Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly infuence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87W·m−1·K−1 for 90nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we fnd a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifes the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient infuences phonon transport along the axis of the NW. Our fndings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices

Review on measurement techniques of transport properties of nanowires

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. © 2013 The Royal Society of Chemistry.This work has been supported by ERC Starting Grant Nano-TEC number 240497, Nanotherm Consolider CSD-2010-00044 project and PHOMENTA project MAT2011-27911. MM and OC wish to acknowledge CSIC and the European Social Fund for financial support by JAE-Pre and JAE-Doc. JRV and AFL acknowledge financial support from Generalitat de Catalunya through Grant SGR2009-01225 and from Marie Curie European Reintegration Grant within the 7th European Community Framework Programme.We also acknowledge institutional support from the Unit of Information Resources for Research at the "Consejo Superior de Investigaciones Científicas" (CSIC) for the article-processing charges contribution.Peer Reviewe

Prediction of the thermal conductivity of Bi2Te3 Nanowire when reducing its diameter

Póster presentado en la 12th European Conference on Thermoelectricity (ECT2014), celebrada en Madrid del 24 al 26 de septiembre de 2014.A Kinetic-Collective model of phonon heat transport is used to calculate the thermal conductivity of bulk Bi2 Te and nanowires with diameters ranging from 350 to 50 nm, oriented in the [110] direction from low temperatures up to above room temperature. This model accounts for the role of normal (momentum-conserving) collisions in thermal transport, and provides a more accurate prediction of the thermal conductivity as shown elsewhere on other materials [1,2]. Within this model, the thermal conductivity is explained as a combination of a kinetic and a collective phonon heat flux with significantly different contributions. Nanowires with 350 and 120 nm in diameter have been grown in the [110] direction. easurements of their thermal conductivity at room temperature agree very well with th e theoretical predictions. Furthermore, the figure of merit ZT is expected to increase considerably for the 120 nm NW under certain values of the applied voltage across the sample, while the 350 nm NW remains with near bulk values.Peer Reviewe

Review on measurement techniques of transport properties of nanowires

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

From kinetic to collective behavior in thermal transport on semiconductors and semiconductor nanostructures

We present a model which deepens into the role that normal scattering has on the thermal conductivity in semiconductor bulk, micro, and nanoscale samples. Thermal conductivity as a function of the temperature undergoes a smooth transition from a kinetic to a collective regime that depends on the importance of normal scattering events. We demonstrate that in this transition, the key point to fit experimental data is changing the way to perform the average on the scattering rates. We apply the model to bulk Si with different isotopic compositions obtaining an accurate fit. Then we calculate the thermal conductivity of Si thin films and nanowires by only introducing the effective size as additional parameter. The model provides a better prediction of the thermal conductivity behavior valid for all temperatures and sizes above 30 nm with a single expression. Avoiding the introduction of confinement or quantum effects, the model permits to establish the limit of classical theories in the study of the thermal conductivity in nanoscopic systems. © 2014 AIP Publishing LLC

Measuring Device and Material ZT in a Thin-Film Si-Based Thermoelectric Microgenerator

Thermoelectricity (TE) is proving to be a promising way to harvest energy for small applications and to produce a new range of thermal sensors. Recently, several thermoelectric generators (TEGs) based on nanomaterials have been developed, outperforming the efficiencies of many previous bulk generators. Here, we presented the thermoelectric characterization at different temperatures (from 50 to 350 K) of the Si thin-film based on Phosphorous (n) and Boron (p) doped thermocouples that conform to a planar micro TEG. The thermocouples were defined through selective doping by ion implantation, using boron and phosphorous, on a 100 nm thin Si film. The thermal conductivity, the Seebeck coefficient, and the electrical resistivity of each Si thermocouple was experimentally determined using the in-built heater/sensor probes and the resulting values were refined with the aid of finite element modeling (FEM). The results showed a thermoelectric figure of merit for the Si thin films of z T = 0.0093, at room temperature, which was about 12% higher than the bulk Si. In addition, we tested the thermoelectric performance of the TEG by measuring its own figure of merit, yielding a result of ZT = 0.0046 at room temperature.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires (vol 8, 12796, 2018)

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