Left shows a sectioned view of the actuator, while the right shows the in-progress SEM image of gold (birght color) electrodeposited onto AAO template (dark)
Schematic of the actuator's surface microstructure
Gold thin-film with nanotextured surface
One of the bottlenecks of nanoporous gold (NPG) actuators is the restriction to work in wet environments. It relies on electrolyte for ion transportation for charges to accumulate at the metal-electrolyte surface to induce changes in surface stress. Moreover, this actuation mechanism is only significant in high surface-to-volume ratio structures. Nanoporous metals often suffer from strain degradation as a result of coarsening of its nanoligaments and reduced surface areas for actuation.
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In the present work, we developed a thin-film gold actuator with distinct microstructure and actuation mechanism from NPG. These features enable the actuator to actuate in ambient air, be free from ligament-coarsening effect, and generate strain and strain rate that are comparable with existing NPGs.
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Microstructure: The actuator is a 7.7mm x 1.9 mm rectangular gold thin film (~ 0.1 um) with high density of short gold nanofibres collapsed on one side, and backed by a solid lacquer layer (~30 um) on the other side. The gold nanofibres were synthesized from inverse templating from a 3D anodic aluminum oxide (AAO) nanohoneycomb template with high pore density (> E10 pores per centimeter square). After etching away the AAO template, the gold nanofibres collapse onto the supporting gold thin film. The pore diameter of AAO template was approximately 50 nm.
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Actuation: The actuation is electrolyte-free. We eject negative ions onto the nanofibre-side of the actuator (see below figure). As negative ion accumulate at the surface of the nanofibres, they repel each other and causes the gold layer to expand. Since clean metal surface are set into compression state under equilibrium, the actuation in the video on the left begins with a fully compressed (curled) state of the gold film. As the actuation is triggered, the thin film expands.
Upon charging, the end-tip velocity was initially fast (~0.8mm/s), but the velocity dropped quickly with time. The maximum end deflection of the 7.7mm long Au film was 1.6mm achieved in 10 minutes.
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Performance: The maximum strain is approximately 0.08%, corresponding to a strain rate of 0.013%/s. This performance is comparable to existing NPG cantilevers working in electrolytes, but is slightly weaker than one nanoporous gold/polyaniline actuator which can actuate in air by Detsi et.al [2] with strain and strain rate at 0.15% and 0.05%/s respectively. The actuation stress of our actuator estimated from Stoney forumla is approximately 20 MPa, which is comparable to ionicpolymer-metal-composite actuators (~16 MPa) and is much higher than conductive polymers (~5 MPa).
The mechanical work of the actuator is proportional to both actuation stress and strain, and it is equivalently important for actuators to achieve good performance in both aspects. However, there is often a trade-off between the two parameters. The strain of the actuator can be further enhanced by increasing the pore density of AAO template, thereby increasing the density of gold nanofibres at the surface. Alternatively, the thickness of lacquer layer can be reduced to impose less stiffness resistance to actuation. However, an increase in strain achieved under this setting is accompanied by lower actuation stress.
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References:
[1] Kwan, KW, Gao, P, Martin, CR, & Ngan, AHW. 2015. Electrical bending actuation of gold-films with nanotextured surfaces, App. Phys. Lett., 106(2): 023701:1-5
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[2] Detsi E, Tolbert SH, Punzhin S, Jeff De Hosson TM. 2016. Metallic muscles and beyond: nanofoams at work. J. Mater. Sci., 51:615 - 634.
Actuation of gold thin-film covered with gold nanofibres.
