Catalysis and manufacturing of two-scale hierarchical nano- and microfiber advanced aerospace fiber-reinforced plastic composites

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Catalysis and Manufacturing of Two-scale Hierarchical Nano- and Microfiber Advanced Aerospace Fiber-Reinforced

Plastic Composites by

Richard Li

S.B., Aeronautics and Astronautics, Massachusetts Institute of Technology (2010) S.M., Aeronautics and Astronautics, Massachusetts Institute of Technology (2013)

Submitted to the Department of Aeronautics and Astronautics in partial fulfillment of the requirements for the degree of

Doctor of Philosophy at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

September 2018

Author...

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Department of-<eronautics and Astronautics

August 23, 2018 .Signature

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Brian L. Wardle Professor of Aeronautics and Astronautics

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Thesis Supervisor

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A. John Hart Associate Professor of Mechanical Engineering

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Jean Botti Chief Execitive Offjcer, VoltAero

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Hamsa Balakrishnan Associate Professor of Aeronautics and Astronautics Chair, Department of Aeronautics and Astronautics Graduate Committee

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

OCT 0

9 2018

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Catalysis and Manufacturing of Two-scale Hierarchical Nano- and Microfiber Advanced Aerospace Fiber-Reinforced

Plastic Composites

by

Richard Li

Submitted to the Department of Aeronautics and Astronautics on August 23, 2018 in partial fulfillment of the

requirements for the degree of Doctor of Philosophy

Abstract

The development of hierarchical nanoengineered "fuzzy fiber" aerospace fiber-reinforced plastic (FRP) composite laminates holds the potential for enabling future generations of lightweight, durable, and multifunctional vehicle structures. By reinforcing the weak matrix-rich regions between individual fibers and plies, the circumferential growth of aligned carbon nanotubes (A-CNTs) on carbon microfibers (CFs) enables new composites with improved strength, toughness, electrical and thermal properties. While these improvements have been empirically demonstrated on alumina fiber FRPs, CNT growth degrades the CFs and sacrifices in-plane FRP properties for the benefits of CNT reinforcement. This thesis presents novel and scalable methods for realizing advanced fuzzy carbon fiber reinforced plastic (fuzzy CFRP) composite laminates with retained CF and interlaminar strength properties. Earth-abundant sodium (Na) is revealed as a new facile catalyst for CNT growth that allows for direct deposition of the catalyst precursor on carbon fabrics without any fiber pretreatments. This new catalyst discovery also enables high-yield CNT growth on a variety of low-temperature substrates. Simultaneously, this finding has led to other novel findings in carbon nanostructure catalysis including a core-shell morphology and the use of other alkali metals (e.g., potassium) for CNT growth. Towards the development of advanced composites, vacuum-assisted resin infusion processes are studied and refined, resulting in high-quality woven and unidirectional fuzzy (via Na-catalysis of CNTs) CFRP laminates. Growth uniformity improvement studies yielded strategies for increasing the quantity of CNT reinforcement within matrix-rich regions. Moreover, a new commercial unidirectional fabric enables the first retention of CF properties concomitant with interlaminar shear strength retention in the fuzzy CFRP architecture. The contributions of this thesis extend beyond CF composites: techniques developed for improving fuzzy CF synthesis were applied towards demonstrating A-CNT growth on SiC woven fabric, desired for creating damage tolerant and multifunctional lightweight vehicle systems. These advances pave the way for improvements in catalysis of nanostructures, electronics interfaces, energy storage devices, and advanced composite materials.

Thesis Supervisor: Brian L. Wardle

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Acknowledgements

This thesis is dedicated to the many friends and family who have supported me along my graduate career. I am particularly indebted to all my labmates from the past through present, my PhD thesis committee, and my mentors both at MIT and in industry, through my NASA fellowship and internships. Also, this work would not have been possible were it not for the invaluable help and creativity of the UROPs who have spearheaded different studies discussed here. Most of all, I can't thank my thesis advisor Prof. Brian L. Wardle enough for providing such an incredible learning experience in the past 9 years. From my days of being a UROP through finishing my PhD and discussing future ambitions, Brian has been a superb mentor, equipping me with the resources to tackle all challenges throughout grad school, and encouraging personal growth through laboratory and hands-on industry experiences. I could not have asked for a more supportive advisor for an incredibly enriching journey.

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List of Figures

1.1 Schematic of a filamentary composite laminate schematic, consisting of stacked laminae (plies) comprised of fibers and matrix [1]. Fiber orientations in each laminae can be arranged to tailor elastic response of the overall laminate... 28 2.1 Schematic illustration of the Fuzzy Fiber Reinforced Plastic (FFRP)

architecture. CNTs circumferentially coat individual filaments within a cloth (right). Cloths containing CNT covered fiber tows can be laid up as composite plies in a polymer matrix (left) [11, 59]...43 2.2 A typical stress strain curve is shown for UD fuzzy CFRP

composite as compared with a baseline unsized CFRP (top) employing noncovalent functionalization and low temperature CNT growth at 480 'C. Both composite moduli (bottom left) and composite strength properties (bottom right) are demonstrated to be preserved and in alignment with micromechanical predictions as indicated through "ROM". Note that the strain axis is offset in the top image to improve visualization of the fuzzy CFRP data...45 4.1 High yields of carbon nanotubes (CNTs) grown on carbon

microfibers dip-coated in household solutions. SEM images showing CNTs grown on 7 pm diameter carbon microfibers

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(NaCi), (c), baking soda (NaHCO3), and (d), lye (NaOH). Insets show TEM images of individual CNTs, all having simlar structure. 5 pm SEM and 50 nm TEM scale bars are the same in all images. ... 56 4.2 CNTs were grown on a variety of substrates by dip-coating with

Na-based solutions, and using the nominal CO2 and C2H2 CVD reaction at 480 'C flowing for 15 min (a), SEM image showing CNTs grown on alumina fibers. (b), SEM image showing CNTs grown on a silicon nitride TEM grid. c, SEM images showing CNTs grown on sized CFs. d, SEM image showing CNTs grown on thermal silicon oxide on Si wafer. TEM images are shown in the inset for each specific substrate with the same scale bar. ... 59 4.3 Temperature study of NaOH catalyst precursor growth using

nominal CO2

/C

2H2 CVD process. (a) Micrographs of CFs subjected

to the CNT growth process at 390 'C, (b) 480 'C, (c) 720 'C, and (d) 820 'C. SEM micrographs are displayed in the top row and TEM images of grown CNTs are shown in the bottom row. CNTs were observed to form across the tested temperature range of 390 'C to 720 *C, with no growth observed at 820 C. ... 59 4.4 NaOH-dip-coated desized carbon fabrics were subjected to the

initial steps of CVD processing for CNT growth and terminated right before the introduction of CO2 and C2H2 at 480 'C. SEM images with EDS mapping was performed showing the formation of catalyst nanoparticles. (a), SEM image showing catalyst particles on the surface of the CF. (b), EDS map showing that the nanoparticles are composed of Na. c, EDS map showing no detection of Fe contamination. d, EDS map showing that the nanoparticles are com posed of 0. ... 60 4.5 CNT growth trajectory for NaOH-drop-cast silicon nitride TEM

