Laminated piezoelectric element and electroacoustic transducer

Abstract

Provided are a laminated piezoelectric element in which a plurality of layers of piezoelectric films are laminated, said laminated piezoelectric element being capable of obtaining high sound pressure even after long-term use, and an electro-acoustic transducer. A laminated piezoelectric element in which a plurality of piezoelectric films are laminated with an adhesive layer interposed therebetween, the adhesive layer having an adhesive region and a gap portion, the adhesive region and the gap portion being provided in one direction, a direction orthogonal to the one direction, a direction at 45 degrees to the one direction, and a direction at 135 degrees to the one direction in a plane of a main surface of the piezoelectric film, respectively, and the gap portion being provided in the one direction, the direction orthogonal to the one direction, the direction at 45 degrees to the one direction, and the direction at 135 degrees to the one direction. A cross-section in the lamination direction of the piezoelectric film of the laminated piezoelectric element is observed with a scanning electron microscope to obtain ten consecutive fields of view, and the ratio d2 / d1 of the average value d1 of the thickness of the piezoelectric film observed in each field of view to the average value d2 of the thickness of the adhesive layer is 0.15-1.0 inclusive. And the ratio of the total area of the gap sections observed in each field of view to the total area of the adhesive layer is 1% or more and less than 40%.

Description

Technical Field

[0001] The present invention relates to a stacked piezoelectric element and an electroacoustic transducer. Background Art

[0002] Piezoelectric elements are used in various applications as so-called exciters (excitons) that, when attached to various objects, cause them to vibrate and produce sound. For example, by attaching an exciter to an image display panel or screen and vibrating it, it can produce sound instead of a speaker.

[0003] As a piezoelectric element, a piezoelectric film having a piezoelectric layer sandwiched between electrode layers and protective layers has been proposed. In addition, a stacked piezoelectric element has been proposed in which a plurality of piezoelectric films are stacked with an adhesive layer interposed therebetween.

[0004] For example, Patent Document 1 describes a piezoelectric film comprising a polymer composite piezoelectric body and electrode layers. The polymer composite piezoelectric body is formed by dispersing piezoelectric particles in a matrix containing a polymer material. The electrode layers are formed on both surfaces of the polymer composite piezoelectric body. The piezoelectric film has a loss tangent at a frequency of 1 kHz, as measured by dynamic viscoelasticity, that reaches a maximum value of 0.1 or greater within a temperature range exceeding 50°C and below 150°C, and has a value of 0.08 or greater at 50°C. Furthermore, Patent Document 1 describes a piezoelectric element formed by stacking multiple layers of piezoelectric films by folding the piezoelectric film one or more times.

[0005] Previous technical literature

[0006] Patent Literature

[0007] Patent Document 1: International Publication No. 2020 / 196850 Summary of the Invention

[0008] Technical issues to be solved by the invention

[0009] A laminated piezoelectric element, made by laminating multiple layers of piezoelectric films, is attached to a diaphragm and vibrates the diaphragm, thereby acting as an exciter to generate sound from the diaphragm. Electroacoustic transducers made by attaching a laminated piezoelectric element to a diaphragm are expected to maintain high sound pressure even after long-term use. Therefore, a high initial sound pressure is required.

[0010] Furthermore, it was found that even when the initial sound pressure is high, there is a problem that the sound pressure decreases over time with long-term use.

[0011] The present inventors conducted research on this issue and found that when power is applied to a laminated piezoelectric element to generate sound, the element undergoes repeated significant expansion and contraction. Because the laminated piezoelectric element is bonded to the vibration plate, it repeatedly experiences significant warping. The inventors discovered that this is due to the differential expansion and contraction across the thickness of the laminated piezoelectric element, which exerts stress and causes defects such as delamination at the interface between the piezoelectric film and the adhesive layer.

[0012] The present invention aims to solve the problems of the prior art and to provide a stacked piezoelectric element and an electroacoustic transducer that can achieve high sound pressure even after long-term use, in a stacked piezoelectric element formed by stacking a plurality of piezoelectric films.

[0013] Means for solving technical problems

[0014] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0015] [1] A stacked piezoelectric element comprising a plurality of piezoelectric films stacked with an adhesive layer interposed therebetween, wherein:

[0016] The adhesive layer has an adhesive region and a gap portion in the in-plane direction of the main surface of the piezoelectric film.

[0017] A cross section of the stacking direction of the piezoelectric film of the stacked piezoelectric element is observed using a scanning electron microscope in one direction within the plane of the main surface of the piezoelectric film, a direction perpendicular to the one direction, a direction at 45 degrees to the one direction, and a direction at 135 degrees to the one direction, obtaining 10 consecutive fields of view, and a ratio d2 / d1 of an average value d1 of the thickness of the piezoelectric film observed in each field of view to an average value d2 of the thickness of the adhesive layer is greater than or equal to 0.15 and less than or equal to 1.0,

[0018] Furthermore, the ratio of the total area of ​​the gap portions observed in each field of view to the total area of ​​the adhesive layer is 1% or more and 40% or less.

[0019] [2] The laminated piezoelectric element according to [1], wherein

[0020] The piezoelectric film includes a piezoelectric layer formed of a polymer composite piezoelectric body containing piezoelectric particles in a matrix containing a polymer material, and electrode layers provided on both surfaces of the piezoelectric layer.

[0021] [3] The laminated piezoelectric element according to [1], wherein the piezoelectric film is folded one or more times to form a plurality of laminated layers.

[0022] [4] An electroacoustic transducer formed by attaching the laminated piezoelectric element according to any one of [1] to [3] to a vibration plate.

[0023] Effects of the Invention

[0024] According to the present invention, it is possible to provide a multi-layer piezoelectric element and an electroacoustic transducer that can achieve high sound pressure even after long-term use in a multi-layer piezoelectric element formed by stacking a plurality of piezoelectric films. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a perspective view schematically showing an example of the laminated piezoelectric element of the present invention.

[0026] Figure 2 is a schematic diagram showing Figure 1 FIG. 1 is a diagram showing an example of an electroacoustic transducer having stacked piezoelectric elements.

[0027] Figure 3 yes Figure 2 A partial enlarged view of .

[0028] Figure 4 It is schematically shown Figure 1 A top view of an adhesive layer included in the laminated piezoelectric element shown.

[0029] Figure 5 yes Figure 2 A cross-sectional view of the electroacoustic transducer shown.

[0030] Figure 6 It is a diagram for explaining the operation of the multi-layer piezoelectric element of the present invention.

[0031] Figure 7 It is a plan view schematically showing another example of the adhesive layer included in the multi-layer piezoelectric element of the present invention.

[0032] Figure 8 It is a plan view schematically showing another example of the adhesive layer included in the multi-layer piezoelectric element of the present invention.

[0033] Figure 9 It is a plan view schematically showing another example of the adhesive layer included in the multi-layer piezoelectric element of the present invention.

[0034] Figure 10 This is a partially enlarged view schematically showing another example of the laminated piezoelectric element of the present invention.

[0035] Figure 11 This is a diagram schematically showing an example of a piezoelectric film included in the multi-layer piezoelectric element of the present invention.

[0036] Figure 12 This is a conceptual diagram for explaining an example of a method for producing a piezoelectric film.

[0037] Figure 13This is a conceptual diagram for explaining an example of a method for producing a piezoelectric film.

[0038] Figure 14 This is a conceptual diagram for explaining an example of a method for producing a piezoelectric film.

[0039] Figure 15 This is a conceptual diagram for explaining an example of a conventional electroacoustic transducer having a laminated piezoelectric element. DETAILED DESCRIPTION

[0040] Hereinafter, the laminated piezoelectric element and the electroacoustic transducer of the present invention will be described in detail based on the preferred embodiments shown in the accompanying drawings.

[0041] Although the description of the constituent elements described below may be based on representative embodiments of the present invention, the present invention is not limited to these embodiments.

[0042] In addition, in this specification, the numerical range expressed using "to" means a range including the numerical values ​​described before and after "to" as the lower limit and the upper limit.

[0043] [Stacked piezoelectric element and electroacoustic transducer]

[0044] The laminated piezoelectric element of the present invention is a laminated piezoelectric element formed by laminating a plurality of piezoelectric films with an adhesive layer interposed therebetween, wherein:

[0045] The adhesive layer has an adhesive region and a gap portion in the in-plane direction of the main surface of the piezoelectric film.

[0046] A cross section of the stacking direction of the piezoelectric film of the stacked piezoelectric element is observed using a scanning electron microscope in one direction within the plane of the main surface of the piezoelectric film, a direction perpendicular to the one direction, a direction at 45 degrees to the one direction, and a direction at 135 degrees to the one direction, obtaining 10 consecutive fields of view, and a ratio d2 / d1 of an average value d1 of the thickness of the piezoelectric film observed in each field of view to an average value d2 of the thickness of the adhesive layer is greater than or equal to 0.15 and less than or equal to 1.0,

[0047] Furthermore, the ratio of the total area of ​​the gap portions observed in each field of view to the total area of ​​the adhesive layer is 1% or more and 40% or less.

[0048] The electroacoustic transducer of the present invention is an electroacoustic transducer in which the piezoelectric element is attached to a vibration plate.

[0049] exist Figure 1 A perspective view schematically showing an example of a laminated piezoelectric element of the present invention is shown in FIG. Figure 2 The schematic diagram shows Figure 1 FIG. 1 is a diagram showing an example of an electroacoustic transducer of the present invention comprising a stacked piezoelectric element. Figure 3Shown in Figure 2 A partial enlarged view of .

[0050] Figures 1 to 3 The piezoelectric element 50 shown is formed by folding a single rectangular piezoelectric film 10 three times in one direction to stack four layers of the piezoelectric film 10. That is, the stacked piezoelectric element 50 is a stacked piezoelectric element in which four layers of the piezoelectric film 10 are stacked.

[0051] exist Figures 1 to 3 Although omitted to simplify the drawing and clearly illustrate the structure of the laminated piezoelectric element 50 , the piezoelectric film 10 has electrode layers on both surfaces of the piezoelectric layer 20 and a protective layer covering the two electrode layers.

[0052] In the following description, the direction in which the piezoelectric film 10 is folded ( Figure 1 The left and right directions in the figure are called folding directions.

