Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1998-06-03
2001-05-15
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06232698
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an energy trap piezoelectric resonator used in various resonators, oscillators, and similar devices and, more particularly, to an energy trap thickness extensional vibration mode piezoelectric resonator constructed to maximize use of harmonics of a thickness extensional vibration mode.
2. Description of the Related Art
Piezoelectric resonators are used in various piezoelectric resonator components such as piezoelectric oscillators and piezoelectric filters. Known piezoelectric resonators of this kind utilize various piezoelectric vibration modes, depending on the frequency desired.
An energy-trap piezoelectric resonator utilizing the second-order wave of a thickness extensional vibration mode is disclosed in Japanese Unexamined Patent Publication No. 117409/1989. This piezoelectric resonator is now described with reference to
FIGS. 17 and 18
.
The piezoelectric resonator shown in
FIGS. 17 and 18
is constructed by stacking ceramic green sheets
51
,
52
made of a piezoelectric material on top of each other and sintering the sheets
51
,
52
together, as shown in the exploded perspective view of
FIG. 17. A
circular excitation electrode
53
is disposed in the center of the ceramic green sheet
51
. The excitation electrode
53
is extended to an end of the ceramic green sheet
51
via an extraction electrode
54
. A circular excitation electrode
55
is disposed in the center of the top surface of the ceramic green sheet
52
. The excitation electrode
55
is extended to an end of the ceramic green sheet
52
via an extraction electrode
56
. As shown in the lower projected view of
FIG. 17
, an excitation electrode
57
is disposed on the bottom surface of the ceramic green sheet
52
. The excitation electrode
57
is extended to an end of the ceramic green sheet
52
via an extraction electrode
58
. It is noted that the electrodes
53
,
55
,
57
are only partially formed and only partially cover the respective surfaces of the green sheets
51
,
52
, respectively at a central portion thereof and do not extend across an entire width or length of the sheets
51
,
52
. That is, the circular electrodes
53
,
55
,
57
are surrounded in all directions by the surfaces of the respective green sheets
51
,
52
.
The ceramic green sheets
51
and
52
are stacked on top of each other and pressure is applied in the direction of thickness thereof. Then, the sheets
51
,
52
are sintered, thus producing a sintered body. The sintered body is then polarized. Thus, a piezoelectric resonator
60
is obtained, as shown in FIG.
18
.
In the piezoelectric resonator
60
, piezoelectric layers
61
and
62
are polarized uniformly in the direction of the arrows, i.e., in the direction of thickness.
When the device shown in
FIG. 18
is driven, the excitation electrodes
53
and
57
are connected together, and an AC voltage is applied between the excitation electrodes
53
,
57
and the excitation electrode
55
. In this way, the piezoelectric resonator
60
is driven to resonate such that the vibration energy is confined to a region where the excitation electrodes
53
,
55
,
57
overlap each other, i.e., a resonating portion A.
The prior art piezoelectric resonator
60
which is constructed to use the harmonics of a thickness extensional vibration mode is designed as an energy-trap piezoelectric resonator as mentioned above. Therefore, in order to function as an energy trap type resonator, this resonator
60
requires vibration-attenuating portions which are located so as to surround the resonating portion A in all directions for attenuating vibrations created therein. More specifically, because the circular electrodes
53
,
55
and
57
are surrounded by surfaces of the respective green sheets
51
,
52
at which vibration-attenuating portions are located, the vibration-attenuating portions have a large size compared with the size of the resonating portion. The large size and arrangement of vibration-attenuating portions in all directions around the electrodes
53
,
55
,
57
and resonating portion A are necessary to sufficiently suppress vibrations. Thus, because large vibration-attenuating portions are required to suppress vibrations, it has been difficult to reduce the size of the piezoelectric resonator
60
.
On the other hand, Japanese Unexamined Patent Publication No. 235422/1990 discloses an energy-trap piezoelectric resonator that uses a piezoelectric ceramic strip and hardly needs extra piezoelectric substrate portions surrounding the resonating portion to attenuate vibrations.
In this device shown in
FIG. 19
, an excitation electrode
72
a
and an excitation electrode
72
b
are located on the top and bottom major surfaces, respectively, of an elongated piezoelectric substrate
71
. The excitation electrodes
72
a
and
72
b
extend along the entire width and part of the length of the piezoelectric substrate
71
, and are disposed opposite to each other with the piezoelectric substrate
71
located therebetween. The electrodes
72
a,
72
b
overlap each other at an approximately central portion of the piezoelectric substrate
71
to define a resonating portion. The excitation electrodes
72
a
and
72
b
extend to longitudinal ends
71
a
and
71
b,
respectively, of the piezoelectric substrate
71
.
When the piezoelectric resonator
70
is excited into a thickness extensional vibration mode, unwanted vibrations occur due to the dimensional relation between the width W and the thickness T of the piezoelectric substrate
71
. Accordingly, Japanese Unexamined Patent Publication No. 235422/1990 discloses that where the fundamental wave is used, W/T=5.33 should be used if the resonance frequency is 16 MHz, and that where the third-order wave is used, setting W/T to approximately 2.87 (where the resonance frequency is approximately 16 MHz) can reduce unwanted spurious waves between resonant and antiresonant frequencies.
As described above, the energy-trap piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 117409/1989 and utilizing the second-order wave of a thickness extensional vibration mode needs large vibration-attenuating portions adjacent to the resonating portion.
Hence, it is difficult to reduce the size of the resonator.
The energy-trap piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 235422/1990 does not require vibration-attenuating portions adjacent to the resonator portion and so a reduction in size can be attained. However, because harmonic waves of a thickness extensional vibration mode are utilized in this resonator, various unwanted spurious waves appear, in addition to the spurious waves between the resonant and antiresonant frequencies. Because this resonator does not have extra portions surrounding the resonating portion, the spurious waves are generated and are not suppressed. As a result, effective and sufficient resonant characteristics can not be achieved in this resonator.
SUMMARY OF THE INVENTION
To overcome the problems described above, the preferred embodiments of the present invention provide an energy trap thickness extensional piezoelectric resonator that maximizes the use of harmonic waves of a thickness extensional vibration mode, has a significantly reduced size, suppresses unwanted spurious vibrations and has excellent resonant characteristics.
The preferred embodiments of the present invention provide an energy trap thickness extensional vibration mode piezoelectric resonator utilizing an nth-order harmonic of a thickness extensional vibration mode. The preferred embodiments of the energy trap piezoelectric resonator preferably include a piezoelectric plate having first and second surfaces arranged opposite to each other, a first excitation electrode and a second excitation electrode provided on the first and second surfaces, respectively, and arranged opposite to each other with the piezoelectric plate located therebetween, at least one internal electrode disposed in the piezoelectric
Inoue Jiro
Kaida Hiroaki
Yamada Mitsuhiro
Budd Mark O.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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