium molecular dynamics simulations, we investigate the thermal transport in van der Waals heterostructures consisting of alternating multilayer graphene and multilayer MoS2. It is found that the thermal conductance at graphene/MoS2 (G/M) interface is much lower than that at graphene/graphene (G/G) and MoS2/MoS2 (M/M) interfaces. This low interfacial thermal conductance is attributed to the low friction at G/M interface, which significantly reduces the contribution of shearing modes to the thermal conductance. It is also found that there is no thermal rectification at the G/M interface as the thermal conductance is independent on the heat flux direction. Moreover, the interfacial thermal conductance can be effectively tuned by cross-plane strain. More specifically, a 5% tensile strain is able to reduce the interfacial thermal conductance by 70%; while a 5% compressive strain is able to increase the thermal conductance by 150%. Unexpectedly, the G/M interfacial thermal conductance is found to increase with increasing the defect density near the interface, which is in strong contrast to the in-plane thermal conductivity. This unexpected increase in thermal conductance can be explained by the enhanced phonon coupling at the G/M interface arising from the enhanced interface friction caused by the defects.

As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.

BiCuSeO is a promising thermoelectric material but needs a complicated synthesis process. In this work, fine-grained BiCuSeO samples with high phase purity were synthesized by a combination of mechanical alloying (MA) and spark plasma sintering (SPS). It is found that BiCuSeO compounds can be formed by the reaction between BiSe and CuO, after BiSe appeared firstly when using Bi, Se, and CuO as raw materials during MA process. The electrical conductivity of the corresponding bulk attains a modest enhancement due to the significantly increased carrier concentration although the carrier mobility was reduced compared with the sample prepared by classical solid-state reaction. The total thermal conductivity is below 0.7 W/mK in the whole measured temperature range, which is attributable to the increased defects and gain boundaries. As a result, a relatively high ZT value of 0.50 was obtained for the pristine BiCuSeO sample. In addition, when Bi was replaced by Ca element, the electrical transport property of the samples was improved, resulting in an enhancement of ZT in the whole measured temperature range.

Fullerence encapsulation has been proven to be a powerful approach to enhance mechanical and electronic properties of the single-walled carbon nanotubes (SWCNTs). However, some discrepancies emerge in recent studies of the fullerence encapsulation effect on the thermal conductivity of SWCNTs. More specifically, most previous theoretical works predicted slightly enhancement of the thermal conductivity, but the recent experiment by Kodama et al. (Nat. Mat. 16, 892 (2017)) observed clear reduction of the thermal conductivity in SWCNTs by fullerence encapsulation. We perform molecular dynamics simulations to revisit this issue, by comparatively investigate the thermal conductivity of the SWCNT (n, n) and the corresponding peapod (n, n) with n = 8, 9, 10, and 11. We find that the fullerence encapsulation can reduce the thermal conductivity of narrower SWCNTs with n = 8 and 9, but it can slightly enhance the thermal conductivity of thicker SWCNTs with n = 10 and 11. The underlying mechanisms for these opposite effects are explored by analyzing the encapsulation induced structural deformation and the variation of the phonon dispersion of the SWCNT. We illustrate that the reduction effect observed in the recent experiment is related to the mechanism for reduction of the thermal conductivity for narrower SWCNTs here, while previous numerical works correspond to the enhancement effect for thicker SWCNTs found in the present work. Our findings shed lights on clarifying the discrepancy between previous numerical predictions and experimental observations on the effect of the fullerence encapsulation on the thermal conductivity of the SWCNT.

Inorganic aerogels have been attracting great interest owing to their distinctive structures and properties. However, the practical applications of inorganic aerogels are greatly restricted by their high brittleness and high fabrication cost. Herein, inspired by the cancellous bone, we have developed a novel kind of hydroxyapatite (HAP) nanowire-based inorganic aerogel with excellent elasticity, which is highly porous (porosity ≈ 99.7%), ultralight (density 8.54 mg/cm3, which is about 0.854% of water density), and highly adiabatic (thermal conductivity 0.0387 W/m·K). Significantly, the as-prepared HAP nanowire aerogel can be used as the highly efficient air filter with high PM2.5 filtration efficiency. In addition, the HAP nanowire aerogel is also an ideal candidate for continuous oil-water separation, which can be used as a smart switch to separate oil from water continuously. Compared with organic aerogels, the as-prepared HAP nanowire aerogel is biocompatible, environmentally friendly, and low-cost. Moreover, the synthetic method reported in this work can be scaled up for large-scale production of HAP nanowires, free from the use of organic solvents. Therefore, the as-prepared new kind of HAP nanowire aerogel is promising for the applications in various fields.

