Multilayer piezoelectric element

Abstract

A multilayer piezoelectric element according to one aspect of the present invention is provided with piezoelectric ceramic layers which are mainly composed of an alkali niobate salt having a perovskite structure, and an internal electrode layer 20 which is arranged between the piezoelectric ceramic layers and has a conductive part 21 that has a silver content of 50% by mass or more, wherein: the area percentage (R1) occupied by the conductive part 21 is 75% to 95%; the internal electrode layer also has interstitial parts 22 in which the conductive part 21 is not present, and which have an average area percentage (R2) per one interstitial part of 5% or less; and at least one of the interstitial parts 22 contains lithium manganate 23.

Description

[Technical Field]

[0001] The present invention relates to a multilayer piezoelectric element. [Background technology]

[0002] A piezoelectric element is an electronic component in which a piezoelectric ceramic (piezoelectric ceramic) is sandwiched between a pair of electrodes. Here, piezoelectricity refers to the property of being able to convert electrical energy and mechanical energy into each other.

[0003] Piezoelectric elements utilize the properties of piezoelectric ceramics mentioned above to convert the voltage applied between a pair of electrodes into mechanical energy such as pressure or vibration, thereby moving other objects or operating themselves. On the other hand, piezoelectric elements can also convert mechanical energy such as vibration or pressure into electrical energy and extract that electrical energy as a voltage between a pair of electrodes.

[0004] Piezoelectric elements have structures in which electrodes are formed only on the surface of the piezoelectric ceramic, as well as multilayer piezoelectric elements, which are constructed by alternately stacking multiple piezoelectric ceramic layers and internal electrode layers. Multilayer piezoelectric elements can be used in applications such as actuators because they allow for large displacement in the stacking direction of the piezoelectric ceramic layers. Multilayer piezoelectric elements are typically manufactured by simultaneously firing the piezoelectric ceramic layers and the internal electrode layers.

[0005] Lead zirconate titanate (Pb(Zr,Ti)O3, PZT) and its solid solutions are widely used as piezoelectric ceramics to constitute these piezoelectric elements. PZT-based piezoelectric ceramics have the advantage of being able to produce piezoelectric elements that can be used in high-temperature environments due to their high Curie temperature, and also have the advantage of being able to produce piezoelectric elements that can efficiently convert electrical energy and mechanical energy due to their high electromechanical coupling coefficient. Furthermore, by selecting an appropriate composition, firing can be done at temperatures below 1000°C, which also has the advantage of reducing the manufacturing cost of piezoelectric elements. In particular, in the aforementioned multilayer piezoelectric elements, the use of low-melting-point materials with a high silver content, in other words, a low content of expensive materials such as platinum and palladium, for the internal electrodes fired together with the piezoelectric ceramics results in a significant cost reduction.

[0006] However, PZT-based piezoelectric ceramics are problematic because they contain lead, a harmful substance, and there is a need for alternative lead-free piezoelectric compositions.

[0007] To date, lead-free piezoelectric ceramics include alkali niobate ((Li,Na,K)NbO3) and bismuth sodium titanate ((Bi 0.5 Na 0.5 Various compositions have been reported, including TiO3, BNT systems, bismuth layered compound systems, and tungsten bronze systems. Among these, alkali niobate-based piezoelectric ceramics are attracting attention as an alternative to PZT systems because they have a high Curie point and a relatively large electromechanical coupling coefficient (Patent Document 1).

[0008] Attempts have been made to sinter this alkali niobate-based piezoelectric ceramic at a low temperature to enable co-firing with an internal electrode having a high silver content ratio, thereby reducing the manufacturing cost of the laminated piezoelectric element. For example, in Patent Document 2, it has been reported that by making the composition of the alkali niobate-based piezoelectric ceramic contain an alkaline earth metal and silver, it was possible to perform co-firing with an internal electrode of Ag0.7Pd0.3. Also, in Patent Document 2, it has been reported that the obtained laminated piezoelectric element exhibited a high electrical resistivity and showed a large displacement amount when a voltage was applied.

