Piezo electric element

Abstract

To provide a piezoelectric element that is resistant to variations in resonance frequency and to degradation in vibration characteristics.SOLUTION: A piezoelectric element PE1 comprises a piezoelectric body 1 and a pair of external electrodes 10, 20. The piezoelectric element 1 includes a pair of opposing main surfaces 3, 5. Each of the pair of external electrodes 10, 20 is positioned on the corresponding main surface 3, 5 of the pair of main surfaces. Viewed from the direction DO in which the pair of main faces 3, 5 face each other, the piezoelectric element 1 exhibits a polygonal shape having an area ratio relative to the area of the smallest circumscribed circle that is 0.8 or more but less than 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric element. [Background technology]

[0002] A known piezoelectric element includes a piezoelectric element having a pair of opposing main surfaces, and a plurality of external electrodes disposed on corresponding ones of the pair of main surfaces (see, for example, Patent Document 1). When viewed from the direction in which the pair of main surfaces face each other, the piezoelectric element has a circular shape. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-012776 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one aspect of the present invention is to provide a piezoelectric element that is less likely to have variations in resonance frequency and vibration characteristics that are less likely to deteriorate. [Means for solving the problem]

[0005] As a result of research and study, the inventors have newly discovered the following information regarding the resonance frequency of a piezoelectric element. The resonant frequency of a piezoelectric element tends to be affected by the shape of the piezoelectric element when viewed from the direction in which the pair of principal surfaces face each other, i.e., the planar shape of the piezoelectric element. When the piezoelectric element has a circular shape when viewed from the direction in which the pair of principal surfaces face each other, the resonant frequency is likely to vary. In contrast, when the piezoelectric element has a polygonal shape when viewed from the direction in which the pair of principal surfaces face each other, the resonant frequency is less likely to vary.

[0006] For example, in a configuration in which the piezoelectric element has a circular shape when viewed from the direction in which the pair of principal surfaces face each other, the circularity may differ between piezoelectric elements due to manufacturing variations. That is, the planar shape of the piezoelectric element may not be a simple circle. Even when the planar shape of the piezoelectric element is circular, the planar shape of the piezoelectric element may be oval rather than a perfect circle. An oval shape includes, for example, an elliptical shape or a rounded rectangular shape. Therefore, in a configuration in which the planar shape of the piezoelectric element is circular, the planar shape of the piezoelectric element may differ between piezoelectric elements, i.e., there may be variation in the planar shape of the piezoelectric element, which causes variation in the resonant frequency between piezoelectric elements.

[0007] For example, in a configuration in which the piezoelectric element has a polygonal shape when viewed from the direction in which the pair of principal surfaces face each other, the number of corners in the planar shape will not change between piezoelectric elements even if manufacturing variations occur. For example, in a configuration in which the piezoelectric element has a hexagonal planar shape, the planar shape of the piezoelectric element will always be a hexagon even if manufacturing variations occur. Therefore, in a configuration in which the planar shape of the piezoelectric element is polygonal, the planar shape of the piezoelectric element is unlikely to differ between piezoelectric elements, i.e., the planar shape of the piezoelectric element is unlikely to vary, and therefore the resonant frequency is unlikely to vary between piezoelectric elements.

[0008] As a result of research and study, the present inventors have newly discovered the following information regarding the vibration characteristics of piezoelectric elements. In a configuration in which the piezoelectric element has a polygonal shape when viewed from the direction in which the pair of main surfaces face each other, the vibration characteristics may be reduced compared to a configuration in which the piezoelectric element has a circular shape when viewed from the direction in which the pair of main surfaces face each other.

[0009] A polygonal shape typically has an area smaller than the area of ​​the smallest enclosing circle of the polygonal shape. That is, a polygonal shape typically has an area whose ratio to the area of ​​the smallest enclosing circle of the polygonal shape is less than 1. In a configuration in which the planar shape of a piezoelectric element is polygonal, the area of ​​the main surface is smaller than in a configuration in which the planar shape of the piezoelectric element is a circle defined by the smallest enclosing circle of the polygonal planar shape. In a configuration in which the planar shape of a piezoelectric element is polygonal, the area of ​​the piezoelectrically active region is smaller than in a configuration in which the planar shape of the piezoelectric element is a circle defined by the above-mentioned smallest enclosing circle. If the area of ​​the piezoelectrically active region is small, the vibration characteristics tend to deteriorate.

[0010] The present inventors have further investigated and researched piezoelectric elements that suppress deterioration of vibration characteristics, and as a result, have found that when the piezoelectric element has a polygonal shape with an area that is equal to or greater than the area of ​​the smallest encompassing circle when viewed from the direction in which the pair of principal surfaces face each other, the vibration characteristics are less likely to deteriorate.

[0011] Based on newly obtained knowledge about the resonance frequency and vibration characteristics, the inventors have come up with the following aspect. A piezoelectric element according to one embodiment includes a piezoelectric body having a pair of opposing main surfaces, and a plurality of external electrodes each arranged on a corresponding one of the pair of main surfaces, and when viewed from the direction in which the pair of main surfaces face each other, the piezoelectric body has a polygonal shape having an area whose ratio to the area of ​​the smallest encompassing circle is greater than or equal to 0.8 and less than 1. [Effects of the Invention]

[0012] One aspect of the present invention provides a piezoelectric element that is less likely to have variations in resonance frequency and vibration characteristics that are less likely to deteriorate. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a plan view of a piezoelectric element according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing a cross-sectional configuration of the piezoelectric element according to the first embodiment. [Figure 3] FIG. 3 is a plan view of the piezoelectric element. [Figure 4] FIG. 4 is a plan view of a piezoelectric element according to a modified example of the first embodiment. [Figure 5] FIG. 5 is a diagram showing a cross-sectional configuration of a piezoelectric element according to a modified example of the first embodiment. [Figure 6] FIG. 6 is a plan view of a piezoelectric element according to another modified example of the first embodiment. [Figure 7] FIG. 7 is a plan view of a piezoelectric element according to the second embodiment. [Figure 8] FIG. 8 is a diagram showing a cross-sectional configuration of a piezoelectric element according to the second embodiment. [Figure 9] FIG. 9 is a plan view of the piezoelectric element. [Figure 10] FIG. 10 is a plan view of a piezoelectric element according to a modified example of the second embodiment. [Figure 11] FIG. 11 is a diagram showing a cross-sectional configuration of a piezoelectric element according to a modified example of the second embodiment. [Figure 12] FIG. 12 is a plan view of a piezoelectric element according to another modified example of the second embodiment. [Figure 13] FIG. 13 is a plan view of the piezoelectric element. [Figure 14] FIG. 14 is a plan view of the piezoelectric element. [Figure 15] FIG. 15 is a diagram showing the relationship between the ratio of the area of ​​the piezoelectric element to the area of ​​the smallest encompassing circle and the electromechanical coupling coefficient. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description, the same elements or elements having the same functions will be denoted by the same reference numerals, and redundant description will be omitted.