[0053] The details will be described later. For example, the piezoelectric film 10 comprises: a piezoelectric layer exhibiting piezoelectricity; electrode layers disposed on both sides of the piezoelectric layer; and a protective layer disposed on the electrode layers. When a voltage is applied to the electrode layers (electrode pair) sandwiching the piezoelectric layer, the piezoelectric layer expands and contracts in accordance with the applied voltage. As a result, the piezoelectric film 10 (piezoelectric layer 20) contracts in the thickness direction. Simultaneously, due to the Poisson's ratio, the piezoelectric film 10 also expands and contracts in the plane direction. This enables the piezoelectric film 10 to exhibit piezoelectric properties.

[0054] By folding the piezoelectric film 10 having such piezoelectric properties, a plurality of piezoelectric films can be stacked to produce the stacked piezoelectric element 50 .

[0055] exist Figure 1 In the illustrated example, the multilayer piezoelectric element 50 includes a stacked portion in which four piezoelectric films 10 are stacked in a plan view, and a protruding portion protruding outward in a planar direction from the stacked portion.

[0056] In the present invention, the laminated portion refers to a portion of the laminated portion that is formed when viewed from above. Figure 1 In the stacked piezoelectric element, when viewed from above (or below), the piezoelectric film overlaps two or more layers. Figure 1 and Figure 2 In the example shown, the region where four layers of the piezoelectric film 10 are stacked is the stacked portion. Figure 1 In the example shown, the laminated portion has a substantially rectangular shape, with short sides extending in the folding direction and long sides extending in a direction perpendicular to the folding direction.

[0057] On the other hand, the protrusion refers to a region that protrudes from the laminated portion in the surface direction and does not overlap with other layers when viewed from above. Figure 1 In the example shown, the right end of the uppermost layer in the figure is a protrusion. Figure 2 and Figure 3 In the example shown, illustration of the protruding portion is omitted.

[0058] In the stacked portion, adjacent layers of the piezoelectric film 10 are bonded to each other via an adhesive layer 19. In the present invention, the adhesive layer 19 includes an adhesive region for bonding the layers to each other and a gap. The adhesive layer 19 will be described in detail later.

[0059] like Figure 1 As shown, the protruding portion includes wires 40 and 42 for connecting the two electrode layers to an external power source. If the piezoelectric film 10 includes a protective layer, through-holes are formed in the protective layer in the protruding region to expose the electrode layers, and connecting portions are provided to electrically connect wires 40 and 42 to the electrode layers, respectively. The through-holes can be formed using any known method, such as laser processing, removal by solvent, or mechanical processing such as mechanical polishing, depending on the material used to form the protective layer.

[0060] The connection portion is filled with a known conductive material such as a conductive metal paste such as silver paste, a conductive carbon paste, or a conductive nano-ink to connect to a wire connected to an external power source.

[0061] In addition, the method for connecting the electrode layer and the lead wire in the protruding portion is not limited, and various known methods can be used.

[0062] As an example, there are methods of connecting a conductor such as copper foil to an electrode layer to lead the electrode to the outside, and methods of forming through holes in a protective layer using a laser or the like and filling the through holes with a conductive material to lead the electrode to the outside.

[0063] And, in Figure 1 In the example shown, a protrusion is provided and the electrode layer and the external power supply are connected at the protrusion. However, the present invention is not limited to this, and the electrode layer and the lead may be electrically connected in the laminated portion.

[0064] Preferred electrode extraction methods include the method described in Japanese Patent Application Laid-Open No. 2014-209724 and the method described in Japanese Patent Application Laid-Open No. 2016-015354.

[0065] like Figure 2 As shown, the laminated piezoelectric element 50 of the present invention is adhered to the vibration plate 102 via the adhesive layer 104 to form the electroacoustic transducer 100 .

[0066] In this electroacoustic transducer 100, a voltage is applied to the electrode layers of the stacked piezoelectric element 50 using an external power source to drive the stacked piezoelectric element 50. When the stacked piezoelectric element 50 is driven, it expands and contracts in the planar direction, bending the vibration plate 102 to which the stacked piezoelectric element 50 is bonded. As a result, the vibration plate 102 vibrates, generating sound. The vibration plate 102 vibrates according to the magnitude of the driving voltage applied to the stacked piezoelectric element 50, generating sound corresponding to the driving voltage applied to the stacked piezoelectric element 50.

[0067] That is, the laminated piezoelectric element 50 can function as an actuator.

[0068] Here, in the stacked piezoelectric element 50 of the present invention, the adhesive layer 19 has an adhesive area and a gap portion in the in-plane direction of the main surface. When the cross section is observed using a scanning electron microscope (SEM), the area ratio of the gap portion in the adhesive layer 19 and the thickness ratio of the piezoelectric film and the adhesive layer are respectively within the specified range.

[0069] In this regard, use Figure 4 and Figure 5 Provide explanation.

[0070] Figure 4 It is schematically shown Figure 1 FIG. 1 is a plan view of an adhesive layer 19 included in a multi-layer piezoelectric element 50 . Figure 5 yes Figure 1 The illustrated cross-sectional view is a cross-sectional view taken along line AA of the multilayer piezoelectric element 50. The plan view is a view viewed from the stacking direction of the multilayer piezoelectric film 10.

[0071] like Figure 4 As shown, the adhesive layer 19 has the following structure: in the long side direction ( Figure 4 In the left-right direction in FIG, the bonding areas 18 and the gap portions 17 are alternately arranged, that is, arranged in a stripe shape.

[0072] When a cross section of the piezoelectric film 10 of the stacked piezoelectric element 50 in which each adhesive layer 19 has the structure described above (having the adhesive region 18 and the gap portion 17 in a stripe shape) is observed using an SEM, it is as follows: Figure 5 As shown, the bonding area 18 and the gap 17 in the bonding layer 19 can be seen.

[0073] In the present invention, a cross section is observed using an SEM in one direction within the plane of the main surface of the piezoelectric film, in a direction perpendicular to the one direction, in a direction at 45° to the one direction, and in a direction at 135° to the one direction, and 10 consecutive fields of view are obtained on each cross section. The ratio d2 / d1 of the average value d1 of the thickness of the piezoelectric film 10 observed in each field of view to the average value d2 of the thickness of the adhesive layer is greater than 0.15 and less than 0.1, and the ratio of the total area of ​​the gap portion observed in each field of view to the total area of ​​the adhesive layer is greater than 1% and less than 40%.

[0074] For example, through the center of the rectangular laminated portion, in a direction parallel to the long side ( Figure 4 The single-point dash line A in the middle), the direction parallel to the short side ( Figure 4 The single dotted line B in the figure) and the direction at 45° to the long side ( Figure 4 The single-dot dash line C in the middle), the direction of 135° with the long side (with Figure 4 The cross section was taken in each direction (a direction perpendicular to the dot-dashed line D in the figure) and observed using SEM to obtain 10 consecutive fields of view. Figure 5 As shown, the adhesive region 18 and the gap 17 of each adhesive layer 19 are observed. On the other hand, in the cross section in the direction of the dot-dashed line B, the entire area of ​​the adhesive layer 19 is observed as the adhesive region 18. Furthermore, in the cross section in the direction of the dot-dashed line C and the cross section in the direction of the dot-dashed line D, the adhesive region 18 and the gap 17 of each adhesive layer 19 are observed.

[0075] In addition, in the above example, cross sections are taken in four directions in such a way that one direction becomes parallel to the long side of the rectangular-shaped stacking portion, but this is not limited to this. The cross sections in the four directions can be cross sections in directions orthogonal to each other and in directions of 45° and 135°, or any direction can be not parallel to the long side and the short side.

[0076] Furthermore, in the above example, any cross section passes through the center of the rectangular laminated portion, but the present invention is not limited thereto, and a cross section at any position may be obtained.

[0077] Specifically, for example, cross-sectional observation can be performed as follows.

[0078] To observe the cross section of the laminated piezoelectric element, cut along the thickness direction. For example, a Leica Biosystems RM2265 can be equipped with an 8mm wide histo blade manufactured by Drukker, and the speed can be set to 1 on the controller scale, and the engagement can be set to 0.25 to 1 μm.

[0079] The cross section thereof is observed by SEM (for example, manufactured by Hitachi High-Tech Corporation., SU8220).

[0080] An example of the observation conditions using the SEM is shown below.

[0081] The samples were treated for conductivity by platinum evaporation, and the working distance was set to 8 mm.

[0082] The observation conditions are SE (secondary-electron) image (Upper), acceleration voltage: 0.5kV, focus adjustment and astigmatism adjustment to produce the clearest image, and automatic brightness adjustment (automatically setting brightness: 0, contrast: 0) when the piezoelectric film becomes the entire screen.

[0083] The imaging magnification was set so that the thickness direction of the stacked piezoelectric element fit within one frame and occupied at least half of the frame. Furthermore, the image was taken so that the bottom piezoelectric film was level with the bottom of the image.

[0084] Ten consecutive fields of view were imaged using an SEM at any position on the cross section of the laminated piezoelectric element. This SEM observation was performed in four directions as described above, resulting in a total of 40 SEM images.

[0085] The total area of ​​adhesive layer 19 and the total area of ​​gaps 17 in adhesive layer 19 were determined in a total of 40 fields of view of the obtained SEM images, and the ratio of the total area of ​​gaps 17 to the total area of ​​adhesive layer 19 was calculated.

[0086] The method for obtaining the total area of ​​the adhesive layer 19 and the total area of ​​the gaps 17 in the adhesive layer 19 from the SEM image is not particularly limited, and they can be obtained using commercially available image processing software such as WinROOF.

[0087] For each field of view of the obtained SEM image, the thickness of each piezoelectric film 10 and the thickness of each adhesive layer 19 (adhesive region 18) were measured at intervals of 1 μm at 10 or more points, and the average values ​​were calculated. The average value d1 of the thickness of each piezoelectric film 10 and the average value d2 of the thickness of each adhesive layer 19 measured in all fields of view were calculated, and their ratio d2 / d1 was calculated.

[0088] In the present invention, the ratio of the total area of ​​the gaps 17 measured as described above to the total area of ​​the adhesive layer 19 is set to 1% or more and 40% or less.

[0089] Furthermore, the ratio d2 / d1 of the thickness (average value) of the piezoelectric film 10 and the adhesive layer 19 is set to 0.15 or more and 1.0 or less.

[0090] As described above, in an electroacoustic transducer formed by attaching a laminated piezoelectric element to a vibration plate, in order to obtain a high sound pressure even after long-term use, it is necessary to increase the initial sound pressure and suppress a decrease in sound pressure over time.

[0091] As a result of studies conducted by the present inventors, it was found that the temporal decrease in sound pressure is caused by the following mechanism.

