Laminated film for piezoelectric elements and piezoelectric elements

Abstract

To provide a piezoelectric element with good piezoelectric properties. [Solution] A laminated film 2 for a piezoelectric element, comprising a first conductive layer 21, a piezoelectric layer 22, a second conductive layer 23, and an insulating layer 24 in that order, wherein the thickness T of the piezoelectric layer 22 P Thickness T of the insulating layer 24 relative to the I Ratio T I / T P A laminated film 2 for piezoelectric elements, wherein the coefficient is 0.92 or less.

Description

[Technical Field]

[0001] The present invention relates to a laminated film for a piezoelectric element, comprising a first conductive layer, a piezoelectric layer, a second conductive layer, and an insulating layer in that order, and to a piezoelectric element formed by winding the laminated film for the piezoelectric element. [Background technology]

[0002] Patent Document 1 discloses a piezoelectric laminate element (piezoelectric element) formed by winding a piezoelectric laminate (laminated film for piezoelectric elements) which is made up of a first conductive layer, a piezoelectric polymer film layer (piezoelectric layer), a second conductive layer, and an insulating layer (insulating layer) in that order. In this piezoelectric element, by using a film that exhibits piezoelectricity in the planar direction of the film, such as a poly-L-lactic acid film (PLLA) or poly-D-lactic acid film (PDLA), for the piezoelectric layer, a substantially constant voltage is continuously output even after the load on the piezoelectric element becomes constant. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6634638 [Overview of the project] [Problems that the invention aims to solve]

[0004] In Examples 1 to 6 of Patent Document 1, the piezoelectric layer and the insulating layer are of equal thickness in all cases, and there is no disclosure of making their thicknesses different. Furthermore, in conventional piezoelectric elements, a film containing alternating layers of piezoelectric material containing PLLA and piezoelectric material containing PDLA is used as the wound film. However, since PDLA is more expensive than PLLA, there is a problem in that it is difficult to reduce the cost of piezoelectric elements.

[0005] The present invention has been made in view of the above problems, and aims to provide a piezoelectric element with good piezoelectric properties, or a piezoelectric element at low cost.

Means for Solving the Problem

[0006] In order to solve the above problems, the present invention includes, for example, the subject matters described in the following items. Item 1. A laminated film for a piezoelectric element formed by laminating a first conductive layer, a piezoelectric layer, a second conductive layer, and an insulator layer in this order, where the thickness T of the piezoelectric layer P and the thickness T of the insulator layer I The ratio T of I / T P is 0.92 or less, a laminated film for a piezoelectric element. Item 2. A laminated film for a piezoelectric element formed by laminating a first conductive layer, a piezoelectric layer, a second conductive layer, and an insulator layer in this order, where the piezoelectric layer contains poly-L-lactic acid, and the insulator layer contains polypropylene, a laminated film for a piezoelectric element. Item 3. The laminated film for a piezoelectric element according to Item 2, where T I / T P is 0.92 or less. Item 4. The laminated film for a piezoelectric element according to Item 1 or 2, where T I / T P is 0.46 or less. Item 5. A piezoelectric element formed by winding the laminated film for a piezoelectric element according to any one of Items 1 to 4. Item 6. The piezoelectric element according to Item 5, having a measured hardness of 80 or less using a Type C Asker rubber hardness meter.

Advantages of the Invention

[0007] According to the present invention, it is possible to provide a piezoelectric element with good piezoelectric properties or a low-cost piezoelectric element.

Brief Description of the Drawings

[0008] [Figure 1]It is a perspective view showing the configuration of a piezoelectric element according to an embodiment of the present invention. [Figure 2] It is a partial cross-sectional view of the piezoelectric element. [Figure 3] It is a graph showing an example of the frequency characteristics of tanδ of the piezoelectric element.

Embodiments for Carrying out the Invention

[0009] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In this specification, expressions such as "containing" and "including" include the concepts of "containing", "including", "substantially consisting of", and "consisting only of".

