单层Cu2X的热电性质

具有低晶格热导率和高热电优值的二维(2D)材料可用于热电器件的制备.本文通过第一性原理和玻尔兹曼输运理论,系统地预测了单层Cu₂X (X=S, Se)的热电性质.研究发现单层Cu₂Se较Cu₂S在室温下具有更低的晶格热导率(1.93 W/(m·K)和3.25 W/(m·K)),这源于其更低的德拜温度和更强的非谐性.单层Cu₂X(X=S, Se)价带顶处的能带简并效应显著增大了其载流子有效质量,导致p型掺杂下具有高的塞贝克系数和低的电导率.在最优掺杂浓度下,单层Cu₂S (Cu₂Se) n型的功率因数16.5 mW/(m·K²)(25.9 mW/(m·K²))远高于其p型的功率因数1.1 mW/(m·K²)(6.6 mW/(m·K²)),且随着温度的提升这一优势将更加明显.温度为700 K时,单层Cu₂S和Cu₂Se在n型最优掺...     

CuGaTe_2和CuInTe_2的电子和热电性质的第一性原理研究

热电材料是通过载流子作用实现热能和电能直接转换的功能材料,在能源、环境、国防等领域具有重要应用.如何提高材料的转换效率是目前热电材料研究的关键.最近发现,三元黄铜矿Ⅰ-Ⅲ-Ⅳ_2(Ⅰ=Ag,Cu;Ⅲ=Al,Ga,In;Ⅳ=S,Se,Te)是一类潜在的高性能热电材料,其结构独特,可通过多种途径优化其性能.本文采用基于密度泛函理论的第一性原理方法系统地研究CuGaTe_2和CuInTe_2的电子特性,为提高其热电效率提供新思路研究发现改进的Becke Johnson-广义梯度近似比广义梯度近似交换关联近似计算的能隙值更接近实验值.基于玻尔兹曼理论研究了体系热电性质,发现通过优化载流子的浓度可以改善体系的热电性.通过拟合计算的晶格热导率发现,在300-800 K,CuGaTe_2和CuInTe_2的晶格热导率和温度成反比,表明其晶格热导率主要来源于声子散射,并且声子散射又是以Umklapp散射为主.CuGaTe_2在700 K的热电优值ZT可以达到0.63,远大于其他Te类材料的ZT值.     

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.     

纳米线阵列横向输运的热电特性研究

利用包络函数的平面波展开法计算准二维纳米线阵列中的电子态,获得电输运系数表达式.同时,通过合理近似考虑边界散射对声子输运的影响,计算得到了晶格热导率.以Si/Ge体系为例,研究了纳米线阵列横向输运的热电特性.结果表明：结构优值与费米能级、纳米线直径及间距等参数相关.通过对结构参数的调整,纳米线阵列的横向输运可有效提高热电性能. 关键词： 热电性能 纳米线阵列 Seebeck系数 晶格热导率     

纳米SiO2复合对Mg3Sb2基材料热电性能的影响

Mg₃(Sb,Bi)2基热电材料由于其优异的热电性能和较低的成本近来受到广泛的关注.本研究通过将纳米SiO₂复合进成分为Mg_3.275Mn_0.025Sb_1.49Bi_0.5Te_0.01的基体相中,考察其热电输运性能的变化及机制.结果表明,当SiO₂复合进Mg₃Sb₂基材料中时,由于引进大量的微小晶界,能有效地散射声子,促使晶格热导率降低,优化热输运性能,如SiO₂体积含量为0.54%时,室温时热导率由复合前的1.24 W/(m·K)降至1.04 W/(m·K),降幅达到15%;同时其对电子也产生强烈的散射作用,导致迁移率和电导率大幅下滑,结果表现为近室温区功率因子剧烈衰减,恶化了电输运性能.电性能相对于热性能较大降低幅度使得材料在整个测试温区的热电优值没有得到改善.纳米SiO₂作为Mg₃Sb₂     

AgyIn3.33–y/3Se5化合物结构和热电性能

本文使用静态扩散法结合常规X射线粉末衍射和电子探针技术,在Ag-In-Se体系中发现了Ag_yIn_3.33-y/3Se₅新化合物.其结构属于三方晶系,空间群为P3m1,是二维层状结构,单层晶胞由9个原子量子层按照Se1-In1-Se2-In2-Se3-Ag/In3-Se4-In4-Se5顺序排布构成,层间由弱范德瓦耳斯力结合.烧结的块体样品表现出强烈的取向性,在平行压力方向上具有极低的晶格热导率(在873 K为0.15 W·m^-1·K^-1).这种本征低的晶格热导率主要源于材料的低声速和低频光学支声子与声学支声子强耦合作用.Ag_yIn_3.33-y/3Se₅样品表现为n型传导,室温下电导率约为4×10~4S·m^-1,Seebeck系数约为-80μV·K^-1,样品在宽温度范围内均表现出较好的电传输性能,在450—800 K范围内的功率因子为5μW·cm^-1...     

