JP2007305907A - Laminated piezoelectric element, and polarization method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laminated piezoelectric element in which while preventing a destruction of the element, a satisfactory polarization near to a saturated polarization is achieved. <P>SOLUTION: For polarizing this laminated piezoelectric element 1 produced by alternatively laminating a piezoelectric ceramics layer 2 and an internal electrode layer 3, by applying a direct current voltage, a DC polarization is performed and thereafter, by applying a pulse voltage, a pulse polarization is performed. By the DC polarization, in at least a part of the piezoelectric ceramics layer 2, a crack which does not penetrate through the piezoelectric ceramics layer 2 in the thickness direction thereof is formed. Next, by performing the pulse polarization, a polarization near to a saturated polarization can be achieved. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a laminated piezoelectric element constituted by alternately laminating piezoelectric ceramic layers and internal electrode layers, and a polarization method thereof, and in particular, approaches a saturated polarization without destroying the laminated piezoelectric element. For technology.

  For example, various types of piezoelectric elements such as actuators, piezoelectric buzzers, sounding bodies, sensors, etc. are being developed, and laminated actuators (laminated piezoelectric elements) constructed by laminating piezoelectric ceramics that are displaced by voltage application via electrodes. Etc. are also proposed. For example, an actuator that uses displacement generated by the piezoelectric effect as a mechanical drive source has advantages such as low power consumption and heat generation, good response, and miniaturization and weight reduction. And practicality is high. In particular, the laminated actuator has features such as high displacement efficiency, high positional accuracy, and high response speed, and therefore is expected to be applied to a wide range of fields such as injectors for fuel injection systems.

  By the way, when manufacturing this type of piezoelectric element, the polarization process is an essential process. The polarization process is performed by applying a high voltage in a certain direction to the fired piezoelectric element. By aligning the polarization direction by the polarization process, the piezoelectric element exhibits piezoelectric characteristics.

Conventionally, as a method of polarization treatment, DC polarization in which a direct high voltage is applied is generally performed. In DC polarization, polarization is performed by applying a DC high voltage, but the device may be destroyed by abrupt distortion or internal stress accompanying polarization, and in order to alleviate this, DC is applied while applying a predetermined pressure. Attempts have been made to perform polarization (see, for example, Patent Document 1). In the invention described in Patent Document 1, a laminated body for a multilayer ceramic piezoelectric element having internal electrodes between upper and lower layers, and a portion in the vicinity of one side surface and a portion in the vicinity of the other side surface parallel to the one side surface. When performing a polarization treatment on a multilayer ceramic piezoelectric element laminate in which electrode forming portions where internal electrodes are formed and electrode non-forming portions where no internal electrodes are formed are alternately positioned above and below, A predetermined polarization voltage is applied between the internal electrodes while uniformly applying a constant load within a range of 50 to 1000 kgf / cm ² between the upper surface and the lower surface. By performing polarization in a state where a load is applied, it is possible to relieve abrupt strain and internal stress associated with polarization, and to prevent occurrence of internal destruction of the laminate.

  As the method for the polarization treatment, so-called pulse polarization is proposed in addition to the DC polarization (see, for example, Patent Document 2 and Patent Document 3). In pulse polarization, polarization is performed by applying a pulse voltage having a pulse-like waveform. The invention described in Patent Document 2 is a polarization method of a piezoelectric ceramic that polarizes by applying a pulse voltage having a pulse voltage waveform to the piezoelectric ceramic, wherein the pulse voltage includes a high frequency component. By applying a pulse voltage containing high-frequency components to polarize, many crystal grains and domains contained in the piezoelectric ceramic can be vibrated, and sufficient polarization processing can be performed in a short time without causing damage to the piezoelectric ceramic. You can do it.

