Path Of Phonons Research Articles

Effect of built-in-polarization field on intrinsic and extrinsic thermal conductivity of InN

The effect of built-in-polarization field on intrinsic and extrinsic lattice thermal conductivity of InN has been investigated theoretically. The built-in-polarization field modifies elastic constant, phonon velocity and Debye temperature of InN. Using these modified parameters, the relaxation time of phonons in various scattering processes at room temperature has been computed as a function of phonon frequency for with and without built-in-polarization field. The result shows that built-in-polarization field enhances the relaxation time of phonons. This implies longer mean free path of phonons. The Callaway model of phonon thermal conductivity has been used to estimate the effect of built-in-polarization field on intrinsic and extrinsic thermal conductivity of InN. The theoretical analysis shows that up to a certain temperature, the polarization field acts as negative effect and reduces the intrinsic and extrinsic thermal conductivity. However, after this temperature, both thermal conductivity are significantly contributed by polarization field. This gives the idea of temperature dependence of polarization effect, which signifies pyro-electric character of InN. The intrinsic thermal conductivity of InN at room temperature including built-in-polarization field is found to be enhanced by 28 %. However, at room temperature, the extrinsic thermal conductivity with built-in-polarization mechanism is found to be enhanced by 15 %. The method, we have developed may be taken in the simulation of heat transport in optoelectronic nitride devices to minimize the self-heating processes and in polarization engineering strategies to optimize the thermoelectric performance of InN alloys.

Tuning thermal transport in ultrathin silicon membranes by surface nanoscale engineering.

A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively.

Simultaneous reconstruction of thermal degradation properties for anisotropic scattering fibrous insulation after high temperature thermal exposures

To probe thermal degradation behavior of fibrous insulation for long-term service, an inverse analysis model was developed to simultaneously reconstruct thermal degradation properties of fibers after thermal exposures from the experimental thermal response data, by using the measured infrared spectral transmittance and X-ray phase analysis data as direct inputs. To take into account the possible influence of fibers degradation after thermal exposure on the conduction heat transfer, we introduced a new parameter in the thermal conductivity model. The effect of microstructures on the thermal degradation parameters was evaluated. It was found that after high temperature thermal exposure the decay rate of the radiation intensity passing through the material was weakened, and the probability of being scattered decreased during the photons traveling in the medium. The fibrous medium scattered more radiation into the forward directions. The shortened heat transfer path due to possible mechanical degradation, along with the enhancement of mean free path of phonon scattering as devitrification after severe heat treatment, made the coupled solid/gas thermal conductivities increase with the rise of heat treatment temperature.

Broad Search of Better Thermoelectric Oxides via First-Principles Computations

ABSTRACTThe advancement of computational tools for material property predictions enables broad search of novel materials for various energy-related applications. However, challenges still exist in accurately predicting the mean free paths (MFPs) of electrons and phonons in a high-throughput frame for thermoelectric property predictions, which largely hinders the computation-driven search for novel materials. In this work, this need is eliminated under the small-grain-size limit, in which these MFPs are restricted by the grain sizes within a bulk material. A new criterion for ZT evaluation is proposed for general nanograined bulk materials and is demonstrated with representative oxides.

Thermal Conductivity in the Triangular-Lattice Antiferromagnet Ba3CoSb2O9

We have measured the thermal conductivity in the ab-plane, κab, and along the c-axis, κc, of single crystals of the S = 1/2 triangular-lattice antiferromagnet Ba3CoSb2O9 in zero field and magnetic fields. In zero field, it has been found that both κab and κc show a similar broad peak around 40 K, suggesting that the thermal conductivity due to phonons is dominant in this compound and that the mean free path of phonons is suppressed so much by strong magnetic-fluctuations due to the magnetic frustration. It has also been found that both κab and κc show a kink at the antiferromagnetic transition temperature TN. In magnetic fields parallel to the ab-plane up to 14 T, magnetic-field dependences of both κab and κc at temperatures below and above TN have been found to be understood taking into account the phonon-magnon scattering in the so-called 120° structure phase and the up-up-down (UUD) phase, while the minima of κab and κc observed in the middle of the UUD phase have not been understood. This suggests the occurrence of some change of the magnetic state in the middle of the UUD phase.

