Multilayer piezoelectric element and electronic device

Abstract

A piezoelectric element according to one aspect of the present invention comprises: a plurality of piezoelectric ceramic layers formed of polarized piezoelectric ceramics; a plurality of active internal electrodes disposed so as to sandwich each of the piezoelectric ceramic layers; a pair of terminal electrodes electrically connected in every other layer to the active internal electrode; a pair of cover layers formed of non-polarized ceramics and disposed so as to sandwich, from the outside in the layering direction, a set of the piezoelectric ceramic layer and the active internal electrode; and a pair of additional electrodes respectively disposed in the pair of cover layers and having a lower continuity than the active internal electrode.

Description

[Technical Field]

[0001] The present invention relates to a multi-layer piezoelectric element and an electronic device. [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] A known piezoelectric element is a multilayer piezoelectric element that includes multiple piezoelectric ceramic layers and internal electrodes disposed between the piezoelectric ceramic layers and electrically connected to every other layer. Multilayer piezoelectric elements have the advantage of being able to obtain large voltages and displacements with relatively small element dimensions.

[0004] Piezoelectric elements undergo deformation during operation, and repeated use can cause tiny cracks to form in the ceramic parts. In multilayer piezoelectric elements, the piezoelectric ceramic layers are thin, so the internal electrodes can be exposed to the external environment through the cracks. If a multilayer piezoelectric element in this state continues to be used in a high-humidity environment, moisture can reach the internal electrodes through the cracks, causing dielectric breakdown.

[0005] As a means for preventing moisture from entering the piezoelectric element, Patent Document 1 discloses providing a coating film made of insulating resin so as to cover the surface of the piezoelectric element.

[0006] Furthermore, Patent Document 2 discloses that, as a means for preventing cracks from occurring during the manufacture or operation of a stacked piezoelectric actuator, the internal electrode is composed of an electrode portion that is partially exposed on the outer periphery of the element and electrically connected to the external electrode, and a dummy portion that is arranged around the electrode portion and is not joined to the electrode portion.

[0007] As a technique for providing electrodes having a function different from that of internal electrodes in a multilayer ceramic element, it is known to place electrodes that do not contribute to capacitance formation in dielectric layers that do not contribute to capacitance formation and are located outside (above and below) the internal electrodes of the uppermost and lowermost layers in a multilayer ceramic capacitor (Patent Documents 3 to 5). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2019-102525 [Patent Document 2] Japanese Patent Application Publication No. 3-270944 [Patent Document 3] Japanese Patent Application Publication No. 7-335473 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-224589 [Patent Document 5] Patent No. 5467702 Summary of the Invention [Problem to be solved by the invention]

[0009] The technique disclosed in Patent Document 1 requires the formation of a coating film, which increases the amount of raw materials required and the number of steps required to manufacture the element, resulting in a rise in manufacturing costs.

[0010] Furthermore, the technology disclosed in Patent Document 2 is effective in suppressing cracks in the piezoelectric ceramic layers located in the areas that do not overlap with the internal electrodes when viewed from the stacking direction, but the effectiveness in suppressing cracks in the areas that overlap with the internal electrodes has not been confirmed. Therefore, it is unclear whether the deterioration of electrical insulation is suppressed when the device is continuously operated in a high-humidity environment.

[0011] Furthermore, Patent Documents 3 to 5 do not disclose that cracks caused by deformation of the element during operation can be suppressed, and the continuity rate of the internal electrodes is also unclear.

[0012] An object of the present invention is to provide a multi-layer piezoelectric element in which the deterioration of electrical insulation properties is suppressed when the element is continuously operated in a high humidity environment. [Means for solving the problem]

[0013] In the course of research to solve the above-mentioned problems, the inventors discovered that by arranging the internal electrodes arranged at the outermost positions in the stacking direction of a multi-layer piezoelectric element so as not to come into contact with the polarized piezoelectric ceramic layers and by making the internal electrodes have more gaps than the internal electrodes arranged further inside, i.e., a lower continuity rate, it is possible to suppress the deterioration of electrical insulation when the element is continuously operated in a high-humidity environment, and thus completed the present invention.