[0010] (Structure of Piezoelectric Element) FIG. 1 is a perspective view showing the configuration of a piezoelectric element 1 according to an embodiment of the present invention, and FIG. 2 is a partial cross-sectional view of the piezoelectric element 1. The piezoelectric element 1 is formed by winding a laminated film for piezoelectric elements (hereinafter referred to as a film) 2 in which a first conductive layer 21, a piezoelectric layer 22, a second conductive layer 23, and an insulator layer 24 are laminated in this order. The end of the first conductive layer 21 protrudes from one laminated surface 11A of the piezoelectric element 1, and the end of the second conductive layer 23 protrudes from the other laminated surface 11B of the piezoelectric element 1.

[0011] Note that the piezoelectric element 1 is further press-processed after winding the film 2 into a cylindrical shape, and the laminated surface (side surface) has a rounded rectangular shape, but the above press-processing may not be performed.

[0012] External electrodes 3 are formed on a part of the laminated surfaces 11A and 11B of the piezoelectric element 1. The external electrodes 3 can be formed by performing metal spraying on the laminated surfaces 11A and 11B, and function as extraction electrodes for extracting the output voltage of the piezoelectric element 1 to the outside. Note that the external electrodes 3 may be formed on the entire surfaces of the laminated surfaces 11A and 11B.

[0013] Note that an insulation margin M is formed between the first conductive layer 21 and the external electrode 3, and between the second conductive layer 23 and the external electrode 3 so that the first conductive layer 21 and the second conductive layer 23 do not short-circuit. The insulation margin M is a portion of the film 2 where neither the first conductive layer 21 nor the second conductive layer 23 is formed.

[0014] The film 2 is a piezoelectric polymer film that exhibits piezoelectricity in the plane direction. When a load is applied to the piezoelectric element 1, stress occurs in the thickness direction and the plane direction of the film 2, and the voltage generated in the piezoelectric layer 22 is taken out from the external electrode 3 through the first conductive layer 21 and the second conductive layer 23. Also, generally, the voltage generated in a piezoelectric element gradually decreases unless the load increases, and the degree of decrease is gentler as the capacitance is larger.

[0015] Furthermore, in the piezoelectric element 1 according to the present embodiment, the thickness T I of the insulator layer 24 is smaller than the thickness T P of the piezoelectric layer 22, and preferably, the ratio T P of the thickness T I of the insulator layer 24 to the thickness T I / T P of the piezoelectric layer 22 is 0.92 or less, and more preferably, the said T I / T P is 0.46 or less. Thus, by making the insulator layer 24 thinner than the piezoelectric layer 22, as shown in the examples described later, the height of the peak of the piezoelectric resonance of the piezoelectric element 1 (the difference between the peak value and the base value, the tanδ difference) can be increased. Also, by making the insulator layer 24 thinner, miniaturization of the piezoelectric element 1 becomes easier. Furthermore, since the capacitance of the piezoelectric element 1 increases, the generated voltage is maintained for a longer time accordingly, and the detection ability as a pressure sensor is enhanced. Therefore, a piezoelectric element 1 with good piezoelectricity can be provided.

[0016] Note that the lower limit value of the thickness T P of the insulator layer 24 is not particularly limited as long as it is greater than 0 μm, but it may be a thickness such that there are no problems with the insulation of the insulator layer 24 and the strength of the film 2.

[0017] Furthermore, the piezoelectric layer 22 is formed from a piezoelectric polymer film containing polylactic acid as the main component, as will be described later. The polylactic acid may contain only one enantiomer of poly-L-lactic acid (L-form, PLLA) and poly-D-lactic acid (D-form, PDLA), or both, but preferably the piezoelectric layer 22 contains PLLA. This reduces the cost of the piezoelectric element 1 compared to using a film in which piezoelectric layers containing PLLA and piezoelectric layers containing PDLA are alternately laminated. In this case, it is preferable that the insulating layer 24 contains polypropylene.

[0018] Furthermore, it is preferable that the piezoelectric element 1, after the formation of the external electrode 3, has a hardness of 80 or less when measured using an Asker rubber hardness tester type C.