Monolayer molybdenum disulfide (MoS2), a new two-dimensional material beyond graphene, has attracted tremendous attention recently. Its applications in nanoelectronic and thermoelectric devices usually require manipulating the thermal transport properties. Using nonequilibrium molecular dynamics simulations, we investigated the effects of lattice defects and mechanical strain on the thermal conductivity of MoS2. We found that the thermal conductivity of monolayer MoS2 can be effectively tuned by introducing even a small amount of lattice defects. For example, a 0.5% concentration of mono-Mo vacancies is able to reduce the thermal conductivity by about 60%. Remarkably, the thermal conductivity of the defected sample can further be tuned by mechanical strain. For example, a 12% tensile strain is able to reduce the thermal conductivity by another 60%. We also found that the tensile strain exerts almost the same impact on the thermal conductivity of both pristine and defective MoS2, which signifies that there...

We perform classical molecular dynamics simulations to investigate the mechanical compression effect on the thermal conductivity of the single-walled carbon nanotube (SWCNT) forest, in which SWCNTs are closely aligned and parallel with each other. We find that the thermal conductivity can be linearly enhanced by increasing compression before the buckling of SWCNT forests, but the thermal conductivity decreases quickly with further increasing compression after the forest is buckled. Our phonon mode analysis reveals that, before buckling, the smoothness of the inter-tube interface is maintained during compression, and the inter-tube van der Waals interaction is strengthened by the compression. Consequently, the twisting-like mode (good heat carrier) is well preserved and its group velocity is increased by increasing compression, resulting in the enhancement of the thermal conductivity. The buckling phenomenon changes the circular cross section of the SWCNT into ellipse, which causes effective roughness at the inter-tube interface for the twisting motion. As a result, in ellipse SWCNTs, the radial breathing mode (poor heat carrier) becomes the most favorable motion instead of the twisting-like mode and the group velocity of the twisting-like mode drops considerably, both of which lead to the quick decrease of the thermal conductivity with further increasing compression after buckling.

Thermal properties of single wall carbon nanotube sheets (SWCNT-sheets) are of significant importance in the area of thermal management, as an isolated SWCNT possesses high thermal conductivity of the value about 3000 W m(-1) K(-1). Here we report an indirect method of estimating the thermal conductivity of a nanometer thick suspended SWCNT-sheet by employing the Raman scattering technique. Tube diameter size is examined by the transmissions electron microscopy study. The Raman analysis of the radial breathing modes predicts narrow diameter size distribution with achiral (armchair) symmetry of the constituent SWCNTs. From the first order temperature coefficient of the A1g mode of the G band along with the laser power dependent frequency shifting of this mode, the thermal conductivity of the suspended SWCNT-sheet is estimated to be about ∼18.3 W m(-1) K(-1). Our theoretical study shows that the thermal conductivity of the SWCNT-sheet has contributions simultaneously from the intratube and intertube thermal transport. The intertube thermal conductivity (with contributions from the van der Waals interaction) is merely around 0.7 W m(-1) K(-1), which is three orders smaller than the intratube thermal conductivity, leading to an abrupt decrease in the thermal conductivity of the SWCNT-sheet as compared to the reported value for isolated SWCNT.

