Effect of phonon focussing on thermal conductivity of silicon

Thermal conductivity of a cubic crystal in the boundary scattering regime is calculated, taking into account the difference between the phonon phase and group velocities. Numerical estimates in the case of silicon indicate appreciable anisotropy in conductivity as a result of phonon focussing, its maximum value being about 90% larger than the minimum. The contributions of the individual polarization branches are found to be more strongly dependent on direction than the total conductivity. It is further observed that the angle between the phonon phase and group velocities can be sometimes as large as 10°, 24° and 18° in the case of the longitudinal and the two transverse acoustic branches, respectively.     

Calculations of anharmonic phonon relaxation times

Anharmonic phonon relaxation times in Ge are calculated using (i) an isotropic continuum model, and (ii) a dispersive model. A complete spectrum of calculated results is presented. Frequency-averaged values for normal- and umklapp-three-phonon relaxation times are also calculated. A comparison is made between our findings and some earlier works, and disagreements are discussed. The results are applied in calculating some results in the theory of lattice thermal conductivity. From February 1976: Department of Theoretical Physics, The University, Newcastle-Upon-Tyne, NE1 7RU U.K.     

Effect of normal processes on thermal conductivity of germanium,silicon and diamond

The effect of normal scattering processes is considered to redistribute the phonon momentum in (a) the same phonon branch — KK-S model and (b) between different phonon branches — KK-H model. Simplified thermal conductivity relations are used to estimate the thermal conductivity of germanium, silicon and diamond with natural isotopes and highly enriched isotopes. It is observed that the consideration of the normal scattering processes involving different phonon branches gives better results for the temperature dependence of the thermal conductivity of germanium, silicon and diamond with natural and highly enriched isotopes. Also, the estimation of the lattice thermal conductivity of germanium and silicon for these models with the consideration of quadratic form of frequency dependences of phonon wave vector leads to the conclusion that the splitting of longitudinal and transverse phonon modes, as suggested by Holland, is not an essential requirement to explain the entire temperature dependence of lattice thermal conductivity whereas KK-H model gives a better estimation of the thermal conductivity without the splitting of the acoustic phonon modes due to the dispersive nature of the phonon dispersion curves.      

Study of lattice thermal conductivity of alpha-zirconium by molecular dynamics simulation

The non-equilibrium molecular dynamics method is adapted to calculate the phonon thermal conductivity of alphazirconium. By exchanging velocities of atoms in different regions, the stable heat flux and the temperature gradient are established to calculate the thermal conductivity. The phonon thermal conductivities under different conditions, such as different heat exchange frequencies, different temperatures, different crystallographic orientations, and crossing grain boundary (GB), are studied in detail with considering the finite size effect. It turns out that the phonon thermal conductivity decreases with the increase of temperature, and displays anisotropies along different crystallographic orientations. The phonon thermal conductivity in [0001] direction (close-packed plane) is largest, while the values in other two directions of [2īī0] and [01ī0] are relatively close. In the region near GB, there is a sharp temperature drop, and the phonon thermal conductivity is about one-tenth of that of the single crystal at 550 K, suggesting that the GB may act as a thermal barrier in the crystal.     

First-principles investigations on elastic,thermodynamic and lattice thermal conductivity of topological insulator LaAs

Topological insulators are always a hot topic owing to their various peculiar physical effects, which are useful in spintronics and quantum information processing. Herein, we systematically investigate the elastic, thermodynamic and lattice thermal conductivity of a new typical topological insulator LaAs by combining the first-principles approach and an iterative solution of the Boltzmann transport equation. The obtained elastic constants and other lattice structural parameters of LaAs are well consistent with the experimental and other theoretical results. For the first time, the lattice thermal conductivity (5.46 W/(m?K)) and mean free path (14.4 nm) of LaAs are obtained,which manifests that the LaAs is more likely to be a desirable thermoelectric material. It is noted that the obtained mode-averaged Grüneisen parameters by different ab initio simulation packages are very similar, suggesting that our results are rather responsible. From the phonon scattering rates of LaAs, we speculate that the reduction of acoustic-optical gap and the larger phonon scattering may jointly result in reduction of thermal conductivity for LaAs. Meanwhile, the temperature dependence curves of the lattice thermal conductivity, heat capacity and phonon mean free path are also presented. We expect our work can provide more information for further experimental studies.     

