WO2018070483A1 - 圧電素子およびその製造方法 - Google Patents

圧電素子およびその製造方法 Download PDF

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Publication number
WO2018070483A1
WO2018070483A1 PCT/JP2017/037030 JP2017037030W WO2018070483A1 WO 2018070483 A1 WO2018070483 A1 WO 2018070483A1 JP 2017037030 W JP2017037030 W JP 2017037030W WO 2018070483 A1 WO2018070483 A1 WO 2018070483A1
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WIPO (PCT)
Prior art keywords
polymer
nonwoven fabric
piezoelectric element
laminate
ceramic particles
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PCT/JP2017/037030
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English (en)
French (fr)
Inventor
安田　健
康武 早川
大平　晃也
柿本　健一
Original Assignee
Ntn株式会社
国立大学法人 名古屋工業大学
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Application filed by Ntn株式会社, 国立大学法人 名古屋工業大学 filed Critical Ntn株式会社
Priority to US16/341,881 priority Critical patent/US20190229255A1/en
Priority to CN201780063178.2A priority patent/CN109997238A/zh
Publication of WO2018070483A1 publication Critical patent/WO2018070483A1/ja

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    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
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    • B32B5/16—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by features of a layer formed of particles, e.g. chips, powder or granules
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N30/00—Piezoelectric or electrostrictive devices
    • H10N30/702—Piezoelectric or electrostrictive devices based on piezoelectric or electrostrictive fibres
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    • D01D5/0007—Electro-spinning

Definitions

  • the present invention relates to a piezoelectric element and a manufacturing method thereof, and more particularly to a piezoelectric element suitable for vibration power generation using environmental vibration.
  • vibration power generation Power generation using vibration among environmental power generation
  • methods such as piezoelectric, electromagnetic induction, and electrostatic induction.
  • the piezoelectric method uses a piezoelectric element as a power generation element, and has an advantage that the structure is simpler than electromagnetic induction and electrostatic induction because the piezoelectric characteristics of the material are used.
  • the characteristics required for the piezoelectric element are high power generation performance, impact resistance and the like.
  • inorganic piezoelectric materials Materials constituting the piezoelectric element are roughly classified into inorganic piezoelectric materials and organic piezoelectric materials. Ceramics having a perovskite crystal structure typified by lead zirconate titanate (PZT) are widely used as inorganic piezoelectric materials, and polyvinylidene fluoride (hereinafter referred to as PVDF), vinylidene fluoride as organic piezoelectric materials. -Ethylene trifluoride copolymer, polylactic acid and the like. Inorganic piezoelectric materials have better power generation performance than organic piezoelectric materials, but are inferior in flexibility and impact resistance.
  • PZT lead zirconate titanate
  • PVDF polyvinylidene fluoride
  • -Ethylene trifluoride copolymer polylactic acid and the like.
  • Inorganic piezoelectric materials have better power generation performance than organic piezoelectric materials, but are inferior in flexibility and impact resistance.
  • Patent Document 1 proposes a composite piezoelectric element in which a piezoelectric layer having a resin and piezoelectric particles is laminated, and the piezoelectric particle concentration between the two first piezoelectric layers is higher than that of the first piezoelectric layer.
  • This is a structure in which a second piezoelectric layer having a low is disposed. The bending resistance of the composite piezoelectric element is improved by reducing the piezoelectric particle concentration of the second piezoelectric layer.
  • Patent Document 2 discloses a piezoelectric sheet including a nonwoven fabric or a woven fabric formed using a fiber including an organic polymer and including an inorganic filler.
  • Non-Patent Document 1 NKN particles are formed on a sheet layer composed of a polyvinyl alcohol (hereinafter referred to as PVA) resin composition in which sodium potassium niobate solid solution (hereinafter referred to as NKN) particles are blended, and a nonwoven fabric composed of fiberized PVDF.
  • PVA polyvinyl alcohol
  • NKN sodium potassium niobate solid solution
  • a piezoelectric element in which held nonwoven layers are alternately laminated and integrated. Since this structure has a porous nonwoven fabric layer, it is considered that the structure is more flexible than the structure of Patent Document 1.
  • both surfaces of the piezoelectric element are sheet layers made of a PVA resin composition containing NKN particles.
  • NKN particles In order to further improve the power generation performance, it is necessary to increase the surface charge density on the surface of the sheet layer by easily filling the PVA resin composition with NKN particles so that the charge can be easily taken out.
  • examination is not carried out in terms of the power generation performance with respect to the thickness of the sheet layer, the thickness of the nonwoven fabric layer, and the number of sheets of the sheet layer and the nonwoven fabric layer.
  • the power generation performance of a laminate composed of a sheet layer and a nonwoven fabric layer has not been studied in Patent Document 2.
  • the present invention has been made to address such problems, and an object of the present invention is to provide a piezoelectric element that can exhibit high power generation performance without impairing flexibility, and a method for manufacturing the same.
  • the piezoelectric element of the present invention is formed by laminating a polymer nonwoven fabric in which piezoelectric ceramic particles are held or blended and a polymer resin sheet in which piezoelectric ceramic particles are blended so that at least one layer of the polymer nonwoven fabric is included.
  • This laminate is a laminate capable of realizing a power generation amount equal to or greater than the power generation amount generated from a laminate in which one layer of the polymer resin sheet is laminated on each of the two main planes of one layer of the polymer nonwoven fabric. It is characterized by.
  • the polymer resin sheet is a sheet having a thickness of 10 to 100 ⁇ m in which 50 to 80% by volume of piezoelectric ceramic particles are blended, and the polymer nonwoven fabric is made of fibers constituting the polymer nonwoven fabric.
  • a non-woven fabric having an average diameter of 0.05 to 5 ⁇ m and a thickness of 10 to 300 ⁇ m of one layer holding or blending 30 to 60% by volume of piezoelectric ceramic particles.
  • the laminate constituting the piezoelectric element of the present invention is characterized in that a plurality of the polymer nonwoven fabrics are laminated, or the polymer nonwoven fabrics and the polymer resin sheets are alternately laminated. In particular, the two main plane sides of the laminate are both the polymer resin sheet.
  • the method for producing a piezoelectric element of the present invention includes a polymer nonwoven fabric in which piezoelectric ceramic particles are held or blended and a polymer resin sheet in which piezoelectric ceramic particles are blended so that at least one layer of the polymer nonwoven fabric is included. It is a method for manufacturing a piezoelectric element including a step of laminating and a step of integrating the laminated body by press-bonding using a press. Further, the polymer nonwoven fabric in which the piezoelectric ceramic particles are held or blended is obtained by an electrospinning method in which the slurry obtained by dispersing the piezoelectric ceramic particles in a solution in which the polymer is dissolved in water or an organic solvent is electrospun. It is a manufactured polymer nonwoven fabric.
  • the piezoelectric element of the present invention is integrated by laminating a polymer resin sheet layer and a polymer nonwoven fabric layer, high piezoelectricity can be expressed without impairing flexibility. Further, since the polymer resin sheet is highly filled with 50 to 80% by volume of piezoelectric ceramic particles, charges are induced on the surface of the piezoelectric element, and the charges can be easily taken out. Furthermore, since the polymer nonwoven fabric layer is highly filled with piezoelectric ceramic particles at 30 to 60% by volume, high piezoelectricity can be expressed without impairing flexibility.
