Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

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Complex thermoelectric materials.

TiO2 photocatalysis and related surface phenomena

Spurred by recent progress in materials chemistry and drug delivery, stimuli-responsive devices that deliver a drug in spatial-, temporal- and dosage-controlled fashions have become possible. Implementation of such devices requires the use of biocompatible materials that are susceptible to a specific physical incitement or that, in response to a specific stimulus, undergo a protonation, a hydrolytic cleavage or a (supra)molecular conformational change. In this Review, we discuss recent advances in the design of nanoscale stimuli-responsive systems that are able to control drug biodistribution in response to specific stimuli, either exogenous (variations in temperature, magnetic field, ultrasound intensity, light or electric pulses) or endogenous (changes in pH, enzyme concentration or redox gradients).

Stimuli-responsive nanocarriers for drug delivery

Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

https://link.springer.com/content/pdf/10.1007%2Fs10853-010-5113-0.pdf

Review of the anatase to rutile phase transformation

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

Polycrystalline samples of Ca0.9−xSrxYb0.1MnO3−δ (x=0, 0.025. 0.05, 0.1, and 0.2) were prepared by a conventional solid-state reaction and their thermoelectric properties were evaluated at 303–973 K. Each of the samples consisted of a single phase with an orthorhombic structure. All the samples showed a metallic conductivity and their electrical resistivity was markedly affected by the distortion of the MnO6 octahedron. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers were electrons over the entire temperature range examined. The highest power factor achieved (0.22 mW m−1 K−2 at 773 K) was shown by the sample with x=0.1. The thermal conductivity was affected by both the crystal distortion and the difference in mass between the Ca2+ and Sr2+ ions. The highest dimensionless figure of merit obtained was 0.09 at 973 K for the sample with x=0.1; this is a result of its low electrical resistivity and its moderate Seebeck coefficient and thermal conductivity.

High-temperature thermoelectric properties of Ca0.9−xSrxYb0.1MnO3−δ(0≤x≤0.2)

Textured bulk ceramics of the layer-structured homologous compound (ZnO)5In2O3 were fabricated by the reactive templated grain growth (RTGG) technique and the thermoelectric properties evaluated. Plate-like ZnSO4·3Zn(OH)2 particles were mixed with In2O3 powders and the mixture was tape cast to form a sheet, such that the ZnSO4·3Zn(OH)2 platelets were aligned parallel to the sheet surface; the sheets were then sintered to form a single phase of (ZnO)5In2O3 with the c-axis perpendicular to the sheet surface. The electrical conductivity, σ, was found to be highly anisotropic in the textured specimen; that is, σ
(in the ab-plane) > σ
(along the c-axis). The textured specimen shows
a porous microstructure and exhibits lower thermal conductivity than a dense ceramic specimen with random orientation. As a result, the thermoelectric figure of merit of the textured (ZnO)5In2O3 specimen is nearly three times as large as that of the randomly oriented specimen.

Thermoelectric properties of highly textured (ZnO)5In2O3 ceramics

Through the systematic addition of B, C, and Fe, additive effects on the stacking fault formation and the morphology of the particles formed during reaction synthesis of β-SiC were investigated in the present study. The whisker content of the synthesized product and the formation mechanism of whiskers were closely related to the stacking fault density. The addition of B inhibited whisker formation probably because of isotropic imperfection and the suppression of surface diffusion. Increase in the reduction force by using an active carbon source and also adding excess carbon led to an increase in stacking fault density through enhanced whisker formation. In the presence of Fe, the synthesized β-SiC whiskers appeared to possess only a small amount of stacking faults. The growth mechanism was different with Fe; i.e., isotropic growth occurred via a dissolution-precipitation reaction through a liquid phase in the Fe-Si system.

Effects of Boron, Carbon, and Iron Content on the Stacking Fault Formation during Synthesis of β‐SiC Particles in the System SiO2‐C‐H2

Grain-boundary grooving was studied on polished surfaces of polycrystalline alumina, after it was annealed at 1370~1600 °C in air. The groove angles and the groove widths were measured by AFM and it was determined that surface diffusion is the dominant mechanism for the mass transport and the ratio of grain-boundary energy to surface energy decreased with increasing temperature. The surface diffusion coefficient calculated was D s = 8.22 × 10 7 × exp [ −577 ± 30(kJ mol −1 ) RT ] which is the smallest among those reported in the literature.

Kunihito Koumoto

Papers

High-temperature thermoelectric properties of Ca0.9−xSrxYb0.1MnO3−δ(0≤x≤0.2)

Thermoelectric properties of highly textured (ZnO)5In2O3 ceramics

Effects of Boron, Carbon, and Iron Content on the Stacking Fault Formation during Synthesis of β‐SiC Particles in the System SiO2‐C‐H2

Grain-boundary grooves and surface diffusion in polycrystalline alumina measured by atomic force microscope

Fabrication and thermoelectric properties of heavily rare-earth metal-doped SrO(SrTiO3)n (n = 1, 2) ceramics