WO1992016975A1

WO1992016975A1 - Piezoelectric or electrostrictive actuators

Info

Publication number: WO1992016975A1
Application number: PCT/GB1992/000464
Authority: WO
Inventors: William Anthony Denne; Francis William Bellion
Original assignee: Domino Printing Sciences Plc; Extec Hybrids Limited
Priority date: 1991-03-20
Filing date: 1992-03-16
Publication date: 1992-10-01
Also published as: GB9105892D0

Abstract

A method of forming a piezoelectric or electrostrictive actuator comprises the step of laying on a substrate a finely divided piezoelectric or electrostrictive material (140) and a binder (141) as a matrix, the rigidity of which is substantially equal to that of the piezoelectric or electrostrictive material itself. The layer formed in this way can be extremely thin and therefore can have a greater piezoelectric effect.

Description

PIEZOELECTRIC OR ELECTROSTRICTIVE ACTUATORS

The present invention relates to piezoelectric or electrostrictive actuators. In this specification, a reference to piezoelectric is also to be taken to include a reference to electrostrictive, unless the context requires otherwise.

Piezoelectric electromechanical actuators have been used in a variety of forms for some time. Most piezoelectric materials exhibit a very small effect and are of little practical use. Some titanates and niobates are ferroelectric and exhibit very large piezoelectric coefficients but even if these materials are used, very large electric fields are required to produce appreciable effects.

A practical actuator may use a thin section of piezoelectric material. A low voltage is applied in use across the thickness of the actuator so that the resulting relatively large movement along the length can be used. However, the piezoelectric material must be sintered at very high temperatures (of the order of 1300°C) , which precludes sintering in situ on most substrates, and so layers of the material have conventionally been made first and then subsequently bonded to a substrate. As an example, thin discs of lead zirconium titanate are bonded to metal discs to form audible sounders. However, the piezoelectric discs are extremely fragile and there is a limit to the minimum thickness, flatness and lateral extent of such plates that is practicable. Some piezoelectric materials such as cadmium sulphide and lithium niobate may be deposited by sputtering, but this is a relatively expensive process and is not generally applicable. Piezoelectric plastics materials such as polyvinylidene difluoride may easily be formed in very thin sheets of large extent, but again the piezoelectric coefficients are very low. According to a first aspect of the present invention, a method of forming a piezoelectric or electrostrictive actuator comprises the step of laying on a substrate a finely divided piezoelectric or electrostrictive material and a binder as a matrix, the rigidity of which is substantially equal to that of the piezoelectric or electrostrictive material itself.

This method allows extremely thin piezoelectric layers, of the order of 10 μ , to be formed with considerable lateral extent. This offers the benefits of permitting high strains with low voltage operation and large displacements along the actuator length, due to the high aspect ratio.

Preferably, the binder is electrically conductive. This ensures that the main voltage drop is across the piezoelectric material, ensuring the maximum piezoelectric effect in use.

The piezoelectric material and binder may be screen printed. This enables accurate control of the thickness and lateral definition of the film. Transfer printing, etched plate printing or other means such as spraying may also be used to deposit the material as a thin film.

Multiple layers of materials may be used, and may be laid down by screen printing, to provide, for example, integral electrodes, or inert layers for the piezoelectric material (which may be equally thin) to react against mechanically and/or to provide environmental protection.

The piezoelectric or electrostrictive material may be in powdered or granular form and may be, for example, lead zirconium titanate (PZT) or lead magnesium niobate (PMN) . Each of these ceramic materials exhibits very large piezoelectric effects and high electromechanical coupling coefficients. The dimensions of the granules may equal the thickness of the actuator layer or sub-micrometre powders may be used.

The binder may be a glass powder or ^■■frit" which when fired will bind the piezoelectric granules close together and to the substrate. The tensile modulus of most glasses is similar to that of PZT and PMN so there is no significant mechanical loss through differential rigidity if these materials are used for the piezoelectric material. Suitable melting point crystalline materials may also be used. The binder and piezoelectric powder may be dispersed in a fluid vehicle for printing and this may be diluted to provide the consistency and film thickness required.

