Photothermal Treatments of Electrospun Lignin and Asphaltene for Manufacturing Carbon Nanofibers
Creators: name: Wong, Danny
Publisher: name: Schulich School of Engineering
Year: 2022
Subjects: subject: Electrospinning
subject: carbon fiber
subject: nanofibers
subject: microwave
subject: intense pulsed light
subject: laser
subject: lignin
subject: asphaltene
subject: Engineering--Materials Science
subject: Engineering--Mechanical
subject: Engineering--Chemical

description: Asphaltene and lignin are traditionally viewed as waste products which are difficult to dispose and manage. Recently, these materials have been explored for value-added materials, primarily due to their carbon-rich structure. This makes them promising precursors for carbon materials. Carbon fiber is one such carbon material possessing high tensile strength and lightweight structure. The high cost has limited applications primarily to high-end automotives, aerospace and aviation. Polyacrylonitrile (PAN), the traditional precursor, accounts for more than 50% of the manufacturing cost of carbon fibers. Using waste products to make carbon fiber can increase economic viability for alternative applications. Conventional furnace-based heating methods also pose technical challenges including substantial greenhouse gas emissions, long processing times and poor financial feasibility.In this thesis, lignin and asphaltene are converted to fibers via electrospinning. Pretreatment methods are first proposed to reduce undesirable impurities and standardize the precursors. Electrospinning solutions and parameters are optimized to produce fibrous mats with limited usage of PAN. Nanoscale diameters can be achieved with electrospinning beyond a critical electric potential identified by finite element analysis (FEA). The electrospun fiber mats show excellent air filtration performance meeting N95 standards. Electromagnetic radiation is applied as a novel, environmentally sustainable and economically viable alternative to convert electrospun fibers to carbon nanofibers. The photothermal heating reduces volatiles in the lignin and asphaltene to develop the desired graphitic structure. Microwave, intense pulsed light (IPL) and laser are each studied because they can reduce the time, energy and cost of making carbon fiber compared to conventional furnace heating. Reductions in greenhouse gas emissions are also verified using preliminary life cycle analysis. Unique properties are identified from each photothermal treatment compared to conventional methods. Microwave-assisted stabilization and carbonization yields lignin-based fibers with a lower average diameter of 84 nm and higher water contact angles making them applicable for oil-water separation. IPL-assisted carbonization of lignin-based fibers yield novel porous and hollow fibers with applications in sensors and gas adsorption owing to the high surface area. Laser treatment of asphaltene-based fibers yield highly graphitic structures that enhances tensile strength by an average of 47.7%. FEA is conducted to model the difficult to measure fiber temperatures during laser treatment. Chemical treatment using hydrogen peroxide as an oxidizing agent is also shown to effectively enhance the yield of lignin-based carbon fiber from 22.4 wt.% to 46.8 wt.% at 1100 °C. A higher overall yield while skipping the most time-intensive thermostabilization step may further increase the overall viability of carbon fibers. Physical, chemical and thermal characterization are presented to verify the each successful manufacturing step.