[0015] (First embodiment) The configuration of a piezoelectric element PE1 according to a first embodiment will be described with reference to Figures 1 to 3. Figure 1 is a plan view of the piezoelectric element according to the first embodiment. Figure 2 is a diagram showing the cross-sectional configuration of the piezoelectric element according to the first embodiment. Figure 3 is a plan view of a piezoelectric body. The piezoelectric element PE1 includes a piezoelectric body 1 and a plurality of external electrodes Ee. The piezoelectric element PE1 is included in, for example, an actuator that can generate a force. The piezoelectric element PE1 may also be included in, for example, a sensor that can detect an acting force. The piezoelectric element PE1 is, for example, resonantly driven.

[0016] The piezoelectric element 1 is, for example, plate-shaped. The piezoelectric element 1 is, for example, thin-plate-shaped. The piezoelectric element 1 includes a pair of main surfaces 3, 5 and multiple side surfaces 7. The pair of main surfaces 3, 5 face each other. The pair of main surfaces 3, 5 face each other, for example, in the thickness direction of the piezoelectric element 1. The direction DO in which the pair of main surfaces 3, 5 face each other includes, for example, the thickness direction of the piezoelectric element 1. The direction DO includes, for example, a direction perpendicular to the main surface 3. The direction DO includes, for example, a direction perpendicular to the main surface 5. The multiple side surfaces 7 connect the pair of main surfaces 3, 5. The multiple side surfaces 7 extend, for example, in the direction DO. The thickness of the piezoelectric element 1 is, for example, 150 to 300 μm. For example, when the main surface 3 includes one of the pair of main surfaces, the main surface 5 may include the other of the pair of main surfaces.

[0017] When viewed from the direction DO, the piezoelectric element 1 has a polygonal shape with an area ratio to the area of ​​the smallest encompassing circle Mec that is equal to or greater than 0.8 and less than 1. The pair of main surfaces 3, 5 have polygonal shapes. When viewed from the direction DO, the piezoelectric element 1 has, for example, a hexagonal shape. This hexagonal shape includes a regular hexagonal shape. The multiple side surfaces 7 include six side surfaces 7a, 7b, 7c, 7d, 7e, and 7f. The main surface 3 includes an outer edge 3e that defines the shape of the main surface 3. The main surface 5 includes an outer edge 5e that defines the shape of the main surface 5. Each of the outer edges 3e, 5e also has a polygonal shape. Each of the outer edges 3e, 5e includes multiple sides. Each of the outer edges 3e, 5e includes, for example, six sides. For example, in a piezoelectric element 1 in which a pair of principal surfaces 3 and 5 are regular hexagonal, the ratio of the area of ​​the piezoelectric element 1 to the area of ​​the smallest encompassing circle Mec when viewed from the direction DO is theoretically (3×31 / 2 ) / (2×π).

[0018] The piezoelectric element 1 is, for example, made of only one piezoelectric layer. In other words, the piezoelectric element PE1 does not need to include internal electrodes disposed within the piezoelectric element 1. The piezoelectric element 1 is made of a piezoelectric ceramic material. The piezoelectric ceramic material includes, for example, PZT [Pb(Zr,Ti)O3], PT(PbTiO3), PLZT [(Pb,La)(Zr,Ti)O3], PZN [Pb(Zn,Nb)O3], or barium titanate (BaTiO3). The piezoelectric element 1 is, for example, made of a sintered ceramic green sheet including the above-mentioned piezoelectric ceramic material.

[0019] A plurality of external electrodes Ee are arranged on the piezoelectric element 1. The plurality of external electrodes Ee are arranged on corresponding ones of the pair of principal surfaces 3, 5. The plurality of external electrodes Ee include an external electrode 10 on the principal surface 3 and an external electrode 20 on the principal surface 5. For example, a region of the piezoelectric element 1 located between the external electrodes 10 and 20 functions as a piezoelectrically active region. The external electrodes Ee are made of a conductive material. The conductive material includes, for example, silver (Ag), palladium (Pd), or a silver-palladium alloy (Ag-Pd). The external electrodes Ee are configured, for example, as a sintered body of a conductive paste including the conductive material. That is, the external electrodes Ee are, for example, a sintered metal film. The external electrodes Ee may be, for example, a physical vapor deposition film. The physical vapor deposition film includes, for example, a sputtered film. The sputtered film includes, for example, a laminate including a chromium (Cr) layer, a nickel-copper alloy (Ni-Cu) layer, and a gold (Au) layer. The sputtered film may include silver (Ag), titanium (Ti), platinum (Pt), a silver-palladium alloy (Ag-Pd), or a nickel-chromium alloy (Ni-Cr). In a configuration in which the plurality of external electrodes Ee are sintered metal films, the thickness of each external electrode Ee is, for example, 1 to 15 μm. In a configuration in which the plurality of external electrodes Ee are physical vapor deposition films, the thickness of each external electrode Ee is, for example, 100 to 300 nm.

[0020] The external electrode 10 is disposed on the main surface 3. The external electrode 10 includes main surfaces 10a and 10b facing each other. The external electrode 10 is in contact with the main surface 3. For example, the external electrode 10 is in direct contact with the main surface 3. For example, the main surface 10a is in contact with the main surface 3. The main surface 10b is exposed. The external electrode 20 is disposed on the main surface 5. The external electrode 20 includes main surfaces 20a and 20b facing each other. The external electrode 20 is in contact with the main surface 5. The external electrode 20 is in direct contact with the main surface 5. For example, the main surface 20a is in contact with the main surface 5. The main surface 20b is exposed. For example, when the external electrode 10 includes a first external electrode, the external electrode 20 may include a second external electrode. The external electrodes Ee are not disposed on the side surfaces 7. The side surfaces 7 are exposed from the external electrodes Ee.

