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.     

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.     

Local composition and carrier concentration in Pb0.7Ge0.3Te and Pb0.5Ge0.5Te alloys from Te NMR and microscopy

Pb_0.7Ge_0.3Te and Pb_0.5Ge_0.5Te alloys, (i) quenched from 923 K or (ii) quenched and annealed at 573 K for 2 h, have been studied by ¹²⁵Te NMR, X-ray diffraction, electron and optical microscopy, as well as energy dispersive spectroscopy. Depending on the composition and thermal treatment history, ¹²⁵Te NMR spectra exhibit different resonance frequencies and spin-lattice relaxation times, which can be assigned to different phases in the alloy. Quenched and annealed Pb_0.7Ge_0.3Te alloys can be considered as solid solutions but are shown by NMR to have components with various carrier concentrations. Quenched and annealed Pb_0.5Ge_0.5Te alloys contain GeTe- and PbTe-based phases with different compositions and charge carrier concentrations. Based on the analysis of non-exponential ¹²⁵Te NMR spin-lattice relaxation, the fractions and carrier concentrations of the various phases have been estimated. Our data show that alloying of PbTe with Ge results in the formation of chemically and electronically inhomogeneous systems. ¹²⁵Te NMR can be used as an efficient probe to detect the local composition in equilibrium as well as non-equilibrium states, and to determine the local carrier concentrations in complex multiphase tellurides.     

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     

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)     

In0.3Co4Sb12-xSex 方钴矿热电材料的制备和热电性能

用熔融退火结合放电等离子烧结法制备了In_0.3Co₄Sb_12-xSe_x(x=0—0.3)方钴矿热电材料,探讨了In的存在形式,系统研究了Se掺杂量对结构和热电性能的影响.结果表明:In可以填充到方钴矿二十面体空洞处,过量In在晶界处形成InSb第二相,Se对Sb的置换使晶格常数减小,In填充上限降低;In_0.3Co₄Sb_12-xSe_x样品呈n型传导,随着Se掺杂量的增大,载流子浓度降低,电导率下降,Seebeck系数增大,功率因子有所降低;由于在结构中引入了质量波动及晶格畸变,适量的Se掺杂可以大幅降低材料晶格热导率;样品In_0.3Co₄Sb₁₂和In_0.3Co₄Sb_11.95Se_0.05的最大ZT值均达到1.0以上. 关键词： 掺杂 填充式方钴矿 热电性能     

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     

Thermoelectric properties of indium-filled skutterudites prepared by combining solvothermal synthesis and melting

Indium-filled skutterudites with nominal compositions of In _x Co₄Sb₁₂ (x=0,0.1,0.2,0.3) were prepared by combining solvothermal synthesis and melting. The bulk samples were characterized by X-ray diffraction and scanning electron microscopy, respectively. The Seebeck coefficient, electrical conductivity, and thermal conductivity were measured from room temperature up to ∼773 K. Hall effect measurements were performed at room temperature. The thermoelectric properties of the samples were significantly influenced by filling In into CoSb₃. The dimensionless thermoelectric figure of merit, ZT, increased with increasing temperature and reached a maximum value of ∼0.79 for In_0.1Co₄Sb₁₂ at 573 K.     

Effects of double filling of Pb and Ba on thermoelectric properties of PbxBayCo4Sb11.5Te0.5 compounds by HPHT

Skutterudite compounds Pb_xBa_yCo₄Sb_11.5Te_0.5 (x≤0.23,y≤0.27) with bcc crystal structure have been prepared by the high pressure and high temperature (HPHT) method. The study explored a chemical method for filling Pb and Ba atoms into the voids of CoSb₃ to optimize the thermoelectric figure of merit ZT in the system of Pb_yBa_xCo₄Sb_11.5Te_0.5. The structure of Pb_xBa_yCo₄Sb_11.5Te_0.5 skutterudites was evaluated by means of X-ray diffraction. The Seebeck coefficient, electrical resistivity and power factor were performed from room temperature to 710 K. Compared with Co₄Sb_11.5Te_0.5, the thermal conductivity of Pb and Ba double-filled samples was reduced evidently. Among all filled samples, Pb_0.03Ba_0.27Co₄Sb_11.5Te_0.5 showed the highest power factor of 31.64 μW cm⁻¹ K⁻² at 663 K. Pb_0.05Ba_0.25Co₄Sb_11.5Te_0.5 showed the lowest thermal conductivity of 2.73 W m⁻¹ K⁻¹ at 663 K, and its maximum ZT value reached 0.63 at 673 K.     

