Piezoelectric/electrostrictive actuator having at least one piezoelectric/electrostrictive film

A piezoelectric/electrostrictive actuator including a ceramic substrate, and at least one piezoelectric/electrostrictive actuator unit formed on at least a portion of at least one surface of the substrate, each piezoelectric/electrostrictive actuator unit having a first electrode film, a piezoelectric/electrostrictive film and a second electrode film which are laminated in the order of description, with the piezoelectric/electrostrictive actuator unit formed on the substrate by heat treatment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a bi-morph or uni-morph type piezoelectric 
or electrostrictive actuator used as or for a displacement-controllable 
element, a solid element motor, an ink Set ejector, a relay, a switching 
element, a camera shutter, a print head, a pump, a fan or blower, and 
other components or devices. The term "actuator" used herein is a member 
capable of transducing or converting an electric energy into a mechanical 
force, displacement or strain. 
2. Discussion of the Prior Art 
Recently, an element whose displacement can be controlled has been widely 
used and increasingly needed in the fields of optics and precision 
positioning or machining operations, for example, for adjusting or 
changing an optical path length or the position of a member or component 
of a device, on the order of fractions of a micron (.mu.m). To meet this 
need, there have been proposed and developed various piezoelectric or 
electrostrictive actuators utilizing a piezoelectric or electrostrictive 
material such as a ferroelectric material, which exhibits the reverse or 
converse piezoelectric effect or the electrostrictive effect, in which the 
application of a voltage or electric field to such a piezoelectric or 
electrostrictive material produces a mechanical displacement. 
Conventionally, the piezoelectric/electrostrictive actuator is structurally 
classified into a mono-morph type, a uni-morph type, a bi-morph type and a 
lamination type. The mono-morph, uni-morph and hi-morph types provide a 
relatively large amount of bending or flexural displacement or deflection 
or distortion owing to the transverse mode of converse piezoelectric or 
electrostrictive effect, namely, due to the strain perpendicular to the 
direction of the electric field produced upon application of a voltage. 
However, these types suffer from inherent problems such as a small 
magnitude of a force generated, a low response speed, a low level of 
electro-mechanical conversion efficiency, and a low degree of operating 
reliability due to the use of an adhesive for bonding the component 
layers. On the other hand, the lamination type utilizes the longitudinal 
mode of converse piezoelectric or electrostrictive effect, namely, the 
strain parallel to the direction of the electric field, and therefore 
assures a large magnitude of the generated force, a high response speed, 
and a high level of electro-mechanical conversion efficiency. However, the 
lamination type suffers from an inherent problem that the amount of 
displacement produced is relatively small. 
The conventional piezoelectric/electrostrictive actuator of the uni-morph 
or bi-morph type also suffers from a relatively low operating reliability, 
which arises from the use of a bonding agent for bonding together the 
component sheets or plates of the actuator such as a 
piezoelectric/electrostrictive element. 
Thus, the conventional piezoelectric or electrostrictive actuators have 
drawbacks as well as advantages, and suffer from some problems that should 
be solved. 
SUMMARY OF THE INVENTION 
It is accordingly, a first object of the present invention to provide a 
piezoelectric/electrostrictive actuator of uni-morph or hi-morph type 
which does not use a bonding adhesive or cement and which undergoes a 
sufficient amount of displacement by application of a relatively low 
voltage, with an improved response to the applied voltage. 
Another object of the invention is to provide such 
piezoelectric/electrostrictive actuator wherein 
piezoelectric/electrostrictive actuator units are formed with high 
integration density on a substrate or support. 
The above objects may be achieved according to the principle of the present 
invention, which provides a piezoelectric/electrostrictive actuator 
comprising a ceramic substrate, and at least one 
piezoelectric/electrostrictive actuator unit formed on at least a portion 
of at least one surface of the substrate. Each 
piezoelectric/electrostrictive actuator unit comprises a first electrode 
film, a piezoelectric or electrostrictive film and a second electrode film 
which are laminated in the order of description. 
