Single crystal piezo (SCP) apparatus and method of forming same

This invention was produced pursuant to U.S. Government Contract No. N66604-99-3-4671 with the Defense Advanced Research Projects Agency (DARPA). The U.S. Government has certain rights in the invention.

The present invention relates to piezoelectric devices and methods of manufacturing same, and more particularly to a single crystal piezo (SCP) apparatus manufactured to impart a compressive prestress thereto to significantly harden the apparatus.

Conventional ceramic piezo wafers, such as PZT, are often constructed for use in connection with bi-morph or unimorph actuators. To use the piezo wafer as an actuator or possibly as an energy harvesting device, it must be strengthened during manufacture by imparting a compressive prestress thereto. One such means for prestressing a wafer of piezoelectric material in a manner that strengthens it is by bonding the layer of piezoelectric material to a “prestressing layer” using a high temperature polyimide adhesive. The compressive stress in the piezoelectric layer is induced by the difference in coefficient of thermal expansion between the piezoelectric material and both the prestressing layer and the adhesive, both of which have much higher coefficients of thermal expansion than the piezoelectric material.

Another approach to hardening a piezoelectric material involves sandwiching a piezoelectric layer of material between two fiber composite panels that are under tension in at least one direction of their plane during the bonding process. The composite panels are held under tension until the bond cures, at which point they are released and a compressive stress is induced in the piezoelectric material.

Until the present time, certain “smart” materials, such as single crystal PZN-PT or PMN-PT, which are known as single crystal piezo (“SCP”) material, have not been used in connection with actuators and/or energy harvesting devices because of the lack of hardness of these materials. This lack of hardness makes SCP material highly prone to fracturing when the SCP material is placed under tension. However, SCP material would otherwise be ideally suited for actuator and energy harvesting applications because of its very high energy density, which is many times that of conventional piezo material.

It is therefore desired to provide a means for manufacturing an actuator or energy harvesting device incorporating an SCP layer of material which is not prone or susceptible to fracturing when placed in tension. Such a hardened SCP material could then be used in a wide variety of tasks such as aerodynamic flow control and structural energy harvesting applications that would otherwise be impossible because of the tendency of the SCP material to fracture when placed in tension.

The present invention is directed to a single crystal piezo (SCP) apparatus and method of manufacturing same which makes the apparatus ideally suited for actuator and energy harvesting applications. In one preferred form the actuator includes a steel layer which is formed having a predetermined curvature such that one surface thereof forms a concave surface. A layer of SCP material is bonded to the steel layer while the steel layer is held in a flattened condition by a suitable tool. Once bonding is complete, the force is removed and the steel layer is allowed to flex back into its predetermined curvature. This causes a compressive prestrain to be applied to the SCP material as the steel layer assumes its original, predetermined curvature.

In one preferred embodiment a layer of uniaxial graphite is also bonded to a surface of the SCP material such that the SCP material is sandwiched between the uniaxial graphite layer and the steel layer. The uniaxial graphite material prevents the axis of the SCP wafer normal to its longitudinal surface in contact with the steel layer from going into tension as the steel layer is allowed to flex back into its predetermined curvature. In effect, the graphite fibers of the uniaxial graphite material serve to keep poison's ratio induced strain in compression and allow the SCP layer to energize the steel layer. Since the uniaxial graphite is orientated in the nonbending direction, any small stiffness is added in the bending direction.

The resulting SCP apparatus is ideally suited for actuator and energy harvesting applications due to its very high energy density. The compressive strength imparted to the SCP material of the apparatus prevents breakage as the SCP layer flexes during operation.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention.

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a side view of an SCP apparatus in accordance with a preferred embodiment of the present invention;

FIG. 2 is an exploded side view of the components of the apparatus of FIG. 1 prior to assembly;

FIG. 3 is a side view of the components of the apparatus of FIG. 2 being bonded together via a suitable tool such as a press;

FIG. 4 is a side view of an alternative preferred embodiment of the present invention;

FIG. 5 is a side view of the embodiment of FIG. 4 with the SCP layer in a shortened condition and being held against the flexible substrates; and

FIG. 6 is a view of the shape that the flexible substrate forms once the SCP layer is bonded thereto.

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

FIG. 1 is a simplified side view of a single crystal piezo (SCP) apparatus 10 in accordance with a preferred embodiment of the present invention. The SCP apparatus 10 generally includes a layer of single crystal piezo (SCP) material 12 which is bonded to a steel layer or substrate 14 via a suitable adhesive 16 . The steel layer 14 typically comprises spring steel, but other suitable materials are brass, graphite epoxy composite and glass epoxy composite. The adhesive typically comprises epoxy with 5% weight glass beads 0.002 inch (0.0508 mm) in diameter, but again, it will be appreciated that any suitable adhesive can be utilized. In a preferred embodiment a layer of uniaxial graphite 18 is also bonded to the SCP material 12 via an adhesive 20 . Preferably, a quantity of conductive epoxy, for example 10 milligrams, is also placed in the center of the adhesive layer 16 to allow for electrical conduction. Adhesive 20 may similarly comprise conductive epoxy. The SCP apparatus 10 can be used, for example, as an actuator to control aerodynamic flow control surfaces on aircraft and other mobile platforms, or just as readily as a structural energy harvesting apparatus for harvesting energy and generating an electrical output as the apparatus flexes.

