Chapter Managing Heat Transfer Issues in Thermoelectric Microgenerators

This chapter deals with heat transfer challenges in the microdomain. It focuses on practical issues regarding this matter when attempting the fabrication of small footprint thermoelectric generators (μTEGs). Thermoelectric devices are designed to bridge a heat source (e.g. hot surface) and a heat sink (e.g. ambient) assuring that a significant fraction of the available temperature difference is captured across the active thermoelectric materials. Coexistence of those contrasted temperatures in small devices is challenging. It requires careful decisions about the geometry and the intrinsic thermal properties of the materials involved. The geometrical challenges lead to micromachined architectures, which silicon technologies provide in a controlled way, but leading to fragile structures, too. In addition, extracting heat from small systems is problematic because of the high thermal resistance associated to heat exchanged by natural convection between the surrounding air and small bare surfaces. Forced convection or the application of a cold finger clearly shows the usefulness of assembling a heat exchanger in a way that is effective and compliant with the mechanical constraints of micromachined devices. Simulations and characterization of fabricated structures illustrate the effectiveness of this element integration and its impact on the trade-off between electrical and thermal behavior of the active materials in device performance

Managing Heat Transfer Issues in Thermoelectric Microgenerators

This chapter deals with heat transfer challenges in the microdomain. It focuses on practical issues regarding this matter when attempting the fabrication of small footprint thermoelectric generators (μTEGs). Thermoelectric devices are designed to bridge a heat source (e.g. hot surface) and a heat sink (e.g. ambient) assuring that a significant fraction of the available temperature difference is captured across the active thermoelectric materials. Coexistence of those contrasted temperatures in small devices is challenging. It requires careful decisions about the geometry and the intrinsic thermal properties of the materials involved. The geometrical challenges lead to micromachined architectures, which silicon technologies provide in a controlled way, but leading to fragile structures, too. In addition, extracting heat from small systems is problematic because of the high thermal resistance associated to heat exchanged by natural convection between the surrounding air and small bare surfaces. Forced convection or the application of a cold finger clearly shows the usefulness of assembling a heat exchanger in a way that is effective and compliant with the mechanical constraints of micromachined devices. Simulations and characterization of fabricated structures illustrate the effectiveness of this element integration and its impact on the trade-off between electrical and thermal behavior of the active materials in device performance

Preparation and characterization of SbSeI thin films

Metal chalcohalides are promising candidates for next-generation technologies that include energy conversion, information storage, and quantum computing. Among them, antimony selenoiodide (SbSeI) has received rising interest for different optoelectronic devices, including photovoltaics, due to its bandgap energy, strong optical absorption, stability, and earth abundant, low-cost, and low toxicity constituents. In this work, SbSeI thin films were prepared through a two-step process. At first, antimony selenide (Sb2Se3) thin films were deposited at 300 °C (Sb2Se3-300) and at room temperature (Sb2Se3-RT) onto molybdenum covered soda-lime glass substrates by a magnetron sputtering method. The formation of SbSeI thin films was performed by isothermally annealing the as-deposited Sb2Se3 thin films in sealed quartz ampoules in the atmosphere of antimony iodide (SbI3) with the presence of 100 Torr of argon pressure. The influence of the annealing temperature and time during the iodization of different types of substrates on the morphology and composition of SbSeI thin films was investigated. The well-oriented and dense single-phase SbSeI thin films with stoichiometric composition and single-crystal micro-columnar structures were achieved by annealing Sb2Se3-RT in SbI3 atmosphere at 250 °C for 5 min under 100 Torr of Ar pressure. The room temperature photoluminescence (RT-PL) of SbSeI exhibited a broad asymmetric PL band with a maximum at 1.67 eV. The low-temperature (T = 8 K) PL study of SbSeI showed a broad and asymmetric PL band at 1.4 eV, being quite distant from the bandgap. This PL band at 1.4 eV with obtained small thermal quenching activation energy of 12.7 meV is proposed to originate from the deep donor-deep acceptor pair (DD-DA) recombination

Transitioning from Si to SiGe Nanowires as Thermoelectric Material in Silicon-Based Microgenerators

The thermoelectric performance of nanostructured low dimensional silicon and sili-con-germanium has been functionally compared device-wise. The arrays of nanowires of both materials, grown by a VLS-CVD (Vapor-Liquid-Solid Chemical Vapor Deposition) method, have been monolithically integrated in a silicon micromachined structure in order to exploit the im-proved thermoelectric properties of nanostructured silicon-based materials. The device archi-tecture helps to translate a vertically occurring temperature gradient into a lateral temperature difference across the nanowires. Such thermocouple is completed with a thin film metal leg in a unileg configuration. The device is operative on its own and can be largely replicated (and in-terconnected) using standard IC (Integrated Circuits) and MEMS (Micro Electro Mechanical Sys-tems) technologies. Despite SiGe nanowires devices show a lower Seebeck coefficient and a higher electrical resistance, they exhibit a much better performance leading to larger open cir-cuit voltages and a larger overall power supply. This is possible due to the lower thermal con-ductance of the nanostructured SiGe ensemble that enables a much larger internal temperature difference for the same external thermal gradient. Indeed, power densities in the μW/cm2 could be obtained for such devices when resting on hot surfaces in the 50–200 °C range under natural convection even without the presence of a heat exchanger.Peer reviewe