Piezoelectric element and its application

Abstract

To provide a technology capable of improving the continuous drive endurance of piezoelectric elements.SOLUTION: A piezoelectric element comprises alternating layers of a piezoelectric ceramic layer containing an alkali-based perovskite oxide with niobium acid as the main component and an internal electrode containing Ni as the main component, the piezoelectric ceramic layer has an active layer located internally in the stacking direction, and the porosity of the active layer is 0.3% or more and 5.1% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to piezoelectric elements. [Background technology]

[0002] Multilayer piezoelectric elements have been known for some time. Generally, piezoelectric elements containing lead-containing PZT (lead zirconate titanate) ceramics are used as such piezoelectric elements. However, lead can have adverse effects on the environment. For this reason, lead-free multilayer piezoelectric elements have been proposed, as described in Patent Documents 1 and 2, for example. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5862983 [Patent Document 2] Patent No. 6094682 Summary of the Invention [Problem to be solved by the invention]

[0004] In general, it is desirable for a piezoelectric element to have minimal deterioration in mechanical strength or displacement characteristics even after continuous driving. However, the piezoelectric elements described in Patent Documents 1 and 2 have room for improvement in terms of durability during continuous driving. For this reason, there has been a demand for technology that can improve the durability during continuous driving of piezoelectric elements. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to one aspect of the present disclosure, there is provided a piezoelectric element. The piezoelectric element is formed by alternately stacking piezoelectric ceramic layers containing an alkali niobate perovskite oxide as a main component and internal electrodes containing Ni as a main component, the piezoelectric ceramic layers having an active layer positioned inside in the stacking direction, the active layer having a porosity of 0.3% or more and 5.1% or less. In this aspect of the piezoelectric element, the porosity of the active layer is 0.3% or more and 5.1% or less, thereby improving the continuous driving durability of the piezoelectric element.

[0007] (2) In the piezoelectric element described in (1) above, the pore diameter of the active layer may be 0.6 μm or more and 3.4 μm or less. In this piezoelectric element, the pore diameter of the active layer is 0.6 μm or more and 3.4 μm or less, thereby further improving the continuous driving durability of the piezoelectric element.

[0008] (3) In the piezoelectric element described in (1) or (2), the piezoelectric ceramic layer may further contain an oxide containing Mn and Ti. In this piezoelectric element, the piezoelectric ceramic layer contains an oxide containing Mn and Ti, which allows the piezoelectric ceramic layer to be dense and highly reliable, and as a result, deterioration of the piezoelectric properties can be suppressed.

[0009] (4) In the piezoelectric element according to any one of (1) to (3), the piezoelectric ceramic layer may further contain an oxide containing Mn, Ti, and Sc. In this piezoelectric element, the piezoelectric ceramic layer contains an oxide containing Mn, Ti, and Sc, which further suppresses deterioration of the insulating properties.

[0010] (5) In the piezoelectric element according to any one of (1) to (4), the alkali niobate perovskite oxide may contain Mn and Ti. Since the alkali niobate perovskite oxide contains Mn and Ti, the piezoelectric element according to this embodiment has superior piezoelectric properties and insulating properties.

[0011] (6) In the piezoelectric element according to any one of (1) to (5), the alkali niobate perovskite oxide may contain Mn, Ti, and Sc. In this piezoelectric element, the alkali niobate perovskite oxide contains Mn, Ti, and Sc, which further improves the insulating properties.

[0012] (7) According to another aspect of the present disclosure, there is provided a component including the piezoelectric element according to any one of (1) to (6) above. The component of this aspect has excellent durability against continuous driving.

[0013] (8) According to another aspect of the present disclosure, there is provided a device including the component described in (7) above. The device of this aspect has excellent durability against continuous driving.

[0014] (9) According to another aspect of the present disclosure, there is provided the device according to (8), which is any one of an actuator, a haptic, a buzzer, and an ultrasonic sensor. The device of this aspect has excellent durability against continuous driving.

[0015] The present disclosure can be realized in various forms, such as a method for manufacturing a piezoelectric element, a method for manufacturing a component including such a piezoelectric element, or a method for manufacturing a device including such a component. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a cross-sectional view schematically illustrating the general configuration of a piezoelectric element. [Figure 2] FIG. 2 is an explanatory diagram for explaining measurement points of porosity and pore diameter. [Figure 3] FIG. 1 is a schematic diagram showing an example of a schematic configuration of an actuator as a device. [Figure 4] FIG. 1 is a cross-sectional view showing an example of a schematic configuration of a haptic device. [Figure 5] FIG. 1 is a cross-sectional view showing an example of a schematic configuration of a buzzer as a device. [Figure 6]1 is a cross-sectional view showing an example of a schematic configuration of an ultrasonic sensor as a device. DETAILED DESCRIPTION OF THE INVENTION

[0017] A. Implementation: FIG. 1 is a cross-sectional view schematically illustrating the general configuration of a piezoelectric element 100 according to an embodiment of the present disclosure. The piezoelectric element 100 of this embodiment includes a laminate 90 having a generally rectangular parallelepiped external shape that is generally square in top view. For convenience, in the drawings of the present disclosure, the thickness of each layer of the laminate 90 is illustrated as being thicker than it actually is, and the number of layers is illustrated as being fewer than it actually is. The piezoelectric element 100 includes a piezoelectric ceramic layer 10, a plurality of internal electrodes 20, and a pair of external electrodes 30.