[0019] (material) The materials of the first conductive layer 21, piezoelectric layer 22, second conductive layer 23, and insulating layer 24 that constitute the film 2, as well as the materials of the external electrode 3, will be described below.

[0020] The materials of the first conductive layer 21 and the second conductive layer 23 are not particularly limited as long as they are conductive, and include, for example, aluminum, zinc, tin, lead, nickel, iron, copper, and alloys thereof.

[0021] The piezoelectric layer 22 is formed by a piezoelectric polymer film. The piezoelectric polymer film contains polylactic acid as its main component. The type of polylactic acid is not particularly limited, and known polylactic acids can be widely used, for example, polylactic acid obtained by condensation polymerization of lactic acid components as raw material monomers. The polylactic acid may contain only one optical isomer of L-lactic acid (L-form, PLLA) and D-lactic acid (D-form, PDLA), or both. When the polylactic acid mainly consists of L-lactic acid (L-form), the content of the D-form is preferably 5.0 mol% or less, more preferably 3.0 mol% or less, even more preferably 1.0 mol% or less, and particularly preferably 0.01 mol% or less. When the polylactic acid mainly consists of D-lactic acid (D-form), the content of the L-form is preferably 5.0 mol% or less, more preferably 3.0 mol% or less, even more preferably 1.0 mol% or less, and particularly preferably 0.01 mol% or less. The glass transition temperature, crystallization temperature, molecular weight, etc., of polylactic acid are not particularly limited.

[0022] The method for producing polylactic acid is not particularly limited; for example, it can be obtained by known manufacturing methods. Polylactic acid can also be obtained from commercially available products. Representative commercially available polylactic acid products include NatureWorks' "4032D" and TotalCorbion's Luminy series, specifically "L175," "LX175," and "LX575."

[0023] As the insulating layer 24, various insulating resins such as polypropylene (PP), polyethylene terephthalate (PET), polyphenylene sulfide (PPS), and polyethylene naphthalate (PEN) can be used.

[0024] The material for the external electrode 3 can be any conductive material, such as individual metals like zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures of multiple metals, or alloys thereof.

[0025] (Summary) As described above, in this embodiment, the thickness T of the insulating layer 24 I Thickness T of the piezoelectric layer 22 P Smaller than, preferably, the thickness T of the piezoelectric layer 22. P Thickness T of the insulating layer 24 relative to the I Ratio T I / T P This is 0.92 or less. As a result, the peak height of the piezoelectric resonance and the capacitance of the piezoelectric element 1 increase, thus enabling good piezoelectric properties. Furthermore, by using a piezoelectric layer 22 containing PLLA, the cost of the piezoelectric element 1 can be reduced.

[0026] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. [Examples]

[0027] The following describes examples of the present invention, but the present invention is not limited to the following examples.

[0028] (Examples 1-3) As the piezoelectric layer 22, a poly-L-lactic acid (PLLA) film having piezoelectric properties in the planar direction of the film was coated with oil to provide an insulating margin M, and then a first conductive layer 21 was formed by aluminum deposition at a deposition resistance of 5Ω / □ (also written as Ω / square) to create a first metallized film. Oil deposition was performed using an ULVAC-manufactured roll-type vacuum deposition apparatus (EWE-060). The thickness of the piezoelectric layer 22 was 6.5 μm.

[0029] Next, a second metallized film was fabricated by applying oil deposition to a polypropylene film to provide an insulating margin M with a width of 2 mm as an insulating layer 24, and then applying aluminum deposition with a deposition resistance of 5 Ω / □ to form a second conductive layer 23. The oil deposition and aluminum deposition were performed using an ULVAC-manufactured roll-type vacuum deposition apparatus (EWE-060). The thickness of the insulating layer 24 was 6 μm in Example 1, 3 μm in Example 2, and 2.5 μm in Example 3.

[0030] The obtained first and second metallized films were slit to a width of 30 mm, and then the first and second metallized films were joined together and wound 395 turns using a 3KAW-N2 automatic winding machine manufactured by Kaito Manufacturing Co., Ltd., at a winding tension of 100 g. Furthermore, this wound laminate was flattened by pressing it at room temperature under a load of 0.58 MPa, thereby fabricating a piezoelectric element 1 with a rounded rectangular laminate surface.