Patent Document 3 discloses that pulse polarization is performed by applying a DC pulse voltage to a piezoelectric body under a condition that the effective voltage is 2/3 or less of the minimum breakdown voltage of the piezoelectric body. In the pulse polarization described in Patent Document 3, since the effective voltage that determines the heat generation of the piezoelectric body is suppressed, dielectric breakdown does not occur during the polarization process, while the peak voltage is set to a magnitude necessary for polarization. Therefore, even with a piezoelectric body having a relatively small insulation resistance, the polarization treatment can be effectively performed.
Japanese Patent Laid-Open No. 2-163983 JP 2004-296784 A JP-A-61-268085

  However, when considering both saturation polarization and prevention of breakdown, the actual situation is that the conventional polarization method (DC polarization or pulse polarization) does not always provide sufficient results. For example, in DC polarization, as described in Patent Document 1 and the like, it is necessary to perform polarization while applying pressure in order to prevent destruction of the element. It is difficult to reach polarization. For example, in a laminated piezoelectric element used in an injector for a fuel injection system, if polarization is insufficient and saturation polarization has not been reached, the polarization progresses with use and the characteristics change over time. . In order to eliminate such inconvenience, if a sufficient DC voltage is applied to the saturation polarization without applying pressure, destruction of the element is inevitable.

  The same applies to pulse polarization. Until now, only the superiority of pulse polarization has been paid attention. However, as a result of investigations by the present inventors, it has been found that device breakdown can easily occur when a voltage necessary for saturation polarization is applied even in pulse polarization. In pulse polarization, a high voltage is momentarily applied when the voltage of a pulse waveform is switched, which causes element destruction. In order to eliminate the element breakdown, the pulse voltage must eventually be lowered. As a result, the polarization becomes insufficient and it becomes difficult to reach the saturation polarization.

  The present invention has been proposed in light of the above-described disadvantages of the prior art, and aims to achieve sufficient polarization close to saturation polarization while preventing element breakdown, and sufficient piezoelectric characteristics. It is an object of the present invention to provide a multilayer piezoelectric element having a piezoelectric property that does not change with time. It is another object of the present invention to provide a polarization method for a laminated piezoelectric element capable of realizing sufficient polarization close to saturation polarization while preventing destruction of the element.

  In order to achieve the above-described object, the multilayer piezoelectric element of the present invention has piezoelectric ceramic layers and internal electrode layers alternately stacked, and at least a part of the piezoelectric ceramic layer has the piezoelectric ceramic layer in the thickness direction. It is characterized by the formation of cracks that do not cross.

  In general, in a multilayer piezoelectric element, it is ideal that a ceramic layer has no cracks, and there is a direction to eliminate the presence of cracks. However, as a result of investigations by the present inventors, it has been found that not all cracks adversely affect the characteristics, and the presence of cracks in a specific direction is indispensable for promoting polarization. That is, in the multilayer piezoelectric element of the present invention, a crack that does not cross the piezoelectric ceramic layer in the thickness direction is formed in at least a part of the piezoelectric ceramic layer, and sufficient polarization occurs due to the presence of the crack. It has been realized. Further, the presence of the crack does not lead to destruction of the element.

  On the other hand, the polarization method of the laminated piezoelectric element according to the present invention is such that, when a laminated piezoelectric element in which piezoelectric ceramic layers and internal electrode layers are alternately laminated is polarized, DC polarization is applied by applying a DC voltage. The pulse polarization is performed by applying a pulse voltage.

  In the present invention, first, DC polarization is performed, but there is no need for saturation polarization by this DC polarization. For example, cracks that do not cross the piezoelectric ceramic layer in the thickness direction are formed in at least a part of the piezoelectric ceramic layer. The condition is such that it is formed. Next, by performing pulse polarization, saturation polarization or a state close to saturation polarization is obtained. According to the study by the present inventors, it has been found that the combination of the DC polarization and the pulse polarization realizes sufficient polarization close to the saturation polarization without causing element destruction.

  According to the multi-layer piezoelectric element of the present invention, the multi-layer piezoelectric element does not cause element destruction that adversely affects the piezoelectric characteristics, has sufficient piezoelectric characteristics by sufficient polarization, and does not change with time. It is possible to provide a piezoelectric element. In addition, according to the polarization method of the multilayer piezoelectric element of the present invention, it is possible to realize sufficient polarization close to saturation polarization while preventing destruction of the element.

  Hereinafter, a multilayer piezoelectric element to which the present invention is applied and a polarization method thereof will be described in detail with reference to the drawings.

  The laminated piezoelectric element 1 of this embodiment is used as an actuator such as an injector for a fuel injection system, for example, and as shown in FIG. 1, piezoelectric ceramic layers 2 and internal electrode layers 3 are alternately laminated. The laminated body 4 and the external electrodes 5 formed on both side surfaces of the laminated body 4 are configured.