[0018] The piezoelectric ceramic layer 10 contains an alkali niobate perovskite oxide as a main component. In this disclosure, "main component" means a component that accounts for 50% or more by volume. A detailed description of the material composition of the piezoelectric ceramic layer 10 will be provided later. The piezoelectric ceramic layer 10 has an inactive layer 12 and an active layer 14. The inactive layer 12 and the active layer 14 are formed by stacking multiple piezoelectric ceramic sheets and sintering them together.

[0019] The inactive layer 12 is located on an end surface S1 in the stacking direction D1. In the present disclosure, the "inactive layer 12" refers to a layer of the piezoelectric ceramic layer 10 that is located outside the layers of the internal electrodes 20 at both ends in the stacking direction D1. Therefore, one surface of the inactive layer 12 in the stacking direction D1 is in contact with the internal electrode 20, and the other surface of the inactive layer 12 in the stacking direction D1 is not in contact with the internal electrode 20. The piezoelectric ceramic layer 10 of this embodiment has two inactive layers 12 located at both ends in the stacking direction D1, but one of the inactive layers 12 may be omitted.

[0020] The active layer 14 is located inside the piezoelectric ceramic layer 10 in the stacking direction D1. The active layer 14 is formed of multiple layers, and each layer is sandwiched between internal electrodes 20 in the stacking direction D1. In this disclosure, the "active layer 14" refers to a layer of the piezoelectric ceramic layer 10 that is located inside the layers of the internal electrodes 20 on both ends in the stacking direction D1. The active layer 14 is displaced when a voltage is applied.

[0021] The multiple internal electrodes 20 are alternately stacked with each layer of the piezoelectric ceramic layer 10. The multiple internal electrodes 20 include multiple electrodes 22 connected to one of the pair of external electrodes 30 and multiple electrodes 24 connected to the other of the pair of external electrodes 30. The piezoelectric ceramic layer 10 and the electrodes 22, 24 of the internal electrodes 20 are stacked in the order of piezoelectric ceramic layer 10, electrode 22, piezoelectric ceramic layer 10, electrode 24, with this structure being repeated according to the number of layers.

[0022] The internal electrode 20 of this embodiment contains Ni (nickel) as its main component. This improves the continuous driving durability of the piezoelectric element 100. Furthermore, since Ni is included as its main component, the manufacturing cost of the piezoelectric element 100 can be suppressed compared to a configuration containing a precious metal such as Pd (palladium) or Pt (platinum) as its main component. The internal electrode 20 may contain other elements such as Ag (silver), Pd (palladium), or Pt (platinum) in addition to the main component. The external electrode 30 of this embodiment is formed, for example, with Au (gold) as its main component. However, it is not limited to Au, and may contain metals such as Pt (platinum) or Ag (silver) as its main component, or may be formed of an alloy such as Ag-Pd (silver-palladium). When a voltage is applied between the pair of external electrodes 30, the piezoelectric ceramic layer 10 expands and contracts, causing the entire piezoelectric element 100 to expand and contract.

[0023] As will be shown in the examples below, the inventors of the present disclosure have found that the continuous driving durability of the piezoelectric element 100 can be improved by controlling the density of the active layer 14 of the piezoelectric ceramic layer 10.

[0024] In this embodiment, the porosity of the active layer 14 is 0.3% or more and 5.1% or less. Such a configuration can improve the continuous driving durability of the piezoelectric element 100. Although the estimated mechanism for improving the continuous driving durability of the piezoelectric element 100 is not clear, the following mechanism is estimated. That is, by setting the porosity of the active layer 14 to 0.3% or more and 5.1% or less, it is possible to reduce residual stress, thereby suppressing deterioration of displacement characteristics. As a result, it is thought that the continuous driving durability can be improved. The porosity of the active layer 14 is more preferably 1.0% or more and 5.1% or less, and even more preferably 1.2% or more and 5.1% or less.

[0025] The pore diameter of the active layer 14 is preferably 0.6 μm or more and 3.4 μm or less. With such a configuration, it is believed that the residual stress can be further reduced, thereby further suppressing deterioration of the displacement characteristics. As a result, the continuous driving durability of the piezoelectric element 100 can be further improved. The pore diameter of the active layer 14 is more preferably 0.6 μm or more and 3.0 μm or less, and even more preferably 0.6 μm or more and 2.5 μm or less. In the present disclosure, "pore diameter" means the average circle-equivalent radius of the pores.

[0026] The porosity and pore size are calculated based on SEM images as explained below.

[0027] FIG. 2 is an explanatory diagram illustrating the measurement locations for porosity and pore diameter. To measure porosity and pore diameter, a cross-section of a mechanically polished sample is first mirror-finished and a backscattered electron image is acquired using a scanning electron microscope (SEM) at an accelerating voltage of 5.0 kV and a magnification of 500x. As indicated by the dashed lines in FIG. 2, the imaging range of the active layer 14 is a 25 μm × 25 μm region R centered on the center positions of the end layers and the central layer of the active layer 14. Note that if the thickness of the active layer is thinner than 25 μm, the length of one side of the imaging range in the stacking direction D1 is set to the same as the thickness of the active layer. More specifically, for example, if the thickness of the active layer is 15 μm, the imaging range is a 15 μm (thickness) × 25 μm (width) region centered on the central position of the active layer. In FIG. 2, dimensions w1 to w4 are indicated to indicate the central position of the active layer 14. The extracted image is then binarized using image analysis, with the ceramic portion painted black and the pores painted white, to clearly distinguish between the two. Gaussian blurring (radius 2 pixels) is applied to the binarization, using a threshold determined by the Huang method. The area of ​​the pores and the area of ​​the ceramic portion are then calculated, and the porosity (%) is calculated using the following formula:

