Electrospun Preparation And Properties Charaterization Of Nanostructured Alumina Fibers

Alumina fibers have been widely investigated due to their unique physical and chemical properties. This dissertation is focused on the preparation of alumina nano-structured fibers by sol-gel method combined with an electrospinning process. The formation mechanism of the alumina fiber is discussed. The analysis was done on the relationship between the microstructure of electrospun alumina fibers and their properties. The contents comprise fabrication and mechanical properties of flexible a-alumina fibers composed of nanosheets, fabrication and insulation properties of α-alumina fibers with CaO-SiO2additive, and fabrication and anti-corrosion properties of flexible and amphiphobic y-alumina mats. The sizes, morphology and microstructures of alumina fibers can be well controlled by adjusting the electrospinning process parameters (system parameters, process parameters and environment parameters). The formation mechanism of the alumina fiber under different conditions is discussed. The effects of structure of alumina fibers on their properties were investigated. This work is not only enriching the alumina nano-structured fibers investigations, but also providing a theoretical foundation and technical support to alumina fibers potential application. The detailed information of the dissertation is listed as follows:1. Fabrication and mechanical properties of flexible a-alumina fibers composed of nanosheetsa-alumina nanofibers with diameters of300-400nm were fabricated by the sol-gel method combined with an electrospinning process. The sol was prepared with aluminum isopropoxide and aluminum nitrate as the precursors, nitric acid as the catalyst, small amounts of anhydrous magnesium sulfate as additive, and ethanol aqueous solution as the solvent. Polyvinylpyrrolidone (PVP K90) was applied to tune sol viscosity. The fibers are composed of α-Al2O3nanosheets with thickness of20-50nm which stack unidirectionally along the axis. Small amounts of anhydrous magnesium sulfate as additive acted to stabilize the γ-Al2O3structure and retard its conversion to α-Al2O3firstly, and subsequently MgAl2O4was formed at the grain boundaries to inhibit grain growth and effectively promoted sintering. The nanofibers have good thermal stability, for their microstructure can be maintained even after heat treatment at1400℃for12h. For a single α-Al2O3fiber with a diameter of ca.350nm, the elastic modulus was23.8GPa, indicating the good flexibility. The good thermal stability and flexibility may extend the service life even when it is subject to stresses and strains resulting from thermal tensions and mechanical contact, which favors their application in catalysis and filtration at high temperature.2. Fabrication and insulation properties of a-alumina fibers with CaO-SiO2additiveCaO-SiO2two-component doped flexible a-alumina fibers were successfully prepared through the sol-gel method combined with an electrospinning process. The sol was prepared with aluminum chloride hexahydrate and aluminum powder as raw materials, calcium oxide-silica (CaO-SiO2) was applied as two-component additive, and water as the solvent. The mixture was heated to90℃and refluxed for5h with magnetic stirring to form a transparent solution. The resulting transparent solution was condensed for5h using a water bath (80℃). Finally, PVP was added with continuously stirring at ambient temperature to form a transparent sol. Small amounts of CaO-SiO2additive was applied to retard the formation of γ-Al2O3and the phase transformation from γ-Al2O3to α-Al2O3, and control the size of the grains during sintering. With the addition of the CaO-SiO2in the system, the size of the particles composed the fibers calcined at1300℃significantly decreased compared to that without additive, which might due to the formation of6-alumina in the samples. The alumina fiber mat calcined at1300℃with the CaO/SiO2ratio of1:10had good flexibility and low thermal conductivity (0.07329W/m·K), which favored its application in insulation.3. Fabrication and anti-corrosion properties of flexible and amphiphobic γ-alumina matsAlumina gel fibers were by the sol-gel method combined with an electrospinnin process. The sol was prepared using aluminum isopropoxide and aluminum nitrate as the precursors, nitric acid and acetic acid as catalysts, water as the solvent, and polyvinylpyrrolidone (PVP) was applied to tune sol viscosity. The γ-alumina mats were obtained after calcination of the gel fibers. Amphiphobic γ-alumina mats were fabricated by (fluoroalkyl)silane (FAS) modification of the obtained γ-alumina mats. When the content of PVP was0.3g, the average fiber diameters were smaller than those with0.4and0.5g PVP, and the resulting larger surface roughness endowed the modified alumina mat better amphiphobicity. At the same time, the modified alumina mat exhibited a hydrophobic property in the pH range from1to14, indicating that it had a certain anti-corrosion resistance. Additionally, the modified alumina mat was flexible and would not rapture even folding it on a paper, which may extend the application of fiber mats in many important industrial fields. Flexible α-alumina nanofibers were fabricated by the sol-gel method combined with an electrospinning process. The sol was prepared with aluminum isopropoxide and aluminum nitrate as the precursors, nitric acid as the catalyst, and ethanol aqueous solution as the solvent. Small amounts of anhydrous magnesium sulfate as additive acted to stabilize the γ-Al2O3structure and retard its conversion to α-Al2O3firstly, and subsequently MgAl2O4was formed at the grain boundaries to inhibit grain growth. Polyvinylpyrrolidone (PVP) was applied to tune sol viscosity. The fibers with diameters of300-400nm are composed of α-Al2O3nanosheets with thickness of20-50nm which stack along [0001] at the axial direction. The fibers exhibited good flexibility and thermal stability. Alumina fibers were fabricated by the sol-gel method combined with an electrospinning process using aluminum chloride hexahydrate (AICl3·6H2O) and aluminum powder as raw materials in water solvent. Calcium oxide-silica (CaO-SiO2) was applied as two-component additive to retard the phase transformation of alumina and control the size of the grains during sintering. With the addition of the CaO-SiO2in the system, the size of the particles that composed the fibers calcined at1300℃significantly decreased compared to that without additive, which might be due to the formation of θ-alumina in the samples. In addition, the fiber had low thermal conductivity and good flexibility, which favored its application in insulation.