JP6297365B2 - Wave-receiving piezoelectric element - Google Patents

Description

  The present invention relates to a piezoelectric element that receives a sound wave, and more particularly, to a wave-receiving piezoelectric element having improved sound wave reception sensitivity.

Polymer piezoelectric bodies including vinylidene fluoride resin that has undergone polarization treatment are (1) more flexible than ceramic piezoelectric bodies, and can be made into any shape that is easy to make thin, large, and long. (2) The hydrostatic piezoelectric strain constant d _h is equal to or less than that, but since the dielectric constant ε is small, the hydrostatic voltage output coefficient (determined by d _h / ε) ( g _h constant) is extremely large, and the thus sensitivity characteristics are excellent; (3) low density, due to low elasticity, acoustic impedance (sound velocity × density), close to the value of the water or a living body, thus water and biological and elements And has a characteristic such that efficient energy propagation is possible. Taking advantage of such characteristics, polymer piezoelectric materials are generally electro-mechanical (acoustic) conversion elements such as speakers, microphones, ultrasonic probes, hydrophones, vibration meters, strain gauges, blood pressure monitors, bimorph fans, etc. As a pyroelectric conversion element, application to a wide range of uses has been proposed or put into practical use.

  As a receiving type piezoelectric element, a microphone that is usually placed and used in these media to receive sound waves propagating in air or gas, and a normal wave to receive sound waves propagating in water or liquid. There are known hydrophones that are used in such media.

  These piezoelectric elements are always required to have improved reception sensitivity. However, the polymer piezoelectric material has a problem that the dielectric constant is small as described above, and the impedance increases when the thickness of the polymer piezoelectric material is increased in order to increase the receiving sensitivity.

Further, reception sensitivity of the sound wave in these piezoelectric elements, when the size of the piezoelectric element is sufficiently smaller than the wavelength of the acoustic wave is represented by the hydrostatic piezoelectric constant (d _h constant), naturally d _h constants The larger the value, the better the sensitivity. In general, d _h constant of the piezoelectric body, given by the following equation as the sum of the contribution of the polarization direction (d ₃₃₎ and two-way contributions perpendicular to the polarization direction (d ₃₁ and d _32).
d _h = d ₃₁ + d ₃₂ + d ₃₃ (1)
In a generally used sheet-like piezoelectric body, the polarization direction is often taken in the thickness direction because of the ease of polarization, and the thickness direction contribution (d ₃₃ ) is orthogonal to it. The two-direction contributions (d ₃₁ and d ₃₂ ) are often opposite in sign, that is, have an opposite effect. Such contradictory effects must be taken into account when designing the piezoelectric element.

D ₃₁ in Patent Document 1, in order to increase the d _h constant (increased sensitivity), even when using a piezoelectric body is polarized in the thickness direction, rather than its constant d _33, which rather perpendicular thereto Alternatively, it is disclosed that an element configuration that positively utilizes the contribution of the d ₃₂ constant, particularly that such an element configuration is formed using a generally hollow cylindrical piezoelectric body (paragraph 0009, FIG. 1). .
That is, as shown in FIG. 1, two opposite side surfaces 2c and 2d having a certain area of a cylindrical piezoelectric body 2 are held between a pair of rigid members 4 and 5 having a larger area, so that the cylindrical piezoelectric body 2 thereby achieving a significant increase in apparent d _h constant by causing axially deformed to increase the effective active area contributing sound pressure. Furthermore, it performed for pressure amplification by the rigid member, blocking the sound pressure to the rigid member surface (the rigid cylinder 6), in many cases, of the d _h constant of the piezoelectric body, the polarization direction of the piezoelectric member 2 The contribution of the thickness direction component (d ₃₃ component) having the opposite action to the component orthogonal to the component (d ₃₁ or d ₃₂ component of the piezoelectric body polarized in the thickness direction), that is, in the thickness direction of the piezoelectric body 2 effects of the sound pressure, there is also the effect of blocking in this aspect contributes to increase the apparent d _h constants (paragraph 0014). However, the above piezoelectric element has a problem that its structure becomes complicated.

