UC Davis

The redox behaviors of nanoscale metal and ceramic materials are affected by various parameters and are important in determining certain properties of the material. This dissertation reports the controlling of oxidation and reduction behaviors in some example metal and ceramic materials at the nanoscale by varying material dimensions, gaseous environments, surface energy densities and application of electric fields and currents. It was suggested in previous studies that iron oxide FeO is thermodynamically unstable under 1000 K with dimensions smaller than 100 nm. In this study, in-situ heating experiments to gradually reduce Fe2O3 nanoparticles under 50 nm and nanochains were conducted in a transmission electron microscope. Electron energy loss spectroscopy and selected area electron diffraction both confirmed previous predictions and also revealed the stabilization of FeO phase in nanochains above a critical length. It provides direct evidence that metal (Ⅱ) oxide with dimensions (particle size) below 100 nm can be stabilized by assembling particles in 1D nanochains. In the case of nickel nanoparticle oxidation, anisotropic growth of nickel oxide nanostructures was observed during in-situ heating of the particles at 800 °C under water vapor atmospheres in an environmental scanning electron microscope. The NiO stoichiometry was confirmed by both energy dispersive X-ray spectroscopy and selected area electron diffraction. Annealing of the nickel particles under different oxygen partial pressures prior to ESEM heating showed that anisotropic NiO growth only takes place at specific locations where local surface energy density is high enough. The results suggested that the oxide growth can be prevented by manipulation of the surface energy by annealing under low oxygen partial pressures. One directional growth of single crystalline nickel oxide nanostructures during nickel particle oxidation is also reported at 650 C and 4 ×10-4 Pa oxygen partial pressure in a transmission electron microscope. In-situ high resolution TEM revealed the layer-by-layer growth of nickel oxide at the Ni/NiO interface while the nickel particle was being consumed. Ledge movement and disconnection migration was observed at the interface that resembled a terrace-ledge-kink growth mechanism. The study demonstrates the applicability of TLK crystal growth mechanism at buried reactive heterophase interfaces. The role of electric current on one directional nickel nanostructure growth in an SEM is also reported in the study. A positive DC bias was applied to a tungsten carbide nanoindenter tip and electric current flow was established by contacting the tip to a nickel particle sitting on a nickel micropillar. Dielectric breakdown of the nickel oxide surface layer was observed prior to nanostructure growth. Nanostructure growth was achieved upon the retraction of the indenter tip. It was demonstrated by both theoretical calculations and finite element modeling that growth was caused by the combination of electromigration and Joule heating. For ceramic materials, the study reports electric field effects on the (100) twist grain boundary core structure of SrTiO3 bicrystals. Nominal field strengths of 50 V/mm and 150 V/mm were applied to the bicrystal in the direction that’s parallel to the grain boundary during high temperature annealing after the formation of the grain boundary. High angle angular dark field imaging displayed different grain boundary structure near the positive and negative electrode side. Electron energy loss spectroscopy and X-ray photoelectron spectroscopy both showed higher oxygen vacancy concentrations, i.e. more local reduction near the negative electrode side. The defect redistribution is caused by the migration of oxygen vacancies to the negative electrode side driven by the external electric field.