Ultrasonic element and array structured ultrasonic element

Abstract

To provide an ultrasonic element that exhibits excellent piezoelectric performance in the ultrasonic band and can be easily and inexpensively manufactured.SOLUTION: An ultrasonic element according to the present invention includes a piezoelectric body having a dome-shaped portion and electrode layers provided on both sides of the dome-shaped portion, the piezoelectric body is made of a piezoelectric sheet including a piezoelectric composite material containing piezoelectric particles and a resin and having a Young's modulus of 10 GPa or more, and the ultrasonic element has a resonant frequency of more than 20 kHz. It is preferable that the Q value observed from the vibration displacement be 5 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an ultrasonic element including a piezoelectric body and an electrode layer, and an ultrasonic element having an array structure. [Background technology]

[0002] Ultrasonic sensors are used in a variety of fields, including in-vehicle ultrasonic sonar, liquid level gauges, powder level meters, medical ultrasonic probes, and industrial ultrasonic probes. For example, in-vehicle airborne ultrasonic sensors transmit ultrasonic waves from the vehicle and measure the time it takes for the wave to be reflected by an obstacle and then received back, thereby detecting the distance to the obstacle and enabling collision prevention. In recent years, tactile devices using ultrasonic waves, known as airborne haptics, have been gaining attention.

[0003] These ultrasonic sensors use piezoelectric elements to transmit and receive ultrasonic waves using the piezoelectric effect. To transmit and receive ultrasonic waves with high accuracy, it is thought that arranging dome-shaped piezoelectric elements in an array structure is the most effective method.

[0004] Traditionally, ceramic types using lead zirconate titanate (PZT) have often been used as piezoelectric elements in ultrasonic sensors. For example, Patent Document 1 discloses a method for producing a dome-shaped piezoelectric element using PZT-based piezoelectric ceramics. However, this manufacturing method involves clamping and pressing powdered piezoelectric ceramic material between dies of the desired shape and firing at 1200°C, which has the drawback of being a complicated and expensive process.

[0005] On the other hand, Patent Document 2 discloses an electroacoustic converter film in which a polymer composite piezoelectric body, which is made by dispersing piezoelectric particles in a viscoelastic matrix made of a polymer material, is molded into a convex shape so that it protrudes from one main surface. However, the electroacoustic converter film exhibits excellent performance in the audio band of 20 Hz to 20 kHz, and has not been sufficiently studied in the ultrasonic band.

[0006] Furthermore, none of the documents describe an array-structured piezoelectric element, and it is likely that manufacturing an array-structured piezoelectric element requires a complicated process of manufacturing individual piezoelectric elements and then arranging the desired number of piezoelectric elements. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-008290 [Patent Document 2] International Publication No. 17 / 030045 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, an ultrasonic element that has excellent performance in the ultrasonic band and can be easily and inexpensively manufactured has not yet been known.

[0009] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an ultrasonic element that exhibits excellent piezoelectric performance in the ultrasonic band and can be manufactured easily and inexpensively. [Means for solving the problem]

[0010] As a result of intensive research into solving the above problems, the present inventors discovered that the above problems can be solved by producing a dome-shaped ultrasonic element having a main layer made of a piezoelectric composite material that exhibits a specific Young's modulus, and thus completed the present invention. That is, the present invention has the following aspects.

[0011] (1) An ultrasonic element comprising a piezoelectric body having a dome-shaped portion and electrode layers provided on both sides of the dome-shaped portion, wherein the piezoelectric body comprises a piezoelectric composite material containing piezoelectric particles and a resin, and is made of a piezoelectric sheet having a Young's modulus of 10 GPa or more, and the resonant frequency of the ultrasonic element is greater than 20 kHz. (2) The ultrasonic element according to (1) above, wherein the Q value observed from vibration displacement is 5 or more. (3) The ultrasonic element according to (1) or (2) above, wherein the resin is polyvinylidene fluoride. (4) An ultrasonic element having an array structure including a plurality of ultrasonic elements according to (1) or (2) above, wherein the piezoelectric bodies of the plurality of ultrasonic elements are formed from a single piezoelectric sheet. (5) The array structure ultrasonic element according to (4) above, wherein the variation in height H from the bottom to the zenith of the dome-shaped portion of each of the ultrasonic elements is within ±5%. (6) The plurality of ultrasonic elements are array-structured ultrasonic elements as described in (4) above, in which the electrode layers provided on at least one surface of the electrode layers of the dome-shaped portion of each of the piezoelectric bodies are provided independently of each other and can be controlled independently of each other. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide an ultrasonic element that has excellent performance in the ultrasonic band, and the ultrasonic element can be manufactured easily and inexpensively. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram of an ultrasonic element according to an embodiment of the present invention. [Figure 2] 1 is a cross-sectional view of an ultrasonic element according to an embodiment of the present invention. [Figure 3] FIG. 1 is a schematic diagram of an ultrasonic element having an array structure according to an embodiment of the present invention. [Figure 4] FIG. 1 is a schematic diagram of an ultrasonic element having an array structure according to an embodiment of the present invention. [Figure 5] FIG. 1 is a schematic diagram of an ultrasonic element having an array structure according to an embodiment of the present invention. [Figure 6] FIG. 1 is a conceptual diagram showing an example of a method for forming a piezoelectric body of an ultrasonic element having an array structure according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail.

