JP6300458B2 - Multilayer piezoelectric element - Google Patents

Description

  The present invention relates to a laminated piezoelectric element.

Conventionally, PZT (PbZrO ₃ —PbTiO ₃ solid solution), which is a ceramic material, has been used as a piezoelectric material. However, since PZT contains lead, it has a low environmental burden and is highly flexible. Molecular piezoelectric materials (piezoelectric films) have been used.
Currently known polymer piezoelectric materials are mainly classified into the following two types. That is, Pauling type polymer represented by nylon 11, polyvinyl fluoride, polyvinyl chloride, polyurea, etc., polyvinylidene fluoride (β type) (PVDF), and vinylidene fluoride-trifluoroethylene copolymer (P (VDF-TrFE)) and two types of ferroelectric polymers represented by (75/25).

In recent years, attention has been paid to the use of polymers having optical activity, such as polypeptides and polylactic acid, in addition to the above-described polymeric piezoelectric materials. It is known that polylactic acid polymers exhibit piezoelectricity only by mechanical stretching operation.
Among the polymers having optical activity, the piezoelectricity of a polymer crystal such as polylactic acid is attributed to a C = O bond permanent dipole that exists in the direction of the helical axis. In particular, polylactic acid has a small volume fraction of side chains with respect to the main chain and a large ratio of permanent dipoles per volume, and can be said to be an ideal polymer among the polymers having helical chirality.
It is known that polylactic acid that exhibits piezoelectricity only by stretching treatment does not require poling treatment and the piezoelectricity does not decrease over several years.

A laminated piezoelectric element in which a plurality of piezoelectric films using a polylactic acid polymer is laminated is also known (for example, see Patent Document 1).
Furthermore, the laminated piezoelectric element includes a plurality of piezoelectric films mainly composed of polylactic acid and having a predetermined stretching axis, and electrodes provided on both main surfaces of each piezoelectric film, and the plurality of piezoelectric films As at least one piezoelectric film, a laminated piezoelectric element having a different stretching axis direction from other piezoelectric films is known (for example, see Patent Document 2).

International Publication No. 2013/054918 Pamphlet International Publication No. 2009/139237 Pamphlet

However, investigations by the present inventors have revealed for the first time that the multilayer piezoelectric element described in Patent Documents 1 and 2 may have a large frequency dependency of sound pressure when used in an acoustic device such as a speaker. did.
Accordingly, an object of the present invention is to provide a laminated piezoelectric element capable of suppressing the frequency dependence of sound pressure, and an acoustic device in which the frequency dependence of sound pressure is suppressed.

Specific means for solving the problems are as follows.
<1> A multilayer piezoelectric element comprising a plurality of piezoelectric films having molecular orientation, wherein the plurality of piezoelectric films have substantially parallel molecular orientation and have a curved portion.
<2> The laminated piezoelectric element according to <1>, wherein the following ratio C is 19/20 to 6/7 in a cross section when the curved portion is cut in a direction in which the curvature is maximized.
Ratio C = Linear distance from one end to the other end of the bending portion / length along the bending portion of the bending portion <3> The thickness of the piezoelectric film is 20 μm to 80 μm, or <1> or <2> Laminated piezoelectric element.
<4> In any one of <1> to <3>, comprising: a laminate in which the piezoelectric film is laminated at least via an electrode layer; and electrode layers provided on both surfaces of the laminate. The laminated piezoelectric element described.
<5> The laminated piezoelectric element according to any one of <1> to <3>, wherein a piezoelectric unit including electrode layers on both surfaces of the piezoelectric film is laminated via an insulating layer.
<6> A plurality of piezoelectric units each including a first electrode layer on one surface of the piezoelectric film and a second electrode layer on the other surface are provided, and the plurality of piezoelectric units includes a first electrode layer and a second electrode unit. The electrode layers are stacked via an insulating layer so as to face each other, and the same potential is applied to the plurality of first electrode layers, and another same potential is applied to the plurality of second electrode layers. The laminated piezoelectric element according to any one of <1> to <3>, wherein the same voltage is applied to the plurality of piezoelectric films.
<7> The plurality of piezoelectric films contain a helical chiral polymer having an optical activity in which the amount of L-form is 95% by mass or more, or the optical activity in which the amount of D-form is 95% by mass or more. The laminated piezoelectric element according to any one of <1> to <6>, comprising a helical chiral polymer having the above.
<8> The piezoelectric film is 50% or less internal haze to visible light, and the piezoelectric constant _{d 14} measured by a displacement method at 25 ° C. is 1 Pm/V least, <1> to the <7> The laminated piezoelectric element according to any one of the above.
<9> The piezoelectric film includes a helical chiral polymer having an optical activity having a weight average molecular weight of 50,000 to 1,000,000, a crystallinity obtained by a DSC method of 20% to 80%, and a microscopic Any one of <1> to <8>, wherein the product of the normalized molecular orientation MORc and the crystallinity when the reference thickness measured by a wave transmission type molecular orientation meter is 50 μm is 25 to 700 The laminated piezoelectric element according to the item.
<10> The laminated piezoelectric element according to <7> or <9>, wherein the helical chiral polymer is a polylactic acid polymer having a main chain including a repeating unit represented by the following formula (1).

<11> An acoustic device comprising the laminated piezoelectric element according to any one of <1> to <10>.

  ADVANTAGE OF THE INVENTION According to this invention, the laminated piezoelectric element which can suppress the frequency dependence of sound pressure, and the audio equipment by which the frequency dependence of sound pressure was suppressed are provided.

1 is a schematic perspective view showing a laminated piezoelectric element according to an example of the present invention. It is a schematic perspective view which shows the laminated piezoelectric element which concerns on another example of this invention. It is AA sectional view taken on the line of FIG. 1A. It is a schematic sectional drawing which shows the curved part of the laminated piezoelectric element which concerns on one Embodiment of this invention. In an Example, it is a schematic perspective view which shows the state before changing a laminated piezoelectric element. In an Example, it is a schematic perspective view which shows the state after deform | transforming a laminated piezoelectric element. It is a graph which shows the relationship between the ratio C and the maximum sound pressure (dB) in an Example.

≪Multilayer piezoelectric element≫
The multilayer piezoelectric element of the present invention includes a plurality of piezoelectric films having molecular orientation (hereinafter also simply referred to as “piezoelectric films”), the molecular orientations of the plurality of piezoelectric films are substantially parallel, and have a curved portion.

As a result of studying the configuration of a laminated piezoelectric element including a plurality of piezoelectric films, the present inventors have arranged a plurality of piezoelectric films so that their molecular orientations are substantially parallel, and the laminated piezoelectric element has a curved portion. It has been found that the frequency dependence of the sound pressure is suppressed by adopting the shape, and the present invention has been completed.
That is, according to the multilayer piezoelectric element of the present invention, the frequency dependence of the sound pressure can be suppressed (for example, when used in an acoustic device).
Moreover, according to the laminated piezoelectric element of the present invention, a high sound pressure can be obtained.

Specifically, the multilayer piezoelectric element of the present invention includes a plurality of piezoelectric films having molecular orientation, and thus can obtain a higher sound pressure than a single-layer piezoelectric film having molecular orientation.
In addition, the multilayer piezoelectric element of the present invention can obtain a higher sound pressure by having a curved portion than in the case without the curved portion.
Here, “the multilayer piezoelectric element has a curved portion” means that at least a part of the main surface of the multilayer piezoelectric element is curved (that is, at least a part of the main surface of the multilayer piezoelectric element is a curved surface). Is).
The “main surface” refers to the surface having the largest area (unlike the end surface).

Furthermore, in the laminated piezoelectric element of the present invention, the molecular orientation of at least one piezoelectric film is made orthogonal to the molecular orientation of other piezoelectric films by making the molecular orientation of a plurality of piezoelectric films substantially parallel (for example, international The frequency dependence of the sound pressure can be suppressed as compared with the laminated piezoelectric element described in the publication No. 2009/139237 pamphlet.
The reason for this is not clear, but is presumed as follows.
That is, when the molecular orientation of at least one piezoelectric film is orthogonal to the molecular orientation of other piezoelectric films, the sound pressure may decrease depending on the frequency due to the displacement of a plurality of piezoelectric films interfering with each other. it is conceivable that. On the other hand, the laminated piezoelectric element of the present invention can suppress the above-described displacement interference (decrease in sound pressure) over a wide frequency range by making the molecular orientations of the plurality of piezoelectric films substantially parallel. It is considered that the frequency dependence of sound pressure is suppressed.

  In the present invention, “substantially parallel” means that when an angle formed by two straight lines is expressed in a range of 0 ° to 90 °, this angle is 0 ° to less than 30 ° (preferably 0 ° to 22 °. 0.5 ° or less, more preferably 0 ° or more and 10 ° or less, still more preferably 0 ° or more and 5 ° or less, and particularly preferably 0 ° or more and 3 ° or less.

In the present invention, “the molecular orientation of the plurality of piezoelectric films is substantially parallel” means that the maximum angle between the molecular orientation of one arbitrarily selected piezoelectric film and the molecular orientation of the remaining piezoelectric films is the maximum. The value is 0 ° or more and less than 30 °.
The maximum value of this angle is preferably 0 ° or more and 22.5 ° or less, more preferably 0 ° or more and 10 ° or less, and further preferably 0 ° or more and 5 ° or less from the viewpoint of obtaining a higher sound pressure. More preferably, it is 0 ° or more and 3 ° or less, and ideally 0 °.

  In addition, since the multilayer piezoelectric element of the present invention has the molecular orientation between the plurality of piezoelectric films substantially parallel, compared to the case where the molecular orientation of at least one piezoelectric film is orthogonal to the molecular orientation of other piezoelectric films. When the laminated piezoelectric element is manufactured, there is an advantage that the angle between the molecular orientations between the piezoelectric films can be easily adjusted.

