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Xiaoxing Xia1 Arman Afshar2 Heng Yang1 Claudio Di Leo2 Julia Greer1

1, California Institute of Technology, Pasadena, California, United States
2, Georgia Institute of Technology, Atlanta, Georgia, United States

Additive manufacturing opens up a new design space to create architected materials with exotic properties not attainable by conventional materials – for example, negative refractive index, simultaneous ultra-high stiffness and recoverability, and photonic and phononic bandgaps. External stimuli like mechanical forces, wetting and dewetting, and magnetic field could be applied to reconfigure the structure of architected materials in order to achieve novel functionalities. Such transformations are usually binary and volatile because they toggle between the “on” and “off” states and require persistent external stimulus to stay in the deformed geometry. In this work, we demonstrate the cooperative beam buckling phenomenon that transforms a Si-coated tetragonal microlattice into an ordered sinusoidal lattice via electrochemically controlled Si-Li alloying reaction. Ex situ scanning electron microscopy reveals the stable structural transformation across multiple length scales from the nanostructured beams to the millimeter-scale bistable domains. In situ optical microscopy visualizes the dynamic cooperative buckling process caused by lithiation-induced Si volume expansion and built-in mechanical instabilities in the lattice architecture. Long-term electrochemical cycling shows buckling and unbuckling of the lattice beams are highly reversible within a voltage range.
We investigate the mechanical dynamics of individual buckling beams and the cooperation of buckling directions among neighboring beams using a coupled chemo-mechanical finite element model, which highlights the interplay between large elastic-plastic deformation and instability-driven buckling. Furthermore, we discover the stochastic distribution of local defects in architected materials play an important role in the formation of domains separated by distorted domain boundaries and analyze such phenomenon with a statistical mechanics model analogous to the Ising model. Our study provides a pathway for predicting dynamic architected material responses according to defect density and distribution, which has not been discussed in previous works. With this understanding, we designed and implanted artificial defects in Si microlattices to deterministically control buckling directions to produce single-domain sinusoidal lattices and to program domain boundaries that form a particular pattern. This new class of electrochemically reconfigurable architected materials provides insights for next-generation battery electrodes with novel stress-relief mechanisms and dynamic mechanical metamaterials with tunable phononic bandgaps.

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