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In this work, the impact of roughness and amorphization on the effective thermal conductivity of silicon nanofilms and nanowires is studied with atomistic simulations. The discrepancy between simulations and experimental measurements shows that it is necessary to consider realistic roughness and amorphization to reach an agreement. We show that subnanometric roughness and specific correlation length can reduce thermal transport by a factor of two in both nanofilms and nanowires; in addition, this reduction is even more pronounced than the one related to the existence of native oxides or amorphous phases on nanostructure edges. Furthermore, an interfacial thermal resistance parallel to the heat flux is observed. This thermal resistance is increasing upon the increase of the amorphous shell thickness, reaching a maximum value for thickness of 6 nm. Our findings could improve the strategy to elaborate nanomaterials with enhanced thermoelectric efficiency by tuning thermal conductivity through the engineering of surfaces. [ABSTRACT FROM AUTHOR]

The thermal conductivity of phononic-like membranes and their analogous nano-wires are computed with both Molecular Dynamics and Monte Carlo simulations. Using both methods, the thermal conductivity of the porous membranes with cylindrical pores is found to be lower than the nano-wire’s one for the same scattering surface to volume. This work confirms experimental observations by Nomura et al. (2015) and Yu et al. (2010) for similar systems. The lower thermal conductivity of membranes is attributed to the different orientation of the scattering surfaces either at the pores or at free surfaces and not to coherent effects. The comparison between the porous membranes and nano-wires with periodic cylindrical modulations shows that thermal transport is hindered further when there are surfaces perpendicular to the heat flow. These results show that coherent effects have no impact on thermal transport in both kind of nano-structures at room temperature. [ABSTRACT FROM AUTHOR]

This article is devoted to heat transfer involving radiation and conduction. Considering a nongray, purely absorbing medium, the radiative heat transfer equation (RTE) and the energy balance equation are both solved with the finite-volume method (FVM). The energy equation is coupled to the RTE through the radiative source term. Several differencing scheme efficiencies are discussed for the case of strong opacities. Validation of the model with various benchmarks simulating high-temperature glass melting, including one- and two-dimensional geometries, is achieved. [ABSTRACT FROM AUTHOR]

Nano-engineering crystalline materials can be used to tailor their thermal properties. By adding new nanoscale phonon scattering centers and controlling their size, one can effectively decrease the phonon mean free path, hence the thermal conductivity of a fully crystalline material. In this Letter, we use the 3ω method in the temperature range of 100–300 K to experimentally report on the more than threefold reduction of the thermal conductivity of an epitaxially grown crystalline germanium thin film with embedded polydispersed crystalline Ge3Mn5 nano-inclusions with diameters ranging from 5 to 25 nm. A detailed analysis of the structure of the thin film coupled with Monte Carlo simulations of phonon transport highlights the role of the nano-inclusions volume fraction in the reduction of the phononic contribution to the thermal conductivity, in particular its temperature dependence, leading to a phonon mean free path that is set by geometrical constraints. [ABSTRACT FROM AUTHOR]

The authors study the thermal conductivity of silicon nanowires by simulation of phonon motion and interactions through a dedicated Monte Carlo model. This model solves the Boltzmann transport equation, taking into account silicon acoustic mode dispersion curves and three phonon interactions (the normal and umklapp processes). The confinement, which limits the thermal conductivity in such structures, is described by diffuse reflection at lateral boundaries of the nanowire without any adjustment by a boundary collision time, which depends on a specularity factor. They compare simulation results to experimental measurements on similar nanostructures. A good agreement is achieved for almost all the considered diameters. [ABSTRACT FROM AUTHOR]

This paper studies thermal transport in nanoporous silicon with a significant specific surface area. First, the equilibrium molecular dynamics approach was used to obtain the dependence of thermal conductivity on a specific surface area. Then, a modified phonon transport kinetic theory-based approach was developed to analyze thermal conductivity. Two models were used to evaluate the phonon mean free path in the porous materials. The first model assumes that the dependence of the mean free path only relies on the specific surface area, and the second one also considers the mean free path variation with the porosity. Both approaches approximate molecular dynamics data well for the smaller porosity values. However, the first model fails for highly porous matrixes, while the second one matches well with molecular dynamics simulations across all considered ranges of the porosities. This work illustrates that the phonon mean free path dependence with the porosity/volume fraction of composite materials is essential for describing thermal transport in systems with significant surface-to-volume ratio. [ABSTRACT FROM AUTHOR]

