Ashley L. Kaiser: Research

Research Focus

In my work, I aim to create experimentally-validated analytical models to enable the predictive design and advanced manufacturing of high-performance materials, which have broad applications in the electronics, healthcare & life sciences, automotive, aviation & aerospace, and energy sectors. My research, which has a strong emphasis on process development and property prediction/tuning, investigates the synthesis and process-structure-property characterization of nanomaterials with a central focus on nanotubes/nanofibers, graphene, polymers, and ceramics, which can be combined to form a variety of material systems with enhanced performance and manufacturability. In pursuit of creating materials by-design, my work employs nanomaterial processing and post-processing techniques, patterning and self-assembly, composite fabrication and testing, multi-scale materials characterization to quantify structural, thermal, mechanical, and electrical properties, and interfacial science and engineering to design high-strength, lightweight, and high-temperature materials, including ultradense nanofiber thin films, carbon nanotube (CNT)-reinforced polymer and ceramic matrix nanocomposites, and 2D nanomaterials for innovative and sustainable technologies.

Substrate adhesion evolves non-monotonically with processing time in millimeter-scale aligned carbon nanotube arrays

The advantaged intrinsic and scale-dependent properties of aligned nanofibers (NFs) and their assembly into 3D architectures motivates their use as dry adhesives and shape-engineerable materials. While controlling NF-substrate adhesion is critical for scaled manufacturing and application-specific performance, current understanding of how this property evolves with processing conditions is limited. In this report, we introduce substrate adhesion predictive capabilities by using an exemplary array of NFs, aligned carbon nanotubes (CNTs), studied as a function of their processing. Substrate adhesion is found to scale non-monotonically with process time in a hydrocarbon environment and is investigated via the tensile pull-off of mm-scale CNT arrays from their growth substrate. CNT synthesis follows two regimes: Mode I ('Growth') and Mode II ('Post-Growth'), separated by growth termination. Within 10 minutes of post-growth, experiments and modeling indicate an order-of-magnitude increase in CNT array-substrate adhesion strength (~40 to 285 kPa) and effective elastic array modulus (~6 to 47 MPa), and a two-orders-of-magnitude increase in single CNT-substrate adhesion force (~0.190 to 12.3 nN) and work of adhesion (~0.07 to 1.5 J/m2), where the iron catalyst is found to remain on the substrate. Growth number decay in Mode I and carbon accumulation in Mode II contribute to the mechanical response, which may imply a change in deformation mechanism. Predictive capabilities of the model are assessed for previously studied NF arrays, suggesting that the current framework can enable the future design and manufacture of high-value NF array applications.

Nanoscale TOC 2021
A. L. Kaiser, et al., Nanoscale (2021).

Morphology control of aligned carbon nanotube pins formed via patterned capillary densification

While a variety of self-assembled carbon nanotube (CNT) structures, such as cell networks, micropillars, and pins have previously been fabricated via the capillary-mediated densification of patterned CNT arrays, predicting the critical pattern size (scr) that separates cell versus pin formation and the corresponding process-morphology scaling relations within the micrometer range are outstanding. Here, facile and scalable mechanical patterning and capillary densification techniques are used to establish scr by elucidating how the effective elastic modulus of aligned CNT arrays during densification governs the resulting pin geometries. Experiments and modeling show that this effective modulus scales with CNT height and is about an order of magnitude smaller for pins as compared to cell networks formed from bulk-scale (i.e. non-patterned) CNT arrays. Patterning therefore results in pins with a lower packing density (commensurate with double the wall thickness) and a larger characteristic length scale than bulk cell networks (i.e. scr ~ 5x cell width). CNT arrays with the randomly-oriented carbon crust removed via oxygen plasma etching yield a higher degree of structural uniformity and better agreement with the proposed elasto-capillary model, which enables the use of capillary densification to predictively design hierarchical and shape-tunable materials for advanced thermal, electronic, and biomedical devices.

