Bi2Sr2Co2Ox (BSC-222) bulk materials have been prepared in air by single hot pressing or partial melting followed by hot pressing. Thermal transport properties of as-prepared samples were compared with BSC-222 samples prepared by single partial melting. Samples prepared through hot pressing have a high bulk density and improved preferential grain orientation. Bulk density and preferential grain orientation are significantly better in samples processed by partial melting followed by hot pressing. In samples prepared by partial melting, the presence of sub-micrometer-sized secondary phases Bi0.75Sr0.25Ox, Sr-based oxide, and CoO, is observed. The lattice contribution to the total thermal conductivity decreases significantly in partially melted BSC-222 samples. By using a classical phonon transport model, this result demonstrates that the decrease in lattice thermal conductivity in partially melted BSC-222 samples may be linked not only to porosity but also to the presence of defects, induced by partial melting process, which may play an important role in phonon scattering.

Thermoelectric generation is one of the strongest candidates for recovering the waste heat from industry and transportation. Some of oxides and silicides are considered to be promising thermoelectric materials because of their high oxidation resistance. Several types of modules using p-type Ca3Co4O9/n-type CaMnO3 and p-type MnSi1.75/n-type Mn3Si4Al2 have been prepared and shown around 4 kW/m2 of maximum power density. The present study described the challenging enhancement of the thermoelectric figure of merit ZT of both oxide and silicide compounds. Introduction of secondary phases and low bulk density using a partial melting method is found to be effective for reducing phonon thermal conductivity in the promising Bi2Sr2Co2Ox. The grain size and distribution of the secondary phases can be controlled by optimizing the parameters of the partial melting method. On the other hand, detailed crystallographic structure of a new n-type Mn3Si4Al2 is clarified and leads to the enhancement of the ZT values by elemental substitution.

The thermoelectric properties of Bi2Sr2Co2Ox (BSC-222) bulk materials prepared by three different processing methods, i.e., conventional sintering, hot pressing, or partial melting, were investigated and compared. The electrical current, temperature difference for Seebeck coefficient, and thermal diffusion were measured in the same direction. The hot pressing and partial melting are effective processing methods for improving the electrical transport property in BSC-222 bulk materials due to an improvement of density in hot-pressed samples or by grain growth during partial melting process. For partially melted samples, a decrease in the thermal conductivity is also observed. The highest dimensionless thermoelectric figure of merit (ZT) values have been obtained in the sample prepared by partial melting method, for which ZT has been increased by a factor of 2.7 by comparison with bulk materials prepared by conventional sintering. At 700 °C in air, ZT value reaches 0.27 for partially melted Bi2Sr2Co2Ox bulk materials. This study shows that optimized electrical and thermal transport properties can be achieved in BSC-222 bulk materials possessing microstructures with both large average grain size and appropriate bulk density.

In order to achieve waste heat recovery using thermoelectric systems, thermoelectric materials showing high conversion efficiency over wide temperature range and high resistance against oxidation are indispensable. A silicide material with good n-type thermoelectric properties and oxidation resistance has been discovered. The composition and crystal structure of the silicide are found out Mn3Si4Al2 (abbreviated as 342 phase) and hexagonal CrSi2 structure, respectively. Element substitution of Mn with 3d transition metals is succeeded. Enhancement of Seebeck coefficient is observed in a Cr-substituted sample. The maximum dimensionless thermoelectric figure of merit ZT is 0.3 at 573 K in air for the Mn2.7Cr0.3Si4Al2 sample. Electrical resistivity of the Mn3Si4Al2 bulk sample holds constant value for 48 h at 873 K in air. This is due to formation of oxide passive layer on the surface of the bulk sample. The 342 phase is a promising n-type material with a good oxidation resistance in the middle temperature range of 500-800 K.

Most state-of-the-art thermoelectric (TE) materials contain heavy elements Bi, Pb, Sb, or Te and exhibit maximum figure of merit, ZT∼1–2. On the other hand, oxides were believed to make poor TEs because of the low carrier mobility and high lattice thermal conductivity. That is why the discoveries of good p-type TE properties in layered cobaltites NaxCoO2, Ca4Co3O9, and Bi2Sr2Co2O9, and promising n-type TE properties in CaMnO3- and SrTiO3-based perovskites and doped ZnO, broke new ground in thermoelectrics study. The past two decades have witnessed more than an order of magnitude enhancement in ZT of oxides. In this article, we briefly review the challenges, progress, and outlook of oxide TE materials in their different forms (bulk, epitaxial film, superlattice, and nanocomposites), with a greater focus on the nanostructuring approach and the late development of the oxide-based TE module.