This paper investigates thermal transport in a nanocomposite system consisting of a porous silicon matrix filled with ionic liquid. Firstly, the thermal conductivity and heat capacity of two imidazolium and one ammonium ionic liquids were evaluated using the photoacoustic approach in piezoelectric configuration and differential scanning calorimetry, respectively. Then, the thermal transport properties of the composite system "ionic liquid confined inside porous silicon matrix" were investigated with the photoacoustic approach in gas-microphone configuration. The results demonstrated a significant enhancement of the thermal conductivity of the composite system when compared to the individual components, i.e. (i) more than two times for pristine porous silicon and (ii) more than eight times for ionic liquids. These results provide new paths for innovative solutions in the field of thermal management, particularly in the development of highly efficient energy storage devices. [ABSTRACT FROM AUTHOR]

Germanium manganese compounds exhibit a variety of stable and metastable phases with different stoichiometries. These materials entail interesting electronic, magnetic, and thermal properties both in their bulk form and as heterostructures. Here, we develop and validate a transferable machine learning potential, based on the high-dimensional neural network formalism, to enable the study of Mn x Ge y materials over a wide range of compositions. We show that a neural network potential fitted on a minimal training set reproduces successfully the structural and vibrational properties and the thermal conductivity of systems with different local chemical environments, and it can be used to predict phononic effects in nanoscale heterostructures. [ABSTRACT FROM AUTHOR]

In this work, we investigated the tensile and compression forces effect on the thermal conductivity of silicon. We used the equilibrium molecular dynamics approach for the evaluation of thermal conductivity considering different interatomic potentials. More specifically, we tested Stillinger-Weber, Tersoff, Environment-Dependent Interatomic Potential, and Modified Embedded Atom Method potentials for the description of silicon atom motion under different strain and temperature conditions. It was shown that the Tersoff potential gives a correct trend of the thermal conductivity with the hydrostatic strain, while other potentials fail, especially when the compression strain is applied. Additionally, we extracted phonon density of states and dispersion curves from molecular dynamics simulations. These data were used for direct calculations of the thermal conductivity considering the kinetic theory approach. Comparison of molecular dynamics and kinetic theory simulations results as a function of strain and temperature allowed us to investigate the different factors affecting the thermal conductivity of the strained silicon. [ABSTRACT FROM AUTHOR]

The majority of the silicon devices contain amorphous phase and amorphous/crystalline interfaces which both considerably affect the transport of energy carriers as phonons and electrons. In this article, we investigate the impact of amorphous phases (both amorphous silicon and amorphous SiO2) of silicon nanoporous membranes on their thermal properties via molecular dynamics simulations. We show that a small fraction of amorphous phase reduces dramatically the thermal transport. One can even create nanostructured materials with subamorphous thermal conductivity, while keeping an important crystalline fraction. In general, the a-SiO2 shell around the pores reduces the thermal conductivity by a factor of five to ten compared to a-Si shell. The phonon density of states for several systems is also given to give the impact of the amorphisation on the phonon modes. [ABSTRACT FROM AUTHOR]

The thermal transport properties of porous silicon and nano-hybrid "porous silicon/water" systems are presented here. The thermal conductivity was evaluated using the equilibrium molecular dynamics technique for porous systems made of spherical voids or water-filled cavities. We revealed large thermal conductivity enhancement in the nano-hybrid systems as compared to their dry porous counterparts, which cannot be captured by effective media theory. This rise of thermal conductivity is related to the increase in the specific surface area of the liquid/solid interface. We demonstrated that the significant difference, more than two folds, in thermal conductivity of pristine porous silicon and "porous silicon liquid–composite" is due to the liquid density fluctuation close to the "solid–liquid interface" (layering effect). This effect is getting more importance for the large specific surface of the interfacial area. Specifically, the enhancement of the effective thermal conductivity is 50% for a specific surface area of 0.3 (1/nm), and it increases further upon the increase in the surface to volume ratio. Our study provides valuable insights into the thermal properties of hybrid liquid/solid nanocomposites and into the importance of confined liquids within nanoporous materials. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The hollowing of silicon membranes to form a lattice of cylindrical holes, also called phononic crystal, has been used by several experimental groups willing to fabricate efficient thermoelectric modules. The idea is to reduce the thermal conductivity without impacting the electronic conductivity. For several a priori identical materials, i.e. thin films containing periodic cylindrical holes, drastically different levels of thermal conductivity reduction have been reported in the literature: from 1–2 W K−1 m−1 to 15–40 W K−1 m−1, i. e. half the thermal conductivity of the plain membrane. These differences may be due to variations in the geometrical patterns, or to the technological processes specific to each group. It is therefore highly desirable to understand which level of reduction can be expected from the basic concept. In this work, we address the question by applying a fully atomistic framework, the approach-to-equilibrium molecular dynamics (AEMD), to study two deca-nanometric patterns used in the literature and reported respectively with a high and low level of thermal conductivity reduction. For both patterns, the thermal conductivity roughly decreases by a factor 2 only compared to the plain membrane. Thanks to Monte Carlo simulations, in agreement with AEMD for the two patterns, we propose that the origin of stronger reductions could be an increase of the surface roughness during the step of hole fabrication. [ABSTRACT FROM AUTHOR]

