Piezoelectric element and piezoelectric device

Abstract

To provide a piezoelectric element that has a high piezoelectric effect and inverse piezoelectric effect, and a piezoelectric device including such a piezoelectric element.SOLUTION: A piezoelectric element comprises: a piezoelectric material that has a first surface and a second surface different from each other; a first electrode that is provided on the first surface; and a second electrode that is provided on the second surface. The piezoelectric material includes a helical chiral polymer crystal having an orientation axis as a crystal axis. The orientation axis is uniaxially oriented to intersect with both the first surface and the second surface. The degree of orientation of the orientation axis in the piezoelectric material is 0.80 or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a piezoelectric element and a piezoelectric device.

Background Art

[0002] Piezoelectric polymers have processability unique to polymer materials and are thus used in various forms. For example, a piezoelectric sheet made of polylactic acid generates an electrical output due to a shear stress applied to the sheet. Therefore, such a piezoelectric sheet is considered for application as an input device provided on, for example, the screen of a smartphone to receive touch input by a finger. However, even when the piezoelectric sheet is disposed on the screen, there is not much room for the piezoelectric sheet to bend, so that a shear stress is hardly generated and a sufficient electrical output cannot be obtained.

[0003] Therefore, Patent Document 1 discloses a stranded piezoelectric element in which a piezoelectric fiber formed from a piezoelectric polymer is wound around and coated on the surface of a conductive fiber formed from a conductive material. In such a stranded piezoelectric element, a piezoelectric fiber containing a piezoelectric polymer uniaxially oriented in the fiber axis direction is used. Patent Document 1 also discloses that the winding angle of the piezoelectric fiber around the conductive fiber is set in an oblique direction of 15° or more and 75° or less. By setting the winding angle within such a range, a shear stress is likely to be generated in the piezoelectric fiber, and a large electrical signal associated with the piezoelectric effect can be taken out via the conductive fiber.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the braided piezoelectric element has a complex structure and high manufacturing difficulty. In addition, since the braided piezoelectric element has a special shape, there is a problem that it is difficult to handle as a piezoelectric element.

Means for Solving the Problems

[0006] The piezoelectric element according to an application example of the present invention includes a piezoelectric body having mutually different first and second surfaces, a first electrode provided on the first surface, a second electrode provided on the second surface, and is provided with the piezoelectric body includes a helical chiral polymer crystal having an orientation axis as a crystal axis, the helical chiral polymer crystal includes a helical structure, and is a polylactic acid crystal having a unit cell with a-axis, b-axis which is the orientation axis, and c-axis parallel to the progress axis of the helical structure as crystal axes, the lengths of the crystal axes satisfy the relationship of b-axis < a-axis < c-axis, the orientation axis is uniaxially oriented so as to intersect both the first surface and the second surface, the degree of orientation of the orientation axis in the piezoelectric body is 0.80 or more and the c-axis is uniaxially oriented in the plane of the piezoelectric body and

[0007] The piezoelectric device according to an application example of the present invention is characterized by including the piezoelectric element according to an application example of the present invention.

Brief Description of the Drawings

[0008] [Fig. 1] It is a perspective view showing the piezoelectric element according to the embodiment. [Fig. 2] It is a schematic diagram showing the molecular structure of the polylactic acid crystal. [Fig. 3] It is a schematic diagram showing the molecular structure of the polylactic acid crystal. [Fig. 4] It is a schematic diagram of the piezoelectric body included in the piezoelectric element shown in FIG. 1. [Fig. 5]An example of a rocking curve profile obtained by fixing the 2θ position at 14.7° for a piezoelectric body containing polylactic acid crystals. [Fig. 6] It is a cross-sectional view showing a piezoelectric device according to an embodiment. [Fig. 7] It is a cross-sectional view showing a piezoelectric device according to an embodiment. [Fig. 8] It is a cross-sectional view showing a piezoelectric device according to an embodiment. [Fig. 9] It is a cross-sectional view showing a piezoelectric device according to an embodiment. [Fig. 10] It is a cross-sectional view showing an example of a manufacturing apparatus for a piezoelectric body. [Fig. 11] It is a perspective view showing a flat screw included in the manufacturing apparatus of FIG. 10. [Fig. 12] It is a schematic view showing a state where a flat screw of FIG. 11 is filled with a material. [Fig. 13] It is a schematic view showing a barrel included in the manufacturing apparatus of FIG. 10. [Fig. 14] It is a flowchart for explaining a method of manufacturing a piezoelectric body. [Fig. 15] It is a graph showing a part of an endothermic melting curve obtained by DSC measurement performed on polylactic acid pellets as raw materials. [Fig. 16] It is a θ-2θ profile obtained by θ-2θ measurement for piezoelectric body test pieces obtained in Examples and each Comparative Example.

Embodiments for Carrying Out the Invention

[0009] Hereinafter, the piezoelectric element and piezoelectric device of the present invention will be described in detail based on the accompanying drawings. 1. Piezoelectric Element First, the piezoelectric element according to the embodiment will be described. FIG. 1 is a perspective view showing a piezoelectric element according to an embodiment. In FIG. 1, as three axes orthogonal to each other, a first axis 1, a second axis 2, and a third axis 3 are set.

[0010] The piezoelectric element 100 shown in FIG. 1 includes a laminate 500 having a piezoelectric body 200, a first electrode 300, and a second electrode 400. The piezoelectric body 200 has a first surface 201 and a second surface 202 that are in a front-back relationship with each other. The first surface 201 and the second surface 202 are each orthogonal to the second axis 2. And the first surface 201 and the second surface 202 are each parallel to the plane including the first axis 1 and the third axis 3. Note that in this specification, "parallel" includes a state shifted within a range of ±10°.

[0011] The first electrode 300 is provided on the first surface 201 of the piezoelectric body 200. The second electrode 400 is provided on the second surface 202 of the piezoelectric body 200.

[0012] Note that the piezoelectric element 100 may include members other than these members. For example, an adhesive layer or the like may be interposed between the piezoelectric body 200, the first electrode 300, and the second electrode 400. Also, a protective film covering the laminate 500, a spacer provided between the first electrode 300 and the second electrode 400 and surrounding the piezoelectric body 200, or the like may be provided.

