CoSb3基Skutterudite化合物拉曼光谱研究

Skutterudite材料因具有特殊的晶体结构成为当前最有前途的热电材料之一,为深入研究填充Skutterudite化合物的热传导机制,对化合物的声子振动模式及置换、填充原子对声子模的影响进行了研究.通过对CoSb3,Co3.5Fe0.5Sb12的偏振拉曼谱研究,确认了一个Ag模,且与理论计算吻合较好.对CoSb3,Co3.5Fe0.5Sb12和La0.6Co3.5Fe0.5Sb12拉曼谱的对比研究表明,单纯Fe置换对拉曼谱影响很小,而La部分填充则使部分拉曼峰明显展宽,分析认为,这一现象主要源于填充原子在晶格空隙中的扰动.     

Unique surface structure resulting in the excellent long-term thermal stability of Fe4Sb12-based filled skutterudites

The evolutions of the phase composition, microstructure, and thermoelectric properties of BaFe₄Sb₁₂ materials at 773 K in vacuum were investigated to evaluate their long-term thermal stabilities. It was observed that the interior of the BaFe₄Sb₁₂ exhibited excellent thermal stability, while the surface decomposed into FeSb₂, Sb, and Ba₅Sb₃. However, the decomposition rate sharply declined with increasing annealing time, and the thicknesses of the decomposition layer was approximately 49 μm after annealing for 30 days. Thermoelectric performances negligibly changed during the annealing process, and the dimensionless figure of merit remained in the range of 0.61 ± 0.02 at 800 K. The unique decomposition layer, wherein Ba₅Sb₃ compounds enriched in the outermost layer and along the FeSb₂ grain boundaries, served as a dense protective cover to effectively retard the Sb sublimation, resulting in excellent long-term thermal stability. This work provides a plausible explanation for the long-term thermal stability of Fe₄Sb₁₂-based filled skutterudites.     

First-principle calculations of elastic and electronic properties of the filled skutterudite CeFe4P12

A theoretical study of elastic and electronic properties of the filled skutterudite CeFe₄P₁₂ is presented, using the full-potential linear muffin–tin orbital (FP-LMTO) method. In this approach the local spin density approximation (LSDA) was used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus, its pressure derivative and elastic constants. Our calculations performed for band structure and density of state show that this compound is an indirect band gap material (Γ–N). The results are compared with previous calculations and experimental data.     

Effects of Tl-filling into the voids and Rh substitution for Co on the thermoelectric properties of CoSb3

In order to enhance the thermoelectric (TE) properties of CoSb₃, we tried to reduce the lattice thermal conductivity (κ_lat) by filling Tl into the voids and substitution of Rh for Co. We prepared polycrystalline samples of Tl_x(Co_1−yRh_y)₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20 and y = 0.1, 0.2) and examined their TE properties from room temperature to 750 K. All the samples indicated negative values of the Seebeck coefficient (S). Both the electrical resistivity and the absolute values of the S decreased with increasing the Tl-filling ratio. The Tl-filling and Rh substitution reduced the κ_lat, due to the rattling and the alloy scattering effects. The minimum value of the κ_lat was 1.54 W m⁻¹ K⁻¹ at 550 K obtained for Tl_0.20(Co_0.8Rh_0.2)₄Sb₁₂. Tl_0.20(Co_0.8Rh_0.2)₄Sb₁₂ exhibited the best TE performance; the maximum value for the dimensionless figure of merit ZT was 0.58 at around 600 K.     

Rapid preparation of high-performance S0.4Co4Sb11.2Te0.8 skutterudites with a highly porous structure

In this study, S_0.4Co₄Sb_11.2Te_0.8 skutterudites with a highly porous structure inside grains are prepared by a one-step hot-pressing (OS-HP) method. The effect of the pressure relief treatment at the heating stage on the micro-morphology and thermoelectric properties of materials is investigated. When the temperature corresponding to the pressure relief treatment is less than 723 K, the grain size dramatically increases from ～1 to ～50 μm, and a large number of pores are distributed inside these large grains. Compared with those samples prepared by the conventional method, the thermal conductivity of samples prepared by the pressure relief treatment is significantly reduced due to the high porosity. The ZT values of samples prepared by the pressure relief treatment are greater than 1.6 at 825 K. This newly developed OS-HP method can be employed for the rapid fabrication of highly porous structured skutterudites with low-melting-point compositions as well as of other material systems.     

