High pressure synthesis and thermoelectric performances of Cu2Se compounds

The Cu₂Se samples were synthesized by high pressure directly at room temperature in several minutes. The composition evolution under high pressure demonstrates that the critical conditions to synthesize Cu₂Se are the pressure of 1 GPa and the reaction time of 5 min. The synthetic pressure can effectively tune the morphology, carrier concentration and the electrical transport properties. The low lattice thermal conductivity less than 0.5 Wm^?1 K^?1 is obtained because of the intrinsic superionic character and the microstructures by high pressure including abundant micropores and lattice defects. A maximum zT of 0.92 at 783 K is achieved for Cu₂Se synthesized at 1 GPa. This work indicates the potentiality of high pressure technique to further enhance the thermoelectric properties of Cu₂Se materials.     

Strong Anisotropic Thermal Conductivity in Polycrystalline Layers of (AgxSn1-xS)1.2(TiS2)2 with Prospects Toward Improved Thermoelectric Performance

Anisotropic tuning is of crucial importance for designing and developing high-performance thermoelectric materials. Here, a prominent anisotropic thermoelectric characteristic of Ag-substituted misfit-layered (SnS)_1.2(TiS₂)₂ alloys is investigated in the perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction. In the in-plane direction, the (Ag_xSn_1-xS)_1.2(TiS₂)₂ alloys possess a highest power factor of 0.86 mW K⁻² m⁻¹ at 520 K, while in the out-of-plane direction the lowest lattice thermal conductivity (0.37 W K⁻¹ m⁻¹) is achieved, which is driven by the natural intercalated structure where the out-of-plane phonon is strongly scattered without affecting the in-plane mobility. Moreover, along the in-plane orientation, the introduced point defects due to the substitution of Sn by Ag trigger a significant reduction of lattice thermal conductivity. In contrast, along the out-of-plane orientation, the decreased carrier concentration enables a large Seebeck coefficient and power factor, ultimately ensuring high thermoelectric performance. The present finding in the misfit-layered chalcogenide opens up a new route to manipulating thermoelectrics via anisotropy engineering.     

Thermoelectric properties of Cr1−xMoxSi2

The thermoelectric properties of Mo-substituted CrSi₂ were studied. Dense polycrystalline samples of Mo-substituted hexagonal C40 phase Cr_1−xMo_xSi₂ (x=0–0.30) were fabricated by arc melting followed by spark plasma sintering. Mo substitution substantially increases the carrier concentration. The lattice thermal conductivity of CrSi₂ at room temperature was reduced from 9.0 to 4.5 W m⁻¹ K⁻¹ by Mo substitution due to enhanced phonon–impurity scattering. The thermoelectric figure of merit, ZT, increases with increasing Mo content because of the reduced lattice thermal conductivity. The maximum ZT value obtained in the present study was 0.23 at 800 K, which was observed for the sample with x=0.30. This value is significantly greater than that of undoped CrSi₂ (ZT=0.13).     

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.     

Effective Mass Enhancement and Thermal Conductivity Reduction for Improving the Thermoelectric Properties of Pseudo-Binary Ge2Sb2Te5

Ge₂Sb₂Te₅ is a famous phase-change memory material for rewriteable optical storage, which is widely applied in the information storage field. The stable trigonal phase of Ge₂Sb₂Te₅ shows potential as a thermoelectric material as well, due to its tunable electrical transport properties and low lattice thermal conductivity. In this work, the carrier concentration and effective mass of Ge₂Sb₂Te₅ are modulated by substituting Te with Se. Meanwhile, the thermal conductivity reduces from 2.48 W m⁻¹ K⁻¹ for Ge₂Sb₂Te₅ to 1.37 W m⁻¹ K⁻¹ for Ge₂Sb₂Te_3.5Se_1.5 at 703 K. Therefore, the thermoelectric figure of merit zT increases from 0.24 for Ge₂Sb₂Te₅ to 0.41 for Ge₂Sb₂Te_3.5Se_1.5 at 703 K. This study reveals that Se alloying is an effective way to enhance the thermoelectric properties of Ge₂Sb₂Te₅.     

