Directional mechanical and thermal properties of single-layer black phosphorus by classical molecular dynamics

Black phosphorus(BP) has received attention due to its own higher carrier mobility and layer dependent electronic properties, such as direct band gap. Interestingly, the single layer black phosphorus(SLBP) has had large popularity in applications related to thermoelectric, optoelectronic, and electronic devices. Here, we investigate the phonon spectrum,thermal conductivities, and stress strain effects. Robust anisotropy was mainly observed in the thermal conductivities together with the alongside zigzag(ZZ) direction value, compared to the armchair(AC) directions. We also investigated the attitude of stress that was anisotropic in both directions, and the stress effects were two times greater across the ZZ path than those in the AC direction at a low temperature. We obtained a Young's modulus of 63.77 and 20.74 GPa in the AC and ZZ directions, respectively, for a strain range of 0.01. These results had good agreement with first principle calculations.Our study here is useful and significant for the thermal tuning of phosphorus-based nanoelectronics and thermalelectric applications of phosphorus.     

Thermal and thermoelectric response from Keldysh formalism with application to gapped Dirac fermions

Based on the Keldysh Green's functions theory, we present a general formula of the thermal and thermoelectric transport. In the clean limit, our formula recovers the previous results obtained from the semiclassical transport theory.In our approach, we propose an appropriate energy current operator and electric current operator, and the unphysical divergence from the direct application of the Kubo formula is eliminated. As an application, we study the thermal and the thermoelectric Hall conductivities of a gapped Dirac fermion model in the presence of impurity scattering.     

形变碳纳米管场效应晶体管的电学性质

在紧束缚理论的基础上,推导出轴向拉伸和扭转形变时碳纳米管（CNT）的能带公式.结果显示拉伸和扭转形变都可以改变CNT的导电性质,在金属型和半导体型之间转变,特别是对于锯齿型CNT,根据n 与3的余数关系,在拉伸和扭转中分别显示出三种不同的变化规律.进一步应用场效应晶体管Natori理论模拟计算形变对CNT场效应晶体管的电流-电压特性的影响,锯齿型CNT根据n 与3的余数关系表现出不同的电流变化趋势,而对于扶手椅型CNT轴向拉伸不改变电流;在扭转形变时,CNT电流急剧升高,特别是扶手椅型CNT.锯齿型CNT和扶手椅型CNT的电流随扭转角度和外电压行为明显不同.在某些特定的扭转角度,电流随扭转角度变化非常显著,显示出锯齿型CNT和扶手椅型CNT发生半导体型与金属型之间的转变. 关键词： 碳纳米管 紧束缚理论 费米能级 能带结构     

The Effect of Higher Derivative Correction on η/s and Conductivities in STU Model

In this paper we study the ratio of shear viscosity to entropy, electrical and thermal conductivities for the R-charged black hole in STU model. We generalize previous works to the case of a black hole with three different charges. Actually we use diffusion constant to obtain ratio of shear viscosity to entropy. By applying the thermodynamical stability we recover previous results. Also we investigate the effect of higher derivative corrections and find that, to the first order, the higher derivative term increases the value of η/s.     

Thermoelastic behavior of a thermoelectric thin-film attached to an infinite elastic substrate

Abstract

This paper examines the thermoelectric behaviour of a thermoelectric thin film bonded to an elastic substrate. A calculation model for thermoelectric thin films is developed based on the singular integral equation method. The interface shear stress is found to exhibit singular behaviour at the ends of the films. Numerical results for the thermal stress distribution in the film and the film/substrate interface are obtained. Effects of film thickness and the substrate to film stiffness ratio on the stress of the film and the stress intensity factor of the interface are identified. The effects of interface electricity conductivity and the elastic–plastic deformation of the film are discussed.     