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(b), 113 s, (c), 225 s, and (d), 900 s after the introduction of CO2 and

C2H2. Na nanoparticles become smaller during growth, with large

particles formed at 0 s which then disappear after 900 s of growth. Each column of TEM images has the same scale bar, and each row of images is for the same specimen. Note that the circles found in the left column are TEM grid pores that facilitate imaging and an aly sis ... . . 61

4.6 NaOH-dip-coated silicon nitride TEM grids were subjected to

reduction only CVD processing (0 min of CO2/C 2H2 flow at 480 'C)

resulting in the formation of Na catalyst nanoparticles. (a) TEM image shows immediate electron beam interaction. (b) TEM image shows complete removal of the nanoparticle from the field of view within 30 s of electron beam exposure. (c) EDS scans show a complete removal of the Na peak corresponding to nanoparticle removal. This shows that catalyst nanoparticles are composed of Na, which is removed by the strong interaction with the TEM electron beam, likely boiling off the metallic Na. EDS spectra were normalized by the dominant Cu Ka (~8.040 keV) background peak that is from the TEM grid holder...62

4.7 CNTs grown from NaOH-dip-coated carbon fabrics were observed

under the TEM to reveal two types of CNTs from Na-based growth. (a) Open-ended CNTs without catalyst particles observed at base or tip and (b) half-filled hollow CNTs were observed. Neither of these features are typically observed with CNT growth from traditional transition metal catalysts ... 63

4.8 The vanishing catalyst phenomenon-snapshots from supplemental

video 1 (SV1) of electron beam-induced etching (core depletion) of a CNT with core-shell morphology grown in situ on a NaOH-coated silicon nitride TEM grid within an environmental TEM.

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TEM images were taken in vacuum at room temperature after CNT growth at 480 0C at several time intervals of electron beam exposure. (a) A nanofiber structure at the beginning of beam exposure. (b) The onset of core depletion from the tip of the tube after 60 s of beam exposure. (c) The continued core depletion towards the base, revealing a hollow carbon tube identical to the CNT found in ex situ-grown specimens, after 120 s of beam exposure. (d) The onset of base Na nanoparticle depletion in addition to the start of base core depletion after 180 s of beam exp osu re. ... . . 64 4.9 NaOH-drop-cast silicon nitride TEM grids were subjected to

480 'C CVD in situ growth processing within the ETEM. Real-time imaging was performed right after CO2 and C2H2 were introduced

at 480 *C for growth durations of: (a) 0 s, (b) 180 s, and (c) 960 s. Extrusions of CNTs can be observed from Na nanoparticles...64 4.10 Structure and chemical composition of core-shell CNT revealed by

EELS line scan. (a) Annular dark field (ADF) image of CNT grown

in situ inside of the ETEM on NaOH-drop-cast silicon nitride grids,

taken immediately after growth. EELS line scan, direction, and position indicated by the white arrow, was performed to reveal the structure and chemical composition of the tube. (b) ADF image of the same CNT taken after the line scan. Note that line scan results in apparent removal of material from the core, which results in position shifts of the maximum intensities of elements in the core in (c). (c) Intensity profiles of the elements (C, 0, and Na) from the EELS line scan reveal a core-shell structure with carbon walls and N a-rich core... . . 65

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4.11 Core-shell CNTs were grown on NaOH-dip-coated alumina fibers using the nominal growth process at 480 'C for 15 min TEM images were taken of the solid nanofibers (a) immediately after growth, (b) after the TEM electron beam hollowed out the core of the CNT, and (c) after a solid nanofiber was annealed at 1000 'C in Ar for 60 min, revealing a hollow CNT. All TEM images have the same scale bar. (d) EDS was taken over each image area and the sodium-to-carbon intensity ratio is found to approach zero after the inert annealing process, leaving a catalyst-free CNT. EDS spectra were normalized by the dominant Cu Ka (~8.040 keV) background peak that is from the TEM grid holder...66

4.12 Modeling the energetics associated with the growth of armchair

(AC) and zigzag (ZZ) edges in a CNT, represented as a periodic

graphene nanoribbon. (a) Atoms 1 and 2 need to be repeatedly added at the AC edge to propagate the edge. (b) Atom 1 needs to be added to initiate the growth of a ZZ edge, followed by the repeated addition of atoms 2 and 3, to propagate the ZZ edge. (c-h), Energy minimized atomic structures corresponding to the sequential addition of carbon atoms (1 and 2), depicted in green, at an AC edge in the presence of a Na atom, depicted in blue within (c-e), and an Fe atom, depicted in cyan within (f-h). (i-p), Energy minimized atomic structures corresponding to the sequential addition of carbon atoms (1, 2, and 3), depicted in green, at a ZZ

edge in the presence of a Na atom, depicted in blue within (i-1) and an Fe atom, depicted in cyan within (m-p). The minimum C-Na and C-Fe bond lengths in the system are indicated on the top right hand corner of each panel. (q) Energy profile of the system as a function of the number of carbon atoms added, corresponding to panels (a) and (b), respectively, for an AC and ZZ edge, for both

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Na (blue) and Fe (red) atoms. The energy changes associated with the rate determining steps are shown. Solid lines depict the ZZ edge and dashed lines depict the AC edge...68 4.13 Ex situ X-ray photoelectron spectroscopy scans of NaOH-drop-cast

TEM grids (exposed to air right after processing and before scans). Scans were performed for binding energy levels corresponding to C is and Na is (where available) peaks for (a), As-received silicon nitride TEM grids, (b), NaOH-drop-cast TEM grids, (c), reduced only NaOH-drop-cast TEM grids, and (d), NaOH-drop-cast TEM grids after 15 min growth processing. Carbon-carbon double bonds, carbon-carbon single bonds, and H-I-H* transition peaks can be observed after 15 min growth, thereby indicating presence of sp2

and sp3 _{bonds consistent with CNT formation. In addition, the}

proportion of NaOH to Na2CO3 diminished after growth,

consistent with ETEM line-scans. Note no Nals peaks were observed for as-received TEM grids, and are thus now shown here...72 4.14 Raman spectra for NaOH-drop-cast silicon nitride TEM grids that

have been subjected to CNT growth for different growth temperatures. D/G ratios of 0.85, 0.91, and 0.81 were found for growths performed at 480 'C, 720 'C, and 390 'C, respectively. The D and G peaks observed are consistent with the presence of sp3 and

sp2 _{b on d s [97]. ...} _{. . 73}

4.15 Controls of as-received substrates (without dip-coating in NaOH) were subjected to the nominal CO2 and C2H2 CVD reaction at 480