[0028] Porosity (%) = {area of ​​pores / (area of ​​ceramic part + area of ​​pores)} x 100

[0029] The porosity of the active layer 14 is the average value of the porosities calculated from SEM images of three locations, namely, the end layers and the center layer of the active layer 14. The pore diameter (μm) is determined by calculating the circle-equivalent radius of each pore from the area of ​​each pore, and the average of the calculated circle-equivalent radii is used as the average circle-equivalent radius. The pore diameter of the active layer 14 is the average value of the average circle-equivalent radii calculated from SEM images of three locations, namely, the end layers and the center layer of the active layer 14.

[0030] The porosity and pore diameter of the active layer 14 can be controlled, for example, by controlling the particle size of the ceramic sheet, the maximum temperature during firing, and the holding time. Specifically, the smaller the particle size of the ceramic sheet and the higher the maximum temperature during firing, the more sintering progresses and the more dense the material becomes, leading to a tendency for the porosity to decrease. The longer the holding time, the smaller the pore diameter tends to be. The particle size of the ceramic sheet can also be adjusted, for example, by controlling the mixing time during mixing using a ball mill, and the longer the mixing time, the smaller the particle size tends to be.

[0031] The piezoelectric ceramic layer 10 contains, as a main component, an alkali niobate perovskite oxide having piezoelectric properties. The alkali niobate perovskite oxide has a perovskite structure. Metal oxides with a perovskite structure are generally represented by the composition formula ABO3 and are composed of a metal element located at the A site, a metal element located at the B site, and oxygen. In an ideal perovskite structure, 12 oxygen atoms are coordinated around the metal element at the A site, and 6 oxygen atoms are coordinated around the metal element at the B site, with this structure repeating periodically to form a crystal. The crystalline phase of the alkali niobate perovskite oxide contained in the piezoelectric ceramic layer 10 is also referred to as the "main phase." The alkali niobate perovskite oxide of this embodiment preferably contains at least one alkali metal (potassium (K), sodium (Na), lithium (Li), etc.) as an alkaline component at the A site, and particularly preferably contains at least one of potassium (K) and sodium (Na). It also preferably contains niobium (Nb) at the B site. It may also contain an alkaline earth metal (at least one of calcium (Ca), strontium (Sr), barium (Ba), etc.) as an alkaline component at the A site. The effects of the present invention can also be achieved even if some of the alkaline components are located at the B site or niobium is located at the A site.

[0032] The alkali niobate perovskite oxide is preferably represented by the following composition formula (1).

[0033] (A1 a M1 b ) c (Nb d1 Mn d2 Ti d3 Zr d4 Sc d5 )O 3+e ···(1)

[0034] Element A1 is at least one of Li, Na, and K which are alkali metals. Element M1 is at least one of Ba, Ca, and Sr which are alkaline earth metals. In the above compositional formula (1), element A1 and element M1 are arranged at the A site of the perovskite structure, and Nb, Mn, Ti, Zr, and Sc are arranged at the B site of the perovskite structure.

[0035] As the values of the coefficients a to e in the above compositional formula (1), among the values for which the perovskite structure is established, preferable values are selected from the viewpoints of the electrical properties or piezoelectric properties of the lead-free piezoelectric composition.

[0036] Specifically, the coefficients a and b satisfy 0 < a < 1 and 0 < b < 1, and a = 0 (a composition containing no alkali metal) and b = 0 (a composition containing none of Ba, Ca, and Sr) are excluded. Also, it is preferable that a + b = 1. The coefficient c with respect to the entire A site satisfies 0.80 < c < 1.10, and preferably satisfies 0.90 ≤ c ≤ 1.05.

[0037] The coefficients d1, d2, d3, d4, and d5 satisfy 0 < d1 < 1, 0 < d2 < 1, 0 < d3 < 1, 0 ≤ d4 < 1, and 0 ≤ d5 < 1. However, d1 = 0 (composition不含Nb), d2 = 0 (composition不含Mn), and d3 = 0 (composition不含Ti) are excluded. The coefficient d4 of Zr and the coefficient d5 of Sc may be zero (i.e., the composition不含Zr or Sc may be acceptable). The coefficient d1 of Nb preferably satisfies 0.830 ≤ d1 ≤ 0.959. The coefficient d2 of Mn preferably satisfies 0.001 ≤ d2 ≤ 0.10. The coefficient d3 of Ti preferably satisfies 0.005 ≤ d3 ≤ 0.10. The coefficient d4 of Zr preferably satisfies 0 ≤ d4 ≤ 0.20. Also, it is preferable that d1 + d2 + d3 + d4 + d5 = 1. Among the coefficients 3 + e of oxygen, the coefficient e is a positive or negative value indicating oxygen deficiency or excess with respect to the coefficient of oxygen which is usually 3. The coefficient 3 + e of oxygen can take a value such that the main phase constitutes a perovskite oxide. A typical value of the coefficient e is e = 0, and 0 ≤ e ≤ 0.1 is preferable. Note that the value of the coefficient e can be calculated from the electrical neutrality condition of the composition of the main phase. However, as the composition of the main phase, a composition slightly deviated from the electrical neutrality condition is also acceptable.

