Excellent thermal stability and thermoelectric properties of Pnma-phase SnSe in middle temperature aerobic environment

SnSe is considered to be a promising thermoelectric material due to a high ZT value and abundant and non-toxic composition elements. However, the thermal stability is an important issue for commercial application. In particular,thermoelectric materials are in the high temperature for a long time due to the working condition. The present work investigates the thermal stability and oxidation resistance of single crystal SnSe thermoelectric materials. The scanning electron microscopy(SEM) and transmission electron microscopy(TEM) results show that the internal of SnSe crystal is not easily oxidized, while the x-ray photoelectron spectroscopy(XPS) results indicate that the surface of SnSe is slight oxidized to SnO_2. Even if the surface is oxidized, the SnSe crystal still exhibits stable thermoelectric properties. Meanwhile,the crystallization quality of SnSe samples can be improved after the appropriate heat treatment in the air, which is in favor of the carrier mobility and can improve the electrical conduction properties of SnSe. Moreover, the decrease of defect density after heat treatment can further improve the Seebeck coefficient and electrical transport properties of SnSe. The density functional theory(DFT) calculation verifies the important role of defect on the electrical conductivity and electron configuration. In summary, appropriate temperature annealing is an effective way to improve the transmission properties of SnSe single crystal thermoelectric materials.     

Synthesis and thermoelectric properties of Bi-doped SnSe thin films

Bi doped n-type SnSe thin films were prepared by chemical vapor deposition (CVD) and their structure and thermoelectric properties were studied. The x-ray diffraction patterns, x-ray photoelectron spectroscopy, and microscopic images show that the prepared SnSe thin films were composed of pure SnSe crystals. The Seebeck coefficients of the Bi-doped SnSe were greatly improved compared to that of undoped SnSe thin films. Specifically, Sn_0.99Bi_0.01Se thin film exhibited a Seebeck coefficient of -905.8μV·K^-1 at 600 K, much higher than 285.5 μV·K^-1 of undoped SnSe thin film. Further first-principles calculations reveal that the enhancement of the thermoelectric properties can be explained mainly by the Fermi level lifting and the carrier pockets increasing near the Fermi level due to Bi doping in the SnSe samples. Our results suggest the potentials of the Bi-doped SnSe thin films in thermoelectric applications.     

Co掺杂对YBa2 Cu3 O7-δ电阻率与塞贝克系数的影响

用固态反应法制备了YBa2Cu3-xCoxO7-δ(x=0.0,0.1,0.2,0.5)样品,研究了Co掺杂对YBa2Cu3O7-δ高温区的电阻率和塞贝克系数的影响。随着Co含量的增加,样品的电阻率和塞贝克系数逐渐增大,从金属性导电转变为p型半导体导电。x=0.2和0.5样品的电导活化能在500K处发生突变,高温区的活化能大于低温区的活化能。通过塞贝克系数与温度的关系,计算出x=0.2和0.5样品的费米能级分别为0.02和0.12 eV。当温度高于650-700K时,氧脱附显著影响样品的电输运性质,导致电阻率和塞贝克系数随温度增加而增大。     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Sb_2Te_3热电薄膜的离子束溅射制备与表征

采用离子束溅射技术交替沉积Sb-Te-Sb多层薄膜后进行高真空热处理,直接制备Sb2Te3薄膜.利用X射线衍射(XRD)仪、霍尔系数测试仪、薄膜Seebeck系数测量系统对所制备的薄膜特性进行表征.XRD测量结果显示,薄膜的主要衍射峰与Sb2Te3标准衍射峰相同,在[101]/[012]晶向取向明显,存在较多的Te杂质峰;霍尔系数测试结果表明,薄膜为p型半导体薄膜,薄膜电阻率较低,其电导率接近于金属电导率,载流子浓度量级为1023cm-3,具有良好的电学性能;Seebeck系数测量结果显示,薄膜具有良好的热电性能,在不同条件下制备的薄膜的Seebeck系数在7.8—62μV/K范围;在所制备的薄膜中,退火时间为6h、退火温度为200℃的薄膜其Seebeck系数达到最大,约为62μV/K,且电阻率最小.     

Large transverse thermoelectric figure of merit in a topological Dirac semimetal

The Seebeck effect encounters a few fundamental constraints hindering its thermoelectric(TE)conversion efficiency.Most notably,there are the charge compensation of electrons and holes that diminishes this effect,and the Wiedemann-Franz(WF)law that makes independent optimization of the corresponding electrical and thermal conductivities impossible.Here,we demonstrate that in the topological Dirac semimetal Cd3As2 the Nernst effect,i.e.,the transverse counterpart of the Seebeck effect,can generate a large TE figure of merit zNT.At room temperature,zNT≈0.5 in a small field of 2 T and it significantly surmounts its longitudinal counterpart for any field.A large Nernst effect is generically expected in topological semimetals,benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities.In this case,heat and charge transport are orthogonal,i.e.,not intertwined by the WF law anymore.More importantly,further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport,but also the anomalous Nernst effect arising from a pronounced Berry curvature.A combination of the topologically trivial and nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.     

