JPH0532927B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a piezoelectric device, such as a ceramic piezoelectric device, which includes a plurality of internal electrodes arranged so as to overlap each other in the thickness direction of ceramics and utilizes bulk waves propagating in the thickness direction. This invention relates to improvements in structures such as resonators or ceramic filters.

BACKGROUND OF THE INVENTION FIG. 1 is a perspective view showing an example of a ceramic filter, which is not yet publicly known, but which led to the invention of the present application. The ceramic filter 1 includes a ceramic 2 and a plurality of internal electrodes 3a...3j, 4a...4j arranged in parallel so as to overlap each other in the thickness direction within the ceramic 2. Among the plurality of internal electrodes 3a...4j, internal electrodes 3a...
3j constitutes one electrode group, and internal electrodes 4a...4
j constitutes the other electrode group, and each internal electrode 3
a...4j are drawn out alternately to the left and right sides in the drawing, and for each electrode group, external electrodes 13a, 13b,
14a and 14b. Ceramics 2 is polarized in the direction of arrow P as shown in the figure, and therefore external electrodes 13a, 13b (14
a, 14b) as input-side external electrodes and apply a voltage, a bulk wave corresponding to the distance between the internal electrodes is excited, and the bulk wave propagates in the thickness direction of the ceramic 2 as shown by the solid line indicated by arrow A. , is transmitted to the other electrode group, and output is extracted from the other external electrodes 14a, 14b (13a, 13b) based on the vibration of the bulk wave.

A piezoelectric device having a configuration in which a plurality of internal electrodes are stacked in parallel in ceramics as shown in Fig. 1 is manufactured by laminating and sintering a large number of ceramic sheets and electrode patterns that become internal electrodes. Therefore, it has the great advantage that it is possible to construct an extremely small piezoelectric device, and piezoelectric devices with various impedances can be obtained by changing the method of forming the internal electrodes and the method of polarization treatment. .

However, since the density difference between the ceramic 2 and air is extremely large, the bulk wave A not only propagates as shown in the schematic front view in FIG. A reflected wave B, which is likely to be reflected by and is therefore called a triple transit echo (TTE), also propagates. Therefore, the amplitude characteristics are greatly affected by the propagation of bulk waves other than the bulk waves of the target frequency, resulting in a disadvantage that the characteristics as designed cannot be obtained.

The above-mentioned problem is not limited to the ceramic filter 1 shown in FIG. 1, but also applies to resonators or delay structures that have a structure in which a large number of internal electrodes are arranged in the ceramic 2 so as to overlap in parallel in the thickness direction. They also exist in devices, and therefore there has been a demand for the removal of unnecessary bulk waves as described above. Also,
In addition, since bulk waves propagate in the thickness direction of filters, resonators, delay lines, etc. that utilize thickness-shear vibration, there has also been a desire to remove unnecessary bulk waves from such devices as well.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a piezoelectric device that eliminates the above-mentioned drawbacks, makes it possible to remove unnecessary bulk waves that become spurious, and provides characteristics as designed.

Composition of the Invention In summary, the present invention includes a ceramic whose at least a portion is polarized, a plurality of internal electrodes that overlap in parallel in the thickness direction within the ceramic, and a bulk that propagates in the thickness direction within the ceramic. A piezoelectric device that uses waves, characterized in that at least one end face in the thickness direction of the ceramic is provided with a bulk wave removal means for removing bulk waves that generate spurious waves other than the utilized bulk waves. It is a device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be clarified by describing embodiments of the present invention with reference to the drawings.

DESCRIPTION OF EMBODIMENTS FIG. 3 is a partially cutaway sectional view for explaining a first embodiment of the present invention, and FIG. 4 is a bottom view showing an end face in the thickness direction of ceramics. Third
As is clear from the figures and FIG. 4, in this embodiment, a plurality of grooves 21 are formed in one end surface 2b in the thickness direction of the ceramic 2 of the ceramic filter 1 shown in FIG. 1 as means for removing unnecessary bulk waves. ing. The bottom 22 of the groove 21 is parallel to the end surface 2b as shown, and the depth d of the groove 21 is
The grooves 21 are formed so as to satisfy the relationship 2d=(n+1/2)λ (where n=0, 1, 2, . . . ), where λ is the wavelength of the bulk wave. Therefore, the bulk wave C reflected by the end face 2d and the bottom 2 of the groove 21
Since the phase of the bulk wave D reflected by the wave D is shifted by half a wavelength, they cancel each other out. Therefore, it can be seen that reflected bulk waves can be effectively removed.