Moreover, in this invention, molecular orientation is calculated | required by measuring molecular orientation degree MOR (Molecular Orientation Ratio).
Details of the method for measuring the degree of molecular orientation MOR will be described later.

In addition, a piezoelectric film having molecular orientation is obtained by, for example, extrusion-molding or the like to obtain a film containing a polymer (for example, a helical chiral polymer having optical activity described later, the same shall apply hereinafter), and then stretching the film. And orienting the polymer.
In the piezoelectric film thus produced, the molecular orientation and the stretching direction are usually substantially parallel.

In the present invention, “sound pressure” refers to sound pressure (dB) in the frequency range of 1500 Hz to 9500 Hz.
In the present invention, “frequency dependency of sound pressure can be suppressed” means that the difference between the maximum sound pressure and the minimum sound pressure in the frequency range of 1500 Hz to 9500 Hz can be reduced.
In the multilayer piezoelectric element of the present invention, the maximum sound pressure in the frequency range of 1500 Hz to 9500 Hz is preferably 80 dB or more.

Although there is no restriction | limiting in particular in the thickness (thickness of one piezoelectric film) of each piezoelectric film with which the laminated piezoelectric element of this invention is equipped, It is preferable that they are 20 micrometers-80 micrometers, and it is more preferable that they are 30 micrometers-70 micrometers. .
When the thickness is 20 μm or more, the mechanical strength is more excellent.
When the thickness is 80 μm or less, the amount of displacement of the piezoelectric film relative to the applied voltage can be further increased.

The total thickness of the multilayer piezoelectric element of the present invention is not particularly limited, but is preferably 600 μm or less, more preferably 500 μm or less, still more preferably 400 μm or less, and particularly preferably 300 μm or less from the viewpoint of obtaining a higher sound pressure. .
The lower limit of the total thickness of the laminated piezoelectric element of the present invention can be appropriately set according to the thickness of each piezoelectric film. For example, the total thickness is preferably 45 μm or more, more preferably 50 μm or more.

In addition, the multilayer piezoelectric element of the present invention only needs to have a curved portion. That is, it may be composed only of a curved portion, or may be composed of a curved portion and a flat portion.
FIG. 1A is a schematic perspective view showing a laminated piezoelectric element 10 which is an example of the laminated piezoelectric element of the present invention.
As shown in FIG. 1A, the laminated piezoelectric element 10 is entirely curved. That is, the laminated piezoelectric element 10 is composed of only a curved portion.
FIG. 1B is a schematic perspective view showing a laminated piezoelectric element 20 which is another example of the laminated piezoelectric element of the present invention.
As shown in FIG. 1B, the laminated piezoelectric element 20 includes a curved portion 22 that is a central portion, a flat portion 24 that is one end, and a flat portion 26 that is the other end.

2 is a cross-sectional view taken along the line AA of FIG. 1A, and more specifically, a schematic cross-sectional view when the laminated piezoelectric element 10 including only a curved portion is cut in a direction in which the curvature is maximized.
In FIG. 2, a linear distance a from one end to the other end of the bending portion and a length b along the bending of the bending portion are defined.
In FIG. 1A, FIG. 1B, FIG. 2, FIG. 4A, and FIG. 4B, illustration of the laminated structure of the laminated piezoelectric element is omitted.

In the present invention, the shape of the curved portion is not particularly limited, but from the viewpoint of obtaining a higher sound pressure, the following ratio C is obtained in a cross section when the curved portion is cut in the direction in which the curvature is maximized (for example, FIG. 2). However, it is preferable that it is 19 / 20-6 / 7, and it is more preferable that it is 9 / 10-6 / 7.
Ratio C = Linear distance from one end to the other end of the bending portion (for example, a linear distance a in FIG. 2) / Length along the bending portion of the bending portion (for example, length b in FIG. 2)

  Here, “the cross section when the curved portion is cut in the direction in which the curvature is maximized” means that the curvature at the apex of the curved portion is maximized among many cross sections that can be assumed as the cross section of the curved portion. Refers to a cross section.

In the present invention, the above-mentioned direction of “length along the bending portion of the bending portion” may be simply referred to as “direction along the bending”.
In FIG. 1A, FIG. 1B, FIG. 4A, and FIG. 4B, the direction along the curve and the short side of the multilayer piezoelectric element are parallel. There is no restriction | limiting in particular in the angle with one side. For example, the direction along the curve and the short side of the multilayer piezoelectric element may be orthogonal, or may be at an angle other than orthogonal and parallel. In addition, the laminated piezoelectric element may be square, or may have a shape other than a rectangle and a square.

  Although there is no restriction | limiting in particular in the length (for example, length b in FIG. 2) along the curve of the above-mentioned curved part, 60 mm-500 mm are preferable, 60 mm-200 mm are more preferable, 60 mm-100 mm are especially preferable.

  When the multilayer piezoelectric element 10 is an element produced by curving (curling) a multilayer piezoelectric element that is not curved, the length along the curvature of the multilayer piezoelectric element 10 is the length before the curve. Equal to the length of the laminated piezoelectric element.

In the laminated piezoelectric element of the present invention, it is preferable that the molecular orientation of the piezoelectric film follows the curved shape in the curved portion. Thereby, a higher sound pressure can be obtained as compared with the case where the molecular orientation of the piezoelectric film does not follow the curved shape.
For example, when focusing on one point in the curved portion, an angle θ between the direction of molecular orientation and the direction along the curve at this point (where the angle θ is defined in the range of 0 ° to 90 °). Is preferably less than 90 °. Thereby, compared with the case where angle (theta) is 90 degrees, a higher sound pressure is obtained.
The angle θ is preferably 0 ° to 80 °, more preferably 0 ° to 70 °, still more preferably 0 ° to 60 °, and particularly preferably 0 ° to 50 °.

In the laminated piezoelectric element of the present invention, it is preferable to include a laminate in which the piezoelectric film is laminated through at least an electrode layer, and electrode layers provided on both surfaces of the laminate.
Thereby, since voltage can be applied for every piezoelectric film, it is excellent in the efficiency of voltage application. Therefore, high sound pressure can be obtained efficiently.
For example, the applied voltage for obtaining the same electric field strength can be reduced as compared with the case where a voltage is applied to the entire single-layer piezoelectric film having the same thickness as the total thickness of the laminated piezoelectric element. Therefore, under the same applied voltage, a higher sound pressure can be obtained as compared with a single-layer piezoelectric film.
The “both sides” means both main surfaces (the same applies hereinafter).

In the laminated piezoelectric element of the present invention, it is preferable that piezoelectric units each having electrode layers on both sides of the piezoelectric film are laminated via an insulating layer.
Thereby, different potentials can be applied to the two electrode layers sandwiching the insulating layer.
Therefore, since the electric field of the same direction can be produced with respect to each piezoelectric film (refer embodiment mentioned later and FIG. 3), the frequency dependence of sound pressure can be suppressed more effectively. Furthermore, in this aspect, since a voltage can be applied to each piezoelectric film, a high sound pressure can be obtained efficiently as described above.
In this aspect, more preferably, a plurality of piezoelectric units each including a first electrode layer on one surface of the piezoelectric film and a second electrode layer on the other surface are provided, and the plurality of piezoelectric units are first to each other. The electrode layer and the second electrode layer are stacked with an insulating layer interposed therebetween, the same potential is applied to the plurality of first electrode layers, and another same potential is applied to the plurality of second electrode layers. This is a mode in which the same voltage is applied to a plurality of piezoelectric films by being applied (see the embodiment described later and FIG. 3).

Here, the concept of “applied potential” includes not only applying a positive potential or a negative potential but also applying a ground potential (0 V).
In short, a potential may be applied to each electrode layer so that a voltage is applied between the electrode layers of each piezoelectric unit.

In the multilayered piezoelectric element of the present invention, each of the plurality of piezoelectric films contains an optically active helical chiral polymer having an L-form amount of 95% by mass or more, or an D-form quantity. It is preferable to contain a helical chiral polymer having an optical activity of 95% by mass or more.
That is, it is preferable that a plurality of piezoelectric films have the same chirality.
Thus, when two types of piezoelectric films having different chiralities are alternately laminated (specifically, in order of L piezoelectric film, R piezoelectric film, L piezoelectric film, R piezoelectric film, etc.) Compared with the case of stacking), a laminated piezoelectric element can be manufactured by using a plurality of the same piezoelectric films, so that the displacement interference between the piezoelectric films is further suppressed, and the frequency dependence of the sound pressure is further suppressed.
Needless to say, since the number of types of piezoelectric films is small, it is easier to manufacture a laminated piezoelectric element.

<Embodiment>
Next, an embodiment of the multilayer piezoelectric element of the present invention will be described with reference to FIG. 3, but the present invention is not limited to the following embodiment.
This embodiment is an example of the preferable aspect mentioned above.

FIG. 3 is a schematic cross-sectional view showing a curved portion of the multilayer piezoelectric element 100, which is an embodiment of the multilayer piezoelectric element of the present invention. This cross section is a cross section cut along a curve.
FIG. 3 also shows a method of applying a voltage to the laminated piezoelectric element 100.

As shown in FIG. 3, in the laminated piezoelectric element 100, the piezoelectric unit 110 </ b> A, the piezoelectric unit 110 </ b> B, and the piezoelectric unit 110 </ b> C are laminated via the insulating layer 112.
The piezoelectric unit 110A includes a piezoelectric film 101A, a first electrode layer 102A provided on one main surface (convex surface) of the piezoelectric film 101A, and a second electrode provided on the other main surface (concave surface) of the piezoelectric film 101A. 104A.
Similarly, the piezoelectric unit 110B is provided on the piezoelectric film 101B, the first electrode layer 102B provided on one main surface (convex surface) of the piezoelectric film 101B, and the other main surface (concave surface) of the piezoelectric film 101B. A second electrode layer 104B.
Similarly, the piezoelectric unit 110C is provided on the piezoelectric film 101C, the first electrode layer 102C provided on one main surface (convex surface) of the piezoelectric film 101C, and the other main surface (concave surface) of the piezoelectric film 101C. A second electrode layer 104C.
In this multilayer piezoelectric element 100, the first electrode layer 102A and the second electrode layer 104C are the outermost layers.