基于第一性原理的Mn纳米线掺杂二维SnSe热电性能研究

本文提出了一种在二维SnSe中掺杂一维Mn纳米线的2D-1D复合结构,并系统地研究了其热电性能。结果表明,一维Mn纳米线将电子态汇聚在纳米线附近,提高了材料的各向异性,降低了电子在某一方向上的散射效应,导致了较高的迁移率和电导率。自旋向上和向下的电子态发生简并,导致了较高的塞贝克系数和电导率。此外,Mn纳米线将晶格热导率降低了约0.17 W·m^?1·K^?1。在200至650 K的温度范围内,3Mn-SnSe具有0.73至3.78的极高ZT值,比本征二维SnSe平均提高了约39.2%。     

基于第一性原理的Mn纳米线掺杂二维SnSe热电性能研究

本文提出了一种在二维SnSe中掺杂一维Mn纳米线的2D-1D复合结构,并系统地研究了其热电性能。结果表明,一维Mn纳米线将电子态汇聚在纳米线附近,提高了材料的各向异性,降低了电子在某一方向上的散射效应,导致了较高的迁移率和电导率。自旋向上和向下的电子态发生简并,导致了较高的塞贝克系数和电导率。此外,Mn纳米线将晶格热导率降低了约0.17 W·m^?1·K^?1。在200至650 K的温度范围内,3Mn-SnSe具有0.73至3.78的极高ZT值,比本征二维SnSe平均提高了约39.2%。     

Seebeck coefficient of Ln<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> perovskites in paramagnetic state

The Seebeck coefficient is a function of carrier concentration and configurational entropy. In this report, we semi-theoretically investigate the Seebeck coefficient of Ln _x Ca_1−x MnO₃ (Ln=Rare-earth) perovskites based on the electronic structure of the 3d orbitals of Mn ions, using the developed Heikes model, Boltzmann transport model, and diffusion model. The results show that the Seebeck coefficient of such a strongly correlated electron system in paramagnetic state is remarkably affected by site degeneracy. As temperature decreases, the evolution of the spin and orbital degrees of freedom together with the change in phonon scattering mode describes the Seebeck coefficient behavior satisfactorily. The phonon drag effect at low temperature is also discussed.     

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.     

铁基超导体的输运性质

在铁基超导体中存在着多种有序态,例如电子向列相和自旋密度波等,从而呈现出丰富的物理现象.输运性质的测量能为认识铁基超导体的低能激发提供极为有用的信息.铁砷超导体由于其电子结构的多能带特性,其电阻率和霍尔系数与温度的关系出现多样性的变化,但在正常态并没有看到有类似铜氧化物超导体的赝能隙打开等奇异行为.在空穴型掺杂的铁基超导体中观测到霍尔系数在低温下变号,对应温区的电阻率上出现一个很宽的鼓包等,可能是从非相干到相干态的转变.热电势行为也表现出与铜氧化物超导体的明显差异,比如铁基超导体的正常态热电势的绝对值反而在最佳掺杂区是最大的,这也许跟强的带间散射有关.能斯特效应表明铁基超导体在Tc以上的超导位相涨落并不明显,与铜氧化物超导体存在明显差别.在铁基超导体上所显示出来的这些反常热电性质,并没有在类似结构的镍基超导体(如LaNiAsO)上观测到,镍基超导体表现得更像一个通常的金属.这些均说明铁基超导体的奇异输运性质与其高温超导电性存在内在的关联,这些因素是建立其超导机理时需要考虑进去的.     

Non-equilibrium energy transport in nano-layer gold coating onto silica

The non-equilibrium energy transport in a metallic material with two layered structure having different material properties results in thermal separation of electrons and lattice sub-systems during a laser short-pulse irradiation. In this case, electron and lattice site temperature distributions become material properties and thickness of each layer dependent. In the present study, energy transport in a nanometer scale gold coating onto silica due to laser short-pulse irradiation is examined. The Seebeck coefficient in the gold coating is predicted and compared to the results obtained from the previous formulation. It is found that the maximum value of electron temperature in the gold coating increases with increasing coating thickness and lattice site temperature results in a peak in the silica in the region close to the coating silica interface. The Seebeck coefficient predicted agrees well with the results of the previous formulation.     