Phonon considerations in the reduction of thermal conductivity in phononic crystals

Periodic porous structures offer unique material solutions to thermoelectric applications. With recent interest in phonon band gap engineering, these periodic structures can result in reduction of the phonon thermal conductivity due to coherent destruction of phonon modes characteristic in phononic crystals. In this paper, we numerically study phonon transport in periodic porous silicon phononic crystal structures. We develop a model for the thermal conductivity of phononic crystal that accounts for both coherent and incoherent phonon effects, and show that the phonon thermal conductivity is reduced to less than 4% of the bulk value for Si at room temperature. This has substantial impact on thermoelectric applications, where the efficiency of thermoelectric materials is inversely proportional to the thermal conductivity.     

Thermal conductivity of VLS-grown rough Si nanowires with various surface roughnesses and diameters

In this paper, we synthesize VLS-grown rough Si nanowires using Mn as a catalyst with various surface roughnesses and diameters and measured their thermal conductivities. We grew the nanowires by a combination vapor-liquid-solid and vapor-solid mechanism for longitudinal and radial growth, respectively. The surface roughness was controlled from smooth up to about 37 nm by the radial growth. Our measurements showed that the thermal conductivity of rough surface Si nanowires is significantly lower than that of smooth surface nanowires and decreased with increasing surface roughness even though the diameter of the smooth nanowire was lower than that of the rough nanowires. Considering both nanowires were grown via the same growth mechanism, these outcomes clearly demonstrate that the rough surface induces phonon scattering and reduces thermal conductivity with this nanoscale-hole-free nanowires. Control of roughness induced phonon scattering in Si nanowires holds promise for novel thermoelectric devices with high figures of merit.     

Semiconducting large bandgap oxides as potential thermoelectric materials for high-temperature power generation?

Semiconducting large bandgap oxides are considered as interesting candidates for high-temperature thermoelectric power generation (700–1,200 °C) due to their stability, lack of toxicity and low cost, but so far they have not reached sufficient performance for extended application. In this review, we summarize recent progress on thermoelectric oxides, analyze concepts for tuning semiconductor thermoelectric properties with view of their applicability to oxides and determine key drivers and limitations for electrical and thermal transport properties in oxides based on our own experimental work and literature results. For our experimental assessment, we have selected representative multicomponent oxides that range from materials with highly symmetric crystal structure (SrTiO₃ perovskite) over oxides with large densities of planar crystallographic defects (Ti _n O_2n?1 Magnéli phases with a single type of shear plane, NbO _x block structures with intersecting shear planes and WO_3?x with more defective block and channel structures) to layered superstructures (Ca₃Co₄O₉ and double perovskites) and also include a wide range of their composites with a variety of second phases. Crystallographic or microstructural features of these oxides are in 0.3–2 nm size range, so that oxide phonons can efficiently interact with them. We explore in our experiments the effects of doping, grain size, crystallographic defects, superstructures, second phases, texturing and (to a limited extend) processing on electric conductivity, Seebeck coefficient, thermal conductivity and figure of merit. Jonker and lattice-versus-electrical conductivity plots are used to compare specific materials and material families and extract levers for future improvement of oxide thermoelectrics. We show in our work that oxygen vacancy doping (reduction) is a more powerful driver for improving the power factor for SrTiO₃, TiO₂ and NbO _x than heterovalent doping. Based on our Seebeck-conductivity plots, we derived a set of highest achievable power factors. We met these best values in our own experiments for our titanium oxide- and niobium oxide-based materials. For strontium titanate-based materials, the estimated highest power factor was not reached; further material improvement is possible and can be reached for materials with higher carrier densities. Our results show that periodic crystallographic defects and superstructures are most efficient in reducing the lattice thermal conductivity in oxides, followed by hetero- and homovalent doping. Due to the small phonon mean free path in oxides, grain boundary scattering in nanoceramics or materials with nanodispersions is much less efficient. We investigated the impact of texturing in Ca₃Co₄O₉ ceramics on thermoelectric performance; we did not find any improvement in the overall in-plane performance of a textured ceramic compared to the corresponding random ceramic.     

