Piezoelectric element, and ultrasonic vibrator and ultrasonic motor equipped with the same

Abstract

To provide a piezoelectric element with small mechanical loss and large displacement when driven at high speed and large amplitude.SOLUTION: A piezoelectric element consists of first and second electrodes provided opposite each other, and a piezoelectric ceramics disposed between the first electrode and the second electrode and in contact therewith, mainly composed of a compound having a perovskite-type structure containing Pb, Zr, Ti and O as constituent elements, wherein the average domain width in the sintered particles 11 observed in the cross section perpendicular to the first and second electrodes is between 100 nm and 500 nm and the area percentage of sintered particles with observed domain 12 in the cross section is 60% or more.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric element, and an ultrasonic vibrator and an ultrasonic motor including the same. [Background technology]

[0002] Piezoelectric elements are used in sensor elements, power generation elements, etc. by utilizing the positive piezoelectric effect, which converts mechanical energy into electrical energy. Piezoelectric elements are also used in vibrators, sound generators, actuators, ultrasonic motors, pumps, etc. by utilizing the inverse piezoelectric effect, which converts electrical energy into mechanical energy. Furthermore, piezoelectric elements are also used in circuit elements, vibration control elements, etc. by utilizing both the positive piezoelectric effect and the inverse piezoelectric effect.

[0003] Among piezoelectric elements, vibrators and ultrasonic motors, etc., are continuously driven under conditions where large amplitudes occur, such as at resonance points, and therefore the elements themselves are prone to heat generation. Heat generation in piezoelectric elements leads to the deterioration or loss of piezoelectric properties, so it is necessary to suppress this. Heat generation in piezoelectric elements is caused by mechanical and electrical losses that occur during operation. For this reason, materials with low mechanical and electrical losses, known as hard piezoelectric materials or hard materials, are used in such piezoelectric elements. For hard piezoelectric materials, a high mechanical quality factor Qm is important as an indicator of low mechanical loss, and a small dielectric tangent tanδ is important as an indicator of low electrical loss.

[0004] As such low-loss hard piezoelectric materials, those based on lead zirconate titanate (Pb(Zr,Ti)O3:PZT) with a perovskite structure and incorporating various elements into the solid solution to reduce loss have been proposed.

[0005] For example, a known piezoelectric ceramic composition with a high mechanical quality factor Qm is one in which Mn or the like is added to a compound containing Pb, Zn, Nb, Ti, Zr, and O as constituent elements and having a perovskite structure (Patent Documents 1 and 2).

[0006] It is also known that a means for increasing the mechanical quality factor Qm of PZT-based piezoelectric ceramics is to set the area ratio of crystal grains with a domain size of 100 nm or less to 30% or more (Patent Document 3). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 54-18400 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-181037 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-92280 Summary of the Invention [Problem to be solved by the invention]

[0008] Piezoelectric elements used in ultrasonic vibrators and ultrasonic motors are required to generate little heat when driven, as well as to have a large amount of displacement. The magnitude of the displacement of a piezoelectric element increases as the piezoelectric d constant increases. However, since the mechanical quality factor Qm and the piezoelectric d constant of a piezoelectric element are generally in a trade-off relationship, it has been difficult to obtain a piezoelectric element with high values ​​for both. In fact, while the piezoelectric ceramic compositions described in Patent Documents 1 to 3 have been able to obtain piezoelectric elements with high mechanical quality factors Qm, the elements often have low values ​​for the piezoelectric d constant.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a piezoelectric element that has small mechanical loss and a large amount of displacement when driven at high speed and with large amplitude. [Means for solving the problem]

[0010] In the course of research into solving the above problems, the inventors focused on the domain structure in sintered particles of PZT-based piezoelectric ceramics and discovered that the above problems could be solved by increasing the width of the domains, which led to the completion of the present invention.

[0011] That is, one aspect of the present invention for solving the above-mentioned problems is a piezoelectric element including first and second electrodes provided opposite to each other, and a piezoelectric ceramic disposed between and in contact with the first and second electrodes, the piezoelectric ceramic containing, as its main component, a compound having a perovskite structure containing Pb, Zr, Ti, and O as constituent elements, and in which the average domain width in sintered particles observed in a cross section perpendicular to the first and second electrodes is 100 nm or more and 500 nm or less, and the area percentage of the sintered particles in which the domains are observed in the cross section is 60% or more.

[0012] Another aspect of the present invention is an ultrasonic vibrator including the above-described piezoelectric element and a pair of block bodies that sandwich the piezoelectric element in one axial direction.

