JPH06252465A - Coating for piezoelectric solid state motor laminated body - Google Patents

Abstract

PURPOSE: To enlarge the operation temperature range of a laminate, to increase durability, to increase outputs and to prolong a life by surrounding the assembly of the laminate, electrodes and silicon adhesive in a cylindrical form which a protection housing. CONSTITUTION: Silicon adhesive is sandwiched between the laminate 102 and the housing 104. Silicon adhesive has low viscosity so that it easily flows into a crevasse viewed in the laminate 102 or around the complicated form parts of the electrodes 108 and 108' and the laminate 102 is completely capsuled, and it is formed of ceramic particles containing a plurality of alumina particles. The alumina particles have high heat conduction characteristics and increase the heat conductivity of the capsuled material. The particle sizes of the alumina particles are effective to be substantially similar to the particle size of a piezoelectric element. Thus, the alumina particles are satisfactorily and physically brought into contact with a piezoelectric material and much heat generated in the laminate 102 is transmitted to the inner part of the wall of the housing 104. Then, heat is radiated.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates generally to solid state motor actuator encapsulation structures, and more particularly to a method of encapsulating a piezoelectric solid state motor stack.

[0002]

2. Description of the Related Art Mass-produced solid-state motor laminates, that is, actuators, are manufactured using piezoelectric discs interleaved with metal foil electrodes. Each piezoelectric disk is expanded, ie axially distorted, by applying a potential to the alternately biased electrodes. The additive sled of stacked disks is typically amplified by a hydraulic system to perform useful movements.

One example of a conventional electromechanical actuator having active elements of electro-expanding material is US Pat. No. 3,50, to Glendon M. Benson.
See 1,099 and 3,635,016.
The Benson patent discloses both a motion amplification structure and a method of making a piezoelectric stack. A sheet of ceramic material is rolled, consolidated and punched to form a ceramic disc. After the cleaning process, alternating sets of successive disc electrodes are placed between these ceramic discs to stack the discs. These stacks are subjected to a pressure cold welding process, followed by a high temperature pressure bonding process after connecting the common electrodes to the two electrode groups. The stacks are poled by applying a DC voltage, then encapsulated with a plastic insulating cover and then finally mounted in the transducer housing.

Various environmental design requirements are important when manufacturing piezoelectric laminates. Of these factors, the operating temperature range of the device and external mechanical stress are the most important.

[0005]

Conventional laminates are limited to a maximum operating temperature of about 75 ° C. measured outside the laminate housing. The heat generated by the laminate itself is a balance with the ambient temperature. For example, one use for laminates is
To operate the valve of the engine. In engines where the contained stacks are commonly attached, typically:
Very high heat is generated. The temperature of the stack reaches a temperature which is 40 to 50 ° C. higher than the measured engine temperature.

On the other hand, in the conventional laminated body, if there is a structural defect, shearing stress or twisting stress is applied to the laminated body during operation and / or installation, so that the laminated body generally causes a failure. Structural failure of the laminate is generally due to fatigue cracking of the ceramic disc. Separation between the disc and the electrodes is also often a problem.

In an attempt to minimize some of the above mentioned problems, piezoelectric laminate insulation has been introduced between the disc / electrode stack and the housing.

Komak G. Onail US Pat. No. 4,0
No. 11,474, a number of methods are disclosed for improving the insulation of the stack to avoid breakdown during operation. Here, it is stated that arc contact is avoided by maintaining contact between the piezoelectric laminate and the insulating material. Onell teaches in its first embodiment to introduce a pressurized insulating fluid, such as oil, into the housing of the piezoelectric stack. The fluid is pressurized to maintain the fluid in contact with the stack while the stack contracts radially or expands axially when an applied voltage is applied.

In his second embodiment, Oneil applied a solid polyurethane coating to the laminate. The coating is held in contact with the stack by a pressurized insulating fluid to prevent separation during operation and the associated arcing.

In a third embodiment by Onail, the laminate and the solid insulating coating are kept in contact by wrapping a filament or tape around the coated laminate. The tape is wrapped around the coating to preload the coating and prevent it from separating from the laminate. This tape is wrapped at intervals to accommodate the expansion of the polyurethane coating during operation of the laminate.

