A study of stimuli responsive bilayered actuators based on transition metal oxides/hydroxides

Abstract

Compact material constructs that can respond to multiple stimuli with certain motions are essential for developing artificial muscles for insect-scale robots. This thesis reports the development of a novel type of high-performing transition metal oxides/hydroxides materials responsive to multiple stimuli including light, heat, humidity and electrical voltages. Compared to other actuating materials, this type of materials can actuate with high stress and responding rates, under very low stimuli requirements. Recently discovered light-induced bilayered actuators comprising a light-responsive actuating layer, namely cobalt-oxides/hydroxides (C-O-H) and nickel hydroxide/oxyhydroxide (N-H-O), supported by a passive layer are versatile in miniaturized robotics applications, owing to their simple, compact construction and wireless, self-contained mode of actuation. However, the chemo-mechanics and quantitative description of their actuation mechanisms are not sufficiently understood. Firstly, based on a recently developed chemo-mechanics model, a novel instability phenomenon leading to extraordinarily large magnitudes of the bending actuation of bilayered actuators is found and experimentally proven. At specific ratios of the elastic moduli and thicknesses of the active and passive layers, and activation volume of the actuation mechanism, the actuation of the active layer will be put into a positive feedback mode where the actuation-induced bending of the cantilever structure triggers a compressive stress in a surface region of the active layer which enhances further contractive actuation of the latter by means of light-induced water de-intercalation. The beneficial instability is observed and analyzed for abovementioned two active material systems that exhibit such a light-induced water de-intercalation mechanism. Experimental results agree well with predictions of the chemo-mechanics model, thus verifying its applicability to design high-performing actuation systems. On the other hand, electrochemical actuating materials that can generate mechanical motions in response to low voltage stimuli are useful as artificial muscles in micro- or insect-scale robots, but such materials tend to have small actuation strain and stress, slow actuation response rate and poor motion controllability. In this thesis, c-disordered δ-MnO2 is discovered to have outstanding actuation performance and maneuverability, due to a volume-changing pseudo-capacitive redox reaction in the neutral electrolyte of Na2SO4. An electrochemo-mechanical model well describes quantitatively the intrinsic actuation properties of δ-MnO2 and the bending motion of bilayered cantilever actuators comprising an active layer of δ-MnO2 supported by a Ni thin-film substrate. Under the potential drive between -0.2 and 0.8 V vs SCE in Na2SO4, δ-MnO2 exhibits an electrochemical driving force of (5.39 ± 0.40) × 10-23 J and activation volume of (6.26 ± 0.40) × 10-31 m3 for the actuation at 298 K, a maximum strain of 1.28% and actuation stress of 71.5 MPa, and maximum energy density of 2.76 MJ/m3, indicating its high potential to be utilized as strong artificial muscles in multi-functional miniaturized actuating devices.