335 research outputs found
Planar and vertical nanostructures for thermoelectric generation
No full textThe reduction of phonon propagation at the nanoscale offers the opportunity to exploit, for thermoelectric applications, materials which are abundant on Earth and technologically feasible, such as silicon, but which have a high bulk thermal conductivity. We propose two strategies for the fabrication of thermoelectric devices based on large collections of silicon nanostructures, and we report the results of thermal conductivity measurements in both cases. At first, we describe the fabrication process of a silicon nanowire thermoelectric generator based on nanowire forests. We describe also a possible solution to reduce the parasitic electrical resistance of the substrate. Then, we illustrate a strategy based on planar silicon nanofabrication techniques. Devices based on suspended nanostructures are shown, and the effect of localized phonon scattering centers on the reduction of the thermal conductivity is reported
Nanostructured Multimetal Granular Thin Films: How to Control Chaos
No full textA simple method for the fabrication of films with an high density of nanometric metal grains is presented and discussed. The method is based on the successive evaporation of different metals (Au, Al and In), with different melting points: after each evaporation, rapid thermal annealing treatments are used to induce agglomeration of the deposited material. The nanograin agglomeration (in particular of the Al film) resulted strongly dependent on the previous nanograin (gold) distribution and concentration. Statistics of gold based nanograin films, deposited on silicon dioxide, are presented for different gold thickness and annealing parameters. Results on Al induced agglomeration of a successively evaporated thin film are shown for different Au nanoparticle distribution and concentration. A final indium deposition, with suitable annealing temperature and time, produces films with a very high density of metal nanoparticles (more than 2000 nanograins/1m2, average radius between 5 and 10 nm)
Note: Improvement of the 3w thermal conductivity measurement technique for its application at the nanoscale
No full textConventional techniques for thermal conductivity measurements can lead to unreliable results when applied to nanostructures because heaters and temperature sensors needed for the measurement cannot have a negligible size and therefore perturb the result. In this paper, we focus on the 3w technique, applied to the evaluation of the thermal conductivity of suspended silicon nanoribbons. We introduce a numerical approach based on the finite element solution of the electrical and thermal transport equations and compare its results with those of conventional methods. We show that with our approach we achieve an excellent fit of the experimental data, in particular, for nanostructured materials
Management of the output electrical power in thermoelectric generators
No full textThermoelectric Generators (TEGs) are devices for direct conversion of heat into electrical power and bear a great potential for applications in energy scavenging and green energy harvesting. Given a heat source, the conversion efficiency depends on the available temperature difference, and must be maximized for optimal operation of the TEG. In this frame, the choice of materials with high thermoelectric properties should be accompanied by the identification of criteria for an optimal exploitation of the electrical power output. In this work, we briefly review the main properties of TEGs, focusing on the electrical power output and the thermal-to-electrical conversion efficiency. Besides, we discuss principles of operation of TEGs enabling the optimization of the electrical power output, based on the suitable choice of the electrical load. In particular, we comparatively present and discuss the conditions for matching the electrical load—yielding to maximum power transfer—and those for maximizing the conversion efficiency. We compare the two conditions applying them to the exploitation of a heat reservoir for energy storage and to the recovery of heat from a heat exchanger. We conclude that the difference between the two conditions is not significant enough to justify the complexity required by the implementation of the maximum efficiency. In addition, we consider the effect of the thermal contact resistance on the electrical power output. Using a simple thermal-electrical model, we demonstrate that the equivalent electrical resistance measured between the terminals of the TEG depends on the thermal exchange. Hence, for maximum power transfer, the electrical load of the TEG should not match its parasitic resistance, but the equivalent electrical resistance in each specific operating conditions, which determine the thermal fluxes. The model can be applied for the development of efficient alternative algorithms for maximum power point tracking
Seebeck coefficient of silicon nanowire forests doped by thermal diffusion
No full textThermoelectric generators made by large arrays of nanowires perpendicular to a silicon substrate, that is, so-called silicon nanowire forests are fabricated on large areas by an inexpensive metal-assisted etching technique. After fabrication, a thermal diffusion process is used for doping the nanowire forest with phosphorous. A suitable experimental technique has been developed for the measurement of the Seebeck coefficient under static conditions, and results are reported for different doping parameters. These results are in good agreement with numerical simulations of the doping process applied to silicon nanowires. These devices, based on doped nanowire forests, offer a possible route for the exploitation of the high power factor of silicon, which, combined with the very low thermal conductivity of nanostructures, will yield a high efficiency of the conversion of thermal to electrical energy
Silicon nanowires: A breakthrough for thermoelectric applications
No full textThe potentialities of silicon as a starting material for electronic devices are well known and largely exploited, driving the worldwide spreading of integrated circuits. When nanostructured, silicon is also an excellent material for thermoelectric applications, and hence it could give a significant contribution in the fundamental fields of energy micro-harvesting (scavenging) and macro-harvesting. On the basis of recently published experimental works, we show that the power factor of silicon is very high in a large temperature range (from room temperature up to 900 K). Combining the high power factor with the reduced thermal conductivity of monocrystalline silicon nanowires and nanostructures, we show that the foreseen figure of merit ZT could be very high, reaching values well above 1 at temperatures around 900 K. We report the best parameters to optimize the thermoelectric properties of silicon nanostructures, in terms of doping concentration and nanowire diameter. At the end, we report some technological processes and solutions for the fabrication of macroscopic thermoelectric devices, based on large numbers of silicon nanowire/nanostructures, showing some fabricated demonstrators
Thermal conductivity of silicon nanowire forests
No full textA large amount of parallel silicon nanowires, placed perpendicularly to a silicon substrate (silicon nanowire forests), have been contacted and assembled in order to fabricate legs of a thermoelectric generator. This paper reports the measurement of the main parameter for thermoelectric applications, which is the thermal conductivity. The reported value, which confirms the strong reduction of the thermal conductivity in nanostructures, is measured on a large amount (>107) of parallel nanowires with a diameter variable in the range 60-120 nm, and takes into account eventual non-uniformities which are unavoidable on surfaces of several mm2. As silicon nanowire forests are very thin, it has been necessary to develop a suitable measurement apparatus. The fabrication of devices based on silicon nanowire forests, the apparatus and the measurement procedure, as well as the the results, are illustrated and discussed
Electrical and noise characterization of suspended silicon wires
No full textWe present the fabrication of suspended silicon nanowires using 1 mum resolution conventional photolithography, anisotropic wet etching and thermal oxidation. A minimum transverse dimension of about 50 nm was achieved in some of the wires. We characterized the wires from the electrical and noise points of view, comparing the behavior at room temperature and at 77 K, and discuss possible hypotheses to explain the observed differences. (C) 2002 Elsevier Science B.V. All rights reserved
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