Abstract
Maximizing the efficiency of thermoelectrics requires maximizing the power factor while also minimizing the lattice thermal conductivity. In recent years, most progress has been made by reducing the lattice thermal conductivity, either by microstructural control or selecting compounds with low thermal conductivity. In this talk I will describe some of our work on the role of spin and orbital degeneracy in oxides containing transition elements and their relationship to the Seebeck coefficient, one of the components of the power factor. Our objective is to select compounds with stereochemistries for transition ions that will maximize the Seebeck coefficient and the power factor.
Lithium manganate LiMn2O4 is an example of a strongly correlated oxide in which the Mn ion can exist in both Mn3+ and Mn4+ states and so offers the opportunity to assess the effects of spin and orbital degeneracies but complications arise because of its susceptibility to Jahn-Teller distortions. I will describe our measurements of its thermopower, electrical conductivity and thermal transport as well as crystal structure from room temperature up to 900 oC. Good agreement with the high temperature Heikes formula for the Seebeck coefficient was observed and the high temperature Seebeck coefficient was found to be -86 μV/K, for equal concentrations of Mn3+/4+ ions in an octahedral crystal field. The other properties of LiMn2O4 will be presented and the prospects for enhancing the Seebeck coefficient by doping will be described.
CV
David Clarke pursues three primary areas of research: high temperature materials, including thermal barrier coatings; materials for electronics and MEMS devices; and electrical and piezoelectric properties of ceramics. The research includes the development and application of novel sensor for measuring stress and temperature with a focus on optical based methods.
He received his Ph.D. in Physics from Cavendish Laboratory, University of Cambridge, England and B.Sc. (First Class Honors) in Applied Sciences from the University of Sussex, England.
Prior to his appointment at Harvard, Clarke was a Professor of Materials at the University of California, Santa Barbara. He also served as Senior Manager of the Materials Department at IBM Research Division in Yorktown Heights, New York; an Associate Professor at Massachusetts Institute of Technology; and had appointments at Rockwell International Science Center and the University of California, Berkeley.
Clarke has been involved in many different materials research and development programs, contributing to ceramics, metals, composites and semiconductors, as well as introducing new approaches for studying the interrelations between microstructure and properties. He is author or co-author of more than 350 papers, holder of 6 patents, and a member of the National Academy of Engineering.
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