Doping effects on the thermoelectric properties of Cu-intercalated Bi2Te2.7Se0.3

We herein report an enhancement of the thermoelectric performance of spark plasma sintered polycrystalline n-type Bi₂Te_2.7Se_0.3 by the intercalation of Cu and the doping of Al on Bi-sites. Through the intercalation of a small amount of Cu (0.008), the reproducibility could be significantly improved, with ZT was enhanced from 0.64 to 0.73 at 300 K due to the reduced lattice thermal conductivity benefiting from intensified point-defect phonon scattering. We also found that Al is an effective doping element for power factor enhancement and for reducing the lattice thermal conductivity of Cu-intercalated Bi₂Te_2.7Se_0.3. With these synergetic effects, an enhanced ZT values of 0.78 at 300 K and 0.81 at 360 K were obtained in 1 at% Al-doped Cu_0.008Bi₂Te_2.7Se_0.3 (Cu_0.008Bi_1.98Al_0.02Te_2.7Se_0.3).     

Thermoelectric performance of monolayer Bi2Te2Se of ultra low lattice thermal conductivity

The electronic structure and thermoelectric properties of monolayer Bi₂Te₂Se were studied by density functional theory and semi-classical Boltzmann transport equation. The band gap with TB-mBJ can be improved for monolayer Bi₂Te₂Se. Monolayer Bi₂Te₂Se have ultra-low thermal conductivity comparing with other well-known two-dimensional materials. The monolayer Bi₂Te₂Se can improve electrical conductivities. ZT increases with increasing temperature for monolayer Bi₂Te₂Se. Comparing to GGA, TB-mBJ has larger ZT value in p-type doping. Monolayer Bi₂Te₂Se have larger ZT comparing with other well-known two-dimensional materials. Our calculated results show that our calculation greatly underestimates ZT value, therefore, monolayer Bi₂Te₂Se should have a higher ZT value.     

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.     

Thermoelectric properties of Bi2Se3/Bi2Te3/Bi2Se3 and Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems

In this work, we develop a theory of thermoelectric transport properties in two-dimensional semiconducting quantum well structures. Calculations are performed for n-type 0.1 wt.% CuBr-doped Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ and p-type 3 wt.% Te-doped Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ quantum well systems in the temperature range 50–600 K. It is found that reducing the well thickness has a pronounced effect on enhancing the thermoelectric figure of merit (ZT). For the n-type Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ with 7 nm well width, the maximum value of ZT is estimated to be 0.97 at 350 K and for the p-type Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ with well width 10 nm the highest value of the ZT is found to be 1.945 at 440 K. An explanation is provided for the resulting higher ZT value of the p-type system compared to the n-type system.     

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.     

Carrier transport in Bi2Se3 topological insulator slab

Electron transport in Bi₂Se₃ topological insulator slabs is investigated in the thermal activation regime (>50 K) both in the absence (ballistic) and presence of weak and strong acoustic phonon scattering using the non-equilibrium Green function approach. Resistance of the slab is simulated as a function of temperature for a range of slab thicknesses and effective doping in order to gain a handle on how various factors interact and compete to determine the overall resistance of the slab. If the Bi₂Se₃ slab is biased at the Dirac point, resistance is found to display an insulating trend even for strong electron–phonon coupling strength. However, when the Fermi-level lies close to the bulk conduction band (heavy electron doping), phonon scattering can dominate and result in a metallic behavior, although the insulating trend is retained in the limit of ballistic transport. Depending on values of the operating parameters, the temperature dependence of the slab is found to exhibit a remarkably complex behavior, which ranges from insulating to metallic, and includes cases where the resistance exhibits a local maximum, much like the contradictory behaviors seen experimentally in various experiments.     

Relating phase transition heat capacity to thermal conductivity and effusivity in Cu2Se

Accurate measurement of thermal conductivity is essential to determine the thermoelectric figure‐of‐merit, zT. Near the phase transition of Cu₂Se at 410 K, the transport properties change rapidly with temperature, and there is a concurrent peak in measured heat capacity from differential scanning calorimetry (DSC). Interpreting the origin as a broad increase in heat capacity or as a transient resulted in a three‐fold difference in the reported zT in two recent publications. To resolve this discrepancy, thermal effusivity was deduced from thermal conductivity and diffusivity measurements via the transient plane source (TPS) method and compared with that calculated from thermal diffusivity and the two interpretations of the DSC data for heat capacity. The comparison shows that the DSC measurement gave the heat capacity relevant for calculation of the thermal conductivity of Cu₂Se. The thermal conductivity calculated this way follows the electronic contribution to thermal conductivity closely, and hence the main cause of the zT peak is concluded to be the enhanced Seebeck coefficient. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

