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Author ORCID Identifier

https://orcid.org/0009-0008-5087-2778

Date Available

11-8-2026

Year of Publication

2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Chemical and Materials Engineering

Faculty

Rodney Andrews

Faculty

Matthew Weisenberger

Faculty

Zachary Hilt

Abstract

Carbon fiber is applicable for many structural applications that require a high strength-to-weight ratio. While the most common precursor to carbon fiber is polyacrylonitrile, carbon fibers made from mesophase pitch have niche applications where high thermal conductivity and exceptional stiffness are required. Mesophase pitch-derived carbon fibers typically stem from coal tar or the bottoms products from petroleum distillation, but there is an opportunity for lower-cost carbon fiber when the precursor pitch is a coal and petroleum oil liquefaction product. The aligned graphitic structure of these high-modulus carbon fibers originates from the orientation of liquid crystalline domains in mesophase pitch during melt spinning. A significant challenge to the production of mesophase pitch-derived carbon fibers is the melt spinning process. The short draw distance (< 1 cm) of the molten filament creates high melt stress, causing greater susceptibility to failure-inducing instabilities. Mitigation of these instabilities is essential for scale-up to multifilament melt spinning on an industrial scale.

Despite prior investigations into mesophase pitch melt spinning, critical gaps remain in understanding how mesophase characteristics and spinning conditions influence stability, particularly under conditions relevant to industrial-scale production. Mitigation of instabilities during melt draw can lead to fewer filament breakages and facilitate the production of carbon fiber. Therefore, this dissertation begins by investigating the mitigation of instabilities during melt spinning by focusing on the properties of the mesophase pitch. Then, challenges in economical scale-up of high modulus carbon fiber are explored by studying spinning stability under various multifilament melt spinning conditions and with coal extract-derived mesophase pitch.

First, this work investigates the modification of coal tar-derived isotropic pitch, which is the amorphous precursor to liquid crystalline mesophase pitch. The isotropic domains in mesophase pitch form a heterogenous, bi-phasic system and are concerning due to their local viscosity differences compared to the adjacent mesophase domains. Previous works have explored how instabilities in mesophase pitch are more prevalent with increasing concentrations of isotropic domains. In this work, the isotropic domain size distribution of a mesophase pitch is varied while holding the isotropic content constant to study changes in the viscous flow through a capillary. Viscosity fluctuations within the capillary over time are correlated with instabilities in melt spinning. The distribution of isotropic domains is shown to change with time as the isotropic domains coalesce at the melt spinning temperature. Second, the storage conditions for the pitch are hypothesized to cause slow oxidation of the pitch, affecting the viscosity and spinning stability over several months. Oxygen content of pitch over time is studied while varying the pitch particle size and storage atmosphere. Correlation between oxygen content and melt spinning stability is investigated for pitches in air at an elevated temperature and at room temperature for long durations.

Then, this work addresses the challenges of scale-up by investigating several processing variables of multifilament melt spinning, including air temperature outside the capillary, spinning temperature, mass flow rate, and spinneret capillary quality. The effects of these process variables on multifilament spinning stability are compared to those observed under similar conditions in single filament melt spinning. Lastly, this work focuses on lower cost precursors for producing mesophase pitch. Waste coal is a focus for this study as it has potential utility due to its high polycyclic aromatic content, high surface area for liquefaction, and no present market value. The properties of mesophase pitches derived from waste coal and petroleum decant oil liquefaction extract are compared to those derived from petroleum decant oil alone. Differences in melt spinning stability between these two mesophase pitches are investigated by contrasting their viscosity, molecular weight distribution, mass loss under melt spinning conditions, and SEM cross sections of the green filaments. The graphitic texture, mechanical properties, and overall yield of the graphitized carbon fibers derived from waste coal extract are compared to those derived from petroleum decant oil alone.

In conclusion, stable multifilament melt spinning was demonstrated, achieving 30 minutes of uninterrupted melt spinning for > 90% of the spinneret capillaries while producing fiber with a diameter of approximately 20 µm. These are the first high modulus carbon fibers obtained utilizing waste coal as a feedstock, with 562 GPa modulus and 1.22 GPa tensile strength, and an overall yield to carbon fiber was improved by 75% compared to carbon fibers derived from petroleum decant oil alone.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2026.192

Archival?

Archival

Funding Information

This research was sponsored in part by the U.S. Department of Energy, Office of Fossil Energy & Carbon Management under contract DE-AC05-00OR22725 with UT-Battelle, LLC., through ORNL project FEAA302 “Scale-up Production of Graphite, Carbon Fibers and other Products from Coal”.  This research was also sponsored by the U.S. Department of Energy under award number DEFE0031796 for project “Coal to Carbon Fiber (C2CF) Continuous Processing for High Value Composites”. 

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