The piezoelectric film may be multiply printed (i.e. laid down in plural layers) to provide protection against pinholes in the film.

Transient layers may be used during the forming of the actuator. The transient layers may be removed by melting, burning, dissolving, disintegrating or other means, and permit the creation of freestanding piezoelectric devices. The substrate may be ceramic, metal, glass, polyimide or any suitable material. It may even be a transient layer to produce loose actuators under very controlled conditions. The piezoelectric film may be compacted by mechanical force, hydraulic pressure, ultrasonic vibration or other means, prior to firing, to ensure good electrical contact between the particles.

Before laying the matrix of the material and binder on the substrate, a release film may be laid on the substrate. Manganese dioxide is a suitable material which can be dissolved out by acid so that the printed layer of piezoelectric material is more easily removed. The release film could alternativley be a rigid layer of low mechanical strength which would shear to release the component. According to a second aspect of the present invention, there is provided a piezoelectric or electrostrictive actuator formed by a method as described above.

According to a third aspect of the present invention, there is provided a piezoelectric or electrostrictive actuator comprising a layer consisting of finely divided piezoelectric or electrostrictive material and a binder in a matrix, the rigidity of which is substantially equal to _tha_t of the piezoelectric or electrostrictive material itself .

Devices may be fabricated which comprise multiple layers of screen printed materials including the actuator of the invention. These multiple layers may include integral electrodes, conductors or inert layers.

The actuator of the present invention has very wide applications. For example, a layer of low dielectric insulation between a conductor and a piezoelectric layer will form a low value capacitance. In this fashion, multilayer structures may be fabricated in which conductors may pass above or beneath piezoelectric layers yet remain both electrically and capacitively isolated.

An initial printing may provide a bottom electrode, followed by printing of the PZT layer and then a top electrode layer. The substrate may act as an inert backing against which the piezoelectric film can react when expanding or contracting. Alternatively, a glass film may be printed either before the initial electrode or after the final electrode to provide a very thin reaction substrate. In the latter case, it is also essential to have a transient layer printed beneath the actuator to allow the actuator free movement after fabrication. Such a transient layer nay be formed in a wide variety of ways. For example, a carbon filled wax or polymer may be used as the transient layer. On firing in an inert or reducing atmosphere, it may be arranged that the carbon filling supports an adjacent layer, which may be glass, until firing is complete. A subsequent firing in air will burn away the carbon deposit leaving the glass reestanding.

The granules of piezoelectric material are net electrically connected and are of very high relative dielectric constant, typically cf the order of several thousand. Significant gaps between particles will result in low value series capacitors. An applied voltage will tend tc be dropped across these rather thar. the piezoelectric σranules. To σuard aαainεt this oossibiiitv. the printing medium is preferably formulated with a minimum of, preferably glass, binder to provide adhesion without porosity. With this arrangement, the surface tension of the molten glass will generate pressure to hold the piezoelectric particles in contact. A range of particle sizes in the piezoelectric material will also assist in providing a compact film of high dielectric constant. The highest possible dielectric binder will also ameliorate the problem. Whilst the electrical connections between granules may effectively be point contacts, the entire granule will be excited by the applied voltage. As mentioned above, the binder may be conductive.

The applications of the invention, especially in conjunction with hybrid electronic circuitry, are wide ranging.

The invention will be further described with reference to the accompanying drawings, in which:-

Fig. 1 is a graph of bending moment vs. proportional thickness of a piezoelectric layer of the invention for different ratios of tensile modulus between the piezoelectric-binder composite and the substrate material;

Fig. 2 is a an elevation of an acoustic sounder;

Fig. 3 is a plan view of the acoustic sounder of Fig. 2; Fig. 4 is a transverse section through a first example of a head of an ink jet printer;

Fig. 5 is a plan view of the head and electrical connections of Fig. 4;

Fig. 6 is a transverse section through a second example of a head of an ink jet printer;

Fig. 7 is a plan view of the head of Fig. 6;

Fig. 8 is a stylised cross-sectional view of a piezoelectric layer and glass matrix; and,

Fig. 9 is an equivalent circuit to the layer of Fig. 8.