具有本征低晶格热导率的硫化银快离子导体的热电性能

硫化银(Ag_2S)是一种典型的快离子导体材料,前期关于Ag_2S的研究主要集中在光电和生物等领域.最近的研究表明, a-Ag_2S具有和金属一样的良好延展性和变形能力.但是, Ag_2S的热电性能尚无公开报道.本工作合成了单相Ag_2S化合物,系统研究了其在300—600 K范围的物相变化、离子迁移特性和电热输运性质.研究发现, Ag_2S在300—600 K温度区间表现出半导体的电输运性质.由于单斜-体心立方相晶体结构转变, Ag_2S的离子电导率、载流子浓度、迁移率、电导率、泽贝克系数等性质在455 K前后出现急剧变化.在550 K, Ag_2S的功率因子最高可达5μW·cm~(–1)·K~(–2). Ag_2S在300—600 K温度区间均表现出本征的低晶格热导率(低于0.6 W·m~(–1)·K~(–1)). S亚晶格中随机分布的类液态Ag离子是导致b-Ag_2S体心立方相具有低晶格热导率的主要原因.在573 K, Ag_2S的热电优值可达0.55,与Ag_2Se, Ag_2Te, CuAgSe等已报道的Ag基快离子导体热电材料的性能相当.     

Si微/纳米带的制备与热电性能

目前,低维材料是热电领域研究的热点,因为块体材料低维化后热电性能会得到显著的改善.块体材料低维化有很多方法,本文基于半导体微加工和聚焦离子束技术制备了尺寸可控的Si微/纳米带,并通过微悬空结构详细研究了不同尺寸Si微/纳米带的热电性能.实验发现:随着Si微/纳米带宽度的减小,材料的热导率发生了显著的降低,从体硅的148 W/(m·K)降低到17.75 W/(m·K)(800 nm);材料的Seebeck系数低于相应的体Si值.热导率的降低主要来源于声子边界散射的增加,这显著抑制了Si材料中声子的传输行为,从而影响热能的传输和转换.在373 K时,800 nm宽的Si微/纳米带的ZT值约达到了0.056,与体硅相比增大了约6倍.聚焦离子束加工技术为将来Si材料提高热电性能提供了新的制备方案,这种技术也可以应用于其他材料低维化的制备.     

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.     

第一性原理研究单层Ge2X4S2(X=P,As)的热电性能

单层Ge₂X₄S₂(X=P, As)是最近预测的一种二维层状材料,它们不仅拥有高的光吸收系数,同时还有较高的载流子迁移率,这意味着它们在光电和热电领域可能有较好的应用前景.本文通过第一性原理和玻尔兹曼输运理论系统地研究了这两种材料的热电性质.结果表明,单层Ge₂P₄S₂和Ge₂As₄S₂在室温下展现较低的晶格热导率,沿armchair方向分别为3.93 W·m^-1·K^-1和3.19 W·m^-1·K^-1, zigzag方向分别为4.38 W·m^-1·K^-1和3.79 W·m^-1·K^-1,这主要是由低的声子群速度、大的格林艾森参数以及小的声子弛豫时间造成的.基于HSE06泛函计算出的能带结构表明单层Ge₂     

Effect of porosity on the thermoelectric efficiency of PbTe

The effect of electron and phonon scattering on nanometer-sized pores on the thermoelectric properties of lead telluride has been studied theoretically. Estimations show that the thermoelectric efficiency can increase by 20–25% at room temperature and by 5–10% at 600 K at the optimal pore size of several nanometers and the porosity of ~10%. An analysis shows that the increase in the thermoelectric efficiency due to additional scattering in the porous material is related to the decrease in the thermal conductivity of the lattice and the increase in the thermoelectric coefficient due to the change in the energy dependence of the relaxation time. To estimate the multiple scattering at high pore concentration, the lattice thermal conductivity by the molecular-dynamics method and the electron free paths in a coherent potential approximation were calculated. It is shown that the inclusion of the multiple scattering slightly influences the thermoelectric properties at noted sizes and pore concentrations.     

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.     

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.     

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.     

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.     

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.     

Excellent thermoelectric performance predicted in Sb2Te with natural superlattice structure

Using first-principles calculations combined with the Boltzmann transport theory, we explore the thermoelectric properties of natural superlattice (SL) structure Sb₂Te. The results show that n-type Sb₂Te possesses larger Seebeck coefficient of 249.59 (318.87) μV/K than p-type Sb₂Te of 219.85 (210.38) μV/K and low lattice thermal conductivity of 1.25 (0.21) W/mK along the in-plane (out-of-plane) direction at 300 K. The excellent electron transport performance is mainly attributed to steeper density of state around the bottom of conduction band. The ultralow lattice thermal conductivity of Sb₂Te is mainly caused by low phonon group velocity and strong anharmonicity. Further analysis shows that the decrease of group velocity comes from flatter dispersion curves which are contributed by the Brillouin-zone folding. The strong anharmonicity is mainly due to the presence of lone-pair electrons in Sb₂Te. Combining such a high Seebeck coefficient with the low lattice thermal conductivity, maximum n-type thermoelectric figure of merit (ZT) of 1.46 and 1.38 could be achieved along the in-plane and out-of-plane directions at room temperature, which is higher than the reported values of Sb₂Te₃. The findings presented here provide insight into the transport property of Sb₂Te and highlight potential applications of thermoelectric materials at room temperature.     

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.     

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.