[0092] When a stacked piezoelectric element made of stacked multi-layer piezoelectric films is attached to a vibration plate and used as an exciter, when electricity is applied to the stacked piezoelectric element to produce sound, the stacked piezoelectric element will repeatedly expand and contract significantly, but because the stacked piezoelectric element is glued to the vibration plate, the stacked piezoelectric element will repeatedly warp significantly.

[0093] Discovery, such as Figure 15 As in the conventional laminated piezoelectric element 250 shown in the left figure, if the piezoelectric film 10 is fully bonded by the adhesive layer 219, Figure 15 As shown in the middle and right figures of the figure, when the laminated piezoelectric element 250 warps, a difference in the amount of expansion and contraction occurs in the thickness direction of the laminated piezoelectric element 250, which causes stress and causes defects such as peeling at the interface between the piezoelectric film 210 and the adhesive layer 219. As a result, it was discovered that the sound pressure decreases over time with long-term use.

[0094] In contrast, in the laminated piezoelectric element 50 of the present invention, the adhesive layer 19 has an adhesive region 18 and gaps 17, and in a cross section observed using an SEM, the ratio of the total area of ​​the gaps 17 to the total area of ​​the adhesive layer 19 is greater than 1% and less than 40%. By having the gaps 17 within this range in the laminated piezoelectric element 50, when power is applied to the laminated piezoelectric element 50 to generate sound, the stress applied when the laminated piezoelectric element 50 is repeatedly warped can be alleviated, and peeling at the interface between the piezoelectric film 10 and the adhesive layer 19 can be suppressed (see FIG. Figure 6 ). As a result, even in the case of long-term use, the decrease in sound pressure over time can be suppressed.

[0095] On the other hand, if the area ratio of gap 17 is too high (exceeding 40%), the expansion and contraction (vibration) of piezoelectric film 10 is not easily transmitted to diaphragm 102, potentially resulting in a decrease in initial sound pressure. By setting the area ratio of gap 17 to 40% or less, the expansion and contraction (vibration) of piezoelectric film 10 is transmitted to diaphragm 102, maintaining a high initial sound pressure. Therefore, an electroacoustic transducer using the laminated piezoelectric element 50 of the present invention can achieve high sound pressure even after long-term use.

[0096] Furthermore, in the stacked piezoelectric element 50 of the present invention, the ratio d2 / d1 of the thickness d1 of the piezoelectric film 10 to the thickness d2 of the adhesive layer 19 is set to 0.15 or greater and 1.0 or less. By setting the ratio d2 / d1 to 0.15 or greater, that is, by setting the thickness of the adhesive layer 19 to a certain level, the adhesion between the piezoelectric film 10 and the adhesive layer 19 can be ensured, and delamination at the interface between the piezoelectric film 10 and the adhesive layer 19 can be suppressed. Furthermore, by setting the ratio d2 / d1 to 1.0 or less, that is, by not making the adhesive layer 19 too thick, the expansion and contraction of the piezoelectric film 10 can be suppressed from being absorbed by the adhesive layer 19 and transmitted to the vibration plate 102. This can increase the initial sound pressure and also increase the sound pressure after use.

[0097] Here, from the perspective of suppressing peeling at the interface between the piezoelectric film 10 and the adhesive layer 19 and increasing the initial sound pressure, the ratio of the total area of ​​the gap 17 to the total area of ​​the adhesive layer 19 is preferably greater than 2% and less than 35%, and more preferably greater than 5% and less than 30%.

[0098] Furthermore, from the viewpoint of increasing the initial sound pressure and the sound pressure after use, the ratio d2 / d1 of the thickness d1 of the piezoelectric film 10 to the thickness d2 of the adhesive layer 19 is preferably 0.2 to 0.75, more preferably 0.3 to 0.5.

[0099] And, in the above example, if Figure 4 As shown, the adhesive layer 19 is provided with adhesive regions 18 and gap portions 17 alternately arranged in the long side direction, i.e., arranged in a stripe-like structure, but as long as the ratio of the total area of ​​the gap portions 17 to the total area of ​​the adhesive layer 19 satisfies the above range, the arrangement pattern of the adhesive regions 18 and the gap portions 17 is not limited to this.

[0100] For example, Figure 7 As in the adhesive layer 19 b shown in the figure, the adhesive regions 18 and the gaps 17 may be arranged alternately in the short-side direction, that is, in a stripe-like structure.

[0101] Alternatively, you can set it as Figure 8 The structure of the adhesive layer 19c shown is a lattice-like structure in which gaps 17 are arranged parallel to the long sides and the short sides.

[0102] Alternatively, you can set it as Figure 9 The structure of the adhesive layer 19d shown is a grid-like structure in which adhesive regions 18 are arranged parallel to the long sides and the short sides.

[0103] And, as Figure 4 and Figure 7As shown in the example, when the bonding area 18 and the gap portion 17 are configured into a stripe-like structure, the width of the bonding area 18 (the width in the arrangement direction of the bonding area 18 and the gap portion 17) is preferably 1 μm to 5000 μm, more preferably 5 μm to 2500 μm, and further preferably 10 μm to 1000 μm.

[0104] The arrangement pattern of the adhesive regions 18 and the gaps 17 in each of the plurality of adhesive layers 19 of the laminated piezoelectric element 50 may be the same or different. For example, one adhesive layer 19 may have the following arrangement: Figure 4 The adhesive region 18 and the gap portion 17 are arranged in a stripe shape in the longitudinal direction, and the other adhesive layer 19 has a structure as shown in FIG. Figure 7 In the illustrated structure, the adhesive regions 18 and the gaps 17 are arranged in stripes in the short-side direction.

[0105] Furthermore, the plurality of adhesive layers 19 may have the same arrangement pattern, and the arrangement positions of the adhesive regions 18 and the gaps 17 may be offset within the plane. Figure 5 As shown in the laminated piezoelectric element 50, the uppermost adhesive layer 19 and the middle adhesive layer 19, the position of the gap 17 can be in the in-plane direction (in Figure 5 The center is the left and right direction) upward offset.

[0106] In addition, Figure 1 In the example shown, the stacked piezoelectric element 50 is a stacked piezoelectric element formed by folding the piezoelectric film 10 three times to stack four layers of piezoelectric films 10, but the present invention is not limited thereto. The stacked piezoelectric element may be a stacked piezoelectric element formed by stacking two or three layers of piezoelectric films, or may be a stacked piezoelectric element formed by stacking five or more layers of piezoelectric films. Figure 10 The same is true for the piezoelectric element shown.

[0107] And, in Figure 1 In the example shown, the laminated piezoelectric element 50 includes a plurality of piezoelectric films laminated by folding a long piezoelectric film 10 one or more times, but the present invention is not limited thereto. Figure 10 As shown, the laminated piezoelectric element may have a structure in which a plurality of single-sheet (slice-shaped) piezoelectric films 10 are laminated.

[0108] Figure 10 In the illustrated laminated piezoelectric element, four piezoelectric films 10 are laminated via an adhesive layer 19. Each of the four piezoelectric films 10 is connected to a power source (not shown).

[0109] exist Figure 10In the stacked piezoelectric element shown, the piezoelectric film 10 is preferably polarized in the thickness direction, and the polarization directions of adjacent piezoelectric films 10 are opposite to each other. Therefore, in adjacent piezoelectric films 10, the first electrode layers 24 and the second electrode layers 26 are opposite to each other. Therefore, regardless of whether it is an AC power supply or a DC power supply, the power supply always supplies power of the same polarity to the opposing electrodes. Therefore, in Figure 10 In the illustrated stacked piezoelectric element, even if the electrodes of adjacent piezoelectric films 10 come into contact with each other, a short circuit does not occur.

[0110] The polarization direction of the piezoelectric film 10 can be detected by a d33 meter, etc. Alternatively, the polarization direction of the piezoelectric film 10 can be known from the polarization processing conditions described later.

[0111] And, in Figure 10 In the example shown, the polarization directions of the adjacent piezoelectric films 10 are opposite to each other, but the present invention is not limited to this, and the polarization directions of the adjacent piezoelectric films 10 may be the same.

[0112] will be as Figure 1 The stacked piezoelectric element formed by folding and stacking long piezoelectric films has the following advantages.

[0113] That is, when multiple slices of piezoelectric films 10 are stacked, the first electrode layer 24 and the second electrode layer 26 need to be connected to a driving power source for each piezoelectric film. In contrast, in a structure in which long strips of piezoelectric films 10 are folded and stacked, the stack can be formed from only one long strip of piezoelectric film 10. Furthermore, in a structure in which long strips of piezoelectric films 10 are folded and stacked, only one power source is required to apply the driving voltage, and electrodes need only be drawn from the piezoelectric film 10 at one location.

[0114] Furthermore, in a structure in which the long piezoelectric films 10 are folded and stacked, the polarization directions of the adjacent piezoelectric films are necessarily opposite to each other (in Figure 3 (See the arrow in the piezoelectric body 20 in FIG. 1 ). Therefore, even if the electrode layers of adjacent piezoelectric films 10 come into contact with each other, a short circuit does not occur.

[0115] And, in Figure 1 In the example shown, the shape of the stacked piezoelectric element 50 (stacked portion) when viewed from above is substantially rectangular, but the shape is not limited thereto and may be various shapes such as square, circular, elliptical, polygonal, or irregular.

[0116] like Figure 1In the example shown, when the shape of the stacked piezoelectric element 50 (stacked portion) is rectangular when viewed from above, it is preferable to have a structure in which the bonding regions 18 and the gaps 17 are alternately arranged in the longitudinal direction. In other words, it is preferable to have a structure in which the bonding regions 18 and the gaps 17 are alternately arranged in the direction of greater curvature.

[0117] Adhesive layer and bonding layer

[0118] As the adhesive (adhesive) used in the adhesive layer 19 (adhesive region 18 ) for bonding the piezoelectric films 10 together, various known adhesives can be used as long as they can bond the adjacent piezoelectric films 10 .

[0119] Furthermore, as the adhesive (adhesive) used in the adhesive layer 104 for adhering the laminated piezoelectric element 50 and the vibration plate 102 , various known adhesives can be used as long as they can adhere the laminated piezoelectric element 50 and the vibration plate 102 .

[0120] Therefore, the adhesive layer 19 (adhesive area 18) and the adhesive layer 104 can be a layer composed of an adhesive that is fluid when bonded and then becomes solid, or a layer composed of an adhesive that is a soft solid in a gel-like (rubber-like) state when bonded and then remains in a gel-like state, or a layer composed of a material having the characteristics of both an adhesive and an adhesive.

