Piezoelectric film

Abstract

Provided is a highly durable piezoelectric film capable of suppressing deterioration of acoustic characteristics due to a long period of use. A piezoelectric film comprising: a piezoelectric layer made of a polymer composite piezoelectric containing piezoelectric particles in a matrix including a polymer material; and electrode layers formed on both surfaces of the piezoelectric layer. If the smaller of a domain ratio X of c-domain and a-domain measured by X-ray diffractometry from one major surface side of the piezoelectric layer and a domain ratio Y of c-domain and a-domain measured by X-ray diffractometry from the other major surface side of the piezoelectric layer is 1.00, the other domain ratio is greater than or equal to 1.05.

Description

Technical Field

[0001] The present invention relates to a piezoelectric film.

Background Art

[0002] In response to the thinning and weight reduction of displays such as liquid crystal displays and organic EL (Electro Luminescence) displays, speakers used in these thin displays are also required to be thinned and lightened. Further, in response to the development of flexible displays using flexible substrates such as plastic, flexibility is also required for speakers used in these flexible displays.

[0003] Therefore, as a speaker that is thin and can be integrated into a thin display or a flexible display without sacrificing lightweight and flexibility, it has been proposed to use a piezoelectric film that is sheet-shaped, flexible, and has the property of expanding and contracting in response to an applied voltage.

[0004] For example, the applicant of the present application has proposed a piezoelectric film (electroacoustic conversion film) disclosed in Patent Document 1 as a piezoelectric film that is sheet-shaped, flexible, and can stably reproduce high-quality sound. The piezoelectric film disclosed in Patent Document 1 has a polymer composite piezoelectric body in which piezoelectric particles are dispersed in a viscoelastic matrix made of a polymer material having viscoelasticity at room temperature, and electrode layers provided so as to sandwich the polymer composite piezoelectric body. The piezoelectric film described in Patent Document 1 preferably has a protective layer formed on the surface of the thin film electrode.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Disclosure of the Invention

Problems to be Solved by the Invention

[0006] When a voltage is applied to such a piezoelectric film, the piezoelectric layer of the film expands and contracts significantly in the in-plane direction. When a piezoelectric film is used as a speaker, the ends of the piezoelectric film are fixed to a support member, and the expansion and contraction of the piezoelectric layer in the in-plane direction is converted into vibrations in the thickness direction, generating sound.

[0007] According to the inventor's research, since the edges of the piezoelectric film are fixed to the support member, the piezoelectric layer within the piezoelectric film warps significantly. This warping means that there is a difference in the degree of expansion and contraction in the thickness direction of the piezoelectric layer, which places a great deal of stress on the piezoelectric layer itself, causing defects such as cracks and delamination within the piezoelectric layer. As a result, there was a problem in that the acoustic properties deteriorated with prolonged use.

[0008] The object of the present invention is to solve the problems of the prior art and to provide a highly durable piezoelectric film that can suppress the deterioration of acoustic properties due to prolonged use. [Means for solving the problem]

[0009] To solve these problems, the present invention has the following configuration. [1] A piezoelectric film having a piezoelectric layer made of a polymer composite piezoelectric material containing piezoelectric particles in a matrix containing a polymer material, and electrode layers formed on both sides of the piezoelectric layer, A piezoelectric film in which, when the smaller of the domain ratio X of c domains to a domains measured by X-ray diffraction from one main surface side of the piezoelectric layer and the domain ratio Y of c domains to a domains measured by X-ray diffraction from the other main surface side of the piezoelectric layer is set to 1.00, the domain ratio of the other side is 1.05 or greater. [2] The piezoelectric film according to [1], wherein the average value of domain ratio X and domain ratio Y is 2 or greater. [Effects of the Invention]

[0010] According to such an invention, a highly durable piezoelectric film capable of suppressing a decrease in acoustic characteristics associated with long-term use can be provided.

Brief Description of the Drawings

[0011] [Figure 1] It is a diagram conceptually showing an example of the piezoelectric film of the present invention. [Figure 2] It is a conceptual diagram for explaining a method of measuring the domain ratio of a piezoelectric layer. [Figure 3] It is a conceptual diagram for explaining a method of measuring the domain ratio of a piezoelectric layer. [Figure 4] It is a conceptual diagram for explaining an example of a method of manufacturing a piezoelectric film. [Figure 5] It is a conceptual diagram for explaining an example of a method of manufacturing a piezoelectric film. [Figure 6] It is a conceptual diagram for explaining an example of a method of manufacturing a piezoelectric film. [Figure 7] It is a diagram conceptually showing an example of a piezoelectric speaker using the piezoelectric film shown in FIG. 1. [Figure 8] It is a conceptual diagram for explaining a method of measuring sound pressure in an embodiment. [Figure 9] It is a graph showing the relationship between 2θ and intensity obtained by measuring an XRD pattern.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, the piezoelectric film of the present invention will be described in detail based on the preferred embodiments shown in the accompanying drawings.

[0013] The description of the constituent elements described below may be made based on representative embodiments of the present invention, but the present invention is not limited to such embodiments. In this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0014] [Piezoelectric Film] The piezoelectric film of the present invention is a piezoelectric film having a piezoelectric layer made of a polymer composite piezoelectric containing piezoelectric particles in a matrix containing a polymer material, and electrode layers formed on both surfaces of the piezoelectric layer, wherein, when the smaller of the domain ratio X of the c domain and the a domain measured by X-ray diffraction from one main surface side of the piezoelectric layer and the domain ratio Y of the c domain and the a domain measured by X-ray diffraction from the other main surface side of the piezoelectric layer is taken as 1.00, the other domain ratio is 1.05 or more.

[0015] Fig. 1 conceptually shows an example of the piezoelectric film of the present invention. The piezoelectric film 10 shown in Fig. 1 has a piezoelectric layer 12 which is a sheet-like object having piezoelectricity, a first electrode layer 16 laminated on one surface of the piezoelectric layer 12, a first protective layer 20 laminated on the first electrode layer 16, a second electrode layer 14 laminated on the other surface of the piezoelectric layer 12, and a second protective layer 18 laminated on the second electrode layer 14.

[0016] As shown in Fig. 1, the piezoelectric layer 12 is made of a polymer composite piezoelectric containing piezoelectric particles 26 in a polymer matrix 24 containing a polymer material. Also, the first electrode layer 16 and the second electrode layer 14 are electrode layers in the present invention. As will be described later, the piezoelectric film 10 (piezoelectric layer 12) is preferably polarized in the thickness direction.

[0017] Such a piezoelectric film 10 is, for example, used in various acoustic devices (acoustic equipment) such as speakers, microphones, and pickups for musical instruments such as guitars, for generating (reproducing) sound by vibration according to an electric signal and converting vibration by sound into an electric signal. In addition, the piezoelectric film can also be used for pressure sensors, power generation elements, etc. Alternatively, the piezoelectric film can also be used as an exciter for vibrating an article to produce sound by contacting and attaching to various articles.

[0018] In the piezoelectric film 10, the second electrode layer 14 and the first electrode layer 16 form an electrode pair. That is, the piezoelectric film 10 has a structure in which both sides of the piezoelectric body layer 12 are sandwiched between the electrode pair, i.e., the first electrode layer 16 and the second electrode layer 14, and this laminate is sandwiched between the first protective layer 20 and the second protective layer 18.

[0019] Thus, in the piezoelectric film 10, the region sandwiched between the first electrode layer 16 and the second electrode layer 14 expands and contracts in response to the applied voltage.

[0020] The first electrode layer 16 and the first protective layer 20, as well as the second electrode layer 14 and the second protective layer 18, are named according to the polarization direction of the piezoelectric layer 12. Therefore, the first electrode layer 16 and the second electrode layer 14, as well as the first protective layer 20 and the second protective layer 18, have basically the same configuration.

[0021] Furthermore, in addition to these layers, the piezoelectric film 10 may also have an insulating layer or the like that covers areas where the piezoelectric layer 12 is exposed, such as the sides, to prevent short circuits, etc.