With further reference to FIG. 1 , an important feature is the formation of the steel layer 14 with a predetermined curvature that produces a concave surface 14 a . During manufacture of the apparatus 10 , as will be described momentarily, a compressive strain is introduced into the SCP material 12 when the metal layer 14 assumes the curvature shown in FIG. 1 . This serves to harden the SCP layer 12 and prevent fracturing. Without the compressive strain imparted to the SCP material 12 , flexing of the steel layer 14 could cause fracturing of the SCP material 12 . As a result, without this characteristic of having an induced compressive strain when the SCP apparatus 10 is at rest (i.e., not experiencing any external force that would place it under tension), the apparatus 10 would not be reliable in applications involving actuators and energy harvesting where repeated flexing of the steel layer 14 would be expected.

Referring to FIG. 2 , the manufacture of the apparatus 10 will now be described. Initially, the metal layer 14 is placed against a work surface 22 of a suitable tool, for example a press having a piston 24 . The metal layer 14 is placed with its concave surface 14 a facing the SCP material 12 . The adhesive layer 16 is shown placed on the concave surface 14 a of the metal layer 14 , but it will be appreciated that the adhesive could just as readily be applied to a lower surface 12 a of the SCP material 12 . Adhesive layer 20 is also shown placed on the SCP material 12 , but again this adhesive could also be placed on a lower surface 18 a of the uniaxial graphite layer 18 .

The press 24 is used to apply a sufficient force to the uniaxial graphite layer 18 and the SCP layer 12 to thus flatten the metal layer 14 down against the work surface 22 as shown in FIG. 3 . The application of force is maintained for a time, for example one hour to twelve hours, sufficient to allow the adhesive layers 16 and 20 to bond to the surfaces with which they are in contact. This securely bonds the SCP layer 12 to the metal layer 14 and the uniaxial graphite layer 18 to the SCP layer 12 . When the piston 24 is released, the steel 14 flexes or “springs” back into its preformed curvature shown in FIG. 1 . As it does this, it imparts a compressive strain into the SCP layer along axis 26 shown in FIG. 3 . However, due to the poison's ratio, the opposite axis (designated by arrow 28 ) of the SCP layer 12 would normally go into tension. The uniaxial graphite layer 18 includes fibers which are arranged horizontally in the drawing of FIG. 3 such that the layer 18 is stiff only in the direction indicated by arrow 28 . Thus, the uniaxial graphite layer 18 serves to keep the poison's ratio induced strain in compression while still allowing the SCP material 12 to experience a compressive strain.

The foregoing apparatus and method effectively induces a compressive strain along the axis 26 shown in FIG. 3 once the steel layer is allowed to flex back into its preformed curvature. This compressive strain serves to significantly harden the SCP material 14 and prevent fracturing of same during flexing of the metal layer 14 when the SCP apparatus 10 is used as an actuator or energy harvesting device.

It will be appreciated that the significant energy density of the SCP layer 12 allows the apparatus 10 to be used in an energy harvesting application, for example, to generate sufficient power to power a radio frequency (RF) transmitter. In an actuator application, the apparatus 10 can be used as a bi-morph or unimorph actuator. The manufacturing process described herein further does not rely on the difference in the coefficient of thermal expansion between various layers of the apparatus, as with certain methods of prestressing piezo materials.

Referring to FIG. 4 , an alternative preferred method in accordance with the present invention is shown. In this method, an SCP layer of material 50 is placed on a flexible, electrically conductive substrate 52 . The substrate 52 has a layer of electrically conductive adhesive 54 thereon. In one preferred implementation, adhesive layer 54 is non-conductive but a microliter of electrically conductive adhesive is placed on the adhesive layer 54 at the center of the substrate 52 .

The substrate 52 and the SCP material 50 are placed in a suitable tool, such as a press 56 , and a voltage is applied across the SCP material 50 via a pair of electrodes 57 temporarily secured to the SCP layer 50 and to the substrate 52 , as shown in FIG. 5 . The applied voltage causes the SCP material 50 to shrink in the direction of arrow 58 . This direction is known in the art as the “3-1 direction”. FIG. 5 illustrates the SCP material 50 in its longitudinally shortened condition. The necessary applied voltage field is typically within the range of about 0.5 megavolts per meter to 2 megavolts per meter.

The SCP layer 50 and substrate 52 are held within the press for a time sufficient to allow the adhesive 54 to fully set. This typically occurs after a time of between about one minute-12 hours, depending on the specific adhesive used.

Referring to FIG. 6 , when the adhesive 54 has fully set, the voltage is removed from the SCP layer 50 . This causes the SCP layer 50 to expand in length along the axis of arrow 58 to its original length, which causes a compressive stress to be induced in the SCP layer by its attachment to the substrate 52 . The substrate 52 will “bow” or bend slightly as a result of the lengthening of the SCP layer 50 . The compressive stress induced significantly hardens and improves the fracture resistance of the SCP layer 50 .

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.