[0014] That is, one aspect of the present invention for solving the above-mentioned problems is a piezoelectric element comprising: a plurality of piezoelectric ceramic layers formed of polarized piezoelectric ceramic; a plurality of active internal electrodes arranged so as to sandwich each of the piezoelectric ceramic layers; a pair of terminal electrodes electrically connected to the active internal electrodes in every other layer; a pair of cover layers formed of non-polarized ceramic and arranged so as to sandwich the set of the piezoelectric ceramic layers and the active internal electrodes from outside in the stacking direction; and a pair of additional electrodes arranged in the pair of cover layers, each of which has a lower continuity rate than the active internal electrodes. the distance between the additional electrode and the adjacent active internal electrode is shorter than the distance between the adjacent active internal electrodes; It is a multilayer piezoelectric element.

[0015] Another aspect of the present invention is a tactile module equipped with the aforementioned stacked piezoelectric element. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a multi-layer piezoelectric element in which the deterioration of electrical insulation properties is suppressed when it is continuously operated in a high humidity environment. [Brief explanation of the drawings]

[0017] [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 3a] FIG. 1 is a cross-sectional view showing the arrangement of additional electrodes that are not electrically connected to other electrodes in a multi-layer piezoelectric element according to one aspect of the present invention. [Figure 3b] FIG. 1 is a cross-sectional view showing the arrangement of additional electrodes electrically connected to active internal electrodes arranged immediately inside the stacked piezoelectric element according to one aspect of the present invention. [Figure 4] Schematic cross-sectional view of the sample for measuring the continuity of the internal electrode [Figure 5] An explanatory diagram of the lengths of each part required to calculate the continuity rate of the internal electrode DETAILED DESCRIPTION OF THE INVENTION

[0018] 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.

[0019] [Stacked piezoelectric element] As shown in FIGS. 1 and 2 , a multilayer piezoelectric element 100 according to one aspect of the present invention (hereinafter, sometimes simply referred to as the “multilayer piezoelectric element according to the first aspect” or the “multilayer piezoelectric element 100”) comprises a plurality of piezoelectric ceramic layers 10 formed of polarized piezoelectric ceramic, a plurality of active internal electrodes 20 arranged so as to sandwich the piezoelectric ceramic layers 10, a pair of terminal electrodes 30 electrically connected to every other active internal electrode 20, a pair of cover layers 40 formed of non-polarized ceramic and arranged so as to sandwich the set of piezoelectric ceramic layers 10 and active internal electrodes 20 from the outer sides in the stacking direction, and a pair of additional electrodes 50 arranged in the cover layers 40, each having a lower continuity than the active internal electrodes 20. Note that among the terminal electrodes 30 shown in FIGS. 1 and 2 and the active internal electrodes 20 shown in FIG. 2, those with the same alphabet (“a” or “b”) have the same polarity (“+” or “−”).

[0020] The piezoelectric ceramic layer 10 is made of a polarized piezoelectric ceramic. The composition, crystal structure, and particle size of the piezoelectric ceramic are not particularly limited, and commonly used ones can be used. Examples of piezoelectric ceramics include lead zirconate titanate (Pb(Zr,Ti)O3, PZT) and solid solutions containing lead zirconate as the main component, alkali niobate ((Li,Na,K)NbO3) and solid solutions containing lead zirconate as the main component, bismuth sodium titanate ((Bi 0.5 Na 0.5 )TiO3, BNT) and solid solutions containing these as the main components, bismuth layer compounds, and tungsten bronze type compounds.

[0021] The active internal electrodes 20 are arranged so as to sandwich the piezoelectric ceramic layers 10. The shape of the active internal electrodes 20 can be determined appropriately depending on the properties required of the multilayer piezoelectric element 100. The material of the active internal electrodes 20 is not limited as long as it is conductive and physically and chemically stable throughout the manufacture and use of the multilayer piezoelectric element 100. Examples include silver (Ag), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), and alloys thereof. Among these, metals containing 50 mass % or more of silver, such as silver and silver-palladium alloys, are preferred because they have high conductivity and can be fired together with the piezoelectric ceramic layers 10 in an air atmosphere.

[0022] The terminal electrodes 30 are a pair of conductors formed on the surface of the multi-layer piezoelectric element 100, and each is electrically connected to every other active internal electrode 20. The shape and arrangement of the terminal electrodes 30 can be determined appropriately depending on the arrangement of the active internal electrodes 20 and the arrangement of the multi-layer piezoelectric element 100 in the circuit. The material of the terminal electrodes 30 is not limited as long as it is highly conductive and physically and chemically stable throughout the manufacture and use of the multi-layer piezoelectric element 100. Examples include silver (Ag), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), and alloys thereof.

