There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Sodium layered oxides NaxCoO2 form one of the most fascinating low-dimensional and strongly correlated systems; in particular P2–NaxCoO2 exhibits various single-phase domains with different Na+/vacancy patterns depending on the sodium concentration. Here we used sodium batteries to clearly depict the P2–NaxCoO2 phase diagram for x≥0.50. By coupling the electrochemical process with an in situ X-ray diffraction experiment, we identified the succession of single-phase or two-phase domains appearing on sodium intercalation with a rather good accuracy compared with previous studies. We reported new single-phase domains and we underlined the thermal instability of some ordered phases from an electrochemical study at various temperatures. As each phase is characterized by the position of its Fermi level versus the Na+/Na couple, we showed that the synthesis of each material, even in large amounts, can be carried out electrochemically. The physical properties of the as-prepared Na1/2CoO2 and Na2/3CoO2 ordered phases were characterized and compared. Electrochemical processes are confirmed to be an accurate route to precisely investigate in a continuous way such a complex system and provide a new way to synthesize materials with a very narrow existence range.

Electrochemical investigation of the P2–NaxCoO2 phase diagram.

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.

The panoscopic approach to high performance thermoelectrics

Thermoelectric effects enable direct conversion between thermal and electrical energy and provide an alternative route for power generation and refrigeration. Over the past ten years, the exploration of high-performance thermoelectric materials has attracted great attention from both an academic research perspective and with a view to industrial applications. This review summarizes the progress that has been made in recent years in developing thermoelectric materials with a high dimensionless figure of merits (ZT) and the related fabrication processes for producing nanostuctured materials. The challenge to develop thermoelectric materials with superior performance is to tailor the interconnected thermoelectric physical parameters — electrical conductivity, Seebeck coefficient and thermal conductivity — for a crystalline system. Nanostructures provide a chance to disconnect the linkage between thermal and electrical transport by introducing some new scattering mechanisms. Recent improvements in thermoelectric efficiency appear to be dominated by efforts to reduce the lattice thermal conductivity through nanostructural design. The materials focused in this review include Bi–Te alloys, skutterudite compounds, Ag–Pb–Sb–Te quaternary systems, half-Heusler compounds and some high-ZT oxides. Possible future strategies for developing thermoelectric materials are also discussed.

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High-performance nanostructured thermoelectric materials

Metal oxides (Ca3Co4O9, CaMnO3, SrTiO3, In2O3), Ti sulfides, and Mn silicides are promising thermoelectric (TE) material candidates for cascade-type modules that are usable in a temperature range of 300–1200 K in air. In this paper, we review previous studies in the field of TE materials development and make recommendations for each material regarding future research. Furthermore, the R&D of TE modules composed of metal oxide materials and the prospect of their commercialization for energy harvesting is demonstrated.

Thermoelectric Ceramics for Energy Harvesting

Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614[CoO2], also known as Ca3Co4O9, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µVK-1, resistivity of ρ= 15 mΩcm and thermal conductivity of κ= 9.8 mWK-1cm-1. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2, which is comparable to the value reported for a polycrystalline sample of NaCo2O4, indicating that the title compound is a potential candidate for a thermoelectric material.

https://iopscience.iop.org/article/10.1143/JJAP.39.L531/pdf

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a super...

Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2

Preparation of Bi2Te3 films by electrodeposition

The crystal structure of a polycrystalline sample of higher manganese silicide (HMS) has been determined by means of the $(3+1)$-dimensional superspace group approach. The structural parameters were refined with a superspace group of $I{4}_{1}/amd(00\ensuremath{\gamma})00ss$ using powder neutron-diffraction data collected at 295 K. The compound belongs to a composite crystal family consisting of [Mn] and [Si] subsystems, with an irrational $c$-axis ratio (misfit parameter) of $\ensuremath{\gamma}={c}_{\text{Mn}}/{c}_{\text{Si}}\ensuremath{\sim}1.74$. Significant in-plane rotational modulation was revealed in the ``chimney''-[Si] subsystem, while positional modulation in the ``ladder''-[Mn] subsystem was only realized along ${c}_{\text{Mn}}$. The electronic structure of the sample was calculated on the basis of a commensurate approximation of the modulated structure using the full potential linearized augmented plane-wave method. The obtained band gap of ${E}_{g}\ensuremath{\sim}0.6\text{ }\text{eV}$ agreed well with the experimentally observed one. It appears that the band gap and density of states of the HMS samples depend on the positional modulation of the Si atoms. The various controversial formulas (for example, ${\text{Mn}}_{4}{\text{Si}}_{7}$, ${\text{Mn}}_{11}{\text{Si}}_{19}$, and so on) of the HMS phases reported thus far can be regarded as commensurate cases of a series of incommensurate ${\text{MnSi}}_{\ensuremath{\gamma}}$ phases in which the $\ensuremath{\gamma}$ value ranges from $\ensuremath{\sim}1.70$ to 1.75.

Yuzuru Miyazaki

Papers

Thermoelectric Ceramics for Energy Harvesting

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2

Preparation of Bi2Te3 films by electrodeposition

Modulated crystal structure of chimney-ladder higher manganese silicides MnSi γ (γ∼1.74)