[0013] Furthermore, another aspect of the present invention is an ultrasonic motor including the above-mentioned piezoelectric element and a sliding member adhered to the piezoelectric element. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a piezoelectric element that has small mechanical loss when driven at high speed and with large amplitude, and has a large displacement amount. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic perspective view showing the structure of a multi-layer piezoelectric element according to one aspect of the present invention; [Figure 2] A-A' cross-sectional view of the multilayer piezoelectric element shown in Figure 1 [Figure 3] FIG. 1 is an explanatory diagram showing a method for measuring a domain width in a piezoelectric element according to one aspect of the present invention. [Figure 4] FIG. 1 is an explanatory diagram showing a method for measuring the area of ​​a sintered particle where a domain is observed in a piezoelectric element according to one aspect of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The configuration and effects of the present invention will be described below, along with the technical concept, with reference to the drawings. However, the mechanism of action includes assumptions, and the correctness of such assumptions does not limit the present invention.

[0017] In this specification, "driving at high speed and large amplitude" of a piezoelectric element means driving at a resonance frequency, or driving under conditions where the vibration velocity measured with a laser Doppler vibrometer is 0.62 m / s or higher.

[0018] [Piezoelectric element] A piezoelectric element according to one aspect of the present invention (hereinafter, sometimes simply referred to as the "first aspect") comprises first and second electrodes disposed opposite each other, and a piezoelectric ceramic disposed between and in contact with the first and second electrodes, the piezoelectric ceramic containing as its main component a compound having a perovskite structure containing Pb, Zr, Ti, and O as constituent elements, in which the average domain width in sintered particles observed in a cross section perpendicular to the first and second electrodes is 100 nm or more and 500 nm or less, and the area percentage of the sintered particles in which the domains are observed within the cross section is 60% or more.

[0019] The first and second electrodes are arranged opposite each other to sandwich the piezoelectric ceramic described below, and voltages of opposite polarities ("+" or "-") are applied to them when the piezoelectric element is driven. The material, shape, and arrangement of the electrodes are not particularly limited as long as they can apply the desired voltage to the piezoelectric ceramic. Examples of electrode materials include silver (Ag), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), and alloys thereof. Examples of electrode shapes and arrangements include those that cover almost the entire specific surface of the piezoelectric ceramic. Furthermore, the electrodes may be formed only on the surface of the piezoelectric element, or may be formed inside the piezoelectric element, such as the internal electrodes in the multilayer piezoelectric element described below.

[0020] When the first side surface is a multi-layer piezoelectric element 100 having the structure shown in FIGS. 1 and 2, the multi-layer piezoelectric element 100 includes connecting conductors 30a and 30b that electrically connect the first electrodes 20a and the second electrodes 20b, respectively, present between the piezoelectric ceramic layers 10. As with the electrodes described above, examples of the material for the connecting conductors include silver (Ag), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), and alloys thereof. The connecting conductors may be disposed on the surface of the multi-layer piezoelectric element 100 as shown in FIGS. 1 and 2, or may be disposed inside the multi-layer piezoelectric element 100 so as to penetrate the piezoelectric ceramic layers 10. In the multi-layer piezoelectric element 100 shown in FIGS. 1 and 2, the first electrodes 20a and the second electrodes 20b are disposed inside the piezoelectric element, and are therefore collectively referred to as internal electrodes 20.

[0021] Furthermore, when the first side surface is the multi-layer piezoelectric element 100, a pair of external electrodes 40a and 40b may be provided on the surface thereof, electrically connected to the connecting conductors 30a and 30b, for applying a voltage to the piezoelectric ceramics located at both ends in the stacking direction, as shown in Figures 1 and 2. The external electrodes may be made of the same material as the connecting conductors described above.

[0022] The first side includes a piezoelectric ceramic disposed between and in contact with the first and second electrodes.

[0023] This piezoelectric ceramic is mainly composed of a compound with a perovskite structure containing the constituent elements Pb, Zr, Ti, and O. This allows for a large displacement to be obtained when a voltage is applied to the piezoelectric element.