[0121] Here, the stacked piezoelectric element 50 generates sound by bending and vibrating the vibration plate 102 through the expansion and contraction of the stacked piezoelectric films 10. Therefore, in the electroacoustic transducer 100, it is preferable that the expansion and contraction of the stacked piezoelectric element 50 be directly transmitted to the vibration plate 102. If a viscous material that dampens vibrations is present between the vibration plate 102 and the stacked piezoelectric element 50, or between the piezoelectric films 10, the efficiency of transmitting the expansion and contraction energy of the stacked piezoelectric element 50 to the vibration plate 102 decreases, resulting in a reduction in the driving efficiency of the electroacoustic transducer 100.

[0122] Taking this into consideration, the adhesive layer 19 that bonds the piezoelectric films to each other and the adhesive layer 104 that bonds the vibration plate to the stacked piezoelectric element are preferably adhesive layers composed of an adhesive. This adhesive layer can provide a stronger and harder adhesive layer than an adhesive layer composed of an adhesive. More preferred adhesive layers include, specifically, those composed of thermoplastic adhesives such as polyester adhesives and styrene-butadiene rubber (SBR) adhesives.

[0123] Adhesion, unlike bonding, is useful when a high bonding temperature is required. Thermoplastic adhesives are preferred because they combine "relatively low temperature, short bonding time, and strong adhesion."

[0124] The thickness of the adhesive layer 19 is not limited, and may be appropriately set according to the material of the adhesive layer 19 so as to obtain a sufficient adhesive force (adhesive force, cohesive force).

[0125] In the stacked piezoelectric element 50, a thinner adhesive layer improves the transmission of expansion and contraction energy (vibration energy) from each piezoelectric film 10 to the vibration plate 102, thereby improving energy efficiency. However, if the adhesive layer is too thick, the rigidity increases, potentially restricting the expansion and contraction of the piezoelectric film 10 (stacked piezoelectric element 50). On the other hand, if the adhesive layer is too thin, the adhesive force weakens, potentially causing separation between the piezoelectric film 10 and the adhesive layer 19.

[0126] Taking this into consideration, the thickness of the adhesive layer 19 after bonding is preferably 0.1 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.1 to 10 μm.

[0127] The thickness of the adhesive layer 104 is not limited, and may be appropriately set according to the material of the adhesive layer 104 so as to obtain sufficient adhesive force (adhesive force, cohesive force).

[0128] In the electroacoustic transducer 100, a thinner adhesive layer can improve the transmission of the expansion and contraction energy (vibration energy) of the laminated piezoelectric element 50 to the vibration plate 102, thereby improving energy efficiency. On the other hand, a thicker and more rigid adhesive layer may restrict the expansion and contraction of the laminated piezoelectric element 50.

[0129] Taking this into consideration, the adhesive layer is preferably thin. Specifically, the thickness of the adhesive layer after bonding is preferably 0.1 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.1 to 10 μm.

[0130] In the electroacoustic transducer 100 , the adhesive layer 104 is provided as a preferred embodiment and is not an essential component.

[0131] Therefore, even if the electroacoustic transducer 100 does not include the adhesive layer 104, a known crimping mechanism, fastening mechanism, or fixing mechanism can be used to fix the vibration plate 102 and the stacked piezoelectric element 50. For example, if the stacked piezoelectric element 50 is rectangular in shape when viewed from above, the electroacoustic transducer can be constructed by fastening the four corners with components such as bolts and nuts, or by fastening the four corners and the center with components such as bolts and nuts.

[0132] However, in this case, when a driving voltage is applied from the power supply, the stacked piezoelectric element 50 expands and contracts independently of the vibration plate 102. Depending on the circumstances, only the stacked piezoelectric element 50 may bend, and the expansion and contraction of the stacked piezoelectric element 50 may not be transmitted to the vibration plate 102. In this way, when the stacked piezoelectric element 50 expands and contracts independently of the vibration plate 102, the vibration efficiency of the vibration plate 102 caused by the stacked piezoelectric element 50 decreases, potentially preventing the vibration plate 102 from vibrating sufficiently.

[0133] Taking this into consideration, the vibration plate 102 and the laminated piezoelectric element 50 are preferably bonded together with an adhesive layer.

[0134] <Vibration Plate>

[0135] Preferably, the vibration plate 102 is flexible. In the present invention, flexibility has the same meaning as generally interpreted as flexibility, indicating the ability to bend and flex, and specifically, the ability to bend and stretch without being broken or damaged.

[0136] The vibration plate 102 is not limited as long as it has flexibility, and various sheet-shaped objects (plate-shaped objects, films) can be used.

[0137] Examples include resin films made of polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polyphenylene sulfide (PPS), polymethyl methacrylate (PMMA), polyetherimide (PEI), polyimide (PI), polyethylene naphthalate (PEN), triacetyl cellulose (TAC), and cyclic olefin resins; foam plastics made of expanded polystyrene, expanded polystyrene, and expanded polyethylene; plywood, corkboard, leather such as kraft leather; various paperboards such as carbon paper and Japanese paper; various corrugated cardboard materials formed by attaching other paperboard to one or both sides of corrugated paperboard; and thin-film metals made of various metals such as stainless steel, aluminum, copper, and nickel, as well as various alloys. Furthermore, the diaphragm 102 may be a composite material formed by laminating films made of these materials.

[0138] Furthermore, as long as they are flexible, display devices such as organic light emitting diode (OLED) displays, liquid crystal displays, micro LED (Light Emitting Diode) displays, and inorganic electroluminescent displays, as well as projector screens, can be preferably used as the vibration plate 102 .

[0139] In the electroacoustic transducer 100 , it is preferable that the electrode layer of the laminated piezoelectric element 50 (piezoelectric film 10 ) and the vibration plate 102 are not electrically connected.

[0140] If the electrode layer of the laminated piezoelectric element 50 is electrically connected to the vibration plate 102 , a short circuit or other failure may occur. Therefore, by adopting a structure in which the electrode layer of the laminated piezoelectric element 50 and the vibration plate 102 are not electrically connected, the risk of failure can be reduced.

[0141] Piezoelectric film

[0142] Hereinafter, the piezoelectric film used in the multi-layer piezoelectric element of the present invention will be described.

[0143] exist Figure 11 A portion of the piezoelectric film 10 is shown in an enlarged manner in FIG.

[0144] Figure 11 The illustrated piezoelectric film 10 includes a piezoelectric layer 20, which is a sheet-like material exhibiting piezoelectricity; a first electrode layer 24 laminated on one surface of the piezoelectric layer 20; a first protective layer 28 laminated on the surface of the first electrode layer 24 opposite the piezoelectric layer 20; a second electrode layer 26 laminated on the other surface of the piezoelectric layer 20; and a second protective layer 30 laminated on the surface of the second electrode layer 26 opposite the piezoelectric layer 20. Specifically, the piezoelectric film 10 has a structure in which the piezoelectric layer 20 is sandwiched between the electrode layers, and the protective layer is laminated on the surface of the electrode layer that is not in contact with the piezoelectric layer.

[0145] In the present invention, various known piezoelectric layers can be used as the piezoelectric layer 20 .

[0146] In the present invention, as in Figure 11 As schematically shown in FIG, the piezoelectric layer 20 is preferably a polymer composite piezoelectric body including piezoelectric particles 36 in a matrix 34 made of a polymer material.

[0147] A polymer material having viscoelasticity at room temperature is preferably used as the material of the polymer composite piezoelectric body 34 (base and binder) constituting the piezoelectric layer 20. In this specification, "room temperature" refers to a temperature range of approximately 0 to 50°C.

[0148] Here, the polymer composite piezoelectric body (piezoelectric layer 20 ) preferably meets the following requirements.

[0149] (i) Flexibility

[0150] For example, when a portable item such as a newspaper or magazine is held after being slightly bent in a document-like manner, it will be continuously subjected to relatively slow and large bending deformation of less than a few Hz from the outside. At this time, if the polymer composite piezoelectric body is hard, a correspondingly large bending stress will be generated, which may cause cracks at the interface between the polymer matrix and the piezoelectric particles, and ultimately lead to destruction. Therefore, the polymer composite piezoelectric body is required to have appropriate softness. In addition, if the strain energy can be diffused to the outside as heat, the stress can be relieved. Therefore, the loss tangent of the polymer composite piezoelectric body is required to be appropriately large.

[0151] In summary, a flexible polymer composite piezoelectric element used as an actuator must be rigid at vibrations between 20 Hz and 20 kHz, but flexible at vibrations below a few Hz. Furthermore, the loss tangent of the polymer composite piezoelectric element must be appropriately high at all vibration frequencies below 20 kHz.

[0152] Furthermore, it is preferable to stack the layers according to the rigidity (hardness, stiffness, spring constant) of the target material (vibration plate) to be attached, so that the spring constant can be easily adjusted. In this case, the thinner the adhesive layer 104 is, the more energy efficiency can be improved.

[0153] Generally, polymer solids exhibit viscoelastic relaxation mechanisms, and as temperature rises or frequency decreases, large-scale molecular motion can be observed, manifesting as a decrease in the storage modulus (Young's modulus) (relaxation) or a maximization of the loss modulus (absorption). Relaxation caused by the micro-Brownian motion of molecular chains in the amorphous region is called primary dispersion, and it can be observed to be very large. The temperature at which this primary dispersion occurs is the glass transition point (Tg), at which point the viscoelastic relaxation mechanism is most pronounced.

[0154] In the polymer composite piezoelectric body (piezoelectric layer 20), by using a polymer material having a glass transition point at room temperature, in other words, a polymer material having viscoelasticity at room temperature, in the matrix, a polymer composite piezoelectric body is achieved that exhibits rigidity for vibrations of 20 Hz to 20 kHz, but flexibility for slow vibrations of several Hz or less. In particular, from the perspective of appropriately exhibiting this behavior, it is preferred to use a polymer material having a glass transition point at room temperature, i.e., 0 to 50°C, in the matrix of the polymer composite piezoelectric body.

[0155] As the polymer material having viscoelasticity at room temperature, various known polymer materials can be used. Preferably, a polymer material is used whose maximum value of the loss tangent Tanδ at a frequency of 1 Hz obtained from a dynamic viscoelasticity test at room temperature, i.e., 0 to 50°C, is 0.5 or greater.

[0156] As a result, when the polymer composite piezoelectric body is slowly bent by an external force, stress concentration at the interface between the polymer matrix and the piezoelectric particles in the maximum bending moment portion is alleviated, and high flexibility can be expected.

