Temperature-dependent mechanical properties of Ti_n+1C_nO₂ (n = 1, 2) MXene monolayers: a first-principles study

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (named as MXenes) have become of the fastest growing family of 2D materials in terms of compositions and their applications in different areas. One of the least explored properties of MXenes is their mechanical properties. While the basic elastic properties of MXenes have been studied by first-principles, the effects of temperature on the elastic properties have never been explored. In this study, we investigate temperature-dependent structural and mechanical properties of the titanium-containing MXenes (Ti_n+1C_nO₂ (n = 1, 2)) based on the first-principles calculations combined with quasi-harmonic approximation. The effective Young's modulus of a single layer of Ti₂CO₂ and Ti₃C₂O₂ is calculated to be 565 and 482 GPa, respectively, at 0 K. By increasing temperature to 1000 K, Young's moduli of Ti₂CO₂ and Ti₃C₂O₂ decrease to 469 GPa and 442 GPa, respectively, which indicates a larger reduction in stiffness in thinner MXenes at higher temperatures. Our calculations of the temperature-dependent bond strengths within MXenes showed that titanium and carbon atoms in Ti₃C₂O₂ form stronger bonds than Ti₂CO₂ and atomic bonds in Ti₂CO₂ lose their stiffness more than Ti₃C₂O₂ with increasing temperatures. The Debye temperature of these monolayers is also calculated to provide a comparison of the thermal conductivity between these monolayers, in which the results show that the Ti₃C₂O₂ has a higher thermal conductivity than Ti₂CO₂. Our calculated electronic properties results of the monolayers are also shown that the electrical conductivity of the monolayers would not change with temperature. Our study extends MXenes applications to high-temperature applications, such as structural composite components and aerospace coatings.