  • the piezoelectric element of the present invention is a laminate that can realize a power generation amount that is greater than the power generation amount generated from a laminate in which one polymer resin sheet is laminated on each of the two main planes of one layer of the polymer nonwoven fabric.
  • the power generation performance can be further improved and maintained.
  • the power generation amount of the piezoelectric element integrated by laminating the polymer nonwoven fabric layer and the polymer resin sheet layer increases as the number of polymer nonwoven fabric layers in the laminate increases. Furthermore, when the number of stacked layers was increased, the power generation amount decreased. That is, it was found that there is an optimum value for the amount of power generation in the number of laminated layers of the polymer nonwoven fabric layer and the polymer resin sheet layer. The present invention is based on such knowledge.
  • FIG. 1 shows a case where the two main plane sides of the laminate are both polymer resin sheets
  • FIG. 1A is an example in which polymer resin sheets and polymer nonwoven fabrics are alternately laminated
  • 1 (b) is an example in which a plurality of polymer nonwoven fabrics are laminated.
  • FIG. 1 is a schematic diagram of a laminated body expressed by enlarging the thickness, and piezoelectric ceramic particles, nonwoven fabric, and the like are conceptually shown.
  • 1A shows a polymer resin sheet 2 in which piezoelectric ceramic particles are blended and a polymer nonwoven fabric 3 in which piezoelectric ceramic particles 4 are held or blended in a nonwoven fabric 5 are alternately laminated.
  • 2a and 2b form the front and back of the laminated body 1a.
  • the laminate having the minimum structure has two pieces of n and one piece of m, and this is a laminate having the minimum structure.
  • a plurality of the polymer nonwoven fabrics 3 are laminated, and the polymer resin sheets 2a and 2b form the front and back of the laminate 1b.
  • the minimum number of laminated bodies is 2 for n and 1 for m.
  • n is a constant 2
  • m is a value of 2, 3, 4,.
  • the laminate 1 is not limited to the laminate shown in FIGS. 1A and 1B, and may be a laminate that is laminated so that at least one layer of the polymer nonwoven fabric 3 is included.
  • a plurality of polymer nonwoven fabrics 3 can be laminated, and this laminate and the polymer resin sheet 2 can be laminated.
  • the amount of power generated by the piezoelectric element composed of the laminate 1 was studied.
  • a piezoelectric element as shown in FIG. 1A, a polymer resin sheet 2 and a polymer nonwoven fabric 3 are alternately laminated, and the polymer resin sheets 2a and 2b form the front and back of the laminate.
  • a laminate 1b was prepared in which a plurality of polymer nonwoven fabrics 3 were laminated and the polymer resin sheets 2a and 2b formed the front and back of the laminate.
  • the polymer resin sheet 2 was prepared by mixing 50% by volume of NKN particles having an average particle diameter of 1 ⁇ m in a PVA resin and having a thickness of 40 ⁇ m.
  • the polymer nonwoven fabric 3 is a nonwoven fabric having a thickness of 40 ⁇ m, produced by an electrospinning method using PVDF slurry containing 50% by volume of NKN particles having an average particle diameter of 1 ⁇ m.
  • the average diameter of the fibers of the polymer nonwoven fabric 3 was prepared in three levels of 0.05, 0.5, and 5 ⁇ m.
  • the laminate 1a is represented by an nm structure, where n is the number of laminated polymer resin sheets 2 and m is the number of laminated polymer nonwoven fabrics 3.
  • n is the number of laminated polymer resin sheets 2
  • m is the number of laminated polymer nonwoven fabrics 3.
  • a power generation amount measurement sample of a piezoelectric element six types of laminates of 2-1 structure, 3-2 structure, 4-3 structure, 5-4 structure, 6-5 structure, and 7-6 structure are provided, and the respective laminates Three levels of samples with average fiber diameters of 0.05, 0.5, and 5 ⁇ m were prepared. The total number of samples prepared is 18 samples.
  • the laminated body 1b is represented by a 2-m structure, where the number of laminated polymer nonwoven fabrics 3 is m, because the number of laminated polymer resin sheets 2 forming the front and back of the laminated body is two.
  • a power generation amount measurement sample of the piezoelectric element five types of laminates of 2-1 structure, 2-3 structure, 2-5 structure, 2-7 structure, and 2-9 structure were used, and the average of the fibers for each of the laminates Three levels of samples with diameters of 0.05, 0.5 and 5 ⁇ m were produced. The total number of samples prepared is 15 samples.
  • FIG. 2 is a schematic diagram showing an example of a polarization method of the laminated body 1a and the laminated body 1b.
  • the laminated body 1 is placed on a grounded sample stage 6, and subjected to polarization treatment by corona discharge generated by applying a DC electric field by a needle-like electrode 7 placed at a distance of 3 mm in the vertical direction from the upper surface of the laminated body 1. An element was produced.
  • the treatment conditions were a room temperature, a voltage of 20 kV, and a treatment time of 10 minutes.
  • FIG. 3 is a diagram of a piezoelectric element for testing. The layer thickness is shown enlarged.
  • 3A is a plan view
  • FIG. 3B is a cross-sectional view taken along the line AA of the piezoelectric element A obtained from the laminate 1a shown in FIG. 1
  • FIG. 3C is a view shown in FIG.
  • FIG. 3 is a cross-sectional view of the piezoelectric element B obtained from the laminated body 1b along the line AA.
  • a silver paste 8 was applied to both surfaces of the piezoelectric elements A and B made of the laminate 1 subjected to polarization treatment, upper and lower electrodes were formed, and a copper foil tape 9 was attached to obtain a piezoelectric element for testing.
  • FIG. 4 is a circuit diagram showing a method for measuring the electric power generated by the piezoelectric element. Using the circuit shown in FIG. 4, a stretching vibration of 170 Hz was applied in the longitudinal direction (the arrow direction shown in FIG. 3) of the piezoelectric elements A and B, and the power generation amount per vibration was measured. The piezoelectric elements A and B were connected to the load resistor 10, and the electric power generated in the load resistor 10 was measured with an oscilloscope 11.
  • FIG. 5A shows the result of the piezoelectric element A obtained from the laminate 1a
  • FIG. 5B shows the result of the piezoelectric element B obtained from the laminate 1b.
  • the power generation amount is expressed as a percentage of the maximum power generation amount, with the maximum power generation amount being 100%.
  • the maximum power generation amount of the piezoelectric element A was a 4-3 structure when the average diameter (fiber diameter) of the fibers was 0.05 and 0.5 ⁇ m, and the power generation amount was 529 nW.
  • the maximum power generation amount of the piezoelectric element B was a 2-5 structure when the average fiber diameter (fiber diameter) was 0.05 and 0.5 ⁇ m, and the power generation amount was 495 nW.
  • the piezoelectric element A was higher in tensile stress and strain in the tensile test than the piezoelectric element B. As a result, the piezoelectric element A is a preferable structure.
  • 1 is the smallest unit. From this minimum unit, the amount of power generation tends to increase as the number of laminated polymer nonwoven fabrics 3 increases. However, this power generation amount does not increase monotonously, but the maximum power generation amount is obtained with the 4-3 structure in the case of the piezoelectric element A and the 2-5 structure in the case of the piezoelectric element B. Thereafter, the laminated layer of the polymer nonwoven fabric 3 is obtained. As the number increases, power generation tends to decrease.
  • the present invention specifies a certain range on both sides of this optimum value, and is a laminate capable of realizing a power generation amount equal to or greater than the power generation amount generated from the laminate 1 of the minimum unit.