Localized self-trapping in two-dimensional molecular lattice with interaction between Wannier-Mott excitons and phonon lattice

We investigate the interactions of lattice phonons with Wannier-Mott exciton, the exciton that has a large radius in two-dimensional molecular lattice, by the method of continuum limit approximation, and obtain that the self-trapping can also appear in two-dimensional molecular lattice with a harmonic and nonlinear potential. The exciton effect on molecular lattice does not distort the molecular lattice but only makes it localized and the localization can also react, again through phonon coupling, to trap the energy and prevents its dispersion.     

Effect of multipolar interaction on the effective thermal conductivity of nanofluids

Nanofluids or liquids with suspended nanoparticles are likely to be the future heat transfer media, as they exhibit higher thermal conductivity than those of liquids. It has been proposed that nanoparticles are apt to congregate and form clusters, and hence the interaction between nanoparticles becomes important. In this paper, by taking into account the interaction between nearest-neighbour inclusions, we adopt the multiple image method to investigate the effective thermal conductivity of nanofluids. Numerical results show that then the thermal conductivity ratio between the nanoparticles and fluids is large, and the two nanoparticles are close up and even touch, and the point-dipole theory such as Maxwell--Garnett theory becomes rough as many-body interactions are neglected. Our theoretical results on the effective thermal conductivity of CuO/water and Al$_{2}$O$_{3}$/water nanofluids are in good agreement with experimental data.     

Dispersive effects and correction term in two-mode phonon conduction model for Ge

A complete analysis of the phonon conductivity κ, still lacking in the literature, is presented in the two-mode conduction model for Germanium. First a method is derived from which the correction term κ_c of the Callaway model is separated into its longitudinal and transverse parts and then the effects of strong phonon dispersion and the role of longitudinal and transverse phonons on κ_c are studied. For this purpose we have also proposed some new empirical expressions for the three phonon relaxation rates τ_3ph⁻¹'s which are valid in the entire temperature range. This improvised model, when applied simultaneously to the phonon conductivity data of both normal and enriched Ge, yields some new results. These are (i) κ_c neglected by the earlier workers in the two-mode phonon conduction model, gives a substantial contribution beyond the conductivity maximum and (ii) the longitudinal phonons are the major carriers of heat at high temperature.     

One phonon resonant Raman scattering in a quantized spherical film

We have presented a theoretical calculation of the differential cross section (DCS) for the electron Raman scattering (ERS) process associated with surface optical (SO) phonon modes in a semiconductor quantized spherical film. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. We study the selection rules for the processes. Singularities are found to be size-dependent and by varying the size of the QDs, it is possible to control the frequency shift in the Raman spectrum. A discussion of the phonon behavior for the films with large and small size is presented. The numerical results are also compared with that of experiments.     

RyMxCO4-xSb12化合物的晶格热导率

系统地研究了离子半径不同的Ba，Ce，Y作为填充原子及Fe，Ni作为置换原子对填充化合物RyMxCO4-xSb12晶格热导率的影响规律．结果表明：在skunemdite结构的sb组成的20面体空洞中，Ba，Ce，Y的填充原子能显著降低其晶格热导率，且晶格热导率降低幅度按Ba，Ce，Y离子半径减小的顺序而增大．Sb组成的20面体空洞部分被Ba，Ce填充时，晶格热导率最小，填充原子的扰动对声子的散射作用最强．在Co位置上Fe和Ni的置换，能显著地降低RyMxCO4-xSb12化合物的晶格热导率，与Fe相比，Ni对晶格热导率的影响更强．     

光频支格波对晶格热容的贡献

讨论晶格的光频支格波对晶格热容的贡献,得到了这部分的热容的表达式。在低温极限情况下,光频支格波对晶格热容的贡献很小,可以忽略,只有频率较低的声频支格波对晶格热容有重要贡献;在高温情况下,复式 和中光频支格波对晶格热容有重要贡献,高温情况下的结果与杜隆－珀蒂定律给出的经典数值一致,该定律在高温时与实际实验符合得很好。     

