Fabrication, characterization, and transport analysis of silicon-based microsystems: Micro fuel cells and micro gas perconcentrators

The field of Micro Electromechanical Systems (MEMS) research has evolved from a technology that miniaturizes existing macroscale systems (pumps, valves, sensors, etc.) to a 'bridging' technology that connects nano/molecular scale phenomena with macro/meso scale systems, Micro total analysis systems (muTAS) and Lap-On-a-Chip (LOC) technology are good examples of this emerging trend, and their applications now span from pharmaceutical and biomedical devices to micro chemical factories and renewable energy sources. While numerous silicon-based microsystems have been developed in academia and industry and many now appear in the market, there are still many challenging and important issues to be resolved in terms of fabrication, performance characterization, and understanding of underlying phenomena. Here, we present a successful implementation of MEMS technologies to create two different microsystems: micro fuel cells (muFC) as small-scale power sources and micro gas preconcentrators (muGPC) for enhancing a detection limit in chemical and biological sensors. Issues related to heat and/or mass transfer in these systems are addressed to help us understand device performance limitations and provide a guideline for their optimal design.; First, we have developed post-CMOS compatible fabrication techniques using electroplated Pt- and Pd-based catalysts for proton exchange membrane (PEM) FCs powered by formic acid, which has fast kinetics and is less prone to fuel crossover. Structural characterization and electrocatalytic analysis demonstrated that the electroplated Pd catalysts facilitate formic acid oxidation at lower potentials and deliver higher oxidation currents compared to pure electroplated Pt catalysts structure. To provide a high energy and power density power source on the millimeter size scale, our micro formic acid fuel cells were operated in an all-passive mode (sitting pool of liquid fuel on anode and quiescent air on cathode) and produced a maximum power density of 12.3 mW/cm2.; Second, we have developed a muGPC that helps gas detectors sniff a trace amount of toxic vapors by increasing their concentration via adsorption and thermal desorption. Integration with electrostatic microvalves enabled the muGPC to achieve extremely small dead volumes and fast injection times while the power and energy required to run a microheater, a key component of the muGPC, were kept small. Another unique feature of the muGPC is the use of an array of microposts coated with thin metal-organic framework adsorbents permitting a small pressure drop across the chamber while maintaining a large surface-area-to-volume ratio. The optimization of designing the muGPC is discussed in the context of maximizing the preconcentration factor while minimizing the energy required for operation.