Each of the piezoelectric films (piezoelectric films 101A, 101B, and 101C) is a piezoelectric film mainly composed of a helical chiral polymer having optical activity in which the amount of L-form is 95% by mass or more.
However, as each piezoelectric film, a piezoelectric film mainly composed of a helical chiral polymer having optical activity in which the amount of R-form is 95% by mass or more can be used. In short, each piezoelectric film in the present embodiment only needs to have the same chirality.

Next, a method for applying a voltage to the laminated piezoelectric element 100 will be described.
In this laminated piezoelectric element 100, the first electrode layer 102A, the first electrode layer 102B, and the first electrode layer 102C are positioned on the same side (the main surface side that is a convex surface) when viewed from each piezoelectric film in each piezoelectric unit. doing. The same potential (for example, plus potential) is applied to the first electrode layer 102A, the first electrode layer 102B, and the first electrode layer 102C.
On the other hand, the second electrode layer 104A, the second electrode layer 104B, and the second electrode layer 104C are located on the same side (the main surface side that is a concave surface) when viewed from each piezoelectric film in each piezoelectric unit. The second electrode layer 104A, the second electrode layer 104B, and the second electrode layer 104C are applied with the same potential (for example, minus potential) different from the potential applied to each first electrode layer.
That is, different potentials are applied to the second electrode layer 104A and the first electrode layer 102B, which are two electrode layers sandwiching the insulating layer 112 (the same applies to the second electrode layer 104B and the first electrode layer 102C). Is).
In this way, a voltage having the same value as the voltage supplied from the voltage supply power source 120 is applied to each piezoelectric film in the laminated piezoelectric element 100. Thereby, the electric field of the same direction arises in each piezoelectric film.

  Although not shown, in the laminated piezoelectric element 100, the molecular orientation of each piezoelectric film follows the curved shape.

Further, the material of each first electrode layer (first electrode layers 102A, 102B, 102C) and each second electrode layer (second electrode layers 104A, 104B, 104C) is not particularly limited. For example, Al, Ag , Au, Cu, Ag—Pd alloy, ITO, ZnO, IZO (registered trademark), conductive polymer, and the like are used.
Details of preferred examples of each electrode layer will be described later.

Further, when an insulating layer having an adhesive function is used as the insulating layer 112, it is possible to ensure insulation between the piezoelectric units while bonding the piezoelectric units.
The two insulating layers 112 included in the laminated piezoelectric element 100 may be the same layer (the same material and the same thickness) or different layers. Details of preferred examples of the insulating layer will be described later.

Next, the operation of the laminated piezoelectric element 100 will be described.
In the laminated piezoelectric element 100, as described above, the same voltage is applied to each of the piezoelectric films by the voltage supply power source 120, thereby generating an electric field in the same direction in each of the piezoelectric films.
Here, since each piezoelectric film has the same chirality, each piezoelectric film is displaced in the same direction by the application of the voltage (electric field).
Thereby, a remarkably high sound pressure can be obtained as compared with only one piezoelectric film, and the molecular orientation of at least one piezoelectric film is made orthogonal to the molecular orientation of other piezoelectric films. The frequency dependence of sound pressure can be significantly reduced.

In the laminated piezoelectric element 100 shown in FIG. 3, three piezoelectric units are laminated, but the number of piezoelectric units is not limited to three.
From the viewpoint of obtaining a higher sound pressure, the total number of piezoelectric units 100 is 600 μm or less (more preferably 500 μm), considering the individual thickness of each piezoelectric unit (or each piezoelectric film). In the following, it is more preferable to adjust in a range of 400 μm or less, particularly preferably 300 μm or less.
For example, when the thickness of the piezoelectric unit (or piezoelectric film) is 20 μm, the number of piezoelectric units is particularly preferably 2 to 10 layers. When the thickness of the piezoelectric unit (or piezoelectric film) is 50 μm, the number of piezoelectric units is particularly preferably 2 to 4 layers.

Although there is no limitation in particular in the method of manufacturing the laminated piezoelectric element (for example, laminated piezoelectric element 100) of this invention, For example, the following method is mentioned.
For example, a piezoelectric film is manufactured by extruding and stretching (details of a preferable manufacturing method of the piezoelectric film will be described later). Electrode layers are formed on both sides of the manufactured piezoelectric film by a known method such as vapor deposition, sputtering, coating, etc. to obtain a piezoelectric unit.
A plurality of the piezoelectric units are prepared, and each is bonded through an insulating layer having an adhesive function to form a laminated piezoelectric element.
By bending (deforming) at least a part of the obtained multilayer piezoelectric element (preferably in the above-mentioned ratio C range) to obtain a curved portion, the multilayer piezoelectric element of the present invention is obtained.
There is no particular limitation on the method of deforming the laminated piezoelectric element, and there is no restriction on the method of deforming the laminated piezoelectric element by applying a force in the direction to bring the distance between both ends of the laminated piezoelectric element closer, or by using a member having a curved surface. For example, a method of deforming the laminated piezoelectric element along the surface may be used.

Alternatively, a multilayer piezoelectric element may be manufactured by preparing a plurality of curved piezoelectric units and then bonding the plurality of piezoelectric units via an insulating layer having an adhesive function.
Here, the curved piezoelectric unit can be manufactured, for example, by first forming electrode layers on both sides of a non-curved piezoelectric film and then bending the piezoelectric film on which the electrode layers are formed. A curved piezoelectric unit can also be manufactured by forming electrode layers on both sides of a curved piezoelectric film.

  Next, each member of the multilayer piezoelectric element of the present invention will be described.

<Piezoelectric film>
The laminated piezoelectric element of the present invention includes a plurality of piezoelectric films having molecular orientation (for example, the above-described piezoelectric films 101A, 101B, 101C).
The thickness of the plurality of piezoelectric films may be the same or different.
Hereinafter, preferred embodiments of the piezoelectric film will be described.

(Piezoelectric constant d ₁₄ )
As the piezoelectric film, the piezoelectric film is preferably a piezoelectric constant d ₁₄ measured by a displacement method at 25 ° C. is 1 Pm/V more.
The “piezoelectric constant d ₁₄ ” is one of piezoelectricity tensors, and is determined from the degree of polarization generated in the direction of shear stress when shear stress is applied in the direction of the stretching axis of the stretched material. Specifically, the generated charge density per unit shear stress is defined as d _14. Numerical piezoelectric constant d ₁₄ indicating that piezoelectricity is higher the larger. When simply referred to as “piezoelectric constant” in the present application, it refers to “piezoelectric constant d ₁₄ ” measured by a displacement method.

The complex piezoelectric constant d ₁₄ is calculated as “d ₁₄ = d ₁₄ ′ −id ₁₄ ″”, and “d ₁₄ ′” and “id ₁₄ ″” are “Rheograph Solid S manufactured by Toyo Seiki Seisakusho Co., Ltd. -1 type ". “D ₁₄ ′” represents the real part of the complex piezoelectric constant, “id ₁₄ ″” represents the imaginary part of the complex piezoelectric constant, and d ₁₄ ′ (real part of the complex piezoelectric constant) represents the piezoelectric in the present embodiment. corresponding to the constant _{d 14.} In addition, it shows that it is excellent in piezoelectricity, so that the real part of complex piezoelectricity is high.

Hereinafter, an example of a method for measuring a piezoelectric constant d ₁₄ due to the displacement method.
The piezoelectric film is cut at 40 mm in the stretching direction (for example, the MD direction) and 40 mm in the direction orthogonal to the stretching direction (for example, the TD direction) to produce rectangular test pieces. Next, the obtained test piece is set on a test stand of a sputtering thin film forming apparatus JSP-8000 manufactured by ULVAC, and the coater chamber is evacuated (for example, 10 ⁻³ Pa or less) by a rotary pump. Thereafter, sputtering is performed on one surface of the test piece for 500 seconds on an Ag (silver) target under the conditions of an applied voltage of 280 V and a sputtering current of 0.4 A). Next, the other surface of the test piece is sputtered for 500 seconds under the same conditions to coat Ag on both sides of the test piece to form an electrode layer of Ag.

  A 40 mm × 40 mm test piece having an Ag electrode layer formed on both sides was cut into 32 mm in a direction of 45 ° with respect to the stretching direction of the piezoelectric film and 5 mm in a direction perpendicular to the direction of 45 °, and 32 mm × Cut out a 5 mm rectangular film. This is a piezoelectric constant measurement sample.

The difference distance between the maximum value and the minimum value of the displacement of the film when a sine wave AC voltage of 10 Hz, 300 Vpp was applied to the obtained piezoelectric constant measurement sample was measured using the Keyence Corporation Laser Spectral Interference Displacement Meter SI- Measured by 1000. The value obtained by dividing the measured displacement (mp-p) by the reference length of the film 30 mm is used as the amount of strain, and the amount of strain is expressed by the electric field intensity ((applied voltage (V)) / (film thickness)) applied to the film. the value obtained by multiplying 2 to the value obtained by dividing a piezoelectric constant d _14.

