Percolation Conduction in Hybrid Thermoelectric Material Consisting of Bi0.88Sb0.12 and Barium Ferrite Particles

A hybrid material consisting of thermoelectric Bi_0.88Sb_0.12 and Ba ferrite Bi_0.88Sb_0.12(BaFe₁₂O₁₉) _x (x = 0, 0.025, 0.04, and 0.08) was synthesized using sintering. Powder x-ray diffraction patterns and scanning electron microscopy images of the hybrid indicate that the BaFe₁₂O₁₉ particles were well distributed in the host Bi_0.88Sb_0.12 phase. The temperature dependence of the electrical resistivity ρ of the host Bi-Sb exhibits metallic behavior. By the addition of Ba ferrite particles, the ρ at 300 K increases intensively, and ρ(Τ) then behaves similarly to a semiconductor. However, it is noted that the thermoelectric power S is unchanged. Inhibition of current and heat flows by a restricted conduction path and the unchanged electromotive force generated by the Seebeck effect in the conduction path can be understood based on a site-percolation model consisting of conducting Bi-Sb and insulating Ba ferrite. The critical volume fraction p _c of this system was estimated experimentally as p _c = 0.68.     

Enhanced Thermoelectric Properties of BiCuSeO/Polyaniline Composites

BiCuSeO/polyaniline (BCSO/PANI) bulk composites have been successfully fabricated by a ball-milling and hot-pressing method. Microstructure analysis shows that BCSO particles are well mixed and dispersed in the PANI matrix. Our results indicate that the Seebeck coefficient can be increased substantially by adding BCSO filler to the PANI matrix, especially for 40 wt.% BCSO (5–87 μV K^?1). Electrical conductivity and thermal conductivity both change slightly with the increasing filler content. The highest figure of merit, ZT, among these bulk composites is 0.004 at 341 K, which is almost 500 times greater than that of pure PANI.     

分子束外延PbTe/Pb_(0.88)Sn_(0.12)Te量子阱持续光电导与导带不连续

从理论和实验上研究了分子束外延生长的PbTe/Pb_(0.88)Sn_(0.12)Te多量子阱结构材料中的持续光电导过程.认为材料中持续光电导的衰减是依赖于隧穿协助的电子-深中心复合过程.理论计算与实验结果一致.通过对持续光电导衰减规律的理论拟合,得到了PbTe/Pb_(0.88)Sn_(0.12)Te量子阱材料的导带不连续值及两类能谷间的能量差.     

Thermoelectric Properties of Nanocrystalline PbTe Synthesized by Mechanical Alloying

In this work, nanocrystalline lead telluride powder was synthesized from high-purity elements by mechanical alloying by means of a planetary ball-milling procedure. The milling medium was tungsten carbide, and the diameter of the balls was varied in order to investigate the effect on the structural features of the material. Phase transformations and crystallite evolution during ball-milling were followed by powder x-ray diffraction (PXRD). The broadened PXRD peaks were analyzed with Voigt functions, revealing small crystalline size and stress introduced during the mechanical alloying process. Transmission electron microscopy (TEM) studies confirmed the material’s nanostructure, as well as the effect of ball diameter on the size of the crystals. Thermoelectric properties are discussed in terms of the Seebeck coefficient and the nominal carrier concentration, as determined by Hall-effect measurements. The enhancement of the Seebeck coefficient is reported to be higher compared with other PbTe-based nanocomposites.     

Direct Evidence of Cation Disorder in Thermoelectric Lead Chalcogenides PbTe and PbS

The extraordinary thermoelectric properties of lead chalcogenides have attracted huge interest in part due to their unexpected low thermal conductivity. Here, it is shown that anharmonicity and large cation disorder are present in both PbTe and PbS, based on elaborate charge density visualization using synchrotron powder X‐ray diffraction (SPXRD) data analyzed with the maximum entropy method (MEM). In both systems, the cation disorder increases with increasing temperature, whereas the Te/S anions appear to be centered on the expected lattice positions. Even at the lowest temperatures of 105 K, the lead ion is on average displaced by ≈0.2 Å from the rock‐salt lattice position, creating a strong phonon scattering mechanism. These findings provide a clue to understanding the excellent thermoelectric performance of crystals with atomic disorder. The SPXRD–MEM approach can be applied in general opening up for widespread characterization of subtle structural features in crystals with unusual properties.     