'C and yielded no CNT. Scanning electron micrographs of CVD-processed substrates are shown in (a) received sized CF, (b) as-received silicon nitride TEM grid, (c) as-received diamond-abraded thermal silica on Si wafer, and (d) as-received thermal

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5.1 HTA40 CF weaves as received (a), and inside quartz tube furnace after K-PSMA functionalization, Fe dipcoating, and CNT growth

(b )...8 4

5.2 HTA40 CF weaves: as received (a), K-PSMA and iron

nitrate-coated HTA40 CF weaves after CNT growth (b), TEM image of individual CNT (c), and K-PSMA and iron nitrate-coated HTA40

CF weaves with CNT growth across tows (d)...85

5.3 Scanning electron micrograph of HTA40 CF weaves were dipped

in a 0.18 M NaOH solution and subjected to 480 'C CVD process

with C02/C 2H2 (top) in accordance with the depiction (bottom). ... 86

5.4 Evolution of vacuum-assisted resin infusion of CF weaves from

initial setups requiring internal vacuum bag sealing for top and bottom plates (a), to elimination of internal bagging for the top plate (b), and to a refined setup requiring only one sided bagging with a halving of the number of required layers (c)...89

5.5 A micrograph of a polished SBS specimen with lighter regions

showing 0 degree tows, grey regions showing 90 degree tows, and dark grey regions showing resin-rich areas...90

5.6 Both baseline and fuzzy CFRP were fabricated according to this

process flow for SBS strength testing... 91 5.7 SBS specimens after trimming down to ASTM compliant coupons

(a), and SBS laminated composite coupon loaded on three-point b en d fixtu re (b)... 92

5.8 Woven CFRP laminates were speckled with paint on the side and

tracked with optical strain mapping during SBS testing (a). Digital image correlation contour plot of shear strain from the side of a

SBS sp ecim en (b) ... 93

5.9 Interlaminar shear strengths are calculated from initial failure load

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load is plotted against deformation for SBS specimens over the full test deform ation (c)... 95 5.10 Infused and cured woven CFRP laminates representing additional

controls such as desized CFRP and CVD-processed CFRP are trimmed (a), and manufactured into SBS specimens that are shown here with dimensions of 2 mm X 4 mm X 12 mm (b). ... 100 5.11 Single filaments of HTA40 weave-extracted sized CFs, fuzzy CFs

(CNT grown on sized CFs), desized CFs, and fuzzy desized CFs are tensile tested in air to assess the effect of CNT growth on CF properties. No statistically significant difference in fiber strength is observed across all fiber groups. Additionally, no statistically significant difference in fiber modulus was observed between sized CF and fuzzy sized CF, as shown elsewhere [59]. Note that the error bars denote one standard error above and below the m ean s [121]...102 5.12 Thermogravimetric analysis was performed for desized and sized

HTA40 carbon fabrics in a nitrogen environment with a temperature ramp rate of 10 'C/min. A significant mass loss was observed for sized CFs starting at -300 'C corresponding to sizing re m o v a l...10 3 5.13 Fiber-matrix interfacial shear strengths obtained through

continuously monitored single fiber fragmentation tests of woven HTA40 fibers embedded within EP502 epoxy. IFSSs were obtained for fibers extracted from baseline (BL) sized weaves, fuzzy (FF) sized CF weaves, baseline desized weaves, and fuzzy desized CF weaves. The full-saturation values are used to calculate IFSS [59],

and numbers in the bars represent # of specimens tested...104 5.14 X-ray micro-CT scans were performed on woven CFRP laminates

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XY (a), YZ (b), and XZ (c) planes were observed with no detection of voids. A three-dimensional volumetric reconstruction (d) did not show any voids in the laminate. White spot denote residual dust from trim m ing specim ens...105 5.15 Load-deformation curves for SBS testing showing higher initial

peak loads but more abrupt failures for baseline and catalyst-coated sized CFRP. Fuzzy, desized, and CVD-processed sized CFRPs exhibit lower initial peak loads but exhibit tougher responses by maintaining load until loading nose travel limits are rea ch e d ... 10 6 5.16 Interlaminar shear strengths are calculated from initial failure load

of woven CFRP SBS specimens with different fabric processing steps for the interior 4 plies. Five specimens were tested for each treatment in accordance with ASTM D2344. The error bars denote one standard deviation. ... 107 6.1 Teflon-coated drying rack used to hold dip-coated fabrics and

allow for im proved catalyst deposition...113 6.2 0.18 M NaOH-dipped desized HTA40 CF weave was dried

horizontally on a "'V" rack and then subjected to nominal C02/C 2H2 growth at 480 'C. Lettering labels correspond to the

location of the image acquisition as marked on the CF weave (top row, middle column). Conformal CNT growth was observed across most externally visible tows with longer CNTs generally found at the edges of the w eave. ... 113 6.3 NaOH dipped woven CFs (both sized and desized) subjected to

480 'C growth with C02/C 2H2 showing CNT growth

nonuniformities in tow overlap regions that were moved apart for imaging, as evidenced by the darker patch in the SEM micrograph (a). For the same samples and process, visually, more CNT growth

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is observed at the leading edge of samples immediately after growth within the tube furnace as indicated by the darker region in the upstream direction (b)...115 6.4 Tows were extracted from a NaOH dip-coated desized CF weave

and placed on top of the weave for CNT growth processing. All exposed surfaces of tows experienced uniform CNT growth, while underlying regions of the tows did not yield CNTs...116 6.5 0.18 M NaOH dip-coated desized CF weaves were subjected to a

reduction-only process consisting of flowing hydrogen and ramping to 480 'C before the process was stopped and removed for SEM analysis. Variation in nanoparticle formation was observed spatially across the sam ple...118 6.6 0.18 M NaOH dip-coated desized CF weaves were subjected to a

reduction-only process consisting of flowing hydrogen and ramping to 480 *C before the process was stopped and removed for SEM analysis. Variation in nanoparticle formation was observed at the leading edge and correlated with the growth num ber of the day ... 118 6.7 0.05 M iron nitrate-dipped alumina and desized CFs are subjected

to a modified C02/C 2H2 CVD process at 550 'C showing

successful, but different morphologies of growth. Both samples shown here were processed in the same run...121 6.8 Fe and C elemental maps of 0.05 M iron nitrate-dip-coated desized

