Effect of C60 fullerene additions on the thermal conductivity of low-density polyethylene films

The thermal conductivity of LDPE + C₆₀ nanocomposites with a fullerene concentration up to 10 wt % is studied in the temperature range 20–80°C. This conductivity is found to nonlinearly decrease with increasing fullerene concentration. The decrease in the thermal conductivity of the composites is considered to be caused by a decrease in the phonon mean free path as a result of an increase in the number of scattering centers. The temperature dependence of the thermal conductivity is found to have a maximum.     

Lattice thermal conductivity of skutterudite compounds

A generalized expression is used on the basis of relaxation time approximation to facilitate calculation of lattice thermal conductivity of dielectric materials as well as skutterudite family consists of compounds of the form AB₃. It is assumed that phonon scattering processes are independent and is represented by frequency dependent relaxation times. The contributions of normal three phonon scattering processes are included explicitly as redistribution of phonon momentum between two oscillation branches is considered. Magnitudes of relaxation times are estimated from the experimental data. The result for CoSb₃ is in reasonably good agreement with the experimental result in the temperature range 1–1000°K. It is observed that redistribution of phonon momentum between two oscillation branches leads to a significant suppression of thermal conductivity maximum and it is observed that for unfilled skutterudite the main dominant mechanism at the thermal conductivity maximum is three phonon normal scattering process.     

Layer and size dependence of thermal conductivity in multilayer graphene nanoribbons

Using nonequilibrium molecular dynamics method (NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers which can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is proportional to the layer size, which is caused by the increase of phonon resonance. The results clearly show the dimensional evolution of thermal conductivity from quasi-one dimension to higher dimensions in graphene nanoribbons.     

Translation-rotation coupling and heat transfer in orientationally-disordered phase of CCl4

The isochoric thermal conductivity of an orientationally-disordered phase of CCl₄ is analysed within a model in which heat is transferred by phonons and above the phonon mobility edge by ”diffusive” modes migrating randomly from site to site. The mobility edge ω₀ is found from the condition that the phonon mean-free path cannot become smaller than half the phonon wavelength. The contributions of phonon-phonon, one-, and two-phonon scattering to the total thermal resistance of solid CCl₄ are calcualted under the assumption that the different scattering mechanisms contribute additively. An increase in the isochoric thermal conductivity with temperature is explained by suppression of phonon scattering at rotational excitations due to a decrease in correlation in the rotation of neighbouring molecules.     

Thermal conductivity and magnetic properties of the B substituted Ca3Co4O9

In this study, we reported the effects of the boron (B) substitution into the Ca site in the Ca₃Co₄O₉ system on the electrical, thermal and magnetic properties between 300 K and 5 K. The results indicated that the B-substitution into the system caused an increase of resistivity due to the decrease on carrier concentration. Thermal conductivity decreased for the x = 0.5 B-substituted sample and then increased with increasing the B-content. Analysis on the thermal conductivity of samples showed that the phonon–phonon interaction term is the dominant component in the total thermal conductivity for all the samples. It was found that the point defect contribution to the thermal conductivity increased by increasing the B-content. The temperature dependence of magnetic susceptibility showed a paramagnetic behavior at room temperature and ferrimagnetic behavior below 20 K for unsubstituted sample. But, the magnetization decreased in the B-substituted samples. The substitution of B into the Ca site destroyed the interlayer coupling, which resulted in the decrease of the ferromagnetic behavior. The susceptibility data was fitted using Curie–Weiss law with temperature independent term and the μ_eff values were calculated to be 1.42 μ_B and 3.89 μ_B for unsubstituted sample and the highest B-substitution, respectively.     

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.      

β型锑烯热电输运性质的第一性原理研究

利用第一性原理与半经典玻尔兹曼方程，计算并分析β型锑烯的声子色散、声子群速度、声子弛豫时间、晶格热导率及不同温度下的塞贝克系数、电导率和电子热导率随化学势的变化；结果表明：β型锑烯由于非平面六角结构，三支声学声子在Γ点附近均呈线性变化；声学声子对整个晶格热导率的贡献高达96.68%，而光学声子仅仅占到3.32%；由于较大的声光带隙（a-o gap）导致LA支在声子群速度和弛豫时间中占据主导地位，从而增大了LA支声子对整个热导的贡献；热电优值随温度的升高而增大，在费米面附近其绝对值最大可达0.275.     

