Our lab is dedicated to the discovery of new actuation materials, actuation mechanisms, and synthesis methods. Our primary focus is on the development of nanoporous actuating materials. These actuators are normally based on precious metals (Au, Pt) synthesized by de-alloying method. The actuators have a nano-scale surface structure and thus a large surface area to store a large amount of electric charges. When an external voltage is applied to such actuator (immersed in an ionic electrolyte), charges would be attracted to the metal's surface which induce redistribution of charges within the metal's suface. The bonding strength between atoms in the first few atomic layers at the surface of the metal would be increased/decreased by the change in induced surface charges. This would then cause a change in surface stress in the metal, leading to macroscopic expansion or contraction of the actuator. The degree of deformation depends on multiple factors including the applied voltage, electrical capacitance of the metal, and the pore density (and thus flexibility). Nanoporous metals possess advantages such as compactnesss, low actuation voltage (several volts, compared to > 1kV for piezoceramics), fast response rate, and relatively high actuation stress (compared to compliant electroactive polymers).
However, one major limitations of nanoporous metals is that it has to be actuated in a wet environment, since the actuation relies on the ions to transport electric charges to the metal's surface. Therefore, many challenges still persist and needs to be overcome before nanoporous actuators can be considered for commercialized actuation applications.
Our research
Development of new actuating materials
Modelling and Simulation
Mechanisms:
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We discovered new actuation mechanisms in nanotextured gold films and nickel-based actuators involving electrostatic repulsion and reduction-oxidation reaction respectively. These mechanisms differ from the actuation mechanism of nanoporous Au/Pt actuators working in electrolyte, which is due to the change is surface stress caused by a difference in induced charges in the space-charge layer at the metal's clean surfaces.
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Synthesis method:
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The main limitations of de-alloying methods are the high material costs and the disordered nano-structure which hampers ion transport speed and actuation rates. We hence propose a "bottom-up" synthesis approach by electrodepositing metals on to pre-synthesized templates with the desired nano-feautres. The resulting surface of the metals would become the inversed images of the templates. This saves the cost for synthesizing de-alloyable ingots, and also enables a higher degree of control over the surface textures. We found that by engineering "ordered" surface textures onto nanoporous actuators, the ion transport speeds, actuation rates, and macroscopic strain were improved. The "templating" method can in principle be applied to any materials that can be electrodeposited onto the template, which makes it possible to synthesize actuators with much wider range of materials to enable further cost reduction.
The major challenges of nanoporous metal actuators mentioned in the contribution issue to 50th anniversary of the Journal of Material Science:
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i. Limited to wet environment
ii. Low actuation rate hampered by low ion conductivity of aqueous electrolyte
iii. Suffer from severe ligament coarsening during electrochemical processes
iv. Large working space required due to the need to operate in a three-component configuration, i.e. the electrolyte, working electrode, and a counter electrode
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In one of our recent publications (2017), we engineered a self-contained assembly with nickel hydroxide/oxyhydroxide actuator which can actuate in air at a high frequency up to 25 Hz. This reduces the volume occupied by the 3-electrode electrochemical cell, and also decreases the damping effect of actuating in liuqid (electrolytes). In two other publications (2015 and 2016), we developed nanoporous nickel actuators with different nanostructures. These actuators did not suffer from ligament coarsening effect as some nanoporous actuators do, due to a protection layer of nickel hydroxide (Ni(OH)2) formed at the surfaces. In addition, this layer of nickel oxides were found to contribute to extra actuation strain apart from the strain due to EDL mechanism, which is well-known in most nanoporous metals.
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Our recent publications are listed below:
Materials:
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We have identified actuation behaviour in non-precious metals and metallic oxides such as nickel, nickel hydroxides, and anodized aluminum oxide. These findings indicate the alternative elements that can be synthesized into nanoporous actuators which may reduces the fabrication costs of conventional de-alloying method.