[0022] When a voltage is applied to the first electrode layer 16 and the second electrode layer 14 of such a piezoelectric film 10, the piezoelectric particles 26 expand and contract in the polarization direction according to the applied voltage. As a result, the piezoelectric film 10 (piezoelectric layer 12) shrinks in the thickness direction. At the same time, due to the poison ratio, the piezoelectric film 10 also expands and contracts in the in-plane direction. This expansion and contraction is approximately 0.01 to 0.1%. In the in-plane direction, the expansion and contraction is isotropic in all directions. The thickness of the piezoelectric layer 12 is preferably about 10 to 300 μm. Therefore, the expansion and contraction in the thickness direction is very small, at most about 0.3 μm. In contrast, the piezoelectric film 10, or piezoelectric layer 12, has a size in the planar direction that is much larger than its thickness. Therefore, for example, if the length of the piezoelectric film 10 is 20 cm, the piezoelectric film 10 will expand or contract by up to approximately 0.2 mm when a voltage is applied. Furthermore, when pressure is applied to the piezoelectric film 10, electricity is generated by the action of the piezoelectric particles 26. By utilizing this, the piezoelectric film 10 can be used for various applications such as speakers, microphones, and pressure sensors, as described above.

[0023] In this invention, the piezoelectric film 10 has a configuration in which, when the smaller of the domain ratio X of c domains to a domains measured by X-ray diffraction from one main surface side of the piezoelectric layer 12 and the domain ratio Y of c domains to a domains measured by X-ray diffraction from the other main surface side of the piezoelectric layer 12 is set to 1.00, the other domain ratio is 1.05 or higher. This point will be described in detail later.

[0024] <Piezoelectric layer> The piezoelectric layer is a layer made of a polymer composite piezoelectric material containing piezoelectric particles in a matrix containing a polymer material, and is a layer that exhibits a piezoelectric effect, expanding and contracting when a voltage is applied.

[0025] In the piezoelectric film 10, the piezoelectric layer 12 is preferably made of a polymer composite piezoelectric material in which piezoelectric particles 26 are dispersed in a polymer matrix 24 made of a polymer material that has viscoelasticity at room temperature. In this specification, "room temperature" refers to a temperature range of approximately 0 to 50°C.

[0026] Here, it is preferable that the polymer composite piezoelectric material (piezoelectric layer 12) has the following requirements.

[0027] (i) Flexibility For example, when a device is held loosely like a newspaper or magazine for portability, it is constantly subjected to large, relatively slow bending deformations from the outside at a frequency of a few Hz or less. In this case, if the polymer composite piezoelectric material is rigid, a large bending stress will be generated, causing cracks to form at the interface between the polymer matrix and the piezoelectric particles, which may eventually lead to fracture. Therefore, a moderate degree of flexibility is required for the polymer composite piezoelectric material. Furthermore, if the strain energy can be diffused to the outside as heat, the stress can be relieved. Therefore, a moderately large loss tangent is required for the polymer composite piezoelectric material.

[0028] (ii) Sound quality Speakers reproduce sound by vibrating piezoelectric particles at frequencies in the audio band of 20Hz to 20kHz. This vibration energy causes the entire polymer composite piezoelectric material (piezoelectric element) to vibrate as a whole. Therefore, the polymer composite piezoelectric material needs to have a suitable hardness to improve the efficiency of vibration energy transmission. Furthermore, if the frequency response of the speaker is smooth, the change in sound quality when the lowest resonant frequency changes due to a change in curvature will also be small. Therefore, the loss tangent of the polymer composite piezoelectric material needs to be reasonably large.

[0029] In summary, polymer composite piezoelectric materials are required to be rigid for vibrations between 20 Hz and 20 kHz, and flexible for vibrations below a few Hz. Furthermore, the loss tangent of the polymer composite piezoelectric material is required to be moderately large for vibrations at all frequencies below 20 kHz.

[0030] Generally, polymer solids possess a viscoelastic relaxation mechanism, where large-scale molecular motion is observed as a decrease in the storage modulus (Young's modulus) (relaxation) or a maximum in the loss modulus (absorption) with increasing temperature or decreasing frequency. Among these, the relaxation caused by micro-Brownian motion of molecular chains in the amorphous region is called the principal dispersion, and a very large relaxation phenomenon is observed. The temperature at which this principal dispersion occurs is the glass transition temperature (Tg), where the viscoelastic relaxation mechanism is most prominently displayed.

[0031] In a polymer composite piezoelectric material (piezoelectric layer 12), using a polymer material with a glass transition temperature at room temperature, in other words, a polymer material that is viscoelastic at room temperature, as the matrix makes it possible to realize a polymer composite piezoelectric material that is rigid for vibrations of 20 Hz to 20 kHz and flexible for slow vibrations of a few Hz or less. In particular, it is preferable to use a polymer material with a glass transition temperature at 1 Hz at room temperature, i.e., 0 to 50°C, as the matrix of the polymer composite piezoelectric material, as this behavior is preferably expressed.

[0032] Various known polymer materials that exhibit viscoelasticity at room temperature can be used. Preferably, a polymer material is used in which the maximum value of the loss tangent Tanδ at a frequency of 1 Hz, as determined by a dynamic viscoelasticity test at room temperature, i.e., 0 to 50°C, is 0.5 or more. As a result, when the polymer composite piezoelectric material is slowly bent by an external force, stress concentration at the interface between the polymer matrix and piezoelectric particles at the point of maximum bending moment is relieved, and high flexibility can be expected.

[0033] Furthermore, for polymer materials that are viscoelastic at room temperature, it is preferable that the storage modulus (E') at a frequency of 1 Hz, as measured by dynamic viscoelasticity measurement, is 100 MPa or more at 0°C and 10 MPa or less at 50°C. This reduces the bending moment generated when the polymer composite piezoelectric material is slowly bent by an external force, while simultaneously allowing it to behave rigidly against acoustic vibrations in the 20Hz to 20kHz range.

[0034] Furthermore, polymer materials that are viscoelastic at room temperature are more preferable if their dielectric constant is 10 or higher at 25°C. This allows for a larger deformation to be expected when a voltage is applied to the polymer composite piezoelectric material, as a higher electric field is applied to the piezoelectric particles in the polymer matrix. However, on the other hand, considering the need to ensure good moisture resistance, it is also preferable for the polymer material to have a dielectric constant of 10 or less at 25°C.

[0035] Examples of polymer materials that have viscoelasticity at room temperature and satisfy these conditions include cyanoethylated polyvinyl alcohol (cyanoethylated PVA), polyvinyl acetate, polyvinylidene chloride coacrylonitrile, polystyrene-vinyl polyisoprene block copolymer, polyvinyl methyl ketone, and polybutyl methacrylate. Commercially available polymer materials such as Hybrar 5127 (manufactured by Kuraray Co., Ltd.) can also be suitably used. Among these, it is preferable to use a polymer material having a cyanoethyl group, and cyanoethylated PVA is particularly preferred. These polymer materials may be used individually or in combination (mixed).

[0036] The polymer matrix 24 using such a polymer material that exhibits viscoelasticity at room temperature may, if necessary, use multiple polymer materials in combination. In other words, the polymer matrix 24 may contain, if necessary, other dielectric polymer materials in addition to viscoelastic materials such as cyanoethylated PVA, for the purpose of adjusting dielectric properties and mechanical properties.

[0037] Examples of addable dielectric polymer materials include fluorine-based polymers such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene copolymer, and polyvinylidene fluoride-tetrafluoroethylene copolymer, vinylidene cyanide-vinyl acetate copolymer, cyanoethylcellulose, cyanoethyl hydroxysaccharose, cyanoethyl hydroxycellulose, cyanoethyl hydroxypullulan, cyanoethyl methacrylate, and cyanoethyl acrylate. Examples include polymers having cyano or cyanoethyl groups, such as cyanoethyl hydroxyethyl cellulose, cyanoethyl amylose, cyanoethyl hydroxypropyl cellulose, cyanoethyl dihydroxypropyl cellulose, cyanoethyl hydroxypropyl amylose, cyanoethyl polyacrylamide, cyanoethyl polyacrylate, cyanoethyl pullulan, cyanoethyl polyhydroxymethylene, cyanoethyl glycidol pullulan, cyanoethyl saccharose, and cyanoethyl sorbitol, as well as synthetic rubbers such as nitrile rubber and chloroprene rubber. Among these, polymer materials having cyanoethyl groups are particularly suitable for use. Furthermore, the dielectric polymer added to the polymer matrix 24 of the piezoelectric layer 12, in addition to a material that has viscoelasticity at room temperature such as cyanoethylated PVA, is not limited to one type, but may be added in multiple types.

