The atomistic study on the thermal expansion behaviors of nanowires

The thermal expansion coefficient of copper nanowire and bulk was investigated using molecular dynamics simulation in the temperature range up to 900 K. Both the size and crystal orientation effects on the thermal expansion coefficients of nanowires were studied. It is noticed that the linear thermal expansion is isotropic for single-crystalline bulk copper and the linear thermal expansion coefficient is exactly one third of the volumetric thermal expansion coefficient irrespective to the crystal orientations. The axial linear thermal expansion coefficient of an infinite long nanowire would be affected by both the surface and axial crystal orientation. Different temperature dependencies of lattice parameter and thermal expansion coefficient were found for nanowires with different width, and there is a transition from positive thermal expansion coefficient at low temperature to negative one at high temperature for the thinnest nanowires along [1 0 0] direction. However, the negative linear thermal expansion is not observed for nanowires along [1 1 0] and [1 1 1] direction. From the examination of the atomic configuration and radial distribution function for the thinnest [1 0 0] nanowires at various temperatures, it is concluded that the negative linear thermal expansion behavior is not caused by melting or the phase transformation.