[0009] On the other hand, in a laminated piezoelectric element, it has been reported that there is a problem that the displacement amount of the element is suppressed because the internal electrode layer restricts the displacement of the piezoelectric ceramic (Patent Document 3). In Patent Document 3, it has been reported that by forming a non-conductive portion composed of a ceramic portion filled with ceramic powder and a void portion in a specific ratio in the internal electrode layer of the laminated piezoelectric element, the inhibition of displacement by the internal electrode layer was suppressed.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] Patent Document 2 describes that the above-mentioned alkali niobate-based piezoelectric ceramics may contain Li2O and SiO2, which are components contributing to the improvement of sinterability, and MnO, which is a component contributing to the improvement of electrical resistance. However, depending on the content of these components, Li3NbO4 having conductivity or lithium manganate having a lower resistivity than alkali niobates may be generated in a large amount in the piezoelectric ceramics, and the electrical insulation may decrease during the use of the device, resulting in a shorter device life.

[0012] Also, as described above, the multilayer piezoelectric element described in Patent Document 2 exhibits a large displacement amount when a voltage is applied. However, as reported in Patent Document 3, the displacement of the piezoelectric ceramic layer may be restricted by the internal electrode layer.

[0013] On the other hand, Patent Document 3 does not describe preventing the shortening of the device life due to the decrease in electrical insulation during the use of the piezoelectric element.

[0014] Therefore, an object of the present invention is to provide a multilayer piezoelectric element that does not contain lead in its constituent components, has a small decrease in electrical insulation during use, and has a large displacement amount when a voltage is applied.

[0015] The inventor of the present invention conducted various studies to solve the above problems. As a result, it was found that the problem can be solved by providing a multilayer piezoelectric element having a piezoelectric ceramic layer mainly composed of an alkali niobate having a perovskite-type structure and a conductive portion mainly composed of silver at a specific area ratio, and an internal electrode layer containing lithium manganate in at least a part of the gap portion where the conductive portion does not exist, and thus the present invention was completed.

[0016] That is, one aspect of the present invention for solving the above problems is that the main component is an alkali niobate having a perovskite-type structure and contains silverThis is a laminated piezoelectric element comprising a piezoelectric ceramic layer, and a conductive portion disposed between the piezoelectric ceramic layers having a silver content of 50% by mass or more, wherein the area percentage (R1) occupied by the conductive portion is 75% or more and 95% or less, and the gap portion where the conductive portion is absent is sized such that the average area percentage (R2) per location is 5% or less, and an internal electrode layer containing lithium manganese oxide is provided in at least one location of the gap portion. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a multilayer piezoelectric element that does not contain lead as a component, exhibits little decrease in electrical insulation performance during use, and has a large displacement when voltage is applied. [Brief explanation of the drawing]

[0018] [Figure 1] Cross-sectional view (XZ plane) showing the structure of a multilayer piezoelectric element according to one aspect of the present invention. [Figure 2] Cross-sectional view (YZ plane) showing the structure of a multilayer piezoelectric element according to one aspect of the present invention. [Figure 3] Perspective view showing a unit cell model of a perovskite-type structure. [Figure 4] A perspective view showing the state of the internal electrode layer in a multilayer piezoelectric element according to one aspect of the present invention. [Figure 5] An explanatory diagram of the dimensions of each part necessary for calculating parameters R1, R2, R3, and R4 indicating the state of the internal electrode layer in a multilayer piezoelectric element according to one aspect of the present invention. [Modes for carrying out the invention]

[0019] The structure and effects of the present invention will be explained below, along with the technical concepts, with reference to the drawings. However, the mechanism of action is based on assumptions, and its accuracy does not limit the present invention.

