Nanowires: A route to efficient thermoelectric devices

Miniaturization of electronic devices aims at manufacturing ever smaller products, from mesoscopic to nanoscopic sizes. This trend is challenging because the increased levels of dissipated power demands a better understanding of heat transport in small volumes. A significant amount of the consumed energy is transformed into heat and dissipated to the environment. Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. Nanowires can meet nicely these two requirements because enhanced phonon scattering at the surface and defects reduces the lattice thermal conductivity while electric conductivity is not deteriorated, leading to an overall remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a promising route to achieving valuable thermoelectric materials at the nanoscale. In this paper, we present an overview of key experimental and theoretical results concerning the thermoelectric properties of nanowires. The focus of this review is put on the physical mechanisms by which the efficiency of nanowires can be improved. Phonon scattering at surfaces and interfaces, enhancement of the power factor by quantum effects and topological protection of electron states to prevent the degradation of electrical conductivity in nanowires are thoroughly discussed

Simultaneous Refractive Index Sensing Using an Array of Suspended Porous Silicon Membranes

We propose a fast and cost-effective method for obtaining a miniaturized array-formatted sensor suitable for multiplexed detection. Our solution is based on the fabrication of multiple µm-sized suspended porous silicon (PSi) membranes working as independent transducers. Our process can potentially integrate an array of up to 1000 sensing spots per cm2 . We also propose a simple and user-friendly optical platform to simultaneously interrogate each element of the array in real-time. The feasibility of this idea was proved performing several sensing experiments where we were able to detect refractive index (RI) variations with different transducers at the same time. An average experimental sensitivity of 685 nm/RIU (Refractive Index Unit) was achieved, with a theoretical limit of detection (LoD) of 10-5 RIU. The analyzed sensing spots displayed similar behavior both in time and in magnitude. We believe that the high capabilities of the sensor presented in this work, along with the sensing mechanism, can be very useful for multi-parametric analysis and multi-target detection of biological samples

Sensitivity Comparison of a Self-Standing Porous Silicon Membrane Under Flow-Through and Flow-Over Conditions

An optical sensor based on a self-standing porous silicon (PS) membrane is presented. The sensor was created by electrochemically etching a heavily doped p-type silicon wafer with an organic electrolyte that contained dimethylformamide. After fabrication, a high-current density close to electropolishing was applied in order to allow the detachment from the substrate using a lift-off method. The PS membrane was integrated in a microfluidic cell for sensing purposes, and reflectance spectra were continuously obtained while the target substance was flowed. A comparison of the bulk sensitivity is achieved when flowing through and over the pores is reported. During the experiments, a maximum sensitivity of 770 nm/RIU measured at 1700 nm was achieved. Experimental sensitivity values are in good agreement with the theoretical calculations performed when flowing through the PS membrane, it means that the highest possible sensitivity of that sensor was achieved. In contrast, a drop in the sensitivity of around 25% was observed when flowing over the PS membrane

Voltage dip generator for testing wind turbines connected to electrical networks

This paper describes a new voltage dip generator that allows the shape of the time profile of the voltage generated to be configured. The use of this device as a tool to test the fault ride-through capability of wind turbines connected to the electricity grid can provide some remarkable benefits: First, this system offers the possibility of adapting the main features of the time–voltage profile generated (dip depth, dip duration, the ramp slope during the recovery process after clearing fault, etc.) to the specific requirements set forth by the grid operation codes, in accordance with different network electrical systems standards. Second, another remarkable ability of this system is to provide sinusoidal voltage and current wave forms during the overall testing process without the presence of harmonic components. This is made possible by the absence of electronic converters. Finally, the paper includes results and a discussion on the experimental data obtained with the use of a reduced size laboratory prototype that was constructed to validate the operating features of this new device

Thermo-Optic Coefficient of Porous Silicon in the Infrared Region and Oxidation Process at Low Temperatures

In this work, a porous silicon nanostructure has been fabricated by electrochemical means and used as a thermal sensor. The thermo-optic effect in the near infrared region has been experimentally studied based on spectroscopy measurements. Values of the thermo-optic coefficient between 3.2 and 7.9·10^{-5} K^{-1} have been obtained, depending on the porosity, reaching a maximum thermal sensitivity of 91 ± 3 pm/°C during the experiments carried out with the fabricated samples. Additionally, the oxidation process of the sensor at temperatures below 500 K has been studied, showing that the growth of the silicon oxide was dependent on the characteristics of the porous layers. Based on the experimental results, a mathematical model was developed to estimate the evolution of the oxidation process as a function of porosity and thickness

ABCD transfer matrix model of Gaussian beam propagation in plano-concave optical microresonators

Plano-concave optical microresonators (PCMRs) are optical microcavities formed of one planar and one concave mirror separated by a spacer. PCMRs illuminated by Gaussian laser beams are used as sensors and filters in fields including quantum electrodynamics, temperature sensing, and photoacoustic imaging. To predict characteristics such as the sensitivity of PCMRs, a model of Gaussian beam propagation through PCMRs based on the ABCD matrix method was developed. To validate the model, interferometer transfer functions (ITFs) calculated for a range of PCMRs and beams were compared to experimental measurements. A good agreement was observed, suggesting the model is valid. It could therefore constitute a useful tool for designing and evaluating PCMR systems in various fields. The computer code implementing the model has been made available online

Team 6: Joint Capability Metamodel-Test-Metamodel Integration with Data Farming

from Scythe : Proceedings and Bulletin of the International Data Farming Community, Issue 2 Workshop 14US adversaries are continuously seeking new ways to threaten US interests at home and abroad. In order to counter these threats, now more than ever, commanders must seek to leverage existing and emerging joint capabilities effectively in a variety of unique contexts. Achieving mission effectiveness in today's joint operational environment demands robust synergy among a wide array of mission-critical Service systems and capabilities