[0023] The cover layer 40 is formed of a non-polarized ceramic and is disposed so as to sandwich the set of piezoelectric ceramic layers 10 and active internal electrodes 20 from the outer sides in the stacking direction. The cover layer 40 functions to protect the outermost active internal electrodes 20 in the stacking direction and suppress dielectric breakdown of the multilayer piezoelectric element 100. The material of the cover layer 40 is not limited as long as it has high electrical insulation and mechanical properties similar to those of the piezoelectric ceramics constituting the piezoelectric ceramic layers. Examples include commonly used insulating materials such as alumina (Al2O3) and cordierite (2MgO·2Al2O3·5SiO2) that do not have piezoelectricity, as well as the various piezoelectric ceramics listed above that are not polarized. To suppress internal stress in the multilayer piezoelectric element 100, it is preferable that the material of the cover layer 40 be the same as that of the piezoelectric ceramics constituting the piezoelectric ceramic layers 10.

[0024] In this specification, whether the ceramic layers in the multi-layer piezoelectric element 100 are formed from polarized piezoelectric ceramics or not is confirmed by the following procedure. First, the multi-layer piezoelectric element 100 is cut along a plane perpendicular to the internal electrodes. There are no particular limitations on the cutting means, and a dicing saw, a cutter, or the like can be used. Next, the cut multi-layer piezoelectric element 100 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, the cut surface after mirror polishing is coated with osmium (Os) to prepare a measurement sample. Next, a backscattered electron (BSE) image of the ceramic layer of the conductive cut surface is obtained using a Schottky scanning electron microscope with an accelerating voltage of 7.00 kV, a working distance (WD) of 4 mm (the distance between the bottom surface of the objective lens and the sample), 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, after identifying sintered particles containing striped domains in the BSE image, the "area tool" is selected as the measurement type in the "ruler tool" function of Acrobat, and the outline of the sintered particle containing the domain is traced to draw a polygon, and the area of ​​the polygon is displayed. This process is repeated for each sintered particle confirmed in the BSE image, and the total area of ​​the resulting polygon is calculated. Finally, the percentage of the total area of ​​the calculated polygons relative to the total area of ​​the BSE image is calculated, and if this percentage is 50% or more, the ceramic layer from which the BSE image was obtained is determined to be made of polarized piezoelectric ceramics.On the other hand, if this percentage is less than 50%, the ceramic layer from which the BSE image was obtained is determined to be made of non-polarized ceramics.

[0025] An additional electrode 50 is disposed in the cover layer 40. The number of additional electrodes 50 disposed in one cover layer 40 may be one (layer) or multiple. Examples of materials for the additional electrode 50 include those similar to those for the active internal electrodes 20. The material for the additional electrode 50 may be the same as or different from that of the active internal electrodes 20, but from the viewpoint of suppressing the generation of internal stress in the multilayer piezoelectric element 100, it is preferable that the material for the additional electrode 50 be the same as that for the active internal electrodes 20.

[0026] The shape of the additional electrode 50 is not particularly limited, but in order to fully exert the crack suppression effect on the cover layer 40, it is preferable that its area be approximately the same as that of the active internal electrode 20, for example, 95% or more and 105% or less of the area of ​​the active internal electrode 20.

[0027] The additional electrode 50 may be arranged either in a manner not electrically connected to other electrodes, as shown in Fig. 3a, or in a manner electrically connected to the active internal electrode 20 arranged immediately inside it, as shown in Fig. 3b. If the additional electrode 50 has the same shape as the active internal electrode 20 arranged immediately inside it and is electrically connected to it, this is preferable because it can effectively prevent poor electrical connection between the active internal electrode 20 and the terminal electrode 30. This is presumably because the additional electrode 50 prevents the electrode material from diffusing or migrating into the cover layer 40 throughout the entire active internal electrode 20 arranged immediately inside it.

[0028] Furthermore, it is preferable that the additional electrodes 50 are arranged so that the distance between adjacent active internal electrodes 20, i.e., the active internal electrodes 20 located immediately inside in the stacking direction, is shorter than the distance between adjacent active internal electrodes 20. This effectively suppresses an increase in the electrical resistance value of the active internal electrodes 20. The reason for this is presumably that the additional electrodes 50 suppress the diffusion or migration of electrode material from the active internal electrodes 20 located immediately inside thereof to the cover layer 40.