A simple thin film on a rigid substrate will provide a mechanical actuator. Calculation shows that for a given overall thickness and excitation voltage, the thrust of an actuator increases with thinness of piezoelectric film as shown in figure 1. The different curves relate to different ratios of tensile modulus between the piezoelectric-binder composite and the substrate material. The practical limit is set by electrical breakdown of the piezoelectric film.

A simple thin membrane consisting of a piezoelectric element and a glass backing may be exploited as a micro- actuator. An important application of this is in ink jet printers. The high resolution attainable with screen printing means that this technique may be used to produce an array of actuators on the same pitch as an array of high resolution nozzles. Connections to these actuators may be made by using screen printing techniques to provide an adequate fan-out.

Clearly there are a multitude of other applications and a few of these are set out briefly below.

A simple actuator may form an earphone, loudspeaker, audio sounder or the like. Conversely, pressure on such a diaphragm will produce a voltage on the piezoelectric element permitting use as a sensitive microphone or pressure gauge.

Diaphragm pumps are possible with the appropriate valve gear.

Two elements of piezoelectric material, possibly on opposite sides of a glass membrane, may be exploited as a transformer. For low ratios, the thickness of the piezoelectric films would provide the transformer ratio. For higher ratios, one element may be thickness polarised and the other length polarised. Multiple strips may provide a stack of elements to provide very high voltages.

A variety of designs of linear and rotary piezoelectric motors are feasible. Punkah type fans are feasible for cooling semiconductor devices. Such devices would have the advantages of low profile, high throughput and operation at low voltage from the chip supply rail. Since heat sinks are merely a device for matching chip dissipation to convection cooling, such fans could replace the more expensive, and unnecessarily bulky, extruded aluminium heat sinks currently in use.

Microswitches could clearly be fabricated using multilayer techniques. These would be electrostatically driven relays with the advantage of extremely high speed of operation, zero current consumption and small size. Low cost resonators may be printed to provide essentially planar inductances of high value. These may be developed for band pass filters of various types, phase shifters in AC circuit controllers and such like.

By producing open network free-standing transducers, it is possible to fabricate precision micropositioning devices incorporating flexure hinges, possibly for ensuring parallel motion. Such devices can be optimised for high speed mirror deflection systems for laser scanning in a variety of applications. By the nature of screen printing, it is straightforward to produce arrays of^" individually addressable devices which may be developed for display driving, position sensitive detectors and phased array acoustic transmitters for ultrasound scanning. The invention may be used to provide extremely robust keyboards for extremely harsh environments. The multilayer technique may be used to produce a digital output with check digits from a single keystroke.

An example of a device manufactured according to the present method is shown in Figures 2 and 3 and consists of an acoustic sounder. A thin layer 1 of piezoelectric material and binder as a matrix is printed in the form of a circular disc onto a thin backing plate 2, which may be made of stainless steel, the printed piezoelectric layer having a diameter less than that of the backing plate. A top electrode 3 in the form of a thin circular disc is then printed on top of the piezoelectric layer 2, the top electrode being laid down in the form of a silver paste, for example, in the form of a circular disc having a diameter less than that of the piezoelectric layer. The three discs 1,2,3 are concentric. A very thin acoustic sounder can be made by this method. The acoustic sounder has application in many areas, for example as an alarm in watches and the like.

The example outlined above of a high resolution ink jet printer will be described in detail with reference to Figures 4 and 5.

The printer is of the slit printer type as described in PCT publication no. W090/12691. The printer has a glass block 4 and a printer body 5 which incorporates an ink feed channel 6. Slits 7 are formed in silicone rubber 8 to provide both valve and nozzle as described in the above patent application.

