Papers by Author: Ludwig J. Gauckler

Abstract: Distinctive microstructure engineering of amorphous to nanocrystalline electroceramic thin films is of high relevance for integration in low to high temperature operating MEMS-devices. Up to now, kinetic rules of nucleation, crystallization and grain growth of precipitation-based ceramic thin films are unknown. In this study, general rules for the crystallization and grain growth kinetics of a pure single-phase metal oxide thin film with only one kind of cation, i.e. ceria, made by spray pyrolysis from a precursor with one single organic solvent is discussed [1,. The near-and long range disorder is studied via Raman, DSC investigation of crystallization enthalpy, XRD, SEM and TEM for amorphous to fully crystalline state. These 400 nm thick-thin films were dense, crack-free and amorphous directly after deposition on a sapphire substrate. Briefly, above deposition temperature crystallization sets in with respect to temperature and persists over a broad temperature range from 400 to 950°C. In this regime, biphasic amorphous-crystallien films exist and grain growth proceeds simultaneously to crystallization. Isothermal grain growth studies showed that after short dwell times of 10-20h stable microstructures established following self-limited grain growth law [. In this state, driving force for the crystallization is the reduction of free enthalpy for phase transformation and interface diffusion prevails. A transition to classical grain curvature-driven parabolic grain growth kinetics appeared once the material reached the fully crystalline state for average grain sizes larger than 140 nm and higher annealing temperatures. Volume diffusion was then activated in addition to the interface diffusion. It was found that once crystallized the material shows independent on processing route equal XRD density and microstrain, as well as Raman characteristics. However, dependent on processing conditions i.e. choice of organic and, according, deposition temperature of the film amorphous states vary and affect strongly crystallization and grain growth history for the biphasic films.

Abstract: Three series of ZnO-based materials with different doping levels were prepared. The correlation between the composition and microstructure, and the roles of main dopants, Bi2O3 and Sb2O3, in the sintering behaviors were proposed. Both Bi2O3 and Sb2O3 evaporated at 1115°C, but the amount of them, in which bismuth is the majority, is not significant. Bi2O3 functioned mainly as liquid during sintering to promote the sintering of ZnO, but it doesn’t mean the materials will be denser. The bismuth-rich phase retracted into small pores during cooling, leaving the big pores as voids at room temperature. More Bi2O3 added would result in less increase in material densities and dramatic decrease in relative densities, and a little bit increase in grain sizes of matrix ZnO. Sb2O3 would react with ZnO matrix into spinel, Zn7Sb2O12, which will pin at the grain boundary of ZnO to control the ZnO grain growth. The more Sb2O3 added, the smaller the grain sizes of ZnO. Appropriate amount of Sb2O3 added will yield denser materials.

Abstract: Nanocrystalline ceria-based thin films are of potential interest for use as gas-sensing layers and electrolytes in micro-Solid Oxide Fuel Cells (micro-SOFC) used for energy supply of next generation portables. In these devices the thin films have to be operated at intermediate to high temperatures (500 - 1000 °C) to be sufficiently high electrical conductive. However, only little is known on the nucleation and grain growth kinetics of pure ceria and its solid solutions when present as nanocrystalline thin film microstructures (average grain size < 100 nm). In this study amorphous, dense and crack-free CeO2 and Ce0.8Gd0.2O1.9-x thin films have been deposited by spray pyrolysis on sapphire. These films were crystallized to biphasic amorphous-nanocrystalline and fully nanocrystalline microstructures upon annealing with respect to time, temperature, heating rate and doping. Nucleation and grain growth kinetics were studied by differential scanning calorimetry, Xray diffraction analysis with in-situ heating chamber and scanning electron microscopy.

Abstract: Protein adsorption onto metal oxide surfaces is an essential aspect of the cascade of biological reactions taking place at all interfaces between implanted materials and the biological environment. The types and amounts of adsorbed proteins mediate subsequent adhesion, proliferation and differentiation of cells. Protein adsorption to surfaces of metal oxides and their kinetics are important in the formation and growth of seashells, one of the toughest natural ceramics, in modern bio-analytical devices as well as in bone and teeth implant technology. This paper describes results obtained in a feasibility study of how to use metal-oxide particles to obtain biosensors with a high turnover. The most important features of proteins are outlined describing them as purpose-built "polymers" from amino acids with specific conformations. Some key aspects of Metaloxide (MeO) surfaces in water and the influence of electrostatic and hydrophobic interaction on protein adsorption are reported. Results concerning the interaction between different proteins and MeO surfaces in water are discussed in detail. Examples of purely electrostatic interactions of proteins with MeO surfaces as well as the influence of hydrophobic interaction are elucidated. An outlook of the implications of the new insights on natural and synthetic materials will be given concerning bio-compatibility, bio-mineralization and self assembly of materials.