A comparison study on the electronic structures,lattice dynamics and thermoelectric properties of bulk silicon and silicon nanotubes

In order to investigate the mechanism of the electron and phonon transport in a silicon nanotube(SiNT),the electronic structures,the lattice dynamics,and the thermoelectric properties of bulk silicon(bulk Si)and a SiNT have been calculated in this work using density functional theory and Boltzmann transport theory.Our results suggest that the thermal conductivity of a SiNT is reduced by a factor of 1,while its electrical conductivity is improved significantly,although the Seebeck coefficient is increased slightly as compared to those of the bulk Si.As a consequence,the figure of merit(ZT)of a SiNT at 1200 K is enhanced by 12 times from 0.08 for bulk Si to 1.10.The large enhancement in electrical conductivity originates from the largely increased density of states at the Fermi energy level and the obviously narrowed band gap.The significant reduction in thermal conductivity is ascribed to the remarkably suppressed phonon thermal conductivity caused by a weakened covalent bonding,a decreased phonon density of states,a reduced phonon vibration frequency,as well as a shortened mean free path of phonons.The other factors influencing the thermoelectric properties have also been studied from the perspective of electronic structures and lattice dynamics.     

Dynamical stability,electronic and thermoelectric properties of quaternary ZnFeTiSi Heusler compound

We investigated the dynamical stability, electronic and thermoelectric properties of the ZnFeTiSi Heusler compound by combining the first-principles calculations and semi-classical Boltzmann transport theory. The phonon dispersion indicates the dynamical stability and the calculated formation energy is negative which confirm the stability of ZnFeTiSi in the Heusler structure. The calculated electronic structures show that ZnFeTiSi is a semiconductor with an indirect band gap of about 0.573 eV using GGA and 0.643 eV by mBJ-GGA potentials at equilibrium lattice parameter (5.90 Å). Seebeck coefficient, electrical conductivity and electronic thermal conductivity were calculated to describe the thermoelectric properties of the ZnFeTiSi compound. It is found that it exhibits high Seebeck coefficient and power factor, making it promising for future thermoelectric applications.     

First principle investigation of the electronic and thermoelectric properties of Mg_2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties(Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor(PF) increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.     

A study of electronic structure and lattice dynamics of CoSb3 skutterudite

The novel filled skutterudite materials have attracted much interest in recent years and experimental studies have revealed that electrical properties (electrical conductivity and Seebeck coefficient) in these materials are dominated by their electronic structure while the effective suppression of thermal conductivity is mainly determined by their lattice dynamics. To clarify the relationship between microstructure and properties in further, we report a systematic study of electronic structures and lattice dynamics of CoSb₃ in this paper using linearized augmented plane waves based on the density functional theory of first principles. By calculating band structure and partial density of states (PDOS), effects of electronic structures of CoSb₃ on electrical properties were investigated. Based on the calculated results of phonon dispersions and phonon density of states of CoSb₃, lattice dynamics of CoSb₃ (heat capacity, Debye temperature, mean free path and lattice thermal conductivity) are discussed in detail. The calculated results are excellently consistent with other work and experimental data.     

Effect of pressure on electronic and thermoelectric properties of magnesium silicide: A density functional theory study

We study the effect of pressure on electronic and thermoelectric properties of Mg_2Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties(Seebeck coefficient, electrical conductivity, electronic thermal conductivity) have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg_2Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity.The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg_2Si at zero pressure. The maximum value of figure of merit is attained 1.83 × 10~(-3) at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results.     

Mg2Si化合物在静水压下的电子输运性能研究

本文基于第一性原理采用全电势线性缀加平面波方法和波尔兹曼理论运算了在静水压下Mg₂Si的电子和热电性能. 研究发现, 对于n型载流子控制Mg₂Si输运性质, 应变达到0.02时, 室温情况下, 热电性能参数得到了明显提高, 其塞贝克系数增幅为26%, 功率因数增幅47%; 高温时, 功率因数增幅45%. 而对于主要载流子为空穴时, 其热电系数最值出现在应变为0.01时. 但其数值与未施加静水压的结构相比提高不多, 表明对于p型Mg₂Si半导体应变对其输运性能的影响不大. 并且结合电子能带结构图解释这些现象.     

