WO2017010135A1

WO2017010135A1 - Piezoelectric sensor

Info

Publication number: WO2017010135A1
Application number: PCT/JP2016/062250
Authority: WO
Inventors: 高橋　渉; 吉川　均
Original assignee: 住友理工株式会社
Priority date: 2015-07-16
Filing date: 2016-04-18
Publication date: 2017-01-19
Also published as: JP2017028323A; JP6696885B2; DE112016000917B4; CN107924986B; DE112016000917T5; CN107924986A

Abstract

This piezoelectric sensor (1) is provided with a piezoelectric element (10) which includes a piezoelectric layer (11) containing elastomer and piezoelectric particles and which includes electrode layers (12a, 12b) containing elastomer and a conductive material. The breaking elongation of the piezoelectric element (10) is equal to or greater than 10%, and the volume resistivity of the electrode layers (12a, 12b) in the natural state and in the elongated state of being elongated in an axial direction by up to 10% from the natural state is not greater than 100 Ω·cm. The piezoelectric element (10) is capable of being elongated/retracted, and has the piezoelectric performance even in the elongated state.

Description

Piezoelectric sensor

The present invention relates to a piezoelectric sensor including an extendable piezoelectric element.

Piezoelectric materials that can convert mechanical energy into electrical energy are widely used for pressure sensors, acceleration sensors, vibration sensors, impact sensors, and the like. Known piezoelectric materials include ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) and polylactic acid, and composites in which polymer particles are filled in a polymer matrix. . For example, Patent Document 1 describes a piezoelectric element in which an electrode made of conductive rubber and a piezoelectric crystal thin film such as PZT are formed on a substrate having stretch elasticity. Patent Document 2 describes a piezoelectric element having a piezoelectric layer made of a fluorinated polymer, an electrode made of a conductive polymer, and a textile substrate. Patent Document 3 describes a piezoelectric element having a composite in which a matrix having a resin and rubber is filled with piezoelectric particles, and an electrode made of conductive rubber. Patent Document 4 describes a piezoelectric element having a piezoelectric sheet in which piezoelectric particles are filled in a resin matrix such as chlorinated polyethylene, and a flexible electrode in which carbon is filled in chlorinated polyethylene. Patent Document 5 describes a piezoelectric element having a composite in which chloroprene rubber is filled with lead titanate powder and an electrode made of silver paste. Patent Document 6 describes a fluctuating load detection sheet having a PVDF piezoelectric film, a pair of electrodes disposed on both sides thereof, and a strain amplification member provided on the electrodes.

JP 2005-347364 A JP-T-2014-529913 JP 2013-225608 A JP 2002-111087 A JP-A-2-32574 JP 2006-153842 A

A piezoelectric element using ceramics such as PZT as a piezoelectric layer as described in Patent Document 1 has a hard piezoelectric layer and poor stretchability. For this reason, when the piezoelectric element is applied to an adherend that expands and contracts, the movement of the adherend tends to be hindered. In addition, the piezoelectric elements described in Patent Documents 2 and 6 use a resin for the piezoelectric layer. For this reason, the piezoelectric layer has flexibility, but lacks stretchability. Even if the piezoelectric layer can be extended, it is difficult to restore the original shape. Therefore, it is difficult to apply the piezoelectric element to an adherend that undergoes expansion and deformation. On the other hand, the piezoelectric elements described in Patent Documents 3 to 5 use a composite of a polymer matrix and piezoelectric particles in the piezoelectric layer. However, when a polymer is contained in the polymer matrix, it has flexibility but poor stretchability. In this regard, in the piezoelectric layer described in Patent Document 5, chloroprene rubber is used for the polymer matrix. For this reason, the piezoelectric layer has stretchability, but the electrode laminated thereon is made of a silver paste with poor stretchability. In this case, the expansion and contraction of the piezoelectric layer is restricted by the electrode, and the stretchability of the entire piezoelectric element is reduced. In addition, since the electrical resistance increases when the electrode is extended, the output is reduced at the time of extension, and the load applied to the piezoelectric layer cannot be accurately detected. This problem is common to the piezoelectric elements described in other patent documents. For example, Patent Document 3 describes the use of conductive rubber for the electrodes. However, Patent Document 3 does not discuss the expansion / contraction performance of the electrode and the behavior of the electrical resistance during expansion. Further, paragraph [0020] of Patent Document 3 describes that the distortion amount of the vibration source is about 5%, and in the examples, an application example in which the distortion amount is 3% is described. In patent document 3, the piezoelectric element is not assumed to be deformed at a relatively large elongation rate of 10% or more.

Thus, conventionally, since it is not assumed to be applied to an adherend that greatly deforms and stretches, the stretchability of the entire piezoelectric element including not only the piezoelectric layer but also the electrodes has not been studied. Therefore, a piezoelectric element that can maintain the piezoelectric performance even in the extended state has not been realized.

This invention is made in view of such a situation, and makes it a subject to provide a piezoelectric sensor provided with the piezoelectric element which can be extended-contracted and can be used even in the extended state.

The piezoelectric sensor of the present invention includes a piezoelectric element having a piezoelectric layer including an elastomer and piezoelectric particles and an electrode layer including an elastomer and a conductive material, and the elongation at break of the piezoelectric element is 10% or more. Is characterized in that the volume resistivity of the stretched state from the natural state to the stretched state by 10% in the uniaxial direction is 100 Ω · cm or less.

The matrix (base material) of the piezoelectric layer and electrode layer constituting the piezoelectric element are both elastomers. The elongation at break of the piezoelectric element is 10% or more. Since the piezoelectric element is flexible and stretchable, even if the piezoelectric element is arranged on an adherend that repeatedly stretches or bends or an adherend that greatly expands and contracts, the movement of the adherend is hardly hindered. Further, even when the adherend has a complicated shape, the piezoelectric element can be arranged along the shape.

The electrode layer has a volume resistivity of 100 Ω · cm or less in a natural state and a stretched state from that state to a state where the electrode layer is stretched by 10% in a uniaxial direction. The natural state means a state in which no load is applied and the body is not deformed. The state of extending 10% in the uniaxial direction means a state in which the length in the uniaxial direction is 1.1 times the natural state. The electrode layer not only has high conductivity in a natural state, but also has a high electrical conductivity with a small increase in electrical resistance even in an extended state extended up to 10% in a uniaxial direction. For this reason, even in the extended state, the output is unlikely to decrease, and the load applied to the piezoelectric layer can be accurately detected. In the present invention, the volume resistivity of the electrode is measured in both a natural state and a state in which the electrode is stretched by 10% in the uniaxial direction. If any volume resistivity is 100 Ω · cm or less, the “natural state and then the uniaxial direction” It is determined that the condition that the volume resistivity of the stretched state until reaching the stretched state by 10% is 100 Ω · cm or less ”is satisfied. In the piezoelectric sensor of the present invention, the piezoelectric element can extend not only in a uniaxial direction but also in a biaxial direction, a diameter expansion direction, and the like.

As described above, according to the piezoelectric sensor of the present invention, the adherend is disposed on the adherend accompanied by deformation such as bending, stretching, and compression, and the adherend is not only deformed but also deformed. The load applied to can be detected. That is, even when the secondary deformation is further performed in the primary deformation state of the adherend, the load applied to the adherend can be detected. In addition, since the piezoelectric sensor of the present invention has a higher sensitivity (S / N ratio (Signal-Noise Ratio)) than a capacitive sensor, it is easy to detect a small load. For example, the piezoelectric element of the piezoelectric sensor of the present invention can be placed directly on the human skin or indirectly through clothes to measure the pulse rate and respiratory rate.

It is a top view of one embodiment of a piezoelectric sensor of the present invention. FIG. 2 is a sectional view taken along the line II-II in FIG. It is a graph of the electromotive voltage in the state which expanded the piezoelectric element of Example 2 1%. It is a graph of the electromotive voltage in the state which expanded the piezoelectric element of Example 2 10%. It is a schematic diagram which shows a dispersed state in case a piezoelectric particle consists of a single particle. It is a schematic diagram which shows a dispersion | distribution state in case a piezoelectric particle consists of aggregates. It is a SEM photograph of powder (single particle) of barium titanate before baking. It is a SEM photograph of powder b (conjugate) of barium titanate after calcination and grinding. It is an up-down direction sectional view of a piezoelectric element manufactured in an example. It is a graph which shows the relationship between the volume ratio of a barium titanate particle, and a generated electric field.

