Additive Manufacturing and in Situ Mechanical and Material Characterization of Metallic Structures at Micro/Nanoscale




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The progress in microscale additive manufacturing (µ-AM) of metals requires engineering of the microstructure for various functional applications. In situ control over the microstructure during three-dimensional (3D) printing is critical to achieve metallic nanostructures with desired properties and eliminate the need for post-processing. In this dissertation, an ambient environment localized pulsed electrodeposition (L-PED) process for direct printing of 3D free-standing and layer-by-layer nanotwinned (nt) metals is introduced. 3D nt-Cu structures were additively manufactured using pulsed electrodeposition at the tip of an electrolyte containing nozzle. For the first time, the control of the microstructure of metals during 3D-printing is reported. In particular, It is shown that through variation of electrochemical process parameters the density and the orientation of the coherent twin boundaries (CTBs), as well as the grain size can be controlled. Ntmetals offer superior mechanical (high ductility and strength) and electrical properties (low electromigration) compared to their nanocrystalline (nc) counterparts. While these properties are advantageous for applications in nanoscale devices, the fabrication of nt-metals has been limited to 2D films or template-based 1D geometries. The focused ion beam (FIB) and transmission electron microscopy (TEM) analysis showed that the printed metal was fully dense, mostly devoid of impurities and microstructural defects, and confirmed the formation of CTBs. Mechanical properties of the 3D printed nt-Cu were characterized by in situ SEM micro-compression experiments. The 3D printed nt-Cu exhibited average flow stress of 960 MPa, which is remarkable for a 3D printed material. The microstructure and mechanical properties of the nt-Cu was compared to nc-Cu printed using the same process under direct current (DC) voltage. The results show that such control over microstructure directly correlates with the mechanical properties of the printed metal. L-PED is introduced as an approach to enable direct deposition of nano-pillars and micro-pillars of metals and alloys. When combined with in situ nanomechanics instrumentation, this approach may enable high-throughput investigation of the process–microstructure–property relationship, in particular for nanocrystalline and nano-twinned metals. The application for 3D printed electronics will be greatly extended when they can be used in extreme environments and especially at elevated temperatures. Therefore, the thermal stability and reliability of the additively manufactured nt-Cu interconnects is investigated. The results show clear correlation between the microstructure and the thermal stability of the printed metal. The microstructures with columnar shaped grains and high density of low-energy CTBs exhibited better thermal stability compared to the ones with high-angle random-oriented grain boundaries. Despite the evolution of microstructure and growth of the grains, printed materials with high density of TBs exhibited high strength of 522 MPa after annealing for 4 hours at 300 °C. Additionally, LED is extended to a larger scale process for patterning of high quality metallic patterns on flexible and nonconductive substrates. The printed metallic patterns are solid with no porosity and impurities, and exhibit high electrical conductivity. Overall, the results indicate the feasibility of using LED 3D-printed high conductivity nt-metal features in a wide range of applications, particularly for the electronics and the next generation integrated circuits.



Electroplating, Nanoelectromechanical systems, Alloys, Microstructure, Three-dimensional printing


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