[0013] The laminate 500 shown in FIG. 1 has a plate shape with a rectangular shape in plan view. Note that the shape of the laminate 500 in plan view is not particularly limited, and it may be a circular shape such as a perfect circle, an ellipse, or an oblong, a polygon such as a quadrilateral, a hexagon, or an octagon, or other shapes. In this specification, "plan view" means viewing from the normal line of the first surface 201 of the piezoelectric body 200.

[0014] A first wiring 700 and a second wiring 800 are connected to the piezoelectric element 100. One end of the first wiring 700 is connected to the first electrode 300. One end of the second wiring 800 is connected to the second electrode 400.

[0015] The other end of the first wiring 700 and the other end of the second wiring 800 are connected to the voltage detection device 600. In the voltage detection device 600, the charges generated by the piezoelectric effect in the piezoelectric body 200 are taken out from the first electrode 300 and the second electrode 400, and the charges are converted into voltage. Based on the voltage value obtained in this way, the external force applied to the piezoelectric body 200 can be obtained, or the power required for the switch operation or power generation can be generated.

[0016] Note that the other end of the first wiring 700 and the other end of the second wiring 800 may be connected to a power supply device (not shown) instead of the voltage detection device 600. Thereby, a voltage can be applied between the first electrode 300 and the second electrode 400, and the inverse piezoelectric effect can be expressed in the piezoelectric body 200.

[0017] Examples of the constituent materials of the first electrode 300 and the second electrode 400 include not only metal materials such as simple substances of metal elements such as gold, silver, platinum, copper, nickel, aluminum, indium, tin, zinc, palladium, etc., alloys or intermetallic compounds containing these metal elements, but also resin materials such as conductive polymers.

[0018] The average thickness of the first electrode 300 and the second electrode 400 is not particularly limited, but is preferably 0.05 μm or more and 500 μm or less, and more preferably 0.50 μm or more and 300 μm or less.

[0019] The average thickness of the piezoelectric body 200 is not particularly limited, but is preferably 10 μm or more, more preferably 30 μm or more and 5 mm or less, and even more preferably 50 μm or more and 1 mm or less. Thereby, the piezoelectric body 200 has sufficient piezoelectric performance.

[0020] The piezoelectric body 200 includes a helical chiral polymer crystal and is a member having piezoelectricity. The piezoelectric body 200 is manufactured by various methods such as, for example, an injection molding method, an extrusion molding method, a stretching method, etc. A helical chiral polymer refers to a polymer having a helical molecular structure and molecular optical activity. And a helical chiral polymer crystal refers to a crystal of such a helical chiral polymer.

[0021] Examples of the helical chiral polymer include, for example, polypeptides, cellulose derivatives, polylactic acid, polypropylene oxide, poly-β-hydroxybutyric acid, etc. The helical chiral polymer crystal used in the present embodiment is composed of polymer chains and has a unit cell with the a-axis, b-axis, and c-axis as crystal axes. In the present embodiment, it is assumed that the lengths of the a-axis, b-axis, and c-axis of the crystal axes are such that b-axis < a-axis < c-axis. In addition, in polymer crystals, it is common for the crystal axis parallel to the long-chain direction of the polymer chain to be long. Therefore, in the present embodiment, the long-chain direction of the polymer chain, that is, the helical axis in the helical structure, is the c-axis.

[0022] Hereinafter, polylactic acid will be described as an example of the helical chiral polymer. As optically active polylactic acid, L-type polylactic acid (PLLA) and D-type polylactic acid (PDLA) are known. Hereinafter, L-type polylactic acid, particularly α-phase L-type polylactic acid, which is stable among the crystal phases of L-type polylactic acid, will be described as an example. In the following description, α-phase L-type polylactic acid is simply referred to as "polylactic acid".

[0023] Figures 2 and 3 are schematic diagrams showing the molecular structures of polylactic acid crystals, respectively. As shown in Figures 2 and 3, the molecular structure of the polylactic acid crystal has a helical structure. The crystal system of the polylactic acid crystal is a monoclinic system, and the length of the a-axis of the unit cell is about 1.066 nm, the length of the b-axis is about 0.616 nm, and the length of the c-axis is about 2.888 nm. Note that Figure 2 shows a plane including the a-axis and c-axis of the polylactic acid crystal, and Figure 3 shows a plane including the a-axis and b-axis of the polylactic acid crystal.

[0024] FIG. 4 is a schematic diagram of the piezoelectric body 200 included in the piezoelectric element shown in FIG. 1. This piezoelectric body 200 may contain polylactic acid crystals, but preferably has polylactic acid crystals as the main material. The content of polylactic acid crystals in the piezoelectric body 200 is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. In addition, the piezoelectric body 200 may contain, for example, amorphous polylactic acid in addition to the polylactic acid crystals. Further, the piezoelectric body 200 may contain L-type polylactic acid in a metastable phase such as the α'-phase or β-phase, in addition to the α-phase L-type polylactic acid described above, or may contain D-type polylactic acid.

[0025] The piezoelectric body 200 according to the present embodiment contains crystals in which polylactic acid is b-axis oriented. The b-axis orientation means a state in which the b-axis of polylactic acid is uniaxially oriented. That is, in the present embodiment, the b-axis serves as the "orientation axis". The direction in which the b-axis is uniaxially oriented is the direction connecting the first electrode 300 and the second electrode 400 of the piezoelectric body 200. Therefore, the b-axis is uniaxially oriented parallel to the second axis 2. As a result, polarization appears in the extending direction of the second axis 2. On the other hand, in the present embodiment, the c-axis of polylactic acid is oriented in the plane of the piezoelectric body 200.

[0026] The orientation state as described above is schematically shown in FIG. 4. In FIG. 4, the helical structure of polylactic acid is represented by a helix SP. The advancing axis of the helix SP is parallel to the c-axis. As shown in FIG. 4, in the piezoelectric body 200, while the c-axis of polylactic acid faces various directions in the plane, the b-axis is uniaxially oriented parallel to the second axis 2.

[0027] As described above, the piezoelectric constant d of a helical chiral polymer crystal in which the b-axis is uniaxially oriented and the crystal system is monoclinic ij is expressed as follows by Equation (1) using a matrix.