CFs grown at 550 *C with C02/C2H2 CVD. An SEM image of the

fuzzy desized CF (left) shows streaks of nanostructures and nanoparticles coinciding with the CF surface grooves. An EDS map for C (middle) co-located with the SEM image shows C everywhere on the fiber as expected, with no distinction on areas

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for the C map coincides with the bright region on the Fe map (right) since the Fe agglomeration covers the CF. Streaks of Fe coincide with the CF surface grooves, indicating more Fe coverage is needed for improved CNT growth uniformity. ... 122 6.9 UD carbon fabrics were dip-coated with the support of porous

mesh sheets to prevent surface tension-induced distortion (left), and also supported on quartz slides to prevent tow separation by thermoplastic weft transformation during CVD (right). ... 125 6.10 Scanning electron micrograph of radially aligned CNTs

successfully grown directly on UD carbon fabrics. ... 126 6.11 Vacuum-assisted epoxy resin infusion setup with UD fuzzy CF

fabric sandwiched between two plates at a specified thickness for control over specimen fiber volume fraction (top), and trimmed UD SBS specimens after laminate cure (bottom)...127 6.12 As-received sized UD CFs, thermally desized UD CFs (heat treated

in Ar at 400 *C for 2 min), and CVD processed sized UD CFs were heated in nitrogen to 1000 C within a thermogravimetric analyzer....128 6.13 X-ray micro-computed tomography of a representative sized UD

CFRP SBS specimen showing cross-sectional views (top left, top right, and bottom left), as well as a 3D reconstruction (bottom right), to assess lam inate quality...129 6.14 SBS load-deformation plot for UD sized CFRP, UD desized CFRP,

and UD fuzzy CFRP (top), as well as the mean interlaminar shear strengths calculated from the peak load (bottom). ... 131 7.1 50-mm-diameter furnace with iron nitrate-dip-coated SiC and

alumina fibers simultaneously CVD processed for CNT growth...138 7.2 Alumina fibers, sized SiC fabrics, and thermally desized SiC

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subjected to a 650 'C CNT growth process. Alumina (left) and sized SiC fabrics (middle) are shown to support growth in contrast to desized SiC fabrics (right). ... 139 7.3 Iron nitrate-dip-coated sized SiC fabrics were subjected to varying

reduction times for growth at 650 *C and observed under the SEM. ... 140 7.4 Sized SiC fabric immersed in an evaporating bath of 0.5 M iron

nitrate in isopropanol solution at 60 'C was grown using 5 min nominal C2H4 CVD at 650 'C, which is expected to support CNT

growth since baseline iron-coated dipped alumina fiber samples that were processed concurrently yielded CNTs. Evidence of CNT alignment is seen between the clumps of catalyst particles. ... 142 7.5 Representative scanning electron micrographs of CNT growths

performed on iron nitrate solution-dipped SiC fibers for which experimental variables are catalyst solution temperature and catalyst solution drying orientation. ... 143

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List of Tables

5.1 Woven carbon fabric processing summary for single fiber and SBS

te stin g ... 9 7 6.1 UD carbon fabric processing summary ... 124

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Abbreviations and Symbols

AC Armchair

Ar Argon

C Carbon

CF Carbon fiber

CFRP Carbon-fiber reinforced polymer (or plastic)

C2H2 Acetylene

C2H4 Ethylene

CO2 Carbon dioxide

CNT Carbon nanotube

CVD Chemical vapor deposition

DFT Density functional theory

EELS Electron energy loss spectroscopy

Ec Longitudinal composite chord modulus

EcROM Longitudinal composite chord modulus from rule of mixtures

Ef Longitudinal modulus of fibers

Effplied Implied longitudinal modulus of fibers

E~n Longitudinal modulus of matrix

EDS Energy dispersive x-ray spectroscopy

ETEM Environmental transmission electron microscope

Fe Iron

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FRC FRP GPa H₂ HTA-40 HTR-40 HTS-45 ID IFSS ILSS K-PSMA MPa MWNT N Na NaCl Na2CO₃ NaHCO₃ NaOH Ni n nm OD PAN PNC ROM sccm SBS

Fiber reinforced composite Fiber reinforced plastic

Gigapascals, 109 kg s-2 m-1 = 109 N m-2

Hydrogen

TohoTenax high tenacity aerospace-grade carbon fiber TohoTenax high tenacity research-grade carbon fiber TohoTenax high tenacity carbon fiber

Inner diameter

Interfacial shear strength Interlaminar shear strength

Potassium polystyrene-alt-maleic anhydride

Megapascals, 106 kg s-2 m-1 = 106 N m-2

Multiwall carbon nanotube Nitrogen Sodium Sodium chloride Sodium carbonate Sodium bicarbonate Sodium hydroxide Nickel

Number of fibers in tow

Nanometer, 10-9 m

Outer diameter

Polyacrylonitrile

Polymer nanocomposite Rule of mixtures

Standard cubic centimeters per minute = cm3 _min-1 ₌_{mL min-}1

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SEM STEM SWNT TEM Vf (or Vf) XPS ZZ Efm pm n

Scanning electron microscopy

Scanning transmission electron microscope Single-wall carbon nanotube

Transmission electron microscopy Fiber volume fraction

X-ray spectroscopy Zigzag

Fiber failure strain

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Chapter 1 Introduction

Advances in materials have historically lay at the heart of many performance leaps in vehicles and structures. Particularly in the aerospace industry, introduction of new materials has ushered in fundamentally new design possibilities for aerospace vehicles that have in turn enabled gains in efficiency. For example, the introduction of aluminum alloys into aircraft enabled new metal monocoque designs that ushered in the jet age. Following World War II came the development of advanced continuous (or long) filamentary composites, a new class of lightweight engineering materials that achieve desired materials properties through combining multiple component materials into a single material with a hybrid composition. Development of increasingly advanced composite materials continues to this day and has brought a new wave of lightweight vehicles to commercial airlines with significant effects to the global transportation infrastructure. Some key examples include the Boeing 787, the Airbus A350, and the Airbus A220, the first commercial passenger aircraft with fuselages composed primarily of carbon fiber reinforced plastic (CFRP) composites. Simultaneously, the emergence of reusable and economically favorable space launch vehicles has been enabled in large part by lightweight composite structures and pressure

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vessels that leave more mass budget for payload and propellant for vehicle recovery than parts made with metal alloys. In short, the mass reduction benefits of composites are quickly and increasingly being realized across numerous aerospace applications and is projected to continue to be quickly integrated into systems of other sectors, such as energy, automotive, and sporting goods. Fiber Out--ane (through-thickness)e. direction IntedaninarIn-plane nei n directions Intralaminar region _LAMINATE Matrix LAMINAE

Figure 1.1 Schematic of a filamentary composite laminate schematic, consisting of stacked laminae (plies) comprised of fibers and matrix [1]. Fiber orientations in each

laminae can be arranged to tailor elastic response of the overall laminate.