  • the piezoelectric element A has a 2-1 structure, a 3-2 structure, a 4-3 structure, a 5-4 structure, a 6-5 structure, and a 7-6 structure, preferably 2-1
  • the structure is a 3-2 structure, a 4-3 structure, a 5-4 structure, and a 6-5 structure, and more preferably a 3-2 structure, a 4-3 structure, and a 5-4 structure.
  • the piezoelectric element B the 2-1 structure, 2-3 structure, 2-5 structure, 2-7 structure, and 2-9 structure are preferable, and the 2-1 structure, 2-3 structure, -5 structure and 2-7 structure.
  • the piezoelectric ceramic particles blended in the polymer resin sheet or the piezoelectric ceramic particles held or blended in the polymer nonwoven fabric may be the same type of piezoelectric ceramic particles or different types of piezoelectric ceramic particles. Similarly, in the polymer resin sheets or the polymer nonwoven fabrics, the piezoelectric ceramic particles may be the same type of piezoelectric ceramic particles or different types of piezoelectric ceramic particles. It is preferable to use piezoelectric ceramic particles having the same composition throughout the laminate constituting the piezoelectric element.
  • the piezoelectric ceramic particles are preferably piezoelectric ceramic particles having a perovskite crystal structure.
  • piezoelectric ceramic particles containing one or more elements of niobium, lead, titanium, zinc, barium, bismuth, zirconium, lanthanum, potassium, sodium, calcium, and magnesium are listed.
  • NKN particles not containing lead or barium titanate particles are more preferable because they are excellent in safety to the human body and the environment.
  • the NKN particles are ceramic particles represented by (Na 0.5 K 0.5 ) NbO 3 .
  • NKN particles can be produced by a solid reaction of sodium carbonate, potassium carbonate and niobium oxide.
  • the average particle size of the piezoelectric ceramic particles is 0.1 ⁇ m to 10 ⁇ m, preferably 0.5 ⁇ m to 5 ⁇ m, more preferably 1 ⁇ m to 2 ⁇ m. If it is less than 0.1 ⁇ m, uniform dispersion in the polymer resin sheet or polymer resin nonwoven fabric is difficult, and if it exceeds 10 ⁇ m, the mechanical strength of the polymer resin sheet or polymer nonwoven fabric is lowered.
  • the average particle diameter in this invention is 50% particle diameter (D50) measured and calculated by the laser diffraction method.
  • the piezoelectric ceramic particles are blended in the polymer resin sheet, it is preferable to use a granulated powder bonded to the piezoelectric ceramic particles using a polymer binder.
  • the polymer binder is preferably a material different from the polymer material constituting the polymer resin sheet.
  • Specific examples of the polymer binder include acrylic, cellulose, PVA, polyvinyl acetal, urethane, and vinyl acetate polymers.
  • the granulation method is not particularly limited, and known methods such as spray granulation, rolling granulation, extrusion granulation, and compression granulation can be used.
  • the average particle diameter of the granulated powder is 10 ⁇ m to 100 ⁇ m, preferably 30 ⁇ m to 50 ⁇ m.
  • the type of the polymer material constituting the polymer resin sheet is not particularly limited, and may be any of thermoplastic resin, thermosetting resin, thermoplastic elastomer, synthetic rubber, and natural rubber.
  • a crystalline resin having a melting point of 150 ° C. or higher or an amorphous resin having a glass transition point of 150 ° C. or higher is more preferable.
  • polymer materials such as PVA, polyvinyl butyral (hereinafter referred to as PVB), polystyrene, polyimide, polyamideimide, polyetherimide, polysulfone, polyphenylsulfone, polyethersulfone, polyarylate, polyphenylene ether, etc. Can be mentioned.
  • the piezoelectric ceramic particles are blended with the polymer material.
  • the polymer resin sheet preferably contains an inorganic filler having no piezoelectricity together with the piezoelectric ceramic particles.
  • an inorganic filler it is preferable to blend a conductive filler for the purpose of facilitating charge transfer in the sheet layer.
  • the conductive filler include graphite, carbon black, carbon nanotube, fullerene, metal powder, carbon fiber, and metal fiber.
  • a reinforcing material can be blended as an inorganic filler in order to improve the mechanical reinforcement of the sheet layer. Examples of the reinforcing material include carbon nanotubes, whiskers, carbon fibers, and glass fibers.
  • the polymer resin sheet preferably contains 50 to 80% by volume of piezoelectric ceramic particles, and the remainder is the polymer material, or the remainder is the polymer material and the inorganic filler having no piezoelectricity,
  • the blending amount of the piezoelectric ceramic particles is more preferably 70 to 80% by volume.
  • the polymer material is preferably blended at least 20% by volume. If the piezoelectric ceramic particle is less than 50% by volume, the piezoelectricity is not improved, and if it exceeds 80% by volume, the mechanical strength of the polymer resin sheet is lowered.
  • the piezoelectric ceramic particles refer to particles before the granulated powder.
  • any method capable of forming a thin sheet can be used.
  • a slurry is prepared by dispersing a filler such as the piezoelectric ceramic particles in water or an organic solvent in which the polymer material is dissolved, and the slurry is applied onto a support to form a thin film.
  • a production method in which water or an organic solvent is removed by drying or the like is preferable.
  • a method for applying the slurry onto the support a known method such as a tape casting method represented by a doctor blade method, a spin coating method, or the like can be used.
  • the thickness of one polymer resin sheet is 10 to 100 ⁇ m, preferably 30 to 50 ⁇ m.
  • the thickness of the polymer resin sheet layer is less than 10 ⁇ m, the mechanical strength of the piezoelectric element is lowered, and when it exceeds 100 ⁇ m, the flexibility is lowered and cracking may occur when the piezoelectric element is vibrated.
  • the polymer nonwoven fabric can be used as long as it is made into a fabric by bonding or entanglement of a fiberized polymer material by thermal, mechanical or chemical action.
  • the average diameter of the fibers constituting the polymer nonwoven fabric is preferably 0.05 to 5 ⁇ m, more preferably 0.5 to 1 ⁇ m.
  • the average diameter is larger than 5 ⁇ m, the porous volume of the nonwoven fabric layer is reduced, so that the power generation performance is lowered.
  • the average diameter is less than 0.05 ⁇ m, the stress applied to the piezoelectric ceramic particles by the fiber is reduced, and the power generation performance is lowered.
  • the average diameter of the fiber in this invention is an average value measured and calculated by the image obtained with the scanning electron microscope.
  • the type of the polymer material used as the polymer nonwoven fabric is not particularly limited, and the presence or absence of piezoelectricity due to the molecular structure is not limited. From the viewpoint of heat resistance, a crystalline resin having a melting point of 150 ° C. or higher or an amorphous resin having a glass transition point of 150 ° C. or higher is preferable, and a resin having excellent flexibility is more preferable. Specific examples include PVA, PVB, PVDF, tetrafluoroethylene-ethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkoxyethylene copolymer, and the like. .
  • the piezoelectric ceramic particles are held or blended in the polymer nonwoven fabric.
  • the polymer nonwoven fabric preferably holds or blends an inorganic filler having no piezoelectricity with the piezoelectric ceramic particles.
  • the inorganic filler it is preferable to hold or blend a conductive filler for the purpose of facilitating charge transfer in the nonwoven fabric layer.
  • the conductive filler include graphite, carbon black, carbon nanotube, fullerene, and metal powder.
