JP3121416B2 - Flexible piezoelectric element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer piezoelectric element having a spray-coated electrode.

[0002]

2. Description of the Related Art A polymer piezoelectric material such as a polarized vinylidene fluoride resin (hereinafter, typically referred to as PVDF) has the following advantages. (2) Hydrostatic pressure piezoelectric strain constant d _h is equal to or less than, but has a small dielectric constant ε, a, d _h / hydrostatic voltage output coefficient determined by epsilon (g _h constant) is extremely large, and the thus sensitivity characteristics are excellent; (3) low density, due to low elasticity, acoustic impedance (sound velocity × density), It has characteristics such as being close to the values of water and living organisms, and therefore having little reflection between water and living organisms and the element, enabling efficient energy transmission. Taking advantage of these characteristics, polymer piezoelectric materials are generally used for electro-mechanical (acoustic) conversion elements such as speakers, microphones, ultrasonic probes, hydrophones, vibrometers, strain gauges, blood pressure monitors, bimorph fans, and the like. As a pyroelectric conversion element, application to a wide range of uses has been proposed or put into practical use.

When a film-shaped or sheet-shaped polymer piezoelectric material (hereinafter, collectively referred to as a "polymer piezoelectric film") is made into an element, electrodes are usually provided on both surfaces thereof. At this time, the electrode is generally made of a vapor-deposited electrode of copper, aluminum, or the like in consideration of the fact that the heat resistance of the polymer piezoelectric body, including the piezoelectric properties imparted by orientation and polarization, is around 100 ° C. Alternatively, these metal foil electrodes attached with an adhesive are used. However, these electrodes have respective problems as described below.

[0004]

First, in addition to the thinness of the generally provided electrode of about 0.02 to 0.1 μm, the vapor deposition electrode has insufficient adhesive strength with the polymer piezoelectric material, and the It is very difficult to solder the leads. For this reason, for a polymer piezoelectric element provided with a vapor deposition electrode, it is necessary to adopt a complicated electrode terminal fixing structure for inputting and outputting electric signals.

On the other hand, metal foil electrodes are generally 6 to 1
A metal foil having a thickness of about 00 μm is
5-40μ consisting of urethane resin, epoxy resin, etc.
It is attached to the polymer piezoelectric film via an adhesive layer of about m, and is formed as an electrode which can be connected to a lead wire by soldering. However, when this metal foil electrode is provided on both surfaces of a polymer piezoelectric film, there is a disadvantage that the important advantage of flexibility of the polymer piezoelectric element is impaired. Further, depending on the use state, the adhesive deteriorates particularly on the surface that is exposed to the outside air, and the durability of the piezoelectric element may become a problem.

Based on the above-mentioned circumstances, in the conventional polymer piezoelectric element, when importance is placed on flexibility, a vapor-deposited electrode is adopted even if a complicated terminal structure is adopted, and on the other hand, when a lead wire connection is required, it is acceptable. Metal foil electrodes have been employed at the expense of flexibility.

A main object of the present invention is to provide a polymer-based piezoelectric element having an electrode structure excellent in lead wire connectivity by soldering without substantially impairing flexibility.

[0008]

The present inventors have continued their researches for the above-mentioned purpose, and as a result, surprisingly, the application to a polymer piezoelectric element having poor heat resistance has not been considered so far. However, the present invention has been found not only to be able to be achieved by setting appropriate conditions, but also to enhance the piezoelectric characteristics of the polymer piezoelectric element, that is, to provide a polymer piezoelectric element with improved sensitivity.

That is, the flexible piezoelectric element of the present invention is characterized in that a spray-coated electrode is provided on at least one surface of a polymer piezoelectric film.

The thermal spray coating generally forms a metal coating by spraying a molten metal material toward a mating base material and solidifying the molten metal on the surface. Generally, the thermal spray coating is almost used for surface treatment of polymer films. Absent. Furthermore, in addition to the inherently low heat resistance of the material, it is possible that thermal spray coating with improved piezoelectric properties is possible for polymer piezoelectric films that have been subjected to polarization treatment to exhibit excellent piezoelectric properties. It was very surprising to the inventors.