Printed on the underside of the glass block 4 are circuitry and actuators consisting of a number of screen printed layers which are sequentially printed. The first layer to be printed is a longitudinal area of a transient layer, which ultimately is removed as described below to leave a void 9. The transient layer can be a carbon filled solution of methyl methacrylate which is screen printed and dried to form a layer 10 μm thick. On top of this layer is printed a double layer of glass 10, which melts at 750°C. The printing is so organised that a uniform height of 50 μm of glass is produced over the entire surface of the block 4, so that a level surface is presented for subsequent printing of the high resolution electrode layer 12. The glass layer 10 and transient layer are fired in an inert atmosphere such as nitrogen. The initial firing temperature is 200°c to depolymerise the methyl methacrylate and leave a porous carbon support matrix for the glass layer. The temperature is raised to 750°C and the glass fuses to form a compact and continuous vitreous layer. On cooling, a layer of finger electrodes 12 is printed in gold resinate. This consists of parallel tracks of electrodes 12 on the pitch of the printheads of the ink jet printer. Typically, there would be twelve tracks per millimetre configured as 50 μm track widths with 33 μm gaps. The gold resinate is fired in air at 400°C whereupon the resinate decomposes and volatises to leave a series of parallel continuous metallic gold conductors approximately 1 μm thick. Gold melts at 1063°C so that, although the resinate fires at 400°C, the metallic gold remains intact during subsequent higher temperature firings. The carbon support burns out from the transient layer during this firing, to leave a freestanding glass bridge over the void 9. The next layer 11 is a longitudinal area of PZT powder mixed with glass frit, which melts at 600°C. The PZT is printed as a double layer to guard against pinholes, and extends continuously across the end portions of the finger electrodes 12. Indeed, it extends from the ink channel up to the front edge of the printer so that the rubber forming comb has a flat area on which to operate. 'This layer is fired at 600°C to consolidate the PZT granules and bond them to the underlying glass and gold finger electrode layers. The final layer thickness is of the order of 20 μm.

The next layer is a further gold electrode layer 13 which acts as a common earth return for the underlying gold finger electrodes 11. This layer 13 is in register with the unsupported glass and PZT bridge and is U-shaped in layout. It is continuous along the bridge, but has return connections at each end parallel to and extending beyond the electrodes of the first layer. The layer 13 is also of gold resinate which is fired in air at 400°C to produce a continuous metallic gold deposit. The next layer 14 is a thin glaze of approximately 5 μm thickness, which is fired at 500^βC. This protects the gold layer 13 from the ink and insulates the finger electrodes 12 from each other so inhibiting flashover. There are also windows in this glaze 14 which permit a suitable fan-out for easy connection to the closely packed finger electrodes 12 and connection to the earth return electrode 13.

In figure 5 there is shown a series of windows 15 in the glaze 14 so arranged that a small length of each gold finger electrode 12 is exposed for electrical connection. The windows 15 on neighbouring electrodes 12 are printed far enough apart to prevent flashover. A final printing of a two-dimensional array of connection pads 16 is made in gold resinate and fired at 400°C in air. The pads 16 are so arranged that each covers one window 15 in the insulating glaze layer 14 and so connects to a finger electrode 12, but extends laterally over a number of finger electrodes 12, electrically isolated by layer 14, to provide a large area for connection. A relatively coarse two-dimensional array of connector pads may connect easily to the layer 16 with relatively poor registration. In this way, connection to the fine pitch finger electrodes 12 may be made with relative ease.

The overlap of a finger electrode 12 with the common electrode 13 defines a piezoelectric actuator. Application of a high voltage will polarise the PZT layer 11, and further signals of similar polarity will cause the layer to expand in thickness and contract in length. This contraction in opposition to the inert glass layer 10 will cause the composite bridge to deform locally in a shallow dish. In conjunction with the rest of the ink jet assembly, this deformation will act as an ink pump. Excitation will suck ink into the channel adjacent to the actuator and de-excitation will pump this ink out through the valve and nozzle assembly 7,8.

Another example of a printhead is shown in Figures 6 and 7. The printer may again be of the slit printer type as described in PCT publication no. WO 90/12691. A printer body 115 incorporates an ink feed channel 116 from which ink is ejected through a silicon rubber valve/nozzle arrangement 118 as described above and in the above mentioned patent application. An assembly 119 including the piezoelectric actuators is bonded to the printer body 115 via an adhesive layer 120. The assembly 119 consists of an inert substrate 114, which is a layer which may be printed glass or a PZT/glass mixture to match the thermal expansion of the printed piezoelectric layer of the actuator to be described below. A first electrode 113 of gold resinate is printed onto the substrate 114 and an insulating dielectric layer 117 is printed onto the first electrode 113 to extend substantially from the rear of the ink feed channel 116 backwards away from the nozzles 118 so that the region over which the piezoelectric actuators operate is restricted to the region over the ink feed channel 116 and particularly the valves/nozzles 118.