  • a reinforcing material can be held or blended in order to improve the mechanical reinforcement of the nonwoven fabric layer. Examples of the reinforcing material include carbon nanotubes and whiskers.
  • “Holding” means that the piezoelectric ceramic particles are fixed between the fibers of the polymer nonwoven fabric, and “mixing” means that the piezoelectric ceramic particles are contained in the fiberized polymer material. .
  • the polymer non-woven fabric retains or mixes 30 to 60% by volume of piezoelectric ceramic particles, and the remainder is the above-described fiberized polymer material, or the remainder is the above-mentioned fiberized polymer material and the inorganic material having no piezoelectricity.
  • a filler is preferable, and the holding or blending amount of the piezoelectric ceramic particles is more preferably 50 to 60% by volume. Further, it is preferable that at least 40% by volume of the fiberized polymer material is blended. If the piezoelectric ceramic particle is less than 30% by volume, the piezoelectricity is not improved, and if it exceeds 60% by volume, the mechanical strength of the polymer nonwoven fabric is lowered.
  • the polymer nonwoven fabric can be produced as long as it can be made into a thin nonwoven fabric using fibers having an average diameter of 0.05 to 5 ⁇ m.
  • the electrospinning method is a method for producing a nonwoven fabric by applying a voltage between a needle of a syringe of an electrospinning apparatus and a collector and injecting slurry in the syringe toward the collector.
  • the shape of the collector is not particularly limited, such as a drum type, a disk type, or a plate type, but a drum type capable of producing a large area nonwoven fabric is preferable. Water or an organic solvent can be removed by drying the obtained nonwoven fabric.
  • the thickness of one polymer nonwoven fabric is 10 to 300 ⁇ m, preferably 120 to 200 ⁇ m. If the thickness of the polymer nonwoven fabric is less than 10 ⁇ m, the piezoelectricity when the piezoelectric element is formed is lowered, and if it exceeds 300 ⁇ m, the piezoelectric nonwoven fabric may be broken when it is vibrated.
  • a sheet-like piezoelectric element can be obtained by integrating the laminate of the polymer resin sheet and the polymer nonwoven fabric.
  • a method of pressure bonding using a press can be used.
  • the method for polarizing a piezoelectric element of the present invention preferably includes a step of applying a DC electric field to the integrated piezoelectric element.
  • Specific examples of the polarization method include a method using corona discharge in the atmosphere, a method of applying a DC electric field in silicone oil heated to 100 to 200 ° C., and the like.
  • the piezoelectric element of the present invention the polymer resin sheet layer and the non-woven fabric layer are integrated, and charge is easily induced on the surface of the piezoelectric element by easily filling the polymer resin sheet layer with piezoelectric ceramic particles. Electric charge can be taken out. Moreover, high piezoelectricity can be expressed without impairing flexibility by highly filling the nonwoven fabric layer with piezoelectric ceramic particles. Furthermore, power generation performance can be improved by optimizing the thickness of the sheet layer, the thickness of the nonwoven fabric layer, and the number of sheets of the sheet layer and the nonwoven fabric layer. For this reason, the piezoelectric element of the present invention can be applied to uses of vibration power generation, current sensors, and voltage sensors, and is particularly suitable for vibration power generation using environmental vibration.
  • NKN particles used as piezoelectric ceramics are Na 2 CO 3 (purity 99.9%), K 2 CO 3 (purity 99.9%), and Nb 2 O 5 (purity 99.9%) as raw material powders.
  • the raw material powder was sufficiently mixed, and the mixture was sintered at 1098 ° C. for 2 hours and then pulverized to prepare a powder having an average particle diameter of 1 ⁇ m. This powder was dispersed in a polyurethane solution as a polymer binder, and granulated powder was prepared by a spray dryer method.
  • the polymer resin sheet was prepared by tape-casting a slurry prepared by dispersing this granulated powder in an aqueous solution in which 7% by mass of PVA was dissolved, on a support.
  • a doctor blade type coating machine manufactured by Imoto Manufacturing Co., Ltd .: IMC-70F0-C type was used for tape casting.
  • the obtained sheet was dried at room temperature to remove water, thereby obtaining a polymer resin sheet.
  • the polymer nonwoven fabric was prepared by electrospinning a slurry in which the NKN particles were dispersed in a dimethyl sulfoxide solution in which PVDF was dissolved.
  • As the electrospinning apparatus IMC-1639 type manufactured by Imoto Seisakusho was used.
  • the concentration of the dimethyl sulfoxide solution in which PVDF was dissolved was 0.11 g / mL, and a slurry in which 50% by volume of NKN particles were dispersed in PVDF was used.
  • a voltage of 18 kV was applied between the syringe needle and the collector. By applying, the slurry in the syringe was injected toward the collector, and a nonwoven fabric was produced.
  • the obtained non-woven fabric was dried at room temperature to remove dimethyl sulfoxide to obtain a polymer non-woven fabric.
  • the pressure is 40 MPa and the temperature is 65 ° C.
  • the laminate was obtained by pressurizing for 3 minutes.
  • Table 1 shows the structure and thickness of the laminate, the amount of NKN retained or blended in the polymer resin sheet and the polymer nonwoven fabric, the thickness of the polymer resin sheet and the polymer nonwoven fabric, and the average diameter of the fibers constituting the polymer nonwoven fabric. And in Table 2.
  • Example 6 the result was that the piezoelectric element having a 4-3 structure as the laminate had the maximum power generation amount. Further, the larger the amount of NKN particles in the polymer resin sheet layer, the larger the amount of power generation. Comparative Example 3 (thickness of polymer resin sheet 1 layer 40 ⁇ m, NKN particle content 90% by volume), comparison In Example 4 (the thickness of one layer of the polymer resin sheet was 5 ⁇ m and the amount of NKN particles was 70% by volume), the amount of power generation could not be measured because the sheet layer was broken. Further, the power generation amount showed a good value when the average diameter of the fibers of the polymer nonwoven fabric layer was in the range of 0.05 to 5 ⁇ m. Table 2 shows the amount of power generation depending on the thickness of the polymer nonwoven fabric layer. Example 13 (the thickness of one layer of the polymer nonwoven fabric was 200 ⁇ m) was the most excellent result.
  • the present invention can be used in the field of vibration power generation using environmental vibration.