[0157] Furthermore, the storage modulus (E') at a frequency of 1 Hz obtained by dynamic viscoelasticity measurement of a polymer material having viscoelasticity at room temperature is preferably 100 MPa or more at 0°C and 10 MPa or less at 50°C.

[0158] This can reduce the bending moment generated when the polymer composite piezoelectric body is gradually bent by an external force, and can also exhibit rigidity against acoustic vibrations of 20 Hz to 20 kHz.

[0159] Furthermore, it is more preferable if the relative dielectric constant of the polymer material having viscoelasticity at room temperature is 10 or greater at 25° C. Thus, when voltage is applied to the polymer composite piezoelectric body, a higher electric field is applied to the piezoelectric particles in the matrix, and a larger deformation can be expected.

[0160] However, on the other hand, in consideration of ensuring good moisture resistance, the relative dielectric constant of the polymer material is preferably 10 or less at 25°C.

[0161] Examples of polymer materials having viscoelasticity at room temperature that satisfy this condition include cyanoethylated polyvinyl alcohol (cyanoethylated PVA), polyvinyl acetate, polyvinylidene chloride acrylonitrile, polystyrene-vinyl polyisoprene block copolymer, polyvinyl methyl ketone, and polybutyl methacrylate. Commercially available products such as HYBRAR 5127 (manufactured by KURARAY CO., LTD.) can also be appropriately used as these polymer materials. Among these, materials having a cyanoethyl group are preferably used as the polymer material, and cyanoethylated PVA is particularly preferred.

[0162] As a polymer material having viscoelasticity at room temperature, a polymer material having a cyanoethyl group is preferably used, and cyanoethylated PVA is particularly preferred. That is, in the present invention, a polymer material having a cyanoethyl group is preferably used as the matrix 34 of the piezoelectric layer 20, and cyanoethylated PVA is particularly preferred.

[0163] In the following description, the above-mentioned polymer materials represented by cyanoethylated PVA are also collectively referred to as "polymer materials having viscoelasticity at room temperature."

[0164] These polymer materials having viscoelasticity at room temperature may be used alone or in combination (mixture) of two or more.

[0165] The base 34 made of such a polymer material having viscoelasticity at room temperature may be made of a plurality of polymer materials in combination as needed.

[0166] That is, for the purpose of adjusting dielectric properties or mechanical properties, in addition to viscoelastic materials such as cyanoethylated PVA, other dielectric polymer materials may be added to the base 34 as needed.

[0167] Examples of dielectric polymer materials that can be added include fluorine-based polymers such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymers, vinylidene fluoride-trifluoroethylene copolymers, polyvinylidene fluoride-trifluoroethylene copolymers, and polyvinylidene fluoride-tetrafluoroethylene copolymers; vinylidene cyanide-vinyl ester copolymers; cyanoethyl cellulose, cyanoethyl hydroxysucrose, cyanoethyl hydroxycellulose, cyanoethyl hydroxyfullerene, cyanoethyl methacrylate, cyanoethyl acrylate, cyanoethyl hydroxyethyl cellulose, cyanoethyl amylose, cyanoethyl hydroxypropyl cellulose, cyanoethyl dihydroxypropyl cellulose, cyanoethyl hydroxypropyl amylose, cyanoethyl polyacrylamide, cyanoethyl polyethyl acrylate, cyanoethyl fullerene, cyanoethyl polyhydroxymethylene, cyanoethyl glycidyl fullerene, cyanoethyl sucrose, and cyanoethyl sorbitol; and synthetic rubbers such as nitrile rubber and chloroprene rubber.

[0168] Among them, polymer materials having a cyanoethyl group can be preferably used.

[0169] Furthermore, the dielectric polymer materials are not limited to one kind in the base 34 of the piezoelectric layer 20 , and multiple kinds may be added.

[0170] In addition, in order to adjust the glass transition point Tg, in addition to dielectric polymer materials, thermoplastic resins such as vinyl chloride resin, polyethylene, polystyrene, methacrylic resin, polybutene and isobutylene, as well as thermosetting resins such as phenolic resin, urea-formaldehyde resin, melamine resin, alkyd resin and mica can also be added to the matrix 34.

[0171] Furthermore, for the purpose of improving adhesion, a tackifier such as rosin ester, rosin, terpene, terpene phenol, or petroleum resin may be added.

[0172] The amount of materials other than the viscoelastic polymer material such as cyanoethylated PVA added to the matrix 34 of the piezoelectric layer 20 is not particularly limited, but is preferably 30% by mass or less in terms of the proportion in the matrix 34 .

[0173] Thus, the properties of the added polymer material can be exhibited without damaging the viscoelastic relaxation mechanism in the matrix 34, so that preferred results can be obtained from the perspectives of high dielectric constant, improved heat resistance, and improved adhesion with the piezoelectric particles 36 and the electrode layer.

[0174] The piezoelectric layer 20 is a layer composed of a polymer composite piezoelectric material containing piezoelectric particles 36 in a matrix 34. The piezoelectric particles 36 are dispersed in the matrix 34. It is preferable that the piezoelectric particles 36 are dispersed in the matrix 34 uniformly (substantially uniformly).

[0175] The piezoelectric particles 36 are composed of ceramic particles having a perovskite or wurtzite crystal structure.

[0176] Examples of ceramic particles constituting the piezoelectric particles 36 include lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), barium titanate (BaTiO3), zinc oxide (ZnO), and a solid solution of barium titanate and bismuth ferrite (BiFe3) (BFBT).

[0177] The particle size of the piezoelectric particles 36 is not limited and may be appropriately selected according to the size of the piezoelectric film 10 and the application of the laminated piezoelectric element 50. The particle size of the piezoelectric particles 36 is preferably 1 to 10 μm.

[0178] By setting the particle size of the piezoelectric particles 36 within this range, a preferable result can be obtained in terms of the piezoelectric film 10 being able to achieve both high piezoelectric characteristics and flexibility.

[0179] The piezoelectric particles 36 in the piezoelectric layer 20 may be uniformly and regularly dispersed in the matrix 34 , or may be irregularly dispersed in the matrix 34 as long as they are uniformly dispersed.

[0180] In the piezoelectric film 10, the amount ratio of the matrix 34 to the piezoelectric particles 36 in the piezoelectric layer 20 is not limited, and can be appropriately set according to the size and thickness in the surface direction of the piezoelectric film 10, the purpose of the stacked piezoelectric element 50, and the characteristics required for the stacked piezoelectric element 50.

[0181] The volume fraction of the piezoelectric particles 36 in the piezoelectric layer 20 is preferably 30 to 80%, more preferably 50% or more, and further preferably 50 to 80%.

[0182] By setting the amount ratio of the matrix 34 to the piezoelectric particles 36 within the above range, a preferable result can be obtained from the viewpoint of achieving both high piezoelectric characteristics and flexibility.

[0183] In the piezoelectric film 10 , the thickness of the piezoelectric layer 20 is not particularly limited and may be appropriately set depending on the application of the stacked piezoelectric element 50 , the number of stacked piezoelectric films in the stacked piezoelectric element 50 , the characteristics required of the piezoelectric film 10 , and the like.

[0184] From the perspective of rigidity such as the strength of the sheet-like material, a thicker piezoelectric layer 20 is more advantageous, but the voltage (potential difference) required to expand or contract the piezoelectric film 10 by the same amount increases.

[0185] The thickness of the piezoelectric layer 20 is preferably 10 to 300 μm, more preferably 20 to 200 μm, and even more preferably 30 to 150 μm.

[0186] By setting the thickness of the piezoelectric layer 20 within the above range, preferable results can be obtained from the perspectives of ensuring both rigidity and appropriate flexibility.

[0187] Furthermore, the piezoelectric layer 20 is preferably polarized (poled) in the thickness direction. When a voltage is applied to the electrode layers (electrode pair) sandwiching the piezoelectric layer 20, the piezoelectric particles 36 in the piezoelectric layer 20 expand and contract in the polarization direction according to the applied voltage. As a result, the piezoelectric film 10 (piezoelectric layer 20) contracts in the thickness direction. Simultaneously, due to the Poisson's ratio, the piezoelectric film 10 also expands and contracts in the plane direction. This enables the piezoelectric film 10 to exhibit piezoelectric properties.

[0188] In the present invention, the piezoelectric layer 20 is not limited to the polymer composite piezoelectric body in which the piezoelectric particles 36 are contained in the matrix 34 made of a polymer material having viscoelasticity at room temperature such as cyanoethylated PVA as described above.

[0189] That is, in the piezoelectric film 10 of the present invention, various known piezoelectric layers can be used as the piezoelectric layer.

[0190] As an example, it is also possible to utilize a polymer composite piezoelectric body containing the same piezoelectric particles 36 in a matrix containing dielectric polymer materials such as the above-mentioned polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, a piezoelectric layer composed of polyvinylidene fluoride, a piezoelectric layer composed of a fluororesin other than polyvinylidene fluoride, and a piezoelectric layer formed by stacking a film composed of poly (L) lactic acid and a film composed of poly (D) lactic acid.

[0191] However, as described above, considering that it can exhibit rigidity to vibrations of 20 Hz to 20 kHz and flexibility to slow vibrations below several Hz, and can obtain excellent acoustic properties and excellent flexibility, it is preferred to use a polymer composite piezoelectric body containing piezoelectric particles 36 in a polymer matrix 34 composed of a polymer material having viscoelasticity at room temperature (for example, the above-mentioned cyanoethylated PVA).

[0192] like Figure 11 As shown, the piezoelectric film 10 has a structure in which a first electrode layer 24 is provided on one side of the piezoelectric layer 20, a first protective layer 28 is provided thereon, and a second electrode layer 26 is provided on the other side of the piezoelectric layer 20, a second protective layer 30 is provided thereon. Here, the first electrode layer 24 and the second electrode layer 26 form an electrode pair.

[0193] That is, the piezoelectric film 10 has a structure in which both surfaces of the piezoelectric layer 20 are sandwiched between the first electrode layer 24 and the second electrode layer 26 , which are an electrode pair, and the stacked body is sandwiched between the first protective layer 28 and the second protective layer 30 .

[0194] In this manner, in the piezoelectric film 10 , the region sandwiched between the first electrode layer 24 and the second electrode layer 26 expands and contracts according to the applied voltage.

[0195] The first electrode layer 24 and the first protective layer 28, as well as the second electrode layer 26 and the second protective layer 30, are added for the convenience of explaining the piezoelectric film 10. Therefore, the first and second have no technical significance in the present invention and are irrelevant to actual usage.

