Nanocomposites, a high performance material exhibit unusual property combinations and unique design possibilities. With an estimated annual growth rate of about 25% and fastest demand to be in engineering plastics and elastomers, their potential is so striking that they are useful in several areas ranging from packaging to biomedical applications. In this unified overview the three types of matrix nanocomposites are presented underlining the need for these materials, their processing methods and some recent results on structure, properties and potential applications, perspectives including need for such materials in future space mission and other interesting applications together with market and safety aspects. Possible uses of natural materials such as clay based minerals, chrysotile and lignocellulosic fibers are highlighted. Being environmentally friendly, applications of nanocomposites offer new technology and business opportunities for several sectors of the aerospace, automotive, electronics and biotechnology industries.

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Nanocomposites: synthesis, structure, properties and new application opportunities

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

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

Thermoelectric energy conversion can be used to capture electric power from waste heat in a variety of applications. The materials that have been shown to have the best thermoelectric properties are compounds containing elements such as tellurium and antimony. These compounds can be oxidized if exposed to the high temperature air that may be present in heat recovery applications. Oxide materials have better stability in oxidizing environments, so their use enables the fabrication of more durable devices. Thus, although the thermoelectric properties of oxides are inferior to those of the compounds mentioned above, their superior stability may expand potential the high temperature application of thermoelectric energy conversion. In this paper, the thermoelectric properties of promising oxide materials are reviewed. The different types of oxides used for thermoelectric applications are compared and approaches for improving performance through doping are discussed.

Oxide materials for high temperature thermoelectric energy conversion

Single crystals of (Ca2CoO3)0.7CoO2 were grown using a modified strontium chloride flux technique, and their electrical and thermal properties were determined. Growth conditions were established for obtaining plate-like single crystals of a relatively large size (5×10×0.05 mm3). At 973 K, thermoelectric power S and electrical resistivity ρ of the specimens are ≈240 μV K−1 and ≈2.3×10−5 Ω m, respectively, and their thermal conductivity κ is nearly 3 W m1 K−1, as determined by mathematical extrapolation. The figure of merit ZT=S2T/ρκ derived therefrom is ≈0.87 at 973 K. The relatively low κ is likely the result of a misfit structure between the CoO2 layer and the Ca2CoO3 slab.

Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure

The electrical conductivity, Seebeck coefficient and thermal conductivity of oxide Ca
 9
Co
 12
O
 28
 with Ca
 2
Co
 2
O
 5
-type structure are 84 S cm
 –1
, 118 µV K
 –1
 and 1.73 W m
 –1
 K
 –1
 respectively at 700 °C, and its figure of merit is 0.67×10
 –4
 K
 –1
, showing that Ca
 9
Co
 12
O
 28
 is a potential material for high temperature thermoelectric energy conversion.

High temperature thermoelectric properties of oxide Ca9Co12O28

A new series of oxides Ca3-xBixCo4O9+δ, (x = 0.0−0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured. The values of Seebeck coefficients of the new oxides are all positive, showing that they are p-type conductors. Both the electrical conductivity and Seebeck coefficients increase with the increasing Bi contents which can be attributed to the increase of carrier mobility due to the larger size of Bi ion. The electrical conductivity, Seebeck coefficient, and the calculated value of the power factor of Ca3-xBixCo4O9+δ (x = 0.5) are 105 S cm-1, 160 μV K-1, and 2.7 × 10-4 W K-2 m-1 at 700 °C, respectively. The thermal conductivity of Ca3-xBixCo4O9+δ (x = 0.5) at room temperature is 1.14 W m-1 K-1 and increase slightly with the increasing temperature. At 700 °C, the figure of merit of Ca3-xBixCo4O9+δ (x = 0.5) is 2.0 × 10-4 K-1.

Synthesis and Thermoelectric Properties of the New Oxide Materials Ca3-xBixCo4O9+δ (0.0 < x < 0.75)

An oxide thermoelectric device was fabricated using Gd-doped Ca3Co4O9 p-type legs and La-doped CaMnO3 n-type legs on a fin. The power factors of p legs and n legs were 4.8×10−4 Wm−1 K−2 and 2.2×10−4 Wm−1 K−2 at 700 °C in air, respectively. With eight p–n couples the device generated an output power of 63.5 mW under the thermal condition of hot side temperature Th=773 °C and a temperature difference ΔT=390 °C. This device proved to be operable for more than two weeks in air showing high durability.

Fabrication of an all-oxide thermoelectric power generator

Synthesis and thermoelectric properties of the new oxide ceramics Ca3−xSrxCo4O9+δ (x=0.0–1.0)

Multilayer Al 2 O 3 /SiC ceramics with a perfect laminated structure were fabricated by extrusion-molding and hot-pressing. The mechanical properties and fracture behavior were evaluated under bending tests. Crack deflection was consistently observed at the interfaces between the Al 2 O 3 layers and the SiC-containing interphase. This led to a graceful failure in combination with a high apparent toughness and an increased fracture energy.

Kazuo Ueno

Papers

High temperature thermoelectric properties of oxide Ca9Co12O28

Synthesis and Thermoelectric Properties of the New Oxide Materials Ca3-xBixCo4O9+δ (0.0 < x < 0.75)

Fabrication of an all-oxide thermoelectric power generator

Synthesis and thermoelectric properties of the new oxide ceramics Ca3−xSrxCo4O9+δ (x=0.0–1.0)

Multilayer Al2O3/SiC ceramics with improved mechanical behavior