Grain-Oriented Ca3Co4O9 Thermoelectric Oxide Ceramics Prepared by Solid-State Reaction

We studied a method to enhance the degree of grain orientation of Ca₃Co₄O₉ thermoelectric oxide ceramics. Ceramic specimens were prepared by solid-state reaction with different growth conditions. Large-grained Ca₃Co₄O₉ powders were obtained by using “heavy-calcination” and “moderate-grinding” steps before pelletizing, and these large-grained powders contributed to the enhancement of the degree of orientation. Scanning electron microscopy (SEM) observation results showed that plate-like crystal grains were stacked up in layers for the heavily calcined ceramics, while no such anisotropic structure was found for those that were lightly calcined. x-Ray diffraction (XRD) analysis also indicated that the specimen obtained by heavy-calcination and moderate-grinding steps had a high degree of (002) orientation. The effect of the heavy-calcination and moderate-grinding steps was clearly evidenced by the electrical resistivity ρ. The electrical resistivity ρ at 700°C for the higher-oriented ceramics was 73% of that for the lower-oriented ceramics. Since ρ was reduced without deterioration of the Seebeck coefficient S, the power factor (S ²/ρ) at 700°C for the former was increased by 29% compared with that for the latter.     

Microstructures and Thermoelectric Properties of Sintered Misfit-Layered Cobalt Oxide

Misfit-layered cobalt oxide Ca₃Co₄O₉ is considered to be a prospective material for thermoelectric conversion. The thermoelectric properties are anisotropic owing to its anisotropic crystal structure. The crystal has preferred thermoelectric properties along the a–b plane. Therefore, the thermoelectric properties are improved and controlled by the degree of orientation of the sintered sample. In the present work, Sr-doped misfit cobalt oxide Ca_2.7Sr_0.3Co₄O₉ was prepared by solid-phase reaction, followed by uniaxial compression molding and sintering at 1173 K. The Seebeck coefficient α, electrical resistivity ρ, and dimensionless figure of merit ZT were measured as a function of the compression pressure applied in the uniaxial molding. α, ρ, and ZT as functions of the degree of orientation and the relative density are experimentally clarified and explained by calculations using the compound model.     

Ca_3Co_4O_9热电材料的研究进展

由于Ca_3Co_4O_9具有良好的化学稳定性及廉价、无毒等优点,已成为氧化物热电材料研究的热点。但其合成难,致密度低,转换效率低等严重阻碍了Ca_3Co_4O_9的实际应用。该文从合成方法、改性手段及烧结工艺等3方面,归纳和分析了层状结构Ca_3Co_4O_9的研究进展,并对Ca_3Co_4O_9今后的研究发展提出一些建议。     

Thermoelectric Properties of Ca3Co4O9/[Ca2(Co0.65Cu0.35)2O4]0.624CoO2 Composites

Ca₃Co₄O₉ is one of the most promising p-type thermoelectric materials because of its high dimensionless figure of merit ZT. However, polycrystalline Ca₃Co₄O₉ ceramics shows lower ZT value than that for single crystal Ca₃Co₄O₉ due to its higher electrical resistivity ρ. Mikami et al. have reported that the addition of Ag to Ca₃Co₄O₉ ceramics could successfully reduce ρ and enhance the power factor. On the other hand, Ohtaki et al. reported that a composite structure could be highly effective to reduce κ for ZnO dually doped with Al and Ga. In this work, we tried to enhance the power factor and reduce κ by forming Ca₃Co₄O₉/[Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624CoO₂ composite structure. As a result, the ZT value for Ca₃Co₄O₉/[Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624CoO₂ composites reached 0.164 at 700 °C, which was 40 % higher than the value for Ca₃Co₄O₉.     

热电Ca3Co4O9薄膜的一致取向生长及其激光感生电压效应

利用脉冲激光沉积(PLD)法,在蓝宝石(0001)平衬底上成功制备了c轴一致取向的Ca3Co4O9薄膜。利用X射线衍射(XRD)分析测定了薄膜的相结构和生长取向。研究了不同衬底温度与不同原位氧压对Ca3Co4O9薄膜结晶质量与生长取向的影响,确定了最佳生长条件。利用这一条件在倾斜Al2O3(0001)衬底上制备了Ca3Co4O9薄膜。研究发现,当Ca3Co4O9薄膜被波长248nm,脉冲宽度20ns的脉冲激光照射时,在薄膜两端存在较大的激光感生热电电压(LITV)信号,峰值电压达到4.4V,其上升沿为36ns,半峰全宽(FWHM)为131ns。可以认为这种激光感生热电电压信号是由于Ca3Co4O9薄膜面内与面间泽贝克(Seebeck)系数张量的各向异性引起的。     

Enhancing the Thermoelectric Properties of Ca3Co4O9 Thin Films by the Addition of a Nanoscale NbN x Second Phase

High-quality Ca₃Co₄O₉ (CCO) thin films have been epitaxially grown on c-cut Al₂O₃ single crystal substrates using pulsed laser deposition (PLD). Different doses of Nb ions were injected into the films using an ion beam injection technique, and a nanoscale NbN _x second phase was generated in the films after annealing in pure N₂. The resistivity and Seebeck coefficient of the films were measured in the temperature range 175–375 K. The results demonstrated that the power factor of the films increases when injected with appropriate quantities of Nb. When the injected Nb concentration was 1.46 × 10²⁰/cm³, the power factor of the film reached 0.17 mW/m K² at room temperature, which is nearly twice as large as that for pure CCO film. A maximum value of 0.22 mW/m K² was obtained at 375 K.     