Decomposition of coherent and incoherent phonon conduction in superlattices and random multilayers

Nonequilibrium molecular dynamics (NEMD) simulations on conceptual binary Lennard-Jones systems show that the thermal conductivity ($\ensuremath{\kappa}$) of a superlattice (SL) can be significantly reduced by randomizing the thicknesses of its layers, by which a SL becomes a random multilayer (RML). Such reduction in $\ensuremath{\kappa}$ is a clear signature of coherent phonon that can be localized in RMLs. We build a two-phonon model that divides the overall heat conduction into coherent and incoherent phonon contributions. In SL both coherent and incoherent phonons contribute to heat conduction, while in RML coherent phonons are localized so only incoherent phonons contribute. This model can fit the length dependence of the thermal conductances predicted in our NEMD simulations very well. The ballistic-limit thermal conductance and the intrinsic mean free path (MFP) of both coherent and incoherent phonons, and the localization length of coherent phonons, are obtained by fitting our model to the NEMD simulation results. The significant increase in $\ensuremath{\kappa}$ of SL with total length is due to the long MFP of coherent phonons, and the lower $\ensuremath{\kappa}$ of RML than SL is caused by the localization of coherent phonons.

Size Effects of Heat Conduction for Silicon Nanograins

The structure model of silicon nanograins was built. And then based the modification of the mean free path of phonons according to the size of nanograins, the expression of thermal conductivity in nanograins was obtained according to the phonon kinetic theory. The dependence of the thermal conductivity of silicon nanograins on size was investigated. The results showed that thermal conductivity of nanograins decrease with the reduction of characteristic sizes when the characteristic sizes of nanograins are comparable to or smaller than the phonon mean free path.

Low-temperature anisotropy of the thermal conductivity of single-crystal nanofilms and nanowires

The effect of phonon focusing on the phonon transport in single-crystal nanofilms and nanowires is studied in the boundary scattering regime. The dependences of the thermal conductivity and the free path of phonons on the geometric parameters of nanostructures with various elastic energy anisotropies are analyzed for diffuse phonon scattering by boundaries. It is shown that the anisotropies of thermal conductivity for nanostructures made of cubic crystals with positive (LiF, GaAs, Ge, Si, diamond, YAG) and negative (CaF2, NaCl, YIG) anisotropies of the second-order elastic moduli are qualitatively different for both nanofilms and nanowires. The single-crystal film plane orientations and the heat flow directions that ensure the maximum or minimum thermal conductivity in a film plane are determined for the crystals of both types. The thermal conductivity of nanowires with a square cross section mainly depends on a heat flow direction, and the thermal conductivity of sufficiently wide nanofilms is substantially determined by a film plane orientation.

Lattice thermal conductivity of crystalline and amorphous silicon with and without isotopic effects from the ballistic to diffusive thermal transport regime

Thermal conductivity of a material is an important physical parameter in electronic and thermal devices, and as the device size shrinks down, its length-dependence becomes unable to be neglected. Even in micrometer scale devices, materials having a long mean free path of phonons, such as crystalline silicon (Si), exhibit a strong length dependence of the thermal conductivities that spans from the ballistic to diffusive thermal transport regime. In this work, through non-equilibrium molecular-dynamics (NEMD) simulations up to 17 μm in length, the lattice thermal conductivities are explicitly calculated for crystalline Si and up to 2 μm for amorphous Si. The Boltzmann transport equation (BTE) is solved within a frequency-dependent relaxation time approximation, and the calculated lattice thermal conductivities in the BTE are found to be in good agreement with the values obtained in the NEMD. The isotopic effects on the length-dependent lattice thermal conductivities are also investigated both in the crystalline and amorphous Si.

Effect of structural features on the thermal conductivity of SiGe-based materials

Approach-to-equilibrium molecular dynamics have been utilized to investigate the thermal transport in SiGe-based materials focusing on the effect of structural changes while the chemical composition has been kept constant. At a Si:Ge ratio of 1:1, the thermal conductivity has been evaluated for crystalline SiGe with a periodic (ZnS-like) and random (alloy) distribution of Si and Ge atoms, for SiGe nanocomposite and for amorphous SiGe. Thermal conductivity has been found to be lowest in amorphous SiGe (0.9 W/mK). In the regime studied here, a non-linear behavior of 1/κ to 1/L z has been observed for amorphous SiGe, while a linear trend is found for all crystalline materials (ZnS-like, alloy and nanocomposite). This has been attributed to a wide spread in the mean free path of phonons dominating the thermal transport in the amorphous system.

