Invited Speaker

Dr. Sepideh Akhbarifar

Dr. Sepideh Akhbarifar

Vitreous State Laboratory and Department of Physics, The Catholic University of America, USA
Speech Title: Transport Mechanisms and Metal-insulator Transition of Lead-yttrium Ruthenate Pyrochlores

Abstract: Aims: Find and understand the metal-to-insulator transition (MIT) in these ceramics. Use of quantum physical models a) to identify and understand the scattering mechanisms underlying electrical (σ) and thermal conductivity (κ) in lead-yttrium ruthenate pyrochlore solid solutions; b) understand the temperature dependence of the Seebeck coefficient (S)

Methods: All materials were synthesized by solid state reaction at high temperature. Thermoelectric properties were measured from 25 to 300ºC.

Results: At 0.2 moles of yttrium (Y) a metal-insulator transition was discovered, which is explained by the Mott-Hubbard model. S(T) is discussed in terms of the effect of minority carrier excitation at lower- and a broadening of the Fermi distribution at higher temperatures. κ(T) showed glass-like behavior in these crystalline pyrochlores. On the metallic side (<0.2 moles Y) ‘electron impurity scattering’ prevailed. On the semiconductor/insulator side between 0.2 and 1.0 moles Y several mechanisms were equally likely. At higher Y concentrations the Mott Variable Range Hopping mechanism was active.

Conclusions: The hitherto unknown thermoelectric properties of lead-yttrium ruthenate pyrochlores were measured, a metal-insulator transition was found, and all properties were explained using quantum-physical models.

Acknowledgements: The author thanks Drs. Werner Lutze, Nicholas A. Mecholsky, David McKeowan, Marek Brandys, Ian Pegg for contributions and discussions.

Biography: Sepideh Akhbarifar is a postdoctoral researcher in physics (materials science) and adjunct faculty in the School of Engineering at The Catholic University of America, Washington, DC, USA. She holds a PhD in physic and has master’s degrees in Nuclear Environmental Protection and in Chemical Engineering. Her scientific interests focus on environmental protection, specifically energy efficiency and clean air. Her research comprises thermoelectric materials, metal-insulator transitions, low CO2 producing cement and other binders (geopolymers), dust removal from air (ultrahigh efficiency cyclones), and geopolymer radioactive waste forms. She was honored as ‘Young Women Inventor 2010’ by Iran's National Elites Foundation. She publishes widely, presents her work at national and international conferences, and holds two patents. She is an active member of several scientific societies, e.g., as a member of the ‘Early Career Subcommittee’ of the Materials Research Society, USA.