JP6696885B2 - Piezoelectric sensor - Google Patents

Description

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

  Piezoelectric materials capable of converting mechanical energy into electric 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 piezoelectric 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 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 fabric base material. 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 including a piezoelectric sheet in which a resin matrix such as chlorinated polyethylene is filled with piezoelectric particles, and a flexible electrode in which chlorinated polyethylene is filled with carbon. Patent Document 5 describes a piezoelectric element having a composite in which lead titanate powder is filled in chloroprene rubber and an electrode made of a silver paste. Patent Document 6 describes a fluctuating load detection sheet including a PVDF piezoelectric film, a pair of electrodes arranged on both surfaces of the PVDF film, and a strain amplification member provided on the electrodes.

JP, 2005-347364, A Special table 2014-529913 gazette JP, 2013-225608, A JP, 2002-111087, A JP-A-2-32574 JP, 2006-153842, A

  In a piezoelectric element using a ceramic such as PZT for the piezoelectric layer as described in Patent Document 1, the piezoelectric layer is hard and the elasticity is poor. Therefore, when the piezoelectric element is applied to an adherend that expands and contracts, the movement of the adherend is likely to be hindered. Further, in the piezoelectric elements described in Patent Documents 2 and 6, resin is used for the piezoelectric layer. Therefore, the piezoelectric layer has flexibility but lacks elasticity. Even if the piezoelectric layer could be stretched, it is difficult to restore the original shape. Therefore, it is difficult to apply the piezoelectric element to an adherend that expands and contracts. On the other hand, the piezoelectric elements described in Patent Documents 3 to 5 use a composite of a polymer matrix and piezoelectric particles for the piezoelectric layer. However, when the polymer matrix contains a resin, it has flexibility but poor stretchability. In this respect, in the piezoelectric layer described in Patent Document 5, chloroprene rubber is used as the polymer matrix. Therefore, the piezoelectric layer has elasticity, but the electrode laminated on the piezoelectric layer is made of silver paste having poor elasticity. In this case, the expansion and contraction of the piezoelectric layer is restricted by the electrodes, and the expansion and contraction of the piezoelectric element as a whole is reduced. Moreover, since the electrical resistance increases when the electrode is extended, the output is reduced during extension, and the load applied to the piezoelectric layer cannot be accurately detected. This problem is common to the piezoelectric elements described in the other patent documents mentioned above. For example, Patent Document 3 describes the use of conductive rubber for the electrodes. However, Patent Document 3 does not consider the expansion / contraction performance of the electrode or the behavior of the electric resistance during expansion. Further, in paragraph [0020] of Patent Document 3, it is described that the strain amount of the vibration source is about 5%, and as described in the example, an application example in which the strain amount is 3% is described. Patent Document 3 does not assume a form in which the piezoelectric element is deformed at a relatively large elongation rate of 10% or more.

  As described above, conventionally, since it is not supposed to be applied to an adherend that undergoes large expansion / contraction deformation, expansion / contraction of the entire piezoelectric element including not only the piezoelectric layer but also the electrode has not been examined. Therefore, a piezoelectric element that can maintain the piezoelectric performance even in the stretched state has not been realized.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a piezoelectric sensor including a piezoelectric element that is expandable and contractible and that can be used even in an expanded state.

  The piezoelectric sensor of the present invention has a piezoelectric layer containing an elastomer and piezoelectric particles, an electrode layer containing an elastomer and a conductive material, and a protective layer laminated on at least the electrode layer of the piezoelectric layer and the electrode layer. The electrode layer is provided with a piezoelectric element, and 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 of being stretched by 10% in a uniaxial direction, and the elastic modulus of the protective layer is It is characterized in that it is smaller than the composite elastic modulus of a set of laminates consisting of a pair of the electrode layers adjacent to the layers and the piezoelectric layer interposed therebetween.

  The matrix (matrix) of the piezoelectric layer and the electrode layer forming the piezoelectric element are both elastomers. Since the piezoelectric element is flexible and expandable, even if the piezoelectric element is arranged on an adherend that repeatedly extends or bends or an adherend that greatly expands and contracts, the movement of the adherend is unlikely to be 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 the natural state to a state in which it is stretched by 10% in the uniaxial direction. The natural state means a state where no load is applied and no deformation occurs. The state of being 10% elongated 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 the natural state, but also has a small increase in electric resistance even in the stretched state in which it is stretched up to 10% in the uniaxial direction, and has high conductivity. Therefore, even in the stretched 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 both in the natural state and in the state of being stretched by 10% in the uniaxial direction. If both volume resistivity are 100 Ω · cm or less, the “natural state and uniaxial direction The volume resistivity in the stretched state up to the state of being stretched by 10% is 100 Ω · cm or less ”. In the piezoelectric sensor of the present invention, the piezoelectric element can be extended not only in the uniaxial direction but also in the biaxial direction, the radial expansion direction, and the like.

  The piezoelectric element has a protective layer laminated on at least the electrode layer. For example, when a force is applied in the stacking direction of the piezoelectric elements (when the piezoelectric elements are compressed), the protective layer expands in the surface direction, so that a shearing force acts on the piezoelectric layers. As a result, a tensile force in the surface direction is applied to the piezoelectric layer in addition to the pressing force in the stacking direction, and the strain of the piezoelectric layer increases. As a result, the amount of charge generated in the piezoelectric layer increases, and the sensitivity of the sensor improves. The sensitivity improving effect of the protective layer is more remarkable as the elastic modulus in the tensile direction of the protective layer is smaller. In this respect, according to the piezoelectric sensor of the present invention, the elastic modulus of the protective layer is smaller than the combined elastic modulus of a set of laminates composed of a pair of electrode layers adjacent to the protective layer and a piezoelectric layer interposed therebetween. .. Therefore, the distortion of the piezoelectric layer is likely to increase, and the sensitivity improving effect of the protective layer is likely to be exhibited. Here, the composite elastic modulus of a set of laminated bodies is the sum of the elastic moduli of the piezoelectric layer and the elastic moduli of the pair of electrode layers.

