JP2011192665A - Additive-free non-stretched piezoelectric material containing pvdf, and piezoelectric sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a piezoelectric material comprising polyvinylidene fluoride wherein contraction resulting from extension is minimized, and to provide a piezoelectric sensor using the piezoelectric material. <P>SOLUTION: The piezoelectric material is obtained by subjecting vinylidene fluoride polymer having a nonpolar α-type crystal structure to additive-free non-stretched polarization. Since it is non-stretched, a piezoelectric material where contraction resulting from extension is minimized, and a piezoelectric sensor using the piezoelectric material can be obtained. The piezoelectric material has an electric field strength of 200-600 MV/m during polarization, and the polarized polymer includes a polar α-type crystal structure. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a piezoelectric body and a piezoelectric sensor, and more particularly to a piezoelectric body including polyvinylidene fluoride (PVDF) and a piezoelectric sensor using the piezoelectric body.

  In 1969, it was discovered that when polyvinylidene fluoride was uniaxially stretched and subjected to polarization treatment, high piezoelectricity was developed in polyvinylidene fluoride. Since then, polyvinylidene fluoride has been widely used as a piezoelectric polymer (or pyroelectric polymer), such as ultrasonic sensors and oscillators, acoustic speakers and microphones, sensors that detect infrared rays from the human body and flames, and acceleration sensors. It is widely used for applications.

  Polyvinylidene fluoride has several crystal structures. Among these, nonpolar α type (II type) obtained by cooling and crystallizing molten polyvinylidene fluoride is considered to be the most stable. The piezoelectricity (pyroelectricity) is obtained by uniaxially stretching polyvinylidene fluoride to transfer the crystalline structure of polyvinylidene fluoride from nonpolar α-type to polar β-type (I-type). This is due to the processing.

  As described above, in general, when polyvinylidene fluoride is used as a piezoelectric body (or pyroelectric body), the extruded polyvinylidene fluoride film is uniaxially stretched at a relatively low temperature and then subjected to polarization treatment. For example, in Patent Document 1, “polyvinylidene fluoride PVDF uniaxially stretched film which is a kind of vinylidene fluoride polymer” is suitably used as the polymer piezoelectric bodies 2 and 4 used in the piezoelectric sensor (page 6). , Paragraph 0022). Similarly, in Patent Document 2, a polyvinylidene fluoride PVDF uniaxially stretched film is used (page 9, paragraph 0040).

  In the case where a piezoelectric sensor (or pyroelectric sensor) is formed using a polyvinylidene fluoride film, the polyvinylidene fluoride may be used by being laminated. For example, in Patent Document 3 (FIG. 4), silver paste is applied to both surfaces of a piezoelectric film made of polyvinylidene fluoride, the upper side is used as a positive electrode layer, and the lower side is used as a negative electrode layer (the other electrode layer). Constructs a stack. In Patent Document 1 (paragraph 0010, FIG. 1), two piezoelectric films made of polyvinylidene fluoride are bonded using an adhesive layer formed of a urethane-based resin having good compatibility with the polyvinylidene fluoride film. And the piezoelectric sensor which enlarged the electrical output by comprising laminated | stacking by a some piezoelectric film is disclosed.

JP 2008-304558 A JP 2009-80090 A Japanese Patent Laid-Open No. 10-332509

However, when the stretched film is laminated or bonded to another thermoplastic resin film or substrate, it is shrunk in the process of thermal lamination at a relatively high temperature or the curing of the adhesive, and has good flatness. Getting a body can be difficult. For example, a uniaxially stretched film tends to shrink in the stretching direction. When shrinkage occurs, the laminate is bent, warped, uneven, peeled, and the like. Therefore, in order to improve this shrinkage, it is necessary to perform aging (equivalent to relaxation heat treatment).
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to obtain a piezoelectric body in which shrinkage is suppressed as compared with conventional ones, and a piezoelectric sensor using the piezoelectric body, from a vinylidene fluoride polymer.

The piezoelectric body according to the first aspect of the present invention for solving the above problem is a piezoelectric body obtained by polarization treatment of a nonpolar α-type vinylidene fluoride polymer having a crystal structure portion; The electric field strength during the polarization treatment is 200 to 600 MV / m; the polymer after the polarization treatment includes a polar α-type crystal structure; and is a piezoelectric body.
“No addition” means that a substance that causes a transition in the crystal form of polyvinylidene fluoride is not added. For example, it means that no ammonium salt or polymethyl methacrylate (PMMA) that causes a transition from nonpolar α-type to β-type is added. Excludes cases where inorganic powders such as organic substances, ceramics, oxides, metals, and carbon that do not affect the crystal structure are added in small amounts as processing aids or stabilizers.
Further, “non-stretching” means that it has not undergone a cold stretching process (stretching process below the melting point). For example, it does not undergo a processing step such as uniaxial stretching in which a film-like resin is stretched several times in one direction (for example, the length direction) or biaxial stretching in which the resin is stretched several times in two directions (for example, the length and width directions). That means.
In the present application, the term “piezoelectric body” includes “pyroelectric body”.
Further, in the piezoelectric body, the main crystal structure portion of the polyvinylidene fluoride after the polarization treatment may be a polar α-type piezoelectric body. The “main crystal structure portion” refers to a main component of the crystal structure portion (ie, more than 50% of the crystal structure portion). For example, the main crystal structure portion of polyvinylidene fluoride being polar α-type means that more than 50% of the crystal structure portion of polyvinylidene fluoride, which is a semicrystalline polymer, is polar α-type.

  If constituted in this way, a polymer of vinylidene fluoride containing a polar α-type crystal structure can be obtained, and the polymer exhibits piezoelectricity that is sufficiently industrially usable. In addition, since this polymer is unstretched, shrinkage | contraction which arises in the film | membrane of the stretched polyvinylidene fluoride can be suppressed.

The piezoelectric body according to the second aspect of the present invention is the piezoelectric body according to the first aspect of the present invention, wherein the vinylidene fluoride polymer is a homopolymer of vinylidene fluoride, tetrafluoroethylene (C ₂ F ₄ ). Copolymer of trifluoroethylene (C ₂ HF ₃ ), copolymer of chlorotrifluoroethylene (C ₂ ClF ₃ ), copolymer of vinyl fluoride (C ₂ H ₃ F), hexafluoropropylene (C ₃ H ₆ ) and any one of the copolymers thereof; tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, hexafluoropropylene, respectively, relative to 100 parts by weight of vinylidene fluoride 10 parts by weight or less; a piezoelectric body.

  If comprised in this way, it can be set as the piezoelectric material which consists of a polymer more suitable as a vinylidene fluoride polymer.