Se替代Te对BiCuTeO电热输运性能的影响

层状氧硫族化合物由于其本征的低晶格热导率和可观的热电性能吸引了广泛关注,其中以BiCuSeO化合物的热电性能最为优异.但是,其同晶型化合物BiCuTeO,由于带隙较小且存在大量本征Cu空位,导致载流子浓度较高,热电性能较差,从而研究较少.针对BiCuTeO存在的上述问题,本文利用Se替代部分Te,以期能够展宽带隙并减少Cu空位,提高其热电性能.采用固相反应结合快速热压烧结制备了BiCuTe_(1-x)Se_xO(x=0, 0.1, 0.2, 0.3和0.4)块体热电材料,并系统地研究了该体系的电热输运性能.研究结果表明,利用Se替代Te,可以使BiCuTeO导电层化学键强度增加、带隙增大、载流子有效质量增加以及载流子散射增强,从而导致载流子浓度和迁移率同时降低,进而电导率随着Se含量增加而剧烈降低, Seebeck系数则显著增大.由于综合电输运性能恶化,功率因子随着Se含量增加而减小,导致热电优值zT随着Se含量增加而降低.最终,Se含量为x=0.1的样品,在室温和723 K时的zT值分别达到约0.3和0.7,仍然在较宽温区内保持较高的zT值.由于Se替代Te改变了BiCuTeO的能带结构,通过载流子浓度优化,有望进一步提高其热电性能.     

Phase transition and high temperature thermoelectric properties of copper selenide Cu2-x Se （0 ≤ x ≤ 0.25）

With good electrical properties and an inherently complex crystal structure, Cu_2-xSe is a potential “phonon glass electron crystal” thermoelectric material that has previously not attracted much interest. In this study, Cu_2-xSe (0 ≤ x ≤ 0.25) compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu_2-xSe were investigated in the temperature range of 300 K—750 K. The results of X-ray diffraction at room temperature show that Cu_2-xSe compounds possess a cubic structure with a space group of Fm3m (#225) when 0.15 < x le 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤ x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu₂Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.     

Conduction mechanism in polycrystalline Na1−xSrxNbO3 niobium bronzes

Electrical conductivity and thermoelectric power of polycrystalline Na_1?xSr_xNbO₃ have been measured between 5 and 300 K for various compositions. The results suggest that the density of states is finite at the Fermi level, whilst the carrier mobility is activated. The observed behavior is attributed to an Anderson localization of states at the Fermi level due to disorder resulting from either a random distribution of Sr²⁺ and Na⁺ ions or from the polycrystalline character of the samples or from both effects.     

Synergistically Enhancing Thermoelectric Performance of n-Type PbTe with Indium Doping and Sulfur Alloying

To achieve high-performance n-type PbTe-based thermoelectric materials, this work provides a synergetic strategy to improve electrical transport property with indium (In) element doping and reduces thermal conductivity with sulfur (S) element alloying. In n-type PbTe, In doping can tune the carrier density in the whole working temperature range, causing the carrier density to increase from 2.18 × 10¹⁹ cm⁻³ at 300 K to 4.84 × 10¹⁹ cm⁻³ at 823 K in Pb_0.98In_0.005Sb_0.015Te. The optimized carrier density can further modulate electrical conductivity and Seebeck coefficient, finally contributing to a substantial increase of power factor, and a maximum power factor increase from 19.7 µW cm⁻¹ K⁻² in Pb_0.985Sb_0.015Te to 28.2 µW cm⁻¹ K⁻² in Pb_0.9775In_0.0075Sb_0.015Te. Based on the optimally In-doped PbTe, S alloying is introduced to suppress phonon propagation by forming a complete solid solution, which could effectively reduce lattice thermal conductivity and simultaneously benefit carrier mobility to maintain high power factor. With S alloying, the minimum lattice thermal conductivity decreases from 0.76 Wm⁻¹ K⁻¹ in Pb_0.985Sb_0.015Te to 0.42 Wm⁻¹ K⁻¹ in Pb_0.98In_0.005Sb_0.015Te_0.88S_0.12. Combining the advantages of both In doping and S alloying, the peak ZT value and averaged ZT (ZT_ave) (300–873 K) are boosted from 1.0 and 0.60 in Pb_0.985Sb_0.015Te to 1.4 and 0.87 in Pb_0.98In_0.005Sb_0.015Te_0.94S_0.06.     

The effect of Te doping on the electronic structure and thermoelectric properties of SnSe

SnSe_1−xTe_x (x=0, 0.0625) bulk materials were fabricated by melting Sn, Se and Te powders and then hot pressing them at various temperatures. The phase compositions of the materials were determined by X-ray diffraction (XRD) and the crystal lattice parameters were refined by the Rietveld method performed with DBWS. XRD analysis revealed that the grains in the materials preferentially grew along the (l 0 0) directions. The structural behavior of SnSe_1−xTe_x (x=0, 0.0625) was calculated using CASTEP package provided by Materials Studio. We found that the band gap of SnSe reduced from 0.643 to 0.608 eV after Te doping. The calculated results were in good agreement with experimental results. The electrical conductivity and the Seebeck coefficient of the as-prepared materials were measured from room temperature to 673 K. The maximum power factor of SnSe is ∼0.7 μW cm⁻¹K⁻² at 673 K.     