In the present invention, a vinylidene cyanide-vinyl acetate copolymer having relatively high heat resistance is preferably used as a polymer piezoelectric material, and a vinylidene fluoride resin piezoelectric material having excellent piezoelectric properties is used. Among them, a VD capable of β-type crystallization under ordinary crystallization conditions is more preferable than a vinylidene fluoride (VDF) homopolymer which requires uniaxial stretching for β-type crystallization suitable for piezoelectricity development.
F-based copolymer (for example, a copolymer of a superior amount of VDF and an inferior amount of vinyl fluoride (VF) trifluoroethylene (TrFE) or tetrafluoroethylene (TFE))
The copolymer of VDF having a superior amount (especially 70 to 80 mol%) and TrFE having an inferior amount (especially 30 to 20 mol%) is most preferably used.

These polymer piezoelectric materials are formed into a film by melt extrusion or the like, and then subjected to a uniaxial stretching or a heat treatment at a softening temperature or lower, and a polarization treatment by application of an electric field at a softening temperature or lower, as necessary. Before the thermal spray coating, the polymer piezoelectric film used in the present invention preferably has a thickness of about 200 to 2000 μm, particularly 300 to 1000 μm. If the thickness of the film is less than 200 μm, the thermal resistance at the time of thermal spraying is poor, and problems such as deformation of the film and variation in piezoelectric characteristics occur. On the other hand, if the thickness exceeds 2000 μm, the flexibility of the film is impaired, and a high voltage is required for polarization, so that an edge discharge is generated and the polarization treatment becomes extremely difficult.

In the most basic form, the flexible piezoelectric element of the present invention has a spray-coated electrode 2 on one surface of the above-described polymer piezoelectric film 1 as shown in FIG. It is also preferable that the surface of the polymer piezoelectric film 1 covered with the electrode 2 or both surfaces of the film 1 be roughened in advance as required.

Examples of the material of the sprayed electrode 2 include copper, aluminum, zinc and the like. Among them, zinc or zinc, which can be sprayed at a relatively low temperature and provides an electrode film having excellent solderability, An alloy of zinc and copper is preferably used. The thickness of the spray-coated electrode 2 can be selected in a range that gives good solderability and does not impair the flexibility of the piezoelectric element, but is generally set in the range of 10 to 100 μm, particularly preferably 20 to 50 μm.

The thermal spraying method used to form the electrode 2 must not excessively heat the polymer piezoelectric film 1 so that the piezoelectric characteristics of the polymer piezoelectric film 1 are not substantially impaired. Arc spraying or gas spraying (plasma spraying) can be used. In particular, wire arc spraying is a preferred embodiment in which the thermal load on the polymer piezoelectric film 1 is smaller than that of other spraying methods. However, even in such a case, the spraying process is performed by forcibly cooling the piezoelectric film 1 as necessary. It is preferable that the heat load be controlled while controlling the heat load.

When the flexible piezoelectric element of the present invention is used as a piezoelectric element, as shown in FIG. 1, it faces the surface of the polymer piezoelectric film 1 opposite to the surface on which the spray coating electrode 2 is provided. It has an electrode 3. The counter electrode 3 may be a spray-coated electrode similar to the spray-coated electrode 2, or may be a vapor-deposited electrode of copper, aluminum or the like when lead wire connection by solder is not so required.

As a preferred embodiment of the flexible piezoelectric element of the present invention, as shown in FIG. 2, an adhesive layer 4 is provided on the surface of the polymer piezoelectric film 1 opposite to the surface on which the thermal spray coating electrode 2 is formed. A structure in which the metal foil electrode 5 is attached can also be adopted. The adhesive layer 4 can be formed of a conductive adhesive in which conductive particles are dispersed. However, an epoxy resin, a urethane resin, a polyester resin, a butadiene resin, a propylene resin having higher adhesive strength can be used. It is preferable to form a layer having a thickness of about 5 to 40 μm with an adhesive such as an acrylic resin.

The metal foil electrode 5 is made of a metal having good conductivity such as copper, aluminum, tin, zinc, gold, silver, platinum and the like, having a thickness of 6 to 200 μm, more preferably 20 to 120 μm, especially 20 to 8 μm.
A 0 μm foil is preferably used.

