Additive manufacturing and characterization of structural and multi-functional metamaterials via a large-area high-resolution stereolithography system
Projection micro-stereolithography (P�SL) is a technique for micro-fabrication that can fabricate complex three-dimensional (3D) microstructures. However, this technique has been limited to printing low-volume structures with high resolution. My dissertation research introduces a large-area projection stereolithography (LAP�SL) system that integrates scanning optics with the existing P�SL to manufacture cm-scale objects with micro-scale architectures. This technique enables the fabrication of structures with features down to the sub-10�m scale and overall size up to hundreds of millimeters, which is particularly suitable for fabricating micro-architected metamaterials with large volumes that can be employed in a broad array of applications.The LAP�SL system is capable of fabricating multi-scale features, making it possible to create lightweight structural materials with feature sizes from a few micrometers to hundreds of millimeters. Herein, we studied the process-structure-property relationships of a class of high-temperature ceramics via LaP�SL. This study, for the first time, achieves high-resolution printing of high-temperature ceramics, with accurate three-dimensional feature sizes on the scale of a few micrometers, opening new opportunities for high-temperature material and device manufacturing. We discovered the size-dependent mechanical properties of high-temperature ceramics and their failure properties corresponding to various feature sizes. Furthermore, the LAP�SL system is capable of fabricating metamaterials containing millions of unit cells, providing a unique experimental platform to study the fracture and damage tolerance of metamaterials. We additively manufactured stretch-dominated architected metamaterials with pre-defined embedded crack, where the size of the unit cells becomes sufficiently small compared to the flaw dimensions. Via combined experimental, X-ray tomography and numerical calculations, we have elucidated the emergence of fracture toughness as a material property in architected metamaterials, which is found to be a property largely influenced by the elastic instabilities of struts members and T-stress, A design map based on a 2-parameter fracture model was developed to guide the design of failure modes in micro-architected metamaterials. Beyond structural materials, my research then extends to the design and additive manufacturing of multi-functional materials and assembly-free devices for directional sensing, underwater transducers and meso-scale robotic systems. Using piezoelectric materials as an example, we demonstrated the process-structure-property relationships of additive manufacturing of multi-functional materials and energy transduction devices. A novel micro-stereolithography system integrated with the blade-casting process was developed and employed to print piezoelectric particles with surface functionalization. These as-printed piezo-active materials can be rationally designed to achieve programmable voltage-strain responses, going beyond the limitations of the intrinsic crystalline structures. The design strategy can be applied to create the next generation of intelligent infrastructure, able to perform a variety of structural and functional tasks, including simultaneous impact absorption and monitoring, three-dimensional pressure mapping and directionality detection. This study demonstrated the ease of implementation and utility of the piezoelectric metamaterials in underwater applications. Underwater transducers consisting of rationally designed metamaterials to accommodate diverse frequency ranges were developed. Through tuning geometry of the micro-architectures, the working range of the underwater transducers can vary from 10kHZ to 4MHz. With this broad frequency range, we developed hydrophones with arbitrary directivity patterns and ultrasonic array with high sensitivity. This study showed the feasibility and applicability of these underwater transducers for noise elimination and underwater object detection. Additionally, leveraging these piezo-active micro-architectures, my research then focused on additive manufacturable piezoelectric actuation metamaterials, with programmable deformation modes, including twisting, bending, shearing and axial strain under uniform electric fields. As a consequence of freeform design and fabrication, different types of metamaterial blocks and actuation modes can be combined into a single-piece material block and construct multiple degree-of-freedom modular actuation elements. The stackable, modular actuation elements allow the generation of complex coupled or decoupled motion without any transmission systems, which increases the energy efficiency of robotic systems generated by the piezoelectric metamaterials.