Japanese Patent No. 3270616

As described above, the conventional piezoelectric element has a problem that the impedance increases with the thickness of the piezoelectric body and the structure becomes complicated. Therefore, the present invention uses a piezoelectric body having an appropriate thickness, can deal with the conflicting effects of d ₃₁ , d ₃₂ , and d ₃₃ with a simple structure, and can further improve the receiving sensitivity. An object of the present invention is to provide a piezoelectric element.

According to the researches of the present inventors, in order to achieve the above-mentioned object, when a solid body having a curved surface is used as a skeleton and a curved surface portion is formed of a piezoelectric body to form a piezoelectric element, the element is formed with a simple structure. It is possible to secure the area of the piezoelectric body and increase the capacitance without increasing the thickness, and when using the piezoelectric body polarized in the thickness direction, it is not the d ₃₃ constant, but rather orthogonal to this. The present inventors have found that an element configuration that actively uses the contribution of the d ₃₁ (or d ₃₂ ) constant can be obtained, and the present invention has been completed.
In the present specification, the “sound wave” should be interpreted as a pressure vibration wave, and is not limited to an audible sound wave. In other words, “sonic waves” are not only elastic waves propagating in the air, which is the audible frequency of humans and animals in the narrow sense, but also all elastic waves propagating in elastic bodies, whether in the gas, liquid, or solid, in a broad sense. The generic name of

The wave receiving piezoelectric element according to the first aspect of the present invention is a wave receiving piezoelectric element 10 as shown in FIGS. 1A to 1C, for example; a piezoelectric body 12 formed on a curved surface; A positive electrode 15a and a negative electrode 15b respectively disposed on both surfaces of the piezoelectric body 12; the wave-receiving piezoelectric element 10 is formed from a solid 10 'surrounded by the curved surface, and the curved surface has a three-dimensional shape. All or part of the surface of 10 ′ is formed, a space is formed on the inner surface side of the curved surface, and the solid 10 ′ is a closed solid. The “curved surface” includes not only a continuously curved surface that is not a plane but also a surface that is curved by a combination of planes by forming two dihedral angles between adjacent two planes in a line segment. Note that it is preferable that the dihedral angle exceeds 90 ° because the curved surface to be formed becomes more gentle. In addition, the curved surface by the combination of two planes adjacent by a point like the vertex of a pyramid is not included.
With this configuration, the piezoelectric body can be polarized by being compressed and expanded in the circumferential direction by the sound wave sw (see FIG. 1B) received from the thickness direction of the piezoelectric body. In addition, the piezoelectric element can be formed with a simple structure. Furthermore, it is possible to increase the area of the piezoelectric body and secure a sufficient electrostatic capacity to form a highly sensitive piezoelectric element. Furthermore, it is possible to positively utilize the contribution of mainly d ₃₁ constant to suppress the contribution of d ₃₃ constant.

The wave receiving piezoelectric element according to a second aspect of the present invention is the wave receiving piezoelectric element according to the first aspect of the present invention, wherein the curved surface is a cylinder, a polygonal column, a cone, a polygonal pyramid, a sphere, a polyhedron. Forming an entire surface or a partial surface of one solid selected from the group consisting of an ellipsoid and a solid formed by a part of these solids.
With this configuration, it is possible to form a wave-receiving piezoelectric element having a simple shape and configuration.

The wave receiving piezoelectric element according to the third aspect of the present invention is the wave receiving piezoelectric element according to the first aspect or the second aspect of the present invention, wherein the wave receiving sensitivity / thickness of the piezoelectric body is: 0.17 to 10 V / Pa / m.
If comprised in this way, the receiving type piezoelectric element excellent in receiving sensitivity can be obtained.

A wave receiving type piezoelectric element according to a fourth aspect of the present invention is the wave receiving type piezoelectric element according to any one of the first to third aspects of the present invention, wherein the space is filled. The filler has a rubber hardness of A0 to A90 based on JIS K6253 or ASKER C 0 to 90 based on SRIS 0101.
With this configuration, since the outside of the piezoelectric forming the outer surface can form a wave receiving type piezoelectric element that can withstand external water pressure or the like, the hydrophone that receives sound waves propagating through the piezoelectric element particularly in water or liquid Can be used as

According to the present invention, a piezoelectric element can be formed with a simple structure. Furthermore, it is possible to increase the area of the piezoelectric body and secure a sufficient electrostatic capacity to form a highly sensitive piezoelectric element. Furthermore, it is possible to positively utilize the contribution of mainly d ₃₁ constant to suppress the contribution of d ₃₃ constant.

