Electrohydrodynamic fabrication of inorganic and hybrid (organic-inorganic) fibers and core -shell structures with micro- and nanometric dimensions

This dissertation describes the fabrication of inorganic and hybrid (organic-inorganic) fibers and core-shell spheres and tubes with micro and submicrometric dimensions. These structures resulted from the application of electrohydrodynamic forces to carefully aged, viscous sol-gel precursors. The contribution of Dr. Larsen and Dr. Dzenis (Engineering Mechanics, UNL) to the field was to extend the electrospinning technique to the inorganic materials field. At a later stage, Dr. Larsen and collaborators made such structures with hollow geometries for the first time. Using this technique, titania-silica, silica, alumina, mullite, zirconia and yttria-stabilized zirconia fibers of quasi-circular cross sections with diameters ranging from 0.15 to 1 μm, and fibers with non-circular cross sections (ribbons) with widths of 20–40 μm and thicknesses of just few microns were produced. The materials initially collected were amorphous, and after an appropriate thermal treatment they formed polycrystalline phases, and in all cases they displayed a remarkable structural stability retaining their individual fibrous identity. Furthermore, using a coaxial nozzle that was previously used to carry out liquid-liquid encapsulations expanded the range of potential applications of this novel technique. The coaxial nozzle allows the generation of compound liquid jets. In this case the shell liquid was the aged sol and the core liquid was an inert, immiscible or slow-mixing liquid such as olive oil, glycerin or water. The result was the formation of well-defined spherical hollow silica spheres and tubes with average diameters from 200 nm to 5 μm, and wall thicknesses of few nanometers to 2 μm. Since the injection of the shell and core liquids is done using independently controlled pumps, the flow rate ratio of both streams can be conveniently adjusted and different wall thicknesses can be obtained. On collection, certain liquid templates evaporate at room temperatures, thereby producing solid hollow structures without further processing. This approach is simple and very general and it is expected that other systems such as dissolved polymers can be used