Date of Award

May 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Member

Amod A Ogale

Committee Member

David Anderson

Committee Member

Igor Luzinov

Committee Member

Mark Roberts

Abstract

Mesophase pitch, a nematic liquid crystal composed of disc-like polyaromatic hydrocarbons, has significant potential as an inexpensive and high-carbon-yielding precursor for structural carbon fibers. Before such potential is realized, the strength of mesophase pitch-based carbon fibers (MPCFs) must be enhanced while retaining their superior modulus and electrical/thermal conductivities. In MPCFs, lattice-related properties are favored by the three-dimensional graphitic development that originates from the preferential order that the liquid crystal molecules achieve during fiber formation. Carbon fibers derived from polyacrylonitrile (PAN), a long-chain polymer, do not possess graphitic microstructure; it is referred to as “turbostratic” carbon. Nevertheless, PAN-based carbon fibers constitute over 90% of global carbon fiber market, because of their higher tensile and compressive strengths (relative to those of MPCFs) that originate from less-perfectly formed carbon structure coupled with less aligned texture that prevents crack propagation. In MPCFs, it is believed that the large graphitic regions are also defect sensitive, and the tensile strength is lowered because it is a defect-limited property.

Previous studies have demonstrated that fiber spinning/process conditions play an important role in the development of carbon fiber microstructure, which in turn determines the strength of the carbon fiber. MPCF processing consists of three main steps, namely melt spinning of mesophase pitch where fiber is formed, oxidative stabilization where fiber is rendered infusible, and high temperature treatment where carbon layer and crystallinity get developed. Studies on the relationships between fiber microstructure and carbon fiber properties have focused on fiber formation because it is in this step that the microstructure starts to develop. The melt spinning step consists of three main operations: melting of pitch, extrusion of molten pitch through spinnerets, and drawdown of filaments. Most discoveries relating the formation of fiber microstructure to MPCF properties have emphasized on processing conditions during the melting/extrusion steps, while the drawing step has not been investigated systematically. Therefore, the primary goal of this dissertation was to study how the microstructure and properties of MPCFs were influenced by changes in drawdown ratio (DDR). The specific objectives were to investigate the effects of DDR on (i) carbon fiber microstructure, (ii) fiber mechanical properties, and (iii) fiber transport properties.

To accomplish these goals, precursor pitch fibers were melt-spun from a synthetic, naphthalene-based mesophase pitch using multiple sets of ultrafine diameter spinnerets with a range of capillary diameters of 50-150 µm. Pitch fibers were oxidatively stabilized in air (220-240 °C), and carbonized at 2100 °C under helium atmosphere. As a result, carbon fiber samples with average diameters in the range 8-16 µm were obtained, with drawdown ratios ranging from about 10 to 200.

The microstructure of carbon fibers was studied by scanning electron microscopy (SEM), Raman spectroscopy and wide-angle x-ray diffraction (WAXD). SEM of the cross-section of fibers revealed a line-origin radial type of transverse microstructure across samples, which meant that transverse microstructure would not constitute a confounding variable in further analysis. Raman spectroscopy revealed that decreasing DDR led to an enhancement of crystallite coherence length along the fiber axis. The coherence length increased from ~ 60 nm for DDR of 190 to ~ 85 nm for DDR of 15, while at the same time reducing the defect density of carbon layer planes. WAXD revealed a 2θ peak position of ~ 26.0° for (002) planes indicating d002 spacing of 0.342 nm and Lc stacking thickness of approximately 20 nm, generally consistent with prior results on mesophase pitch-based carbon fibers produced at 2100 °C. However, significant differences in crystalline features resulting from different DDRs could not be detected, likely due to instrumental limitations.

Tensile properties of carbon fibers were measured by single filament tensile testing. Between samples of equivalent fiber diameter and heat treatment temperature yet contrasting DDR, tensile strength was favored by decreasing DDR. Tensile strength was found to increase from 1.4 ± 0.2 GPa for DDR ~ 190 to 2.3 ± 0.3 GPa for DDR ~ 15, for carbon fibers with nominal diameter of 8 µm. Limited samples were tested by the tension-recoil testing to estimate compressive strength. The compressive strength displayed no measurable change, and remained about 800 MPa as the DDR was reduced from 190 to 15. The behavior of tensile strength with DDR is somewhat anomalous with that observed for carbon fibers derived from PAN for which a higher DDR is known to enhance tensile strength, via the improvement of molecular orientation along fiber axis caused by more pronounced drawing. Weibull analysis revealed that for samples of comparable fiber diameter, the Weibull modulus increases with low DDR, which means that the distribution of tensile strength is narrower.

The electrical conductivity of single carbon fibers was experimentally measured, and the thermal conductivity was estimated using the Issi-Lavin electrical/thermal conductivity correlation. Limited measurements of thermal diffusivity of carbon fiber/polymer matrix composites were conducted using a laser/light flash analysis (LFA) technique. For the experimental carbon fibers the electrical resistivity was measured at ~ 6 µΩ∙m and the estimated thermal conductivity at ~ 200 W/m∙K. These measurements confirm that the conductivity of these MPCFs were a whole order of magnitude better than those of PAN-based fibers. However, no significant difference could be detected in the electrical or thermal conductivity as a function of the different DDRs. This is generally consistent with x-ray results that indicated no significant changes in graphitic crystallinity. In summary, while decreasing drawdown ratio used during pitch fiber spinning did not deteriorate electrical/thermal conductivity, the tensile strength was observed to increase with decreasing DDR.

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