Thermoelectric properties of unoxidized graphene/Bi2Te2.7Se0.3 composites synthesized by exfoliation/re‐assembly method

Nanocomposites of n‐type thermoelectric Bi₂Te_2.7Se_0.3 (BTS) and unoxidized graphene (UG) were prepared from the exfoliated BTS and UG nanoplatelets. Polycrystalline BTS ingots were exfoliated into nanoscoll‐type crystals by chemical exfoliation, and were re‐assembled with UG nanoplatelets. The composites were chemically reduced by hydrazine hydrate and sintered by a spark‐plasma‐sintering method. The thermoelectric properties of the sintered composites were evaluated and exhibited decreased carrier concentration and increased thermal conductivity due to the embedded graphene. The peak ZT values for the UG/BTS‐US and UG/BTS‐EX composites were ～0.8 at the UG concentration of 0.05 wt%. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

熔体旋甩法合成n型(Bi_(0.85)Sb_(0.15))_2(Te_(1-x)Se_x)_3化合物的微结构及热电性能

采用熔体旋甩结合放电等离子烧结(MS-SPS)技术制备了单相n型四元(Bi0.85Sb0.15)2(Te1-xSex)3(x=0.15,0.17,0.19,0.21)化合物,并对所得样品的微结构和热电传输性能进行了系统研究.样品自由断裂面的场发射扫描电子显微镜及抛光面的背散射电子成分分析表明:块体材料晶粒细小,晶粒排列紧密,成分分布均匀且相结构单一,样品中存在大量10—100nm的层状结构.随着Se含量x的增加,样品的电导率和热导率逐渐增加,而Seebeck系数逐渐降低.相比商业应用的区熔材料,MS-SPS方法合成的高Se组成的样品均在425K后表现出更高的ZT值,其中(Bi0.85Sb0.15)2(Te0.83Se0.17)3样品具有最高的ZT值,在360K可达到0.96,并在320—500K均保持较高的ZT值,500K时其ZT值相比区熔材料提高了48%.此外,通过调节Se的含量,可以有效地调控材料的ZT峰值出现的温度段,这对多级或梯度热电器件的制备具有重要意义.     

(Bi1-x Agx)2(Te1-ySey)3粉体的机械合金化制备及其放电等离子烧结体的热电输运特性

采用机械合金化制备了n型(Bi_1-xAg_x)₂(Te_1-ySe_y)₃合金粉体,对其进行XRD分析表明Bi,Te,Ag,Se单质粉末,经2h球磨后实现了合金化;SEM分析表明随着机械合金化时间延长粉体颗粒变得均匀、细小,颗粒尺寸在微米至亚微米数量级.采用放电等离子烧结制备了块体样品,研究了合金成分和球磨时间对热电性能的影响.结果表明材料的热电性能与掺杂元素有密切关系,Ag有利于提高功率因子和降低晶格热导率,球磨10h的(Bi_0.99Ag_0.01)₂(Te_0.96Se_0.04)₃合金粉末的烧结块体具有最大的功率因子和最低的晶格热导率,并在323K取得最高ZT值0.52. 关键词： 1-xAg_x)₂(Te_1-ySe_y)₃合金')" href="#">(Bi_1-xAg_x)₂(Te_1-ySe_y)₃合金 机械合金化 放电等离子烧结 热电性能     

TeI4掺杂量对n型Bi2Te3基烧结材料热电性能的影响

采用区熔法结合放电等离子体快速烧结(SPS)技术制备了n型Bi₂Te₃基热电材料.在300—500K的温度范围内测量了各热电性能参数，包括电导率(σ)、塞贝克系数(α)和热导率(κ)，研究了掺杂剂TeI₄的含量(质量百分比分别为0，0.05，0.08，0.10，0.13和0.15wt%)对热电性能的影响.结果表明：试样的载流子浓度(n)随TeI₄含量增加而增大，使电导率增大、塞贝克系数的绝对值先增大而后减小，从而导致品质因子(α²σ)呈先增加后降低的变化趋势；同时，由于异质离子(I^-)以及载流子对声子的散射作用增强，可显著降低其晶格热导率.烧结材料的性能优值(ZT=α²σT/κ)对应于TeI₄含量为0.08wt%有其最大值，约为0.92.此外，烧结材料的抗弯强度增加至80MPa左右，从而可以显著改善材料的可加工性以及元器件的使用可靠性. 关键词： 2Te₃')" href="#">Bi₂Te₃ 放电等离子体快速烧结 热电性能     

High-temperature specific heat of Cu5Bi2B4O14

Cu₅Bi₂B₄O₁₄ single crystals are grown by spontaneous crystallization from a melt of a CuO, Bi₂O₃, and B₂O₃ mixture. The specific heat of the crystals in the temperature range 396–633 K is measured by differential scanning calorimetry. Using the experimental data, the thermodynamic properties of the Cu₅Bi₂B₄O₁₄ solid compound are calculated.     

