Thermoelectric and thermal transport properties of complex oxide thin films, heterostructures and superlattices
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Over the years, the search for high performance thermoelectric materials has been dictated by the “phonon glass and electron crystal (PGEC)” paradigm, which suggests that low band gap semiconductors with high atomic number elements and high carrier mobility are the ideal materials to achieve high thermoelectric figure of merit. Complex oxides provide alternative mechanisms such as large density of states and strong electron correlation for high thermoelectric efficiency, albeit having low carrier mobility. Due to vast structural and chemical flexibility, they provide a fertile playground to design high efficiency thermoelectric materials. Further, developments in oxide thin film growth methods have enabled synthesis of high quality, atomically precise low dimensional structures such as heterostructures and superlattices. These materials and structures act as excellent model systems to explore nanoscale thermal and thermoelectric transport, which will not only expand the frontier of our knowledge, but also continue to enable cutting edge applications. Jayakanth Ravichandran is currently an Assistant Professor in the Mork Family Department of Chemical Engineering and Materials Science, University of Southern California. He received his B.Tech, and M.Tech in Metallurgical and Materials Engineering from Indian Institute of Technology, Kharagpur, India in 2007, and completed his Ph.D. in Applied Science and Technology from University of California, Berkeley in 2011. He was a post-doctoral fellow at Columbia University (2012–2014) and Harvard University (2014) before his current appointment. Currently, his research group’s interests include design, synthesis, characterization, and transport properties of complex ionic materials, and thin film science and technology. Often, the materials investigated in his group have relevance to electronic, photonic and energy applications. Jayakanth Ravichandran
I. INTRODUCTION
Thermoelectricity is a process of direct inter-conversion of thermal and electrical energy using a solid-state engine. The efficiency of a thermoelectric engine is related to a material dependent figure of merit, ZT and the Carnot efficiency. Z is given as a2r/j, where a is Seebeck coefficient or thermopower, a2r is power factor, and r and j are electrical and thermal conductivity respectively.1 Due to conflicting inter-relationships between these three parameters, every material has an optimal Z, and significant fraction of research on thermoelectrics has been focused on identifying high efficiency thermoelectric materials and optimizing their ZT.1,2 The two key
Contributing Editor: Terry M. Tritt a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.419
methods for optimizing Z are reduction of lattice or phonon thermal conductivity without affecting the electrical properties,3 and tuning the carrier concentration (very rarely band structure) by doping or alloying.4 While a thermoelectric engine possesses several advantages over other waste heat recovery technologies, such as lack o
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