Julia R. Greer

Ultralight Metallic Microlattices

A route is developed for fabricating extremely low-density, hollow-strut metallic lattices. Ultralight (<10 milligrams per cubic centimeter) cellular materials are desirable for thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping. We present ultralight materials based on periodic hollow-tube microlattices. These materials are fabricated by starting with a template formed by self-propagating photopolymer waveguide prototyping, coating the template by electroless nickel plating, and subsequently etching away the template. The resulting metallic microlattices exhibit densities ρ ≥ 0.9 milligram per cubic centimeter, complete recovery after compression exceeding 50% strain, and energy absorption similar to elastomers. Young’s modulus E scales with density as E ~ ρ2, in contrast to the E ~ ρ3 scaling observed for ultralight aerogels and carbon nanotube foams with stochastic architecture. We attribute these properties to structural hierarchy at the nanometer, micrometer, and millimeter scales.

Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses

Amorphous metallic alloys, or metallic glasses, are lucrative engineering materials owing to their superior mechanical properties such as high strength and large elastic strain. However, their main drawback is their propensity for highly catastrophic failure through rapid shear banding, significantly undercutting their structural applications. Here, we show that when reduced to 100 nm, Zr-based metallic glass nanopillars attain ceramic-like strengths (2.25 GPa) and metal-like ductility (25%) simultaneously. We report separate and distinct critical sizes for maximum strength and for the brittle-to-ductile transition, thereby demonstrating that strength and ability to carry plasticity are decoupled at the nanoscale. A phenomenological model for size dependence and brittle-to-homogeneous deformation is provided.

Strong, lightweight, and recoverable three-dimensional ceramic nanolattices

Compressive, ductile ceramic nanolattices Ceramics are strong and stiff, but their limited ability to stretch like putty or steels makes them unsuitable for many engineering applications. Meza et al. constructed ceramic nanolattices from aluminum oxide, in which the beams are designed to stretch rather than bend. A key parameter in lattice design is the ratio of the wall thickness to the beam radius. When that ratio is small enough, compressing the beams does not break them. That way, the nanolattices can be highly compressed and recover to something close to their original shape when the stress is removed. Science, this issue p. 1322 Hollow, three-dimensional alumina nanolattices absorb energy and recover after substantial compression. Ceramics have some of the highest strength- and stiffness-to-weight ratios of any material but are suboptimal for use as structural materials because of their brittleness and sensitivity to flaws. We demonstrate the creation of structural metamaterials composed of nanoscale ceramics that are simultaneously ultralight, strong, and energy-absorbing and can recover their original shape after compressions in excess of 50% strain. Hollow-tube alumina nanolattices were fabricated using two-photon lithography, atomic layer deposition, and oxygen plasma etching. Structures were made with wall thicknesses of 5 to 60 nanometers and densities of 6.3 to 258 kilograms per cubic meter. Compression experiments revealed that optimizing the wall thickness-to-radius ratio of the tubes can suppress brittle fracture in the constituent solid in favor of elastic shell buckling, resulting in ductile-like deformation and recoverability.

Fabrication and deformation of three-dimensional hollow ceramic nanostructures

Creating lightweight, mechanically robust materials has long been an engineering pursuit. Many siliceous skeleton species--such as diatoms, sea sponges and radiolarians--have remarkably high strengths when compared with man-made materials of the same composition, yet are able to remain lightweight and porous. It has been suggested that these properties arise from the hierarchical arrangement of different structural elements at their relevant length scales. Here, we report the fabrication of hollow ceramic scaffolds that mimic the length scales and hierarchy of biological materials. The constituent solids attain tensile strengths of 1.75 GPa without failure even after multiple deformation cycles, as revealed by in situ nanomechanical experiments and finite-element analysis. We discuss the high strength and lack of failure in terms of stress concentrators at surface imperfections and of local stresses within the microstructural landscape. Our findings suggest that the hierarchical design principles offered by hard biological organisms can be applied to create damage-tolerant lightweight engineering materials.

Electronic-Mechanical Coupling in Graphene from in situ Nanoindentation Experiments and Multiscale Atomistic Simulations

We present the in situ nanoindentation experiments performed on suspended graphene devices to introduce homogeneous tensile strain, while simultaneously carrying out electrical measurements. We find that the electrical resistance shows only a marginal change even under severe strain, and the electronic transport measurement confirms that there is no band gap opening for graphene under moderate uniform strain, which is consistent with our results from the first-principles informed molecular dynamics simulation.