While advanced filamentary composites offer desirable and highly tailorable mass-normalized strength and stiffness properties, they also exhibit drawbacks over traditional alloy materials due to their layered construction in which high-aspect-ratio fibers are held together by a relatively weak matrix material, usually a polymer such as an epoxy (Figure 1.1). Currently, most filamentary composites are prepared by stacking component plies comprising axially-aligned continuous fibers bound by preimpregnated semicured resin, or prepreg, one layer at the time, resulting in a fiber-reinforced preform with

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Consequently, these microscopic polymer-rich regions between plies and fibers - typically consisting of relatively brittle thermoset epoxies in aerospace applications - are left without reinforcement, and are thus susceptible to crack initiation and propagation. Particular load cases such as impact and fatigue where this failure mode becomes especially problematic drive engineers to overdesign structures and thus not realize the full potential weight savings that CF composites could otherwise bring. Additionally, the electrical conductivity of a structure decreases when switching from aluminum alloys to CF/epoxy composites, rendering composites disadvantageous for certain flight requirements such as lightning strike protection. In the latter example, aircraft manufacturers resort to adding relateively dense copper meshes, further subtracting from the potential weight savings that composites could offer.

These shortcomings were identified early on before the widespread introduction of composites into commercial aircraft, and many ideas have been attempted to address them albeit with mechanical properties compromises. For instance, z-pinning [2] and 3D braiding/weaving [3] techniques have helped to reinforce the through-thickness direction of composite laminates but at the expense of in-plane properties of the resulting composites. Such microscale approaches for reinforcement result in alterations to the existing composite architectures by either volumetrically displacing fibers running within a plane or damaging fiber reinforcement in those directions. As a result, nano-length-scale approaches for integrating mechanical reinforcement into the interstitial expanses of the matrix between fibers are of significant interest.

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1.1. Motivation: Fuzzy Aerospace FRPs

Nanoengineering offers unique opportunities for reinforcing the brittle matrices of advanced filamentary composites without affecting the existing microfiber architecture while simultaneously introducing additional functionalities such as increased thermal and electrical conductivity. Introduction of numerous nanofillers, that is, nanostructured particulate additives, has been investigated by several groups, with additives ranging from various oxide nanoparticles [4], graphene [5], nanostructured boron nitride fillers [6], and perhaps most-studied: carbon nanotubes (CNTs). One additive technology of particular interest is integration of aligned carbon nanotubes [7] (A-CNTs) into the composite matrix as aligned high-aspect-ratio nanoscale fibers that could serve to reinforce the microscopic matrix interstices between microfibers. Aligned CNTs exhibit high mass-specific strength and stiffness properties, as well as large surface areas for toughening of matrices. They are of similar chemical composition to the carbon microfibers that already make up the composite architecture, making them particularly appealing candidates for nano-level reinforcement in existing CFRP. Previous work has already demonstrated the toughening effect arising from integration of CNTs into polymers [8]. Consequently, various architectures have been investigated that integrate CNTs into microfiber composites in order to develop laminates with enhanced mechanical and multifunctional properties. One architecture of great interest involves radially-aligned CNTs circumferentially coating the surface of the microfibers to create a "fuzzy-fiber" reinforced plastic (FFRP) hierarchical architecture with both inter- and intraply reinforcement. This architecture has been successfully implemented and investigated on alumina fibers previously to create alumina fiber/epoxy FRP composites with integrated CNT reinforcement that have significant improvements in mechanical, electrical,

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and thermal characteristics [9-11]. For lightweight and high-performance structures such as airframes, there exists high interest in implementing this same architecture with CFs to create fuzzy carbon fiber reinforced plastics (fuzzy CFRP) featuring the same types of improvements. The most versatile and common method of CNT growth on CF is through chemical vapor deposition (CVD), in which microfibers are deposited with catalyst nanoparticles and heated in the presence of hydrocarbon gas (e.g., C2H2 and

C2H4) to preferentially decompose carbon onto the surface of catalyst particles

in the form of CNTs. However, scaleable CVD methods for direct growth of CNTs onto CFs have been challenging to date, resulting in loss of strength in the underlying microfiber and thus in-plane strength reductions in the resulting CFRP composites [12]. As a result, methods for growing high yields of CNTs onto CFs must be developed and the effect of the process on the CF strengths must be assessed. Immediately prior to this thesis, promising CNT growth methods have been developed and shown to preserve CF strengths on the single fiber [13] and ply levels [14]. Further, scale up to laminates must be demonstrated to assess inter- and intralaminar effects of CNTs on CFRPs.

1.2. Thesis Outline

This thesis explores the development of fuzzy CFRPs with preserved in-plane properties (from preserved CF properties) through research efforts that include fundamentals of CNT growth, manufacturing of fuzzy CF laminates, and the assessment of interlaminar mechanical properties of such laminates.

In Chapter 2, an overview of nanoengineered composites is presented with a focus on the FFRP architecture. A review of past challenges and efforts underlying growth of CNTs on CFs is discussed followed by solutions implemented on the single-fiber level and subsequently the unidirectional (UD) ply level.

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In Chapter 3, the objectives of this thesis set forth to motivate the development of fuzzy CFRP laminates with uncompromised microfiber properties and the methods of interest for effective scale-up in the laboratory setting are presented.

In Chapter 4, the discovery of a new catalyst system that simplifies the process for growing CNTs directly on CFs, as well as a variety of substrates, at relatively low temperatures while still preserving the CF properties is presented. For the first time, sodium (Na) is identified to be an active catalyst for high-yield synthesis of CNTs at low temperatures, and confirmation analyses are presented to verify the identity of CNTs and catalyst particles. In

situ TEM analysis is also presented to visualize the formation of CNTs and

shed insight into this novel catalyst for carbon nanostructure synthesis. Additionally, the possibility of other alkali metals for CNT growth is mentioned along with opportunities for future research vectors into CNT catalysis incorporating alkali metals.

In Chapter 5, the scaling up of CNT growth on CFs to enable production of laminates is presented. Discussions on catalyst deposition methods and CNT growth on woven fabrics are presented. Vacuum-assisted resin infusion (VARI) processing and laminate quality assessments are shown, followed by interlaminar shear strength testing. Informed by the initial differences in short-beam shear (SBS) failure between baseline and fuzzy CFRP laminates, additional controls were developed and tested to elucidate contributions to the observed failure behavior. In conjunction, both single-fiber and fiber-matrix interface studies were conducted to isolate CNT reinforcement contributions.

