Paz Vaqueiro

Recent developments in nanostructured materials for high-performance thermoelectrics

This highlight discusses recent trends in the search for new high-efficiency thermoelectric materials. Thermoelectric materials offer considerable attractions in the pursuit of a more efficient use of existing energy resources, as they may be used to construct power-generation devices that allow useful electrical power to be extracted from otherwise waste heat. Here, we focus on the significant enhancements in thermoelectric performance that have been achieved through nanostructuring. The principal factor behind the improved performance appears to be increased phonon scattering at interfaces. This results in a substantial reduction in the lattice contribution to thermal conductivity, a low value of which is a key requirement for improved thermoelectric performance.

Gallium-sulfide supertetrahedral clusters as building blocks of covalent organic-inorganic networks

The synthesis and characterization of novel covalent organic-inorganic architectures containing organically functionalized supertetrahedra is described. The structures of these unique materials consist of one-dimensional zigzag chains or of honeycomb-type layers, in which gallium-sulfide supertetrahedral clusters and dipyridyl ligands alternate.

Synthesis of yttrium aluminium garnet by the citrate gel process

We describe the synthesis of polycrystalline yttrium aluminium garnet (YAG) by the citrate gel process. The conditions for obtaining the pure phase have been determined and the YAG powders were characterized by X-ray diffraction, IR spectroscopy and transmission electron microscopy. YAG powders were obtained at significantly lower temperatures than by other conventional techniques. Moreover, the particle size of the powders prepared through this process is in the range 20–70 nm, depending on the thermal treatment. The lattice parameter of the YAG powders was also determined and we observed that the lattice parameter depends on the particle size. An explanation for this behaviour is provided.

Annealing dependence of magnetic properties in nanostructured particles of yttrium iron garnet prepared by citrate gel process

Abstract Yttrium iron garnet (YIG) particles were first synthesized using a low-temperature method based on the citrate gel process. The annealing temperature dependence of the magnetic behaviour was investigated by means of transmission 57 Fe Mossbauer spectrometry and DC magnetic measurements. The as-prepared particles behaves as an amorphous-like structure. Annealing treatments cause aggregation and crystallization of particles. For low annealing temperatures, one can suggest a nanostructured behaviour of the YIG particles, which progressively disappears when the annealing temperature increases. Depending on the annealing temperature, YIG nanostructured particles of different size, in the range 50–700 nm were obtained. The values of the saturation magnetization as well as the Curie temperature for the YIG particles are close to those observed for well-crystallized YIG.

Arrays of Chiral Nanotubes and a Layered Coordination Polymer Containing Gallium–Sulfide Supertetrahedral Clusters

Organically functionalised supertetrahedral clusters: Two novel coordination polymers, consisting of chiral helical nanotubes and of composite layers, have been obtained by linkage of gallium-sulfide supertetrahedral clusters and dipyridyl ligands (see picture).

Zero-Dimensional Units of Ligand-Bridged Gallium-Sulfide Supertetrahedra

The synthesis and characterization of the first anions containing two gallium-sulfide supertetrahedra linked via an organic moiety are described.

A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery

The new thermoelectric material BiOCuTe exhibits an electrical conductivity of 224 S cm−1 and a Seebeck coefficient of +186 μV K−1 at 373 K, together with an extremely low lattice thermal conductivity of ∼0.5 W m−1 K−1. This results in a ZT of 0.42 at 373 K, which increases to 0.66 at the maximum temperature investigated, 673 K.

Synthesis of yttrium iron garnet nanoparticles via coprecipitation in microemulsion

We describe a new technique using microemulsions to produce ultrafine precursors of yttrium iron garnet. A coprecipitation of hydroxide or carbonate precursors was made in a W/O microemulsion medium. These precursors (ca. 3 nm in size), when heated above 700 °C, transformed to YIG phase. Once prepared, YIG nanoparticles were characterized by X-ray diffraction, transmission electron microscopy and dc magnetic measurements.

Synthesis, structural characterisation and thermoelectric properties of Bi1−xPbxOCuSe

The effect of Pb2+ doping on the structure and thermoelectric properties of BiOCuSe (also known as BiCuSeO or BiCuOSe) is described. With increasing Pb2+ content, the expansion of the unit cell results in a weakening of the bonding between the [Bi2(1−x)Pb2xO2]2(1−x)+ and the [Cu2Se2]2(1−x)− layers. The electrical resistivity and Seebeck coefficient decrease in a systematic way with growing Pb2+ levels. The thermal conductivity rises due to the increase of the electronic contribution with doping. The power factor of materials with a 4–5% Pb2+ content takes values of ca. 8 μW cm−1 K−2 over a wide temperature range. ZT at 673 K is enhanced by ca. 50% when compared to values found for other dopants, such as Sr2+ or Mg2+.

Ball milling as an effective route for the preparation of doped bornite: synthesis, stability and thermoelectric properties

Bornite, Cu5FeS4, is a naturally-occurring mineral with an ultralow thermal conductivity and potential for thermoelectric power generation. We describe here a new, easy and scalable route for synthesising bornite, together with the thermoelectric behaviour of manganese-substituted derivatives, Cu5Fe1−xMnxS4 (0 ≤ x ≤ 0.10). The electrical and thermal transport properties of Cu5Fe1−xMnxS4 (0 ≤ x ≤ 0.10), which are p-type semiconductors, were measured from room temperature to 573 K. The stability of bornite was investigated by thermogravimetric analysis under inert and oxidising atmospheres. Repeated measurements of the electrical transport properties confirm that bornite is stable up to 580 K under an inert atmosphere, while heating to 890 K results in rapid degradation. Ball milling leads to a substantial improvement in the thermoelectric figure of merit of unsubstituted bornite (ZT = 0.55 at 543 K), when compared to bornite prepared by conventional high-temperature synthesis (ZT < 0.3 at 543 K). Manganese-substituted samples have a ZT comparable to that of unsubstituted bornite.