We perform molecular dynamics simulations to investigate the reduction of the thermal conductivity by kinks in silicon nanowires. The reduction percentage can be as high as 70% at room temperature. The temperature dependence of the reduction is also calculated. By calculating phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal conductivity: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, i.e a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. Our work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelectric materials., Comment: Nano. Lett. accepted (2013)

Graphene, as a passivation layer, can be used to protect the black phosphorus from the chemical reaction with surrounding oxygen and water. However, black phosphorus and graphene heterostructures have low efficiency of heat dissipation due to its intrinsic high thermal resistance at the interfaces. The accumulated energy from Joule heat has to be removed efficiently to avoid the malfunction of the devices. Therefore, it is of significance to investigate the interfacial thermal dissipation properties and manipulate the properties by interfacial engineering on demand. In this work, the interfacial thermal conductance between few-layer black phosphorus and graphene is studied extensively using molecular dynamics simulations. Two critical parameters, the critical power Pcr to maintain thermal stability and the maximum heat power density Pmax with which the system can be loaded, are identified. Our results show that interfacial thermal conductance can be effectively tuned in a wide range with external strains and interracial defects. The compressive strain can enhance the interfacial thermal conductance by one order of magnitude, while interface defects give a two-fold increase. These findings could provide guidelines in heat dissipation and interfacial engineering for thermal conductance manipulation of black phosphorus-graphene heterostructure-based devices., Comment: 33 pages, 22 figures

Polycrystalline In4Se3−x thermoelectric materials were prepared by a combined process of mechanical alloying and hot pressing (MA-HP), and the effect of Se deficiency on their thermoelectric properties was studied. Starting from elemental In and Se powders, In4Se3 compound was synthesized after mechanical alloying for 1 h. Single-phase In4Se3 compound was obtained in the sample without Se deficiency, while some In impurity was detected in Se-deficient samples. With increasing Se deficiency x, the carrier concentration n increased, and the electrical resistivity and absolute value of Seebeck coefficient of In4Se3−x decreased rapidly. All of the In4Se3−x samples (x > 0) had higher ZT than the stoichiometric In4Se3, with In4Se2.65 showing the highest ZT because of its low thermal conductivity. The maximum ZT reached 0.93 at 700 K.

Starting from elemental powders of Ag, Sn, Pb, Sb, and Te, Sn-substituted single-phase LAST (AgPbmSbTem+2) materials were synthesized by a combined process of mechanical alloying and plasma-activated sintering. The effect of Sn content on thermoelectric (TE) properties of the Sn-substituted LAST materials was investigated in detail. With increase of Sn/Pb ratio, the fraction of SnTe in the (Pb,Sn)Te-based solid solution increased, and the lattice spacing decreased accordingly. The electrical conductivity, thermal conductivity, and power factor also increased with increasing substitutional Sn content. It was found that the composition AgPb9Sn9SbTe20 had superior TE properties compared with other compositions, and the maximum ZT of 0.70 was achieved at 675 K for this composition.

We conducted extensive molecular dynamics simulations to investigate the thermal conductivity of polycrystalline hexagonal boron-nitride (h-BN) films. To this aim, we constructed large atomistic models of polycrystalline h-BN sheets with random and uniform grain configuration. By performing equilibrium molecular dynamics (EMD) simulations, we investigated the influence of the average grain size on the thermal conductivity of polycrystalline h-BN films at various temperatures. Using the EMD results, we constructed finite element models of polycrystalline h-BN sheets to probe the thermal conductivity of samples with larger grain sizes. Our multiscale investigations not only provide a general viewpoint regarding the heat conduction in h-BN films but also propose that polycrystalline h-BN sheets present high thermal conductivity comparable to monocrystalline sheets.

Single-layer graphene is so flexible that its flexural mode (also called the ZA mode, bending mode, or out-of-plane transverse acoustic mode) is important for its thermal and mechanical properties. Accordingly, this review focuses on exploring the relationship between the flexural mode and thermal and mechanical properties of graphene. We first survey the lattice dynamic properties of the flexural mode, where the rigid translational and rotational invariances play a crucial role. After that, we outline contributions from the flexural mode in four different physical properties or phenomena of graphene -- its thermal conductivity, thermal expansion, Young's modulus, and nanomechanical resonance. We explain how graphene's superior thermal conductivity is mainly due to its three acoustic phonon modes at room temperature, including the flexural mode. Its coefficient of thermal expansion is negative in a wide temperature range resulting from the particular vibration morphology of the flexural mode. We then describe how the Young's modulus of graphene can be extracted from its thermal fluctuations, which are dominated by the flexural mode. Finally, we discuss the effects of the flexural mode on graphene nanomechanical resonators, while also discussing how the essential properties of the resonators, including mass sensitivity and quality factor, can be enhanced., Comment: Journal of Physics: Condensed Matter, topical review, published

