Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor – Automatic and/or material-triggered control
Reexamination Certificate
1999-12-17
2001-07-10
Yao, Sam Chuan (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Surface bonding means and/or assembly means therefor
Automatic and/or material-triggered control
C156S358000, C156S359000, C156S498000, C156S538000, C156S580000
Reexamination Certificate
active
06257293
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing piezoelectric devices. More particularly, the present invention is directed to an automated, high-volume method and apparatus for manufacturing multi-layered high deformation piezoelectric actuators and sensors.
2. Description of the Prior Art
The present invention is a unique method and apparatus for automatically manufacturing piezoelectric actuators and sensors, principally pre-stressed high deformation actuators and sensors. The disclosed invention provides a method of manufacturing high deformation actuators which is fast, reliable, precise and easy as compared with prior manufacturing methods.
Piezoelectric materials change shape when a voltage potential is applied across their faces. Piezoelectric materials used in conventional configurations have limited application because of the relatively small amount of displacement which the piezoelectric material undergoes during electrical excitation. In order to increase the amount of displacement which can be developed by the piezoelectric material (typically a thin ceramic wafer) the material may be “pre-stressed”. Prior methods of pre-stressing the ceramic wafer include bonding a metallic foil (for example aluminum, stainless steel or the like), under tension, to the ceramic with an adhesive (such as LaRC-SI™ or the like), thus creating what is known as THUNDER (THin layer composite UNimorph ferroelectric Driver and sEnsoR) as disclosed in U.S. Pat. No. 5,632,841. Other methods, such as the “Rainbow” method as disclosed in U.S. Pat. No. 5,471,721, use a chemical reduction process to pre-stress the ceramic wafer. The present invention provides a method and apparatus for producing pre-stressed piezoelectric actuators such as THUNDER and the like. The following disclosure principally describes the preferred embodiment of the invention and its use in manufacturing THUNDER. It will be understood, however, that the present invention, or modifications thereof, may be used to manufacture other types of multi-layer piezoelectric actuators and sensors.
Prior methods of manufacturing THUNDER include inefficient, low output methods which rely heavily on human labor. The prior method of manufacturing THUNDER actuators is as follows: THUNDER actuators are commonly constructed in a “sandwich” configuration with each actuator having a plurality of layers, including first and second metal pre-stress layers, first and second adhesive layers, and a PZT ceramic wafer with electrodes disposed on both major faces. Initially, all of the layers are manually cut to their desired shape. A razor blade or similar instrument is used to cut the ceramic wafer; and a paper cutter, scissors or a razor blade is typically used to cut the adhesive and metal pre-stress layers to size.
Before the “sandwich” can be constructed the two major faces of the ceramic wafer, one major surface of the first metal pre-stress layer and one major surface of the second metal pre-stress layer are sprayed with a primer coating of LaRC-SI™ using an air brush or the like. LaRC-SI™ is a soluble imide developed by the National Aeronautics and Space Administration which is manufactured by NASA in accordance with the process disclosed in U.S. Pat. No. 5,639,850. Initially, one side of the ceramic wafer is sprayed with LaRC-SI™. The coated ceramic wafers are placed on a release cloth-covered aluminum tray. The aluminum tray, release cloth and ceramic wafers are placed in an oven at approximately 70 deg. C., where they remain until the LaRC-SI™ dries. The tray and its contents are subsequently removed from the oven, and the LaRC-SI™ coating process is repeated a second time for the same side of the ceramic wafer. After the second coat is dry, the ceramic wafers are turned over and two coats of LaRC-SI™ adhesive are applied to the opposing major surface using the above described process. The same process is then repeated for the first and second metal pre-stress layers, however only one major surface of the metal pre-stress layers is coated. Because LaRC-SI™ is a dielectric, and in a finished THUNDER actuator the adhesive layer is disposed between a metal pre-stress layer and the ceramic wafer, it is sometimes necessary to roughen a major surface of the metal prestress layers using sandpaper so that intermittent electrical contact is made between the metal prestress layers and the electrodes.
After the LaRC-SI™ coating on the ceramic wafers and the first and second metal pre-stress layers are dry, the “sandwich” is ready for assembly. The first metal pre-stress layer, which is usually the bottom layer in the “sandwich”, typically comprises steel, stainless steel, beryllium alloy or other metal. Placed adjacent the first pre-stress layer in the “sandwich” is the first adhesive layer which is typically LaRC-SI™ material in a thin film form. The PZT piezoelectric ceramic wafer which is electroplated on its two opposing faces is placed on top of the first adhesive layer. A second adhesive layer, also comprising LaRC-SI™ material or the like, is positioned on top of the ceramic wafer, and a second metal pre-stress layer, which typically comprises aluminum foil or the like, is placed on top of the second adhesive layer thereby completing the “sandwich”. As the layers are stacked in the desirable configuration a “dot” of glue is placed between each adjacent layer to prevent slippage of adjacent layers of the “sandwich” during the manufacturing process. Prior THUNDER actuators have been constructed using various numbers of adhesive layers and/or metal pre-stress layers, depending on the desired pre-stressing characteristics.
The “sandwich” building process is repeated until a desirable number of composite structures have been assembled. Each assembled composite structure is placed on a heating tray. The heating tray comprises an aluminum plate, a first layer of fiberglass, and a first layer of release cloth. The first layer of fiberglass is positioned on top of the aluminum plate, and the first layer of release cloth is placed on top of the first layer of fiberglass. The composite structures are positioned on the heating tray, and a second layer of release cloth is placed on the composite structures. A second layer of fiberglass is placed on the second layer of release cloth. A heat resistant sealant tape is disposed around the perimeter of the heating tray to hold the first and second layers of release cloth, the first and second layers of fiberglass and the composite structures in place. A sheet of Kaptan™ is placed over the secured heating tray, and the entire assembly is placed in an autoclave. A vacuum line is inserted under the Kaptan™ sheet; and the Kaptan™ sheet pulls the composite structures against the heating tray as a vacuum is drawn through the vacuum line.
While in the autoclave, the ceramic wafer, the first and second adhesive layers and the first and second pre-stress layers are simultaneously heated to a temperature above the melting point of the adhesive material (typically several hundred degrees Fahrenheit). Due to the relatively large mass of the autoclave, it may take several hours to heat the entire inner chamber to a sufficient temperature. The temperature is then maintained above the LaRC-SI™ melting point for approximately an hour. Natural convection currents, set up within the chamber, transfer heat to the individual composite structures. In some situations, if natural convection is not sufficient, forced convection, using fans or pumps are used. After sufficient heating, the autoclave and the composite structures are allowed to cool, thereby re-solidifying and setting the adhesive layers. The cooling process typically takes several hours due to the high temperature within the autoclave. During the cooling process the ceramic wafer becomes compressively stressed, due to the higher coefficient of thermal contraction of the materials of the pre-stress layers than for the material of the ceramic wafer. Also, due to the greater thermal contracti
Clark Stephen E.
Face, Jr. Samuel A.
Bolduc David J.
Clark Stephen E.
Face International Corp.
Yao Sam Chuan
LandOfFree
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