The higher the piezoelectric constant, the more useful the displacement of the material with respect to the voltage applied to the piezoelectric film, and conversely, the greater the voltage generated against the force applied to the piezoelectric film.
Specifically, is preferably at least 1 Pm/V piezoelectric constant _{d 14} is measured by a displacement method at 25 ° C., more preferably at least 4pm / V, more preferably not less than 6pm / V, even more preferably more than 8pm / V. The upper limit of the piezoelectric constant is not particularly limited, but is preferably 50 pm / V or less and more preferably 30 pm / V or less from the viewpoint of balance with transparency and the like.

Moreover, as a piezoelectric film in this invention, it is preferable to use the piezoelectric film whose internal haze with respect to visible light is 50% or less from a transparency viewpoint.
As the piezoelectric film, more preferably, not more than 50% internal haze to visible light, and a piezoelectric film piezoelectric constant d ₁₄ is 1 Pm/V or more as measured by a displacement method at 25 ° C..

(Internal haze)
The transparency of the piezoelectric film can be evaluated by visual observation or haze measurement, for example.
The piezoelectric film preferably has an internal haze with respect to visible light (hereinafter also simply referred to as “internal haze”) of 50% or less, more preferably 20% or less, and even more preferably 15% or less.
The lower the internal haze of the piezoelectric film, the better. However, from the viewpoint of balance with the piezoelectric constant, etc., it is preferably 0.01% to 13%, more preferably 0.1% to 5%. preferable.

In the present invention, “inside haze” refers to haze excluding haze due to the shape of the outer surface of the piezoelectric film.
The “internal haze” herein is a value when measured at 25 ° C. in accordance with JIS-K7105 with respect to a piezoelectric film having a thickness of 0.05 mm.

More specifically, the internal haze in the present invention (hereinafter also referred to as “internal haze H1”) refers to a value measured as follows.
That is, first, a haze (hereinafter, also referred to as “haze H2”) of a laminate in which only silicon oil is sandwiched between two glass plates, and a piezoelectric film whose surface is uniformly wetted with silicon oil are applied to the glass plate. The haze (hereinafter also referred to as “haze H3”) of the laminated body sandwiched between two sheets is measured at 25 ° C. in accordance with JIS-K7105, and then the internal haze H1 is obtained according to the following formula. .
Internal haze (H1) = Haze (H3) -Haze (H2)

  The haze H2 and the haze H3 can be measured using, for example, a haze measuring machine [TC Density Co., Ltd., TC-HIII DPK].

  Moreover, the piezoelectric film in the present invention includes a helical chiral polymer having optical activity having a weight average molecular weight of 50,000 to 1,000,000, the crystallinity obtained by DSC method is 20% to 80%, and It is preferable that the product of the normalized molecular orientation MORc and the crystallinity is 40 to 700 when the reference thickness measured by a microwave transmission type molecular orientation meter is 50 μm.

(Molecular orientation MOR, normalized molecular orientation MORc)
The piezoelectric film in the present invention has molecular orientation as described above.
As an index representing the molecular orientation, there is a “molecular orientation degree MOR”.
The molecular orientation degree MOR (Molecular Orientation Ratio) is a value indicating the degree of molecular orientation, and is measured by the following microwave measurement method. That is, a sample (piezoelectric film) is placed in a microwave resonance waveguide of a known microwave molecular orientation measuring device (also referred to as a microwave transmission type molecular orientation meter) in the direction of the microwave in the sample surface (film). (Surface) is vertical.
Then, in a state where the sample is continuously irradiated with the microwave whose vibration direction is biased in one direction, the sample is rotated by 0 to 360 ° in a plane perpendicular to the traveling direction of the microwave, and the microwave transmitted through the sample is transmitted. The degree of molecular orientation MOR can be determined by measuring the strength.

In the present invention, the normalized molecular orientation MORc can also be obtained by the following formula. Here, the normalized molecular orientation MORc is an MOR value when the reference thickness tc is 50 μm.
MORc = (tc / t) × (MOR-1) +1
(Tc: reference thickness to be corrected, t: sample thickness)

  The normalized molecular orientation MORc can be measured with a known molecular orientation meter such as a microwave molecular orientation meter MOA-2012A or MOA-6000 manufactured by Oji Scientific Instruments Co., Ltd., at a resonance frequency in the vicinity of 4 GHz or 12 GHz.

  As described later, the normalized molecular orientation MORc can be controlled mainly by, for example, heat treatment conditions (heating temperature and heating time) before stretching of the uniaxially stretched film, stretching conditions (stretching temperature and stretching speed), and the like.

The normalized molecular orientation MORc can be converted into a birefringence Δn obtained by dividing the retardation amount (retardation) by the thickness of the film. Specifically, retardation can be measured using RETS100 manufactured by Otsuka Electronics Co., Ltd. MORc and Δn are approximately in a linear proportional relationship, and when Δn is 0, MORc is 1.
For example, when the below-mentioned optically active polymer is a polylactic acid polymer and the birefringence Δn is measured at a measurement wavelength of 550 nm, 2.0, which is the lower limit of the preferred range of the normalized molecular orientation MORc, is birefringence Δn 0. .005 can be converted. The lower limit of the preferred range of the product of the normalized molecular orientation MORc and the crystallinity of the piezoelectric film, 40, can be converted to a product of the birefringence Δn of the piezoelectric film and the crystallinity of 0.1.

In the piezoelectric film of the present invention, the normalized molecular orientation MORc is preferably 3.5 to 15.0, more preferably 4.0 to 15.0, and 6.0 to 10.0. Is more preferable, and it is still more preferable that it is 7-10.0.
If the normalized molecular orientation MORc is in the range of 3.5 to 15.0, there are many polymer chains (for example, polylactic acid molecular chains) arranged in the stretching direction, and as a result, the rate of formation of oriented crystals increases. High piezoelectricity can be expressed.

(Crystallinity)
The piezoelectric film in the present invention preferably has a crystallinity of 20% to 80%.
Here, the crystallinity can be determined by the DSC method.
The crystallinity of the piezoelectric film is preferably 25% to 70%, more preferably 30% to 50%.
If the crystallinity of the piezoelectric film is 20% to 80%, the piezoelectric film has good balance between piezoelectricity and transparency, and when the piezoelectric film is stretched, whitening and breakage hardly occur and it is easy to manufacture.

(Product of normalized molecular orientation MORc and crystallinity)
In the piezoelectric film of the present invention, the product of the crystallinity of the piezoelectric film and the normalized molecular orientation MORc is preferably 25 to 700, more preferably 40 to 700, still more preferably 75 to 680, and still more preferably 90 to 660. More preferably, it is 125-650, More preferably, it is 180-350.
If the product of the crystallinity and the normalized molecular orientation MORc is in the range of 40 to 700, the balance between piezoelectricity and transparency of the piezoelectric film is better, and the dimensional stability is higher.

  Moreover, the piezoelectric film in the present invention includes a helical chiral polymer having optical activity having a weight average molecular weight of 50,000 to 1,000,000, the crystallinity obtained by DSC method is 20% to 80%, and It is preferable that the product of the normalized molecular orientation MORc and the crystallinity is 40 to 700 when the reference thickness measured by a microwave transmission type molecular orientation meter is 50 μm.

  Next, the preferable range of the components of the piezoelectric film in the present invention will be described.

(Helical chiral polymer with optical activity (optically active polymer))
The piezoelectric film preferably contains a helical chiral polymer having optical activity (hereinafter also referred to as “optically active polymer”).
Here, the helical chiral polymer having optical activity (optically active polymer) refers to a polymer having molecular optical activity in which the molecular structure is a helical structure.

Examples of the optically active polymer include a polypeptide, a cellulose derivative, a polylactic acid resin, polypropylene oxide, poly (β-hydroxybutyric acid), and the like.
Examples of the polypeptide include poly (glutarate γ-benzyl), poly (glutarate γ-methyl), and the like.
Examples of the cellulose derivative include cellulose acetate and cyanoethyl cellulose.

The optical purity of the optically active polymer is preferably 95.00% ee or more, more preferably 96.00% ee or more, from the viewpoint of improving the piezoelectricity of the piezoelectric film, and 99.00%. More preferably, it is ee or more, and even more preferably 99.99% ee or more. Desirably, it is 100.00% ee.
By setting the optical purity of the optically active polymer within the above range, it is considered that the packing property of the polymer crystal that exhibits piezoelectricity is enhanced, and as a result, the piezoelectricity is enhanced.

Here, the optical purity of the optically active polymer is a value calculated by the following formula.
Optical purity (% ee) = 100 × | L body weight−D body weight | / (L body weight + D body weight)
That is, “the amount difference (absolute value) between the amount of L-form [mass%] of optically active polymer and the amount of D-form [mass%] of optically active polymer” is expressed as The value obtained by multiplying (multiplying) the value obtained by dividing (dividing) the amount [mass%] by the total amount of the amount [mass%] of the optically active polymer D-form [mass%] and multiplying it by 100. And

  In addition, the value obtained by the method using a high performance liquid chromatography (HPLC) is used for the quantity [mass%] of the L body of an optically active polymer and the quantity [mass%] of the D body of an optically active polymer.

  Among the above optically active polymers, a compound having a main chain containing a repeating unit represented by the following formula (1) is preferable from the viewpoint of increasing optical purity and improving piezoelectricity.

  Examples of the compound having the repeating unit represented by the formula (1) as a main chain include polylactic acid polymers. Among them, polylactic acid is preferable, and L-lactic acid homopolymer (PLLA) or D-lactic acid homopolymer (PDLA) is most preferable. The polylactic acid polymer in the present embodiment refers to “polylactic acid (polymer compound composed only of repeating units derived from a monomer selected from L-lactic acid and D-lactic acid)”, “L-lactic acid or D-lactic acid”. “Copolymer of lactic acid and a compound copolymerizable with L-lactic acid or D-lactic acid” or a mixture of both.

