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.     

Thermoelectric properties of indium-filled skutterudites prepared by combining solvothermal synthesis and melting

Indium-filled skutterudites with nominal compositions of In _x Co₄Sb₁₂ (x=0,0.1,0.2,0.3) were prepared by combining solvothermal synthesis and melting. The bulk samples were characterized by X-ray diffraction and scanning electron microscopy, respectively. The Seebeck coefficient, electrical conductivity, and thermal conductivity were measured from room temperature up to ∼773 K. Hall effect measurements were performed at room temperature. The thermoelectric properties of the samples were significantly influenced by filling In into CoSb₃. The dimensionless thermoelectric figure of merit, ZT, increased with increasing temperature and reached a maximum value of ∼0.79 for In_0.1Co₄Sb₁₂ at 573 K.     

Sm填充skutterudite化合物中填充原子扰动效应研究

结合Rietveld结构解析和拉曼光谱对单相多晶的Sm原子填充的skutterudite化合物Sm_yFe_xCo_4-xSb₁₂进行了分析.Rietveld精确化结果表明：Sm_yFe_xCo_4-xSb₁₂化合物具有填充式skutterudite结构,Sm原 关键词： 方钴矿 Rietveld结构解析 拉曼散射 扰动     

Ga填充n型方钴矿化合物的结构及热电性能

用熔融退火结合放电等离子烧结(SPS)技术制备了具有不同Ga填充含量的Ga_xCo₄Sb₁₂方钴矿化合物,研究了不同Ga含量对其热电传输特性的影响规律. Rietveld结构解析表明,Ga占据晶体学2a空洞位置,Ga填充上限约为0.22,当Ga的名义组成x≤0.25时,样品的电导率、室温载流子浓度N_p随Ga含量的增加而增加,Seebeck系数随Ga含量的增加而减小. 室温下霍尔测试表明,每一个Ga授予框架0.9个电子,比Ga的氧化价态Ga³⁺小得多. 由于Ga离子半径相对较小,致使Ga填充方钴矿化合物的热导率κ及晶格热导率κ_L较其他元素填充的方钴矿化合物低. 当x＝0.22时对应的样品在300K时的热导率和晶格热导率分别为3.05Wm^-1·K^-1和 2.86Wm^-1·K^-1.在600K下Ga_0.22Co_4.0Sb_12.0样品晶格热导率达到最小,为1.83Wm^-1·K^-1,最大热电优值Z,在560K处达1.31×10^-3K^-1. 关键词： skutterudite化合物 Ga原子填充 结构 热电性能     

High‐temperature thermoelectric properties of p‐type skutterudites Ybx Co3FeSb12

Partially filled polycrystalline p‐type skutterudites of nominal compositions Yb_x Co₃FeSb₁₂ were synthesized and their thermoelectric properties characterized. The compositions and filling fractions were confirmed with a combination of Rietveld refinement and elemental analysis. The thermoelectric properties were evaluated from 300 K to 810 K. The Seebeck coefficient and resistivity increase while the thermal conductivity decreases with increasing Yb content. A maximum ZT value of 0.85 was obtained at 810 K. This work is part of a continuing effort to enhance the thermoelectric properties of p‐type skutterudites, as this class of materials continues to be of interest for thermoelectrics applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and impedance spectroscopy of Pr1? x Sr x Fe0.8Co0.2O3? δ ( x =0.1, 0.2, 0.3) thin films grown by laser ablation

Polycrystalline samples of Pr_1−x Sr _x Fe_0.8Co_0.2 O_3−δ (x=0.1, 0.2, 0.3) (PSFC) were prepared by the combustion synthesis route at 1200°C. The structure of the polycrystalline powders was analysed with X-ray powder diffraction data. The X-ray diffraction (XRD) patterns were indexed as the orthoferrite similar to that of PrFeO₃ having a single-phase orthorhombic perovskite structure (Pbnm). Pr_1−x Sr _x Fe_0.8Co_0.2O_3−δ (x=0.1, 0.2, 0.3) films have been deposited on yttria-stabilized zirconia (YSZ) single-crystal substrates at 700°C by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM). All films are polycrystalline with a marked texture and present pyramidal grains in the surface with different size distributions. Electrochemical impedance spectroscopy (EIS) measurements of PSFC/YSZ single crystal/PSFC test cells were conducted. The Pr_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ film at 850°C presents a lower area specific resistance (ASR) value, 1.65 Ω cm², followed by the Pr_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (2.29 Ω cm² at 850°C) and the Pr_0.9Sr_0.1Fe_0.8Co_0.2O_3−δ films (5.45 Ω cm² at 850°C).     