微型层式热电模块制冷特性研究

本文建立了微型层式热电模块的仿真模型,讨论了不同的层数、层厚度,层导热系数对层式热电模块制冷效果的影响。研究发现:二层热电臂结构最优,冷、热端热电臂导热系数梯度越大,制冷效果越好,且热电臂厚度在不同导热系数梯度下存在最优值。另外,在微型层式热电模块中,接触效应会显著影响其制冷性能,因此在实际设计及应用研究中不可忽略。     

Unraveling the effect of uniaxial strain on thermoelectric properties of Mg_2Si: A density functional theory study

In this work, the effect of uniaxial strain on electronic and thermoelectric properties of magnesium silicide using density functional theory(DFT) and Boltzmann transport equations has been studied. We have found that the value of band gap increases with tensile strain and decreases with compressive strain. The variations of electrical conductivity,Seebeck coefficient, electronic thermal conductivity, and power factor with temperatures have been calculated. The Seebeck coefficient and power factor are observed to be modified strongly with strain. The value of power factor is found to be higher in comparison with the unstrained structure at 2% tensile strain. We have also calculated phonon dispersion, phonon density of states, specific heat at constant volume, and lattice thermal conductivity of material under uniaxial strain. The phonon properties and lattice thermal conductivity of Mg_2Si under uniaxial strain have been explored first time in this report.     

The magneto-thermoelectric effect of graphene with intra-valley scattering

We present a qualitative and quantitative study of the magneto-thermoelectric effect of graphene. In the limit of impurity scattering length being much longer than the lattice constant, the intra-valley scattering dominates the charge and thermal transport. The self-energy and the Green's functions are calculated in the self-consistent Born approximation. It is found that the longitudinal thermal conductivity splits into double peaks at high Landau levels and exhibits oscillations which are out of phase with the electric conductivity. The chemical potential-dependent electrical resistivity, the thermal conductivities, the Seebeck coefficient, and the Nernst coefficient are obtained. The results are in good agreement with the experimental observations.     

Influence of a circulating current on the thermal conductivity of heterogeneous systems

An approach describing the influence of thermoelectric effects and boundary conditions on the thermal conductivity of heterogeneous systems is developed. At a certain configuration of a heterogeneous system, circulating electric currents appearing in the system are shown to influence markedly the effective thermal conductivity. The maximum growth of the thermal conductivity is to be expected in heterogeneous semiconductors and semimetal systems with opposed thermoelectromotive forces.     

Zur Quantentheorie der Transportprozesse im Magnetfeld. II

Applying Kubo's formulae and damping theoretical arguments the electric, thermoelectric and thermal transport coefficients for a system of free and independent electrons and fixed scatterers in the presence of an external magnetic field are calculated. Assumingδ-type scatterers it is shown that in the quantum regionkT?ω?ζ the Wiedemann-Franz law holds for the oscillating symmetric conductivities, whereas in the case of the antisymmetric ones quantum mechanical deviations from this law occur. For strong magnetic fields and arbitrary scattering potential the symmetric transversal transport coefficients can be expressed by Titeica type formulae.     

Calculation of the thermoelectric characteristics of n-and p-type PbTe using a three-band electron spectrum

In this paper we study the thermoelectric properties of n-and p-type PbTe theoretically in a wide temperature interval of 300 to 900 K. A three-band model of the PbTe electron-energy spectrum was used in these calculations for the first time. The full set of the relevant kinetic characteristics is calculated, including the electrical and thermal conductivities, as well as the Seebeck coefficient and the thermoelectric figure-of-merit. The calculated thermoelectric quantities are in good agreement with the available experimental data.     

Tuning the thermal conductivity of strontium titanate through annealing treatments

Strontium titanate(SrTiO_3) is a promising n-type material for thermoelectric applications. However, its relatively high thermal conductivity limits its performance in efficiently converting heat into electrical power through thermoelectric effect.This work shows that the thermal conductivity of SrTiO_3 can be effectively reduced by annealing treatments, through an integrated study of laser flash measurement, scanning electron microscopy, Fourier transform infrared spectroscopy, x-ray absorption fine structure, and first-principles calculations. A phonon scattering model is proposed to explain the reduction of thermal conductivity after annealing. This work suggests a promising means to characterize and optimize the material for thermoelectric applications.     

