Large, visible macroscopic strain is first reported for nanoporous oxide actuators.
Nanowire-forest Ni actuator with nanoporous side facing up
Synthesized AAO template appearning in half-transparent.
Nanowire forest Ni bi-layer actuator
Motivation: Some recurring issues preventing nanoporous metal actuators from becoming commercialized artificial muscles lies in its slow actuation rate and high fabrication cost. The high cost is associated with the use of noble, expensive precursor alloys to synthesize the actuators using a material-removal process known as de-alloying. The slow actuation rate, often requiring hundreds of seconds of reaction time, can be attributed to the fact that randomly porous structure is unfavorable for ions migrating in and out of the nanostructure, delaying the time for charges to accumulate at metal/electrolyte surface to induce actuation.
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Synthesis: Here we present a bilayer cantilever Ni actuator comprised of nanowire-forest layer backed by a thin nickel substrate. The 3D nanowire structure consisted of primary nanowires (~230 nm in dia.) bundled together and evenly separated by smaller subnanowires (107 nm) to form a dual-level nanowire forest. The nanowire forest structure is synthesized from Ni electrodeposition onto an anodized aluminum oxide (AAO) template. In contrast to the high purity Al we used in the past for anodization to create 3D templates, we added copper impurities into the Al foil. Oxygen bubbles form at the sites of Cu impurities during anodization, leaving cavities in the oxide and connecting up neighboring pore channels.
A microstructure favorable for actuation: The subnanowires has several benefits with respect to actuation.
(i) Firstly, it acts as spacers between the wires, preventing them from collapsing into random stacks and reducing the surface-to-volume ratio, and stabilizing the open channels for ions to transport deep into the nanowire stacks.
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(ii) Secondly, the subnanowires carry the in-plane stress and strengthen the material in the bending direction. It also connect up the strain arising from individual primary nanowires, allowing local strain to be coupled into macroscopic bending in the material.
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Performance:
(i) Macropscopic strain visible to naked eyes: The 18.5 mm long actuator demonstrated large end-deflection as 3.8 mm within a narrow potential of 0.6 V. This actuation is more significant than the nanohoneycomb Ni that we have developed (end-deflection of ~ 60µm). This is first time macroscopic displacement visible by naked eyes is reported for metal-based actuators other than de-alloyed nanoporous Au/Pt.
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(ii) Fast actuation rates: Although the strain in the actuator did not exceed that of the state-of-the-art nanoporous Pt actuator, it promotes much faster response rate. At lowest potential scan rate (5 mV/s), the nanowire-forest Ni achieved a strain of 0.04% in ~120 sec. The strain is 8 times smaller than the nanoporous Au-Pt actuator (0.35%) by Jin et.al [2] but the strain rate (0.091%/s) is 13 times faster (Au-Pt actuator: (2.2x 10^(-4)%/s). At extremes cases, we achieved 0.011% strain at the highest scan rate of 5000 mV/s. It only took 0.12 sec to reach maximum deflection, and this response rate is 13300 times faster than the nanoporous Au-Pt in Jin et.al [2].
The nanowire-forest Ni actuator is an excellent example that nanoporous actuators can exhibit high actuation rates (maximum~ 3 Hz) and are not necessarily limited by the low ion transport speed by having a highly ordered nanoporous structure.
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The following videos show the actuation at two different actuation rates:
(0.042 Hz left, 2.5 Hz right)
Microstructure of the nanowire-forest
(iii) High mechanical work density comparable to human muscles: The relatively low strain is due to the smaller surface areas available for reaction due to larger ligament sizes (~230 nm for primary wires and ~ 107 nm for subwires) compared with nanoporous Au-Pt (~5nm). [2] However, the mechanical work capacity of the current actuator (~32 kJ/m^(3)) is comparable with human skeletal muscles (~40 kJ/m^(3)) [2] and falls in the range of most artificial muscles (16 - 1000 kJ/m^(3)).
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References: ​
[1] Cheng C, Weissmüller J, Ngan HW. 2016. Fast and Reversible Actuation of Metallic Muscles Composed of Nickel Nanowire-Forest. Advanced Materials, 28(26):5315 -5321
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[2] Jin HJ, Wang XL, Parida S, Wang K, Seo M, Weissmüller J. 2010. Nanoporous Au-Pt alloys as large strain electrochemical actuator. Nano Lett. , 10(1):187-194