Microvalves employed to control the flow of fluid are presently in use, with several designs falling within a class known as micro electromechanical systems or “MEMS.” It will be appreciated that such microvalves are preferably driven thermally or electrostatically. In either case, slots or other types of openings are placed in an open or closed position, respectively, preferably within a shutter-type configuration so as to permit or prevent fluid from flowing therethrough. Accordingly, the greatest amount of opening can be accomplished by a minimal amount of movement.
It has been determined, however, that the prior art microvalves have an undesirable amount of power drain associated therewith. This is because the power required to open the microvalve must be maintained in order to keep it in the open position. By latching the movable portion of a microvalve shutter in the open position, the need for continuous power thereto would be eliminated. One example of latching is disclosed in U.S. Pat. No. 5,837,394 to Schumm, Jr., where a detent or ratchet is provided to assist in holding a sliding portion of a semiconductor microactuator in either the open or closed position. As seen therein, though, separate actuators are utilized to move the sliding portion in each direction. In this way, the actuators must overcome the resistance provided by the detent/ratchet so that the sliding portion is able to move into the desired position. This clearly requires a greater force from the actuators, and therefore a greater amount of power to the actuators. Further, it will be seen that the '394 patent relates specifically to electrically activated, thermally responsive semiconductor valves that include and contain a cantilever deformable element which deforms on heating by electrical resistance.
It will further be appreciated that while microvalves of the type disclosed herein may be utilized in any number of environments, one specific application has been in the field of metal-air batteries. Metal-air batteries have decided advantages over other types of electrochemical cells such as typical alkaline (zinc/manganese dioxide) or lithium batteries. The metal-air batteries utilize a gas reactant, such as oxygen or air, which does not have to be stored in the battery like a solid reactant. The gas reactant may enter the cell through vents or holes in the battery case. Thus, metal-air batteries are able to provide a higher energy density (watts per unit mass) that may result in a relatively higher power output and a relatively lower weight. This is particularly useful in applications in which a small, light battery is desired so that more energy is provided in the same size package or the same amount of energy in a smaller package. Metal-air batteries are also environmentally safe and generally leakage-free.
Metal-air batteries are comprised of one or more electrochemical cells. Each cell typically includes a metal anode and an air cathode with a separator electrically isolating the two, where an electrolyte is present in the anode, cathode and separator. The metal anode usually comprises a fine-grained metal powder, such as, but not limited to, zinc, aluminum or magnesium, blended together with an aqueous electrolyte, such as potassium hydroxide, and a gelling agent into a paste. The air cathode is a catalytic structure designed to facilitate the reduction of oxygen and typically comprises active carbon, a binder and a catalyst, which are formed into a sheet together with a metal current collector. The air cathode also commonly incorporates a hydrophobic polymer, such as polytetrafluoroethylene or polypropylene, directly into the cathode sheet and/or as a coextensive film. The hydrophobic polymer prevents the electrolyte from passing through the cathode and leaking from the cell.
In a metal-air battery, oxygen, through a series of reactions, reacts with the metal in the cell producing electrical current. In a zinc-air cell, for example, oxygen enables a charge/discharge reaction at the cathode (positive electrode):½O2+H2O+2e−2OH−.
Meanwhile, a charge/discharge reaction occurs at the anode (negative electrode):Zn+2OH−ZnO+H2O+2e−.
Hence, the zinc-air cell has an overall reaction:Zn+½O2ZnO.
Typically, metal-air batteries utilize ambient air, which contains approximately 21% oxygen, as the reactant for the cells. The ambient air enters through ventilation holes in the housing. In the housing, the oxygen in the ambient air reacts with the cells. The oxygen-depleted air then exits the housing. Thus, ambient air enters or is drawn into the housing in a flow sufficient to achieve the desired power output.