A Power-Generation Test for Oxide-Based Thermoelectric Modules Using p-Type Ca3Co4O9 and n-Type Ca0.9Nd0.1MnO3 Legs

Metal oxides are considered to be promising thermoelectric (TE) materials, especially for high-temperature power-generation applications, because they have many advantages such as low price, light weight, thermal stability, nontoxicity, and high oxidation resistance. For these reasons, oxide-based TE modules were fabricated using p-type pure Ca₃Co₄O₉ and n-type Ca_0.9Nd_0.1MnO₃ legs for power generation at temperatures in excess of 1000?K. This study involved the use of Ag sheets with a Ag paste as electrode materials and alumina plates as a substrate for the modules. The p-type pure Ca₃Co₄O₉ legs were manufactured by spark plasma sintering, and the n-type Ca_0.9Nd_0.1MnO₃ legs were sintered by a conventional process at atmospheric pressure. From a unicouple, a power density as high as 93.2?mW/cm² under a temperature condition of ??T?=?727?K (T _hot?=?1175?K) was obtained. This high power density is believed to be a result of the modified contact of the electrode (notch process) and the optimized material properties (the SPS process and a dopant effect) along with the high ??T obtained in this study (reduced thermal losses because of good packing of thermal insulation). Areas of concern for future research include the following: (1) the measured open-circuit voltage from the present unicouples was only 94.3% of the theoretical voltage, and (2) the internal resistance value was as high as 490% of the theoretical resistance.     

Ca_3Co_4O_9热电材料前驱体制备及性能表征

分别采用固相反应法和空间俘获法制备了Ca3Co4O9前驱体,并经冷等静压后常压烧结工艺制备了块体陶瓷样品。利用TG-DSC、XRD、SEM等方法对前驱体进行了表征,同时测试了陶瓷样品的热电性能。结果表明:空间俘获法可获得纯相、晶粒较小的前驱粉体,同时降低了合成温度,而且合成样品的Seebeck系数和功率因子明显高于固相反应法。在976 K获得最大Seebeck系数和功率因子分别为179μV/K、7.5×10–7 W/(cm·K2),较固相反应优化60%、16倍;电阻率略有升高。     

Impact of Dynamic Interlayer Interactions on Thermal Conductivity of Ca3Co4O9

The phonon thermal conductivity of misfit-layered Ca₃Co₄O₉ has been calculated by perturbed molecular dynamics using a classical force field. Detailed numerical analyses reveal that, in spite of its smaller cross-sectional area, the CoO₂ layer transports more heat than the thicker rock salt (RS) layer, although its local thermal conduction is more suppressed than in another layered cobaltite, Na _x CoO₂. The origins of these differences have been elucidated through careful examination of the atomic arrangements in each layer. Since thermal conduction in the RS layer can be reduced without deteriorating electronic properties for which the CoO₂ layer is responsible, it is suggested that the RS layer should be modified to further suppress the overall in-plane thermal conductivity. Computational experiments with increasing number of Ca–O planes in the RS layer showed the opposite trend to what can be predicted based on the misfit between two dissimilar layers. Further analyses to reveal the origin of these unexpected results provide yet another strategy to further decrease the thermal conductivity, namely to control the dynamic interference between atoms across the interface between two layers.     

铜掺杂量对钴酸钙热电性能的影响研究

采用sol-gel法制备了掺Cu的钴酸钙(Ca3Co4O9)热电材料,研究了Cu掺杂量对其物相、电导率σ、Seebeck系数S和功率因子P的影响。结果表明:随着Cu掺杂量的增加,试样中Ca3Co4O9的含量下降,但试样的电导率增加;试样的Seebeck系数和功率因子先增加后下降。试样(Ca0.90Cu0.10)3Co4O9在973 K时的功率因子最大,为15.3×10–4 W.m–1.K–2。     

Microstructures and Thermoelectric Properties of Melt-Spun Zn x Sb3 Ribbons

Melt-spun Zn _x Sb₃ ribbons were fabricated with weight compositions of x = 3.6, 3.9, and 4.2 through a single-wheel process and were annealed for 2 h at 673 K. The microstructures of the ribbons were investigated using transmission electron microscopy, together with energy-dispersive x-ray analysis. The main structure consisted of β-Zn₄Sb₃ phase, which mainly coexisted with ZnSb phase for x < 4 and with Zn phase for x > 4. The analyzed composition of the β-Zn₄Sb₃ phase deviated from the stoichiometric composition of 4:3 for all the ribbons. Nanosized voids and zinc inclusions were randomly distributed throughout the β-Zn₄Sb₃ phase. The thermoelectric characteristics of the ribbons were revealed by measuring the Seebeck coefficient, electrical conductivity, power factor, dimensionless figure of merit, and thermal conductivity. The power factor and dimensionless figure of merit increase with increasing x and temperature because either the electrical conductivity or Seebeck coefficient increases.     