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.     

Evidence of d-wave pairing in the thermal conductivity of YBa2Cu3O7−d and Bi2Sr2CaCu2O8 single crystals

The electronic contribution to the thermal conductivity of a d _x ²?_y ²-wave superconductor is calculated within a variational method. By considering the scattering of electrons by point defects and acoustic phonons, the behavior of the thermal conductivity of YBa₂Cu₃O_7?δ and Bi₂Sr₂CaCu₂O₈ single crystals can be well described with realistic values of the physical parameters. Besides, the values obtained for the transport electron-phonon coupling constant λ _tr lie in the same range of magnitude than those derived from electrical resistivity measurements. b]References     

Transverse Seebeck effect in Bi2Sr2CaCu2O8

2 Cu₃O₇ has been observed in “off-c-axis” grown Bi₂Sr₂CaCu₂O₈ films. The films were prepared by pulsed laser deposition from Bi₂Sr₂CaCu₂O₈ targets and grown with a tilt angle of 10 degrees between the film c-axis and the film surface on off-axis cut SrTiO₃ substrates. From the thermoelectric response upon laser irradiation the anisotropy of the thermopower is estimated. Received: 15 April 1996/ Accepted:19 November 1996     

Luminescence centers in Pb2Bi6O11 and Sn2Bi6O11 ceramics

We have studied the luminescence spectra and luminescence excitation spectra of Pb₂Bi₆O₁₁ and Sn₂Bi₆O₁₁ ceramics at 80 K. We have used the Alentsev-Fock to decompose the spectra into elementary components. We have established that the luminescence spectra of Pb₂Bi₆O₁₁ and Sn₂Bi₆O₁₁ ceramics contain three elementary bands each with maxima at 2.60, 2.32, 12.93 eV and 2.62, 2.30, 2.00 eV. Comparison of the data obtained with the results of a study of the luminescence spectra for a series of bismuth-containing oxide compounds suggest that luminescence of Pb₂Bi₆O₁₁ and Sn₂Bi₆O₁₁ is due to radiative processes in structural complexes containing a bismuth ion in a nearest-neighbor oxygen environment. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 597–600, September–October, 2006.     

Specific features in the behavior of the effective mass and mobility in n-(Bi, Sb)2(Te, Se, S)3 solid solutions

The thermoelectric properties of the multicomponent solid solutions Bi_2?x Sb_xTe_3?y?z Se_yS_z with substitutions of atoms in both sublattices of Bi₂Te₃ were studied. The data obtained in studies of the galvanomagnetic effects in weak magnetic fields were used to properly take into account the change in the carrier scattering mechanisms due to the substitutions Sb → Bi, Se, and S → Te in the solid solutions. The mobility μ₀ with inclusion of the degeneracy, the effective density-of-states mass m/m ₀, and the lattice thermal conductivity κ_L were calculated. An analysis was carried out for the quantities μ₀, m/m ₀, and κ_L in the solid solutions under study as functions of the composition, carrier concentration, and temperature.     

Changes in the electronic properties of Bi2X3 (X: Se, Te) single crystals due to intercalation

Bi₂Se₃ and Bi₂Te₃ are layered semiconductors n-type and p-type, respectively, which belong to the family of thermoelectric materials. In this work we examine the insertion of Cu in Bi₂Te₃ and Bi₂Se₃ single crystals through an intercalation reaction. The inserted Cu acting as donor enhances the n-type character of Bi₂Se₃ while changing the native p-type character of Bi₂Te₃ to n-type. The spatial distribution of the intercalated species was monitoring by X-ray microanalysis and microscopic IR reflectivity measurements. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Optimization of thermoelectric properties of Bi2Se3 using insertion techniques

In this work we investigate the optimization of the performance of thermoelectric materials using insertion techniques. Our results indicate that in cases such as Bi₂Se₃ compounds, where insertion of foreign species in the lattice is possible due to the particular crystal structure, the insertion technique could be proven to be a valuable easy to apply and cost effective technique, an alternative to doping, that can produce homogeneous materials with fine tuning of the electronic and thermoelectric properties. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.