FIG. 2A is a partially cutaway perspective view showing a cylindrical solid 10 ′ that is a skeleton of the wave receiving piezoelectric element 10. FIG. 2B is a partially cutaway perspective view showing actual dimensions of the created wave receiving piezoelectric element 10. FIG. 4C is a partial cross-sectional view of the wave receiving piezoelectric element 10 on a plane parallel to the bottom surface 11b. It is the photograph of the receiving type piezoelectric element 10 actually completed. It is an example of a three-dimensional shape that can be a skeleton of a receiving piezoelectric element. It is a block diagram of a wave receiving type piezoelectric element created by laminating polymer piezoelectric bodies in parallel. It is a hydrophone circuit diagram directly connected to the impedance conversion circuit. It is a photograph which shows the outline of a hydrophone. It is a hydrostatic pressure sensitivity measurement principle figure. It is a figure which shows an acoustic sensitivity measurement system. It is a figure which shows the frequency characteristic of acoustic sensitivity. It is a principle diagram of a hydrostatic pressure piezoelectric strain constant measuring apparatus. It is a figure explaining the external pressure concerning a cylinder. It is a block diagram of a wave receiving type piezoelectric element using a flat piezoelectric material.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same or similar reference numerals, and redundant description is omitted. Further, the present invention is not limited to the following embodiment.

  As shown in FIGS. 1A to 1C, the wave receiving piezoelectric element according to the first embodiment of the present invention includes a piezoelectric body 12 formed in a cylindrical shape as a curved surface; A positive electrode 15a and a negative electrode 15b are provided respectively on the surface. The cylindrical shape forms the outer periphery of the wave receiving piezoelectric element 10. The upper surface 11a and the bottom surface 11b of the case 11 form a cylindrical upper surface and a bottom surface, and the cylindrical shape is closed from above and below. Thus, the wave receiving piezoelectric element of the present application is a solid whose solid structure is closed, and part or all of the three-dimensional surface is formed of a piezoelectric body formed on a curved surface. Furthermore, a space is formed on the inner surface side of the piezoelectric body.

With reference to FIGS. 1A to 1C, the case where the wave receiving piezoelectric element 10 of the present application is formed from a cylindrical (column) solid 10 ′ will be further described. FIG. 1A is a partially cutaway perspective view showing a cylindrical solid 10 ′ which is a skeleton of the receiving piezoelectric element 10. The cylindrical solid body 10 ′ is a cylinder formed by winding a PVDF piezo film 12 as a piezoelectric body around a bobbin-shaped metal case 11. The case 11 forms an upper surface 11a and a bottom surface 11b of a cylinder, and the upper surface 11a and the bottom surface 11b are supported by a central column 11c. The PVDF piezo film 12 forms a cylindrical side surface. Since the upper surface 11a and the bottom surface 11b have outward protrusions from the support column 11c, a space is formed on the inner surface side of the PVDF piezo film 12. The top surface 11a and the bottom surface 11b close the space in the column from above and below to form a closed cylindrical solid 10 ′. Thus, a closed solid is used for the skeleton of the receiving piezoelectric element 10.
Al electrodes (15a, 15b) are vapor-deposited on both surfaces of the PVDF piezo film 12 as positive and negative electrodes in a range smaller than the entire surface of the film 12 (for example, with margins on the top, bottom, left and right of the surface). In the case of a cylindrical piezoelectric element as shown in FIG. 1, vapor deposition may be continuously performed in the circumferential direction.

FIG. 1B is a partially cutaway perspective view showing the actual dimensions of the created wave receiving piezoelectric element 10. As shown in FIG.1 (b), it is preferable that the outer surface of the PVDF piezo film 12 is coat | covered with the polyolefin film 14 as a protective film. FIG. 1C is a partial cross-sectional view of the wave-receiving piezoelectric element 10 on a plane parallel to the bottom surface 11b. As shown in FIG. 1C, the wave receiving type piezoelectric element 10 has a support 11c as a center, a space (or filler) around it, an Al vapor deposition electrode 15a (positive electrode), a PVDF piezo film 12, an Al vapor deposition electrode. 15b (negative electrode) and polyolefin film 14 are laminated in this order. Note that either of the Al vapor deposition electrodes 15a and 15b may be a positive electrode, and one may be a positive electrode and the other a negative electrode.
FIG. 2 is a photograph of a wave-receiving piezoelectric element that is actually completed by connecting lead wires to positive and negative electrodes, and is created using screws, flat plates, nuts, and the like.
As shown in FIG. 1B, the space formed on the inner surface side of the PVDF piezo film 12 may be filled with polyurethane 13 as a filler. The PVDF piezo film 12 is supported from the inside by the polyurethane 13 so that it can withstand water pressure from the outside, and the wave receiving piezoelectric element 10 can be used as a hydrophone.