The piezoelectric/electrostrictive actuator of this invention wherein each 
piezoelectric/electrostrictive actuator unit consists of a laminar 
structure as described above provides a large amount of displacement by 
application of a relative low voltage applied thereto, with a high 
response to the applied voltage. Further, the laminar 
piezoelectric/electrostrictive actuator units may be formed with improved 
integration density on the substrate. Although the 
piezoelectric/electrostrictive actuator of the present invention which 
includes the laminated films integrally formed on the substrate is more or 
less similar in construction to the conventional bulk type laminar 
actuator, the present actuator is capable of undergoing a sufficient 
amount of flexural or bending displacement or distortion owing to the 
transverse mode of converse piezoelectric or electrostrictive effect 
produced upon application of an electric filed, and generating an 
accordingly large force, while assuring improved operating response. 
Further, the electrode films and piezoelectric/electrostrictive film of the 
laminar structure are integrally laminated on the substrate, without a 
bonding adhesive as conventionally used for bonding thin component sheets 
of the known actuator of the uni-morph or hi-morph type. According to the 
present invention, discrete bonding layers present between the substrate, 
electrode films and piezoelectric or electrostrictive film, are 
eliminated. Additionally, bonding adhesive materials mixed within the 
layers of the electrode and/or piezoelectric film are eliminated. The 
materials which are eliminated correspond to those understood by the 
artisan which are considered adhesive materials. As defined in Chemical 
Dictionary, Morikita Publishing Company and Glossary of Chemical Terms, 
Van Nostrand Reinhold Company, an adhesive is a substance applied to an 
interface between two other substances (similar or dissimilar substances), 
for forming a bond between contact surfaces of the two other substances. 
Three fundamental requirements of an adhesive are fluidity, wettability on 
a surface of a solid, and solidification. Other requirements include: 
small volumetric shrinkage upon solidification; freedom from loss of 
bonding stability due to internal stresses which arise from variations in 
temperature and humidity; high resistance to water, heat and aging; and 
high freedom from creep under load for a long time, and high shock 
resistance. Adhesives are classified, in terms of fluidity and 
solidification characteristics, into (1) solution and emulsion type, (2) 
hot-melt type, (3) monomer or pre-polymer type, and (4) pressure-sensitive 
type. Typical examples of these types respectively include: (1) 
chloroprene, SBR, Natural rubber, polyvinyl acetate, polyacrylic acid 
ester, polyvinyl alcohol, carboxymethyl cellulose, (2) polyvinyl acetate, 
polyamide, polyvinyl butyral, (3) urea-, phenol-, cresol-, epoxy-, and 
alkyd-based adhesives, and cyano-acrylic acid ester, and (4) polyacrylic 
acid ester, polyvinyl ether. Additional examples include rubber, rosin, 
asphalt, semi-synthetics including various inorganic cements, and glass 
materials (e.g., water glass) which provide a bonding force after 
solidifying via cooling from a high-temperature molten state. 
Due to elimination of the bonding adhesive, the present 
piezoelectric/electrostrictive actuator has improved operating reliability 
for a prolonged period of use, and the displacement to be produced by the 
actuator is subject to a minimum amount of drift. 
The laminar structure according to the present invention permits the 
piezoelectric/electrostrictive actuator units to be easily formed with a 
relatively high density on the same surface of the substrate. 
According to a finding of the applicants, for obtaining a large amount of 
flexural or bending displacement and the accordingly large magnitude of 
the force generated, the thickness of the present actuator is preferably 
300 .mu.m or less, and more preferably 150 .mu.m or less, and the bending 
strength of the ceramic substrate is preferably at least 1200 
kgf/cm.sup.2, and more preferably at least 1500 kgf/cm.sup.2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 1, there is shown an example of the piezoelectric 
or electrostrictive actuator of the present invention, wherein a 
piezoelectric/electrostrictive actuator unit is formed on one of opposite 
major surfaces of a generally elongate rectangular ceramic substrate 2. 
