Lightweight materials that are simultaneously strong and stiff are desirable for a range of applications from transportation to energy storage to defense. Micro-and nanolattices represent some of the lightest fabricated materials to date, but studies of their mechanical properties have produced inconsistent results that are not well captured by existing lattice models. We performed systematic nanomechanical experiments on four distinct geometries of solid polymer and hollow ceramic (Al 2 O 3 ) nanolattices. All samples tested had a nearly identical scaling of strength (ߪ ௬ ) and Young's modulus ‫)ܧ(‬ with relative density (ߩ̅ ), ranging from ߪ ௬ ∝ ߩ̅ ଵ.ସହ to ߩ̅ ଵ.ଽଶ and ‫ܧ‬ ∝ ߩ̅ ଵ.ସଵ to ߩ̅ ଵ.଼ଷ , revealing that changing topology alone does not necessarily have a significant impact on nanolattice mechanical properties. Finite element analysis was performed on solid and hollow lattices with structural parameters beyond those realized experimentally, enabling the identification of transition regimes where solid-beam lattices diverge from existing analytical theories and revealing the complex parameter space of hollow-beam lattices. We propose a simplified analytical model for solid-beam lattices that provides insight into the mechanisms behind their observed stiffness, and we investigate different hollow-beam lattice parameters that give rise to their aberrant properties. These experimental, computational and theoretical results uncover how architecture can be used to access unique lattice mechanical property spaces while demonstrating the practical limits of existing beam-based models in characterizing their behavior.

This paper presents the design and fabrication of 3‐dimensional hollow metallic nanolattices using 2‐photon lithography, shown in the figure. The ability to fabricate structures of any geometry, with resolution down to 150 nm, provides opportunities to engineer structures spanning multiple length scales with potential to capitalize on combined structural and material size effects for use in many technological applications.

Ordered cellular solids have higher compressive yield strength and stiffness compared to stochastic foams. The mechanical properties of cellular solids depend on their relative density and follow structural scaling laws. These scaling laws assume the mechanical properties of the constituent materials, like modulus and yield strength, to be constant and dictate that equivalent-density cellular solids made from the same material should have identical mechanical properties. We present the fabrication and mechanical properties of three-dimensional hollow gold nanolattices whose compressive responses demonstrate that strength and stiffness vary as a function of geometry and tube wall thickness. All nanolattices had octahedron geometry, a constant relative density, q $ 5%, a unit cell size of 5-20 lm, and a constant grain size in the Au film of 25-50 nm. Structural effects were explored by increasing the unit cell angle from 30 deg to 60 deg while keeping all other parameters constant; material size effects were probed by varying the tube wall thickness, t, from 200 nm to 635 nm, at a constant relative density and grain size. In situ uniaxial compression experiments revealed an order of magnitude increase in yield stress and modulus in nanolattices with greater lattice angles, and a 150% increase in the yield strength without a concomitant change in modulus in thicker-walled nanolattices for fixed lattice angles. These results imply that independent control of structural and material size effects enables tunability of mechanical properties of three-dimensional architected metamaterials and highlight the importance of material, geometric, and microstructural effects in small-scale mechanics.

MATERIALS BY DESIGN: USING ARCHITECTURE IN MATERIAL DESIGN TO REACH NEW PROPERTY SPACES 1123MRS BULLETIN • VOLUME 40 • DECEMBER 2015 • www.mrs.org/bulletin S Similarly, phononic crystals (PnCs) are artifi cial periodic structures composed of elastic materials, in which mechanical waves within a specifi c frequency bandwidth are forbidden from propagating. This prohibited frequency range is termed the phononic bandgap (PnBG) and enables sound, and often heat, to be controlled, allowing for the creation of more effi cient fi lters, waveguides, and resonant cavities.The combination of geometry-including lattice type, topology, and scale-with material properties determines the ultimate behavior of architected materials. Beyond controlling wave propagation and mechanical properties, structural design and microstructural parameters are signifi cant for many applications, including battery electrodes, supercapacitors, and other electrochemical (e.g., electrochromic) devices.

The use of bulk metallic glasses (BMGs) as the flexspline in strain wave gears (SWGs), also known as harmonic drives, is presented. SWGs are unique, ultra-precision gearboxes that function through the elastic flexing of a thin-walled cup, called a flexspline. The current research demonstrates that BMGs can be cast at extremely low cost relative to machining and can be implemented into SWGs as an alternative to steel. This approach may significantly reduce the cost of SWGs, enabling lower-cost robotics. The attractive properties of BMGs, such as hardness, elastic limit and yield strength, may also be suitable for extreme environment applications in spacecraft.