  As described above, according to the piezoelectric sensor of the present invention, the piezoelectric sensor according to the present invention is arranged on an adherend that is deformed by bending, stretching, compressing, etc., and the adherend is not deformed but is also deformed. The load applied to can be detected. That is, the load applied to the adherend can be detected even when the secondary deformation further occurs in the primary deformed state of the adherend. Further, the piezoelectric sensor of the present invention has a higher sensor sensitivity (S / N ratio (Signal-Noise Ratio: signal noise ratio)) than a capacitance-type sensor, so that a small load can be easily detected. For example, the piezoelectric element of the piezoelectric sensor of the present invention can be placed directly on the skin of the human body or indirectly via clothes to measure the pulse rate and respiration rate.

It is a top view of one embodiment of the piezoelectric sensor of the present invention. FIG. 2 is a sectional view taken along line II-II of FIG. 1. 9 is a graph of electromotive voltage in a state where the piezoelectric element of Reference Example 2 is expanded by 1%. 9 is a graph of electromotive voltage in a state where the piezoelectric element of Reference Example 2 is expanded by 10%. It is a schematic diagram which shows the dispersion state when a piezoelectric particle consists of a single particle. It is a schematic diagram which shows the dispersion state when a piezoelectric particle consists of an aggregate. It is a SEM photograph of barium titanate powder (single particle) before baking. It is an SEM photograph of barium titanate powder b (combined body) after baking and crushing. FIG. 3 is a vertical cross-sectional view of the piezoelectric element manufactured in the example. 6 is a graph showing the relationship between the volume ratio of barium titanate particles and the generated electric field.

  Hereinafter, embodiments of the piezoelectric sensor of the present invention will be described. It should be noted that the piezoelectric sensor of the present invention is not limited to the following forms, and may be implemented in various forms with modifications and improvements that can be made by those skilled in the art without departing from the scope of the present invention. You can

  The piezoelectric sensor of the present invention includes a piezoelectric element having a piezoelectric layer containing an elastomer and piezoelectric particles, and an electrode layer containing 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 a flexible elastomer having a 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 acid 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 may be used. Examples of the modified elastomer include carboxyl group-modified nitrile rubber (X-NBR) and carboxyl group-modified hydrogenated nitrile rubber (XH-NBR).

The electromotive field (V / m) generated when a load is applied to the piezoelectric layer depends on the piezoelectric strain constant (C / N) of the piezoelectric layer, the dielectric constant (F / m), and the applied load (N / m ² ). Is expressed by the following equation (a).
Electromotive field = piezoelectric strain constant / dielectric constant × load ... (a)
From the viewpoint 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 use an elastomer having a relatively small relative dielectric constant. For example, urethane rubber, silicone rubber, NBR, H-NBR and the like 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. As a compound having piezoelectricity, a ferroelectric having a perovskite crystal structure is known, and examples thereof include barium titanate, strontium titanate, potassium niobate, sodium niobate, lithium niobate, and 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 kind or two kinds or more of these may be used.

  The particle size of the piezoelectric particles is not particularly limited. For example, by using a plurality of types of piezoelectric particle powders having different average particle diameters, it is possible to mix piezoelectric particles having a large particle diameter and piezoelectric particles having a small particle diameter in the elastomer. In this case, the piezoelectric particles having a small particle size enter between the piezoelectric particles having a large particle diameter, and the pressure is easily transmitted to the piezoelectric particles. As a result, 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. When the aggregate including a plurality of piezoelectric particles is included, it becomes easy to balance the flexibility and the piezoelectricity. For example, when a large amount of piezoelectric particles is mixed with an elastomer, the piezoelectric property is improved, but the volume ratio of the elastomer is reduced, so that the flexibility is decreased. On the other hand, when the amount of the piezoelectric particles is small, the volume ratio of the elastomer is large, so that the flexibility is improved but the piezoelectricity is decreased. According to the study by the present inventor, it has been confirmed that the flexibility of the piezoelectric layer, specifically, the elongation at break is large, so that the change in electromotive voltage is small even after repeated expansion and contraction, that is, expansion and contraction durability is improved. ing. Therefore, it is desirable to secure the desired piezoelectricity by reducing the compounding 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 dispersed state in which the piezoelectric particles are single particles. FIG. 6 schematically shows a dispersed state when the piezoelectric particles are composed 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, normally, the piezoelectric particles 80 are blended in a large amount to bring the piezoelectric particles 80 close to the close-packed structure, thereby ensuring the connection between the piezoelectric particles 80. On the other hand, as shown in FIG. 6, when a lump-shaped aggregate 82 in which a plurality of piezoelectric particles 80 are aggregated is mixed, the shape causes a steric hindrance, and the piezoelectric particles 80 are not adjacent to each other even if a dense packing structure is not formed. You can build connections. That is, desired piezoelectricity can be ensured even if the volume ratio of the piezoelectric particles 80 is small. This makes it easy to satisfy all of piezoelectricity, flexibility, and stretch durability. For example, a piezoelectric sensor is provided with a piezoelectric element having a piezoelectric layer containing an elastomer and piezoelectric particles, and an electrode layer containing an elastomer and a conductive material, and the piezoelectric particle includes a collection of a plurality of piezoelectric particles. It is good to set. With this configuration, a flexible and highly sensitive piezoelectric sensor can be realized.

  Examples of the aggregate in which a plurality of piezoelectric particles are aggregate include aggregates in which individual particles are aggregated by electrostatic force or the like, and aggregates in which individual particles are chemically bonded. The latter bonded body is preferable from the viewpoint that individual particles are difficult to separate and a connected structure of piezoelectric particles is easily constructed. The method for producing the bound body is not particularly limited, but for example, it can be produced by firing a powder consisting of single particles and then pulverizing. The difference between aggregates and binders 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 they are separated, and are aggregates when they are not separated. Here, the good solvent refers to a polar solvent that does not easily settle when the piezoelectric particles are dispersed. Specifically, any solvent having an SP value (solubility parameter) of 8 or more and 13 or less and capable of dissolving the elastomer may be used. For example, 2-methoxyethanol may be mentioned.