[0021] The external electrode 10 includes an outer edge 10e that coincides with the outer edge 3e included in the main surface 3. For example, the outer edge 10e coincides with the outer edge 3e over the entire periphery of the external electrode 10. When viewed from the direction DO, the external electrode 10 has a polygonal shape. When viewed from the direction DO, the external electrode 10 has, for example, a hexagonal shape. This hexagonal shape also includes a regular hexagonal shape. In a piezoelectric element PE1 in which the outer edge 10e coincides with the outer edge 3e over the entire periphery of the external electrode 10, the external electrode 10 may cover the entire main surface 3. An opening that exposes a portion of the main surface 3 may be formed in the external electrode 10. 1, the outer edge 3e and the outer edge 10e are intentionally misaligned to make it easier to understand the configuration of the piezoelectric element PE1. However, in an actual piezoelectric element PE1, the outer edge 3e and the outer edge 10e coincide with each other, as shown in FIG.

[0022] The external electrode 20 includes an outer edge 20e that coincides with the outer edge 5e included in the main surface 5. For example, the outer edge 20e coincides with the outer edge 5e over the entire periphery of the external electrode 20. When viewed from the direction DO, the external electrode 20 has a polygonal shape. When viewed from the direction DO, the external electrode 20 has, for example, a hexagonal shape. This hexagonal shape also includes a regular hexagonal shape. In a piezoelectric element PE1 in which the outer edge 20e coincides with the outer edge 5e over the entire periphery of the external electrode 20, the external electrode 20 may cover the entire main surface 5. An opening that exposes a part of the main surface 5 may be formed in the external electrode 20. 1, the outer edge 5e and the outer edge 20e are intentionally misaligned to make it easier to understand the configuration of the piezoelectric element PE1. However, in an actual piezoelectric element PE1, the outer edge 5e and the outer edge 20e coincide with each other, as shown in FIG.

[0023] The configuration of a piezoelectric element PE1a according to a modification of the first embodiment will be described with reference to FIGS. 4 and 5. FIG. 4 is a plan view of the piezoelectric element according to this modification. FIG. 5 is a diagram showing the cross-sectional configuration of the piezoelectric element according to this modification. The piezoelectric element PE1a according to this modification is generally similar to or the same as the piezoelectric element PE1 described above, but the piezoelectric element PE1a differs from the piezoelectric element PE1 in the configuration of the multiple external electrodes Ee. The following mainly describes the differences between the piezoelectric element PE1 and the piezoelectric element PE1a. Like the piezoelectric element PE1, the piezoelectric element PE1a includes a piezoelectric body 1 and a plurality of external electrodes Ee. The plurality of external electrodes Ee include an external electrode 10 and an external electrode 20. The piezoelectric element PE1a is, for example, resonantly driven.

[0024] The external electrode 10 includes an electrode portion 11 disposed on a part of the main surface 3 . The electrode portions 11 are arranged on only a part of the main surface 3. The electrode portions 11 have a polygonal shape. When viewed from the direction DO, the electrode portions 11 have, for example, a hexagonal shape. The electrode portion 11 includes an outer edge 11e1 that coincides with the outer edge 3e and an outer edge 11e2 that is located on the main surface 3. The outer edge 11e1 is continuous with the outer edge 11e2. The outer edge 11e1 coincides with, for example, five sides included in the outer edge 3e. When viewed from the direction DO, the angle α1 formed between the outer edge 11e2 and the outer edge 3e is other than a right angle. The angle α1 is, for example, an acute angle. For example, when the outer edge 11e2 includes the first outer edge, the outer edge 11e1 may include the second outer edge. In Fig. 4, the outer edge 3e and the outer edge 11e1 are intentionally misaligned to make the configuration of the piezoelectric element PE1a easier to understand, as in Fig. 1. However, in an actual piezoelectric element PE1a, the outer edge 3e and the outer edge 11e1 coincide with each other, as shown in Fig. 5.

[0025] The external electrode 20 is arranged on the main surface 3, the main surface 5, and the side surface 7. The external electrode 20 is in contact with the main surface 3, the main surface 5, and the side surface 7. For example, the external electrode 20 is in direct contact with the main surface 3, the main surface 5, and the side surface 7. The external electrode 20 includes an electrode portion 21 arranged on the remainder of the main surface 3, an electrode portion 23 arranged on the main surface 5, and an electrode portion 25 arranged on one of the plurality of side surfaces 7. For example, when the electrode portion 11 includes a first electrode portion, the electrode portion 21 may include a second electrode portion, the electrode portion 23 may include a third electrode portion, and the electrode portion 25 may include a fourth electrode portion.

[0026] The electrode portion 21 is arranged only on the remaining portion of the main surface 3. The electrode portion 21 is spaced apart from the electrode portion 11. The electrode portion 21 has a polygonal shape. When viewed from the direction DO, the electrode portion 21 has, for example, a trapezoidal shape. The electrode portion 21 includes an outer edge 21e1 that coincides with the outer edge 3e and an outer edge 21e2 that is located on the main surface 3. The outer edge 21e1 is continuous with the outer edge 21e2. The outer edge 21e2 faces the outer edge 11e2. The outer edge 21e1 coincides with, for example, three sides included in the outer edge 3e. When viewed from the direction DO, the angle α2 formed between the outer edge 21e2 and the outer edge 3e is other than a right angle. The angle α2 is, for example, an obtuse angle. For example, when the outer edge 21e2 includes the first outer edge, the outer edge 21e1 may include the second outer edge. In Fig. 4, the outer edge 3e and the outer edge 21e1 are intentionally misaligned to make it easier to understand the configuration of the piezoelectric element PE1a, as in Fig. 1. However, in an actual piezoelectric element PE1a, the outer edge 3e and the outer edge 21e1 coincide with each other, as shown in Fig. 5.