[0024] Here, whether the piezoelectric ceramic is composed mainly of a compound having a perovskite structure is confirmed by the following procedure. First, the piezoelectric ceramic is pulverized to prepare a powder sample. In this case, it is preferable to pulverize after removing parts other than the piezoelectric ceramic, such as the electrodes and coating. However, in the case of a piezoelectric element such as the above-mentioned multilayer piezoelectric element, which is difficult to separate the piezoelectric ceramic part from other parts and in which the proportion of the piezoelectric ceramic part is higher than the other parts, the element itself may be pulverized to prepare a powder sample. Next, the diffraction line profile of the obtained powdered sample is measured using an X-ray diffraction (XRD) device using Cu-Kα radiation, and if the ratio of the strongest diffraction line intensity in the diffraction profile derived from other structures to the strongest diffraction line intensity in the profile derived from the perovskite structure is 10% or less, the piezoelectric ceramic is determined to be composed primarily of a compound having a perovskite structure. Note that if the piezoelectric element is pulverized without separating the piezoelectric ceramic portion from the other portions to obtain a powdered sample, and a peak clearly originating from a portion other than the piezoelectric ceramic, such as an electrode, is observed in the diffraction line profile, that peak is excluded from the comparison of the strongest line intensities described above.

[0025] Furthermore, whether the compound having a perovskite structure contains Pb, Zr, Ti, and O as constituent elements is confirmed by the following procedure. First, a powdered sample determined by the above-mentioned method to contain a compound having a perovskite structure as its main component is subjected to composition analysis using a high-frequency inductively coupled plasma (ICP) optical emission spectrometer, an ion chromatograph, or an X-ray fluorescence (XRF) analyzer. Next, if the presence of Pb, Zr, and Ti is confirmed as a result of composition analysis, the compound having a perovskite structure is determined to contain the elements mentioned above and O. If the presence of Zn, Nb, and Mn is also confirmed by this composition analysis, the compound having a perovskite structure is determined to further contain Zn, Nb, and Mn as constituent elements, as described below.

[0026] The compound having the perovskite structure preferably further contains Zn, Nb, and Mn as constituent elements. Thereby, the compound mainly becomes Pb(Zr,Ti)O3-Pb(Zn,Nb)O3-Pb(Mn,Nb)O3, and when a voltage is applied to the piezoelectric element, a larger displacement can be obtained and the mechanical loss during driving is reduced.

[0027] More preferably, the compound having the perovskite structure is represented by the following general formula (1). Thereby, a larger displacement amount and a smaller mechanical loss can be achieved. xPb(Zr 1-a Ti a )O3-yPb(Zn 1 / 3 Nb 2 / 3 )O3 -zPb(Mn 1 / 3 Nb 2 / 3 )O3…(1) However, a, x, y, and z in the formula are real numbers satisfying 0.45 ≦ a ≦ 0.60, 0 < x ≦ 0.85, 0 ≦ y < 0.99, 0.01 < z < 0.10, and x + y + z = 1.0, respectively.

[0028] In the following description, the compound represented by the general formula (1) may be referred to as "PZT-PZN-PMnN", and when individually referring to each part connected by a hyphen (-) in the general formula (1), they may be referred to as "PZT", "PZN", and "PMnN", respectively.

[0029] Here, that the compound having the perovskite structure is represented by the general formula (1) is confirmed by the following procedure. First, from the results of the above-described composition analysis, the contents of Zr, Ti, Zn, Nb, and Mn are calculated in atomic % or mole %. Next, the ratio of the content of Ti to the total amount of Zr and Ti is defined as "a". Next, the contents of Zn, Nb, and Mn in the perovskite structure are determined so that the ratio of the Nb content to the Zn content and the ratio of the Nb content to the Mn content are both 2. At this time, if the ratio of the Nb content to the total amount of Zn and Mn exceeds 2, the excess Nb is not included in the perovskite structure. On the other hand, if the ratio of the Nb content to the total amount of Zn and Mn is less than 2, in other words, if the ratio of the total amount of Zn and Mn to the Nb content exceeds 1 / 2, the total amount of Nb is divided proportionally according to the Zn and Mn content ratio to determine the amount of Nb included in PZN and PMnN, and Zn and Mn equivalent to 1 / 2 of each amount of Nb are included in the perovskite structure, and Zn and Mn in excess of this amount are not included in the perovskite structure. Next, the ratio of the total amount of Zr and Ti to the total of the determined contents of Zn, Nb, and Mn in the perovskite structure, and the total amount of Zr and Ti (hereinafter simply referred to as the "total amount of B sites") is defined as "x," the ratio of the total amount of Zn and Nb in PZN to the total amount of B sites is defined as "y," and the ratio of the total amount of Mn and Nb in PMnN to the total amount of B sites is defined as "z." Then, if the obtained values ​​of "a," "x," "y," and "z" all satisfy the general formula (1), the compound having the perovskite structure is determined to be one represented by the general formula (1).

