Date of Award

8-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Luo, Jian

Committee Member

Hudson , JoAn

Committee Member

Lee , Burtrand

Committee Member

Luzinov , Igor

Abstract

The primary objective of this thesis research is to greatly advance the knowledge of the formation mechanism and thermodynamic stability of a unique class of surficial amorphous films (SAFs) and analogous intergranular films (IGFs) via critical experiments and modeling. These quasi-liquid IGFs and SAFs films exhibit the following distinct characteristics: 1) self-regulating (equilibrium) thicknesses on the order of 1 nm, 2) structures and compositions that are neither found nor stable as bulk phases, and 3) thermodynamic stability below the bulk solidus lines.
The first part of this study examines anisotropic wetting of ZnO single crystals by Bi2O3-rich liquid with and without the presence of SAFs. For Bi2O3 on the ZnO surfaces wherein nanometer-thick SAFs are present in equilibrium with partial-wetting drops, the measured contact angle decreases with increasing temperature, but it levels off above 860 °C. In contrast, the contact angle is virtually a constant on the surfaces where SAFs are not present. This observation provides a critical piece of evidence supporting a generalized Cahn critical point wetting model. The observed 'residual' contact angle of 4-6° and the extended SAF stability in the solid-liquid co-existence regime are quantitatively explained on the basis of an attractive van der Waals London dispersion force.
In the second part of this study, vanadia-based SAFs were observed on TiO2 surfaces in V2O5-TiO2 binary system and six V2O5-TiO2-based ternary oxide systems (Ti-V-X-O; X = P, Na, K, Nb, Mo or W). This is a model 'monolayer' catalyst system. It is demonstrated that the film appearance and thickness can be tailored via co-doping or changing the equilibration temperature. Furthermore, the observed discontinuous changes in film thickness, hysteresis and bimodal thickness distributions indicate a first-order monolayer-to-multilayer adsorption transition, the existence of which was revealed for the first time to our knowledge. In addition to the theoretical significance, such interfacial transitions are of practical importance because they can cause abrupt changes in transport kinetics and material properties.
This study contributes to the general high-temperature wetting and adsorption theories for oxides by critically testing several hypotheses in the above-mentioned systematical in-situ wetting and high-resolution transmission electron microscopy experiments. Moreover, the characterization of more than 850 independent films in V2O5-TiO2 based systems as functions of equilibration temperature, anneal time, thermal treatment history, co-doping, overall composition, and TiO2 phase and orientation represents the most systematic measurement of similar interfacial films to date, providing a critical dataset for establishing thermodynamic models for SAFs and analogous IGFs. Further studies should be conducted to exploit SAFs with self-selecting thickness in applications such as supported oxide catalysts, ultrathin dielectric films, and morphology control of nanocrystals.

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