Microstructures and thermoelectric properties of GeSbTe based layered compounds

Microstructures and thermoelectric properties of Ge₁Sb₂Te₄ and Ge₂Sb₂Te₅ chalcogenide semiconductors have been investigated to explore the possibility of their thermoelectric applications. The phase transformation from the face-centered cubic to hexagonal structure was observed in Ge₂Sb₂Te₅ compounds prepared by the melt spinning technique. The Seebeck coefficient and electrical resistivity of the alloys were increased due to the enhanced scattering of charge carriers at grain boundaries. The maximum power factors of the rapidly solidified Ge₁Sb₂Te₄ and Ge₂Sb₂Te₅ attained 0.975×10^-3 W m^-1K^-2 at 750 K and 0.767×10^-3 W m^-1K^-2 at 643 K respectively, higher than those of water quenched counterparts, implying that thermoelectric properties of GeSbTe based layered compounds can be improved by grain refinement. The present results show this class of chalcogenide semiconductors is promising for thermoelectric applications. PACS  84.60.Rb; 81.05.Hd; 72.20.Pa; 64.70.Kb; 61.66.Fn     

(Sb2Te3)0.75(1－x)(Bi2Te3)0.25(1－x)(Sb2Se3)x机械合金化粉体的制备及其冷压烧结样品的热电性能研究

采用机械合金化法制备了p型赝三元(Sb₂Te₃-Bi₂Te₃-Sb₂Se₃)合金粉体，对其进行XRD分析表明Te，Bi，Sb，Se单质粉末，经100h球磨后实现了合金化；SEM分析表明所得机械合金化粉体材料颗粒均匀、细小，颗粒尺寸在10nm到100nm量级.使用这种粉体制备了冷压烧结块体样品，在室温下测量了温差电动势率(α)和电导率(σ)，研究了烧结温度对材料热电性能的影响，结果表明在低于300℃的烧结温区，样品室温下的热电性能随烧结温度的升高不断提高，功率因子(α²σ)由未烧结样品的0.59μW cm^-1K^-2升高到在300℃下烧结样品的15.9μW cm^-1K^-2，这一结果对确定材料的最佳烧结温度具有重要意义. 关键词： 赝三元热电材料 机械合金化 冷压 烧结     

Improving the thermoelectric properties of thick Sb2Te3 film via Cu doping and annealing deposited by DC magnetron sputtering using a mosaic target

Thick Cu-doped Sb₂Te₃ films were deposited on flexible substrate by DC magnetron sputtering from a mosaic Cu–Sb₂Te₃ target. The Cu-doped Sb₂Te₃ films were vacuum annealed to improve their thermoelectric properties. Density functional theory was used to clarify the internal mechanism of the Cu doped into the Sb₂Te₃ system. The results showed that Cu substitution on a Sb site induced electronic states or impurity peaks of Sb₂Te₃ at a valence band maximum. The carrier concentration of the Cu-doped Sb₂Te₃ films increased as the Cu-doped concentration increased. However, the crystallite size and Seebeck coefficient of the Cu-doped Sb₂Te₃ films decreased as the Cu-doped concentration increased. Post-annealing treatment improved the microstructure and thermoelectric properties of the Cu-doped Sb₂Te₃ films. The maximum electrical conductivity and power factor values of 754.20 S/cm at 50 °C and 1.56 10⁻³ W/mK² at 100 °C were obtained in the annealed film with a Cu-doped concentration of 3 at%.     

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.     

相变存储材料Ge1Sb2Te4和Ge2Sb2Te5薄膜的结构和电学特性研究

采用射频磁控溅射方法制备了两种用于相变存储器的Ge₁Sb₂Te₄和Ge₂Sb₂Te₅相变薄膜材料,对其结构、电学输运性质和恒温下电阻随时间的变化关系进行了比较和分析.X射线衍射(XRD)和原子力显微镜(AFM)的结果表明:随着退火温度的升高,Ge₁Sb₂Te₄薄膜逐步晶化,由非晶态转变为多晶态,表面出现均匀的、 关键词： 硫系相变材料 1Sb₂Te₄')" href="#">Ge₁Sb₂Te₄ 2Sb₂Te₅')" href="#">Ge₂Sb₂Te₅     

High thermoelectric performance and low thermal conductivity in K-doped SnSe polycrystalline compounds

SnSe single crystal showed a high thermoelectric zT of 2.6 at 923 K mainly due to an extremely low thermal conductivity 0.23 W m⁻¹ K⁻¹. It has anisotropic crystal structure resulting in deterioration of thermoelectric performance in polycrystalline SnSe, providing a low zT of 0.6 and 0.8 for Ag and Na-doped SnSe, respectively. Here, we presented the thermoelectric properties on the K-doped K_xSn_1−xSe (x = 0, 0.1, 0.3, 0.5, 1.5, and 2.0%) polycrystals, synthesized by a high-temperature melting and hot-press sintering with annealing process. The K-doping in SnSe efficiently enhances the hole carrier concentration without significant degradation of carrier mobility. We find that there exist widespread Se-rich precipitates, inducing strong phonon scattering and thus resulting in a very low thermal conductivity. Due to low thermal conductivity and moderate power factor, the K_0.001Sn_0.999Se sample shows an exceptionally high zT of 1.11 at 823 K which is significantly enhanced value in polycrystalline compounds.     

