Thermal conductivity reduction in thermoelectric nanowires

Comunicación presentada en la 12th European Conference on Thermoelectricity (ECT2014), celebrada en Madrid del 24 al 26 de septiembre de 2014.Peer Reviewe

Ordered three-dimensional interconnected nanoarchitectures in anodic porous ​alumina

Three-dimensional (3D) nanostructures combine properties of nanoscale materials with the advantages of being macro-sized pieces when the time comes to manipulate, measure their properties or make a device. However, the amount of compounds with the ability to self-organize in ordered 3D nanostructures is limited. Therefore, template-based fabrication strategies become the key approach towards 3D nanostructures. Here we report the simple fabrication of a template based on anodic ¿aluminium oxide, having a well-defined, ordered, tunable, homogeneous 3D nanotubular network in the sub 100-nm range. The 3D templates are then employed to achieve 3D, ordered nanowire networks in ¿Bi2Te3 and polystyrene. Finally, we demonstrate the photonic crystal behaviour of both the template and the polystyrene 3D nanostructure. Our approach may establish the foundations for future high-throughput, cheap, photonic materials and devices made of simple commodity plastics, metals and semiconductors.We would like to acknowledge financial support from ERC 2008 Starting Grant ‘Nano-TEC’ number 240497.Peer Reviewe

Silicon‐Germanium (SiGe) Nanostructures for Thermoelectric Devices: Recent Advances and New Approaches to High Thermoelectric Efficiency

Silicon and germanium present distinct and interesting transport properties. However, composites made of silicon‐germanium (SiGe) have resulted in a breakthrough in terms of their transport properties. Currently, these alloys are used in different applications, such as microelectronic devices and integrated circuits, photovoltaic cells, and thermoelectric applications. With respect to thermoelectricity, in the last decades, Si0.8Ge0.2 has attracted significant attention as an energy harvesting material, for powering space applications and other industrial applications. This chapter focuses on the recent advances and new approaches in silicon‐germanium (Si1−xGex) nanostructures for thermoelectric devices with high thermoelectric efficiency obtained through magnetron sputtering

Seleniuro de Plata: un candidato a la alta eficiencia termoeléctrica

Ponencia presentada en las IV Jornadas de Jóvenes Investigadores, celebradas en Madrid el 9 de julio de 2015.El actual interés en los materiales termoeléctricos se centra en su capacidad de transformar una diferencia de temperatura en una diferencia de Voltaje (efecto Seebeck). Igualmente a la creación de una diferencia de temperatura debida a un voltaje eléctrico (efecto Peltier), siendo posible utilizarlos como fuentes y sumideros de calor en aplicaciones industriales como un medio alternativo de refrigeración y enfriamiento. Entre las principales ventajas de los dispositivos termoeléctricos sobre los demás sistemas de refrigeración ¿ sistemas de compresión ¿ se encuentra una mayor fiabilidad en el tiempo de uso al no contar con partes móviles, la no utilización de gases de efecto invernadero y la ausencia de vibración debido a que son dispositivos de estado sólido con un tamaño reducido llegando a ser muy eficientes en aplicaciones locales.Peer Reviewe

Cylindrical Three-Dimensional Porous Anodic Alumina Networks

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.The European Research Council and the EU-H2020 program are gratefully acknowledged for co-funding this work through the projects Tonality (ERC-2014-PoC) and Marie Skłodowska-Curie Fellow (706094–TONSOPS). We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

Advances in Scanning Thermal Microscopy Measurements for Thin Films

One of the main challenges nowadays concerning nanostructured materials is the understanding of the heat transfer mechanisms, which are of the utmost relevance for many specific applications. There are different methods to characterize thermal conductivity at the nanoscale and in films, but in most cases, metrology, good resolution, fast time acquisition, and sample preparation are the issues. In this chapter, we will discuss one of the most fascinating techniques used for thermal characterization, the scanning thermal microscopy (SThM), which can provide simultaneously topographic and thermal information of the samples under study with nanometer resolution and with virtually no sample preparation needed. This method is based on using a nanothermometer, which can also be used as heater element, integrated into an atomic force microscope (AFM) cantilever. The chapter will start with a historical introduction of the technique, followed by the different kinds of probes and operation modes that can be used. Then, some of the equations and heating models used to extract the thermal conductivity from these measurements will be briefly discussed. Finally, different examples of actual measurements performed on films will be shown. Most of these results deal with thermoelectric thin films, where the thermal conductivity characterization is one of the most important parameters to optimize their performance for real applications

Nanoscale Measurement of Thermal Conductivity of Organic and Inorganic Nanowires embedded in a matrix