[0038] According to the aspect in which the alkali niobate perovskite-type oxide is represented by the above composition formula (1), the piezoelectric properties and insulation properties are excellent. Although the estimation mechanism of this is not clear, the following mechanism is estimated. That is, it is considered that when a predetermined amount of Mn is added, Mn solid-solves into the Nb site as an acceptor, and the insulation property is improved by forming oxygen vacancies. Also, it is considered that when a predetermined amount of Ti is added, the crystal structure changes and the piezoelectric properties are improved.

[0039] For the alkali niobate perovskite-type oxide, in the above composition formula (1), it is preferable to satisfy 0 < d5 < 1. That is, it is preferable that d5 = 0 (composition不含Sc) is excluded. Also, the coefficient d5 of Sc preferably satisfies 0.0002 ≤ d5 ≤ 0.10.

[0040] In the case where the alkali niobate perovskite-type oxide satisfies 0 < d5 < 1 in the above composition formula (1), that is, in the case containing Sc, the insulation can be further improved. Specifically, since the generation of carriers is suppressed by Sc acting as an acceptor, it is considered that the insulation is further improved.

[0041] Furthermore, in the case where the alkali niobate perovskite-type oxide satisfies 0 < d5 < 1 in the above composition formula (1), the content ratio of Sc to Ti (Sc / Ti) is preferably 0.004 or more and 8 or less in terms of molar ratio. When the content ratio of Sc to Ti is within this range, the piezoelectric properties are further excellent, and the insulation under high-temperature conditions is further excellent.

[0042] The alkali niobate perovskite-type oxide is more preferably represented by the following composition formula (2). The following composition formula (2) corresponds to a subordinate concept of the above composition formula (1).

[0043] (K a1 Na a2 Li a3 Ba b1 Ca b2 Sr b3 ) c (Nb d1 Mn d2 Ti d3 Zr d4 Sc d5 )O 3+e ···(2)

[0044] The above compositional formula (1) and the above compositional formula (2) are equivalent, where a1 + a2 + a3 = a and b1 + b2 + b3 = b. The coefficient a1 of K satisfies 0 < a1 ≤ 0.7, and preferably satisfies 0.095 ≤ a1 ≤ 0.665. The coefficient a2 of Na satisfies 0 < a2 ≤ 0.9, and preferably satisfies 0.285 ≤ a2 ≤ 0.855. The coefficient a3 of Li satisfies 0 ≤ a3 ≤ 0.2, and preferably satisfies 0 ≤ a3 ≤ 0.1. The coefficient b1 of Ba satisfies 0 ≤ b1 ≤ 0.2, and preferably satisfies 0 ≤ b1 ≤ 0.1. The coefficient b2 of Ca satisfies 0 ≤ b2 ≤ 0.2, and preferably satisfies 0 ≤ b2 ≤ 0.1. The coefficient b3 of Sr satisfies 0 ≤ b3 ≤ 0.2, and preferably satisfies 0 ≤ b3 ≤ 0.1.

[0045] According to the embodiment in which the alkali niobate perovskite-type oxide is represented by the above compositional formula (2), the piezoelectric properties are further improved. Specifically, by setting the coefficients a and b within the above ranges, the crystal structure is further optimized, and it is considered that the piezoelectric properties are further improved.

[0046] In addition, the alkali niobate perovskite oxide according to this embodiment may contain other elements as needed. For the purposes of improving piezoelectric properties and insulation properties, controlling sintering temperature, suppressing crystal grain growth, etc., for example, a composition containing at least one of Ta, Ni, Cu, V, Cr, Fe, Co, Zn, Y, Mo, Ru, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, W, Re, Os, Ir, Ag, Si may be added.

[0047] In addition to the alkali niobate perovskite oxide, which is the main component, the piezoelectric ceramic layer 10 preferably further contains an oxide containing Mn and Ti. In the following description, the crystalline phase of the oxide containing Mn and Ti contained in the piezoelectric ceramic layer 10 is also referred to as the "subphase." Therefore, the piezoelectric ceramic layer 10 preferably contains a subphase containing an oxide containing Mn and Ti in addition to the main phase. The subphase is dispersed in a dotted pattern within the main phase. Generally, alkali niobate perovskite oxides have a cubic particle shape, which makes voids prone to occur and makes densification difficult. However, by incorporating a crystalline phase of an oxide containing Mn and Ti, a dense and highly reliable ceramic can be obtained.

[0048] The amount of the subphase relative to the main phase is preferably 0.1 to 20% by volume, and more preferably 0.5 to 5% by volume. From the viewpoint of affinity with the main phase, the subphase preferably contains a Mn-Ti-O-based oxide (e.g., MnTi2O4, Mn2TiO4, MnTiO3-based oxide). When the subphase contains a Mn-Ti-O-based oxide, deterioration of piezoelectric properties can be suppressed and reliability can be further improved.

[0049] The Mn—Ti—O-based oxide is an oxide containing Mn (manganese) and Ti (titanium), and is represented, for example, by the following composition formula (3).

