JP2001127582A - Piezoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a piezoelectric element by which a high performance oscillator is realized, which is excellent in thermal stability with a narrow common difference. SOLUTION: The piezoelectric element 10 includes two piezoelectric body layers 12 and 14. The layers 12 and 14 are formed by a piezoelectric material which includes Sr, Bi, Nb and O as main components. A first vibration electrode 16 is formed between the layers 12 and 14 and the second and the third vibration electrodes 18 and 20 are formed on the outer surface. An energy confinement region is formed in an area where the electrodes are superimposed between the vibration electrodes 18, 20 and the electrode 16. A straight line in the energy confinement region, which is in parallel with the surfaces of the electrodes 16, 18 and 20, is cut at two points where the line is intersected with the outer periphery of the confinement region. Its maximum length is made L and a distance between the second and the third vibration electrodes is t. When an n-th order thickness vertical higher harmonic vibration is excited, the value of nL/t is set to be less than 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric element,
In particular, for example, the present invention relates to an energy trap type piezoelectric element using thickness vibration.

[0002]

2. Description of the Related Art Heretofore, there has been proposed an energy trapping type piezoelectric element in which a vibration electrode is formed in a piezoelectric body to excite higher-order vibrations of thickness vibration. It has been revealed that this type of piezoelectric element is hardly affected by Poisson's ratio, unlike an element that excites fundamental vibration of thickness vibration. Generally, materials having excellent thermal stability, such as lead titanate, have a Poisson's ratio of less than 1/3, and it is impossible to confine the frequency-reduced energy of the fundamental wave of thickness vibration. Regarding high-order thickness vibration waves excited by the structure, it is possible to achieve good frequency-reduction-type energy confinement even by using such a thermally stable material, and as a technology that can commercialize high-performance piezoelectric elements. Attention has been paid.

[0003]

However, the energy confinement element of the thickness vibration higher-order wave excited by the vibrating electrode structure has a different optimum electrode structure depending on the material system, and an optimum electrode size for each material system. Need to decide. In other words, it is known that in an energy trap type element, a resonance frequency of spurious vibration called an harmonic overtone exists near a resonance frequency of a main vibration. Excitation is not achieved when the area is reduced, but the maximum electrode diameter at which the anharmonic overtone is not excited varies depending on the material system, so it is necessary to determine the maximum electrode diameter for each material. A non-oriented bismuth layered compound ceramic material has excellent thermal stability and a small electromechanical coupling coefficient, which can be expected to realize a high-performance oscillator with a narrow tolerance. Was not found.

[0004] Therefore, a main object of the present invention is to provide a piezoelectric element capable of realizing a high-performance oscillator having excellent thermal stability and a narrow tolerance.

[0005]

According to the present invention, Sr, B
a plurality of piezoelectric layers made of a piezoelectric material containing i, Nb and O as main components, and at least three vibrating electrodes formed via the piezoelectric layers so as to face each other;
A piezoelectric element that excites an nth-order thickness longitudinal harmonic vibration in an energy confinement region formed in a region where at least three vibrating electrodes overlap each other. When the maximum length when cut at two points crossing the outer periphery of the confinement region is L, and when the distance between the vibration electrode located on the uppermost layer and the vibration electrode located on the lowermost layer is t, nL / A piezoelectric element, wherein the value of t is less than 10. In such a piezoelectric element, SrB
i ₂ Nb ₂ O ₉ can be used.

A piezoelectric material containing Sr, Bi, Nb and O as main components, such as SrBi ₂ Nb ₂ O ₉ , has a low electromechanical coupling coefficient and is a thermally stable material. When a piezoelectric element that excites an nth-order thickness longitudinal harmonic is manufactured using such a material, the value of nL / t is 1
By setting the value to be less than 0, it is possible to realize good energy confinement without superimposing an unharmonic overtone. Here, if the planar shape of the energy confinement region is circular, L is its diameter; if it is elliptical, L is its major axis; if it is rectangular, L is its diagonal length. It is.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments of the present invention with reference to the drawings.