[0015] (ultrasonic element) Fig. 1 is a schematic diagram of an ultrasonic element according to one embodiment of the present invention. As shown in Fig. 1, ultrasonic element 1 is an ultrasonic element including a piezoelectric body 2 having a dome-shaped portion 2a and electrode layers 3 provided on both sides of dome-shaped portion 2a, wherein piezoelectric body 2 includes a piezoelectric composite material containing piezoelectric particles and resin, and is made of a piezoelectric sheet 4 having a Young's modulus of 10 GPa or more, and ultrasonic element 1 has a resonant frequency greater than 20 kHz.

[0016] The ultrasonic element of this embodiment has excellent energy efficiency because the piezoelectric body 2 has a dome-shaped portion, making it possible to convert in-plane vibration into the thickness direction.

[0017] The ultrasonic element of this embodiment preferably has a Q value of 5 or more, more preferably 8 or more, as observed from vibration displacement. If the Q value is equal to or greater than the lower limit, sufficient sound pressure is obtained, which is preferable. Furthermore, the Q value is preferably 30 or less. If the Q value is equal to or less than the upper limit, a decrease in sound pressure can be suppressed when the resonant frequency fluctuates due to dimensional tolerances during fabrication or temperature changes during operation. This is particularly effective in the case of ultrasonic elements with an array structure, which will be described later.

[0018] The Q value can be calculated using the following formula, where fr is the resonance frequency with the largest vibration displacement, f1 is the frequency lower than fr and where the vibration displacement is 1 / √2, and f2 is the frequency higher than fr and where the vibration displacement is 1 / √2, and Δf is defined as f2 - f1. Q=Δf / fr

[0019] FIG. 2 is a cross-sectional view of the ultrasonic element 1 of FIG. 1 cut in the thickness direction. In FIG. 2, the electrode layers 3 provided on both sides of the dome-shaped portion 2a and the hatching of the cross section are omitted. The size of the dome-shaped portion 2a is not particularly limited, but the diameter D of the bottom surface of the dome-shaped portion 2a is preferably 1 mm to 10 mm, and more preferably 4 mm to 8 mm. When the diameter D is equal to or greater than the lower limit, dimensional accuracy, i.e., errors in the resonant frequency, can be suppressed, which is preferable. When the diameter D is equal to or less than the upper limit, sound pressure increases when designing the ultrasonic element, which is preferable. Furthermore, the height H from the bottom surface to the zenith surface of the dome-shaped portion 2a is preferably 0.1 mm to (the diameter D / 2) mm. When the height H is within the above range, vibrations in the in-plane direction are suppressed and vibrations in the ultrasonic transmission direction are improved, which is preferable, thereby obtaining sufficient sound pressure. The size of the dome shape is merely an example, and may be appropriately set depending on the size and performance of the final product. For example, when the diameter D is 2 mm or more, the height H is preferably 0.1 mm to 1 mm.

[0020] The piezoelectric body 2 in the ultrasonic element 1 of this embodiment includes a piezoelectric composite material containing piezoelectric particles and a resin, and is formed from a piezoelectric sheet 4 having a Young's modulus of 10 GPa or more, so that the ultrasonic element 1 has a resonance frequency greater than 20 kHz. Therefore, the ultrasonic element 1 of this embodiment has excellent performance in the ultrasonic band.

[0021] The piezoelectric sheet must have a Young's modulus of 10 GPa or more. If the Young's modulus is less than 10 GPa, the piezoelectric sheet will be too hard and vibration fluctuation will be insufficient, resulting in a decrease in sound pressure.

[0022] The Young's modulus can be calculated, for example, by measuring the piezoelectric resonance with an impedance analyzer to determine the resonance frequency in the longitudinal direction, and by measuring the length and density of the sample.

[0023] The piezoelectric particles constituting the piezoelectric composite material are not particularly limited as long as they have piezoelectric properties, and may be lead zirconate titanate (PZT:Pb(Zr,Ti)O3), lead titanate (PbTiO3), or lead lanthanum titanate ((Pb,La)TiO3), or may be substances to which impurity atoms such as Nb or Mn are added. In addition, PNN:Pb(Ni 1 / 3 Nb 2 / 3 )O3·PMN:Pb(Mg 1 / 3 Nb 2 / 3 )O3·PZN:Pb(Zn 1 / 3 Nb 2 / 3 Materials in which PbTiO3 is solid-solved with PZT or PbTiO3 (e.g., PNN-PZT, PMN-PT, PZN-PT) are also acceptable. From the viewpoints of piezoelectric performance, particle properties, and environmental toxicity, lead-free materials are also acceptable.

