A method for forming ferroelectric wafers is provided. A prestress layer is placed on the desired mold. A ferroelectric wafer is placed on top of the prestress layer. The layers are heated and then cooled causing the ferroelectric wafer to become prestressed. The prestress layer may include reinforcing material and the ferroelectric wafer may include electrodes or electrode layers may be placed on either side of the ferroelectric layer. Wafers produced using this method have greatly improved output motion

Bryant, Robert G.

Fox, Robert L.

Hellbaum, Richard F.

English

NASA Technical Reports Server

United States Patent 1191 [ i l l  Patent Number: 5,63 2,84 1 
Hellbaum et al. [4s] Date of Patent: May 27, 1997 
1731 
THIN LAYER COMPOSITE UNIMORFW 
FERROELECTRIC DRIVER AND SENSOR 
Inventors: Richard F. Hellbaum. Hampton; 
Robert G. Bryant, Poquoson; Robert 
L. Fox, Hayes. al l  of Va. 
Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration. Washington. D.C. 
Appl. No.: 416598 
Filed: Apr. 4, 1995 
Int. CL6 ..................................................... HOlL 41/22 
U.S. Cl. ......................... 15W245; 29/25.35; 156/160; 
310/311; 264/229 
Field of Search ..................................... 156/160. 163. 
156/165. 245; 29/25.35; 310/330. 331. 
332. 340, 345. 367, 369,370.371. 311: 
264/229 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,631,383 1Y1971 Zilinskas ............................ 29LZ5.35 X 
5,042,493 8/1991 S i t 0  et al. ......................... 29125.35 X 
Primary Examiner-Michael W. Ball 
Assistant Examiner-Francis J. Lorin 
Attorney, Agent, or Firm-Kimberly A. Chasteen 
[571 ABSTRACT 
A method for forming ferroelectric wafers is provided. A 
prestress layer is placed on the desired mold. A ferroelectric 
wafer is placed on top of the prestress layer. The layers are 
heated and then cooled. causing the ferroelectric wafer to 
become prestressed. The prestress layer may include rein- 
forcing material and the ferroelectric wafer may include 
electrodes or electrode layers may be placed on either side 
of the ferroelectric layer. Wafers produced using this method 
have greatly improved output motion. 
8 Claims, 8 Drawing Sheets 
K- id 
https://ntrs.nasa.gov/search.jsp?R=19980037610 2020-06-18T00:20:52+00:00Z
U S .  Patent May 27, 1997 Sheet 1 of 8 5,632,841 
US. Patent May 27,1997 Sheet 2 of 8 5,632,841 
U.S. Patent May 27, 1997 Sheet 3 of 8 5,632,84 1 
U S .  Patent May 27, 1997 Sheet 4 of 8 
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US.  Patent May 27,1997 Sheet 5 of 8 5,632,841 
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US.  Patent May 27,1997 Sheet 6 of 8 5,632,841 
US.  Patent May 27, 1997 Sheet 7 of 8 5,632,84 1 
US.  Patent May 27,1997 Sheet 8 of 8 5,632,841 
5,632.84 1 
1 2 
THIN LAYER COMPOSITE UNIMORF’H 
FERROELECTRIC DRIVER AND SENSOR 
OFUGIN OF THE INVENTION 
electrode layer on either side of the ferroelectric layer prior 
to heating the assembly. 
BIUEF DEScRIpnoN OF THE DRAWINGS 
The invention described herein was made by employees 
of the United States Government and may be used by and for 
the Government for governmental purposes without the 
payment of any royalties thereon or therefor. 
FIG. 1 is a perspective view of the preferred embodiment 
FIG. 2 is a cross sectional view of the preferred embodi- 
Prior to bonding the layers: 
ment after cooling of the layers; 
FIG. 3 is a cross sectional view of an alternate embodi- 
ment of the present invention; BACKGROUND OF THE INVFNITON 
1. Technical Field of the Invention 
The present invention relates generally to ferroelectric 
devices. and more particularly to ferroelectric devices pro- 
viding large mechanical output displacements. 
FIG. 4 is a cross sectional view of an alternate embodi- 
FIG. 5 is a cross sectional view showing the manufactur- 
FIG. 6 is perspective view showing an alternate embodi- 
FIG. 7 is a top view showing a plurality of prestressed 
FIG. 8 is a side view showing three of the prestressed 
merit Of the present invention; 
15 ing process of the present invention; 
2. Discussion of the Related Art 
prior art methods include ‘Rainbow’ piezoelectric actua- 
tors and sensors. more conventional piezoelectric actuators 
and sensors. and electro-magnetic actuators. 
Conventional piezoelectric actuators exhibit limited 
mechanical displacements. The output of conventional wafers in  a stacked configuration. 
piezoelectric devices is limited by the material’s basically 
low piezoelectric displacement constant. Thus conventional 
devices of reasonable thickness (i.e. on the order of a few 14 
merit Of the present invention; 
wafers connected to form a larger wafer: and 20 
DETAILED OF THE PREFERRED 
EMBODIMENT 
LJ 
millimeters) offer Ody micrometer-sized mechanical Output FIG. 1 shows a piezoelectric device 10 according to the 