Thermal conductivity of UC1 − xNx from 100 to 1000 °K

The thermal diffusivities of UC_{1 ? x}N_x of several compositions were measured from 100 to 1000 °K by a laser flash method. The thermal conductivity was separated into electronic and phonon components by assuming the constant Lorenz number. The phonon conductivity showed an anomalous behaviour against composition at low temperatures. The total thermal conductivity of UC_{1 ? x}N_x showed a minimum above 300 °K at an intermediate composition which moved to higher carbon content with increasing temperature. This behaviour was explained by the temperature dependence of the lattice and electronic components.     

Thermal conductivity and infrared spectra study of polycrystalline La0.67Ca0.33MnO3

The thermal conductivity and infrared transmission spectra of La_0.67Ca_0.33MnO₃ are investigated systematically. The thermal conductivity increases abruptly as the temperature is decreased through the Curie temperature. The effective charge carriers number and the frequency of transverse optic phonon corresponding to the stretching mode also increase dramatically along with decreasing temperature near Curie temperature. Combining our observation with previous reported results, we ascribe the abrupt change of thermal conductivity to the itinerating of the charge carriers due to the remarkable reduction of Jahn–Teller distortion below the Curie temperature.     

First-principles analysis of phonon thermal transport properties of two-dimensional WS_2/WSe_2 heterostructures

The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.     

Isotherms of thermal conductivity in PbTe-MnTe solid solutions

The temperature dependences of the thermal conductivity λ of PbTe-MnTe solid solutions (0–4 mol % MnTe) are measured in the range 170–670 K. The data obtained are used in constructing the isotherms of the lattice thermal conductivity λ_l and in estimating the effective cross section for phonon scattering by Mn impurity atoms. It is found that all the isotherms exhibit an anomalous increase in λ_l in the concentration range 1.25–2.0 mol % MnTe, which disagrees with the usually observed decrease in λ_l with an increase in the impurity concentration. It is assumed that the anomalous increase in λ_l manifests itself after attainment of the percolation threshold when a continuous chain of overlapping deformation fields produced by individual atoms (an infinite cluster) is formed in the crystal. In the crystal lattice, stresses are partly compensated and phonon scattering decreases. The assumption is made that the effect observed has a universal character.     

Ca2+掺杂对CdO多晶热电性能的影响

以CaCO₃作为Ca²⁺源, 利用传统固相烧结法制备了Cd_1-xCa_xO (x=0, 0.01, 0.03, 0.05) 多晶块体样品并研究了Ca²⁺掺杂对CdO高温热电性能的影响. CaCO₃的掺入会导致CdO多晶载流子浓度降低, 使Cd_1-xCa_xO的电阻率ρ和塞贝克系数的绝对值|S|增大、电子热导率κ_e减小. 同时, 在CdO中掺入CaCO₃会引入点缺陷和气孔并可抑制CdO晶粒长大、晶界增多, 从而增加了对声子的散射, 使样品的声子热导率κ_p减小. 由于总热导率的大幅降低, Cd_0.99Ca_0.01O多晶样品在1000 K时的热电优值ZT可达0.42, 比本征CdO提高了约27%, 为迄今n型氧化物热电材料报道的最好结果之一.     

On the thermal conductivity of diamond under changes to its isotopic character

We show that the 50% increase of the room-temperature thermal conductivity of single-crystal diamond [1] upon increasing its isotopic purity is unlikely to be due to a decrease in phonon-isotope scattering alone. In addition, removal of the¹³C isotope can sharpen the phonon dispersion in a heretofore unappreciated way and thereby decrease the phonon-phonon Umklapp scattering enough to cause the observed results. In addition, the order-of-magnitude increase [1] in the UV damage threshold in the isotopically purer diamond is probably caused by similar effects. In fact, all physical processes wherein phonon wave-vector or energy thresholds are important should be affected by this mechanism.     

