Piezoelectric element

Details Piezoelectric element Piezoelectric element

1999-03-17

2001-04-17

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S330000, C310S332000, C310S366000

Reexamination Certificate

active

06218770

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric element, more particularly to a piezoelectric element used in technological fields such as data processing and communications and functioning as a resonator, a filter and the like.
2. Description of the Related Art
A piezoelectric element includes, for instance, a piezoelectric member made of piezoelectric ceramic. This type of piezoelectric member is adapted to vibrate in a square type vibration mode or a thickness vibration mode and is widely used.
FIG. 10
shows a piezoelectric element
1
adapted to vibrate in a square type vibration mode. The piezoelectric element
1
includes a square plate-like piezoelectric member
2
, and electrodes
3
and
4
are provided on the major surfaces of the piezoelectric member
2
.
The piezoelectric member
2
is polarized in the direction indicated by an arrow
5
, and square type vibrations are excited by applying an electrical field between the electrodes
3
and
4
via terminals
6
and
7
.
FIG. 11
shows a piezoelectric element
8
utilizing an unstiffened vibration mode. The piezoelectric element
8
includes a thin plate-like piezoelectric member
9
, and electrodes
10
and
11
are provided on the major surfaces of the piezoelectric member
9
.
The piezoelectric member
9
is polarized in the direction indicated by an arrow
12
, and longitudinal vibration in the unstiffened mode is excited by applying an electrical field between the electrodes
10
and
11
via terminals
13
and
14
.
FIG. 12
shows a piezoelectric element
15
utilizing a stiffened vibration mode. The piezoelectric element
15
includes a thin plate-like or rod-like piezoelectric member
16
, and electrodes
17
and
18
are provided on the major surfaces of the piezoelectric member
16
.
The piezoelectric member
16
is polarized in the direction indicated by the arrow
19
, and longitudinal vibration in a stiffened mode is excited by applying an electrical field between the electrodes
17
and
18
via terminals
20
and
21
.
However, the piezoelectric elements
1
,
8
and
15
shown in
FIG. 10
, FIG.
11
and
FIG. 12
respectively have the following disadvantages.
Firstly, the piezoelectric element
1
shown in
FIG. 10
is comparatively large, which prevents a circuit or device incorporating such element
1
from being made small. For instance, when the piezoelectric element
1
is used as an AM middle frequency filter in a 455 kHz radio, the piezoelectric member
2
must be a square plate-like piezoelectric member having approximate dimensions of 5 mm×5 mm, which is a considerable barrier to miniaturizing an electronic circuit or device incorporating such piezoelectric member
2
.
Furthermore, in the case of the piezoelectric element
8
shown in
FIG. 11
, when the piezoelectric member
9
has a thin plate-like shape which is approximately 4 mm in length in order to make the piezoelectric element
8
small, a wide filter pass band cannot be achieved.
Furthermore, in the piezoelectric element
15
shown in
FIG. 12
, when the piezoelectric member
16
has thin rod-like shape of, for instance, approximately 4 mm in length in order to make the piezoelectric element
15
small, impedance is too high and the impedance cannot easily be matched with the impedance of a circuit incorporating the element
15
.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric element which has a very small size, is easily impedance-matched to a circuit, and has high performance.
A preferred embodiment of the present invention provides a piezoelectric element, including a plate-like or rod-like piezoelectric member, and at least one interdigital electrode including two comb-shaped electrodes and provided on at least one surface of the piezoelectric member, wherein the piezoelectric member is polarized or aligned by applying an electrical field between the two comb-shaped electrodes, and the piezoelectric element is adapted to vibrate in a stiffened mode by applying an electrical field between the two comb-shaped electrodes.
According to the above described structure and arrangement, since the piezoelectric element is vibrated in a stiffened mode and thereby expanded and contracted in the longitudinal direction thereof, the piezoelectric element of preferred embodiments of the present invention can be made small in size, enabling an electronic device incorporating such element to be very small also, and is easily impedance-matched to a circuit, and achieves a wide pass band when, for instance, used as a filter or the like.
In the above described piezoelectric element, the interdigital electrodes are preferably respectively provided on at least two surfaces of the piezoelectric member. The two surfaces of the piezoelectric member are preferably substantially parallel to each other.
According to the above described structure and arrangement, the piezoelectric member can be more efficiently polarized and aligned, and in addition, it can be more efficiently excited. Furthermore, when interdigital electrodes are provided on two surfaces of a piezoelectric member so as to be substantially parallel to each other, it is easier to form the interdigital electrodes than when the interdigital electrodes are provided on two surfaces which are not substantially parallel. Moreover, such surfaces can be more easily polished prior to forming the interdigital electrodes thereon.
In the above described piezoelectric element, one of the comb-shaped electrodes constituting the interdigital electrode provided on one of the two surfaces of the piezoelectric member, is preferably electrically connected to the other of the comb-shaped electrodes constituting the interdigital electrode provided on the other of the two surfaces of the piezoelectric member.
According to the above described structure and arrangement, the piezoelectric member can be more efficiently excited.
In the above described piezoelectric element, one of the surfaces of the piezoelectric element on which the interdigital electrode is provided is preferably a polished surface.
According to the above described structure and arrangement, the polished surface is smooth, making it possible to effectively use photolithography to form the interdigital electrodes.
In the above described piezoelectric element, a distance between two of the electrode fingers of the comb-shaped electrodes of the interdigital electrode may be different from a distance between the other two of the electrode fingers of the comb-shaped electrodes of the interdigital electrode. In this case, the distance between two of the electrode fingers of the comb-shaped electrodes of the interdigital electrode located at a center portion of the piezoelectric member is preferably larger than the distance between the other two of the electrode fingers of the comb-shaped electrodes of the interdigital electrode located at an end portion of the piezoelectric member.
According to the above described structure and arrangement, the portions where the distance between the electrode fingers is large has high impedance, with a resulting improvement in the coefficient of electromechanical coupling, while the portions where the distance between the electrode fingers is small helps to prevent increased impedance of the piezoelectric element. When the distance between electrode fingers is made greater in the center portion of the piezoelectric member and smaller at the end portion of the piezoelectric member, the level of polarization is increased in the center, as the center is a portion which considerably influences the coefficient of electromechanical coupling, thereby further raising the coefficient of electromechanical coupling and achieving greater effectiveness.


REFERENCES:
patent: 2540194 (1951-02-01), Ellett
patent: 2540412 (1951-02-01), Adler
patent: 3071841 (1963-01-01), Brussaard et al.
patent: 3114849 (1963-12-01), Poschenrieder
patent: 3252017 (1966-05-01), Bartels
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