  Here, as the ceramic material constituting the piezoelectric ceramic layer 2 of the laminate 4, it is possible to use all ceramic materials having piezoelectric characteristics, for example, a piezoelectric ceramic material fired in a reducing atmosphere can be used. It is.

Examples of the piezoelectric ceramic material fired in a reducing atmosphere include a lead zirconate titanate-based piezoelectric ceramic material. The lead zirconate titanate-based piezoelectric ceramic material is a piezoelectric ceramic material mainly composed of a composite oxide containing Pb, Ti, and Zr as constituent elements. Here, the composite oxide is composed of, for example, lead titanate (PbTiO ₃ ), lead zirconate (PbZrO ₃ ), and zinc / lead niobate [Pb (Zn _1/3 Nb _2/3 ) O ₃ ]. Or a composite oxide obtained by substituting a part of Pb with Sr, Ba, Ca, or the like in the ternary composite oxide.

  Specific examples of the composition include composite oxides represented by the following formula (1) or (2). In these formulas (1) and (2), the oxygen composition is obtained stoichiometrically, and deviation from the stoichiometric composition is allowed in the actual composition. .

Pb _a [(Zn _1/3 Nb _2/3 ) _x Ti _y Zr _z ] O ₃ (1)
(However, 0.96 ≦ a ≦ 1.03, 0.05 ≦ x ≦ 0.15, 0.25 ≦ y ≦ 0.5, 0.35 ≦ z ≦ 0.6, and x + y + z = 1.)

(Pb _a-b Me _b ) [(Zn _1/3 Nb _2/3 ) _x Ti _y Zr _z ] O ₃ (2)
(However, 0.96 ≦ a ≦ 1.03, 0 <b ≦ 0.1, 0.05 ≦ x ≦ 0.15, 0.25 ≦ y ≦ 0.5, 0.35 ≦ z ≦ 0.6 X + y + z = 1, and Me in the formula represents at least one selected from Sr, Ca, and Ba.)

  The composite oxide has a so-called perovskite structure, and Pb and the substitution element Me in the formula (2) are located at a so-called A site of the perovskite structure. Zn, Nb, Ti, and Zr are located at the so-called B site of the perovskite structure.

One of the major features of the aforementioned lead zirconate titanate-based piezoelectric ceramic material is that it is fired under reducing firing conditions. When the piezoelectric ceramic material is fired in an oxidizing atmosphere, it is necessary to use a noble metal as an electrode material of an internal electrode of the piezoelectric element, for example. On the other hand, if a piezoelectric ceramic material fired under reducing firing conditions is used, an inexpensive electrode material such as Cu or Ni can be used for the internal electrode. The reducing firing conditions are, for example, a firing temperature of 800 ° C. to 1200 ° C. and an oxygen partial pressure of 1 × 10 ⁻¹⁰ to 1 × 10 ⁻⁶ atm.

  The piezoelectric ceramic layer 2 is formed by firing a green sheet containing the piezoelectric ceramic material. The thickness of the fired piezoelectric ceramic layer 2 is, for example, about 80 μm to 100 μm. The thickness of the piezoelectric ceramic layer 2 is not limited to this, and may be designed so as to obtain desired piezoelectric characteristics (displacement amount, etc.) according to the number of laminated layers.

  On the other hand, the internal electrode layer 3 is formed by printing a conductive paste containing a noble metal such as silver or palladium as a conductive material in a predetermined pattern on the green sheet by a method such as screen printing. Further, when a piezoelectric ceramic material fired in the reducing atmosphere is used as the piezoelectric ceramic material, it is also possible to use a conductive paste containing a base metal such as Cu as a conductive material. The thickness of the internal electrode layer 3 is, for example, about 0.5 μm to 5 μm, but is not limited thereto.

  The internal electrode layers 3 are alternately drawn on both side surfaces of the laminate 4 and are electrically connected to external electrodes 5 formed on the respective side surfaces. Therefore, by applying a voltage to the external electrode 5, a voltage is applied to each piezoelectric ceramic layer 2 between the internal electrode layers 3, and the laminate 4 is displaced in the lamination direction of the piezoelectric ceramic layer 2. The external electrode 5 is formed, for example, by printing a conductive paste containing silver as a conductive material and baking it.