[Piezoelectric body]
As the piezoelectric body of the present application, a polymer piezoelectric body that is excellent in flexibility, can be easily formed into a thin film, a large area, and a long length and can be formed in any shape and form is preferable. As the polymer piezoelectric material, a vinylidene fluoride resin piezoelectric material having excellent piezoelectric characteristics is preferable. Among them, vinylidene fluoride (VDF) single weight that needs to be uniaxially stretched for β-type crystallization suitable for piezoelectricity expression is preferable. Coalescence is preferred. In addition, a VDF copolymer that can be crystallized in β-type under normal crystallization conditions (for example, a superior amount of VDF and an inferior amount of vinyl fluoride (VF), trifluoroethylene (TrFE), or tetrafluoroethylene (TFE). And a copolymer of a dominant amount (especially 70 to 80 mol%) of VDF and an inferior amount (especially 30 to 20 mol%) of TrFE is most preferably used. A vinylidene cyanide-vinyl acetate copolymer having a relatively high heat resistance is also preferably used.

  These polymer-based piezoelectric materials are formed as films or sheets after film formation by melt extrusion or the like, and if necessary, subjected to polarization treatment by uniaxial stretching or heat treatment below the softening temperature, and application of an electric field below the softening temperature. Is done. In the polymer piezoelectric material used in the present invention, these films or sheets can be used as a single layer, or the polarization direction may be the same or they may be laminated in the opposite direction via an intermediate electrode layer. The number of stacked layers is not limited in principle, but examples include a stacked body of about 2 to 20 layers.

The thickness of the piezoelectric body (thickness in the polarization direction) is 10 μm to 2 mm, more preferably 40 μm to 1 mm. When the thickness is 10 μm or more, it is possible to suppress a decrease in the shape self-holding property, and it is possible to reduce the necessity of relying on the shape retention of other structural materials. It is possible to avoid that the stress generated inside becomes small and sufficient sensitivity cannot be obtained. If it is 2 mm or less, it can be avoided that the rigidity becomes too large and the molding process becomes difficult, and the rigidity becomes moderately small and the molding process becomes easy. Furthermore, when it is 40 μm or more, the shape self-holding property is further increased, which is preferable.
There is no particular limitation on the size of the piezoelectric body (the size when the curved piezoelectric body is expanded into a flat plate shape), and the piezoelectric body can be manufactured and formed as all or part of a three-dimensional surface. Any size is possible.

[electrode]
As the positive electrode and the negative electrode arranged on both surfaces of the piezoelectric body, a vapor deposition electrode, a foil electrode attached with an adhesive, or a metal spray electrode as generally used can be used. However, preferably, it satisfies the requirements such as (1) does not hinder the compressive deformation of the piezoelectric body, and (2) it is easy to shape the piezoelectric body formed on the curved surface. An electrode capable of ensuring flexibility such as a conductive paint is suitable. For example, Al vapor deposition electrode and copper foil electrode can be mentioned.
The thickness of the electrode is 30 nm to 70 μm, more preferably 60 nm to 40 μm. When the thickness is 30 nm or more, the value of electric resistance is small, and when it is 70 μm or less, flexibility can be maintained.

[Protective film]
It is preferable to provide a protective film such as a coating for protecting the electrode on the outer surface of the piezoelectric body. An example of such a protective film is a resin coating that has good waterproofness and water vapor barrier properties, and has a similar acoustic impedance and compatibility with the polymer piezoelectric material. For example, a polyolefin film, a PVDF film, a vinylidene chloride film, a polyester film, a polyurethane coating, an epoxy coating, and the like can be given.