[0027] The electrode portion 23 includes an outer edge 23e that coincides with the outer edge 5e included in the main surface 5. For example, the outer edge 23e coincides with the outer edge 5e around the entire periphery of the electrode portion 23. When viewed from the direction DO, the electrode portion 23 has a polygonal shape. When viewed from the direction DO, the electrode portion 23 has, for example, a hexagonal shape. This hexagonal shape also includes a regular hexagonal shape. In Fig. 4, the outer edge 5e and the outer edge 23e are intentionally misaligned to make the configuration of the piezoelectric element PE1a easier to understand, as in Fig. 1. However, in an actual piezoelectric element PE1a, the outer edge 5e and the outer edge 23e coincide with each other, as shown in Fig. 5.

[0028] The electrode portion 25 is disposed, for example, on the side surface 7a. The electrode portion 25 is disposed, for example, only on the side surface 7a. Of the multiple side surfaces 7, the side surfaces 7b to 7f, excluding the side surface 7a, are exposed from the electrode portion 25. The electrode portion 25 includes an outer edge that coincides with the outer edge of the side surface 7a when viewed from a direction perpendicular to the side surface 7a. The electrode portion 25 connects the electrode portion 21 and the electrode portion 23. The electrode portion 25 is physically and electrically connected to the electrode portion 21 and the electrode portion 23. The electrode portion 21 and the electrode portion 25 are connected to each other on the edge between the main surface 3 and the side surface 7a. The electrode portion 23 and the electrode portion 25 are connected to each other on the edge between the main surface 5 and the side surface 7a. For example, the region of the piezoelectric element 1 located between the electrode portion 11 and the electrode portion 25 functions as a piezoelectrically active region.

[0029] Outer edges 11e2 and 21e2 extend, for example, in the direction in which the ridge between the main surface 3 and the side surface 7a extends. Of the main surface 3, regions 3a exposed from electrode portions 11 and 21 extend in the direction in which the ridge between the main surface 3 and the side surface 7a extends. When viewed from direction DO, the direction in which region 3a extends intersects with the direction in which the ridge between the main surface 3 and the side surface 7b extends at an angle other than a right angle. When viewed from direction DO, the direction in which region 3a extends intersects with the direction in which the ridge between the main surface 3 and the side surface 7f extends at an angle other than a right angle.

[0030] The configuration of a piezoelectric element PE1b according to another modified example of the first embodiment will be described with reference to FIG. 6. FIG. 6 is a plan view of the piezoelectric element according to this modified example. The piezoelectric element PE1b according to this modified example is generally similar to or the same as the piezoelectric element PE1a described above, but the piezoelectric element PE1b differs from the piezoelectric element PE1a in the configuration of the plurality of external electrodes Ee. The following mainly describes the differences between the piezoelectric element PE1a and the piezoelectric element PE1b. The piezoelectric element PE1b is, for example, resonantly driven. Like the piezoelectric element PE1a, the piezoelectric element PE1b includes the piezoelectric body 1 and a plurality of external electrodes Ee. The plurality of external electrodes Ee include an external electrode 10 and an external electrode 20.

[0031] The external electrode 10 includes an electrode portion 11. When viewed from the direction DO, the electrode portion 11 has, for example, a heptagonal shape. The external electrode 20 includes an electrode portion 21, an electrode portion 23, and an electrode portion 25. When viewed from the direction DO, the electrode portion 21 has, for example, a triangular shape. The electrode portion 25 is disposed on a pair of adjacent side surfaces among the multiple side surfaces 7. The electrode portion 25 is disposed, for example, on a portion of each of the pair of adjacent side surfaces 7a, 7b. The electrode portion 25 is disposed, for example, only on a portion of each of the pair of adjacent side surfaces 7a, 7b. The electrode portion 25 covers the edge between the pair of side surfaces 7a, 7b. Electrode portion 21 and electrode portion 25 are connected to each other on a portion of the edge between main surface 3 and side surface 7a and on a portion of the edge between main surface 3 and side surface 7b. Electrode portion 23 and electrode portion 25 are connected to each other on a portion of the edge between main surface 5 and side surface 7a and on a portion of the edge between main surface 5 and side surface 7b. In Fig. 6, to make it easier to understand the configuration of piezoelectric element PE1b, outer edge 3e and outer edge 11e1 are intentionally misaligned, as in Fig. 4. In Fig. 6, to make it easier to understand the configuration of piezoelectric element PE1b, outer edge 3e and outer edge 21e1 are intentionally misaligned, and outer edge 5e and outer edge 23e are intentionally misaligned, as in Fig. 4.

[0032] Second Embodiment The configuration of a piezoelectric element PE2 according to the second embodiment will be described with reference to FIGS. 7 to 9. FIG. 7 is a plan view of the piezoelectric element according to the second embodiment. FIG. 8 is a diagram showing the cross-sectional configuration of the piezoelectric element according to the second embodiment. FIG. 9 is a plan view of a piezoelectric body. The piezoelectric element PE2 according to the second embodiment is generally similar to or the same as the piezoelectric element PE1 described above, but the piezoelectric element PE2 differs from the piezoelectric element PE1 in terms of the configuration of the piezoelectric body 1. The following mainly describes the differences between the piezoelectric elements PE1 and PE2. Like the piezoelectric element PE1, the piezoelectric element PE2 includes a piezoelectric body 1 and a plurality of external electrodes Ee. The plurality of external electrodes Ee include an external electrode 10 and an external electrode 20. The piezoelectric element PE2 is, for example, resonantly driven.

[0033] When viewed from the direction DO, the piezoelectric element 1 has, for example, an octagonal shape. This octagonal shape includes a regular octagon. The multiple side surfaces 7 include eight side surfaces 7a, 7b, 7c, 7d, 7e, 7f, 7g, and 7h. Each of the outer edges 3e and 5e includes, for example, eight sides. In FIG. 7, to make it easier to understand the configuration of the piezoelectric element PE2, the outer edges 3e and 10e are intentionally offset, and the outer edges 5e and 20e are also intentionally offset. However, in an actual piezoelectric element PE2, as shown in FIG. 8, the outer edges 3e and 10e coincide with each other, and the outer edges 5e and 20e coincide with each other. For example, in a piezoelectric element 1 having a pair of main surfaces 3, 5 each having a regular octagonal shape, the ratio of the area of ​​the piezoelectric element 1 to the area of ​​the smallest encompassing circle Mec when viewed from the direction DO is theoretically (2×2 1 / 2 ) / π.

