Reduction of phonon mean free path: From low-temperature physics to room temperature applications in thermoelectricity

It has been proposed for a long time now that the reduction of the thermal conductivity by reducing the phonon mean free path is one of the best way to improve the current performance of thermoelectrics. By measuring the thermal conductance and thermal conductivity of nanowires and thin films, we show different ways of increasing the phonon scattering from low-temperature up to room-temperature experiments. It is shown that playing with the geometry (constriction, periodic structures, nano-inclusions), from the ballistic to the diffusive limit, the phonon thermal transport can be severely altered in single crystalline semiconducting structures; the phonon mean free path is in consequence reduced. The diverse implications on thermoelectric properties will be eventually discussed.     

Thermal conductivity of single crystals with a fluorite structure: Cadmium fluoride

The thermal conductivity of Ca, Sr, Ba, and Cd difluoride single crystals and the CdF₂ samples doped by 3 mol % NdF₃, 15 mol % HoF₃, and 10 mol % ErF₃ has been studied using the method of steady longitudinal heat flow in the temperature range 50–300 K. The thermal conductivity of the matrices of these compounds decreases in the order CaF₂-SrF₂-BaF₂-CdF₂. The temperature dependences of the phonon mean free path for these crystals have been calculated from experimental data and exhibit different behaviors. It has been assumed that the intense phonon scattering observed in the undoped CdF₂ sample is caused by the specific features of the processes of phonon-phonon scattering. The formation of heterovalent solid solutions of cadmium difluoride and rare-earth trifluorides is accompanied by a drastic decrease in the thermal conductivity and a change in its character from that typical of dielectric single crystals to that typical of glassy materials.     

多约束纳米结构的声子热导率模型研究

纳米技术的快速发展使得对微纳尺度导热机理的深入研究变得至关重要. 理论和实验都表明, 在纳米尺度下声子热导率将表现出尺寸效应. 基于声子玻尔兹曼方程和修正声子平均自由程的方法得到了多约束纳米结构的声子热导率模型, 可以描述多个几何约束共同作用下热导率的尺寸效应. 不同几何约束对声子输运的限制作用可以分开计算, 总体影响则通过马西森定则进行耦合. 对于热流方向的约束, 采用扩散近似的方法求解声子玻尔兹曼方程; 对于侧面边界约束, 采用修正平均自由程的方法计算边界散射对热导率的影响. 得到的模型能够预测纳米薄膜(法向和面向)及有限长度方形纳米线的热导率随相应特征尺寸的变化. 与蒙特卡罗模拟及硅纳米结构热导率实验值的对比验证了模型的正确性.     

氮化铝陶瓷低温热导率的实验研究

根据轴向稳态热流法原理 ,实验研究了 Al N陶瓷的低温热导率 ,为高温超导制冷机直接冷却提供了必要的实验数据。 Al N的热导率在 30～ 170 K之间 ,随温度的升高而增大 ,在 10 0 K时达到 5 8.36 W/ (m· K)。根据平均自由程理论 ,对影响氮化铝陶瓷热导率的微观因素进行了分析 ,影响本次测试氮化铝样品低温热导率的主要因素为声子与缺陷之间的散射。     

Anomalous thermal conductivity of polyacetylene

Measurements of the specific heat and of the thermal conductivity of pure and iodine doped polyacetylene from liquid helium to room temperature are reported. The thermal conductivity rises linearly from 3 to 50 K and with about the third power of the temperature from 50 to 300 K. The kink at 50 K corresponds to a very unusual minimum of the phonon mean free path, probably caused by resonant scattering of fast thermal phonons (which travel along the chains) on low frequency interchain modes. These results suggest that the polymer chains are oriented parallel to the well-known fibers in polyacetylene.     

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.     

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.     

