Thermal Conductivity Of Nanostructured Materials Research Articles

Effects of acoustic softening on thermal conductivity beyond group velocity

We model the highly reduced thermal conductivity of nanostructured materials observed in nanoribbons. For highly scaled structures, such as wires with diameters on the order of 20nm, physical effects beyond classical boundary scattering, including acoustic softening, become important. To date, work on acoustic softening has focused on reductions in group velocity. However, a reduction in the group velocity implies that the phonon dispersion is modified. Here, we investigate how changes in the phonon dispersion manifest in the mean free path, heat capacity, and group velocity. Including these effects in the modeling of thermal conductivity, we find that softening increases low-temperature thermal conductivity while reducing high temperature thermal conductivity. We further compare the model to experimental data.

Controlling Thermal Conductivity in Mesoporous Silica Films Using Pore Size and Nanoscale Architecture.

This work investigates the effect of wall thickness on the thermal conductivity of mesoporous silica materials made from different precursors. Sol-gel- and nanoparticle-based mesoporous silica films were synthesized by evaporation-induced self-assembly using either tetraethyl orthosilicate or premade silica nanoparticles. Since wall thickness and pore size are correlated, a variety of polymer templates were used to achieve pore sizes ranging from 3-23 nm for sol-gel-based materials and 10-70 nm for nanoparticle-based materials. We found that the type of nanoscale precursor determines how changing the wall thickness affects the resulting thermal conductivity. The data indicate that the thermal conductivity of sol-gel-derived porous silica decreased with decreasing wall thickness, while for nanoparticle-based mesoporous silica, the wall thickness had little effect on the thermal conductivity. This work expands our understanding of heat transfer at the nanoscale and opens opportunities for tailoring the thermal conductivity of nanostructured materials by means other than porosity and composition.

Peculiarity of lattice thermal conductivity of nanostructured materials based on titanium and aluminum. molecular dynamics method

Peculiarity of lattice thermal conductivity of nanostructured materials based on titanium and aluminum. molecular dynamics method

Thermal conductivity engineering of bulk and one-dimensional Si-Ge nanoarchitectures

Various theoretical and experimental methods are utilized to investigate the thermal conductivity of nanostructured materials; this is a critical parameter to increase performance of thermoelectric devices. Among these methods, equilibrium molecular dynamics (EMD) is an accurate technique to predict lattice thermal conductivity. In this study, by means of systematic EMD simulations, thermal conductivity of bulk Si-Ge structures (pristine, alloy and superlattice) and their nanostructured one dimensional forms with square and circular cross-section geometries (asymmetric and symmetric) are calculated for different crystallographic directions. A comprehensive temperature analysis is evaluated for selected structures as well. The results show that one-dimensional structures are superior candidates in terms of their low lattice thermal conductivity and thermal conductivity tunability by nanostructuring, such as by diameter modulation, interface roughness, periodicity and number of interfaces. We find that thermal conductivity decreases with smaller diameters or cross section areas. Furthermore, interface roughness decreases thermal conductivity with a profound impact. Moreover, we predicted that there is a specific periodicity that gives minimum thermal conductivity in symmetric superlattice structures. The decreasing thermal conductivity is due to the reducing phonon movement in the system due to the effect of the number of interfaces that determine regimes of ballistic and wave transport phenomena. In some nanostructures, such as nanowire superlattices, thermal conductivity of the Si/Ge system can be reduced to nearly twice that of an amorphous silicon thermal conductivity. Additionally, it is found that one crystal orientation, 100, is better than the 111 crystal orientation in one-dimensional and bulk SiGe systems. Our results clearly point out the importance of lattice thermal conductivity engineering in bulk and nanostructures to produce high-performance thermoelectric materials.

Ballistic Energy Transport in Oligomers.