[0024] Examples of the non-lead material include bismuth ferrite particles (BF: BiFeO3), barium titanate (BT: BaTiO3) particles, NBT:(Na 1 / 2 Bi 1 / 2 The preferred materials are NbO3, niobate particles, or tantalate particles. Materials containing a combination of these, such as BF-BT and NBT-BT, may also be used. In addition to these main components, other elements (such as Ni, Mn, and Co) may be incorporated into the solid solution to the extent that the crystal structure is not altered, optimizing piezoelectric performance, durability, and heat resistance. Niobates and tantalates have the general formula ANbO3·A(Nb,Ta)O3·ATaO3 (A = Li·Na·K), with the Li·Na·K ratio and Nb:Ta ratio being selectable depending on the desired performance. The crystal structure may be a perovskite structure, such as KNN:(K,Na)NbO3, or a pseudo-ilmenite structure, such as LiNbO3. When the crystal structure is a perovskite structure, excellent piezoelectric performance can be imparted by controlling the composition at what is called the morphotropic phase boundary (MPB), which is the boundary between different crystal systems (monoclinic and orthorhombic, or orthorhombic and tetragonal, etc.). Furthermore, these particles may be particles of a single composition, or particles of a plurality of compositions may be mixed and used.

[0025] The particle size of the piezoelectric particles is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1 μm or more. The upper limit of the particle size of the piezoelectric particles is not particularly limited, but may be, for example, 100 μm or less, 80 μm or less, or 50 μm or less. The numerical range of the particle size of the piezoelectric particles may be, for example, 0.1 to 100 μm, 0.5 to 80 μm, or 1 to 50 μm. It is also possible to prepare and use a mixture of particles having different particle size distributions, rather than particles having a single particle size.

[0026] In this specification, the "particle size" of a piezoelectric particle refers to the length determined from the particle image of the primary particle of the piezoelectric particle in a two-dimensional image taken with a scanning electron microscope (SEM). The value of the piezoelectric particle size is the average value obtained from 50 or more piezoelectric particles randomly selected from the above-mentioned idiomorphic particles to be measured.

[0027] The resin constituting the piezoelectric composite material is not particularly limited, and may be a polymer, an oligomer, or a monomer, a thermosetting resin, a thermoplastic resin, or an active energy ray-curable resin.

[0028] [Thermosetting resin] The thermosetting resin is a resin that has the property of becoming substantially insoluble and infusible when cured by means of heat, radiation, a catalyst, etc. For example, it may be a known and commonly used resin used in molding materials, etc. Specific examples include novolac-type phenolic resins such as phenol novolac resin and cresol novolac resin; phenolic resins such as unmodified resol phenolic resin and resol-type phenolic resin such as oil-modified resol phenolic resin modified with tung oil, linseed oil, walnut oil, etc.; bisphenol-type epoxy resins such as bisphenol A epoxy resin and bisphenol F epoxy resin; novolac-type epoxy resins such as aliphatic chain-modified bisphenol-type epoxy resin, novolac epoxy resin, cresol novolac epoxy resin; epoxy resins such as biphenyl-type epoxy resin and polyalkylene glycol-type epoxy resin; resins having a triazine ring such as urea resin and melamine resin; vinyl resins such as (meth)acrylic resin and vinyl ester resin; unsaturated polyester resin, bismaleimide resin, polyurethane resin, diallyl phthalate resin, silicone resin, resin having a benzoxazine ring, cyanate ester resin, etc., and they may be polymers, oligomers, or monomers.

[0029] The thermosetting resin may be used with a curing agent. The curing agent used in this case can be used in a known and commonly used combination with the thermosetting resin. For example, when the thermosetting resin is an epoxy resin, any compound commonly used as a curing agent can be used, such as an amine compound, an amide compound, an acid anhydride compound, or a phenolic compound. Specific examples of amine compounds include diaminodiphenylmethane, diethylenetriamine, triethylenetetramine, diaminodiphenylsulfone, isophoronediamine, imidazole, BF3-amine complex, and guanidine derivatives. Examples of amide compounds include dicyandiamide and polyamide resins synthesized from a linoleic acid dimer and ethylenediamine. Examples of acid anhydride compounds include phthalic anhydride, trimellitic anhydride, pyromellitic anhydride, maleic anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylnadic anhydride, hexahydrophthalic anhydride, and methylhexahydrophthalic anhydride. Phenol compounds include phenol novolac resin, cresol novolac resin, aromatic hydrocarbon formaldehyde resin modified phenol resin, dicyclopentadiene phenol addition type resin, phenol aralkyl resin (Zylok resin), polyhydric phenol novolac resin synthesized from polyhydric hydroxy compounds and formaldehyde, such as resorcinol novolac resin, naphthol aralkyl resin, trimethylolmethane resin, tetraphenylolethane resin, naphthol novolac resin, naphthol-phenol co-condensed novolac resin, naphthol-c Examples of suitable curing agents include polyhydric phenol compounds such as resole-co-condensed novolac resins, biphenyl-modified phenolic resins (polyhydric phenol compounds in which phenol nuclei are linked by bismethylene groups), biphenyl-modified naphthol resins (polyhydric naphthol compounds in which phenol nuclei are linked by bismethylene groups), aminotriazine-modified phenolic resins (polyhydric phenol compounds in which phenol nuclei are linked by melamine, benzoguanamine, etc.), and alkoxy-group-containing aromatic ring-modified novolac resins (polyhydric phenol compounds in which phenol nuclei and alkoxy-group-containing aromatic rings are linked by formaldehyde). These curing agents may be used alone or in combination of two or more.