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Abstract

柔軟性を損ねることなく高い発電性能を発現できる圧電素子およびその製造方法を提供する。圧電セラミックス粒子４が保持または配合された高分子不織布３と、圧電セラミックス粒子が配合された高分子樹脂シート２とを、高分子不織布が少なくとも１層含まれるように積層された積層体１からなる圧電素子である。この積層体は、高分子不織布１層の２つの主平面側に高分子樹脂シートがそれぞれ１層ずつ積層された積層体より発生する発電量以上の発電量を実現できる積層体である。

Description

圧電素子およびその製造方法
　本発明は、圧電素子とその製造方法に関し、特に環境振動を利用した振動発電に好適な圧電素子に関する。
　近年、身近に存在する振動、太陽光、室内光、電波等のエネルギーを採取し、電力変換する環境発電が注目されており、電子機器等の自立型電源への適用が進みつつある。環境発電のうち振動を利用する発電は振動発電と呼ばれ、圧電、電磁誘導、静電誘導等の方式がある。
　圧電方式は発電素子として圧電素子を用いるものであり、材料の圧電特性を利用することから電磁誘導、静電誘導に比べて構造が単純であることに利点がある。圧電素子に求められる特性は高い発電性能、耐衝撃性等である。
　圧電素子を構成する材料は無機圧電材料、有機圧電材料に大別される。無機圧電材料としては、チタン酸ジルコン酸鉛（ＰＺＴ）に代表されるペロブスカイト型結晶構造を有するセラミックスが広く用いられており、有機圧電材料としてはポリフッ化ビニリデン（以下、ＰＶＤＦという）、フッ化ビニリデン－三フッ化エチレン共重合体、ポリ乳酸等が挙げられる。無機圧電材料は有機圧電材料に比べて発電性能に優れるが、柔軟性、耐衝撃性は劣る。
　無機圧電材料と有機圧電材料を複合化させることで、高い発電性能と、柔軟性、耐衝撃性を併せ持つ圧電素子を作製する試みもなされている。例えば特許文献１では、樹脂と圧電粒子とを有する圧電体層を積層した複合圧電素子が提案されており、２つの第１圧電体層の間に、この第１圧電体層よりも圧電粒子濃度が低い第２圧電体層が配置された構造である。第２圧電体層の圧電粒子濃度を低くすることにより、複合圧電素子の曲げ耐性を向上させている。また、特許文献２では、有機ポリマーを含むファイバーを用いて形成された不織布または織布を含み、かつ、無機フィラーを含む圧電性シートが開示されている。
　非特許文献１では、ニオブ酸ナトリウムカリウム固溶体（以下、ＮＫＮという）粒子が配合されたポリビニルアルコール（以下、ＰＶＡという）樹脂組成物からなるシート層と、繊維化したＰＶＤＦからなる不織布にＮＫＮ粒子が保持されている不織布層と、を交互に積層し一体化した圧電素子が提案されている。この構造は多孔質の不織布層を有するため、特許文献１の構造よりさらに柔軟性が高いと考えられる。
特開２０１５－５０４３２号 ＷＯ２０１５／００５４２０
M. Kato, K. Kakimoto, Materials Letters, 156, 183-186(2015).
　しかしながら、非特許文献１では、圧電素子の両面がＮＫＮ粒子を配合したＰＶＡ樹脂組成物からなるシート層である。発電性能をさらに向上させるには、ＰＶＡ樹脂組成物にＮＫＮ粒子を高充填することでシート層表面の面電荷密度を高め、電荷を取り出し易くする必要がある。また、シート層の厚み、不織布層の厚み、シート層と不織布層の層数に関して発電性能がどうなるかの点で検討がされていない。シート層および不織布層からなる積層体の発電性能については特許文献２においても検討されていない。
　本発明はこのような問題に対処するためになされたものであり、柔軟性を損ねることなく高い発電性能を発現できる圧電素子およびその製造方法の提供を目的とする。
　本発明の圧電素子は、圧電セラミックス粒子が保持または配合された高分子不織布と、圧電セラミックス粒子が配合された高分子樹脂シートとを、上記高分子不織布が少なくとも１層含まれるように積層された積層体からなる圧電素子である。この積層体は、上記高分子不織布１層の２つの主平面側に上記高分子樹脂シートがそれぞれ１層ずつ積層された積層体より発生する発電量以上の発電量を実現できる積層体であることを特徴とする。 
　上記高分子樹脂シートは、５０～８０体積％の圧電セラミックス粒子が配合されている１層の厚さが１０～１００μｍのシートであり、上記高分子不織布は、この高分子不織布を構成する繊維の平均直径が０.０５～５μｍであり、３０～６０体積％の圧電セラミックス粒子が保持または配合されている１層の厚さが１０～３００μｍの不織布であることを特徴とする。
　本発明の圧電素子を構成する積層体は、上記高分子不織布が複数枚積層されているか、または、上記高分子不織布と上記高分子樹脂シートとが交互に積層されていることを特徴とする。特に、上記積層体の２つの主平面側がいずれも上記高分子樹脂シートであることを特徴とする。
　本発明の圧電素子の製造方法は、圧電セラミックス粒子が保持または配合された高分子不織布と、圧電セラミックス粒子が配合された高分子樹脂シートとを、上記高分子不織布が少なくとも１層含まれるように積層させる工程と、上記積層された積層体をプレスを用いて圧着することで一体化する工程とを含む圧電素子の製造方法である。また、上記圧電セラミックス粒子が保持または配合された高分子不織布は、高分子を水または有機溶剤に溶解させた溶液に上記圧電セラミックス粒子を分散することで得たスラリーを電界紡糸する電界紡糸法によって製造された高分子不織布であることを特徴とする。
　本発明の圧電素子は、高分子樹脂シート層と高分子不織布層を積層することで一体化させているので、柔軟性を損ねることなく高い圧電性を発現できる。また、高分子樹脂シートに圧電セラミックス粒子が５０～８０体積％と高充填されているので、圧電素子表面に電荷が誘起され、容易に電荷を取り出すことができる。さらに、高分子不織布層に圧電セラミックス粒子が３０～６０体積％と高充填されているので、柔軟性を損ねることなく高い圧電性を発現できる。
　本発明の圧電素子は、高分子不織布１層の２つの主平面側に高分子樹脂シートがそれぞれ１層ずつ積層された積層体より発生する発電量以上の発電量を実現できる積層体であるので、発電性能をより向上させ維持することができる。
圧電素子の断面図の一例を示す図である。 積層体の分極方法の一例を示す図である。 供試用の圧電素子の図である。 圧電により発生する電力を測定する方法を示す回路図である。 発電量測定結果を示す図である。
　高分子不織布層と高分子樹脂シート層とを積層することで一体化させた圧電素子の発電量について研究した結果、積層体における高分子不織布の積層枚数が増加するに従って発電量が増加すること、さらに積層枚数を増加すると逆に発電量が減少するとの現象が見られた。すなわち、高分子不織布層と高分子樹脂シート層との積層枚数には発電量に関して最適値が存在することが分かった。本発明はこのような知見に基づくものである。
　本発明の圧電素子の断面図の一例を図１に示す。図１は、積層体の表裏２つの主平面側がいずれも高分子樹脂シートとなる場合であり、図１（ａ）は高分子樹脂シートと高分子不織布とを交互に積層した例であり、図１（ｂ）は高分子不織布を複数枚積層した例である。なお、図１は厚さを拡大して表した積層体の模式図であり、圧電セラミックス粒子、不織布等は概念的に表している。