  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 individual piezoelectric particles. Here, as the diameter (d2) of the aggregate, a median diameter measured by a laser diffraction / scattering type particle diameter distribution measuring device is adopted. 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. An aggregate is one that satisfies 2d1 <d2.

  The piezoelectric particles may be chemically bonded to each other by surface-treating the piezoelectric particles. As a method of surface-treating the piezoelectric particles, a method of preliminarily reacting the surface treatment agent having a functional group capable of reacting with the elastomer polymer with the piezoelectric particles and mixing the piezoelectric particles with the elastomer polymer, or the surface of the piezoelectric particles Examples thereof include a method in which after dissolving with an acid, an alkali or subcritical water to generate a hydroxyl group, the hydroxyl group is 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 if they repeatedly expand and contract. Further, since the piezoelectric particles are less likely to be peeled from the elastomer, the fluctuations of the physical properties and the output from the initial values are reduced. Therefore, the output is stabilized and the sag resistance of the piezoelectric layer is improved. Further, since the breaking elongation of the piezoelectric layer becomes large, it is possible to suppress the deterioration of the piezoelectric performance due to local breakage at the time of stretching. As a result, high piezoelectric performance can be maintained even in the stretched state.

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

  The piezoelectric layer may include reinforcing particles having a relative dielectric constant smaller than that of the piezoelectric particles, in addition to the elastomer and the piezoelectric particles. The dielectric constant of the reinforcing particles is preferably 100 or less, more preferably 30 or less, provided that it is smaller than that of the piezoelectric particles.

  Since the external force is easily transmitted to the piezoelectric particles in the structure in which the piezoelectric particles having a large relative permittivity are connected, the improvement of the piezoelectric strain constant of the above-mentioned formula (a) can be expected. However, the coupling of the piezoelectric particles having a large relative dielectric constant increases the dielectric constant of the piezoelectric layer as a whole. On the other hand, when the piezoelectric layer contains both the piezoelectric particles and the reinforcing particles, the connection between the piezoelectric particles having a large relative dielectric constant is divided by the interposition of the reinforcing particles having a smaller relative dielectric constant. As a result, it is possible to suppress an increase in the dielectric constant of the piezoelectric layer as a whole. On the other hand, since the particle connecting structure is maintained by the reinforcing particles and the piezoelectric particles, the piezoelectric strain constant can be maintained. That is, when the piezoelectric layer contains the reinforcing particles, the dielectric constant of the entire piezoelectric layer can be made smaller than the case where only the piezoelectric particles are contained while maintaining the piezoelectric strain constant. Therefore, a large electromotive field can be obtained by the above-mentioned formula (a).

  As the reinforcing particles, particles having high electric resistance are desirable. When the electric resistance of the reinforcing particles is large, the dielectric breakdown strength of the piezoelectric layer becomes large. This makes it possible to shorten the processing time by applying a high electric field in the polarization processing of the piezoelectric layer described later. In addition, it is possible to reduce the number of piezoelectric elements that are destroyed during the polarization process, which improves productivity.

  Further, 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, the cross-linking agent, the additive, the impurity ions ionized by the moisture in the air, etc. become hard to move, and the electric resistance of the piezoelectric layer increases. The chemical bond between the reinforcing particles and the elastomer can be realized by, for example, surface treating the reinforcing particles. As a method of surface treatment, a method of preliminarily reacting a reinforcing agent with a surface treating agent having a functional group capable of reacting with an elastomer polymer, and mixing the reinforcing particles with the elastomer polymer, or the surface of the reinforcing particle is treated with an acid or an alkali. Alternatively, there may be mentioned a method in which the compound is dissolved in subcritical water to generate a hydroxyl group and then mixed with an elastomer polymer having a functional group capable of reacting with the hydroxyl group. When the reinforcing particles are chemically bonded to the elastomer, the reinforcing particles are less likely to be misaligned even after repeated expansion and contraction. Further, since the reinforcing particles are difficult to be peeled from the elastomer, the physical properties and the output are less likely to vary from the initial values. Therefore, the output is stabilized and the sag resistance of the piezoelectric layer is improved. Further, since the breaking elongation of the piezoelectric layer becomes large, it is possible to suppress the deterioration of the piezoelectric performance due to local breakage at the time of stretching. As a result, high piezoelectric performance can be maintained even in the stretched state.

  The type of reinforcing particles is not particularly limited. For example, particles of titanium dioxide, silica, oxides such as 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 easily transmitted to the piezoelectric particles. From the viewpoint of facilitating the transmission of force to the piezoelectric particles, increasing the piezoelectric strain constant of the piezoelectric layer in the above formula (a), and increasing the electromotive field, the reinforcing particles have an elastic modulus higher than that of the matrix elastomer. It is better to use larger particles. For example, metal oxide particles such as titanium dioxide are preferable because of their small relative permittivity and large effect of improving dielectric breakdown resistance. As a method for producing metal oxide particles, the sol-gel method is preferable because particles having low crystallinity and a small relative dielectric constant can be obtained.

  The piezoelectric layer is manufactured by curing a composition obtained by adding powder of piezoelectric particles, a cross-linking agent, and the like to an elastomer polymer under predetermined conditions. Then, the piezoelectric layer is polarized. 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 studies by the present inventors, it was confirmed that in a thin film piezoelectric element, the smaller the cross-sectional area of the piezoelectric layer perpendicular to the tensile direction, the greater the sensitivity to the applied load. Therefore, it is desirable that the piezoelectric layer be thin. For example, the thickness of the piezoelectric layer is preferably 200 μm or less, more preferably 100 μm or less. On the other hand, if it is too thin, dielectric breakdown tends to occur during polarization treatment. Therefore, the thickness of the piezoelectric layer is preferably 10 μm or more, more preferably 20 μm or more.

<Electrode layer>
As the elastomer forming the electrode layer, one or more selected from cross-linked rubber and thermoplastic elastomer may be used as in the elastomer of the piezoelectric layer. Acrylic rubber, silicone rubber, urethane rubber, urea rubber, fluororubber, H-NBR and the like can be cited as elastomers having a relatively small elastic modulus and flexibility and having good adhesion to the piezoelectric layer.