Enhanced Thermoelectric Properties of Cu2SnSe3 by (Ag,In)‐Co‐Doping

Dense bulk samples of (Ag,In)‐co‐doped Cu₂SnSe₃ have been prepared by a fast and one‐step method of combustion synthesis, and their thermoelectric properties have been investigated from 323 to 823 K. The experimental results show that Ag‐doping at Cu site remarkably enhances the Seebeck coefficient, reduces both electrical and thermal conductivities, and finally increases the figure of merit (ZT) value. The ZT of the Cu_1.85Ag_0.15SnSe₃ sample reaches 0.80 at 773 K, which is improved by about 70% compared with the unadulterated sample (ZT = 0.46 at 773 K). First principle calculation indicates that Ag‐doping changes the electronic structure of Cu₂SnSe₃ and results in larger effective mass of carriers, thus enhancing the Seebeck coefficient and reducing the electrical conductivity. The low electrical conductivity caused by Ag‐doping can be repaired by accompanying In‐doping at Sn site, and by (Ag,In)‐co‐doping the thermoelectric properties are further promoted. The (Ag,In)‐co‐doped sample of Cu_1.85Ag_0.15Sn_0.9In_0.1Se₃ shows the maximum ZT of 1.42 at 823 K, which is likely the best result for Cu₂SnSe₃‐based materials up to now. This work indicates that co‐doping may provide an effective solution to optimize the conflicting material properties for increasing ZT.     

Thermoelectric Properties of Two-Phase PbTe with Indium Inclusions

The thermoelectric figure of merit (zT) can be increased by introduction of additional interfaces in the bulk to reduce the thermal conductivity. In this work, PbTe with a dispersed indium (In) phase was synthesized by a matrix encapsulation technique for different In concentrations. x-Ray diffraction analysis showed single-phase PbTe with In secondary phase. Rietveld analysis did not show In substitution at either the Pb or Te site, and this was further confirmed by room-temperature Raman data. Low-magnification (~1500×) scanning electron microscopy images showed micrometer-sized In dispersed throughout the PbTe matrix, while at high magnification (150,000×) an agglomeration of PbTe particles in the hot-pressed samples could be seen. The electrical resistivity (ρ) and Seebeck coefficient (S) were measured from 300 K to 723 K. Negative Seebeck values showed all the samples to be n-type. A systematic increase in resistivity and higher Seebeck coefficient values with increasing In content indicated the role of PbTe-In interfaces in the scattering of electrons. This was further confirmed by the thermal conductivity (κ), measured from 423 K to 723 K, where a greater reduction in the electronic as compared with the lattice contribution was found for In-added samples. It was found that, despite the high lattice mismatch at the PbTe-In interface, phonons were not scattered as effectively as electrons. The highest zT obtained was 0.78 at 723 K for the sample with the lowest In content.     

Low-Temperature Synthesis and Thermoelectric Properties of n-Type PbTe

n-Type PbTe compounds were synthesized at temperatures as low as 430°C. After synthesis, the materials were ground, cold pressed, and sintered at 600°C. The effect of this low-temperature synthesis on the structural features and thermoelectric properties of as-prepared and PbI₂-doped materials was investigated for the first time. The Seebeck coefficient, and electrical and thermal conductivity were measured in the temperature range 2 K ≤ T ≤  610 K. The results show that all materials exhibit n-type conduction and the thermoelectric properties are improved by doping. ZT values reach 0.5 at 610 K, and the discrepancies with the literature are discussed.     