In the embodiment shown in FIGS. 3 and 4, the grooves 21 are formed as shown, but preferably the total area of the bottoms 22 of the plurality of grooves 21 is the remaining area of the end surface 2b. By making it equal to
The bulk waves reflected by the end face 2b and the bottom 22 of the groove 21 can be made equal, and therefore unnecessary bulk waves can be effectively removed. However, since bulk waves propagate in the thickness direction only in the ceramic parts where the internal electrodes overlap each other, the grooves 21 are formed only in the parts corresponding to the parts where the internal electrodes overlap each other. Good too. It should be noted that the shape of the grooves 21 and the method of dispersion thereof are not limited to those shown in the drawings, and may be arbitrarily determined.

FIG. 5 is a schematic front view for explaining a second embodiment of the invention. As is clear from FIG. 5, one end surface 2b in the thickness direction of the ceramic 2 is used as a bulk wave removing means here to form an internal electrode 4.
It is formed on a plane that is not parallel to i and 4j (shown by imaginary lines). Therefore, the bulk wave E is the end face 2
b as shown in the figure, and the internal electrodes 4i, 4j
reflected in a direction that does not exist. Therefore, it can be seen that spurious caused by reflected bulk waves can be effectively reduced.

In the embodiment shown in FIG. 5, the angle indicated by θ, that is, the inclination angle θ of the end face 2b, and the reflection coefficient have a relationship as shown in the graph in FIG. Therefore, if θ≧10°, the reflection coefficient is θ
= 80% smaller than 0°. Therefore, it is preferable that the inclination angle θ is 10° or more.

In addition, in the second embodiment of the present invention, in addition to simply tilting the end surface 2b as shown in FIG. The end surface 2b can be formed at the end surface 2b. For example, as shown in FIG.
b may be formed into a curved surface like the side surface of a cylinder,
Alternatively, it may be treated like a spherical surface.

FIG. 8 is a schematic front view for explaining a third embodiment of the invention. In the embodiment shown in FIG. 8, as unnecessary bulk wave removal means, recesses 3 are formed randomly on one end surface 2b of the ceramic 2 in the thickness direction.
1 is formed. The average depth x of the plurality of recesses 31 is x≧, where λ is the wavelength of the bulk wave.
It is selected to be about λ/2. By forming the grooves 31 so that the average depth x satisfies the above relationship, the bulk waves that have proceeded straight to the end surface 2b are transmitted through the plurality of grooves 3.
1, the light is reflected and scattered in a disorderly manner.
Therefore, it can be seen that bulk waves reflected toward the internal electrodes can be effectively removed.

Note that the grooves 31 may be formed after the ceramics 2 are sintered, or may be formed simultaneously when the ceramic green sheets are laminated and pressed before sintering.

FIG. 9 is a schematic front view for explaining a fourth embodiment of the invention. In the embodiment shown in FIG. 9, a sound absorbing layer 41 having an acoustic impedance comparable to that of the ceramic 2 is attached to one end surface 2b of the ceramic 2 in the thickness direction as a bulk wave removing means. In the case of PZT type ceramics 2, the sound absorption layer 41 has a specific gravity of about 8, so a metal powder with a relatively high specific gravity such as PZT powder or lead, or an oxide such as PbO or Nb ₂ O ₅ is used as the sound absorbing layer 41. By using a sound absorbing material mixed with the like, it is possible to achieve an acoustic impedance comparable to that of the ceramics 2. In this embodiment, a sound absorbing layer 41 having an acoustic impedance equivalent to that of ceramics is used.
is attached to one end surface 2b in the thickness direction of the ceramics 2, so that the bulk waves propagating toward the one end surface 2b in the thickness direction are effectively absorbed by the sound absorbing layer 41. Therefore, generation of unnecessary bulk waves can be effectively prevented.

In the embodiment shown in FIG. 9, the sound absorbing layer 41 does not need to be attached over the entire thickness direction end face 2b as shown in the figure, but may be attached only to a portion where bulk waves may propagate.

As described above, this invention includes four types of bulk wave removing means, but the bulk wave removing means explained in each embodiment may be combined as appropriate, thereby more effectively removing reflected bulk waves. Waves can be removed. As an example, a specific experimental example will be described in which the bulk wave removal means in the embodiment shown in FIG. 8 and the embodiment shown in FIG. 9 are combined.