[0028]

Equation

[0029] From the above formula (1), in the piezoelectric body 200, the piezoelectric longitudinal effect represented by the piezoelectric constant d 22 and the piezoelectric transverse effect represented by the piezoelectric constant d 21 can be used. Therefore, it can be seen that the piezoelectric element 100 according to the present embodiment utilizes at least one of the piezoelectric longitudinal effect represented by the piezoelectric constant d 22 and the piezoelectric transverse effect represented by the piezoelectric constant d 21 .

[0030] In the helical chiral polymer crystal included in the piezoelectric body 200, the symmetry of the molecules in the crystal is represented by the space group P1211. This space group is C2 when represented in the Schoenflies notation 2 . According to the piezoelectric body 200 including a helical chiral polymer crystal having such molecular symmetry, the above-described piezoelectric longitudinal effect and piezoelectric transverse effect can be effectively utilized. The piezoelectric longitudinal effect and the piezoelectric transverse effect are parallel or orthogonal to the direction of the applied stress with respect to the direction connecting the first electrode 300 and the second electrode 400. Therefore, the piezoelectric effect or the inverse piezoelectric effect is more easily utilized in a device or the like than the piezoelectric shear effect in which the applied stress is a shear stress

[0031] For example, when an external force is applied to the piezoelectric body 200 in the direction indicated by the arrow labeled longitudinal strain in FIG. 1, a voltage is detected by the voltage detection device 600 due to the piezoelectric longitudinal effect. Based on this voltage value, the external force applied in the direction of the arrow labeled longitudinal strain can be detected. The piezoelectric constant d 22 is one of the tensors of the piezoelectric coefficient, and corresponds to the charge density generated per unit stress when an external force is applied in the direction of the arrow labeled longitudinal strain in FIG. 1 to generate a stress

[0032] Also, when an external force is applied to the piezoelectric body 200 in the direction indicated by the arrow labeled transverse strain in FIG. 1, a voltage is detected by the voltage detection device 600 due to the piezoelectric transverse effect. Based on this voltage value, the external force applied in the direction of the arrow labeled transverse strain can be detected. The piezoelectric constant d 21It is one of the tensors of piezoelectric constants. When an external force is applied in the direction of the arrow marked with transverse strain in FIG. 1 to generate stress, it corresponds to the charge density generated per unit stress.

[0033] In the piezoelectric element 100 according to the present embodiment, the degree of orientation of the b-axis in the piezoelectric body 200 is 0.80 or more. Thereby, the voltage obtained from the charges generated in the first electrode 300 and the second electrode 400 by the above-described longitudinal piezoelectric effect or transverse piezoelectric effect, or the stress generated by the longitudinal inverse piezoelectric effect or transverse inverse piezoelectric effect when a voltage is applied between the first electrode 300 and the second electrode 400 can be increased.

[0034] The degree of orientation of the b-axis in the piezoelectric body 200 is measured as follows. First, a sample of the piezoelectric body 200 is fixed to the sample holder of an X-ray diffractometer. Then, θ-2θ measurement by X-ray diffraction is performed on the sample to obtain a θ-2θ profile.

[0035] Next, for the peak of the orientation axis whose degree of orientation is to be evaluated, an azimuthal angular distribution intensity (rocking curve profile) is obtained. As the X-ray source, characteristic X-rays of CuKα1 with a wavelength of 1.5404 Å are used, the output tube voltage is 40 kV, and the output tube current is 300 mA. The measurement range is the angle ψ = -85 to 85° in the tilt direction.

[0036] A peak derived from the orientation axis is recognized in the rocking curve profile. Therefore, assuming that the degree of orientation of the orientation axis is F, the degree of orientation F is defined by the following formula (2). F = (180 - α) / 180 … (2)

[0037] In the above formula (2), α is the full width at half maximum (FWHM) of the peak derived from the orientation axis in the rocking curve profile.

[0038] FIG. 5 is an example of a rocking curve profile obtained by fixing the 2θ position at 14.7° for the piezoelectric body 200 containing polylactic acid crystals.

[0039] In the rocking curve profile shown in Fig. 5, a peak is observed at the position where ψ = 0°. This is the peak derived from the orientation axis (b-axis). The full width at half maximum of the peak at ψ = 0° is estimated to be 30°. Therefore, the degree of orientation F of the b-axis in the piezoelectric body 200 from which the rocking curve profile shown in Fig. 5 was obtained is estimated to be 0.83 as an example.

[0040] As described above, the piezoelectric element 100 according to the present embodiment includes a piezoelectric body 200, a first electrode 300, and a second electrode 400. The piezoelectric body 200 has a first surface 201 and a second surface 202 that are different from each other. The first electrode 300 is provided on the first surface 201. The second electrode 400 is provided on the second surface 202. The piezoelectric body 200 includes a helical chiral polymer crystal having an orientation axis as a crystal axis. In the present embodiment, the orientation axis is the b-axis. The b-axis is uniaxially oriented so as to intersect both the first surface 201 and the second surface 202. And in the piezoelectric element 100 according to the present embodiment, the degree of orientation of the b-axis in the piezoelectric body 200 is 0.80 or more.

[0041] According to such a configuration, since the degree of orientation of the b-axis in the piezoelectric body 200 is sufficiently high, the voltage obtained by the longitudinal piezoelectric effect or the transverse piezoelectric effect, or the stress generated by the inverse longitudinal piezoelectric effect or the inverse transverse piezoelectric effect when a voltage is applied can be increased. For this reason, a high-performance piezoelectric element 100 can be obtained. And by using such a piezoelectric element 100, a high-performance piezoelectric device can be obtained.

[0042] Further, the helical chiral polymer crystal contained in the piezoelectric body 200 is preferably a polylactic acid crystal. Polylactic acid has high piezoelectricity, relatively high mechanical strength, and good moldability. Therefore, by including polylactic acid crystals, a piezoelectric body 200 excellent in dimensional accuracy and durability can be obtained even with a complex shape. As a result, a high-performance piezoelectric element 100 can be realized.

[0043] In a helical chiral polymer crystal, the uniaxial orientation of the b-axis can be identified by obtaining and analyzing the X-ray diffraction profile of the piezoelectric body 200. Specifically, first, θ-2θ measurement using an X-ray diffractometer is performed on the piezoelectric body 200 to obtain a θ-2θ profile.