[Filler]
As shown in FIG. 1A, a space is formed on the inner surface side of the piezoelectric body 12.
The internal space formed in the solid constituting the wave receiving type piezoelectric element is preferably a vacuum if the piezoelectric body can withstand the pressure difference between the inside and outside. Or, as a filler, any gas (air, nitrogen, etc.) is filled, any liquid (water, etc.) is filled, or an elastic resin such as polyurethane or a foamed resin such as urethane foam is optional. It is also possible to fill the buffer material. When the wave receiving type piezoelectric element is used as a hydrophone, any filler that can support hydrostatic pressure may be used.
In addition, although gas, liquid, and solid were mentioned as a filler, what is necessary is just to select these suitably according to the environment where external pressure differs, for example, the environment where external pressure differs, such as a shallow area | region and a deep area | region, if it is underwater. Alternatively, the thickness of the piezoelectric body may be adjusted to withstand external pressure.
When the solid filler is based on JIS K6253 (durometer type A (Shore A)), the rubber hardness is preferably A0 to A90. Alternatively, when based on SRIS 0101 (standard of the Japan Rubber Association, soft rubber (expanded rubber), spring type Asker C type), the rubber hardness (sponge hardness) is preferably ASKER C0 to C90.

[3D]
The wave receiving piezoelectric element of the present application includes a piezoelectric body formed on a situation. The piezoelectric body formed on the curved surface forms a three-dimensional surface that becomes a skeleton of the receiving piezoelectric element. Therefore, the piezoelectric body is polarized by being compressed and expanded in the circumferential direction by the sound wave received from the thickness direction. For example, the piezoelectric body 12 forms the side surface of the cylinder shown in FIG. Alternatively, it may be the side surface of the cone shown in FIG. 3B or the side surface of the truncated cone shown in FIG. Moreover, the perimeter of a side surface may be formed and a part (for example, half circumference) may be sufficient.
Alternatively, the piezoelectric body 12 may form the surface of a sphere shown in FIG. 3D, or may form the surface of an ellipsoid. The piezoelectric body may form the entire surface or a part of the surface. In the case of a sphere or an ellipsoid, compression / extension in the circumferential direction of the piezoelectric body is not limited to one direction, and is possible in any direction, which is more preferable.
Note that the solid shape may be a solid formed by a part of a solid such as a sphere or a cylinder like the hemisphere or half pipe shown in FIG.
In this way, the curved surface formed by the piezoelectric body constitutes a continuously curved, non-planar surface, and forms the entire surface or a partial surface of a closed solid.

Alternatively, the curved surface formed by the piezoelectric body may be a curved surface formed by a combination of planes by forming two dihedral angles between two adjacent planes. For example, the piezoelectric body 12 forms the side surface of the prism shown in FIG. Alternatively, the side surface of the pyramid shown in FIG. Moreover, the perimeter of a side surface may be formed and a part (for example, half circumference) may be sufficient.
In addition, it is preferable that the dihedral angle exceeds 90 ° because the curved surface formed of the piezoelectric body becomes gentler. Therefore, a pentagonal or higher prism and a pentagonal or higher pyramid are preferable.
Furthermore, a polyhedron can be given as a solid surrounded by a curved surface. For example, the piezoelectric body 12 may form the surface of the polyhedron shown in FIG. 3 (h) or the surface of the polyhedron shown in FIG. 3 (i). In addition, the piezoelectric body may form the entire surface of the polyhedron or a part of the surface.

The solid shown in FIG. 3 is an example, and the solid that forms the skeleton of the receiving piezoelectric element of the present application may be a solid that can form a curved surface and can form a space on the inner surface side of the piezoelectric body. .
When the piezoelectric body forms a part of the three-dimensional surface, the remaining surface may be formed of a rigid member such as a metal as shown in FIG. By using the case 11 shown in FIG. 1A, a stronger solid can be easily formed.
Moreover, you may provide the support | pillar etc. for maintaining a shape inside the solid | solid like the case 11 shown to Fig.1 (a).
The material of the rigid member is not particularly limited as long as the material has a strength that can maintain a solid body.

[Receiving sensitivity and piezoelectric thickness]
The wave receiving piezoelectric element of the present application preferably satisfies 0.17 ≦ wave receiving sensitivity / thickness ≦ 10 V / Pa / m in the relationship between the wave receiving sensitivity and the thickness of the piezoelectric body. A wave receiving type piezoelectric element satisfying this range can increase wave receiving sensitivity while ensuring capacitance.