In Chapter 6, efforts to improve three-dimensional growth uniformity of CNTs on CFs are presented, including parametric studies of low-temperature CNT growth from iron (Fe) nanoparticle catalysts on CFs, and variations in

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process conditions. Further, to isolate and increase the CNT fill fraction

-defined as the ratio of CNT lengths to interlaminar spacing - within

interlaminar regions, CNT growth was successfully migrated onto UD carbon fabrics, which were subsequently made into UD fuzzy CFRP laminates. Improved CNT interlaminar fill-fraction over initial woven fuzzy CFRP samples in Chapter 5 are demonstrated. Comparisons between UD baseline and UD fuzzy CFRP laminates are made.

In Chapter 7, the methods for CNT growth on CFs developed in this thesis

are applied towards another microfiber of interest for structural applications.

CNT growth on silicon carbide (SiC) fibers is demonstrated through

parametric coating and growth studies, along with methods for process tuning to achieve alignment of CNTs on such microfibers.

In Chapter 8, key contributions of this thesis are summarized and recommendations for implementation of fuzzy CFRP laminates in support of mechanical and interlaminar characterizations are made. Possible areas for investigation of alkali-metal-based CNT growth catalysts are identified as both immediate and longer-term next steps towards realizing improvements across various industries including aerospace, electronics, and energy.

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Chapter 2 Background

While composites are being increasingly used across a host of industries, their full potential for weight savings and performance enhancement in mass-critical structures remains untapped due to damage tolerance considerations. In particular, failure within the matrix-rich interlaminar regions of composites, results in laminate-level failure modes such as delamination. Integration of aligned nanoscale fibers into these weak interlaminar matrix regions is a promising approach for improving the toughness and strength of advanced filamentary composites and thus unlocking the ability to streamline structural design with composites. This chapter provides an overview of current mechanical limitations in composites, nanoengineered approaches to address these issues with a focus on fuzzy fiber architectures, and challenges towards scaling these methods to laminate-level production. This review contextualizes the overall objectives and approaches taken in this thesis. Additional review of prior works relevant to the specific approaches explored in this thesis is provided within individual chapters.

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2.1. Overview of Carbon Fiber Reinforced Plastic

(CFRP) Composites

Fiber-reinforced composites (FRCs) are a subset of heterogeneous materials, a class of materials that integrate multiple dissimilar component materials to produce a new material that averages the materials properties of its components or sometimes even results in improved materials properties over its components through synergistic effects. In the case of FRCs, fibers that are generally stiff and/or strong in tension are embedded within a matrix, generally with attractive shear properties, allowing for load sharing between adjacent fibers and resulting in an overall stiff material structure. The choice of fiber and matrix is highly engineered based on the application of interest, weighing factors including mechanical performance, environment effects, manufacturability/ geometry, cost, and electromagnetic interference/shielding. Two basic fiber morphologies are short (chopped fibers usually thought of as fillers) and continuous, or long-fiber composites, used in demanding structural applications. In the aerospace sector, common structural microfibers include glass fibers (density ~2.5 g/cm 3_{) and CFs}

(density ~1.8 g/cm 3_{). In terms of matrices, common airframe structures use}

low-density thermoset polymers such as epoxies (density ~1.1 g/cm 3_{) for}

their desirable shear properties (> 30 GPa), or thermoplastic matrices for their repairability. For high-temperature applications, metal or pyrolyzed carbon matrices are common and used in conjunction with CFs. Predominantly for airframes or primary structures of vehicles where lightweight, stiff, and damage tolerant parts are needed, carbon/epoxy (sometimes called graphite/epoxy) are typically employed, with a majority of aerospace structural applications using continuous fiber CFRP. Additionally, by tuning the stacking sequence of plies of aligned fibers and the relative orientation of fibers from ply to ply, the elastic and strength response of CFRPs can be

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highly tailored [15] for applications such as passive gust alleviation [16]. Composite fabrication also enables complex part geometries to be built up without significant subtractive manufacturing, by wrapping preforms over molds one layer at a time.

Various form factors of carbon fiber include tow, UD plies, and weaves, while several different matrix infiltration techniques exist, together enabling production of a wide variety of CFRP architectures. Aerospace-grade CFs are commonly derived through pultrusion and subsequent carbonization of a polyacrylonitrile-based fibers to yield what is called an "ex-PAN" CF [17]. In this thesis, the term CF will be used to refer exclusively to ex-PAN CFs unless otherwise noted. During conversion into carbon, the filaments are processed together under tension over sets of rollers in tows (also referred to as rovings) typically consisting of thousands to tens of thousands of fibers. The fibers are then treated via, for example, an electrolytic bath to impart oxygenated surface moieties over the surface of the fiber. Finally the fiber is coated with a thin polymer layer, or sizing, to protect the underlying surface chemistry critical for matrix adhesion and to avoid self-abrading of the CF during handling [18, 19]. For dry fabrics, sized CF tows can be pulled into a weaving machine to create a woven fabric of varying patterns (e.g., plain weave, satin harness weaves are common), a UD fabric (e.g., tows that are spread and stitched transversely with other types of microfibers), or even a three-dimensional woven preform in which each layer is stitched or woven together. For other fabrics, sized CF tows are pulled into a preimpregnation machine that applies uncured or semicured resin to the tow and continuously joins them transversely to make flat sheets of preimpregnated UD uncured laminae, commonly referred to as prepreg. Woven prepregs can also be fabricated through a combined implementation of the two aforementioned processes. From these available carbon textile types, some manufacturing methods in aerospace include wet layup, prepreg layup, and vacuum-assisted

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resin infusion (VARI). In wet layup, dry fabrics are placed on a mold, resin is poured onto the fabric, excess is "squeegeed" out of the fabric, and then the steps are repeated sequentially until the full desired laminate thickness is reached before then finally being vacuum bagged and cured either at room or elevated temperatures. In prepreg layup, prepreg layers are peeled from separation film (used for storage in freezers and transport) and stacked in the desired orientations until the full desired laminate thickness is reached, followed by vacuum bagging and then curing at elevated temperatures either at ambient pressures (for what are called out-of-autoclave, OoA, materials) or at high pressures (for what are called autoclave materials). In a VARI or vacuum-assisted resin transfer molding process (VARTM), dry fabrics are stacked and sealed within a closed or open mold before being connected to a vacuum source, which is used to draw in a low-viscosity resin (under ambient or positive pressure) into the opposite end of the preform. A valve placed between the resin and the fabric is then opened to controllably infuse the fabric with resin.