[0196] In the present invention, the piezoelectric film 10 may have, in addition to these layers, for example, an adhesive layer for bonding the electrode layer and the piezoelectric layer 20 together, and an adhesive layer for bonding the electrode layer and the protective layer together.

[0197] The adhesive may be an adhesive or a binder. Furthermore, the adhesive may preferably be the same material as the polymer material after the piezoelectric particles 36 are removed from the piezoelectric layer 20 (i.e., the same material as the matrix 34). Furthermore, the adhesive layer may be provided on both the first electrode layer 24 side and the second electrode layer 26 side, or may be provided on only one of the first electrode layer 24 side and the second electrode layer 26 side.

[0198] In the piezoelectric film 10, the first protective layer 28 and the second protective layer 30 cover the first electrode layer 24 and the second electrode layer 26, while also providing the piezoelectric layer 20 with appropriate rigidity and mechanical strength. Specifically, while the piezoelectric layer 20, comprised of the matrix 34 and the piezoelectric particles 36, exhibits excellent flexibility in response to gradual bending deformation, it may lack rigidity or mechanical strength depending on the intended use. To compensate for this deficiency, the piezoelectric film 10 is provided with the first protective layer 28 and the second protective layer 30.

[0199] The first protective layer 28 and the second protective layer 30 have the same structure except for their different positions. Therefore, in the following description, when there is no need to distinguish between the first protective layer 28 and the second protective layer 30, both are collectively referred to as protective layers.

[0200] The protective layer is not limited, and various sheet-shaped materials can be used. As an example, various resin films can be preferably exemplified.

[0201] Among them, resin films composed of polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polyphenylene sulfide (PPS), polymethyl methacrylate (PMMA), polyetherimide (PEI), polyimide (PI), polyethylene naphthalate (PEN), triacetyl cellulose (TAC) and cyclic olefin resins are preferably used due to their excellent mechanical properties and heat resistance.

[0202] The thickness of the protective layer is not limited. Furthermore, the thicknesses of the first protective layer 28 and the second protective layer 30 are basically the same, but may be different.

[0203] If the rigidity of the protective layer is too high, not only will the expansion and contraction of the piezoelectric layer 20 be restricted, but the flexibility will also be impaired. Therefore, except when mechanical strength or good handling as a sheet is required, a thinner protective layer is more advantageous.

[0204] In the piezoelectric film 10 , if the thickness of the protective layer is not more than twice the thickness of the piezoelectric layer 20 , a preferable result can be obtained from the viewpoints of achieving both rigidity and appropriate flexibility.

[0205] For example, when the thickness of the piezoelectric layer 20 is 50 μm and the protective layer is made of PET, the thickness of the protective layer is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 25 μm or less.

[0206] In the piezoelectric film 10, a first electrode layer 24 is formed between the piezoelectric layer 20 and the first protective layer 28, and a second electrode layer 26 is formed between the piezoelectric layer 20 and the second protective layer 30. The first electrode layer 24 and the second electrode layer 26 are provided to apply a voltage to the piezoelectric layer 20 (piezoelectric film 10).

[0207] The first electrode layer 24 and the second electrode layer 26 are basically the same except for their positions. Therefore, in the following description, when there is no need to distinguish between the first electrode layer 24 and the second electrode layer 26, both are collectively referred to as electrode layers.

[0208] In the present invention, the material forming the electrode layer is not limited, and various conductors can be used. Specifically, metals such as carbon, palladium, iron, tin, aluminum, nickel, platinum, gold, silver, copper, titanium, chromium and molybdenum, their alloys, laminates and composites of these metals and alloys, and indium tin oxide can be exemplified. Alternatively, conductive polymers such as PEDOT / PPS (polyethylene dioxythiophene-polystyrene sulfonic acid) can also be exemplified. Among them, copper, aluminum, gold, silver, platinum and indium tin oxide can be preferably exemplified as the electrode layer. Among them, copper is more preferred from the viewpoints of conductivity, cost and flexibility.

[0209] Furthermore, the method for forming the electrode layer is not limited, and known methods such as various vapor deposition methods (vacuum film forming methods) such as vacuum evaporation and sputtering, film formation by plating, and methods of bonding foils formed of the above materials can be used.

[0210] Among them, thin films of copper, aluminum, etc. formed by vacuum deposition can be preferably used as the electrode layer because they can ensure the flexibility of the piezoelectric film 10. Among them, thin films of copper formed by vacuum deposition can be particularly preferably used.

[0211] The thickness of the electrode layer is not limited. Furthermore, the thicknesses of the first electrode layer 24 and the second electrode layer 26 are substantially the same, but may be different.

[0212] Here, as with the protective layer, if the rigidity of the electrode layer is too high, not only will the expansion and contraction of the piezoelectric layer 20 be restricted, but the flexibility will also be impaired. Therefore, as long as the resistance does not become too high, the thinner the electrode layer, the more advantageous it is.

[0213] In the piezoelectric film 10 , it is preferable that the product of the thickness of the electrode layer and the Young's modulus is smaller than the product of the thickness of the protective layer and the Young's modulus because flexibility is not seriously impaired.

[0214] For example, in the case of a combination in which the protective layer is PET (Young's modulus: approximately 6.2 GPa) and the electrode layer is composed of copper (Young's modulus: approximately 130 GPa), if the thickness of the protective layer is 25 μm, the thickness of the electrode layer is preferably less than 1.2 μm, more preferably less than 0.3 μm, and preferably less than 0.1 μm.

[0215] As described above, the piezoelectric film 10 has a structure in which the piezoelectric layer 20 , in which piezoelectric particles 36 are dispersed in the matrix 34 containing a polymer material, is sandwiched between the first electrode layer 24 and the second electrode layer 26 , and this stacked body is further sandwiched between the first protective layer 28 and the second protective layer 30 .

[0216] Such a piezoelectric film 10 preferably has a maximum value of loss tangent (Tan δ) at a frequency of 1 Hz obtained by dynamic viscoelasticity measurement at room temperature, and more preferably has a maximum value of 0.1 or more at room temperature.

[0217] Thus, even if the piezoelectric film 10 is subjected to relatively slow and large bending deformation of several Hz or less from the outside, the strain energy can be effectively diffused to the outside as heat, thereby preventing cracks from occurring at the interface between the polymer matrix and the piezoelectric particles.

[0218] The storage modulus (E′) of the piezoelectric film 10 at a frequency of 1 Hz obtained by dynamic viscoelasticity measurement is preferably 10 to 30 GPa at 0° C. and 1 to 10 GPa at 50° C. This condition also applies to the piezoelectric layer 20 .

[0219] As a result, the storage modulus (E') of the piezoelectric film 10 can have a large frequency dispersion at room temperature. That is, it can exhibit rigidity for vibrations of 20 Hz to 20 kHz and flexibility for vibrations of several Hz or less.

[0220] Furthermore, the product of the thickness of the piezoelectric film 10 and the storage modulus (E') at a frequency of 1 Hz obtained by dynamic viscoelasticity measurement is preferably 1.0×10 5 ~2.0×10 6 N / m, 1.0×10 at 50°C 5 ~1.0×10 6 N / m. Note that this condition also applies to the piezoelectric layer 20 .

[0221] Thus, the piezoelectric film 10 can have appropriate rigidity and mechanical strength within a range that does not impair flexibility and acoustic characteristics.

[0222] Furthermore, the piezoelectric film 10 preferably has a loss tangent (Tan δ) of 0.05 or greater at 25° C. and a frequency of 1 kHz in a master curve obtained by dynamic viscoelasticity measurement. This condition also applies to the piezoelectric layer 20 .

[0223] This makes the frequency characteristics of the speaker using the piezoelectric film 10 smoother, and also reduces the amount of change in sound quality when the lowest resonance frequency f0 changes with changes in the curvature of the speaker.

[0224] In the present invention, the storage modulus (Young's modulus) and loss tangent of the piezoelectric film 10 and the piezoelectric layer 20 may be measured by a known method. For example, the dynamic viscoelasticity measuring device DMS6100 manufactured by SII Nano Technology Inc. may be used for measurement.

[0225] As an example of measurement conditions, the following can be cited: measurement frequency is 0.1Hz~20Hz (0.1Hz, 0.2Hz, 0.5Hz, 1Hz, 2Hz, 5Hz, 10Hz and 20Hz), measurement temperature is -50~150℃, heating rate is 2℃ / min (in nitrogen environment), sample size is 40mm×10mm (including clamping area), and chuck spacing is 20mm.

[0226] In the laminated piezoelectric element 50 , a power source (external power source) for applying a driving voltage to expand and contract the piezoelectric film 10 , that is, for supplying driving power, is connected to the first electrode layer 24 and the second electrode layer 26 of the piezoelectric film 10 .

[0227] The power source is not limited and can be either a DC power source or an AC power source. Furthermore, the driving voltage can be appropriately set to a voltage that can appropriately drive the piezoelectric film 10 according to the thickness and forming material of the piezoelectric layer 20 of the piezoelectric film 10 .

[0228] As described above, the piezoelectric film 10 includes: a piezoelectric layer 20 formed of a polymer composite piezoelectric material containing piezoelectric particles 36 in a matrix 34 made of a polymer material; and electrode layers (a first electrode layer 24 and a second electrode layer 26) provided on both surfaces of the piezoelectric layer 20. Furthermore, the piezoelectric film 10 includes protective layers (a first protective layer 28 and a second protective layer 30) provided on each electrode layer.

[0229] When a voltage is applied to the first electrode layer 24 and the second electrode layer 26 of the piezoelectric film 10 having such a piezoelectric layer 20, the piezoelectric particles 36 expand and contract in the polarization direction according to the applied voltage. As a result, the piezoelectric film 10 (piezoelectric layer 20) contracts in the thickness direction. Simultaneously, due to the Poisson's ratio, the piezoelectric film 10 also expands and contracts in the in-plane direction. This expansion and contraction is approximately 0.01 to 0.1%.

[0230] As described above, the thickness of the piezoelectric layer 20 is preferably about 10 to 300 μm. Therefore, the expansion and contraction in the thickness direction is very small, and is only about 0.3 μm at most.

[0231] In contrast, the piezoelectric film 10, or the piezoelectric layer 20, has a dimension in the planar direction that is significantly greater than its thickness. Therefore, for example, if the length of the laminated portion of the laminated piezoelectric element is 20 cm, the piezoelectric film 10 expands and contracts in the planar direction by a maximum of approximately 0.2 mm when voltage is applied.