  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. It may be appropriately selected from conductive carbon materials such as metal carbide particles, metal nanowires made of silver, gold, copper, platinum, nickel, etc., carbon black, carbon nanotubes, graphite, and graphene. Alternatively, particles coated with a metal such as silver-coated copper particles may be used. As the conductive material, one kind of these may be used alone, or two or more kinds may be mixed and used. The electrode layer may contain, as other components, a cross-linking agent, a dispersant, a reinforcing material, a plasticizer, an antiaging agent, a coloring agent and the like.

  The volume resistivity of the electrode layer is 100 Ω · cm or less in both the natural state and the stretched state from the stretched state up 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 in 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 greatly increases due to extension is used, the output in the natural state and the output in the extended state are significantly different, which causes a problem that the load cannot be accurately detected. Therefore, by combining a flexible piezoelectric layer that is stretchable and can maintain piezoelectricity even when stretched, and a flexible electrode layer that is stretchable and can maintain conductivity even when stretched, it can be used even in the stretched state. 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 blending amount of the conductive material increases, the volume resistivity of the electrode layer can be reduced, but the flexibility decreases. For example, when Ketjen Black (registered trademark) is used as the conductive material, it is desirable that the compounding 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 stacking 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 side of the piezoelectric layer in the thickness direction. When the piezoelectric particles are polarized in the plane direction that intersects the thickness direction of the piezoelectric layer, the pair of electrode layers may be arranged separately on one face that intersects 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 thereof.

  The breaking elongation of the piezoelectric element is preferably 10% or more. It is more preferable that it is 30% or more. In the present 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 at a tensile speed of 100 mm / min.

  The elastic modulus of the piezoelectric element is preferably 10 MPa or more and 500 MPa or less. In the present 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 of test piece type 2 at a pulling speed of 100 mm / min.

It is desirable that the piezoelectric element satisfy the following expression (I) in a state in which the piezoelectric element is expanded 10% in the uniaxial direction. The following formula (I) is an index indicating flexibility and whether or not it can be used even when stretched. That is, the piezoelectric element satisfying the following expression (I) is flexible and can generate an electromotive voltage by deformation even when it is expanded. On the other hand, when the following formula (I) is not satisfied, the change in electromotive voltage during expansion is large, and accurate sensing becomes difficult.
0.5 <V2 / V1 (I)
[In the formula (I), V1 is the electromotive voltage (V) of the piezoelectric element in the natural state, and V2 is the electromotive voltage (V) of the piezoelectric element in the state of being stretched by 10% in the uniaxial direction. ]
The electromotive voltage V1 in the natural state may be measured as follows. First, the piezoelectric element is installed in a repulsion elasticity 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 that is suspended with a suspension length of 2000 mm is pendulum-moved with a swing width (distance from the test piece in the horizontal direction) of 15 mm to 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 the average value of the five peak values of the electromotive voltage is taken as the electromotive voltage V1 in the natural state. Further, the piezoelectric element was installed in a repulsion elasticity tester (same as above) in a state of being stretched by 10% in a uniaxial direction, and an average value of five peak values of electromotive force measured by the same method as above was measured in a stretched state. The electromotive voltage V2 may be V2.

  The piezoelectric element has a protective layer in addition to the piezoelectric layer and the electrode layer. The protective layer is arranged so as to be laminated on at least the electrode layer of the piezoelectric layer and the electrode layer. For example, the protective layer may be arranged on one or both of the outer sides of the laminated body of the piezoelectric layer and the electrode layer in the laminating direction. When a plurality of units in which a piezoelectric layer is interposed between a pair of electrode layers are laminated, a protective layer may be arranged between the electrode layers adjacent in the laminating direction.

  It is desirable that the protective layer be stretchable together with the piezoelectric layer and the electrode layer. For the protective layer, it is desirable to use one or more selected from crosslinked rubbers and thermoplastic elastomers. By disposing the protective layer made of an elastomer, it is possible to secure the insulation of the piezoelectric element and suppress the destruction of the piezoelectric element due to mechanical stress from the outside. Further, the extension of the protective layer can increase the strain of the piezoelectric layer and improve the sensitivity of the sensor.

  Examples of elastomers having a relatively small elastic modulus and flexibility and having good adhesion to the electrode layer include natural rubber, isoprene rubber, butyl rubber, acrylic rubber, silicone rubber, urethane rubber, urea rubber, fluororubber and NBR. In order to reduce the change in the sensitivity of the sensor when it is repeatedly used, it is desirable that the protective layer has excellent sag resistance. Further, since the protective layer plays a role of protecting the piezoelectric element from external mechanical stress, it is desirable that the protective layer has excellent wear durability and tear durability. Further, in order to prevent the protective layer from breaking and the piezoelectric element from breaking during extension, it is desirable that the breaking elongation of the protective layer is larger than the breaking elongation of the piezoelectric layer.

  As described above, when a force is applied in the stacking direction of the piezoelectric elements (when the piezoelectric elements are compressed), the protective layer expands in the surface direction, so that a shearing force acts on the piezoelectric layers. As a result, a tensile force in the surface direction is applied to the piezoelectric layer in addition to the pressing force in the stacking direction, and the strain of the piezoelectric layer increases. As a result, the amount of charge generated in the piezoelectric layer increases, and the sensitivity of the sensor improves. The sensitivity improving effect of the protective layer is more remarkable as the elastic modulus of the protective layer in the tensile direction is smaller. In this respect, according to the piezoelectric sensor of the present invention, the elastic modulus of the protective layer is smaller than the composite elastic modulus of a set of laminates composed of a pair of electrode layers adjacent to the protective layer and a piezoelectric layer interposed therebetween. . Therefore, the strain of the piezoelectric layer is likely to increase, and the sensitivity improving effect of the protective layer is likely to be exhibited.