Thermal and Electrical Conductivities of Porous Si Membranes

The microstructure of materials affects thermal and electrical transport as well as the physical properties. The effects of the microstructure on both thermal and electrical transport in silicon membranes with periodic microporous structures produced from silicon-on-insulator wafers using microfabrication processes were studied. The in-plane thermal and electrical conductivities of the Si membranes were measured simultaneously by using a self-heating method. The measured thermal conductivity was compared with the result from the periodically laser-heating method. The thermal and electrical conductivities were much lower in the porous membranes than in the non-porous membrane. The measured thermal conductivity was much lower than expected based on values determined using classical models. A significant phonon size effect was observed even in microsized structures, and the mean free path for phonons was very long. It was concluded that phonon transport is quasi-ballistic and electron transport is diffuse in microporous Si structures. It was suggested that the microstructure had a different effect on thermal and electrical transport.

Phonon interference and thermal conductance reduction in atomic-scale metamaterials

We introduce and model a three-dimensional (3D) atomic-scale phononic metamaterial producing two-path phonon interference antiresonances to control the heat flux spectrum. We show that a crystal plane partially embedded with defect-atom arrays can completely reflect phonons at the frequency prescribed by masses and interaction forces. We emphasize the predominant role of the second phonon path and destructive interference in the origin of the total phonon reflection and thermal conductance reduction in comparison with the Fano-resonance concept. The random defect distribution in the plane and the anharmonicity of atom bonds do not deteriorate the antiresonance. The width of the antiresonance dip can provide a measure of the coherence length of the phonon wave packet. All our conclusions are confirmed both by analytical studies of the equivalent quasi-1D lattice models and by numerical molecular dynamics simulations of realistic 3D lattices.

High thermal conductivity of chain-oriented amorphous polythiophene

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ∼ 4.4 W m(-1) K(-1) (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

PROPAGATION OF PHONONS IN TOPOLOGICAL SUPERCONDUCTORS INDUCED BY STRAIN FIELDS INSTANTONS

We show that for a multiple-connected space the low energy strain fields excitations are given by instantons, which are controlled by the phase of the superconductor. Dirac fermions with a chiral mass and a superconducting pairing field propagates effectively in a multiple-connected space. When the elastic strain field response is probed one finds that it is given by the Pointriagin characteristic. As a result the space–time metric is modified. Applying an external stress field we observe that the phonon path bends in the transverse direction to the initial direction.

22pm1-E2 多結晶構造におけるフォノンの平均自由行程のシミュレーション

We have calculated the mean free path (MFP) of phonons associated with boundary scattering in polycrystalline nanostructures. A numerical transmission model, based on the kinetic theory and the Landauer's formula, was developed to obtain the phonon MFP. The MFP in a simple cubic polycrystalline nanostructure was calculated and compared with that of the previously-formulated analytical model derived from Matthiesen's rule with combining scatterings effect of super lattice and nanowire. The results show that the MFP of our calculations is around 0.6〜0.9 times that of the model. It is due to the limitation of the rule on combining two anisotropic scatterings. We have also revealed how the geometric complexity and grain-size distribution influence the MFP by performing simulations for nanostructures generated with Voronoi diagram. It is shown that the MFP in complex nanostructures with a realistic grain-size distribution is at most 20% longer than that in a mono grain-size structure with the same average grain size.

A comparison study on the electronic structures, lattice dynamics and thermoelectric properties of bulk silicon and silicon nanotubes

In order to investigate the mechanism of the electron and phonon transport in a silicon nanotube (SiNT), the electronic structures, the lattice dynamics, and the thermoelectric properties of bulk silicon (bulk Si) and a SiNT have been calculated in this work using density functional theory and Boltzmann transport theory. Our results suggest that the thermal conductivity of a SiNT is reduced by a factor of 1, while its electrical conductivity is improved significantly, although the Seebeck coefficient is increased slightly as compared to those of the bulk Si. As a consequence, the figure of merit (ZT) of a SiNT at 1200 K is enhanced by 12 times from 0.08 for bulk Si to 1.10. The large enhancement in electrical conductivity originates from the largely increased density of states at the Fermi energy level and the obviously narrowed band gap. The significant reduction in thermal conductivity is ascribed to the remarkably suppressed phonon thermal conductivity caused by a weakened covalent bonding, a decreased phonon density of states, a reduced phonon vibration frequency, as well as a shortened mean free path of phonons. The other factors influencing the thermoelectric properties have also been studied from the perspective of electronic structures and lattice dynamics.