[0044] For example, when the b-axis of polylactic acid is uniaxially oriented, when θ-2θ measurement by X-ray diffraction is performed on the piezoelectric body 200 to obtain a θ-2θ profile, the obtained θ-2θ profile has a peak in the range where the diffraction angle 2θ is 13.5° or more and less than 15.5°. This peak position is the peak position by θ-2θ measurement using the characteristic X-ray of CuKα1 with a wavelength of 1.5404 Å.

[0045] The fact that the b-axis of polylactic acid is uniaxially oriented is confirmed by having a peak in the range of 13.5° or more and less than 15.5° in the θ-2θ profile and observing this peak at the position of ψ = 0° in the rocking curve profile. When the b-axis of polylactic acid is uniaxially oriented, as described above, in the polylactic acid crystal, the piezoelectric longitudinal effect represented by the piezoelectric constant d 22 or its inverse effect, and at least one of the piezoelectric transverse effect represented by the piezoelectric constant d 21 or its inverse effect can be utilized. According to the piezoelectric longitudinal effect or its inverse effect, for example, an actuator that generates thickness vibration, a sensor that converts the stress in the direction including the component of the second axis 2 into voltage, etc. can be realized. According to the piezoelectric transverse effect, for example, an actuator that generates stretching vibration, a sensor that converts the stress in the direction including the components of the first axis 1 and the third axis 3 into voltage, etc. can be realized. Such devices are highly practical in terms of the relationship between the electrode arrangement and the stress generation direction, and thus are more user-friendly than when using the piezoelectric shear effect.

[0046] In particular, poly(lactic acid) crystals include a helical structure and have a unit cell with the a-axis, b-axis, and c-axis as crystal axes. The lengths of the crystal axes satisfy the relationship b-axis < a-axis < c-axis, and the c-axis is parallel to the advancing axis of the helical structure. In the piezoelectric body 200 included in the piezoelectric element 100 according to the present embodiment, the orientation axis is the b-axis.

[0047] According to such a configuration, in the piezoelectric body 200, at least one of the piezoelectric longitudinal effect represented by d 22 or its inverse effect, and the piezoelectric transverse effect represented by d 21 or its inverse effect can be utilized. Thereby, a user-friendly device can be realized.

[0048] In the present embodiment, the orientation axis of the helical chiral polymer crystal is the b-axis of the poly(lactic acid) crystal. However, the orientation axis is not limited to the b-axis and may be the a-axis or the c-axis. That is, in the poly(lactic acid) crystal, the a-axis may be the orientation axis, or the c-axis may be the orientation axis. Even in such a case, although the piezoelectric constants are different, a piezoelectric element capable of utilizing the piezoelectric longitudinal effect or the piezoelectric transverse effect can be realized using the first electrode 300 and the second electrode 400 described above, similar to the case where the b-axis is the orientation axis.

[0049] The piezoelectric body 200 is preferably an injection molded body. If it is an injection molded body, the piezoelectric body 200 having a desired shape can be efficiently manufactured. Therefore, the manufacturing efficiency of the piezoelectric element 100 can be increased, and cost reduction can be easily achieved. The injection molded body includes, in addition to the molded body molded by an injection molding machine, a shaped body manufactured by injecting and depositing a raw material from a nozzle by a 3D printer described later.

[0050] In FIG. 4, the b-axis is uniaxially oriented parallel to the second axis 2, while the c-axis is plane-oriented, that is, randomly oriented within the plane of the piezoelectric body 200. The c-axis may be uniaxially oriented within the plane of the piezoelectric body 200. For example, the c-axis may be uniaxially oriented parallel to the third axis 3. Thereby, the piezoelectric constant d described above 21The piezoelectric transverse effect represented by becomes larger along the first axis 1. As a result, a higher-performance piezoelectric element 100 can be realized.

[0051] 2. Piezoelectric Device Next, a piezoelectric device according to an embodiment will be described. FIGS. 6 to 9 are cross-sectional views showing a piezoelectric device according to an embodiment. In FIGS. 6 to 9, the x-axis, y-axis, and z-axis are set as three mutually orthogonal axes. Each axis is represented by an arrow, with the tip side being "plus" and the base end side being "minus". In the following description, for example, the "x-axis direction" includes both the plus direction and the minus direction of the x-axis. Also, in the following description, the z-axis plus side may be described as "up" and the z-axis minus side may be described as "down".

[0052] The piezoelectric device 1000A shown in FIGS. 6 and 7 includes a substrate 1100, a support portion 1200, and the piezoelectric element 100 according to the above-described embodiment. The substrate 1100 has a long axis extending along the x-axis and a short axis extending along the y-axis, and has a plate shape in which the z-axis direction is the thickness direction. And the end portion on the minus side of the x-axis is supported by the support portion 1200. Thereby, the substrate 1100 shown in FIGS. 6 and 7 is a so-called cantilever beam. Also, the piezoelectric element 100 is disposed on the upper surface of the substrate 1100. And the first electrode 300 and the second electrode 400 of the piezoelectric element 100 are connected to the voltage detection device 2000.

[0053] When, for example, a bending deformation in the z-axis direction occurs in the piezoelectric device 1000A, a tensile force in the x-axis direction is applied to the piezoelectric element 100. Then, due to the piezoelectric transverse effect represented by the piezoelectric constant d 21 charges are generated in the first electrode 300 and the second electrode 400, respectively. And in the voltage detection device 2000, a voltage is detected based on the charges generated in the first electrode 300 and the second electrode 400.

[0054] FIG. 6 shows the state where the piezoelectric device 1000A is not bent. Further, FIG. 7 shows the state where the piezoelectric device 1000A is bent.

[0055] In FIG. 6, the piezoelectric device 1000A is in a natural state. On the other hand, when an external force is applied to the piezoelectric device 1000A, as shown in FIG. 7, the piezoelectric element 100 extends in the x-axis direction. As a result, a voltage is detected by the voltage detection device 2000. Therefore, the piezoelectric device 1000A functions as a force sensor such as a pressure sensor or a tactile sensor.

