Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

3 pages, 3 figures.Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40 nm diameter Bi2Te3 nanowires/alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10–7 m2 s–1 at 150 K to 6.9×10–7 m2 s–1 at 300 K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.G.C. would like to acknowledge financial support from JPL and DOE. M.S.M.G. acknowledges a fellowship awarded by the MCYT (Spain) in the Ramon y Cajal Program.Peer reviewe

Seebeck coefficient enhancement in electrodeposited Bi2te3-ySey films with additives and pH variations on the electrochemical bath

Trabajo presentado en el International Workshop on Electrodeposited Nanostructures (EDNANO), celebrado en Sofia (Bulgaria), del 16 al 18 de marzo de 2017Peer reviewe

Non-contact methods for thermal properties measurement

Many of the renewable and sustainable energy technologies employ novel nanomaterials. For instance, thermal storage and thermoelectric conversion are in constant progress due to the emergence of new structures such as carbon-based materials, bulk nanostructures, 2D novel materials or nanowires. Thermal properties play a significant role to all these energy technologies as key parameters to evaluate the performance and efficiency of those materials in the final device. Understanding the effects of nanostructuring on thermal properties becomes critical, since a reduction in the thermal conductivity due to increased phonon scattering at interfaces is usually expected. Therefore, the determination of the thermal properties remains a critical aspect of material development effort, and measurement techniques are continuously developed or improved. Among those, non-contact heating methods are of importance since they bypass a frequent source of errors characteristic to contact-based thermal measurements, namely the thermal contact resistances, which can be dominant in nanoscale materials. Non-contact heating techniques are usually based on photothermal phenomenon, where heating is generated typically by incident radiation. This paper reviews non-contact heating measurement methods, providing an overview of basic principles for measurement along with associated theoretical model necessary for data reduction and their main applications. The techniques are categorized as time domain and frequency domain techniques, where the thermal response of the sample under study is analyzed as a function of time and frequency, respectively. Both types of methods study the transient response of the sample from a pulsed or modulated heating, and typical measurement output is thermal diffusivity. In addition, other non-contact techniques are also discussed, such as those based on steady-state response, from which the thermal conductivity is directly obtained, or those using AFM probe in the non-contact mode. Finally, main advantages and disadvantages of these techniques are summarized along with their associated uncertainties.The authors would like to acknowledge the financial support from ERC StG NanoTEC 240497. Authors also acknowledge CSIC through the Intramural INFANTE and MICINN through the CONSOLIDER-INGENIO 2010program (grant number CSD2010-00044) projects. D.A.B.-T. acknowledges Fulbright fellowship. M.S.M.-G. would like to thank her Salvador Madariaga fellowship from MECD.Peer reviewe

Numerical and Experimental Investigation of Nanostructure-Based Asymmetric Light Transmission Interfaces for Solar Concentrator Applications

Research in asymmetric light transmission interfaces has been recently gaining traction. While traditionally considered for optical circuitry applications, there is a new interest to use these interfaces in luminescent solar concentrators. Previous studies have shown that applying them to the top surface of a concentrator could mitigate surface losses. This paper presents experimental results for proof-of-concept asymmetric light transmission interfaces that may have potential applications in luminescent solar concentrators. The interfaces and the underneath substrate were created in a single step from polydimethylsiloxane using silicon molds fabricated on <100> wafers via anisotropic wet etching. The resulting structures were pyramidal in shape. Large surface areas of nanostructures repeating at 800 nm, 900 nm, and 1000 nm were tested for backward and forward transmission using a spectrometer. Results showed a 21%, 10%, and 0% average transmissivity difference between the forward and backward directions for each periodicity, respectively. The trends seen experimentally were confirmed numerically via COMSOL simulations

Effectiveness of Energy Transfer versus Mixing Entropy in Coupled Mechanical–Electrical Oscillators