[0034] The configuration of a piezoelectric element PE2a according to a modified example of the second embodiment will be described with reference to Figures 10 and 11. Figure 13 is a plan view of the piezoelectric element according to this modified example. Figure 14 is a diagram showing the cross-sectional configuration of the piezoelectric element according to this modified example. The piezoelectric element PE2a according to this modified example is generally similar to or the same as the piezoelectric element PE1a described above, but the piezoelectric element PE2a differs from the piezoelectric element PE1a in terms of the configuration of the piezoelectric body 1. The following mainly describes the differences between the piezoelectric elements PE1a and PE2a. Like the piezoelectric element PE1a, the piezoelectric element PE2a includes the piezoelectric body 1 and a plurality of external electrodes Ee. The plurality of external electrodes Ee include an external electrode 10 and an external electrode 20. The piezoelectric element PE2a is, for example, resonantly driven.

[0035] When viewed from the direction DO, the piezoelectric element 1 has, for example, an octagonal shape. This octagonal shape includes a regular octagon. The multiple side surfaces 7 include eight side surfaces 7a, 7b, 7c, 7d, 7e, 7f, 7g, and 7h. Each of the outer edges 3e and 5e includes, for example, eight sides.

[0036] The external electrode 10 includes an electrode portion 11. When viewed from the direction DO, the electrode portion 11 has, for example, a hexagonal shape. The electrode portion 11 includes an outer edge 11e1 and an outer edge 11e2. The outer edge 11e1 is continuous with the outer edge 11e2. The outer edge 11e1 coincides with, for example, seven sides included in the outer edge 3e. When viewed from the direction DO, the angle α1 is other than a right angle. 10, the outer edge 3e and the outer edge 11e1 are intentionally misaligned to make the configuration of the piezoelectric element PE2a easier to understand, as in FIG. 7. However, in an actual piezoelectric element PE2a, the outer edge 3e and the outer edge 11e1 coincide with each other, as shown in FIG.

[0037] The external electrode 20 includes an electrode portion 21 , an electrode portion 23 , and an electrode portion 25 . When viewed from direction DO, electrode portion 21 has, for example, a trapezoidal shape. Electrode portion 21 includes outer edges 21e1 and 21e2. Outer edge 21e1 is continuous with outer edge 21e2. Outer edge 21e1 coincides with, for example, three sides included in outer edge 3e. When viewed from direction DO, angle α2 is not a right angle. The electrode portion 23 includes an outer edge 23e. When viewed from the direction DO, the electrode portion 23 has, for example, an octagonal shape. This octagonal shape also includes a regular octagonal shape. The electrode portion 25 is disposed on the side surface 7a, for example. Of the multiple side surfaces 7, the side surfaces 7b to 7h, excluding the side surface 7a, are exposed from the electrode portion 25. 10, to make it easier to understand the configuration of piezoelectric element PE2a, outer edge 3e and outer edge 21e1 are intentionally misaligned, and outer edge 5e and outer edge 23e are intentionally misaligned, as in Fig. 7. However, in an actual piezoelectric element PE2a, outer edge 3e and outer edge 21e1 coincide with each other, and outer edge 5e and outer edge 23e coincide with each other, as shown in Fig. 11.

[0038] Outer edges 11e2 and 21e2 extend, for example, in the direction in which the ridge between the main surface 3 and the side surface 7a extends. Of the main surface 3, regions 3a exposed from electrode portions 11 and 21 extend in the direction in which the ridge between the main surface 3 and the side surface 7a extends. When viewed from direction DO, the direction in which region 3a extends intersects with the direction in which the ridge between the main surface 3 and the side surface 7b extends at an angle other than a right angle. When viewed from direction DO, the direction in which region 3a extends intersects with the direction in which the ridge between the main surface 3 and the side surface 7h extends at an angle other than a right angle.

[0039] The configuration of a piezoelectric element PE2b according to another modified example of the second embodiment will be described with reference to Fig. 12. Fig. 12 is a plan view of the piezoelectric element according to this modified example. The piezoelectric element PE2b according to this modified example is generally similar to or the same as the piezoelectric element PE1b described above, but the piezoelectric element PE2b differs from the piezoelectric element PE1b in terms of the configuration of the piezoelectric body 1. The following mainly describes the differences between the piezoelectric elements PE1b and PE2b. Like the piezoelectric element PE1b, the piezoelectric element PE2b includes the piezoelectric body 1 and a plurality of external electrodes Ee. The plurality of external electrodes Ee include an external electrode 10 and an external electrode 20. The piezoelectric element PE2b is, for example, resonantly driven.

[0040] When viewed from the direction DO, the piezoelectric element 1 has, for example, an octagonal shape. This octagonal shape includes a regular octagon. The multiple side surfaces 7 include eight side surfaces 7a, 7b, 7c, 7d, 7e, 7f, 7g, and 7h. Each of the outer edges 3e and 5e includes, for example, eight sides.

[0041] The external electrode 10 includes an electrode portion 11. When viewed from the direction DO, the electrode portion 11 has, for example, a nonagonal shape. The external electrode 20 includes an electrode portion 21, an electrode portion 23, and an electrode portion 25. When viewed from the direction DO, the electrode portion 21 has, for example, a triangular shape. The electrode portion 25 is, for example, arranged on a portion of each of a pair of adjacent side surfaces 7a, 7h. The electrode portion 25 is, for example, arranged only on a portion of each of a pair of adjacent side surfaces 7a, 7h. The electrode portion 25 covers the edge between the pair of side surfaces 7a, 7h. In Fig. 12, to make it easier to understand the configuration of piezoelectric element PE2b, outer edge 3e and outer edge 11e1 are intentionally misaligned, as in Fig. 10. In Fig. 12, to make it easier to understand the configuration of piezoelectric element PE2b, outer edge 3e and outer edge 21e1 are intentionally misaligned, and outer edge 5e and outer edge 23e are intentionally misaligned, as in Fig. 10.