  The elastic modulus can be obtained as the slope of a stress-elongation (strain) curve in which the vertical axis represents stress and the horizontal axis represents elongation (strain). However, in the case of an elastic body, the slope changes as the strain increases, and therefore the value of the elastic modulus differs depending on which strain region the slope is obtained. Since conventional piezoelectric ceramics typified by PZT and piezoelectric resins typified by PVDF and polylactic acid can be used only in a region where the expansion rate is extremely small, the elastic modulus in the region where the strain amount is extremely small should be considered. Good. However, since the piezoelectric sensor of the present invention is flexible and expandable, it is necessary to design in consideration of the elastic modulus in the region where the expansion rate is large (the strain is large).

For example, the protective layer is elastically deformable in a region having an elongation of 25% or less, and the elastic modulus of the protective layer in the region is preferably less than 50 MPa. This can be 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 less than 20 MPa, and more preferably less than 10 MPa.

Further, the sensitivity improving effect of 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 including the pair of electrode layers and the piezoelectric layer interposed therebetween can be elastically deformed in the region where the expansion rate is 25% or less, and the expansion rate 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 expression (β-1). It is more preferable to satisfy the following expression (β-2). 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 in a state of being stretched by 10% or more.

  The Poisson's ratio of the elastomer is approximately 0.5. Therefore, 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. Therefore, the larger the thickness of the protective layer, the greater the effect of increasing the strain of the piezoelectric layer and the greater the effect of improving the sensitivity of the sensor. On the other hand, as the thickness of the protective layer increases, the size of the piezoelectric element increases. Therefore, the thickness of the protective layer may be appropriately set according to the installation place and the application. For example, the thickness may be 5 μm or more and 5 mm or less.

<Piezoelectric sensor>
An embodiment of the 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 this embodiment. FIG. 2 shows a sectional view taken along line II-II of FIG. In FIG. 1, the protective layer 13a is shown transparently. As shown in FIGS. 1 and 2, the piezoelectric sensor 1 includes a piezoelectric element 10 and a control circuit section 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 polarized, and the barium titanate particles are polarized in the thickness direction (vertical direction) of the piezoelectric layer 11. The electrode layer 12a contains acrylic rubber, conductive carbon black, and carbon nanotubes. The electrode layer 12a has a square thin film shape. The electrode layer 12 a is arranged on the upper surface of the piezoelectric layer 11. The 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 12b is arranged on the lower surface of the piezoelectric layer 11. The wiring 20b is connected to the right end of the electrode layer 12b. Seen from above, the piezoelectric layer 11 and the electrode layers 12a and 12b have the same size. The volume resistivity in the natural state of the electrode layers 12a and 12b is 0.2 Ω · cm, and the volume resistivity in the state of 10% extension 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 the 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 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 forming the piezoelectric element 10 is an elastomer. The protective layers 13a and 13b are also made of elastomer. The breaking elongation of the piezoelectric element 10 is 10% or more. Therefore, the piezoelectric element 10 is flexible and expandable. Therefore, even if the piezoelectric element 10 is arranged on an adherend that extends or bends, the movement of the adherend is less likely to be hindered. 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 the natural state and in the stretched state up to the state where 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 small increase in electric resistance even in a stretched state of being stretched up to 10% in the uniaxial direction, and have high conductivity. Therefore, even in the stretched state, the output does not easily decrease, and the load applied to the piezoelectric layer 11 can be accurately detected.

  As described above, according to the piezoelectric sensor 1, the piezoelectric sensor 1 is arranged on an adherend that is deformed by bending, stretching, compressing, etc., and is applied to the adherend not only when the adherend is not deformed but also when deformed. The load can be detected. That is, the load applied to the adherend can be detected even when the secondary deformation further occurs in the primary deformed state of the adherend.

  Since the piezoelectric sensor 1 has a higher sensor sensitivity (S / N ratio) than a capacitance type sensor, it is easy to detect a small load. Further, since the load can be detected by the voltage value or the current value, the circuit configuration can be simplified as compared with the case where the load is detected from the capacitance. Further, since the piezoelectric element 10 does not need to be energized, a power source for driving is also unnecessary. By the way, if the capacitance of the piezoelectric element 10 is also measured, it is possible to add the function of the capacitance sensor 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.

<Production of piezoelectric layer>
[Piezoelectric layers 1 to 4]
First, 100 parts by mass of a carboxyl group-modified hydrogenated nitrile rubber polymer (“Telvane (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. The blending amount of the barium titanate powder with respect to 100 parts by mass of the polymer content 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, as shown in Tables 1 and 2 below. The mass of the piezoelectric layer 4 is 800 parts by mass. Then, the kneaded product was repeatedly passed through a triple roll 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 manufacture piezoelectric layers 1 to 4 having a thickness of 50 μm.

[Piezoelectric layer 5]
Except that a polyurethane polymer (“N5139” manufactured by Tosoh Corporation) was used as an elastomer and 2 parts by mass of polyisocyanate (“Coronate (registered trademark) HX” manufactured by Tosoh Corporation) was used as a cross-linking agent. The piezoelectric layer 5 was manufactured in the same manner as the piezoelectric layer 2.

[Piezoelectric layer 6]
First, 100 parts by mass of a mixed liquid of liquid A and liquid B of silicone rubber polymer (“KE-1935” manufactured by Shin-Etsu Chemical Co., Ltd.) as an elastomer were mixed with powder of barium titanate (same as above). Was added and kneaded. Next, the kneaded product was repeatedly passed through a triple roll five times to obtain a slurry. Then, the obtained slurry was applied onto a substrate by a bar coating method. This was heated at 150 ° C. for 1 hour to manufacture a piezoelectric layer 6 having a thickness of 50 μm.

[Piezoelectric layer 7]
A piezoelectric layer 7 was manufactured 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 cross-linking agent and titanium dioxide sol as reinforcing particles to the slurry used for manufacturing the piezoelectric layer 2 and kneading with an air stirrer, the slurry was subjected to bar coating by a bar coating method. It was applied on the material. This was heated at 150 ° C. for 1 hour to manufacture piezoelectric layers 9 to 11 having a thickness of 50 μm. As shown in Table 2 below, the compounding amount of 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 tetra-i-propoxytitanium as 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 were added to the obtained chelate, and the mixture was stirred, and after the addition was completed, the temperature was raised to 40 ° C. and the mixture was further stirred for 2 hours. did. Then, it was allowed to stand overnight at room temperature to obtain a titanium dioxide sol.

