Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-11-02
2002-10-08
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S328000
Reexamination Certificate
active
06462464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stacked piezoelectric device adapted to extend and contract along the height of the stack upon energization and a method of fabrication thereof.
2. Description of the Related Art
The following-described configuration of the conventional stacked piezoelectric device is well known.
As shown in
FIG. 14
, a stacked piezoelectric device
9
comprises a piezoelectric stack formed of piezoelectric layers
931
and
932
, internal electrode layers
941
and
942
alternately formed between the piezoelectric layers
931
and
932
with alternate positive and negative voltages applicable to the piezoelectric layers
931
and
932
, and a pair of side electrodes
91
formed on the sides
901
and
902
of the piezoelectric stack.
In the piezoelectric stack, the internal electrode layers
941
are arranged to be exposed to the side
901
, while the internal electrode layers
942
are arranged to be exposed to the other side
902
.
A side electrode
91
is formed on each of the sides
901
and
902
of the piezoelectric stack in such a manner as to energize the ends of the internal electrode layers
941
and
942
exposed thereto. The other side electrode electrically connected with the ends of the internal electrode layers
942
is hidden and therefore not visible in FIG.
14
.
The conventional stacked piezoelectric device
9
shown above has the problem that cracking tends to occur in the N portion of
FIG. 15
in the direction toward the side
901
from the end of the internal electrode layer
941
(A similar problem is also affects the other side
902
, not shown).
As shown in
FIG. 15
, the end portion
944
of the internal electrode layer
942
not exposed to the side
901
has a progressively tapered section in the piezoelectric stack. The end portion
943
of the internal electrode layer
941
is exposed to the side
901
.
Though not shown, the end portion of the internal electrode layer
942
is exposed to the side
902
, while the end of the internal electrode layer
941
is not exposed to the side
902
of the piezoelectric stack but located within the piezoelectric stack with the section thereof progressively tapered.
As a result, the piezoelectric layers
931
and
932
are divided into a portion M sandwiched between the internal electrode layer
941
and the internal electrode layer
942
, and a portion N in contact with either the internal electrode layer
941
or
942
.
Upon application of a voltage from the internal electrode layers
941
and
942
to the piezoelectric layers
931
and
932
, the portion M sandwiched between the internal electrode layers
941
and
942
can be displaced along the height of the stack. The portion N, however, cannot be displaced, as it is in contact with only one of the internal electrode layers
941
and
942
.
Stress develops in the portion L indicated by dashed line in
FIG. 15
constituting the boundary between the portions M and N which is in contact with the portion displaced and the portion not displaced.
Thus, the piezoelectric stack may be damaged by cracking occurring from the end portion
942
toward the side
901
.
This damage occurs especially after the stacked piezoelectric device is used for a long time or in a harsh operating environment, and has been a major cause of device deterioration.
Also, in the conventional stacked piezoelectric device
9
, the internal electrode layers
941
and
942
are configured on a part of the piezoelectric layers
931
and
932
. For this reason, a complicated and troublesome process control is required to form the internal electrode layers
941
and
942
of a predetermined size at exact positions on the piezoelectric layers
931
and
932
at the time of manufacture, and therefore simplification of the process control is desirable.
In order to obviate this problem, a method has been proposed to form each internal electrode layer over the entire surface of the corresponding piezoelectric layer.
In this configuration, the internal electrode layers and the piezoelectric layers have substantially the same area. Also, each side electrode is configured in such a manner that the ends of alternate ones of the internal electrode layers are covered with an insulative portion, and the other ends are electrically connected by a conductive portion covering the insulative portions, so that each piezoelectric layer is sandwiched between internal electrode layers of different polarities.
This configuration, however, still has the problem of durability of the piezoelectric device.
Specifically, in view of the fact that the stacked piezoelectric device is displaced along the height of the stack, stress acts on the side electrodes along the height of the stack. Since the conductive portions are formed only at the required points, the mechanical strength of the conductive portions is so low that they can easily become separated from the internal electrode layers.
As described above, with the configuration having conductive portions to energize the internal electrode layers formed over the entire surface of the piezoelectric layers, it is difficult to produce a piezoelectric device high in durability.
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the problems of the prior art described above, and the object thereof is to provide a stacked piezoelectric device having a high durability and a method of fabrication thereof with a simplified production process control.
According to a first aspect of the invention, there is provided a stacked piezoelectric device comprising:
a piezoelectric stack including piezoelectric layers adapted to extend and contract in accordance with a voltage applied thereto and internal electrode layers for supplying the applied voltage, the piezoelectric layers and the internal electrode layers being stacked alternately with each other; and
a first side electrode arranged on one side of the piezoelectric stack and a second side electrode arranged on the other side of the piezoelectric stack, the side electrodes being so configured that the internal electrode layers adjacent to each other with a piezoelectric layer therebetween are energized to different polarities;
wherein the piezoelectric layers and the internal electrode layers are configured to have substantially the same area;
wherein said internal electrode layers have the ends thereof exposed to the sides of the piezoelectric stack;
wherein the first side electrode has a first insulating portion formed to cover each of the ends of alternate ones of the internal electrode layers exposed to one side of the piezoelectric stack, a first conductive portion being arranged over the first insulating portions along the height of the piezoelectric stack;
wherein the first side electrode energizes alternate ones of the internal electrode layers;
wherein the second side electrode has a second insulating portion formed to cover each of the ends of alternate ones of the internal electrode layers not formed with the first insulating portion on the other side of the piezoelectric stack, a second conductive portion being arranged over the second insulating portions along the height of the piezoelectric stack;
wherein the second side electrode energizes alternate ones of the internal electrode layers;
wherein the first and second insulating portions are formed of an insulative resin; and
the first and second conductive portions are formed of a conductive resin.
The most notable feature of the present invention is that the piezoelectric layers and the internal electrode layers are configured to have substantially the same area, that each internal electrode layer has an end thereof exposed to a side of the piezoelectric stack, that the first and second side electrodes include the first and second insulating portions, respectively, covering the ends of the internal electrode layers and the first and second conductive portions arranged on the first and second insulating portions, respectively, and that the first and second insulating p
Kobayashi Masayuki
Mitarai Shinya
Mizuno Isao
Murai Atsushi
Sato Kazuhide
Denso Corporation
Dougherty Thomas M.
Nixon & Vanderhye PC
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