　図１（ａ）は、圧電セラミックス粒子が配合された高分子樹脂シート２と、圧電セラミックス粒子４が不織布５内に保持または配合された高分子不織布３とが交互に積層され、高分子樹脂シート２ａおよび２ｂが積層体１ａの表裏を形成している。高分子樹脂シート２の積層枚数をｎ、高分子不織布３の積層枚数をｍとした場合、積層体１ａにおける枚数の関係式としては、ｎ＝ｍ＋１となる。また、最小構造の積層体はｎが２枚で、ｍが１枚であり、これが最小構造の積層体となる。
　図１（ｂ）は、上記高分子不織布３が複数枚積層され、高分子樹脂シート２ａおよび２ｂが積層体１ｂの表裏を形成している。この場合においても、最小積層体はｎが２枚で、ｍが１枚である。積層体１ｂの枚数の関係式としては、ｎが定数２であり、ｍは高分子不織布３の積層枚数に応じて２、３、４、・・・の値となる。
　積層体１は、図１（ａ）および（ｂ）に示される積層体のみに限定されることなく、高分子不織布３が少なくとも１層含まれるように積層された積層体であればよい。例えば高分子不織布３を複数枚積層して、この積層体と高分子樹脂シート２とを積層することができる。
　積層体１からなる圧電素子の発電量について研究した。圧電素子としては、図１（ａ）に示すように、高分子樹脂シート２と高分子不織布３とが交互に積層され、高分子樹脂シート２ａおよび２ｂが積層体の表裏を形成している積層体１ａ、および図１（ｂ）に示すように、高分子不織布３が複数枚積層され、高分子樹脂シート２ａおよび２ｂが積層体の表裏を形成している積層体１ｂを準備した。
　高分子樹脂シート２は、ＰＶＡ樹脂中に平均粒子径１μｍのＮＫＮ粒子が５０体積％配合され、１枚の厚さが４０μｍのシートを準備した。
　高分子不織布３は、平均粒子径１μｍのＮＫＮ粒子が５０体積％配合されたＰＶＤＦスラリーを用いて電界紡糸法により作製した、１枚の厚さが４０μｍの不織布である。高分子不織布３の繊維の平均直径は０．０５、０．５、５μｍの３水準を準備した。
　積層体１ａは、高分子樹脂シート２の積層枚数をｎ、高分子不織布３の積層枚数をｍとして、ｎ－ｍ構造で表される。圧電素子の発電量測定試料として、２－１構造、３－２構造、４－３構造、５－４構造、６－５構造、および７－６構造の６種類の積層体とし、それぞれの積層体について繊維の平均直径が０．０５、０．５、５μｍと異なる３水準の試料を作製した。準備した試料合計は１８試料である。
　積層体１ｂは、積層体の表裏を形成している高分子樹脂シート２の積層枚数が２枚であるので、高分子不織布３の積層枚数をｍとして、２－ｍ構造で表される。圧電素子の発電量測定試料として、２－１構造、２－３構造、２－５構造、２－７構造、および２－９構造の５種類の積層体とし、それぞれの積層体について繊維の平均直径が０．０５、０．５、５μｍと異なる３水準の試料を作製した。準備した試料合計は１５試料である。
　積層体１ａおよび積層体１ｂをそれぞれ１３ｍｍ×２８ｍｍの大きさに切断し、プレスにて圧力４０ＭＰａ、温度６５℃の条件で３分間加圧してシート状の積層体とした。
　図２は積層体１ａおよび積層体１ｂの分極方法の一例を示した模式図である。アースされた試料台６上に積層体１を置き、積層体１の上面から垂直方向に３ｍｍの距離に設置した針状電極７によって直流電界を印加して生じるコロナ放電によって分極処理をして圧電素子を作製した。処理条件は室温にて、電圧２０ｋＶ、処理時間１０分間とした。
　図３は供試用の圧電素子の図である。層厚さは拡大して表した。図３（ａ）は平面図であり、図３（ｂ）は図１に示す積層体１ａより得られた圧電素子ＡのＡ－Ａ断面図であり、図３（ｃ）は図１に示す積層体１ｂより得られた圧電素子ＢのＡ－Ａ断面図である。分極処理をした積層体１からなる圧電素子ＡおよびＢの両面に銀ペースト８を塗布し、上部および下部電極を形成し、銅箔テープ９を取り付けて供試用の圧電素子とした。
　図４は圧電により発生する電力を測定する方法を示す回路図である。図４に示す回路を用いて、圧電素子ＡおよびＢの長手方向（図３に示す矢印方向）に１７０Ｈｚの伸縮振動を与え、１振動当たりの発電量を測定した。圧電素子ＡおよびＢは負荷抵抗１０に接続され、負荷抵抗１０に発生する電力はオシロスコープ１１によって測定した。
　測定結果を図５に示す。図５（ａ）は積層体１ａより得られた圧電素子Ａ、図５（ｂ）は積層体１ｂより得られた圧電素子Ｂの結果をそれぞれ示す。なお、発電量は最大発電量を１００％として、この最大発電量に対する百分率で表した。圧電素子Ａの最大発電量は繊維の平均直径（繊維径）が０．０５および０．５μｍのときの４－３構造であり、発電量は５２９ｎＷであった。圧電素子Ｂの最大発電量は繊維の平均直径（繊維径）が０．０５および０．５μｍのときの２－５構造であり、発電量は４９５ｎＷであった。
　また、圧電素子Ａは圧電素子Ｂよりも引張試験における引張応力および歪が大きくなった。この結果、圧電素子Ａが好ましい構造である。
　図５に示すように、１枚の高分子不織布３の２つの主平面側に２枚の高分子樹脂シート２がそれぞれ１枚ずつ積層された２－１構造の積層体１ａおよび１ｂが積層体１の最小単位となる。この最小単位より、高分子不織布３の積層枚数が増加するに従い発電量は増加傾向を示す。しかし、この発電量は単調に増加するのではなく、圧電素子Ａの場合は４－３構造、圧電素子Ｂの場合は２－５構造で最大発電量となり、それ以降は高分子不織布３の積層枚数が増加するに従い発電量が減少傾向を示す。すなわち、高分子樹脂シート２と高分子不織布３との積層枚数には発電量に関して最適値が存在する。
　本発明は、この最適値の両側一定範囲を特定するもので、上記最小単位の積層体１より発生する発電量以上の発電量を実現できる積層体とする。具体的には、圧電素子Ａの場合は、２－１構造、３－２構造、４－３構造、５－４構造、６－５構造、および７－６構造であり、好ましくは２－１構造、３－２構造、４－３構造、５－４構造、および６－５構造であり、より好ましくは３－２構造、４－３構造、および５－４構造である。また、圧電素子Ｂの場合は、２－１構造、２－３構造、２－５構造、２－７構造、および２－９構造であり、好ましくは２－１構造、２－３構造、２－５構造、および２－７構造である。
　高分子樹脂シートに配合される圧電セラミックス粒子、または高分子不織布内に保持または配合される圧電セラミックス粒子は、同一種類の圧電セラミックス粒子であっても、異なる種類の圧電セラミックス粒子であってもよい。同様に高分子樹脂シート同士、または高分子不織布同士においても、圧電セラミックス粒子は、同一種類の圧電セラミックス粒子であっても、異なる種類の圧電セラミックス粒子であってもよい。圧電素子を構成する積層体全体が同一組成を有する圧電セラミックス粒子を用いることが好ましい。
　圧電セラミックス粒子は、ペロブスカイト型結晶構造を有する圧電セラミックス粒子であることが好ましい。例えば、ニオブ、鉛、チタン、亜鉛、バリウム、ビスマス、ジルコニウム、ランタン、カリウム、ナトリウム、カルシウム、マグネシウムの元素のうち、１種以上を含む圧電セラミックス粒子が挙げられる。これらの中で、鉛を含有しないＮＫＮ粒子、またはチタン酸バリウム粒子が人体および環境への安全に優れる点からより好ましい。ＮＫＮ粒子は、（Ｎａ0.5Ｋ0.5）ＮｂＯ3で代表されるセラミックス粒子である。ＮＫＮ粒子は炭酸ナトリウムと、炭酸カリウムと、酸化ニオブの固体反応によって製造することができる。
