Effect of thermal annealing on the microstructure and thermoelectric properties of nano-TiN/Co4Sb11.5Te0.5 composites

The microstructure and thermoelectric properties of nano-TiN (1 vol%) dispersed Co₄Sb_11.5Te_0.5 skutterudite composites were systematically investigated after different thermal annealing time at 773 K in vacuum. Interior pores and surface bulges are formed clearly in the annealed sample. After 120 h annealing, the porosity of the sample increases by 10 %. However, the elemental composition and the phase composition remain stable after thermal annealing. The electrical conductivity and the thermal conductivity simultaneously decline due to the higher porosity in the annealed sample, but the thermal conductivity reduces more remarkably. In addition, the Seebeck coefficient changes slightly after annealing. As a result, the ZT value increases by 20 % after 120 h annealing. The results show that the annealing treatment can improve the ZT value of the TiN/Co₄Sb_11.5Te_0.5 nanocomposites to some extent, but it will also degrade the structure of the sample, and cause the decrease of the sample’s mechanical prosperities.     

The thermoelectric properties of Co4Sb12-xTex synthesized at different pressure

Skutterudite compounds Co₄Sb_12-xTe_x (0.1 ≤ ×≤0.8) was synthesized successfully by high temperature and high pressure (HTHP) method and characterized with X-ray diffractometry and thermoelectric properties measurements. The samples prepared by HTHP are nearly with the single phase CoSb₃. The electrical resistivity, Seebeck coefficient and thermal conductivity were all depending on synthetic pressure and the Te content of the Skutterudite compounds were performed at room temperature. As our expected, the Seebeck coefficient increased with an increase of the synthetic pressure and the thermal conductivity decreased with an increase of the synthetic pressure. These results indicated that HTHP technique may be helpful to prepare thermoelectric materials with enhanced thermoelectric properties.     

快淬和球磨时间对p型(Bi_(0.25)Sb_(0.75))_2Te_3合金热电性能的影响

通过快淬-机械球磨-放电等离子烧结工艺制备了p型(Bi0.25Sb0.75)2Te3块体热电材料.在300~523K温度范围内对其电导率、Seebeck系数和热导率进行了测试,并系统研究了快淬后球磨时间对合金热电性能的影响.研究结果表明,随着球磨时间的延长,样品的电导率呈先降后升的趋势,Seebeck系数变化并不明显,而热导率随球磨时间的延长逐渐下降.球磨20h的样品在室温下具有最高的热电优值,最大值达到0.96,机械抗弯强度达到91MPa.     

纳米SiO_2包覆提高填充方钴矿热电材料热稳定性

采用熔融结合放电等离子体烧结制备了单相Ba0.3In0.2Ni0.05Co3.95Sb12块体材料,并用湿化学包覆处理方法制备了纳米SiO2包覆的Ba0.3In0.2Ni0.05Co3.95Sb12/SiO2纳米复合材料,重点研究了两种(Ba,In)双原子填充方钴矿基块体材料在300~723~300K范围热循环2000次过程中其显微结构、化学成分和热电性能的演变特征.结果发现,纳米SiO2包覆可以显著提高(Ba,In)双原子填充方钴矿块体材料的热稳定性,单相Ba0.3In0.2Ni0.05Co3.95Sb12块体材料晶界处的显微结构和化学成分发生显著变化,Ba0.3In0.2Ni0.05Co3.95Sb12/SiO2纳米复合材料几乎没有影响,纳米SiO2起着稳定晶界和抑制晶内元素扩散与挥发的作用.热循环过程中,两种材料在300和500K时的综合热电性能ZT值相近,变化很小;800K时,单相块体材料的ZT值呈逐渐降低趋势,纳米复合材料由于受热导率的复杂变化影响其ZT值,随淬火次数增加而逐渐增大,淬火1800次时ZT值达到1.12,甚至高于未淬火的Ba0.3In0.2Co3.95Ni0.05Sb12/SiO2纳米复合材料(1.08)和淬火1800次后的单相Ba0.3In0.2Co3.95Ni0.05Sb12块体材料(1.10)的ZT值.     

Ag_2Te掺杂对p型Bi_(0.5)Sb_(1.5)Te_3块状合金热电性能的影响

采用真空熔炼、机械球磨及放电等离子烧结技术(SPS)制备得到了(Ag2Te)x(Bi0.5Sb1.5Te3)1-x(x=0,0.025,0.05,0.1)系列样品,性能测试表明,Ag2Te的掺入可以显著改变材料的热电性能变化趋势,掺杂样品在温度为450~550K范围内具有较未掺杂样品更优的热电性能.适当量的Ag2Te掺入能够有效地提高材料的声子散射,降低材料的热导率.在测试温度范围内,(Ag2Te)0.05(Bi0.5Sb1.5Te3)0.95具有最低的晶格热导,室温至575K范围内保持在0.2~0.3W/(m·K)之间,在575K时,(Ag2Te)0.05(Bi0.5Sb1.5Te3)0.95试样具有最大热电优值ZT=0.84,相较于未掺杂样品提高了约20%.     

