The effects of morphology and temperature on the tensile characteristics of carbon nitride nanothreads †
Very recently synthesized carbon nitride nanothreads (CNNTs) by compressing crystalline pyridine show outperform diamond nanothreads in chemical and physical properties. Here, using first-principles-based ReaxFF molecular dynamics (MD) simulations, a comprehensive investigation on the mechanical characteristics of seven experimentally synthesized CNNTs has been performed. All CNNTs exhibit unique tensile properties that change with molecular morphology, atomic arrangement and the distribution of nitrogen in the skeleton. The CNNTs with more effective covalent bonds at cross-sections are more mechanically robust. Surprisingly, a tiny CNNT with periodic unit structures of 5462-cage shows extreme ductility because of the formation of a linear polymer via 4-step dissociation-and-reformation of bonds at extremely low temperatures in the range of 1–15 K; however, it shows brittle failure at one cross-section with low ductility at higher temperatures similar to other CNNTs at different temperatures; this offers a feasible way to design a kind of lightweight material that can be used in ultra-low temperature conditions, for example, the harsh deep space environment. The results also show that temperature significantly affects the fracture stress and rupture strain but not the effective stiffness. The analysis of atomic bond orders and bond lengthening reveals that the unique nonlinear elasticity of CNNTs is attributed to the occurrence of local bond transformations. This study provides physical insights into the tensile characteristics of CNNTs for the design and application of CNNT-based nanostructures as multifunctional materials.