The piezoelectric sensor according to the third aspect of the present invention, for example, as shown in FIG. 2, insulates the signal electrode 71 from the first ground electrode 61 and prevents the first surface 41a and the first surface 41a. A first piezoelectric body 41 having a second surface 41b in a front-back relationship; a first ground electrode 61 disposed on the first surface 41a of the first piezoelectric body 41; and a first piezoelectric body 41 And a signal electrode 71 disposed on the second surface 41b. The first piezoelectric body 41 is a piezoelectric body according to the first aspect or the second aspect of the present invention.
In the present application, the term “piezoelectric sensor” includes “pyroelectric sensor”, and further includes a sensor using piezoelectricity and pyroelectricity (for example, “ultrasonic transmission / reception sensor”).

  If comprised in this way, since a piezoelectric material is non-stretched, shrinkage | contraction will be suppressed and the curvature and peeling by the difference of the shrinkage | contraction rate of a piezoelectric material and the laminated | stacked electrode can be suppressed.

  In the piezoelectric sensor according to the fourth aspect of the present invention, in the piezoelectric sensor according to the third aspect of the present invention, for example, as shown in FIG. 2, the signal electrode 71 is an outer edge portion OT of the first piezoelectric body 41. The first ground electrode 61 is disposed so as to cover a portion where the signal electrode 71 is projected on the first surface 41a of the first piezoelectric body 41; A step 81 is provided at a portion IN where the signal electrode 71 is projected on the surface of the first ground electrode 61 on which the first piezoelectric body 41 is disposed and the surface on which the first piezoelectric body 41 is disposed. The stress generated in the portion where 71 is arranged is increased.

  With this configuration, since a step is provided on a surface different from the surface on which the first piezoelectric body of the first ground electrode is disposed, the stress generated in the portion where the signal electrode is disposed by being pushed from the step side. Becomes larger.

  The piezoelectric sensor according to the fifth aspect of the present invention is the piezoelectric sensor according to the third aspect or the fourth aspect of the present invention described above, for example, as shown in FIG. An adhesive layer 51 disposed on a surface in which the surface of the adhesive layer 51 is disposed on the front and back; a second piezoelectric body 42 disposed on the surface of the adhesive layer 51 on which the signal electrode 71 is disposed and on the surface in the relationship of front and back. And a second ground electrode 62 disposed on a surface of the second piezoelectric body 42 on which the adhesive layer 51 is disposed and a surface in a front-back relationship. Furthermore, the adhesive layer 51 is insulative; the second piezoelectric body 42 is a piezoelectric body according to the first aspect or the second aspect of the present invention, and has a polarity opposite to that of the first piezoelectric body 41. The piezoelectric body is oriented.

  With this configuration, the second piezoelectric body having a polarity opposite to that of the first piezoelectric body is provided with the insulating adhesive layer interposed therebetween, so that the electrical output from the piezoelectric sensor can be increased.

  In the piezoelectric sensor according to the sixth aspect of the present invention, in the piezoelectric sensor according to the fifth aspect of the present invention, for example, as shown in FIG. 3, the adhesive layer 51 is made of a material having a smaller longitudinal elastic modulus than the signal electrode 72. In the signal electrode 72, the stress generated in the portion IN where the signal electrode 72 of the first piezoelectric body 41 and the second piezoelectric body 42 is disposed is the stress generated in the outer edge portion OT where the signal electrode 72 is not disposed. It is formed thicker to be larger.

  If comprised in this way, since the signal electrode is formed thickly, the stress which arises in the part by which the signal electrode is arrange | positioned easily in a 2 layer piezoelectric material can be enlarged.

  According to the present invention, since the vinylidene fluoride polymer is not stretched, it is possible to obtain a piezoelectric body in which shrinkage is suppressed as compared with a piezoelectric body made of a stretched vinylidene fluoride polymer. Furthermore, by using the piezoelectric body, it is possible to obtain a piezoelectric sensor that suppresses warping and peeling when laminated.

It is a conceptual diagram of the apparatus 20 which performs corona polling. It is sectional drawing in the surface orthogonal to the longitudinal direction of the piezoelectric sensor 100 which concerns on 2nd Embodiment. It is sectional drawing of the piezoelectric sensor 101 which has formed the signal electrode 72 thickly, without forming a level | step difference with a protrusion. It is a top view explaining the external shape of the piezoelectric sensor 100, and the shape of the protrusion 81, and shows the example by which the protrusion 81 is arrange | positioned continuously. It is an end view explaining the shape of the signal electrode 71 of the piezoelectric sensor 100 shown in FIG. 4, and shows an example in which the signal electrode 71 is continuously arranged. It is a top view explaining the external shape of the piezoelectric sensor 102, and the shape of the protrusion 82, and the example in which the protrusion 82 is arrange | positioned intermittently is shown. It is an end view explaining the shape of the signal electrode 73 of the piezoelectric sensor 102 shown in FIG. 6, and shows an example in which the signal electrode 73 is intermittently arranged. 1 is a perspective view for explaining an electronic stringed instrument 200 using a piezoelectric sensor 100. FIG. 4 is an exploded enlarged view showing a relationship between a top 220 of an electronic stringed musical instrument 200 using the piezoelectric sensor 100 and the piezoelectric sensor 100. FIG. It is a table | surface which shows the piezoelectric constant of Example 1- Example 6, and the comparative example 2 and the comparative example 3. FIG. The X-ray diffraction spectrum of the polyvinylidene fluoride film of Example 1 is shown. The X-ray diffraction spectrum of the polyvinylidene fluoride film of Example 2 is shown. The X-ray diffraction spectrum of the polyvinylidene fluoride film of Example 5 is shown. The X-ray diffraction spectrum of the polyvinylidene fluoride film of Comparative Example 1 is shown. The X-ray diffraction spectrum of the polyvinylidene fluoride film of Comparative Example 2 is shown. The X-ray-diffraction spectrum of the polyvinylidene fluoride film of the comparative example 3 is shown.

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

  In the piezoelectric body and piezoelectric sensor of the present invention, for the purpose of suppressing “shrinkage” caused by stretching, an electric field strength of 100 to 600 MV / m is obtained after molding without adding vinylidene fluoride polymer and stretching as follows. This is realized by a configuration in which a polarization process is applied.

A piezoelectric body according to the first embodiment of the present invention will be described. The piezoelectric body according to the first embodiment is melt-extruded (nothing) into a polyvinylidene fluoride pellet without adding a specific compound (for example, a quaternary ammonium salt) that causes a transition in crystal form. Addition), after forming into a film, it is obtained by performing a polarization treatment by corona poling without going through a stretching process such as uniaxial stretching or biaxial stretching (no stretching).
“Melting extrusion” refers to extrusion molding of a molten resin from a discharge port (slit) of an extrusion mold.