[0011]

SUMMARY OF THE INVENTION The present invention is an improvement over conventional encapsulation techniques. The effects of expanding the operating temperature range of the laminate, increasing durability, increasing output and extending life are achieved by the present invention.

In one aspect of the invention, a piezoelectric solid state motor is provided. This solid state motor comprises a plurality of piezoelectric elements assembled in a stack. Each element has two opposing flat surfaces. Each of the plurality of electrodes has a flat section that contacts the flat surfaces of two adjacent elements of the stack. A silicone adhesive is attached to the piezoelectric element. A plurality of alumina particles are arranged on the silicone adhesive. The protective housing cylindrically surrounds the combination of the laminate, electrodes and silicone adhesive.

In another aspect of the invention, a method of encapsulating a piezoelectric solid state motor is provided.
The method includes the following steps. That is, the laminate is encapsulated with a low viscosity silicone adhesive having thermal properties. The silicone adhesive is cured at a first temperature for a first predetermined time to fix the silicone adhesive. Thereafter, the silicone adhesive is cured at a second temperature for a second predetermined time to further establish adhesion between the piezoelectric element and the silicone adhesive.

[0014]

The present invention will be described in detail below with reference to the accompanying drawings. In general, the method of the present invention for encapsulating a piezoelectric stack is designed for an automated manufacturing process to form high quality, durable solid state motor stacks. The piezoelectric encapsulation step of the method of the present invention improves the technique of forming a laminate with fast response and high driving force using careful cleaning and inspection operations.

The present invention relates to an encapsulation structure for a piezoelectric solid state motor laminate and a method for encapsulating such a laminate structure. However, for example, the terms solid state motor stack and electro-expansion actuator are synonymous. Piezoelectric solid-state motor laminates are generally referred to as "laminates" throughout this specification.

The present invention not only overcomes the above-mentioned drawbacks of the prior art, but also provides the features and advantages described below.

FIG. 1 illustrates a housed solid state motor stack 100 according to the present invention. The electrode / ceramic element laminate 102 is arranged in the center of the housing 104. The laminated body 102 includes a plurality of piezoelectric elements 103. These elements 1
03 can take various geometric shapes. In the preferred embodiment, the element 103 is substantially circular. Therefore, the element 103 is also referred to as a disk. Each disk is formed substantially parallel to each other 2
It is preferred to have two opposing surfaces. Two sets of electrodes 108, 108 'are alternately interleaved on these disks.

The basic structure of each electrode 108, 108 'will be described with reference to FIG. Each individual electrode 108, 108 '
Includes a flat section 220 and an elongated section 222. The flat section 220 of each electrode is adapted to contact a substantial portion of the flat surface of two adjacent discs.

The electrodes 108, 108 'are metal foils that have been cut or stamped and formed using known techniques. These electrodes are formed of a copper alloy such as brass or a known equivalent material.

The housing 104 is made of hardened steel and has a cylindrical shape and has a hollow cylindrical cavity for accommodating the laminate. 2 on top of housing 104
Two through ports 106, 106 'are provided,
The electrodes 108, 108 'can be taken out from the housing. Each electrode is bundled together by connectors 109, 109 '.

The housing 104 also includes a set of screws 110 used to attach the stack and housing to, for example, the head of an engine. Housing 10
4 has a trapezoidal region 112. When viewed from above in FIG. 1, this trapezoidal region 112 has a hexagonal cross section. This hexagonal shape, not shown, is used to tighten and loosen the piezoelectric solid state motor housing on the engine head.

The silicone encapsulant is injected to fill the cavity between the stack and the housing and the gap between adjacent electrodes. Further, the end surface 114 of the housing 104 and the exposed surface of the last ceramic element 115 are attached to the end of the laminate housing 104 by a diaphragm 116.
Must be simultaneously polished to facilitate proper alignment. The diaphragm 116 is then laser welded to the surface 114. As a result, the end of the stack (that is, the end surface of the element 115) can be brought into contact with the diaphragm 116.

The diaphragm 116 has a thickness of about 0.1 m.
It is preferably formed of m stainless steel. This steel diaphragm acts to protect the laminate from external contamination. In addition, the diaphragm prevents the electrode / element stack from rotating in the housing, but it is thin and flexible, allowing the stack to move axially with little constraint. The diaphragm and housing preferably have a hardness of about 30.
It is Rockwell C.

