Jon F. Ihlefeld, Ph.D.
Principal Member of the Technical Staff
Electronic, Optical, and Nano Materials Department
Sandia National Laboratories
Host: Prof. Jerry Floro
Talk Title: Developing and Understanding Functional Electronic and Thermal Properties in Complex Oxides
Date: Tuesday, March 17, 2015
Time: 4:00 to 5:00
Room: Wilsdorf 101
Refreshments: 3:30 to 4:00
Two topics of engineering functionality into complex oxides will be discussed in this presentation. The first will be developing an understanding of the role of thermodynamic equilibrium on the performance of oxide-based thermoelectric materials. We will discuss the role of oxygen and cation vacancies on the power factor (thermopower and electrical conductivity) of a relatively high ZT, n-type thermoelectric oxide ceramic, Nb-doped SrTiO3, under equilibrium conditions. The profound role of these point defects on the thermoelectric performance will be assessed as a function of temperature between 300K and 1200K and as a function of oxygen partial pressure between 10-23 and 10-2 atmospheres at elevated temperatures (1073 and 1173K) – those for which these materials are commonly suggested have a stability advantage over conventional semiconductor thermoelectrics. It will be shown that there is a colossal change, 104 µW/m-K, in power factor from 2 µW/m-K to 4x10-4 µW/m-K for a 10% Nb-doped ceramic at 1173K that is measured by only varying the oxygen partial pressure from 10-23 to 10-2 atm, demonstrating the importance of oxygen stoichiometry and defect compensation on performance. These results have significant impacts on the design considerations of thermoelectric modules utilizing oxide materials.
Second, we will discuss means to deterministically engineer phonon thermal conduction properties in ferroelectric materials by manipulating the density of coherent ferroelastic domain boundaries. It will be shown that crystallographically coherent 71° domain walls in epitaxial BiFeO3 and 90° domain walls in polycrystalline Pb(Zr,Ti)O3 thin films can scatter heat-carrying phonons at room temperature. Utilizing bilayer PZT films, where the ferroelastic domain walls are labile under applied fields, we will show how modifying the domain structure can be used to alter thermal conductivity. For Pb(Zr0.3Ti0.7)O3 films a repeatable 11% change in thermal conductivity at room temperature can be achieved with the application and removal of an electric field. Utilizing piezoforce response and in operando channeling contrast scanning electron microscopies, we can identify the domain wall reconfiguration responsible for the observed thermal property tuning. The rapid and reversible response and this demonstration of a voltage tunable thermal conductivity at room temperature without passing through phase transitions, physically separating components, or altering chemical composition opens a pathway to develop phononic devices and may also be exploited for low input energy nanoscale temperature control.
Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.