　圧電セラミックス粒子の平均粒子径は０．１μｍ～１０μｍ、好ましくは０．５μｍ～５μｍ、より好ましくは１μｍ～２μｍである。０．１μｍ未満では、高分子樹脂シートまたは高分子樹脂不織布への均一分散が困難であり、１０μｍを超えると高分子樹脂シートまたは高分子不織布の機械的強度が低下する。なお、本発明における平均粒子径は、レーザー回析法により測定・算出された５０％粒子径（Ｄ５０）である。
　圧電セラミックス粒子を高分子樹脂シートに配合する場合、圧電セラミックス粒子に高分子バインダーを用いて結合させた造粒粉とすることが好ましい。高分子バインダーは、高分子樹脂シートを構成する高分子材料とは異なる材料であることが好ましい。高分子バインダーとして具体的には、アクリル系、セルロース系、ＰＶＡ系、ポリビニルアセタール系、ウレタン系、酢酸ビニル系高分子などが挙げられる。造粒粉を用いることにより、圧電セラミックス粒子の高充填化が可能となる。造粒の方法は特に限定されるものではなく、噴霧造粒、転動造粒、押出し造粒、圧縮造粒等、公知の方法を用いることができる。造粒粉の平均粒子径は１０μｍ～１００μｍ、好ましくは３０μｍ～５０μｍである。
　高分子樹脂シートを構成する高分子材料は、その種類が特に限定されるものではなく、熱可塑性樹脂、熱硬化性樹脂、熱可塑性エラストマー、合成ゴム、天然ゴムのいずれであってもよい。圧電素子の耐熱性を上げるためには、融点が１５０℃以上の結晶性樹脂、またはガラス転移点が１５０℃以上の非晶性樹脂がより好ましい。具体的には、ＰＶＡ、ポリビニルブチラール（以下、ＰＶＢという）、ポリスチレン、ポリイミド、ポリアミドイミド、ポリエーテルイミド、ポリサルホン、ポリフェニルサルホン、ポリエーテルサルホン、ポリアリレート、ポリフェニレンエーテル等の高分子材料が挙げられる。
　上記高分子材料に上記圧電セラミックス粒子が配合される。高分子樹脂シートは上記圧電セラミックス粒子と共に圧電性を有しない無機充填剤を配合することが好ましい。無機充填剤を配合する場合、シート層内での電荷移動を容易にする目的で導電性フィラーを配合することが好ましい。導電性フィラーとしては、黒鉛、カーボンブラック、カーボンナノチューブ、フラーレン、金属粉末、炭素繊維、金属繊維等が挙げられる。また、無機充填剤として、シート層の機械的強化を向上させるために補強材を配合できる。補強材としてはカーボンナノチューブ、ウィスカー、炭素繊維、ガラス繊維等が挙げられる。
　高分子樹脂シートは、圧電セラミックス粒子を５０～８０体積％含有し、残部が上記高分子材料とするか、または残部が高分子材料および圧電性を有しない上記無機充填剤であることが好ましく、圧電セラミックス粒子の配合量は７０～８０体積％であることがより好ましい。高分子樹脂シートに圧電セラミックス粒子が高充填されていると高分子樹脂シート層の表面に電荷が誘起され易くなる。また、高分子樹脂シートにおいて、上記高分子材料は少なくとも２０体積％配合されていることが好ましい。圧電セラミックス粒子が５０体積％未満では圧電性が向上せず、８０体積％を超えると高分子樹脂シートの機械的強度が低下する。なお、配合割合の算出において、圧電セラミックス粒子は、上記造粒粉とする前の粒子をいう。
　高分子樹脂シートの製造方法は、薄いシート状とできる方法であれば使用できる。本発明においては、上記高分子材料を溶解させた水または有機溶剤中に、上記圧電セラミックス粒子等の充填材を分散させてスラリーを製造し、このスラリーを支持体上に塗布して薄膜とし、水または有機溶剤を乾燥等により除去する製造方法が好ましい。スラリーを支持体上に塗布する方法はドクターブレード法に代表されるテープキャスト法、スピンコート法等、公知の方法を用いることができる。
　高分子樹脂シート１枚の厚さは、１０～１００μｍ、好ましくは３０～５０μｍである。高分子樹脂シート層の厚みが１０μｍ未満では圧電素子としたときの機械的強度が低下し、１００μｍを超えると柔軟性が低下して圧電素子に振動を与えた際にクラックを生じる場合がある。
　高分子不織布は、繊維化した高分子材料を熱・機械的または化学的な作用によって接着または絡み合わせることで布にしたものであれば使用できる。高分子不織布を構成する繊維の平均直径は０.０５～５μｍであることが好ましく、０.５～１μｍがより好ましい。平均直径が５μｍより大きいと、不織布層のポーラスの体積が減少するため発電性能が低下する。また、平均直径が０.０５μｍ未満では、繊維が圧電セラミックス粒子に与える応力が小さくなり、発電性能が低下する。なお、本発明における繊維の平均直径は、走査型電子顕微鏡で得られた画像により測定・算出された平均値である。
　高分子不織布となる高分子材料は、その種類が特に限定されるものではなく、また、分子構造に起因する圧電性の有無は問わない。耐熱性の点から、融点が１５０℃以上の結晶性樹脂、またはガラス転移点が１５０℃以上の非晶性樹脂が好ましく、柔軟性に優れるものがより好ましい。具体的には、ＰＶＡ、ＰＶＢ、ＰＶＤＦ、四フッ化エチレン－エチレン共重合体、四フッ化エチレン－六フッ化プロピレン共重合体、四フッ化エチレン－パーフルオロアルコキシエチレン共重合体等が挙げられる。
　高分子不織布に上記圧電セラミックス粒子が保持または配合される。高分子不織布は上記圧電セラミックス粒子と共に圧電性を有しない無機充填剤を保持または配合することが好ましい。無機充填剤としては、不織布層内での電荷移動を容易にする目的で導電性フィラーを保持または配合することが好ましい。導電性フィラーとしては、黒鉛、カーボンブラック、カーボンナノチューブ、フラーレン、金属粉末等が挙げられる。また、無機充填剤として、不織布層の機械的強化を向上させるために補強材を保持または配合できる。補強材としてはカーボンナノチューブ、ウィスカー等が挙げられる。ここで保持されるとは、高分子不織布の繊維間に圧電セラミックス粒子が固定されることであり、配合するとは、繊維化した高分子材料の内部に圧電セラミックス粒子が含有されていることをいう。
　高分子不織布は、圧電セラミックス粒子を３０～６０体積％保持または配合し、残部が上記繊維化した高分子材料とするか、または残部が上記繊維化した高分子材料および圧電性を有しない上記無機充填剤であることが好ましく、圧電セラミックス粒子の保持または配合量は５０～６０体積％であることがより好ましい。また、繊維化した高分子材料は少なくとも４０体積％配合されていることが好ましい。圧電セラミックス粒子が３０体積％未満では圧電性が向上せず、６０体積％を超えると高分子不織布の機械的強度が低下する。
　高分子不織布の製造方法は、平均直径が０.０５～５μｍの繊維を用いて薄い不織布とできる方法であれば使用できる。本発明においては、高分子材料を水または有機溶剤に溶解させた溶液に上記圧電セラミックス粒子を分散することで得たスラリーを用いて、電界紡糸法により製造することが好ましい。電界紡糸法は、電界紡糸装置のシリンジのニードルと、コレクターとの間に電圧を印加し、シリンジ内のスラリーをコレクターに向かって射出することで不織布を作製する方法である。コレクターの形状はドラム型、ディスク型、プレート型等、特に限定されるものではないが、大面積の不織布を作製できるドラム型が好ましい。得られた不織布は乾燥させることで水または有機溶剤を除去できる。