[0042] The polygonal shape of the piezoelectric element 1 will now be described in detail. The inventors prepared a number of samples S1 to S5 with different piezoelectric element shapes and checked the electromechanical coupling coefficient of each sample S1 to S5. The electromechanical coupling coefficient was determined by a resonance-antiresonance method, for example, in accordance with the test method for disc-shaped vibrators in standard EM-4501A of the Japan Electronics and Information Technology Industries Association (JEITA). Each of the samples S1 to S5 is a lot including multiple specimens. Each lot includes four specimens. As will be described later, the specimens of the samples S1 to S5 are piezoelectric elements having the same configuration except for the shape of the piezoelectric element.

[0043] Each specimen in sample S1 includes a piezoelectric element 101 and a pair of external electrodes. As shown in FIG. 13, the piezoelectric element 101 includes a pair of opposing main surfaces 103, 105. When viewed from the direction in which the pair of main surfaces 103, 105 face each other, the piezoelectric element 101 has a hexagonal shape. When viewed from the direction in which the pair of main surfaces 103, 105 face each other, the piezoelectric element 101 does not have a regular hexagonal shape. The thickness of the piezoelectric element 101 is 0.175 mm. The thickness of each external electrode is 0.010 mm. FIG. 13 is a plan view of the piezoelectric element. The area of ​​the piezoelectric element 101 when viewed from the direction in which the pair of principal surfaces 103 and 105 face each other, i.e., the area of ​​each of the principal surfaces 103 and 105, is 48.4 mm 2 The area of ​​each external electrode when viewed from the direction in which the pair of principal surfaces 103 and 105 face each other is 48.4 mm 2 When viewed from the direction in which the pair of principal surfaces 103 and 105 face each other, the radius of the smallest inclusive circle Mec of the piezoelectric element 101 is 4.5 mm, and the area of ​​the smallest inclusive circle Mec is 63.6 mm. 2 When viewed from the direction in which the pair of principal surfaces 103 and 105 face each other, the piezoelectric element 101 has a hexagonal shape with an area whose ratio to the area of ​​the smallest encompassing circle Mec is 0.76.

[0044] Each specimen of sample S2 has the same configuration as the piezoelectric element PE1 described above, except for the planar shape of the piezoelectric body (external electrodes). The thickness of the piezoelectric body 1 is 0.175 mm. The thickness of each of the external electrodes 10, 20 is 0.010 mm. The area of ​​the piezoelectric element 1 as viewed from the direction DO, i.e., the area of ​​each of the main surfaces 3 and 5, is 50.9 mm 2 The area of ​​each of the external electrodes 10 and 20 as viewed from the direction DO is 50.9 mm, which is the same as the area of ​​each of the main surfaces 3 and 5. 2 When viewed from the direction DO, the radius of the smallest inclusive circle Mec of the piezoelectric element 1 is 4.5 mm, and the area of ​​the smallest inclusive circle Mec is 63.6 mm 2 When viewed from the direction DO, the piezoelectric element 101 has a hexagonal shape with an area whose ratio to the area of ​​the smallest encompassing circle Mec is 0.80.

[0045] Each specimen of sample S3 has the same configuration as the above-described piezoelectric element PE1, except for the planar shape of the piezoelectric body (external electrodes). The thickness of the piezoelectric body 1 is 0.175 mm. The thickness of each of the external electrodes 10, 20 is 0.010 mm. The area of ​​the piezoelectric element 1 as viewed from the direction DO, i.e., the area of ​​each of the main surfaces 3 and 5, is 52.9 mm 2 The area of ​​each of the external electrodes 10 and 20 as viewed from the direction DO is 52.9 mm, which is the same as the area of ​​each of the main surfaces 3 and 5. 2 When viewed from the direction DO, the radius of the smallest inclusive circle Mec of the piezoelectric element 1 is 4.5 mm, and the area of ​​the smallest inclusive circle Mec is 63.6 mm 2 When viewed from the direction DO, the piezoelectric element 101 has a hexagonal shape with an area whose ratio to the area of ​​the smallest encompassing circle Mec is 0.83.

[0046] Each specimen of sample S4 has the same configuration as the above-mentioned piezoelectric element PE2, except for the planar shape of the piezoelectric body (external electrodes). The thickness of the piezoelectric body 1 is 0.175 mm. The thickness of each of the external electrodes 10, 20 is 0.010 mm. The area of ​​the piezoelectric element 1 as viewed from the direction DO, i.e., the area of ​​each of the main surfaces 3 and 5, is 57.6 mm 2 The area of ​​each of the external electrodes 10 and 20 as viewed from the direction DO is 57.6 mm , which is the same as the area of ​​each of the main surfaces 3 and 5. 2 When viewed from the direction DO, the radius of the smallest inclusive circle Mec of the piezoelectric element 1 is 4.5 mm, and the area of ​​the smallest inclusive circle Mec is 63.6 mm 2When viewed from the direction DO, the piezoelectric element 101 has an octagonal shape with an area whose ratio to the area of ​​the smallest encompassing circle Mec is 0.91.

[0047] Each specimen in sample S5 includes a piezoelectric element 201 and a pair of external electrodes. As shown in FIG. 14, the piezoelectric element 201 includes a pair of opposing main surfaces 203, 205. When viewed from the direction in which the pair of main surfaces 203, 205 face each other, the piezoelectric element 201 has a perfect circular shape. The thickness of the piezoelectric element 201 is 0.175 mm. The thickness of each external electrode is 0.010 mm. FIG. 14 is a plan view of the piezoelectric element. The radius of the piezoelectric element 201 is 4.5 mm when viewed from the direction in which the pair of principal surfaces 203 and 205 face each other. The area of ​​the piezoelectric element 201 when viewed from the direction in which the pair of principal surfaces 203 and 205 face each other, i.e., the area of ​​each of the principal surfaces 203 and 205, is 63.6 mm. 2 The area of ​​each external electrode when viewed from the direction in which the pair of principal surfaces 203 and 205 face each other is 63.6 mm 2 , the same as the area of ​​each of the principal surfaces 203 and 205. 2 When viewed from the direction in which the pair of principal surfaces 203, 205 face each other, the area of ​​the smallest inclusive circle of the piezoelectric element 201 is the same as the area of ​​the piezoelectric element 201. When viewed from the direction in which the pair of principal surfaces 203, 205 face each other, the piezoelectric element 101 has a circular shape having an area whose ratio to the area of ​​the smallest inclusive circle is 1.