Evaluation of Thermoelectric Properties of Ag0.366Sb0.558Te

Ternary AgSbTe₂ materials are frequently reported to show a promising thermoelectric performance, due to the intrinsically low lattice thermal conductivity and complex valence band structure. However, stoichiometric AgSbTe₂ is found to be thermodynamically unstable and would partially decompose into Ag₂Te and Sb₂Te₃ during thermal cycling. Instead, Ag_0.366Sb_0.558Te is the composition for stabilizing the single-phase according to the Ag₂Te-Sb₂Te₃ phase diagram, while the thermoelectric transport properties have rarely been reported and are the focus of this work. Sn/Sb substitution is found to effectively increase not only the carrier concentration from ≈5 × 10¹⁹ cm⁻³ to ≈4 × 10²¹ cm⁻³, but also the density-of-states effective mass, leading to an enhanced Seebeck coefficient along with a decreased carrier mobility. Single parabolic band (SPB) model with acoustic phonon scattering enables a good understanding on the charge transport. The increased carrier concentration effectively suppresses the bipolar effect at high temperatures. As a result, a peak zT of ≈1.3 and an average of ≈0.9 are achieved.     

Improved thermoelectric performance of n-type Mg3Sb2–Mg3Bi2 alloy with Co element doping

n-type Mg₃Sb₂–Mg₃Bi₂ alloy shows as a potential new thermoelectric material (TE) and has been widely researched recently. The pure phase n-type Mg_3·20(Sb_0·3Bi_0.7)_1.99Te_0.01 were prepared by adjusting Mg content with the Bi impurity phase being effectively suppressed. Then, Co element was doped into the pure phase and the electrical conductivity of samples were improved. With a high power factor of 20.3 μW cm⁻¹K⁻² for Mg_3·185Co_0·015(Sb_0·3Bi_0.7)_1.99Te_0.01 at 525 K. Additionally, it was found that the phonon scattering is enhanced due to the larger atomic mass of Co comparing to Mg and the lattice thermal conductivity is reduced. As a result, a high ZT value of ~ 1.03 at 525 K is achieved for the Mg_3·185Co_0·015(Sb_0·3Bi_0.7)_1.99Te_0.01.     

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     

Dynamical nonlinear refraction in PbTe/Pb1−xSnxTe MQWs

An analysis of nonlinear refraction due to the dynamical free carrier filling of minibands in PbTe/Pb_1−xSn_xTe (X = 0.2) MQEs on (111) BaF₂ substrates is presented. The results are compared with those for bulk Pb_1−xSn_xTe layers. The abrupt substantial increase of n(I) in MQWs is connected with crossing of the minibands in oblique valleys of Pb_1−xSn_xTe wells by the Fermi quasi-level.     

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).     

Temperature-dependent thermal expansion of cast and hot-pressed LAST (Pb–Sb–Ag–Te) thermoelectric materials

The thermal expansion for two compositions of cast and hot-pressed LAST (Pb–Sb–Ag–Te) n-type thermoelectric materials has been measured between room temperature and 673 K via thermomechanical analysis (TMA). In addition, using high-temperature X-ray diffraction (HT-XRD), the thermal expansion for both cast and hot-pressed LAST materials was determined from the temperature-dependent lattice parameters measured between room temperature and 623 K. The TMA and HT-XRD determined values of the coefficient of thermal expansion (CTE) for the LAST compositions ranged between 20 × 10^?6 K^?1 and 24 × 10^?6 K^?1, which is comparable to the CTE values for other thermoelectric materials including PbTe and Bi₂Te₃. The CTE of the LAST specimens with a higher Ag content (Ag_0.86Pb₁₉Sb_1.0Te₂₀) exhibited a higher CTE value than that of the LAST material with a lower Ag content (Ag_0.43Pb₁₈Sb_1.2Te₂₀). In addition, a peak in the temperature-dependent CTE was observed between room temperature and approximately 450 K for both the cast and hot-pressed LAST with the Ag_0.86Pb₁₉Sb_1.0Te₂₀ composition, whereas the CTE of the Ag_0.43Pb₁₈Sb_1.2Te₂₀ specimen increased monotonically with temperature.     

