Committee Members:

Prof. Satish Kumar, Advisor, MSE

Prof. Donggang Yao, MSE

Prof. Karl Jacob, MSE

Prof. Mary Lynn Realff, MSE

Prof. Paul Russo, MSE

Prof. Sankar Nair, ChBE

Prof. Guoliang Liu, CHEM, Virginia Tech

Development of High-Performance, Lightweight, Low-Cost Carbon Fibers for Sustainability""

Abstract:

This thesis focuses on the development of high-performance carbon fibers for light weighting automobiles to achieve sustainable developmental goals (SDG) of clean energy, climate action and infrastructure. Replacing steel and aluminum used in current day automobiles by carbon fibers leads to low-weight structures (theoretical reduction by ~70 % and 35% for steel and aluminum, respectively) with higher fuel savings and lower emissions. Two pathways were studied to design high performance carbon fibers that could have the potential for such replacement and their widespread use in the future. The first approach towards the goal involved designing porous carbon fibers with low-density and multifunctional capability using blends of poly(acrylonitrile) (PAN) with sacrificial polymers and block copolymers. The second route explored the use of low-cost precursor, asphaltene, to derive carbon fibers.

Porous carbon fibers from gel-spun blends of PAN with sacrificial polymers were the initial systems studied to characterize the mechanical performance upon introduction of porosity. Orientation imparted during gel-spinning of the PAN-sacrificial polymer blends was synergistically combined with the micro-phase separation of the polymer pairs, to tailor the porous morphology of these fibers and their resultant mechanical properties. From five sacrificial polymers, three systems including poly(acrylic acid) (PAA), poly(methyl methacrylate) (PMMA), poly(styrene-co-acrylonitrile) (SAN), were selected due to their intermediate and higher degree of compatibility with PAN, discerned through UV-vis spectroscopy. Porous morphology of the different carbon fibers produced was characterized through scanning electron microscopy (SEM) and the differences shown due to the sacrificial polymers studied. The variation in pore size caused by the differences in compatibility between PAN and the sacrificial polymer was evaluated experimentally through blend rheology and theoretically using interaction parameter values. The tensile properties of porous carbon fibers from the three systems were compared to that of the non-porous PAN based carbon fibers processed under similar conditions. Specific tensile modulus of the porous carbon fibers derived from PAN-PAA and PAN-PMMA was found to be 15 to 40 % higher than that for the PAN based carbon fibers. Additionally, influence of sacrificial polymer content (5-20 wt%) on the mechanical, structural and morphological properties was studied for the PAN-SAN fiber systems with porous channeled morphology.

This study showed that gel-spinning of bicomponent PAN/PMMA-b-PAN fibers provided a versatile means for tuning the mechanical and electrochemical properties of porous carbon fibers, thus allowing for their potential use as structural energy storage materials. Herein, by gel-spinning polymer precursors of poly(acrylonitrile) (PAN) and poly(methyl methacrylate)-block­-poly(acrylonitrile) (PMMA-b-PAN), we produced a series of carbon fibers and systematically studied the morphological, mechanical, and electrochemical properties. Porous carbon fibers with block copolymer (BCP) in the sheath exhibited the best tensile properties with a strength of 1.1 GPa, modulus of ~190 GPa, and electrochemical capacitance of 11 F/g at 10 mV/s when pyrolyzed at 1315 °C under tension. Without tension and at a pyrolysis temperature of 800 °C, the fibers with BCP as both the sheath and core components achieved the highest electrochemical capacitance of 70 F/g at 10 mV/s and highest surface area of 264 m2/g. The characteristic correlation length of PMMA-b-PAN was calculated through thermodynamically governed computational method and compared with the pore size in the experimentally obtained carbon fibers.

Through the second approach, conditions to successfully gel-spin blends of low-cost precursor of asphaltene with PAN were established. Functionalization of asphaltenes with nitric acid (f-Asp) enabled their complete solubility in dimethylacetamide (DMAc) unlike the as-received asphaltene powder and further resulted in obtaining relatively homogeneous spinning solutions with PAN. Mechanical properties and structural parameters of precursor f-Asp/PAN fibers with their blend ratio ranging from 30/70 to 60/40, were studied. Carbon fibers with maximum tensile strength and modulus of 1.1 GPa and 181 GPa, were obtained from the gel-spun f-Asp/PAN blends. The study suggests that carbon fibers with reasonable mechanical properties can be derived from low-cost asphaltene through the scalable route of solution/gel-spinning.

The two approaches studied in the thesis present ways to develop high-performance carbon fibers that would have the potential to light weighting automobiles and thus, steer towards a future of achieving SDG.