[0029] The effect of arranging the additional electrodes 50 in the above-mentioned preferred manner becomes significant when the active internal electrodes 20 and the additional electrodes 50 are made of a metal with a silver content of 50 mass % or more. This is thought to be because silver is an element that easily diffuses or migrates in ceramics.

[0030] The additional electrode 50 has a lower continuity ratio than the active internal electrode 20. This prevents deterioration of electrical insulation when the multilayer piezoelectric element 100 is continuously operated in a high-humidity environment. This is presumably because the gaps in the internal electrode, where no conductive material is present, act to relieve stress generated in the cover layer 40 due to displacement during operation. To maximize this effect, the continuity ratio of the additional electrode 50 is preferably 85% or less, and more preferably 80% or less. There is no particular lower limit for the continuity ratio of the additional electrode 50. However, to ensure sufficient protection against deteriorating factors such as moisture that may penetrate into the multilayer piezoelectric element 100, the continuity ratio is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0031] Here, the continuity ratio of the active internal electrodes 20 and the additional electrodes 50 is determined by the following method. First, using the method described above, it is confirmed whether the ceramic layers in the multilayer piezoelectric element 100 are formed of polarized piezoelectric ceramics. The multilayer piezoelectric element 100 determined to have a polarized piezoelectric ceramic layer 10 and an unpolarized cover layer 40 is cut across the terminal electrodes 30 in a plane perpendicular to the plane perpendicular to the stacking direction, and the cut surface is polished to expose the internal electrodes to prepare a measurement sample. A schematic diagram of the cross section (measurement surface) of the measurement sample is shown in Figure 4. Next, the measurement surface of the measurement sample is observed with an optical microscope to identify the internal electrodes, and the piezoelectric ceramic layer 10 and the cover layer 40 are identified based on the results of confirming the polarization state of the ceramic layer using the BSE image described above. The internal electrodes that are in contact with the outside of the piezoelectric ceramic layer 10 and located at the outermost position in the stacking direction are designated as active internal electrodes 20M1 and 20M2 for measuring the continuity ratio. Of the internal electrodes observed in the cover layer 40, those located at the outermost position in the stacking direction are designated as additional electrodes 50M1 and 50M2 for measuring the continuity ratio. Next, the active internal electrodes 20M1 and 20M2 for measuring the continuity ratio are observed over their entire lengths with an optical microscope to identify the conductive portions 21, which are recognized as portions having a metallic luster, and the gap portions 22, which are recognized as portions where the conductive portions 21 are interrupted. The additional electrodes 50M1 and 50M2 are also observed in the same manner to identify the conductive portions 51 and the gap portions 52. Next, for each internal electrode, as shown in FIG. 5, the total length (L A ), and the length of each gap (L a1 , L a2 , …, L an ) are measured. Here, "n" means the number of gaps 22 present in the internal electrode. Next, the continuity ratio C of each internal electrode is calculated from each measured length using the following formula (1). Finally, the average value of the continuity ratios of the active internal electrodes 20M1 and 20M2 is calculated and used as the continuity ratio of the active internal electrode 20. On the other hand, the average value of the continuity ratios of the additional electrodes 50M1 and 50M2 is calculated and used as the continuity ratio of the additional electrode 50.

[0032]

number

[0033] The multilayer piezoelectric element 100 has a thickness of t p , the thickness of the cover layer 40 is t c When t c ≦2.5t p It is preferable that the following condition is satisfied. Furthermore, it is preferable that the ratio of the dimension in the stacking direction to the minimum dimension in the direction perpendicular to the stacking direction of the multilayer piezoelectric element 100 is 0.5 or less. In a multilayer piezoelectric element having such a shape or structure, the distance from the internal electrode surface to the element surface is short, so cracks that reach the internal electrode surface are likely to occur. For this reason, the additional electrode 50, which is arranged in the cover layer 40 and has a lower continuity rate than the active internal electrode 20, has a significant crack-suppressing effect.

