Molecular Simulations and Lattice Dynamics Determination of Stillinger-Weber GaN Thermal Conductivity

The Bulk Thermal Conductivity of Stillinger-Weber (SW) Wurtzite GaN in the [0001] Direction at a Temperature of 300 K is Calculated using Equilibrium Molecular Dynamics (EMD), Non-Equilibrium MD (NEMD), and Lattice Dynamics (LD) Methods. While the NEMD Method Predicts a Thermal Conductivity of 166 ± 11 W/m·K, Both the EMD and LD Methods Predict Thermal Conductivities that Are an Order of Magnitude Greater. We Attribute the Discrepancy to Significant Contributions to Thermal Conductivity from Long-Mean Free Path Phonons. We Propose that the Grüneisen Parameter for Low-Frequency Phonons is a Good Predictor of the Severity of the Size Effects in NEMD Thermal Conductivity Prediction. for Weakly Anharmonic Crystals Characterized by Small Grüneisen Parameters, Accurate Determination of Thermal Conductivity by NEMD is Computationally Impractical. the Simulation Results Also Indicate the GaN SW Potential, Which Was Originally Developed for Studying the Atomic-Level Structure of Dislocations, is Not Suitable for Prediction of its Thermal Conductivity

Observation of Reduced Thermal Conductivity in a Metal-Organic Framework Due to the Presence of Adsorbates

Whether the presence of adsorbates increases or decreases thermal conductivity in metal-organic frameworks (MOFs) has been an open question. Here we report observations of thermal transport in the metal-organic framework HKUST-1 in the presence of various liquid adsorbates: water, methanol, and ethanol. Experimental thermoreflectance measurements were performed on single crystals and thin films, and theoretical predictions were made using molecular dynamics simulations. We find that the thermal conductivity of HKUST-1 decreases by 40 – 80% depending on the adsorbate, a result that cannot be explained by effective medium approximations. Our findings demonstrate that adsorbates introduce additional phonon scattering in HKUST-1, which particularly shortens the lifetimes of low-frequency phonon modes. As a result, the system thermal conductivity is lowered to a greater extent than the increase expected by the creation of additional heat transfer channels. Finally, we show that thermal diffusivity is even more greatly reduced than thermal conductivity by adsorption

Hybridization from Guest-Host Interactions Reduces the Thermal Conductivity of Metal-Organic Frameworks

We experimentally and theoretically investigate the thermal conductivity and mechanical properties of polycrystalline HKUST-1 metal–organic frameworks (MOFs) infiltrated with three guest molecules: tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), and (cyclohexane-1,4-diylidene)dimalononitrile (H4-TCNQ). This allows for modification of the interaction strength between the guest and host, presenting an opportunity to study the fundamental atomic scale mechanisms of how guest molecules impact the thermal conductivity of large unit cell porous crystals. The thermal conductivities of the guest@MOF systems decrease significantly, by on average a factor of 4, for all infiltrated samples as compared to the uninfiltrated, pristine HKUST-1. This reduction in thermal conductivity goes in tandem with an increase in density of 38% and corresponding increase in heat capacity of ∼48%, defying conventional effective medium scaling of thermal properties of porous materials. We explore the origin of this reduction by experimentally investigating the guest molecules’ effects on the mechanical properties of the MOF and performing atomistic simulations to elucidate the roles of the mass and bonding environments on thermal conductivity. The reduction in thermal conductivity can be ascribed to an increase in vibrational scattering introduced by extrinsic guest-MOF collisions as well as guest molecule-induced modifications to the intrinsic vibrational structure of the MOF in the form of hybridization of low frequency modes that is concomitant with an enhanced population of localized modes. The concentration of localized modes and resulting reduction in thermal conductivity do not seem to be significantly affected by the mass or bonding strength of the guest species

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Thermal conductivity of compound semiconductors: Interplay of mass density and acoustic-optical phonon frequency gap

The thermal conductivities of model compound semiconductors, where the two species differ only in mass, are predicted using lattice dynamics calculations and the Boltzmann transport equation. The thermal conductivity varies non-monotonically with mass ratio, with a maximum value that is four times higher than that of a monatomic semiconductor of the same density. The very high thermal conductivities are attributed to a reduction in the scattering of optical phonons when the acoustic-optical frequency gap in the phonon dispersion approaches the maximum acoustic phonon frequency. The model system predictions compare well to predictions for realcompound semiconductors under appropriate scaling, suggesting a universal behavior and a strategy for efficient screening of materials for high thermal conductivity.</p