[0030] [Thermoplastic resin] Examples of thermoplastic resins include well-known and commonly used resins used in molding materials, etc. Specific examples include polyethylene resin, polypropylene resin, polymethyl methacrylate resin, polyvinyl acetate resin, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, polyvinyl chloride resin, polystyrene resin, polyacrylonitrile resin, polyamide resin, polycarbonate resin, polyacetal resin, polyethylene terephthalate resin, polyphenylene oxide resin, polyphenylene sulfide resin, polysulfone resin, polyether sulfone resin, polyether ether ketone resin, polyallylsulfone resin, thermoplastic polyimide resin, thermoplastic urethane resin, polyaminobismaleimide resin, polyamideimide resin, polyetherimide resin, bismaleimide triazine resin, polymethylpentene resin, fluorinated resin, liquid crystal polymer, olefin-vinyl alcohol copolymer, ionomer resin, polyarylate resin, acrylonitrile-ethylene-styrene copolymer, acrylonitrile-butadiene-styrene copolymer, and acrylonitrile-styrene copolymer. At least one thermoplastic resin can be selected and used, but two or more thermoplastic resins can also be used in combination depending on the purpose.

[0031] When used for piezoelectric applications, the resin preferably exhibits a high dielectric constant, and the resin is preferably a polymer having an electron-withdrawing group, and examples thereof include fluorine-containing polymers such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, and vinylidene fluoride-trifluoroethylene copolymer, cyanoethylated polyvinyl alcohol, vinylidene cyanide-vinyl acetate copolymer, cyanoethyl cellulose, cyanoethyl hydroxysucrose, cyanoethyl hydroxycellulose, cyanoethyl hydroxypullulan, cyanoethyl methacrylate, cyanoethyl acrylate, and cyanoethyl methacrylate. Preferred are polymers having a cyano group or a cyanoethyl group, 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 sucrose, and cyanoethyl sorbitol, and polyvinylidene fluoride is particularly preferred because it further improves the Q value.

[0032] (Ultrasonic element array structure) An ultrasonic element having an array structure may be configured by arranging a plurality of ultrasonic elements of this embodiment. Figures 3 to 5 are schematic diagrams of an ultrasonic element having an array structure according to one embodiment of the present invention. Figure 3 shows the front surface of an ultrasonic element having an array structure, and Figure 4 shows the back surface of an ultrasonic element having an array structure. Figure 5 shows the front surface of an ultrasonic element having an array structure, but the electrode layer is not shown.

[0033] The array-structured ultrasonic element 11 shown in Figures 3 to 5 includes a plurality of ultrasonic elements 12, and each ultrasonic element 12 includes a piezoelectric body 13 having a dome-shaped portion 13a and electrode layers 14 provided on both sides of the dome-shaped portion 13a. The array-structured ultrasonic element 11 is characterized in that the piezoelectric bodies 13 of the plurality of ultrasonic elements 12 are formed from a single piezoelectric sheet 15. Therefore, a plurality of ultrasonic elements 12 can be manufactured at once. Conventionally, to manufacture an array-structured ultrasonic element, it was necessary to form a plurality of individually manufactured piezoelectric elements in an array, which resulted in large man-hours and manufacturing costs, but the ultrasonic element of the present invention can solve these problems.

[0034] The ultrasonic element 11 of the array structure has two or more ultrasonic elements 12 connected in a row. This makes it possible to obtain superior sound pressure compared to when there is only one ultrasonic element 12. For the multiple ultrasonic elements 12, the variation in height H from the bottom to the zenith of the dome-shaped portion 13a of each ultrasonic element 12 is preferably within ±5%, and more preferably within ±3%. Having the variation within this range is preferable because it suppresses deviations in the resonant frequency and eliminates the need for redundancy in the drive circuit and receive circuit.

[0035] In the ultrasonic element 11 having an array structure according to the present embodiment, the electrode layers 14 provided on the front and back surfaces of the dome-shaped portion 13a of each piezoelectric body 13 may be common ground electrodes provided in common to all piezoelectric bodies 13, but are preferably individual electrodes. Specifically, as shown in FIG. 3, it is preferable that the electrode layers 14 provided on at least one surface of each piezoelectric body 13 are provided independently of each other and can be controlled independently of each other. This allows the ultrasonic element 11 having an array structure to be configured so that each ultrasonic element 12 emits ultrasonic waves in any direction, or can be configured so that some ultrasonic elements 12 emit ultrasonic waves and other ultrasonic elements receive ultrasonic waves. It is preferable to employ a patterned electrode structure as the electrode layer 14 provided on one surface of each piezoelectric body 13.