  The above-mentioned “polylactic acid” is a polymer in which lactic acid is polymerized by an ester bond and is connected for a long time, a lactide method via lactide, and a direct polymerization method in which lactic acid is heated in a solvent under reduced pressure and polymerized while removing water It is known that it can be manufactured by, for example. Examples of the “polylactic acid” include L-lactic acid homopolymer, D-lactic acid homopolymer, block copolymer including at least one polymer of L-lactic acid and D-lactic acid, and L-lactic acid and D-lactic acid. Examples include graft copolymers containing at least one polymer.

  Examples of the “compound copolymerizable with L-lactic acid or D-lactic acid” include glycolic acid, dimethyl glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 2-hydroxypropanoic acid, 3-hydroxypropanoic acid, 2- Hydroxyvaleric acid, 3-hydroxyvaleric acid, 4-hydroxyvaleric acid, 5-hydroxyvaleric acid, 2-hydroxycaproic acid, 3-hydroxycaproic acid, 4-hydroxycaproic acid, 5-hydroxycaproic acid, 6-hydroxycaprone Acid, 6-hydroxymethyl caproic acid, hydroxycarboxylic acid such as mandelic acid, glycolide, cyclic ester such as β-methyl-δ-valerolactone, γ-valerolactone, ε-caprolactone, oxalic acid, malonic acid, succinic acid, Glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, urine Polyvalent carboxylic acids such as decanedioic acid, dodecanedioic acid and terephthalic acid, and anhydrides thereof, ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3- Butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, 3-methyl-1,5-pentanediol, neo Examples thereof include polyhydric alcohols such as pentyl glycol, tetramethylene glycol and 1,4-hexanedimethanol, polysaccharides such as cellulose, and aminocarboxylic acids such as α-amino acids.

  The “copolymer of L-lactic acid or D-lactic acid and a compound copolymerizable with the L-lactic acid or D-lactic acid” includes a block copolymer or graft copolymer having a polylactic acid sequence capable of forming a helical crystal. Can be mentioned.

  The concentration of the structure derived from the copolymer component in the optically active polymer is preferably 20 mol% or less. For example, when the optically active polymer is a polylactic acid polymer, the number of moles of the structure derived from lactic acid in the polylactic acid polymer and the structure derived from a compound copolymerizable with lactic acid (copolymer component) It is preferable that the copolymer component is 20 mol% or less based on the total.

  Examples of the polylactic acid polymer include a method obtained by direct dehydration condensation of lactic acid described in JP-A-59-096123 and JP-A-7-033861, or US Pat. No. 2,668,182. And ring-opening polymerization using lactide, which is a cyclic dimer of lactic acid, described in US Pat. No. 4,057,357 and the like.

  Furthermore, the optically active polymer obtained by each of the above production methods has an optical purity of 95.00% ee or more. For example, when polylactic acid is produced by the lactide method, the optical purity is improved by crystallization operation. It is preferable to polymerize lactide having an optical purity of 95.00% ee or higher.

  When the piezoelectric film of the present invention contains an optically active polymer, the content of the optically active polymer contained in the piezoelectric film is preferably 80% by mass or more.

(Weight average molecular weight of optically active polymer)
The optically active polymer preferably has a weight average molecular weight (Mw) of 50,000 to 1,000,000.
If the lower limit of the weight average molecular weight of the optically active polymer is 50,000 or more, the mechanical strength when the optically active polymer is molded is sufficient. The lower limit of the weight average molecular weight of the optically active polymer is preferably 100,000 or more, and more preferably 200,000 or more.
On the other hand, when the upper limit of the weight average molecular weight of the optically active polymer is 1,000,000 or less, it becomes easier to form the optically active polymer (for example, to form a film shape by extrusion molding or the like). The upper limit of the weight average molecular weight is preferably 800,000 or less, and more preferably 300,000 or less.

  Further, the molecular weight distribution (Mw / Mn) of the optically active polymer is preferably 1.1 to 5, and more preferably 1.2 to 4, from the viewpoint of the strength of the piezoelectric film. Furthermore, it is preferable that it is 1.4-3.

The weight average molecular weight Mw and molecular weight distribution (Mw / Mn) of the optically active polymer are measured by gel permeation chromatograph (GPC) by the GPC measurement method under the following conditions.
-GPC measuring device-
Waters GPC-100
-Column-
Made by Showa Denko KK, Shodex LF-804
-Sample preparation-
The optically active polymer is dissolved in a solvent (for example, chloroform) at 40 ° C. to prepare a sample solution having a concentration of 1 mg / ml.
-Measurement conditions-
0.1 ml of the sample solution is introduced into the column at a solvent [chloroform], a temperature of 40 ° C., and a flow rate of 1 ml / min.

  The sample concentration in the sample solution separated by the column is measured with a differential refractometer. A universal calibration curve is prepared with a polystyrene standard sample, and the weight average molecular weight (Mw) and molecular weight distribution (Mw / Mn) of the optically active polymer are calculated.

As the polylactic acid polymer which is an example of the optically active polymer, commercially available polylactic acid may be used. For example, PURASORB (PD, PL) manufactured by PURAC, LACEA (H-100, manufactured by Mitsui Chemicals, Inc.) H-400) and the like.
In order to increase the weight average molecular weight (Mw) of the polylactic acid polymer to 50,000 or more when using a polylactic acid polymer as the optically active polymer, the optically active polymer is added by the lactide method or the direct polymerization method. It is preferable to manufacture.

(Stabilizer)
The piezoelectric film in the present invention preferably contains a compound having a weight average molecular weight of 200 to 60000 having at least one functional group selected from the group consisting of a carbodiimide group, an epoxy group, and an isocyanate group as a stabilizer.
Thereby, the hydrolysis reaction of the optically active polymer can be suppressed, and the wet heat resistance of the obtained piezoelectric film can be further improved.
Regarding the stabilizer, the description in paragraphs 0039 to 0055 of International Publication No. 2013/054918 pamphlet can be appropriately referred to.

(Other ingredients)
The piezoelectric film in the present invention is a known resin such as polyvinylidene fluoride, polyethylene resin and polystyrene resin, inorganic fillers such as silica, hydroxyapatite and montmorillonite, and phthalocyanine as long as the effects of the present invention are not impaired. Other components such as a crystal nucleating agent may be contained.
Regarding the inorganic filler and the crystal nucleating agent, the description in paragraphs 0057 to 0059 of International Publication No. 2013/054918 can be referred to as appropriate.

(Piezoelectric film manufacturing method)
Although there is no restriction | limiting in particular in the manufacturing method of the piezoelectric film which has molecular orientation in this invention, For example, the film containing polymer | macromolecule (For example, the helical chiral polymer | macromolecule which has the optical activity mentioned later. The following is the same) by extrusion molding etc. The film can then be produced by stretching the film to orient the polymer.

  Hereinafter, a preferable manufacturing method (manufacturing method A and manufacturing method B) when the piezoelectric film contains at least an optically active polymer will be described.

-Manufacturing method A-
In the production method A, a non-crystalline sheet containing an optically active polymer (and other components such as a stabilizer if necessary) is crystallized to obtain a pre-crystallized sheet (also referred to as a crystallization raw material). A first step and a second step of stretching the pre-crystallized sheet mainly in a uniaxial direction.

In production method A, the raw material of the piezoelectric film is obtained by melt-kneading an optically active polymer such as the polylactic acid polymer described above (and other components such as a stabilizer such as a carbodiimide compound, if necessary). Can be obtained. Specifically, the optically active polymer and other components used as needed are mixed using a melt-kneader [Toyo Seiki Co., Ltd., Labo Plast Mill], mixer rotation speed 30 rpm to 70 rpm, 180 ° C. to 250 ° C. It is preferable to melt and knead for 5 to 20 minutes under the above conditions.
As a result, a blend of an optically active polymer and a stabilizer, a blend of a plurality of types of helical chiral polymers, a blend of a helical chiral polymer and other components such as an inorganic filler, and the like can be obtained.

  In general, by increasing the force applied to the film during stretching, the orientation of the optically active polymer is promoted and the piezoelectric constant increases, crystallization proceeds, and the crystal size increases, so that the haze tends to increase. Also, the dimensional deformation rate tends to increase due to an increase in internal stress. If force is simply applied to the film, crystals that are not oriented like spherulites are formed. Crystals with low orientation, such as spherulites, increase haze, but are unlikely to contribute to an increase in piezoelectric constant. Therefore, in order to form a film having a high piezoelectric constant and a low haze and dimensional deformation rate, it is necessary to efficiently form an oriented crystal that contributes to the piezoelectric constant with a small size that does not increase haze.

  In the production method A, for example, the inside of the sheet is pre-crystallized to form fine crystals before stretching, and then stretched. As a result, the force applied to the film during stretching can be efficiently applied to the polymer part having low crystallinity between the microcrystals, and the helical chiral polymer is efficiently oriented in the main stretching direction. be able to. Specifically, finely oriented crystals are formed in the polymer portion where the crystallinity between the microcrystals is low, and at the same time, the spherulites generated by the precrystallization are broken down to form spherulites. The desired lamellar crystals are oriented in the stretching direction in a daisy chain connected to tie molecular chains, whereby a desired value of MORc can be obtained. For this reason, a sheet having a low haze and dimensional deformation rate can be obtained without greatly reducing the piezoelectric constant.

  In order to control the normalized molecular orientation MORc, the crystallinity of the crystallization raw material is adjusted by the heat treatment time and the heat treatment temperature in the first step, and the stretching speed and the stretching temperature in the second step. is important. As described above, the helical chiral polymer is a polymer having molecular optical activity. The amorphous sheet containing the helical chiral polymer and the carbodilite compound may be commercially available or may be produced by a known film forming means such as extrusion. The amorphous sheet may be a single layer or a multilayer.