When the stack is installed on the head of an engine, a piston is typically abutted against diaphragm 116. During installation, the stack is screwed onto the head of the engine and the diaphragm 116 transfers stress to the housing. Without the diaphragm, friction between the distal ceramic element and the piston causes the stack to rotate. As the laminate rotates, the elements are subjected to shear and separation forces, which will rupture the silicone encapsulant. Such structural defects adversely affect the operation of the stack.

The section of the housing designated 118 forms the anchorage for the stack to rest on. About 1000
When a driving voltage of V is applied to the electrodes 108 and 108 ′, the laminated body expands axially against the fixed base 118,
The diaphragm 116 is moved outward to produce the motion. Therefore, almost all of the axial displacement occurs against the diaphragm 116, with little axial displacement at the end adjacent the section 118.

FIG. 3 is a sectional view taken along the line AA of the piezoelectric solid state motor laminate and the housing of FIG. The silicone adhesive is shown sandwiched between the laminate and the housing. In the preferred embodiment, the silicone adhesive was manufactured by Dow Corning, Inc. under product number Q3-6632. The silicone adhesive is effectively low in viscosity so that it easily flows into the crevasse found in the laminate and around the complex features of the electrode to ensure complete encapsulation of the laminate. The viscosity of the silicone adhesive at 25 ° C. is 5700 cps and the specific gravity is 2.14. Silicone adhesive is made of multiple aluminum oxide (alumina)
Composed of particles. These alumina particles have high thermal conductivity properties, increasing the thermal conductivity of the encapsulating material. Further, it is effective that the particle size of the alumina particles is substantially the same as the particle size of the piezoelectric material. For example, the particle size of the alumina particles is about 1-5 microns and the particle size of the piezoelectric material is 4-5 microns. Therefore, the alumina particles make good physical contact with the piezoelectric material, which causes most of the heat generated in the stack to be conducted and dissipated into the interior of the housing wall, as well as the adhesion of the encapsulating material to the piezoelectric material. Is also promoted.

General Laminate Encapsulation Process 300
Are shown in FIGS. As shown in block 302, the stack undergoes a cleaning process. In one embodiment, excess gloss and contaminants are removed from the exterior surface of a laminate assembled by conventional grit blasting techniques. The laminate is ultrasonically cleaned in a methanol bath. The cleaned laminate is then heat dried at block 304. The heating process removes volatile contaminants. This heating process is performed in a desiccator such as a vacuum oven using a pressure of about -98 kPa for 10
It consists of heating the laminate at a temperature of 0 ° C. for 2 hours. The stack is then cooled at room temperature as shown in block 306.

Following block 306, a mold release material is applied to the inner wall of the housing. The release material inhibits the silicone encapsulation material from adhering to the inner wall of the housing. Thus, the release material allows the encapsulating material to move with the laminate as it is operated. In addition, the release material allows the encapsulant to expand / contract with changes in temperature, avoiding damage to the encapsulant. In a preferred embodiment, the mold release material is part number MS- by Miller Steferson.
145 is a fluorocarbon suspension release agent manufactured as 145.

The stack is then aligned in the housing at block 310 with the modified alignment fixture. The alignment fixture must include a cylindrical case that includes a bottom / end section. As shown in FIG. 1, one end of the actuator has two openings to allow passage of electrical leads. Further, the inside diameter of the align fixture case must be slightly larger than the outside diameter of the stack to allow proper encapsulation. A stopper must then be inserted into the hole in the base of the alignment fixture to seal around the electrical leads, as shown in block 312. Note that the composition of the align fixture case and port stopper is not critical to the invention. However, their composition must be such that there is no contamination or chemical reaction of the laminate.

At block 314, the prepared alignment fixture is placed in an air potting fixture located in the vacuum chamber. The air potting fixture then applies a small axial load to the stack and the alignment fixture is removed. This small axial load is a force in the range of about 445 to 1112.5 Newtons.

After the alignment fixture is removed, at block 320, a large axial load is applied to the stack. With this load used to compress the laminate,
Brass electrodes can be formed on the irregular surface of a disk. Therefore, essentially no space is formed between successive disks, increasing stiffness. In the preferred embodiment, this force is approximately equal to 8900 Newtons. The chamber is then sealed, as shown in block 322. A vacuum is then created in the chamber, as shown in block 324. This vacuum draws a sufficient amount of air so that the elastomer fills the voids left in the laminate when the vacuum is released. Preferably, a vacuum of -100 kPa is drawn.