DESCRIPTION OF SYMBOLS 1: Piezoelectric sensor, 10: Piezoelectric element, 11: Piezoelectric layer, 12a, 12b: Electrode layer, 13a, 13b: Protective layer, 20a, 20b: Wiring, 30: Control circuit part.
40: Piezoelectric element, 41: Piezoelectric layer, 42a, 42b: Electrode layer, 43a, 43b: Protective layer.
80: Piezoelectric particles, 81: Elastomer, 82: Combined piezoelectric particles.

Hereinafter, embodiments of the piezoelectric sensor of the present invention will be described. In addition, the piezoelectric sensor of the present invention is not limited to the following forms, and may be implemented in various forms that have been modified or improved by those skilled in the art without departing from the spirit of the present invention. Can do.

The piezoelectric sensor of the present invention includes a piezoelectric element having a piezoelectric layer including an elastomer and piezoelectric particles and an electrode layer including an elastomer and a conductive material.

<Piezoelectric layer>
As the elastomer constituting the piezoelectric layer, one or more selected from crosslinked rubber and thermoplastic elastomer may be used. As elastic elastomers with relatively small elastic modulus, urethane rubber, silicone rubber, nitrile rubber (NBR), hydrogenated nitrile rubber (H-NBR), acrylic rubber, natural rubber, isoprene rubber, ethylene-propylene-diene rubber (EPDM) Ethylene-vinyl acetate copolymer, ethylene-vinyl acetate-acrylic ester copolymer, butyl rubber, styrene-butadiene rubber, fluororubber, epichlorohydrin rubber, chloroprene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, and the like. Further, an elastomer modified by introducing a functional group or the like may be used. Examples of the modified elastomer include carboxyl group-modified nitrile rubber (X-NBR), carboxyl group-modified hydrogenated nitrile rubber (XH-NBR), and the like.

The electromotive force (V / m) generated when a load is applied to the piezoelectric layer depends on the piezoelectric strain constant (C / N), dielectric constant (F / m), and applied load (N / m ² ) of the piezoelectric layer. Is represented by the following equation (a).
Electromotive field = piezoelectric strain constant / dielectric constant × load (a)
In terms of increasing the electromotive field, it is desirable that the dielectric constant of the piezoelectric layer be small. In this case, it is desirable to employ an elastomer having a relatively low relative dielectric constant. For example, urethane rubber, silicone rubber, NBR, H-NBR, etc. are suitable as the elastomer having a relative dielectric constant of 15 or less (measurement frequency: 100 Hz).

Piezoelectric particles are particles of a compound having piezoelectricity. Ferroelectric materials having a perovskite crystal structure are known as piezoelectric compounds, for example, barium titanate, strontium titanate, potassium niobate, sodium niobate, lithium niobate, potassium sodium niobate , Lead zirconate titanate (PZT), barium strontium titanate (BST), bismuth lanthanum titanate (BLT), bismuth strontium tantalate (SBT), and the like. As the piezoelectric particles, one or more of these may be used.

The particle size of the piezoelectric particles is not particularly limited. For example, when a plurality of types of piezoelectric particle powders having different average particle sizes are used, a large particle size piezoelectric particle and a small particle size piezoelectric particle can be mixed in the elastomer. In this case, piezoelectric particles having a small particle diameter enter between piezoelectric particles having a large particle diameter, and pressure is easily transmitted to the piezoelectric particles. Thereby, the piezoelectric strain constant of the piezoelectric layer is increased, and the electromotive voltage can be increased.

The piezoelectric particles may be single particles or an aggregate of a plurality of particles. In the case of including an aggregate composed of a plurality of piezoelectric particles, it becomes easy to balance the flexibility and the piezoelectricity. For example, when a large amount of piezoelectric particles is blended in the elastomer, the piezoelectricity is improved, but the flexibility is lowered because the volume ratio of the elastomer is reduced. On the other hand, when the amount of the piezoelectric particles is small, the volume ratio of the elastomer increases, so that flexibility is improved, but piezoelectricity is lowered. According to the study of the present inventors, it was confirmed that the change in electromotive force is reduced even when the expansion and contraction is repeated, that is, the expansion and contraction durability is improved by increasing the flexibility of the piezoelectric layer, specifically, the elongation at break. ing. For this reason, it is desirable to ensure the desired piezoelectricity by reducing the blending amount of the piezoelectric particles as much as possible.

In order to obtain high piezoelectricity, the connection between piezoelectric particles is important. FIG. 5 schematically shows a dispersion state when the piezoelectric particles are made of single particles. FIG. 6 schematically shows a dispersion state in the case where the piezoelectric particles are made of an aggregate. As shown in FIG. 5, the piezoelectric particles 80 are filled in an elastomer 81. Each piezoelectric particle 80 has a substantially spherical shape. For this reason, usually, the connection between the piezoelectric particles 80 is ensured by blending a large amount of the piezoelectric particles 80 and bringing them close to the close-packed structure. On the other hand, as shown in FIG. 6, when a massive aggregate 82 in which a plurality of piezoelectric particles 80 are aggregated is blended, the shape becomes a steric hindrance, and the piezoelectric particles 80 can be connected to each other even if a dense filling structure is not obtained. Connection can be established. That is, desired piezoelectricity can be ensured even if the volume ratio of the piezoelectric particles 80 is small. Thereby, it becomes easy to satisfy all of piezoelectricity, flexibility and stretch durability. For example, the piezoelectric sensor includes a piezoelectric element having a piezoelectric layer including an elastomer and piezoelectric particles, and an electrode layer including an elastomer and a conductive material, and the piezoelectric particles include an aggregate in which a plurality of piezoelectric particles are aggregated. It is good to. According to this configuration, a flexible and highly sensitive piezoelectric sensor can be realized.

Examples of aggregates in which a plurality of piezoelectric particles are aggregated include aggregates in which individual particles are aggregated by an electrostatic force or the like, and aggregates in which individual particles are chemically bonded. The latter combination is preferred from the viewpoint that individual particles are difficult to separate and that a connection structure of piezoelectric particles can be easily constructed. Although the manufacturing method of a conjugate | bonded_body is not specifically limited, For example, after baking the powder which consists of a single particle, it can grind | pulverize and can manufacture. The difference between the aggregate and the conjugate can be analyzed as follows. First, the piezoelectric layer is heated to remove the elastomer component. Next, the remaining piezoelectric particles are dispersed in a good solvent and subjected to ultrasonic treatment. As a result, it is determined that the particles are aggregates when separated into individual particles, and are combined when not separated. Here, the good solvent refers to a polar solvent that hardly precipitates when the piezoelectric particles are dispersed. Specifically, any solvent that has an SP value (solubility parameter) of 8 or more and 13 or less and can dissolve the elastomer may be used. An example is 2-methoxyethanol.

An aggregate of a plurality of piezoelectric particles can be defined as a particle having a diameter larger than twice the average particle diameter of each piezoelectric particle. Here, as the diameter (d2) of the aggregate, a median diameter measured by a laser diffraction / scattering particle size distribution measuring apparatus is employed. As the average particle diameter (d1) of the piezoelectric particles, a scanning electron microscope (SEM) photograph of the aggregate is taken, and the average value of the maximum diameters of 100 or more piezoelectric particles arbitrarily selected so as not to be biased is adopted. To do. And what satisfies 2d1 <d2 is an aggregate.

The elastomer and the piezoelectric particles may be chemically bonded by surface-treating the piezoelectric particles. As a method of surface-treating the piezoelectric particles, a surface treatment agent having a functional group capable of reacting with an elastomer polymer is reacted with the piezoelectric particles in advance, and the piezoelectric particles are mixed with the elastomer polymer. Examples include a method in which a hydroxyl group is generated by dissolving with an acid, an alkali, or subcritical water, and then mixed with an elastomer polymer having a functional group capable of reacting with the hydroxyl group. When the piezoelectric particles are chemically bonded to the elastomer, the piezoelectric particles are unlikely to be displaced even when the expansion and contraction is repeated. In addition, since the piezoelectric particles are difficult to peel from the elastomer, fluctuations from the initial values of physical properties and output are reduced. Therefore, the output is stabilized and the sag resistance of the piezoelectric layer is improved. In addition, since the elongation at break of the piezoelectric layer is increased, it is possible to suppress a decrease in piezoelectric performance due to local fracture during elongation. As a result, high piezoelectric performance can be maintained even in the extended state.

