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  5. Ultralight Mechanical Metamaterials with Ordered Hierarchies

Ultralight Mechanical Metamaterials with Ordered Hierarchies

Jianchao Ye (15-LW-083)

Abstract

We have developed a new class of low-density materials with properties not found in nature (metamaterials) with an unprecedented range in size scale and with arbitrary hierarchical three-dimensional geometries. These materials are 107 times larger than their smallest structural feature at the nanoscale, with architectures nested throughout 7 levels of length scale, creating hierarchies from tens of centimeters down to nanometers. At the macroscale, these metamaterials simultaneously achieve ultrahigh tensile elasticity (>20%) not found in their relatively brittle metallic constituents, and a near-constant specific strength. Creation of these materials is enabled by a high-resolution, large-area additive manufacturing technique with scalability not achievable by two-photon polymerization or traditional stereolithography. With overall part sizes approaching tens of centimeters, these unique metamaterials can impact a broad array of applications ranging from energy absorbing materials to flexible batteries to armor and high-temperature materials design and fabrication. We have expanded our knowledge and capability in designing and creating ultralow density architected materials that are both strong and tough. The ability to capitalize upon the remarkable properties that manifest themselves at the microscale and nanoscale by creating billets of material at a substantial size will enable researchers across multiple scientific disciplines to take advantage of these properties.

Background and Research Objectives

There has been a lengthy research and engineering pursuit to create lightweight, mechanically robust, and efficient materials with interconnected porosity. These porous materials are desirable for a wide range of applications including thermal insulation, active cooling, catalyst supports, energy storage, infiltration, and biomaterials. However, these required outstanding properties have remained elusive on the bulk scale, constrained by the inherent coupling of material properties and the lack of suitable processes to generate the artificial materials. The critical challenge is the significant degradation of material property as the relative density is reduced. Our goal was to develop a new class of novel materials based on multiple hierarchies and unusual nanometer-scale size effects with vastly superior properties that is largely unclaimed at ultralow relative density. We intended to develop a new class of metamaterials (materials engineered to have properties not found in nature) with ordered hierarchies that offer extremely low mass densities (lighter than aerogels) with unprecedented properties. We investigated how fractal hierarchical orders and nanoscale size serve to change the scaling of material properties against density.

Scientific Approach and Accomplishments

We have demonstrated a new class of ultralow density hierarchical metamaterials that can be potentially used on a variety of constituents, including polymer, metal, and graphene. This work has been featured on the October 2017 cover of Nature Materials.1 With this project, we have addressed a major scientific and engineering challenge of architected materials. Architected metamaterials has been a rapidly growing field in the last few years with several major breakthroughs demonstrating remarkable properties derived from microscale and nanometer-scale three-dimensional architectures.2–6 However, the applicability of these types of metamaterials is significantly limited by their scalability to incorporate the lucrative properties from miniature architectures. For example, three-dimensional architected lattices comprised of a periodic array of hollow tubes with properties influenced by a nanoscale wall thickness have only been demonstrated on a maximum size scale of 200 μm using two-photon lithography,3,7–9 which significantly limits their applicability. These limitations make these remarkable properties inaccessible in real-world applications where scalability to relevant sizes and optimal properties are equally crucial.

We have developed scalable, metallic, mechanical metamaterials that simultaneously achieve high strength and ultralow density, as well as unprecedented high compressive and tensile ductility that reveal themselves at a length scale 107 times larger than their smallest nanoscale feature sizes within the structure. These scalable metamaterials contain hierarchical topologies whose feature size spans seven orders of magnitude in length scale from tens of nanometers to tens of centimeters. Creation of the metamaterial with this unprecedented scalability (Figure 1) is enabled by a new type of additive manufacturing technique that is capable of miniaturized architectures over large areas, combined with nanoscale post-processing. The resulting nickel–phosphorous metamaterials are stretchable and compressible, with ultrahigh tensile (~20%) and compressive ductile-like deformation (>50%) that is not observed in any reported lightweight metal foams or lattices. These hierarchical materials exhibit an optimized, near-constant specific strength in both compression and tension even at relative densities below 0.2%. The structural hierarchy connects macroscale architectures to nanoscale features through successive reduction of feature size by approximately a factor of ten between each hierarchy. With overall part sizes approaching tens of centimeters, these unique metamaterials will be accessible for a broad array of applications.


Figure 1. polymer metamaterial template (a,c). large-area, high-resolution additive manufacturing of hierarchical metamaterials (b). optical microscope images of bulk hierarchical lattice material with a network of hierarchical stretch-dominated octet unit cells (d–j).
Figure 1. Polymer metamaterial template (a,c). Large-area, high-resolution additive manufacturing of hierarchical metamaterials (b). Optical microscope images of bulk hierarchical lattice material with a network of hierarchical stretch-dominated octet unit cells (d–j).
 