The development of nanocomposite materials with desired heat management properties, including nanowires, layered semiconductor structures, and self-assembled monolayer (SAM) junctions, attracts broad interest. Such materials often involve polymeric/oligomeric components and can feature high or low thermal conductivity, depending on their design. For example, in SAM junctions made of alkane chains sandwiched between metal layers, the thermal conductivity can be very low, whereas the fibers of ordered polyethylene chains feature high thermal conductivity, exceeding that of many pure metals. The thermal conductivity of nanostructured materials is determined by the energy transport between and within each component of the material, which all need to be understood for optimizing the properties. For example, in the SAM junctions, the energy transport across the metal-chain interface as well as the transport through the chains both determine the overall heat conductivity, however, to separate these contributions is difficult. Recently developed relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy is capable of studying energy transport in individual molecules in the time domain. The transport in a molecule is initiated by exciting an IR-active group (a tag); the method records the influence of the excess energy on another mode in the molecule (a reporter). The energy transport time can be measured for different reporters, and the transport speed through the molecule is evaluated. Various molecules were interrogated by RA 2DIR: in molecules without repeating units (disordered), the transport mechanism was expected and found to be diffusive. The transport via an oligomer backbone can potentially be ballistic, as the chain offers delocalized vibrational states. Indeed, the transport regime via three tested types of oligomers, alkanes, polyethyleneglycols, and perfluoroalkanes was found to be ballistic, whereas the transport within the end groups was diffusive. Interestingly, the transport speeds via these chains were different. Moreover, the transport speed was found to be dependent on the vibrational mode initiating the transport. For the difference in the transport speeds to be explained, the chain bands involved in the wavepacket formation were analyzed, and specific optical bands of the chain were identified as the energy transporters. For example, the transport initiated in alkanes by the stretching mode of the azido end group (2100 cm(-1)) occurs predominantly via the CH2 twisting and wagging chain bands, but the transport initiated by the C=O stretching modes of the carboxylic acid or succinimide ester end groups occurs via C-C stretching and CH2 rocking bands of the alkane chain. Direct formation of the wavepacket within the CH2 twisting and wagging chain bands occurs when the transport is initiated by the N═N stretching mode (1270 cm-1) of the azido end-group. The transport via optical chain bands in oligomers involves rather large vibrational quanta (700-1400 cm(-1)), resulting in efficient energy delivery to substantial distances. Achieved quantitative description of various energy transport steps in oligomers, including the specific contributions of different chain bands, can result in a better understanding of the transport steps in nanocomposite materials, including SAM junctions, and lead towards designing systems for molecular electronics with a controllable energy transport speed.

Adjoint-based deviational Monte Carlo methods for phonon transport calculations

In the field of linear transport, adjoint formulations exploit linearity to derive powerful reciprocity relations between a variety of quantities of interest. In this paper, we develop an adjoint formulation of the linearized Boltzmann transport equation for phonon transport. We use this formulation for accelerating deviational Monte Carlo simulations of complex, multiscale problems. Benefits include significant computational savings via direct variance reduction, or by enabling formulations which allow more efficient use of computational resources, such as formulations which provide high resolution in a particular phase-space dimension (e.g., spectral). We show that the proposed adjoint-based methods are particularly well suited to problems involving a wide range of length scales (e.g., nanometers to hundreds of microns) and lead to computational methods that can calculate quantities of interest with a cost that is independent of the system characteristic length scale, thus removing the traditional stiffness of kinetic descriptions. Applications to problems of current interest, such as simulation of transient thermoreflectance experiments or spectrally resolved calculation of the effective thermal conductivity of nanostructured materials, are presented and discussed in detail.

Phonon transmission acrossMg2Si/Mg2Si1−xSnxinterfaces: A first-principles-based atomistic Green's function study

Phonon transmission across interfaces of dissimilar materials has been studied intensively in recent years by using atomistic simulation tools due to its importance in determining the effective thermal conductivity of nanostructured materials. The atomistic Green's function method with interatomic force constants from the first-principles calculations has evolved to be a promising approach to study phonon transmission in many material systems that have not been well studied. However, the direct first-principles calculation for interatomic force constants becomes infeasible when the system involves atomic disorder. Mass approximation is usually used, but its validity has not been tested. In this paper, we employ the higher-order force constant model to extract harmonic force constants from the first-principles calculations, which originates from the virtual crystal approximation but considers the local force-field difference. As a feasibility demonstration of the proposed method that integrates higher-order force constant model from the first-principles calculations with the atomistic Green's function, we study the phonon transmission in the ${\mathrm{Mg}}_{2}\mathrm{Si}/{\mathrm{Mg}}_{2}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ systems. When integrated with the atomistic Green's function, the widely used mass approximation is found to overpredict phonon transmission across the ${\mathrm{Mg}}_{2}{\mathrm{Si}/\mathrm{Mg}}_{2}\mathrm{Sn}$ interface. The difference can be attributed to the absence of local strain field-induced scattering in the mass approximation, which makes the high-frequency phonons less scattered. The frequency-dependent phonon transmission across an interface between a crystal and an alloy, which often appears in high-efficiency ``nanoparticle in alloy'' thermoelectric materials, is studied. The interfacial thermal resistance across ${\mathrm{Mg}}_{2}\mathrm{Si}/{\mathrm{Mg}}_{2}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ interface is found to be weakly dependent on the composition of Sn when the composition of $\mathrm{x}$ is less than 40% but increases rapidly when it is larger than 40% due to the transition of high-frequency phonon DOS in ${\mathrm{Mg}}_{2}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ alloys. This work could have a great impact on the design of novel nanostructures with tunable thermal properties.