[0008]

FIG. 1 is an illustrative view showing one example of a piezoelectric element of the present invention, and FIG. 2 is an illustrative view showing a side view thereof. The piezoelectric element 10 includes, for example, two piezoelectric layers 12, 1
4 inclusive. The piezoelectric layers 12 and 14 are formed of a piezoelectric ceramic material containing Sr, Bi, Nb and O as main components. As such a piezoelectric ceramic material, for example, a material containing SrBi ₂ Nb ₂ O ₉ as a main component is used. A circular first vibrating electrode 16 is formed substantially at the center between the two piezoelectric layers 12 and 14. This first
The vibrating electrode 16 is drawn out to one end of the laminated piezoelectric layers 12 and 14. Further, a circular second vibration electrode 18 is formed on the outer surface of one of the piezoelectric layers 12 so as to face the first vibration electrode 16. The second vibrating electrode 18 is drawn out to the end opposite to the first vibrating electrode 16. The third vibration electrode 20 is provided on the outer surface of the other piezoelectric layer 14 so as to face the first vibration electrode 16.
Is formed. The third vibration electrode 20 is drawn out to the same end as the second vibration electrode 18. First vibrating electrode 1
6, the second vibrating electrode 18 and the third vibrating electrode 20
Each is formed to have the same diameter.

This piezoelectric element 10 is used as an energy trapping type piezoelectric element, and its energy trapping region is between the first vibrating electrode 16 and the second vibrating electrode 18 and the first vibrating electrode 16. And the third vibrating electrode 20, and a portion where the vibrating electrodes 16, 18, 20 overlap is an energy confinement region. Here, the vibrating electrodes 16, 18, 20 in the energy trapping region
Let L be the maximum length when a straight line parallel to is cut at a point that intersects the outer periphery of the energy confinement region. In the piezoelectric element 10 shown in FIG. 1, the diameter of the circular portions of the three vibrating electrodes 16, 18, 20 is L. Also, the second vibration electrode 18
The distance between the second vibration electrode 20 and the third vibration electrode 20 is represented by t. At this time, when exciting the nth-order thickness longitudinal vibration, the value of nL / t is set to be less than 10.

In this piezoelectric element 10, SrBi ₂ Nb ₂
By using a piezoelectric material containing Sr, Bi, Nb and O as main components such as O _9, it can be used as a high-performance oscillator having excellent thermal stability and a small tolerance due to a small electromechanical coupling coefficient. A piezoelectric element can be obtained.
Further, by setting the value of nL / t to less than 10, it is possible to obtain a piezoelectric element in which an unharmonic overtone is not excited.

[0011]

EXAMPLE First, SrCO was used as a starting material._Three, Bi_Two
O_Three, Nb_TwoO_FiveAnd MnCO _ThreeWas prepared. these
Raw material of SrBi_TwoNb_TwoO₉+ 1wt% MnO_TwoIn table
Weighing and mixing were performed so as to obtain the composition to be obtained. This mixture
After calcining the powder at 800-1000 ° C, doctor blade
The sheet was formed by the method. Obtained in this way
As shown in FIG. 3, the second vibrating electrode
Pattern 30, a third vibrating electrode pattern 32 and
And a sheet on which a pattern 34 for a first vibrating electrode is formed
36, 38 and 40 were formed. These patterns 30,
32 and 34 were formed by printing platinum electrodes. further,
A plurality of sheets 42 are sandwiched between the sheets 36 and 40.
Between the sheets 38 and 40.
Sandwiched. These sheets 36, 38, 40, 42, 4
4 are laminated, and 200 GPa (2 × 10⁷kg / m^Two)of
Pressure bonding and firing at 1250-1300 ° C for 1-5 hours
did. After that, the fired product is polarized in the direction shown by the arrow in FIG.
Then, a piezoelectric element having a structure as shown in FIGS. 1 and 2 is obtained.
Was.

In the thus obtained piezoelectric element, the diameter of the energy confining region, that is, the diameter of the circular portions of the three vibrating electrodes is 0.5 to 2.
It was set to 5 mm. The total thickness of the fired element, that is, the distance between the second vibrating electrode 18 and the third vibrating electrode 20 was set to 0.4 mm. And
As shown in FIG. 4, the second and third vibrating electrodes 18, 2
The impedance analyzer 50 was connected between the first vibrating electrode 16 and the first vibrating electrode 16 to excite piezoelectric vibration.

Using such a piezoelectric element, the frequency characteristics of impedance were measured in order to evaluate the energy confinement of the second harmonic. FIG. 5 is a graph showing impedance-frequency characteristics when the electrode diameter is 2 mm. Here, since n = 2 mm, L = 2 mm, and t = 0.4 mm, the value of nL / t corresponds to 10. It can be seen that in such a piezoelectric element, the unharmonic overtone is superimposed on the resonance frequency of the main vibration.

FIG. 6 shows the relationship between the index nL / t, the peak-to-valley ratio of the main vibration (the ratio between the resonance impedance and the anti-resonance impedance), and the peak-to-valley ratio of the anharmonic overtone.
It was shown to. As can be seen from FIG. 6, the value of nL / t is 10
From the above, it can be seen that the unharmonic overtone starts to be superimposed. When the piezoelectric element is used as an oscillator, the unharmonic overtone causes abnormal oscillation or oscillation stop. Further, when a piezoelectric element is used as a filter, problems such as a decrease in attenuation are caused.