Next, the application of the adhesive will be described in detail. Silicone adhesives are supplied by the manufacturer as component A and component B. The silicone adhesive is mixed in a 1: 1 weight ratio based on the manufacturer's suggested instructions. When mixed thoroughly, the mixture will appear gray in appearance. To ensure a uniform product mix, A
The components B and B must be thoroughly mixed before they are mixed together. When components A and B are thoroughly mixed, the resulting elastomer must be deflated. The elastomer is deflated by conventional techniques.

At block 328, the elastomer mixture is injected into the desiccator through a glass tube, and
Pouring into the space between the stack and the inner wall of the case. At block 330, the vacuum in the chamber is released and the remaining voids in the laminate are filled with silicone elastomer. Thus, when the vacuum is released, the elastomer fills the voids in the laminate and improves the stiffness properties of the laminate.

The elastomer is then cured at block 332. In the preferred embodiment, the first curing step is conducted at 100 ° C. for about 2 hours. During this first curing stage, the encapsulating material is fixed and has the appearance of a hard rubber material. Then, the second curing stage is about 145-1.
It is carried out at 50 ° C. for about 4 hours. This second step establishes the adhesion of the silicone encapsulant to the piezoelectric material. In addition, a binder such as silane is used to further enhance the adhesive properties of the encapsulating material.

At block 336, axial loads are released from the stack. After that, the laminate is in block 3
At 34, it is cooled to room temperature for 1 hour.

The elastomer is applied so as to sufficiently cover the electrodes and the piezoelectric disc to prevent arcing. Further, the elastomer electrically isolates the laminate from the housing.

At block 338, the stack is removed from the air fixture inside the vacuum chamber. The coating of silicone encapsulant is block 340.
Trimmed as shown in. This trimming exposes the end faces of the assembled stack, allowing direct translation of translational motion from the stack to the diaphragm member during operation. Otherwise, the untrimmed end coating of elastomer acts as a compliant layer.

Block 342 represents the polarisation process. This poling process consists of applying a voltage signal to the stack at room temperature. For example, the voltage signal is ramped up from 0 volts to 800 volts and then ramped down from 800 volts to 0 volts. This poling technique is conventional and based on ceramic materials.

As shown in block 344, the end face of the housing and the exposed face of the piezoelectric disk are simultaneously ground to facilitate proper alignment of the steel diaphragm with the end of the laminate housing assembly. The diaphragm is then laser welded to the end face of the housing as shown at block 346. The diaphragm is preferably made of stainless steel and its thickness is about 0.25 mm. The steel diaphragm functions to protect the laminate from external contamination. In addition, the diaphragm prevents the electrode disc / stack from rotating within the housing.

An example of the operation of the present invention will be described below. The main function of the piezoelectric actuator is to give an actuation. This is achieved by applying a high potential to each of the piezoelectric discs. In response to receiving electrical energy, each disk “stretches” or expands axially, while “shrinks” or contracts radially. When the individual disks of the stack expand, so does stack 130. Further, removal of electrical energy from the disc causes the disc to return to its original form. This expansion property of piezoelectric materials makes ceramics desirable for many applications requiring operation.

As noted above, silicone adhesives exhibit excellent physical properties, including but not limited to: The low viscosity of silicone adhesives results in good adhesion to piezoelectric discs. In addition, this low viscosity allows the crevasses of the laminate to be filled with adhesive to give good stiffness. Therefore, the actuator can be largely displaced by this good stiffness characteristic. Alumina particles exhibit good thermal conductivity properties, allowing the laminate to be used in a variety of heat-critical applications. Further, the encapsulating material protects the laminate from external impacts and allows the laminate to be used in harsh conditions. All of these result in an actuator with excellent reliability.

In addition to the excellent physical properties associated with silicone adhesives, many assembly properties are also improved. The adhesive also adheres to the laminate without a primer (undercoat). Furthermore, the actuator can be manufactured quickly, since only one application of adhesive is required.

Other features, objects and advantages of the invention will be apparent from the above description, the accompanying drawings and the claims.