The blending amount of the piezoelectric particles may be determined by taking into account the flexibility of the piezoelectric layer, and thus the piezoelectric element, and the piezoelectric performance of the piezoelectric layer. When the amount of the piezoelectric particles is increased, the piezoelectric performance of the piezoelectric layer is improved, but the flexibility is lowered. Therefore, it is desirable to adjust the blending amount of the piezoelectric particles so that desired flexibility can be realized in the combination of the elastomer and the piezoelectric particles to be used.

The piezoelectric layer may contain reinforcing particles having a relative dielectric constant smaller than that of the piezoelectric particles, in addition to the elastomer and the piezoelectric particles. The relative permittivity of the reinforcing particles is preferably 100 or less, and more preferably 30 or less, on condition that the relative permittivity of the reinforcement particles is smaller than that of the piezoelectric particles.

Since the structure in which piezoelectric particles having a large relative dielectric constant are connected is easy to transmit external force to the piezoelectric particles, an improvement in the piezoelectric strain constant of the above-described formula (a) can be expected. However, when the piezoelectric particles having a large relative dielectric constant are connected, the dielectric constant of the entire piezoelectric layer is increased. On the other hand, when both the piezoelectric particles and the reinforcing particles are included in the piezoelectric layer, the connection between the piezoelectric particles having a large relative dielectric constant is divided by the intervening reinforcing particles having a smaller relative dielectric constant. Thereby, the raise of the dielectric constant as the whole piezoelectric layer can be suppressed. On the other hand, since the connection structure of the particles is maintained by the reinforcing particles and the piezoelectric particles, the piezoelectric strain constant can be maintained. That is, when the reinforcing particles are included in the piezoelectric layer, the dielectric constant of the entire piezoelectric layer can be made smaller than when only the piezoelectric particles are included while maintaining the piezoelectric strain constant. Therefore, a large electromotive field can be obtained by the above-described formula (a).

As the reinforcing particles, particles having a large electric resistance are desirable. When the electrical resistance of the reinforcing particles is large, the dielectric breakdown strength of the piezoelectric layer is increased. Thereby, in the polarization process of the piezoelectric layer which will be described later, the processing time can be shortened by applying a high electric field. In addition, since the number of piezoelectric elements that are destroyed during the polarization process can be reduced, productivity is improved.

Also, it is desirable that the reinforcing particles are chemically bonded to the elastomer. In this case, since a network of reinforcing particles is formed in the elastomer, impurity ions obtained by ionizing a crosslinking agent, an additive, moisture in the air, and the like are difficult to move, and the electric resistance of the piezoelectric layer is increased. The chemical bond between the reinforcing particles and the elastomer can be realized, for example, by surface-treating the reinforcing particles. As a surface treatment method, a surface treatment agent having a functional group capable of reacting with an elastomer polymer is reacted with the reinforcing particles in advance, and the reinforcing particles are mixed with the elastomer polymer. Or the method of mixing with the elastomer polymer which has a functional group which can react with a hydroxyl group after melt | dissolving with subcritical water and producing | generating a hydroxyl group etc. is mentioned. When the reinforcing particles are chemically bonded to the elastomer, the reinforcing particles are unlikely to be displaced even if the expansion and contraction are repeated. Further, since the reinforcing particles are difficult to peel off from the elastomer, fluctuations from the initial values of physical properties and output are reduced. Therefore, the output is stabilized and the sag resistance of the piezoelectric layer is improved. In addition, since the elongation at break of the piezoelectric layer is increased, it is possible to suppress a decrease in piezoelectric performance due to local fracture during elongation. As a result, high piezoelectric performance can be maintained even in the extended state.

The type of reinforcing particles is not particularly limited. For example, particles such as oxides such as titanium dioxide, silica, and barium titanate, rubber, and resin can be used. However, when relatively soft particles such as rubber particles are included, the applied load may be attenuated by the resin particles and may not be transmitted to the piezoelectric particles. From the viewpoint of facilitating transmission of force to the piezoelectric particles, increasing the piezoelectric strain constant of the piezoelectric layer in the above-described formula (a), and increasing the electromotive force, the reinforcing particles have an elastic modulus higher than that of the matrix elastomer. It is better to use large particles. For example, metal oxide particles such as titanium dioxide are preferable because they have a small relative dielectric constant and a large effect of improving dielectric breakdown resistance. As a method for producing metal oxide particles, a sol-gel method is preferable because particles having low crystallinity and a low relative dielectric constant can be obtained.

The piezoelectric layer is manufactured by curing a composition obtained by adding a powder of a piezoelectric particle or a crosslinking agent to an elastomer polymer under predetermined conditions. Thereafter, the piezoelectric layer is subjected to polarization treatment. That is, a voltage is applied to the piezoelectric layer to align the polarization direction of the piezoelectric particles in a predetermined direction.

As a result of investigation by the present inventors, it was confirmed that in a thin film piezoelectric element, the smaller the cross-sectional area perpendicular to the tensile direction of the piezoelectric layer, the greater the sensitivity to the applied load. Therefore, the thinner piezoelectric layer is desirable. For example, the thickness of the piezoelectric layer is preferably 200 μm or less, and more preferably 100 μm or less. On the other hand, if it is too thin, it tends to break down during polarization treatment. For this reason, the thickness of the piezoelectric layer is desirably 10 μm or more, and more desirably 20 μm or more.

<Electrode layer>
As the elastomer constituting the electrode layer, one or more selected from cross-linked rubber and thermoplastic elastomer may be used in the same manner as the elastomer of the piezoelectric layer. Examples of the elastomer having a relatively small elastic modulus and good adhesion to the piezoelectric layer include acrylic rubber, silicone rubber, urethane rubber, urea rubber, fluorine rubber, and H-NBR.

The type of conductive material is not particularly limited. For example, metal particles made of silver, gold, copper, nickel, rhodium, palladium, chromium, titanium, platinum, iron, and alloys thereof, metal oxide particles made of zinc oxide, titanium oxide, etc., titanium carbonate, etc. What is necessary is just to select suitably from electroconductive carbon materials, such as metal nanowire which consists of metal carbide particle | grains, silver, gold | metal | money, copper, platinum, nickel, etc., carbon black, a carbon nanotube, graphite, and graphene. Alternatively, particles coated with a metal such as silver-coated copper particles may be used. As the conductive material, one of these can be used alone, or two or more can be mixed and used. In addition, the electrode layer may contain a crosslinking agent, a dispersing agent, a reinforcing material, a plasticizer, an antiaging agent, a coloring agent, and the like as other components.

The volume resistivity of the electrode layer is 100 Ω · cm or less both in the natural state and in the stretched state from the stretched state to 10% in the uniaxial direction. More preferably, it is 10 Ω · cm or less. When the electric resistance of the electrode layer is large, the electromotive voltage generated in the piezoelectric layer drops at the electrode layer, and the output voltage becomes small. That is, the S / N ratio of the sensor decreases. In addition, when an electrode layer whose electric resistance is greatly increased by extension is used, the output in the natural state and the output in the extended state are greatly different, which causes a problem that the load cannot be accurately detected. Therefore, it can be used even in a stretched state by combining a flexible piezoelectric layer that can stretch and maintain piezoelectricity even when stretched and a flexible electrode layer that can stretch and maintain conductivity even when stretched. A piezoelectric element can be realized.

The blending amount of the conductive material may be appropriately determined so that the electrode layer can achieve a desired volume resistivity. When the amount of the conductive material is increased, the volume resistivity of the electrode layer can be reduced, but the flexibility is lowered. For example, when Ketjen Black (registered trademark) is used as the conductive material, it is desirable that the blending amount of the conductive material is 5 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the elastomer.