Here, the dielectric breakdown strength among the physical properties of polyvinylidene fluoride will be described. According to "Engineering polymer-from engineering plastics to highly functional resins-" ¹⁾ , the dielectric breakdown strength of polyvinylidene fluoride (test method ASTM D-149) is 52-74 kV / mm (= 52-74 MV / m), or 260 V / mil (= 10.2 MV / m, 1 mil = 0.0254 mm). That is, if it is attempted to perform polarization treatment beyond this range, polyvinylidene fluoride causes dielectric breakdown.
¹⁾ “Engineering Polymers: From Engineering Plastics to Highly Functional Resins” (Publisher: Chemical Industry Daily, published on March 29, 1996, p.525, p.532)
However, the present inventors have found that polyvinylidene fluoride can be polarized even at an electric field strength of 100 to 600 MV / m, far exceeding the above range. As a result, it was confirmed that even if it is an additive-free and non-stretched polyvinylidene fluoride, piezoelectricity that can be sufficiently used industrially is exhibited.

First, melt extrusion of polyvinylidene fluoride will be described. Polyvinylidene fluoride pellets are melted (at a temperature of 200 to 280 ° C.) and molded into, for example, a film having a width of 250 mm and a thickness of 40 μm using a single screw extruder having a diameter of 40 mmφ equipped with a T die. As soon as it is extruded from the die of a single screw extruder, the film is cooled to 40-140 ° C. on a chill roll. In addition, it is preferable that the thickness of the film extruded is 10-2000 micrometers.
Thus, the specific compound (for example, quaternary ammonium salt etc.) which causes the transition of the crystal structure is not added to the polyvinylidene fluoride pellets (that is, no addition). The crystal structure of the polyvinylidene fluoride (after melt extrusion) at this point is nonpolar α-type.

  Next, the polarization treatment of the melt-extruded polyvinylidene fluoride film will be described. The polarization process is performed by corona poling using the apparatus 20 shown in FIG. The apparatus 20 includes a first roll 21 that sends the melt-extruded film 10, a second roll 22 that sends the film 10 from the first roll, and a third roll 23 that sends the film 10 from the second roll. Furthermore, the tip electrode 31 arrange | positioned so that the film 10 which contacted the 2nd roll 22 may be covered without contact is provided. During corona poling, the second roll 22 functions as a counter electrode of the tip electrode 31. That is, the second roll 22 becomes a polarization roll. The second roll 22 when functioning as a counter electrode is hereinafter referred to as an electrode 32. Further, the tip electrode 31 and the electrode 32 are connected via a DC high voltage power source 33. The DC high-voltage power supply 33 can reverse the polarity of the tip electrode 31 and the electrode 32.

The tip electrode 31 polarizes the film 10 with a direct current electric field between the counter electrode (electrode 32) while holding the charge generated by the corona discharge generated at the tip on the surface of the film 10. The tip electrode 31 preferably has a tip to cause an effective corona discharge. As an example of the pointed electrode, in addition to a needle-like electrode (literally an electrode having a needle-like tip) such as stainless steel or tungsten, a wire-like electrode (that is, approximately the same length as the width of the film 10 in the width direction of the film 10). A wire-like electrode extending in this manner is also preferably used.
It is also possible to perform polarization treatment with the electrode 32 using an electrode that is in direct contact with the film 10 instead of such a non-contact type electrode. However, in the case where there is a risk of dielectric breakdown of the film 10, a non-contact electrode is preferable in order to avoid the accompanying power supply shutdown.

The distance between the tip of the pointed electrode 31 and the electrode 32 is preferably about 5 to 30 mm. If the interval is too small, the dielectric breakdown of the film 10 is liable to occur. If it is too large, corona discharge is suppressed and the polarization treatment effect is reduced. Moreover, when the pointed electrode 31 is a wire-like electrode, it is desirable to provide it at a density of about 0.5 to 2 pieces / cm 2, and when the pointed electrode 31 is a needle-like electrode, it is desirable to have a density of about 0.5 to 3 pieces / cm ² .

  In order to set the temperature of the film 10 to an appropriate temperature, the second roll 22 (electrode 32) functions as a heater. In order to avoid abrupt heating of the film 10, the first roll 21 may be a preheating roll having a temperature lower than that of the heater. Alternatively, preheating means such as an infrared heater (not shown) may be provided upstream of the first roll 21.

Corona while adjusting the supply speed of the film of the first roll 21, the second roll 22 and the third roll 23 of the apparatus 20 so as to keep the film from sagging but without stretching the film (ie, unstretched) Polarization is performed by polling.
In addition, you may perform a polarization process from both surfaces of the film 10 using a pointed electrode. Moreover, you may use the existing apparatuses (for example, the apparatus etc. which fix a cut film, and perform a polarization process) other than the apparatus 20. FIG.

  When the polarization process is performed using the apparatus 20 shown in FIG. 1, the film 10 is supplied to the second roll 22 at a specific speed. The film 10 is subjected to a polarization treatment by the action of a direct current high electric field between the tip electrode 31 and the electrode 32 connected to the direct current high voltage power source 33 in a region in contact with the electrode 32 (second roll 22). The polarized film 10 leaves the second roll 22 at a speed adjusted so as not to generate the sagging of the film. However, the film 10 is polarized without being stretched without being stretched. Further, after being subjected to post-treatment such as heat treatment for dimensional stabilization as required, it is wound on a winding roll (not shown).

The higher the electric field strength during corona poling, the better the film does not cause dielectric breakdown. On the other hand, if it is too low, the polarization treatment becomes insufficient and sufficient piezoelectricity cannot be obtained. In addition, in order to express piezoelectricity from about 100MV / m at room temperature, it is good to perform a polarization process in the range of 100-600MV / m. However, in order to obtain sufficient piezoelectricity, it is preferably 200 to 550 MV / m, particularly preferably 300 to 500 MV / m.
The temperature of the second roll 22 that functions as a heater during corona poling (substantially the same as the temperature of the film 10) is desirably a glass transition temperature (Tg) to a melting point (Tm), preferably 0 to 140 ° C., More preferably, it is room temperature-130 degreeC.

  By the above method, a polyvinylidene fluoride film that exhibits high piezoelectricity without stretching can be obtained. The polarized polyvinylidene fluoride film is considered to contain a polar α-type crystal structure as shown in the examples described later.

Here, the crystal structure of polyvinylidene fluoride will be described.
When the molten polyvinylidene fluoride is cooled and crystallized, non-polar α-type polyvinylidene fluoride is obtained. The nonpolar α-type crystal structure adopts a 2/1 helical TGTG ′ conformation. The molecule is a polar molecule due to the dipole moment of the C—F ₂ and C—H ₂ bonds in the crystal. However, since the molecule has a center of symmetry, the dipole moment cancels out, and the crystal is a nonpolar crystal. Become.

  Furthermore, in the present embodiment, it is considered that the crystal structure portion of polyvinylidene fluoride has been changed from the nonpolar α-type to the polar α-type by performing the polarization treatment, as shown in Examples described later. The polar α-type molecular chain is a 2/1 helix that adopts the TGTG ′ conformation, like the nonpolar α-type. However, since the dipoles are aligned in the same direction by rotation, the crystals are polar crystals.