We report magnetic properties of [Ca2CoO3-δ]0.62CoO2 (Ca349) powders with various average size and the Bi- and Sr-doping effects on thermoelectric properties for the magnetically grain-aligned and densified Ca349 thick films. Magnetic anisotropy at 300 K depended on the initial average size of Ca349 powders and decreased with the decrease in the size. This presumably suggests that distortion of crystal structure was induced by a ball-milling process and led to the change of magnetic anisotropy. On the Bi- and Sr-doping effects, an obvious enhancement of thermoelectric properties did not emerge in the case of the Sr-doping, whereas the enhancement was observed for the Bi-doped Ca349 thick films. However, a drastic decrease of magnetic anisotropy was caused by the Bi-doping. For usage of the p-type layer in multilayered thermoelectric module, tuning of the Bi-doping levels in which both enhancement of thermoelectric properties and a certain level of magnetic anisotropy are achieved is required.

The Ca0.9Yb0.1MnO3/Ag composites(the ratio of Ag to Ca0.9Yb0.1MnO3 was 0, 4.7, 9.4, and 18.8 wt %) were prepared by wet milling various amounts of Ca0.9Yb0.1MnO3 and Ag2O powder mixtures followed by sintering in order to improve the mechanical properties of Ca0.9Yb0.1MnO3 for n-type legs of thermoelectric oxide devices. The obtained composites consisted of two phases such as Ca0.9Yb0.1MnO3 and metallic silver from the X-ray diffraction (XRD) analysis. The scanning electron microscope (SEM) analysis indicated that the Ag particles, the size of which was within 5 μm, were homogeneously dispersed in Ca0.9Yb0.1MnO3 matrix for all the composites. The σf of 18.8 wt% Ag composite became 251 MPa, which was 2 times larger value than that of Ca0.9Yb0.1MnO3. The power factor (S2ρ) was slightly improved by the addition of silver particles. The maximum S2ρ, i.e. 0.26 mWm-1K-2 at 573 K was obtained for 18.8 wt% Ag composite.

We report thermoelectric (TE) properties of mono-layer thick films of p-type [Ca2CoO3-δ]0.62CoO2 (Ca349) and n-type compounds of Ca0.9La0.1MnO3 (Mn113) and LaNi0.98Mo0.02O3 (Ni113) for the improvement of TE performance in multi-layered TE modules. In the case of magnetically aligned Ca349 film, the improvement of TE properties due to the reduction of resistivity (ρ) was achieved by optimization of hot-pressing temperature and ρ was sensitive to relative density of the Ca349 layer. For the decrease in ρ of the n-type layers, two different approaches, the choice of Ni113 as a n-type compound and usage of fine powders of Mn113, were attempted. Both approaches were effective for reducing ρ even in the sintering at ∼900°C.

Layered CoO2 materials are excellent candidates for potential thermoelectric applications. Their single crystals show good p-type thermoelectric properties at temperatures higher than 800K in air.Recently, the mechanism of thermoelectric properties was clarified through a discussion of electronic and crystallographic structure. In order to fabricate thermoelectric modules possessing good power-generation properties, thermoelectric materials and metallic electrodes must be connected with low contact resistance and high mechanical strength.It has been found that good junctions can be formed using Ag paste including p- and n-type oxide powders.The role of spin entropy contributions to thermopower will be presented, in connection with strong electron correlation and triangular lattices.

Hot-forged Ca3Co4O9 (Co349) ceramics were synthesized using large-grained powders prepared by a flux-growth method, and their thermoelectric properties and degree of grain alignment were evaluated. Neutron-diffraction experiments evidenced the effect of grain size on the development of the c-axis grain alignment. The optimum grain size was around 7 μm in our hot-forging method. The electrical resistivity (ρ) in the direction parallel to the pressed-plane was more reduced at higher degrees of orientation. Since ρ was reduced without lowering the Seebeck coefficient (S), the power factor (PF = S2/ρ) of the Co349 sample was improved and reached 0.8 mW/mK2 at 1073 K using Co349 grains with average size of around 7 μm. The thermal conductivity (κ) in the direction parallel to the pressed-plane slightly increased with the increase of the grain size, however the improvement of PF owing to use of large-grained powder outweighed this negative impact on the κ component of the thermoelectric figure of merit (Z = S2/ρκ).