Thermal Conductivity of Gel-Grown Barium Oxalate at 326 and 335 K

Single crystals of barium oxalate have been grown by gel method using agar-agar gel as media of growth at ambient temperature. The grown crystal crystallizes under monoclinic structure. Thermal conductivity of gel-grown barium oxalate crystals as a function of temperature has been studied at 326 and 335 K by using divided bar method. The thermal conductivity of barium oxalate crystal at 326 K was found 3.685 W m−1 K−1 and 3.133 W m−1 K−1 at 335 K. The reduction of thermal conductivity with the rise in temperature may be due to reduction in mean free path of phonons in the solid.

Thermal balance and quantum heat transport in nanostructures thermalized by local Langevin heat baths

Modeling of thermal transport in practical nanostructures requires making tradeoffs between the size of the system and the completeness of the model. We study quantum heat transfer in a self-consistent thermal bath setup consisting of two lead regions connected by a center region. Atoms both in the leads and in the center region are coupled to quantum Langevin heat baths that mimic the damping and dephasing of phonon waves by anharmonic scattering. This approach treats the leads and the center region on the same footing and thereby allows for a simple and physically transparent thermalization of the system, enabling also perfect acoustic matching between the leads and the center region. Increasing the strength of the coupling reduces the mean-free path of phonons and gradually shifts phonon transport from ballistic regime to diffusive regime. In the center region, the bath temperatures are determined self-consistently from the requirement of zero net energy exchange between the local heat bath and each atom. By solving the stochastic equations of motion in frequency space and averaging over noise using the general fluctuation-dissipation relation derived by Dhar and Roy [J. Stat. Phys. 125, 801 (2006)], we derive the formula for thermal current, which contains the Caroli formula for phonon transmission function and reduces to the Landauer-Büttiker formula in the limit of vanishing coupling to local heat baths. We prove that the bath temperatures measure local kinetic energy and can, therefore, be interpreted as true atomic temperatures. In a setup where phonon reflections are eliminated, the Boltzmann transport equation under gray approximation with full phonon dispersion is shown to be equivalent to the self-consistent heat bath model. We also study thermal transport through two-dimensional constrictions in square lattice and graphene and discuss the differences between the exact solution and linear approximations.

Application of the Callaway theory to analysis of thermal transport by phonons in ceramic and biomorphic composites

By applying the Klemens–Callaway expression for the thermal conductivity in the framework of the Debye's model of phonons we present an analysis of the thermal transport by phonons in ceramic composites Si3N4/BN of various fibre structure (including plaited fibre configurations denoted [0/45] and [0/90]) and biomorphic composites (or ecocomposites) SiC/Si based on beech matrices filled with Si of two different volume percentage (2% and 26%). We obtained a good agreement with the Debye–Klemens–Callaway theory of the thermal conductivity at low temperatures for the ceramic composites Si3N4/BN with the plaited fibre configurations. A good fitting of the Callaway expression for the thermal conductivity coefficient in the whole temperature range can be made to our experimental results for the biomorphic composites SiC/Si. The latter fitting allowed us to assess the magnitude of the mean free path of phonons scattered by the boundaries of grains. We also estimated that the contribution of the conduction electrons to the heat transfer at low temperatures is 5–7% of the effective thermal conductivity in case of SiC/Si.

Phonon-glass-like behavior of magnetic origin in single-crystal Tb2Ti2O7

We report a study on the thermal conductivity ($\ensuremath{\kappa}$) of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ single crystals at low temperatures. It is found that in zero field this material has an extremely low phonon thermal conductivity in a broad temperature range. The mean free path of phonons is even smaller than that of amorphous materials and is 3--4 orders of magnitude smaller than the sample size at 0.3 K. The strong spin fluctuation of the spin-liquid state is discussed to be the reason of the strong phonon scattering. The magnetic-field dependence of $\ensuremath{\kappa}$ and comparison with Y${}_{2}$Ti${}_{2}$O${}_{7}$ and TbYTi${}_{2}$O${}_{7}$ confirm the magnetic origin of this phonon-glass-like behavior.