[0038] In addition to dielectric polymer materials, thermoplastic resins such as vinyl chloride resin, polyethylene, polystyrene, methacrylic resin, polybutene, and isobutylene, as well as thermosetting resins such as phenolic resin, urea resin, melamine resin, alkyd resin, and mica, may also be added to the polymer matrix 24 for the purpose of adjusting the glass transition temperature Tg. Furthermore, tackifiers such as rosin esters, rosin, terpenes, terpene phenols, and petroleum resins may be added to improve tackiness.

[0039] In the polymer matrix 24 of the piezoelectric layer 12, there are no particular limitations on the amount of material other than viscoelastic polymer materials such as cyanoethylated PVA added, but it is preferable that the amount is 30% by mass or less in proportion to the polymer matrix 24. This allows the properties of the added polymer material to be expressed without impairing the viscoelastic relaxation mechanism in the polymer matrix 24, resulting in favorable outcomes such as increased dielectric constant, improved heat resistance, and improved adhesion with the piezoelectric particles 26 and electrode layer.

[0040] The piezoelectric layer 12 contains piezoelectric particles 26 in such a polymer matrix 24. The piezoelectric particles 26 consist of ceramic particles having a perovskite or wurtzite crystal structure. Examples of ceramic particles constituting the piezoelectric particles 26 include lead zirconate titanate (PZT), lead zirconate lanthanate titanate (PLZT), barium titanate (BaTiO3), zinc oxide (ZnO), and a solid solution of barium titanate and bismuth ferrite (BiFe3) (BFBT). These piezoelectric particles 26 may be used individually or in combination (mixed).

[0041] There are no restrictions on the particle size of such piezoelectric particles 26; they can be appropriately selected according to the size of the piezoelectric film 10 and the intended use of the piezoelectric film 10. The particle size of the piezoelectric particles 26 is preferably 1 to 10 μm. By setting the particle size of the piezoelectric particles 26 within this range, favorable results can be obtained in that the piezoelectric film 10 can achieve both high piezoelectric properties and flexibility.

[0042] In Figure 1, the piezoelectric particles 26 in the piezoelectric layer 12 are irregularly dispersed in the polymer matrix 24, but the present invention is not limited thereto. That is, the piezoelectric particles 26 in the piezoelectric layer 12 may be regularly dispersed in the polymer matrix 24, as long as they are preferably uniformly dispersed.

[0043] In the piezoelectric film 10, there are no restrictions on the ratio of polymer matrix 24 to piezoelectric particles 26 in the piezoelectric layer 12. It can be set appropriately according to the size and thickness of the piezoelectric film 10 in the planar direction, the application of the piezoelectric film 10, and the properties required of the piezoelectric film 10. The volume fraction of piezoelectric particles 26 in the piezoelectric layer 12 is preferably 30 to 80%, more preferably 50% or more, and therefore even more preferably 50 to 80%. By setting the ratio of polymer matrix 24 to piezoelectric particles 26 within the above range, favorable results can be obtained, such as achieving both high piezoelectric properties and flexibility.

[0044] In a preferred embodiment, the piezoelectric film 10 described above has a piezoelectric layer 12 in which piezoelectric particles are dispersed in a viscoelastic matrix containing a polymer material that is viscoelastic at room temperature. However, the present invention is not limited thereto, and a polymer composite piezoelectric material, which is used in known piezoelectric elements and consists of piezoelectric particles dispersed in a matrix containing a polymer material, can be used as the piezoelectric layer.

[0045] In the piezoelectric film 10, there are no particular limitations on the thickness of the piezoelectric layer 12, and it can be set appropriately according to the application of the piezoelectric film 10, the required characteristics of the piezoelectric film 10, etc. While a thicker piezoelectric layer 12 is advantageous in terms of rigidity, such as the stiffness of the sheet-like material, the voltage (potential difference) required to stretch or contract the piezoelectric film 10 by the same amount becomes larger. The thickness of the piezoelectric layer 12 is preferably 10 to 300 μm, more preferably 20 to 200 μm, and even more preferably 30 to 150 μm. By setting the thickness of the piezoelectric layer 12 within the above range, favorable results can be obtained in terms of achieving both rigidity and appropriate flexibility.

[0046] <Protective layer> In the piezoelectric film 10, the first protective layer 20 and the second protective layer 18 cover the second electrode layer 14 and the first electrode layer 16, and also serve to impart appropriate rigidity and mechanical strength to the piezoelectric layer 12. That is, in the piezoelectric film 10, the piezoelectric layer 12, which consists of a polymer matrix 24 and piezoelectric particles 26, exhibits excellent flexibility against slow bending deformation, but depending on the application, it may lack sufficient rigidity and mechanical strength. The piezoelectric film 10 is provided with the first protective layer 20 and the second protective layer 18 to compensate for this.

[0047] There are no restrictions on the first protective layer 20 and the second protective layer 18, and various sheet-like materials can be used. As an example, various resin films are preferably exemplified. In particular, resin films made of polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polyphenylene sulfite (PPS), polymethyl methacrylate (PMMA), polyetherimide (PEI), polyimide (PI), polyethylene naphthalate (PEN), triacetylcellulose (TAC), and cyclic olefin resins are preferably used due to their excellent mechanical properties and heat resistance.

[0048] There are no restrictions on the thickness of the first protective layer 20 and the second protective layer 18. Furthermore, while the thicknesses of the first protective layer 20 and the second protective layer 18 are basically the same, they may be different. Here, if the rigidity of the first protective layer 20 and the second protective layer 18 is too high, it will not only restrict the expansion and contraction of the piezoelectric layer 12, but also impair its flexibility. Therefore, unless mechanical strength or good handling properties as a sheet material are required, it is advantageous for the first protective layer 20 and the second protective layer 18 to be as thin as possible.

[0049] The thickness of the first protective layer 20 and the second protective layer 18 is preferably 3 μm to 100 μm, more preferably 3 μm to 50 μm, even more preferably 3 μm to 30 μm, and particularly preferably 4 μm to 10 μm. In the piezoelectric film 10, if the thickness of the first protective layer 20 and the second protective layer 18 is twice or less the thickness of the piezoelectric body layer 12, favorable results can be obtained in terms of achieving both rigidity and appropriate flexibility. For example, if the piezoelectric layer 12 has a thickness of 50 μm and the first protective layer 20 and the second protective layer 18 are made of PET, the thickness of the first protective layer 20 and the second protective layer 18 is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 25 μm or less.

[0050] <Electrode layer> In the piezoelectric film 10, a first electrode layer 16 is formed between the piezoelectric layer 12 and the first protective layer 20, and a second electrode layer 14 is formed between the piezoelectric layer 12 and the second protective layer 18. The first electrode layer 16 and the second electrode layer 14 are provided to apply a voltage to the piezoelectric layer 12 (piezoelectric film 10).

[0051] In the present invention, there are no limitations on the materials used to form the first electrode layer 16 and the second electrode layer 14, and various conductors can be used. Specifically, examples include metals such as carbon, palladium, iron, tin, aluminum, nickel, platinum, gold, silver, copper, titanium, chromium, and molybdenum, alloys thereof, laminates and composites of these metals and alloys, and indium tin oxide. Among these, copper, aluminum, gold, silver, platinum, and indium tin oxide are preferably exemplified as materials for the first electrode layer 16 and the second electrode layer 14.

[0052] Furthermore, there are no restrictions on the method for forming the first electrode layer 16 and the second electrode layer 14. Various known methods can be used, such as vacuum deposition, ion-assisted deposition, vapor deposition methods (vacuum film formation methods) such as sputtering, film formation by plating, or methods of attaching foils formed from the above materials.

[0053] There are no restrictions on the thickness of the first electrode layer 16 and the second electrode layer 14. Furthermore, while the thicknesses of the first electrode layer 16 and the second electrode layer 14 are basically the same, they may be different.

[0054] Here, similar to the first protective layer 20 and the second protective layer 18 mentioned above, if the rigidity of the first electrode layer 16 and the second electrode layer 14 is too high, it will not only restrict the expansion and contraction of the piezoelectric layer 12 but also impair its flexibility. Therefore, the thinner the first electrode layer 16 and the second electrode layer 14 are, the better, as long as the electrical resistance does not become too high. In other words, it is preferable that the first electrode layer 16 and the second electrode layer 14 are thin-film electrodes.

