Corrugated Two-dimensional Material Enabled Flexoelectricity for Cryogenic Actuator Technology

The objective of this project is to advance NASA’s research portfolio of cryogenic actuator technology (CAT) by creating a novel, self-lubricating actuator based on the converse flexoelectric behavior in the well-known transition metal dichalcogenide (TMD), molybdenum disulphide (MoS2). As a ubiquitous solid lubricant that is familiar to the aerospace community for its dry and vacuum-compatible rheological properties, typically in the form of sputtered coatings, the principal investigator (PI) investigated the optimal design and configuration of MoS2 thin films as the active material in a new class of space-compatible actuators, which the PI terms “flexoelectric cryogenic actuator technology” or FCAT. Specifically, the PI is leveraging the thus-far underappreciated phenomena of the converse flexoelectric (as opposed to piezoelectric) effect to enable self-lubricating, high stroke actuation capability under cryogenic and vacuum conditions. As a result of MoS2’s unique extreme environment tribological properties as well as highly deformable nature from its atomically-thin layered structure, this novel actuator device will not only enable longer service life, but also deliver significantly higher performance at low operating temperatures under vacuum. To develop novel actuator technology based on MoS2, the PI has investigated the converse flexoelectricity induced out-of-plane electromechanical response of MoS2 thin film. To better understand the underlying mechanism, the PI also included tungsten diselenide (WSe2) thin film, another member of the TMD group with similar atomic structure and properties of MoS2. The investigation yielded strongest electromechanical response from TMD monolayer till to date. Furthermore, the PI conducted an in-depth investigation to study the underlying factors to dictate the electromechanical response of TMD at atomically thin limit. The result established converse flexoelectricity as a lucrative phenomenon at nanoscale which can even surpass the conventional piezoresponse of MoS2 or WSe2. Based on the fundamental studies, the PI developed converse flexoelectric actuators with MoS2 thin film. The actuator exhibited significant out-of-plane displacement (approximately 67 times larger actuation than the thickness of MoS2) at the device’s resonance frequency. This actuation is superior by at least 10 times to previously reported nanoscale flexo-actuators and is comparable to state-of-the-art piezo-actuators. Moreover, the developed device demonstrated stable operation under extreme environments such as 10 K cryogenic and 1010 repetitive cyclic conditions, out-performing conventional piezo-actuators at the same conditions. This verified the potential of 2D converse flexoelectric actuators not only for aerospace applications, but also for wide range of commercial applications which have been dominated by piezo-actuators.