[0020] [Multilayer piezoelectric element] A multilayer piezoelectric element 100 according to one aspect of the present invention (hereinafter sometimes simply referred to as "this aspect") comprises a piezoelectric ceramic layer 10, an internal electrode layer 20 disposed between the piezoelectric ceramic layers 10, and a connecting conductor 30 that electrically connects the internal electrode layers 20 every other layer, as schematically shown in the cross-sectional views in Figures 1 and 2. Note that among the internal electrode layers 20 shown in Figures 1 and 2 and the connecting conductor 30 shown in Figure 1, those with the same letter ("a" or "b") indicate that they have the same polarity ("+" or "-"). Furthermore, in the multilayer piezoelectric element 100 shown in Figure 1, the connecting conductor 30 is formed on its surface, but the connecting conductor 30 may also be formed inside the multilayer piezoelectric element 100, penetrating the piezoelectric ceramic layer 10.

[0021] As shown in Figure 2, this side surface may have side margin portions 40 located between the Y-axis side surfaces and the internal electrode layer 20, and cover portions 50 located on the Z-axis top and bottom surfaces. Furthermore, this side surface may have external electrodes (not shown) formed on its surface to electrically connect the connecting conductor 30 and the drive circuit.

[0022] The following describes in detail each component of the multilayer piezoelectric element 100.

[0023] (Piezoelectric ceramic layer) The piezoelectric ceramic layer 10 mainly consists of an alkali niobate having a perovskite-type structure.

[0024] The main component, alkali niobate, is an oxide having a perovskite structure, containing at least one alkali metal element selected from the group consisting of lithium (Li), sodium (Na), and potassium (K), as well as niobium (Nb) as constituent elements. Here, the perovskite structure is a crystalline structure having A sites located at the vertices of the unit cell, O sites located at the face centers of the unit cell, and B sites located in an octahedron with the O sites as vertices, as shown in Figure 3. In the alkali niobate of this embodiment, alkali metal ions are located at the A sites, niobium ions at the B sites, and oxide ions at the O sites. In addition, each site may contain various ions other than those mentioned above.

[0025] Here, confirmation that the piezoelectric ceramic layer 10 mainly consists of an alkali niobate having a perovskite-type structure is performed by the following procedure. First, the diffraction line profile of the piezoelectric ceramic layer 10 exposed on the surface of the multilayer piezoelectric element 100, or the powder obtained by pulverizing the multilayer piezoelectric element 100, is measured using an X-ray diffraction (XRD) apparatus that uses Cu-Kα rays. The method for exposing the piezoelectric ceramic layer 10 on the surface of the multilayer piezoelectric element 100 is not particularly limited, and methods such as cutting or polishing the piezoelectric element can be employed. Furthermore, the means for pulverizing the multilayer piezoelectric element 100 is not particularly limited, and a hand mill (mortar and pestle) can be used. Next, when the ratio of the strongest diffraction line intensity in the diffraction profile derived from other structures to the strongest diffraction line intensity in the profile derived from the perovskite-type structure in the obtained diffraction line profile is 10% or less, it is determined that the piezoelectric ceramic layer 10 is mainly composed of a compound having a perovskite-type structure. At this time, if XRD measurement is performed on the powder obtained by crushing the multilayer piezoelectric element 100, peaks of the metal constituting the internal electrode layer 20 will also be detected, so these are excluded before comparing the diffraction line intensities as described above. Next, the piezoelectric ceramic layer 10, or the powder prepared therefrom, which is determined to mainly consist of a compound having a perovskite structure, is subjected to the measurement of the proportion of each element it contains using inductively coupled plasma (ICP) emission spectroscopy, ion chromatography, or XRF fluorescence analysis. Based on the measurement results, if both the total content of alkali metal elements and the niobium content, expressed in mole percent (or atomic percent), are greater than the content of other elements, it is determined that the main component, a compound having a perovskite structure, is an alkali niobate salt.

[0026] The piezoelectric ceramic layer 10 may contain other components as long as it is mainly composed of an alkali niobate having a perovskite-type structure. These other components may be solid-dissolved in any of the A, B, and O sites of the perovskite structure described above, or they may exist as a separate phase between the sintered particles of the main component.