[0055] The thickness of the first electrode layer 16 and the second electrode layer 14 is thinner than the protective layer, preferably 0.05 μm to 10 μm, more preferably 0.05 μm to 5 μm, even more preferably 0.08 μm to 3 μm, and particularly preferably 0.1 μm to 2 μm.

[0056] In this case, for the piezoelectric film 10, it is preferable that the product of the thickness of the first electrode layer 16 and the second electrode layer 14 and the Young's modulus is less than the product of the thickness of the first protective layer 20 and the second protective layer 18 and the Young's modulus, as this does not significantly impair the flexibility. For example, in the case of a combination where the first protective layer 20 and the second protective layer 18 are made of PET (Young's modulus: approximately 6.2 GPa) and the first electrode layer 16 and the second electrode layer 14 are made of copper (Young's modulus: approximately 130 GPa), if the thickness of the first protective layer 20 and the second protective layer 18 is 25 μm, then the thickness of the first electrode layer 16 and the second electrode layer 14 is preferably 1.2 μm or less, more preferably 0.3 μm or less, and most preferably 0.1 μm or less.

[0057] As described above, the piezoelectric film 10 preferably has a structure in which a piezoelectric layer 12, which is made by dispersing piezoelectric particles 26 in a polymer matrix 24 containing a polymer material that is viscoelastic at room temperature, is sandwiched between a first electrode layer 16 and a second electrode layer 14, and this laminate is further sandwiched between a first protective layer 20 and a second protective layer 18. In such a piezoelectric film 10, it is preferable that the maximum value of the loss tangent (Tanδ) at a frequency of 1 Hz, as measured by dynamic viscoelasticity, exists at room temperature, and it is more preferable that the maximum value is 0.1 or greater at room temperature. As a result, even if the piezoelectric film 10 is subjected to relatively slow, large bending deformation of a few Hz or less from the outside, the strain energy can be effectively diffused to the outside as heat, thus preventing cracks from forming at the interface between the polymer matrix and the piezoelectric particles.

[0058] The piezoelectric film 10 preferably has a storage modulus (E') at a frequency of 1 Hz, as measured by dynamic viscoelasticity, of 10 to 30 GPa at 0°C and 1 to 10 GPa at 50°C. The same conditions apply to the piezoelectric layer 12. As a result, the piezoelectric film 10 can have a large frequency dispersion in its storage modulus (E') at room temperature. That is, it can behave rigidly for vibrations between 20 Hz and 20 kHz, and flexiblely for vibrations below a few Hz.

[0059] Furthermore, the piezoelectric film 10 has a product of its thickness and its storage modulus (E') at a frequency of 1 Hz, measured by dynamic viscoelasticity measurement, which is 1.0 × 10⁻⁶ at 0°C. 6 ~2.0×10 6 N / m, 1.0 × 10 at 5°C 5 ~1.0×10 6 It is preferable that the density is N / m. The same condition applies to the piezoelectric layer 12. This allows the piezoelectric film 10 to possess appropriate rigidity and mechanical strength without impairing its flexibility and acoustic properties.

[0060] Furthermore, it is preferable that the piezoelectric film 10 has a loss tangent (Tanδ) of 0.05 or higher at 25°C and a frequency of 1 kHz in the master curve obtained from dynamic viscoelasticity measurement. The same conditions apply to the piezoelectric layer 12. This results in a smoother frequency response for the speaker using the piezoelectric film 10, and also reduces the amount of change in sound quality when the lowest resonant frequency f0 changes due to a change in the curvature of the speaker.

[0061] In this invention, the storage modulus (Young's modulus) and loss tangent of the piezoelectric film 10 and piezoelectric layer 12 can be measured by known methods. For example, they can be measured using the DMS6100 dynamic viscoelasticity measuring instrument manufactured by SII Nanotechnology Co., Ltd. As an example of measurement conditions, the measurement frequency is 0.1Hz to 20Hz (0.1Hz, 0.2Hz, 0.5Hz, 1Hz, 2Hz, 5Hz, 10Hz, and 20Hz), the measurement temperature is -50 to 150℃, the heating rate is 2℃ / min (in a nitrogen atmosphere), the sample size is 40mm x 10mm (including the clamp area), and the distance between chucks is 20mm.

[0062] In addition to the piezoelectric layer, electrode layer, and protective layer, the piezoelectric film 10 may also have, for example, electrode lead-out sections for leading electrodes from the first electrode layer 16 and the second electrode layer 14, and an insulating layer that covers the area where the piezoelectric layer 12 is exposed to prevent short circuits, etc.

[0063] As for the electrode lead-out portion, the electrode layer and protective layer may have a portion that protrudes convexly outward in the planar direction of the piezoelectric layer, or a portion of the protective layer may be removed to form a hole, and a conductive material such as silver paste may be inserted into this hole to electrically connect the conductive material and the electrode layer, thereby forming the electrode lead-out portion. Furthermore, each electrode layer is not limited to having only one electrode lead-out section, and may have two or more electrode lead-out sections. In particular, in the case where a portion of the protective layer is removed and a conductive material is inserted into the hole to form an electrode lead-out section, it is preferable to have three or more electrode lead-out sections in order to more reliably ensure current flow.

[0064] Here, the piezoelectric film 10 of the present invention has a domain ratio of 1.05 or greater when the smaller of the domain ratio X of c domains and a domains measured by X-ray diffraction from one main surface side of the piezoelectric layer 12 and the domain ratio Y of c domains and a domains measured by X-ray diffraction from the other main surface side of the piezoelectric layer 12 is set to 1.00.

[0065] As mentioned above, when a voltage is applied to a piezoelectric film, the piezoelectric layer of the piezoelectric film expands and contracts significantly in the in-plane direction. However, since the edges of the piezoelectric film are fixed to the support members, the piezoelectric layer inside the piezoelectric film warps significantly. When warping occurs, differences in the degree of expansion and contraction occur in the thickness direction of the piezoelectric layer. These differences in the degree of expansion and contraction within the piezoelectric layer place a great deal of stress on the piezoelectric layer itself, causing defects such as cracks and delamination inside the piezoelectric layer. As a result, there was a problem in that the acoustic characteristics, such as the sound pressure when the same electrical signal is applied, i.e., the conversion efficiency between electrical signals and vibrations (sound), decreased with prolonged use.

[0066] In contrast, the inventors have found that in a piezoelectric layer made of a polymer composite piezoelectric material, if there is a polarization in the thickness direction, i.e., a bias in the ratio of c-domains to a-domains, it can mitigate the difference in the degree of expansion and contraction caused by the warping of the piezoelectric film, thereby reducing stress on the piezoelectric layer itself. Therefore, the piezoelectric film of the present invention provides a polarization in the thickness direction of the piezoelectric layer by setting the ratio of the c-domain to a-domain ratio X=c-domain / a-domain on one side of the piezoelectric layer to 1.05 or more, thereby mitigating the difference in the degree of expansion and contraction caused by the warping of the piezoelectric film and reducing stress on the piezoelectric layer itself. As a result, the piezoelectric film of the present invention can suppress the occurrence of defects such as cracks and delamination inside the piezoelectric layer even after prolonged use, suppress the deterioration of acoustic properties such as sound pressure (conversion efficiency between electrical vibration and vibration (sound)) caused by defects, and increase durability.

[0067] The following describes the c-domain and a-domain of the piezoelectric layer. As described above, in piezoelectric films that use a polymer composite piezoelectric material, which is formed by dispersing piezoelectric particles in a polymer matrix, as the piezoelectric layer, ferroelectric materials such as PZT are used as the piezoelectric particles. The crystal structure of this ferroelectric material is divided into many domains with different directions of spontaneous polarization. In this state, the spontaneous polarization of each domain and the piezoelectric effect that arises from it cancel each other out, so no piezoelectric properties are observed overall. Therefore, in conventional piezoelectric films, the piezoelectric layer is subjected to electrical polarization treatment such as poling, and an electric field above a certain value is applied from the outside to align the direction of spontaneous polarization in each region. The electrically polarized piezoelectric particles then exhibit a piezoelectric effect in response to the external electric field. As a result, the piezoelectric film expands and contracts in the planar direction and vibrates in a direction perpendicular to the surface in response to the applied voltage, thereby converting vibration (sound) into an electrical signal.

