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

Organic-inorganic nanohybrid materials have been attracting much attention for the creation of high-performance materials. Development of materials with organized nanostructures is a fascinating research subject in ceramics, especially in the field of hybrid materials. Supramolecular organization is one of the key approaches for the creation of nanomaterials. This technique can be combined with the sol-gel process for the development of organic-inorganic nanohybrids. Here, several approaches for the preparation of organic-inorganic nanohybrids are reviewed. In particular, bioinspired colloidal nanohybrids created by the supramolecular organization and the sol-gel process are highlighted. Firstly, development of liposomal nanohybrid materials using amphiphilic organoalkoxysilanes is described. This hybrid has a unique structure since the inner structure is lipid bilayer vesicle, whereas the outer structure is analogous to silica particle. Preparation of hybrid hollow capsules by layer-by-layer assembly and sol-gel method using colloid templating is also described. These capsules can act as stimuli responsive materials with on-demand release functions. The nanohybrid colloidal materials are expected to show promise in areas ranging from drug and gene delivery to cell membrane modeling.

Organic–Inorganic Hybrid Materials Prepared Through Supramolecular Assembly

Thermoelectric materials with high Z's have been searched for recently among semiconducting oxide ceramics for the purpose of power generation at high temperatures in an air atmosphere. Metallic oxides consisting of non-transition metal elements which mould possibly have large band widths and high carrier mobility have been the target materials. One possible oxide group is that having In/sub 2/O/sub 3/ as a main component. In the present study, homologous compounds of (ZnO)mIn/sub 2/O/sub 3/ (m=integer) with layer structures were synthesized by reaction-sintering the powder mixtures of ZnO and In/sub 2/O/sub 3/ at 1823 K for 2 h in air. Their thermoelectric characteristics, i.e. electrical conductivity, Seebeck coefficient, and thermal conductivity were measured at 500-1000 K. Their Z depended on the composition and the microstructure.

http://ieeexplore.ieee.org/iel3/4070/11983/00553286.pdf

Thermoelectric characteristics of homologous compounds with layer structures in the ZnO-In 2 O 3 system

We fabricated highly textured Ca/sub 3/Co/sub 4/O/sub 9/-based ceramics by using plate-like particles as a template in order to extract highly anisotropic thermoelectric properties. Plate-like powder and fine powder of (Ca/sub 2.7/Na/sub 0.15/Bi/sub 0.15/)Co/sub 4/O/sub 9/ were synthesized and wet-mixed at the ratio of 1:4. The slurry was tape-cast into a form with the platelets aligned parallel to the tape surface. The tapes were stacked, and sintered or hot-pressed at 1193 K. The resultant ceramics showed highly preferred orientation with the {001} plane parallel to the tape surface. The electrical conductivity in the direction of the ab-plane was improved to 1.8/spl times/10/sup 4/ S/m at 1073 K by the texture given. The maximum power factor reached 5.9/spl times/10/sup -4/ W/mK/sup 2/ at the same temperature.

Thermoelectric properties of Ca/sub 3/Co/sub 4/O/sub 9/-based ceramics textured by templated grain growth method

A novel process (nano-block integration) is proposed and we attempted to fabricate new composites with artificial superlattices alternatively stacking the conductive layers (octahedral CoO/sub 2/) and insulating layers (silica, etc.). Cobalt hydroxides, Co(OH)/sub 2/, with well controlled thickness ( nano scale) and crystallized morphologies by adjusting the conditions of an aqueous solution are achieved and subsequently oxidized to form CoO/sub 2/ sheets with the oxidization agents. A dense silica layer is prepared on CoO/sub 2/ sheets via the hydrolysis of organosilane. These identical operations are repeated several times and the thin film of composites is obtained on a glass substrate. After an appropriate treatment, the thermoelectric properties of the film are investigated.

Kunihito Koumoto

Papers

Organic–Inorganic Hybrid Materials Prepared Through Supramolecular Assembly

Thermoelectric characteristics of homologous compounds with layer structures in the ZnO-In 2 O 3 system

Thermoelectric properties of Ca/sub 3/Co/sub 4/O/sub 9/-based ceramics textured by templated grain growth method

Investigation on the assessment of nano-block integration process for novel thermoelectric materials

High thermoelectric properties of shear-exfoliation-derived TiS2-AgSnSe2 nano-composites via ionized impurity scattering