表面共振圆环结构对Si纳米线热传导的调控

利用声子的波动性,在纳米线表面引入共振结构,可以有效阻碍声子输运.为进一步优化共振结构,本文基于平衡态分子动力学(EMD)方法,研究表面共振圆环结构的高度和宽度对Si纳米线热输运性质的影响.结果表明：随着共振圆环高度的增加,Si纳米线的热导率逐渐减小,最大减幅可达61.9%.当高度达到2nm以后,热导率基本不再变化.共振圆环宽度则对热导率的影响较小.声子色散关系中出现的平带,证实了共振圆环引起的声子共振效应;最低共振频率的变化说明了共振圆环中的声子波长决定了共振圆环高度对纳米线热导率的最大影响程度.研究进一步发现,随着共振圆环高度的增加,声学支声子对热导率贡献的比重变小.本文研究结果对高效热电材料和隔热材料的微纳结构设计提供了一种新的思路.     

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.     

Phonon propagation through photonic crystals—media with spatially modulated acoustic properties

The thermal conductivity κ of photonic crystals differing in degree of optical homogeneity (single crystals of synthetic opals) was measured in the 4.2–300 K temperature range. The thermal conductivity revealed, in addition to the conventional decrease in comparison with solid amorphous SiO₂ characteristic of porous solids, a noticeable decrease for T<20 K, the range wherein the phonon wavelength in amorphous SiO₂ approaches the diameters of the contact areas between the opal spheres. This effect is enhanced in the case of phonon propagation along the SiO₂ sphere chains (six directions in the cubic opal lattice). The propagation of light waves (photons) through a medium with spatially modulated optical properties (photonic crystals) is presently well studied. The propagation of acoustic waves through a medium with spatially modulated acoustic properties (phononic crystals) may also reveal specific effects, which are discussed in this paper; among them are, e.g., the ballistic mode of phonon propagation and waveguide effects.     

硅功能化石墨烯热导率的分子动力学模拟

采用Tersoff势函数与Lennard-Jones势函数,结合速度形式的Verlet算法和Fourier定律,对单层和两层硅功能化石墨烯沿长度方向的导热性能进行了正向非平衡态分子动力学模拟.通过模拟发现,硅原子的加入改变了石墨烯声子的模式、平均自由程和移动速度,使得单层硅功能化石墨烯模型的热导率随着硅原子数目的增加而急剧地减小.在300 K至1000 K温度变化范围内,单层硅功能化石墨烯的热导率呈下降趋势,具有明显的温度效应.对双层硅功能化石墨烯而言,少量的硅原子嵌入,起到了提高热导率的作用,但当硅原子数目达到一定数量后,材料的导热性能下降.     

The effect of normal phonon-phonon scattering processes on the maximum thermal conductivity of isotopically pure silicon crystals

The effect of normal phonon-phonon scattering processes on the thermal conductivity of silicon crystals with various degrees of isotope disorder is considered. The redistribution of phonon momentum in normal scattering processes is taken into account within each oscillation branch (the Callaway generalized model), as well as between different oscillation branches of the phonon spectrum (the Herring mechanism). The values of the parameters are obtained that determine the phonon momentum relaxation in anharmonic scattering processes. The contributions of the drift motion of longitudinal and transverse phonons to the thermal conductivity are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons in the Herring relaxation model represents an efficient mechanism that limits the maximum thermal conductivity in isotopically pure silicon crystals. The dependence of the maximum thermal conductivity on the degree of isotope disorder is calculated. The maximum thermal conductivity of isotopically pure silicon crystals is estimated for two variants of phonon momentum relaxation in normal phonon-phonon scattering processes.     

Heat transfer in the plastic phases I and II of cyclopentane

Thermal conductivity of solid cyclopentane C₅H₁₀ has been measured at isochoric conditions in the plastic phases I and II for samples of different densities. Isochoric thermal conductivity is nearly constant in phase II and increases with temperature in phase I. Such behaviour is attributed to weakening of the translational orientational coupling which, in turn, leads to a decrease of phonon scattering on rotational excitations. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both low-frequency phonons and diffusive modes.     

The applicability of the Debye model to thermal conductivity

The thermal conductivity of KBr crystals containing known concentrations of KNO₂ is studied at low temperatures. The conductivity of the doped crystals is greatly reduced and can be described with a simple relaxation time using the model of the thermal conductivity proposed byDebye. Crystals of KBr containing small concentrations of KI show a decrease in thermal conductivity very similar to that observed in mixed crystals with KCl as host lattice. This observation supports the model proposed byWagner that quasi localized modes act as phonon scatterers.