A conventional approach to making miniature or microscale reactor components (e.g., heat exchangers, microreactors, separators, etc.) relies on silicon as a base material and MEMS fabrication as manufacturing processes. These Si-based microfluidic devices, however, often fail in applications involving medium-to-high-temperature operations due to lack of robust fluidic interconnects and a high-yield bonding process required to make those devices. Here we explore additive manufacturing (AM), also known as metal 3D printing, as an alternative platform to produce small scale microfluidic devices that can operate at temperature much higher than what polymers can withstand. Binder jet printing (BJP), is utilized to make stainless steel (SS) preconcentrators (PCs) with submillimeter internal features. Small-scale PCs can increase the concentration of gaseous analytes or serve as an inline injector for micro gas chromatography system (micro-GC) or portable gas sensor applications. Normally, parts printed by BJP are highly porous and thus unsuitable as fluidic components due to leaks. By adding to SS316 powder sintering additives such as boron nitride (BN), which reduces the liquid temperature, we produce near full-density SS PCs at sintering temperatures much lower than the SS melting temperature and importantly without any measurable shape distortion. Next, we leverage high initial porosity and decoupling of printing and sintering in BJP to fabricate 3D-printed heterogeneous metal/ceramic structures that are functionally graded. Functionally-graded materials (FGMs) are particularly challenging to produce in AM because of the material and processing incompatibilities caused by the thermal shrinkage/expansion mismatch and residual stress issues. We introduce a selective-reactive sintering (SRS) process to locally tune the electrical properties of BJP-derived SS parts. The SRS process utilizes reactive gaseous environments such as oxygen during sintering and partially converts metal powders to more resistive metal oxides. The combination of BJP and SRS allows the portion of the resulting structures to possess much higher electrical resistance than the other regions, facilitating efficient electrothermal conversion for heat exchanger or reactor applications. Moreover, the heterogeneous nature of the metal/metal oxide structures significantly increases the temperature coefficient of resistance (TCR) compared to that of the raw metal. Large TCR values of the heterogeneous FGM structure makes it highly sensitive to the temperature variation, i.e., useful as resistance-temperature detectors (RTD) or anemometer-like flow sensors at medium-to-high temperatures. The second part of the dissertation is to address the intrinsic limitation of nanosphere lithography (NSL). NSL is known as one of the most inexpensive and widespread nanopatterning approaches, and in conjunction with metal-assisted chemical etching (MACE), can create an array of vertically-aligned silicon nanowires (VA-SiNWs) for various applications including highly-sensitive gas detectors and Li-ion battery anodes. However, VA-SiNWs obtained from NSL and MACE are limited in their size and spacing because the array pitch and wire diameter are inherently linked to the original nanosphere size. Here, we present deformable soft lithography using controlled deformation of elastomeric substrates and subsequent contact printing transfer to systematically control the lattice spacing and arrangements of the nanosphere array. The unique aspect of our approach is to utilize a custom-made radial stretching apparatus that allows the nanospheres to be stretched without disrupting original hexagonal arrangements over large areas. This is different from the patterns obtained from the more conventional uniaxial or biaxial stretching method whose anisotropic nature breaks the hexagonal symmetry.
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