Physical properties at the nanoscale are novel and different from those in bulk materials.
Physical properties at the nanoscale are novel and different from those in bulk materials. Over the last few ...decades, there has been an ever growing interest in the fabrication of nanowire structures for a wide variety of applications including energy generation purposes. Nevertheless, the study of their transport properties, such as thermal conductivity, electrical conductivity or Seebeck coefficient, remains an experimental challenge. For instance, in the particular case of nanostructured thermoelectrics, theoretical calculations have shown that nanowires offer a promising way of enhancing the hitherto low efficiency of these materials in the conversion of temperature differences into electricity. Therefore, within the thermoelectrical community there has been a great experimental effort in the measurement of these quantities in actual nanowires. The measurements of these properties at the nanoscale are also of interest in fields other than energy, such as electrical components for microchips, field effect transistors, sensors, and other low scale devices. For all these applications, knowing the transport properties is mandatory. This review deals with the latest techniques developed to perform the measurement of these transport properties in nanowires. A thorough overview of the most important and modern techniques used for the characterization of different kinds of nanowires will be shown.
3D interconnected nanowire scaffoldings are shown to increase the thermoelectric efficiency in comparison to similar diameter 1D nanowires and films grown under similar electrodeposition conditions. ...Bi2Te3 3D nanonetworks offer a reduction in thermal conductivity (κT) while preserving the high electrical conductivity of the films. The reduction in κT is modeled using the hydrodynamic heat transport equation, and it can be understood as a heat viscosity effect due to the 3D nanostructuration. In addition, the Seebeck coefficient is twice that of nanowires and films, and up to 50% higher than in a single crystal. This increase is interpreted as a nonequilibrium effect that the geometry of the structure induces on the distribution function of the phonons, producing an enhanced phonon drag. These thermoelectric metamaterials have higher performance and are fabricated with large areas by a cost-effective method, which makes them suitable for up-scale production.
Vanadium tetrasulfide (VS4, called patronite as a mineral) is a one-dimensional compound with promising properties for energy conversion applications. However, it has been scarcely investigated ...because of its complex synthesis. In this work, we report a detailed investigation about the formation mechanism of VS4 (V4+(S2 2–)2) as well as its structural, transport, and photoelectrochemical properties. To this aim, VS4 films were grown by a solid–gas reaction process between vanadium films and sulfur at temperatures between 350 and 450 °C during different reaction times. Film characterization (X-ray diffraction, energy-dispersive analysis of X-ray, micro-Raman spectroscopy, and scanning electron microscopy) reveals the formation of monoclinic VS4 nanorods (I2/C) as single crystalline phase in very short reaction times (t < 5 h). Optical characterization was carried out by reflectance and transmittance measurements to obtain the optical absorption coefficient (α = 104 cm–1 at photon energies higher than 1.6 eV). From these measurements, a direct optical band gap of 1.35 ± 0.05 eV is obtained. Additionally, VS4 films were used as photoanodes of a photoelectrochemical cell (PEC) with a platinum foil as counter electrode and a Ag/AgCl reference electrode to characterize the VS4/electrolyte (aqueous 0.5 M Na2SO3) interface. Finally, the evolved hydrogen under 200 mW/cm2 white light illumination over the VS4/interface at 0.3 V (Ag/AgCl) bias potential was quantified by a quadrupole mass spectrometer (QMS) reaching fluxes of ∼20 μmol/h.
The synthesis of a conformal three-dimensional nanostructure based on porous anodic alumina with transversal nanopores on wires is herein presented. The resulting three-dimensional network exhibits ...the same nanostructure as that obtained on planar geometries, but with a macroscopic cylindrical geometry. The morphological analysis of the nanostructure revealed the effects of the initial defects on the aluminum surface and the mechanical strains on the integrity of the three-dimensional network. The results evidence the feasibility of obtaining 3D porous anodic alumina on non-planar aluminum substrates.
In article number 1700012, Marisol Martin‐Gonzalez and co‐workers report a novel technological approach to deposit highly efficient thermoelectric films via the pulsed hybrid reactive magnetron ...sputtering (PHRMS). PHRMS is a single‐step fabrication process with the ability to get even at room temperature high values of power factor and low values of thermal conductivity for films with the β‐Cu2Se phase and transferable to industry even for 2D materials.