The piezoelectric/electrostrictive actuator unit is a laminar structure 
consisting of a first electrode film 4, a piezoelectric/electrostrictive 
film 6 and a second electrode film 8, which are integrally formed on the 
substrate in the order of description. The first and second electrode 
films 4, 8 have respective terminal portions 4a, 8a which extend beyond 
the appropriate end face of the piezoelectric/electrostrictive film 6. The 
terminal portion 8a covers a part of the end face of the film 6. In 
operation of the actuator, a voltage is applied between the first and 
second electrode films 4, 8 through the terminal portions 4a, 8a. 
FIG. 2 shows an example of the piezoelectric/electrostrictive actuator in 
which two piezoelectric/electrostrictive actuator units are provided on 
the respective opposite major surfaces of the substrate 2. Each portion 
consists of the first and second electrode films 4, 8 and the 
piezoelectric/electrostrictive film 6 sandwiched by the two films 4, 6. 
The piezoelectric/electrostrictive actuator units (4, 6, 8) are 
structurally integrated with the substrate 2, by heat treatment. 
Five different forms of the actuator which includes a plurality of 
piezoelectric/electrostrictive actuator units on the substrate 2 are 
illustrate in FIGS. 3 through 7, respectively. The 
piezoelectric/electrostrictive actuator units (4, 6, 8) are arranged in 
the direction which is either perpendicular or parallel to the major 
surfaces of the substrate 2, i.e., the plane of the substrate. 
In the examples of FIGS. 3, 4 and 5, the plurality of 
piezoelectric/electrostrictive actuator units (4, 6, 8) are formed in 
parallel with each other on the same major surface of the elongate 
substrate 2, such that he piezoelectric/electrostrictive actuator units 
(4, 6, 8) are spaced from each other in the longitudinal direction of the 
substrate 2. In the actuator of FIGS. 3 and 4, the 
piezoelectric/electrostrictive actuator units (4,6,8) are separated from 
each other by rectangular slots 10 formed in respective portions of the 
ceramic substrate 2, each located between the adjacent 
piezoelectric/electrostrictive actuator units. The actuator of FIG. 3 has 
insulating films 14 which cover a rear part of the exposed end face of the 
piezoelectric/electrostrictive film 6, for electrically insulating the 
first and second electrode films 4, 8. In the actuator of FIG. 5, the 
ceramic substrate 2 has a plurality of elongate rectangular holes 12 
formed therethrough at a suitable pitch in the longitudinal direction, so 
as to define a plurality of beam portions 2a. On each of the beam portions 
2a of the substrate 2, there is formed a piezoelectric/electrostrictive 
actuator unit each consisting of the first electrode film 4, the 
piezoelectric/electrostrictive film 6 and the second electrode film 8. 
In the example of FIG. 6, the two piezoelectric/electrostrictive actuator 
units are superposed on each other on the same major surface of the 
ceramic substrate 2. More specifically, the first 
piezoelectric/electrostrictive actuator unit (4, 6, 8) is formed on the 
substrate 2, and the second piezoelectric/electrostrictive actuator unit 
(4, 6, 8) is formed on the first piezoelectric/electrostrictive actuator 
unit, such that the two first electrode films 4, 4, sandwich the two 
piezoelectric/electrostrictive films 6, 6 and the single common second 
electrode film 8, which separates the two films 6,6 from each other. The 
two piezoelectric/electrostrictive actuator units and the substrate 
constitute an integrally formed lamination. 
The piezoelectric/electrostrictive actuator illustrated in FIG. 7 uses a 
relatively large ceramic substrate 2, on which a plurality of 
piezoelectric/electrostrictive actuator units (4, 6, 8) are formed in two 
rows parallel to the length of the substrate, such that the 
piezoelectric/electrostrictive actuator units of each row are disposed at 
desired spacing distances. Each piezoelectric/electrostrictive actuator 
unit is a laminar structure consisting of the first and second electrode 
films 4, 8 and the piezoelectric/electrostrictive film 6 sandwiched 
between the two electrode films 4, 8, as described above. 
