Alan J. Jacobsen

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.

Additive manufacturing of polymer-derived ceramics

Printing ceramics into complex shapes Some materials, such as thermoplastics and metals, are naturally suited to being 3D printed because the individual particles can be fused together by applying heat. In contrast, ceramics do not fuse together the same way. Eckel et al. developed a way to pattern specific preceramic monomers using either 3D printing or stereolithography into complex, curved, and porous shapes. Upon heating, they observed almost no shrinkage, and the formed parts showed exceptional thermal stability. Science, this issue p. 58 Preceramic monomers can be patterned, using stereolithography or 3D printing, into complex shapes and cellular architectures. The extremely high melting point of many ceramics adds challenges to additive manufacturing as compared with metals and polymers. Because ceramics cannot be cast or machined easily, three-dimensional (3D) printing enables a big leap in geometrical flexibility. We report preceramic monomers that are cured with ultraviolet light in a stereolithography 3D printer or through a patterned mask, forming 3D polymer structures that can have complex shape and cellular architecture. These polymer structures can be pyrolyzed to a ceramic with uniform shrinkage and virtually no porosity. Silicon oxycarbide microlattice and honeycomb cellular materials fabricated with this approach exhibit higher strength than ceramic foams of similar density. Additive manufacturing of such materials is of interest for propulsion components, thermal protection systems, porous burners, microelectromechanical systems, and electronic device packaging.

Microlattices as architected thin films: Analysis of mechanical properties and high strain elastic recovery

Ordered periodic microlattices with densities from 0.5 mg/cm3 to 500 mg/cm3 are fabricated by depositing various thin film materials (Au, Cu, Ni, SiO2, poly(C8H4F4)) onto sacrificial polymer lattice templates. Youngs modulus and strength are measured in compression and the density scaling is determined. At low relative densities, recovery from compressive strains of 50% and higher is observed, independent of lattice material. An analytical model is shown to accurately predict the transition between recoverable “pseudo-superelastic” and irrecoverable plastic deformation for all constituent materials. These materials are of interest for energy storage applications, deployable structures, and for acoustic, shock, and vibration damping.

Toward Lighter, Stiffer Materials

Lightweight cellular (porous) materials are assembled from prefabricated building blocks. [Also see Report by Cheung and Gershenfeld] For hundreds or even thousands of years, humans have developed ever lighter and stronger materials, including alloys, polymers, and composites. Recently, these efforts have been joined by a different approach to lightweight materials: the introduction of carefully engineered open structure into solid materials to create cellular materials (see the figure). On page 1219 of this issue, Cheung and Gershenfeld (1) present a fabrication method for cellular materials that enables them to reversibly assemble cellular composite materials with tailored properties.

Scalable 3D Bicontinuous Fluid Networks: Polymer Heat Exchangers Toward Artificial Organs

A scalable method for fabricating architected materials well-suited for heat and mass exchange is presented. These materials exhibit unprecedented combinations of small hydraulic diameters (13.0-0.09 mm) and large hydraulic-diameter-to-thickness ratios (5.0-30,100). This process expands the range of material architectures achievable starting from photopolymer waveguide lattices or additive manufacturing.

Inertial stabilization of flexible polymer micro-lattice materials

Soft micro-lattice materials with different lattice geometries were fabricated using a self-propagating photopolymer waveguide process. The parent polymer was characterized by dynamic mechanical analysis and the glass transition temperature shifted with equivalent strain rate. Quasi-static and dynamic compression tests were subsequently carried out to investigate the inertial stabilization of lattice member buckling as a function of strain rate and structural geometry (e.g. relative density and lattice aspect ratio). A high-speed digital camera was used to record the progression of deformation and failure events during compression. The micro-lattice structures exhibited super compressibility and increased strength. The observed strength increase, particularly for high aspect ratio and high strain rate, was attributed to inertial stabilization.

Research Update: Enabling ultra-thin lightweight structures: Microsandwich structures with microlattice cores

We achieve the benefits of large-scale structural hierarchy at the micro-scale by utilizing a self-propagating photopolymer waveguide process to form ultra-thin sandwich structures. A single step forms the microlattice sandwich core and bonds the core to both facesheets, minimizing adhesive mass and manufacturing time, with core thicknesses <2 mm, facesheet thicknesses ranging from 12.7 to 300 μm, areal densities 0.030–0.041 g cm−2, and flexural rigidity per unit width up to 0.62 Nm. This work extends the lightweighting benefit of sandwich structures to lower thicknesses and areal densities that were previously the exclusive domain of monolithic materials.

Effects of material heterogeneities on the compressive response of thiol-ene pyramidal lattices

A process of directed UV photo-curing was previously developed for producing periodic thiol-ene lattices, with potential for use in lightweight structures. The present study probes the compressive response of two families of such lattices: with either one or two layers of a pyramidal truss structure. The principal goals are to assess whether the strengths of the lattices attain levels predicted by micromechanical models and to ascertain the role of lattice heterogeneities. These goals are accomplished through characterization of the lattice geometries via X-ray computed tomography and optical microscopy, measurements of the mechanical properties of the constituent thiol-ene and those of the lattices, and strain mapping on the lattices during compressive loading. Comparisons are also made with the properties of the thiol-ene alone, produced in bulk form. We find two lattice heterogeneities: (i) variations in strut diameter, from smallest at the top surface where the incident UV beam impinges on the monomer bath to largest at the bottom surface; and (ii) variations in physical and mechanical properties, with regions near the top surface being stiffest and strongest and exhibiting the highest glass transition temperature. Finally, we find that the measured strengths of the lattices are in accord with the model predictions when the geometric and material property variations are taken into account in the micromechanical models.

Multiphysics simulation of microstructure formation by self-propagating photopolymer waveguides

A 3D multiphysics solver which combines beam propagation with chemical kinetics for simulating self-propagating photopolymer waveguides is presented. The solver is shown to predict various experimentally observed phenomena and is used to simulate the formation of micro-truss structures consisting of intersecting photopolymer waveguides.