--First step (pre-crystallization step)-
The pre-crystallized sheet can be obtained by heat-treating a non-crystalline sheet containing the optically active polymer (and other components such as a stabilizer as required) to crystallize it.
In addition, a raw material containing an optically active polymer (and other components such as a stabilizer, if necessary) is heated to a temperature higher than the glass transition temperature of the optically active polymer by extrusion molding or the like to form a sheet. After the extrusion molding, the pre-crystallized sheet having a predetermined crystallinity can be obtained by rapidly cooling the sheet extruded by a caster.

  Also, 1) a pre-crystallized pre-crystallized sheet may be sent to a stretching step (second step) described later and set in a stretching apparatus to be stretched (off-line heat treatment), or 2) by heat treatment A non-crystallized amorphous sheet is set in a stretching apparatus, heated in a stretching apparatus to be pre-crystallized, and then continuously sent to a stretching process (second process) for stretching. Good (heat treatment by in-line).

The heating temperature T for pre-crystallizing the amorphous sheet is not particularly limited, but the glass transition temperature of the optically active polymer is enhanced in terms of enhancing the piezoelectricity and transparency of the piezoelectric film produced by the production method A. It is preferable that the relationship between Tg and the following formula is satisfied and the crystallinity is set to 3% to 70%.
Tg−40 ° C. ≦ T ≦ Tg + 40 ° C.
(Tg represents the glass transition temperature of the optically active polymer)

  Here, the glass transition temperature of the optically active polymer is increased by using a differential scanning calorimeter (DSC) with a temperature increase rate of 10 ° C./min with respect to the measurement target (for example, optically active polymer). It refers to the glass transition temperature (Tg) obtained as the inflection point of the curve from the melting endothermic curve.

  The heating time for pre-crystallization or the heating time for crystallization when extruding into a sheet form satisfies the desired crystallinity and is a normalized molecule of the piezoelectric film after stretching (after the second step) The product of the orientation MORc and the crystallinity of the stretched piezoelectric film is preferably 40 to 700, more preferably 125 to 650, and even more preferably 250 to 350. As the heating time becomes longer, the degree of crystallinity after stretching increases, and the normalized molecular orientation MORc after stretching also increases. When the heating time is shortened, the crystallinity after stretching also decreases, and the normalized molecular orientation MORc after stretching tends to decrease.

  When the degree of crystallization of the pre-crystallized sheet before stretching increases, the sheet becomes harder and a greater stretching stress is applied to the sheet, so that the portion with relatively low crystallinity in the sheet also becomes more oriented, and after stretching The normalized molecular orientation MORc is also increased. On the contrary, when the crystallinity of the pre-crystallized sheet before stretching becomes low, the sheet becomes soft and the stretching stress becomes more difficult to be applied to the sheet, so that the portion with relatively low crystallinity in the sheet becomes weakly oriented. The normalized molecular orientation MORc after stretching is also considered to be low.

  The heating time varies depending on the heating temperature, the thickness of the sheet, the molecular weight of the resin constituting the sheet, the type or amount of the additive. Further, the substantial heating time for crystallizing the sheet is the preheating when preheating at a temperature at which the amorphous sheet crystallizes in preheating that may be performed before the stretching step (second step) described later. This corresponds to the sum of the time and the heating time in the precrystallization step before preheating.

  The heating time of the amorphous sheet or the heating time in the case of crystallization when extruding into a sheet is usually 5 seconds to 60 minutes, and 1 minute to 30 minutes from the viewpoint of stabilizing the manufacturing conditions. But you can. For example, when pre-crystallizing an amorphous sheet containing a polylactic acid polymer as an optically active polymer, it is preferably heated at 20 ° C. to 170 ° C. for 5 seconds to 60 minutes, for 1 minute to 30 minutes. But you can.

In order to efficiently impart piezoelectricity, transparency, and high dimensional stability to the stretched sheet, it is important to adjust the crystallinity of the pre-crystallized sheet before stretching. That is, the reason why the piezoelectricity and dimensional stability are improved by stretching is that the stress due to stretching is concentrated in a portion where the crystallinity in the pre-crystallized sheet presumed to be in a spherulitic state is relatively high. While the piezoelectric d ₁₄ is improved by being oriented while being broken, it is considered that the stretching stress is applied to a portion having a relatively low crystallinity through the spherulite to promote the orientation and improve the piezoelectric d _14. Because.

  In the case where the crystallization degree of the sheet after stretching or the annealing treatment described later is performed, the crystallization degree after the annealing treatment is set to 20% to 80%, preferably 40% to 70%. Therefore, the degree of crystallinity of the pre-crystallized sheet immediately before stretching is set to 3% to 70%, preferably 10% to 60%, and more preferably 15% to 50%.

  What is necessary is just to perform the crystallinity degree of a pre-crystallization sheet | seat similarly to the measurement of the crystallinity degree of the piezoelectric film in this embodiment after extending | stretching.

  The thickness of the pre-crystallized sheet is mainly determined by the thickness of the piezoelectric film to be obtained by stretching in the second step and the stretching ratio, but is preferably 50 μm to 1000 μm, more preferably about 200 μm to 800 μm. .

--Second step (stretching step)-
The stretching method in the stretching step, which is the second step of production method A, is not particularly limited, and various stretching methods such as uniaxial stretching, biaxial stretching, and solid phase stretching described later can be used.
By passing through this second step, a piezoelectric film having molecular orientation can be suitably obtained. Furthermore, a piezoelectric film having a large main surface area can be obtained.

  By stretching the pre-crystallized sheet mainly in one direction, the molecular chains of the optically active polymer (eg, polylactic acid polymer) contained in the pre-crystallized sheet are aligned in one direction and aligned at high density. It is estimated that a piezoelectric film having higher piezoelectricity can be obtained.

  The stretching temperature of the pre-crystallized sheet is 10 ° C. to 20 ° C. from the glass transition temperature of the pre-crystallized sheet when the pre-crystallized sheet is stretched only by a tensile force, such as a uniaxial stretching method or a biaxial stretching method. It is preferable that the temperature range be as high as about ° C.

  The stretching ratio in the stretching treatment is preferably 3 to 30 times, and more preferably 4 to 15 times.

  When the pre-crystallized sheet is stretched, preheating may be performed immediately before stretching in order to facilitate stretching of the sheet. This preheating is generally performed in order to soften the sheet before stretching and facilitate stretching, so that the sheet before stretching is crystallized and does not harden the sheet. It is normal. However, in the manufacturing method A, since precrystallization is performed before stretching, the preheating may be performed also as precrystallization. Specifically, preheating and precrystallization can be performed by performing preheating at a temperature higher or longer than the normal temperature in accordance with the heating temperature and heat treatment time in the precrystallization step described above.

--- Annealing process-
From the viewpoint of improving the piezoelectric constant, it is preferable that the piezoelectric film after the stretching process (after the second step) is subjected to a constant heat treatment (hereinafter also referred to as “annealing process”). Note that in the case where crystallization is mainly performed by the annealing treatment, the preliminary crystallization performed in the above-described preliminary crystallization step may be omitted.

  The temperature of the annealing treatment is preferably about 80 ° C to 160 ° C, and more preferably 100 ° C to 155 ° C.

  The temperature application method for the annealing treatment is not particularly limited, and examples thereof include a method of directly heating using a hot air heater or an infrared heater, a method of immersing a piezoelectric film in a heated liquid such as heated silicon oil, and the like.

  At this time, if the piezoelectric film is deformed by linear expansion, it is difficult to obtain a practically flat film. Therefore, a certain tensile stress (for example, 0.01 MPa to 100 Mpa) is applied to the piezoelectric film, and the piezoelectric film becomes slack. It is preferable to apply the temperature while avoiding it.

  The temperature application time for the annealing treatment is preferably 1 second to 60 minutes, more preferably 1 second to 300 seconds, and further preferably heating in the range of 1 second to 60 seconds. When annealing is performed for more than 60 minutes, the degree of orientation may decrease due to the growth of spherulites from the molecular chains of the amorphous portion at a temperature higher than the glass transition temperature of the piezoelectric film. May decrease.

  The piezoelectric film annealed as described above is preferably cooled rapidly after the annealing treatment. In the annealing treatment, “quickly cool” means that the annealed piezoelectric film is immersed in, for example, ice water immediately after the annealing treatment and cooled to at least the glass transition point Tg or less. It means that no other treatment is included between the immersion.

  The rapid cooling method can be performed by immersing the annealed piezoelectric film in a coolant such as ethanol, methanol, or liquid nitrogen containing water, ice water, ethanol, or dry ice, or by spraying a liquid spray with a low vapor pressure and using latent heat of vaporization. The method of cooling is mentioned. In order to continuously cool the piezoelectric film, it is possible to rapidly cool the piezoelectric film by bringing it into contact with a metal roll controlled to a temperature not higher than the glass transition temperature Tg of the piezoelectric film. Further, the number of times of cooling may be only once, or may be two or more. Furthermore, annealing and cooling can be alternately repeated.

-Manufacturing method B-
As a preferable production method in the case where the piezoelectric film contains at least an optically active polymer, in addition to the production method A, the following production method B can be mentioned.
Production method B includes a step of stretching a sheet containing an optically active polymer (and other components such as a stabilizer as necessary) mainly in a uniaxial direction and a step of annealing treatment in this order. It may be. The step of stretching and the step of annealing can be the same as the step in the manufacturing method A.
Moreover, in the manufacturing method B, the preliminary crystallization process in the manufacturing method A does not need to be implemented.

<Electrode layer>
As described above, the multilayer piezoelectric element of the present invention preferably includes an electrode layer (for example, the first electrode layers 102A, 102B, and 102C and the second electrode layers 104A, 104B, and 104C described above). The electrode layer is preferably provided on the main surface of the piezoelectric film.
Examples of the material for the electrode layer include Al, Ag, Au, Cu, an Ag—Pd alloy, ITO, ZnO, IZO (registered trademark), and a conductive polymer.
Each electrode layer may contain only one type of the above material or two or more types.
Each electrode layer may contain components having no electrical conductivity (such as a binder component) in addition to the above materials, as long as the electrical conductivity of the electrode layer can be maintained.