[Brief description of drawings]

1 is a side cross-sectional view showing a piezoelectric solid state motor laminate housed and encapsulated according to the present invention.

FIG. 2 shows an electrode with flat and elongated sections.

FIG. 3 is a cross-sectional top view taken along line AA of the housed piezoelectric laminate of FIG.

FIG. 4 is a flow chart showing the basic steps of a method of applying an elastomer to the encapsulation of a piezoelectric laminate according to the present invention.

FIG. 5 is a flow chart showing the basic steps of a method of applying an elastomer for encapsulating a piezoelectric laminate according to the present invention.

[Explanation of symbols]

 100 Solid State Motor Laminate 102 Electrode / Ceramic Element Laminate 103 Piezoelectric Element 104 Housing 108, 108 'Electrode 109, 109' Connector 110 Screw 112 Unit Area 116 Diaphragm 114 Surface 118 Fixing Base

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Claims (17)

[Claims] 1. A plurality of piezoelectric elements assembled in a stack, each piezoelectric element having two opposing flat surfaces, and said flat surface of two adjacent elements of said stack. A plurality of electrodes each having a flat section in contact, a silicone adhesive adhered to the piezoelectric element, and a plurality of ceramic particles disposed in the silicone adhesive, the particle size of which is that of the piezoelectric element. A piezoelectric solid-state motor comprising: ceramic particles having a particle size substantially equal to each other; and a protective housing that cylindrically surrounds the combination of the laminate, the electrode, and the silicone adhesive. 2. The silicone adhesive is initially a low viscosity liquid and is distributed around and throughout the laminate to fill the area between the laminate and the housing and then the silicone adhesive. The solid state motor according to claim 1, wherein the agent is hardened and adheres to the piezoelectric element. 3. The solid state motor of claim 2, wherein the particle size of the alumina particles is substantially equal to the particle size of the piezoelectric element. 4. The solid state motor of claim 1, wherein each said electrode comprises an elongated section formed with said flat section so as to extend outwardly from said stack. 5. The piezoelectric element and the flat section of the electrode have a substantially circular shape, and the elongate section extends outwardly from the periphery of the flat section, the elongate section of a given plurality of electrodes. The solid state motor of claim 4, wherein the are fixed together. 6. The solid state motor of claim 5, wherein the two opposing flat surfaces of the element are formed substantially parallel to each other. 7. The solid state motor of claim 6, wherein the elongated section extends outwardly from the periphery of the flat section at a substantially right angle. 8. The solid state motor according to claim 2, wherein the composition of the ceramic particles includes alumina. 9. The solid state motor of claim 8 wherein said alumina particles have a particle size of about 1-5 microns and said piezoelectric element has a particle size of about 4-5 microns. 10. A method of encapsulating a piezoelectric solid-state motor having a plurality of elements alternately assembled with electrodes and assembled in a laminated body, wherein the laminated body is made of a low-viscosity silicone adhesive having thermal characteristics. Encapsulating, curing the silicone adhesive at a first temperature for a first predetermined time to fix the silicone adhesive, and thereafter, curing the silicone adhesive at a second temperature at a second predetermined time. Curing for a period of time to establish further adhesion between the piezoelectric element and the silicone adhesive. 11. The encapsulating step further comprises adding a plurality of ceramic particles to the silicone adhesive, the particle size of the ceramic particles being substantially equal to the particle size of the piezoelectric element. The method according to claim 10. 12. The method of claim 11, wherein the silicone adhesive is cured at the first temperature of 110 ° C. for the first predetermined time of 2 hours. 13. The silicone adhesive is about 145-1.
The method of claim 11, wherein the method is cured at the second temperature of 50 ° C. for the first predetermined time of 4 hours. 14. The encapsulating step further comprises grit blasting the laminate to remove debris, cleaning the laminate to remove unwanted contaminants, and under vacuum. 14. The method of claim 13, comprising the step of applying a silicone adhesive to the laminate. 15. The method of claim 14, wherein the encapsulating and curing steps are performed while applying a predetermined amount of force to the laminate. 16. The method of claim 15 including the step of aligning the stack in a protective housing. 17. The method according to claim 16, wherein in the encapsulating step, the area between the laminate and the housing is filled with silicone adhesive.