  Based on the first principles calculation combined with quasi-harmonic
approximation, in this work we focus on the analysis of temperature dependent
lattice geometries, thermal expansion coefficients, elastic constants and
ultimate strength of graphene and graphyne. For the linear thermal expansion
coefficient, both graphene and graphyne show a negative region in the low
temperature regime. This coefficient increases up to be positive at high
temperatures. Graphene has superior mechanical properties, with Young modulus
E11=371.0 N/m, E22=378.2 N/m and ultimate tensile strength of 119.2 GPa at room
temperature. Based on our analysis, it is found that graphene's mechanical
properties have strong resistance against temperature increase up to 1200 K.
Graphyne also shows good mechanical properties, with Young modulus E11=224.7
N/m, E22=223.9 N/m and ultimate tensile strength of 81.2 GPa at room
temperature, but graphyne's mechanical properties have a weaker resistance with
respect to the increase of temperature than that of graphene.
