Abstract
The thermoelectric performance of nanostructured low dimensional silicon and silicon-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 improved thermoelectric properties of nanostructured silicon-based materials. The device architecture 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 interconnected) using standard IC (Integrated Circuits) and MEMS (Micro-ElectroMechanical Systems) 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 circuit voltages and a larger overall power supply. This is possible due to the lower thermal conductance 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.
Highlights
The advent of Internet of Things (IoT) [1,2] and the digital transformation of our society is adding another dimension to the energy issue that fossil fuels and renewables cannot cope with
In our particular unileg approach [16,17,18,19,20], free-standing arrays of p-type silicon nanowires (NWs) arrays have been chosen as the main thermoelectric material, and the thermoelectric circuit is completed with a metal thin film as second thermoelectric material placed on top of ancillary connecting structures
In an attempt to go beyond material characterization, this paper focuses in integrating and assessing the performance of such SiGe NWs using similar Si NWs counterparts as a benchmark at working device level, and tracing back the performance outcome to the thermal, electrical and thermoelectric properties of both types of nanowires
Summary
The advent of Internet of Things (IoT) [1,2] and the digital transformation of our society is adding another dimension to the energy issue that fossil fuels and renewables cannot cope with. Powering trillions of sensors [3,4] amounts to a good deal of energy, but each one of those sensors will just require a tiny bit of it to be functionally autonomous. Batteries are currently providing this micro-energy autonomy. Replacing billions of batteries will not be feasible logistically in near future IoT scenarios and their disposal will generate a huge environmental impact. The renewable alternative to batteries is energy harvesting, by which tiny to moderate amounts of energy can be retrieved from environmental sources such as light, heat or electromagnetic radiation [5]. Thermoelectric generators are a completely solid-state robust approach to energy harvesting featuring no moving parts [6,7,8] that can provide energy autonomy in those
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