FeMnAl合金低温声子热导率的反常行为

我们在4.2—30K测量了具有电阻负温度系数的晶态Fe-25Mn-5Al-0.2C合金的热导率,发现其声子热导率主要随温度线性变化,可能是由于Al的加入使电子平均自由程减小,从而对声子散射减弱所致。 关键词：     

Intrinsically minimal thermal conductivity in cubic I-V-VI2 semiconductors

We report measurements of the thermal conductivity of high-quality crystals of the cubic I-V-VI2 semiconductors AgSbTe2 and AgBiSe2. The thermal conductivity is temperature independent from 80 to 300 K at a value of approximately 0.70 W/mK. Heat conduction is dominated by the lattice term, which we show is limited by umklapp and normal phonon-phonon scattering processes to a value that corresponds to the minimum possible, where the phonon mean free path equals the interatomic distance. Minimum thermal conductivity in cubic I-V-VI2 semiconductors is due to an extreme anharmonicity of the lattice vibrational spectrum that gives rise to a high Grüneisen parameter and strong phonon-phonon interactions. Members of this family of compounds are therefore most promising for thermoelectric applications, particularly as p-type materials.     

Thermal conductivity of NbSe3

The thermal conductivity, κ, of NbSe₃ has been measured by novel self-heating techniques that allowed the electric field dependence of κ to also be measured. Measurements were made from 35 K to room temperature. Above the charge density wave transitions, the phonon thermal conductivity is 4–7 times the electron thermal conductivity, and it rises smoothly below the transitions, indicating that phonon-phonon scattering predominates. Phonon mean free parths have been estimated at 187 A° at 60 K and 60 A° at 150 K. No clear anomalies were observed at the phase transitions, giving upper limits to changes in the phonon mean free path. No field dependence of κ was observed.     

金属纳米颗粒的热导率

本文使用统计模拟方法对金属纳米颗粒的电子平均自由程进行了计算,并考察了纳米颗粒的晶格比热和声子平均群速度,最后应用动力学理论对纳米颗粒的电子热导率和声子热导率分别进行了求解.研究结果表明:具有相同特征尺寸的方形、球形纳米颗粒的无量纲电子(或声子)平均自由程比较接近.金属纳米颗粒的电子热导率远大于声子热导率;电子、声子热导率随着直径减小呈现降低趋势,而电子热导率的颗粒尺度依赖性比声子热导率更为明显;随着颗粒直径进一步减小,声子热导率与电子热导率趋于同一数量级.当纳米颗粒特征尺寸大于4倍块材电子(或声子)平均自由程,其电子(或声子)热导率的颗粒尺度依赖性将减弱.     

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.     

基于第一性原理分子动力学的填充方钴矿热输运性质及微观过程的研究

对于重要热电材料之一的填充方钴矿材料,其低热导率的成因存在两种观点:1)填充原子的局域振动引起共振散射降低热导率;2)填充原子的引入加强了三声子倒逆过程来降低热导率.本文采用含有限温度效应的第一性原理分子动力学方法模拟了YbFe_4Sb_(12)的动力学过程,并通过温度相关有效势场方法得到了充分包含非线性作用的等效非谐力常数,研究了微扰近似下的声子输运性质.结果显示,在填充原子振动全部参与三声子倒逆散射过程的近似下,相比于纯方钴矿体系,声子寿命大幅地降低,填充原子的振动是热阻的重要来源.但即便如此,理论计算结果与实验的晶格热导率之间仍存在明显偏离.不同填充原子振动之间的较弱关联性质也揭示其明显偏离经典的声子图像,表现为一种强烈的局域特征振动模式,并以此散射其他晶格声子,因而对热阻的贡献也超出了传统三声子的理论框架.通过将填充原子Yb振动模式的寿命进行共振散射形式的修正,可以使晶格热导率与实验结果符合较好.以上结果表明,YbFe_4Sb_(12)的低晶格热导率是由声子间相互作用以及具有局域振动特征的共振散射两方面因素导致.     

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.     