[0030] As long as the piezoelectric ceramic contains the aforementioned elements as constituent elements and is primarily composed of a compound having a perovskite structure, it may also contain other additive elements or compounds. Examples of additive elements include Ca, Sr, Ba, Ag, La, Ce, and Bi, which are solid-solubilized in the A site of the perovskite structure represented by ABO3, and Mg, Fe, Co, Ni, Ta, and W, which are solid-solubilized in the B site. Examples of compounds include a glassy grain boundary phase derived from components added to lower the sintering temperature.

[0031] In a first aspect, when a cross section of the piezoelectric ceramic perpendicular to the first and second electrodes is observed, the average domain width present in the sintered particles is 100 nm or more and 500 nm or less. This results in a piezoelectric element with a large mechanical quality factor Qm and piezoelectric d constant. While the reason for this is unclear, it is presumed that a large average domain width of 100 nm or more suppresses the movement of domain walls, which are the boundaries between domains, during operation of the piezoelectric element, reducing mechanical losses due to this movement, and that a moderate average domain width of 500 nm or less allows the sintered particles to contain a certain amount of domain walls, improving the displacement capacity due to the excellent electric field response of the domain walls. To obtain a larger mechanical quality factor Qm, the average domain width is preferably 105 nm or more, and more preferably 110 nm or more. On the other hand, to obtain a larger piezoelectric d constant, the average domain width is preferably 450 nm or less, and more preferably 400 nm or less. For the reasons mentioned above, the average domain width is preferably 105 nm or more and 450 nm or less, and more preferably 110 nm or more and 400 nm or less.

[0032] In a first aspect, when a cross section perpendicular to the first and second electrodes of the piezoelectric ceramic is observed, the area percentage of sintered grains in which domains are observed within the cross section is 60% or more. This results in a piezoelectric element with a large mechanical quality factor Qm. Although the reason for this is unclear, it is presumed that this is due to the fact that domain walls that appear in the cross section are less likely to move when the piezoelectric element is driven than those that do not appear in the cross section. In order to obtain a larger mechanical quality factor Qm, the area percentage is preferably 65% ​​or more, and more preferably 70% or more. On the other hand, since a higher area percentage is preferable, the upper limit is not limited and may be 100%.

[0033] In the first aspect, when a cross section perpendicular to the first and second electrodes in the piezoelectric ceramic is observed, the density of the domain walls present in the sintered particles is 2.5 lines / μm 2or less. This results in a piezoelectric element with a large mechanical quality factor Qm. The reason for this is not clear, but considering that sintered particles having domains of the same width have a lower domain wall density when the dimension of the domain wall in the longitudinal direction is larger, i.e., when the contained domain wall length is longer, it is presumed that the low density of the domain walls in the sintered particles and the long contained domain wall length increase the energy required for the domain walls to move, making it difficult for the domain walls to move when the piezoelectric element is driven.

[0034] Here, the average domain width in a cross section perpendicular to the first and second electrodes of the piezoelectric ceramic, the area percentage of the sintered grains in which domains are observed within the cross section, and the domain wall density in the sintered grains are determined by the following procedure. First, the piezoelectric element is cut along a plane perpendicular to the first and second electrodes. The cutting means is not particularly limited, and a dicing saw, a cutter, or the like can be used. Next, the cut piezoelectric ceramic is embedded in epoxy resin so that the cut surface is exposed, and then the cut surface is mirror-polished using colloidal silica. Next, in order to impart conductivity to the cut surface of the piezoelectric ceramic, the polished surface is coated with osmium (Os) to prepare a measurement sample. Next, a backscattered electron (BSE) image of the piezoelectric ceramic part is obtained using a Schottky scanning electron microscope on the mirror-finished cut surface, with an accelerating voltage of 7.00 kV, a working distance (WD) between the bottom surface of the objective lens and the sample of 4 mm, and a magnification of 10,000x. Next, the image file of the obtained BSE image is saved in PDF format, and then the PDF file is opened in Acrobat (manufactured by Adobe). Next, three sintered particles observed in the BSE image were randomly selected from those in which striped domains were clearly visible from end to end. These were used as measurement particles. Then, for each measurement particle, the width of the striped domains 12 present inside was measured at five different locations in the domains 12, as shown in Figure 3. Measurements were performed by selecting the "Distance Tool" as the measurement type in the "Measurement Tool" function of Acrobat, setting one end of the domain 12 as the starting point (see Figure 3(a)), moving the cursor to the other end of the domain 12 (see Figure 3(b)), and setting the position where the line segment displayed on the other end follows the domain boundary (domain wall 13) as the ending point (see Figure 3(c)), and reading the displayed distance. Next, the average value of the 15 domain width data obtained from each particle to be measured is calculated, and the value obtained by dividing this by 10,000 is taken as the average domain width of the piezoelectric ceramic. Next, select the "area tool" as the measurement type in the "ruler tool" function of Acrobat, and for all sintered particles observed in the BSE image in which striped domains are clearly visible from end to end within the domain, trace the outline to draw a polygon and display the area of ​​the polygon, as shown in Figure 4. In Figure 4, sintered particles 11 in which striped domains 12 are clearly visible from end to end within the domain 12 within the width direction are displayed with spots. The sum of the displayed areas is then divided by the area of ​​the entire BSE image and multiplied by 100 to obtain the area percentage of sintered particles in which domains are observed within the cross section of the piezoelectric ceramic. Next, three particles are randomly selected from the sintered particles 11 in which the domains 12 are clearly observed from end to end, and the domain boundaries (domain walls 13) present in each selected particle are counted and summed. The total value is then calculated as the sum of the areas (μm 2 ) is the domain wall density in the sintered particle.