In operation of the piezoelectric/electrostrictive actuators as illustrate 
above, a voltage is applied between the first and second electrode films, 
4, 8, so that the piezoelectric/electrostrictive film 6 is exposed to an 
electric field. As a result, the piezoelectric/electrostrictive film 6 is 
subject to strains due to the transverse mode of converse piezoelectric or 
electrostrictive effect produced by the electric field whereby the film 6 
undergoes a flexural or bending displacement and produces a force in the 
direction perpendicular to the plane of the ceramic substrate 2. 
In the piezoelectric/electrostrictive actuator constructed according to the 
present invention, the electrode films 4, 8 and the 
piezoelectric/electrostrictive film 6 are formed of suitable electrode and 
piezoelectric or electrostrictive materials, so as to provide a laminar 
structure of the piezoelectric/electrostrictive actuator unit, which is 
integrated, by heat treatment, with the ceramic substrate 2 that functions 
as an oscillating or otherwise operating plate. Namely, the 
piezoelectric/electrostrictive actuator unit is bonded to the ceramic 
substrate in the fabrication process, without using a bonding adhesive. 
There will be descried the fabrication process of the present actuator. 
Initially, the first electrode film 4, piezoelectric/electrostrictive film 
6 and second electrode film 8 are formed on the ceramic substrate 2, by a 
method selected from the various known film forming techniques, which 
include thick-film forming processes such as screen printing, coating 
processes such as dipping, spinning or spraying, and thin-film forming 
processes such as sputtering, ion-plating, vacuum vapor deposition, CVD 
and plating. Although the method of forming these films 4, 6, 8 is not 
limited to those indicated above, the screen printing and dipping methods 
are preferred for forming the piezoelectric/electrostrictive film 6, since 
these methods may use a paste or slurry whose major component consists of 
a piezoelectric or electrostrictive material, so as to give the film 6 the 
properties desired for the actuator. The films 4, 6, 8 may be usually 
patterned in a screen printing or photolithographic process. However, the 
films may be formed to desired shapes by removing unnecessary portions of 
the applied masses of the electrode and piezoelectric or electrostrictive 
materials, by laser machining, or slicing or other mechanical machining 
process. In particular, it is desirable to concurrently machine the 
ceramic substrate and the initially applied films into desired shapes, by 
the laser or mechanical machining process. This process facilitates the 
patterning of the piezoelectric/electrostrictive actuator units that are 
formed with high integration density on the substrate. 
The construction and the shape or configuration of the actuator are not 
limited to those illustrated in the drawings, but may be suitably 
determined depending upon the application of the relevant actuator. For 
example, the actuator may have a polygonal shape such as triangle, square, 
pentagon or hexagon, a generally round shape such as circle, ellipse or 
annulus, or any special shapes, which include a comb-like or lattice-like 
cellular configuration, and a combination of the two or more shapes 
indicated above. The principle of the present invention can be 
advantageously embodied as an actuator having two or more 
piezoelectric/electrostrictive actuator units formed on the same surface 
of the substrate, as illustrate in FIGS. 3-7. In particular, the comb-like 
arrangements of the piezoelectric/electrostrictive actuator units as shown 
in FIGS. 3-5 are advantageous for increased integration density, and 
improved displacement and force characteristics. 
Where the actuator has a relatively large number of 
piezoelectric/electrostrictive actuator units on the same substrate, the 
spacing pitch of the piezoelectric/electrostrictive actuator units is 
preferably 3000 .mu.m or less, more preferably 1000 .mu.m or less, and 
most preferably 500 .mu.m or less, so that the actuator has a considerably 
high integration density of the piezoelectric/electrostrictive actuator 
units. 