Temperature dependence of thermal properties of Ag<Subscript>8</Subscript>In<Subscript>14</Subscript>Sb<Subscript>55</Subscript>Te<Subscript>23</Subscript> phase-change memory materials

The dependence of thermal properties of Ag₈In₁₄Sb₅₅Te₂₃ phase-change memory materials in crystalline and amorphous states on temperature was measured and analyzed. The results show that in the crystalline state, the thermal properties monotonically decrease with the temperature and present obvious crystalline semiconductor characteristics. The heat capacity, thermal diffusivity, and thermal conductivity decrease from 0.35 J/g K, 1.85 mm²/s, and 4.0 W/m K at 300 K to 0.025 J/g K, 1.475 mm²/s, and 0.25 W/m K at 600 K, respectively. In the amorphous state, while the dependence of thermal properties on temperature does not present significant changes, the materials retain the glass-like thermal characteristics. Within the temperature range from 320 K to 440 K, the heat capacity fluctuates between 0.27 J/g K and 0.075 J/g K, the thermal diffusivity basically maintains at 0.525 mm²/s, and the thermal conductivity decreases from 1.02 W/m K at 320 K to 0.2 W/m K at 440 K. Whether in the crystalline or amorphous state, Ag₈In₁₄Sb₅₅Te₂₃ are more thermally active than Ge₂Sb₂Te₅, that is, the Ag₈In₁₄Sb₅₅Te₂₃ composites bear stronger thermal conduction and diffusion than the Ge₂Sb₂Te₅ phase-change memory materials.     

Thermoelectric properties in Mn-doped Bi2Se3

Using n-type and p-type Mn-doped Bi₂Se₃ single crystals, a thin-film-type thermoelectric (TE) module was fabricated and the TE characteristics were investigated. The Seebeck coefficient at room temperature was about 100 μV K⁻¹ with different sign for both materials. From the Seebeck coefficient and resistivity values, the electric power of our TE module was evaluated to be 90 μW for a single couple at the temperature difference of 10 K. This value is compared to that (∼21 μW) of commercialized TE device. Nevertheless, the actual power was measured to be quite small around 0.74 μW, which is much higher than other homemade TE power level. This small power is attributed to the high electrical contact resistance between the TE material and the heat source and sink. Assuming the contact resistance level ∼0.1 Ω similar to that of commercialized TE devices, the electric power should be about 41 μW, which is almost 2 times higher than that in commercialized TE devices. These results propose that the Mn-doped Bi₂Se₃ system is another promising TE material, which can be replaced with the commercialized Bi₂Te₃ system.     

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).     

Bi-Sb-Te基热电薄膜温差电池离子束溅射制备与表征

本文采用离子束溅射Bi/Te和Sb/Te二元复合靶,直接制备n型Bi₂Te₃热电薄膜和p型Sb₂Te₃热电薄膜.在退火时间同为1 h的条件下,对所制备的Bi₂Te₃薄膜和Sb₂Te₃薄膜进行不同温度的退火处理,并对其热电性能进行表征.结果表明,在退火温度为150 ℃时,制备的n型Bi₂Te₃关键词： 薄膜温差电池 2Te₃薄膜')" href="#">Sb₂Te₃薄膜 2Te₃薄膜')" href="#">Bi₂Te₃薄膜 离子束溅射     

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.     

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.     

基于Si掺杂Sb2Te3薄膜的相变存储器研究

采用磁控三靶(Si，Sb及Te)共溅射法制备了Si掺杂Sb₂Te₃薄膜，作为对比，制备了Ge₂Sb₂Te₅和Sb₂Te₃薄膜，并且采用微加工工艺制备了单元尺寸为10μm×10μm的存储器件原型来研究器件性能.研究表明，Si掺杂提高了Sb₂Te₃薄膜的晶化温度以及薄膜的晶态和非晶态电阻率，使得其非晶态与晶态电阻率之比达到10⁶，提高了器件的电阻开/关比；同Ge₂Sb₂Te₅薄膜相比，16at% Si掺杂Sb₂Te₃薄膜具有较低的熔点和更高的晶态电阻率，这有利于降低器件的RESET电流.研究还表明，采用16at% Si掺杂Sb₂Te₃薄膜作为存储介质的存储器器件原型具有记忆开关特性，可以在脉高3V、脉宽500ns的电脉冲下实现SET操作，在脉高4V、脉宽20ns的电脉冲下实现RESET操作，并能实现反复写/擦，而采用Ge₂Sb₂Te₅薄膜的相同结构的器件不能实现RESET操作. 关键词： 相变存储器 硫系化合物 2Te₃薄膜')" href="#">Si掺杂Sb₂Te₃薄膜 SET/RESET转变     