[0036] As shown in Fig. 3, when each electrode layer 14 on one surface is an individual electrode, the electrode layer 14 provided on the other surface may be a common ground electrode, or may be an individual electrode as shown in Fig. 4. Furthermore, when the electrode layers 14 on both surfaces of each ultrasonic element 12 are common ground electrodes, the cost of forming the electrode layers can be reduced. In this case, multiple ultrasonic elements 12 are driven simultaneously.

[0037] (Method of manufacturing ultrasonic element) Both the ultrasonic element 1 and the array-structured ultrasonic element 11 described above can be manufactured by a manufacturing method including a molding process for molding a piezoelectric body having a dome-shaped portion using a piezoelectric sheet made of a piezoelectric composite material containing piezoelectric particles and resin, and a vapor deposition process for forming electrodes by vapor-depositing metal on both sides of the dome-shaped portion of the piezoelectric body.

[0038] <Preparing the piezoelectric sheet> The piezoelectric sheet is produced by coating a piezoelectric composite material on a base film and then peeling off the base film.

[0039] As a preliminary step to coating, a resin is dissolved in an organic solvent to prepare a resin liquid, and then piezoelectric particles are added and stirred to prepare a coating liquid in which the particles are dispersed. The organic solvent is not particularly limited, and various organic solvents can be used, such as methyl ethyl ketone and cyclohexane. The piezoelectric particles are not particularly limited, but the above-mentioned piezoelectric particles are preferred from the viewpoint of piezoelectric performance.

[0040] The piezoelectric composite material may contain other compounds as needed, and may contain external lubricants, internal lubricants, antioxidants, flame retardants, light stabilizers, UV absorbers, silane, titanate, or aluminate coupling agents, reinforcing materials such as glass fiber or carbon fiber, fillers, various colorants, etc. It is also possible to use stress-reducing agents (stress relaxation agents) such as silicone oil, liquid rubber, rubber powder, butadiene copolymer rubbers such as methyl acrylate-butadiene-styrene copolymers and methyl methacrylate-butadiene-styrene copolymers, and silicone compounds, as long as the effects of the invention are achieved.

[0041] When the resin is a thermosetting resin, the above-mentioned curing agent can be used. The amount of the curing agent is not particularly limited, but for example, when the curable resin is an epoxy resin, it is preferable to use an amount such that the active groups in the curing agent are 0.7 to 1.5 equivalents per total equivalent of the epoxy groups in the epoxy resin, in order to obtain good properties of the cured product.

[0042] If necessary, a curing accelerator can be used in combination with the thermosetting resin in the piezoelectric composite material. For example, when the curable resin is an epoxy resin, various curing accelerators can be used, such as phosphorus compounds, tertiary amines, imidazoles, organic acid metal salts, Lewis acids, and amine complex salts.

[0043] If necessary, a curing catalyst may be used in combination with the thermosetting resin at appropriate times, and examples of such a catalyst include known and commonly used thermal polymerization initiators and active energy ray polymerization initiators.

[0044] The piezoelectric composite material can be obtained by mixing piezoelectric particles, a resin, and, if necessary, other ingredients. There are no particular limitations on the mixing method, and the materials can be mixed by a known, commonly used method.

[0045] When the resin is a thermosetting resin, a common method is to thoroughly mix the thermosetting resin, piezoelectric particles, and other components as needed using a mixer or the like, and then knead using a three-roll mill or the like to obtain a fluid liquid composition, or to thoroughly mix a predetermined amount of thermosetting resin, the tantalate particles of the embodiment, and other components as needed using a mixer or the like, and then melt-knead using a mixing roll, extruder, or the like, and then cool to obtain a solid composition. When a curing agent, catalyst, or the like is added, it is preferable that the curable resin and the compound are thoroughly and uniformly mixed, and it is even more preferable that the piezoelectric particles are also uniformly dispersed and mixed.

[0046] When the resin is a thermoplastic resin, a common method is to premix the thermoplastic resin, piezoelectric particles, and, if necessary, other components using various mixers such as a tumbler or a Henschel mixer, and then melt-knead them using a mixer such as a Banbury mixer, a roll, a Brabender, a single-screw kneading extruder, a twin-screw kneading extruder, a kneader, a mixing roll, etc. The melt-kneading temperature is not particularly limited, but can be in the range of 240 to 320°C.

[0047] When preparing the piezoelectric composite material, the mixing ratio of the piezoelectric particles to the non-volatile content of the resin is not particularly limited, but may be, for example, in the range of 0.1 to 1800 parts of piezoelectric particles, or 10 to 900 parts, per 100 parts by mass of the non-volatile content of the resin.

