High-efficiency mechanical energy storage and retrieval using interfaces in nanowires.

By molecular dynamics simulations, we demonstrate a new concept for mechanical energy storage and retrieval using surface energy as reservoir in body-centered cubic (bcc) tungsten nanowire, achieving a combination of unique features such as large and constant actuation stress (>3 GPa), exceptionally large actuation strain (>30%) and energy density, and >98% energy storage efficiency. The underlying mechanism is a shear-dominant diffusionless transformation akin to martensitic transformation, but driven by surface rather than bulk free energies, and enabled by motion of coherent twin boundary, whose migration has been shown to possess ultralow friction in bcc metals. Aside from energy storage, such surface-energy driven displacive transformations are important for phase transformation and energy-matter control at the nanoscale.