  As described in the background art, when conventional polyvinylidene fluoride is used as a piezoelectric body, nonpolar α-type polyvinylidene fluoride is stretched (for example, uniaxially stretched). Then, the crystal structure part changes from nonpolar α type to β type. The β-type crystal structure has a planar zigzag TT conformation. In the β-type crystal, the molecular chain in the crystal has a dipole moment in the vertical direction and is oriented in the same direction in the crystal, so that the crystal is a polar crystal. Thus, since the β-type crystal has polarity based on spontaneous polarization, high piezoelectricity was obtained by performing polarization treatment thereon.

  As described above, the piezoelectric body made of the polyvinylidene fluoride film according to the first embodiment of the present invention is manufactured without going through a stretching process. However, it has high piezoelectricity as shown in the examples described later. Since this polyvinylidene fluoride film is not stretched, it becomes a film in which shrinkage due to stretching is suppressed.

Furthermore, anisotropy of physical properties due to stretching occurs in a piezoelectric body made of a uniaxially stretched polyvinylidene fluoride film. For example, the x-axis is taken in the uniaxial stretching direction, the y-axis is perpendicular to it, and the z-axis is perpendicular to the film surface, and the x, y, and z axes are determined. When a piezoelectric constant indicating polarization in the z-axis direction when stress or strain is applied in the x-axis direction is d ₃₁ , and a piezoelectric constant in the z-axis direction when stress or strain is applied in the y-axis direction is d ₃₂ , polyvinylidene fluoride In the piezoelectric body made of a uniaxially stretched film, d ₃₁ is about 10 times or more than d ₃₂ . When this piezoelectric body is used as a vibration film of a speaker, when a voltage is applied to both surfaces of the film, the amplitude in the x-axis and y-axis directions is extremely different, so the film stretching direction must be properly managed. However, since the polyvinylidene fluoride film according to the first embodiment of the present invention is non-stretched, it becomes a piezoelectric film having less anisotropy than a uniaxially stretched film. Therefore, if the film is used for a piezoelectric sensor, the reliability of the sensor can be further improved. In addition, when the film is used for an acoustic transducer or the like, the design becomes easy.

  Further, the thickness and form of the uniaxially stretched film (and biaxially stretched film) are limited. When the piezoelectric body according to the first embodiment of the present invention is used, an extremely thin film, a thick sheet, It becomes possible to give a high piezoelectricity to a pipe, a spherical or other complicated shaped molded article, a coating film, and the like.

In the above embodiment, as the vinylidene fluoride polymer, a homopolymer of vinylidene fluoride (same polymer, polyvinylidene fluoride) has been described as an example. However, the crystal structure portion in the vinylidene fluoride polymer is nonpolar α-type. If so, it may be a copolymer (copolymer) of vinylidene fluoride. For example, a copolymer of vinylidene fluoride (C ₂ H ₂ F ₂ ) and tetrafluoroethylene (C ₂ F ₄ ), a copolymer of vinylidene fluoride and trifluoroethylene (C ₂ HF ₃ ), fluoride Copolymer of vinylidene and chlorotrifluoroethylene (C ₂ ClF ₃ ), copolymer of vinylidene fluoride and vinyl fluoride (C ₂ H ₃ F), vinylidene fluoride and hexafluoropropylene (C ₃ H ₆ ) And a copolymer. The copolymer is preferably about 10 parts by weight or less per 100 parts by weight of vinylidene fluoride.

A piezoelectric sensor according to a second embodiment of the present invention will be described. The piezoelectric sensor according to the second embodiment is configured using the piezoelectric body according to the first embodiment.
FIG. 2 is a cross-sectional view taken along a plane orthogonal to the longitudinal direction of the piezoelectric sensor 100 according to the second embodiment. The piezoelectric sensor 100 includes a plate-like piezoelectric body 41 as a first piezoelectric body having a first surface 41a and a second surface 41b, a plate-like piezoelectric body 42 as a second piezoelectric body, and a piezoelectric element. And an adhesive layer 51 that is sandwiched between the bodies 41 and 42 and bonds the piezoelectric bodies 41 and 42 to each other. A ground electrode 61 as a first ground electrode and a ground electrode 62 as a second ground electrode are formed so as to sandwich the piezoelectric bodies 41 and 42 bonded by the adhesive layer 51. Further, the signal electrode 71 is formed on the second surface 41 b side of the first piezoelectric body 41, that is, on the adhesive layer 51 side. In the piezoelectric sensor 100, the signal electrode 71 is formed at a position in contact with the second surface 41 b of the first piezoelectric body 41 inside the adhesive layer 51, but between the piezoelectric bodies 41 and 42, that is, in the adhesive layer 51. What is necessary is just to form. However, it is preferable to arrange it at a position close to the signal detection surface on which the load acts in detecting the rise of the signal and the minute signal. In the piezoelectric sensor 100, the upper surface shown in FIG. 2 is the signal detection surface, and therefore the signal electrode 71 is disposed on the first piezoelectric body 41 side. Between the signal electrode 71 and the ground electrodes 61 and 62, the piezoelectric bodies 41 and 42 which are insulators are disposed, so that the capacitor between the signal electrode 71 and the ground electrodes 61 and 62 can be charged, respectively. Composed.

  The piezoelectric bodies 41 and 42 are disposed with opposite polarities. That is, when a load is applied in the plate thickness direction, the positively charged side is disposed on the adhesive layer 51 side, and the negatively charged side is disposed on the ground electrodes 61 and 62 side. As described above, by arranging the piezoelectric bodies 41 and 42 with the polarities reversed, it has a stored amount of electricity to be added, and an electric output almost twice as large as that of a single piezoelectric body can be obtained. it can. Depending on the embodiment, the negatively charged side may be disposed on the adhesive layer 51 side.

  The signal electrode 71 is disposed at the central portion IN avoiding the outer edge portion OT of the piezoelectric sensor 100. Here, the outer edge portion OT is a range affected by the external noise of the piezoelectric sensor 100, and typically varies with the thickness of the piezoelectric sensor 100, but typically has the same width as the thickness of each of the piezoelectric bodies 41 and 42. Have. For example, when the thickness of the piezoelectric sensor 100 is 0.3 mm and the thickness of each of the piezoelectric bodies 41 and 42 is 0.1 mm, it is in the range of 0.1 mm from the end, and if the distance from the end is increased, the external noise Can be removed more. By arranging the signal electrode 71 so as to avoid the outer edge portion OT in this way, external noise does not reach the signal electrode 71 unless it enters deeper than the thickness using the thin piezoelectric sensor 100 as a path. Becomes larger. Therefore, the influence of external noise on the signal electrode 71 from the width direction of the piezoelectric sensor 100 (lateral direction in FIG. 2) can be substantially eliminated. Here, the signal electrode 71 is described as being disposed in a range excluding only the outer edge portion OT, which is a range affected by external noise. However, the signal electrode 71 can avoid the outer edge portion OT while avoiding the outer edge portion OT. You may arrange | position in the center part IN of a narrow range.