Póster presentado en la 12th European Conference on Thermoelectricity (ECT2014), celebrada en Madrid del 24 al 26 de septiembre de 2014.In this abstract, we present thermal conductivity measurements of inorganic and organic nanowires. These measurements have been carried out with a Scanning Thermal Microscope (SThM) working in 3¿ mode. This technique has been proved to be a successful method to evaluate the thermal conductivity of single nanowires without the need of removing the matrix at which they are embedded. On the one hand, regarding inorganic nanowires, a thermal conductivity of 1.37±0.20W/m·K have been determined for nanowires made of Bi2Te3 with 350nm diameter [1]. On the other hand, measurements of the thermal conductivity of polymeric nanowires made of P3HT embedded in a matrix have been studied in dependence with the diameter of the nanowire. In this work, a reduction of the thermal conductivity of the nanowire is observed as its diameter becomes lower, which can be correlated with its different polymer crystalline orientations [2]. The thermal conductivity of the nanowires varies drastically from 2.29±0.15W/m·K to 0.5±0.24W/m·K when the diameter of the P3HT nanowire is reduced from 350nm to 120nm [2]. Moreover, a finite element model with COMSOL was also developed to validate the results of the thermal conductivity of the nanowires obtained from the analysis of the 3¿ signal of the thermal probe and the use of the effective medium theory. The 3¿-SThM technique is a powerful technique to determine the thermal properties of individual nanowires and study how this property changes in comparison to bulk structures or as a dependence of its diameter size, among others.Peer Reviewe

Thermoelectric properties of Bi2Te3 nanowire array in thickness direction

Comunicación presentada en la 12th European Conference on Thermoelectricity (ECT2014), celebrada en Madrid del 24 al 26 de septiembre de 2014.Theoretical studies have predicted a possibility of increasing thermoelectric efficiency of nanos tructure materials, owing to quantum confinement effect on the charge carriers and lattice vibrations. On the other hand, Bi2Te3 is well known to be the most efficient thermoelectric material that can be operated around room temperatureThis work has been supported by the bilateral Spanish–Japanese NANOTHERMA projects of Strategic International Reserch Cooperative Program and KAKENHI for Challenging Exploratory.Peer Reviewe

Tailoring magnetic anisotropy at will in 3d interconnected nanowire networks

The control of magnetic anisotropy has been the driving force for the development of magnetic applications in a wide range of technological fields from sensing to spintronics. In recent years, the possibility of tailoring the magnetic properties goes together with a need for new 3D materials to expand the applications to a new generation of devices. Herein, the possibility of designing the magnetic anisotropy of 3D magnetic nanowire networks is shown just by modifying the geometry of the structure or by composition. It is also shown that this is possible when the magnetic properties of the structure are governed by magnetostatic anisotropy. The present approach can guide systematic tuning of the magnetic easy axis and coercivity in the desired direction at the nanoscale. Importantly, this can be achieved on virtually any magnetic material, alloy, or multilayers that can be prepared inside porous alumina. These results are promising for engineering novel magnetic devices that exploit tailored magnetic anisotropy using metamaterials concept

Optimization of bismuth telluride films and nano-wire arrays via electrodeposition for thermoelectric applications

Comunicación presentada en el 3rd Early Stage Researchers Workshop in Nanoscience, celebrado en Madrid el 27 y 28 de junio de 2013.Due to the current world’s demand for energy, there is a great interest in thermoelectricity, which offers the possibility of increasing the sustainability of our electrical system. Thermoelectric materials can convert heat into electricity and vice versa, and thus they offer a way of recovering wasted heat produced in engines, industrial processes and others into usable power. However, one of the main problems for their actual use is their low efficiency in this conversion. This efficiency is directly related with what is called the thermoelectric figure of merit, described by ZT=(S2·σ·T)/κ ,where S, σ, κ, and T stand for the Seebeck coefficient, electrical and thermal conductivities, and the absolute temperature, respectively. Given that in classical physics S, σ, and κ, are correlated, the improvement of the efficiency is not straightforward. Nevertheless, in 1993 a theoretical work suggested that the efficiency could be greatly enhanced by reducing the dimensionality of the structures under studied and working in the nano-scale. Therefore, much experimental effort has been done to achieve these kind of structures and in some cases, an enhancement of the ZT value has been achieved, although this has not been due to the quantum confinement to the charge carriers, as it was theoretically predicted, but to an increase of the κ due to the increased number of interface boundaries in nanostructures. Among the most efficient thermoelectric materials used for applications at room temperature, bismuth telluride (Bi2Te3) and its different alloys stand out, with a ZT for Bi2Te3of around 1 at RT [2]. We present here an optimized method of obtaining films and nanowire arrays via electrochemical deposition in a conventional three-electrode cell. Different ways of improving the quality of the obtained films have been studied (working electrode, constant and pulsed potentials, different chemical baths, etc.) in order to obtain highly oriented (110) films, which are the most favorable for out-of-plane applications. Then, nanostructuration has been achieved by changing the working electrode to porous alumina templates and realizing the electrochemical deposition inside the pores. The samples produced have been characterized using SEM, EDX, AFM, XRD, and Raman spectrometry, and in the case of the films, their transport properties have also been measured.Peer Reviewe