[0034] [Method of manufacturing a multilayer piezoelectric element] The multilayer piezoelectric element according to the first aspect is manufactured by, for example, mixing raw material powders in a predetermined ratio to obtain a raw material mixed powder, calcining the raw material mixed powder to obtain a calcined powder mainly composed of a piezoelectric ceramic composition, mixing the calcined powder with a binder and a dispersion medium to prepare a slurry, forming the slurry into a sheet-like green sheet, printing a paste containing an internal electrode material onto the green sheet in the shape of the internal electrodes, stacking a predetermined number of the green sheets with the printed paste and pressing them together to obtain a raw formed body, removing the binder from the raw formed body and firing the body to obtain a multilayer sintered body containing the piezoelectric ceramic and the internal electrodes, applying a conductive paste to the surfaces of the multilayer sintered body where the internal electrodes are exposed and then firing the applied conductive paste to form terminal electrodes, and applying a high voltage between the terminal electrodes to polarize the piezoelectric ceramic. Each of these steps is described in detail below.

[0035] (Preparation of raw material mixed powder) The powder of the compound used as the raw material is not limited in composition and particle size, as long as it can produce a desired piezoelectric ceramic upon firing.

[0036] 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 employed. When wet mixing using a ball mill is employed as the mixing method, for example, zirconia balls may be used, and the mixture may be stirred for about 8 to 60 hours in a ball mill using an organic solvent such as ethanol as a dispersion medium, and then the organic solvent may be evaporated and dried.

[0037] (Preparation of calcined powder) Next, the raw material powder mixture is calcined to obtain a calcined powder. The calcination is carried out under conditions that allow the raw material powders to react with each other and obtain a predetermined piezoelectric ceramic composition. One example is firing in air at a temperature of 700°C to 1000°C for 1 to 10 hours. The calcined powder may be used directly to prepare a slurry, but it is preferable to crush it using a ball mill, stamp mill, or the like prior to this in order to obtain a smooth green sheet through a uniform slurry.

[0038] When a powder of a ready-made piezoelectric ceramic composition is used, the above-mentioned mixing and calcination steps are omitted, and the commercially available powder is subjected to the below-described treatment for calcined powder.

[0039] (Preparation of Slurry) Next, the calcined powder is mixed with a binder and a dispersion medium to prepare a slurry. The binder used should be one that can maintain the shape of the green sheet (described later) and volatilizes without leaving carbon or other residues during firing or the binder removal process that precedes firing. Examples of binders that can be used include polyvinyl alcohol, polyvinyl butyral, cellulose, urethane, and vinyl acetate. The amount of binder used is not particularly limited, but since it will be removed in a later process, it is preferable to keep it as small as possible within the range that achieves the desired moldability and shape retention in order to reduce raw material costs.

[0040] The dispersion medium used should be one that does not cause aggregation of the calcined powder and binder and can be easily removed by volatilization or the like after forming the green sheet, which will be described later. Examples of the dispersion medium that can be used include water and alcohol-based solvents.

[0041] Components for adjusting the properties of the slurry, such as dispersants, plasticizers, and thickeners, may be added to the slurry.

[0042] The slurry may also contain components that are incorporated into the piezoelectric ceramic composition during firing, as described below, or components that form precipitates between sintered particles of the piezoelectric ceramic.

[0043] The method for mixing the calcined powder, binder, and dispersion medium is not particularly limited as long as the components are mixed uniformly while preventing the inclusion of impurities. One example is ball mill mixing.

[0044] When manufacturing a multilayer piezoelectric element having a cover layer with a composition different from that of the piezoelectric ceramic layer, a slurry containing powder of the ceramic composition that will form the cover layer is prepared in addition to the above-mentioned slurry.

[0045] (Production of green sheets) The resulting slurry is then molded into a green sheet by a commonly used method such as a doctor blade method.

[0046] (Printing of paste containing material for internal electrodes) Next, a paste containing an internal electrode material is printed on the resulting green sheet in the shape of the internal electrode. At this time, the printed pattern for the additional electrode is formed so that the continuity rate after firing is lower than that of the active internal electrode. Examples of forming methods include (1) using a paste for forming the additional electrode to which a powder for forming gaps has been added, (2) forming the printed pattern of the additional electrode to have gaps, and (3) using a material for the internal electrode that diffuses a large amount into the piezoelectric ceramic layer during firing, such as a metal containing 50 mass% or more of silver, and thinning the printing thickness of the pattern of the additional electrode so that the gaps are formed by the diffusion.