motion. ‘Rainbow’ actuators, ‘Moonies’. UnhCXPhiC. and present invention prior to being processed, The device 
bimovhic piezoelectric actuators exhibit greater mechanical includes four layers, a piezoelectric layer 12. a prestressing 
output motion. However even the thinnest ceramic Wafers. layer 14 and two electrode layers 1Sa and 18b. The piezo- 
which exhibit the maximum observed output motion. pro- 3o electric layer 12 a n  be ma& from a disk of piezoelectric 
vide a displacement l h h d  to aPFo-telY 1 Of material commercially available from Aura Ceramics 
motion in the 2-direction for a device that is 3 4  Cm long. (C39OO material) or Morgan Maaox. Alternatively. this 
Additionally % thick ceramic devices are extremely layer can be made from piezoelectric material that was first 
brittle and fragile SO that *eY are Pone to breakage and ground to a fine powder and subsequently consolidated into 
require special handling. ~eVi0US methods Of forming 35 a layer by compression bonding the powder with an adhe- 
‘R~ihbow’ actuators include an additional chemical reduc- sive such as a polyfi&, as shown in ‘ ~ ~ ~ g h .  Soluble. 
tion process which releases lead vapors from the wafer into Aromatic, mermoplastic Copoly~des”.  sere N ~ .  08/359. 
the atmosphere. 752. filed Dec. 16,1994. Note that in the latter approach. the 
It is accordingly an object of the present invention to adhesive binder m a k e s  up a very small fraction. typically 5 
provide a ferroelectric actuator with improved mechanical 40 percent by weight. of the finished piezoelectric layer 12. 
displacement. This latter approach is attractive since the required bonding 
operation can Simply be Performed simultaneously With 
ferroelectric actuator with improved durability. other needed bonding operations discussed in the next 
45 roelectric materials. including piezostrictive materials may ferroelectric actuator with improved machinability. 
It is another object of the present invention to provide a The prestressing layer 14 can be made of a mechanically method for producing a ferroelectric actuator which is more adhesive such as a polyimide. Thermoplastics. 
mosets and braze alloys may also be used for this layer 14. environmentally safe than previous methods. 
It is yet another object of the present invention to accom- 5o Additionally. multiple prestress layers 14 may be to 
plish the foregoing objects in a simple manner. provide increased prestress. The adhesive is wet-coated or a 
Additional objects and advantages of the present inven- thin film is melted onto one surface of the piezoelectric layer 
tion are apparent from the drawings and specification which 12 and then bonded to it at an elevated temperature which is 
follow. dependent on the adhesive being used and allows the mate- 
55 rial to undergo cure. drying. and/or melt flow. The layer of 
adhesive thus formed is typically twelve microns thick but SUMMARY OF THE INVENTION 
According to the present invention. the foregoing and can range in  thickness from a few microns to several nun. 
additional objects are obtained by providing a method for Bonding of the layers occurs at a high temperature. which 
producing ferroelectric devices. First. a mold is selected for depends upon the adhesive but is typically 200’-350 O C.. so 
the device. A prestress layer i s  placed on the mold and a 60 that when the two-layer composite matrix cools to room 
ferroelectric layer is placed on top of the prestress layer. temperature. the differential thermal compression rates of 
These layers are bonded together by heating and then the layers automatically impart the desired mechanical pre- 
cooling the assembled device. The prestress layer may be an stress into the layers. as shown in FIG. 2. If desired, the 
adhesive and may include reinforcing material. The ferro- prestressing layer 14 of adhesive can be reinforced primarily 
electric layer may be a piezoelectric material, a piezostric- 65 to allow augmenting the level of prestress, but also for 
tive material or a composite. The ferroelectric layer includes mechanical toughness and decreased hysteresis. To accom- 
surface electrodes which may be applied by including an plish this. a thin layer of reinforcing material 16 is fused or 
It is another object of the present invention to provide a 
It is another object of the present invention to provide a paragraph‘ In addition to piezoelectric materials* Other fer- 
be used to form this layer. 
5,63234 1 
3 4 
bonded onto (FIG. 3). or into (FIG. 4). the prestressing layer piezoelectric wafer 12, if necessary. A sheet of high tem- 
14. Examples of reinforcing materials include, but are not perature material 22. such as KaptonO (DuPont), is placed 
limited to. plastics. ceramics. metals and combinations of over the stack and is sealed using high temperature bagging 