  The ground electrodes 61 and 62 are typically disposed to have the same width as that of the piezoelectric sensor 100, that is, the width of the piezoelectric bodies 41 and 42. Since the ground electrodes 61 and 62 sandwich the insulating piezoelectric bodies 41 and 42, the ground electrodes 61 and 62 have a function as a shield. Therefore, by arranging the ground electrodes 61 and 62 wider than the signal electrode 71, the influence of external noise on the signal electrode 71 from the thickness direction (vertical direction in FIG. 2) of the piezoelectric sensor 100 is substantially reduced. Can be removed. In the piezoelectric sensor 100, the ground electrodes 61 and 62 are arranged to have the same width as the piezoelectric bodies 41 and 42, but the range where the ground electrodes 61 and 62 are arranged is at least the signal electrode 71. A range that covers the corresponding portion is preferable because it is possible to almost eliminate the influence of external noise, and a wider area can more reliably remove the influence of external noise. Here, the “portion corresponding to the signal electrode 71” is a predetermined surface (here, the portion where the signal electrode 71 is disposed) when projected in the plate thickness direction (vertical direction in FIG. 2) of the piezoelectric sensor 100. A portion of the projected image formed on the outer surface of the ground electrode 61.

Further, in the piezoelectric sensor 100, the protrusion 81 is provided on the surface (hereinafter also referred to as “outer surface”) opposite to the surface (hereinafter also referred to as “inner surface”) where the ground electrode 61 is in contact with the piezoelectric body 41. Provided, a part of the outer surface is convex to form a step. The protrusion 81 may be inserted with a thin plate, or may be formed on the outer surface of the ground electrode 61 by printing by a known printing technique. The protrusion 81 is typically installed at a portion corresponding to the signal electrode 71, but it is not necessarily installed at all of the portion corresponding to the signal electrode 71 as long as it partially overlaps the projection image portion. It does not have to be. However, it is preferable to make the protrusion 81 slightly larger than the signal electrode 71 in order to absorb the positional deviation in manufacturing, because variations in sensitivity of the piezoelectric sensor 100 can be prevented.
If the protrusion 81 is formed by inserting a thin plate or printing, it is possible to form the protrusion 81 with a hard material, which is preferable because the action of the protrusion 81 can be reliably obtained. The protrusion 81 is formed, for example, by solidifying after printing a conductive paste containing a metal such as silver or carbon. When a conductive paste containing a metal is used, the conductivity is increased. When a conductive paste containing carbon is used, there is no deterioration such as oxidation, and the price is low. Further, the protrusion 81 may be a metal foil or a carbon foil. When a metal foil is used, it is easy to process and has high toughness, so it is difficult to break. In addition, when carbon foil is used, the hardness is high and the weight is light.

  The height of the step differs depending on the size, shape, application, etc. of the piezoelectric sensor 100, but is 5 μm or more, preferably 30 μm or more. When the level difference is 5 μm or less, there is a possibility that the effect of the level difference cannot be obtained due to variations in manufacturing. If there is a step of 30 μm or more, the load is more reliably received at the stepped portion. If the level difference is too large, the level difference part is likely to be damaged, or the flexibility unique to the piezoelectric body is lost. Furthermore, the height of the step is preferably 0.5 to 1 mm or less for manufacturing reasons.

  A protective layer 91 is formed to cover the ground electrode 61 and the protrusion 81. The protective layer 91 is a layer for protecting the ground electrode 61 and the protrusion 81 from the outside, and is formed of, for example, polyimide. Further, a protective layer 92 similar to the protective layer 91 is formed so as to cover the ground electrode 62. A rubber layer 93 as an elastic layer is further formed on the protective layer 92. The rubber layer 93 is a layer formed of rubber, soft plastic, or the like in order to attenuate vibrations from below when the piezoelectric sensor 100 is placed. The thickness of the piezoelectric sensor 100 is, for example, 250 μm to 300 μm between the protective layers 91 and 92, and the thickness of the rubber layer 93 is about 500 μm.

  Next, the operation of the piezoelectric sensor 100 will be described. The piezoelectric sensor 100 is placed on a flat surface (not shown) with the rubber layer 93 facing down. A load acting body (not shown) that applies a surface load to the piezoelectric sensor 100 is brought into contact with the upper portion (the protective layer 91 side) of the piezoelectric sensor 100. The load acting body has a plane wider than the protrusion 81, and a surface load acts on the piezoelectric sensor 100 from the plane. The surface load is a distributed load applied to the piezoelectric sensor 100 from a rigid surface on which a large load acts, and is typically a distributed load from a plane that is substantially rigid with respect to the piezoelectric sensor 100. It is. A surface load acts on the piezoelectric sensor 100 from the load acting body due to a displacement such as vibration of the load acting body. The surface load acting on the piezoelectric sensor 100 from the load acting body acts greatly on the step due to the protrusion 81, and the load acting on the peripheral edge portion becomes small. Therefore, in the piezoelectric bodies 41 and 42, the stress generated in the portion where the step is provided, that is, the portion IN where the signal electrode 71 is disposed is larger than the stress generated around the portion, and the portion IN where the signal electrode 71 is disposed. Produces a high potential.

  The signal electrode 71 is insulated by the piezoelectric bodies 41 and 42 and covered in the plate thickness direction (vertical direction) by the ground electrodes 61 and 62 with the piezoelectric bodies 41 and 42 interposed therebetween. Since the ground electrodes 61 and 62 are grounded, the ground electrodes 61 and 62 have a shielding action and reduce the influence of external noise. Furthermore, since the signal electrode 71 is disposed at the center portion IN avoiding the outer edge portion OT of the piezoelectric sensor 100, the influence of external noise on the signal electrode 71 from the width direction (lateral direction in FIG. 1) of the piezoelectric sensor 100 is achieved. Is reduced.

  FIG. 3 shows a piezoelectric sensor 101 which is a modification of the piezoelectric sensor 100 (see FIG. 2). In the piezoelectric sensor 101 shown in FIG. 3, the signal electrode 72 is formed thick without forming a step with the protrusion. The signal electrode 72 is typically formed of a metal such as copper, and the adhesive layer 51 around the signal electrode 72 is formed of, for example, a urethane resin that is compatible with polyvinylidene fluoride as the piezoelectric body 41. In general, the longitudinal elastic modulus of metal is larger than that of resin. Therefore, in the portion IN where the thick signal electrode 72 is arranged, the rigidity in the plate thickness direction (vertical direction in FIG. 3) becomes higher than the outer edge portion OT where the signal electrode 72 is not arranged. Therefore, when a surface load is applied to the piezoelectric sensor 101 from a load acting body (not shown), the stress generated in the piezoelectric bodies 41 and 42 of the portion IN where the signal electrode 72 is disposed and the rigidity is high is generated in the outer edge portion OT around the portion. It becomes larger than the stress that occurs. For example, the signal electrode 72 has a thickness of 1/5 or more, preferably 1/4 or more of the thickness between the protective layers 91 and 92. As a result, a high potential is generated in the portion IN where the signal electrode 72 is disposed.