We investigate the in-plane and cross-plane thermal conductivities of molybdenum disulfide (MoS2) using non-equilibrium molecular dynamics simulations. We find that the in-plane thermal conductivity of monolayer MoS2 is about 19.76 W mK(-1). Interestingly, the in-plane thermal conductivity of multilayer MoS2 is insensitive to the number of layers, which is in strong contrast to the in-plane thermal conductivity of graphene where the interlayer interaction strongly affects the in-plane thermal conductivity. This layer number insensitivity is attributable to the finite energy gap in the phonon spectrum of MoS2, which makes the phonon-phonon scattering channel almost unchanged with increasing layer number. For the cross-plane thermal transport, we find that the cross-plane thermal conductivity of multilayer MoS2 can be effectively tuned by applying cross-plane strain. More specifically, a 10% cross-plane compressive strain can enhance the thermal conductivity by a factor of 10, while a 5% cross-plane tensile strain can reduce the thermal conductivity by 90%. Our findings are important for thermal management in MoS2 based nanodevices and for thermoelectric applications of MoS2.

We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally-obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2., Comment: Two supplemental documents for LAMMPS users can be found in this submission. (1). mos2.sw: Stillinger-Weber potential script for MoS2 in LAMMPS. (2). pair_sw.cpp: the modified source file for LAMMPS

The upper limit of the thermal conductivity and the mechanical strength are predicted for the polyethylene chain, by performing the {\it ab initio} calculation and applying the quantum mechanical non-equilibrium Green's function approach. Specially, there are two main findings from our calculation: (1). the thermal conductivity can reach a high value of 310 W/K/m in a 100 nm polyethylene chain at room temperature; (2). the Young's modulus in the polyethylene chain is as high as 374.5 GPa, and the polyethylene chain can sustain $32.85% \pm 0.05%$ (ultimate) strain before undergoing structural phase transition into gaseous ethylene., Comment: published in J. Appl. Phys. (2012)

This paper investigates the morphology and in-plane thermal conductivity of hybrid graphene sheets (HGSs), which consist of un-hydrogenated and single-side or double-side hydrogenated strips, via molecular dynamics simulation. The study shows that the hydrogenation styles and hydrogen coverage significantly affect the morphology and thermal conductivity of HGSs. The thermal conductivity of HGSs decreases dramatically, compared to that of pure graphene sheets, and the magnitude falls in the range of 30%-75%. Such differences are explained by conducting the phonon spectra analysis. Published version

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect (substitution of C by Si atom) is investigated by non-equilibrium Green's function (NEGF) formalism combined with first-principle calculations density-functional theory with local density approximation. An efficient correction to the force constant matrix is presented to solve the conflict between the long-range character of the {\it ab initio} approach and the first-nearest-neighboring character of the NEGF scheme. In nanoribbon with a vacancy defect, the thermal conductance is very sensitive to the position of the vacancy defect. A vacancy defect situated at the center of the nanoribbon generates a saddle-like surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance. For the Si defect, the position of the defect plays no role for the value of the thermal conductance, since the defective region is limited within a narrow area around the defect center., accepted by APL

We perform a theoretical investigation on the thermal conductivity of single-walled boron nitride nanotubes (SWBNT) using the kinetic theory. By fitting to the phonon spectrum of boron nitride sheet, we develop an efficient and stable Tersoff-derived interatomic potential which is suitable for the study of heat transport in sp2 structures. We work out the selection rules for the three-phonon process with the help of the helical quantum numbers $(\kappa, n)$ attributed to the symmetry group (line group) of the SWBNT. Our calculation shows that the thermal conductivity $\kappa_{\rm ph}$ diverges with length as $\kappa_{\rm ph}\propto L^{\beta}$ with exponentially decaying $\beta(T)\propto e^{-T/T_{c}}$, which results from the competition between boundary scattering and three-phonon scattering for flexure modes. We find that the two flexure modes of the SWBNT make dominant contribution to the thermal conductivity, because their zero frequency locates at $\kappa=\pm\alpha$ where $\alpha$ is the rotational angle of the screw symmetry in SWBNT., Comment: accepted by PRB