[0048] The results of confirming the electromechanical coupling coefficient for each of samples S1 to S5 are shown in FIG. 15. FIG. 15 is a diagram showing the relationship between the ratio of the area of ​​the piezoelectric element to the area of ​​the smallest encompassing circle and the electromechanical coupling coefficient. The horizontal axis represents the ratio of the area of ​​the piezoelectric element to the area of ​​the smallest encompassing circle (area of ​​piezoelectric element / area of ​​smallest encompassing circle) when viewed from the direction in which a pair of main surfaces included in the piezoelectric element face each other. The vertical axis represents the electromechanical coupling coefficient. The dashed line represents a third-order polynomial approximation curve. The electromechanical coupling coefficients of the four specimens of sample S1 are 36.484, 35.950, 35.706, and 35.602, respectively. The electromechanical coupling coefficients of the four specimens of sample S2 are 50.151, 49.751, 49.597, and 49.382, respectively. The electromechanical coupling coefficients of the four specimens of sample S3 are 55.920, 55.704, 55.987, and 55.725, respectively. The electromechanical coupling coefficients of the four specimens of sample S4 are 59.261, 59.124, 59.170, and 59.182, respectively. The electromechanical coupling coefficients of the four specimens of sample S5 are 59.601, 59.223, 59.158, and 59.033, respectively. As shown in FIG. 15, in a piezoelectric element in which the piezoelectric body has a polygonal shape with an area whose ratio to the area of ​​the smallest encompassing circle Mec is less than 0.8 when viewed from the direction in which a pair of principal surfaces included in the piezoelectric body face each other, the electromechanical coupling coefficient is significantly reduced.

[0049] In a configuration in which the piezoelectric element 1 has a polygonal shape when viewed from the direction DO, the number of corners in the planar shape does not change between piezoelectric elements PE1 even if manufacturing variations occur. For example, in a configuration in which the piezoelectric element 1 has a hexagonal planar shape, the planar shape of the piezoelectric element 1 remains hexagonal even if manufacturing variations occur. For example, in a configuration in which the piezoelectric element 1 has an octagonal planar shape, the planar shape of the piezoelectric element 1 remains octagonal even if manufacturing variations occur. Therefore, in a configuration in which the planar shape of the piezoelectric element 1 is polygonal, the planar shape of the piezoelectric element 1 is unlikely to differ between the piezoelectric elements PE1, i.e., there is unlikely to be variation in the planar shape of the piezoelectric element 1. There is unlikely to be variation in the resonant frequency between the piezoelectric elements PE1. There is also unlikely to be variation in the resonant frequency between the piezoelectric elements PE1a, PE1b, PE2, PE2a, or PE2b other than the piezoelectric element PE1.

[0050] In each of the piezoelectric elements PE1, PE1a, PE1b, PE2, PE2a, and PE2b, the piezoelectric body 1 has a polygonal shape with an area ratio to the area of ​​the smallest encompassing circle Mec of 0.8 or more when viewed from the direction DO. In each of the piezoelectric elements PE1, PE1a, PE1b, PE2, PE2a, and PE2b, the electromechanical coupling coefficient is unlikely to decrease significantly. As a result, in each of the piezoelectric elements PE1, PE1a, PE1b, PE2, PE2a, and PE2b, the vibration characteristics are unlikely to decrease.

[0051] In each of the piezoelectric elements PE1 and PE2, the external electrode 10 includes an outer edge 10e that coincides with the outer edge 3e included in the main surface 3. The external electrode 20 includes an outer edge 20e that coincides with the outer edge 5e included in the main surface 5. Therefore, each of the piezoelectric elements PE1 and PE2 tends to have a large piezoelectrically active area. As a result, the vibration characteristics of each of the piezoelectric elements PE1 and PE2 are even less likely to deteriorate.

[0052] In each of the piezoelectric elements PE1a, PE1b, PE2a, and PE2b, the external electrode 10 includes an electrode portion 11, and the external electrode 20 includes a plurality of electrode portions 21, 23, and 25. Therefore, the piezoelectric elements PE1a, PE1b, PE2a, and PE2b can be electrically connected only on the main surface 3. As a result, the piezoelectric elements PE1a, PE1b, PE2a, and PE2b can be easily mounted on other devices.

[0053] In each of the piezoelectric elements PE1a, PE1b, PE2a, and PE2b, the angles α1 and α2 are other than right angles. When each of the piezoelectric elements PE1a, PE1b, PE2a, and PE2b is driven, stress tends to concentrate at the corner defined by the outer edges 11e1 and 11e2 of the electrode portion 11 and the corner defined by the outer edges 21e1 and 21e2 of the electrode portion 21. If stress concentrates at the corner defined by the outer edges 11e1 and 11e2 of the electrode portion 11 or the corner defined by the outer edges 21e1 and 21e2 of the electrode portion 21, cracks may occur in the piezoelectric element 1 from the edge between the main surface 3 and the side surface 7b or the edge between the main surface 3 and the side surface 7f. In contrast, a configuration in which angles α1 and α2 are other than right angles tends to reduce the stress acting on the ridge between the principal surface 3 and the side surface 7b and the ridge between the principal surface 3 and the side surface 7f from the angle defined by outer edges 11e1 and 11e2 of electrode portion 11 and the angle defined by outer edges 21e1 and 21e2 of electrode portion 21. Therefore, each of piezoelectric elements PE1a, PE1b, PE2a, and PE2b suppresses cracks from occurring in the piezoelectric element 1 from the ridge between the principal surface 3 and the side surface 7b or the ridge between the principal surface 3 and the side surface 7f.

[0054] In each of the piezoelectric elements PE1a, PE1b, PE2a, and PE2b, the electrode portion 23 includes an outer edge 23e that coincides with the outer edge 5e included in the main surface 5. Therefore, each of the piezoelectric elements PE1a, PE1b, PE2a, and PE2b tends to have a large piezoelectrically active area. As a result, the vibration characteristics of each of the piezoelectric elements PE1a, PE1b, PE2a, and PE2b are even less likely to deteriorate.