Anharmonic relaxation of phonons

Using the Ziman scheme of considering a drifting Planck’s distribution, as the eigenvector of the linearized phonon collision operator, the anharmonic relaxation of phonons is discussed. The earlier arbitrariness in the phonon-phonon coupling parameters is removed by formulating explicit expressions for different allowed processes in terms of measurable quantities. Low and high temperature approximations of relaxation rates are also discussed: the results differ from earlier calculations. At low temperatures superthermal or high frequency phonons, which have temperature-independent and equal N- and U-relaxation rates, play important roles in thermal conduction in pure insulators. Presently working as University of Wales Fellow in the Applied Physics Department, UWIST Cathays Park, Cardiff CF1 3NU, U.K.     

Mechanism for thermal conductivity in energetic displacement cascades

Abstract

Temperature relaxation inside and outside the energetic displacement cascade in fast neutron-irradiated metals is consistently described. The characteristic time of the energy transfer between phonons and electrons in the damaged area is calculated. The space–time temperature distribution in the cascade damaged area in Fe and Ni is presented. The electron–phonon coupling is shown to play an important role in the evolution of the damaged area due to non-equilibrium between the local phonon and electron systems at the beginning of the cooling phase of the displacement cascade.     

一维晶格振动声子谱的全量子理论和"不变量本征算符"方法

建立一维晶格振动声子谱的全量子理论,其哈密顿量是自动包含了Born-von-Karmann边界条件的环链量子哈密顿量,然后用不变本征算符方法简捷地求出其声子谱.     

Shape,size and phonon scattering effect on the thermal conductivity of nanostructures

A phenomological model is described here to study the effect of size, shape and phonon scattering on the thermal conductivity of nanostructures. Using the classical model proposed by Guisbiers et al (Phys. Chem. Chem. Phys. 12, 7203 (2010), J. Phys. Chem. C 112, 4097 (2008)) in terms of the melting temperature of nanostructures, the expression for variation of thermal conductivity is obtained in terms of shape and size parameter. An additional term is included in the expression of thermal conductivity to consider the impact of phonon scattering due to the surface roughness with a decrease in size. The expression of thermal conductivity is obtained for spherical nanosolids, nanowires and nanofilms. The thermal conductivity is found to decrease in nanostructures in comparison with the counterpart bulk material. The values of thermal conductivity obtained from the present model are found to be close to the available experimental data for different values of roughness parameter which verifies the suitability of the model.     

The effect of phonon anisotropic scattering on the thermal conductivity of silicon thin films at 300K and 400K

Previous studies have shown that anisotropy in phonon transport exist because of the difference in phonon dispersion relation due to different lattice directions, as observed by a difference in in-plane and cross-plane thermal conductivities. Our current work intends to study the effect of anisotropy scattering on silicon thermal conductivity at 300 K and 400 K. We adopt the Henyey and Greenstein probability density function in our phonon Monte Carlo simulation to investigate the effect of highly forward and backward scattering events. The impact of applying the anisotropy scattering using this approach is discussed in detail. While the forward and backward scattering will increase and decrease thermal conductivity respectively, the extent of the effect is non-linear such that forward scattering has a more obvious effect on thermal conductivity than backward scattering.     

A method to calculate the thermal conductivity of HMX under high pressure

A method based on Debye theory is developed to calculate the thermal conductivity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The phonon–phonon interaction model is built up for solid HMX. The phonon lifetime formula is derived by the phonon–phonon scattering mechanism, and the thermal conductivity tensor is derived by the phonon dispersion model. The thermal conductivities of α/β/δ-HMX are calculated in the temperature range 0–700?K and pressure range of 0–10?GPa. The phonon softening process of HMX is investigated. We have proven that the Debye frequency and thermal conductivity tend to 0 at the phonon softening point. A physical picture of the phonon–phonon interaction, phonon lifetime and phonon softening is built up.