[0027] For example, the piezoelectric ceramic layer 10 may further contain at least one alkaline earth metal element selected from the group consisting of calcium (Ca), strontium (Sr), and barium (Ba), as well as silver (Ag). This results in a dense piezoelectric ceramic layer 10 with small sintered particle sizes, exhibiting excellent piezoelectric properties. From this perspective, the total content of alkaline earth metal elements is preferably more than 0.2 mol%, more preferably 0.3 mol%, and even more preferably 0.5 mol%, when the content of elements (often in an ionic state) in the B site of the main component, alkali niobate, is taken as 100 mol%,. Similarly, the silver content is preferably more than 0.5 mol%, more preferably 0.7 mol%, and even more preferably 1.0 mol%, relative to 100 mol% of the elements in the B site. On the other hand, in order to further improve the electrical insulation of the piezoelectric ceramic layer 10, enable use under high electric fields, and extend the lifespan of the element, the total content of the alkaline earth metal elements is preferably 5.0 mol% or less, more preferably 3.0 mol% or less, and even more preferably 1.0 mol% or less. Similarly, for the same reasons, the silver content is preferably 5.0 mol% or less, more preferably 4.0 mol% or less, and even more preferably 3.0 mol% or less. Furthermore, for the reasons mentioned above, the total content of alkaline earth metal elements and the content of silver are preferably such that, with respect to 100 mol% of the elements in the B site, the total content of alkaline earth metal elements is greater than 0.2 mol% and 5.0 mol%, and the silver content is greater than 0.5 mol% and 5.0 mol%, more preferably the total content of alkaline earth metal elements is greater than 0.3 mol% and 3.0 mol%, and the silver content is greater than 0.7 mol% and 4.0 mol%, and even more preferably the total content of alkaline earth metal elements is greater than 0.5 mol% and 1.0 mol%, and the silver content is greater than 1.0 mol% and 3.0 mol%.

[0028] Furthermore, the piezoelectric ceramic layer 10 may further contain manganese (Mn). This improves the electrical insulation properties of the piezoelectric ceramic layer 10, resulting in a long-life multilayer piezoelectric element 100. From this point of view, the manganese content is preferably 0.2 mol% or more, more preferably 0.3 mol% or more, and even more preferably 0.5 mol% or more, when the content of elements (often in an ionic state) in the B site of the main component alkali niobate is taken as 100 mol%,. On the other hand, from the viewpoint of making the piezoelectric ceramic layer 10 excellent in piezoelectric properties, the manganese content is preferably 2.0 mol% or less, more preferably 1.5 mol% or less, and even more preferably 1.0 mol% or less, relative to 100 mol% of the elements in the B site. Furthermore, for the reasons mentioned above, the manganese content is preferably 0.2 mol% to 2.0 mol%, more preferably 0.3 mol% to 1.5 mol%, and even more preferably 0.5 mol% to 1.0 mol%, based on 100 mol% of the elements in the B site.

[0029] Furthermore, the piezoelectric ceramic layer 10 may further contain silicon (Si). This densifies the piezoelectric ceramic layer 10, and the excess Li that could not be dissolved in the main component reacts with silicon to produce compounds such as Li2SiO3 and Li4SiO4, thereby suppressing the formation of conductive compounds such as Li3NbO4. From this point of view, the silicon content is preferably 0.1 mol% or more, more preferably 0.5 mol% or more, and even more preferably 1.0 mol% or more, when the elemental content (often in an ionic state) in the B site of the alkali niobate, which is the main component, is taken as 100 mol%. On the other hand, from the point of suppressing the amount of non-piezoelectric phases contained in the piezoelectric ceramic layer 10, the silicon content is preferably 3.0 mol% or less, more preferably 2.5 mol% or less, and even more preferably 2.0 mol% or less, relative to 100 mol% of the element in the B site. Furthermore, for the reasons mentioned above, the silicon content is preferably 0.1 mol% to 3.0 mol%, more preferably 0.5 mol% to 2.5 mol%, and even more preferably 1.0 mol% to 2.0 mol%, based on 100 mol% of the elements in the B site.

[0030] In addition to these components, the piezoelectric ceramic layer 10 may optionally contain at least one first transition element selected from Sc, Ti, V, Cr, Fe, Co, Ni, Cu, and Zn. By including these elements in appropriate amounts, it is possible to adjust the firing temperature of the multilayer piezoelectric element 100, control grain growth, and extend the lifespan at high electric fields.

