Liquid-Gas Surface Tension Voltage Dependence During Electrowetting on Dielectric of 5-90 nm Gold Nanofluids

This article investigates the effective liquid-gas surface tension changes of water and 5-90nm gold nanofluids measured during electrowetting on dielectric experiments. The Young-Laplace equation for sessile droplets in air was solved to fit the experimental droplet shape and determine the effective liquid-gas surface tension at each applied voltage. A good agreement between experimental droplet shapes and the predictions was observed for all the liquids investigated in applied range of 0-30V. The measured liquid-gas effective surface tensions of water and gold nanofluid decreased with voltage. At a given voltage, the effective liquid-gas surface tension of gold nanofluids initially decreased as the size of gold nanoparticles increased from 5 nm to 50 nm. Then, for 70nm and 90nm particle gold nanofluids, the effective liquid-gas surface tension started increasing too. The size of nanoparticles, and the applied voltage have a significant effect on variation of the effective liquid-gas surface tension with variations as much as 93% induced by voltage at a given particle size and 80% induced by particle size at a given voltage

Novel Measurement Methods for Thermoelectric Power Generator Materials and Devices

Thermoelectric measurements are notoriously challenging. In this work, we outline new thermoelectric characterization methods that are experimentally more straightforward and provide much higher accuracy, reducing error by at least a factor of 2. Specifically, three novel measurement methodologies for thermal conductivity are detailed: steady‐state isothermal measurements, scanning hot probe, and lock‐in transient Harman technique. These three new measurement methodologies are validated using experimental measurement results from standards, as well as candidate materials for thermoelectric power generation. We review thermal conductivity measurement results from new half‐Heusler (ZrNiSn‐based) materials, as well as commercial (Bi,Sb)2(Te,Se)3 and mature PbTe samples. For devices, we show characterization of commercial (Bi,Sb)2(Te,Se)3 modules, precommercial PbTe/TAGS modules, and new high accuracy numerical device simulation of Skutterudite devices. Measurements are validated by comparison to well‐established standard reference materials, as well as evaluation of device performance, and comparison to theoretical prediction obtained using measurements of individual properties. The new measurement methodologies presented here provide a new, compelling, simple, and more accurate means of material characterization, providing better agreement with theory

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

Electrical conductivity measurements of films

Póster presentado en la 12th European Conference on Thermoelectrics (ECT2014), celebrada en Madrid del 24 al 26 de septiembre de 2014.The characterization of the electrical conductivity of thin films is mandatory in all materials but particularly in thermoelectricity, its measurement is crucial in order to be able to determine the power factor and the figure of merit. A technique that could be used to carry out electrical measurements on thin films is the four probe technique. However, the spreading of the current due to the electrical field or the influence of the electrical contact resistances complicate the determination and analysis of the electrical conductivity of the film. In order to overcome these problems, we carried out a mesa attack on Bi2Te3 films grown by electrodeposition technique, which are hold on a Si substrate with a gold layer of 150nm. The goal is fabricating pillars whose later film resistivity analysis could approach to the 1D electrical model, which cannot be taken into account when dealing with a big film area. For that purpose, a lithography process was done on films with different thicknesses, which consist of a pattern of disks with different diameters ranging between 120µm and 60µm diameter. After the lithography, we evaporated 100nm of gold on top of the disks that would act as the top electrode. Then, we removed the photoresist and we performed a mesa attack with diluted nitric acid (1:2.5). As a result, we obtained pillars as the ones showed in Figure 1. On these pillars, I-V curves with the four probe technique were taken and the resistances of the pillars were determined. Representing the resistance of the pillars versus the inverse of the area of the pillar, the total resistivity of the films, which includes the contact resistances between the electrodes and the pillar, is obtained. Finally, to determine the electrical contact resistance and obtain the pure electrical resistivity of the film, these measurements were performed on different pillar thicknesses. The experimental results have been double-checked with COMSOL simulations of the four probe experiments carried out here, observing a good agreement between them.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

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

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

Enhancement of thermoelectric efficiency of doped PCDTBT polymer films

8 pags.; 8 figs.; 1 sch.This journal is © The Royal Society of Chemistry. Conjugated polymers may be used as thermoelectric materials due to their low thermal conductivity and have the advantageous characteristics of conventional polymers, such as low weight, non-toxicity and low cost. Here, a detailed investigation into the thermoelectric properties of PCDTBT films is reported. Moreover, in order to improve the thermoelectric properties of this polymer, FeCl3 is used as a doping agent. For the most optimally doped film reported in this work, a power factor value of 24 μW m−1 K−2 is obtained at 150 °C. The different films were characterized by wide-angle X-ray scattering (WAXS) experiments at different temperatures. In order to see the temperature effect, the thermoelectric power factor is measured as a function of temperature from (from RT to 150 °C). Thermal conductivity at room temperature is calculated with two independent methods which give values in agreement within the margin of uncertainty. The results obtained show promise and give insight to motivate future investigation into these types of carbazole derivates.This work has been supported by ERC Starting Grant Nano-TEC number 240497, Nanotherm Consolider CSD-2010-00044 project and PHOMENTA project MAT2011-27911. We gratefully acknowledge funding from the US Department of Energy, Office of Basic Energy Sciences through the S3TEC Energy Frontiers Research Center (T. B.-T.), and NSF IRES grant #1028071 and the Fulbright for financial support towards this work (A. A. W., D.-A. B.-T and T. B.-T.).Peer Reviewe

A Review on Principles and Applications of Scanning Thermal Microscopy (SThM)

As the size of materials, particles, and devices shrinks to nanometer, atomic, or even quantum scale, it is more challenging to characterize their thermal properties reliably. Scanning thermal microscopy (SThM) is an emerging method to obtain local thermal information by controlling and monitoring probe–sample thermal exchange processes. In this review, key experimental and theoretical components of the SThM system are discussed, including thermal probes and experimental methods, heat transfer mechanisms, calibration strategies, thermal exchange resistance, and effective heat transfer coefficients. Additionally, recent applications of SThM to novel materials and devices are reviewed, with emphasis on thermoelectric, biological, phase change, and 2D materials