[0056] The piezoelectric device 1000B shown in FIGS. 8 and 9 is the same as the piezoelectric device 1000A shown in FIGS. 6 and 7, except that it further includes a support portion 1300. That is, the piezoelectric device 1000B includes a substrate 1100, support portions 1200 and 1300, and the piezoelectric element 100 according to the above-described embodiment. The end portion on the minus side of the x-axis of the substrate 1100 is supported by the support portion 1200. The end portion on the plus side of the x-axis of the substrate 1100 is supported by the support portion 1300. As a result, the substrate 1100 shown in FIGS. 8 and 9 is a so-called simply supported beam.

[0057] When the piezoelectric device 1000B is bent, a tensile force in the x-axis direction is applied to the piezoelectric element 100. Then, due to the transverse piezoelectric effect represented by the piezoelectric constant d 21 charges are generated on the first electrode 300 and the second electrode 400, respectively. Then, the voltage detection device 2000 detects a voltage based on the charges generated on the first electrode 300 and the second electrode 400.

[0058] FIG. 8 shows the state where the piezoelectric device 1000B is not bent. Further, FIG. 9 shows the state where the piezoelectric device 1000B is bent.

[0059] In FIG. 8, the piezoelectric device 1000B is in a natural state. On the other hand, when an external force is applied to the piezoelectric device 1000B, as shown in FIG. 9, the piezoelectric element 100 contracts in the x-axis direction. As a result, a voltage is detected by the voltage detection device 2000. Therefore, the piezoelectric device 1000B functions as a force sensor such as a pressure sensor or a tactile sensor.

[0060] Note that the piezoelectric devices 1000A and 1000B may have the illustrated unimorph structure or a bimorph structure. In the bimorph structure, the piezoelectric element 100 is disposed not only on the upper surface of the substrate 1100 but also on the lower surface. Then, the polarization direction of the piezoelectric body 200 is set so that voltages with opposite signs are detected between the piezoelectric element 100 disposed on the upper surface and the piezoelectric element 100 disposed on the lower surface. Thereby, in the bimorph structure, compared with the unimorph structure, the voltage value detected by the voltage detection device 2000 when displaced by the same displacement amount can be increased.

[0061] Examples of the substrate 1100 include a metal plate, a ceramic plate, a silicon plate, a glass plate, a resin plate, and the like.

[0062] As described above, the piezoelectric devices 1000A and 1000B have been described. Examples of piezoelectric devices include various force sensors such as tactile sensors and force sensors, various switches, power generation elements, actuators, vibration generating elements, ultrasonic motors, inkjet heads, fuel injection injectors, atomization devices, micromirrors, etc., in addition to the above-described devices.

[0063] 3. Manufacturing Apparatus for Piezoelectric Body Next, an example of an apparatus for manufacturing the piezoelectric body 200 will be described.

[0064] FIG. 10 is a cross-sectional view showing an example of an apparatus for manufacturing a piezoelectric body. FIG. 11 is a perspective view showing a flat screw included in the manufacturing apparatus of FIG. 10. FIG. 12 is a schematic view showing a state in which the flat screw of FIG. 11 is filled with a material. FIG. 13 is a schematic view showing a barrel included in the manufacturing apparatus of FIG. 10.

[0065] In FIGS. 10 to 13, the X-axis, Y-axis, and Z-axis are set as three mutually orthogonal axes. Each axis is represented by an arrow, with the tip side being "plus" and the base end side being "minus". In the following description, for example, the "X-axis direction" includes both the plus direction and the minus direction of the X-axis. Also, the X-axis direction and the Y-axis direction are in the horizontal plane, and the Z-axis direction is in the vertical direction.

[0066] The three-dimensional shaping apparatus 9 shown in FIG. 10 is a manufacturing apparatus for a polymer piezoelectric material injection molded body according to an embodiment, and is a so-called 3D printer. The three-dimensional shaping apparatus 9 includes an injection unit 921, a stage unit 922, and a control unit 918.

[0067] The injection unit 921 heats the pellet 919, which is the raw material, to obtain a melt 919a, and injects the obtained melt 919a. The stage unit 922 receives the injected melt 919a to obtain a deposit 919b and a three-dimensional shaped object 919c formed by laminating the deposits 919b. The control unit 918 controls various operations of the injection unit 921 and various operations of the stage unit 922.

[0068] The injection unit 921 shown in FIG. 10 includes a hopper 92, a supply pipe 93, a flat screw 94, a barrel 95, a motor 96, and a nozzle 910 (injection part).

[0069] The hopper 92 is a container for accommodating the pellet 919. The supply pipe 93 is a pipe connecting the hopper 92 and the flat screw 94.

[0070] Also, the injection unit 921 has a screw case 940. The flat screw 94 is accommodated in a space formed between the screw case 940 and the barrel 95. The pellet 919 accommodated in the hopper 92 is supplied to this space via the supply pipe 93.

[0071] The flat screw 94 is connected to the rotating shaft of the motor 96 and rotates by the power of the motor 96. Further, as shown in FIGS. 11 and 12, the flat screw 94 has a spiral notch 94b that extends from the circumferential surface 94a to the central portion 94c. The pellets 919 supplied into the screw case 940 are compressed while moving through the space portion 920 formed by the notch 94b and the barrel 95 as the flat screw 94 rotates.

[0072] The barrel 95 incorporates a heater 97. The pellets 919 compressed in the space portion 920 are melted (plasticized) by the heat of the heater 97 and become the melt 919a. Therefore, the flat screw 94 and the barrel 95 constitute a melting portion 945 that heats and melts the pellets 919.

[0073] As shown in FIG. 13, the barrel 95 has a communication hole 95a extending in the Z-axis direction and a plurality of grooves 95b formed on the surface facing the flat screw 94. The communication hole 95a is provided at a position corresponding to the central portion 94c of the flat screw 94. Further, a nozzle 910 is provided on the minus Z-axis side of the communication hole 95a. The nozzle 910 has a nozzle hole 910a. The communication hole 95a is connected to the nozzle hole 910a. For this reason, the melt 919a that has moved while being pressurized up to the central portion 94c of the flat screw 94 is ejected from the nozzle hole 910a via the grooves 95b and the communication hole 95a in sequence, using the pressure as the driving force.

[0074] Further, the nozzle 910 incorporates a heater 99. The temperature of the melt 919a passing through the nozzle hole 910a is controlled to a desired temperature by the heater 99.