The reason why the flexible piezoelectric element having the structure shown in FIG. 2 is preferably used is that when the above-described metal foil electrode 5 is used, the soldering is preferably performed on both surfaces together with the spray-coated electrode 2 provided on the opposite surface. Lead wire connection is ensured,
This is because when the metal foil electrode 5 is adhered to one surface of the polymer piezoelectric film, the decrease in flexibility is significantly less than when it is adhered to both surfaces.

According to another preferred embodiment of the present invention, a laminated piezoelectric element as shown in FIG. 3 is formed as a bimorph diaphragm. In this laminated piezoelectric element, a pair of polymer piezoelectric films 1 having a spray-coated electrode 2 formed on one surface thereof are provided on both sides of a metal foil electrode 5 forming a central layer with an adhesive layer 4 interposed therebetween. They are attached so as to be outside each other. As described in the example of FIG. 2, when only one metal foil electrode 5 is used,
While a decrease in flexibility due to this can be tolerated, good thermal spray connection electrodes 2 provided on both sides assure good lead wire connectivity by soldering. The structure of FIG. 3 is vertically symmetric with respect to the metal foil electrode 5 whose flexibility is located at the center, and has an advantage that the vibration characteristics are also symmetric. In obtaining the structure shown in FIG. 3, it is of course possible to first adhere the polymer piezoelectric film 1 to both surfaces of the metal foil electrode 5 and then form the spray-coated electrodes 2 on both surfaces by thermal spraying.

In the laminated piezoelectric element shown in FIG. 3, the polarization direction p of the upper and lower polymer piezoelectric films 1 is set to the same direction as shown in FIG.
Repeats the movement similar to the fan movement of the fan. This laminated piezoelectric element is suitably flexible as a bimorph diaphragm because it is moderately flexible and, in addition, the spray-coated electrode 2 is firmly coated on the piezoelectric film 1.

[0022]

EXAMPLES Examples of preparation and evaluation of the piezoelectric element of the present invention will be described below. In the following examples, the following characteristics were evaluated for the created piezoelectric elements.

The piezoelectric properties: was determined by measuring the hydrostatic piezoelectric constant (d _h constant) in the following manner. The sample is immersed in silicon oil placed in a pressure-resistant container, and the charge Q (Coulomb (C)) of the sample is measured while applying a pressure P (Newton (N) / m ² ) from a nitrogen gas source to the container. Then, the charge increase dQ with respect to the pressure rise dP near the gauge pressure of 2 kg / cm ² was obtained, and was calculated by the following equation.

D _h = (dQ / dP) / A The unit is C / N. Here, A is the electrode area (m ² )
It is.

Flexibility: As shown in FIG.
1cm required to apply a load W to the sample at a speed of 5 mm / min at the center of the fulcrum and to bend it by 2 mm.
The load value per width was determined and defined as the deflection load (g / cm).

Electrode peel strength: An epoxy-based adhesive (Araldite AW106 (resin): HV) was applied to a copper foil having a thickness of 35 μm.
953U (hardener) = 1: 1, Ciba-Geigy Japan Co., Ltd.) was applied, bonded to the electrode forming surface of the piezoelectric element, and pressed at 90 ° C. under a pressure of 100 kgf / cm ² for 20 minutes. Subsequently, the JIS C
A 90 degree peel test was performed at a speed of 50 mm / min in accordance with -6481. The electrode peel strength is determined by the degree of inconvenience such as cracking or peeling of the electrode around the solder when a lead wire is connected to the electrode by solder or when the element is subsequently handled, that is, the lead wire connection by solder. It is empirically known to show a good correlation with sex.

In the tests for flexibility and electrode peel strength,
STROGRA manufactured by Toyo Seiki Seisaku-sho as a testing machine
PH-R2 was used.

Example 1 A piezoelectric element having the structure shown in FIG. 2 was formed as follows.

