  The conversion of other forms of energy into mechanical work through the
geometrical extension and retraction of nanomaterials has a wide variety of
potential applications, including for mimicking biomotors. Here, using
molecular dynamic simulations, we demonstrate that there exists an intrinsic
energy conversion mechanism between thermal energy and mechanical work in the
telescopic motions of double-walled carbon nanotubes (DWCNTs). A DWCNT can
inherently convert heat into mechanical work in its telescopic extension
process, while convert mechanical energy into heat in its telescopic retraction
process. These two processes are thermodynamically reversible. The underlying
mechanism for this reversibility is that the entropy changes with the
telescopic overlapping length of concentric individual tubes. We find also that
the entropy effect enlarges with the decreasing intertube space of DWCNTs. As a
result, the spontaneously telescopic motion of a condensed DWCNT can be
switched to extrusion by rising the system temperature above a critical value.
These findings are important for fundamentally understanding the mechanical
behavior of concentric nanotubes, and may have general implications in the
application of DWCNTs as linear motors in nanodevices.