A tunable green alkaline-earth silicon-oxynitride solid solution (Ca1? x Sr x )Si2O2N2:Eu2+ and its application in LED

A series of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ (x=0.0–0.97, y=0.03) phosphors were synthesized by high-temperature solid-state reaction. The XRD patterns confirm the formation of a solid solution of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺. An intense tunable green light is observed with the increasing ratio of Sr/Ca. With an increase in x, the excitation and emission spectra show a redshift and blueshift, respectively, due to large centroid shift and small Stokes shift. The temperature dependent luminescence is also investigated in the temperature range of 77–450 K. The Huang–Rhys factor and the thermal-quenching temperature are determined. Intense green LEDs were successfully fabricated based on the (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ phosphor and near-ultraviolet (∼395 nm) GaN/blue (460 nm) InGaN chips. All the results indicate that the solid solution (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ is a promising phosphor applicable to near-UV and blue LEDs for solid-state lighting.     

In0.3Co4Sb12-xSex 方钴矿热电材料的制备和热电性能

用熔融退火结合放电等离子烧结法制备了In_0.3Co₄Sb_12-xSe_x(x=0—0.3)方钴矿热电材料,探讨了In的存在形式,系统研究了Se掺杂量对结构和热电性能的影响.结果表明:In可以填充到方钴矿二十面体空洞处,过量In在晶界处形成InSb第二相,Se对Sb的置换使晶格常数减小,In填充上限降低;In_0.3Co₄Sb_12-xSe_x样品呈n型传导,随着Se掺杂量的增大,载流子浓度降低,电导率下降,Seebeck系数增大,功率因子有所降低;由于在结构中引入了质量波动及晶格畸变,适量的Se掺杂可以大幅降低材料晶格热导率;样品In_0.3Co₄Sb₁₂和In_0.3Co₄Sb_11.95Se_0.05的最大ZT值均达到1.0以上. 关键词： 掺杂 填充式方钴矿 热电性能     

Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

An EIS study of La2 − x Sr x NiO4 + δ SOFC cathodes

La_2 − x Sr _x NiO_{4 +  δ} materials were investigated as cathodes for the electrochemical reduction of oxygen on a Ce_1.9Gd_0.1O_1.95 (CGO10) electrolyte using cone-shaped electrodes together with electrochemical impedance spectroscopy. The area-specific resistance (ASR) of the La_2 − x Sr _x NiO_{4 +  δ} nickelates towards the reduction of oxygen is equal to the ASR of perovskites; however, it is not as low as for the best Fe–Co-based perovskites. The lowest ASR is found for the compound La_1.75Sr_0.25NiO_{4 +  δ} with an ASR of 23.8 Ωcm² measured on a cone-shaped electrode in air at 600 °C. It is suggested that difference in oxide ionic conductivity of the nickelates is the main cause for the different activity of the nickelates towards the electrochemical reduction of oxygen.     

Physicochemical properties of LiAl x Mn2 − x O4 and LiAl0.05Mn1.95O4 − y F y cathode material by the citric acid-assisted sol–gel method

LiAl _x Mn_2 − x O₄ and LiAl_0.05Mn_1.95O_4 − y F _y spinel have been successfully synthesized by citric acid-assisted sol–gel method. The structure and physicochemical properties of this as-prepared powder were investigated by electronic conductivity test, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge test in detail. The electronic conductivity decreases with increasing of the content of doped Al. XRD patterns show that the diffraction of LiAl_0.05Mn_1.95O_4 − y F _y samples is similar, with all the peaks indexable in the Fd3m space group, and a little impurity appears in the LiAl_0.05Mn_1.95O_3.8F_0.2 sample. SEM reveals that all LiAl_0.05Mn_1.95O_4 − y F _y powders have the uniform, nearly cubic structure morphology with narrow size distribution which is less than 500 nm. Galvanostatic charge–discharge test indicates that LiAl_0.05Mn_1.95O₄ has the highest discharge capacity and electrochemical performance among all LiAl _x Mn_2 − x O₄ samples after 50 cycles, and the initial discharge capacity of LiAl_0.05Mn_1.95O_4 − y F _y (y = 0, 0.02, 0.05, 0.1) is 123.9, 124.6, 124.9, and 125.0 mAh g⁻¹, respectively, and their capacity retention ratios are 94.2%, 94.9%, 91.7%, and 89.9% after 50 cycles, respectively. EIS indicates that LiAl_0.05Mn_1.95O_3.98F_0.02 have smaller charge transfer resistance than that of LiAl_0.05Mn_1.95O₄ corresponding to the extraction of Li⁺ ions.     

