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THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Monday, February 25, 2019
12:00 PM
in Petit 102B
will be held the
DISSERTATION PROPOSAL DEFENSE
for
Katherine “Katie” Young
"The Impact of Synthesis Conditions and Crystal Quality of Two-Dimensional Materials for Permeation Barriers"
Committee Members:
Prof. Eric Vogel, Advisor, MSE
Prof. Preet Singh, MSE
Prof. Mark Losego, MSE
Dale Hitchcock, Ph.D., Savannah River National Lab
Abstract:
Permeation barriers, for applications such as gas separation and corrosion inhibition, would ideally combine properties, such as high strength and chemical stability, with low permeation and thickness. Two-dimensional materials possess many of these qualities and could serve as the thinnest possible permeation barriers. Two-dimensional materials are defined by an extremely small thickness, with much larger lateral dimensions. For example, graphene is an atomically thin, two-dimensional allotrope of graphite. Pristine graphene is extremely strong for its size, chemically inert in many environments, and impermeable to all gases. For instance, it has already shown excellent corrosion resistance for short-term experiments, but there have been contradictory reports in the literature regarding the impact of defects and the associated permeation mechanisms. Likewise, its use as a gas separation barrier hinges on permeation through induced pores, but permeation through intrinsic defects is also possible and less understood.
Hexagonal boron nitride (hBN) is another two-dimensional material that may be even more advantageous than graphene because it not only has low permeability and high strength, but is even more chemically stable at high temperatures, as well as electrically insulating. MXenes are another two-dimensional material that was recently discovered and may also have relevant properties. MXenes are derived from MAX bulk phases, where the “M” is an early transition metal in an hcp lattice, the “X” is C or N located at octahedral interstitials, and the “A” is a group 13-16 element that holds the M-X layers together. Once the “A” phase is selectively etched, the MXene remains. MXenes have also shown promising results for gas selectivity for gas separation. However, the effects of intrinsic defects and crystal quality on permeation in hBN and MXenes are even less understood than graphene because they are newer.
The crystal quality of two-dimensional materials is heavily dependent on the detailed conditions used for their synthesis. Chemical vapor deposition (CVD) is a promising synthesis technique for large-area, uniform coverage of 2D materials; however, it can lead to the formation of unwanted defects, such as grain boundaries and increased permeability. The development of a detailed and fundamental understanding of the impact of synthesis conditions on defects in the materials and the associated permeation mechanisms is lacking.
This work specifically proposes to study the effects of synthesis parameters and crystal quality on the permeation mechanisms of graphene, hBN, and MXenes. Though graphene has already been studied extensively, a complete analysis on the effects of grain boundaries, point defects, and the graphene-metal interaction is still needed to completely understand the permeation mechanism. There have been significantly fewer studies of the permeation properties of hBN and MXenes. All three materials will be synthesized via a bottom-up chemical vapor deposition method to form large-area, uniform thin films. While synthesis of graphene and hBN is relatively mature, there have been very few reports of bottom-up synthesis of MXenes. Therefore, an additional outcome of this work will be the developments of new processes for MXene synthesis. The synthesis parameters, as well as pre- and post- synthesis treatments, will be used to develop 2D materials with varying levels of defects (point and grain boundaries). The structure and defect density of the resulting materials will be characterized using a variety of techniques, including: Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Permeation and corrosion experiments will be performed, and the results correlated to the structure of the material. Finally, after studying individual materials as barriers, heterostructures will be synthesized and studied to understand the combined effects of materials. The development of this fundamental understanding will lead to engineering superior materials for permeation and corrosion applications.