[0075] The stage unit 922 shown in FIG. 10 includes a plate 911, a first stage 912, a second stage 913, a base portion 914, a first drive portion 915, a second drive portion 916, and a third drive portion 917.

[0076] The plate 911 has an upper surface 911a facing the positive Z-axis side. This upper surface 911a is a support surface that receives and deposits the melt 919a ejected from the nozzle hole 910a. As a result, a deposit 919b is obtained on the upper surface 911a. And as the deposits 919b are stacked, a three-dimensional object 919c of a desired shape is formed. The plate 911 is placed on the first stage 912.

[0077] The first stage 912 can move the plate 911 in the X-axis direction by the power of the first drive unit 915. The first stage 912 is placed on the second stage 913.

[0078] The second stage 913 moves the first stage 912 in the Y-axis direction by the power of the second drive unit 916. Thereby, the second stage 913 can move the plate 911 in the Y-axis direction.

[0079] The base part 914 moves the second stage 913 in the Z-axis direction by the power of the third drive unit 917. Thereby, the base part 914 can move the plate 911 in the Z-axis direction.

[0080] Therefore, the stage unit 922 functions as a so-called XYZ stage that moves the plate 911 to an arbitrary position in three-dimensional space. By moving the plate 911 in this way, the relative position of the nozzle hole 910a with respect to the plate 911 can be moved without changing the position of the nozzle 910. Therefore, while ejecting the melt 919a from the nozzle hole 910a, by moving the position of the plate 911 three-dimensionally, the melt 919a can be deposited three-dimensionally. As a result, a three-dimensional object 919c of an arbitrary shape can be formed.

[0081] Further, as described above, the melting section 945 has the flat screw 94. The flat screw 94 efficiently compresses the pellets 919 which are raw materials. Thereby, for example, uniaxial orientation of the b-axis of polylactic acid is realized with a higher degree of orientation. Also, compared with other screws, the flat screw 94 can efficiently knead and compress even with a smaller amount of pellets 919 in a short time. For this reason, the time during which the pellets 919 melt can be shortened, and deterioration of the melt 919a due to heating, oxidation, etc. is less likely to occur.

[0082] Note that although the flat screw 94 has many advantages for the above reasons, a screw showing the same action may replace the flat screw 94 as long as it shows the same action.

[0083] The inner diameter of the nozzle hole 910a is not particularly limited, but is preferably 0.1 mm or more and 3.0 mm or less, and more preferably 0.2 mm or more and 0.5 mm or less. By setting the inner diameter of the nozzle hole 910a within the above range, sufficient pressure can be applied to the melt 919a, and a good kneaded state can be maintained. As a result, uniaxial orientation of the b-axis of polylactic acid in the piezoelectric body 200 can be realized with a higher degree of orientation.

[0084] In this embodiment, the plate 911 having the upper surface 911a is moved three-dimensionally, but the nozzle 910, in other words, the injection unit 921 may be moved three-dimensionally. Also, among the three axes of the X-axis, Y-axis, and Z-axis, the plate 911 may be moved by one axis or two axes by the stage unit 922, and the nozzle 910 may be moved by the remaining axis.

[0085] Also, although the three-dimensional shaping apparatus 9 is a so-called 3D printer, the piezoelectric body manufacturing apparatus is not limited to a 3D printer, and may be, for example, an injection molding machine. The injection molding machine is provided with a mold having a cavity of an arbitrary shape. Then, the melt 919a injected from the nozzle 910 is filled into the cavity. Thereafter, by releasing the filling from the mold, an injection molded body is obtained.

[0086] The control unit 918 shown in FIG. 10 is electrically connected to the injection unit 921 and the stage unit 922. The control unit 918 controls the operations of the injection unit 921 and the stage unit 922 while coordinating with each other.

[0087] The control unit 918 can be realized by a computer having a processor such as a CPU (Central Processing Unit), memories such as RAM (Random Access Memory) and ROM (Read Only Memory), and an interface such as USB (Universal Serial Bus). Programs and data are stored in the memory. The processor reads and executes the program from the memory to control the operations of the injection unit 921 and the stage unit 922.

[0088] 4. Method for manufacturing a piezoelectric body Next, an example of a method for manufacturing the piezoelectric body 200 will be described.

[0089] FIG. 14 is a flowchart for explaining the method for manufacturing a piezoelectric body. In the following description, the method using the three-dimensional modeling apparatus 9 shown in FIGS. 10 to 13 will be described as an example.

[0090] The manufacturing method shown in FIG. 14 includes a melting step S102 of obtaining a melt 919a, an injection step S104 of injecting the melt 919a, and a shaping step S106 of obtaining a three-dimensional molded object 919c. Hereinafter, each step will be sequentially described.

[0091] 4.1. Melting step In the melting process S102, pellets 919 containing a helical chiral polymer are supplied to the space formed between the screw case 940 and the barrel 95. The pellets 919 are melted while moving through the space portion 920 formed by the flat screw 94, which is the melting part 945, and the barrel 95, and become the melt 919a. The melt 919a is compressed as the flat screw 94 rotates.

[0092] 4.2. Injection process In the injection process S104, the melt 919a is injected from the nozzle 910. Since the melt 919a is compressed by the flat screw 94, it is injected by that pressure. As a result, since the melt 919a is injected at a high pressure, a force that presses in the injection direction D1 is applied to the deposit 919b formed by the melt 919a depositing on the upper surface 911a. For this reason, when the deposit 919b contains, for example, polylactic acid as a helical chiral polymer, the advancing axis of the helix SP that the molecular structure of polylactic acid has, that is, the longest c-axis in the unit cell of polylactic acid, is oriented along the upper surface 911a. On the other hand, the shortest b-axis in the unit cell of polylactic acid is uniaxially oriented in a direction intersecting the upper surface 911a. By cooling the deposit 919b in a state where the helical chiral polymer is oriented in this way, a three-dimensional molded object 919c, that is, the piezoelectric body 200, is obtained.

[0093] The temperature of the nozzle 910 (injection part) is preferably equal to or higher than the melting point of the helical chiral polymer. Thereby, until immediately before injection, the helical chiral polymer contained in the melt 919a can be maintained in a good molten state. As a result, for example, a piezoelectric body 200 in which the b-axis of polylactic acid is uniaxially oriented and has a high degree of orientation can be obtained.