[0050] MnTiO y ···(3)

[0051] The coefficient y satisfies 2≦y≦8. The Mn-Ti-O-based oxide preferably has a spinel structure or an inverse spinel structure, and examples of Mn-Ti-O-based oxides include MnTi2O4 and Mn2TiO4. This allows for a more reliable piezoelectric ceramic to be obtained. Furthermore, the above-mentioned standard numerical ranges may be exceeded as long as the structure can be maintained and changes in the compound's properties are within acceptable limits. For example, the Mn-Ti-O-based oxide may have a structure represented by the following composition formula (4) or (5).

[0052] Mn f Ti 2g O 4±j ···(4) Mn 2h Ti i O 4±j ···(5)

[0053] In the above compositional formulas (4) and (5), the coefficients f, g, h, and i are respectively 0.80 < f < 1.2, 0.8 < g < 1.2, 0.8 < h < 1.2, and 0.8 < i < 1.2, and the coefficient j is a value indicating oxygen deficiency or excess. Also, other elements may be contained as necessary. For example, it is possible to contain elements such as Na, K, Zr, Ba, Ca, Sr, Ni, Cu, Ag, and Sc.

[0054] From the viewpoint of suppressing the decrease in insulation, the Mn-Ti-O-based oxide preferably contains Mn (manganese) and further contains Sc (scandium). That is, it is more preferable that the piezoelectric ceramic layer 10 further contains an oxide containing Mn, Ti, and Sc in addition to the alkali niobate perovskite-type oxide. The Mn-Ti-O-based oxide is more preferably an oxide containing Sc in a proportion of 0.05 mol% or more and 5 mol% or less from the viewpoint of further improving the insulation.

[0055] Also, in addition to the Mn-Ti-O-based oxide, the piezoelectric ceramic layer 10 may be composed of a composite of MnO x oxides such as MnO, MnO2, and Mn3O4, and Mn-Nb-O-based oxides (for example, Mn4Nb2O9, MnNb2O6-based oxides), etc., and Sc may be added to the MnO x oxides, oxides such as Mn-Nb-O-based oxides.

[0056] An example of a method for manufacturing the piezoelectric element 100 of this embodiment is described below. First, multiple types of raw material powders necessary for forming the main phase are prepared, and these raw material powders are weighed out to obtain the desired composition. The raw material powders may be oxides, carbonates, or hydroxides of the elements contained in the main phase, or compounds containing multiple elements contained in the main phase. Ethanol is added to a mixture of the weighed raw material powders, and the mixture is wet-mixed using a ball mill, preferably for 15 hours or more, to obtain a slurry. The resulting slurry is dried, and the resulting mixed powder is calcined, for example, in an air atmosphere at a temperature of 600 to 1000°C for 1 to 10 hours, to obtain a calcined powder of the main phase.

[0057] A dispersant, a binder, and an organic solvent (e.g., toluene) are added to the obtained calcined powder of the main phase, and the mixture is pulverized and mixed using a ball mill or the like to obtain a slurry. At this time, by controlling the mixing time using the ball mill or the like, it is possible to obtain ceramic sheets with various particle sizes. Thereafter, the slurry is processed into a sheet shape using a doctor blade method or the like to produce a ceramic green sheet. Thereafter, an electrode layer that will become the internal electrode 20 is formed on one side of the ceramic green sheet using a conductive paste for the internal electrode, for example, by screen printing.

[0058] A plurality of ceramic green sheets with electrode layers formed thereon are stacked so that the electrode layers are exposed alternately from both sides, and ceramic green sheets without electrode layers are stacked on both the front and back sides of the resulting laminate 90. The resulting laminate 90 is pressed together to obtain a laminate 90 in which ceramic green sheets and electrode layers are alternately stacked. The laminate 90 is cut into a desired shape and then subjected to a binder removal process, for example, by holding the laminate 90 in an N2 atmosphere at a temperature of 200 to 400°C for 2 to 10 hours.

[0059] After the binder removal process, the laminate 90 is fired for 2 to 5 hours at a temperature of, for example, 1000 to 1200°C in a reducing atmosphere in which the pressure is controlled to be one order of magnitude or more lower than the equilibrium oxygen partial pressure of Ni / NO. Thereafter, the laminate is annealed for 10 hours at, for example, 800°C in a N2 atmosphere. The annealing process allows the laminate 90 to have a stable piezoelectric ceramic structure.

[0060] After the fired laminate 90 has its side surfaces polished appropriately, a pair of external electrodes 30 are formed on the side surfaces of the laminate 90 by, for example, sputtering, using, for example, Au. Instead of sputtering, a conductive paste containing, for example, Ag, Cu, or the like may be applied and baked at approximately 600°C to 900°C. The pair of external electrodes 30 are formed facing each other with the laminate 90 interposed therebetween. The laminate 90 on which the external electrodes 30 have been formed is then subjected to polarization. In this manner, a piezoelectric element 100 can be obtained in which piezoelectric ceramic layers 10 and internal electrodes 20 are alternately stacked.

[0061] The above manufacturing method is merely an example, and various other steps and processing conditions can be adopted for manufacturing the piezoelectric element 100. For example, if the piezoelectric ceramic layer 10 includes a subphase, multiple types of raw material powders necessary for forming the subphase may be prepared, and these raw material powders may be used to appropriately produce powdered subphase calcined powder. The obtained subphase calcined powder may then be appropriately mixed with the main phase calcined powder in a predetermined ratio to form the desired piezoelectric element 100.

