Takuma Hori

Crystal structure dependent thermal conductivity in two-dimensional phononic crystal nanostructures

Thermal phonon transport in square- and triangular-lattice Si phononic crystal (PnC) nanostructures with a period of 300 nm was investigated by measuring the thermal conductivity using micrometer-scale time-domain thermoreflectance. The placement of circular nanoholes has a strong influence on thermal conductivity when the periodicity is within the range of the thermal phonon mean free path. A staggered hole structure, i.e., a triangular lattice, has lower thermal conductivity, where the difference in thermal conductivity depends on the porosity of the structure. The largest difference in conductivity of approximately 20% was observed at a porosity of around 30%. This crystal structure dependent thermal conductivity can be understood by considering the local heat flux disorder created by a staggered hole structure. Numerical simulation using the Monte Carlo technique was also employed and also showed the lower thermal conductivity for a triangular lattice structure. Besides gaining a deeper understanding ...

Effective phonon mean free path in polycrystalline nanostructures

We have calculated the mean free path (MFP) of phonons associated with grain boundary scattering in polycrystalline nanostructures, by developing a Monte Carlo ray tracing transmission model that can be applied to arbitrary geometries. The calculations for various log-normal grain-size distributions realized by Voronoi diagrams and genetic algorithms show that the boundary-scattering MFP in a polycrystalline nanostructure is 20%–30% longer than that in a simple cubic structure with the same average grain size (defined by matching grain volumes). The impact on thermal conductivity is quantified for nanocrystalline silicon by using Matthiessens rule to combine boundary scattering with intrinsic phonon-phonon scattering. The result reveals that the thermal conductivity depends strongly on the average grain size but only weakly on the breadth of the grain-size distribution, and thus, the simple cubic structure is a reasonable approximation for the polydisperse grain structure of actual materials.

Importance of local force fields on lattice thermal conductivity reduction in PbTe1−xSex alloys

Lattice thermal conductivity of PbTe1−xSex alloyed crystals has been calculated by molecular-dynamics simulations with anharmonic interatomic force constants (a-IFCs) obtained from first principles. The a-IFCs of pure PbTe and PbSe were calculated by the real-space displacement method with care of the stability for molecular-dynamics simulations. An empirical mixing rule of a-IFCs has been developed to account for both mass and local force-field differences in alloys. The obtained alloy-fraction dependence of lattice thermal conductivity reduction agrees well with the experiments. The comparative study shows that the local force-field difference significantly impacts the lattice thermal conductivity.

Probing and tuning inelastic phonon conductance across finite-thickness interface

Phonon transmission across an interface between dissimilar crystalline solids is calculated using molecular dynamics simulations with interatomic force constants obtained from first principles. The results reveal that although inelastic phonon-transmission right at the geometrical interface can become far greater than the elastic one, its contribution to thermal boundary conductance (TBC) is severely limited by the transition regions, where local phonon states at the interface recover the bulk state over a finite thickness. This suggests TBC can be increased by enhancing phonon equilibration in the transition region for instance by phonon scattering, which is demonstrated by increasing the lattice anharmonicity.

Thermal conductivity of bulk nanostructured lead telluride

Thermal conductivity of lead telluride with embedded nanoinclusions was studied using Monte Carlo simulations with intrinsic phonon transport properties obtained from first-principles-based lattice dynamics. The nanoinclusion/matrix interfaces were set to completely reflect phonons to model the maximum interface-phonon-scattering scenario. The simulations with the geometrical cross section and volume fraction of the nanoinclusions matched to those of the experiment show that the experiment has already reached the theoretical limit of thermal conductivity. The frequency-dependent analysis further identifies that the thermal conductivity reduction is dominantly attributed to scattering of low frequency phonons and demonstrates mutual adaptability of nanostructuring and local disordering.

