A normally closed, piezoelectrically actuated microvalve is being developed as a prototype of valves in microfluidic systems and other microelectromechanical systems (MEMS) intended for operation in outer space. Terrestrial MEMS in which such valves could also prove useful include implantable pumps to administer precisely metered medications, controllers for tightly regulating flows of chemicals in semiconductor-manufacturing processes, and flow controllers for environmental and biological monitoring systems.
Like other devices originally intended for use aboard spacecraft, the present microvalve must be designed to withstand the extreme mechanical stresses of launch, to operate reliably over a wide temperature range and in the presence of ionizing radiation, and to operate reliably after a long time in storage or transit under the aforementioned temperature and radiation conditions. Additional requirements with regard to its specific function include an extremely low leak rate and immunity to disruption by small contaminant particles that may slip into the gaps between actuated valve sealing surfaces.
The design of this valve (see figure) is based partly on the designs of larger, commercially available diaphragm valves. Although the dimensions of this valve exceed the dimensions of most microelectromechanical devices, one is justified making this valve somewhat larger than a typical microvalve because the leak rate of a valve tends to decrease with increasing sealing area.
The valve begins as three separate parts: the base (which includes the seat), the diaphragm, and the actuator. The base, which is micromachined out of silicon wafer, contains the inlet and the outlet. The seat area on the inner (upper in the figure) surface of the base is textured with two sets of sealing rings: one centered on the inlet, the other centered on the outlet. The diaphragm, also micromachined out of a silicon wafer, features circular corrugations near its outer diameter and a central circular boss that covers both openings in the seat. The actuator consists of a stack of piezoelectric disks in a rigid housing machined out of a silicon wafer. A Ti/Pt/Au layer is evaporatively deposited on the faying surfaces of the three parts, then the parts are heated and pressed together to join the pieces with metal-to-metal diffusion bonds.
To apply a large sealing force on the two openings to ensure that the valve is normally closed, the piezoelectric stack is compressed into a slightly contracted condition during the bonding process. Application of a voltage across the stack causes the stack to contract further; this action lifts the diaphragm away from the seat, thereby creating a narrow channel between the inlet and outlet.
The geometry of the seal is expected to impart substantial immunity to disruption by small contaminant particles. The sealing rings are about 20 μm high and are numerous and closely spaced. Because they are many and dense, they still provide a large sealing area despite the valleys between them. At the same time, any small particles that are entrained in the fluid controlled by the valve and that come to rest in the seal area are expected to become trapped in the valleys between the rings, so that they will not disrupt the seal. Also, any single scratch that might occur would likely affect only a limited number of rings and thus be unlikely to create an open path from the inlet to the outlet.
This work was done by Indrani Chakraborty, William Tang, David Bame, and Tony Tang of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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