Composite piezoelectric multilayer element and method of manufacturing such an element

The invention relates to a composite piezoelectric multilayer element of which, in operation, only a part is piezoelectrically active, and to a method of manufacturing such an element. In accordance with the invention, the piezoelectrically inactive part has larger dimensions in one or more directions than the part which, in operation, is piezoelectrically active when a voltage is applied across the electric connections of the element. Moreover, the inactive part is built up of ceramic foils and electrode layers whose thickness and composition correspond to the thickness and composition of the part of the element which, in operation, is piezoelectrically active. The element in accordance with the invention can be manufactured in a simple and inexpensive manner.

BACKGROUND OF THE INVENTION 
The invention relates to a composite piezoelectric multilayer element of 
which, in operation, only a part is piezoelectrically active, as well as 
to a method of manufacturing such a multilayer element. The invention 
relates, in particular, to actuators based on a piezoelectric multilayer 
element of which, in operation, a part is piezoelectrically active and 
another part is not. 
An actuator of the type mentioned in the opening paragraph is disclosed in 
EP-A-0448349. Said actuator is built up of a number of stacked foils of a 
ceramic material having piezoelectric properties and of electrode layers, 
said foils and electrode layers being stacked so that, in operation, a 
part of the ceramic foils is not piezoelectrically active. The thickness 
of the ceramic foils employed in the known actuator is so small that when 
use is made of electrode layers whose dimensions correspond to those of 
the foils, there is a great risk of flashover between closely spaced 
electrode layers when the actuator is in operation. For this reason, the 
electrode layers between the ceramic foils in the known actuator have a 
smaller circumferential dimension than the ceramic foils. However, in 
actuators of this type having larger dimensions, this may lead to 
deformations during operation, because a part of the foils is not 
piezoelectrically active. 
In EP-A-0448349, it is proposed to solve this problem by building up the 
actuator from sub-units, either presintered or not, between which 
sub-units intermediate layers of a synthetic resin material or a soldering 
material are provided which have the same or smaller dimensions than the 
electrode layers. The slits formed in this process between the edge 
portions of the sub-units can be filled with a filler material such as a 
thermocuring resin. The parts of the known actuator which, in operation, 
are not piezoelectrically active, only serve as electrical insulators. 
From the contents of EP-A-0448349, however, it cannot be derived how a 
piezoelectric multilayer element of which, in operation, only a part is 
piezoelectrically active while the inactive part is mechanically loaded, 
for example for rigidly fixing it and for the transmission of motions, 
would have to be built up to preclude undesirable deformations in the 
manufacture as well as in operation, and how such a multilayer should be 
manufactured. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a multilayer element of the 
type mentioned in the opening paragraph which, in operation, does not 
exhibit undesirable deformations and which can be manufactured in a simple 
manner without the development of deformations in the manufacture or in 
operation, and which also has advantages in other respects. 
In accordance with the invention, this object is achieved by a composite 
multilayer element, which is characterized in that the piezoelectrically 
inactive part has larger dimensions in one or more directions than the 
part which, in operation, is piezoelectrically active when a voltage is 
applied across the electric connections of the element, and in that said 
other part is built up of ceramic foils and electrode layers whose 
thickness and composition correspond to the thickness and composition of 
the part of the element which, in operation, is piezoelectrically active. 
It is noted that "built up of ceramic foils and electrode layers whose 
thickness and composition correspond . . . " is to be understood to mean 
herein that both the composition of the ceramic foils and the electrode 
layers and the thickness of these layers in both the piezoelectrically 
active and the piezoelectrically inactive part of the composite multilayer 
element in accordance with the invention are chosen to be such that at 
least the sinter behavior of both parts of the element is the same or 
substantially the same. Preferably, the thickness and composition of the 
ceramic foils and the electrode layers is the same. The difference in 
piezoelectric behavior can be brought about, for example, by providing 
electrode layers, in the part which in operation is piezoelectrically 
inactive, in accordance with such a pattern that, upon applying an 
electric voltage across the electric connections of the multilayer 
element, a voltage difference between the electrode layers in the inactive 
part is not created. To this end, the electrode layers of the inactive 
part are preferably interrupted in accordance with a pattern. 
The parts of the element having larger dimensions, for example a larger 
thickness, can for example be used to rigidly fix the element in a device 
and to transmit motion to another part. For reasons relating to energy, 
the piezoelectrically active part is preferably thinner than the 
mechanically loaded parts which are piezoelectrically inactive. 
An example of such an element is an actuator of which the rigidly fixed 
part has a larger thickness than the part which, in operation, is 
piezoelectrically active when a voltage is applied across the electric 
connections of the actuator. Also the part of the actuator which serves to 
transmit motion and which engages a part of a device which includes the 
actuator, can have, for example, a larger thickness and/or width than the 
part which, in operation, is piezoelectrically active. In actuators of 
this type, less energy is required to bring about a movement than in 
actuators of which all parts have dimensions which are actually required 
only to withstand mechanical forces such as clamping forces. 