<Piezoelectric element>
The piezoelectric element is formed by laminating a piezoelectric layer and an electrode layer. For example, the pair of electrode layers may be arranged apart from each other in the polarization direction of the piezoelectric particles in the piezoelectric layer. When the piezoelectric particles are polarized in the thickness direction of the piezoelectric layer, a pair of electrode layers may be arranged one on each of the two surfaces in the thickness direction of the piezoelectric layer. In the case where the piezoelectric particles are polarized in the plane direction intersecting with the thickness direction of the piezoelectric layer, the pair of electrode layers may be arranged separately on one surface intersecting with the thickness direction of the piezoelectric layer. The electrode layer may be formed on the entire surface of the piezoelectric layer, or may be formed on only a part.

The breaking elongation of the piezoelectric element is 10% or more. More preferably, it is 30% or more. In this specification, the elongation at break is a value of elongation at break measured by a tensile test specified in JIS K6251: 2010. The tensile test is performed using a dumbbell-shaped No. 5 test piece and a tensile speed of 100 mm / min.

The elastic modulus of the piezoelectric element is desirably 10 MPa or more and 500 MPa or less. In this specification, the elastic modulus is a value calculated from a stress-elongation curve obtained by a tensile test specified in JIS K7127: 1999. The tensile test is performed using a test piece type 2 test piece and a tensile speed of 100 mm / min.

The piezoelectric element desirably satisfies the following formula (I) in a state where the piezoelectric element is stretched by 10% in the uniaxial direction. The following formula (I) is an index indicating flexibility and whether or not it can be used at the time of extension. That is, a piezoelectric element satisfying the following formula (I) is flexible and can generate an electromotive force by deformation even when it is extended. On the other hand, when the following formula (I) is not satisfied, the change in the electromotive voltage when it expands is large, and accurate sensing becomes difficult.
0.5 <V2 / V1 (I)
[In Formula (I), V1 is an electromotive voltage (V) of the piezoelectric element in a natural state, and V2 is an electromotive voltage (V) of the piezoelectric element in a state of being extended by 10% in a uniaxial direction. ]
The electromotive voltage V1 in the natural state may be measured as follows. First, the piezoelectric element is installed in a rebound resilience tester manufactured by Kobunshi Keiki Co., Ltd. in a natural state without stretching. Next, a steel ball having a diameter of 14 mm and a mass of 300 g suspended with a suspension length of 2000 mm is caused to make a pendulum movement with a swing width (distance from the test piece in the horizontal direction) of 15 mm and collide with the piezoelectric element. Then, the peak value of the electromotive voltage generated at the time of collision is measured with an oscilloscope (“TPS2012B” manufactured by Tektronix). This is repeated five times, and an average value of five times of the peak value of the electromotive voltage is set as the electromotive voltage V1 in the natural state. In addition, the piezoelectric element was installed in a rebound resilience tester (same as above) in a state where the piezoelectric element was stretched by 10% in the uniaxial direction. The electromotive voltage V2 is sufficient.

The piezoelectric element may have a protective layer in addition to the piezoelectric layer and the electrode layer. The protective layer may be disposed so as to be stacked on at least the electrode layer of the piezoelectric layer and the electrode layer. For example, a protective layer may be disposed on one or both of the laminate direction outer side of the laminate of the piezoelectric layer and the electrode layer. In addition, when a plurality of units in which a piezoelectric layer is interposed between a pair of electrode layers are stacked, a protective layer may be disposed between electrode layers adjacent in the stacking direction.

The protective layer is preferably stretchable together with the piezoelectric layer and the electrode layer. It is desirable to use at least one kind selected from a crosslinked rubber and a thermoplastic elastomer for the protective layer. By disposing the protective layer made of elastomer, it is possible to ensure the insulation of the piezoelectric element and suppress the destruction of the piezoelectric element due to external mechanical stress. Further, as will be described later, the extension of the protective layer increases the strain of the piezoelectric layer, thereby improving the sensitivity of the sensor.

Examples of elastomers having a relatively small elastic modulus and good adhesion to the electrode layer include natural rubber, isoprene rubber, butyl rubber, acrylic rubber, silicone rubber, urethane rubber, urea rubber, fluorine rubber, NBR, and the like. In order to reduce the change in sensitivity of the sensor after repeated use, it is desirable that the protective layer has excellent sag resistance. In addition, since the protective layer plays a role of protecting the piezoelectric element from external mechanical stress, it is desirable that the protective layer is excellent in wear durability and tear durability. Also, in order to prevent the protective layer from breaking and the piezoelectric element from being broken when stretched, it is desirable that the breaking elongation of the protective layer is larger than the breaking elongation of the piezoelectric layer.

For example, when a force is applied in the stacking direction of the piezoelectric elements (when the piezoelectric elements are compressed), a shearing force acts on the piezoelectric layer by extending the protective layer in the surface direction. Thereby, in addition to the pressing force in the stacking direction, a tensile force in the surface direction is applied to the piezoelectric layer, and the distortion of the piezoelectric layer increases. As a result, the amount of charge generated in the piezoelectric layer is increased, and the sensitivity of the sensor is improved. The sensitivity improvement effect by the protective layer is more remarkable as the elastic modulus in the tensile direction of the protective layer is smaller. It is desirable that the elastic modulus of the protective layer is smaller than the combined elastic modulus of a pair of laminates that are adjacent to the protective layer and that include a pair of electrode layers and a piezoelectric layer interposed therebetween. Here, the composite elastic modulus of a set of laminated bodies is the sum of the elastic modulus of the piezoelectric layer and the elastic modulus of the pair of electrode layers.

Elastic modulus can be obtained as the slope of a stress-elongation (strain) curve with stress on the vertical axis and elongation (strain) on the horizontal axis. However, in the case of an elastic body, since the inclination changes with an increase in strain, the value of the elastic modulus differs depending on in which strain region the inclination is obtained. Conventional piezoelectric ceramics represented by PZT and piezoelectric resins represented by PVDF and polylactic acid can only be used in a region where the elongation is extremely small. That's fine. However, since the piezoelectric sensor of the present invention is flexible and can be expanded and contracted, it is necessary to design in consideration of an elastic modulus in a region where the elongation rate is large (the strain is large).

For example, the protective layer can be elastically deformed in a region where the elongation rate is 25% or less, and the elastic modulus of the protective layer in the region is desirably smaller than 50 MPa. This is expressed by the following equation (α). The elastic modulus of the protective layer in the region where the elongation rate is 25% or less is preferably less than 20 MPa, and more preferably less than 10 MPa.

Further, the sensitivity improvement effect by the protective layer is more remarkable as the difference between the elastic modulus in the tensile direction of the protective layer and the elastic modulus in the tensile direction of the piezoelectric layer is smaller. Therefore, the protective layer and the pair of laminates composed of the pair of electrode layers and the piezoelectric layer interposed therebetween can be elastically deformed in a region where the elongation is 25% or less, and further, the elongation is 10%. It is desirable that the elastic modulus of the protective layer and the combined elastic modulus of the set of laminates in the region of 25% or less satisfy the following formula (β-1). It is more preferable that the following formula (β-2) is satisfied. When the protective layer and the set of laminates satisfy the formula (β-1) or the formula (β-2), the sensitivity of the sensor can be improved even when the protective layer is extended by 10% or more.

The Poisson's ratio of the elastomer is about 0.5. For this reason, when the protective layer is made of an elastomer, the force applied in the thickness direction acts as the force in the surface direction as it is. For this reason, the greater the thickness of the protective layer, the greater the distortion increasing effect of the piezoelectric layer, and the greater the sensitivity improving effect of the sensor. On the other hand, when the thickness of the protective layer is increased, the piezoelectric element is increased. For this reason, what is necessary is just to set the thickness of a protective layer suitably according to an installation place or a use. For example, it may be 5 μm or more and 5 mm or less.

<Piezoelectric sensor>
An embodiment of a piezoelectric sensor of the present invention will be described with reference to the drawings. FIG. 1 shows a top view of the piezoelectric sensor of the present embodiment. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. In FIG. 1, the protective layer 13a is shown in a transparent manner. As shown in FIGS. 1 and 2, the piezoelectric sensor 1 includes a piezoelectric element 10 and a control circuit unit 30. The piezoelectric element 10 includes a piezoelectric layer 11, a pair of electrode layers 12a and 12b, and a pair of

protective layers

13a and 13b. The breaking elongation of the piezoelectric element 10 is 50%.