The next layer 111 to be printed is a mixture of PZT powder or granules with grass frit, as described above, on top of which is printed a layer of finger electrodes 112. As shown in the plan view of Figure 7, the finger electrodes 112 are separated from one another by a silicon seal 121. Also shown in the'plan view is an ink feed tube 122.

A further explanation of the considerations involved in determining the characteristics of the layer incorporating electrically-conductive glass will be made with reference to Figures 8 and 9. Figure 8 is a magnified and stylised cross-section through a printed layer including piezoelectric granules 140 in an electrically- conductive glass binder 141, the piezoelectric granules/ glass layer being formed between opposed electrodes 142, 143. On applying a voltage, a current passes through the layer. As the glass is in the form of a convoluted thin film, the ratio of length of the film (i.e. from one electrode 142 to the other electrode 143) to its cross- sectional area (i.e. the area presented to current travelling along the region between the granules 140) is high which ensures a relatively high shunt resistance , and low dissipation of the current.

When a voltage is applied across the piezoelectric layer, the piezoelectric granules become polarised. The induced charge on the opposed edges of the granules may conduct across the separating glass membranes 144 which in this connection have a large cross sectional area relative to the short conduction path so that the intergranular connection is of relatively low resistance. The equivalent circui is shown in Figure 9 where R, is the shunt resistance offered to current flowing through the layer and R₂ is the average intergranular resistance of the glass membranes 144 between respective PZT granules. C is the average capacitance of a PZT granule. As will be well understood, the time constant to charge the actuator is R₂C. Thus, the rise time for actuation of the PZT layer and thermal dissipation in the layer can be adjusted by varying the amount of glass in the matrix, since if relatively less glass is used, the shunt resistance , becomes relatively higher and the intergranular resistance R₂ becomes relatively lower, and vice versa. Therefore, preferably, the glass binder is electrically conductive so that current through the printed piezoelectric layer defines a uniform electric field and conduction between piezoelectric granules will transmit transfer of charge between the granules, ensuring that the maximum piezoelectric effect is obtained.

Claims

1. A method of forming a piezoelectric or electrostrictive actuator comprising the step of laying on a substrate a finely divided piezoelectric or electrostrictive material (140) and a binder (141) as a matrix, the rigidity of which is substantially equal to that of the piezoelectric or electrostrictive material itself.

2. A method according to claim 1, wherein the binder is electrically conductive.

3. A method according to claim 1 or claim 2, wherein the material and binder are screen printed.

4. A method according to any of claims 1 to 3, wherein the piezoelectric material and binder are dispersed in a fluid vehicle for printing.

5. A method according to any of claims 1 to 4, wherein the binder is glass.

6. A method according to any of claims 1 to 5, further comprising the step of printing at least one layer of another material.

7. A method according to claim 6, further comprising the step of printing a transient layer which is subsequently removed.

8. A method according to any of claims l to 7, wherein, prior to laying down the matrix layer, a layer of a release film is laid down on the substrate so that the piezoelectric layer is more easily removed after printing.

9. A method according to claim 8, wherein the release film is manganese dioxide.

10. A piezoelectric or electrostrictive actuator, manufactured according to the method of any of claims l to 9.

11. A piezoelectric or electrostrictive actuator, comprising a layer (1;11;111) consisting of a finely divided piezoelectric or electrostrictive material and a binder in a matrix, the rigidity of which is substantially equal to that of the piezoelectric or electrostrictive material itself.

12. An actuator according to claim 11, wherein the binder is electrically conductive.

13. An actuator according to claim 11 or claim 12, wherein the binder is glass.

14. An actuator according to any of claims 11 to 13, comprising at least one further layer.

15. An actuator according to claim 14, wherein the at least one further layer constitutes an electrode

(2,3;12,13;112,113) .

16. A head for an ink jet printer including an actuator according to any of claims 10 to 15.