The phonon thermal conductivity in diamond nanowires (DNW) is studied by molecular dynamics simulation. It is found that the thermal conductivity in narrower DNW is lower and does not show obvious temperature dependence; a very small value (about 2.0 W/m/K) of thermal conductivity is observed in ultra-narrow DNW, which may be of potential applications in thermoelectric devices. These two phenomena are probably due to the dominant surface effect and phonon confinement effect in narrow DNW. Our simulation reveals a high anisotropy in the heat transport of DNW. Specifically, the thermal conductivity in DNW along [110] growth direction is about five times larger than that of [100] and [111] growth directions. The anisotropy is believed to root in the anisotropic group velocity for acoustic phonon modes in DNW along three different growth directions., Comment: accepted by PRB

The topological effect on thermal conductivity is investigated through the comparison among graphene nanoribbons, carbon nanotubes and the Mobius-like graphene strips (MGS), by molecular dynamics simulation. It is found that the thermal conductivity of MGS is less than one half of that of graphene nanoribbons. The underlying mechanism whereby MGS acquire such low thermal conductivity may be attributable to the enhanced phonon-phonon scattering and localization property, which are induced by the nontrivial topology of Mobius strip. Moreover by counting in the dimensions of MGS, a lower length/width ratio reduces its thermal conductivity, as the phonon-phonon scattering and localization within might be further elevated., accepted by Journal of Applied Physics

Thermal conductivity of graphene nanoribbons (GNR) with length 106~{\AA} and width 4.92~{\AA} after isotopic doping is investigated by molecular dynamics with quantum correction. Two interesting phenomena are found: (1) isotopic doping reduces thermal conductivity effectively in low doping region, and the reduction slows down in high doping region; (2) thermal conductivity increases with increasing temperature in both pure and doped GNR; but the increasing behavior is much more slowly in the doped GNR than that in pure ones. Further studies reveal that the physics of these two phenomena is related to the localized phonon modes, whose number increases quickly (slowly) with increasing isotopic doping in low (high) isotopic doping region., Comment: 6 figs

A generalized Langevin equation with quantum heat baths [quantum molecular dynamics (QMD)] for thermal transport is derived with the help of nonequilibrium Green's function (NEGF) formulation. The exact relationship of the quasiclassical approximation to NEGF is demonstrated using Feynman diagrams of the nonlinear self-energies. To leading order, the retarded self-energies agree but QMD and NEGF differ in lesser/greater self-energies. An implementation for general systems using Cholesky decomposition of the correlated noises is discussed. Some means of stabilizing the dynamics is given. Thermal conductance results for graphene strips and carbon nanotubes are presented. The ``quantum correction'' method is critically examined.

We investigate the phonon thermal conductance of graphene regarding the graphene sheet as the large-width limit of graphene strips in the ballistic limit. We find that the thermal conductance depends weakly on the direction angle $\theta$ of the thermal flux periodically with period $\pi/3$. It is further shown that the nature of this directional dependence is the directional dependence of group velocities of the phonon modes in the graphene, originating from the $D_{6h}$ symmetry in the honeycomb structure. By breaking the $D_{6h}$ symmetry in graphene, we see more obvious anisotropic effect in the thermal conductance as demonstrated by dimerite., Comment: enlarged version, in PRB

We point out that the origin of the commonly occurred boundary temperature jump in the application of No\'se-Hoover heat bath in molecular dynamics is related to the edge modes, which are exponentially localized at the edge of the system. If heat baths are applied to these edge regions, the injected thermal energy will be localized thus leading to a boundary temperature jump. The jump can be eliminated by shifting the location of heat baths away from edge regions. Following this suggestion, a very good temperature profile is obtained without increasing any simulation time, and the accuracy of thermal conductivity calculated can be largely improved., Comment: accepted by PRB, brief report, references added, typo corrected