Effect of Nonlinearity of the Phonon Spectrum on the Thermal Conductivity of Nanostructured Materials Based on Bi–Sb–Te

A theoretical investigation of the lattice thermal conductivity of nanostructured materials based on Bi–Sb–Te is presented. The calculations were based on relaxation time approximation and took into account both the real phonon spectra, obtained from first-principles by use of density functional theory, and the anisotropy of phonon relaxation time. Phonon relaxation time data were determined from experimental values of the lattice thermal conductivity. The decrease of the thermal conductivity caused by the nanostructure was compared with results from calculations based on the linear Debye approach. Estimation showed that phonon boundary scattering can lead to a 55% decrease of thermal conductivity for a grain size of ~20 nm in the Debye approximation. Taking the nonlinearity of the acoustic phonon spectrum into account leads to a 20% larger decrease of the thermal conductivity because of boundary scattering. The reason is that consideration of the real phonon spectrum increases the relative contribution to thermal conductivity of acoustic phonons with low frequencies that are scattered more strongly at nanograin boundaries. Similarly, estimation of lattice thermal conductivity reduction as a result of phonon scattering by nanoinclusions gave an 8% larger decrease when the real phonon spectrum was used rather than the linear Debye approximation. For such a substantial decrease of lattice thermal conductivity, the effect of the optical phonons was estimated; it was shown that optical phonons can reduce the change of thermal conductivity as a result of grain boundary scattering by no more than 10%. Finally, the minimum lattice thermal conductivity was estimated to be 0.07 W/m K because of acoustic modes (0.09 W/m K in the Debye approach) and 0.14 W/m K when the contribution of optical modes was also taken into consideration.

Effect of lattice mismatch on phonon transmission and interface thermal conductance across dissimilar material interfaces

When phonons transport across a material interface, they experience reflection, transmission, and mode conversion, which results in a local temperature jump at the interface and thus dramatically changes the thermal conductivity of nanostructured materials. Phonon transmission across lattice-matched interfaces has been studied extensively in recent years with the atomistic Green's function (AGF) approach, which usually uses one unit cell to represent the cross section along the interface. However, modeling phonon transmission across realistic material interfaces is much more challenging because realistic interfaces are usually lattice-mismatched ones with atomic reconstruction, defects, and species mixing, which demands a larger cross-sectional area for the AGF simulation. In this paper, an integrated molecular dynamics (MD) and AGF approach is developed to study the phonon transmission across lattice-mismatched interfaces. MD simulation is used to simulate atomic reconstruction close to the interface. The recursive AGF approach is then employed to calculate frequency-dependent phonon transmission across lattice-mismatched interfaces with defects and species mixing, which addresses the numerical challenge in calculating phonon transmission for a relatively large cross-sectional area with reduced computational cost. The study of the relaxed interface formed from two semi-infinite bulk materials shows that lattice mismatch increases the lattice disorder and decreases the adhesion energy, which in turn lowers phonon transmission and reduces the interface thermal conductance across lattice-mismatched interfaces. Low-frequency phonons can be significantly scattered by increasing the defect size across the interface, while high-frequency phonons can be scattered almost completely (phonon transmission 0.1) across an alloyed layer as thin as 2.27 nm. The effect of lattice mismatch on phonon transmission becomes smaller for interfaces with defects and species mixing. The effect of annealing temperature on the Si/Ge interface thermal conductance was studied. A significant reduction of the Si/Ge interface thermal conductance was observed for a lattice-mismatched interface when annealed at high temperature, which agrees well with the available experimental data in literature.

Effect of boundary scattering on the thermal conductivity of a nanostructured semiconductor material based on the Bi x Sb2 − x Te3 solid solution

This paper reports on a theoretical and experimental investigation of the behavior of the electrical and thermal conductivities of a nanostructured material based on BixSb2 − xTe3 solid solutions. The effect of boundary scattering has been taken into account by introducing the scattering mechanism with a constant mean free path equal to the nanoparticle size. A comparison with the results of the measurements has demonstrated that one can describe satisfactorily the experimental dependences of the electrical and thermal conductivities on the nanoparticle size by using only the parameters of the initial solid solution and its pure constituents. The estimates have revealed that the lattice thermal conductivity of nanostructured materials can be reduced by 20–30% as compared to the initial solid solution with nanoparticle sizes of the order of 20 nm, which should produce a favorable effect on the magnitude of the thermoelectric figure-of-merit.