[0047] To improve the adhesive strength to the piezoelectric ceramic layer after firing, glass frit or powder having the same composition as the powder of the ceramic composition contained in the green sheet may be added to the paste containing the internal electrode material.

[0048] (Production of the raw body) Next, a predetermined number of green sheets printed with a paste containing the internal electrode material are stacked in a predetermined order, and the green sheets are pressed together to produce a raw formed body. The stacking and pressing can be performed by a conventional method, such as pressing the stacked green sheets together in the stacking direction while heating them, and thermocompression bonding by the action of a binder.

[0049] (Production of laminated sintered body) The resulting green body is then fired to obtain a laminated sintered body. Prior to firing, the binder may be removed from the green body. In this case, binder removal and firing may be performed consecutively using the same firing apparatus. The conditions for binder removal and firing may be appropriately set taking into consideration the volatilization temperature and content of the binder, the sinterability of the ceramic composition, and the heat resistance of the internal electrode material. Examples of binder removal conditions include firing in an air atmosphere at a temperature of 300°C to 500°C for 5 to 20 hours. Examples of firing conditions include firing in an air atmosphere at a temperature of 800°C to 1100°C for 1 to 5 hours. To obtain multiple laminated piezoelectric ceramics from a single green body, the green body may be divided into several blocks prior to firing.

[0050] (Formation of terminal electrodes) Next, a conductive paste is applied to the surface of the resulting laminated sintered body where the internal electrode layers are exposed, and then baked to form terminal electrodes.

[0051] (Polarization treatment) Finally, a high voltage is applied between the connecting conductors to perform polarization treatment, resulting in a multilayer piezoelectric element. The conditions for the polarization treatment are not particularly limited, as long as they align the spontaneous polarization direction in each piezoelectric ceramic layer without causing damage such as cracks in the multilayer sintered compact. One example is application of an electric field of 1 kV / mm to 6 kV / mm at a temperature of 100°C to 180°C.

[0052] [Electronic equipment] An electronic device according to another aspect of the present invention (hereinafter sometimes simply referred to as "electronic device according to second aspect") is equipped with the multilayer piezoelectric element according to the first aspect. The electronic device according to the second aspect is equipped with a multilayer piezoelectric element that is inhibited from decreasing in electrical insulation when continuously operated in a high-humidity environment, and therefore has excellent durability. Furthermore, in an electronic device according to the second aspect that is equipped with a preferred embodiment of the multilayer piezoelectric element according to the first aspect, poor electrical connection between the active internal electrodes and the terminal electrodes is effectively inhibited, and therefore large displacements can be stably obtained.

[0053] In the electronic device according to the second aspect, known circuits for driving the piezoelectric elements and mechanisms for transmitting the displacement of the piezoelectric elements can be used as appropriate. [Example]

[0054] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0055] Example 1 As a powder of piezoelectric ceramic composition, the composition formula Li 0.06 Na 0.52 K 0.42A calcined powder represented by NbO3 was prepared. 0.65 mol% Li2CO3, 1.3 mol% SiO2, 0.5 mol% CaCO3, 0.5 mol% MnCO3, and a polyvinyl butyral binder were added to 100 mol% of this calcined powder and mixed using a wet ball mill. The resulting mixed slurry was formed using a doctor blade to obtain a 70 μm thick green sheet. An Ag-Pd alloy paste (Ag / Pd mass ratio = 9 / 1) was screen-printed onto the green sheet to obtain a green sheet with an active internal electrode pattern and a green sheet with an additional electrode pattern. Eleven green sheets with active internal electrode patterns were stacked so that every other layer had a different pattern arrangement. Green sheets with additional electrode patterns were placed on the top and bottom of the stack. A green sheet without an electrode pattern was placed on the top side of the resulting stack (the side where the additional electrode pattern was exposed). The sheet stack was then heated and pressed together under a pressure of approximately 50 MPa to obtain a laminate. This laminate was then singulated, debindered in air, and subsequently sintered at 980°C for 2 hours in air to obtain a sintered body (a laminated sintered body). A conductive paste containing Ag was applied to the surface of the sintered body where the internal electrodes were exposed, and the temperature was raised to 600°C for sintering to form a pair of terminal electrodes. Finally, a 3.0 kV / mm electric field was applied between the pair of terminal electrodes for 3 minutes in a constant temperature oven at 80°C to obtain a multilayer piezoelectric element according to Example 1. The dimensions of the resulting multilayer piezoelectric element were 20 mm in length, 10 mm in width, and 0.7 mm in thickness.

