MSE Colloquium - Prof. Alexandra Navrotsky
Date: February 15, 2013 from 2:00 pm to 3:00 pm EST
Location: Room 214, S.W. Mudd
Contact: For further information regarding this event, please contact Wesley Hattan by sending email to wjh2121@columbia.edu or by calling 2128547860.
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Size-induced Shifts in the Thermodynamics of Nanophase Oxides and Implications for Materials Science, Geochemistry and Environmental Science

Alexandra Navrotsky

Peter A. Rock Thermochemistry Laboratory and NEAT ORU

UC Davis

                It is well established that difference in surface energies can alter the relative free energies of different polymorphs, causing size driven thermodynamic crossovers in phase stability at the nanoscale. It has also been shown that, because oxyhydroxides generally have smaller surface energies than oxides, dehydration equilibria, e.g. goethite to hematite plus water, can shift to higher temperature by as much as 100 K at the nanoscale. A general formulation of the effect of particle size on chemical equilibria among solid phases is that increasing surface area will favor the phase assemblage of lower surface energy. There is now new thermochemical evidence for strong thermodynamic shifts in the position of oxidation-reduction (redox) equilibria in oxides at the nanoscale. Using calorimetric data on surface energies in the cobalt-oxygen system, we show that the thermodynamic phase field in oxygen fugacity-temperature space of the divalent rocksalt oxide CoO is substantially narrowed at the nanoscale, bringing the reduction to Co metal to higher oxygen fugacity and the oxidation to Co3O4 spinel to lower oxygen fugacity at a given temperature. Metals generally have lower surface energy than oxides and we present evidence that spinels have lower surface energy than rocksalt oxides. Thus the contraction of the stability field of the divalent oxide, MO, relative to the metal, M, and the spinel M3O4, is probably a general phenomenon.  In the iron-oxygen system, wustite, Fe1-xO, is thermodynamically unstable with respect to iron and magnetite, Fe3O4, below the melting point of bulk wustite (1650 K) for particle sizes below 16 nm, in contrast to being stable above 850 K for the bulk. It is important to realize that the effects discussed here are thermodynamic in nature, and these thermodynamic driving forces set the stage for differences in kinetics and chemical reactivity at the nanoscale.  Similar effects are seen in manganese oxides, and their  shifts in redox equilibria and hydration state are linked in a complex manner.

                Nanoparticles are ubiquitous in the environment, where initially formed fine grained precipitates coarsen only very slowly. These size-driven phase stability changes have implications for material preparation and characterization, catalysis and the splitting of water in the presence of transition metal oxide nanoparticles, as well as for environmental, geological, and biological redox reactions. These changes can affect the transport , solubility and bioavailability of heavy metals in the environment. A further complication is the formation of nanoscale clusters (polyoxometallates) for metals ranging from aluminium to uranium.

BRIEF BIOGRAPHY, Alexandra Navrotsky

Alexandra Navrotsky was educated at the Bronx High School of Science and the University of Chicago (B.S., M.S., and Ph.D. in physical chemistry). After postdoctoral work in Germany and at Penn State University, she joined the faculty in Chemistry at Arizona State University, where she remained till her move to the Department of Geological and Geophysical Sciences at Princeton University in 1985. She chaired that department from 1988 to 1991 and has been active in the Princeton Materials Institute. In 1997, she became an Interdisciplinary Professor of Ceramic, Earth, and Environmental Materials Chemistry at the University of California at Davis and was appointed Edward Roessler Chair in Mathematical and Physical Sciences in 2001. 

Her research interests have centered about relating microscopic features of structure and bonding to macroscopic thermodynamic behavior in minerals, ceramics, and other complex materials. She has made contributions to mineral thermodynamics; mantle mineralogy and high pressure phase transitions; silicate melt and glass thermodynamics; order-disorder in spinels; framework silicates; and other oxides; ceramic processing; oxide superconductors; nanophase oxides, zeolites, nitrides, perovskites; and the general problem of structure-energy-property systematics. The main technical area of her laboratory is high temperature reaction calorimetry. She is director of the UC Davis Organized Research Unit on Nanomaterials in the Environment, Agriculture and Technology (NEAT-ORU)..

Alexandra Navrotsky's research interests lie at the intersection of solid state chemistry, geochemistry, and materials science. The fundamental question that gives unity to a diverse set of studies (over six hundred papers) on materials ranging from oxide superconductors to silicates deep in the Earth's mantle is, "Why does a given structure form for a specific composition, pressure and temperature?" The "why" involves relating thermodynamic properties, structural parameters, and chemical bonding in a systematic fashion. At Arizona State University in the 1970s and 80s, at Princeton from 1985 to 1997, and at UC Davis since 1997, Navrotsky has built a unique high temperature calorimetry facility, the Peter A. Rock Thermochemsitry Laboratory, designed and improved the instrumentation, and developed and applied methods for measuring the energetics of crystalline oxides, of glasses, amorphous, nanophase, and porous materials, of hydrous phases and carbonates, and of nitrides, oxynitrides, and sulfides. The thermochemical data obtained are essential to understanding materials compatibility and reactivity in both technological and geological application, but, more fundamentally, the energetics offer insight into chemical bonding, order-disorder reactions, and phase transitions.

Honors include an Alfred P. Sloan Fellowship (1973); Mineralogical Society of America Award (1981); American Geophysical Union Fellow (1988); Vice-President, Mineralogical Society of America (1991-1992), President (1992-1993); Geochemical Society Fellow (1997). She spent five years (1986-1991) as Editor, Physics and Chemistry of Minerals, and serves on numerous advisory committees and panels in both government and academe. She was elected to the National Academy of Sciences in 1993. In 1995 she received the Ross Coffin Purdy Award from the American Ceramic Society and was awarded the degree of Doctor Honoris Causa from Uppsala University, Sweden.  In 2002 she was awarded the Benjamin Franklin Medal in Earth Science. In 2004, she was elected a Fellow of The Mineralogical Society (Great Britain) In 2005 she was awarded the Urey Medal (the highest career honor of the European Association of Geochemistry). In 2005, she received the Spriggs Phase Equilibria Award. In 2006, the International Association of Chemical Thermodynamics presented her with the Rossini Lectureship Award, and she was also awarded the American Geophysical Union's Harry H. Hess Medal. In 2009 she is receiving the Roebling Medal of the Mineralogical Society of America, its highest award. In 2011 she was elected to the American Philosophical Society.