[0031] Furthermore, the piezoelectric ceramic layer 10 may optionally contain at least one second transition element selected from Y, Mo, Ru, Rh, and Pd. By including these elements in appropriate amounts, it is possible to adjust the firing temperature of the multilayer piezoelectric element 100, control grain growth, and extend the lifespan at high electric fields.

[0032] Furthermore, the piezoelectric ceramic layer 10 may contain at least one selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, W, Re, Os, Ir, Pt, and Au, which are third transition elements, if necessary. By containing these elements in appropriate amounts, it is possible to adjust the firing temperature of the multilayer piezoelectric element 100, control grain growth, and extend the life under a high electric field.

[0033] Of course, in the present embodiment, the piezoelectric ceramic layer 10 may contain a plurality of types among the aforementioned first transition elements, second transition elements, and third transition elements.

[0034] The alkali niobate is preferably represented by the following compositional formula (1) in terms of exhibiting excellent piezoelectric characteristics and obtaining a long-life element when used under a high electric field. (Ag u M2 v (K 1-w-x Na w Li x ) 1-u-v ) a (Sb y Ta z Nb 1-y-z )O3…(1) However, M2 in the formula represents at least one alkaline earth metal element selected from the group consisting of calcium (Ca), strontium (Sr), and barium (Ba). Also, u, v, w, x, y, z, and a are numerical values that satisfy the respective inequalities represented by 0.005 < u ≦ 0.05, 0.002 < v ≦ 0.05, 0.007 < u + v ≦ 0.1, 0 ≦ w ≦ 1, 0.02 < x ≦ 0.1, 0.02 < w + x ≦ 1, 0 ≦ y ≦ 0.1, 0 ≦ z ≦ 0.4, and 1 < a ≦ 1.1.

[0035] Here, the fact that the alkali niobate is represented by the above compositional formula (1) is confirmed by the following procedure. First, the piezoelectric ceramic layer 10, or the powder prepared therefrom, which has been confirmed to mainly consist of an alkali niobate having a perovskite structure according to the procedure described above, is subjected to high-frequency inductively coupled plasma (ICP) emission spectroscopy, ion chromatography, or X-ray fluorescence (XRF) analysis to measure the content of silver (Ag), calcium (Ca), strontium (Sr), barium (Ba), potassium (K), sodium (Na), lithium (Li), antimony (Sb), tantalum (Ta), and niobium (Nb). Next, the total number of moles of antimony, tantalum, and niobium is calculated, and the ratio of the number of moles of each of the aforementioned elements to this total is calculated. Then, if the proportions of each element obtained fall within the range of the above compositional formula (1), it is determined that the alkali niobate is represented by the above compositional formula (1).

[0036] (Internal electrode layer) As schematically shown in Figure 4, the internal electrode layer 20 has a conductive portion 21 having a silver content of 50% by mass or more, and a gap portion 22 where the conductive portion 21 is absent, the proportion occupied by the conductive portion 21 being 75% to 95% in terms of area percentage, the size of the gap portion 22 being 5% or less in terms of average area percentage per location, and lithium manganate 23 being contained in at least one location of the gap portion 22.

[0037] The internal electrode layer 20 has a conductive portion 21 made of a metal with a silver content of 50% by mass or more. By having a silver content of 50% by mass or more in the conductive portion 21, the amount of expensive metals such as platinum and palladium used can be reduced, thereby lowering the material cost of the multilayer piezoelectric element 100. In addition, as the proportion of silver, which has excellent conductivity, increases, the electrical resistivity decreases, and the electrical loss when driving the multilayer piezoelectric element 100 is reduced. Examples of metals with a silver content of 50% by mass or more include silver-palladium alloys. The silver content in the conductive portion 21 is preferably 70% by mass or more, and more preferably 80% by mass or more.

