Two-dimensional Transition Metal Carbides as Precursor Materials for Applications in Ultra-high Temperature Ceramics

In this dissertation, we investigate the potential of two-dimensional (2D) transition metal carbides, known as MXenes, as precursor materials for the development of ultra-high temperature ceramics (UHTCs), with a focus on Ti₃C₂T_x MXene. MXenes are distinguished by their unique combination of 2D structure, high surface area, and chemically active basal planes, making them ideal candidates for a wide range of high-performance applications. This study focuses on the phase transformation, grain growth, surface texturing, and electrocatalytic behavior of Ti₃C₂T_x MXene films when subjected to high-temperature annealing, along with their role as sintering aids in UHTCs.

We present the transformation of 2D Ti₃C₂T_x flakes into ordered vacancy carbides of three-dimensional (3D) TiC_y phases at temperatures above 1000°C. Using X-ray diffraction and ex-situ annealing (up to 2000°C in a tube furnace and spark plasma sintering), we investigate the resulting nano-lamellar and micron-sized cubic grain morphologies. Single-flake Ti₃C₂T_x films retain a lamellar morphology after annealing, while multi-layer clay-like MXene transforms into irregular cubic grains.

In addition to investigating the structural evolution, we examine the influence of cationic intercalation on grain growth and texture. Specifically, Ca²⁺ ions lead to highly templated growth along the (111) crystal plane, significantly altering carbon diffusion and metal atom migration during annealing. We show that this preferential growth influences properties with hydrogen evolution reactions (HER) as an example functionality. We observe that with Ca²⁺-intercalated Ti₃C₂T_x films, exhibit an overpotential of 594 mV and a current density of -13 mA/cm² due to increased surface area and dominant texturing.

Additionally, we investigate the use of MXenes in self-assembly with ceramic materials such as ZrB₂, facilitated by optimizing zeta potentials. MXenes, with their functionalized hydrophilic surfaces and negative zeta potentials, serve as sintering aids and reinforcements in UHTC composites. The introduction of Ti₃C₂T_x to ZrB₂ enables improved sinterability, achieving 96% relative density compared to 89% for pure ZrB₂. Furthermore, the addition of MXenes leads to a core-shell microstructure with (Zr,Ti)B₂ solid-solution interfaces, enhanced mechanical properties such as a 36% increase in hardness, and reductions in oxygen content. These findings establish MXenes as promising materials for the development of advanced UHTCs, suitable for extreme environments.

Through a combination of experimental techniques, and theoretical estimations, and advanced characterizations, this dissertation provides critical insights into the role of MXenes in both phase transformation and mechanical reinforcement, thereby laying the foundation for future studies and opening new avenues for applications of MXene derived carbides and the design of high-performance UHTCs.