Thus, the bismuth layered compound material S
Sr, Bi, Nb and O, such as rBi ₂ Nb ₂ O ₉
In a piezoelectric element of a type in which a higher-order wave of thickness vibration is excited by a vibrating electrode structure using a material containing as a main component, the size of the electrode is designed so that the value of nL / t is less than 10. A high-performance piezoelectric element having excellent heat resistance and a narrow tolerance can be obtained.

In the piezoelectric element shown in FIG. 1, the shape of the energy confinement electrode is circular, but may be, for example, elliptical, square or rectangular, or other types. It may be square. In these cases, L indicates the longest part, the ellipse indicates the length of the major axis, and the square or rectangle indicates the diagonal length. Further, as shown in FIG. 7, a strip-shaped electrode including a lead portion may be used. In this case, the portion where the first, second and third vibrating electrodes 16, 18, and 20 overlap is an energy confinement region. Further, the number of vibrating electrodes is not limited to three, and four or more vibrating electrodes may be formed.
In this case, the overlapping part of the plurality of vibrating electrodes becomes the energy trapping region.

In the above embodiment, SrBi ₂ Nb
_As a composition mainly composed of ₂ O ₉ , a composition containing Mn was used. For example, a composition containing Si or W, or a part of the Sr position or the Nb position was replaced with another element. Things can be used. In addition, a composition of a piezoelectric material containing Sr, Bi, Nb, and O as a main component may be added with an aid such as a glass component in order to improve the sinterability of ceramics.

In the present invention, among the at least three vibrating electrodes formed via the piezoelectric layer, the two vibrating electrodes located on the outermost side in the thickness direction need not be formed on the outermost part of the whole element. But, for example,
A piezoelectric layer may be formed outside these vibrating electrodes in the thickness direction. However, these two outermost vibrating electrodes are external electrodes, that is, they are formed on the outermost part of the entire device,
It is preferable that there is no piezoelectric layer on the outside. For example, when this piezoelectric element is used as an oscillator or a filter, it is necessary to adjust the resonance frequency to a desired value. The resonance frequency of a piezoelectric element is determined by the type of piezoelectric material and vibrating electrodes that make it up, the thickness of the element, etc.The final adjustment is to apply a paint or the like on the vibrating electrode and dry it. A method of applying a mass load to the electrode is often used. By using the outermost vibrating electrode as an external electrode and exposing it to the outside, such a resonance frequency adjusting method can be easily used. Similarly, a spurious vibration may be suppressed by applying a mass load to the vibrating electrode with a paint or the like. In such a case, the vibrating electrode is preferably an external electrode. As described above, if the outermost vibration electrode is an external electrode, it is possible to easily work on various vibration electrodes.

[0019]

By using the piezoelectric element of the present invention, a high-performance piezoelectric resonator having excellent heat resistance and a narrow band can be obtained.

[Brief description of the drawings]

FIG. 1 is an illustrative view showing one example of a piezoelectric element of the present invention;

FIG. 2 is an illustrative view showing a side surface of the piezoelectric element shown in FIG. 1;

FIG. 3 is an illustrative view showing a step of manufacturing a piezoelectric element;

FIG. 4 is an illustrative view showing an evaluation circuit for evaluating a piezoelectric element;

FIG. 5 is a graph showing frequency characteristics of impedance when the diameter of the energy confinement region is 2 mm.

FIG. 6 is a graph showing a peak-to-valley ratio of impedance when nL / t is changed in the piezoelectric element shown in the example.

FIG. 7 is an illustrative view showing another example of the piezoelectric element of the present invention;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Piezoelectric element 12, 14 Piezoelectric layer 16 First vibration electrode 18 Second vibration electrode 20 Third vibration electrode

Claims (2)

[Claims] 1. A plurality of piezoelectric layers made of a piezoelectric material containing Sr, Bi, Nb and O as main components, and at least three vibrating electrodes formed via the piezoelectric layers so as to face each other. A piezoelectric element that excites an nth-order thickness longitudinal harmonic vibration in an energy confinement region formed in a region where at least three vibration electrodes overlap, wherein the energy confinement region is parallel to a surface of the vibration electrode. 2 intersects the straight line in
When the maximum length when cut at one point is L and the distance between the vibration electrode located on the uppermost layer and the vibration electrode located on the lowermost layer is t, the value of nL / t is less than 10. A piezoelectric element, characterized in that: 2. The piezoelectric element according to claim 1, wherein the main component is SrBi ₂ Nb ₂ O ₉ .