Reinvestigation of the thermoelectric properties of Ag8GeTe6

High‐density polycrystalline samples (above 98% of the theoretical density) of Ag₈GeTe₆ were prepared by solid‐state reactions of Ag₂Te, GeTe, and Te, followed by hot‐pressing. The thermoelectric properties were measured at temperatures ranging from room temperature to around 700 K. The thermal conductivity values were extremely low (0.25 Wm^–1 K^–1 at room temperature), and consequently Ag₈GeTe₆ exhibited a relatively high thermoelectric figure of merit, ZT = 0.48 at 703 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoelectric characteristics of nanostructured hydrochemically deposited films of the Pb<Subscript>0.975</Subscript>Sn<Subscript>0.025</Subscript>Se substitutional solid solution

The photoelectric characteristics of Pb_0.975Sn_0.025Se solid solution films prepared by the hydrochemical codeposition of PbSe and SnSe with the subsequent heat treatment in air at 573–700 K have been investigated. The thermal and optical band gaps, the temperature coefficient of the optical band gap, the dark resistance, the volt-watt sensitivity, and the range of spectral sensitivity have been determined in the temperature range of 220–300 K. It has been found that, after heat treatment below 573 K, the films of the Pb_0.975Sn_0.025Se solid solutions possess metallic conductivity, while being heat treated at elevated temperatures, they become semiconductors with p-type conductivity. The composition of the solid solution is independent of the heat treatment temperature; it is formed during deposition.     

p型Ag0.5(Pb8－xSnx)In0.5Te10化合物的制备及其热电性能

采用高温熔融缓冷和放电等离子烧结工艺制备了p型Ag_0.5(Pb_8-xSn_x)In_0.5Te₁₀五元化合物.研究了Sn含量对化合物载流子传输特性及热电性能的影响规律.结果表明：在Ag_0.5(Pb_8-xSn_x)In_0.5Te₁₀(x 关键词： 0.5(Pb_8-xSn_x)In_0.5Te₁₀')" href="#">Ag_0.5(Pb_8-xSn_x)In_0.5Te₁₀ 合成 载流子 热电性能     

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.     

Ga填充n型方钴矿化合物的结构及热电性能

用熔融退火结合放电等离子烧结(SPS)技术制备了具有不同Ga填充含量的Ga_xCo₄Sb₁₂方钴矿化合物,研究了不同Ga含量对其热电传输特性的影响规律. Rietveld结构解析表明,Ga占据晶体学2a空洞位置,Ga填充上限约为0.22,当Ga的名义组成x≤0.25时,样品的电导率、室温载流子浓度N_p随Ga含量的增加而增加,Seebeck系数随Ga含量的增加而减小. 室温下霍尔测试表明,每一个Ga授予框架0.9个电子,比Ga的氧化价态Ga³⁺小得多. 由于Ga离子半径相对较小,致使Ga填充方钴矿化合物的热导率κ及晶格热导率κ_L较其他元素填充的方钴矿化合物低. 当x＝0.22时对应的样品在300K时的热导率和晶格热导率分别为3.05Wm^-1·K^-1和 2.86Wm^-1·K^-1.在600K下Ga_0.22Co_4.0Sb_12.0样品晶格热导率达到最小,为1.83Wm^-1·K^-1,最大热电优值Z,在560K处达1.31×10^-3K^-1. 关键词： skutterudite化合物 Ga原子填充 结构 热电性能     

Preparation and thermoelectric power of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and a stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ have been prepared and characterized by X-ray powder diffraction analysis. At room temperature the carrier concentration values are n?=?1.1?×?10¹⁹?cm^?3 for Bi₂Se₃ and n?=?0.53?×?10¹⁹?cm^?3 for SnBi₄Se₇. The thermoelectric power has been measured over the temperature range 90–420?K. The thermoelectric power of Bi₂Se₃ is higher than that for SnBi₄Se₇, which shows that the Sn impurity has an acceptor character. Therefore, doping Bi₂Se₃ with tin atoms does not improve thermoelectric properties of this material, due to decrease the value of the power factor σS ². Transport properties of the studied polycrystalline samples are characterized by a mixed transport mechanism of free carriers. It is necessary to add more than one Sn atom to the Bi₂Se₃ compound in order to suppress the electron concentration by one electron. Such behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant substitutional defect, Sn_Bi, tin atoms also form uncharged defects, corresponding to seven-layer lamellae of the composition Se–Bi–Se–Sn–Se–Bi–Se which corresponds to the structure of the SnBi₂Se₄.     

