Study On The Mechanical Properties In Diamond Nanothreads With Molecular Dynamics Simulations

Recently synthesized diamond nanothreads(DNTs),collecting desired properties of both inorganic nanostructures and hydrocarbon molecular structures,are an interesting group of carbon-based materials.Using full atomistic first-principles based Reax FF molecular dynamics(MD)simulations,a comprehensive study on tensile and bending mechanical characteristics of fifteen energy-favorable DNTs is performed.All the DNTs show unique tensile and bending mechanical properties that markedly vary with morphology and arrangement of carbon polygons.A straight DNT composed of purely carbon hexagons shows brittle fracture in the temperature range of 1-2000 K,whereas with regard to another hexagon-dominated DNT and helically coiled DNT with the largest coiled radius,a thermal-induced brittle-to-ductile transition is uncovered at 2000 K.Particularly,the coiled DNT subjected to tensile loading/unloading shows a clear mechanical hysteresis loop.There are obvious changes in the total bond order and bond length of local specific chemical bonds,which leads to the occurrence of mechanical hysteresis loop.In the tensile stressstrain response,the bending diamond nanothreads with dehydrogenation have strong tensile strength and tensile toughness.For the third type of diamond nanothreads with spiral structure,the tensile stiffness significantly lower than the structure 1 to 10,while,its tensile toughness is almost as they are.Bending MD simulations show a morphology-dominated bending stiffness.As a result of asymmetry in the surface,bending stiffness is also strongly bending direction sensitive.Dehydrogenation does not change the morphologies and stability of DNTs,but significantly affect the tensile mechanical responses;the tensile stiffness,toughness and ductility can be enhanced by approximately 1-fold,2-folds and 3-folds as much of their pristine counterparts,respectively,however,the failure strain is reduced at any degree of dehydrogenation.Similarly,bending stiffness also closely connects with dehydrogenation.A transition of bending stiffness in two specific dehydrogenationfree DNTs occurring at critical curvatures is detected as a consequence of local bond transformations.Moreover,bending stiffness in different bending directions can differ by around 8-folds,originating from the distinct surface morphologies.The findings provide a critical knowledge of mechanical properties of DNTs for practical application.The fundamental knowledge of mechanical characteristics is critically significant for design and application of DNTs for the next generation of micro-/nano-devices and multifunctional materials.