Enhanced thermoelectric properties in Co4Sb12 − xTex alloys prepared by HPHT

Skutterudite compounds Co₄Sb_12 ? xTe_x with bcc crystal structure were prepared by high pressure and high temperature (HTHP) method. The study explored chemical doping with Te at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system Co₄Sb_12 ? xTe_x. The electrical resistivities, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 300–710 K. We found that the presence of Te substantially decreased the electrical resistivity without any detrimental effect on the Seebeck coefficients, which improved the power factor. Among all the samples, Co₄Sb_11.5Te_0.5 shows the highest power factor of 35.3 µw/(cmK²) at 710 K, and the maximum ZT value reaches 0.67 at 710 K.     

Te掺杂对SryCo4Sb12-xTex化合物热电性能的影响

采用熔融法结合SPS烧结技术合成了Sr_yCo₄Sb_12-xTe_x化合物, 并探讨了Te掺杂对化合物热电性能的影响. 采用XRD及EPMA确定了相组成及化学成分, 并测试了材料的高温热电性能. 实验结果表明, 虽然Te掺杂降低了Sr在CoSb₃中的填充量, 但是与具有相近Sr填充量的基体相比, Te掺杂提高了材料的载流子浓度和电导率, 同时也提高了塞贝克系数; Te掺杂由于引入了电子-声子散射, 进一步降低了材料的晶格热导率, 并且随着Te掺杂量的增加, 晶格热导率的降低幅度提高; 对x=0.05的样品Sr_0.18Co₄Sb_11.95Te_0.05, 在850K时, 材料的最大ZT值接近1.0, 与具有相近填充量的基体材料相比, ZT值提高了35％.     

Crystallization behavior and thermoelectric characteristics of the electrodeposited Sb2Te3 thin films

Crystallization behavior of the electrodeposited Sb₂Te₃ film was characterized and the effect of the amorphous-crystalline transition on the Seebeck coefficient was evaluated. The as-electrodeposited Sb₂Te₃ film was amorphous and exhibited the Seebeck coefficient of 268-322 μV/K, which was much larger than the value of the crystalline Sb₂Te₃ film. When annealed at temperatures above 100 °C, the Seebeck coefficient of the Sb₂Te₃ film dropped significantly to 78-107 μV/K due to the amorphous-crystalline transition at 94 °C. The thermal stability of the electrodeposited Sb₂Te₃ film was improved by the addition of Cu, and the crystallization temperature of the CuSbTe film increased up to 149.5 °C.     

Effects of Se substitution on the thermoelectric performance of n-type Co4Sb11.3Te0.7−xSex skutterudites

A series of double-substituted Co₄Sb_11.3Te_0.7?xSe_x skutterudites have been fabricated by combining the solid state reaction and the spark plasma sintering method, and the effects of Se substitution on the thermoelectric properties are characterized by measurements of the electrical conductivity, the Seebeck coefficient and the thermal conductivity in the temperature range of 300–800 K. Doping Se into the Co₄Sb_11.3Te_0.7?xSe_x matrix suppresses the carrier concentration, and the electrical conductivity actually decreases with the Se content. However, moderate Se doping is effective in enhancing the thermoelectric performance of the n-type Co₄Sb_11.3Te_0.7?xSe_x, because of the resulted dramatically decreased thermal conductivity. Analyses indicate that the heightened point-defect scattering induced by Se doping together with the electron–phonon scattering induced by Te doping is responsible for the reduction of lattice thermal conductivity of these compounds.     