[0062] According to another aspect of the present disclosure, a component including the piezoelectric element 100 is provided. The component of this aspect has excellent durability against continuous driving, thereby suppressing a decrease in reliability. The component is not particularly limited, but examples thereof include components for various applications such as vibration detection, pressure detection, oscillation, and piezoelectric device applications. Furthermore, according to another aspect of the present disclosure, a device including such a component is provided. The device of this aspect has excellent durability against continuous driving, thereby suppressing a decrease in reliability. The device is not particularly limited, but examples thereof include actuators, haptics, buzzers, ultrasonic sensors, etc.

[0063] FIG. 3 is a schematic diagram illustrating an example of the overall configuration of an actuator 200 as the device 150. The actuator 200 is not particularly limited, and may be used, for example, in a valve opening / closing control unit of a mass flow controller or a stage driving unit of a precision positioning device to displace a driven object. The actuator 200 shown in FIG. 3 includes an actuator body 205, terminals (driving terminals 226, 227 and output terminals 246, 247, 248), a mesh 230, a strain gauge 240, a seat 250, and a cap 260, and expands and contracts when a voltage is applied. The actuator 200 includes a plurality of piezoelectric elements 100, and the strain gauge 240 is connected to the actuator body 205 via the mesh 230. The plurality of piezoelectric elements 100 are arranged in series in the stacking direction D1, and their end faces are bonded together with an adhesive. Note that the actuator body 205 may be formed of a single piezoelectric element 100 instead of a plurality of piezoelectric elements 100. The strain gauge 240 is provided with strain gauge output terminals 246, 247, and 248. As a result, the actuator 200 equipped with the strain gauge 240 can receive a signal of the displacement of the piezoelectric element 100 via the output terminals 246, 247, and 248 of the strain gauge 240.

[0064] FIG. 4 is a cross-sectional view showing an example of a schematic configuration of a haptic 300 as device 150. Haptic 300 is not particularly limited, but may include, for example, a piezoelectric element 100 and a vibrating body in contact therewith. Haptic 300 shown in FIG. 4 is a device including a touch-sensitive display 310 that provides tactile feedback in response to an actuation signal. Touch-sensitive display 310 forms the upper surface of housing 320 and is in contact with display 330. Housing 320 contains piezoelectric element 100 bonded to substrate 340. Substrate 340 bends when piezoelectric element 100 contracts as a result of charge accumulation in piezoelectric element 100 or in response to an external force applied to touch-sensitive display 310, as indicated by the open arrow.

[0065] FIG. 5 is a cross-sectional view showing an example of the schematic configuration of a buzzer 400 as the device 150. The buzzer 400 is not particularly limited, and may include, for example, a piezoelectric element 100 and a diaphragm in contact therewith. The buzzer 400 shown in FIG. 5 has a substantially disc-shaped exterior and includes a plastic case 410, a piezoelectric diaphragm 420 built into the case 410, and the piezoelectric element 100 in contact with the piezoelectric diaphragm 420. The case 410 is integrally formed with a cylindrical support ring 412 and a circular top plate 414 arranged to close an opening on one end of the support ring 412. A circular sound emission hole 430 is formed in the center of the top plate 414 and penetrates in the thickness direction. The piezoelectric diaphragm 420 is a plate material that is circular in plan view, and is fitted and joined to the support ring 412 so as to close the opening on the other end. A resonance space 440 is formed inside the case 410 between the top plate 414 and the piezoelectric diaphragm 420. The resonance space 440 resonates in response to the vibration of the piezoelectric diaphragm 420. When a voltage is applied to the piezoelectric element 100, the piezoelectric diaphragm 420 vibrates, and sound waves generated from the piezoelectric diaphragm 420 are radiated to the outside through the sound emission holes 430.

[0066] 6 is a cross-sectional view showing an example of a schematic configuration of an ultrasonic sensor 500 as the device 150. The ultrasonic sensor 500 is not particularly limited, but may include, for example, a piezoelectric element 100 and a diaphragm in contact therewith. When a voltage is applied to the piezoelectric element 100, the diaphragm vibrates to emit ultrasonic waves, and the piezoelectric element 100 outputs a voltage when the diaphragm receives the ultrasonic waves. The ultrasonic sensor 500 shown in FIG. 6 includes a bottomed cylindrical case 510 having an opening 512 at one axial end, the piezoelectric element 100 fixed to the inside of the bottom surface of the bottomed cylindrical case 510, an acoustic matching layer 520 fixed to the outside of the bottom surface of the bottomed cylindrical case 510 so as to face the piezoelectric element 100, a base member 530 closing the opening 512 of the bottomed cylindrical case 510, and a pair of input / output terminals 540, 550 fixed to the base member 530 and electrically connected to the piezoelectric element 100. [Example]

[0067] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0068] 1. Sample Preparation (Preparation of calcined powder) A calcined powder of the main phase containing an alkali niobate perovskite-type oxide and a calcined powder of the subphase containing an oxide containing Mn and Ti were produced. The main phase raw materials were selected from K2CO3, Na2CO3, Nb2O5, TiO2, ZrO2, MnCO3, BaCO3, and Sc2O3 powders and weighed out to satisfy the above composition formula (2): a1 = 0.46, a2 = 0.47, a3 = 0, b1 = 0.07, b2 = 0, b3 = 0, c = 1.00, d1 = 0.86, d2 = 0.03, d3 = 0.03, d4 = 0.07, and d5 = 0.01. Ethanol was added to the weighed raw material powders, and the mixture was wet-mixed in a ball mill using 3 mm diameter Zr balls for 15 hours to produce a slurry. The resulting slurry was then dried. The dried mixed powder was calcined at 900°C for 5 hours in an air atmosphere to obtain calcined powder of the main phase. The necessary subphase raw materials were selected and weighed from MnCO3, TiO2, and Sc2O3 powders. MnTi2O4 with 0.1 mol% Sc added was used as the oxide containing Mn and Ti. Ethanol was added to the weighed raw material powder, and the mixture was wet-mixed in a ball mill for 15 hours to obtain a slurry. The resulting slurry was dried. The dried mixed powder was calcined at 1300°C for 5 hours in an air atmosphere to obtain a calcined powder of the subphase.