[0048] The content of the piezoelectric particles relative to the total mass (100 mass%) of the piezoelectric composite material may be 30 mass% or more, 50 to 90 mass%, or 60 to 85 mass%.

[0049] The prepared coating solution is applied to a base film. The coating method is not particularly limited, and the coating solution can be applied to the base film by a dip method, a spin coating method, a spray coating method, a blade method, or any other method. The coating solution can be applied to the base film by using a coating device such as a spin coater, a slit coater, a die coater, a blade coater, a comma coater, a screen printer, a doctor blade, an applicator, or a spray, to form a uniform coating film of a predetermined thickness on the base film. The base film may be any film that allows easy peeling off afterwards, and examples thereof include polyester films such as PET.

[0050] The organic solvent is evaporated from the resulting coating. The drying temperature is not limited to the evaporation of the organic solvent, and can be appropriately selected based on the boiling point of the solvent, the heat resistance temperature of the release film, etc. For example, the drying temperature may be 50 to 120°C, 60 to 100°C, or 70 to 90°C. The drying time can be appropriately selected depending on the drying temperature, and may be, for example, 1 to 100 minutes.

[0051] The base film is peeled off from the dried coated product to obtain a piezoelectric sheet.

[0052] <Molding process> For example, the piezoelectric sheet is heated and compressed to form a piezoelectric body having a dome-shaped portion. The method of heat compression molding is not particularly limited, and known resin film processing methods can be used. Figure 6 shows a conceptual diagram illustrating an example of a method for forming piezoelectric bodies of an ultrasonic element with an array structure according to one embodiment of the present invention. A piezoelectric sheet 15 is placed between an upper mold 21a having a plurality of recesses corresponding to the surface of the dome-shaped portion 13a of each piezoelectric body 13 and a lower mold 21b having a plurality of protrusions corresponding to the back surface of the dome-shaped portion 13a of each piezoelectric body 13, and compression molding is performed. During this process, a metal plate spacer is sandwiched between the upper mold 21a and the lower mold 21b to adjust the thickness to the desired value. Then, the mixture is pressed at 70 to 250°C and 0.1 to 10 MPa to obtain multiple piezoelectric bodies 13 each having a dome-shaped portion 13a. The pressure and temperature during molding can be appropriately selected depending on the piezoelectric composite material. The method for forming a piezoelectric body having a dome-shaped portion is not limited to the above. When using a thermosetting resin that does not deform when heated and pressed, a gap is provided to form a dome shape, and an uncured resin composition is poured into the gap and then heated and cured to obtain a dome-shaped piezoelectric body. It is also possible to obtain a dome-shaped piezoelectric body using a 3D printer.

[0053] <Electrode formation process> The method for forming the electrode layer is not particularly limited, and can be appropriately selected from known methods such as vacuum deposition, sputtering, CVD, plating, etc. Alternatively, the electrode layer can be formed by applying a conductive paste such as silver paste, followed by drying and baking as necessary.

[0054] The material for forming the electrode layer is not particularly limited, and any conductive material can be used. Specific examples include C, Pt, Au, Pd, Ag, Cu, Al, Ni, Fe, Sn, Cr, and alloys thereof. Among these, C, Pt, Au, Pd, Ag, Cu, Al, and Ni are preferably used, and Pt, Au, Ag, Cu, and Al are particularly preferably used.

[0055] The electrode layer may be pattern-deposited on each dome-shaped piezoelectric element. The method for pattern-depositing is not particularly limited, and any known method, such as masking and vapor deposition, may be appropriately selected and used.

[0056] (Application) The ultrasonic element and array-structured ultrasonic element of this embodiment can be used as a replacement for conventional airborne ultrasonic sensors, and because they are thinner than airborne ultrasonic elements using bulk ceramics, they can be installed at a more flexible location. Furthermore, because piezoelectric elements with an array structure can be easily fabricated, they can be suitably used as airborne haptic devices and ultrasonic levitation devices. [Example]

[0057] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0058] <Production of piezoelectric particles> (Production Example 1) A mixture was obtained by mixing 40.0 g of niobium oxide (Nb2O5 ceramics grade, Mitsui Mining & Smelting Co., Ltd.), 27.2 g of molybdenum oxide (MoO3, Nippon Inorganic Chemical Industry Co., Ltd.), 11.0 g of sodium carbonate (soda ash light, Tokuyama Corporation), and 35.1 g of potassium carbonate (FG-F20, AGC Corporation) in an absolute mill (Osaka Chemical Co., Ltd.) for 30 seconds. The resulting mixture was divided and filled into four alumina crucibles and fired at 800°C for 5 hours in a ceramic electric furnace. Next, the obtained fired material was placed in 250 mL of pure water, treated in an ultrasonic cleaner for 10 minutes, then left to stand for 30 minutes, and decanted. This ultrasonic cleaning was repeated four times, and then dried to obtain KNN powder. Composition analysis was performed using ICP-AES to determine the molar ratio of Nb as 1, with the molar ratio of oxygen being 3. The formula was then expressed as (K 0.43 Na 0.57 ) 0.94 The compound was identified as NbO3 and had a perovskite structure by X-ray diffraction measurement. 1-x Na x) KNN particles (average particle diameter 8 μm) of the ABO3 perovskite type represented by NbO3 were obtained.