The length-scale breakdown of our multi-scale metallic metamaterials with hierarchical architectures distributed over seven orders of magnitude is shown in Figure 1. On the macroscale (~5 cm), the bulk metallic metamaterial is comprised of a network of hierarchical stretch-dominated octahedron-tetrahedron (octet) unit cells that are designed to carry load via axial stress, shown in Figure1(d). Each stretch-dominated unit cell from the hierarchical lattice network contains approximately 200-mm hierarchical strut members that are comprised of a network of stretch-dominated unit cells organized in the (110) orientation, shown in Figure 1(h). At the lowest hierarchy, these materials are made out of nickel–phosphorous thin-walled hollow tubes with a thickness ranging from 50 to 700 nm. The hierarchical features within the material constitute three-dimensional nanostructured metamaterials with the highest scalability ever created.

Our approach to multi-scale metamaterial designs is based on assembling microscale filaments along a path defined by the architecture at a higher length scale. Such schemes allows for diverse, wide-ranging microscopic architectures with different deformation mechanisms at multiple length scales assembled into a larger-scale object with tunable properties. Combinations of stretch-bend architectures can be created by constructing one hierarchy with a stretch-dominated unit cell and another with a differing bend-dominated unit cell (Figure 2).


Figure 2. schematic illustration of the structural hierarchy of cubo-octet bend–stretch metamaterials with nanoscale nickel-phosphorous constituent material (a). each strut member of the octet unit cell is a thin-shelled hollow tube. scanning electron microscopy images of an as-fabricated bend–stretch-dominated cubic hierarchical metamaterial with hierarchical feature breakdown at each relevant length scale (b–d).
Figure 2. Schematic illustration of the structural hierarchy of cubo-octet bend–stretch metamaterials with nanoscale nickel-phosphorous constituent material (a). Each strut member of the octet unit cell is a thin-shelled hollow tube. Scanning electron microscopy images of an as-fabricated bend–stretch-dominated cubic hierarchical metamaterial with hierarchical feature breakdown at each relevant length scale (b–d).
 

The fabrication of the multi-scale metamaterial is enabled by a scalable, microscale additive manufacturing technique combined with post-processing methods. Large Area Projection Micro Stereolithography combines an addressable spatial light modulator with a coordinated optical scanning system to produce microscale architectures over a sustainably larger area. This enables microscale features fabricated within an area over five orders of magnitude larger. Using the base polymer hierarchical template of Figure 1(a), we coated the structures with a thin layer of nickel phosphorous to create features at the lowest hierarchy via electroless nickel plating. Once the nickel shell grew to a desired thickness, the polymer structure was chemically removed, leaving behind nanoscale wall thickness with feature thickness from 50 nm to 1 mm as the lowest hierarchy. With fractal hierarchical design, hierarchical nickel–phosophorous metamaterials with relative density of 0.1 to 0.012% were created.

In contrast to single-ordered lattice materials, we have found that hierarchical metamaterials at the same ultralow density have a vastly tunable strength–density scaling relationship subject to three-dimensional topologies at each length-scale level. This allows achieving the maximum strength of a metamaterial for a given density through optimizing the equivalent strength at each hierarchical level, which is characterized by the critical structural parameters from each hierarchy. That is, the nanoscale wall thickness t, hollow tube diameter d1, and the hierarchical strut parameters at the larger length scales and beyond—for example, diameter di and length li (i ≥ 2). Figure 3(a) shows the main results from our theoretical analysis of stretch-dominated multi-scale metamaterial. These structural parameters collectively determine the failure modes at different levels of hierarchies, which collectively determine the overall stability of the mechanical metamaterial. The maximum yield strength (indicated by the red line) occurs when the minimal macroscopic loads that trigger the yield stress, the local buckling at nanoscale wall thickness, and Euler buckling of the hierarchical filaments from each hierarchy approach the same value. Tunable deformation modes are illustrated in Figure 3(b–d), corresponding to the failure map in Figure 3(a). These optimized fractal-like, stretch-dominated metamaterials exhibit unprecedented scaling between yield strength and relative density with a scaling power of 1.3, shown in Figure 3(e). This far exceeds first-order octet metamaterials, which have a scaling power over 2.5 observed in nickel–phosophorous microlattices at relative densities below 0.2%.10 Consequently, the hierarchical lattice yield stress is two orders of magnitude higher than that of the ultralow density first-order periodic lattices. These high compressibility and diverse elastic deformation modes of the metamaterials are attributed to the ability to tune the failure modes within each level of hierarchy: local buckling of the walls, Euler buckling of the first-order tubes, yielding and fracturing of the first-order struts, and Euler buckling of the second-order lattice, thereby optimizing the global failure load of the multi-scale lattice.


Figure 3. tunable compressive response of fractal-like metamaterial. failure map of ultralight hierarchical octet lattice as a function of first- and second-order design parameters (a). the red curve shows optimal strength at a given relative density. uniaxial compression stress–strain curves of samples fabricated from points in (a) is shown in (b–d). an ashby chart comparing performance of our fractal lattice with state-of-the-art low-density materials (e).
Figure 3. Tunable compressive response of fractal-like metamaterial. Failure map of ultralight hierarchical octet lattice as a function of first- and second-order design parameters (a). The red curve shows optimal strength at a given relative density. Uniaxial compression stress–strain curves of samples fabricated from points in (a) is shown in (b–d). An Ashby chart comparing performance of our fractal lattice with state-of-the-art low-density materials (e).
 