Regardless of the fabrication method employed, one of primary the drawbacks to the adoption of composites over metal alloys is the unique failure modes associated with a layered material held together by a relatively brittle matrix. Unlike the intralaminar regions where fibers typically comprise about 50-60% of the volume fraction, the regions between plies are typically matrix-rich, and do not contain a significant fraction (typically none) of fibers spanning across the interfaces. These unreinforced neat resin interlaminar areas are thus common areas for crack initiation and propagation, leading to the separation of plies in what is known as delamination. Such failures compromise the material structure, as the plies are no longer effectively bonded together in a way that permits them to carry loads in concert through combined action. As a result, care is often taken during processing and

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the edges of composite pieces. In terms of design, aerospace composite parts are typically over-designed to handle conservative load cases that cause delamination such as impact events [20, 21]. For instance, minimum gauge thickness for composite fuselages of commercial aircraft are driven by common impact events from ground service equipment [22], not the substantially thinner gauge that would be required for the fuselage to just hold cabin pressure and bear aerodynamic loads.

To enable more damage-tolerant CFRP structures and to toughen and strengthen the matrix-rich regions of composites, many previous efforts have investigated using microscale reinforcements in the through-thickness direction, with notable tradeoffs in mechanical performance. Z-pinning is a technique in which microscale pins are physically through laminates, effectively nailing the plies together and providing an increase in fracture toughness [2, 23]. However, the damage to Cs local to the pins results in

degraded in-plane strengths by about 60% for every 10% pin volume fraction [3]. 3D braiding is another concept in which a fraction of the carbon filaments in a preform run vertically in the z-direction to stitch plies together, resulting in an increase in mode I fracture toughness [24, 25]. While this technique averts localized damage to carbon filaments, a fraction of in-plane fibers is redirected and sacrificed towards the through-thickness direction. Localized pinching and misalignment of tows also contributes to a reduction of in-plane composite strength by as much as 50%. Meanwhile, 3D stitching is another technique that holds prepreg or dry fabrics together by sewing high strength microfibers in the through-thickness direction [26-31]. While this architecture provides out-of-plane reinforcement and impact damage resistance, fiber distortion can contribute up to a 20% degradation of in-plane strengths [25]. Herein lies the motivation to develop composites with superior interlaminar toughness and strength, without reducing planar performance [3].

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2.2. Nano-scale Reinforcement of CFRP

Nanoengineered composites offer a unique opportunity to reinforce the matrix-rich regions of composites without changing the morphology of the microfiber architecture. Because the spacing of CFs within a ply is generally less than 10 microns, with the spacing of fibers across interlaminar regions in excess of 10 microns, the use of nanoscale reinforcements represents a length-scale-appropriate approach for reinforcing the interstitial matrix-rich regions between fibers and plies. Previous work has examined the inclusion of various nanofillers to toughen polymer matrix resins, including thermoplastic

[32], glass [33], nanoclays [34], and nanofibers [35-37], and ceramics [4, 38, 39]. One nanoscale additive of great interest is carbon nanotubes (CNTs), a carbon allotrope consisting of seamless, tubular, filamentous structures comprised of sp2-hybridized hexagonally-patterned carbon atoms analogous to graphite. As conceptually rolled-up sheets of graphene, CNTs possess superlative mass-normalized strength [40], stiffness, electrical, and thermal transport properties [41, 42], making them useful multifunctional additives. In addition, the one-dimensional geometry of CNTs offers large surface-area-to-volume ratios advantageous for activating a variety of toughening modes within the matrix in which they are embedded, such as CNT/matrix pullout, fiber bridging, etc. [43, 44]. CNTs also can be grown to consist of multiple concentric walls, which adds an additional toughening mechanism of inter wall pull-out (also known as "sword-in-sheath" pull-out) [45]. Through combinations of these mechanisms, CNT reinforcement of bulk polymers has already been demonstrated, resulting in observed increases in toughness [8], elastic behavior [46, 47], and electrical and thermal conductivities [48] in resultant polymer nanocomposites (PNCs). Notably, and analogous to microfiber composites, alignment and higher packing of CNTs (such as through transverse mechanical densification of aligned CNTs, or A-CNTs)

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results in tailored non-isotropic properties [46, 48]. These experimentally demonstrated improvements motivate efforts to integrate A-CNTs into the weak polymer-rich regions of FRCs /FRPs.

A variety of methods have been employed to integrate CNTs into the polymer matrix of FRPs. Many efforts have focused on mixing CNTs into resins prior to matrix infiltration but have been meet with significant challenges including nonuniformity of dispersion, increase in resin viscosity with CNT content, damage to CNTs under high shear, and lack of control over CNT orientation [49, 50], which is desirable to preferentially increase toughness for out-of-plane loading [51-53]. Other efforts have included spraying of randomly-oriented CNTs [54] onto the surface of dry fabrics and electrophoretic deposition of CNTs prior to matrix infiltration [55], however a lack of alignment substantially limits the degree of reinforcement. An approach called nanostitching addresses this issue by transferring and sandwiching pregrown aligned forests, or vertically aligned arrays, of A-CNTs (grown and separated from a flat silicon wafer substrate) between plies of prepreg during composite layup to create a hieraarchical nanoengineered composite architecture [45, 56]. Experimentally, nanostitching has been shown to bridge the interlaminar region and maintain through-thickness alignment during cure, resulting in an -50% increase in Mode I toughness [56], and more tortuous crack paths, desirable for increasing interlaminar fracture toughness. This demonstrated the efficacy of CNTs for increasing the toughness of the interlaminar region of composites. Further improvements in composite fracture toughness, however, could potentially be realized through reinforcement of the intralaminar regions as well.

The "fuzzy fiber" architecture is another advanced hierarchical nanoengineered composite that enables both interlaminar and intralaminar reinforcement of the matrix. In this architecture, radially aligned CNTs are grown on the surface of fibers prior to matrix infiltration through a thermal

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chemical vapor deposition (CVD). Because the CNTs emerge from each fiber and thus span the gaps between fibers, matrix reinforcement occurs not only between adjacent plies but also within each ply between adjacent microfibers and tows as shown in Figure 2.1. Consequently, this results in the desired orientation and placement of CNTs throughout the composite by avoiding matrix incorporation complications and by preserving CNT properties. Additionally, different textile form factors such as tows and weaves with CNTs grown on them can still be fabricated into composites through wet layup or vacuum-assisted resin infusion techniques. Prior work with growth of CNTs on alumina woven fabrics showed that CNTs remain on the surface of the fibers during infusion, and that the A-CNT alignment assists the resin flow through capillarity-assisted wetting [57]. Such alumina/epoxy FFRPs have shown a range of materials properties improvements over baseline alumina/epoxy composites including a 75% improvement in steady-state Mode I fracture toughness. Additionally, while baseline alumina/epoxy composites are normally thermally and electrically insulating, incorporation of CNTs has been shown to enable a percolating conductivity network across the composite, resulting in an over six orders of magnitude increase in electrical and thermal conductivity [58], even within the intralaminar region, uniquely afforded by the inclusion of the continuous network of CNTs.

