Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-07-28
2002-03-12
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S357000, C029S025350
Reexamination Certificate
active
06356008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of polarizing a piezoelectric body, and more particularly, to a method of polarizing a piezoelectric body having a structure in which a plurality of piezoelectric layers and internal electrodes are alternately laminated, wherein the piezoelectric layers on both sides of each internal electrode are polarized in opposite directions.
2. Description of the Related Art
Conventionally, a piezoelectric resonator has been provided in which design flexibility of characteristics is substantial, a spurious response is small, and the difference &Dgr;f between the resonance and anti-resonance frequencies can be increased (Japanese Unexamined Patent Application Publication No. 10-4330). The piezoelectric resonator includes a plurality of piezoelectric layers and internal electrodes which are alternately laminated, and the piezoelectric layers on both sides of each internal electrode are polarized in opposite directions. For piezoelectric resonators having such a structure, the polarization degrees of the piezoelectric layers have a substantial effect on the characteristics of the resonator. Therefore, the amounts of scatter in polarization degrees within each device and between devices must be minimized as much as possible.
The polarization treatment of this type of monolithic piezoelectric bodies is carried out as shown in
FIG. 1. A
piezoelectric body
1
is a block-shaped piezoelectric ceramic. Here, the piezoelectric body
1
composed of four piezoelectric layers
1
a
to
1
d
is shown for explanation, and may include at least five layers. Internal electrodes
2
a
to
2
c
are provided between the piezoelectric layers
1
a
to
1
d
, respectively. The internal electrodes
2
a
to
2
c
are led out alternately to the outer surfaces of the piezoelectric body
1
and connected to external electrodes
3
and
4
, respectively. By applying a direct current electric field between the external electrodes
3
and
4
, the piezoelectric layers
1
b
and
1
c
on both sides of the internal electrode
2
b
are polarized in opposite directions, respectively, as indicated by the arrows P to obtain a predetermined polarization degree.
However, according to the method shown in
FIG. 1
, the electric field is concentrated at the end portions of the internal electrodes
2
a
to
2
c
, which causes the polarization degree distribution to be non-uniform.
FIG. 2
shows an example of the polarization degree distribution, in which the slanted lines represent the polarization degrees. As seen in
FIGS. 1 and 2
, when the electric fields are applied to the piezoelectric body
1
in the thickness direction, the polarization degree is substantially increased in the four corners of the piezoelectric body
1
. Thus, a uniform polarization degree distribution cannot be obtained. As a result, if the piezoelectric body having such a non-uniform polarization degree distribution is cut in substantially rectangular elements for use as completed devices, the peripheral portions of the piezoelectric body cannot be used. That is, the use range (yield of the piezoelectric body) is substantially limited.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a method of polarizing a piezoelectric body in which the polarization degree distribution of a monolithic piezoelectric body produced by the method is substantially uniform, and the yield is, therefore, greatly improved.
According to a first preferred embodiment of the present invention, a method of polarizing a piezoelectric body having a plurality of piezoelectric layers and internal electrodes which are alternately laminated, includes the steps of applying an electric field to the piezoelectric body to polarize the body uniformly in the thickness direction thereof, and applying electric fields in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby only the piezoelectric layer on one side of the internal electrode is polarization-inverted.
According to a preferred embodiment of the present invention, a method of polarizing a piezoelectric body having a plurality of piezoelectric layers and internal electrodes are alternately laminated includes the steps of applying a first electric field to the piezoelectric body to polarize the body uniformly in the thickness direction thereof, applying a second electric field in the reverse direction with respect to the direction of the first electric field, whereby the piezoelectric body is polarization-inverted uniformly in the thickness direction, and applying electric fields in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby only the piezoelectric layer on one side of the internal electrode is polarization- inverted.
Further, according to a third preferred embodiment of the present invention, a method of polarizing a piezoelectric body having a plurality of piezoelectric layers and internal electrodes which are alternately laminated, includes the steps of applying electric fields in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby the piezoelectric layers on both sides of the internal electrode are polarized in the opposite directions, and applying electric fields in the reverse directions with respect to the directions of the above electric fields to the piezoelectric layers on both sides of the internal electrode, respectively, whereby the piezoelectric layers on both sides of the internal electrode are polarization-inverted.
According to the first preferred embodiment of the present invention, after an electric field is applied to the piezoelectric electric body to polarize the body uniformly in the thickness direction, electric fields are applied in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby the piezoelectric layer only on one side of the internal electrode is polarization-inverted. By polarization-inverting the piezoelectric body, the phenomenon in which the polarization degree is increased in the four corners of the piezoelectric body is substantially suppressed, and the non-uniformity of the initial polarization degree distribution is substantially reduced. Therefore, when the substantially rectangular elements produced by cutting the piezoelectric body are used, the available range is significantly widened and the yield is greatly enhanced.
FIG. 3
illustrates the change of a polarization degree distribution, caused by initial polarization and polarization-inversion.
By the initial polarization, the polarization degrees increase in the end portions of the piezoelectric body with a large difference &Dgr;P
1
between the polarization degrees in the end portions and the approximate center portion. Subsequently, the polarization-inversion is carried out, so that the polarization axial direction is inverted, and simultaneously, the difference &Dgr;P
2
between the end portions and the approximate center portion is reduced. The electric field intensity at the polarization-inversion is substantially equal to that at the initial polarization, and the polarization time is shorter than that of the initial polarization. The maximum polarization degrees P
max
at the initial polarization and the polarization-inversion are equal to each other.
FIG. 4
shows the variation of a polarization degree when positive and negative electric fields are applied to a piezoelectric body.
First, when a positive electric field is applied at point I, the polarization degree is increased to point II. When the application of the electric field is stopped, the polarization degree is stabilized at point III. The polarization degree at the point III is a residual polarization degree. Subsequently, when an electric field is applied in the negative direction, the polarization degree is decreased to substantially zero (point IV
Keating & Bennett LLP
Medley Peter
Ramirez Nestor
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