- Post Doctoral
MIT Unit Affiliation:
- Aeronautics and Astronautics
- Mechanical Engineering
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
I am currently a postdoctoral associate at the Department of Aeronautics and Astronautics at Massachusetts Institute of Technology. I was a National Defense Science & Engineering Graduate (NDSEG) Fellow in the Department of Mechanical Engineering at Massachusetts Institute of Technology, and received my Ph.D. degree in 2016. My dissertation focused on carbon nanotube (CNT) arrays and investigated CNT array structure and morphology, impact of CNT-CNT proximity effects, and manufacturing and property prediction for aligned CNT architectures, and concluded that the behavior of CNT arrays is governed by their 3D morphology and confinement effects. My current work focuses on advanced material design and manufacturing guided by nano-to-macro modeling and simulation of the structure and resulting physical properties of architectures comprised of 1D and 2D nanomaterials. My research interests include nanoscale energy transport, modeling and simulation of nanostructured materials, and low-dimensional carbon based materials and their physical properties.
Expected End Date of Post Doctoral Position:
The intrinsic and scale-dependent properties of nanofibers (NFs), nanowires, and nanotubes have made them the focus of many application-specific nanostructured materials studies. However, various NF morphology and proximity effects can lead to > 1000x reductions in the performance of NF-based material architectures, such as bulk materials and structures comprised of scalable aligned NF arrays. The physical and chemical origins of these effects, along with the concomitant structure-property mechanisms of materials comprised of aligned NFs, are not currently known and cannot be properly integrated into existing theories. This originates, in part, from an incomplete understanding of the morphology of real NF systems, particularly in three-dimensions.
Through experiments, theory, and multi-scale simulation, this dissertation presents a framework capable of modeling the stochastic 3D morphology of a relevant NF system, carbon nanotubes (CNTs), assembled into aligned CNT (A-CNT) arrays. New descriptions of the multi-wall A-CNT morphology demonstrate that the CNT tortuosity, quantified via sinusoidal amplitude-wavelength waviness ratio (w), decreases significantly from w ~ 0.2 to 0.1 as the CNT volume fraction (Vf) is increased from Vf ~ 1 to 20%. Using these new relations, a 3D stochastic morphology description is presented, and used to quantify the mechanical behavior of A-CNT arrays, A-CNT polymer matrix nanocomposites (A-PNCs), and A-CNT carbon matrix nanocomposites (A-CMNCs) via a mechanics analysis that was previously applied to carbon nanocoils. Focusing on deformations in the A-CNT axial reinforcement direction, torsion and shear deformation mechanisms, which are governed by the low (< 1 GPa) intrinsic shear modulus of the CNTs, are shown to have an effective compliance contribution of > 90% in the experimental A-CNT w regime, and are inferred to be the physical mechanisms responsible for the previously observed ~ 100x increase in the A-CNT effective indentation modulus as Vf is increased from ~ 1 to 20%. In the case of A-PNCs, the polymer matrix effectively eliminates the torsion compliance contribution, so that the observed ~ 2x enhancement in the effective axial elastic modulus of A-PNCs as Vf is increased from ~ 1 to 20% is explained. The geometry of the graphitic crystallites that comprise the pyrolytic carbon (PyC) matrix of A-CMNCs is found to not evolve significantly at pyrolysis temperatures of 1000 to 1400C, and crystallite size estimates from Raman spectroscopy reveal that the Tuinstra-Koenig correlation disagrees with the sizes measured by x-ray diffraction, suggesting a new amorphization transition crystallite size of 6 nm instead of 2 - 3 nm. In the case of A-CMNCs, CNT reinforcement is shown to lower the energy barrier (inferred through the pyrolysis temperature) for meso-scale self-organization of the graphitic crystallites of the PyC matrix, while having no effect on the PyC matrix on the atomic scale. Mechanical property analysis and modeling indicates that the aerospace materials selection criterion of the A-CMNCs can be enhanced to > 8 GPa x (g/cm^3)^-2 at Vf > 20% (experimentally we observe a value of ~ 5 GPa x (g/cm^3)^-2 at Vf ~ 10%). A-CMNCs introduced in this work have the potential to outperform state-of-the-art superhard materials, such as diamond (~ 7.8 GPa x (g/cm^3)^-2) and cubic boron nitride (~ 5.2 GPa x (g/cm^3)^-2).
Using the structure-property prediction tools developed in this thesis, precise tailoring and prediction of application-specific performance of aligned NF based architectures is enabled, and specific new understanding of A-CNT systems is established. Future paths of study that enable the design and manufacture of several classes of next-generation materials are recommended.
5 Recent Papers:
C. A. Amadei*, I. Y. Stein*, G. J. Silverberg, B. L. Wardle, and C. D. Vecitis. Fabrication and morphology tuning of graphene oxide nanoscrolls. Nanoscale 8, 6783 (2016). http://dx.doi.org/10.1039/C5NR07983G
I. Y. Stein, and B. L. Wardle. Mechanics of aligned carbon nanotube polymer matrix nanocomposites simulated via stochastic three-dimensional morphology. Nanotechnology 27, 035701 (2016). http://dx.doi.org/10.1088/0957-4484/27/3/035701
I. Y. Stein, D. J. Lewis, and B. L. Wardle. Aligned carbon nanotube array stiffness from stochastic three-dimensional morphology. Nanoscale 7, 19426 (2015). http://dx.doi.org/10.1039/C5NR06436H
J. Lee*, I. Y. Stein*, S. S. Kessler, and B. L. Wardle. Aligned Carbon Nanotube Film Enables Thermally Induced State Transformations in Layered Polymeric Materials. ACS Applied Materials & Interfaces 7, 8900 (2015). http://dx.doi.org/10.1021/acsami.5b01544
I. Y. Stein, N. Lachman, M. E. Devoe, and B. L. Wardle. Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays. ACS Nano 8, 4591 (2014). http://dx.doi.org/10.1021/nn5002408