Phonon transport analysis of silicon germanium alloys using molecular dynamics simulations

The phonon transport properties and the lattice thermal conductivity of silicon germanium alloy crystals have been investigated based on phonon gas model by using classical molecular dynamics simulations. The attenuation of the mode-dependent phonon relaxation time due to alloying and its dependence on the alloy fraction were quantified by projecting the molecular dynamics phase space trajectory onto the normal mode of the alloyed crystal. By empirically approximating the group velocities from the extended dispersion relations, the lattice thermal conductivity was calculated based on the phonon gas model under relaxation time approximation. The obtained reduction in the lattice thermal conductivity caused by alloying agrees well with that of the experiment and direct non-equilibrium molecular dynamics calculations. The phonon-mean-free-path dependent contribution to thermal conductivity suggests that the effect of nanostructuring can have non-monotonic dependence on the alloy fraction.

Origin of anomalous anharmonic lattice dynamics of lead telluride

The origin of the anomalous anharmonic lattice dynamics of lead telluride is investigated using molecular dynamics simulations with interatomic force constants (IFCs) up to quartic terms obtained from first principles. The calculations reproduce the peak asymmetry of the radial distribution functions and the double peaks of transverse optical phonon previously observed with neutron diffraction and scattering experiments. They are identified to be due to the extremely large nearest-neighbor cubic IFCs in the [100] direction. The outstanding strength of the nearest-neighbor cubic IFCs relative to the longer-range ones explains the reason why the distortion in the radial distribution function is local.

Thermal conductivity reduction in silicon fishbone nanowires

Semiconductor nanowires are potential building blocks for future thermoelectrics because of their low thermal conductivity. Recent theoretical works suggest that thermal conductivity of nanowires can be further reduced by additional constrictions, pillars or wings. Here, we experimentally study heat conduction in silicon nanowires with periodic wings, called fishbone nanowires. We find that like in pristine nanowires, the nanowire cross-section controls thermal conductivity of fishbone nanowires. However, the periodic wings further reduce the thermal conductivity. Whereas an increase in the wing width only slightly affects the thermal conductivity, an increase in the wing depth clearly reduces thermal conductivity, and this reduction is stronger in the structures with narrower nanowires. Our experimental data is supported by the Callaway-Holland model, finite element modelling and phonon transport simulations.

Influence of mass contrast in alloy phonon scattering

We have investigated the effect of mass contrast on alloy phonon scattering in mass-substituted Lennard-Jones crystals. By calculating the mass-difference phonon scattering rate using a modal analysis method based on molecular dynamics, we have identified the applicability and limits of the widely-used mass-difference perturbation model in terms of magnitude and sign of the mass difference. The result of a phonon -mode-dependent analysis reveals that the critical phonon frequency, above which the mass-difference perturbation theory fails, decreases with the magnitude of the mass difference independently of its sign. This gives rise to a critical mass contrast, above which the mass-difference perturbation model noticeably underestimates the lattice thermal conductivity.

Thermal Conductivity Measurement of Vertically Aligned Single-Walled Carbon Nanotubes Utilizing Temperature Dependence of Raman Scattering

We present a method for measuring the thermal conductivity and the thermal contact resistance between the film and the substrate of vertically-aligned single-walled carbon nanotubes (VA-SWNTs) grown on Si substrate by ACCVD (Alcohol Catalytic Chemical Vapor Deposition) method, utilizing temperature dependence of the Raman spectrum obtained from SWNTs. The method utilizes the excitation laser of the Raman system to heat the VA-SWNT film and measure the temperature simultaneously. The method finds the thermal conductivity of the VA-SWNT film to be around 1 Wm−1 K−1 and the thermal contact resistance between the substrate and the film to be around 10−5 ∼10−6 m2 KW−1 . The obtained film thermal conductivity is converted into equivalent thermal conductivity of an individual SWNT, whose value is several tens of Wm−1 K−1 , and is more than an order of magnitude smaller than the reports on individual SWNTs.Copyright