Thermoelectric films on flexible substrates are of interest for the integration of thermoelectric in wearable devices. In this work, copper selenide films are achieved by a novel low‐temperature ...technique, namely pulsed hybrid reactive magnetron sputtering (PHRMS
)
. A brief introduction to the basic chemistry and physics involved during growth is included to explain its fundamentals. PHRMS is a single‐step, room temperature (RT), fabrication process carried out in another ways conventional vacuum sputtering system. It does not require high‐temperature post‐annealing to obtain films with great thermoelectric performance. It is, therefore, compatible with polymeric substrates like Kapton tape. Several sets of films covering a large exploratory compositional range (from Cu/Se = 1 to 9) are deposited and their microstructure and thermoelectric properties are analyzed at RT. Power factors as high as 1.1 mW m
−1
K
−2
in the in‐plane direction and thermal conductivities as low as κ = 0.8 ± 0.1 W m
−1
K
−1
in the out‐of‐plane direction have been obtained for β‐Cu
2
Se films. Consequently, a figure of merit of 0.4 at RT can be estimated under the assumption that for this polycrystalline cubic phase no additional anisotropy in the thermoelectric properties is introduced by the planar configuration. Moreover, PHRMS is also industrially scalable and compatible with the in‐line fabrication of other selenides.
Bismuth tellurium selenide (Bi2Te3-ySey) films have been electrochemically grownin a conventional three electrode cell. The addition of different components to the bath, namely Sodium Lignosulfonate ...(SLS) and Ethylendiaminetetraacetic Acid (EDTA) and their influence in morphology, stoichiometry, structure and Seebeck coefficientwas studied in order to improve the thermoelectric performance of Bi2Te3-ySey films. Films grown with SLS presented high crystallographic orientation and improved morphology, while those grown in the presence of EDTA had higher concentration of bismuth. The combination of both additives gave rise to stoichiometric Bi2Te2.7Se0.3films with denser morphology, higher orientation and higher Seebeck coefficients (60% larger) when compare with films grown without additives.
LiNbO3 is a distinguished multifunctional material where ferroelectric domain engineering is of paramount importance. This degree of freedom of the spontaneous polarization remarkably enhances the ...applicability of LiNbO3, for instance in nonlinear photonics. However, conventional electrical poling suffers from several drawbacks. Namely, the lithographic patterning of electrodes is cumbersome and expensive, and the spatial resolution is constrained. In this work, we report the first method for all-optical domain inversion of LiNbO3 crystals using continuous-wave visible light. While we mainly focus on iron-doped LiNbO3, the applicability of the method is also showcased in undoped congruent LiNbO3. The technique is outstandingly simple, low-cost and readily accessible. It relies on ubiquitous elements: a light source with low/moderate power, basic optics and a conductive surrounding medium, e.g. water. Light-induced domain inversion is unequivocally demonstrated and characterized by combination of several experimental techniques: pyroelectric trapping of charged microparticles, selective chemical etching, surface topography profilometry, scanning electron microscopy and 3D Cerenkov microscopy. The influence of light intensity, exposure time, laser spot size and surrounding medium is thoroughly studied. Overall, our all-optical method offers straightforward implementation of LiNbO3 ferroelectric domain engineering, potentially sparking new research endeavors aimed at novel optoelectronic applications of photovoltaic LiNbO3 platforms.
We show here for the first time the use of a cold sintering process (CSP) to sinter CoSb3-based thermoelectric materials. CSP at 150 {\deg}C for 90 min under a uniaxial pressure of 750 MPa yields ...pieces with a relative density of 86 %, which is increased to around 92 % after a post-annealing at temperatures > 500 {\deg}C in Ar atmosphere. The reported CSP produces Te doped-CoSb3 nanocomposites with similar morphological and structural characteristics to the starting nanopowders obtained by ball milling in air atmosphere. The post-thermal treatment induces grain coalescence and grain growth, crystallite size growth as well as compositional changes in the nanocomposite, decreasing the amount of the main phase, CoSb3, and increasing the weight of secondary phase, CoSb2, up to a 30 wt% at 600 {\deg}C. Remarkably, the average valence for the Co, Sb and Te absorbing atoms is neither transformed by the sintering process nor by the subsequent heat treatment. The functional response of the sintered thermoelectric nanocomposites exhibits a maximum figure of merit of 0.12(3) at room temperature for the nanocomposites sintered by CSP with a subsequent post-annealing at 500 {\deg}C. This is mainly due to its low thermal conductivity in comparison with similar powders sintered by other approaches, and it is explained by the morphological and structural properties. These findings represent an attractive alternative for obtaining efficient thermoelectric skutterudites by a scalable and cost-effective route.
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