[0069] (Preparation of laminate) To the calcined powder of the main phase, 1.5% by volume of the calcined powder of the subphase was added, followed by the addition of a dispersant, binder, and toluene, and the mixture was mixed in a ball mill using φ3 mm Zr balls to obtain a slurry. The ball mill mixing time (hours) was controlled to vary the particle size (μm) of the ceramic sheet. Ceramic green sheets were then produced by processing into sheets using a doctor blade method. Ceramic sheets for use as the inactive layer 12 and the active layer 14 were produced at levels A to D shown in Table 1 below. The particle size (d50) refers to the median diameter, which is the cumulative 50% particle size based on the number of particles.

[0070] [Table 1]

[0071] Next, a conductive paste for the internal electrodes, made of Ni, was applied to the ceramic green sheets using screen printing. A plurality of ceramic green sheets with the printed conductive paste were stacked and thermocompression bonded together so that the conductive paste was exposed alternately on both sides. The inactive layers 12 were stacked on the top and bottom of the laminate 90. Each inactive layer 12 had a thickness of approximately 60 μm. Each active layer 14 had a thickness of approximately 30 μm. Each internal electrode 20 had a thickness of approximately 2 μm. There were 31 active layers 14. The laminate 90 was then obtained by cutting it to a depth of 8 mm, a width of 8 mm, and a height of 1.05 mm along the stacking direction D1.

[0072] The obtained laminate 90 was subjected to a binder removal treatment by being held at 300°C for 5 hours in an N2 atmosphere. The laminate 90 after the binder removal treatment was placed on an alumina setter and heated at a maximum temperature of 1000°C to 1070°C and an oxygen partial pressure of 10 -12 The laminate 90 was fired in a reducing atmosphere at 800°C for 5 hours. The firing times for Samples 10 to 13 were changed from 5 hours to the times shown in Table 3 below. The higher the maximum firing temperature, the more sintered the ceramic sheet, resulting in densification and a smaller porosity. As shown in Table 2 below, the ceramic sheets A to D listed above were used in combination as the inactive layer 12 and the active layer 14. The maximum firing temperature and holding time for the laminate 90 were adjusted to achieve various porosity and pore diameter values. The laminate was then annealed at 800°C for 10 hours in a N2 atmosphere.

[0073] After polishing the sides of the resulting fired laminate 90, external electrodes 30 made of Au were formed on both sides by sputtering, and then polarization treatment was performed for 10 minutes under conditions of a DC voltage of 4 kV / mm and a temperature of 50°C to obtain piezoelectric elements 100 of samples 1 to 13.

[0074] 2.Measurement method For the piezoelectric elements 100 of samples 1 to 13, the composition of the main phase was analyzed and the porosity (%) of the active layer 14 was determined. For the piezoelectric elements 100 of samples 5 and 10 to 13, the pore diameter (μm) of the active layer 14 was also determined. In addition, for the piezoelectric elements 100 of samples 1 to 13, a 1 million cycle drive test was performed and the S / E reduction rate (%) was determined.

[0075] (1) Analysis of the composition of the main and subphases The analysis was carried out using an EPMA (electron probe microanalyzer). Images of the sample were taken at 5000x magnification, and quantitative analysis was carried out on three randomly selected particles of the main phase and subphase from the obtained images. The average value was taken as the composition of the alkali niobate perovskite-type oxide, which is the main component, and the subphase.

[0076] (2) Porosity and pore size A scanning electron microscope (SEM) was used to capture backscattered electron images of the cross-section of the mechanically polished specimen at an accelerating voltage of 5.0 kV and a magnification of 500x. The active layer 14 was imaged within a 25 μm × 25 μm area, centered on the center of each of the first, 16th, and 31st layers of the active layer 14 (4 mm wide, 15 μm thick along the stacking direction D1). The extracted images were analyzed using image processing software. The open-source software ImageJ was used for image analysis. The actual size of the image obtained using the above method was first entered into ImageJ, followed by Gaussian blurring (radius 2 pixels), and the binarization threshold was determined using the Huang method. The ceramic portion was colored black and the pores were colored white to clearly distinguish them. The areas of the black and white regions of the binarized image were calculated, and the porosity (%) was calculated using the above formula. The porosity of the active layer 14 was determined as the average value of the porosities calculated for the first, 16th, and 31st layers. The pore diameter (μm) was determined by calculating the circle-equivalent radius of each pore from the area of ​​each pore, and the average of the calculated circle-equivalent radii was used as the average circle-equivalent radius. The pore diameter of the active layer 14 was determined as the average value of the average circle-equivalent radii calculated for the images of the first, 16th, and 31st layers. For example, the porosity of the active layer 14 in Example 5 was 2.5% for the first layer, 2.4% for the 16th layer, and 2.3% for the 31st layer, and the pore diameters were 1.0 μm for the first layer, 0.9 μm for the 16th layer, and 1.1 μm for the 31st layer. It was considered that there was little variation between layers in either the porosity or the pore diameter.