[Estimation of receiver sensitivity]
The verification method of the received wave sensitivity used in the examples of the present application will be described in detail by taking a hydrophone provided with a flat PVDF polymer piezoelectric material as an example.
A hydrophone was created by directly connecting an impedance conversion circuit to a 12 mmφ piezoelectric element (FIG. 4). The hydrostatic pressure sensitivity was calculated by calculating the gradient of the output voltage with respect to the pressure change generated by pressurizing it in a pressure vessel, and a method for setting the value as the received wave sensitivity (acoustic sensitivity) was examined.

[Configuration of piezoelectric element]
As shown in FIG. 4, a copper foil electrode was applied to a P (VDF / TrFE) piezoelectric body having a thickness of 500 μm and laminated in parallel to obtain a piezoelectric element.
When than g _h constant (0.14Vm / N) to estimate the piezoelectric sensitivity (reception sensitivity), the following formula (2) (s: piezoelectric sensitivity, t: thickness).
s = g _h × t = 70 μV / Pa (2)

[Impedance conversion circuit]
FIG. 5 is a hydrophone circuit diagram directly connected to the impedance conversion circuit.
An impedance conversion circuit using an FET source follower having an input resistance of 1 GΩ was directly connected to the element shown in FIG. The measured value of the capacitance of the element is 30 pF (31 pF in calculation). From the time constant determined by multiplication with the input resistance value, the low-frequency cutoff frequency is 5.3 Hz (see FIG. 5).
FIG. 6 is a photograph showing an overview of the hydrophone.
A hydrophone was created by molding a piezoelectric element and an impedance conversion circuit with urethane rubber. The cable length is 5 m.

[Measuring method of hydrostatic pressure sensitivity and hydrostatic pressure sensitivity]
If the hydrophone is pressurized quasi-statically, the gradient of the voltage generated at that time with respect to the pressure change is considered to coincide with the acoustic sensitivity within the range of the linearity of the piezoelectric element. Based on this idea, the conventional method for measuring the _dh constant of a piezoelectric body was modified, and a method for measuring the hydrostatic pressure piezoelectric constant was devised, and a simple method for estimating acoustic sensitivity (received sensitivity). We examined its usefulness.
As a method for applying pressure quasi-statically, a hydrophone was set in a pressure vessel filled with a liquid leaving a slight gap, and pressurized by injecting a compressed gas. As the liquid, N ₂ gas was used as a compressed gas of a fluorine-based one having excellent insulating properties. The pressure gradient of the output voltage is defined as hydrostatic pressure sensitivity. (D In the _h constant measurement, the generated charge is measured by a charge amplifier.)
In order to pressurize to about 0.5 MPa, the generated voltage becomes about 40 V and exceeds the amplifier power supply voltage, and in order to adjust the time constant to the boosting speed, an attenuating capacitor was connected in parallel. Therefore, the actual sensitivity needs to be corrected by the capacitance ratio.
FIG. 7 shows the principle of hydrostatic pressure sensitivity measurement.
As a result of measurement using this method, the hydrostatic pressure sensitivity was 74 μV / Pa, −202.6 dB (re 1 V / μPa).

[Frequency characteristics of acoustic sensitivity]
FIG. 8 shows a measurement system when the acoustic sensitivity of the hydrophone is actually measured, and FIG. 9 shows the result. It can be confirmed that the level shows a good agreement with the hydrostatic pressure sensitivity. In addition, we think that there is no attenuation by a long cable.
Although a dip is seen near 35 kHz in FIG. 9, it is considered that the relationship between the outer diameter of the hydrophone (D = 20 mm) and the wavelength in water (λ = 42 mm) at that time, D≈λ / 2. .
As described above, it was shown that the acoustic sensitivity (received sensitivity) can be estimated by measuring the hydrostatic pressure piezoelectric sensitivity.

  First, as an example of a receiving piezoelectric element having a piezoelectric body formed on a curved surface, a mechanism that can increase the receiving sensitivity while securing capacitance is described using the cylindrical hydrophone shown in FIG. To do.