A composite multilayer element in accordance with the invention can be 
manufactured by a method which comprises the following steps: 
manufacturing a foil of a ceramic material having piezoelectric properties, 
applying electrode layers on to the foil, 
stacking a number of foils carrying electrode layers, 
pressing the stack of foils to form a green multilayer body, 
if necessary, mechanically shaping the green multilayer body, which method 
is characterized in accordance with the invention in that a composite 
piezoelectric multilayer element is manufactured by producing two or more 
green multilayer bodies having different shapes from ceramic foils and 
electrode layers of corresponding thickness and composition, and, 
subsequently, interconnecting them by stacking the green multilayer bodies 
in a suitable manner and, next, pressing, drying and sintering the 
composite green body thus formed, whereafter the composite multilayer 
element thus formed is provided with electric connections on one or more 
sides, said operations being carried out in such a way that, in operation, 
only a part of the element is piezoelectrically active when a voltage is 
applied across the electric connections. 
To achieve the desired difference in piezoelectric behavior, the electrode 
layers in the piezoelectrically inactive part of the element can be 
provided on the ceramic foils in accordance with such a pattern that, upon 
application of an electric voltage across the electric connections of the 
finished product, a voltage difference cannot develop between the ceramic 
layers in the inactive part. For this purpose, the relevant electrode 
layers, for example, may be interrupted in accordance with a pattern. 
By means of the invention, it is achieved that undesirable deformations of 
the element occur neither in the manufacture, for example during 
sintering, nor in operation. The multilayer element in accordance with the 
invention essentially comprises a monolithic ceramic block without 
intermediate layers of a material other than the ceramic material of the 
foils. This means that the finished multilayer element does not comprise 
non-ceramic connection layers, such as soldering metal, glass or synthetic 
resin, between the piezoelectrically active and piezoelectrically inactive 
parts in the finished product. This is considered to be an important 
advantage because it enables the green multilayer bodies assembled into an 
inventive element to be sintered simultaneously. In this case, no 
additional. steps are necessary to form the connection between the green 
multilayer bodies. 
Research leading to the invention has revealed that the required strength 
and dimensional accuracy of the multilayer element cannot be obtained by 
mechanically reducing the dimensions of a specific part in a sintered 
multilayer body in one or more directions, for example by grinding out 
apertures or a similar action. 
In accordance with a preferred embodiment of the method in accordance with 
the invention, one or more of the green multilayer bodies are given a 
suitable shape before being interconnected. This can be carried out, for 
example, by punching, drilling, laser shaping and the like. 
In the manufacture of the composite green multilayer body, permanent or 
non-permanent binder layers can be provided between the individual green 
multilayer bodies, if necessary. Permanent binder layers must be made of a 
ceramic material whose composition corresponds to that of the ceramic 
materials of the foils from which the green multilayer bodies are built 
up. A non-permanent binder layer may be made, for example, from the 
organic binder of the ceramic material, which is burned out in the 
sintering operation. Surprisingly, it has been found that if the green 
bodies are pressed on to each other at an elevated temperature, such a 
binder layer is generally not necessary to obtain a proper bond between 
parts of the multilayer element originating from various green bodies. 
In accordance with a further embodiment of the method in accordance with 
the invention, the composite multilayer element is divided into smaller 
elements, for example, by sawing, cutting with a laser or a similar 
action, before electric connections are provided. 
These and other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter.

DETAILED DESCRIPTION 
FIG. 1 shows a composite green multilayer body 1 which is composed of a 
green multilayer body 2 and a green multilayer body 3. Said green 
multilayer bodies 2 and 3, having, for example, a largest dimension of 
50.times.50 mm and a thickness of 0.5 mm, can be manufactured by means of 
any method which is known per se. First, a ceramic foil of a ceramic 
material having piezoelectric properties is manufactured in a thickness, 
for example, of 20 .mu.m. Such a foil can be obtained by suspending a 
quantity of powder of metal oxides, for example PbO, TiO.sub.2 and 
ZrO.sub.2, in a composition and ratio suitable for obtaining piezoelectric 
properties, in an alcohol, such as isopropanol. Such mixtures are known 
per se to those skilled in the art and do not form the subject of the 
invention. 
The suspension thus formed is intensively ground in a ball mill for a 
length of time, for example 6 hours. After the grinding operation, the 
suspending agent is removed by drying the mixture at an elevated 
temperature, for example 50.degree.-120 .degree. C. The dried mixture is 
subsequently calcined in air for 2-24 hours at 600.degree.-1,000.degree. 
C. (calcining). Subsequently, an organic binder is added to said calcined 
mixture. The mixture is then suspended in alcohol, for example 
isopropanol, and ground in a ball mill for a length of time, for example 
5-20 hours. The ground suspension is subsequently used to cast a foil. 
