Wednesday 8:00 AM

August 14, 2024

Room: 404

Location: Hilton Austin

A design of experiments (DoE) was performed to develop response surfaces relating elastic modulus and yield strength of five equation-based lattice topologies to volume fraction and cell size over ranges of 0.1 to 0.65 and 5-25mm respectively. Generating this type of multi-dimensional property data for metamaterials, like lattices and other cellular solids, is extremely challenging and experiments in the literature typically cover a small parameter space or a single variable to reduce the cost and time. An I Optimal DoE was performed to define the properties of the lattice specimens generated in nTop, built on a FormLabs Form 3L, and tested in quasi-static compression; this DoE captures curvature in the response surfaces for each topology using relatively few specimens. The value of this testing approach, results of the experiment, and generalization of the data to other materials is discussed.

Tensile testing is a commonly used method for characterizing both traditionally and additively manufactured (AM) materials. Testing AM cellular solids like lattice structures, because of the size and number of unit cells needed to accurately capture the bulk response of the lattice, requires larger test specimen using more material. Contemporary lattice specimen for tensile testing are built with solid grip regions to isolate the lattice in the gauge region from any grip effects. Building integral, non-reusable grips makes testing unnecessarily expensive and wasteful. This work demonstrates that samples need not be built as monolithic parts, and instead reusable or disposable grips can be used; reducing build time, material use, and overall cost. Multiple specimen geometries and attachment methods between the grip and gauge regions are explored in quasi-static conditions using Finite Element Analysis and physical testing.

As cloud computing's role expands, data center energy consumption, especially for cooling, becomes a significant challenge. This project aims to develop an energy-efficient two-phase cooling system for data centers, targeting less than 5% of total energy use for cooling and eliminating water consumption. We propose a novel two-phase wick structure with thermal resistance below 0.01 K/W, directly printed on chips without thermal interface materials using an innovative inter-material. Topology optimization of the heatsink design enhances cooling for high-power chips, crucial for AI applications. We've optimized wick porosity and microstructure through process development studies, validated by using SEM, optical microscopy, and CT-scan. An optimized manifold design using vat polymerization ensures uniform flow distribution. Test artifacts with varied dimensions and material compatibility tests were fabricated to finalize design parameters and identify optimal coolant materials.

The advance of additive manufacturing makes it possible to design spatially varying lattice structures with complex geometric configurations. The effective elastic properties of these periodic lattice structures are known to deviate significantly from the isotropic behavior where orthotropic material symmetry is often assumed. This paper addresses the need for a robust homogenization method for evaluating the anisotropy of periodic lattice structures including an understanding of how the elastic properties transform under rotation. Here, periodic boundary conditions are applied on two-material RVE finite element models to evaluate the complete homogenized stiffness tensor. A constrained multi-output regression approach is proposed to evaluate the elasticity tensor components under any assumed material symmetry model. This approach is applied to various lattice structures including scaffold and surface-based TPMS and Strut-based lattices. Our approach is used to assess the accuracy of rotation for assumed anisotropic and orthotropic homogenized material models over a range of lattice structures.

A combination of solid structure and lattice is shown to be stiffer and stronger than either lattice structure or solid structure in isolation. Boundary conditions are known to have a significant effect on the mechanical performance of lattice structures, which are said to have “a high strength-to-weight ratio.” Contemporary lattice structure design, evaluation, and testing efforts pattern lattice through a volume and/or attempt to reduce unit cell size of the lattice such that the lattice has its far field properties. Finite Element Analysis and quasi-static uniaxial compression experiments are used to evaluate six different specimen geometries of equal mass. Six replicates each of a solid column, a thin walled cylinder, a Schwarz D-surface equation-based lattice structure, and combinations of thin-walled cylinder supported by D-surface lattice specimens are tested. The lattice-supported cylinder is shown to have the highest stiffness and ultimate strength.

Architected materials can be used to elicit and direct fluid flow with the ability to control multiphase interfaces [1]. If the characteristic length scale of unit cells comprising a lattice is small (below 1-2 mm), the distribution of a liquid through that lattice is dominated by surface tension effects. Thus, architected porous media can be designed and manufactured to leverage capillary flow and gas-liquid interfaces in three dimensions. We discuss design considerations for such materials and illustrate examples of applications, such as fabrication of novel electrochemical devices, structured composite materials, CO2 absorbers, and electrospray ionization. *** This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 within the LDRD program 22-ERD-029. LLNL-ABS-810682. *** [1] Dudukovic, N.A., Fong, E.J., Gemeda, H.B., DeOtte, J.R., Ceron, M.R., Moran, B.D., Davis, J.T., Baker, S.E. and Duoss, E.B., 2021. Cellular fluidics. Nature, 595(7865), pp.58-65.

Current additive manufacturing processes allow for repeatable fabrication of complex structures, which fosters design flexibility and innovation. This customizability is pivotal for creating efficient sound absorbers, which rely on the precise engineering of pore geometries to meet specific acoustic needs. Here, we showcase the effectiveness of using additive manufacturing to craft cellular materials for multifunctional noise reduction applications. Using traditional fused filament fabrication (FFF) and stereolithography (SLA) workflows, we characterize the sound absorption performance of various open-celled geometries, including triply periodic minimal surfaces (TPMS) and non-periodic spinodoids. Our techniques enable us to tailor the sound absorption performance by controlling the pore size. We test the acoustic performance using a two-microphone impedance tube, and our results show robust sound absorption behavior, especially in gyroid-type TPMS geometries. Our study suggests that 3D printed sound absorbers could be instrumental in mitigating noise pollution, particularly in the context of aircraft engine liners.

Rechargeable Zn-based batteries are highly promising sustainable post-lithium electrochemical energy storage technologies. Pure Zn is the active material of choice, but its cyclability is impaired by a range of morphochemical issues: dendrite growth, surface passivation and hydrogen formation. In the literature, diverse surface modification and structuring approaches have been proposed to mitigate such issues. Single Point Exposure (SPE) in Laser Powder Bed Fusion (LPBF) is a so far unexplored route to anode engineering, that exhibits unique potentialities for the formation of complex electrode architectures with tuned porosity. In this work we describe the development of a flexible LPBF system, enabled by accurate calibration of the laser temporal emission profile, to deposit strut-based pure Zn lattice structures. A comprehensive set of samples was characterized in terms of their physical (apparent density, roughness and thickness) and electrochemical characteristics, disclosing the influence of process parameters on their activity and stability as battery electrodes.