Performance limits of microactuation with vanadium dioxide as a solid engine.

Miniaturization of the steam engine to the microscale is hampered by severe technical challenges. Microscale mechanical motion is typically actuated with other mechanisms ranging from electrostatic interaction, thermal expansion, and piezoelectricity to more exotic types including shape memory, electrochemical reaction, and thermal responsivity of polymers. These mechanisms typically offer either large-amplitude or high-speed actuation, but not both. In this work we demonstrate the working principle of a microscale solid engine (μSE) based on the phase transition of VO2 at 68 °C with large transformation strain (up to 2%), analogous to the steam engine invoking large volume change in a liquid-vapor phase transition. Compared to polycrystal thin films, single-crystal VO2 nanobeam-based bimorphs deliver higher performance of actuation both with high amplitude (greater than bimorph length) and at high speed (greater than 4 kHz in air and greater than 60 Hz in water). The energy efficiency of the devices is calculated to be equivalent to thermoelectrics with figure of merit ZT = 2 at the working temperatures, and much higher than other bimorph actuators. The bimorph μSE can be easily scaled down to the nanoscale, and operates with high stability in near-room-temperature, ambient, or aqueous conditions. On the basis of the μSE, we demonstrate a macroscopic smart composite of VO2 bimorphs embedded in a polymer, producing high-amplitude actuation at the millimeter scale.