DMSE Doctoral Thesis Defense

Interfacial and Physical Confinement Effects on the Structure and Properties of Aligned Carbon Nanotube Architectures

Ashley Kaiser

Wednesday, April 14, 2021

10:00 – 11:00 AM EST

Zoom link: Contact dmse-gradoffice@mit.edu for the Zoom link.

The advantaged mass-specific, intrinsic, and scale-dependent properties of aligned nanofibers, such as aligned carbon nanotubes (A-CNTs), and their ability to be densified into high volume fraction (commonly, vol%) 3D architectures motivates their use as shape-engineerable materials and composite reinforcement. While controlling CNT adhesion to the growth substrate and packing density in A-CNT arrays is essential for improving material properties towards bulk-scale manufacturing and application-specific performance, experimental and theoretical approaches to date have not addressed the mechanisms or scaling of CNT-substrate adhesion with post-processing conditions. Also unexplored is fully describing how the nano-, meso-, and micro-scale structures of aerospace-grade composite matrices are affected by high levels of A-CNT confinement (inter-CNT spacings on the order of nm). To understand these effects, this thesis studies the CNT-substrate strength that governs the manufacturing of shape-engineered CNT arrays, and the physical confinement of A-CNTs in the composite matrix that scales performance. In addition to tuning CNT-substrate strength via thermal post-processing, four types of nanocomposites are synthesized to study confinement effects as a function of A-CNT vol%: A-CNT-polymer nanocomposites (PNCs) with aerospace-grade epoxy, bismaleimide (BMI), and phenolic resin matrices, and an A-CNT-carbon nanocomposite (A/C-NC) with a phenolic-derived pyrolytic carbon (PyC) matrix.

A thermal post-growth process to anneal mm-tall aligned CNT arrays on their growth substrate (Fe/Al2O3/SiO2/Si wafers) at temperatures from Tp=700°C, the CNT synthesis temperature, up to 950°C in helium is used to study CNT-substrate interfacial strength. The bulk CNT array-substrate pull-off strength measured via tensile testing shows that the array fails progressively, similar to microfiber bundles, and evolves non-monotonically with Tp throughout three Regimes: first increasing from as-grown (~0.04 MPa) to ~0.13 MPa up to Tp = 735°C (Regime I), then up to ~0.35 MPa up to Tp = 800°C (Regime II), and then decreasing back to ~0.13 MPa up to Tp = 950°C (Regime III). The force-strain relation from tensile testing is modeled analytically based on microfiber bundle modeling and Weibull statistics, which considers the statistical failure of individual CNTs as they either debond from the substrate in Regimes I/II or break during pull-off, leaving ~2-micron long CNTs attached to the substrate in Regimes II/III. Morphological and chemical analysis indicates that the Fe catalyst remains on the substrate after CNT array pull-off, the CNT array structural quality is maintained with Tp, and higher Tp may graphitize the disordered carbon at the CNT roots to increase the substrate adhesion strength up to 800°C. At higher temperatures, thermally-induced marring and evolution of the substrate layers are observed in Regime III, where CNT-substrate pull-off strength reduces relative to Regime II.

Key results regarding A-CNT PNCs and A/C-NCs include process development leading to the first successful fabrication of fully infused, microvoid-free BMI, epoxy, and phenolic PNCs and A/C-NCs with high volume fractions of biaxially mechanically densified mm-tall A-CNT array reinforcement (1 to 30 vol%, corresponding to inter-CNT spacings of ~70 to 6 nm, respectively), showing that CNT-polymer and CNT-carbon confinement effects exist at the < 10 nm scale. The development of a polymer infiltration model based on Darcy’s law accurately predicts the time for uncured resin to fully infuse into CNT arrays during capillary-assisted PNC fabrication, which corroborates experimental observations via X-ray micro-computed tomography and microscopy that diluted resin with up to ~8x lower viscosity than neat resin is required for full infusion into dense A-CNT arrays (10-30 vol%). The cured PNCs maintain consistent vertical CNT alignment and glass transition temperature, and the decomposition onset temperature is constant for epoxy PNCs, increases by ~8°C for BMI PNCs, and increases by ~40°C for phenolic PNCs up to 30 vol% A-CNTs. Quasi-static nanoindentation yields a ~2x increase in the axial indentation modulus for 30 vol% BMI and epoxy PNCs compared to neat resin, with no change in transverse modulus, showing enhanced anisotropic mechanical properties with high A-CNT vol%. A/C-NCs are then fabricated by pyrolyzing phenolic PNCs with up to 4 cycles of polymer infiltration and pyrolysis (PIP), where 4 PIP cycles decrease porosity by ~10% and increase bulk density by ~5% compared to 1 PIP cycle. From PyC to 30 vol% A/C-NCs, the 4-PIP-cycle bulk density decreases from ~1.14 g/cm³ to ~0.80 g/cm³, and porosity increases from ~47% to ~74%. The average graphitic crystallite size increases from ~2.9 nm in PyC up to ~7.54 nm in 30 vol% A/C-NCs. Finally, Vickers microhardness testing in the axial CNT direction shows agreement with prior mechanical modeling and data at low vol% CNTs, where specific hardness increases from ~3.3 GPa/(g/cm³) for PyC to ~6.7 GPa/(g/cm³) for 30 vol% A/C-NCs, demonstrating that A/C-NCs are an advantaged superhard lightweight material. Using the process-structure-property relations developed in this thesis, more precise tailoring of application-specific performance for aligned CNT architectures is enabled, new understanding of A-CNT systems is established, and future paths of study that enable the design and manufacture of next-generation materials are recommended.

Thesis Supervisors

Brian L. Wardle, Professor, Aeronautics and Astronautics, Massachusetts Institute of Technology

Thesis Committee

Lorna J. Gibson, Matoula S. Salapatas Professor, Materials Science and Engineering, Massachusetts Institute of Technology

Carl V. Thompson, Stavros Salapatas Professor, Materials Science and Engineering, Massachusetts Institute of Technology