　高分子不織布１枚の厚さは、１０～３００μｍ、好ましくは１２０～２００μｍである。高分子不織布の厚みが１０μｍ未満では圧電素子としたときの圧電性が低下し、３００μｍを超えると圧電素子に振動を与えた際に高分子不織布の内部で破断が生じる場合がある。
　本発明の圧電素子は、上記高分子樹脂シートと高分子不織布との積層体を一体化させることで、シート状の圧電素子が得られる。一体化はプレスを用いて圧着する方法等が挙げられる。
　また、本発明の圧電素子の分極方法は、上記一体化された圧電素子に直流電界を印加する工程を含むことが好ましい。具体的な分極方法として、大気中にてコロナ放電を用いる方法、１００～２００℃に加熱したシリコーンオイル中にて直流電界を印加する方法等が挙げられる。
　本発明の圧電素子は、高分子樹脂シート層と不織布層を一体化させており、高分子樹脂シート層に圧電セラミックス粒子を高充填することで、圧電素子表面に電荷が誘起され易く、容易に電荷を取り出すことができる。また、不織布層に圧電セラミックス粒子を高充填することで、柔軟性を損ねることなく高い圧電性を発現できる。さらに、シート層の厚み、不織布層の厚み、シート層と不織布層の層数を最適化することで発電性能を向上させることができる。このため、本発明の圧電素子は振動発電、電流センサー、電圧センサーの用途に適用でき、特に環境振動を利用した振動発電に好適である。
実施例１～１４、および比較例１～７
　圧電セラミックスとして用いたＮＫＮ粒子は、Ｎａ2ＣＯ3（純度９９.９％）、Ｋ2ＣＯ3（純度９９.９％）、Ｎｂ2Ｏ5（純度９９.９％）を原料粉末とし、この原料粉末を十分に混合し、混合物を１０９８℃で２時間焼結した後、解砕することで平均粒子径１μｍの粉末を作製した。この粉末を高分子バインダーであるポリウレタン溶液に分散させて、スプレードライヤー法により造粒粉を作製した。
　高分子樹脂シートは、ＰＶＡを７質量％溶解させた水溶液に、この造粒粉を分散させることで調整したスラリーを、支持体上にテープキャストすることで作製した。テープキャストにはドクターブレード型塗工機（株式会社井元製作所製：ＩＭＣ－７０Ｆ０－Ｃ型）を用いた。得られたシートを室温で乾燥させることで水を除去し、高分子樹脂シートとした。
　高分子不織布はＰＶＤＦを溶解させたジメチルスルホキシド溶液に上記ＮＫＮ粒子を分散させたスラリーを電界紡糸することで作製した。電界紡糸装置は井元製作所製：ＩＭＣ－１６３９型を用いた。ＰＶＤＦを溶解させたジメチルスルホキシド溶液の濃度を０.１１ｇ／ｍＬとし、ＰＶＤＦに対して５０体積％のＮＫＮ粒子を分散させたスラリーを用い、シリンジのニードルと、コレクターとの間に１８ｋＶの電圧を印加することで、シリンジ内のスラリーをコレクターに向かって射出し、不織布を作製した。得られた不織布は室温で乾燥させることでジメチルスルホキシドを除去し、高分子不織布とした。
　高分子樹脂シート、高分子不織布をそれぞれ１３ｍｍ×２８ｍｍの大きさに切断し、交互に積層させた後、または高分子不織布を複数枚積層させた後、プレスにて圧力４０ＭＰａ、温度６５℃の条件で３分間加圧することで積層体を得た。積層体の構造および厚さ、高分子樹脂シートおよび高分子不織布に保持または配合されるＮＫＮ量、高分子樹脂シートおよび高分子不織布の厚さ、高分子不織布を構成する繊維の平均直径について表１および表２に示す。
　得られた積層体の裏表両面に、図３に示すように、銀ペースト８を塗布し、上部および下部電極を形成し、銅箔テープ９を取り付けて圧電素子を得た。この圧電素子を用いて、図４に示す回路を用いて、圧電素子の長手方向（図３に示す矢印方向）に１７０Ｈｚの伸縮振動を与え、１振動当たりの発電量を測定した。結果を表１および表２に示す。
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
　実施例６に示すように、積層体として４－３構造の圧電素子が最大の発電量となる結果であった。また、高分子樹脂シート層中のＮＫＮ粒子配合量は多いほど発電量が大きい値であったが、比較例３（高分子樹脂シート１層の厚み４０μｍ、ＮＫＮ粒子配合量９０体積％）、比較例４（高分子樹脂シート１層の厚み５μｍ、ＮＫＮ粒子配合量７０体積％）ではシート層の破断が生じたため発電量の測定ができなかった。また、発電量は、高分子不織布層の繊維の平均直径が０.０５～５μｍの範囲で良好な値を示した。表２に高分子不織布層の厚みによる発電量を示した。実施例１３（高分子不織布１層の厚み２００μｍ）が最も優れる結果であった。
　本発明は、環境振動を利用した振動発電の分野で使用できる。
　１　積層体
　２　高分子樹脂シート
　３　高分子不織布
　４　圧電セラミックス粒子
　５　不織布
　６　試料台
　７　針状電極
　８　銀ペースト
　９　銅箔テープ
　１０　負荷抵抗
　１１　オシロスコープ

Claims (7)

  1. 　圧電セラミックス粒子が保持または配合された高分子不織布と、圧電セラミックス粒子が配合された高分子樹脂シートとを、前記高分子不織布が少なくとも１層含まれるように積層された積層体からなる圧電素子であって、
    　前記積層体は、前記高分子不織布１層の２つの主平面側に前記高分子樹脂シートがそれぞれ１層ずつ積層された積層体より発生する発電量以上の発電量を実現できる積層体であることを特徴とする圧電素子。
  2. 　前記高分子不織布は、この高分子不織布を構成する繊維の平均直径が０.０５～５μｍであり、３０～６０体積％の圧電セラミックス粒子が保持または配合されている１層の厚さが１０～３００μｍの不織布であり、
    　前記高分子樹脂シートは、５０～８０体積％の圧電セラミックス粒子が配合されている１層の厚さが１０～１００μｍのシートであることを特徴とする請求項１記載の圧電素子。
  3. 　前記積層体は、前記高分子不織布が複数枚積層されていることを特徴とする請求項１記載の圧電素子。
  4. 　前記積層体の２つの主平面側がいずれも前記高分子樹脂シートであることを特徴とする請求項３記載の圧電素子。
  5. 　前記積層体は、前記高分子不織布と前記高分子樹脂シートとが交互に積層されていることを特徴とする請求項１記載の圧電素子。
  6. 　前記積層体の２つの主平面側がいずれも前記高分子樹脂シートであることを特徴とする請求項５記載の圧電素子。
  7. 　圧電セラミックス粒子が保持または配合された高分子不織布と、圧電セラミックス粒子が配合された高分子樹脂シートとを、前記高分子不織布が少なくとも１層含まれるように積層させる工程と、
    　前記積層された積層体をプレスを用いて圧着することで一体化する工程とを含む圧電素子の製造方法であって、
    　前記圧電セラミックス粒子が保持または配合された高分子不織布は、高分子を水または有機溶剤に溶解させた溶液に前記圧電セラミックス粒子を分散することで得たスラリーを電界紡糸する電界紡糸法によって製造された高分子不織布であることを特徴とする請求項１記載の圧電素子の製造方法。
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