Made of lead titanate ceramics 2, diameter 1mm
A ceramic filter was prepared in which a plurality of internal electrodes were arranged in the ceramic 2 so that the inter-electrode spacing was 100 μm, the propagation distance was 200 μm, and the number of electrode pairs was 4.5. When recesses larger than λ/2 were formed randomly and a sound absorbing layer 41 was further attached, an output frequency characteristic as shown by the solid line in FIG. 10 could be obtained. For comparison, the characteristics before treatment are shown in FIG. 10 by broken lines. As is clear from the comparison of both characteristic curves, the ceramic filter before treatment generates a large number of spurious signals, whereas
It can be seen that the ceramic filter of this invention has a characteristic curve almost as designed.

Effects of the Invention As described above, according to the present invention, the ceramic is provided with a ceramic at least partially polarized, and a plurality of internal electrodes that overlap in parallel in the thickness direction within the ceramic. In piezoelectric devices that utilize propagating bulk waves,
At least one end face in the thickness direction of the ceramic is provided with a bulk wave removal means for removing bulk waves that generate spurious waves other than the utilized bulk waves, so that the influence of unnecessary bulk waves can be effectively eliminated. , and thus a piezoelectric device having the properties as described can be obtained.

It should be pointed out that the present invention is applicable not only to filters but also to various piezoelectric devices such as resonators and delay elements.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an example of the ceramic filter that led to the invention. FIG. 2 is a schematic front view for explaining the problems of the ceramic filter shown in FIG. 1. 3 and 4 show a partially cutaway sectional view and a bottom view for explaining the first embodiment of the present invention.
FIG. 5 is a partially cutaway front view for explaining a second embodiment of the invention. FIG. 6 is a diagram showing the relationship between the inclination angle θ and the reflection coefficient in the embodiment shown in FIG. 7 is a schematic front view showing a modification of the embodiment shown in FIG. 5. FIG. FIG. 8 is a partially cutaway front view for explaining a third embodiment of the present invention. FIG. 9 is a partially cutaway front view for explaining a fourth embodiment of the present invention. FIG. 10 shows the embodiment shown in FIG. 8 and the embodiment shown in FIG.
FIG. 2 is a diagram for explaining the characteristics of a ceramic filter that is a combination of the embodiment shown in the figure. In the figure, 2 is a ceramic, 2a, 2b are end faces of the ceramic in the thickness direction, 3a...3j, 4a
...4j is an internal electrode, 21 is a groove as a bulk wave removing means, 22 is a bottom of the groove, 31 is a recessed portion as a bulk wave removing means, and 41 is a sound absorbing layer as a bulk wave removing means.

Claims (1)

[Claims] 1. Comprising a ceramic at least partially polarized and a plurality of internal electrodes overlapping in parallel in the thickness direction within the ceramic, utilizing bulk waves propagating in the thickness direction within the ceramic. A piezoelectric device, characterized in that at least one of the end faces in the thickness direction of the ceramic is provided with a bulk wave removing means for removing bulk waves that generate spurious waves other than the utilized bulk waves. 2. The bulk wave removing means is a plurality of grooves having bottoms parallel to the end face in the thickness direction of the ceramic, and the depth d of the grooves is 2d=(n+1/2)λ.
(However, n is 0 or an integer, and λ is the wavelength of the bulk wave)
The piezoelectric device according to claim 1, which is selected so as to satisfy the following. 3. The piezoelectric device according to claim 2, wherein the total area of the bottoms of the plurality of grooves is equal to the total area of the remaining portion of the end face where the grooves are formed. 4. The piezoelectric device according to claim 1, wherein the bulk wave removing means is a curved surface formed on an end face in the thickness direction of the ceramic or a plane that is not parallel to the internal electrode. 5. The bulk wave removing means is a plurality of recesses randomly formed on the end face in the thickness direction of the ceramic, and the average depth x of the recesses satisfies x≧λ/2.
The piezoelectric device according to claim 1, wherein the piezoelectric device is selected so as to satisfy (λ is the wavelength of the bulk wave). 6. The piezoelectric device according to claim 1, wherein the bulk wave removing means comprises a sound absorbing layer attached to the end face in the thickness direction of the ceramic, and the sound absorbing layer has an acoustic impedance comparable to that of the ceramic. Device.