• Energy Research

AbstractThree-dimensional anodic alumina templates (3D-AAO) are an astonishing framework with open highly ordered three-dimensional skeleton structures. Since these templates are architecturally different from conventional solids or porous templates, they teem with opportunities for engineering thermal properties. By establishing the mechanisms of heat transfer in these frameworks, we aim to create materials with tailored thermal properties. The effective thermal conductivity of an empty 3D-AAO membrane was measured. As the effective medium theory was not valid to extract the skeletal thermal conductivity of 3D-AAO, a simple 3D thermal conduction model was developed, based on a mixed series and parallel thermal resistor circuit, giving a skeletal thermal conductivity value of approximately 1.25 W·m−1·K−1, which matches the value of the ordinary AAO membranes prepared from the same acid solution. The effect of different filler materials as well as the variation of the number of transversal nanochannels and the length of the 3D-AAO membrane in the effective thermal conductivity of the composite was studied. Finally, the thermal conductivity of two 3D-AAO membranes filled with cobalt and bismuth telluride was also measured, which was in good agreement with the thermal model predictions. Therefore, this work proved this structure as a powerful approach to tailor thermal properties.

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 reviewed

© 2015 Elsevier Ltd. All rights reserved. The effect of the addition of a surfactant, sodium lignosulfonate (SLS), on the thermoelectric properties of tellurium films prepared by electrochemical deposition is studied. The growth mechanism is found to have an important role in the thermoelectric properties since the grain size of the films is sharply reduced when the surfactant is added to the solution. For this reason, the electrical resistivity of the tellurium films when the surfactant is not added is 229 μΩ·m, which is lower than 798 μΩ·m with SLS. The Seebeck coefficient values are not influenced, with values in the vicinity of 285 μV/K for both solutions. The power factor resulted higher values than previous works, reaching values of 280 μW/m·K2 (without SLS) and 82 μW/m·K2 (with SLS) at room temperature. Finally, the thermal conductivity was measured by means of the Photoacoustic technique, which showed values of the order of 1 W/m·K for both solutions, which is a factor of 3 less than the bulk value of tellurium. A notable observation is that the power factor and the thermal conductivity of electrodeposited tellurium films have the same order of magnitude of bismuth telluride films grown by electrodeposition. The figure of merit is estimated to be approximately one order of magnitude higher than the bulk value, 0.09 without SLS and 0.03 with SLS, both at room temperature. The authors would like to acknowledge the financial support from ERC StG NanoTEC240497 and national project PHOMENTA MAT2011-27911. Peer Reviewed

9 figuras The formation of Ni1–xZnxO rock salt solid solution is obtained by thermal treatments applied to NiO nanoparticles supported on ZnO micrometric particles. The high vapor pressure of ZnO produces a Zn-rich atmosphere during thermal treatment. The Zn ions tend to be adsorbed by the highly reactive NiO nanoparticles. When heated to temperatures high enough, over 500 °C, the Zn ions react with the NiO nanoparticles forming the rock salt material. By variation of the treatment temperature, the composition of this rock salt structure can be varied through the whole solubility range. The authors express their thanks to the MICINN (Spain)projects MAT 2010-21088-C03-01, MICINN ACI PLAN E (JAPON) ref: PLE2009-0073, and to the European Comission ERC-2008-Stg: 240497 for their financial support. Peer reviewed

Zinc oxide (ZnO) films have been grown on gold (111) by electrodeposition using two different OH− sources, nitrate and peroxide, in order to obtain a comparative study between them. The morphology, structural and optical characterization of the films were investigated depending on the solution used (nitrate and peroxide) and the applied potential. Scanning electron microscopy pictures show different morphologies in each case. X-ray diffraction confirms that the films are pure ZnO oriented along the (0002) direction. ZnO films have been studied by photoluminescence to identify the emission of defects in the visible range. A consistent model that explains the emissions for the different electrodeposited ZnO films is proposed. We have associated the green and yellow emissions to a transition from the donor OH− to the acceptor zinc vacancies (VZn−) and to interstitial oxygen (Oi0), respectively. The orange-red emission is probably due to transitions from the conducting band to Oi− and OZn0 defects and the infrared emission to transition from these Oi−/2− and OZn0/− defects to the valence band.

This work presents an approach for measuring cross plane electrical contact resistances directly using Kelvin Probe Microscopy. With this technique we were able to measure the electrical contact resistances of a cross section of a thermoelectric thin film made of Bi2Te3 sandwiched between two gold electrodes. On the one hand, the bottom gold electrode, which is located on top of the silicon substrate, was used as a cathode in electro-deposition process to grow the sample. On the other hand, the gold electrode on top was made via physical evaporation. The electrical contact resistances measured at both interfaces were 0.11 ± 0.01Ω and 0.15 ± 0.01Ω, respectively. These differences are related to differences between the top and bottom gold/bismuth-telluride film, obtaining smaller contact resistance where the film was grown by electro-deposition.

In the beginning of the 21st century, the world is facing the major challenge of finding energy sources to satisfy the ever-increasing energy consumption while preserving the environment. In the race to search alternative energy sources, thermoelectric generators are called to play their role in the improvement of the efficiency of the actual energy system by harvesting nowadays wasted heat. This review deals with the novel aspects of nano-structuring of thermoelectric materials, from the so called 3D nanobulk materials down to the incorporation of 0D quantum dots in thermoelectric structures. The improvement in the efficiency of nanoengineering thermoelectrics benefits mainly from the reduction in the thermal conductivity. Other promising trends in thermoelectricity are also reviewed, such as, novel nano-structures, trending materials (polymers, thermionic materials or Zintl phases), spin caloritronics, thermoelectricity in atomic and molecular junctions, or recent developments in theoretical calculations. Finally the review ends with a brief review on recent thermoelectric devices. Authors want to thank the ERC 2008 Starting Grant Nano-TEC number 240497 for financial support. Peer Reviewed