Juan C. Nino
Department of Materials Science and Engineering
University of Florida

Engineering and Dynamics of Defects in Conducting Oxides: The Case of Ceria

Location: EB1 Room 1011

Friday, March 24th 2017 - 11:00 am

Engineering the point and extended defects within functional materials is an essential component in the optimization of their properties and performance. Focusing on rare-earth doped ceria as a model system, two cases of defect engineering are here showcased. First, the phenomenon of bias induced mixed ionic and electronic conductivity in gadolinia doped ceria (GDC) bulk ceramics and thin films deposited using RF sputtering is demonstrated. Electrochemical impedance spectroscopy is used to track the defect dynamics via changes in the electrical conductivity of the compounds under applied dc biases as a function of frequency and temperature. It is found that the application of dc bias produces a reversible decrease in both the grain and grain boundary resistances for GDC, and allows for the injection of electrons, thus leading to mixed conduction. Here we showcase this unique behavior and analyze the data through residual optimization of equivalent circuits models in order to gain insight on the underlying conduction and diffusion mechanisms at play. As a second example, using atom probe tomography we show that in Nd-doped ceria the dopants and oxygen vacancies segregate at grain boundaries, with the compositional difference extending up to 5-6 nm from the structural center of the grain boundary. This high concentration of dopants and defect-defect interactions at the grain boundaries makes them highly resistive and causes a drop in the overall ionic conductivity and performance. To overcome this problem, we demonstrate that in ceria sintered under a reducing atmosphere and lower temperatures compared to conventional sintering, the preferential dopant segregation is curtailed and a corresponding increase in the ionic conductivity of doped ceria is obtained. Opportunities to apply and extend these defect engineering approaches to other functional ceramics will be discussed.

Dr. Juan Claudio Nino, is an Endowed Professor in the Department of Materials Science and Engineering at University of Florida (UF) in Gainesville, FL. He obtained his bachelor's degree in Mechanical Engineering in 1997 at Los Andes University (Bogota, Colombia). He was a Lecturer at the Colombian Engineering School before joining The Pennsylvania State University in 1998, where he completed his doctoral degree in Materials Science and Engineering in 2002. After a postdoctoral appointment focusing on ferroelectric thin films at the Materials Research Institute (State College, PA), he joined UF in fall 2003 as an Assistant Professor. Since, he has established the Nino Research Group (NRG) with main focus of the investigation of advanced energy materials towards enhancing their efficiency, performance, and sustainability. He received tenure with promotion to Associate Professor in 2008, and was promoted to Professor in 2013. His research at NRG on ceramics, polymers, bio-inspired materials, and their composites has resulted in 120 publications in the field and five patents. Current research focus includes optimization and development of advanced functional materials for: (a) energy conversion and storage, (b) high frequency and high temperature electronics, (c) neural networks, and (d) semiconductors and scintillators for radiation detection. He is a recipient of the CAREER and the American Competitiveness and Innovation awards by the US National Science Foundation. In 2009 he received the J Bruce Wagner Jr Young Investigator Award from the Electrochemical Society. In 2014 he received the Fulbright US Scholar Innovation and Technology Award from the US Department of State. In 2016 he served as Expert within the Division of Materials Research at the US National Science Foundation. He is an Associate Editor for the Journal of the American Ceramics Society, and a Coordinating Editor for the Journal of Electroceramics. He was recently selected as a 2017-2018 Jefferson Science Fellow by the US National Academies and the US Department of State.

North Carolina State University