"Phase Evolution of MXenes: From room Temperature Stability to High-temperature Phase Transformations" 

Krutarth Kamath, MSE PhD Candidate 

Advisors: Professor Babak Anasori & Professor Lia Stanciu

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ABSTRACT

MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, has expanded from the single composition Ti3C2Tx in 2011 to ~50 compositions. Although nascent, MXene is taking center stage in 2D materials because of its exceptional properties owing to wide design and compositional tunability. Yet behind the spotlight lie numerous scientific challenges. One of the most pressing challenges is the chemical and temperature stability of MXenes. We will try to fundamentally understand the factors at play here, rendering MXenes thermodynamically metastable in nature. MXene flake-level defects, features, non-stoichiometry, and external stimuli contribute to susceptibility to attack by electron acceptors O2 and H2O, leading to deterioration in ambient aqueous and atmospheric media. Negatively charged MXenes, albeit forming stable colloidal solutions in water, are highly prone to degradation by hydrolysis. Oxygen presence, whether as surface terminations or substitutional atoms within the carbon sublattice, plays a crucial role in driving the structural deterioration of MXenes across all temperatures. Furthermore, vacancy clusters and edges exhibit pronounced reactivity, serving as catalytic sites for oxidation and hydrolysis reactions. We will also understand the three-stage transformation process observed by MXenes at elevated temperatures up to 1500 °C. The order-to-disorder phase transition of MXenes to cubic bulk carbides sets the stage for leveraging highly compositionally diverse MXenes as building blocks for creating processible ultra-high temperature ceramics.