[0094] The temperature of the nozzle 910 only needs to be equal to or higher than the melting point of the helical chiral polymer, but is preferably set 10°C or more higher than the melting point, more preferably 20°C or more higher than the melting point, set below the thermal decomposition temperature of the helical chiral polymer, and even more preferably 30°C or more higher than the melting point and 10°C or more lower than the thermal decomposition temperature.

[0095] For example, the melting point of the α-phase L-type polylactic acid is about 182°C, and the thermal decomposition temperature is about 250°C. Therefore, the temperature of the nozzle 910 for injecting the melt 919a containing polylactic acid is preferably set to 192°C or higher, more preferably set to 202°C or higher and less than 250°C, and even more preferably set to 212°C or higher and 240°C or lower.

[0096] The temperature of the barrel 95 is not particularly limited, but is preferably within a range of ±30°C with respect to the temperature of the nozzle 910, and more preferably within a range of ±15°C. Thereby, the kneading property by the flat screw 94 is enhanced, and finally, for example, a piezoelectric body 200 having a particularly high degree of orientation of the b-axis of polylactic acid can be manufactured.

[0097] 4.3. Shaping process In the shaping process S106, the injected melt 919a is received on the upper surface 911a of the plate 911. Thereby, a deposit 919b is obtained on the upper surface 911a, and a three-dimensional shaped object 919c is obtained by stacking it.

[0098] Also, when the upper surface 911a is relatively moved in the X-Y plane with respect to the nozzle 910 while injecting the melt 919a, the c-axis of polylactic acid tends to be oriented along the moving direction. Therefore, when shaping the piezoelectric body 200, for example, the nozzle 910 may be relatively reciprocated in the Y-axis direction while being slightly shifted in the X-axis direction with respect to the upper surface 911a. Thereby, a piezoelectric body 200 in which the c-axis of polylactic acid is uniaxially oriented can be manufactured.

[0099] The temperature of the upper surface 911a, that is, the temperature of the plate 911, is preferably a temperature equal to or higher than the phase transition temperature of the helical chiral polymer and lower than the melting point. The phase transition temperature is the temperature at which the helical chiral polymer undergoes a phase transition from a metastable phase to a stable phase. Thereby, the cooling of the deposit 919b can be performed in a temperature range in which no phase transition to the metastable phase occurs, that is, a temperature range maintained in the stable phase. As a result, for example, the degree of orientation of the b-axis of polylactic acid can be sufficiently increased.

[0100] Also, when the helical chiral polymer is polylactic acid, in particular, it is preferable to set the temperature of the upper surface 911a (support surface) according to the phase transition temperature from the metastable phase to the stable phase. Specifically, the α' phase is known as the metastable phase of polylactic acid, and the α phase is known as the stable phase. Therefore, the temperature of the upper surface 911a is preferably set to be equal to or higher than the phase transition temperature from the α' phase to the α phase and lower than the melting point of polylactic acid. Thereby, the rapid cooling of the melt 919a is suppressed, and the generation of the metastable phase is suppressed. As a result, a piezoelectric body 200 mainly composed of a stable phase with high piezoelectricity and chemical stability can be manufactured.

[0101] The temperature of the upper surface 911a, that is, the temperature of the plate 911, only needs to be equal to or higher than the phase transition temperature of polylactic acid and lower than the melting point, but is preferably set to be 10°C or more higher than the phase transition temperature and 10°C or more lower than the melting point of polylactic acid, and more preferably set to be 20°C or more higher than the phase transition temperature and 30°C or more lower than the melting point of polylactic acid. Specifically, the phase transition temperature of polylactic acid from the α' phase to the α phase is about 96°C. Therefore, the temperature of the plate 911 is preferably set to be 106°C or more and 172°C or less, and more preferably set to be 116°C or more and 152°C or less.

[0102] As described above, the piezoelectric element and the piezoelectric device of the present invention have been described based on the illustrated embodiments. However, the piezoelectric element and the piezoelectric device of the present invention are not limited to the above embodiments. For example, each part of the above embodiments may be replaced with any configuration having the same function, or any component may be added to the above embodiments.

Example

[0103] Next, specific examples of the present invention will be described. 5. Manufacture of piezoelectric body test piece (Example) First, as an example of the helical chiral polymer, pellets of L-type polylactic acid (PLLA) manufactured by BMG Co., Ltd. were prepared. The melting point of this polylactic acid was 182°C, the thermal decomposition temperature was 250°C, and the phase transition temperature from the α' phase to the α phase was 96°C.

[0104] Next, poly(lactic acid) pellets were loaded into the three-dimensional modeling apparatus shown in FIG. 10, and the melt was injected while being folded back in a certain direction to obtain a piezoelectric test piece composed of a deposit having a two-layer structure. The obtained piezoelectric test piece was a disk with a diameter of 30 mm.

[0105] Note that the inner diameter of the nozzle was 0.3 mm, the temperature of the nozzle was 225°C, the temperature of the barrel was 215°C, and the temperature of the plate was 120°C.

[0106] (Comparative Example 1) A piezoelectric test piece was obtained in the same manner as in the example, except that the temperature of the plate was changed to 50°C.

[0107] (Comparative Example 2) A piezoelectric test piece was obtained in the same manner as in the example, except that the temperature of the plate was changed to 80°C.

[0108] (Comparative Example 3) A piezoelectric test piece was obtained in the same manner as in the example, except that the temperature of the plate was changed to 100°C.

[0109] 6. Evaluation of Raw Materials and Piezoelectric Test Pieces 6.1. Differential Scanning Calorimetry (DSC) of Raw Materials First, DSC measurement was performed on the raw poly(lactic acid) pellets. For the measurement, a differential scanning calorimeter "Q1000" manufactured by TA Instruments was used. In the DSC measurement, a melting endotherm curve was obtained while changing the temperature in three processes: the first heating process, the cooling process, and the second heating process. Specifically, in the first heating process, the temperature was raised from -20°C to 270°C at a heating rate of 5°C / min. In the cooling process, the temperature was lowered from 270°C to -20°C at a cooling rate of 5°C / min. In the second heating process, the temperature was raised from -20°C to 270°C at a heating rate of 5°C / min.