[Example 1]
As the piezoelectric body, a PVDF uniaxially stretched polarizing film (manufactured by Kureha Co., Ltd., Kureha KF Piezo Film) was used. Thickness: 110 μm, width: 10 mm, length: 19 mm (stretching direction). As the electrode, an aluminum (Al) vapor deposition electrode was used. Polyurethane was used as the filler. Polyolefin was used for the protective film.
FIG. 2 is a photograph of the hydrophone (Example 1) created this time.
The element structure is as shown in FIGS. A cylindrical case 11 was formed with a PVDF piezo film 12 on a metallic case 11, filled with polyurethane 13, a lead wire was drawn out, and a protective film of a polyolefin film 14 was coated on the outermost layer. Al vapor deposition electrodes were deposited on both sides of the PVDF piezo film 12 leaving some margins on the top, bottom, left and right of the film. The case 11 was actually formed of screws, flat plates, and nuts. The rubber hardness of the filled polyurethane was A28.

[Hydrostatic pressure piezoelectric strain constant measuring device]
FIG. 10 shows a principle diagram of a hydrostatic piezoelectric strain constant measuring apparatus for estimating the received wave sensitivity. This device is for measuring the hydrostatic piezoelectric constant d _h of the piezoelectric body. Pressurized placed sample (piezoelectric elements) in an inert liquid, the gradient becomes d _h for the pressure of the polarization density generated at that time. FIG. 10 is a simplified diagram of the principle diagram of FIG.

[Theoretical reception sensitivity]
The d _h permittivity (ε ₀ ε ') by dividing to the hydrostatic piezoelectric output constant g _h becomes, is this multiplied by the piezoelectric thickness t, the theoretical wave-receiving sensitivity s.
That is,
s = g _h × t (3)
This s and the actually measured reception sensitivity show a good agreement.
On the other hand, (3)
_{s = g h × t = d} h / (ε 0 ε ') × t = d h × S / (ε 0 ε' × S / t)
Therefore,
s = d _h × S / C (4)
Thus, if C is known, the theoretical sensitivity can be obtained using equation (4) (S: electrode area, C: capacitance).

[Receiving sensitivity]
Cylindrical piezoelectric element (Example 1) and plate-shaped piezoelectric element (piezoelectric film, FIG. 12) was determined theoretical reception sensitivity measures the d _h of. The comparison results are shown in Table 2. The cylindrical shape is about 35 times larger.
Here, regarding the piezoelectric element of Example 1, it is estimated that the elastic modulus of polyurethane having a hardness of A28, which is a filling material in the cylinder, is about 1 to 2 orders of magnitude smaller than PVDF, and the shape of the case is taken into consideration. We consider the mechanism of increasing sensitivity by considering the pressure distribution in an infinitely long cylindrical wall with no filler material.
FIG. 11 shows a cylinder to which external pressure is applied (a curved surface of a piezoelectric body to which external pressure is applied).
In the case where the wavelength is sufficiently larger than the element size, it is only necessary to consider a state in which the hydrostatic pressure is applied with zero internal pressure and external pressure P. Assuming that the inner diameter of the cylinder is a and the outer shape is b, the stress σ _r in the radial direction in the wall surface and the stress σ _{θ in the} circumferential direction (the axial direction is not considered)
σ _r = −b ² (1-a ² / r ² ) P (5)
σ _θ = −b ² (1 + a ² / r ² ) P (6)
Thus, the voltage V generated inside and outside the cylindrical wall is expressed using piezoelectric output constants g ₃₁ and g ₃₃ .
V = ∫ (σ _θ g ₃₁ + σ _r g ₃₃ ) dr (definite integral from integral a to b)
= − {G ₃₁ + (b−a) g ₃₃ / (a + b)} bP (7)
It becomes. Therefore, since the sensitivity s is V / P,
s = − {g ₃₁ + (b−a) g ₃₃ / (a + b)} b (8)
It becomes.
In the case of PVDF, the signs of g ₃₁ and g ₃₃ are opposite and in the canceling direction. However, when the thickness of the piezoelectric body is thin, since (b−a) / (b + a) << 1, the contribution of g ₃₁ It can be seen that the generated voltage is increased by applying an outline that is sufficiently larger than the thickness. When the sensitivity in the case of this element is calculated by Equation (8), s = 634 μV / Pa. Actually, it is necessary to discuss the compression elastic modulus of the filler in the cylinder and the elastic modulus of the protective layer, but the mechanism for increasing the sensitivity is qualitatively understood.
Note that the axial stress (the contribution of g _32), if the hydrophone cylindrical as shown in FIG. 1, the shaft within (center axis, strut) is supported by it all the pressure exerted on the top and bottom rigid Therefore, it is considered that no stress in this direction is generated in the piezoelectric body. Thus, the contribution of g ₃₂ may be cut off using a rigid body. Note that it is preferable that the shaft is not rigid and is soft to some extent, even if g ₃₂ contributes, it contributes in the + direction (increased sensitivity).