Resilient 3D hierarchical architected metamaterials

Significance Fractal-like architectures exist in natural materials, like shells and bone, and have drawn considerable interest because of their mechanical robustness and damage tolerance. Developing hierarchically designed metamaterials remains a highly sought after task impaired mainly by limitations in fabrication techniques. We created 3D hierarchical nanolattices with individual beams comprised of multiple self-similar unit cells spanning length scales over four orders of magnitude in fractal-like geometries. We show, through a combination of experiments and computations, that introducing hierarchy into the architecture of 3D structural metamaterials enables the attainment of a unique combination of properties: ultralightweight, recoverability, and a near-linear scaling of stiffness and strength with density. Hierarchically designed structures with architectural features that span across multiple length scales are found in numerous hard biomaterials, like bone, wood, and glass sponge skeletons, as well as manmade structures, like the Eiffel Tower. It has been hypothesized that their mechanical robustness and damage tolerance stem from sophisticated ordering within the constituents, but the specific role of hierarchy remains to be fully described and understood. We apply the principles of hierarchical design to create structural metamaterials from three material systems: (i) polymer, (ii) hollow ceramic, and (iii) ceramic–polymer composites that are patterned into self-similar unit cells in a fractal-like geometry. In situ nanomechanical experiments revealed (i) a nearly theoretical scaling of structural strength and stiffness with relative density, which outperforms existing nonhierarchical nanolattices; (ii) recoverability, with hollow alumina samples recovering up to 98% of their original height after compression to ≥50% strain; (iii) suppression of brittle failure and structural instabilities in hollow ceramic hierarchical nanolattices; and (iv) a range of deformation mechanisms that can be tuned by changing the slenderness ratios of the beams. Additional levels of hierarchy beyond a second order did not increase the strength or stiffness, which suggests the existence of an optimal degree of hierarchy to amplify resilience. We developed a computational model that captures local stress distributions within the nanolattices under compression and explains some of the underlying deformation mechanisms as well as validates the measured effective stiffness to be interpreted as a metamaterial property.

Fabrication and Microstructure Control of Nanoscale Mechanical Testing Specimens via Electron Beam Lithography and Electroplating

It has been demonstrated that the mechanical properties of materials change significantly when external dimensions are confined to the nanoscale. Currently, the dominant fabrication method for mechanical testing specimens with nanometer dimensions is by using focused ion beam (FIB) milling, which results in inevitable Ga(+) induced damage to the microstructure. Here, we report a FIB-less fabrication technique to create arrays of vertically oriented gold and copper nanopillars based on patterning polymethylmethacrylate by electron beam lithography and subsequent electroplating into the prescribed template. This fabrication process is capable of producing a wide range of microstructures: from single crystals and nanotwinned, to bi-, poly-, and nanocrystalline mechanical testing specimens with diameters from 750 down to 25 nm with the diameter range below 100 nm previously inaccessible by FIB.

Influence of Homogeneous Interfaces on the Strength of 500 nm Diameter Cu Nanopillars

Interfaces play an important role in crystalline plasticity as they affect strength and often serve as obstacles to dislocation motion. Here we investigate effects of grain and nanotwin boundaries on uniaxial strength of 500 nm diameter Cu nanopillars fabricated by e-beam lithography and electroplating. Uniaxial compression experiments reveal that strength is lowered by introducing grain boundaries and significantly rises when twin boundaries are present. Weakening is likely due to the activation of grain-boundary-mediated processes, while impeding dislocation glide can be responsible for strengthening by twin boundaries.

Nanometallic Glasses: Size Reduction Brings Ductility, Surface State Drives Its Extent

We report tensile experiments on Ni80P20 metallic glass samples fabricated via a templated electroplating process and via focused ion beam milling, which differed only in their surface energy states: Ga-ion-irradiated and as-electroplated. Molecular dynamics simulations on similar Ni80Al20 systems corroborate the experimental results, which suggest that the transition from brittle to ductile behavior is driven by sample size, while the extent of ductility is driven by surface state.

Size-dependent mechanical properties of molybdenum nanopillars

We report the deformation behavior of single crystalline molybdenum nanopillars in uniaxial compression, which exhibits a strong size effect called the “smaller is stronger” phenomenon. We show that higher strengths arise from the increase in the yield strength rather than through postyield strain hardening. We find the yield strength at nanoscale to depend strongly on sample size and not on the initial dislocation density, a finding strikingly different from that of the bulk metal.