The single-layer black phosphorus is characteristic for its puckered structure, which has leaded to highly anisotropy in its optical, electronic, and mechanical properties. We report, using the non-equilibrium Green's function approach and the first-principles method, that the anisotropy in the thermal conduction is very weak in the single-layer black phosphorus -- the difference between two in-plane directions is less than 4%. Our phonon calculations disclose that the two in-plane acoustic phonon branches have lower group velocities in the direction perpendicular to the pucker, as the black phosphorus is softer in this direction, leading to a weakening effect for the thermal conductance in the perpendicular direction. However, the out-of-plane acoustic phonon branch behaviors abnormally; i.e., the group velocity of this phonon branch is higher in the perpendicular direction, although the single-layer black phosphorus is softer in this direction. The abnormal behavior of the out-of-plane phonon branch is closely related to the highly anisotropic Poisson's ratio in the single-layer black phosphorus. As a result of the counteraction between the in-plane phonon modes and the out-of-plane phonon mode, the thermal conductance in the perpendicular direction is weaker than the parallel direction, but the anisotropy is pretty small., Nanotechnology, published

This paper investigates the thermal conductivity of silicene nanosheets (SiNSs) via molecular dynamic simulation. It shows that the thermal conductivity of pristine SiNSs is about 25–30 W mK −1 and exhibits anisotropic behaviour. Moreover, it is found that isotopic doping is efficient in reducing the thermal conductivity of SiNSs. When SiNSs are randomly doped with 30 Si at the doping percentage of 50%, a maximum reduction of about 20% is obtained. This reduction can be increased when the dopants are arranged into a superlattice pattern. The thermal conductivity of these superlattice-structured SiNSs changes non-monotonically as the thickness of their lattice layers decreases. Detailed analysis of the phonon spectra demonstrates that the competing mechanism between the interface scattering and confinement effects of the phonon modes is responsible for this non-monotonical behaviour.

The extreme reduction of the thermal conductivity by defects and folds in carbon nanocoils (CNCs) are first demonstrated using nonequilibrium molecular dynamics simulations. The thermal conductivity for two different defect types with five different folds in the CNCs is extensively studied and the maximum reduction can be up to 70% at both room temperature and 600 K by comparison of the corresponding straight single-walled carbon nanotubes. We reveal that the phonon scattering by coupled defects and folds can govern the reduction of the thermal conductivity by calculating phonon polarization vectors.

We apply united-atom (UA) nonequilibrium molecular dynamics simulations to predict the thermal conductivity for amorphous polyethylene at room temperature with chain length (CL) increasing from 4 to 1260. The thermal conductivity is determined by the competition of the two mechanisms: (1) The collision among the UA beads and (2) the phonon vibration. The collision mechanism mainly dominates CL≤7, while the phonon vibration mechanism mainly determines CL ≥ 140. The competition between the two mechanisms determines 12 ≤ CL ≤ 140. In particular, the thermal conductivity for 8 ≤ CL ≤ 11 cannot be obtained accurately due to the aggregation of the beads.

We perform molecular dynamics (MD) simulations to investigate the effect of polar surfaces on the thermal transport in zinc oxide (ZnO) nanowires. We find that the thermal conductivity in nanowires with free polar (0001) surfaces is much higher than in nanowires that have been stabilized with reduced charges on the polar (0001) surfaces, and also hexagonal nanowires without any transverse polar surfaces. From normal mode analysis, we show that the higher thermal conductivity is due to a shell-like reconstruction that occurs for the free polar surfaces. This shell-like reconstruction suppresses twisting motion in the nanowires such that the bending phonon modes are not scattered by the other phonon modes, and leads to substantially higher thermal conductivity in the ZnO nanowire with free polar surfaces. Furthermore, the auto-correlation function of the normal mode coordinate is utilized to extract the phonon lifetime, which leads to a concise explanation for the higher thermal conductivity in ZnO nanowires with free polar surfaces. Our work demonstrates that ZnO nanowires without polar surfaces, which exhibit low thermal conductivity, are more promising candidates for thermoelectric applications than nanowires with polar surfaces (and also high thermal conductivity)., Comment: Nanoscale,published