Electrical and electrochemical behaviour of several LiFe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> solid solutions as cathode materials for lithium ion batteries

Several olivine phosphates were investigated in the last years as cathode materials for secondary lithium ion batteries. Among these compounds, LiFe _x Co_1 − x PO₄ solid solutions might be interesting candidates because they should combine the high potential value of Co³⁺/Co²⁺ (higher than 4.5 V vs Li⁺/Li) with the relatively high charge–discharge rate of LiFePO₄. Solid solutions were prepared by solid-state route and characterised by X-ray powder diffraction, cyclic voltammetry, impedance spectroscopy and the Hebb–Wagner method. The results show that also low amount of iron induces high electronic conductivity in the solid solutions.     

Effects of La–Zn substitution on microstructure and magnetic properties of strontium ferrite nanofibers

Sr_1−x La _x Zn _x Fe_12−x O₁₉/poly(vinylpyrrolidone) (PVP) (0.0≤x≤0.5) precursor nanofibers were prepared by the sol–gel assisted electrospinning method from starting reagents of metal salts and PVP. Subsequently, the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers with diameters of around 100 nm were obtained by calcination of the precursor at 800 to 1000°C for 2 h. The precursor and resultant Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometer and vibrating sample magnetometer. The grain sizes of Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers are in a nanoscale from 40 to 48 nm corresponding to the calcination temperature from 800 to 1000°C. With La–Zn substitution content increase from 0 to 0.5, the grain size and lattice constants for the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers obtained at 900°C show a steady reduction trend. With variations of the ferrite particle size arising from the La–Zn substitution, the nanofiber morphology changes from the necklace-like structure linking by single elongated plate-like particles to the structure building of multi-particles on the nanofiber cross-section. The specific saturation magnetization of Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers initially increases with the La–Zn content, reaching a maximum value 72 A m² kg⁻¹ at x=0.2, and then decreases with a further La–Zn content increase up to x=0.5, while the coercivity exhibits a continuous reduction from 413 (x=0) to 219 kA m⁻¹ (x=0.5). The mechanism for the La–Zn substitution and the nanofiber magnetic property are analyzed.     

Local structure and electrochemistry of LiNi y Mn y Co1 − 2y O2 electrode materials for Li-ion batteries

A series of LiNi _x Mn _y Co _z O₂ (x = y, z = 1 − 2y) oxides have been synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Layered LiNi _y Mn _y Co_1 − 2y O₂ materials with high homogeneity and crystallinity were synthesized using the wet-chemical method assisted by carboxylic acid as the polymeric agent. The long range and local structural properties are investigated with experiments including X-ray diffraction, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The evolution of the structure is discussed as a function of the cobalt content that confers layer-like behavior on the framework. Electrochemical performance of LiNi _y Mn _y Co_1 − 2y O₂ oxides is tested in cells using nonaqueous 1 M LiPF₆ dissolved in ethylene carbonate–diethyl carbonate. Charge–discharge profiles are investigated as a function of the rate capability and the voltage window. A relation is found between the gravimetric capacity and the cation disorder of the positive electrode as indicated by structural analysis. Fast lithium extraction attributed to the larger interslab space has been observed in the cobalt-rich oxides. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007.     

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     

Thermoelectric figure-of-merit in <Emphasis Type="Italic">p</Emphasis>-type bismuth- and antimony-chalcogenide-based solid solutions above room temperature

The specific features of the variation in the thermoelectric figure-of merit Z for p-type bismuth- and antimony-chalcogenide-based solid solutions p-(Bi,Sb)₂(Te,Se)₃ have been analyzed with allowance made for the data amassed in the investigation of thermoelectric and galvanomagnetic properties. It has been shown that, in the samples with optimum carrier concentrations, the increase in Z in the multicomponent p-Bi_{2 − x} Sb _x Te_{3 − y} Se _y composition (x = 1.3, y = 0.06) in the temperature range 300–370 K is mediated by the high carrier mobility and the low lattice thermal conductivity. The higher effective mass of the density of states and the larger slope of the temperature dependence of the mobility as compared to the other compositions bring about an increase in Z in the p-Bi_{2 − x} Sb _x Te₃ solid solution for x = 1.6 in the temperature range 370–550 K. The increase in the figure-of merit reached in the compositions under study stems also from the increasing contraction of constant-energy ellipsoids along the binary and bisector directions and from the change in the angle θ between the principal axes of the ellipsoids and the crystallographic axes.     