Thermal conductance of superlattice junctions

<p>We use molecular dynamics simulations and the lattice-based scattering boundary method to compute the thermal conductance of finite-length Lennard-Jones superlattice junctions confined by bulk crystalline leads. The superlattice junction thermal conductance depends on the properties of the leads. For junctions with a superlattice period of four atomic monolayers at temperatures between 5 and 20 K, those with mass-mismatched leads have a greater thermalconductance than those with mass-matched leads. We attribute this lead effect to interference between and the ballistic transport of emergent junction vibrational modes. The lead effect diminishes when the temperature is increased, when the superlattice period is increased, and when interfacial disorder is introduced, but is reversed in the harmonic limit.</p

Thermal conductance of the junction between single-walled carbon nanotubes

<p>The thermal conductances of the carbon nanotube (CNT) junctions that would be found in aCNT aerogel are predicted using molecular dynamics simulations. At a temperature of 300 K, the thermal conductance of a perpendicular junction converges to 40 pW/K as the CNT lengths approach 100 nm. The key geometric parameter affecting the thermal conductance is the angle formed by the two CNTs. At pressures above 1 bar, the presence of a surrounding gas leads to an effective increase in the junction thermal conductance by providing a parallel path for energy flow.</p

Strongly anisotropic in-plane thermal transport in single-layer black phosphorene.

<p>Using first principles calculations, we predict the thermal conductivity of the two-dimensional materials black phosphorene and blue phosphorene. Black phosphorene has an unprecedented thermal conductivity anisotropy ratio of three, with predicted values of 110 W/m-K and 36 W/m-K along its armchair and zigzag directions at a temperature of 300 K. For blue phosphorene, which is isotropic with a zigzag structure, the predicted value is 78 W/m-K. The two allotropes show strikingly different thermal conductivity accumulation, with phonons of mean free paths between 10 nm and 1 μm dominating in black phosphorene, while a much narrower band of mean free paths (50-200 nm) dominate in blue phosphorene. Black phosphorene shows intriguing potential for strain-tuning of its thermal conductivity tensor.</p

Predicting alloy vibrational mode properties using lattice dynamics calculations, molecular dynamics simulations, and the virtual crystal approximation

<p>The virtual crystal (VC) approximation for mass disorder is evaluated by examining two modelalloy systems: Lennard-Jones argon and Stillinger-Weber silicon. In both material systems, the perfect crystal is alloyed with a heavier mass species up to equal concentration. The analysis is performed using molecular dynamics simulations and lattice dynamics calculations. Mode frequencies and lifetimes are first calculated by treating the disorder explicitly and under the VC approximation, with differences found in the high-concentration alloys at high frequencies. Notably, the lifetimes of high-frequency modes are underpredicted using the VC approximation, a result we attribute to the neglect of higher-order terms in the model used to include point-defect scattering. The mode properties are then used to predict thermal conductivity under the VC approximation. For the Lennard-Jones alloys, where high-frequency modes make a significant contribution to thermal conductivity, the high-frequency lifetime underprediction leads to an underprediction of thermal conductivity compared to predictions from the Green-Kubo method, where no assumptions about the thermal transport are required. Based on observations of a minimum mode diffusivity, we propose a correction that brings the VC approximationthermal conductivities into better agreement with the Green-Kubo values. For the Stillinger-Weber alloys, where the thermal conductivity is dominated by low-frequency modes, the high-frequency lifetime underprediction does not affect the thermal conductivity prediction and reasonable agreement is found with the Green-Kubo values</p

Effect of film thickness on the thermal resistance of confined semiconductor thin films

The thermal resistance of semiconductor thin films is predicted using lattice dynamics (LD) calculations and molecular dynamics (MD) simulations. We consider Si and Gefilms with thicknesses, LF, between 0.2 and 30 nm that are confined between larger extents of the other species (i.e., Ge/Si/Ge and Si/Ge/Si structures). The LD predictions are made in the classical limit for comparison to the classical MD simulations, which are performed at a temperature of 500 K. For structures with LF2 nm, the MD-predicted thermal resistances are independent of the film thickness for the Ge/Si/Ge structures and increase with increasing film thickness for the Si/Ge/Si structures. We attribute these results to phonontransport that is ballistic in the Ge/Si/Ge structures and more diffusive in the Si/Ge/Si structures based on comparisons to the LD predictions, which assume ballisticphonontransport. We find that this difference between the structures cannot be predicted by comparing the mode-averaged phonon mean free path to the film thickness. It can be predicted, however, by considering the frequency dependence of the phonon mean free paths.</p