[0056] (Comparative Example 1) A multilayer piezoelectric element according to Comparative Example 1 was obtained in the same manner as in Example 1, except that green sheets without electrode patterns were placed instead of the green sheets with additional electrode patterns, i.e., two green sheets without electrode patterns were placed on the upper side (the side where the active internal electrode patterns are exposed) and one on the lower side of the stack of green sheets with active internal electrode patterns.

[0057] (Comparative Example 2) A multi-layer piezoelectric element according to Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that the amount of Ag—Pd alloy paste used in forming the internal electrode pattern was tripled.

[0058] <Evaluation> [Polarization of ceramic layer] The obtained multilayer piezoelectric elements were examined for the presence or absence of polarization in the ceramic layers using the method described above. As a result, it was confirmed that in the multilayer piezoelectric element according to Example 1, only the ceramic layers on both sides in contact with the internal electrode located at the outermost position in the stacking direction were not polarized, while the other ceramic layers were polarized. On the other hand, in the multilayer piezoelectric elements according to Comparative Examples 1 and 2, it was confirmed that only the ceramic layers located outside the internal electrode located at the outermost position in the stacking direction were not polarized, while the other ceramic layers were polarized. From these results, it can be seen that in the multilayer piezoelectric element according to Example 1, the internal electrodes located at the outermost positions in the stacking direction are additional electrodes disposed in the cover layers, whereas in the multilayer piezoelectric elements according to Comparative Examples 1 and 2, the internal electrodes located at the outermost positions in the stacking direction are active internal electrodes in contact with the piezoelectric ceramic layers.

[0059] Furthermore, for each of the multi-layer piezoelectric elements according to the examples and comparative examples, a cross section perpendicular to the internal electrode was observed with an optical microscope, and the thicknesses of the piezoelectric ceramic layer and the cover layer were measured based on the results of confirming the presence or absence of polarization in the ceramic layer. The thickness of the piezoelectric ceramic layer was approximately 50 μm, and the thickness of the cover layer was approximately 100 μm. Furthermore, for the multi-layer piezoelectric element according to Example 1, the distance between the additional electrode and the active internal electrode located immediately inside it was measured and found to be approximately 50 μm.

[0060] [Continuity rate of internal electrodes] The continuity ratios of the internal electrodes and additional electrodes of each of the obtained multilayer piezoelectric elements were determined by the method described above. As a result, in the multilayer piezoelectric element according to Example 1, the continuity ratio of the active internal electrodes was 94%, while the continuity ratio of the additional electrodes was 79%. On the other hand, the continuity ratios of the active internal electrodes of the multilayer piezoelectric elements according to the comparative examples, which did not have additional electrodes, were 80% in Comparative Example 1 and 87% in Comparative Example 2.

[0061] [Piezoelectric properties] The piezoelectric characteristics of each stacked piezoelectric element obtained were measured as displacement performance d * 33 First, a unipolar sine wave with a maximum electric field of 8 kV / mm at approximately 100 Hz was applied to the multilayer piezoelectric element, and the displacement of the multilayer piezoelectric element was measured using a laser Doppler displacement meter. The obtained displacement of the multilayer piezoelectric element was then divided by the thickness of the piezoelectric ceramic layer (distance between electrodes) and the maximum voltage calculated from the maximum electric field, as well as the number of piezoelectric ceramic layers that make up the multilayer piezoelectric element, to determine the displacement performance d per unit voltage of one piezoelectric ceramic layer. * 33 The obtained displacement performance d * 33 The displacement performance d * 33 The results are shown in Table 1 as ratios with the value of 100.

[0062] [Measurement of changes in electrical insulation over time (average life)] Each of the obtained multilayer piezoelectric elements was placed in a thermo-hygrostat chamber at 60°C and 90% relative humidity, and an AC electric field of 6 kV / mm was applied between the terminal electrodes at a frequency of 50 Hz. The time until the current flowing between the terminal electrodes reached 1 mA or more was measured. The average value of this time for 10 elements was taken as the average lifespan. The obtained average lifespans are shown in Table 1 as a ratio, with the average lifespan of the multilayer piezoelectric element of Comparative Example 2 taken as 100.