Bi–Ca–Co–O polycrystalline materials with a layered structure were prepared. The synthesis of the sintered specimens from two starting compositions, Bi1.8Ca2Co2Oxand Bi2.5Ca2.5Co2Ox, revealed the latter is preferentially formed at high temperature(850 °C). The increase in sintering time was shown by growth of large platelike grains (up to 50 μm in diameter and several micrometers in thickness). The reaction mechanisms during the heat treatment and the preferential formation of the Bi2.5Ca2.5Co2Ox phase were observed by x-ray diffraction, thermogravimetry–differential thermal analysis, and scanning electron microscopy. These techniques supposed the presence of a liquid phase at high temperature, origin of a highly kinetic phase formation, and the growth of large grains. Interestingly the liquid phase reaction promotes an efficient stacking and sliding of grains during hot-forging treatment, and highly (00l) oriented materials were prepared. A relationship between thermoelectric performance, texture strength, and microstructure is clarified.

A new adhesive material has been developed in order to obtain practically usable thermoelectric modules composed of oxide thermoelectric legs. The thermoelectric module composed of 8-pair oxide legs has been fabricated. Both hot- and cold-sides of the module were covered by alumina plates. Open circuit voltage VO and maximum power Pmax reach 0.38 V and 0.30 W, respectively at 803 K of a hot-side temperature TH and 362 K of a temperature differential ΔT between TH and cold-side temperature TC. Generating power was repeated 11 times at 873-993 K of TH and at 200-290 K of ΔT. The module was cooled down to room temperature after each generation. At third measurement internal resistance RI of the module increased by 30 %. This is due to destruction of junctions because of thermal strain. No deterioration, however, was observed in thermoelectric properties for the oxide legs.

Having recently succeeded in synthesizing large single crystals of (Ca2CoO3)CoO2 (Co-349) with superior thermoelectric properties using a modified flux method, we have prepared a composite material of Co-349 powder and single crystals and examined its thermoelectric properties. The electrical conductivity σ of this composite, which contained 20 wt.% single crystals, was higher than that of a sample without the single crystals. While the achievable effect has yet to be fully realized, improved grain alignment and the effect of current bypassing grain boundaries through the large single crystals in the composite are thought to cause the increasing σ, which consequently results in an enhanced thermoelectric figure of merit of about 0.56 at 973 K in air.

Polycrystalline samples of Ca1-xBixMnO3 (0.02 ≤ x ≤ 0.20) were studied by means of x-ray diffraction, electrical resistivity (ρ), thermoelectric power (S), and thermal conductivity (κ) at high temperature. Bi doping leads to the lattice parameters a, b, and c increasing. And the ρ and the absolute value of S decrease rapidly with Bi doping. The largest power factor, S2/ρ, is obtained in the x = 0.04 sample, which is 3.6×10−4 Wm−1 K−2 at 400 K. The figures of merit (Z = S2/ρκ) for this sample and 1.0×10−4 and 0.86 × 10−4 K−1 at 600 and 1000 K, respectively.

We have prepared (Ca1−x−yMxBiy)3Co4Oz (M = Mg, Sr, and Ba) thin films by a combinatorial approach using a solution process. In the systems of (Ca1−x−yMxBiy)3Co4Oz (M = Mg, Sr, and Ba), solid solution range was determined to be × < 0.8 (M = Sr, y = 0), x < 1.0 (M = Mg, y = 0), x = 0.0 (M = Ba, y = 0), and x < 0.4 (M = Bi, x = 0). No solid solution range was obtained for the substitution of Ba for Ca site. The in-plane compressive stress in the CoO2 sublattice is controllable by the cation substitution for Ca in the (Ca2CoO3) sublattice. With increasing in-plane stress, the magnitude of thermoelectric power and resistivity increased.

SiO2 thin films containing fine Fe particles were prepared by a co-sputtering method. α-Fe particles were then formed by heat-treating the film at 650°C to 700°C. The average size of the particles was controlled by changing heat-treatment time and temperature. The magnetic properties of these samples were investigated and discussed in terms of the size of the particles. It was found that the coercivity of the sample containing α-Fe particles (average particle size = 24nm) is as large as 36 Oe which is much higher than that of the bulk Fe.