[0068] Incidentally, the direction of spontaneous polarization of each domain in the crystal structure of ferroelectric materials (hereinafter simply referred to as the domain direction) is oriented not only in the thickness direction of the piezoelectric film, but also in various other directions such as the plane direction. Therefore, even if an electrical polarization treatment is performed by applying a higher voltage, for example, it is not possible to make all domains oriented in the plane direction oriented in the thickness direction to which the electric field is applied. In other words, it is not possible to completely eliminate 90° domains.

[0069] Generally, X-ray diffraction (XRD) is used to analyze the crystal structure of such piezoelectric layers (piezoelectric particles), and XRD is used to investigate how atoms are arranged inside the crystal.

[0070] Here, the c domain is the domain in the thickness direction of the piezoelectric film, corresponding to the peak intensity of the (002) plane. The c domain is the tetragonal peak around 43.5° in the XRD pattern obtained by XRD analysis. The a domain is the domain in the in-plane direction of the piezoelectric film, corresponding to the peak intensity of the (200) plane. The a domain is the tetragonal peak around 45° in the XRD pattern obtained by XRD analysis. XRD analysis can be performed using an X-ray diffractometer (such as the PANalytical X'Pert PRO).

[0071] The following describes the method for measuring domain ratios. First, as shown in Figure 2, XRD analysis is performed by irradiating one surface 12a of the piezoelectric layer 12 with X-rays (indicated by the arrow in Figure 2) to measure the c-domain and a-domain, and the domain ratio X (= c-domain / a-domain) is calculated. Next, as shown in Figure 3, XRD analysis is performed by irradiating the other surface 12b of the piezoelectric layer 12 with X-rays (indicated by the arrow in Figure 3) to measure the c-domain and a-domain, and the domain ratio Y (= c-domain / a-domain) is calculated.

[0072] Of the measured domain ratios X and Y, the smaller value is set to 1.00, and the ratio of the domain ratio with the larger value is calculated. That is, the value obtained by dividing the domain ratio with the larger value by the domain ratio with the smaller value is calculated. Hereafter, the value obtained by dividing the domain ratio with the larger value by the domain ratio with the smaller value will be called the ratio Z. Such measurements can be performed at five arbitrary points in the piezoelectric layer, spaced at least 10 mm apart, in the planar direction (perpendicular to the thickness direction), and the average value of ratio Z can be calculated.

[0073] If the piezoelectric film is folded and laminated, the layers will be separated to form a sheet for XRD analysis.

[0074] Here, from the viewpoint of durability, the ratio Z is preferably 1.05 to 1.86, and more preferably 1.09 to 1.48. If the ratio Z is too high, the side with the smaller domain ratio will hardly expand or contract, which may restrict the expansion or contraction of the opposite side as well, potentially reducing the initial sound pressure.

[0075] Furthermore, a higher proportion of domains in the thickness direction of the piezoelectric film (c-domains) results in higher piezoelectric properties, and from the viewpoint of being able to improve the conversion efficiency between electrical signals and vibrations (sound), a high ratio of c-domains to a-domains (domain ratio X and domain ratio Y) is preferable. Accordingly, the average value of domain ratio X and domain ratio Y is preferably 2 or more, more preferably 3 to 4.1, and even more preferably 3.4 to 4.0.

[0076] Furthermore, if the proportion of planar domains (a-domains) is high, applying a drive voltage can cause movement of the 90° domain walls, leading to distortion hysteresis and potentially resulting in distortion in the reproduced sound. From this perspective, setting the average value of domain ratio X and domain ratio Y within the above range is preferable because it reduces the 90° domain motion when a drive voltage is applied, thereby reducing distortion in the reproduced sound.

[0077] An example of a method for manufacturing the piezoelectric film 10 will be described below with reference to Figures 4 to 6.

[0078] First, as shown in Figure 4, a sheet-like material 34 is prepared in which a first electrode layer 16 is formed on a first protective layer 20. This sheet-like material 34 can be manufactured by forming a copper thin film or the like as the first electrode layer 16 on the surface of the first protective layer 20 by vacuum deposition, sputtering, plating, or the like. If the first protective layer 20 is very thin and difficult to handle, a first protective layer 20 with a separator (temporary support) may be used as needed. A separator such as PET with a thickness of 25 μm to 100 μm can be used. The separator can be removed after the second electrode layer 14 and the second protective layer 18 are heat-pressed together, but before any material is laminated onto the first protective layer 20.

[0079] On the other hand, a polymer material that will serve as the matrix is ​​dissolved in an organic solvent, and piezoelectric particles 26 such as PZT particles are added and stirred to prepare a paint by dispersion. There are no restrictions on organic solvents other than those mentioned above; various organic solvents can be used.

[0080] After preparing the sheet-like material 34 and the paint, the paint is cast (applied) onto the sheet-like material 34, and the organic solvent is evaporated to dry it. This creates a laminate 36, as shown in Figure 5, which has a first electrode layer 16 on top of the first protective layer 20, and a piezoelectric layer 12 formed on top of the first electrode layer 16. Note that the first electrode layer 16 refers to the electrode on the substrate side when applying the piezoelectric layer 12, and does not indicate the vertical positional relationship in the laminate.

[0081] There are no restrictions on the casting method of this paint; all known methods (coating devices), such as slide coaters and doctor knives, can be used.

[0082] As described above, in the piezoelectric film 10, dielectric polymer materials may be added to the polymer matrix 24 in addition to viscoelastic materials such as cyanoethylated PVA. When adding these polymer materials to the polymer matrix 24, it is sufficient to dissolve the polymer materials that are added to the paint as described above.

[0083] After fabricating a laminate 36 having a first electrode layer 16 on a first protective layer 20 and a piezoelectric layer 12 formed on the first electrode layer 16, preferably the piezoelectric layer 12 is subjected to electrical polarization treatment (poling).

[0084] Electrical polarization treatment can be used to switch the domains in the thickness direction that are facing the opposite direction to the applied electric field (180° domains), thereby causing 180° domain motion and aligning the direction of the domains in the thickness direction.

[0085] There are no restrictions on the method for polarization treatment of the piezoelectric layer 12, and known methods can be used. By adjusting the electric field strength, temperature, etc. during polarization treatment, the domain ratio (= c domains / a domains) in the piezoelectric layer can be adjusted. Before this polarization treatment, the surface of the piezoelectric layer 12 may be smoothed using a heating roller or the like through a calendering process. This calendering process allows the thermocompression bonding process described later to proceed smoothly.

[0086] While the piezoelectric layer 12 of the laminate 36 is subjected to polarization treatment in this manner, a sheet-like material 38 is prepared, in which a second electrode layer 14 is formed on the second protective layer 18. This sheet-like material 38 can be manufactured by forming a copper thin film or the like as the second electrode layer 14 on the surface of the second protective layer 18 by vacuum deposition, sputtering, plating, or the like. Next, as shown in Figure 6, the sheet-like material 38 is laminated onto the laminate 36, which has undergone polarization treatment of the piezoelectric layer 12, with the second electrode layer 14 facing the piezoelectric layer 12. Furthermore, the laminate of the laminated body 36 and the sheet-like material 38 is heat-pressed together using a heating press device, a pair of heating rollers, etc., so as to sandwich the second protective layer 18 and the first protective layer 20.

[0087] The heating temperature during heat sealing is preferably 50°C to 80°C, and more preferably 60°C to 70°C. The heating time is preferably 10 seconds to 60 seconds, and more preferably 20 seconds to 40 seconds.

[0088] Furthermore, in the present invention, mechanical polarization treatment may be performed in addition to, or instead of, electrical polarization treatment. Mechanical polarization treatment is a process in which shear stress is applied to the piezoelectric layer 12 of the laminate of the laminate 36 and the sheet-like material 38, thereby reducing the proportion of a-domains oriented in the plane direction and increasing the proportion of c-domains oriented in the thickness direction.

[0089] The reason why applying shear stress to the piezoelectric layer 12 increases the proportion of c-domains is presumed to be as follows. When shear stress is applied to the piezoelectric layer 12 (piezoelectric particles 26), the piezoelectric particles 26 are forced to stretch in the vertical direction (thickness direction). In this process, 90° domain motion occurs, and the a-domains, which are oriented in the surface direction, become c-domains, oriented in the thickness direction. Furthermore, the orientation of the c-domains, which are oriented in the thickness direction, does not change. As a result, it is estimated that the proportion of a-domains decreases and the proportion of c-domains increases.