Thermal co-evaporation of Sb2Te3 thin-films optimized for thermoelectric applications

Antimony telluride (Sb₂Te₃) is a chalcogenide material used in thermoelectric applications. The deposition of thin films of Sb₂Te₃ requires a precisely controlled process to achieve a desirable high thermoelectric figure-of-merit. The optimization of the thermal co-evaporation process for p-type Sb₂Te₃ thin-film onto plastic substrates (Kapton© polyimide) for thermoelectric applications is reported. The influence of deposition parameters and composition on thermoelectric properties was studied, seeking optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy all confirmed the formation of Sb₂Te₃ thin films. Seebeck coefficient (up to 190 μVK⁻¹), in-plane electrical resistivity (8-15 μΩm), carrier concentration (1 × 10¹⁹-7 × 10¹⁹ cm⁻³) and Hall mobility (120-180 cm²V⁻¹s⁻¹) were measured at room temperature for the best Sb₂Te₃ thin-films.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

AC conductivity and dielectric properties of Sb2Te3 thin films

Sb₂Te₃ films of different thicknesses, in the thickness range 300-620 nm, were prepared by thermal evaporation. X-ray analysis showed that the as-deposited Sb₂Te₃ films are amorphous while the source powder and annealed films showed a polycrystalline nature. The AC conductivity and dielectric properties of Sb₂Te₃ films have been investigated in the frequency range 0.4-100 kHz and temperature range 303-373 K. The AC conductivity σ_AC(ω) was found to obey the power law ω^s where s?1 independent of film thickness. The temperature dependence of both AC conductivity and the exponent s can be reasonably well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and the dielectric loss ε₂ are frequency and temperature dependent and thickness independent. The maximum barrier height W_M calculated from dielectric measurements according to the Guintini equation agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The effect of annealing at different temperatures on the AC conductivity and dielectric properties was also investigated. Values of σ_AC, ε₁ and ε₂ were found to increase with annealing treatment due to the increase of the degree of ordering of the investigated films. The Cole-Cole plots for the as-deposited and annealed Sb₂Te₃ films have been used to determined the molecular relaxation time τ. The temperature dependence of τ indicates a thermally activated process.     

Nano composite Si2Sb2Te film for phase change memory

Growth-dominant Sb₂Te material with large crystal grain is converted to the nano composite material after Si doping. The increase of Si content in Si_xSb₂Te material helps to further diminish the grain size, form more uniform grain distribution, and enhance the thermal stability of the amorphous phase. Si₂Sb₂Te crystallizes into a nano composite structure [amorphous Si + crystalline Sb₂Te (< 20 nm grain size)] without any Te or Sb phase segregation, which ensures better operation stability for the application in T-shaped phase change memory device. Comparing to Ge₂Sb₂Te₅ film, Si₂Sb₂Te film shows better data retention ability (10 years at 397 K). Meanwhile, electrical measurements prove that phase change memory cell based on Si₂Sb₂Te film also has low power consumption than that of the Ge₂Sb₂Te₅ film based cell.     

Structure study on the thermoelectric materials CexCo4Sb12 (x = 0.5, 1.0) by neutron diffraction

In order to understand the relationship between crystal structure and thermoelectric properties, the neutron diffraction patterns of the thermoelectric materials with the nominal composition Ce_xCo₄Sb₁₂ (x = 0.5, 1.0) were measured at room temperature, the data were fitted by the Rietveld profile refinement method using the Fullprof2k program. It is found that the sample Ce_0.5Co₄Sb₁₂ is composed of two phases, its major phase is CoSb₃ with skutterudite-structure and the Ce atom is not incorporated into the lattice, the impurity is monoclinic CoSb₂. In the case of Ce_1.0Co₄Sb₁₂, the major phase is filled skutterudite, about 10% of the 2a site is occupied by Ce atom, the second phase is monoclinic CoSb₂.     

Preparation and electrical transport properties of In filled and Te-doped CoSb3 skutterudite

Nanopowders with nominal compositions of Co₄Sb_11.5Te_0.5 and In_0.5Co₄Sb_11.5Te_0.5 were prepared via hydrothermal synthesis at 180 °C for 48 h, then heat treated and finally hot pressed at 625 °C and 80 MPa for 1 h in vacuum to form bulk samples. The phase compositions of the samples were determined by X-ray diffraction. Hall Effect measurement of the samples was carried out at room temperature. The fracture surface of the samples was observed by field emission scanning electron microscopy. The electrical conductivity and the Seebeck coefficient of the samples were measured from room temperature to around 748 K. The In-filled and Te-doped CoSb₃ sample with longer time annealing before hot pressing had much better electrical transport properties with the highest power factor of 38.4 μWcm^?1 K^?2 around 573 K.     

