High-temperature stability and phase transformations of titanium carbide (Ti3C2T x ) MXene

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are under increasing pressure to meet technological demands in high-temperature applications, as MXenes can be considered to be one of the few ultra-high temperature 2D materials. Although there are studies on the stability of their surface functionalities, there is currently a gap in the fundamental understanding of their phase stability and transformation of MXenes' metal carbide core at high temperatures (>700 degrees C) in an inert environment. In this study, we conduct systematic annealing of Ti3C2T x MXene films in which we present the 2D MXene flake phase transformation to ordered vacancy superstructure of a bulk three-dimensional (3D) Ti2C and TiC y crystals at 700 degrees C <= T <= 1000 degrees C with subsequent transformation to disordered carbon vacancy cubic TiC y at higher temperatures (T > 1000 degrees C). We annealed Ti3C2T x MXene films made from the delaminated MXene single-flakes as well as the multi-layer MXene clay in a controlled environment through the use of in situ hot stage x-ray diffraction (XRD) paired with a 2D detector (XRD2) up to 1000 degrees C and ex situ annealing in a tube furnace and spark plasma sintering up to 1500 degrees C. Our XRD2 analysis paired with cross-sectional scanning electron microscope imaging indicated the resulting nano-sized lamellar and micron-sized cubic grain morphology of the 3D crystals depend on the starting Ti3C2T x form. While annealing the multi-layer clay Ti3C2T (x) MXene creates TiC y grains with cubic and irregular morphology, the grains of 3D Ti2C and TiC y formed by annealing Ti3C2T x MXene single-flake films keep MXenes' lamellar morphology. The ultrathin lamellar nature of the 3D grains formed at temperatures >1000 degrees C can pave way for applications of MXenes as a stable carbide material 2D additive for high-temperature applications.

Automated summary

The study presents the phase transformation of Ti3C2Tx MXene films from 2D MXene flakes to ordered vacancy superstructure of 3D Ti2C and TiCy crystals at temperatures between 700°C and 1000°C, followed by transformation to disordered carbon vacancy cubic TiCy at higher temperatures. The resulting nano-sized lamellar and micron-sized cubic grain morphology of the 3D crystals depends on the starting Ti3C2Tx form, demonstrating potential applications of MXenes as stable carbide material additives for high-temperature applications.