題目：From Voltage-Driven Phase Transition Towards Electrocaloric Devices
報告人：Emmanuel Defay（Luxembourg Institute of Science and Technology）
Emmanuel Defay is the head of the Nanotech Unit at Luxembourg Institute of Science and Technology (70 researchers) since 2021 and the head of the Ferroic Materials for Transducers group (25 researchers) since 2014. He obtained his PhD degree in INSA Lyon (Fr.) in 1999 on piezoelectric materials. He then spent 15 years at CEA LETI (Grenoble, Fr.) to develop microsystems. He stayed for two years (2010-2012) at the University of Cambridge as invited scholar to study the electrocaloric effect. His research lies between materials science, engineering and thermodynamics, all of them being required to create innovation in bringing these materials into realistic applications. He wrote 2 books, 150 papers and filed 40 patents. He has a balanced portfolio between competitive projects (9 projects) and collaborative projects with industry (8 projects) and a large experience in supervising young researchers (25 people in the group today, 18 alumni, all in all 21 PhD students and 11 post docs). Besides, he is very much involved in scientific events for general audience in Luxembourg.
Caloric materials stand for a more and more convincing alternative to standard vapour compression systems for cooling, as recently suggested in the literature with for instance a colossal barocaloric effect in plastic crystals (Lloveras et al., Nat Comms. 2019) or electrocaloric prototypes with large variation of temperature (Torello et al., Science 2020). In this lecture, we will focus on electrocaloric materials in which the caloric effect is triggered with an external electric field. We will first show how to observe this effect experimentally. A simple, though powerful, free energy description of the effect will enable understanding the main aspects of what a good electrocaloric material is. We will then detail the role played by voltage driven phase transitions in electrocalorics. The main prototypical materials such as lead scandium tantalate will be disclosed. Finally, applicative aspects will be described with a focus on heat pumps and coolers that recently caught attention.