非化学计量比AgSbTe2+x化合物制备及热电性能

采用高纯元素直接熔融、淬火并结合放电等离子烧结方法制备了非化学计量比AgSbTe_2+x(x=0—0.05)系列样品,研究了不同Te含量在300—600 K范围内对样品热电性能的影响规律.结果表明:随着Te含量的增加,Ag⁺离子空位浓度增加,空穴浓度和电导率大幅度提高,Seebeck系数减小.热导率随Te过量程度的增加略有增加,但所有Te过量样品的晶格热导率均介于0.32—0.49 W/mK之间,低于化学计量比样品的值,接近理论最低晶格热导率.AgS 关键词： 2')" href="#">AgSbTe₂ 非化学计量比 热电性能 热导率     

Color-electric conductivity in a viscous quark-gluon plasma

Several different transport processes, such as heat, momentum, and charge transports, may occur simultaneously in a thermal plasma system. The corresponding transport coefficients are heat conductivity, shear viscosity, and electric conductivity. In the present study, we investigate the color-electric conductivity of the quark-gluon plasma(QGP) in the presence of shear viscosity, focusing on the connection between the charge transport and momentum transport. To achieve this goal, we solve the viscous chromohydrodynamic equations obtained from the QGP kinetic theory associated with the distribution function modified by shear viscosity. According to the solved color fluctuations of hydrodynamic quantities, we obtain the induced color current through which the color-electric conductivity is derived. Numerical analysis shows that the conductivity properties of the QGP are mainly demonstrated by the longitudinal part of the color-electric conductivity. Shear viscosity has an appreciable impact on real and imaginary parts of the color-electric conductivity in some frequency regions.     

Thermoelectric properties of <Emphasis Type="Italic">n</Emphasis>-Type Mg<Subscript>2</Subscript>Si–Mg<Subscript>2</Subscript>Sn solid solutions with different grain sizes

Influence of the grain sizes on thermoelectric parameters of pressurized solid solutions of the composition Mg₂Si_0.8Sn_0.2 was studied. The Seebeck coefficient, electric conductivity, thermal conductivity, and Hall coefficient were determined. Decreasing the grain size to the nanoscale was found to decrease the mobility at low temperatures and resulted in a peculiar temperature dependence of the electric conductivity, but did not lead to a decrease in the thermo EMF. It was found that the grain size had no effect on the thermoelectric efficiency of the investigated solid solution in the operating temperature range.     

不同周期结构硅锗超晶格导热性能研究

构造了均匀、梯度、随机3种不同周期分布的硅/锗(Si/Ge)超晶格结构.采用非平衡分子动力学(NEMD)方法模拟了硅/锗超晶格在3种不同周期分布下的热导率,并研究了样本总长度和温度对热导率的影响.模拟结果表明:梯度和随机周期Si/Ge超晶格的热导率明显低于均匀周期结构超晶格;在不同的周期结构下,声子分别以波动和粒子性质输运为主;均匀周期超晶格热导率具有显著的尺寸效应和温度效应,而梯度、随机周期Si/Ge超晶格的热导率对样本总长度和温度的依赖性较小.     

Thermal properties of two-dimensional materials

Two-dimensional(2D) materials, such as graphene, phosphorene, and transition metal dichalcogenides(e.g., Mo S2 and WS2), have attracted a great deal of attention recently due to their extraordinary structural, mechanical, and physical properties. In particular, 2D materials have shown great potential for thermal management and thermoelectric energy generation. In this article, we review the recent advances in the study of thermal properties of 2D materials. We first review some important aspects in thermal conductivity of graphene and discuss the possibility to enhance the ultra-high thermal conductivity of graphene. Next, we discuss thermal conductivity of Mo S2 and the new strategy for thermal management of Mo S2 device. Subsequently, we discuss the anisotropic thermal properties of phosphorene. Finally, we review the application of 2D materials in thermal devices, including thermal rectifier and thermal modulator.