Thermoelectric properties of n-type Sr x M y Co4Sb12 (M=Yb, Ba) double-filled skutterudites

Alkaline-earth (AE) and rare-earth (RE) atoms are usually used as void fillers in the caged compound CoSb₃ to improve the thermoelectric performance of the filled system. Polycrystalline single-filled Sr_0.21Co₄Sb₁₂, double-filled Sr _x Yb _y Co₄Sb₁₂, and Sr _x Ba _y Co₄Sb₁₂ skutterudites have been synthesized. Rietveld structure refinement confirms that both Sr and Yb occupy the Sb-icosaedron voids in skutterudite frame work. In this paper, we report the high-temperature thermoelectric properties including electrical conductivity, Seebeck coefficient, and thermal conductivity. Double filling of the Sr–Yb combinations shows a stronger suppression on lattice thermal conductivity than that of Sr–Ba combination. Furthermore, the double-filled Sr _x Yb _y Co₄Sb₁₂ skutterudites exhibit a much higher power factor than the Sr-filled system. The maximum power factor for Sr_0.22Yb_0.03Co₄Sb_12.12 reaches 41 μW cm⁻¹ K⁻² at room temperature and 57.5 μW cm⁻¹ K⁻² at 850 K, respectively. The enhanced thermoelectric figures of merit are 1.32 for Sr _x Yb _y Co₄Sb₁₂ and 1.22 for Sr _x Ba _y Co₄Sb₁₂ at 850 K, respectively.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Ag偏离化学计量比Ag1－xPb18SbTe20材料的热电传输性能

采用熔融-淬火-放电等离子体烧结制备了Ag偏离化学计量比Ag_1-xPb₁₈SbTe₂₀(x=0，0.25，0.50，0.75)样品，研究了Ag含量对样品热电传输性能的影响.结果表明，随Ag含量降低，样品中出现少量第二相Sb₂Te₃，样品载流子浓度增加到5×10¹⁸cm^-3后不再增加.样品载流子迁移率随Ag含量降低先降低后增加，随着温度增加，载流子散射机理由电离杂质散射转变为声学波散射.随Ag含量降低，样品电导率增加而Seebeck系数降低，热导率增加. 关键词： 热电材料 mSbTe_m+2')" href="#">AgPb_mSbTe_m+2 SPS 散射机理     

Nature of gallium deep centres in lead telluride based semiconductors

Doped with Ga lead telluride was taken as a model object to explain the nature of group-III deep levels in IV-VI semiconductors and to elucidate the vapour phase doping mechanism. For this goal, interaction of various gallium-containing molecules with defect-free crystal as well as with native defects in PbTe was considered. Formation energies for different point defects created in PbTe as a result of interaction the Ga₂Te molecules, Ga₂ dimers and single Ga atoms with a host crystal were calculated using density functional theory. Particularly Ga_Pb and Ga_i together with formation of accompanied self interstitials Pb_i in various charge states were examined. In addition we propose the new type of defects - the impurity complex (2Ga)_Pb which looks like <111>-oriented gallium dumbbell. Calculations suggest the double donor behaviour and DX-like properties of this defect together with extremely low formation energy values. Namely, (2Ga)_Pb centres are preferably formed under Ga₂Te doping while (Ga₂)_Pb+Pb_i ones are formed under Ga₂ or Ga doping. In all cases, formation energies are negative and resulting defect concentration is determined by reaction kinetics only. Mechanisms of the lead vacancy compensation with the vapour phase doping are considered as well.