Synthesis and high temperature transport properties of Te-doped skutterudite compounds

Te-doped skutterudite compounds Co₄Sb_12?x Te _x (x?=?0.4–0.7) have been fabricated by solid state reaction method and spark plasma sintering. The scanning electron microscope images indicate all samples are compact and the average particle size increases with the Te doping fraction. The carrier concentration and electrical conductivity exhibit positive doping fraction dependence, and a maximum electrical conductivity of 16.29?×?10⁴?Sm^?1 is obtained at 300?K for Co₄Sb_11.3Te_0.7. The values of the power factor (x?=?0.4–0.6) are greater than 4.0?×?10^?3?Wm^?1?K^?2 at the temperature range of 650–800?K, larger than previous literature reports. The lattice thermal conductivity decreases monotonously over the whole investigated temperature range and exhibits a negative doping fraction dependence except for Co₄Sb_11.3Te_0.7. The resultant dimensionless figure of merit of all the samples increases monotonously over the whole investigated temperature range, and a maximum value of 0.95 is achieved at 800?K for Co₄Sb_11.4Te_0.6.     

Effects of sintering temperature on the microstructure and thermoelectric properties of mesostructured Co<Subscript>4</Subscript>Sb<Subscript>11.5</Subscript>Te<Subscript>0.5</Subscript> skutterudites dispersed nano-TiN

The mesostructured skutterudites Co₄Sb_11.5Te_0.5?+?nano-TiN composites are prepared through ball milling and spark plasma sintering (SPS). The influence of the various SPS temperatures within the range of 813–933 K on the microstructure and thermoelectric properties are focus in this work. The average grain sizes of the skutterudites increase from ~?110 to ~?500 nm with the increasing SPS temperature, while the densities of composites decrease from 7.02 to 6.26 g cm^?3. Additionally, the phase of CoTe₂ is detected in the samples sintered at 903–933 K. With the SPS temperature increasing from 813 to 903 K, the electrical conductivity and thermal conductivity increase simultaneously, and then decrease when SPS temperature rises to 933 K. The absolute value of Seebeck coefficient shows no clear changes when SPS temperature is not higher than 873 K but then slightly decreases with the increasing of SPS temperature. At last, the optimum SPS temperature is determined as 873 K, the ZT value of 1.07 at 800 K for the sample SPSed at 873 K is obtained, which is 11.5% higher than that of the sample SPSed at 903 K.     

Effect of the cooling rate on the thermoelectric properties of p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.94Se0.06)3 after melting in the bismuth-telluride system

The p-type (Bi_0.25Sb_0.75)₂Te₃ and n-type Bi₂(Te_0.94Se_0.06)₃ ingots were prepared by cooling at various cooling rates C after melting so that they have an intermediate state between the polycrystalline and Bridgman ingots which lowers their thermal conductivity κ, where C was changed from 0.10 to 2375 K/min in an evacuated glass tube. When the ingots were cooled at C = 0.50 K/min under the uniaxial temperature gradient of 5 K/cm, it was observed that the c axis of some grains points to the freezing direction. The electrical resistivity ρ, Seebeck coefficient α and κ of ingots were measured at 298 K along the freezing direction, so that ρ and κ at C = 0.50 K/min were lower by 20-30% and 9% than those of the corresponding Bridgman ingots. The thermoelectric figure of merits ZT(=α²T/ρκ) estimated for the p- and n-type ingots then reached high values of 1.27 and 1.25 at 298 K, respectively.     

Thermoelectric properties of Zn4Sb3 prepared by hot pressing

Polycrystalline specimens of the thermoelectric material Zn₄Sb₃ were prepared by the hot-pressing method at various temperatures and pressures and their thermoelectric properties were evaluated in a temperature range from 298 K to 673 K. A single phase of Zn₄Sb₃ was obtained in the samples prepared at 673 K with a pressure above 150 MPa, whereas ZnSb was placed in the Zn₄Sb₃ matrix for the samples prepared at 100 MPa. The electrical transport properties of the single phase compound showed p-type conduction and metallic transport behavior based on the temperature dependence. The sample produced at 673 K under a pressure of 200 MPa exhibited the highest ZT value of 1.36 at 673 K. This study suggests that the dense and single-phase Zn₄Sb₃ compound is a route to achieve a high thermoelectric performance.     

Effects of growth-charge purity and perfection of Bi0.5Sb1.5Te3 single crystals on their thermoelectric properties

Bi_0.5Sb_1.5Te₃ crystals, undoped and doped with 4 mol % Bi₂Se₃, were grown by the floating-crucible technique, using starting materials of 99.9999 or 99.99% purity. The perfection of the crystals (mosaic blocks, second-phase inclusions, dislocation density) was correlated with growth conditions. The elemental composition of several samples was determined by electron probe x-ray microanalysis. The room-temperature electrical conductivity, thermal conductivity, thermoelectric power, carrier concentration, and effective mass and mobility of holes were determined. In the range from 100 to 400 K, the electrical conductivity, thermoelectric power, thermal conductivity, and thermoelectric figure of merit of the crystals were measured.