Thermoelectric Properties of Sb2Te3-Based Nanocomposites with Graphite

Semiconductors - Antimony-telluride-based nanocomposite samples containing different weight fractions of graphite (Sb2Te3 + x% graphite, where x = 0.0, 0.5, 1.0, and 2.5%) are synthesized and...     

Thermoelectric Properties of Sintered n-Type and p-Type Tellurides

Characterization of powder-metallurgically manufactured (Bi _x Sb_1?x )₂(Te _y Se_1?y )₃ thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type (x = 0.2, y = 1) as well as n-type (x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed.     

High-resolution transmission electron microscopy study of Ca(3)Co(4)O(9)

The crystal structure of Ca(3)Co(4)O(9) was investigated using high-resolution transmission electron microscopy (HRTEM) and the image-simulation method. The c(*) was 10.8A and the b parameters were 4.56A for the Ca(2)CoO(3) block and 2.82A for the CoO(2) sheet. The [110] zone axis HRTEM images confirmed that Ca(3)Co(4)O(9) has a modulated layered structure with modulation. For the first time, the atomic positions of the Ca and Co atoms in the Ca(2)CoO(3) block were identified, corresponding to three rows of dark spots in the [110] direction. The observed HRTEM images for Ca(2)CoO(3) agreed well with the calculated images based on the structural model obtained by the Rietveld refinement method.     

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi₂Te₃-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi₂Te₃ nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix, yielding transport in the basal plane. Polycrystalline, textured Sb₂Te₃ and Bi₂Te₃ thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb₂Te₃ films revealed low charge carrier density of 2.6?×?10¹⁹?cm^?3, large thermopower of 130???V?K^?1, and large charge carrier mobility of 402?cm²?V^?1?s^?1. Bi₂(Te_0.91Se_0.09)₃ and (Bi_0.26Sb_0.74)₂Te₃ nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50?nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab?initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi₂Te₃ nanomaterials are discussed.     

Thermoelectric Properties of Indium-Added Skutterudites In x Co4Sb12

The high-temperature thermoelectric properties of In _x Co₄Sb₁₂ (0.05 ≤ x ≤ 0.40) skutterudite compounds were investigated in this study. The phase states of the samples were identified by x-ray diffraction analysis and field-emission scanning electron microscopy at room temperature. InSb and CoSb₂ were found as secondary phases in samples with x = 0.10 to 0.40. The filling limit of In into the CoSb₃ cages of In _x Co₄Sb₁₂ was in the range 0.05 < x < 0.10. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the In _x Co₄Sb₁₂ samples were measured from room temperature to 773 K. The Seebeck coefficient of all samples was negative. Reduction of the thermal conductivity by In addition resulted in a high thermoelectric figure of merit (ZT) of 0.67 for In_0.35Co₄Sb₁₂ at 600 K.     

Phase Stability and Thermoelectric Properties of CuFeS2-Based Magnetic Semiconductor

Recently, based on measurement results below 400 K, we suggested that chalcopyrite CuFeS₂-based alloys hold promise as thermoelectric materials. In this study, we have investigated the phase stability of such compounds and measured their thermoelectric properties at temperatures above 400 K. Thermogravimetric data indicate that the samples synthesized by a spark plasma sintering method were stable up to 700 K, above which sulfur deficiency becomes prominent. The electrical resistivity of the electron-doped samples showed metallic behavior up to 700 K. The Seebeck coefficients show large negative values of about ?300 μV/K above 400 K. As a result, the power factor of Cu_0.97Fe_1.03S₂ is ~1 mW/K²m in the temperature range of 400 K to 600 K.     

Thermoelectric Properties and Conduction Mechanism of CaCu3Ti4O12 Ceramics at High Temperatures

The Seebeck coefficient and electrical conductivity of CaCu₃Ti₄O₁₂ (CCTO) ceramics were measured and analyzed in the high temperature range of 300°C to 800°C, and then the electrical conduction mechanism was investigated by using a combination of experimental data fitting and first-principles calculations. The Seebeck coefficient of the CCTO ceramic sintered at 1050°C is negative with largest absolute value of ～650 μV/K at 300°C, and the electrical conductivity is 2–3 orders greater than the value reported previously by other researchers. With increasing sintering temperature, the Seebeck coefficient decreases while the electrical conductivity increases. The temperature dependence of the electrical conductivity follows the rule of adiabatic hopping conduction of small polarons. The calculated density of states of CCTO indicates that the conduction band is mainly contributed by the antibonding states of Cu 3d electrons, therefore small-polaron hopping between CuO₄ square planar clusters was proposed. Possible ways to further improve the thermoelectric properties of CCTO are also discussed.