  In the multilayer piezoelectric element 1 having the above-described configuration, polarization treatment is performed before use in order to exhibit the piezoelectric characteristics of each piezoelectric ceramic layer 2. In the multilayer piezoelectric element 1 of the present embodiment, saturation polarization, Alternatively, the piezoelectric ceramic layer 2 is characterized by having cracks as a result of realizing polarization close to saturation polarization.

  FIG. 2A schematically shows the crack C. FIG. In the case of the example shown in FIG. 2A, the horizontal direction is substantially along the boundary between the piezoelectric ceramic layer 2 and the internal electrode layer 3 or along the boundary between the piezoelectric ceramic layers 2 in a portion where the internal electrode layer 3 does not exist. Cracks C are formed (in the horizontal direction in the figure). Here, it is important that the crack C is formed in a substantially horizontal direction along the boundary between the piezoelectric ceramic layer 2 and the internal electrode layer 3 as shown in FIG. When the crack C is formed in the thickness direction of the piezoelectric ceramic layer 2 as shown in b) and the piezoelectric ceramic layer 2 is formed so as to cross the thickness direction, the piezoelectric ceramic layer 2 in which the crack C is formed is sandwiched. A short circuit between the pair of internal electrode layers 3 formed in a shape may become a problem.

  The length of the crack C is arbitrary. For example, even if the length of the crack C is long, there is no problem. If it says strongly, it is preferable that the said crack C has stopped in the inactive part shown to Fig.2 (a). If the crack C stops at the inactive portion, the crack C has no influence on the performance of the multilayer piezoelectric element 1. On the other hand, the number of cracks C is also arbitrary, but the number of piezoelectric ceramic layers 2 having cracks C is preferably 3% or more of the total number of laminated piezoelectric ceramic layers 2. For example, when the number of stacked piezoelectric ceramic layers 2 is 300, it is preferable that the number of piezoelectric ceramic layers 2 having cracks C is approximately 10 or more. If the number of piezoelectric ceramic layers 2 having cracks C is too small, the stress is not sufficiently released, and element destruction may occur during polarization.

  The aforementioned crack C is formed as a result of proper polarization treatment. Therefore, in the following, the polarization process of the multilayer piezoelectric element 1 will be described.

  The polarization process is performed by applying a predetermined voltage between the external electrodes 5, a voltage is applied in the stacking direction of the piezoelectric ceramic layer 2 and the internal electrode layer 3, and the direction of the polarization vector of the piezoelectric ceramic layer 2 is changed. Piezoelectric characteristics are manifested by aligning in this direction. As a polarization processing method, DC polarization for applying a direct current voltage or pulse polarization for applying a pulse voltage is known. In the present embodiment, by combining the DC polarization and the pulse polarization, a stacked piezoelectric element is used. Let 1 be saturated or close to saturation without breaking.

  FIG. 3 shows an applied voltage profile [FIG. 3A] and an applied pressure profile [FIG. 3B] in the polarization process of the present embodiment. In the present embodiment, first, DC polarization is performed on the multilayer piezoelectric element 1. DC polarization is performed by applying a DC voltage. At this time, DC polarization is performed without pressurizing the laminate 4. If the DC polarization is performed until the saturation polarization is reached with the pressure released, the element may be destroyed. Therefore, it is a conventional idea to press the laminated body 4 to suppress cracks and eliminate the element destruction. On the other hand, in the present embodiment, the DC polarization is performed in an open state without applying pressure, thereby actively forming a crack that does not lead to element destruction or a short circuit, thereby promoting the polarization. .

  Therefore, the DC polarization does not require conditions necessary for saturation polarization, and may be performed under such conditions that the cracks are formed. If the DC polarization is performed under conditions that cause saturation polarization, the stacked body 4 is not pressurized, and element breakdown is easily caused. Therefore, for example, as shown in FIG. 3, the voltage is increased more slowly than in the case of normal DC polarization, and after reaching a predetermined voltage, the voltage is decreased in a short time. If the ultimate voltage in the DC polarization is too low, the generation of cracks is insufficient, and it may be difficult to make the state close to saturation polarization by subsequent pulse polarization. From a practical viewpoint, it is preferable to set the driving voltage to be equal to or higher than that of the multilayer piezoelectric element 1. The temperature at the time of DC polarization does not need to be high (about 150 ° C.), for example, and may be room temperature (about 25 ° C.).