Nano Futures TOC
A. L. Kaiser, et al., Nano Futures (2019).

Process-morphology scaling relations quantify self-organization in capillary densified nanofiber arrays

Capillary-mediated densification is an inexpensive and versatile approach to tune the application-specific properties and packing morphology of bulk nanofiber (NF) arrays, such as aligned carbon nanotubes. While NF length governs elasto-capillary self-assembly, the geometry of cellular patterns formed by capillary densified NFs cannot be precisely predicted by existing theories. This originates from the recently quantified orders of magnitude lower than expected NF array effective axial elastic modulus (E), and here we show via parametric experimentation and modeling that E determines the width, area, and wall thickness of the resulting cellular pattern. Both experiments and models show that further tuning of the cellular pattern is possible by altering the NF-substrate adhesion strength for aligned NFs (here carbon nanotubes) grown via chemical vapor deposition, which could enable the broad use of this facile approach to predictably pattern NF arrays for high value applications.

Physical Chemistry Chemical Physics TOC
A. L. Kaiser, et al., Phys. Chem. Chem. Phys. (2018).

Mesoscale evolution of non-graphitizing pyrolytic carbon in aligned carbon nanotube carbon matrix nanocomposites

Polymer-derived pyrolytic carbons (PyCs) are highly desirable building blocks for high-strength low-density ceramic meta-materials, and reinforcement with nanofibers is of interest to address brittleness and tailor multi-functional properties. The properties of carbon nanotubes (CNTs) make them leading candidates for nanocomposite reinforcement, but how CNT confinement influences the structural evolution of the PyC matrix is unknown. Here, the influence of aligned CNT proximity interactions on nano- and mesoscale structural evolution of phenol-formaldehyde-derived PyCs is established as a function of pyrolysis temperature (Tp) using X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. Aligned CNT PyC matrix nanocomposites are found to evolve faster at the mesoscale by plateauing in crystallite size at Tp ~800 °C, which is more than 200 °C below that of unconfined PyCs. Since the aligned CNTs used here exhibit ~80 nm average separations and ~8 nm diameters, confinement effects are surprisingly not found to influence PyC structure on the atomic-scale at Tp ≤ 1400 °C. Since CNT confinement could lead to anisotropic crystallite growth in PyCs synthesized below ~1000 °C, and recent modeling indicates that more slender crystallites increase PyC hardness, these results inform fabrication of PyC-based meta-materials with unrivaled specific mechanical properties.

J. Mater. Sci TOC
I. Y. Stein, A. L. Kaiser, et al., J. Mater. Sci. (2017).

Related Press Releases:

MIT News: Stronger than Steel: MIT Homepage Spotlight on June 13. (2021) MIT News - Stronger than Steel MIT News: Scene at MIT: Happy Nanoween. MIT Homepage Spotlight on October 31. (2018) MIT News - Carbon Nanotube Halloween MIT News: Getting to the heart of carbon nanotube clusters. (2018) MIT News - Carbon Nanotube Cells MIT News: Glassy carbon, now with less heat. (2017) MIT News - Glassy Carbon

Relevant Publications:

• L. H. Acauan, A. L. Kaiser, and B. L. Wardle, Carbon 177, 1 (2021).

• A. L. Kaiser, D. L. Lidston, S. C. Peterson, L. H. Acauan, S. A. Steiner III, R. Guzman de Villoria, A. R. Vanderhout, I. Y. Stein, and B. L. Wardle, Nanoscale 13, 261 (2021).

• A. L. Kaiser, I. Y. Stein, K. Cui, and B. L. Wardle, Nano Futures 3, 011003 (2019).

• A. L. Kaiser, I. Y. Stein, K. Cui, and B. L. Wardle, Physical Chemistry Chemical Physics 20, 3876 (2018).

• I. Y. Stein, A. L. Kaiser, A. J. Constable, L. Acauan, and B. L. Wardle, Journal of Materials Science 52, 13799 (2017).

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