The piezoelectric layer 11 contains X-NBR and barium titanate particles. The piezoelectric layer 11 has a square thin film shape. The piezoelectric layer 11 is subjected to polarization treatment, and the barium titanate particles are polarized in the thickness direction (vertical direction) of the piezoelectric layer 11. The electrode layer 12a includes acrylic rubber, conductive carbon black, and carbon nanotubes. The electrode layer 12a has a square thin film shape. The electrode layer 12 a is disposed on the upper surface of the piezoelectric layer 11. A wiring 20a is connected to the right end of the electrode layer 12a. The electrode layer 12b is made of the same material as the electrode layer 12a and has a square thin film shape. The electrode layer 12 b is disposed on the lower surface of the piezoelectric layer 11. A wiring 20b is connected to the right end of the electrode layer 12b. When viewed from above, the piezoelectric layer 11 and the electrode layers 12a and 12b have the same size. The volume resistivity in a natural state of the electrode layers 12a and 12b is 0.2 Ω · cm, and the volume resistivity in a state where the electrode layers 12a and 12b are elongated by 10% in the left-right direction (uniaxial direction) is 0.1 Ω · cm. The protective layer 13a is made of silicone rubber and has a square thin film shape. The protective layer 13a is larger than the piezoelectric layer 11 and the electrode layers 12a and 12b, and covers the piezoelectric layer 11 and the electrode layers 12a and 12b from above. The protective layer 13b is made of silicone rubber and has a square thin film shape. The protective layer 13b is larger than the piezoelectric layer 11 and the electrode layers 12a and 12b, and covers the lower surface of the electrode layer 12b. The electrode layer 12a and the control circuit unit 30 are electrically connected by a wiring 20a. The electrode layer 12b and the control circuit unit 30 are electrically connected by the wiring 20b. When a load is applied to the piezoelectric element 10, electric charges are generated in the piezoelectric layer 11. The generated electric charge is detected by the control circuit unit 30 as a change in voltage or current. Thereby, the applied load is detected.

In the present embodiment, the matrix of the piezoelectric layer 11 and the electrode layers 12a and 12b constituting the piezoelectric element 10 are all elastomers. The

protective layers

13a and 13b are also made of an elastomer. The elongation at break of the piezoelectric element 10 is 10% or more. Therefore, the piezoelectric element 10 is flexible and can be expanded and contracted. For this reason, even if it arrange | positions to the to-be-adhered body which expands or bends the piezoelectric element 10, it is hard to inhibit a motion of an to-be-adhered body. Further, even when the adherend has a complicated shape, the piezoelectric element 10 can be arranged along the shape.

The electrode layers 12a and 12b have a volume resistivity of 100 Ω · cm or less in a natural state and a state in which the electrode layers 12a and 12b are stretched 10% in the uniaxial direction. That is, the electrode layers 12a and 12b not only have high conductivity in a natural state, but also have a high conductivity with a small increase in electrical resistance even in an extended state that extends up to 10% in a uniaxial direction. For this reason, even in the extended state, the output is unlikely to decrease, and the load applied to the piezoelectric layer 11 can be accurately detected.

Thus, according to the piezoelectric sensor 1, it arrange | positions to the to-be-adhered body accompanying deformation | transformation, such as bending, extension, and compression, and adds to an to-be-adhered body also at the time of a deformation | transformation as well as the state to which an to-be-adhered body is not deform | transforming. The load can be detected. That is, even when the secondary deformation is further performed in the primary deformation state of the adherend, the load applied to the adherend can be detected.

Piezoelectric sensor 1 has a higher sensitivity (S / N ratio) than a capacitive sensor, so it is easy to detect a small load. Further, since the load can be detected by a voltage value or a current value, the circuit configuration can be simplified as compared with the case where the load is detected from the capacitance. Further, since energization to the piezoelectric element 10 is unnecessary, a power source for driving is not necessary. Incidentally, if the capacitance of the piezoelectric element 10 is also measured, a function as a capacitance type sensor can be added to the piezoelectric sensor 1. For example, a static load such as a surface pressure distribution can be detected by a change in capacitance, and a dynamic load such as vibration can be detected by a change in voltage.

Next, the present invention will be described more specifically with reference to examples.

<Manufacture of piezoelectric layer>
[Piezoelectric layers 1 to 4]
First, 100 parts by mass of a carboxyl group-modified hydrogenated nitrile rubber polymer (“Terban (registered trademark) XT8889” manufactured by LANXESS) as an elastomer was dissolved in acetylacetone to prepare a polymer solution. Next, barium titanate powder (“BT9DX-400” manufactured by Kyoritsu Material Co., Ltd.) as piezoelectric particles was added to the prepared polymer solution and kneaded. As shown in Tables 1 and 2 below, the blending amount of the barium titanate powder with respect to 100 parts by mass of the polymer is 650 parts by mass for the piezoelectric layer 1, 480 parts by mass for the piezoelectric layer 2, and 350 for the piezoelectric layer 3. In the mass part, the piezoelectric layer 4 was 800 parts by mass. Subsequently, the kneaded material was repeatedly passed through three rolls five times to obtain a slurry. Then, 5 parts by mass of tetrakis (2-ethylhexyloxy) titanium as a cross-linking agent was added to the obtained slurry and kneaded with an air stirrer, and then the slurry was applied onto a substrate by a bar coating method. This was heated at 150 ° C. for 1 hour to produce piezoelectric layers 1 to 4 having a thickness of 50 μm.

[Piezoelectric layer 5]
Except for using a polyurethane polymer (“N5139” manufactured by Tosoh Corporation) as an elastomer and using 2 parts by mass of polyisocyanate (“Coronate (registered trademark) HX” manufactured by Tosoh Corporation) as a crosslinking agent, A piezoelectric layer 5 was manufactured in the same manner as the piezoelectric layer 2.

[Piezoelectric layer 6]
First, barium titanate powder (same as above) was added to 100 parts by mass of a mixture of liquid A and liquid B of silicone rubber polymer (“KE-1935” manufactured by Shin-Etsu Chemical Co., Ltd.) as an elastomer. Was added and kneaded. Next, the kneaded product was repeatedly passed through three rolls five times to obtain a slurry. And the obtained slurry was apply | coated on the base material by the bar-coat method. This was heated at 150 ° C. for 1 hour to produce a piezoelectric layer 6 having a thickness of 50 μm.

[Piezoelectric layer 7]
A piezoelectric layer 7 was produced in the same manner as the piezoelectric layer 5 except that 1050 parts by mass of lead zirconate titanate powder (“PZT-ALT” manufactured by Hayashi Chemical Industry Co., Ltd.) was used as the piezoelectric particles.

[Piezoelectric layer 8]
A piezoelectric layer 8 was manufactured in the same manner as the piezoelectric layer 5 except that 350 parts by mass of potassium niobate powder (“Piezofine” manufactured by Furuuchi Chemical Co., Ltd.) was used as the piezoelectric particles.

[Piezoelectric layers 9 to 11]
After adding 5 parts by mass of tetrakis (2-ethylhexyloxy) titanium as a crosslinking agent and titanium dioxide sol as reinforcing particles to the slurry used for the production of the piezoelectric layer 2 and kneading with an air stirrer, the slurry is obtained by a bar coating method. It was applied on the material. This was heated at 150 ° C. for 1 hour to produce piezoelectric layers 9 to 11 having a thickness of 50 μm. As shown in Table 2 below, the blending amount of the titanium dioxide sol with respect to 100 parts by mass of the polymer content of the slurry was 1 part by mass for the piezoelectric layer 9, 5 parts by mass for the

piezoelectric layer

10, and 20 parts by mass for the piezoelectric layer 11. .

The titanium dioxide sol was manufactured as follows. First, 0.02 mol of acetylacetone was added to 0.01 mol of tetrai-propoxytitanium, an organometallic compound, for chelation. Next, 0.083 mol of isopropyl alcohol, 0.139 mol of methyl ethyl ketone, and 0.08 mol of water are added to the resulting chelated product, and the mixture is stirred. After the addition, the mixture is heated to 40 ° C. and further stirred for 2 hours. did. Then, it was allowed to stand at room temperature overnight to obtain a titanium dioxide sol.