The electrode layer preferably has transparency.
Specifically, the electrode layer preferably has an internal haze with respect to visible light of 50% or less, more preferably 40% or less, further preferably 30% or less, and 20% or less. It is particularly preferred.
From the viewpoint of transparency, the material of the electrode layer is preferably ITO, ZnO, IZO (registered trademark), conductive polymer, or the like.

Although there is no restriction | limiting in particular in the thickness of an electrode layer, 10 nm-1000 nm are preferable, 30 nm-600 nm are more preferable, 50 nm-300 nm are especially preferable.
When the thickness is 10 nm or more, the conductivity can be further increased.
Further, when the thickness is 1000 nm or less, the piezoelectricity of the entire laminated piezoelectric element is maintained higher.

  The electrode layer can be formed by a known method such as vapor deposition, sputtering, or coating formation (including drying and baking as necessary).

<Insulating layer>
As described above, the multilayer piezoelectric element of the present invention preferably includes at least one insulating layer (for example, the above-described insulating layer 112; the same applies hereinafter).
The thickness of the insulating layer (for example, the insulating layer 112; the same applies hereinafter) is preferably 5 μm to 30 μm, and more preferably 7 μm to 25 μm.
When the thickness is 5 μm or more, the insulating property is more excellent.
On the other hand, when the thickness is 30 μm or less, the ratio of the piezoelectric film in the entire laminated piezoelectric element is increased, and the performance as the entire laminated piezoelectric element is further improved.

  The insulating layer preferably has a dielectric constant measured at 1 MHz of 3.4 or less, and more preferably 3.0 or less.

The insulating layer preferably has transparency.
Specifically, the insulating layer preferably has an internal haze for visible light of 50% or less, more preferably 40% or less, further preferably 30% or less, and 20% or less. It is particularly preferred.

The insulating layer preferably has adhesiveness (that is, it also functions as an adhesive layer).
Thereby, the effect as an insulating layer mentioned above can be acquired, bonding piezoelectric units.

  The insulating layer having adhesiveness is preferably an insulating layer that can ensure a peel strength of 3 N / m or more with the piezoelectric film.

  Examples of the insulating layer having an adhesive function include a layer formed using a known adhesive having a low dielectric constant (for example, a relative dielectric constant measured at 1 MHz is 3.4 or less).

  Examples of the resin that is the main component of the adhesive include acrylic resin, urethane resin, cellulose-based, vinyl acetate resin, ethylene-vinyl acetate resin, epoxy resin, nylon-epoxy, vinyl chloride resin, chloroprene rubber, cyano. Acrylate, silicone, modified silicone, aqueous polymer-isocyanate, styrene-butadiene rubber, nitrile rubber, acetal resin, phenol resin, polyamide resin, polyimide resin, melamine resin, urea resin, bromine resin, starch Polyester resin, polyolefin resin, etc. are used.

Moreover, it is also preferable to use an ultraviolet curable adhesive as the adhesive.
Moreover, as an adhesive agent, the adhesive agent which has a viscosity of 250-490 mPa * s in the state before drying is preferable.

In addition, examples of the insulating layer having an adhesive function include known adhesive sheets having a low dielectric constant (for example, a relative dielectric constant measured at 1 MHz is 3.4 or less).
Examples of the adhesive sheet include OCA (transparent double-sided adhesive sheet) and OCR (transparent adhesive resin).

<Other members>
The laminated piezoelectric element of the present invention may include other members as necessary.
Examples of other members include a protective layer, a support layer laminated to support a curved shape, wiring connected to the electrode layer, and a conductive paste layer connecting the electrode layer and the wiring.

The laminated piezoelectric element of the present invention described above can be used suitably for acoustic devices such as speakers, microphones, earphones and the like because high sound pressure is obtained and the frequency dependence of sound pressure is suppressed. .
The audio equipment here includes not only an audio equipment as a single unit, but also a mobile phone, a portable information terminal, a portable game machine, various display devices (liquid crystal display device, organic EL display device, etc.), personal computer, electronic dictionary, Audio equipment mounted on vehicles and the like is also included.

≪Sound equipment≫
The acoustic device of the present invention includes a laminated piezoelectric element.
For this reason, according to the acoustic device of the present invention, the frequency dependence of the sound pressure is suppressed.
The acoustic device of the present invention preferably includes support members (for example, a support member 202 and a support member 204 in FIG. 4B described later) that support both ends in the direction along the curvature of the multilayer piezoelectric element. Accordingly, when the acoustic device is operated (that is, when a voltage is applied to the laminated piezoelectric element), the both end portions become fixed ends, and the central portion (curved portion) can be vibrated.
The support member may be a support frame that supports the both end portions.
The acoustic device of the present invention is configured to include other members as appropriate, such as a voltage application power source for applying a voltage to the laminated piezoelectric element, as necessary.

  Hereinafter, the embodiment of the present invention will be described more specifically by way of examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist of the present invention.

[Example 1]
<< Production of multilayer piezoelectric element >>
A laminated piezoelectric element having the same layer configuration as that of the laminated piezoelectric element 100 shown in FIG. 3 was produced.
Details will be described below.

<Production of piezoelectric film>
For 100 parts by weight of polylactic acid (PLA) (registered trademark LACEA, H-400, weight average molecular weight Mw: 200,000) manufactured by Mitsui Chemicals, Inc. as a helical chiral polymer (optically active polymer) having optical activity Then, 1 part by weight of the following stabilizer SI (Stagexol I (trade name), bis-2,6-diisopropylphenylcarbodiimide, manufactured by Rhein Chemie) as a stabilizer (B) [carbodiimide compound] is added and dry blended. The raw material was prepared.

The prepared raw material is put into an extrusion molding machine hopper, extruded from a T-die while being heated to 220 ° C. to 230 ° C., and brought into contact with a 55 ° C. cast roll for 0.5 minutes to produce a 150 μm thick pre-crystallized sheet. Filmed (pre-crystallization step). The crystallinity of the pre-crystallized sheet was measured and found to be 5.63%.
The obtained pre-crystallized sheet was stretched by roll-to-roll while being heated to 70 ° C. at a stretching speed of 1650 mm / min, and uniaxially stretched in the MD direction up to 3.3 times (stretching step). The thickness of the obtained film was 50 μm.
Thereafter, the uniaxially stretched film was roll-rolled and contacted for 60 seconds on a roll heated to 130 ° C. for annealing treatment (annealing step) to obtain a piezoelectric film.

  In this example, the molecular orientation and the uniaxial stretching direction are parallel (the angle formed by both is 0 °).

<Measurement of physical properties of piezoelectric film>
The following measurements were performed on the obtained piezoelectric film.

(Mw, Mw / Mn, optical purity, chirality of optically active polymer (PLA))
The Mw, Mw / Mn, optical purity, and chirality of the optically active polymer (PLA) contained in the piezoelectric film are determined in the same manner as described in paragraphs 0126 to 0128 of International Publication No. 2013/054918. It was measured.
As a result, Mw was 200,000, Mw / Mn was 2.87, optical purity was 98.5% ee, and chirality was L.

(Melting point, crystallinity)
The piezoelectric film was accurately weighed 10 mg, and heated to 140 ° C. at a heating rate of 500 ° C./min using a differential scanning calorimeter (DSC-1 manufactured by Perkin Elmer), and further heated to 10 ° C. The melting curve was obtained by raising the temperature to 200 ° C under the conditions of / min. The melting point Tm and crystallinity were obtained from the obtained melting curve.
As a result, the melting point Tm was 164.6 ° C., and the crystallinity was 39.8%.

(Internal haze)
Haze (H2) is measured by sandwiching only silicon oil (Shin-Etsu Chemical Co., Ltd., Shin-Etsu Silicone (trademark), model number: KF96-100CS) between two glass plates in advance. The haze (H3) was measured by sandwiching the piezoelectric film coated on the two glass plates, and the internal haze (H1) of the piezoelectric film of this example was obtained by taking these differences as in the following formula.
Internal haze (H1) = Haze (H3) -Haze (H2)
The haze (H2) and the haze (H3) were measured by measuring light transmittance in the thickness direction using the following apparatus under the following measurement conditions.
Measuring device: manufactured by Tokyo Denshoku Co., Ltd., HAZE METER TC-HIIIDPK
Sample size: width 3mm x length 30mm, thickness 0.05mm
Measurement conditions: Conforms to JIS-K7105 Measurement temperature: Room temperature (25 ° C.)

  As a result of the measurement, the internal haze (H1) of the piezoelectric film was 0.0%.

(Standardized molecular orientation MORc)
The normalized molecular orientation MORc of the piezoelectric film was measured with a microwave molecular orientation meter MOA-6000 manufactured by Oji Scientific Instruments. Here, the reference thickness tc was set to 50 μm.
As a result, the normalized molecular orientation MORc of the piezoelectric film was 4.73.

(Product of MORc and crystallinity)
The product of the MORc and the crystallinity was 188.

(Piezoelectric constant d ₁₄ by displacement method)
By the aforementioned measurement method (displacement method) were measured piezoelectric constant d ₁₄ of the piezoelectric film.
As a result, the piezoelectric constant _{d 14} due to the displacement method of the piezoelectric film was 6.2pm / V.