[0077] (3) 1 million cycle drive test A drive test was performed using unipolar drive, in which an electric field was continuously applied in the positive direction. Specifically, for each sample, the maximum electric field E max A positive electric field of +3 kV / mm was applied at room temperature as a sine wave with a frequency of 1 kHz. A drive test was performed by repeating 0 kV / mm → 3 kV / mm → 0 kV / mm as one cycle for a total of 1 million cycles.

[0078] (4) Measurement method for S / E reduction rate The S / E reduction rate was calculated for each sample before and after 1 million cycles of driving. More specifically, the S / E reduction rate was calculated for each sample before and after 1 million cycles of driving. max A positive electric field of +3 kV / mm was applied to the sample at room temperature as a sine wave with a frequency of 0.1 Hz, and the displacement S along the stacking direction D1 was measured using a laser Doppler vibrometer. The displacement S was divided by the electric field E to calculate the S / E value for each electric field. The S / E reduction rate (%) was calculated using the S / E values ​​before and after 1 million cycles of driving using the following formula. The S / E value calculated for the sample after the first cycle of the 1 million cycle driving test was defined as the "S / E value before driving durability," and the S / E value calculated for the sample after 1 million cycles of driving was defined as the "S / E value after driving durability."

[0079] S / E reduction rate = {1 - [(S / E value after drive endurance) / (S / E value before drive endurance)]} x 100

[0080] 3.Results The composition of the alkali niobate perovskite oxide, which is the main phase, and the composition of the subphase were the same as those used at the time of weighing. The sample preparation conditions and measurement results are shown in Tables 2 and 3 below. In the evaluations shown in Table 2, "○" indicates that the sample has particularly excellent overall properties, "△" indicates that the sample has excellent overall properties, and "×" indicates that the sample has poor overall properties.

[0081] [Table 2]

[0082] [Table 3]

[0083] The results shown in Tables 2 and 3 reveal the following. In all samples with an active layer 14 porosity of 0.3% or more and 5.1% or less, the S / E reduction rate was less than 20%, which was favorable. Furthermore, in all samples with an active layer 14 porosity of 1.2% or more and 5.1% or less, the S / E reduction rate was 10% or less, which was even better. Note that the S / E reduction rate could not be measured for sample No. 9, in which the active layer 14 porosity was 7.8%. These results demonstrate that the continuous drive durability of the piezoelectric element 100 can be improved by setting the active layer 14 porosity to 0.3% or more and 5.1% or less. Furthermore, in all samples with an active layer 14 pore diameter of 0.6 μm or more and 3.4 μm or less, the S / E reduction rate was less than 14%, which was particularly favorable. Therefore, it was demonstrated that the continuous drive durability of the piezoelectric element 100 can be further improved by setting the active layer 14 pore diameter to 0.6 μm or more and 3.4 μm or less.

[0084] The present invention is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments and examples corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate. [Explanation of symbols]

[0085] 10...piezoelectric ceramic layer, 12...inactive layer, 14...active layer, 20...internal electrode, 22, 24...electrodes, 30...external electrode, 90...laminated body, 100...piezoelectric element, 150...device, 200...actuator, 205...actuator body, 226, 227...driving terminals, 230...mesh, 240...strain gauge, 246, 247, 248...output terminal, 250...seat, 260...cap, 300...haptics, 310...ta Touch-sensitive display, 320...housing, 330...display, 340...substrate, 400...buzzer, 410...case, 412...support ring, 414...top plate, 420...piezoelectric diaphragm, 430...sound emission hole, 440...resonance space, 500...ultrasonic sensor, 510...bottomed cylindrical case, 512...opening, 520...acoustic matching layer, 530...base member, 540, 550...input / output terminal, D1...stacking direction, R...area, S1...end surface

Claims

1. A piezoelectric element in which piezoelectric ceramic layers containing an alkali niobate perovskite oxide as a main component and internal electrodes containing Ni as a main component are alternately stacked, The piezoelectric ceramic layer has an active layer located inside in the stacking direction, the porosity of the active layer is 0.3% or more and 5.1% or less; A piezoelectric element characterized by:

2. 2. The piezoelectric element according to claim 1, The pore diameter of the active layer is 0.6 μm or more and 3.4 μm or less. A piezoelectric element characterized by:

3. 2. The piezoelectric element according to claim 1, the piezoelectric ceramic layer further contains an oxide containing Mn and Ti; A piezoelectric element characterized by:

4. 2. The piezoelectric element according to claim 1, the piezoelectric ceramic layer further contains an oxide containing Mn, Ti, and Sc; A piezoelectric element characterized by:

5. 2. The piezoelectric element according to claim 1, The alkali niobate perovskite oxide contains Mn and Ti. A piezoelectric element characterized by:

6. 2. The piezoelectric element according to claim 1, The alkali niobate perovskite oxide contains Mn, Ti, and Sc. A piezoelectric element characterized by:

7. A component comprising the piezoelectric element according to any one of claims 1 to 6.

8. An apparatus comprising the component of claim 7.

9. The device of claim 8 , wherein the device is one of an actuator, a haptic, a buzzer, and an ultrasonic sensor.

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