[Piezoelectric layers 12, 13]
A slurry in which reinforcing particles are dispersed is added to the slurry used for the production of the piezoelectric layer 2, and 5 parts by mass of tetrakis (2-ethylhexyloxy) titanium as a cross-linking agent is added and kneaded with an air stirrer. Was applied on the substrate. This was heated at 150 ° C. for 1 hour to produce

piezoelectric layers

12 and 13 having a thickness of 50 μm. The blending amount of the slurry in which the reinforcing particles are dispersed with respect to 100 parts by mass of the polymer content of the slurry was 5 parts by mass for the

piezoelectric layer

12 and 20 parts by mass for the piezoelectric layer 13 as shown in Table 2 below.

The slurry in which the reinforcing particles are dispersed was manufactured as follows. First, a titanium dioxide powder (anatase type, Wako Pure Chemical Industries, Ltd., product code 205-01715) as a reinforcing particle was added to a polymer solution prepared by dissolving a carboxyl group-modified hydrogenated nitrile rubber polymer (same as above) in acetylacetone. ) And kneaded. Next, the kneaded product was repeatedly passed through three rolls five times to obtain a slurry in which reinforcing particles were dispersed.

[Piezoelectric layer 14]
The piezoelectric layer 14 was formed in the same manner as the piezoelectric layers 1 to 4 except that 480 parts by mass of the powder a (a “BTD-UP” manufactured by Nippon Kagaku Kogyo Co., Ltd.) of barium titanate particles was used as the piezoelectric particles. Manufactured.

[Piezoelectric layer 15]
The piezoelectric layer 15 was manufactured in the same manner as the piezoelectric layers 1 to 4 except that 480 parts by mass of the powder b of the combination of barium titanate particles as the piezoelectric particles was used. The combined powder b of the barium titanate particles used was a barium titanate powder (single particle powder, “BT-UP2” manufactured by Nippon Chemical Industry Co., Ltd.) for 180 minutes at 1050 ° C. Manufactured by grinding.

FIG. 7 shows an SEM photograph of the barium titanate powder (single particles) before firing. FIG. 8 shows an SEM photograph of barium titanate powder b (combined body) after firing and pulverization. As shown in FIGS. 7 and 8, it can be confirmed that a combined body formed by aggregating a plurality of barium titanate particles is produced by firing and pulverizing.

[Piezoelectric layer a]
For comparison, a piezoelectric layer having a thickness of 40 μm made of PVDF (manufactured by Kureha Elastomer Co., Ltd.) was used as the piezoelectric layer a.

[Piezoelectric layer b]
For comparison, a piezoelectric layer in which barium titanate particles are dispersed in an epoxy resin is defined as a piezoelectric layer b. The piezoelectric layer b was manufactured as follows. First, 100 parts by mass of bisphenol A ("jER (registered trademark) 828" manufactured by Mitsubishi Chemical Corporation) is added with 4.8 parts by mass of a phenol novolac resin ("BRG # 558" manufactured by Showa Denko KK) as a curing agent. Next, 480 parts by mass of barium titanate powder (same as above) was added to the prepared polymer solution and kneaded, and the kneaded product was passed through three rolls five times to obtain a slurry. The obtained slurry was applied onto a substrate by a bar coating method, and this was heated at 150 ° C. for 1 hour to produce a piezoelectric layer b having a thickness of 50 μm.

<Manufacture of electrode layer>
[Electrode layer 1]
First, 100 parts by mass of an epoxy group-containing acrylic rubber polymer (“Nipol (registered trademark) AR42W” manufactured by Nippon Zeon Co., Ltd.) as an elastomer was dissolved in butyl cellosolve acetate to prepare a polymer solution. Next, 10 parts by mass of conductive carbon black ("Ketjen Black EC600JD" manufactured by Lion Corporation) and 16 parts by mass of carbon nanotubes ("VGCF (registered trademark)" manufactured by Showa Denko KK) were added to the prepared polymer solution. Part and 12 parts by mass of a polyester acid amide amine salt as a dispersant were added and dispersed with a bead mill to prepare a conductive paint. Subsequently, the conductive paint was applied on a polyethylene terephthalate (PET) film subjected to a release treatment by a bar coating method. This was heated at 150 ° C. for 1 hour to produce an electrode layer having a thickness of 20 μm.

[Electrode layer 2]
An electrode layer 2 was produced in the same manner as the electrode layer 1 except that a conductive paint was prepared without blending carbon nanotubes and a dispersant.

[Electrode layer 3]
The conductive carbon black was changed from “Ketjen Black EC600JD” manufactured by Lion Corporation to “# 3050B” manufactured by Mitsubishi Chemical Corporation, except that a conductive paint was prepared without blending carbon nanotubes and a dispersant. The electrode layer 3 was produced in the same manner as the electrode layer 1.

[Electrode layer 4]
A silver paste (“Dotite (registered trademark) D-362” manufactured by Fujikura Kasei Co., Ltd.) was applied onto the release-treated PET film by a bar coating method. This was heated at 150 ° C. for 1 hour to produce an electrode layer 4 having a thickness of 20 μm.

<Manufacture of protective layer>
[Protective layer]
Liquid A and B of silicone rubber polymer (“KE1935” manufactured by Shin-Etsu Chemical Co., Ltd.) are mixed at the same mass, degassed by vacuum degassing, and then released onto a PET film that has been subjected to release treatment. The coating method was applied. This was heated at 150 ° C. for 1 hour to produce a protective layer having a thickness of 10 μm.

<Manufacture of piezoelectric elements>
Various piezoelectric elements were manufactured as follows by appropriately combining the manufactured piezoelectric layer, electrode layer, and protective layer. First, electrode layers were respectively arranged on two surfaces (upper surface and lower surface) in the thickness direction of the piezoelectric layer, and the piezoelectric layer and the electrode layer were pressure-bonded using a laminator (“LPD3223” manufactured by Fuji Pla Co., Ltd.). Next, the protective layer which performed the excimer process previously was laminated | stacked on the electrode layer, and the protective layer and the electrode layer were crimped | bonded using the laminator (same as the above). For excimer treatment, an excimer lamp light source “FLAT EXCIMER” manufactured by Hamamatsu Photonics Co., Ltd. was used. A direct current power source is connected to the electrode layer of the laminate comprising the obtained protective layer / electrode layer / piezoelectric layer / electrode layer / protective layer, and an electric field of 10 V / μm is applied to the piezoelectric layer for 1 hour to perform polarization treatment. It was. FIG. 9 shows a vertical sectional view of the manufactured piezoelectric element. As shown in FIG. 9, the piezoelectric element 40 is formed by laminating a protective layer 43a, an electrode layer 42a, a piezoelectric layer 41, an electrode layer 42b, and a protective layer 43b in order from the top. The manufactured piezoelectric element has a square-shaped detection part of 30 mm in length and width.

<Evaluation of piezoelectric element>
Tables 1 and 2 show the configuration, characteristics, and evaluation results of the manufactured piezoelectric elements. In Tables 1 and 2, the methods for measuring ε (relative dielectric constant), volume resistivity, elastic modulus, elongation at break, electromotive force, and stretching durability are as follows.

[Relative permittivity of elastomer]
A molded body made only from a polymer without blending piezoelectric particles and reinforcing particles was placed in a sample holder (Solartron, model 12962A), and a dielectric constant measurement interface (manufactured by the company, model 1296) and a frequency response analyzer ( The relative permittivity was measured (frequency: 100 Hz) using a 1255B type manufactured by the same company.