Electrostatic energy harvesters convert kinetic energy into electrical energy via variable capacitors. Efforts to improve their power output are hampered by a lack of understanding of the fundamental limit for energy conversion efficiency. In heat engines, the theoretical limit of conversion efficiency is intrinsically related to entropy and the second law of thermodynamics. Laying the foundation for similar concepts for kinetic energy harvesters may be necessary for establishing a conversion efficiency limit. Thus, the mixing entropy concept is borrowed from statistical mechanics and is adapted here, for the first time, to characterize the energy transfer between coupled mechanical–electrical oscillators. The investigated system is composed of a spring-mass coupled to an inductance-capacitor circuit via a variable capacitor. Combining the two subsystems (electrical and mechanical) generates entropy, referred to as mixing entropy. A non-dimensional study of the governing equations of the systems and their energy terms is carried out. Trends in mixing entropy are compared with trends in the total energy of the system, assuming a conservative system, weak coupling between electrical and mechanical domains, and identical natural frequency of the two oscillators. It is found that mixing entropy can predict the peak in effectiveness of the energy transfer between the two domains. For the cases studied, the maximum mixing entropy and effectiveness values occur when the ratio of the mechanical domain energy to the total energy of the system is 67%. The maximum effectiveness is independent of the initial conditions and depends on the squared ratio of the natural frequency of the nominal coupling capacitor to the natural frequency of the mechanical system

Improvement of Seebeck coefficient in as-grown Bi2Te3-ySey electrodeposited films by the addition of additives and bath optimization

Bi2.0Te2.7Se0.3 is the most widely used thermoelectric n-type leg for large-scale cooling applications. However, as-grown electrodeposited Bi2Te3-ySey films typically have a Seebeck coefficient around a third of the bulk value, which sometimes can be improved with thermal annealings. In this work, we report as-grown Bi2Te3-ySey films having a Seebeck coefficient of approximately half the value of bulk without additional thermal treatments. All samples reported were deposited in baths containing no additives, sodium lignosulfonate (SLS), or both SLS plus ethylenediaminetetraacetic acid (EDTA) with a concentration of 1 M HNO3 (pH of 0 ± 0.1) and 0.6 M HNO3 (pH of 0.3 ± 0.1). For each scenario, the deposition was carried out at two different temperatures (0 °C and 5 °C). Seebeck values of −120 μV K−1 have been measured for as-grown films with an optimum morphology and stoichiometry (Bi2Te2.7Se0.3), which is ∼50% of the value obtained for this composition in bulk and the highest among as-grown electrochemically deposited films reported to date. These results are an incentive to revisit and further explore the chemistry behind the electrodeposition of bismuth telluride selenide films to improve the performance of electrodeposited thermoelectric films.M.M-G. acknowledges NSF IRES 1028071 support. D.-A. B.-T. acknowledge Fulbright fellowship support. M.M.G acknowledges the INFANTE, MAT2017-86450-C4-3-R and the ERC Starting grant NANO-Tec projects. O.C.C. wants to thank the financial support of Ramon y Cajal grant of the Spanish Government. The authors acknowledge the service from the X-SEM Laboratory at IMN, and funding from MINECO under project CSIC13-4E-1794 with support from EU (FEDER, FSE).Peer reviewe

Time dependent simulations of thermoelectric thin films and nanowires for direct determination of their efficiency with COMSOL Multi-physics

Comunicación presentada en la COMSOL conference, celebrada en Rotterdam en noviembre de 2013.Peer reviewe

An Open-Source Monte Carlo Ray-Tracing Simulation Tool for Luminescent Solar Concentrators with Validation Studies Employing Scattering Phosphor Films

Luminescent solar concentrators enhance the power output of solar cells through wave-guided luminescent emission and have great potential as building-integrated photovoltaics. Luminescent solar concentrators with a variety of geometries and absorbing–emitting materials have been reported in the literature. As the breadth of available experimental configurations continues to grow, there is an increasing need for versatile Monte Carlo ray-tracing simulation tools to analyze the performance of these devices for specific applications. This paper presents the framework for a Monte Carlo ray-tracing simulation tool that can be used to analyze a host of three-dimensional geometries. It incorporates custom radiative transport models to consider the effects of scattering from luminescent media, while simultaneously modeling absorption and luminescent emission. The model is validated using experimental results for three-dimensional planar and wedge-shaped luminescent solar concentrators employing scattering phosphor films. Performance was studied as a function of length, wavelength, and the angle of incidence of incoming light. The data for the validation studies and the code (written using the Python programming language) associated with the described model are publically available