Phase transition characteristics of Ge–Sb–Te pseudobinary alloys in laser irradiation measurement

Optical switching and structural transformation of GeTe–Sb₂Te₃ pseudobinary alloys, Ge₂Sb₂Te₅, Ge₁Sb₂Te₄, and Ge₁Sb₄Te₇, were studied for data storage application. As-deposited Ge₂Sb₂Te₅, Ge₁Sb₂Te₄, and Ge₁Sb₄Te₇ thin films were amorphous and they crystallized to FCC and HCP upon heat treatment. Crystallization was accelerated by increasing the proportion of Sb₂Te₃ rather than GeTe in Ge–Sb–Te compounds; this was observed by reflectivity changes under nanosecond laser irradiation in static tester. The different crystallization kinetics according to composition might be affected by the structural incompatibility of GeTe under the ‘Umbrella Flip’ theory.     

Temperature dependence of the Raman spectra of Bi2Te3 and Bi0.5Sb1.5Te3 thermoelectric films

The temperature dependence of the Raman spectra of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ thermoelectric films was investigated. The temperature coefficients of the E_g(2) peak positions were determined as –0.0137 cm^–1/°C and –0.0156 cm^–1/°C, respectively. The thermal expansion of the crystal caused a linear shift of the Raman peak induced by the temperature change. Based on the linear relation, a reliable and noninvasive micro‐Raman scattering method was shown to measure the thermal conductivity of the thermoelectric films. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

The effect of (00l) crystal plane orientation on the thermoelectric properties of Bi2Te3 thin film

Highly (00l)-oriented pure Bi₂Te₃ films with in-plane layered grown columnar nanostructure have been fabricated by a simple magnetron co-sputtering method. Compared with ordinary Bi₂Te₃ film and bulk materials, the electrical conductivity and Seebeck coefficient of such films have been greatly increased simultaneously due to raised carrier mobility and electron scattering parameter, while the thermal conductivity has been decreased due to phonon scattering by grain boundaries between columnar grains and interfaces between each layers. The power factor has reached as large as 33.7 μW cm⁻¹ K⁻², and the out-of-plane thermal conductivity is reduced to 0.86 W m⁻¹ K⁻¹. Our results confirm that tailoring nanoscale structures inside thermoelectric films effectively enhances their performances.     

Scanning tunneling microscope light emission spectra of polycrystalline Ge2Sb2Te5 and Sb2Te3

We have observed scanning tunneling microscope light emission (STM-LE) spectra of Ge₂Sb₂Te₅ and Sb₂Te₃. Although these chalcogenide alloys exhibit band gaps less than 0.5 eV, the STM-LE was observed with a narrow spectral width at a photon energy of 1.5 eV for both materials. By analyzing its bias voltage, polarity, and temperature dependencies combined with recently reported theoretical electronic structures, we concluded that the STM-LE is excited by electronic transitions taking place in the local electronic structure having a direct gap-like shape with a band gap of 1.5 eV, commonly found in the electronic structures of both materials.     

Effect of heat treatment on the structure and the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> thin films and composites based on them

This work considers the effect of vacuum annealing on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 thin film and Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb_0.9Bi_1.1Te_2.9Se_0.1–C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution, which determine the type of conductivity of Sb_0.9Bi_1.1Te_2.9Se_0.1 granules. The power factors are estimated for films of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution and films of Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)₂(Te,Se)₃ solid solutions.     

In situ AFM and Raman spectroscopy study of the crystallization behavior of Ge2Sb2Te5 films at different temperature

Using in situ atomic force microscope (AFM) and Raman spectroscopy, the real-time crystallization properties of Ge₂Sb₂Te₅ films at different temperature were characterized. The given AFM topograph and phase images revealed that the structure of amorphous Ge₂Sb₂Te₅ films began to change at a temperature of as low as 100 °C. When the temperature reached 130 °C, some crystal fragments had formed at the film surface. Heating up to 160 °C, the size of the visible crystal fragments increased, but decreased at a higher temperature of 200 °C. When the Ge₂Sb₂Te₅ film was cooled down to room temperature (RT) from 200 °C, the crystal fragments divided into crystal grains due to the absence of heating energy. The Raman spectra at different temperature further verified the structure evolution of the Ge₂Sb₂Te₅ film with temperature. This work is of significance for the preparation of Ge₂Sb₂Te₅ films and the erasing of data.