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.     

Thermoelectric transport coefficients of n-doped CaTiO3, SrTiO3 and BaTiO3: A theoretical study

Boltzmann transport equations and density functional theory calculations were employed to calculate the thermoelectric transport coefficients of CaTiO₃, SrTiO₃ and BaTiO₃. It was found that BaTiO₃ has the largest Seebeck coefficient and power factor. Then the transport coefficients were analyzed using the ‘Tight Binding Model’. The band narrowing, caused by the increasing lattice constants from CaTiO₃ to BaTiO₃, was the main reason for the increasing Seebeck coefficients and the decreasing electrical conductivity. The calculated electrical conductivity and electronic thermal conductivity were in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 2.45 for these oxides as degenerate semiconductors. Our theoretical results are helpful for seeking high performance thermoelectric oxides.     

Ag-ZnO纳米复合热电材料的制备及其性能研究

ZnO是一类具有潜力的热电材料, 但其较大声子热导率影响了热电性能的进一步提高. 纳米复合是降低热导率的有效途径. 本文以醋酸盐为前驱体, 溶胶-凝胶法制备了Ag-ZnO纳米复合热电材料. 扫描电镜照片显示ZnO颗粒呈现多孔结构, Ag纳米颗粒分布于ZnO的晶粒之间. Ag-ZnO纳米复合材料的电导率比未复合ZnO材料高出100倍以上, 而热导率是未复合ZnO材料的1/2. 同时, 随着Ag添加量的增加, 赛贝克系数的绝对值逐渐减小. 综合以上原因, 添加7.5%mol Ag的Ag-ZnO纳米复合材料在700 K时的热电优值达到0.062, 是未复合ZnO材料的约25倍. 在ZnO基体中添加导电金属颗粒有利于产生导电逾渗通道, 提高材料体系的电导率, 但同时导致赛贝克系数的绝对值减小. 总热导率的差异来源于声子热导率的差异. 位于ZnO晶界的纳米Ag颗粒, 有利于降低声子热导率. 关键词： 热电材料 ZnO 纳米复合 热导率     

Graphene for Thermoelectric Applications: Prospects and Challenges

Thermoelectric power generators require high-efficiency thermoelectric materials to transform waste heat into usable electrical energy. An efficient thermoelectric material should have high Seebeck coefficient and excellent electrical conductivity as well as low thermal conductivity. Graphene, the first truly 2D nanomaterial, exhibits unique properties which suit it for use in thermoelectric power generators, but its application in thermoelectrics is limited by the high thermal conductivity and low Seebeck coefficient resulting from its gapless spectrum. However, with the possibility of modification of graphene's band structure to enhance Seebeck coefficient and the reduction of its thermal conductivity, it is an exciting prospect for application in thermoelectric power generation. This article examines the electronic, optical, thermal, and thermoelectric properties of graphene systems. The factors that contribute to these material properties in graphene systems like charge carriers scattering mechanisms are discussed. A salient aspect of this article is a synergistic perspective on the reduction of thermal conductivity and improvement of Seebeck coefficient of graphene for a higher thermoelectric energy conversion efficiency. In this regard, the effect of graphene nanostructuring and doping, forming of structural defects, as well as graphene integration into a polymer matrix on its thermal conductivity and Seebeck coefficient is elucidated.     

Lattice stability and the effect of Co and Re on the ideal strength of Ni:First-principles study of uniaxial tensile deformation

Using first-principles density functional calculations, lattice stability of γ-Ni under [001], [110], and [111] uniaxial tensions and the effect of alloying elements Co and Re on the uniaxial tensile behavior of γ-Ni were studied in this paper.With elastic constants and phonon spectra calculations, we examined the mechanical stability and phonon stability of Ni during the uniaxial tensions along the three characteristic directions. The results show that the mechanical stability and phonon stability of a lattice occurs before the maximum stress–strain point under the [001] and [111] tension, respectively.The effects of Co and Re on the ideal tensile strength of γ-Ni show a significant directivity: Co and Re have little effect on the stresses in [001] and [111] directions, but increases the ideal strength of the system in the weakest uniaxial tensile direction. Moreover, the strengthening effect of Re is significantly better than that of Co. By further analyzing electronic structure, it is found that the effect of alloying elements on the uniaxial tensile behavior of γ-Ni comes from their interactions with host atoms.     