  Next, a specific shape of the piezoelectric sensor 100 described so far will be described with reference to FIG. FIG. 4 is a plan view for explaining the outer shape of the piezoelectric sensor 100 and the shape of the protrusion 81, and the steps are continuously formed. In FIG. 4, the protective layer is omitted.

  The piezoelectric sensor 100 shown in FIG. 4 has an elongated shape. That is, it is long in the arrow X direction and short in the arrow Y direction. In the piezoelectric sensor 100, an extension portion 96 is provided at one end in the longitudinal direction X (the left end in FIG. 4) to install a voltage extraction port to the outside.

  In the piezoelectric sensor 100, a step extending in the longitudinal direction X is formed by the protrusion 81. Since the step is formed long in the longitudinal direction X, the piezoelectric sensor 100 that detects a signal with high sensitivity over almost the entire length of the step is obtained. However, in the width direction, that is, the short direction Y, the step is disposed so as to avoid the outer edge portion, and also in the longitudinal direction X, it is disposed so as to avoid the tip (the right end in FIG. 4). As will be described later, this is because it is arranged at a portion corresponding to the signal electrode 71 (see FIG. 5) arranged so as to avoid the outer edge portion.

  FIG. 5 is an end view for explaining the shape of the signal electrode 71 of the piezoelectric sensor 100 shown in FIG. In the piezoelectric sensor 100, the signal electrode 71 is formed over almost the entire length. However, in the width direction, that is, in the short direction Y, the outer edge portion is avoided and the center portion is disposed, and in the longitudinal direction X, the distal end (right end in FIG. 5) is avoided. Thus, by arranging the signal electrode 71 while avoiding the outer edge portion, the influence of external noise is reduced. The ground electrodes 61 and 62 (see FIG. 2) cover at least a portion corresponding to the signal electrode 71 and are typically disposed so as to sandwich the piezoelectric bodies 41 and 42 entirely in a direction perpendicular to the paper surface. The Further, in the piezoelectric sensor 100, as shown in FIGS. 4 and 5, an extension portion 96 is formed by extending the first piezoelectric body 41 and the second piezoelectric body. Although not shown, the adhesive layer 51 (see FIG. 2) is also extended to the extension portion 96. As shown in FIG. 5, the signal electrode 71 is also extended to the extension portion 96. Further, as indicated by broken lines in FIG. 5, the ground electrodes 61 and 62 are also extended.

  In the extension portion 96, the signal electrode 71 and the ground electrodes 61 and 62 are arranged at different positions when viewed from the direction perpendicular to the first surface 41 a (see FIG. 2) of the piezoelectric body 41. Thus, if the signal electrode 71 and the ground electrodes 61 and 62 are arranged in different parts, the terminals can be easily taken out.

  A signal terminal and a ground terminal (not shown) are respectively attached to the extensions 96 of the piezoelectric sensor 100. The signal terminal is disposed in a portion where the signal electrode 71 is extended, and the ground terminal is disposed in a portion where the ground electrodes 61 and 62 are extended. Therefore, the signal terminal is electrically connected to the signal electrode 71, and the ground terminal is electrically connected to the ground electrodes 61 and 62. The signal terminal and the ground terminal are shielded to reduce the influence of external noise.

  Next, with reference to FIG. 8 and FIG. 9, a specific usage example of the piezoelectric sensor 100 described so far will be described. 8 and 9 are diagrams for explaining an electronic stringed instrument 200 using the piezoelectric sensor 100. FIG. 8 is a perspective view of the electronic stringed instrument 200, and FIG. 9 shows a relationship between the top 220 of the electronic stringed instrument 200 and the piezoelectric sensor 100. FIG. Although the electronic stringed instrument 200 is described as an electronic guitar, it may be an electronic violin, an electronic cello, or other electronic stringed instrument. In addition, an electronic stringed instrument that listens to performances through headphones, for example, may be used without generating a loud sound called a so-called silent instrument.

  The top 220 of the electronic stringed instrument 200 always supports a vibrating string to produce sound. The top 220 has an elongated substantially triangular prism shape, one side surface is formed into a flat surface, and the other two side surfaces have a convex curve. The top 220 is placed with the planar side surface facing the body 260.

  The piezoelectric sensor 100 is inserted between the top 220 and the body 260. The piezoelectric sensor 100 is thin and easy to insert the piezoelectric sensor 100 because the polyvinylidene fluoride film according to the first embodiment of the present invention is used as the piezoelectric bodies 41 and 42 (see FIG. 2). A shield wire 140 is connected to the signal terminal and the ground terminal of the piezoelectric sensor 100. The shielded wire 140 is preferably a shielded shielded wire that is not easily affected by external noise. The shield wire 140 is connected to an acoustic device (not shown) such as an amplifier and a speaker that reproduces the sound of the electronic stringed instrument 200 and a recording device (not shown).

  Although the piezoelectric sensor according to the second embodiment of the present invention using the piezoelectric body according to the first embodiment of the present invention has been described above, the piezoelectric sensor according to the present invention is limited to the above-described example. is not. Below, the piezoelectric sensor as a modification is demonstrated.

Each of the piezoelectric bodies 41 and 42 shown in FIGS. 2 and 3 may be formed by a single layer of a polyvinylidene fluoride film, or may be formed by stacking a plurality of polyvinylidene fluoride films.
When forming a multilayer with a polyvinylidene fluoride film, other thermoplastic resins having good compatibility with the polyvinylidene fluoride film may be used as an adhesive or an adhesive layer. The other thermoplastic resin refers to, for example, a polyamide resin, a polyester resin, an acrylic resin, a urethane resin, and the like. In addition, since the shrinkage | contraction of the film itself is suppressed for the polyvinylidene fluoride film of this invention, when it makes it a multilayer, the curvature by the shrinkage | contraction of a film, peeling, etc. can be suppressed.

  In FIGS. 2 and 3, the piezoelectric sensor has been described as an example in which two layers of the piezoelectric bodies 41 and 42 and the ground electrodes 61 and 62 are disposed. However, only one layer of the piezoelectric body and the ground electrode may be disposed. Or three or more layers may be provided.

  Further, in FIG. 2, only one layer of the signal electrode 71 is formed, but a signal electrode in contact with the first piezoelectric body 41 and a signal electrode in contact with the second piezoelectric body 42 may be formed.

  Furthermore, instead of inserting a thin plate like the projection 81 shown in FIG. 2, the protective layer 91 may be formed thicker only at the portion corresponding to the signal electrode 71. In particular, when the protective layer 91 is hard, typically harder than the piezoelectric bodies 41 and 42 and hardly deformed, the protective layer 91 may be formed thick. If a part of the protective layer 91 is thickened to form a step, the step can be easily manufactured.