these materials such as aluminum sheet stock and carbon tape 24 to produce a vacuum bag. The assembly is placed 
fibers. Bonding of the reinforcing material 16 can occur 5 into an oven and the air in the KaptonO bag 22 is evacuated 
simultaneously with the bonding of the piezoelectric to the through vacuum port 26. The oven is heated to 3 0 °  C. to 
prestressing layer. melt the adhesive and bond the assembly. Upon cooling, the 
The adhesive layer allows the thin ceramic wafer to be cut assembly undergoes further prestressing. and the resulting 
to shape without chipping or fracturing using conventional piezoelectric device is removed from the vacuum bag and 
cutting tools like scissors and pattern cutters allowing tailor- 10 mold. 
made shapes rather than molded shapes. This method Although the ferroelectric wafers are typically poled as 
enables techniques to be used which allow the pattern received from the vendor, they must be repoled following 
building of 3-dimensional objects from the consolidation of thermal treatment in the prestress process. The poling is 
the 2-dimensional flat ceramic shapes. These layers can also done at an elevated temperature with a DC voltage sufficient 
offer additional environmental protection which allows 15 to induce dipole orientation. After poling. the wafer is 
these devices to be used in harsh conditions. If the adhesive cooled to room temperature in the presence of the electric 
layer used is a good dielectric, the chances of internal and field to preserve the induced orientation. The DC field 
external arcing due to the applied voltage are reduced. strength employed in the polarization is selected to obtain 
OPtiI‘IlW polarization without exceeding the field at which 
to the present invention contains two ele&odes 1& and 2o dielectric breakdown occurs in the material at a given p o h g  
186. The electrodes 18a and 186 can be of the more temperature- 
conventional vacuum-deposited metal type, and can be The amount and type of input voltage per unit of 
applied onto the piezoelectric layer 12 prior to application of deflection, motion, force and output voltage. current, or 
the prestressing layer 14.Alternatively, the electrodes can be power conversion can be adjusted by varying the thickness 
a graphite or metal-plus-adhesive mixture such as silver 25 andor number of layers of the piezoelectric, the number of 
epoxy. which is an excellent conductor. This alternate tech- layers and/or thickness of the prestress layer. the prestress 
nique has the advantage that the conductive adhesive mix- material. the piezoelectric composition and the curvature 
ture can be coated onto the piezoelectric layer 12 and and shape of the molding surface. By varying the curvature 
subsequently bonded to the piezoelectric layer 12. simulta- of the mold, the prestress imparted to the finished piezo- 
neous with the bonding of the prestressing 14 and piezo- 30 electric device is varied. By varying the thickness or number 
electric layers 12. Multiple or patterned electrodes may also of prestress material layers or by varying the material used, 
be used if necessary for the desired application. the output motion and mechanical force can also be varied. 
The above teachings, may be combined to simplify the During processing, the piezoelectric and prestressing layers 
manufacture of piezoelectric devices. Complete devices can move with respect to each other and upon cooling bond 
be produced by assembling separate layers of different 35 together With additional Prestress- This method of making 
materials. such as the appropriate -tures of adhesively devices has shown substantial increase of output motion of 
coated powdered piezoelectric material plus adhesive for the Otherwise identical Piezoelectric devices. 
piezoelectric layer, conductive adhesive f a  the electrodes. A cylindrical bender mode may be emphasized by pre- 
and the adhesive by itself or as reinforcement for the 4o stressing in only one direction which can be done by bending 
prestressing layer, followed by a single high-temperature the layers over a cylindrical molding surface during the 
bonding operation as described in ‘Tough. Soluble. heating cycle. On cooling, the prestressing layer 14. being 
Aromatic. Thermoplastic Copolyimides”. Ser. No. 08/359, under the piezoelectric layer 12 has a tighter radius of 
752, filed Dec. 16, 1994. curvature and prestresses more in one direction thus forming 
are WiCdY 
process to ensure that the finished piezoelectric device has shapes Other than C h ~ l a r  as shown in 
its prestressing layer in tension which places the piezoelec- A number of individual. polygon-shaped piezoelectric 
tric material in  the desired compression. The amount of devices 28 can be grouped into a large-area mosaic by 
prestress in the piezoelectric material can be tailored during joining their appropriate edges as shown in FIG. 7. One 