[0090] In this way, by performing mechanical polarization treatment, the proportion of a-domains is reduced and the proportion of c-domains is increased, thereby increasing the domain ratio.

[0091] In this invention, it is preferable to perform a mechanical polarization treatment after the electrical polarization treatment. The 90° domain motion caused by mechanical polarization treatment becomes more likely when the 180° domain wall is eliminated. Therefore, by inducing 180° domain motion through electrical polarization treatment, eliminating the 180° domain wall and creating a state where 90° domain motion is more likely to occur, and then performing mechanical polarization treatment, 90° domain motion can be induced, causing the a-domains oriented in the surface direction to be oriented in the thickness direction and become c-domains, thereby increasing the proportion of c-domains.

[0092] As a method for applying shear stress to the piezoelectric layer 12 as a mechanical polarization treatment, one example is to press a roller against one surface side of the laminate of the laminate 36 and the sheet-like material 38. When applying shear stress to the piezoelectric layer 12 using a roller, there are no particular limitations on the type of roller; rubber rollers, metal rollers, etc., can be used as appropriate.

[0093] Furthermore, there are no particular limitations on the value of the shear stress applied to the piezoelectric layer 12; it can be set appropriately according to the performance required for the piezoelectric film, the material and thickness of each layer of the piezoelectric film, etc. As an example, it is preferable to set the shear stress applied to the piezoelectric layer 12 to 0.3 MPa to 0.5 MPa.

[0094] The shear stress on the piezoelectric layer 12 may be determined by dividing the applied shear load by the cross-sectional area parallel to the shear load, or by detecting the tensile strain or compressive strain caused by tensile or compressive stress and calculating the shear stress from the detection result.

[0095] Furthermore, when applying shear stress to the piezoelectric layer 12 using a roller, the temperature of the laminate and the roller is preferably 20°C to 130°C, and more preferably 50°C to 100°C. If the temperature is too high, the polymer material becomes too soft, making it difficult to transmit the shear force, and if the temperature is too low, the polymer material becomes too hard, making it difficult to change the domain ratio. It is thought that by maintaining the temperature at which the polymer material is moderately soft, it becomes easier to change the domain ratio.

[0096] In this invention, in order to create a bias in the domain ratio (=c domain / a domain) between one main surface side and the other main surface side, that is, to make the ratio Z 1.05 or more, the process includes a step of heating only one main surface side of the piezoelectric film after the thermal compression bonding of the laminate 36 and the sheet-like material 38 and the polarization treatment. At this time, it is preferable that the other main surface side is not heated. By heating only one main surface side of the piezoelectric film, the proportion of c domains of the piezoelectric particles 26 in the piezoelectric layer 12 on the heated side decreases, and the domain ratio (=c domain / a domain) on one main surface side becomes smaller. This makes it possible to create a bias in the domain ratio (=c domain / a domain) between one main surface side and the other main surface side.

[0097] The heating method for heating one main surface is not particularly limited and can be carried out using a heating press device, a pair of heating rollers, etc. Furthermore, it is preferable to cool the other main surface in order to prevent it from being heated.

[0098] From the viewpoint of creating a bias in the domain ratio (= c domains / a domains) between one main surface and the other, it is necessary to raise the heating temperature and lengthen the heating time to some extent. On the other hand, if the heating temperature is too high and / or the heating time is too long, the proportion of c domains may become too low, or the temperature of the other main surface may rise, potentially reducing the domain ratio of the other main surface as well. From the above viewpoint, the heating temperature in the step of heating one main surface is preferably 90°C to 150°C, and more preferably 100°C to 120°C. The heating time is preferably 100 seconds to 600 seconds, and more preferably 120 seconds to 300 seconds.

[0099] The piezoelectric film of the present invention can be manufactured by the above steps. The manufactured piezoelectric film may also be cut into a desired shape after the above steps.

[0100] Furthermore, the above process can also be carried out using a web-like material, that is, a long, continuous sheet wound up, while being transported, rather than a sheet-like material. It is also possible for both the laminate 36 and the sheet-like material 38 to be in a web-like form and heat-pressed together as described above. In that case, the piezoelectric film 10 is manufactured in a web-like form at this point.

[0101] Furthermore, a special adhesive layer may be provided when bonding the laminate 36 and the sheet-like material 38. For example, an adhesive layer may be provided on the surface of the second electrode layer 14 of the sheet-like material 38. The most suitable adhesive layer is made of the same material as the polymer matrix 24. It is also possible to apply the same material to the surface of the second electrode layer 14 and bond them together.

[0102] Figure 7 shows a conceptual diagram of an example of a flat-plate type piezoelectric speaker utilizing the piezoelectric film 10 of the present invention. This piezoelectric speaker 40 is a flat-plate type piezoelectric speaker that uses the piezoelectric film 10 of the present invention as a diaphragm that converts electrical signals into vibrational energy. The piezoelectric speaker 40 can also be used as a microphone, sensor, etc.

[0103] The piezoelectric speaker 40 is composed of a piezoelectric film 10, a case 42, a viscoelastic support 46, and a frame 48. Case 42 is a thin enclosure made of plastic or the like, with one side open. Examples of enclosure shapes include rectangular parallelepiped, cubic, and cylindrical. Furthermore, the frame 48 is a frame material that engages with the open surface side of the case 42, and has a through hole in the center that is the same shape as the open surface of the case 42. The viscoelastic support 46 has appropriate viscosity and elasticity, and supports the piezoelectric film 10. By applying a constant mechanical bias to any point on the piezoelectric film, it efficiently converts the expansion and contraction motion of the piezoelectric film 10 into forward and backward motion (motion perpendicular to the film surface). Examples include nonwoven fabrics such as wool felt and wool felt containing PET, as well as glass wool.

[0104] The piezoelectric speaker 40 is constructed by housing a viscoelastic support 46 inside a case 42, covering the case 42 and the viscoelastic support 46 with a piezoelectric film 10, and fixing the frame 48 to the case 42 while pressing the periphery of the piezoelectric film 10 against the upper end surface of the case 42 with a frame 48.

[0105] In this piezoelectric speaker 40, the viscoelastic support 46 has a height (thickness) greater than the height of the inner surface of the case 42. Therefore, in the piezoelectric speaker 40, the peripheral portion of the viscoelastic support 46 is held in a state where it is pressed downward by the piezoelectric film 10, resulting in a thinner thickness. Also, in the peripheral portion of the viscoelastic support 46, the curvature of the piezoelectric film 10 changes rapidly, and a rising portion is formed on the piezoelectric film 10 that slopes downward toward the periphery of the viscoelastic support 46. Furthermore, the central region of the piezoelectric film 10 is pressed by the rectangular prism-shaped viscoelastic support 46, and (omitted) becomes planar.

[0106] When the piezoelectric speaker 40 applies a driving voltage to the first electrode layer 16 and the second electrode layer 14, the piezoelectric film 10 stretches in the in-plane direction. To absorb this stretching, the viscoelastic support 46 causes the rising portion of the piezoelectric film 10 to change angle in the upward direction. As a result, the piezoelectric film 10, which has a planar portion, moves upward. Conversely, when the piezoelectric film 10 contracts in the in-plane direction due to the application of a driving voltage to the first electrode layer 16 and the second electrode layer 14, the rising portion of the piezoelectric film 10 changes its angle in the direction of tilting (towards becoming closer to a plane) in order to absorb this contraction. As a result, the piezoelectric film 10 having a planar portion moves downward. The piezoelectric speaker 40 generates sound through the vibration of the piezoelectric film 10.

[0107] Furthermore, in the piezoelectric film 10 of the present invention, the conversion from expansion and contraction motion to vibration can also be achieved by holding the piezoelectric film 10 in a curved state. Therefore, the piezoelectric film 10 of the present invention can function as a flexible piezoelectric speaker even if it is simply held in a curved state, rather than being a rigid, flat piezoelectric speaker 40 as shown in Figure 7.