  Further, the protrusions may be intermittently arranged like the protrusions 82 of the piezoelectric sensor 102 of FIG. If it does in this way, a level difference will be formed intermittently. In FIG. 6, the protective layer is omitted. Since the step is intermittently formed, the stress generated in the piezoelectric bodies 41 and 42 (see FIG. 2) at the portion where the step is formed becomes larger, a high potential is generated, and the sensitivity is improved. As described above, when the load acting body (not shown) is not continuous, a step corresponding to the shape of the load acting body is formed, and when the load acting body is continuous, an appropriate interval is provided and intermittent. A step may be formed. If the step is intermittent, the stress generated in the piezoelectric bodies 41 and 42 corresponding to the step increases, and a high potential is generated.

FIG. 7 shows the shape of the signal electrode 73 of the piezoelectric sensor 102 shown in FIG. As shown in FIG. 7, the signal electrode 73 of the piezoelectric sensor 102 may be intermittently disposed and connected by a conductive wire 97. When the signal electrode 73 is arranged at a position corresponding to the step and the conductive electrode 97 connects between the signal electrodes 73, as a result, the portion where the conductive line 97 is arranged becomes a recess with respect to the step. Therefore, even if the piezoelectric sensor 102 receives a surface load, the stress generated in the portion is reduced. Therefore, mixing of unnecessary signals on the conductive line 97 is reduced.
Further, when the load acting body is tilted in the longitudinal direction of the piezoelectric sensor 102 and the signal electrodes 73 are intermittently arranged short in the longitudinal direction, the potentials are individually detected by the respective signal electrodes 73. As compared with the signal electrode 71 (see FIG. 5) arranged in this manner, it can be detected with high sensitivity.
In the piezoelectric sensor 102, the ground electrodes 61 and 62 (see FIG. 2) may be formed only in a portion corresponding to the signal electrode 73. However, the ground electrodes 61 and 62 may be continuously formed in the longitudinal direction X of the piezoelectric sensor 102. preferable. If the ground electrodes 61 and 62 are continuously formed in the longitudinal direction X, the shield in the plate thickness direction of the piezoelectric sensor 102 can be maintained higher and hardly affected by external noise. In the piezoelectric sensor 102, an extension 96 similar to that of the piezoelectric sensor 100 is formed, and the signal electrode 73 is connected by a conductive wire 97. The signal electrode 73 is extended even if connected by the conductive wire 97 in this way.

2 and 3, the plate-shaped piezoelectric sensor has been described. However, a piezoelectric sensor having a curved surface (for example, a parabolic plate shape) may be configured by utilizing the flexibility of the polymer piezoelectric material. . Furthermore, although the elongated piezoelectric sensor has been described with reference to FIGS. 4, 5, 6, and 7, a large area piezoelectric sensor may be configured by taking advantage of the characteristics of the film. Thus, the piezoelectric sensor according to the present invention is not limited to an electronic stringed instrument, and can be used for other purposes.
Moreover, although the Example was demonstrated as a piezoelectric sensor, the piezoelectric material which concerns on this invention also shows pyroelectricity naturally. Therefore, it can be a pyroelectric sensor. Furthermore, it is good also as a sensor (an ultrasonic transmission / reception sensor etc.) using piezoelectricity or pyroelectricity.

  As described above, when a piezoelectric sensor is configured using the piezoelectric body according to the first embodiment of the present invention, warpage and peeling can be suppressed even when stacked, and thus a piezoelectric sensor with stable quality is obtained. And the manufacturing yield can be increased.

The fact that the piezoelectric body according to the first embodiment of the present invention has piezoelectricity will be described using examples and comparative examples.
Example 1 to Example 6 and Comparative Example 1 to Comparative Example 3 shown in FIG. 10 were adjusted as follows. For polyvinylidene fluoride in Examples 1 to 6 and Comparative Examples 1 to 3, KF # 1000 manufactured by Kureha Corporation was used.
<Example 1 to Example 6>
Polyvinylidene fluoride pellets were melted at 225 ° C. using a 40 mmφ single-screw extruder equipped with a T die, and then a film was formed to have a width of 250 mm and a thickness of 40 μm, and cooled to 70 ° C. Next, the DC voltage shown in FIG. 10 was applied to the pointed electrode 31 (a plurality of needle-like electrodes) using the apparatus 20 shown in FIG. The application time was about 12 seconds. The film supply speed was 1 m / min. The temperature of the 2nd roll 22 which functions as a heater at the time of a polarization process was 100 degreeC.
<Comparative Example 1>
It is a polyvinylidene fluoride film prepared in the same manner as in the Examples, and is a film of polyvinylidene fluoride after melt extrusion (before polarization treatment).
<Comparative example 2>
It is a polyvinylidene fluoride film prepared in the same manner as in Examples, and is a polyvinylidene fluoride film that has been subjected to polarization treatment after being uniaxially stretched after melt extrusion. The thickness of the film before stretching was about 160 μm.
<Comparative Example 3>
It is a polyvinylidene fluoride film prepared in the same manner as in the Examples, and is a polyvinylidene fluoride film that has been subjected to polarization treatment after melt extrusion and biaxial stretching using a simultaneous biaxial stretching machine. The thickness of the film before stretching was about 320 μm.

The items in each table shown in FIG. 10 are as follows.
The voltage Vp is a voltage value applied to the pointed electrode (needle electrode).
The estimated Ep is the electric field strength (calculated value) during the polarization process. In this embodiment, corona poling is performed, so that the electric field strength is determined by dividing the applied voltage by the thickness of the film, excluding the voltage drop of air. That is, the estimated Ep was obtained by the following equation. The voltage drop was 7.5 kV at 1 μA, and the value based on the result of monitoring the current during polarization was excluded as the voltage drop.
[Estimated Ep = (applied voltage−voltage drop) / film thickness]
d ₃₁ is a piezoelectric stress constant in the length direction of the polymer piezoelectric film.
d ₃₂ is a piezoelectric stress constant in the width direction of the polymer piezoelectric film.
d ₃₃ is a piezoelectric stress constant in the thickness direction of the polymer piezoelectric film.

[Measurement method of piezoelectric stress constant]
The piezoelectric stress constant of the obtained polymer piezoelectric film was measured as follows.
d ₃₁ (with respect to the stress in the length direction) was measured using Rheograph solid manufactured by Toyo Seiki Seisakusho.
d ₃₂ (with respect to the stress in the width direction) was measured using a Rheograph solid manufactured by Toyo Seiki Seisakusho.
d ₃₃ (with respect to the stress in the thickness direction) was calculated by applying a stress in the thickness direction at a constant speed using a pneumatic press, and measuring the generated charge with a charge amplifier.