[0035] [Manufacturing method of piezoelectric element] The piezoelectric element according to the first aspect is manufactured by, for example, mixing powders of compounds containing one or more elements selected from Pb, Zr, and Ti to obtain a mixed powder containing the elements, calcining the mixed powder to obtain a calcined powder, forming the calcined powder into a predetermined shape to obtain a compact, firing the compact to obtain a sintered body, forming electrodes and / or connecting conductors on the surface of the sintered body, and applying a high voltage between the electrodes or connecting conductors to perform a polarization process. This manufacturing method is described below.

[0036] The composition and particle size of the compound powder used as the raw material are not limited as long as it can produce the desired piezoelectric ceramic upon firing. The compound constituting the powder may contain additional elements other than those mentioned above. Examples of compounds that can be used include Pb-containing compounds such as PbO and Pb3O4, Zr-containing compounds such as ZrO2, and Ti-containing compounds such as TiO2. Furthermore, when Zn, Nb, and Mn are incorporated into the perovskite structure to obtain a higher mechanical quality factor Qm and piezoelectric d constant, ZnO may be used as the Zn-containing compound, Nb2O5 as the Nb-containing compound, and MnCO3 as the Mn-containing compound.

[0037] The method for mixing the raw material powders is not particularly limited as long as the powders are mixed uniformly while preventing impurities from being mixed in, and either dry mixing or wet mixing may be used. When wet mixing using a ball mill is used, mixing may be carried out for, for example, about 8 to 24 hours.

[0038] The calcination conditions are not limited as long as they allow the raw materials to react to produce a compound with a perovskite structure having the desired composition, and may be, for example, in an air atmosphere at 700°C to 1000°C for 2 to 8 hours. If the calcination temperature is too low or the calcination time is too short, there is a risk that unreacted raw materials or intermediate products will remain. Conversely, if the calcination temperature is too high or the calcination time is too long, there is a risk that the compound with the desired composition will not be obtained due to the volatilization of Pb and Zn, or that the product will solidify and become difficult to disintegrate, resulting in reduced productivity.

[0039] The calcined powder may be mixed with components to be incorporated into the perovskite structure during firing, as described below, or components to form precipitates between sintered particles of the piezoelectric ceramic, before molding. In this case, the same mixing method as that for the raw material powders can be used.

[0040] When ZnO powder is added to the calcined powder, the average domain width described above is expanded, making it easier to obtain the piezoelectric element according to the first aspect. In order to keep the average domain width of the piezoelectric element within a predetermined range, the amount of ZnO powder added is preferably 0.1% by mass to 1.0% by mass, and more preferably 0.3% by mass to 0.5% by mass, of the calcined powder.

[0041] The calcined powder can be formed by any method commonly used for forming ceramic powders, such as uniaxial pressing of the powder, extrusion molding of a clay containing the powder, or casting of a slurry in which the powder is dispersed.

[0042] Here, when the piezoelectric element is the multi-layer piezoelectric element 100 shown in FIGS. 1 and 2, the following molding method can be adopted.

[0043] First, the calcined powder is mixed with a binder or the like to form a slurry or a clay, which is then formed into a sheet to obtain a green sheet containing the calcined powder. Conventional methods such as the doctor blade method and extrusion molding can be used to form the sheet.

[0044] Next, an electrode pattern that will become the internal electrode 20 after firing is formed on the green sheet containing the calcined powder. The electrode pattern may be formed by a conventional method, and a method of printing or applying a paste containing an electrode material is preferable from the standpoint of cost. When forming the electrode pattern by printing or applying, the paste may contain a powder (co-material) or glass frit having the same composition and crystalline structure as the piezoelectric ceramic after firing in order to improve the adhesive strength to the piezoelectric ceramic after firing.