[0031] Next, while maintaining the load, zinc metal was sprayed onto the entire surface of the laminated surfaces 11A and 11B of the piezoelectric element 1, forming an external electrode 3 with a thickness of approximately 0.7 mm and a width of 6 mm in a strip shape. After that, the press load was released to fabricate the piezoelectric element 1. The measured hardness using an Asker rubber hardness tester type C was 67.

[0032] Thickness T of the piezoelectric layer 22 in Examples 1-3 I Thickness T of the insulating layer 24 P , and their ratio T I / T P The results were as shown in Table 1.

[0033] [Table 1]

[0034] (Evaluation results) The piezoelectric properties of the piezoelectric elements in Examples 1 to 3 were evaluated. Specifically, lead wires were soldered to the external electrodes 3 of piezoelectric element 1, and the frequency characteristics of the tanδ of the piezoelectric element were measured using an impedance analyzer. Specifically, a Keysight impedance analyzer E4990A was fitted with a test fixture 16047E, and the two lead wires soldered to piezoelectric element 1 were inserted into the test fixture. An AC voltage of 0.5V was applied from the impedance analyzer's built-in power supply, and a frequency sweep was performed in a bandwidth including the range of 1kHz to 100kHz to measure the frequency characteristics of piezoelectric element 1. The capacitance of piezoelectric element 1 was read as a representative value at 1kHz.

[0035] For measurement conditions other than those described herein, the standards of JIS C 5101-16:2009, "4.2.2 Capacitance" and "4.2.3 Dielectric Loss Tangent (tanδ)," were followed.

[0036] Figure 2 is a graph showing an example of the frequency characteristics of tanδ of a piezoelectric element. In this graph, piezoelectric resonance appears between 10kHz and 100kHz, and tanδ has a peak. The height of this peak (the difference between the peak value and the base value of tanδ), which is the tanδ difference, was quantified as the strength of the piezoelectricity of the piezoelectric element. The evaluation results for Examples 1 to 3 are shown in Table 2.

[0037] [Table 2]

[0038] Thus, in Examples 1-3, both capacitance and tanδ difference were good. Therefore, the thickness T of the piezoelectric layer P The thickness T of the insulating layer relative to the I Ratio T I / T P It was found that good piezoelectric properties can be obtained by setting the value to 0.92 or less. [Industrial applicability]

[0039] This invention can be applied to vibrating bodies such as sensors that utilize piezoelectric properties, and electrical components such as piezoelectric actuators. [Explanation of Symbols]

[0040] 1. Piezoelectric element 2. Film (Laminated film for piezoelectric elements) 3 External electrode 11A Laminated surface 11B Laminated surface 21 First conductive layer 22 Piezoelectric layer 23 Second conductive layer 24 Insulator layer M Insulation margin

Claims

1. A laminated film for a piezoelectric element, comprising a first conductive layer, a piezoelectric layer, a second conductive layer, and an insulating layer, in that order, The thickness T of the piezoelectric layer Ｐ The thickness T of the insulating layer relative to the thickness T Ｉ ratio T Ｉ / T Ｐ A laminated film for piezoelectric elements having a coefficient of 0.92 or less.

2. A laminated film for a piezoelectric element, comprising a first conductive layer, a piezoelectric layer, a second conductive layer, and an insulating layer, in that order, The piezoelectric layer contains poly-L-lactic acid, A laminated film for a piezoelectric element, wherein the insulating layer contains polypropylene.

3. Said T Ｉ / T Ｐ A laminated film for a piezoelectric element according to claim 2, wherein the coefficient is 0.92 or less.

4. Said T Ｉ / T Ｐ A laminated film for a piezoelectric element according to claim 1 or 2, wherein the coefficient is 0.46 or less.

5. A piezoelectric element formed by winding a laminated film for piezoelectric elements according to any one of claims 1 to 3.

6. The piezoelectric element according to claim 5, wherein the measured hardness using an Asker rubber hardness tester type C is 80 or less.

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