After drying of the foil, an electrode layer having a thickness, for 
example, of 0.5-3 .mu.m is provided on the foil by means of screen 
printing. For this purpose, use can be made, for example, of a 
conventional silver-palladium-containing screen printing paste. After 
drying of the paste, the foils carrying electrode layers are stacked and, 
subsequently, pressed to form a green multilayer body 2, for example at a 
pressure of 8-10 kgf/mm.sup.2. 
A green multilayer body 3 is obtained by manufacturing a green multilayer 
body in the same manner and, subsequently, providing the desired recesses 
or windows 4 and 5 by means of punching or otherwise. In this case, the 
electrode layers provided on the ceramic foil preferably exhibits 
patterned interruptions. The green multilayer bodies 2 and 3 are united by 
stacking them in a suitable manner and pressing the composite green 
multilayer body 1 at a pressure, for example, of 8-10 kgf/mm.sup.2, at an 
elevated temperature, if necessary. In practice it was found that if the 
pressing operation was carried out at temperatures between 50.degree. and 
100.degree. C., the presence of a binder layer between the green bodies 2 
and 3 was not necessary to obtain a good adhesion. Further treatments to 
which the assembly is subjected include, successively, drying, calcining 
to remove binder residue and sintering in an oxygen-containing atmosphere 
at an elevated temperature, generally approximately 1,200.degree. C. If 
necessary, the sintered multilayer element thus obtained can subsequently 
be divided into smaller elements. Subsequently, external electric 
connections are provided. Next, the elements are polarized in a suitable 
manner. 
FIG. 2 is a sectional view, not to scale, of an actuator in accordance with 
the invention. Said actuator comprises parts 6 and 7 having larger 
dimensions in the thickness direction, and a part 8 having a smaller 
dimension in the thickness direction. Part 6 can be used, for example, for 
rigidly fixing, and part 7 can be used to transmit a movement to another 
part of the construction of which the actuator forms a part. This can be, 
for example, a printer. The movement is brought about by the part 8, 
which, upon applying a voltage across the electric connections 9 and 10, 
is extended or shortened, dependent upon the direction of polarization and 
the sign of the voltage. 
To focus the attention, a number of white and black layers are 
schematically indicated in the drawing, said white layers of the actuator 
indicating ceramic material and the black layers indicating electrode 
layers. The electrode layers of body 2 (see FIG. 1) of the element extend 
from one of the electric connections to the beginning of a thickened part 
6 or 7 to which the other connection is secured. Said thickened parts 6 
and 7 comprise a number of electrode layers which are interrupted in 
accordance with a pattern. Said patterned interruption precludes the 
development of a voltage difference between the intermediate ceramic 
layers when a voltage is applied across the electric connections 9 and 10. 
As a result, said parts 6 and 7 remain piezoelectrically inactive when a 
voltage is applied across the electric connections. Surprisingly, it has 
been found that this patterned interruption of the electrode layers, and 
the double thickness of the ceramic foils between the electrode layers at 
both ends of the multilayer element, do not give rise to deformation 
during sintering or in operation. 
The actuator shown in a longitudinal sectional view in FIG. 2 can be 
obtained, for example, by dividing a sintered multilayer element, as shown 
in FIG. 1, into smaller elements, for example, by sawing. Two of a number 
of possible saw cuts are shown in FIG. 1. The part situated between the 
saw cuts A and B can be treated further to form an actuator as shown in 
FIG. 2. To this end, electric connections 9 and 10 in the form of metal 
layers, for example of gold, are provided on the element by means of 
vacuum evaporation, chemical vapor deposition, sputtering or the like. The 
electric connections 9 and 10 are connected to the electrode layers, which 
are situated between the ceramic foils in part 8. Subsequently, the 
element is polarized in such a manner that, upon applying a voltage across 
the electrode layers 9 and 10, part 8 is extended or shortened only in the 
directions indicated by the arrow. If the distance between the parts 6 and 
7 is approximately 5 mm, for example, a reduction in length of 2.5 .mu.m 
can be obtained by applying a suitable voltage across the electric 
connections 9 and 10. In this case, the piezoelectrically active part 8 
may have a thickness, for example, of 0.5 mm and a length of 5 mm, the 
thickened, inactive parts 6 and 7 may each have a thickness of 0.9 mm and 
a length of 2.5 mm. 
FIG. 3 is a schematic, side view (not to scale) of another embodiment of an 
actuator in accordance with the invention. This actuator can be obtained 
by connecting a green body 2, as shown in FIG. 1, to a green body 3 on 
both sides and subsequently carrying out the same operations as in the 
manufacture of the actuator shown in FIG. 2. When a voltage is applied 
across the electric connections 11 and 12, a piezoelectric effect in the 
form of an extension or contraction occurs only in the part 13 between the 
thickened parts 14 and 15. 
FIG. 4 is a plan view of a ceramic foil 17 on which an electrode layer 16 
having patterned interruptions is provided. This can be used to build up a 
green body 3, as shown in FIG. 1, which green body is utilized to 
manufacture the piezoelectrically inactive parts of the actuators shown in 
FIGS. 2 and 3.