(Bi0.88Ce0.12)2Ti2O7薄膜的结构及C-V特性

采用化学溶液分解法(CSD)在p型Si<100>衬底上制备了(Bi0.88Ce0.12)2Ti2O7薄膜,分别借助X线光电子能谱仪、紫外-可见分光光度计研究了薄膜的化学特性、紫外-可见吸收光谱等结构性能.结果表明,在烧绿石相Bi2Ti2O7薄膜中掺杂Ce3+取代部分Bi3+后,薄膜在高温退火下仍能保持原有的相结构.利用HP4192A型阻抗分析仪测试薄膜的电容-电压(C-V)特性,计算出600℃、650℃、700℃、750℃退火条件下薄膜的介电常数分别为144、190、214、176,固定电荷密度值分别为3.44×1011cm-2、5.82×1011cm-2、5.58×1011cm-2和2.49×1010cm-2.     

Properties of p- and n-Type PbTe Microwires for Thermoelectric Devices

In thermopower measurements, microwires fabricated from as-purchased bulk PbTe exhibits p-type behavior between room temperature and ～600 K. At higher temperatures, it undergoes majority carrier inversion and exhibits n-type behavior. We report on the preparation and properties of potassium oxide and Zn-doped PbTe microwires, which exhibit stable p- and n-type behavior, respectively, between room temperature and 725 K. Thermoelectric figures of merit (ZT) are reported for device components prepared from bundles of such p- and n-type microwires in a glass matrix.     

Effects of Al/Sb Double Doping on the Thermoelectric Properties of Mg2Si0.75Sn0.25

Al/Sb double-doped Mg₂Si_0.75Sn_0.25 materials were prepared by liquid–solid reaction synthesis and the hot-pressing technique. The effects of Al/Sb double doping on the thermoelectric properties were investigated at temperatures between room temperature and 900 K, and the resistivity and Hall coefficient were investigated at 80 K to 900 K. Al/Sb double-doped samples were found to be n-type semiconductors in the investigated temperature range. The absolute Seebeck coefficient (α), resistivity (ρ), and thermal conductivity (κ) for Al/Sb double-doped samples at room temperature were in the ranges of 152.5 μV K^?1 to 109.2 μV K^?1, 2.92 × 10^?5 Ω m to 1.29 × 10^?5 Ω m, and 2.50 W K^?1 m^?1 to 2.86 W K^?1 m^?1, respectively. The absolute values of α increased with increasing temperature up to a maximum, and decreased thereafter. This could be attributed to mixed carrier conduction in the intrinsic region. κ decreased linearly with increasing temperature to a minimum near the intrinsic region, then increased rapidly because of bipolar components. The highest ZT value measured was 0.94 at 850 K for Mg_1.9975Al_0.0025Si_0.75Sn_0.2425Sb_0.0075. Sb doping was effective for enhancement of ZT, because of a remarkable increase in the carrier concentration. However, Al doping was almost ineffective for enhancing ZT.     

Effect of Ce-Doping on Thermoelectric Properties in PbTe Alloys Prepared by Spark Plasma Sintering

Ce-doped Pb_1−x Ce _x Te alloys with x = 0, 0.005, 0.01, 0.015, 0.03, and 0.05 were prepared by induction melting, ball milling, and spark plasma sintering techniques. The structure and thermoelectric properties of the samples were investigated. X-ray diffraction (XRD) analysis indicated that the samples were of single phase with NaCl-type structure for x less than 0.03. The lattice parameter a increases with increasing Ce content. The lower Ce-doped samples (x = 0.005 and 0.01) showed p-type conduction, whereas the pure PbTe and the higher doped samples (x = 0, 0.015, 0.03, and 0.05) showed n-type conduction. The lower Ce-doped samples exhibited a much higher absolute Seebeck coefficient, but the higher electrical resistivity and higher thermal conductivity compared with pure PbTe resulted in a lower figure of merit ZT. In contrast, the higher Ce-doped samples exhibited a lower electrical resistivity, together with a lower absolute Seebeck coefficient and comparable thermal conductivity, leading to ZT comparable to that of PbTe. The lowest thermal conductivity (range from 0.99 W m⁻¹ K⁻¹ at 300 K to 0.696 W m⁻¹ K⁻¹ at 473 K) was found in the alloy Pb_0.95Ce_0.05Te due to the presence of the secondary phases, leading to a ZT higher than that of pure PbTe above 500 K. The maximum figure of merit ZT, in the alloy Pb_0.95Ce_0.05Te, was 0.88 at 673 K.     