[0232] The vibration plate 102 is adhered to the piezoelectric film 10 (laminated piezoelectric element 50 ) via an adhesive layer. Therefore, the vibration plate 102 bends due to the expansion and contraction of the piezoelectric film 10 , and as a result, the vibration plate 102 vibrates in the thickness direction.

[0233] The vibration in the thickness direction generates sound from the vibration plate 102. That is, the vibration plate 102 vibrates according to the magnitude of the voltage (driving voltage) applied to the piezoelectric film 10, and generates sound according to the driving voltage applied to the piezoelectric film 10.

[0234] Furthermore, by adjusting the mass of the piezoelectric film 10 (laminated piezoelectric element 50) according to the spring constant of the vibration plate 102, the sound pressure level can be increased. A large mass of the laminated piezoelectric element 50 causes deflection of the vibration plate 102, potentially suppressing the vibration of the vibration plate 102 during driving. On the other hand, a small mass of the laminated piezoelectric element 50 increases the resonant frequency, potentially suppressing the vibration of the vibration plate 102 at low frequencies. Taking these considerations into account, it is preferable to appropriately adjust the mass of the laminated piezoelectric element 50 according to the spring constant of the vibration plate 102.

[0235] <Method for Manufacturing Piezoelectric Film>

[0236] Below, reference Figures 12 to 14 , an example of a method for manufacturing the piezoelectric film 10 is described.

[0237] First, prepare Figure 12 The sheet 11a shown in FIG. 1 has a first electrode layer 24 formed on the surface of the first protective layer 28. Figure 14 Schematically shown in FIG. 1 is a sheet-like object 11 c in which the second electrode layer 26 is formed on the surface of the second protective layer 30 .

[0238] The sheet 11a can be produced by forming a copper thin film or the like as the first electrode layer 24 on the surface of the first protective layer 28 by vacuum deposition, sputtering, plating, etc. Similarly, the sheet 11c can be produced by forming a copper thin film or the like as the second electrode layer 26 on the surface of the second protective layer 30 by vacuum deposition, sputtering, plating, etc.

[0239] Alternatively, a commercially available sheet in which a copper thin film or the like is formed on a protective layer may be used as the sheet 11 a and / or the sheet 11 c .

[0240] The sheet-shaped object 11a and the sheet-shaped object 11c may be the same or different.

[0241] If the protective layer is very thin and difficult to handle, a protective layer with a separator (temporary support) can be used as needed. PET with a thickness of 25 to 100 μm can be used as the separator. The separator can be removed after the electrode layer and protective layer are thermocompressed.

[0242] Then, if Figure 13 As shown, a coating material (coating composition) to become the piezoelectric layer 20 is applied onto the first electrode layer 24 of the sheet 11a and then cured to form the piezoelectric layer 20. Thus, a piezoelectric laminate 11b is produced in which the sheet 11a and the piezoelectric layer 20 are stacked.

[0243] The piezoelectric layer 20 can be formed using various methods depending on the material forming the piezoelectric layer 20 .

[0244] As an example, first, the polymer material such as the cyanoethylated PVA is dissolved in an organic solvent, and then the piezoelectric particles 36 such as PZT particles are added and stirred to prepare a coating material.

[0245] The organic solvent is not limited, and various organic solvents such as dimethylformamide (DMF), methyl ethyl ketone (MEK), and cyclohexanone can be used.

[0246] After preparing the sheet 11a and the coating, the coating is cast (coated) on the sheet 11a, and the organic solvent is evaporated and dried. Figure 13 As shown in FIG. 1 , a piezoelectric laminate 11 b is produced in which the first electrode layer 24 is provided on the first protective layer 28 and the piezoelectric layer 20 is stacked on the first electrode layer 24 .

[0247] The coating material can be cast by any method without limitation, and any known method (coating device) such as a bar coater, a slide coater, or a doctor knife can be used.

[0248] Alternatively, if the polymer material is a substance that can be heated and melted, a melt can be prepared by heating and melting the polymer material, adding the piezoelectric particles 36 thereto, and then forming the melt by extrusion molding or the like. Figure 12 The sheet 11a shown is extruded in a thin sheet and cooled, thereby producing a sheet as shown in FIG. Figure 13 The piezoelectric stack 11b is shown.

[0249] Furthermore, as described above, in the piezoelectric layer 20 , in addition to the polymer material having viscoelasticity at room temperature, a polymer piezoelectric material such as PVDF may be added to the base 34 .

[0250] When adding these polymeric piezoelectric materials to the matrix 34, the polymeric piezoelectric materials to be added can be dissolved in the coating material or added to a heated and melted polymeric material having viscoelasticity at room temperature and then heated and melted.

[0251] After forming the piezoelectric layer 20, a rolling process may be performed as needed. The rolling process may be performed once or multiple times.

[0252] As is well known, calendaring is a process in which a surface to be processed is heated and pressed by a hot press or a heated roller to perform flattening or the like.

[0253] Next, the piezoelectric layer 20 of the piezoelectric stack 11b is subjected to a polarization treatment (polling). The polarization treatment of the piezoelectric layer 20 may be performed before the rolling treatment, but is preferably performed after the rolling treatment.

[0254] The method for polarizing the piezoelectric layer 20 is not limited, and known methods can be used. For example, electric field polarization, in which a DC electric field is directly applied to the object to be polarized, and corona poling treatment can be exemplified. In addition, when electric field polarization is performed, the second electrode layer 26 can be formed before the polarization treatment, and the electric field polarization treatment can be performed using the first electrode layer 24 and the second electrode layer 26.

[0255] Furthermore, in the piezoelectric film 10 of the present invention, polarization is performed not along the surface direction of the piezoelectric layer 20 but along the thickness direction.

[0256] Then, if Figure 14 As shown in FIG. 1 , a sheet 11 c prepared in advance is stacked on the piezoelectric layer 20 side of the polarized piezoelectric stack 11 b so that the second electrode layer 26 faces the piezoelectric layer 20 .

[0257] Furthermore, a heat press device and a heating roller are used to heat-press the laminated body by sandwiching the first protective layer 28 and the second protective layer 30, thereby laminating the piezoelectric laminate 11b and the sheet 11c, thereby producing a laminated body as shown in FIG. Figure 11 The piezoelectric film 10 is shown.

[0258] Alternatively, the piezoelectric laminate 11b and the sheet 11c may be bonded together using an adhesive and preferably further pressure-bonded to produce the piezoelectric film 10. The adhesive in this case may be made of the same material as that of the base of the piezoelectric layer 20.

[0259] The piezoelectric film 10 can be manufactured using slice-shaped sheets 11 a and 11 c or the like, or can be manufactured using a roll-to-roll process.

[0260] The produced piezoelectric film can also be cut into desired shapes according to various applications.

[0261] The piezoelectric film 10 fabricated in this manner is polarized not in the plane direction but in the thickness direction, and exhibits significant piezoelectric properties even without undergoing a stretching treatment after polarization. Consequently, the piezoelectric film 10 exhibits no in-plane anisotropy in its piezoelectric properties and expands and contracts isotropically in all plane directions when a driving voltage is applied.

[0262] <Method for Manufacturing Laminated Piezoelectric Element>

[0263] Next, Figure 1 A method for producing a stacked piezoelectric element in which the long piezoelectric film shown is folded multiple times will be described.

[0264] First, a long piezoelectric film is prepared, and an adhesive is applied to the entire first layer of the laminated portion during folding and cured. Alternatively, an adhesive sheet is attached.

[0265] Next, according to the desired configuration pattern, a portion of the adhesive (adhesive) or adhesive sheet is removed to form a gap portion. The method for removing the adhesive or adhesive sheet is not particularly limited. For example, there can be cited a method of removing the adhesive by irradiating a laser beam (carbon dioxide laser, etc.) to a position serving as the gap portion, or a method of cutting an incision in the adhesive or adhesive sheet along the depth direction by pressing (for example, when the thickness of the adhesive or adhesive sheet is 15 μm, the incision is cut to 13 to 14.5 μm in the thickness direction), and then peeling off the adhesive layer or adhesive sheet of that portion. Thus, for example, a gap portion having the following characteristics is formed. Figure 4 、 Figures 7 to 9 The adhesive layer 19 of the adhesive area 18 and the gap 17 is shown.

[0266] Next, the piezoelectric film is folded once, and the region serving as the first layer and the region serving as the second layer of the piezoelectric film are bonded together using an adhesive layer.

[0267] Next, an adhesive (binder) is applied to the entire area of ​​the folded laminate portion serving as the second layer of the piezoelectric film and cured. Alternatively, an adhesive sheet is adhered.

[0268] Then, similarly to the above, after removing a portion of the adhesive (adhesive) or adhesive sheet according to the desired configuration pattern to form a gap portion, the piezoelectric film is folded once and the area serving as the second layer and the area serving as the third layer of the piezoelectric film are bonded with an adhesive layer.

[0269] By repeating the formation of the adhesive layer and the folding of the piezoelectric film in this manner, a stacked piezoelectric element can be produced in which a long piezoelectric film is folded multiple times.

[0270] While the multi-layer piezoelectric element and electroacoustic transducer of the present invention have been described in detail above, the present invention is not limited to the above examples, and various improvements and modifications can of course be made without departing from the spirit of the present invention.

[0271] Example

[0272] Below, enumerate specific embodiment of the present invention, the present invention is described in more detail.In addition, the present invention is not limited to this embodiment, as long as do not depart from the purpose of the present invention, then can suitably change the material shown in the following embodiment, usage, ratio, processing content, processing sequence etc.

[0273] [Comparative Example 1]

[0274] <Fabrication of Piezoelectric Film>

[0275] Through the above Figures 12 to 14 The piezoelectric film was fabricated using the method shown.

[0276] First, cyanoethylated PVA (manufactured by CR-V Shin-Etsu Chemical Co., Ltd.) was dissolved in dimethylformamide (DMF) at the following composition ratio. PZT particles were then added to this solution as piezoelectric particles at the following composition ratio, and the mixture was stirred with a propeller mixer (2000 rpm) to prepare a coating for forming a piezoelectric layer.

[0277] PZT particles………………………………300 parts by mass

[0278] Cyanoethylated PVA…………………………30 parts by mass

[0279] DMF……………………………………70 parts by mass

[0280] The PZT particles used were obtained by sintering commercially available PZT raw material powder at 1000 to 1200° C., followed by pulverization and classification to obtain an average particle size of 2 μm.

[0281] Meanwhile, a single sheet was prepared in which a 0.1 μm thick copper film was vacuum-deposited onto a 4 μm thick PET film. Specifically, in this example, the first and second electrode layers were 0.3 μm thick copper-deposited films, and the first and second protective layers were 4 μm thick PET films.