Titanium Doping to Enhance Thermoelectric Performance of 19-Electron VCoSb Half-Heusler Compounds with Vanadium Vacancies

The 19-electron VCoSb compounds are actually composites of an off-stoichiometric half-Heusler phase and impurities. Here the compositional adjustment is systematically studied in V_1−xCoSb to obtain single-phase V_0.955CoSb. Hall measurements suggest that such a V vacancy, as well as Ti doping, can optimize the carrier concentration, which decreases from ≈11.3 × 10²¹ cm⁻³ for VCoSb to ≈6.3 × 10²¹ cm⁻³ for V_0.755Ti_0.2CoSb. Low sound velocity contributes to the intrinsically low lattice thermal conductivity for VCoSb-based materials. The high Ti-dopant content results in enhanced point-defect scattering, which further decreases the lattice thermal conductivity. Finally, the optimized n-type V_0.855Ti_0.1CoSb is found to reach a peak ZT of ≈0.7 at 973 K. The work demonstrates that the VCoSb-based half-Heuslers are promising thermoelectric materials.     

Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering

SiGe alloy is widely used thermoelectric materials for high temperature thermoelectric generator applications. However, its high thermoelectric performance has been thus far realized only in alloys synthesized employing mechanical alloying techniques, which are time-consuming and employ several materials processing steps. In the current study, for the first time, we report an enhanced thermoelectric figure-of-merit (ZT) ∼ 1.1 at 900 °C in n-type Si₈₀Ge₂₀ nano-alloys, synthesized using a facile and up-scalable methodology consisting of rapid solidification at high optimized cooling rate ∼ 3.4 × 10⁷ K/s, employing melt spinning followed by spark plasma sintering of the resulting nano-crystalline melt-spun ribbons. This enhancement in ZT > 20% over its bulk counterpart, owes its origin to the nano-crystalline microstructure formed at high cooling rates, which results in crystallite size ∼7 nm leading to high density of grain boundaries, which scatter heat-carrying phonons. This abundant scattering resulted in a very low thermal conductivity ∼2.1 Wm⁻¹K⁻¹, which corresponds to ∼50% reduction over its bulk counterpart and is amongst the lowest reported thus far in n-type SiGe alloys. The synthesized samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy, based on which the enhancement in their thermoelectric performance has been discussed.     

Si含量对高锰硅化合物相组成及热电性能的影响研究

采用高频感应熔融、退火结合放电等离子烧结方法制备高锰硅(HMS)化合物MnSi1.70+x(x=0,0.05,0.1,0.15),系统研究了Si含量变化对材料相组成、微结构和热电性能的影响规律.结果表明,当x0.1时,样品由HMS和贫Si的MnSi金属相两相组成,随着Si含量x的增加,MnSi相相对含量减小;当x=0.1时,所得样品为单相HMS化合物;当x0.1时,样品由HMS和过量Si两相组成.随着x的增加,由于样品中高电导的金属相MnSi含量逐渐减少,样品的电导率逐渐下降,而Seebeck系数随之增加.室温下样品载流子浓度和有效质量随x增大逐渐减小,而迁移率逐渐增加.MnSi和Si杂相与HMS相比均为高热导相,因此当x=0.1时,由于样品为单相HMS,从而表现出最低热导率和最高ZT值.MnSi1.80样品在800K时热导率最小值达到2.25W·m-1K-1,并在850K处获得最大ZT值(0.45).     

Co1-xNixSb3-ySey热电输运中晶界和点缺陷的耦合散射效应

结合机械合金化与放电等离子烧结工艺制备了Ni和Se共掺的细晶方钴矿化合物Co_1-xNi_xSb_3-ySe_y,研究了晶界和点缺陷的耦合散射效应对CoSb₃热电输运特性的影响.通过Ni掺杂优化载流子浓度提高功率因子.在x=0.1时,功率因子达到最大值1750μWm^-1K^-2(450℃),是没有掺Ni试样的两倍.晶界和点缺陷的耦合散射机理使晶格热导率急剧下降,其中Co_0.9Ni_0.1Sb_2.85Se_0.15的室温晶格热导率降低至1.67Wm^-1K^-1,接近目前单填充效应所能达到的最低值1.6Wm^-1K^-1,其热电优值ZT在450℃时达到最大值0.53.将Callaway-Von Baeyer点缺陷散射模型嵌入到Nan-Birringer有效介质理论模型,对晶界散射和点缺陷散射的耦合效应对热导率的影响进行了定量分析,模型计算与实验结果符合.理论模型计算表明,当晶粒尺寸下降到50nm同时掺杂引入点缺陷散射后,Co_0.9Ni_0.1Sb_2.85Se_0.15的晶格热导率下降到0.8Wm^-1K^-1. 关键词： 3')" href="#">CoSb₃ Ni和Se掺杂 热电性能 耦合散射效应