First, a VDF / TrFE (75/25 molar ratio) copolymer (manufactured by Kureha Chemical Industry Co., Ltd.) was heated at a die temperature of 265 ° C.
After extruding the sheet at 125 ° C. for 13 hours,
Under an electric field of 5 MV / m, a polarization treatment was performed for a total of 1 hour including a holding time of 5 minutes at 123 ° C.
A μm polymer piezoelectric film 1 was obtained.

After one surface of the polymer piezoelectric film 1 was roughened, it was adhered to a 35 μm thick copper foil via a 20 μm thick polyester adhesive layer 4.

Next, the other surface of the polymer piezoelectric film 1 stuck in this manner is sandblasted with an alumina abrasive having a grain size of # 60 to roughen the surface, and then an electric wire spraying machine (Kato Metallicon Co., Ltd.) Made, DK type metal spraying machine E
Using a mold, spraying was performed under the conditions of an air pressure of 5 kg / cm ² and a voltage of 15 V to form a zinc sprayed electrode 2 having a thickness of about 40 μm, thereby obtaining a piezoelectric element.

Example 2 A piezoelectric element having the structure shown in FIG. 1 was formed as follows.

A zinc sprayed electrode 2 was formed on both sides of the same polymer piezoelectric film 1 as in Example 1 by the same method as in Example 1 to obtain a piezoelectric element.

When a 90 ° peel test was performed on this piezoelectric element, the peel strength of the electrode was 1.08 kg / c.
m.

COMPARATIVE EXAMPLE 1 A 0.03 μm-thick aluminum vapor-deposited film was formed on both surfaces of the polymer piezoelectric film 1 formed in Example 1 to obtain a piezoelectric element. When a 90 degree peel test was performed on this piezoelectric element, the peel strength of the electrode was 0.0
It was 3 kg / cm and could be easily peeled off.

Comparative Example 2 A 35 μm-thick copper foil was attached to both surfaces of the polymer piezoelectric film 1 formed in Example 1 via a 20 μm-thick polyester-based adhesive layer to obtain a piezoelectric element.

COMPARATIVE EXAMPLE 3 A piezoelectric element was obtained by using copper as a deposition metal instead of aluminum in Comparative Example 1.

Each electrode of the piezoelectric element obtained in each of the above examples was subjected to a soldering test. As a result, the zinc sprayed electrodes and copper foil electrodes of Examples 1 and 2 and Comparative Example 2 exhibited good lead wire connectivity with good solder. However, the copper vapor-deposited electrode of Comparative Example 3 was peeled or cracked from the peripheral portion of the solder when the lead wire was connected by soldering.

[0039] Then, for each piezoelectric element, the piezoelectric properties (d _h constant) and flexible (deflection load) below the results were measured was obtained.

[0040]

[0041]

As described above, according to the present invention, by providing a spray-coated electrode on at least one surface of a polymer piezoelectric film, a piezoelectric property (high sensitivity) is superior to that of a device provided with a conventional vapor deposition electrode. Furthermore, a piezoelectric element having good lead wire connection strength and flexibility by soldering can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of one embodiment of a piezoelectric element of the present invention.

FIG. 2 is a schematic sectional view of another embodiment of the piezoelectric element of the present invention.

FIG. 3 is a schematic cross-sectional view showing another embodiment of the piezoelectric element of the present invention together with its connection state.

FIG. 4 is an explanatory diagram of a method for measuring the flexibility of a piezoelectric element in Examples and Comparative Examples.

[Explanation of symbols]

 1: Polymer piezoelectric film 2: Spray coated electrode 3: Counter electrode 4: Adhesive layer 5: Metal foil electrode

Claims (4)

(57) [Claims] 1. A flexible piezoelectric element comprising a polymer piezoelectric film or sheet provided with a spray-coated electrode on at least one surface. 2. The device according to claim 1, wherein the spray-coated electrode is made of zinc. 3. The element according to claim 1, wherein a metal foil electrode is attached to a surface of the polymer piezoelectric film or sheet opposite to the surface on which the spray-coated electrode is provided. The two sheets of flexible piezoelectric element surface having their spray coated electrode is formed by laminating stuck together so that the outside with a 4. A structure according to any one each of claims 1 to 3 Multilayer piezoelectric element.