Thermal conductivity of undoped trans-polyacetylene films

The low temperature variation of the thermal conductivity of transpolyacetylene is reported. The thermal conductivity, which though it is comparable to that of a semi-crystalline polymer at 100 K, varies linearly with the temperature below this temperature. The temperature variation of the phonon mean free path is found to be similar to that of an oriented polyethylene.     

Effect of phonon scattering mechanisms on the lattice thermal conductivity of skutterudite-related compound

We have prepared the skutterudite-related compounds FeCo_3Sb_{12} and La_{0.75}Fe_3CoSb_{12} with different average grain sizes (about 0.8 and 3.9μm) by hot pressing. Samples were characterized by XRD, EPMA and SEM. The lattice thermal conductivity was investigated in the temperature range from room temperature to 200℃. Based on the Debye model, we analyse the change in lattice thermal conductivity due to various phonon scattering mechanisms by examining the relationship between the weighted phonon relaxation time τ(ω/ω_D)^2 and the reduced phonon frequency ω/ω_D. The effect of grain boundary scattering to phonon is negligible within the range of grain sizes considered in this study. The large reduction in lattice thermal conductivity of FeCo_3Sb_{12} compound contributes to the electron－phonon scattering. As for La_{0.75}Fe_3CoSb_{12} compound, the atoms of La filled into the large voids in the structure of the skutterudite produce more significant electron－phonon scattering as well as more substitute of Fe at Co site at the same time. Moreover, the point-defect scattering appears due to the difference between the atoms of La and the void. In addition, the scattering by the rattling of the rare-earth atoms in the void is another major contribution to the reduced lattice thermal conductivity. Introducing the coupling of the electron－phonon scattering with the point-defect scattering and the scattering by the rattling of the rare-earth atom is an effective method to reduce the lattice thermal conductivity of the skutterudite-related compounds by substitution of Fe for Co and the atoms of La filled in the large voids in the skutterudite structure.     

The effect of surface roughness on lattice thermal conductivity of silicon nanowires

A theoretic model is presented to take into account the roughness effects on phonon transport in Si nanowires (NWs). Based on the roughness model, an indirect Monte Carlo (MC) simulation is carried out to predict the lattice thermal conductivities of the NWs with different surface qualities. Through fitting the experimental data with the MC predictions, the scattering strength on phonons from the boundary, umklapp phonon-phonon processes and impurities can be estimated. It is found that the scattering on phonons by the roughness cell boundaries in a rough nanowire can reduce the phonon mean free path to be smaller than the nanowire diameter, the Casimir limit of the phonon mean free path in a flat nanowire for phonons engaged in completely diffused boundary scattering processes.     

Magnetic susceptibility and electrical resistivity of some Th2Zn17-type rare-earth zinc phases

We report measurements of the magnetic susceptibility and electrical resistivity of the iostructural compounds RE₂Zn₁₇ (RE=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). The composition dependence of the lattice parameter and effective moment indicate that all the RE ions are trivalent except Yb which is divalent. Magnetic order is observed in compounds where RE=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er. A second transition is seen for RE=Pr, Ho, Sm and Tb. Superzone boundary effects are observed in the electrical resistivity of these four alloys as well as in Er₂Zn₁₇. Resistivity measurements reveal concentrated Kondo behavior (or 4f instability) of Ce in Ce₂Zn₁₇.     

A molecular dynamics simulation: the effect of finite size on the thermal conductivity in a single crystal silicon

Non-equilibrium molecular dynamics (NEMD) simulations are performed to calculate thermal conductivity. The environment-dependent interatomic potential (EDIP) potential on crystal silicon is adopted as a model system. The issues are related to nonlinear response, local thermal equilibrium and statistical averaging. The simulation results by non-equilibrium molecular dynamics show that the calculated thermal conductivity decreases almost linearly as the film thickness reduced at the nanometre scale. The effect of size on the thermal conductivity is also obtained by a theoretic analysis of the kinetic theory and formulas of the heat capacity. The analysis reveals that the contributions of phonon mean free path (MFP) and phonon number in a finite cell to thermal conductivity are very important.