Nuevo método para la producción de nanofibras continuasanálisis experimental y aplicaciones

Resumo

In the present PhD Thesis we introduce Continuous Laser Spinning (FLC), a novel technique to produce continuous glass nanofibers. It was invented, patented and developed in our research group. We focused this doctoral thesis on the experimental development of FLC and the investigation of two interesting applications of these glass nanofibers: the study of their mechanical properties in order to explore their availability for fiber-reinforced composites and the production of CO2 capture filters to reduce CO2 emissions. We combined a detailed theoretical and experimental analysis on Continuous Laser Spinning and the single processes involved in it in order to develop the experimental system from the basic initial concept until we succeed on the production of glass nanofibers. The system initially considered consisted on the use of a single laser beam, a supersonic air jet and silica as precursor material. Then, this thesis systematically deals with all the development process, showing the results obtained on each stage with a discussion about the modifications gradually implemented in the system in order to improve the results. We also include an extensive analysis using Design of Experiments (DOE) methodology on the influence of some processing parameters on the obtained nanofibers, particularly on their final diameters. The experimental set up and conditions for the production of nanofibers with 370 nm of diameter were determined. FLC is finally able to produce fibers from this size up to 30 µm by just changing some of these processing parameters. It was also tried with E-glass as precursor material and we were able to produce nanofibers as thin as 80 nm diameter; however, these fibers are not continuous. Moreover, we present in this chapter different analysis of glass fibers mainly on their microstructure and external aspect. Glass nanofibers were utilized as precursor material for the production of CO2 capture filters since they present an extremely high surface to volume ratio. Moreover, these nanofibers constitute 3D structures that prevent agglomeration so all their surface is available to react with atmosphere and hence to capture CO2. A comprehensive experimental study was carried out in order to optimize the production of lithium orthosilicate nanofibers: We tested different precursor materials with lithium content (in the form of powder), different methods for obtaining a convenient mix of fibers and powder, and different temperatures for the chemical reaction. The best option is the use of lithium hydroxide monohydrate, the evaporation method, and 30 min at 900 °C on inert atmosphere. This process yields a mat of fibers capable of capturing a 31 % in weight of CO2 and they can be totally regenerated for their consecutive reutilization. Finally, mechanical properties of glass nanofibers were investigated. Different advanced techniques were employed to determine their Ultimate Tensile Strength (UTS) and Young´s modulus. Glass nanofibers directly obtained by Continuous Laser Spinning presented poorer mechanical properties than expected; therefore, we tried different thermal treatments with the aim of improving them and the best one was determined. The growth of nanocrystals as a consequence of these thermal treatments on the inner structure of glass nanofibers was evidenced. Further studies on their mechanical properties reflected a significant improve on the values of UTS and Young´s modulus. On the other hand, we demonstrated that the electron beam induces a plastic behavior of the glass nanofibers. An UTS of 3.4 GPa and a Young´s Modulus of 62 GPa was determined and an extraordinary flexural behavior was proven.