Heat transport in bulk semiconductors is well understood, and during the last few years, it has been shown that it can be computed accurately from ab initio calculations. However, describing heat transport in micro- and nanodevices used in applications remains challenging. In this paper, we propose a method, based on the propagation of wave packets, for solving the phonon Boltzmann transport equation parametrized with ab initio calculations. It allows computing the thermal conductivity of micro- and nano-sized systems, without adjustable parameters, and for any materials. The accuracy and applicability of the method are demonstrated by computing the cross plane thermal conductivity of cubic and hexagonal silicon thin films as a function of their thickness. [ABSTRACT FROM AUTHOR]

A High-Throughput Time-Domain ThermoReflectance (HT-TDTR) technique was developed to perform fast thermal conductivity measurements with minimum user actions required. This new setup is based on a heterodyne picosecond thermoreflectance system. The use of two different laser oscillators has been proven to reduce the acquisition time by two orders of magnitude and avoid the experimental artefacts usually induced by moving the elements present in TDTR systems. An amplitude modulation associated to a lock-in detection scheme is included to maintain a high sensitivity to thermal properties. We demonstrate the capabilities of the HT-TDTR setup to perform high-throughput thermal analysis by mapping thermal conductivity and interface resistances of a ternary thin film silicide library FexSiyGe100- x- y (20 < x, y < 80) deposited by wedge-type multi-layer method on a 100 mm diameter sapphire wafer offering more than 300 analysis areas of different ternary alloy compositions. [ABSTRACT FROM AUTHOR]

Evidence for an excellent compromise between structural stability and low thermal conductivity has been achieved with tilted Bi2Te3 nanowires. The latter ones were recently fabricated and there is a need in modeling and characterization. The structural stability and the thermal conductivity of Bi2Te3 nanowires along the tilted [015]* direction and along the [010] direction have been explored. For the two configurations of nanowires, the effect of the length and the cross section on the thermal conductivity is discussed. The thermal conductivity of infinite size tilted nanowire is 0.34W/m K, significantly reduced compared to nanowire along the [010] direction (0.59W/m K). This reveals that in Bi2Te3 nanowires the structural anisotropy can be as important as size effects to reduce the thermal conductivity. The main reason is the reduction of the phonon mean free path which is found to be 1.7 nm in the tilted nanowires, compared to 5.3nm for the nanowires along the [010] direction. The fact that tilted Bi2Te3 nanowire is mechanically stable and it has extremely low thermal conductivity suggests these nanowires as a promising material for future thermoelectric generation application. [ABSTRACT FROM AUTHOR]

The thermal conductivity of a series of structural, diameter, and core/shell modulated silicon nanowires is computed by molecular dynamics simulations using the nonequilibrium method. The purpose of this study is to appraise the impact of surface roughness, amorphous parts integration, and diameter modulation on the thermal conductivity reduction. The addition of both amorphous regions and nanoconstrictions allows us to reach a thermal conductivity close to that of the amorphous pristine nanowires, while preserving a certain degree of crystallinity. This could result in interesting properties for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this work is to predict the in-plane and cross-plane thermal properties of crystalline silicon films. Several thicknesses from 20 nm to 6 μm and mean temperatures between 20 and 500 K have been investigated. Heat transport properties in silicon films have been studied through three different techniques: a semi-analytical method based upon the Kinetic Theory, a deterministic solution of the Boltzmann Transfer Equation (BTE) through the Discrete Ordinate Method and a statistical handling of the BTE by means of Monte Carlo Method. Each technique requires a model for the bulk material dispersion curves and the collision times of the different scattering processes. The three techniques have been validated through their correct prediction of silicon bulk thermal conductivity. Comparisons with in-plane thermal conductivity calculations and measurements have been also discussed. Thus, the cross-plane thermal conduction properties have been predicted. The expected temperature and thickness variations of the thermal conduction properties have been observed: the cross-plane thermal conduction appears to be less efficient than the in-plane thermal conduction, which proves that a significant anisotropy exists. [Copyright &y& Elsevier]