[0063] The structures of the multi-layer piezoelectric elements described above and the various properties measured for them are summarized in Table 1.

[0064] [Table 1]

[0065] Comparing Example 1 with Comparative Example 1 and Comparative Example 2, it can be said that in a multi-layer piezoelectric element, when the continuity ratio of the internal electrodes located at the outermost part in the stacking direction is 80% or less, the average life span when driven in a high-humidity environment is significantly extended. This is thought to be because the internal electrodes located at the outermost part in the stacking direction and having a low continuity ratio act as stress relaxation layers during operation, thereby suppressing the occurrence of cracks in the cover layer located outside them.

[0066] Furthermore, a comparison between Example 1 and Comparative Example 1 reveals that in a multilayer piezoelectric element in which an internal electrode with a low continuity ratio is arranged at the outermost portion in the stacking direction, a large piezoelectric constant can be obtained by using the internal electrode as an additional electrode and arranging an active internal electrode with a high continuity ratio inside it. This is thought to be because the high continuity ratio of the active internal electrodes allows a sufficient voltage to be applied to the piezoelectric ceramic layers during polarization and operation of the element.

[0067] Furthermore, a comparison between Example 1 and Comparative Example 2 reveals that in a multilayer piezoelectric element in which the internal electrodes are formed of a metal containing 50% by mass or more of silver, a large piezoelectric constant can be obtained by arranging additional electrodes at the outermost positions in the stacking direction, thereby reducing the amount of paste used to form the internal electrodes, in other words, even when the internal electrodes are formed thin. This is thought to be due to the effects of two factors: the presence of additional electrodes suppressing the diffusion and migration of silver from the active internal electrodes located at the outermost positions in the stacking direction, thereby maintaining a high continuity ratio; and the thin thickness of the internal electrodes reduces the constraint force on the displacement of the piezoelectric ceramic layers. [Industrial Applicability]

[0068] According to the present invention, it is possible to provide a multi-layer piezoelectric element in which deterioration of electrical insulation is suppressed when continuously operated in a high-humidity environment. Since such a multi-layer piezoelectric element has a long life, the present invention is useful in that electronic devices using the element have excellent durability and can be used for a long period of time. This advantage is particularly noticeable in electronic devices using elements such as tactile modules, in which the ratio of the dimension in the stacking direction to the minimum dimension in the direction perpendicular to the stacking direction is small. Furthermore, according to a preferred embodiment of the present invention, the present invention is also useful in that it can provide electronic devices that can stably obtain large displacements. [Explanation of symbols]

[0069] 100 Multilayer piezoelectric element 10 Piezoelectric ceramic layer 20, 20a, 20b, 20M1, 20M2 active internal electrode 30, 30a, 30b terminal electrode 40 Cover Layer 50, 50M1, 50M2 additional electrodes 21, 51 Conductive part 22, 52 Gap

Claims

1. a plurality of piezoelectric ceramic layers formed of polarized piezoelectric ceramics; a plurality of active internal electrodes arranged so as to sandwich each of the piezoelectric ceramic layers; a pair of terminal electrodes electrically connected to the active internal electrodes in every other layer; a pair of cover layers formed of unpolarized ceramics and arranged so as to sandwich the piezoelectric ceramic layer and the set of active internal electrodes from the outside in the stacking direction; and a pair of additional electrodes, each of which has a lower continuity than the active internal electrodes, disposed in the pair of cover layers; Equipped with The multilayer piezoelectric element has a structure in which the distance between the additional electrode and the adjacent active internal electrode is shorter than the distance between adjacent active internal electrodes.

2. 2. The multi-layer piezoelectric element according to claim 1, wherein the active internal electrodes and the additional electrodes are made of a metal containing 50% or more by mass of silver.

3. The thickness of the piezoelectric ceramic layer is t ｐ , the thickness of the cover layer is t ｃ When this is the case, t ｃ ≦2.5t ｐ 3. The multi-layer piezoelectric element according to claim 1, wherein the following is satisfied:

4. 3. The multi-layer piezoelectric element according to claim 1, wherein the ratio of the dimension in the stacking direction to the minimum dimension in the direction perpendicular to the stacking direction is 0.5 or less.

5. 3. An electronic device incorporating the multilayer piezoelectric element according to claim 1.

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