[0045] 1 and 2, there is also an example of a multilayer piezoelectric element having a structure different from that shown in Figures 1 and 2, in which connecting conductors that electrically connect internal electrodes are disposed in through-holes (vias) that penetrate the piezoelectric ceramic layers. When manufacturing a multilayer piezoelectric element of this structure, prior to forming the electrode pattern, through-holes are formed in the obtained green sheet by punching or laser irradiation, and the through-holes are filled with electrode material before or after forming the electrode pattern. Although there are no particular limitations on the filling method, a method in which a paste containing the electrode material is printed is preferred from the standpoint of cost.

[0046] Next, a predetermined number of green sheets on which electrode patterns have been formed are stacked and the sheets are bonded together to obtain a molded body. The stacking and bonding may be performed by a conventional method, and a method in which the green sheets are thermocompressed together using a binder is preferred from the viewpoint of cost.

[0047] The compact obtained by the above procedure is fired after removing the binder as needed. The firing conditions can be appropriately set taking into consideration the sinterability of the calcined powder and, if present, the durability of the electrode material. When firing a compact containing copper (Cu) or nickel (Ni) as the internal electrode material, a reducing or inert firing atmosphere is preferred to prevent oxidation. Examples of firing conditions for a compact not containing either copper (Cu) or nickel (Ni) as the internal electrode material include firing in an air atmosphere at 900°C to 1200°C for 1 hour to 5 hours. If the firing temperature is too low or the firing time is too short, densification may be insufficient, resulting in a piezoelectric ceramic that does not have the desired characteristics. Conversely, if the firing temperature is too high or the firing time is too long, the volatilization of Pb or Zn may cause compositional deviation, or the generation of coarse particles may result in a deterioration of the characteristics. Furthermore, if the compact contains an internal electrode material, melting or diffusion of the electrode material may prevent the piezoelectric ceramic or piezoelectric element from achieving the desired characteristics. To avoid such inconveniences caused by an excessively high firing temperature and to reduce material costs by using a low-melting-point material for the internal electrode, it is preferable to set the firing temperature to 1100°C or lower. When multiple piezoelectric ceramics or piezoelectric elements are to be obtained from a single compact, the compact may be divided into several blocks prior to firing.

[0048] In the first aspect, when firing is performed in the atmosphere during the manufacturing process, increasing the firing temperature tends to increase the average domain width. An example of a firing temperature at which a piezoelectric element with a large average domain width can be obtained is 1050°C or higher. On the other hand, if the firing temperature is not too high, the area percentage of sintered grains in which domains are observed tends to be large. An example of a firing temperature at which a piezoelectric element with a large area percentage of sintered grains in which domains are observed can be 1100°C or lower. From these points of view, an example of a firing temperature in the atmosphere at which a piezoelectric element with a large average domain width and a large area percentage of sintered grains in which domains are observed can be 1050°C or higher and 1100°C or lower.

[0049] Next, electrodes and / or connecting conductors are formed on the surface of the obtained sintered body. If the sintered body does not contain internal electrodes, a pair of electrodes is formed on its surface. These electrodes become the first and second electrodes of the piezoelectric element, respectively. On the other hand, if the sintered body contains internal electrodes, a pair of connecting conductors is formed on its surface. In this case, a pair of external electrodes electrically connected to each connecting conductor may be further formed on the surface of the sintered body as the first electrode and the second electrode. To form the electrodes and / or connecting conductors, conventional methods can be used, such as applying or printing a paste containing an electrode material onto the surface of the piezoelectric ceramic and baking it, or vapor-depositing an electrode material onto the surface of the piezoelectric ceramic.

[0050] Next, a high voltage is applied between the formed electrodes or connecting conductors to polarize the piezoelectric ceramic. The conditions for the polarization treatment are not particularly limited as long as they can align the direction of spontaneous polarization without causing damage such as cracks in the piezoelectric ceramic. One example is applying an electric field of 1 kV / mm to 5 kV / mm at a temperature of 100°C to 180°C.