[0108] A piezoelectric speaker utilizing the piezoelectric film 10 of the present invention can be rolled up or folded and stored in a bag or the like, taking advantage of its excellent flexibility. Therefore, the piezoelectric film 10 of the present invention makes it possible to realize a piezoelectric speaker that is easily portable, even if it is of a certain size. Furthermore, the piezoelectric film 10 of the present invention is excellent in flexibility and pliability, and there is no anisotropy in the piezoelectric properties within the plane. Therefore, the piezoelectric film 10 of the present invention exhibits little change in sound quality regardless of the direction in which it is bent, and also exhibits little change in sound quality with respect to changes in curvature. Accordingly, piezoelectric speakers utilizing the piezoelectric film 10 of the present invention offer a high degree of freedom in installation location and, as described above, can be attached to various items. For example, by attaching the piezoelectric film 10 of the present invention to clothing or other garments, or to portable items such as bags, in a curved state, a so-called wearable speaker can be realized.

[0109] Furthermore, the piezoelectric film of the present invention can also be used as a speaker for a display device by attaching it to a flexible display device such as a flexible organic EL display device or a flexible liquid crystal display device.

[0110] As described above, the piezoelectric film 10 of the present invention expands and contracts in the planar direction when a voltage is applied, and vibrates suitably in the thickness direction due to this expansion and contraction in the planar direction. Therefore, when used in a piezoelectric speaker or the like, it exhibits good acoustic characteristics that enable the output of high-sound-pressure sound. The piezoelectric film 10 of the present invention, which exhibits such excellent acoustic properties, i.e., high expandability due to piezoelectricity, also functions well as a piezoelectric vibrating element (exciter) that vibrates a vibrating object such as a diaphragm when multiple films are laminated together. Because the piezoelectric film 10 of the present invention has high durability, it exhibits high durability even when laminated to form a piezoelectric vibrator. Furthermore, when laminating the piezoelectric film 10, if there is no possibility of a short circuit, the piezoelectric film does not need to have a second protective layer 18 and / or a first protective layer 20. Alternatively, a piezoelectric film without a second protective layer 18 and / or a first protective layer 20 may be laminated via an insulating layer.

[0111] As an example, a speaker may be constructed by attaching a laminate of piezoelectric film 10 to a diaphragm, causing the diaphragm to vibrate with the laminate of piezoelectric film 10 to produce sound. In other words, in this case, the laminate of piezoelectric film 10 acts as a so-called exciter, producing sound by vibrating the diaphragm. By applying a driving voltage to the laminated piezoelectric film 10, each piezoelectric film 10 expands and contracts in the planar direction, and the expansion and contraction of each piezoelectric film 10 causes the entire laminate of piezoelectric films 10 to expand and contract in the planar direction. The expansion and contraction of the laminate of piezoelectric films 10 in the planar direction causes the diaphragm to which the laminate is attached to bend, and as a result the diaphragm vibrates in the thickness direction. This vibration in the thickness direction causes the diaphragm to generate sound. The diaphragm vibrates in accordance with the magnitude of the driving voltage applied to the piezoelectric film 10, and generates sound corresponding to the driving voltage applied to the piezoelectric film 10. Therefore, in this case, the piezoelectric film 10 itself does not emit sound.

[0112] Even if each individual piezoelectric film 10 has low rigidity and small elasticity, stacking the piezoelectric films 10 increases the rigidity, resulting in a larger overall elasticity for the laminate. As a result, the laminate of piezoelectric films 10 can sufficiently flex the diaphragm with a large force, even if the diaphragm has a certain degree of rigidity, causing the diaphragm to vibrate sufficiently in the thickness direction and generate sound from the diaphragm.

[0113] In a laminate of piezoelectric films 10, there is no limit to the number of layers of piezoelectric films 10. For example, the number of layers should be set appropriately to obtain a sufficient amount of vibration, depending on the rigidity of the vibrating diaphragm. Furthermore, if it has sufficient elasticity, a single piezoelectric film 10 of the present invention can be used as a similar exciter (piezoelectric vibration element).

[0114] There are no limitations on the diaphragm that vibrates with the laminate of piezoelectric film 10 of the present invention, and various sheet-like materials (plate-like materials, films) can be used. Examples include resin films made of polyethylene terephthalate (PET), foamed plastics made of expanded polystyrene, paper materials such as corrugated cardboard, glass plates, and wood. Furthermore, devices such as display devices may be used as diaphragms, as long as they can be sufficiently flexible.

[0115] In the laminate of piezoelectric films 10, it is preferable to bond adjacent piezoelectric films together with an adhesive layer (adhesive). Furthermore, it is preferable to bond the laminate of piezoelectric films 10 and the diaphragm with an adhesive layer. There are no restrictions on the adhesive layer; various types that can bond objects together are available. Therefore, the adhesive layer may consist of either an adhesive or a bonding agent. Preferably, an adhesive layer consisting of a bonding agent is used, which provides a solid and hard adhesive layer after bonding. The same applies to the laminate formed by folding a long piezoelectric film 10, which will be described later.

[0116] In a laminate of piezoelectric films 10, there are no restrictions on the polarization direction of each piezoelectric film 10 in the laminate. As mentioned above, the polarization direction of the piezoelectric film 10 in the present invention is the polarization direction in the thickness direction. Therefore, in a laminate of piezoelectric films 10, the polarization direction may be the same for all piezoelectric films 10, or there may be piezoelectric films with different polarization directions.

[0117] In this case, in the laminate of piezoelectric films 10, it is preferable to laminate the piezoelectric films 10 such that the polarization directions of adjacent piezoelectric films 10 are opposite to each other. In the piezoelectric film 10, the polarity of the voltage applied to the piezoelectric layer 12 corresponds to the polarization direction. Therefore, whether the polarization direction is from the second electrode layer 14 to the first electrode layer 16 or from the first electrode layer 16 to the second electrode layer 14, the polarity of the second electrode layer 14 and the polarity of the first electrode layer 16 are made the same in all laminated piezoelectric films 10. Therefore, by aligning the polarization directions of adjacent piezoelectric films 10, even if the thin film electrodes of adjacent piezoelectric films 10 come into contact with each other, the contacting thin film electrodes have the same polarity, thus eliminating the risk of a short circuit.

[0118] The laminate of piezoelectric films 10 may be configured by folding a long piezoelectric film 10 once or more, preferably multiple times, to laminate multiple piezoelectric films 10. The configuration in which a long piezoelectric film 10 is folded and laminated has the following advantages. In other words, in a laminate formed by stacking multiple cut-sheet-shaped piezoelectric films 10, the second electrode layer 14 and the first electrode layer 16 of each piezoelectric film need to be connected to a power supply. In contrast, in a configuration in which a long piezoelectric film 10 is folded and stacked, the laminate can be formed with only one long piezoelectric film 10. Furthermore, in a configuration in which a long piezoelectric film 10 is folded and stacked, only one power supply is needed to apply the driving voltage, and the electrodes from the piezoelectric film 10 only need to be connected at one point. Furthermore, in a configuration where long piezoelectric films 10 are folded and laminated, the polarization directions of adjacent piezoelectric films 10 inevitably end up being opposite to each other.

[0119] Although the piezoelectric film of the present invention has been described in detail above, the present invention is not limited to the examples described above, and various improvements and modifications may be made without departing from the spirit of the present invention. [Examples]

[0120] The present invention will be described in more detail below with reference to specific embodiments. However, the present invention is not limited to these embodiments, and the materials, amounts used, proportions, processing content, and processing procedures shown in the following embodiments can be modified as appropriate without departing from the spirit of the present invention.

[0121] [Example 1] Sheet-like materials 34 and 38 were prepared by sputtering a 100 nm thick copper thin film onto a 4 μm thick PET film. In other words, in this example, the first electrode layer 16 and the second electrode layer 14 are 100 nm thick copper thin films, and the first protective layer 20 and the second protective layer 18 are 4 μm thick PET films. Furthermore, in order to obtain good handling during the process, PET films with a thickness of 50 μm (temporary support PET) were used, and after the sheet-like material 38 was heat-pressed, the separators of each protective layer were removed.

[0122] Meanwhile, cyanoethylated PVA (CR-V, manufactured by Shin-Etsu Chemical Co., Ltd.) was dissolved in methyl ethyl ketone (MEK) in the following composition ratio. Then, PZT particles were added to this solution in the following composition ratio and dispersed using a propeller mixer (rotation speed 2000 rpm) to prepare a coating for forming the piezoelectric layer 12. ·PZT particles 300 parts by mass • Cyanoethylated PVA ·········15 parts by mass ·MEK·················85 parts by mass The PZT particles used were obtained by sintering commercially available PZT raw material powder at 1000-1200°C, and then crushing and classifying it to an average particle size of 5 μm.