The electrode and piezoelectric/electrostrictive films 4, 6, 8 formed on 
the ceramic substrate 2 by the selected method as described above may be 
either heat-treated in different steps for integration with the substrate 
2 after each of these films is formed, or alternatively concurrently 
heat-treated in one step for integration with the substrate after at least 
two or all of the films are formed into a laminar structure on the 
substrate 2. However, the heat-treatment is not essential, when the film 8 
is formed by ion-plating, sputtering, vacuum vapor deposition, CVD or 
plating. The heat-treatment temperature for integration of the films 4, 6, 
8 with the ceramic substrate 2 is generally within a range between 
800.degree. C. and 1400.degree. C., preferably within a range between 
1100.degree. C. and 1400.degree. C. To avoid a change in the composition 
of the piezoelectric/electrostrictive material during heat-treatment of 
the film 6, it is desirable to control the heat-treatment atmosphere, by 
heating with the evaporation source of the piezoelectric/electrostrictive 
material. 
The ceramic composition for the ceramic substrate 2 may be either an oxide 
or a non-oxide insulating or dielectric material, which has a high value 
of mechanical strength and which can be heat-treated. Preferably, the 
major component of the ceramic composition consists of at least one 
material selected from among aluminum oxide, magnesium oxide, zirconium 
oxide, aluminum nitride and silicon nitride. For the substrate to exhibit 
excellent characteristics with a relatively small thickness, it is 
desirable that the ceramic composition for the substrate contains aluminum 
oxide or zirconium oxide as a major component. Further, the content of a 
glass material, such as silicon oxide or dioxide (SiO, SiO.sub.2), 
contained in the ceramic composition is preferably no more than 10% by 
weight, and more preferably no more than 3% by weight. The upper limit of 
the silicon oxide or dioxide is important for preventing reaction thereof 
with the piezoelectric or electrostrictive material during heat-treatment 
process, which reaction influences the characteristics of the actuator 
produced. 
According to a finding of the applicants, to assure a high level of 
operating response and a large amount of flexural or bending displacement 
or distortion, the thickness of the ceramic substrate 2 is preferably no 
more than 100 .mu.m, more preferably no more than 50 .mu.m, and most 
preferably no more than 30 .mu.m, and the Young's modulus of the substrate 
is preferably between 1.5.times.10.sup.6 kg/cm.sup.2 and 
4.5.times.10.sup.6 kg/cm.sup.2, and more preferably between 
2.0.times.10.sup.6 kg/cm.sup.2 and 4.0.times.10.sup.6 kg/cm.sup.2. 
The green sheet for the ceramic substrate 2 may be fired before the films 
4, 6, 8 are formed on the substrate, or alternatively at least one of the 
films may be formed on the green sheet before the green sheet is co-fired 
with the films. However, the films are desirably applied to the sintered 
ceramic substrate 2, so as to minimize the undesirable warpage or bucking, 
and dimensional error of the substrate. 
Like the piezoelectric/electrostrictive actuator as a whole, the ceramic 
substrate 2 may have a suitable shape or configuration, depending upon the 
application of the actuator. For example, the substrate 2 has a polygonal 
shaped such as triangle or square, a generally round shaped such as 
circle, ellipse and annulus, or any special shapes which include a 
comb-like or lattice-like shape, and a combination of the two or more 
shapes indicated above. 
The electrode films 4, 8, in particular, the lower electrode film 4, may be 
formed of any material which is resistant to a high-temperature oxidizing 
atmosphere and which does not cause a reaction with the materials of the 
piezoelectric/electrostrictive film 6, which reaction may change the 
composition of the film 6, when the film 6 is heat-treated. Described more 
specifically, the electrode films 4, 8 may consist essentially of at least 
material selected from the group consisting of: (a) at least one noble 
metal from the group consisting of platinum, rhodium, palladium and 
iridium; (b) an alloy including at least one these noble metals, such as 
platinum-palladium, platinum-gold, platinum-silver, platinum-rhodium, 
platinum-iridium, and palladium-silver; (c) a mixture of at least one of 
those noble metals and at least one additive selected from the group 
selected from aluminum, copper, bismuth, titanium, zirconium, silver, 
magnesium, vanadium, iron, palladium and lead, or at least one compound 
thereof; and (d) a mixture of the above-indicated alloy and at least one 
additive indicated above or at least one compound thereof. The electrode 
films 4, 8 should not include any inorganic bonding adhesive or agent such 
as glasses, and are heat-treated with the substrate 2 at a suitable 
temperature so that the films 4, 8 and the substrate 2 are integrally 
laminated. Where the upper electrode film 8 is formed on the 
piezoelectric/electrostrictive film 6 after the film 6 is fired or 
sintered, the film 8 may be formed of an electrically conductive material 
which includes at least one material other than the materials indicated 
above at (a) through (d). Such electrically conductive material includes 
at least one material selected from the group consisting of gold, silver, 
copper, aluminum, nickel, titanium and chromium. It is particularly 
desirable to use a mixture of the ceramic material for the substrate 2 and 
the ceramic material for the piezoelectric/electrostrictive film 6, as a 
ceramic additive to be added to the material for the electrode films 4, 8. 