[0282] A previously prepared coating material for forming a piezoelectric layer was applied to the first electrode layer (copper vapor-deposited film) of the sheet using a knife coater. The coating material was applied so that the thickness of the coating film after drying would be 50 μm.

[0283] Next, the sheet coated with the coating was heated and dried on a hot plate at 120°C to evaporate the DMF. This produced a piezoelectric laminate having a copper first electrode layer on a PET first protective layer, and a 91.8 μm thick piezoelectric layer (polymer composite piezoelectric layer) thereon.

[0284] The produced piezoelectric layer was polarized in the thickness direction.

[0285] On the polarized piezoelectric laminate, a sheet-like object obtained by vapor-depositing a copper thin film on a PET film was laminated so that the second electrode layer (copper thin film side) faced the piezoelectric layer.

[0286] Next, the piezoelectric laminate and the sheet-like material were thermally pressed together at 120° C. using a laminating device, thereby gluing and adhering the piezoelectric layer and the second electrode layer to produce a laminated body. Figure 11 The piezoelectric film shown has a thickness of 100 μm.

[0287] <Fabrication of Laminated Piezoelectric Elements>

[0288] The piezoelectric film was cut into a rectangular shape of 140 mm x 150 mm. A piezoelectric element was fabricated by folding the cut piezoelectric film three times along a 140 mm side to form a four-layer stack of piezoelectric films. The stacked portion had a planar shape of 30 mm x 150 mm.

[0289] The stacked piezoelectric films were bonded together with an adhesive layer (acrylic adhesive). The thickness of the adhesive layer was set to 5 μm. In addition, a gap portion was provided in the adhesive layer. The gap portion was formed into a linear shape with a spacing of 100 μm and a width of 100 μm in the 150 mm direction of the adhesive layer of 30 mm × 150 mm (refer to Figure 4 ).

[0290] Furthermore, an electrode lead portion is formed in a region protruding from the laminated portion.

[0291] By using the above-mentioned SEM cross-sectional observation, the ratio of the average value d1 of the thickness of the piezoelectric film in the stacked piezoelectric element of the prepared comparative example 1 to the average value d2 of the thickness of the adhesive layer (hereinafter also referred to as the thickness ratio) d2 / d1 and the ratio of the total area of ​​the gap portion to the total area of ​​the adhesive layer (hereinafter also referred to as the area ratio) were calculated.

[0292] The thickness ratio d2 / d1 of the manufactured multi-layer piezoelectric element of Comparative Example 1 was 0.05, and the area ratio was 20%.

[0293] [Examples 1-2]

[0294] A laminated piezoelectric element was produced in the same manner as in Comparative Example 1 except that the thickness of the adhesive layer was changed to 15 μm and 30 μm, respectively.

[0295] The thickness ratio d2 / d1 of the manufactured multi-layer piezoelectric element of Example 1 was 0.15, and the area ratio was 20%.

[0296] The thickness ratio d2 / d1 of the manufactured multi-layer piezoelectric element of Example 2 was 0.30, and the area ratio was 20%.

[0297] [Comparative Example 2]

[0298] A laminated piezoelectric element was produced in the same manner as in Comparative Example 1 except that the thickness of the piezoelectric layer was 41.8 μm, the thickness of the piezoelectric film was 50 μm, the thickness of the adhesive layer was 15 μm, and no gap was provided.

[0299] The thickness ratio d2 / d1 of the manufactured multi-layer piezoelectric element of Comparative Example 2 was 0.30, and the area ratio was 0%.

[0300] [Comparative Example 3]

[0301] A laminated piezoelectric element was produced in the same manner as in Comparative Example 2, except that a gap portion was provided in the adhesive layer.

[0302] The gaps were formed in a linear shape with a width of 100 μm and an interval of 100 μm in the 150 mm direction of the adhesive layer of 30 mm × 150 mm (refer to Figure 4 ).

[0303] The thickness ratio d2 / d1 of the manufactured multi-layer piezoelectric element of Comparative Example 3 was 0.30, and the area ratio was 50%.

[0304] [Examples 3 to 7]

[0305] A laminated piezoelectric element was produced in the same manner as in Comparative Example 3 except that the width and interval of the gap portions were changed so that the area ratios became 20%, 1%, 5%, 35%, and 40%, respectively.

[0306] The thickness ratio d2 / d1 of the laminated piezoelectric elements of Examples 3 to 7 was all 0.30.

[0307] (Examples 8 to 10)

[0308] Laminated piezoelectric elements were produced in the same manner as in Example 3 except that the thickness of the adhesive layer was set to 5 μm, 30 μm, and 50 μm, respectively.

[0309] The thickness ratios d2 / d1 of the laminated piezoelectric elements of Examples 8 to 10 produced were 0.10, 0.60, and 1.00, respectively.

[0310] [Comparative Example 4]

[0311] A stacked piezoelectric element was produced in the same manner as in Comparative Example 1, except that the thickness of the piezoelectric layer was set to 5.9 μm, the thickness of the protective layer was set to 2 μm, the thickness of the electrode layer was set to 0.05 μm, the thickness of the piezoelectric film was set to 10 μm, and the thickness of the adhesive layer was set to 15 μm.

[0312] The thickness ratio d2 / d1 of the manufactured multi-layer piezoelectric element of Comparative Example 4 was 1.50, and the area ratio was 20%.

[0313] [Example 11]

[0314] A laminated piezoelectric element was produced in the same manner as in Comparative Example 4 except that the thickness of the adhesive layer was set to 5 μm.

[0315] [evaluate]

[0316] The stacked piezoelectric elements of the examples and comparative examples were bonded to a vibration plate to create an electroacoustic transducer. A 500mm x 450mm, 0.8mm thick, aluminum (A5052) plate was used as the vibration plate. The transverse direction of the vibration plate was aligned with the longitudinal direction of the stacked piezoelectric element, and the center of the stacked portion of the stacked piezoelectric element was aligned with the center of the vibration plate before bonding. An acrylic adhesive was used as the adhesive layer bonding the stacked piezoelectric element to the vibration plate.

[0317] A sine sweep signal with a frequency of 100 Hz to 5 kHz and a voltage of 50 Vrms was applied to the laminated piezoelectric element at one end of the longitudinal direction of the diaphragm. The sound pressure was measured using a microphone positioned 1 meter from the center of the diaphragm. The average sound pressure value within the frequency range of 100 Hz to 5 kHz was calculated. The average of the maximum values ​​within the frequency range of 100 Hz to 5 kHz was taken as the average sound pressure value.

[0318] Regarding the sound pressure, the initial sound pressure and the sound pressure after continuous driving for 72 hours were measured, and the sound pressure difference was calculated.

[0319] The results are shown in Table 1.

[0320] [Table 1]

[0321]

[0322] As can be seen from Table 1, the sound pressure after continuous driving is higher in the examples of the present invention than in the comparative examples. This is because the decrease in sound pressure over time due to long-term use can be suppressed, and the initial sound pressure is high.

[0323] Furthermore, as can be seen from the comparison of Examples 3 to 7, the ratio of the area of ​​the gap portion to the total area of ​​the adhesive layer is preferably 5% to 35%.

[0324] Furthermore, as can be seen from the comparison between Examples 3 and 8 to 10, the thickness ratio d2 / d1 of the piezoelectric film to the adhesive layer is preferably 0.1 to 0.6.

[0325] From the above, it can be seen that the effect of the present invention is obvious.

[0326] Industrial applicability

[0327] The stacked piezoelectric element of the present invention can be preferably used as various sensors such as acoustic wave sensors, ultrasonic sensors, pressure sensors, tactile sensors, strain sensors and vibration sensors (especially suitable for infrastructure inspections such as crack detection or manufacturing site inspections such as foreign matter intrusion detection); audio equipment such as microphones, pickups, speakers and exciters (as specific uses, examples include noise cancellers (for cars, trams, airplanes, robots, etc.), artificial vocal cords, buzzers for preventing pests and animal invasions, furniture, wallpaper, photos, helmets, goggles, headrests, signs, robots, etc.); tactile technology for cars, smart phones, smart watches, games, etc.; ultrasonic transducers such as ultrasonic probes and hydrophones; actuators for preventing water droplet adhesion, transportation, stirring, dispersion, grinding, etc.; shock-absorbing materials (dampers) for containers, vehicles, buildings, sports equipment such as skis and rackets; and vibration power generation devices for roads, floors, mattresses, chairs, shoes, tires, wheels and computer keyboards.

[0328] Explanation of symbols

[0329] 10-piezoelectric film, 11a, 11c-sheet-like object, 11b-piezoelectric stack, 17-gap portion, 18-bonding region, 19, 19b to 19d-bonding layer, 20-piezoelectric layer, 24-first electrode layer, 26-second electrode layer, 28-first protective layer, 30-second protective layer, 34-substrate, 36-piezoelectric particles, 40, 42-wires, 50-stacked piezoelectric element, 100-electroacoustic transducer, 102-vibration plate, 104-adhesive layer.

Claims

1. A stacked piezoelectric element comprising a plurality of piezoelectric films stacked with an adhesive layer interposed therebetween, wherein: The adhesive layer has an adhesive region and a gap in the in-plane direction of the main surface of the piezoelectric film. A cross section of the stacking direction of the piezoelectric film of the stacked piezoelectric element is observed using a scanning electron microscope in one direction within the plane of the main surface of the piezoelectric film, a direction perpendicular to the one direction, a direction at 45 degrees to the one direction, and a direction at 135 degrees to the one direction, obtaining 10 consecutive fields of view, wherein a ratio d2 / d1 of an average value d1 of the thickness of the piezoelectric film observed in each field of view to an average value d2 of the thickness of the adhesive layer is greater than or equal to 0.15 and less than or equal to 1.0, Furthermore, a ratio of a total area of ​​the gap portions observed in each field of view to a total area of ​​the adhesive layer is 1% or more and 40% or less.

2. The laminated piezoelectric element according to claim 1, wherein The piezoelectric film includes a piezoelectric layer formed of a polymer composite piezoelectric body containing piezoelectric particles in a matrix containing a polymer material, and electrode layers provided on both surfaces of the piezoelectric layer. 3 . The stacked piezoelectric element according to claim 1 , wherein the piezoelectric film is stacked in multiple layers by folding the piezoelectric film one or more times. 4 . An electroacoustic transducer, comprising: attaching the multi-layer piezoelectric element according to claim 1 to a vibration plate.

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