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Presented in this Master’s project is a microfluidic valve design, a fabrication
technology, and results from test devices. The microvalve will be a component in a
Direct Methanol Fuel Cell developed by Carnegie Mellon University.
The designed microfluidic valve consists of two electrostatically actuated
membranes hydraulically coupled together. The substrate below the membranes has a
curved profile to reduce the pull-in voltage – a phenomenon known as “zipper actuation”.
Three microvalves can be actuated peristaltically in series to create a pump. The
membranes require no steady-state power and use no rectifying valves.
Test devices are fabricated as proof-of-concept experiments for the microvalves.
The test devices have a single mesh-membrane suspended over a flat-bottomed cavity in
the silicon. As a model for zipper actuation, the silicon cavities are fabricated with a
“step” in the silicon to compare to completely flat cavities. Comparisons of test device
performance are made with and without the hydraulic fluid in the silicon cavity.
The test devices are fabricated in a custom single-oxide layer, single metal-layer
process. Stress analysis is presented for the candidate materials: thermal oxide, spin-on
glass, platinum, and aluminum. Ultimately, 1 µm thermal oxide and 0.11 µm platinum
are used to fabricate the devices. The mesh is released from the substrate and sealed with
polymer to form the complete membrane.
The theoretical analysis of the membrane indicated that to actuate it the applied
voltage is around 500 V. That voltage is impractical for this work because it is greater
than the breakdown voltage of the oxide and much greater than the operational voltage of
the fuel cell. At 100 V the theory indicates that the pump displaces less than 5% of its
volume.
Although the fabricated beams in the mesh should lie flat when released, they still
curl enough to touch the substrate. Consequently, this inhibited characterization of pull-in
voltage. Future work entails improving the fabrication process to achieve working test
devices and performing numerical simulations to analyze membrane behavior.
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Full
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