The use of this ceramic mixture increases adhesion between the films, 
without relying on a bonding adhesive as described above. 
While the thickness of the first and second electrode films 4, 8 of each 
piezoelectric/electrostrictive actuator unit are suitably selected 
depending upon the application of the actuator, the thickness of each 
electrode film is generally no more than 15 .mu.m, and preferably no more 
than 5 .mu.m. 
The piezoelectric/electrostrictive film 6 used for the present actuator may 
be formed of any piezoelectric or electrostrictive material which produces 
strain due to the reverse piezoelectric effect or the electrostrictive 
effect, as well known in the art. The piezoelectric or electrostrictive 
material may be a crystalline or noncrystalline material, a semi-conductor 
material, or a dielectric or ferroelectric ceramic material. Further, the 
piezoelectric or electrostrictive material may either require a treatment 
for initial polarization or poling, or may not require such a polarization 
treatment. 
The piezoelectric/electrostrictive film 6 may consist essentially of a 
compound or a mixture or solid solution of compounds, which compound or 
compounds is/are selected from the group consisting of: lead zirconate 
titanate (PZT); lead magnesium niobate (PMN); lead nickel niobate (PNN); 
lead manganese niobate; lead antimony stannate; lead titanate; lead 
zirconate; barium titanate; lead niobate; and barium niobate. Further, the 
composition of the film 6 may include a material which consists 
essentially of at least one oxide or compound of lanthanum (La), barium 
(Ba), niobium (Nb), zinc (Zn), nickel (Ni), lithium (Li), cerium (Ce), 
cadmium (Cd), cobalt (Co), chromium (Cr), antimony (Sb), iron (Fe), 
yttrium (Y), tantalum (Ta), tungsten (W), strontium (Sr), magnesium (Mg), 
calcium (Ca), bismuth (Bi), tin (Sn) and manganese (Mn). Like the 
electrode films 4, 8, the piezoelectric/electrostrictive film 6 is 
heat-treated without any inorganic or organic bonding agents of adhesives, 
so that the film 6 is integrated with the substrate 2 and the electrode 
films 4, 8. 
In view of the construction of the piezoelectric/electrostrictive actuator 
according to the present invention, the piezoelectric constant 
.vertline.d.sub.31 .vertline. of the material used for the 
piezoelectric/electrostrictive film 6 is desirably at least 
50.times.10.sup.-12 C/N! and more desirably at least 100.times.10.sup.-12 
C/N!, for assuring excellent operating characteristics of the actuator. 
Further, the thickness of the film 6 is preferably no more than 100 .mu.m, 
more preferably no more than 50 .mu.m, and most preferably no more than 30 
.mu.m, for reducing the required level of a voltage to be applied to the 
film 6 through the films 4, 8. 
While the presently preferred embodiments of the 
piezoelectric/electrostrictive actuator of this invention have been 
described in detail by reference to the drawings, it is to be described in 
detail by reference to the drawings, it is to be understood that the 
invention is not limited to the details of the illustrated embodiments. 
It is also to be understood that the present invention may be embodied with 
various changes, modifications and improvements, which may occur to those 
skilled in the art, without departing from the spirit and scope of the 
invention defined in the following claims.