Phonon anharmonicity and thermal conductivity of two-dimensional van der Waals materials: A review

Two-dimensional(2D) van der Waals(vdW) materials have extraordinary thermal properties due to the effect of quantum confinement, making them promising for thermoelectric energy conversion and thermal management in microelectronic devices.In this review, the mechanism of phonon anharmonicity originating from three-and four-phonon interactions is derived. The phonon anharmonicity of 2D vdW materials, involving the Grüneisen parameter, phonon lifetime, and thermal conductivity, is summarized and de...     

颗粒线度和非谐效应对传感器电极材料导电性能及其热稳定性的影响

本文考虑到原子的非简谐振动,应用固体物理理论和方法,研究了氧传感器多孔电极材料导电性能及热稳定性随温度和颗粒线度的变化规律,探讨原子非简谐振动和材料颗粒线度的影响.结果表明:(1)多孔Pt电极材料的电导率随温度的升高而非线性减小;电导率随颗粒半径的增大而非线性增大;电导率随时间增长而减小,但变化极小;(2)多孔Pt电极材料的电导率远小于Pt纳米材料的电导率,也小于块状Pt电极材料的电导率,且颗粒越小,颗粒线度效应越显著;(3)电导率的温度稳定性系数随温度的升高和颗粒线度的减小以及表面层参数的增大而非线性减小,即温度越高、颗粒线度越小、表面层参数越大,电极材料导电性的热稳定性越好;(4)表面层的存在使电导率降低,且降低情况与温度和颗粒线度有关,即颗粒越小,温度越低,电导率下降越明显;非简谐效应对电极材料的电导率几乎没有影响.     

高温高压下铁热导率的第一性原理研究

铁在高压高温下的热导率是研究地球动力学和热演化的关键参数.在以往的研究中,铁的热导率主要归结于电子热导率,我们发现铁在高压下晶格振动对热导率的贡献不可忽略.本文利用晶格动力学和玻尔兹曼输运理论计算了铁的声子色散、Hugoniot状态方程和热导率.预测了铁在核幔边界附近温度约为3500K,在地球内核边界条件约为6500K.考虑晶格振动的热导率在地球核幔边界附近为112W/m K,在内核边界条件约为200W/mK.     

Anomalous layer thickness dependent thermal conductivity of Td-WTe2 through first-principles calculation

The thickness dependent in-plane thermal conductivity of layered Tungsten ditelluride (WTe₂) is investigated by first-principles calculation. With the layer number increasing from one to infinite, the thermal conductivity displays a decrease to increase trend. The underlying mechanism is attributed to the change of the phonon dispersion relations. As the layer number increases, optical phonon branches shift downward, which provide more channels for the Umklapp scattering, and result in the decrease of the thermal conductivity. Furthering increasing the layer number makes those low-frequency optical phonon branches having high group velocity and leads to the increase of the lattice thermal conductivity.     

RE plasma deposition of amorphous silicon-germanium alloys:evidence for a chemisorption-based growth process

Amorphous silicon-germanium alloys (a-Si_1-xGe_x :H,F) have been deposited by glow-discharge decomposition of silicon tetrafluoride (SiF₄) and germane (GeH₄) in mixture with hydrogen. The effect of total RF power and deposition temperature on the characteristics of the gas phase and the grown material has been evaluated. Also, the effect of the dopant addition has been studied by adding PH₃ and B₂H₆ to the feeding mixture. Gas-phase characterization has been performed with optical emission spectroscopy for the analysis of the emitting species, mass spectrometry for the analysis of the stable species, and Langmuir electrical probes for the evaluation of electron density and temperature. The deposition rate has been measured in situ by laser interferometry. The deposited films have been analyzed with infrared and visible-range absorption spectroscopy and with X-ray microanalysis for the measurement of the chemical composition and optical properties. A kinetic analysis of the results points to a deposition model in which dissociative chemisorption of the gaseous reactants (SiF₄, GeH₄) plays a relevant role. There is also strong evidence that the chemisorption process is of the anionic type for SiF₄ and of the cationic type for GeH₄     

Superconducting PrOs4Sb12: a thermal conductivity study

The superconducting state of the heavy fermion PrOs4Sb12 is studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The field and temperature dependence of the thermal conductivity confirm multiband superconductivity and point to fully open gaps on the whole Fermi surface.     