[0110] A part of the obtained endothermic melting curve is shown in Fig. 15. Fig. 15 is a graph showing a part of the endothermic melting curve obtained by DSC measurement on the raw material polylactic acid pellets. As shown in Fig. 15, in the first heating process, the melting point was observed at 181.75°C. In the second heating process, the phase transition temperature from the metastable α' phase to the stable α phase was observed at 95.91°C.

[0111] 6.2. X-ray Diffraction of Piezoelectric Specimens Next, for the piezoelectric specimens obtained in the examples and each comparative example, θ-2θ measurement was performed by the convergent beam method using an X-ray diffractometer. For the X-ray diffraction, characteristic X-rays of CuKα1 with a wavelength of 1.5404 Å were used.

[0112] Fig. 16 is a θ-2θ profile obtained by θ-2θ measurement for the piezoelectric specimens obtained in the examples and each comparative example.

[0113] As shown in Fig. 16, no peak was observed in the θ-2θ profile obtained from the piezoelectric specimen obtained in Comparative Example 1. Therefore, it was found that the polylactic acid was not crystallized in the piezoelectric specimen obtained in Comparative Example 1.

[0114] In the θ-2θ profiles obtained from the piezoelectric specimens obtained in Comparative Examples 2 and 3, peaks were observed at around 2θ = 16.5°. On the other hand, a peak was also observed at around 2θ = 16.7° in the θ-2θ profile obtained from the piezoelectric specimen obtained in the example. These peaks are considered to be derived from the (110) plane / (200) plane.

[0115] On the other hand, as shown in Fig. 16, a peak was also observed at around 2θ = 14.7° in the θ-2θ profile obtained from the piezoelectric specimen obtained in the example. This peak is considered to be derived from the (010) plane corresponding to the b-axis of polylactic acid. Therefore, it was found that the piezoelectric specimen obtained in the example contains crystals in which uniaxial orientation of the b-axis has occurred.

[0116] Also, for the piezoelectric test piece obtained in the example, a rocking curve profile was obtained when the 2θ position was fixed at 14.7°, and the orientation degree of the b-axis was calculated by the above-described calculation method. As a result, the orientation degree of the b-axis was 0.83.

[0117] 6.3. Orientation Degree of Piezoelectric Test Piece Next, separately from the piezoelectric test piece obtained in the example, a piezoelectric test piece prepared so as to have a reduced orientation degree was prepared as Comparative Example 4. In Comparative Example 4, a piezoelectric test piece was obtained in the same manner as in the example except that the temperature of the plate was changed to 105°C. The orientation degree of the b-axis in the piezoelectric test piece obtained in Comparative Example 4 was 0.75.

[0118] Next, a piezoelectric device having a unimorph structure shown in FIG. 6, in which the piezoelectric test piece obtained in the example and the piezoelectric test piece obtained in Comparative Example 4 were respectively incorporated, was fabricated. Then, the displacement amount when the same voltage was applied was measured.

[0119] As a result, in the piezoelectric test piece obtained in the example, a displacement amount much larger than that of the piezoelectric test piece obtained in Comparative Example 4 was obtained.

[0120] Also, the same test as above was conducted on piezoelectric test pieces having the orientation degree of the b-axis set to 0.85 and 0.90 by increasing the temperature of the nozzle. However, similar to the piezoelectric test piece obtained in the example, a displacement amount much larger than that of the piezoelectric test piece obtained in Comparative Example 4 was obtained.

[0121] From the above, it became clear that by setting the orientation degree of the b-axis to 0.80 or more, the piezoelectricity of the piezoelectric body can be sufficiently enhanced, and the piezoelectric effect and the inverse piezoelectric effect can be enhanced.

Explanation of Reference Signs

[0122] 1... First axis, 2... Second axis, 3... Third axis, 9... Three-dimensional shaping apparatus, 92... Hopper, 93... Supply pipe, 94... Flat screw, 94a... Circumferential surface, 94b... Notch, 94c... Central portion, 95... Barrel, 95a... Communication hole, 95b... Groove, 96... Motor, 97... Heater, 99... Heater, 100... Piezoelectric element, 200... Piezoelectric body, 201... First surface, 202... Second surface, 300... First electrode, 400... Second electrode, 500... Laminate, 600... Voltage detection device, 700... First wiring, 800... Second wiring, 910... Nozzle, 910a... Nozzle hole, 911... Plate, 911a... Upper surface, 912... First stage, 913... Second stage, 914... Base portion, 915... First drive unit, 916... Second drive unit, 917... Third drive unit, 918... Control unit, 919... Pellet, 919a... Melt, 919b... Deposit, 919c... Three-dimensional shaped object, 920... Space portion, 921... Injection unit, 922... Stage unit, 940... Screw case, 945... Melting portion, 1000A... Piezoelectric device, 1000B... Piezoelectric device, 1100... Substrate, 1200... Support portion, 1300... Support portion, 2000... Voltage detection device, D1... Injection direction, S102... Melting process, S104... Injection process, S106... Shaping process, SP... Helix

Claims

1. A piezoelectric body having a first surface and a second surface different from each other, A first electrode provided on the first surface, A second electrode provided on the second surface, Comprising, The piezoelectric body includes a helical chiral polymer crystal having an orientation axis as a crystal axis, The helical chiral polymer crystal includes a helical structure and is a polylactic acid crystal having a unit cell with crystal axes of an a-axis, a b-axis which is the orientation axis, and a c-axis parallel to the advancing axis of the helical structure, The lengths of the crystal axes satisfy the relationship b-axis < a-axis < c-axis, The orientation axis is uniaxially oriented so as to intersect both the first surface and the second surface, The degree of orientation of the orientation axis in the piezoelectric body is 0.80 or more, The piezoelectric element, wherein the c-axis is uniaxially oriented in the plane of the piezoelectric body.

2. When θ-2θ measurement by X-ray diffraction is performed on the piezoelectric body to obtain a θ-2θ profile, The piezoelectric element according to claim 1, wherein the θ-2θ profile has a peak in a range where the diffraction angle 2θ is 13.5° or more and less than 15.5°.

3. The piezoelectric element according to claim 1 or 2, wherein the piezoelectric body is an injection molded body.

4. A piezoelectric device comprising the piezoelectric element according to any one of claims 1 to 3.

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