Ab initio study of structural,electronic and dynamical properties of MgAuSn

The structural and electronic properties of MgAuSn in the cubic AlLiSi structure have been studied, using density functional theory within the local density approximation. The calculated lattice constant for MgAuSn is found to be in good agreement with its experimental value. Our calculated electronic structure is also compared in detail with a recent tight-binding. A linear-response approach to density-functional theory is used to calculate the phonon spectrum and density of states for MgAuSn.     

Mg2Si的电子结构和热电输运性质的理论研究

利用全势线性缀加平面波法,对Mg₂Si的几何结构和电子结构进行了计算,得到了稳定的晶格参数以及能带和电子态密度.能带结构表明,Mg₂Si为间接带隙半导体,禁带宽度为020 eV.在此基础上利用玻尔兹曼输运理论和刚性带近似计算了材料的电导率、Seebeck系数和功率因子.结果表明,在温度为700 K时p型和n型掺杂的Mg₂Si功率因子达到最大时的最佳载流子浓度分别为7749×10¹⁹ cm^-3和 关键词： 2Si')" href="#">Mg₂Si 全势线性缀加平面波法 热电输运性质     

金属有机物化学气相沉积生长GaN薄膜的室温热电特性研究

采用金属有机物化学气相沉积技术生长了不同掺杂浓度的GaN薄膜, 并且通过霍尔效应测试和塞贝克效应测试, 表征了室温下GaN薄膜的载流子浓度、迁移率和塞贝克系数. 在实验测试的基础上, 计算了GaN薄膜的热电功率因子, 并且结合理论热导率确定了室温条件下GaN薄膜的热电优值(ZT). 研究结果表明: GaN薄膜的迁移率随着载流子浓度的增加而减小, 电导率随着载流子浓度的增加而增加; GaN 薄膜材料的塞贝克系数随载流子浓度的增加而降低, 其数量级在100–500 μV/K范围内; GaN薄膜材料在载流子浓度为1.60×10¹⁸ cm^-3时, 热电功率因子出现极大值4.72×10^-4 W/mK²; 由于Si杂质浓度的增加, 增强了GaN薄膜中的声子散射, 使得GaN薄膜的热导率随着载流子浓度的增加而降低. GaN薄膜的载流子浓度为1.60×10¹⁸ cm^-3时, 室温ZT达到极大值0.0025.     

低维纳米材料量子热输运与自旋热电性质 ——非平衡格林函数方法的应用

低维材料不断涌现的新奇性质吸引着科学研究者的目光. 除了电子的量子输运行为之外, 人们也陆续发现和确认了热输运中显著的量子行为, 如 热导低温量子化、声子子带、尺寸效应、瓶颈效应等. 这些小尺度体系的热输运性质可以很好地用非平衡格林函数来描述. 本文首先介绍了量子热输运的特性、声子非平衡格林函数方法及其在低维纳米材料中的研究进展; 其次回顾了近年来在 一系列低维材料中发现的热-自旋输运现象. 这些自旋热学现象展现了全新的热电转换机制, 有助于设计新型的热电转换器件, 同时也给出了用热产生自旋流的新途径; 最后介绍了线性响应理论以及在此理论框架下结合声子、电子非平衡格林函数方法进行的一些有益的探索. 量子热输运的研究对热效应基础研究以及声子学器件、能量转换器件的发展有着不可替代的重要作用.     