Thermoelectric Properties of Melt-Spun Zn x Sb3 Ribbons

Melt-spun ribbons composed of Zn _x Sb₃ (3.4????x????4.3) were fabricated through a single-wheel melt-spinning process at wheel velocities of 0.6?m?s^?1 to 4.2?m?s^?1 and annealed for 2?h at 673?K. The structures were investigated using x-ray diffraction. The dimensionless figure of merit ZT, Seebeck coefficient, and electrical conductivity were measured to estimate the power factor and thermal conductivity. ??-Zn₄Sb₃ in the as-spun ribbons coexisted with ZnSb or Zn at 0.6?m?s^?1, while it coexisted with ??-Zn₃Sb₂ in x????3.8 at 4.2?m?s^?1, where ??-Zn₃Sb₂ disappeared in the annealed ribbons. The Seebeck coefficient in the as-spun and annealed ribbons tended to decrease slightly with increasing x at all the wheel velocities. At 0.6?m?s^?1, the ZT and power factor of as-spun and annealed ribbons increased with increasing x at x?<?4.0 because of increase in the electrical conductivity. At 4.2?m?s^?1, ZT was smaller than that at 0.6?m?s^?1 because the electrical conductivity was small in the as-spun ribbons and the thermal conductivity was large in the annealed ribbons.     

Structure and Thermoelectric Properties of CoSi-Based Film Composites

Semiconductors - The properties of Co–Si thin films grown by the thermal sintering of Co and Si layers are studied. Co and Si layers are produced by chemical vapor deposition. To form cobalt...     

Thermoelectric Properties of Polycrystalline SrZn2Sb2 Prepared by Spark Plasma Sintering

Compact polycrystalline samples of SrZn₂Sb₂ [space group $ P\overline{3} m1 $ , a = 4.503(1) Å, c = 7.721(1) Å] were prepared by spark plasma sintering. Thermoelectric performance, Hall effect, and magnetic properties were investigated in the temperature range from 2 K to 650 K. The thermoelectric figure of merit ZT was found to increase with temperature up to ZT = 0.15 at 650 K. At this temperature the material showed a high Seebeck coefficient of +230 μV K^?1, low thermal conductivity of 1.3 W m^?1 K^?1, but rather low electrical conductivity of 54 S cm^?1, together with a complex temperature behavior. SrZn₂Sb₂ is a diamagnetic p-type conductor with a carrier concentration of 5 × 10¹⁸ cm^?3 at 300 K. The electronic structure was calculated within the density-functional theory (DFT), revealing a low density of states (DOS) of 0.43 states eV^?1 cell^?1 at the Fermi level.     

Enhanced Thermoelectric Performance of p-Type Mg3Sb2 for Reliable and Low-Cost all-Mg3Sb2-Based Thermoelectric Low-Grade Heat Recovery

Bi₂Te₃-based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low-grade heat into electric power. The recently developed n-type Mg₃Sb₂-based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi₂Te₃ module with a reliable and low-cost all-Mg₃Sb₂-based thermoelectric device if a good p-type Mg₃Sb₂ material can be obtained to match its n-type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p-type Mg₃Sb₂ is significantly improved through Na and Yb doping in Mg_1.8Zn_1.2Sb₂. Moreover, p- and n-type Mg₃Sb₂ are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all-Mg₃Sb₂-based unicouple. The decent conversion efficiency (≈5.5% at the hot-side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg₃Sb₂-based thermoelectric generators for low-grade heat recovery.     

Enhancement of the Thermoelectric Performance of Bi0.4Sb1.6Te3 Alloys by In and Ga Doping

We report an enhancement of the thermoelectric figure of merit in polycrystalline In- and Ga-doped Bi_0.4Sb_1.6Te₃ compounds. Via the controlled doping of In or Ga, the lattice thermal conductivity was effectively reduced by strong point-defect phonon scattering while the power factor was not significantly changed due to the similarity of the density of states near the valence-band maximum between undoped and In- or Ga-doped compositions. An enhanced ZT of 1.2 at 320 K was obtained in 0.5 at.% In-doped Bi_0.4Sb_1.6Te₃ compound by these synergetic effects.