烧结温度对La0.9Sr0.1FeO3热电性能的影响

利用传统的固相反应分别在1250℃,1300℃,1350℃.烧结条件下制备出钙钛矿结构的La_0.9Sr_0.1FeO₃陶瓷样品.样品的XRD粉末衍射结果显示不同烧结温度的La_0.9Sr_0.1FeO₃陶瓷样品都是单相的正交结构,同时晶胞体积随着烧结温度的升高而减小.从样品的SEM结果看出,随着烧结温度的升高,晶粒逐渐变大,并且晶粒间的空隙逐渐减小,样品更加致密.在室温到800℃的 关键词： 铁酸镧陶瓷 热电性能 烧结温度     

Understanding the role of defects in influencing the thermoelectric properties of SnSe

As compared to single crystals, polycrystalline SnSe shows a considerable decline in its ZT value. Optimization of carrier concentration by the way of chemical doping is useful but creates point defect and vacancies that are often overlooked. Here we study polycrystalline Sn_0.95M_0.05Se (M = Co, Ni, In) with an aim to understand the role of defects. The overall crystal structure and microstructure of SnSe is not much affected with substitution as evident from X-ray diffraction and scanning electron microscopy study. Rietveld refinement confirms the single phase nature of the all compositions and provides unit cell parameters. Analysis of the stoichiometry reveals the presence of cation vacancies. Optical spectroscopy indicates a degradation of the in-direct gap and Urbach band tail-width fitting confirms the presence of localized states within the gap. Electrical resistivity and Seebeck coefficient are adversely affected by defects, but thermal conductivity decreases by almost 50% of SnSe value.     

Anisotropic thermoelectric transport properties in polycrystalline SnSe_2

It is reported that SnSe_2 consisting of the same elements as SnSe, is a new promising thermoelectric material with advantageous layered structure. In this work, the thermoelectric performance of polycrystalline SnSe_2 is improved through introducing SnSe phase and electron doping(Cl doped in Se sites). The anisotropic transport properties of SnSe_2 are investigated. A great reduction of the thermal conductivity is achieved in SnSe_2 through introducing SnSe phase, which mainly results from the strong SnSe_2–SnSe inter-phase scattering. Then the carrier concentration is optimized via Cl doping, leading to a great enhancement of the electrical transport properties, thus an extraordinary power factor of ～5.12 μW·cm~(-1)·K~(-2) is achieved along the direction parallel to the spark plasma sintering(SPS) pressure direction( P). Through the comprehensive consideration on the anisotropic thermoelectric transport properties, an enhanced figure of merit ZT is attained and reaches to ～ 0.6 at 773 K in SnSe_2-2% SnSe after 5% Cl doping along the P direction, which is much higher than ～ 0.13 and ～ 0.09 obtained in SnSe_2-2% SnSe and pristine SnSe_2 samples, respectively.     

Giant Nernst effect and bipolarity in the quasi-one-dimensional metal Li0.9Mo6O17

The Nernst coefficient for the quasi-one-dimensional metal, Li{0.9}Mo{6}O{17}, is found to be among the largest known for metals (ν?500 μV/KT at T～20 K), and is enhanced in a broad range of temperature by orders of magnitude over the value expected from Boltzmann theory for carrier diffusion. A comparatively small Seebeck coefficient implies that Li{0.9}Mo{6}O{17} is bipolar with large, partial Seebeck coefficients of opposite sign. A very large thermomagnetic figure of merit, ZT～0.5, is found at high field in the range T≈35-50 K.     

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.     

Narrow band in the intermetallic compounds MNiSn (M=Ti,Zr, Hf)

The presence of a narrow band below conduction band of nonmagnetic compounds MNiSn (M=Ti, Zr, Hf) is assumed to explain their low temperature properties such as the heat capacity, IR optics, electronics transport. A computation of the Seebeck coefficient supplies support for this assumption.     

Synthesis and high temperature thermoelectric transport properties of Si-based type-I clathrates

N-type Si-based type-I clathrates with different Ga content were synthesized by combining the solid-state reaction method, melting method and spark plasma sintering (SPS) method. The effects of Ga composition on high temperature thermoelectric transport properties were investigated. The results show that at room temperature, the carrier concentration decreases, while the carrier mobility increases slightly with increasing Ga content. The Seebeck coefficient increases with increasing Ga content. Among all the samples, Ba_7.93Ga_17.13Si_28.72 exhibits higher Seebeck coefficient than the others and reaches -135~μ V.K^-1 at 1000 K. The sample prepared by this method exhibits very high electrical conductivity, and reaches 1.95× 10⁵S.m^-1 for Ba_8.01Ga_16.61Si_28.93 at room temperature. The thermal conductivity of all samples is almost temperature independent in the temperature range of 300--1000~K, indicating the behaviour of a typical metal. The maximum {ZT} value of 0.75 is obtained at 1000~K for the compound Ba_7.93Ga_17.13Si_28.72.     

Electric and thermal transport properties of topological insulator candidate LiMgBi

We report the transport properties of a topological insulator candidate, LiMgBi. The electric resistivity of the title compound exhibits a metal-to-semiconductor-like transition at around 160 K and tends to saturation below 50 K. At low temperatures, the magnetoresistance is up to ~260% at 9 T and a clear weak antilocalization effect is observed in the low magnetic-field region. The Hall measurement reveals that LiMgBi is a multiband system, where hole-type carriers (n_h~10¹⁸ cm^-3) play a major role in the transport process. Remarkably, LiMgBi possess a large Seebeck coefficient (~440 μV/K) and a moderate thermal conductivity at room temperature, which indicate that LiMgBi is a promising candidate in thermoelectric applications.