[0059] <Preparation of Resin Composition> (Production Example 2) A polymer solution was obtained by dissolving 30% by mass of cyanoethylated polyvinyl alcohol (Cyanoresin (registered trademark) CR-V, manufactured by Shin-Etsu Chemical Co., Ltd.) in 70% by mass of N,N-dimethylformamide. 7.33 g of this polymer solution and 13.5 g of the particles obtained in Production Example 1 were placed in a vial and mixed for 2 minutes using a mixer (Thinky Corporation, Awatori Rentaro) to obtain a resin composition.

[0060] (Production Example 3) A mixture of polyvinylidene fluoride (manufactured by Kureha Corporation, model number #850) and the particles obtained in Production Example 1, adjusted so that the particle volume fraction was 60 vol%, was kneaded for 2 minutes using a Labost Mill Mixer (manufactured by Toyo Seiki Seisakusho Co., Ltd., model number 4C150) at a temperature of 220°C and a screw rotation speed of 60 ppm to obtain a resin composition.

[0061] (Production Example 4) 3.54 g of silicone resin (KE-106, manufactured by Shin-Etsu Chemical Co., Ltd.), 0.35 g of curing agent (CAT-RG, manufactured by Shin-Etsu Chemical Co., Ltd.), and 26.1 g of the particles obtained in Production Example 1 were stirred at 2000 rpm for 120 seconds using a planetary mixer ARV-200 (manufactured by Thinky Corporation) to obtain a resin composition.

[0062] <Fabrication of ultrasonic elements> Example 1 In this example, the resin composition (40 vol% polyvinylidene fluoride / 60 vol% KNN particles) prepared in Production Example 3 was compressed in a compression molding machine at 230°C, 1 minute preheating, 1 minute compression, and 100 kgf / cm² to obtain a 0.5 mm thick piezoelectric sheet. The piezoelectric sheet was further compressed three times at 240°C, 30 seconds preheating, 30 seconds compression, and 100 kgf / cm². The compressed piezoelectric sheet was then compressed using a mold capable of forming a dome-shaped portion at 240°C, 30 seconds preheating, 30 seconds compression, and 100 kgf / cm² to obtain a piezoelectric body. The resulting piezoelectric body had a dome-shaped portion with a diameter D of 5.0 mm at the bottom and a height H of 0.33 mm from the bottom to the ceiling.

[0063] (Example 2, Comparative Example 1) In Example 2, the resin compositions obtained in Production Examples 2 and 3 were applied to a 50 μm-thick release film (Toyobo Co., Ltd., SP8002K210) using a Baker-type applicator (Tester Sangyo Co., Ltd., SA-201) to form a piezoelectric sheet, with a thickness of 50 μm after drying. The piezoelectric sheet was then dried in a dryer at 80°C for 10 minutes and peeled from the release film. The piezoelectric sheet was compressed at 80°C for 30 seconds under a pressure of 100 kgf / cm2 using the mold used in Example 1, to obtain a piezoelectric body. The resulting piezoelectric body had a dome-shaped portion, with a diameter D at the bottom of 5.0 mm and a height H from the bottom to the ceiling of 0.33 mm. In Comparative Example 1, a piezoelectric body was obtained in the same manner as in Example 2, except that a mold with a dome-shaped portion of a different size was used. The obtained piezoelectric body had a dome-shaped portion with a diameter D of 2.2 mm and a height H from the bottom to the ceiling of 0.50 mm.

[0064] Next, an Al electrode having a thickness of 100 nm was deposited on the lower surface (concave surface) of the dome-shaped portion of the obtained piezoelectric body using a deposition device to form the lower electrode 30. This resulted in a laminate having the lower electrode 30 on the lower surface of the dome-shaped portion of the piezoelectric body.

[0065] Next, using a corona discharge system (ELC-01N model, manufactured by Element Co., Ltd.), the lower electrode 30 was grounded, and the laminate obtained above was heated to 100°C, and a voltage of -7 kV was applied to the surface of the dome-shaped piezoelectric body to perform a polarization process.

[0066] Furthermore, an Al electrode having a thickness of 100 nm was deposited on the upper surface (convex surface) of the dome-shaped portion of the piezoelectric body after polarization treatment using a deposition device to form upper electrode 40, thereby obtaining an ultrasonic element.

[0067] [Evaluation method] The obtained ultrasonic element was evaluated according to the following method.

[0068] (density) The density of the piezoelectric sheet was calculated by Archimedes' method.