Martensitic transformation of TiNiPd high-temperature shape memory alloys: A first-principles study

Heat of formation, elastic property and electronic structure of TiNiPd high-temperature shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane-wave method. The results show that the heat of formation difference between austenite and martensite plays an important role in the martensitic transformation. The effect of Pd content on the martensitic transformation temperature and transformation type is clarified based on the elastic constants of the B2 phases. High martensitic transformation temperature can be attributed to a low shear resistance C′. Furthermore, the mechanism of the effect of Pd addition on elastic constants is explained on the basis of the electronic structure.     

Structural relaxation and anharmonicity in polymer electrolytes: Effect of the thermal history

Summary The effects of the storage at room temperature of PEO-KSCN polymer electrolytes have been studied by differential scanning calorimetry (DSC) and dynamical mechanical analysis. It has been revealed that, over the explored time interval, the annealing causes small variations in the anharmonic and relaxation properties of the samples, which are to be ascribed to changes in the relative amount of the phases building up the structure. The elastic and anelastic characteristics show a well-defined dependence on the degree of crystallinity of the polymer, which grows slightly with increasing annealing time. The application of a simplified version of a quasi-harmonic model and of the Kolrausch-Williams-Watts stretched exponential function permits to describe the temperature behaviour of the elastic modulusE′ and to obtain an anharmonicity parameter characterizing the polymeric system. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Role of helium in the sliding and mechanical properties of a vanadium grain boundary:A first-principles study

The effects of helium （He） on the sliding and mechanical properties of a vanadium （V） E5（310）/[001] grain boundary （GB） have been investigated using a first-principles method. It has been found that He was energetically favorable sitting at the GB region with a segregation energy of -0.27 eV, which was attributed to the special atomic configurations and charge density distributions of the GB. The maximal sliding energy barrier of the He-doped GB was calculated to be 1.73 J/m^2, 35% larger than that of the clean GB. This suggested that the presence of He would hinder the V GB mobility. Based on the thermodynamic criterion, the total energy calculations indicated that the embrittlement of V GB would be enhanced by He segregation.     

The effect of pressure on the thermal conductivity of ordered and disordered alloys

A comparison is made between the results from experimental studies into the effect of hydrostatic pressures of up to 400 MPa in the temperature range 273–523 K on the phonon thermal conductivity of CdSnAs₂ alloy and the effective thermal conductivity of granite. Pressure was demonstrated to lead to a change in the degree of structural ordering of the studied specimens.     

Models of a structural phase transition with general anharmonicity and disorder

It is shown how the self-consistent phonon Ansatz leads to a new class of exactly soluble models of a structural phase transition. Both nonpolynomial anharmonicity and disorder are analyzed in detail. In the classical limit, the thermodynamics is obtained and sufficient conditions on the anharmonicity are given to ensure a soft-mode phase transition. Diagonal disorder has been studied numerically. It is found that in three dimensions a pronounced mobility edge, separating localized and delocalized phonon states, may exist.     

The curvature in conductivity plots of alkali halides as a consequence of anharmonicity

By assuming that the creation of a vacancy is accompanied by a local dilatation, the free energy of formation has been determined. Taking the thermal expansivity into account, the corresponding formation enthalpy and entropy were found to be temperature dependent. The same holds for migration enthalpy and entropy. It is shown that this temperature dependence of the above quantities must be partially responsible for the curvature in the conductivity plot of alkali, and silver, halides which has been experimentally observed. For KCl a conductivity deviation of 12% could be attributed to the above effect.     

The curvature in conductivity plots of silver halides as a consequence of anharmonicity

At high temperatures the conductivity plot of silver halides shows a pronounced upward curvature to higher conductivity values. This curvature has been attributed to an excessive decrease of the Gibbs formation energy g^f per Frenkel defect. Such a decrease is known to arise from Lidiard-Debye-Huckel interactions and expansivity. However a quantitative evaluation for AgCl and AgBr has shown that the said effect is not sufficient to explain the experimental data. The present paper proposes a further reason for the excessive decrease of g^f; it is due to an excessive fall of the bulk modulus B at temperatures near the melting point. For AgCl a fall by approx. 10% would give the needed decrease of g^f.