[Capacitance]
Here, the capacitance of the flat plate in the case where the receiving sensitivity is the same is examined. In order to obtain the same sensitivity with a flat plate, the piezoelectric body needs to be about 35 times thicker. When the capacitance value for the same size (9 mm × 18 mm) is calculated, it is as shown in Table 3, and the capacitance is about 35 times smaller than that of the cylinder.

  It was shown that a cylindrical hydrophone using a thin piezo film formed on a curved surface can increase the receiving sensitivity while ensuring the capacitance.

[Receiving sensitivity and piezoelectric thickness]
Table 4 further shows a comparison between the cylindrical hydrophone (Example 1) shown in FIG. 2 and various plate-like piezoelectric elements (Comparative Examples 1 to 3). FIG. 12 shows the piezoelectric elements used in Comparative Example 1 and Comparative Example 2. FIG. 4 shows the piezoelectric element used in Comparative Example 3.
As shown in Comparative Examples 1 to 3, in order to increase the sensitivity of the hydrophone, it is preferable that the piezoelectric body is thicker. However, the capacitance decreases at the same time.
In Example 1, the sensitivity could be increased without increasing the thickness of the piezoelectric body (securing the capacitance). Even when compared with Comparative Example 1 having the same thickness, the sensitivity is about 34 times. Moreover, since sensitivity can be remarkably increased, even if the thickness of the piezoelectric body is increased to some extent (even if the capacitance is slightly reduced), excellent sensitivity can still be maintained.
Here, considering the relationship between the received wave sensitivity and the thickness of the piezoelectric body, in Example 1, which was able to increase the received wave sensitivity while securing the capacitance, the sensitivity to the thickness was compared with Comparative Examples 1 to 3. It turns out that it is remarkably large compared.

10 (Cylindrical) wave receiving piezoelectric element 10 'Solid 11 as skeleton of wave receiving piezoelectric element Case 11a Upper surface 11b Bottom surface 11c Post 12 Piezoelectric body, PVDF piezo film 13 Polyurethane 14 Polyolefin film 15a Positive electrode, Al vapor deposition electrode (Correct)
15b Negative electrode, Al evaporated electrode (negative)
20 (Parallel lamination) Received piezoelectric element 21 Polymer piezoelectric body 22 Copper foil electrode 30 (Plate) Received piezoelectric element sw Sound wave

Claims (4)

A receiving piezoelectric element;
A vinylidene fluoride resin piezoelectric body formed on a curved surface;
A positive electrode and a negative electrode respectively disposed on both surfaces of the vinylidene fluoride resin piezoelectric body;
The vinylidene fluoride resin piezoelectric material has a thickness of 10 μm to 2 mm,
The wave-receiving piezoelectric element is formed from a solid surrounded by the curved surface,
The curved surface forms all or part of the surface of the solid, and a space is formed on the inner surface side of the curved surface,
The solid is a closed solid,
Received piezoelectric element. The curved surface forms an entire surface or a partial surface of one solid selected from the group consisting of a cylinder, a polygonal cylinder, a cone, a polygonal pyramid, a sphere, a polyhedron, an ellipsoid, and a solid formed by a part of these solids. To
The wave receiving type piezoelectric element according to claim 1. The receiving sensitivity / thickness of the vinylidene fluoride resin piezoelectric body is 0.17 to 10 V / Pa / m.
The wave receiving type piezoelectric element according to claim 1 or 2. Comprising a filler filling the space;
The filler has a rubber hardness of A0 to A90 based on JIS K6253 or ASKER C 0 to 90 based on SRIS 0101.
The wave-receiving type piezoelectric element according to any one of claims 1 to 3.