[0055] Although the embodiments of the present invention have been described above, the present invention is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention.

[0056] When viewed from the direction DO, the piezoelectric body 1 may have, for example, a heptagonal shape, or a polygonal shape with more corners than an octagon.

[0057] The outer edge 10e does not have to coincide with the outer edge 3e. For example, the outer edge 10e may be spaced apart from the outer edge 3e and positioned more inward than the outer edge 3e when viewed from the direction DO. The outer edge 20e does not have to coincide with the outer edge 5e. For example, the outer edge 20e may be spaced apart from the outer edge 5e and positioned more inward than the outer edge 5e when viewed from the direction DO. The outer edge 11e1 does not have to coincide with the outer edge 3e. For example, the outer edge 11e1 may be spaced apart from the outer edge 3e and positioned more inward than the outer edge 3e when viewed from the direction DO. The outer edge 21e1 does not have to coincide with the outer edge 3e. For example, when viewed from the direction DO, the outer edge 21e1, except for the edge connected to the electrode portion 25, may be spaced apart from the outer edge 3e and positioned more inward than the outer edge 3e. Outer edge 23e does not have to coincide with outer edge 5e. For example, when viewed from direction DO, outer edge 20e, except for the edge connected to electrode portion 25, may be spaced apart from outer edge 5e and positioned more inward than outer edge 5e.

[0058] As can be understood from the above description of the embodiments and modifications, the present specification includes disclosure of the following aspects. (Appendix 1) a piezoelectric element including a pair of opposing main surfaces; a plurality of external electrodes respectively disposed on corresponding principal surfaces of the pair of principal surfaces, When viewed from the direction in which the pair of principal surfaces face each other, the piezoelectric element has a polygonal shape having an area whose ratio to the area of ​​a smallest encompassing circle is 0.8 or more and less than 1. (Appendix 2) 2. The piezoelectric element according to claim 1, wherein the polygonal shape includes a regular hexagonal shape. (Appendix 3) 2. The piezoelectric element according to claim 1, wherein the polygonal shape includes a regular octagonal shape. (Appendix 4) 4. The piezoelectric element according to claim 1, wherein each of the plurality of external electrodes includes an outer edge that coincides with an outer edge included in a corresponding one of the pair of main surfaces. (Appendix 5) the piezoelectric element includes a plurality of side surfaces connecting the pair of main surfaces, the plurality of external electrodes include a first external electrode and a second external electrode, the first external electrode includes a first electrode portion disposed on a part of one of the pair of principal surfaces, The second external electrode is a second electrode portion disposed on a remainder of the one major surface and spaced apart from the first electrode portion; a third electrode portion disposed on the other of the pair of principal surfaces; A piezoelectric element described in any one of Appendices 1 to 3, including a fourth electrode portion that is arranged on at least one of the plurality of side surfaces and connects the second electrode portion and the third electrode portion. (Appendix 6) the first electrode portion and the second electrode portion each include a first outer edge facing each other, and a second outer edge that is continuous with the first outer edge and coincides with an outer edge included in the one main surface, 6. The piezoelectric element of claim 5, wherein, when viewed from a direction perpendicular to the one principal surface, the angle formed between the first outer edge included in each of the first electrode portion and the second electrode portion and the outer edge included in the one principal surface is other than a right angle. (Appendix 7) 7. The piezoelectric element according to claim 5, wherein the third electrode portion includes an outer edge that coincides with an outer edge included in the other main surface. [Explanation of symbols]

[0059] 1...piezoelectric element, 3...main surface, 3e...outer edge, 5...main surface, 5e...outer edge, 7...side surface, 10...external electrode, 10e...outer edge, 11...electrode portion, 11e1, 11e2...outer edge, 20...external electrode, 20e...outer edge, 21...electrode portion, 21e1, 21e2...outer edge, 23...electrode portion, 23e...outer edge, 25...electrode portion, DO...direction in which a pair of main surfaces included in the piezoelectric element face each other, Ee...external electrode, Mec...minimum enclosing circle, PE1, PE1a, PE1b, PE2, PE2a, PE2b...piezoelectric elements.

Claims

1. a piezoelectric element including a pair of opposing main surfaces; a plurality of external electrodes respectively disposed on corresponding principal surfaces of the pair of principal surfaces, When viewed from the direction in which the pair of principal surfaces face each other, the piezoelectric element has a polygonal shape having an area whose ratio to the area of ​​a smallest encompassing circle is 0.8 or more and less than 1.

2. The piezoelectric element according to claim 1 , wherein the polygonal shape includes a regular hexagonal shape.

3. The piezoelectric element according to claim 1 , wherein the polygonal shape includes a regular octagonal shape.

4. 4. The piezoelectric element according to claim 1, wherein each of the plurality of external electrodes includes an outer edge that coincides with an outer edge included in a corresponding one of the pair of main surfaces.

5. the piezoelectric element includes a plurality of side surfaces connecting the pair of main surfaces, the plurality of external electrodes include a first external electrode and a second external electrode, the first external electrode includes a first electrode portion disposed on a part of one of the pair of principal surfaces, The second external electrode is a second electrode portion disposed on a remainder of the one major surface and spaced apart from the first electrode portion; a third electrode portion disposed on the other of the pair of principal surfaces; The piezoelectric element according to any one of claims 1 to 3, further comprising: a fourth electrode portion arranged on at least one of the plurality of side surfaces and connecting the second electrode portion and the third electrode portion.

6. the first electrode portion and the second electrode portion each include a first outer edge facing each other, and a second outer edge that is continuous with the first outer edge and coincides with an outer edge included in the one main surface, 6. The piezoelectric element according to claim 5, wherein, when viewed from a direction perpendicular to the one principal surface, an angle formed between the first outer edge included in each of the first electrode portion and the second electrode portion and the outer edge included in the one principal surface is other than a right angle.

7. The piezoelectric element according to claim 5 , wherein the third electrode portion includes an outer edge that coincides with an outer edge included in the other main surface.

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