[0051] [Ultrasonic vibrator] An ultrasonic vibrator according to another aspect of the present invention (hereinafter, sometimes simply referred to as the "second aspect") includes a piezoelectric element according to the first aspect and a pair of block bodies that sandwich the piezoelectric element in one axial direction. This ultrasonic vibrator is known as a Langevin-type vibrator. The Langevin-type vibrator may be a so-called bolt-fastened Langevin vibrator in which the block bodies are fastened to the piezoelectric element by bolts to sandwich the piezoelectric element. The Langevin-type vibrator generates ultrasonic vibrations by supplying electrical energy to the piezoelectric element and operates to transmit the ultrasonic vibrations to the outside via the block bodies. By including a piezoelectric element according to the first aspect on the second aspect, the second aspect becomes a vibrator that generates less heat when driven at high speed and with large amplitude and can be driven stably for a long period of time. Furthermore, by including a piezoelectric element according to the first aspect on the second aspect, the second aspect becomes a vibrator that can vibrate with large amplitude.

[0052] The material of the block used in the second aspect is not particularly limited as long as it can efficiently transmit the ultrasonic vibrations generated by the piezoelectric element, and for example, titanium alloy, aluminum alloy, SUS, etc. can be used.

[0053] [Ultrasonic motor] An ultrasonic motor according to another aspect of the present invention (hereinafter, sometimes simply referred to as the "third aspect") includes the piezoelectric element according to the first aspect and a sliding body bonded to the piezoelectric element. The ultrasonic motor operates by causing the sliding body bonded to the piezoelectric element to trace a predetermined trajectory due to vibration of the piezoelectric element, which then comes into contact with the driven body, thereby moving the driven body. By including the piezoelectric element according to the first aspect in the third aspect, the motor generates less heat when driven at high speed and with large amplitude, and can operate stably for a long period of time. Furthermore, by including the piezoelectric element according to the first aspect in the third aspect, the trajectory traced by the sliding body becomes larger, and the amount of movement of the driven body per vibration increases, resulting in a motor that can move the driven body at a higher speed.

[0054] The material of the sliding body used in the third aspect is not particularly limited as long as it can be displaced in response to the vibration of the piezoelectric element and has excellent wear resistance, and ceramic materials such as alumina and silicon nitride are preferably used.

[0055] In the third aspect, the means for bonding the piezoelectric element and the sliding body is not particularly limited as long as they are capable of displacing integrally without peeling off during driving. One example is bonding with an adhesive. [Example]

[0056] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0057] Example 1 As starting materials, high-purity Pb3O4, ZrO2, TiO2, ZnO, Nb2O5 and MnCO3 powders were prepared, and each powder was mixed with a compound having the general formula 0.79Pb(Zr 1 / 2 Ti 1 / 2)O3-0.16Pb(Zn 1 / 3 Nb 2 / 3 )O3-0.05Pb(Mn 1 / 3 Nb 2 / 3 The materials were weighed to obtain a calcined powder having a perovskite structure represented by the formula (I)O3, and wet-mixed in a ball mill using zirconia balls. After mixing, the dispersion medium was removed from the mixed powder, which was then calcined in air at 820°C for 3 hours to obtain a calcined powder. 0.3 mass% of ZnO powder was added to the calcined powder, which was then crushed. An acrylic binder was then added, and the mixture was uniaxially pressed under a load of 2 tf to obtain a disk-shaped compact with a diameter of 10 mm. The resulting disk-shaped compact was fired in air at 1100°C for 2 hours to obtain a sintered body (piezoelectric ceramic). Ag paste was applied to both sides of the sintered body, which was then heated to 600°C and baked to form a pair of connecting conductors and external electrodes. The sintered body after electrode formation was subjected to polarization treatment in silicone oil at 150°C at an electric field strength of 2.2 kV / mm for 15 minutes to obtain a piezoelectric element according to Example 1.

[0058] Example 2 The amount of starting raw material powder was determined by the general formula 0.79Pb(Zr 1 / 2 Ti 1 / 2 )O3-0.13Pb(Zn 1 / 3 Nb 2 / 3 )O3-0.08Pb(Mn 1 / 3 Nb 2 / 3 A piezoelectric element according to Example 2 was obtained in the same manner as in Example 1, except that a calcined powder represented by )O3 was obtained.

[0059] (Comparative Example 1) The amount of starting raw material powder was determined by the general formula 0.79Pb(Zr 0.51 Ti 0.49 )O3-0.21Pb(Zn 1 / 3 Nb 2 / 3 A piezoelectric element according to Comparative Example 1 was obtained in the same manner as in Example 1, except that a calcined powder represented by the formula (I) was obtained, ZnO was not added to the calcined powder, and the calcination temperature of the disk-shaped molded body was set to 1000°C.

[0060] (Comparative Example 2) The amount of starting raw material powder was determined by the general formula 0.79Pb(Zr 0.51 Ti 0.49 )O3-0.18Pb(Zn 1 / 3 Nb 2 / 3 )O3-0.03Pb(Mn 1 / 3 Nb 2 / 3 A piezoelectric element according to Comparative Example 2 was obtained in the same manner as in Example 1, except that a calcined powder represented by )O3 was obtained.