Remarkably, we have found unprecedented tensile elasticity on these hierarchical metamaterials, a behavior not seen in bulk metal alloy. Figure 4(a–c) compares results of uniaxial tensile tests of bend–stretch hierarchical lattices with three representative nanoscale wall thicknesses of 700, 150, and 60 nm. The bulk hierarchical bend–stretch metamaterials with a 70-nm wall thickness exhibit a maximum elastic strain limit over 20%, a value not found in any low-weight metal alloys or nanolattices.11 These lattices even maintain their highly stretchable properties after multiple loading cycles prior to fracture, shown in Figure 4(d). This extreme elasticity is a result of combined rotation of bending dominated hierarchical ligaments and stretching of first-order hollow tubes with nanoscale wall thickness. When the material is stretched, the second-order bend-dominated hierarchical ligaments rotate about their nodes and start to unfold from their original position. The collective rotation of second- and first-order strut members under global tensile stress can be seen from the elongation of voids within the second- and first-order lattices in Figure 4(a–c), with a progressively larger amount of elongation in metamaterials with reduced wall thickness. The largest amount of tensile elastic limit was seen on hierarchical metamaterials with a 70-nm wall thickness. In contrast, at a coating thickness exceeding 500 nm, the uniaxial tensile behavior resembles that of conventional metallic foams where fracture is observed at a strain less than 7%. Figure 4(e) compares hierarchical lattice tensile properties, with other recently reported ultralight materials11,12 and lightweight metal alloys foams.13–16 In a stretch-dominated lattice, with a tube wall thickness of 70 nm, the material exhibits a tensile fracture strain less than 10%, much lower than the maximum fracture strain in hierarchical metamaterial.


Figure 4. uniaxial tensile responses of hybrid hierarchical metamaterials (a–c). mechanical data and still frames from the uniaxial tensile tests on a thick-walled hierarchical with wall thickness of 700, 150 and 60 nm (a–c). cyclic tensile stress–strain response of the super-elastic material prior to permanent fracture (d). specific ultimate tensile strength versus elongation to fracture of hierarchical bend–stretch lattice material compared with non-hierarchical stretch-dominated nickel lattices and liter
Figure 4. Uniaxial tensile responses of hybrid hierarchical metamaterials (a–c). Mechanical data and still frames from the uniaxial tensile tests on a thick-walled hierarchical with wall thickness of 700, 150 and 60 nm (a–c). Cyclic tensile stress–strain response of the super-elastic material prior to permanent fracture (d). Specific ultimate tensile strength versus elongation to fracture of hierarchical bend–stretch lattice material compared with non-hierarchical stretch-dominated nickel lattices and literature data on low-density metal alloys (e).
 

Impact on Mission

Our work has expanded the understanding and capability in designing and creating ultralow density, strong and tough architected materials. Our ultralight, high-surface-area materials have application in several Laboratory mission areas including energy security, bioscience and bioengineering, and high-energy-density science. These materials can be used for, among other applications, thermal insulation for increased energy efficiency, energy storage in solar cells, biomedical implants, and targets for Inertial Confinement Fusion experiments. The research, therefore, supports a variety of Livermore strategic focus areas including inertial fusion science and technology and energy and climate security, as well as the advanced materials and manufacturing core competency. The ability to capitalize upon the remarkable properties that manifest themselves at the microscale and nanoscale by creating billets of material at a substantial size will enable researchers across multiple scientific disciplines to take advantage of these properties. It thus has a broad array of impact applications for LLNL mission areas including energy-absorbing materials, flexible batteries, armor, and high-temperature materials design and fabrication.

Conclusion

We found that unprecedented material properties reveal themselves at the macroscale (107 times larger than the smallest feature in the structure) as a result of the hierarchical combinations of architectures at multiple successive length scales down to the nanoscale. Despite their relatively brittle nickel-alloy constituent, these metallic metamaterials exhibit super-elasticity in both compression and tension, with tensile yield strain as large as 20%, going beyond the elastic limit of their nickel-alloy constituent and any other ultralight lattices. Our methodology can be extended to a variety of metamaterial property designs where combinations of micro-architectures at successive length scales will collectively give rise to new combinations of properties not seen in bulk-scale materials. With the possibility of incorporating precise control of topological architectures across all these scales, we enter into a paradigm where nanoscale material properties can be harnessed and made accessible in large-scale objects, opening a wide range of applications for these mechanical metamaterials.  

References

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Publications and Presentations

  • Zheng, X. Y., et al., "Multi-scale metallic metamaterials." Nat. Mater. 15, 1100 (2016). LLNL-JRNL-677190.