Enhanced thermoelectric properties of binary CoSb3 by embedding FeCl3-intercalated graphene nanosheets

Filling the icosahedral lattice holes of the cage-compound CoSb₃ with alkali, alkaline-earth and rare-earth metal atoms can endow CoSb₃ with the transport characteristic of "phonon-glass-electron-crystal". However, the filler atoms greatly increase the cost in raw material as well as fabrication complexity. In this work, we embedded FeCl₃-intercalated graphene nanolayers into polycrystalline CoSb₃ by a solution-dispersion method combined with spark plasma sintering technology. The graphene nanosheets were mainly distributed in grain boundaries. Due to the increased carrier concentration and mobility, the electrical conductivity was improved. Meanwhile, due to the enhanced interfacial phonon scattering, the lattice thermal conductivity was effectively suppressed. The thermoelectric figure of merit of the nanocomposite reached 0.6 at a mass content of 0.2 % FeCl₃-intercalated graphene nanolayers, outperforming all reported values of binary CoSb₃ based nanocomposites, and even comparable to that of commercially used filled p-type Fe₃CoSb₁₂. This work provides a promising strategy for low-cost fabrication of p-type skutterudites.     

Electronic and Magnetic Properties in the Novel Skutterudite ThPt4Ge12

Theoretical calculations on ThPt₄Ge₁₂ filled skutterudite were performed. The calculated energy bands yielded indication of metallic behavior, while the projected density of states provided indication of a hybridization formed by Th f-, Pt d- and Ge p-states. The absence of a detected minigap provides support that this material is not a good thermoelectric material. Furthermore, Crystal Orbital Overlap Population yielded indication of the absence of a ferromagnetic instability.      

Thermoelectric Properties of Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Skutterudite Materials with Partial In Filling and Excess In Additions

The properties of Co₄Sb₁₂ with various In additions were studied. X-ray diffraction revealed the presence of the pure δ-phase of In_0.16Co₄Sb₁₂, whereas impurity phases (γ-CoSb₂ and InSb) appeared for x = 0.25, 0.40, 0.80, and 1.20. The homogeneity and morphology of the samples were observed by Seebeck microprobe and scanning electron microscopy, respectively. All the quenched ingots from which the studied samples were cut were inhomogeneous in the axial direction. The temperature dependence of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) was measured from room temperature up to 673 K. The Seebeck coefficient of all In-added Co₄Sb₁₂ materials was negative. When the filler concentration increases, the Seebeck coefficient decreases. The samples with In additions above the filling limit (x = 0.22) show an even lower Seebeck coefficient due to the formation of secondary phases: InSb and CoSb₂. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples decreases with temperature. The central region of the In_0.4Co₄Sb₁₂ ingot shows the lowest thermal conductivity, probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine-dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT (S ² σT/κ) value of 0.96 at 673 K has been obtained with an In addition above the filling limit.     

乙醇sol-gel法制备二元Skutterudite化合物纳米粉体

首次采用乙醇sol-gel法制备二元Skutterudite化合物CoSb3纳米粉体.以CoCl2·6H2O和SbCl3为初始原料,乙醇为溶剂,柠檬酸为络合剂,经过溶胶-凝胶过程,将干燥凝胶研磨成粉,然后置于管式气氛炉中进行还原热处理.研究结果表明,当Sb/Co和C6H8O7/Co的摩尔配比适当,且还原气氛为H2,温度为300～600℃,最终可得到平均粒径尺寸在30nm左右的CoSb3纳米粉体.     

S0.05Co4Sb11.6Te0.4 skutterudite introduced into graphene at high pressure and high temperature and its thermoelectric performance enhancement

A series of graphene-S_0.05Co₄Sb_11.6Te_0.4 (C_x-S_0.05Co₄Sb_11.6Te_0.4) composite polycrystalline skutterudite materials with graphene stoichiometric ratio of x = 0, 0.05, 0.10, 0.20 have been successfully prepared by high pressure and high temperature (HPHT) technology. Graphene is a two-dimensional material with large carrier mobility and large specific surface area. Through microscopic observation, it is found that graphene is attached to the grains in the sample. At the same time, with the increase of graphene content, grain growth is inhibited. Graphene addition reduces the thermal conductivity of skutterudite by increasing grain boundaries and achieves the purpose of optimizing its thermoelectric properties. At the same time, there are numerous lattice defects and distortions introduced in skutterudite synthesized by HPHT technology. Finally, the samples were synthesized under the conditions of 1.5 GPa and 900 K. The lattice thermal conductivity of the graphene composite sample C_x-S_0.05Co₄Sb_11.6Te_0.4 with x = 0.10 reaches a minimum of 0.99 at 773 K, and the zT value of this sample is 1.25 at 773 K, which is greater than pure S_0.05Co₄Sb_11.6Te_0.4, its zT value is 1.00 at 773 K. Compared with the method of synthesizing skutterudite under normal pressure, the HPHT technology can dramatically reduce the reaction time from several days to less than 30 min, while forming a high-pressure airtight reaction environment, which can effectively prevent the volatilization and oxidation of samples during the reaction process, thus providing a convenient method for synthesizing thermoelectric materials quickly and efficiently.