As shown in FIG. 10, in Example 1 where the electric field strength (estimated Ep) is small, the piezoelectric constant is also small. However, in Example 2 where the electrolytic strength exceeds 100 MV / m, d ₃₁ is 7.9, and in Examples 3 to 6 where the electrolytic strength exceeds 200 MV / m, d ₃₁ exceeds 10. Thus, it can be seen that even a non-added, non-stretched polyvinylidene fluoride film becomes a piezoelectric body that exhibits high piezoelectricity that can be used industrially by applying a high voltage. As reference values, Comparative Example 2 shows the piezoelectric constant of the uniaxially stretched film, and Comparative Example 3 shows the piezoelectric constant of the biaxially stretched film. In Comparative Example 2, d ₃₁ (31.5) is 10 times or more as large as d ₃₂ (1.3) (that is, the anisotropy is large). However, in Example _6, a _{d 32} of 6.7 whereas _{d 31} it is 10.2, it can be seen that the anisotropy is small.

11 to 13 show the X-ray diffraction spectra of the polyvinylidene fluoride films of Example 1, Example 2, and Example 5. FIG.
FIG. 15 is an X-ray diffraction spectrum of the polyvinylidene fluoride film of Comparative Example 2, and shows the spectrum before the polarization treatment. That is, the spectrum after uniaxial stretching and before polarization treatment. It is known that when a polyvinylidene fluoride film is uniaxially stretched, the crystal structure is changed from a non-polar α-type to a polar β-type. In FIG. 15, only one peak that is considered to be caused by the β type is detected in the vicinity of 2θ: 21 deg. However, the spectrum of FIGS. 11 to 13 shows a waveform that is clearly different from the spectrum of FIG. 15, and the films of Examples 1 to 6 have crystal structures having characteristics different from the crystal structures obtained by uniaxial stretching. I know that there is.

  FIG. 16 shows an X-ray diffraction spectrum of Comparative Example 3, that is, a biaxially stretched polyvinylidene fluoride film after polarization treatment. It is known that when a poly (vinylidene fluoride) film is biaxially stretched, the crystalline α-type and β-type are mixed at a ratio of approximately half. In FIG. 16, two peaks are detected in the vicinity of 2θ: 20 deg and in the vicinity of 2θ: 21 deg. However, the spectra of FIGS. 11 to 13 show waveforms that are clearly different from those of FIG. 16, and the films of Examples 1 to 6 are different from the crystal structure obtained by polarization treatment after biaxial stretching. It can be seen that the crystal structure has characteristics.

FIG. 14 shows an X-ray diffraction spectrum of Comparative Example 1, that is, a polyvinylidene fluoride film after melt extrusion, before polarization treatment. It is known that when a polyvinylidene fluoride film is cooled and crystallized from a molten state, a nonpolar α-type crystal structure is obtained. In FIG. 14, two small peaks are detected before and after 2θ: 18 deg, and a main peak is detected in the vicinity of 2θ: 20 deg. These are peaks 100 (α), 020 (α), and 110 (α) peculiar to nonpolar α-type crystals.
On the other hand, in the spectra of FIGS. 11 to 13, changes appear in each peak as the applied voltage increases, and particularly the reduction of the peaks of 100 (α) and 020 (α) is remarkable, and the crystal structure changes. I understand that. From these spectra, it can be seen that the nonpolar α-type crystal structure has transitioned to the polar α-type. That is, the polyvinylidene fluoride film according to the first embodiment of the present invention includes a polar α-type crystal structure.

DESCRIPTION OF SYMBOLS 10 Film 20 Apparatus 21 1st roll 22 2nd roll 23 3rd roll 31 Pointed electrode 32 Electrode 33 DC high voltage power supply 41, 42 Piezoelectric body 41a 1st surface 41b 2nd surface 51 Adhesive layer 61, 62 Ground electrode 71, 72, 73 Signal electrodes 81, 82 Protrusions 91, 92 Protective layer 93 Rubber layer 96 Extension 97 Conductive wires 100, 101, 102 Piezoelectric sensor 140 Shielded wire 200 Electronic stringed instrument 220 Top 260 Body OT Outer edge portion IN Central portion

Claims (6)

A piezoelectric body obtained by polarizing a vinylidene fluoride polymer having a non-polar α-type crystal structure;
The polymer is polarized with no addition, no stretching;
The electric field strength during the polarization treatment is 200 to 600 MV / m;
The polarized polymer comprises a polar α-type crystal structure;
Piezoelectric body. The vinylidene fluoride polymer is a homopolymer of vinylidene fluoride, a copolymer with tetrafluoroethylene (C ₂ F ₄ ), a copolymer with trifluoroethylene (C ₂ HF ₃ ), or chlorotrifluoroethylene (C ₂ ClF ₃ ). A copolymer of any one of: a copolymer with vinyl fluoride (C ₂ H ₃ F), a copolymer with hexafluoropropylene (C ₃ H ₆ );
Tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride and hexafluoropropylene are each contained in an amount of 10 parts by weight or less based on 100 parts by weight of vinylidene fluoride;
The piezoelectric body according to claim 1. A first piezoelectric body having a second surface that insulates the signal electrode from the first ground electrode and has a first surface and a second surface that is in a front-back relationship with respect to the first surface;
The first ground electrode disposed on the first surface of the first piezoelectric body;
The signal electrode disposed on the second surface of the first piezoelectric body;
The first piezoelectric body is the piezoelectric body according to claim 1 or claim 2;
Piezoelectric sensor. The signal electrode is disposed so as to be covered with the second surface while avoiding an outer edge portion of the first piezoelectric body;
The first ground electrode is disposed so as to cover a portion of the first surface of the first piezoelectric body on which the signal electrode is projected;
A step is provided in a portion where the signal electrode is projected on the surface of the first ground electrode on which the first piezoelectric body is disposed, and the signal electrode of the first piezoelectric body is provided with a step. Increase the stress generated in the place of placement;
The piezoelectric sensor according to claim 3. An adhesive layer disposed on a surface of the signal electrode on which the first piezoelectric body is disposed and a surface in a front-back relationship;
A second piezoelectric body disposed on the surface of the adhesive layer on the front and back side of the surface on which the signal electrode is disposed;
A second ground electrode disposed on the surface of the second piezoelectric body on which the adhesive layer is disposed and a surface in a front-back relationship;
The adhesive layer is insulating;
The second piezoelectric body is the piezoelectric body according to claim 1 or 2, wherein the first piezoelectric body has a polarity opposite to that of the first piezoelectric body;
The piezoelectric sensor according to claim 3 or 4. The adhesive layer is formed of a material having a smaller longitudinal elastic modulus than the signal electrode;
In the signal electrode, the stress generated in the portion where the signal electrode of the first piezoelectric body and the second piezoelectric body is disposed is larger than the stress generated in the outer edge portion where the signal electrode is not disposed. Formed thick;
The piezoelectric sensor according to claim 5.