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Mechanical Metamaterials: Design, Fabrication, and Performance--Christopher Spadaccini
Mechanical Metamaterials: Design, Fabrication, and Performance--Christopher Spadaccini
Pages 85-98
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From page 85... ...
The performance of these mechanical metamaterials is controlled by geometry at multiple length scales rather than by chemical composition alone. We have demonstrated designer properties of these mechanical metamaterials in polymers, metals, ceramics, and combinations thereof, yielding properties such as ultrastiff lightweight materials, negative stiffness, and negative thermal expansion.
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From page 86... ...
Mechanical metamaterials result from exactly the opposite approach. A classic example of architectural control and the resulting unique material performance is the octet truss stretch-dominated lattice (Deshpande et al.
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From page 87... ...
Consequently, the red angled component pulls inward the center of the flexure element, which makes up the sidewall of the unit cell, while simultaneously pushing the corners of the unit cell outward. When arranged in a lattice connected at the midpoint of each sidewall, the corners grow into the void space while the sidewalls are pulled inward, resulting in an overall contraction of the lattice and hence negative thermal expansion.
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From page 88... ...
A negative thermal expansion metamaterial again serves as an example. In a typical implementation, we begin with a unit cell with three phases randomly distributed throughout the space: a high thermal expansion constituent material, a relatively lower thermal expansion constituent material, and void space.
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From page 89... ...
There are significant limitations to TO methods, including a lack of knowledge about practical manufacturing constraints in the algorithm, a propensity to converge to a local minimum solution rather than a global one, and, for more sophisticated design problems, the need for expensive high-performance computing resources. FABRICATION Physical realization of mechanical metamaterials requires a suite of fabrication processes with unique capabilities.
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From page 90... ...
. We have also recently developed a scanning version of this concept, enabling us to rapidly fabricate structures approaching 10 cm in size while maintaining features as small as 10 microns.
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From page 91... ...
Our modifications include automated sample injection during deposition to tailor the material composition, the use of dynamic electrodes to controllably vary the electric field profile on the deposition plane and precisely pattern geometries, and in-depth process modeling to predict the deposition parameters required to achieve a specific packing structure. Postprocessing Techniques Postprocessing techniques can help expand the palette of usable materials and/or improve final component properties.
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From page 92... ...
A hollow tube ceramic lattice, shown in the third column, was formed via ALD and similar polymer removal. This structure represents the light est fabricated material in this test series, with a relative density of 0.025 percent and wall thickness less than 50 nm.
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From page 93... ...
Al2O3 = aluminum oxide; ALD = atomic layer deposition; Ni-P = nickel phosphorus. SOURCE: Zheng et al.
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From page 94... ...
. measured scaling relationship between these two parameters is approximately linear across all constituent material types, all relative density regimes, and regardless of hollow tube or solid strut configurations clearly demonstrates the impact of the stretch-dominated architecture.
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From page 95... ...
Supercapacitors with high compressibility and durability, for example, may become possible. FUTURE DIRECTIONS By combining mechanical metamaterials with inverse design methods and custom micro- and nanoadditive manufacturing techniques, we have been able
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From page 96... ...
. FIGURE 11 Image of printed graphene aerogel, positioned atop a quarter to show size.
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From page 97... ...
There are many potential future directions for advancing the state of the art, including through continued exploration of size-scale effects and efforts that push the boundaries of multimaterial design and fabrication. This could lead to unique new materials with groundbreaking mechanical properties and electrical or photonic functionality all in one.
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From page 98... ...
1997. Design of materials with extreme thermal expansion using a three-phase topology optimization method.
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