[Piezoelectric layers 12, 13]
The slurry in which the reinforcing particles were dispersed was added to the slurry used for manufacturing the piezoelectric layer 2, 5 parts by mass of tetrakis (2-ethylhexyloxy) titanium as a cross-linking agent was further added, and the mixture was kneaded with an air stirrer. Was applied onto the base material. This was heated at 150 ° C. for 1 hour to manufacture piezoelectric layers 12 and 13 having a thickness of 50 μm. As shown in Table 2 below, the compounding amount of the slurry in which the reinforcing particles were 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.

  The slurry in which the reinforcing particles were dispersed was manufactured as follows. First, in a polymer solution prepared by dissolving a carboxyl group-modified hydrogenated nitrile rubber polymer (same as above) in acetylacetone, powder of titanium dioxide as reinforcing particles (anatase type, Wako Pure Chemical Industries, Ltd., product code 205-01715). ) Was added and kneaded. Next, the kneaded product was repeatedly passed through a triple roll 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 of a barium titanate particle binder (“BTD-UP” manufactured by Nippon Kagaku Kogyo Co., Ltd.) 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 barium titanate particle combination was used as the piezoelectric particles. The powder b of the barium titanate particles used was a barium titanate powder (single particle powder, “BT-UP2” manufactured by Nippon Kagaku Kogyo Co., Ltd.) that was baked at 1050 ° C. for 180 minutes and then ball milled. It was crushed and manufactured.

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

[Piezoelectric layer a]
For comparison, a 40 μm-thick piezoelectric layer made of PVDF (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 referred to as a piezoelectric layer b. The piezoelectric layer b was manufactured as follows. First, to 100 parts by mass of bisphenol A (“jER (registered trademark) 828” manufactured by Mitsubishi Chemical Co., Ltd.), 4.8 parts by mass of phenol novolac resin (“BRG # 558” manufactured by Showa Denko KK) was added as a curing agent. Then, 480 parts by mass of barium titanate powder (same as above) was added to the prepared polymer solution and kneaded, and then the kneaded material was repeatedly passed through a three-roll mill five times to form a slurry. Then, the obtained slurry was applied onto a substrate by a bar coating method and heated at 150 ° C. for 1 hour to produce a piezoelectric layer b having a thickness of 50 μm.

<Production 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 (“Ketjenblack EC600JD” manufactured by Lion Co., Ltd.) and 16 parts by mass of carbon nanotubes (“VGCF (registered trademark)” manufactured by Showa Denko KK) were added to the prepared polymer solution. Parts and 12 parts by mass of a polyester acid amide amine salt as a dispersant were added and dispersed by a bead mill to prepare a conductive paint. Subsequently, the conductive coating material was applied onto a polyethylene terephthalate (PET) film that had been subjected to a mold 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 the conductive paint was prepared without adding the carbon nanotubes and the dispersant.

[Electrode layer 3]
The conductive carbon black was changed from "Ketjen Black EC600JD" manufactured by Lion Co., Ltd. to "# 3050B" manufactured by Mitsubishi Chemical Co., Ltd., except that a conductive coating was prepared without adding carbon nanotubes and a dispersant. The electrode layer 3 was manufactured 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 on the PET film subjected to the mold release treatment by a bar coating method. This was heated at 150 ° C. for 1 hour to manufacture an electrode layer 4 having a thickness of 20 μm.

<Production of protective layer>
[Protective layer]
Liquid A and liquid B of a silicone rubber polymer (“KE1935” manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed in the same mass, degassed in vacuum to remove air bubbles, and then a bar was placed on a PET film that had been subjected to mold release treatment. It was applied by the coating method. This was heated at 150 ° C. for 1 hour to produce a protective layer having a thickness of 10 μm.

<Manufacture of piezoelectric element>
Various piezoelectric elements were manufactured as follows by appropriately combining the manufactured piezoelectric layers, electrode layers, and protective layers. First, the electrode layers were respectively arranged on the 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 Plastics Co., Ltd.). Next, a protective layer that had been subjected to an excimer treatment in advance was laminated on the electrode layer, and the protective layer and the electrode layer were pressure bonded using a laminator (same as above). An excimer lamp light source “FLAT EXCIMER” manufactured by Hamamatsu Photonics KK was used for the excimer treatment. A polarization treatment was performed by connecting a DC power supply to the electrode layer of the obtained laminate of protective layer / electrode layer / piezoelectric layer / electrode layer / protective layer and applying an electric field of 10 V / μm to the piezoelectric layer for 1 hour. It was FIG. 9 shows a vertical cross-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 this order from the top. The manufactured piezoelectric element has a square-shaped detection unit having a length of 30 mm and a width of 30 mm.

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

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

[Relative permittivity of piezoelectric particles and reinforcing particles]
Piezoelectric particles or reinforcing particles were blended with an elastomeric polymer whose relative dielectric constant was known by measurement to produce a composite. At this time, various composites having different compounding amounts were produced, and the relative permittivity of each of them was measured by the same method as the relative permittivity of the elastomer. Then, the relative permittivity of the blended particles was calculated by the following equation (b).
Log ε = V _f Log ε _f + V _p Log ε _p (b)
[Ε: relative permittivity of composite, V _f : volume ratio (%) of particles, ε _f : relative permittivity of particles, V _p : volume ratio (%) of elastomer, ε _p : relative permittivity of elastomer. ]
[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 obtain a test piece, and marked lines were provided at positions 20 mm apart in the length direction. A terminal made of copper foil was attached to the position of the marked line, and the electric resistance between the marked lines was measured. Based on the measured electrical resistance value and the size of the test piece, the volume resistivity was calculated by the following equation (c), and the volume resistivity in the natural state of the electrode layer was used.
Volume resistivity (Ω · cm) = electrical resistance value (Ω) × cross-sectional area of test piece (cm ² ) / distance between marked lines (cm) (c)
(2) Volume resistivity in the stretched state A tensile tester (manufactured by Shimadzu Corporation) was used to stretch the test piece of the electrode layer in the length direction. The electrical resistance between the marked lines was measured in a state where the test piece was extended by 10%, and the volume resistivity was calculated by the above equation (c), which was taken as the volume resistivity when the electrode layer was extended by 10%. Similarly, when the test piece was stretched by 50%, the volume resistivity was calculated in the same manner, and was taken as the volume resistivity when the electrode layer was stretched by 50%. The cross-sectional area of the test piece in the stretched state was calculated assuming the Poisson's ratio of the test piece to be 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 of test piece type 2 at 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 to calculate the elongation at break. The tensile test was performed using a dumbbell-shaped No. 5 type test piece at a tensile speed of 100 mm / min.