  Although cracks are formed in the piezoelectric ceramic layer 2 due to the DC polarization, the cracks formed do not cross the piezoelectric ceramic layer 2 in the thickness direction as described above, unlike, for example, how cracks are generated during driving. It will not cause element destruction or short circuit.

  Although pulse polarization is performed after DC polarization, the waveform of the pulse voltage applied at the time of pulse polarization is arbitrary. For example, it may be a rectangular wave or a sine wave. In this embodiment, a pulse voltage is applied while pressurizing the laminate 4. This is due to the following reason.

  Usually, in pulse polarization, a pulse voltage is applied without applying pressure to promote polarization. In this case, if a pulse voltage is applied under conditions necessary for saturation polarization, there is a possibility that element destruction may occur as in the case of DC polarization. In order to eliminate this, it is necessary to apply the pulse electrode while pressurizing the laminated body 4. However, when polarization is performed only by pulse polarization, it becomes difficult to reach saturation polarization after pressurization. . On the other hand, in the case of the present embodiment, since the polarization of the piezoelectric ceramic layer 2 has been advanced by DC polarization in advance, the saturation polarization can be sufficiently reached even when a pulse voltage is applied while applying pressure. In addition, since pressure is applied, element destruction can be prevented.

  As conditions for the pulse polarization, first, it is preferable to perform the pulse polarization while applying a pressure higher than the pressure applied when the multilayer piezoelectric element 1 is used as an actuator. If the pressurization pressure of the laminate 4 is too small, there is a risk of element destruction. Second, the pulse voltage is preferably equal to or higher than the driving voltage of the multilayer piezoelectric element 1. If the pulse voltage is too low, it may be difficult to achieve a state close to saturation polarization.

  Third, the frequency of the pulse voltage to be applied is preferably set to be equal to or higher than the driving frequency of the multilayer piezoelectric element 1, and more preferably set to an appropriate frequency in consideration of temperature. When the piezoelectric ceramic layer 2 is polarized, the temperature is also an important factor. The higher the temperature, the more the polarization is promoted. On the other hand, in the pulse polarization, the laminate 4 generates heat according to the frequency of the applied pulse voltage. The heat generation increases as the frequency of the applied pulse voltage increases. The temperature necessary for promoting the polarization is about 60 ° C. to 180 ° C., and the frequency of the pulse voltage necessary for this is 30 Hz to 200 Hz. That is, the frequency of the pulse voltage is preferably 30 Hz to 200 Hz.

  As described above, DC polarization and pulse polarization are used in combination, and by performing pulse polarization in a state where cracks are formed by DC polarization, sufficient polarization (saturation polarization or polarization close to saturation polarization without causing element destruction) ) Can be realized. That is, in the multilayer piezoelectric element, cracks that are substantially perpendicular to the lamination direction (cracks that are substantially parallel to the interface between the piezoelectric ceramic layer 2 and the internal electrode layer 3) do not have an adverse effect during driving, and the element is free. By applying a DC voltage to form the crack, and then applying a pulse voltage while applying pressure, the stacked piezoelectric element can be brought close to saturation polarization without breaking.

  Hereinafter, specific examples of the present invention will be described based on experimental results.

Production of Multilayer Piezoelectric Element An internal electrode layer was formed on a green sheet containing a 6 mm × 6 mm PZT (lead zirconate titanate) piezoelectric ceramic material, and 300 layers were stacked to form a laminate. The composition of the PZT piezoelectric ceramic material is (Pb _0.965 Sr _0.03 ) [(Nb _2/3 Zn _1/3 ) _0.10 Ti _0.430 Zr _0.460 ] O ₃ + Ta ₂ O ₅ (0. 4% by mass). The internal electrode layer was formed by printing a conductive paste containing Ag / Pd (70:30) as a conductive material in a predetermined pattern. The interlayer (thickness of each piezoelectric ceramic layer) was 80 μm.

Against polarized fabricated laminated piezoelectric element was first subjected to polarization treatment by only DC polarization. In this case, when a DC voltage exceeding the DC voltage of 2.5 kV / mm [applied voltage / element size (mm)] is applied, there is a case where the device is destroyed. Therefore, when the DC voltage was set to 200 V (2.5 kV / mm and DC pressure was applied at 150 ° C. under pressure, the displacement change rate before and after driving was about 4.5%, and the polarization was insufficient. I found out.