<Superscript>29</Superscript>Si MAS-NMR study of oxide conductors derived from the apatite-type La<Subscript>9.33</Subscript>Si<Subscript>6</Subscript>O<Subscript>26</Subscript> compound

The preparation of (La_9.33−2x/3Sr _x □_0.67−x/3)Si₆O₂₄O₂ (0 ≤ x ≤ 2) samples with different amounts of cation vacancies is reported. Structure and unit-cell parameters were deduced by Rietveld analysis of XRD patterns. Structural features that enhance oxygen conductivity in Sr-doped apatites are discussed. Up to three components were detected in ²⁹Si MAS-NMR spectra which change with the amount and distribution of cation vacancies. In general, oxygen conductivity increases with the amount of vacancies at La1 (6h) sites, passing through a maximum for x = 0.4. In the case of activation energy, a minimum is detected near x = 1.2, indicating that entropic and enthalpic change in different ways. The presence of cation vacancies should enhance oxygen hopping along c-axis; however, the analysis of the frequency dependence of conductivity suggests that oxygen motions are produced along three axes.     

Effect of the spin and valence states of cobalt ions on the kinetic properties of Ho1 ? xSrxCoO3 ? δ and Er1 ? xSrxCoO3 ? δ compounds

The electrical conductivity and Seebeck effect in ceramics based on cobaltites Ho_{1 − x} Sr _x CoO_{3 − δ} (x = 0.65, 0.75, 0.85, 0.95) and Er_{1 − x} Sr _x CoO_{3 − δ} (x = 0.75, 0.85, 0.95) with a perovskite-like structure have been investigated in the temperature range T > 77 K. All the compounds under study are characterized by the variable-range-hopping conductivity with the temperature dependence of the electrical resistivity corresponding to the Mott law. It has been found that, in the Ho_0.35Sr_0.65CoO_{3 − δ} compound, thermally excited Co³⁺ ions contribute to the electrical conductivity with an increase in temperature to 250 K. The Seebeck coefficient of the systems studied decreases as the strontium concentration and temperature increase. It has been shown that, for an adequate explanation of this behavior, proper allowance must be made for the splitting of the 3d levels, as well as for the charge disproportionation of the cobalt ions.     

Ionic and electronic transport in perovskite-type La(Ga,M)O3−δ (M=Mg, Cr, Fe, Co, Ni, Nb)

Oxygen ion conduction in La_0.9Sr_0.1Ga_1−xM_xO_3−δ (M=Cr, Fe; x=0 – 0.20), LaGa_1−xM_xO_3−δ (M=Co, Ni; x=0.20 – 0.60), LaGa_1−x−yCo_xMg_yO_3−δ (x=0.35 – 0.60; y=0.10 – 0.25) and LaGa_0.85−xMg_0.15(Nb_0.33Mg_0.66)_xO_3−δ (x=0 – 0.20) is reported. At temperatures below 1200 K the ionic conductivity of La(Ga,M)O_3−δ (M=Co, Ni) increases with increasing oxygen nonstoichiometry, but is lower than for La(Ga,Mg)O_3−δ and (La,Sr)GaO_3−δ. Co-doping with Nb and Mg was found to result in decreasing ionic transport in La(Ga,Nb,Mg)O_3−δ due to blocking of oxygen sites by Nb⁵⁺. Small additions of Fe to the B-site of La_0.9Sr_0.1GaO_3−δ increase the ionic conductivity, whereas substitution of Cr for Ga has the opposite effect. Incorporation of transition metal cations into the Ga site leads to a higher p-type electronic conductivity in all studied perovskites. Paper presented at the 6th Euroconference on Solid Sate Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Electrical conductivity and chemical stability of perovskite-type BaCe0.8-xTixY0.2O3-δ

Here we report the synthesis, chemical stability, and electrical conductivity of Ti-doped perovskite-type BaCe_0.8-x Ti _x Y_0.2O_3-δ (x = 0.05, 0.1, 0.2, and 0.3; BCTY). Samples were synthesized by conventional solid state (ceramic) reaction from corresponding metal salts and oxides at elevated temperature of 1,300–1,500 °C in air. The powder X-ray diffraction confirmed the formation of a simple cubic perovskite-type structure with a lattice constant of a = 4.374(1), 4.377(1), and 4.332(1) ? for x = 0.05, 0.1, and 0.2 members of BCTY, respectively. Like BaCe_0.8Y_0.2O_3-δ (BCY), Ti substituted BCTY was found to be chemically not stable in 100% CO₂ and form BaCO₃ at elevated temperature. The bulk electrical conductivity of BCTY decreased with increasing Ti content and the x = 0.05 member exhibited the highest conductivity of 2.3 × 10⁻³ S cm⁻¹ at 650 °C in air, while a slight increase in the conductivity, especially at low temperatures (below 600 °C), was observed in humidified atmospheres.