[Electromotive force]
The electromotive force was measured by a method similar to the pendulum test defined in JIS K 6255: 2013. First, the piezoelectric element was installed in a natural state in a repulsion elasticity tester manufactured by Kobunshi Keiki Co., Ltd. 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 pendulum-moved with a swing width (distance from the test piece in the horizontal direction) of 15 mm to 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 electromotive voltage V1 in the natural state. Further, the piezoelectric element was installed in a repulsion elasticity tester (same as above) in a state of being stretched by 10% in a uniaxial direction, and an average value of five peak values of electromotive force measured by the same method as above was measured in a stretched state. Of the electromotive voltage V2.

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

  First, the piezoelectric elements of Reference Examples 1 to 8 in which the piezoelectric layer does not contain reinforcing particles will be described. As shown in Table 1, according to the piezoelectric elements of Reference 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 in the natural state and at 10% elongation, and 5 Ω · cm or less at 50% elongation. As a result, the electrode layers constituting the piezoelectric elements of Reference Examples 1 to 8 satisfy the condition that the volume resistivity in the natural state and in the stretched state from that to the state of being stretched by 10% in the uniaxial direction are 100 Ω · cm or less. I can judge. Further, the value of V2 / V1 in the piezoelectric elements of Reference Examples 1 to 8 is larger than 0.5%, which satisfies the condition of the above-mentioned formula (I). The rate of change in electromotive voltage after repeated expansion and contraction was also 150% or less, and it was confirmed that the change in electromotive voltage was small even after repeated expansion and contraction, and that the expansion and contraction durability was excellent. If the piezoelectric element has a large elastic modulus, the movement of the adherend may be hindered. In this respect, the elastic modulus of the piezoelectric elements of Reference Examples 1 to 8 is 500 MPa or less. Therefore, as indicated by the circles in Table 1, it was confirmed that the piezoelectric elements of Reference Examples 1 to 8 had good followability with respect to the adherend and were not likely to hinder 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 return to its original shape after stretching. For this reason, the electromotive voltage in the stretched state could not be measured, and the stretch durability could not be evaluated. Further, in the piezoelectric element of Reference Example 16, since the compounding amount of the piezoelectric particles was large, the elastic modulus of the piezoelectric element was large and the elongation at break was less than 10%. For this reason, the electromotive voltage in the stretched state could not be measured, and the stretch durability could not be evaluated. Further, in the piezoelectric element of Comparative Example 3, the volume resistivity of the electrode layer significantly increased during extension, so that the electromotive voltage significantly decreased. Further, in the piezoelectric element of Comparative Example 4 having the electrode layer made of a silver paste, the volume resistivity of the electrode layer was greatly increased during the extension to be in the insulating state, so that the electromotive voltage in the extension state can be measured. Therefore, the stretch durability could not be evaluated.

  Next, the piezoelectric elements of Reference Examples 9 to 13 in which the piezoelectric layer contains reinforcing particles will be described. As shown in Table 2, the configurations of the piezoelectric elements of Reference Examples 9 to 13 are the same as the configurations of the piezoelectric element of Reference Example 3 except that reinforcing particles are mixed in the piezoelectric layer. Therefore, similarly to the piezoelectric element of Reference Example 3, the piezoelectric elements of Reference Examples 9 to 13 have a small change in electromotive voltage even after repeated expansion and contraction, and have excellent extension / contraction durability. Further, in the piezoelectric elements of Reference Examples 9 to 13, the electromotive voltage in the natural state was larger than that of the piezoelectric element of Reference Example 3. This is a great effect due to the addition of the reinforcing particles. Further, the reinforcing particles have hydroxyl groups on the surface and are chemically bonded to the elastomer. Therefore, the rate of change in electromotive voltage after repeated expansion and contraction became smaller.

  Next, the piezoelectric elements of Reference Examples 14 and 15 using a bonded body in which individual particles are chemically bonded as the piezoelectric particles will be described. As shown in Tables 1 and 2, the configurations of the piezoelectric elements of Reference Examples 14 and 15 are the same as the configurations of the piezoelectric element of Reference Example 3 except that the used piezoelectric particles are different. According to the piezoelectric elements of Reference Examples 14 and 15, as compared with the piezoelectric element of Reference Example 3 using barium titanate particles (single particles), the elastic modulus was small and the elongation at break was large. On the other hand, the electromotive voltages of the piezoelectric elements of Reference Examples 14 and 15 were larger than that of the piezoelectric element of Reference Example 3. The expansion / contraction durability of the piezoelectric elements of Reference Examples 14 and 15 was at the same level as that of the piezoelectric element of Reference Example 3. As described above, according to the piezoelectric elements of Reference Examples 14 and 15, flexibility could be significantly improved while ensuring high piezoelectricity. This is because the use of the aggregate of piezoelectric particles facilitates the formation of a connecting structure between the piezoelectric particles, so that high piezoelectricity can be obtained without increasing the compounding 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, a large electric field is generated even at a low filling rate as compared with the single particle 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 electromotive voltage generated when vibration is applied to the piezoelectric element of Reference Example 2 is shown. FIG. 3 is a graph of an electromotive voltage in the case where vibration is applied in the thickness direction in a state where the piezoelectric element is expanded by 1% in one direction in the plane direction. FIG. 4 is a graph of electromotive voltage when the piezoelectric element is extended in one direction of the surface by 10% and vibration is applied in the thickness direction. In FIG. 3 and FIG. 4, the electromotive voltage is shown by a thick line and the load is shown by a thin line. For the piezoelectric element, a fatigue durability tester “APC-1000” manufactured by Asahi Seisakusho was used, and a sine wave vibration with a load pp of 1.7 N was applied.