  Next, polarization treatment was performed only by pulse polarization. Even in the case of pulse polarization, if a pulse voltage was applied with pressure applied, the element was broken. Pulse polarization was attempted with a very slow boosting speed, but when the voltage increased and the hold state was reached, the device was destroyed in a few seconds.

  Therefore, next, DC polarization was performed with the pressure released, and then pulse polarization was performed while pressure was applied. In DC polarization, the ultimate voltage was 200 V, and the holding time at the ultimate voltage was 0 second. That is, the pressure was reduced immediately after the pressure increase. Due to this DC polarization, cracks were formed in the device along the interface between the piezoelectric ceramic layer and the internal electrode layer. For pulse polarization, a pulse voltage of 200 V (2.5 kV / mm) was applied while applying a pressure of 37 MPa. The shape of the pulse waveform was a trapezoidal wave of 0.2-1-0.2 msec. As a result, the displacement change rate before and after driving was about 0.2%, and it was confirmed that the polarization was advanced to a level close to the saturation polarization. In addition, no sample was broken after the application of the pulse voltage.

  Furthermore, the temperature conditions during DC polarization were changed, pulse polarization was performed at pulse voltages of 200 V (2.5 kV / mm) and 175 V (2.2 kV / mm), and the displacement characteristics were examined using a laser Doppler displacement meter. The results are shown in FIG. When the pulse voltage at the time of pulse polarization is 2.2 kV / mm, the amount of displacement d33 tends to slightly decrease if the temperature at the time of DC polarization is low. On the other hand, it was found that when the pulse voltage at the time of pulse polarization is 2.5 kV / mm, a sufficient displacement d33 is shown regardless of the DC polarization temperature.

It is a perspective view which shows schematic structure of a lamination type piezoelectric element. It is a schematic diagram which shows a mode that a crack enters, (a) is a schematic diagram of the crack formed along the interface of a piezoelectric ceramic layer and an internal electrode layer, (b) is the crack of the crack which entered the direction crossing a piezoelectric ceramic layer It is a schematic diagram. It is a figure which shows the applied voltage profile and applied pressure profile in the polarization method to which this invention is applied. It is a characteristic view which shows the relationship between DC polarization temperature and displacement amount d33 about pulse voltage 200V (2.5 kV / mm) and 175V (2.2 kV / mm).

Explanation of symbols

1 laminated piezoelectric element, 2 piezoelectric ceramic layer, 3 internal electrode layer, 4 laminate, 5 external electrode

Claims (8)

  Piezoelectric ceramic layers and internal electrode layers are alternately laminated, and at least part of the piezoelectric ceramic layer is formed with a crack that does not cross the piezoelectric ceramic layer in the thickness direction. element.   2. The multilayer piezoelectric element according to claim 1, wherein the number of piezoelectric ceramic layers in which cracks are formed is 3% or more of the number of laminated piezoelectric ceramic layers. When polarizing a stacked piezoelectric element in which piezoelectric ceramic layers and internal electrode layers are alternately stacked,
A polarization method for a laminated piezoelectric element, wherein a DC voltage is applied to perform DC polarization, and then a pulse voltage is applied to perform pulse polarization.   4. The method for polarizing a multilayer piezoelectric element according to claim 3, wherein a crack that does not cross the piezoelectric ceramic layer in the thickness direction is formed in at least a part of the piezoelectric ceramic layer by the DC polarization.   5. The method for polarizing a laminated piezoelectric element according to claim 3, wherein the DC voltage applied during the DC polarization is equal to or higher than the driving voltage of the laminated piezoelectric element.   6. The method for polarizing a laminated piezoelectric element according to claim 3, wherein a pulse voltage applied during the pulse polarization is equal to or higher than a driving voltage of the laminated piezoelectric element.   7. The laminate according to claim 3, wherein a pulse frequency of a pulse voltage applied at the time of the pulse polarization is a frequency at which the temperature becomes 100 ° C. to 200 ° C. due to self-heating. Method of piezoelectric piezoelectric element. 8. The method for polarizing a multilayer piezoelectric element according to claim 3, wherein the DC polarization is performed without applying pressure, and the pulse polarization is performed while applying pressure.

Images