[0061] (Comparative Example 3) The amount of starting raw material powder was determined by the general formula 0.79Pb(Zr 1 / 2 Ti 1 / 2 )O3-0.17Pb(Zn 1 / 3 Nb 2 / 3 )O3-0.04Pb(Mn 1 / 3 Nb 2 / 3 A piezoelectric element according to Comparative Example 3 was obtained in the same manner as in Example 1, except that the calcined powder represented by the formula (I) was obtained.

[0062] <Evaluation> [Average domain width, area percentage of sintered particles where domains are observed, and domain wall density in sintered particles] For each of the obtained piezoelectric elements, the average domain width, the area percentage of the sintered grains where domains were observed, and the domain wall density in the sintered grains were measured and calculated using the methods described above. The results are shown in Table 1.

[0063] [Mechanical quality factor Qm] For each of the obtained piezoelectric elements, the relationship between frequency and impedance was measured using an impedance analyzer, and the mechanical quality factor Qm was calculated by the resonance-antiresonance method. The results are shown in Table 1.

[0064] [Piezoelectric constant d 33 ] For each piezoelectric element obtained, d 33 The piezoelectric constant d 33 The results are shown in Table 1.

[0065] [Table 1]

[0066] Comparing the Examples and Comparative Examples, piezoelectric elements according to the Examples, in which the average domain width of the sintered particles observed in a cross section perpendicular to the first and second electrodes is 100 nm to 500 nm and the area percentage of the sintered particles in which the domains are observed in the cross section is 60% or more, have a high mechanical quality factor Qm and a high piezoelectric d constant compared to elements with a smaller average domain width and a smaller area percentage of the sintered particles. These results suggest that piezoelectric ceramics containing Pb, Zr, Ti, and O as constituent elements and a compound having a perovskite structure as their main component can be obtained by setting the average domain width of the sintered particles observed in a cross section perpendicular to the first and second electrodes to 100 nm to 500 nm and setting the area percentage of the sintered particles in which the domains are observed in the cross section to 60% or more. [Industrial Applicability]

[0067] According to the present invention, it is possible to provide a piezoelectric element that has small mechanical loss and large displacement when driven at high speed and large amplitude. When driven at high speed and large amplitude, such a piezoelectric element generates less heat than conventional piezoelectric elements and can obtain larger amplitudes. Therefore, the piezoelectric element according to the present invention can be suitably used in ultrasonic vibrators, ultrasonic motors, and the like that are driven at high speed and large amplitudes. [Explanation of symbols]

[0068] 100 Multilayer piezoelectric element 10 Piezoelectric ceramic layer 11 Sintered particles 12 Domains 13 Domain Wall 20 Internal electrode 20a First electrode 20b Second electrode 30a, 30b Connecting conductor 40a, 40b external electrode

Claims

1. first and second electrodes disposed opposite to each other; and disposed between and in contact with the first and second electrodes; A compound having a perovskite structure containing Pb, Zr, Ti, and O as constituent elements. The main component is the average domain width in the sintered particle as observed in a cross section perpendicular to the first and second electrodes; is 100 nm or more and 500 nm or less, and The area percentage of the sintered grains in which the domains are observed occupying the cross section is 60% or more. is Piezoelectric ceramics A piezoelectric element comprising:

2. The domain wall density in the sintered particles is 2.5 / μm ２ 2. The piezoelectric element according to claim 1, wherein:

3. 3. The piezoelectric element according to claim 1, wherein the compound having a perovskite structure further contains Zn, Nb, and Mn as constituent elements.

4. 4. The piezoelectric element according to claim 1, wherein the compound having a perovskite structure is represented by the following general formula (1): xP﹡(Zr 1-a Till a )O 3 -yP｡(Cn) 1 / 3 N﹂ 2 / 3 )O 3 -zPb (Mn) 1 / 3 Nb 2 / 3 )O 3 …(1) In the formula, a, x, y, and z are real numbers that satisfy the following conditions: 0.45≦a≦0.60, 0<x≦0.85, 0≦y<0.99, 0.01<z<0.10, and x+y+z=1.

0.

5. 5. An ultrasonic vibrator comprising: the piezoelectric element according to claim 1; and a pair of blocks sandwiching the piezoelectric element in one axial direction.

6. 5. An ultrasonic motor comprising: the piezoelectric element according to claim 1; and a sliding body bonded to the piezoelectric element.

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