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

<Study 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 stretched state were measured. The structure 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]
Liquid A and liquid B of a silicone rubber polymer (“KE2004-5” manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed in the same mass, defoamed in vacuum to remove air bubbles, and then on a PET film that had been subjected to mold release treatment. Was applied by the 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 liquid B of a silicone rubber polymer (“KE1935” manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed in the same mass, degassed in vacuum to remove air bubbles, and then a bar was placed on a PET film that had been subjected to mold release treatment. It was applied by the coating method. 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 layers used in the piezoelectric elements of Reference 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 structure 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 voltage were measured according to the methods described above. The composite elastic modulus of the laminated body is a value obtained by separately calculating the elastic modulus of the piezoelectric layer and the elastic modulus of the electrode layer and adding them. The electromotive voltage in the 20% stretched state is an average value of five peak values of the electromotive force measured by installing the piezoelectric element in the uniaxial direction by 20% in a repulsion elasticity tester (same as above).

  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-described elastic modulus equation (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 formula (β-1) and the formula (β-2). Therefore, in the piezoelectric elements of Examples 17 and 18, the electromotive voltage was larger than that of the piezoelectric element of Reference Example 17 having no protective layer. In the piezoelectric elements of Examples 17 and 18, it can be seen that the effect of increasing the strain of the piezoelectric layer by the protective layer is fully exerted. Further, in the piezoelectric element of Example 18 in which the thickness of the protective layer was 1 mm, the electromotive voltage was larger than that of the piezoelectric element of Reference Example 15 in which the thickness of the protective layer was 10 μm. It is considered that this is because the effect of increasing the strain of the piezoelectric layer is increased due to the larger thickness of the protective layer. On the other hand, in the piezoelectric element of Reference Example 18, the protective layer 3 satisfies the equation (α) of the elastic modulus described above, but does not satisfy the equation (β-1). Therefore, the electromotive voltage of the piezoelectric element of Reference Example 18 was at the same level as that of the piezoelectric element of Reference Example 17 having no protective layer. In addition, in the piezoelectric element of Comparative Example 6 having the piezoelectric layer made of PVDF, the laminated body exceeded the elastic region when the expansion ratio was 10% or more. That is, it was confirmed that the piezoelectric element of Comparative Example 6 has a flexible protective layer, but cannot be used for the purpose of greatly expanding because the piezoelectric layer has poor flexibility.

  INDUSTRIAL APPLICABILITY The piezoelectric sensor of the present invention can be applied to an adherend that stretches or bends (repeated expansion and contraction and bending), and thus is a wearable device that measures the pulse rate, respiration rate, etc. without disturbing the natural movement of the living body. It is suitable as a biological information sensor. In addition, it can be used not only in the unstretched state but also in the stretched state (measurable), so it can be used in joints that require expansion and contraction in humans and robots, and in the process in which the sensor installation surface stretches and returns during the manufacturing process. You can It is also suitable as a pressure sensor for robots (including industrial and communication), medical care, nursing care, health care, sports equipment, automobiles, and the like.

  The piezoelectric sensor of the present invention is particularly suitable for application as a human-machine-interface (HMI) for human contact. For example, if it is placed on a mattress, a seat of a wheelchair, etc., it is possible to acquire information on the pulse, position and movement. In addition, it is placed and hit on sports equipment, such as sportswear and other equipment that touches the body (wearable such as shoes and gloves) and sports equipment such as balls, bats, rackets, various armor, weight training, and running equipment. By measuring the position, its strength, weight (acceleration), etc., it is possible to quantify the effect of training without impairing the feel at impact. Of course, the invention can be applied not only to sports and medical care but also to everyday items (clothes, hats, glasses, shoes, belts, masks, pendants, etc.). The digitized data and information can be sent to an IOT (Internet of Things) device as a control means.

1: piezoelectric sensor, 10: piezoelectric element, 11: piezoelectric layer, 12a, 12b: electrode layer, 13a, 13b: protective layer, 20a, 20b: wiring, 30: control circuit section.
40: piezoelectric element, 41: piezoelectric layer, 42a, 42b: electrode layer, 43a, 43b: protective layer.
80: Piezoelectric particles, 81: Elastomer, 82: Combined body of piezoelectric particles.

Claims (6)

A piezoelectric element having a piezoelectric layer containing an elastomer and piezoelectric particles, an electrode layer containing an elastomer and a conductive material, and a protective layer laminated on at least the electrode layer of the piezoelectric layer and the electrode layer,
The electrode layer has a volume resistivity of 100 Ω · cm or less in a natural state and a stretched state up to a state in which the electrode layer is stretched uniaxially by 10%.
Modulus of the protective layer is rather smaller than the composite elastic modulus of a set of stack of piezoelectric layers interposed therebetween and a pair of the electrode layer adjacent to the protective layer,
A piezoelectric sensor, wherein the protective layer is elastically deformable in a region having an elongation of 25% or less and satisfies the following expression (α) .
  The piezoelectric sensor according to claim 1, wherein the piezoelectric particles include an aggregate in which a plurality of piezoelectric particles are aggregated.   The piezoelectric sensor according to claim 1 or 2, wherein the elastomer and the piezoelectric particles are chemically bonded to each other in the piezoelectric layer.   The piezoelectric sensor according to claim 3, wherein the piezoelectric particles are surface-treated.   The piezoelectric sensor according to claim 1, wherein the piezoelectric layer contains reinforcing particles having a relative dielectric constant of 100 or less.   The piezoelectric sensor according to claim 5, wherein the reinforcing particles are metal oxides.