[Relative permittivity of piezoelectric particles and reinforcing particles]
A composite was manufactured by blending piezoelectric particles or reinforcing particles with an elastomeric polymer whose relative dielectric constant was known by measurement. At this time, various composites having different blending amounts were produced, and the relative dielectric constant was measured for each of the composites in the same manner as the measurement of the relative dielectric constant of the elastomer. Then, the relative dielectric constant of the blended particles was calculated by the following formula (b).
Log ε = V _f Log ε _f + V _p Log ε _p (b)
[Ε: dielectric constant of the composite, V _f : particle volume ratio (%), ε _f : particle dielectric constant, V _p : elastomer volume ratio (%), ε _p : elastomer dielectric constant. ]
[Volume resistivity of electrode layer]
(1) Volume resistivity in a natural state An electrode layer having a thickness of 20 μm was cut into a strip shape having a width of 10 mm and a length of 40 mm to form a test piece, and a marked line was attached to a position separated by 20 mm in the length direction. A copper foil terminal was attached to the marked line position, and the electrical resistance between the marked lines was measured. Based on the measured electric resistance value and the dimension of the test piece, the volume resistivity was calculated by the following formula (c), and the volume resistivity in the natural state of the electrode layer was obtained.
Volume resistivity (Ω · cm) = Electric resistance value (Ω) × Cross sectional area of test piece (cm ² ) / Distance between marked lines (cm) (c)
(2) Volume resistivity in the stretched state Using a tensile tester (manufactured by Shimadzu Corporation), the test piece of the electrode layer was stretched in the length direction. In a state where the test piece was extended by 10%, the electrical resistance between the marked lines was measured, and the volume resistivity was calculated according to the previous equation (c), which was taken as the volume resistivity when the electrode layer was extended by 10%. The volume resistivity was calculated in the same manner for the case where the test piece was extended by 50%, and was taken as the volume resistivity when the electrode layer was extended by 50%. The cross-sectional area of the test piece in the extended state was calculated on the assumption that the Poisson's ratio of the test piece was 0.5.

[Elastic modulus]
The piezoelectric element was subjected to a tensile test specified in JIS K 7127: 1999, and the elastic modulus was calculated from the obtained stress-elongation curve. The tensile test was performed using a test piece type 2 test piece with a tensile speed of 100 mm / min.

[Elongation at break]
The piezoelectric element was subjected to a tensile test specified in JIS K 6251: 2010, and the elongation at break was calculated. The tensile test was performed using a dumbbell-shaped No. 5 test piece with a tensile speed of 100 mm / min.

[Electromotive voltage]
The electromotive force was measured by a method similar to the pendulum type test defined in JIS K 6255: 2013. First, the piezoelectric element was installed in a rebound resilience tester manufactured by Kobunshi Keiki Co., Ltd. in a natural state. Next, a steel ball having a diameter of 14 mm and a mass of 300 g suspended with a suspension length of 2000 mm was caused to make a pendulum movement with a swing width (distance from the test piece in the horizontal direction) of 15 mm and collide with the piezoelectric element. Then, the peak value of the electromotive voltage generated at the time of collision was measured with an oscilloscope (“TPS2012B” manufactured by Tektronix). This was repeated five times, and the average value of the five peak values of the electromotive voltage was taken as the natural state electromotive voltage V1. In addition, the piezoelectric element was installed in a rebound resilience tester (same as above) in a state where the piezoelectric element was stretched 10% in the uniaxial direction, and the average value of five times of the peak value of the electromotive force measured by the same method as described above was The electromotive voltage V2 was

[Stretch durability]
The piezoelectric element was subjected to an expansion / contraction test, and the expansion / contraction durability was evaluated by a change in electromotive force before and after the test. In the expansion / contraction test, a cycle in which the piezoelectric element was expanded 10% in one direction of the plane and then restored was repeated 10,000 times. Stretching was performed at a rate of 2 cycles / second. Then, the electromotive force of the piezoelectric element before and after the test was measured by the method for measuring the electromotive voltage in the natural state described above, and the rate of change with respect to the initial electromotive voltage was calculated by the following equation (d).
Rate of change in electromotive voltage (%) = V1 / V3 × 100 (d)
[V1: Initial (natural state) electromotive voltage (V), V3: Electromotive voltage after expansion / contraction test (V). ]

First, the piezoelectric elements of Examples 1 to 8 in which reinforcing particles are not included in the piezoelectric layer will be described. As shown in Table 1, according to the piezoelectric elements of Examples 1 to 8, the breaking elongation of the piezoelectric elements was 40% or more. The volume resistivity of the electrode layer was 3 Ω · cm or less at the natural state and 10% elongation, and 5 Ω · cm or less at 50% elongation. As a result, the electrode layers constituting the piezoelectric elements of Examples 1 to 8 satisfy the condition that the volume resistivity in the natural state and the expanded state from the state to 10% in the uniaxial direction is 100 Ω · cm or less. I can judge. In the piezoelectric elements of Examples 1 to 8, the value of V2 / V1 is greater than 0.5%, which satisfies the condition of the above-described formula (I). Further, the rate of change in electromotive voltage after repeated expansion and contraction was 150% or less, and it was confirmed that the change in electromotive voltage was small even after repeated expansion and contraction and that the stretch durability was excellent. Further, when the elastic modulus of the piezoelectric element is large, there is a possibility that the movement of the adherend is hindered. In this regard, the elastic modulus of the piezoelectric elements of Examples 1 to 8 is 500 MPa or less. Therefore, as indicated by a circle in Table 1, it was confirmed that the piezoelectric elements of Examples 1 to 8 had good followability to the adherend and hardly hindered the movement of the adherend.

On the other hand, in the piezoelectric element of Comparative Example 1 having a piezoelectric layer made of PVDF and the piezoelectric element of Comparative Example 5 having a piezoelectric layer having an epoxy resin as a matrix, the elastic modulus is large as shown in Table 2, It did not restore to its original shape after stretching. For this reason, the electromotive voltage in an extended state could not be measured, and the stretch durability could not be evaluated. Further, in the piezoelectric element of Comparative Example 2, since the blending amount of the piezoelectric particles was large, the elastic modulus of the piezoelectric element was increased and the elongation at break was less than 10%. For this reason, the electromotive voltage in an extended state could not be measured, and the stretch durability could not be evaluated. Moreover, in the piezoelectric element of Comparative Example 3, the volume resistivity of the electrode layer was greatly increased at the time of expansion, so that the electromotive voltage was greatly reduced. In addition, in the piezoelectric element of Comparative Example 4 having the electrode layer made of silver paste, the volume resistivity of the electrode layer was greatly increased at the time of expansion, and the electromotive voltage in the expanded state could be measured. Therefore, the stretch durability was not evaluated.

Next, the piezoelectric elements of Examples 9 to 13 in which reinforcing particles are included in the piezoelectric layer will be described. As shown in Table 2, the configurations of the piezoelectric elements of Examples 9 to 13 are the same as the configuration of the piezoelectric element of Example 3 except that reinforcing particles are blended in the piezoelectric layer. Therefore, like the piezoelectric element of Example 3, the piezoelectric elements of Examples 9 to 13 have a small change in electromotive voltage even after repeated expansion and contraction and are excellent in expansion and contraction durability. In the piezoelectric elements of Examples 9 to 13, the electromotive voltage in the natural state was larger than that of the piezoelectric element of Example 3. This is a great effect due to the incorporation of reinforcing particles. The reinforcing particles have a hydroxyl group on the surface and are chemically bonded to the elastomer. For this reason, the rate of change in electromotive voltage after repeated expansion and contraction is further reduced.

Next, the piezoelectric elements of Examples 14 and 15 using a bonded body in which individual particles are chemically bonded as piezoelectric particles will be described. As shown in Tables 1 and 2, the configurations of the piezoelectric elements of Examples 14 and 15 are the same as the configurations of the piezoelectric element of Example 3 except that the piezoelectric particles used are different. According to the piezoelectric elements of Examples 14 and 15, the elastic modulus was small and the elongation at break was large compared to the piezoelectric element of Example 3 using barium titanate particles (single particles). On the other hand, the electromotive voltage of the piezoelectric elements of Examples 14 and 15 was larger than that of the piezoelectric element of Example 3. The expansion / contraction durability of the piezoelectric elements of Examples 14 and 15 was equivalent to that of the piezoelectric element of Example 3. Thus, according to the piezoelectric elements of Examples 14 and 15, the flexibility could be greatly improved while ensuring high piezoelectricity. This is because, when an aggregate of piezoelectric particles is used, a connection structure between the piezoelectric particles is easily formed, so that high piezoelectricity can be obtained without increasing the blending amount of the piezoelectric particles.