Free flow of ambient air through the metal-air cell, however, creates several problems that may lower the efficiency of a metal-air cell or even cause the cell to fail prematurely. First, ambient air that enters the electrochemical cell will continue to react with the anode regardless of whether the cell is providing electrical energy to a load. Thus, the capacity of the cell will continue to decrease unless air is excluded while the cell is not providing electrical energy to a load. Another problem with allowing free flow of ambient air as the reactant is the difficulty in maintaining the proper humidity in the battery. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 50–60%. If the ambient humidity is greater than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will absorb water from the air through the cathode and fail due to a condition called flooding, which may also cause the battery to leak. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air battery, the metal-air cells will release water vapor from the electrolyte through the air cathode and fail due to drying out. Further, impurities such as carbon dioxide (CO2) present in the ambient air may decrease the energy capacity of the cell. Thus, a metal-air cell will operate more efficiently and longer if the flow of ambient air is controlled so that the air enters the cell only when the cell is providing electrical energy to a load.
Air exchange control systems for metal-air batteries have been designed to control the flow of ambient air into and out of metal-air cells for the following reasons: (1) to prevent the cell from continuing to react; (2) to prevent changes in the cell humidity; and, (3) to prevent CO2 from entering the cell when the battery is not providing electrical energy to a load. Some designs, for example, use a mechanism physically operated by the user where a valve or vent cover is attached to a switch that turns an electrical device “on” so that when the switch moves, the cover moves. See, e.g., U.S. Pat. No. 2,468,430, issued to Derksen on Apr. 26, 1949; U.S. Pat. No. 4,913,983 entitled “Metal-Air Battery Power Supply” and issued to Cheiky on Apr. 3, 1990; and, H. R. Espig & D. F. Porter, Power Sources 4: Research and Development in Non-Mechanical Electrical Power Sources, Proceedings of the 8th International Symposium held at Brighton, September 1972 (Oriel Press) at p. 342. In these designs, however, the air exchange system requires the physical presence of the operator and an electrical device that has a switch compatible with the battery air exchange system.
Automatic air exchange systems that are contained within the battery and operate without the presence of a user, however, typically provide significant parasitic drains on the energy capacity of the cell that may also shorten the life of the cell. One design, such as the one disclosed in U.S. Pat. No. 4,177,327 entitled “Metal-Air Battery Having Electrically Operated Air Access Vent Cover” and issued to Mathews et al. on Dec. 4, 1979, utilizes a vent cover associated with an electrically operated bimetallic actuator to close the air access vents when the battery was not in use to prevent ambient air from entering the housing when the battery is not in use. This is accomplished by applying a current to the bimetallic actuator so that the two materials thereof heat up, whereby the different thermal expansion coefficients thereof cause the system to bend up or down. The electrical actuator, however, provides a substantial parasitic drain on the metal-air cells and diminishes the life of the cell.
Additionally, U.S. Pat. No. 5,304,431 entitled “Fluid Depolarized Electrochemical Battery with Automatic Valve” and issued to Brooke Schumm, Jr. on Apr. 19, 1994; U.S. Pat. No. 5,449,569 entitled Fluid Depolarized Battery with Improved Automatic Valve” and issued to Brooke Schumm, Jr. on Sep. 12, 1995; and U.S. Pat. No. 5,541,016 entitled “Electrical Appliance with Automatic Valve Especially for Fluid Depolarized Electrochemical Battery” and issued to Brooke Schumm, Jr. on Jul. 30, 1996, disclose a design incorporating a thermally responsive semiconductor microactuator disposed over a fluid entrance inlet to permit ambient air to enter the cell when the battery is supplying electrical power to a load. In this design, electrical energy on the order of milliwatts is dissipated to heat a resistive element that opens a thermally responsive valve and keeps that valve open while the battery is in use. Thus, as described hereinabove with respect to the '394 patent, the design also provides a continuous parasitic drain on the cell that decreases the life of the cell.
Therefore, there exists a need for a microvalve, particularly one utilized as an air exchange system in a metal-air battery, that minimizes the parasitic drain on the cell. There also exists a need to minimize the size of microvalves used with a metal-air battery so that it fits within a standard battery package and maximizes the volume of the battery that is available for providing electrical energy. It is also desirable that such microvalves be mass produced to decrease costs, as well as enable large numbers of batteries to be assembled containing them as an air exchange system.