While the alumina/epoxy FFRP serves as a model system that motivates the same implementation of CNTs into other microfiber systems, efforts at creating fuzzy CFRPs of interest for aerospace applications have been met with multiple challenges. Unlike alumina fibers, CFs exhibit poor wettability by, and poor adhesion to, common transition-metal catalyst precursors and catalysts such as Fe required for CVD growth of CNTs upon their surfaces. Previous efforts have employed etching techniques to promote adhesion but have been shown to result in damage to the CF surface, where the majority of

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-- fibe-rs covered with alitened CNT, plies pliesTows of CNT-covered fibers

Figure 2.1 Schematic illustration of the Fuzzy Fiber Reinforced Plastic (FFRP) architecture. CNTs circumferentially coat individual filaments within a cloth (right).

Cloths containing CNT covered fiber tows can be laid up as composite plies in a polymer matrix (left) [11, 59].

tensile strength [60, 61]. Both Sager et al. and Qian et al. have demonstrated unaligned CNT growth on CFs, and found increases in the interfacial shear strength (IFSS) of the CF-epoxy interface, however CF strength losses of 37% and 55% were concomitantly observed, respectively. Kepple et al. demonstrated that unaligned CNT growth increases laminate fracture toughness by 50%, but does not characterize the effect on the CF tensile properties. Previous studies have shown that the CF (and thus CFRP composite) strength loss is contributed by the detrimental interactions between Fe catalysts and the underlying CF at high CVD temperatures, which may result in surface pitting and damage to the CFs [12, 62]. Mathur et al. found a 54% decrease in composite flexural strength for UD CFRP with 3.5 wt% of CNTs, and attributed this result to defects arising from catalyst and CF surface interactions. Further, Steiner III et al. revealed that just exposure to the high temperatures typically required for CVD growth of CNTs alone (> 550 *C) can incur damage to CFs, even under inert atmospheres [13]. In an effort to prevent CVD-induced damage to CFs, Herceg et al. dispersed

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prefabricated PNC powders on CF tows wetted with resin at room temperature and wound them to form fuzzy UD prepregs [63]. However, voids in manufacturing, and microscale polymer nanocomposite (PNC) particle agglomerations in the interlaminar regions cloud any conclusive CNT reinforcement effects. Interestingly, for reasons that were not understood, Anthony et al. showed that applying an in situ potential difference between the CFs and a counter electrode during high temperature CVD results in preservation of CF strength. However, the high voltage requirement (300 V), long growth time used (60 min), and low CNT coverage on the fiber surface observed present scalability challenges [64].

To circumvent the dominant sources of CF strength loss during CVD growth of CNTs on CFs, one strategy developed by Steiner III et al. involved improving the wettability of CFs for attaching Fe nanoparticle catalyst precursor without a need for deleterious chemical etching via the use of a noncovalent functionalization of the fiber surface [65]. The potassium salt of polystyrene-alt-(maleic anhydride) (K-PSMA) was developed as an ion exchange coating applied prior to dip-coating of the CFs with an Fe catalyst solution [66]. Around the same time, Steiner III et al. demonstrated a low-temperature thermal CVD process in which CNTs are grown on CFs at 480 'C using the methods described by Magrez et al. [67]. Tensile strength tests of fibers processed at the single-fiber-level revealed these two strategies could be combined to enable CVD growth of CNTs on CFs in a way that preserves the

mechanical properties of the underlying CF [68].

Leveraging the ability to preserve the materials properties of the underlying carbon fiber during CNT growth, Li et al. developed refined processing techniques to achieve conformal CNT growth on full tows and further developed a process for fabricating the first-ever described UD fuzzy CFRP composites at high fiber volume fractions (64%), with properties

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Baseline Fuzzy CFRP CFRP 25 2 :131-131 G--0_U ₁₃₁_G~a 0 5000 10000 15000 20000

Composite Strain, pstrain Load

160 - 3.5 j 140 ' IT 3.0 120-100 _~~ioo~ _2.0-- _~ 80 1.51 U&20

&

05 0 - - 0.0 ...

Figure 2.2 A typical stress strain curve is shown for UD fuzzy CFRP composite as compared with a baseline unsized CFRP (top) employing noncovalent functionalization and low temperature CNT growth at 480 *C. Both composite

moduli (bottom left) and composite strength properties (bottom right) are demonstrated to be preserved and in alignment with micromechanical predictions as

indicated through "ROM". Note that the strain axis is offset in the top image to improve visualization of the fuzzy CFRP data.

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CFRP tows [69, 70]. As shown in Figure 2.2, the resultant fuzzy CFRP

composites were found to retain in-plane strength and stiffness properties of the composites, with fuzzy CFRP composites exhibiting improved surface crack arrest. Moreover, fiber-matrix interface characterizations revealed that fuzzy CFs do not result in any changes to the microfiber matrix IFSS when compared to unsized (uncoated and bare) CFs, a result that is advantageous for maintaining in-plane damage tolerance of composites. It is important to note that the IFSS is designed to be at an intermediate and optimized value that enables sufficient load sharing between fibers during elastic loading [71], and fiber pullout upon failure to result in a toughened composite response to failure [72].

While these results show promise in preserving filament properties at the filament and ply level, challenges remain in implementing the fuzzy fiber

architecture on a laminate level. Interlaminar characterizations and

assessments have yet to be demonstrated using a technique that enables direct CNT growth on CFs without damaging the underlying fiber. Moreover, the strategy described above requires that sizing on the CFs be removed prior to CNT growth and involves the use of an additional functional coating prior to catalyst application. Hence, a more scalable technique that requires less preprocessing steps for CNT growth is desired.

2.3. Summary

This chapter has highlighted the advantages of nanoengineered

composites with particular focus on fuzzy fiber reinforced hierarchical composite architectures, particularly fuzzy CFRP for aerospace applications, as an approach towards improving toughness and strength of CFRP laminates. Challenges to implementing fuzzy CFRP laminates with preserved

Catalysis and manufacturing of two-scale hierarchical nano- and microfiber advanced aerospace fiber-reinforced plastic composites

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Acknowledgements

Table of Contents

List of Figures

/C

List of Tables

Abbreviations and Symbols

Chapter 1

Introduction

1.1. Motivation: Fuzzy Aerospace FRPs

1.2. Thesis Outline

Chapter 2

Background

2.1. Overview of Carbon Fiber Reinforced Plastic

(CFRP) Composites

2.2. Nano-scale Reinforcement of CFRP

&

2.3. Summary