Galvanomagnetic and Thermomagnetic Properties of a Nonideal Xenon Plasma at Megabar Pressures in Megagauss Magnetic Fields

A method has been developed for the calculation of tensors of the electrical conductivity, Seebeck coefficient, and thermal conductivity of a nonideal plasma in a magnetic field within a unified approach where the kinetic coefficients are calculated together with the equation of state of the nonideal plasma within a quasichemical model. The calculations have shown that the Seebeck coefficient in xenon reaches 3 mV/K, which is slightly smaller than that in hydrogen or deuterium, and the figure of merit appears to be insignificantly higher in xenon. Consequently, the transition from hydrogen (deuterium) to xenon does not result in the expected noticeable improvement of thermoelectric properties. This is due to lower values of the Seebeck coefficient and electrical conductivity, as well as to a fast increase in the thermal conductivity of neutral xenon with an increase in its density. It has been shown for the first time that there is a density range where all components of the Seebeck tensor in xenon change their sign because of the Ramsauer minimum in the cross section for scattering of electrons on neutral atoms in the region of comparable values of cyclotron and transport frequencies of electrons.     

PbTe纳米晶高压热电性能的研究

 利用自建的400 t四柱双缸液压机,研究了PbTe纳米晶在0~0.8 GPa压力范围内热电性能随压力的变化。实验结果表明：PbTe纳米晶电导率随压力的增加而增加,而热电动势随压力的增加而减小,两者随压力的变化具有可逆性;PbTe纳米晶具有极高的热电动势,在常压下达到565 μV/K,在0.8 GPa压力下,材料的电导率为常压下的4倍,热电动势仅降低20%,功率因子则达到常压下的3倍。研究表明,高压能显著提高PbTe纳米晶的热电性能。利用第一性原理计算了0.4~4.0 GPa压力范围内材料的简约费米能级,计算结果与实验结果相吻合。     

Velocities of sound and the densities of phonon states in a uniformly strained flat graphene sheet

The effect of elastic strain on the mechanical and physical properties of graphene has been intensively studied in recent years. Using the molecular dynamics method, a surface has been built in the three-dimensional space of components of the plane strain tensor bounds the region of the structural stability of a flat graphene sheet without considering thermal vibrations and the influence of boundary conditions. The velocities of sound and the densities of phonon states in graphene subjected to an elastic strain within the region of the structural stability have been calculated. It has been shown that one of the velocities of sound becomes zero near the stability boundary of a flat graphene sheet. During biaxial tension of graphene, there is no gap in its phonon spectrum; however, it forms under uniaxial tension along the zigzag or armchair directions and also under combined tensile and compressive strains.     

Band engineering and precipitation enhance thermoelectric performance of SnTe with Zn-doping

We have systematically studied the thermoelectric properties in Zn-doped Sn Te.Strikingly,band convergence and embedded precipitates arising from Zn doping,can trigger a prominent improvement of thermoelectric performance.In particular,the value of dimensionless figure of merit z T has increased by 100% and up to ~ 0.5 at 775 K for the optimal sample with 2% Zn content.Present findings demonstrate that carrier concentration and effective mass play crucial roles on the Seebeck coefficient and power factor.The obvious deviation from the Pisarenko line(Seebeck coefficient versus carrier concentration) due to Zn-doping reveals the convergence of valence bands.When the doping concentration exceeds the solubility,precipitates occur and lead to a reduction of lattice thermal conductivity.In addition,bipolar conduction is suppressed,indicating an enlargement of band gap.The Zn-doped Sn Te is shown to be a promising candidate for thermoelectric applications.     

Electronic,optical and thermoelectric investigations of Zintl phase AE3AlAs3 (AE = Sr,Ba): First-principles calculations

First principles calculations were performed to investigate the electronic, optical and thermoelectric properties of Zintl orthorhombic phase AE₃AlAs₃ (AE?=?Sr, Ba) compounds using the full potential linearized augmented plane wave method. The exchange-correlation potential is treated with the generalized gradient approximation (GGA) and modified Becke-Johnson potential (TB-mBJ) to improve the electronic structure calculations. These two compounds are semiconductors have direct band gaps. The optical transitions are investigated via dielectric function along with other related optical constants such as refractive index and absorption coefficient. Thermoelectric properties are examined using the combination of electronic structure and Boltzmann transport theory. In detail, the calculated results of Seebeck coefficient, electrical and thermal conductivity, figure of merit and power factor are reported as a function of temperature.