[0069] (Young's modulus) The same piezoelectric sheet as that inserted into the mold in each example was polarized, electrodes were deposited on both sides of the sheet, and the sheet was cut into strips using an NT cutter to prepare test specimens. Using the test specimens, the Young's modulus was calculated from the longitudinal resonance frequency f, the length L of the test specimen, and the density of the test specimen using an Impedance Analyzer 4294A.

[0070] (resonant frequency) The resonance frequency was calculated based on the Young's modulus and density using finite element method software Femtet (Murata Software). Ultrasonic measurements, which will be described later, confirmed that the calculated resonance frequency was equivalent.

[0071] (sound pressure measurement) A Type 4156N microphone (manufactured by ACO Co., Ltd.) was placed 30 cm above the top surface of the dome of the dome-shaped ultrasonic element to measure the sound pressure. A sine wave voltage of 10 Vpp was applied to the ultrasonic element, and the frequency was swept to determine the frequency at which the sound pressure was maximum as the resonant frequency, and the sound pressure at the resonant frequency was measured. The sound pressure when the dome-shaped ultrasonic elements are arranged in an array structure was calculated as a relative value by multiplying the sound pressure when one dome-shaped ultrasonic element with a height of 0.33 mm is driven by four (i.e., assuming there are four dome-shaped ultrasonic elements with a height of 0.33 mm).

[0072] (Q value measurement) The Q value can be calculated from "Q = Δf / fr", where fr is the resonance frequency with the largest vibration displacement, f1 is the frequency lower than fr at which the vibration displacement is 1 / √2, and f2 is the frequency higher than fr at which the vibration displacement is 1 / √2, where Δf = f2 - f1. The vibration displacement was measured using the same method as the sound pressure measurement method described above, except that the microphone Type 4156N was replaced with a laser trap vibrometer LV-1800 (manufactured by Ono Sokki Co., Ltd.).

[0073] [Table 1]

[0074] As shown in Table 1, sufficient ultrasonic sound pressure could not be obtained with a piezoelectric sheet with a small Young's modulus. It became clear that a Young's modulus of 10 GPa or more is required to obtain ultrasonic sound pressure.

[0075] It was also found that a high ultrasonic sound pressure can be obtained when the Q value of the ultrasonic element is high. In particular, it was found that the ultrasonic element of Example 1 is particularly suitable for oscillating only sound waves of a specific frequency.

[0076] (Fabrication of array-structured ultrasonic elements) Except for preparing and using a mold capable of producing four dome shapes, the mold was produced in the same manner as in Example 1. As a comparative example, a mold in which the four dome shapes varied was used.

[0077] [Table 2]

[0078] The ultrasonic element with an array structure of Example 3 had a small variation in the height of the dome-shaped portion of each piezoelectric body of the multiple ultrasonic elements with a dimensional accuracy of ±4.5%, and was able to obtain excellent sound pressure. In contrast, the ultrasonic element with an array structure of Comparative Example 2 had a large variation in the height of the dome-shaped portion of each piezoelectric body of the multiple ultrasonic elements with a dimensional accuracy of ±6%, and was unable to obtain sufficient sound pressure. This is presumably because in Comparative Example 2, the difference in the resonance frequency of each ultrasonic element was large, and so sounds were canceled out by opposite-phase frequencies, resulting in a decrease in sound pressure. [Explanation of symbols]

[0079] 1...ultrasonic element, 2...piezoelectric body, 2a...dome-shaped portion, 3...electrode layer, 4...piezoelectric sheet, 11...array-structured ultrasonic element, 12...ultrasonic element, 13...piezoelectric body, 13a...dome-shaped portion, 14...electrode layer, 15...piezoelectric sheet, 21a...upper mold, 21b...lower mold.

Claims

1. An ultrasonic element comprising a piezoelectric body having a dome-shaped portion and an electrode layer provided on both sides of the dome-shaped portion, the piezoelectric body is a piezoelectric sheet including a piezoelectric composite material containing piezoelectric particles and a resin and having a Young's modulus of 10 GPa or more; The ultrasonic element has a resonant frequency greater than 20 kHz.

2. 2. The ultrasonic element according to claim 1, wherein the Q value observed from vibration displacement is 5 or more.

3. The ultrasonic element according to claim 1 or 2, wherein the resin is polyvinylidene fluoride.

4. An ultrasonic element having an array structure including a plurality of ultrasonic elements according to claim 1, An ultrasonic element having an array structure, wherein each of the piezoelectric bodies of a plurality of the ultrasonic elements is formed from a single piezoelectric sheet.

5. The array-structure ultrasonic element according to claim 4 , wherein the variation in height H from the bottom to the apex of the dome-shaped portion of each of the ultrasonic elements is within ±5%.

6. The array-structured ultrasonic element according to claim 4, wherein the plurality of ultrasonic elements are arranged such that the electrode layers provided on at least one surface of the electrode layers of the dome-shaped portion of each of the piezoelectric bodies are provided independently of each other and can be controlled independently of each other.

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