manufacture in order to maximize the output motion and 5o useful method for edge attachment is the use of a single 
efficiency of the final device. The material layers may be reinforcing layer that covers the entire mosaic. 
formed on a curve-shaped mold. Certain applications may require more mechanical output 
A description typical of fabricating a prestressed device force than can be provided by a single device 10. TWO or 
10 by hand is provided here and shown in FIG. 5. A molding more devices 10 Can then be used in an efficient tandem 
surface 20 is selected for the amount of curvature needed to 55 arrangement bY uniting their dome-like shapes in a ‘stacked- 
provide the desired prestress. The prestress reinforcing layer spoons’ configuration. FIG. 8 shows three devices in this 
16 of aluminum foil is then placed on top of the molding stacked configuration. To allow unimpeded bending of the 
surface 20. Next the adhesive prestress layer 14 made from individual devices during operation the devices can be 
a polyfi& as described in ‘Tou&. Soluble, &omtic. bonded to each other using a compliant layer 30, such as an 
Thermoplastic Copolyimi&s”, Ser. NO. 08/359.752. filed 60 elastomer. i.e. silicone rubber. which allows one layer to 
Dec. 16, 1994 is placed on top of the reinforcing layer 16. move relative to the other. In such an actuator stack, the 
The electrode layer of silver is vacuum deposited on the individual devices 10 remain electrically isolated from each 
lower surface of the piezoelectric wafer 12 (this step is other; one or more of the devices 10 can act as a motion 
unnecessary if pre-electroded piezoelectric wafers are being feedback sensor. 
used). The piezoelectric wafer 12 is placed on top of the 65 When made using a matrix composite fabrication method 
adhesive prestress layer 14. Finally, an electrode layer 180 shown in ‘Tough, Soluble. Aromatic. Thermoplastic 
of silver is vacuum deposited on the upper surface of Copolyimides”, Ser. No. 08/359,752, liled Dec. 16. 1994, 
In one embodiment, the piezoelectric device 10 according 
Provisions should be made during the manufacturing 45 the bender. These cylindrical mode 
6. 
5.632.84 1 
5 6 
large flexible sheets may be produced for use in low- cooling the prestress and ferroelectric layers whereby the 
frequency actuator applications (i.e. noise canceling devices heating and cooling steps induce a prestress into the 
or loud speakers). This can be accomplished by using large layered structure. 
flat molds for consolidation or a continuous roll process. 2. The method of claim 1 wherein the prestress layer 
Molded pads can be bonded together by heating them to 5 includes reinforcing 
soften and/or cure the binder adhesive while pressing them 3. ne method of 1 wherein the ferroelectric layer 
together. includes surface electrodes. 
Ferroelectric devices made from the present method can 4. n e  method of claim 1 further comprising: 
be used in pumps, valves. brakes. motors, sensors. actuators. 1o 
optics, acoustic transducers, and active structures. 
Many improvements, modifications, and additions will be 
apparent to the skilled artisan without departing from the 
spirit and scope of the present invention as described herein 
and defined in the following claims. 
ing: is a piezoelectric material. 
is a piezostrictive material. 
is formed frorn a composite. 
placing a first electrode layer 
placing a second electrode layer on top of the ferroelectric 
5. The method of claim 1 wherein the prestress layer is an 
What is claimed is: 
1. Amethod for producing ferroelectric devices, compris- 6. The method Of wherein the layer 
7. The method of claim 1 wherein the ferroelectric layer selecting a mold for the device; 
8. The method of claim 1 wherein the ferroelectric layer placing a prestress layer on the mold; 
placing a ferroelectric layer on the prestress layer to create 
heating the prestress and ferroelectric layers; and 
the prestress 
and the ferroelectric layer; and 
layer. 
15 adhesive. 
20 
layer structure; 
* * * * *  
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE OF CORRECTION 
PATENTNO. : 5,632,841 
DATED : May 27, 1997 
INVENTOR(S): Richard F.  Hellbaum, Robert G. Bryant, and Robert L. Fox 
It is certified that error appears in the above-indentified patent and that said Letters Patent is hereby 
corrected as shown below: 
Please correct the listing of Inventors as follows: 
On the title page, Item 
1751 Inventors: Richard F. Hellbaum, Hampton; Robert G. Bryant, 
Poquoson; Robert L. Fox, Hayes, Antony Jalink, Jr., 
Newport News, Wayne W. Rohrbach, Yorktown; and 
Joycelyn 0. Simpson, Hampton, all of Va. 
Signed and Sealed this 
Seventh Day of October, 1997 
A ?test: 
Attesting OfJicer 


Thin Layer Composite Unimorph Ferroelectric Driver and Sensor

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