<Production of piezoelectric unit>
A 175 mm × 85 mm rectangular piezoelectric film having a dimension of 175 mm in the direction formed by 45 ° with respect to the stretching direction and 85 mm in a direction orthogonal to the direction formed by 45 ° was cut out from the piezoelectric film. .
An ITO layer having a thickness of 100 nm was formed on both sides of the cut out piezoelectric film by sputtering.
Thus, a piezoelectric unit having a configuration in which an ITO layer as an electrode layer was provided on both surfaces of the piezoelectric film was obtained.

<Production of laminated piezoelectric element>
Three piezoelectric units were prepared.
The three piezoelectric units correspond to the piezoelectric unit 110A, the piezoelectric unit 110B, and the piezoelectric unit 110C in the multilayer piezoelectric element 100.
Next, a voltage application wiring was attached to each ITO layer of the three piezoelectric units.
Next, the three piezoelectric units are arranged so that the molecular orientation (stretching direction) between the three piezoelectric films is parallel (that is, the angle formed by the three molecular orientations is 0 °). ), An adhesive (UV curing type liquid crystal sealing material “UV struct bond” manufactured by Mitsui Chemicals, Inc.), and then the adhesive is UV cured by irradiating with ultraviolet rays (UV) to form a laminated piezoelectric element (Laminated piezoelectric element before bending) was obtained.
In Table 1 below, the above structure of the laminated piezoelectric element is expressed as “laminated three piezoelectric films”.
Here, the cured layer obtained by curing the adhesive corresponds to the insulating layer 112 in the laminated piezoelectric element 100.

The thickness of the cured layer was 15 μm.
The relative dielectric constant (1 MHz) of the cured layer was 3.0 using an LCR meter “ELC-131D” manufactured by CUSTOM.

The laminated piezoelectric element obtained above (hereinafter referred to as “laminated piezoelectric element 200A”) was attached to the experimental apparatus shown in FIGS. 4A and 4B.
As shown in FIG. 4A, this experimental apparatus has a pair of support members (support member 202 and support member 204) for supporting both ends of the multilayer piezoelectric element 200A and a voltage for applying a voltage between the pair of support members. The power supply 220 for voltage application, the support | pillar 210 which supports the end of the support member 202 and the end of the support member 204, and the support stand 212 to which the end of the support | pillar 210 was fixed were provided.
Each of the support member 202 and the support member 204 is provided so as to be movable along the support column 210, and thereby the distance between them can be changed. Further, the support member 202 and the support member 204 are provided with a mechanism that sandwiches the end portion of the piezoelectric unit on the opposite sides.

First, as shown in FIG. 4A, both ends in the width direction (that is, the long side) of the laminated piezoelectric element 200A were supported by the support member 202 and the support member 204 of the experimental apparatus, respectively.
Further, each wiring connected to each electrode layer of the laminated piezoelectric element 200A was connected to a voltage application power source 220.
Although not shown in FIG. 4A, the connection method (voltage application method) between each electrode layer of the multilayer piezoelectric element 200A and the voltage application power source 220 is the connection method (FIG. 3) in the multilayer piezoelectric element 100 described above. And the same. That is, the voltage supplied from the voltage application power source 220 is applied to each piezoelectric film, and an electric field in the same direction is generated in each piezoelectric film.

Next, the laminated piezoelectric element 200A was bent (deformed) by applying a force F to the laminated piezoelectric element 200A by moving the supporting member 202 and the supporting member 204 so as to approach each other. Here, force F was applied in a direction parallel to the short side direction of the multilayer piezoelectric element. That is, the multilayer piezoelectric element was bent (deformed) while maintaining the state in which the long sides of the multilayer piezoelectric element were parallel to each other.
As a result, a laminated piezoelectric element 200B having a curved portion shown in FIG. 4B was obtained.
In the laminated piezoelectric element 200B, the molecular orientation of the piezoelectric film follows the curved shape. The aforementioned angle θ is 45 °.
In FIG. 4B, the curved shape of the laminated piezoelectric element 200B is exaggerated from the actual one so as to make the feature of the shape easy to see.
The curved shape of the multilayer piezoelectric element 200B was a shape in which the ratio C represented by the following formula was 19/20 in the cross section when the curved portion was cut in the direction in which the curvature becomes maximum.
Ratio C = Linear distance from one end of the bending portion to the other end / the length along the bending portion of the bending portion

  In the multilayer piezoelectric element 200B, the length along the curve of the bending portion was set to 75 mm.

<< Evaluation of multilayer piezoelectric element >>
A voltage of 100 V to 400 V is applied to each piezoelectric film in the laminated piezoelectric element 200B from a voltage application power source 220 obtained by amplifying the output signal of the frequency transmitter, and the sound pressure at this time is made by Ono Sokki Co., Ltd. The high-performance sound level meter “LA-5560” was used, and the measurement was performed with Lc in the C-weighted sound pressure mode with an emphasis on frequency characteristics.
More specifically, the sound pressure is measured by placing the multilayer piezoelectric element 200B and the LA-5560 in an anechoic box and adjusting the distance (closest distance) between the multilayer piezoelectric element 200B and the sound level meter to 100 mm. I went there.
The sound pressure was measured in the frequency range of 1500 Hz to 9500 Hz.
Based on the measurement results, the maximum sound pressure and the difference between the maximum sound pressure and the minimum sound pressure (sound pressure difference = maximum sound pressure−minimum sound pressure) in the frequency range of 1500 Hz to 9500 Hz were obtained.
The results are shown in Table 1 below.

[Comparative Example 1]
In Example 1, the molecular orientation of the central piezoelectric film (the piezoelectric film corresponding to the piezoelectric film 101B in FIG. 3) and the molecular orientation of the other two piezoelectric films are orthogonal to each other (the angle between the molecular orientations is The same evaluation as in Example 1 was performed except that three piezoelectric units were bonded to each other at 90 °.
The results are shown in Table 1 below.

[Comparative Example 2]
In Example 1, the same evaluation as in Example 1 was performed except that the laminated piezoelectric element was not curved (that is, the ratio C was set to 1/1).
The results are shown in Table 1 below.

As shown in Table 1, in the laminated piezoelectric element of Example 1 in which the molecular orientations of the three piezoelectric films were parallel and had a curved portion, the maximum sound pressure was high and the sound pressure difference was reduced. . That is, in Example 1, the frequency dependence of the sound pressure was reduced.
On the other hand, in Comparative Example 1 in which the molecular orientations were orthogonal, the sound pressure difference was large and the frequency dependence of the sound pressure was large.
In Comparative Example 2 in which the piezoelectric element was not bent, no sound pressure was obtained.

[Examples 2 to 4]
In Example 1, the ratio C was changed to 9/10 (Example 2), 6/7 (Example 3), and 3/4 (Example 4) by changing the force applied to the laminated piezoelectric element. Evaluation similar to Example 1 was performed except having changed.
FIG. 5 is a graph showing the relationship between the ratio C and the maximum sound pressure (dB) in Examples 1 to 4.
As shown in FIG. 5, it was confirmed that a particularly high maximum sound pressure was obtained when the ratio C was in the range of 19/20 to 6/7.
Moreover, in Examples 2-4, since the molecular orientation of the three piezoelectric films is parallel as in Example 1, the effect of reducing the frequency dependence of sound pressure is the same as in Example 1. Can be expected.

10, 20, 100, 200A, 200B Multilayer piezoelectric element 22 Curved portions 24, 26 Planar portions 101A, 101B, 101C Piezoelectric films 102A, 102B, 102C First electrode layers 104A, 104B, 104C Second electrode layers 110A, 110B, 110C Piezoelectric unit 112 Insulating layer a Linear distance b from one end of the bending portion to the other end b Length along the bending portion of the bending portion

Claims (7)

A plurality of piezoelectric films having molecular orientation, the molecular orientation of the plurality of piezoelectric films is substantially parallel, and has a curved portion,
The piezoelectric film includes a plurality of piezoelectric units each having a first electrode layer on one surface and a second electrode layer on the other surface, the plurality of piezoelectric units including a first electrode layer, a second electrode layer, Are stacked via an insulating layer in an opposing arrangement,
By applying the same potential to the plurality of first electrode layers, and applying another same potential to the plurality of second electrode layers, the same voltage is applied to the plurality of piezoelectric films,
The plurality of piezoelectric films have the same chirality , and do not have a structure in which the plurality of piezoelectric films are displaced in the same direction by application of voltage and two types of piezoelectric films having different chiralities are alternately laminated. ,
A laminated piezoelectric element in which the following ratio C is 19/20 to 6/7 in a cross section when the curved portion is cut in a direction in which the curvature becomes maximum .
Ratio C = Linear distance from one end of the bending portion to the other end / the length along the bending portion of the bending portion The multilayer piezoelectric element according to claim 1, wherein the piezoelectric film has a thickness of 20 μm to 80 μm. The plurality of piezoelectric films contain a helical chiral polymer having optical activity in which the amount of L-form is 95% by mass or more, or helical chiral having optical activity in which the amount of D-form is 95% by mass or more The laminated piezoelectric element according to claim 1, comprising a polymer. The piezoelectric film is 50% or less internal haze to visible light, and the piezoelectric constant d ₁₄ measured by a displacement method at 25 ° C. is 1 Pm/V least any one of claims 1 to 3 The laminated piezoelectric element according to the item. The piezoelectric film includes a helical chiral polymer having an optical activity having a weight average molecular weight of 50,000 to 1,000,000, a crystallinity of 20% to 80% obtained by a DSC method, and a microwave transmission type 5. The product of any one of claims 1 to 4 , wherein a product of the normalized molecular orientation MORc and the crystallinity when the reference thickness measured by a molecular orientation meter is 50 μm is 25 to 700. 6. Laminated piezoelectric element. The multilayer piezoelectric element according to claim 3 or 5 , wherein the helical chiral polymer is a polylactic acid polymer having a main chain including a repeating unit represented by the following formula (1).
An acoustic device comprising the laminated piezoelectric element according to any one of claims 1 to 6 .