The European Ceramic Society
YCN Newsletter 16 : YCN-Research in Spot – Micro-harvesters of heat for the Internet of Things by Carlos Nunez Lobato
In the coming years we will witness an ever-growing network of interconnected sensors embedded in our everyday objects and devices.
This wireless infrastructure commonly known as the Internet of Things (IoT) aims to tackle some of the biggest challenges of society through ubiquitous data exchange, like optimizing energy saving or remote healthcare monitoring. However, powering vast amounts of low power IoT sensors still faces strong environmental concerns such us the use of billions of disposable batteries. Due to the excess free heat from the environment and even from our own bodies, miniaturized energy harvesters using thermoelectric (TE) materials have become increasingly relevant in an attempt to push for renewable sources.
The thermoelectric or Seebeck effect is a phenomenon whereby an electrical voltage is generated upon an application of a heat gradient to a material. However, most materials are rather inefficient at this direct heat-to-electricity conversion, since optimizing TE performance means balancing rather opposing material characteristics. For instance, efficient TEs should display both large electrical conductivities – such as metals – while maximizing the temperature difference experienced when the material is heated i.e. exhibit poor thermal conductance – just like insulators. Semiconductors and ceramics can be designed such as to meet some of these strange requirements. Nonetheless most efficient TE materials at room temperature are based on Bi2Te3 semiconductors and the scarcity of tellurides hinders their large scalability.
In my work we aim to understand the behavior of very thin films of alternative and more abundant TE materials, namely SrTiO3-based oxide perovskites. SrTiO3 is an insulator but when doped with small amounts of Niobium it becomes conducting and hence a good candidate for thermoelectricity. We synthesize these nanometer sized films using Pulsed Laser Deposition, a physical deposition where a laser ablates a sintered ceramic target of perovskites. In this case, doped SrTiO3 atoms are evaporated onto the surface of a substrate placed on a heater at high temperatures (700oC) (fig 1a). Once film growth is performed, we fabricate devices that enable characterization of the thermoelectric transport parameters in the film, thereby measuring e.g. electrical conductivity, Seebeck voltage (fig 1b,c) or carrier density.
Our goal with these TE films is thus twofold: (1) to control the thermoelectric properties of these materials using stimuli, such as the very self-strain of the film upon deposition and even applying external electric fields & (2) explore TE thin films grown on top of suitable silicon platforms, in order to facilitate the nanofabrication of these devices in the semiconductor industry.
Merging the field of oxide thermoelectrics with nanoscale thin films opens in this sense promising and exciting strategies to integrate them as room temperature energy micro-harvesters into todays and the futures network of the IoT.
Figure 1.(a) Pulsed Laser Deposition of a thin perovskite film using a Nb-doped SrTiO3 ceramic target and a Si substrate. Inset displays the relative size of the chip with respect to a coin (b) Measurement of the Seebeck effect when the sample is on a hot plate VSeebeck=1.7 mV @ΔT=20oC above room temperature (c) Illustration of the thermoelectric voltage generated upon a heat gradient and close-up of fig 1(b)
Carlos Nunez Lobato
Technical University of Denmark (DTU)
Fysikvej Building 310 2800 Kgs. Lyngby
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