FIG. 10 shows the relationship between the volume ratio of barium titanate particles and the generated electric field. As shown in FIG. 10, in the case of the combined body used in the piezoelectric layer 14, it can be seen that a large electric field is generated even at a low filling rate as compared with the single particles used in the piezoelectric layer 1. Similarly, in the case of the combined body used in the piezoelectric layer 15, it can be seen that a large electric field is generated even at a low filling rate as compared with the single particles before firing.

As an example, a graph of an electromotive voltage generated when vibration is applied to the piezoelectric element of Example 2 is shown. FIG. 3 is a graph of an electromotive voltage when vibration is applied in the thickness direction in a state where the piezoelectric element is extended by 1% in one direction of the plane direction. FIG. 4 is a graph of an electromotive voltage when vibration is applied in the thickness direction in a state where the piezoelectric element is extended by 10% in one direction of the plane direction. In FIGS. 3 and 4, the electromotive voltage is indicated by a thick line, and the load is indicated by a thin line. A sine wave-like vibration with a load pp of 1.7 N was applied to the piezoelectric element using a fatigue durability tester “APC-1000” manufactured by Asahi Seisakusho.

As shown in FIGS. 3 and 4, it can be seen that the piezoelectric element maintains the piezoelectric performance even in the extended state, and can detect the applied load.

<Examination of protective layer in piezoelectric element>
Piezoelectric elements were manufactured by changing the type and thickness of the protective layer, and the electromotive voltages in the natural state and the extended state were measured. The configuration of the piezoelectric element is protective layer / electrode layer / piezoelectric layer / electrode layer / protective layer, and the manufacturing method is as described above. The following three types were used as the protective layer.

[Protective layer 1]
On the PET film that has been subjected to mold release treatment after mixing A liquid and B liquid of silicone rubber polymer (“KE2004-5” manufactured by Shin-Etsu Chemical Co., Ltd.) with the same mass, vacuum degassing to remove bubbles It was applied by a bar coating method. This was heated at 150 ° C. for 1 hour to produce a protective layer 1 having a thickness of 1 mm.

[Protective layer 2]
Liquid A and B of silicone rubber polymer (“KE1935” manufactured by Shin-Etsu Chemical Co., Ltd.) are mixed at the same mass, degassed by vacuum degassing, and then released onto a PET film that has been subjected to release treatment. The coating method was applied. This was heated at 150 ° C. for 1 hour to produce a protective layer 2 having a thickness of 1 mm. The protective layer 2 is different in thickness of the protective layer used in the piezoelectric elements of Examples 1 to 15 described above.

[Protective layer 3]
A commercially available NBR sheet (product code “07-012-02-04”, thickness 2 mm) was used.

Table 3 shows the measurement results of the configuration of the piezoelectric element, the composite elastic modulus of the laminate, the elastic modulus and breaking elongation of the protective layer, and the electromotive voltage of the piezoelectric element. The elastic modulus, elongation at break, and electromotive force were measured according to the methods described above. The composite elastic modulus of the laminate is a value obtained by separately obtaining and adding the elastic modulus of the piezoelectric layer and the elastic modulus of the electrode layer. The electromotive voltage in the 20% stretched state is an average value of five times of the peak value of the electromotive force measured by installing the piezoelectric element in a rebound resilience tester (same as above) in a state where the piezoelectric element is stretched by 20% in one axis direction.

As shown in Table 3, the elastic modulus of the protective layers 1 and 2 is smaller than 10 MPa, and the protective layers 1 and 2 satisfy the above elastic modulus formula (α). The piezoelectric element of Example 17 having the protective layer 1 and the piezoelectric element of Example 18 having the protective layer 2 both satisfy the expressions (β-1) and (β-2). Therefore, in the piezoelectric elements of Examples 17 and 18, the electromotive voltage was larger than that of the piezoelectric element of Example 16 having no protective layer. In the piezoelectric elements of Examples 17 and 18, it can be seen that the effect of increasing the distortion of the piezoelectric layer by the protective layer is fully exhibited. In addition, in the piezoelectric element of Example 18 having a protective layer thickness of 1 mm, the electromotive voltage was larger than that of the piezoelectric element of Example 15 having a protective layer thickness of 10 μm. This is presumably because the effect of increasing the distortion of the piezoelectric layer is increased by the increase in the thickness of the protective layer. On the other hand, in the piezoelectric element of the reference example, the protective layer 3 satisfies the above-described elastic modulus formula (α), but does not satisfy the formula (β-1). For this reason, the electromotive voltage of the piezoelectric element of the reference example was the same level as that of the piezoelectric element of Example 16 having no protective layer. Moreover, in the piezoelectric element of Comparative Example 6 having a piezoelectric layer made of PVDF, the laminate exceeded the elastic region when the elongation ratio was 10% or more. That is, although the piezoelectric element of Comparative Example 6 has a flexible protective layer, it was confirmed that the piezoelectric element cannot be used for applications that greatly expand because the piezoelectric layer has poor flexibility.

Since the piezoelectric sensor of the present invention can be applied to an adherend that stretches or bends (repeatedly expands and contracts and bends), it is a wearable that measures a pulse rate, a respiration rate, etc. without disturbing the natural movement of the living body. It is suitable as a simple biological information sensor. In addition, it can be used not only in an unstretched state but also in a stretched state (measurable), so it can also be used in joints that require expansion and contraction in humans and robots, and in processes where the sensor installation surface extends and returns during the manufacturing process. Can do. Further, it is suitable as a pressure sensor for robots (including industrial and communication), medical use, nursing care, health use, sports equipment, and automobiles.

The piezoelectric sensor of the present invention is particularly suitable for application as a human-machine interface (HMI) that comes into contact with people. For example, if it is placed on a mattress, a wheelchair seat, etc., information on the pulse, position, and movement can be acquired. Also, placed on sports equipment such as sportswear such as sportswear (wearable such as shoes and gloves) and sports equipment such as balls, bats, rackets, various armor, weight training, traveling equipment, etc. By measuring the position, its strength, weight (acceleration), etc., the training effect can be quantified without impairing the hit feeling. Of course, the present invention is not limited to sports and medical care, and can be similarly applied to daily goods (clothes, hats, glasses, shoes, belts, masks, pendants, etc.). Digitized data and information can be sent to an IOT (Internet of Things) device as a control means.

Claims

A piezoelectric element having a piezoelectric layer including an elastomer and piezoelectric particles, and an electrode layer including an elastomer and a conductive material;
The elongation at break of the piezoelectric element is 10% or more,
The piezoelectric layer is characterized in that the electrode layer has a volume resistivity of 100 Ω · cm or less in a natural state and a state in which the electrode layer extends to 10% in a uniaxial direction.
The piezoelectric sensor according to claim 1, wherein the piezoelectric element satisfies the following formula (I) in a state where the piezoelectric element extends by 10% in a uniaxial direction.
0.5 <V2 / V1 (I)
[In Formula (I), V1 is an electromotive voltage (V) of the piezoelectric element in a natural state, and V2 is an electromotive voltage (V) of the piezoelectric element in a state of being extended by 10% in a uniaxial direction. ]
3. The piezoelectric sensor according to claim 1, wherein the piezoelectric element has a protective layer stacked on at least the electrode layer of the piezoelectric layer and the electrode layer.
4. The piezoelectric according to claim 3, wherein the elastic modulus of the protective layer is smaller than a combined elastic modulus of a pair of laminated bodies that are adjacent to the protective layer and include the pair of electrode layers and the piezoelectric layer interposed therebetween. Sensor.
The piezoelectric sensor according to any one of claims 1 to 4, wherein the piezoelectric particles include an aggregate in which a plurality of piezoelectric particles are aggregated.
6. The piezoelectric sensor according to claim 1, wherein the elastomer and the piezoelectric particles are chemically bonded in the piezoelectric layer.
The piezoelectric sensor according to claim 6, wherein the piezoelectric particles are surface-treated.
The piezoelectric sensor according to any one of claims 1 to 7, wherein the piezoelectric layer includes reinforcing particles having a relative dielectric constant of 100 or less.
The piezoelectric sensor according to claim 8, wherein the reinforcing particles are a metal oxide.