[0123] On the first electrode layer 16 (thin copper film) of the previously prepared sheet-like material 34, a coating for forming the previously prepared piezoelectric layer 12 was applied using a slide coater. The coating was applied so that the film thickness after drying would be 100 μm. Next, the MEK was evaporated by heating and drying the sheet-like material 34 coated with paint on a hot plate at 120°C, thereby forming a laminate 36.

[0124] The fabricated piezoelectric layer was subjected to calendering using a heated roller.

[0125] Next, the laminate 36 was inserted between conductive plates that were placed parallel to each other at a distance of 1 mm, and an electric field was generated between the conductive plates by connecting one of the conductive plates to earth and applying a DC voltage of 6 kV to the other, thereby performing an electrical polarization treatment.

[0126] After electrical polarization treatment, a sheet-like material 38 was laminated onto the laminate 36 with the second electrode layer 14 (copper thin film side) facing the piezoelectric layer 12, and then thermocompressed at 70°C.

[0127] Next, the main surface of the laminate of the laminate 36 and the sheet-like material 38 on the second electrode layer 14 (sheet-like material 38) side was subjected to heat treatment. The heat treatment was performed using a hot plate. The heating temperature was 100°C and the heating time was 120 seconds.

[0128] Based on the above, the piezoelectric film 10 was fabricated.

[0129] <Measuring Domain Ratios> The crystal structure of piezoelectric particles 26 in the piezoelectric layer 12 of the fabricated piezoelectric film was measured by X-ray diffraction (XRD) using an X-ray diffractometer (PANalytical X'Pert PRO Cu source, 45kV, 40mA). The sample was fixed on an adsorption sample stage, and measurements were performed with an incident angle of 0.5° to the sample surface.

[0130] In the obtained XRD pattern, first, the intensity between 45.5° and 46.0° was averaged to determine the baseline intensity B (see Figure 9). Next, the value obtained by subtracting B from the maximum intensity at the peak of the (002) plane around 43.5° was defined as the c domain. Next, the value obtained by subtracting B from the maximum intensity at the peak of the (200) plane around 45° was defined as the a domain, and the domain ratio = c domain / a domain was calculated. Through these measurements, the domain ratio was measured on both sides of the piezoelectric layer, and the ratio Z between the domain ratio X on one main surface and the domain ratio Y on the other main surface was calculated. The ratio Z was calculated at five arbitrary points, and the average value was calculated.

[0131] The domain ratio X on the main surface of the first electrode layer 16 was 4.34. The domain ratio Y on the main surface of the second electrode layer 14 was 4.00. The ratio Z was 1.085. The average of the domain ratios X and Y was 4.17.

[0132] [Example 2] A piezoelectric film was prepared in the same manner as in Example 1, except that the heating temperature for the heat treatment after heat sealing was changed to 110°C and the heating time to 200 seconds.

[0133] [Example 3] A piezoelectric film was prepared in the same manner as in Example 1, except that the heating temperature for the heat treatment after heat sealing was changed to 120°C and the heating time to 360 seconds.

[0134] [Examples 4-6] Piezoelectric films were prepared in the same manner as in Examples 1 to 3, except that the thickness of the piezoelectric layer was set to 50 μm.

[0135] [Examples 7-9] Piezoelectric films were prepared in the same manner as in Examples 1 to 3, except that the thickness of the piezoelectric layer was set to 10 μm.

[0136] [Example 10] A piezoelectric film was fabricated in the same manner as in Example 5, except that the heat treatment after thermocompression bonding was performed on the main surface on the first electrode layer side.

[0137] [Example 11] A piezoelectric film was prepared in the same manner as in Example 4, except that the heating temperature for the heat treatment after heat compression bonding was changed to 150°C and the heating time to 600 seconds.

[0138] [Comparative Examples 1-3] Piezoelectric films were prepared in the same manner as in Examples 1, 4, and 7, except that no heat treatment was performed after thermocompression bonding.

[0139] [evaluation] Using the fabricated piezoelectric film, a piezoelectric speaker, as shown in Figure 7, was constructed. First, a rectangular test piece measuring 210 x 300 mm (A4 size) was cut from the fabricated piezoelectric film. As shown in Figure 7, the cut piezoelectric film was placed on a 210 x 300 mm case containing glass wool as a viscoelastic support, and then the edges were held down with a frame to apply appropriate tension and curvature to the piezoelectric film, thereby fabricating a piezoelectric speaker as shown in Figure 7. The case depth is 9 mm, and the density of the glass wool is 32 kg / m³. 3 The thickness before assembly was set to 25 mm. In addition, all piezoelectric speakers were fabricated with the lower electrode side of the piezoelectric film facing the viscoelastic support side.

[0140] A 1kHz sine wave was input to the fabricated piezoelectric speaker via a power amplifier, and the sound pressure was measured using a microphone 50 placed 50cm away from the center of the speaker, as shown in Figure 8. The input voltage was set to 20Vrms when the piezoelectric layer thickness was 50μm, and for other thicknesses, the input voltage was increased or decreased in proportion to the thickness during measurement. Sound pressure was measured twice: 30 seconds after output was started from the piezoelectric speaker (initial), and 36 hours after output was started from the piezoelectric speaker (after the endurance test). Table 1 shows the initial sound pressure, the sound pressure after the endurance test, and the difference between the initial and endurance test sound pressures (degradation). The results are shown in Table 1.

[0141] [Table 1]

[0142] Table 1 shows that the piezoelectric film of the present invention exhibits superior durability, with less decrease in sound pressure after the durability test against the initial sound pressure compared to the comparative example. Furthermore, a comparison between Examples 1 and 2, Examples 4 and 5, and Examples 7 and 8 shows that a ratio Z of 1.09 or higher is preferable. Furthermore, a comparison between Examples 2 and 3, Examples 5 and 6, and Examples 8 and 9 reveals that a ratio Z of 1.86 or less is preferable. Furthermore, a comparison between Example 5 and Example 10 shows that similar effects can be obtained regardless of which side of the piezoelectric layer is subjected to heat treatment. Furthermore, a comparison between Examples 4-6 and Example 11 reveals that setting the average domain ratio to 2 or higher is preferable because it increases the initial sound pressure. The effects of the present invention are clear from the results above. [Industrial applicability]

[0143] The piezoelectric film of the present invention can be suitably used as various sensors such as sound wave sensors, ultrasonic sensors, pressure sensors, tactile sensors, strain sensors, and vibration sensors (particularly useful for infrastructure inspection such as crack detection and manufacturing site inspection such as foreign object contamination detection), acoustic devices such as microphones, pickups, speakers, and exciters (specific applications include noise cancellers (used in cars, trains, airplanes, robots, etc.), artificial vocal cords, buzzers for preventing insect and animal intrusion, furniture, wallpaper, photographs, helmets, goggles, headrests, signage, robots, etc.), haptics used in automobiles, smartphones, smartwatches, games, etc., ultrasonic transducers such as ultrasonic probes and hydrophones, actuators used for preventing water droplet adhesion, transportation, stirring, dispersion, polishing, etc., vibration damping materials (dampers) used in containers, vehicles, buildings, sports equipment such as skis and rackets, and vibration power generation devices used in roads, floors, mattresses, chairs, shoes, tires, wheels, and computer keyboards, etc. [Explanation of Symbols]

[0144] 10 Piezoelectric film 12 Piezoelectric layer 14 Upper electrode layer 16 Lower electrode layer 18 Upper protective layer 20 Lower protective layer 24 Polymer Matrix 26 Piezoelectric particles 34, 38 Sheet-like material 36 Laminate 40 Piezoelectric speaker 42 cases 46 Viscoelastic support 48 Frame 50 Microphones

Claims

1. A piezoelectric film having a piezoelectric layer made of a polymer composite piezoelectric material containing piezoelectric particles in a matrix containing a polymer material, and electrode layers formed on both sides of the piezoelectric layer, A piezoelectric film in which the ratio of the domain ratios obtained by dividing the larger of the two domain ratios, X (the ratio of c-domains to a-domains measured by X-ray diffraction from one main surface side of the piezoelectric layer) and Y (the ratio of c-domains to a-domains measured by X-ray diffraction from the other main surface side of the piezoelectric layer) by the smaller of the two domain ratios, is 1.05 or greater.

2. The piezoelectric film according to claim 1, wherein the average value of domain ratio X and domain ratio Y is 2 or more.

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