[0038] The silver content in the internal electrode layer 20 can be confirmed by performing elemental analysis of the internal electrode layer 20 using various measuring instruments and calculating the mass ratio of silver to all detected elements. Examples of measuring instruments that can be used include energy dispersive X-ray spectrometers (EDS) or wavelength dispersive X-ray spectrometers (WDS) attached to scanning electron microscopes (SEM) or transmission electron microscopes (TEM), electron probe microanalyzers (EPMA), and laser-irradiated inductively coupled plasma mass spectrometers (LA-ICP-MS).

[0039] The proportion of the conductive portion 21 in the internal electrode layer 20 is 75% to 95% in terms of area percentage. By setting the area percentage (hereinafter sometimes simply referred to as "R1") to 75% or more, a sufficient voltage can be applied to the piezoelectric ceramic layer 10 when driving the multilayer piezoelectric element 100, thereby obtaining a large displacement. From this point of view, it is preferable that R1 be 80% or more. On the other hand, by setting R1 to 95% or less, the restraining force caused by the internal electrode layer 20 when the piezoelectric ceramic layer 10 is displaced is reduced, thereby obtaining a large displacement. From this point of view, it is preferable that R1 be 93% or less. Also, for the reasons mentioned above, it is preferable that R1 be 80% to 93%.

[0040] The internal electrode layer 20 has gaps 22 where there are no conductive parts 21. The size of the gaps 22 is such that the average area percentage per location (hereinafter sometimes simply referred to as "R2") of the entire internal electrode layer 20 is 5% or less. This allows a sufficient voltage to be applied to the piezoelectric ceramic layer 10 when driving the multilayer piezoelectric element 100, thereby obtaining a large displacement. From the perspective of applying a larger voltage to both ends of the piezoelectric ceramic layer 10, it is preferable that R2 be 3% or less. The lower limit of R2 is not limited, but in multilayer piezoelectric elements 100 obtained by a general manufacturing method, where the aforementioned R1 is 75% or more and 95% or less, it is often 0.01% or more. The ratio of the total area of ​​the gaps 22 to the area of ​​the internal electrode layer 20 is 5% or more and 25% or less in area percentage, that is, the value obtained by subtracting the aforementioned R1 from 100%. Therefore, the area percentage of the total area of ​​the gap 22 corresponding to the preferred R1 value mentioned above is between 7% and 20%.

[0041] The gap 22 contains lithium manganate 23 at least in one location. This reduces the decrease in electrical insulation performance of the multilayer piezoelectric element 100 during use. This is thought to be suppressed by the following mechanism. Lithium manganate 23 is a compound with lower electrical resistivity than alkali niobate, which is the main component of the piezoelectric ceramic layer 10. If such a compound is present in the piezoelectric ceramic layer 10, it causes a decrease in the electrical resistivity of the piezoelectric ceramic layer 10 and the multilayer piezoelectric element 100 as a whole during use. On the other hand, when lithium manganate 23 is present in the gap 22 of the internal electrode layer 20, although the electrical resistivity of the gap 22 decreases compared to when alkali niobate is present, the internal electrode layer 20 including the gap 22 is originally conductive, so the adverse effect on the electrical resistivity of the multilayer piezoelectric element 100 is limited. Therefore, the presence of lithium manganate 23 in the gap 22 of the internal electrode layer 20 reduces the amount of lithium manganate in the piezoelectric ceramic layer 10, thereby suppressing the decrease in the electrical resistivity of the piezoelectric ceramic layer 10 and the entire multilayer piezoelectric element 100 during use.

[0042] The lithium manganate 23 contained in the gap 22 can be represented by various compositional formulas such as Li2MnO3, LiMn2O4, and LiMnO2, but the aforementioned effect will be exhibited regardless of which compound is used.

[0043] The area ratio (hereinafter sometimes simply referred to as "R3") of lithium manganate 23 contained in the gap 22 to the total area of ​​the internal electrode layer 20 is preferably 1% or more, and more preferably 2% or more. This significantly reduces the decrease in the electrical insulation performance of the multilayer piezoelectric element 100 during use. The upper limit of R3 is not limited, but as mentioned above, the upper limit of the ratio of the total area of ​​the gap 22 to the area of ​​the internal electrode layer 20 is 25%, and the preferred upper limit is 20%, so it will inevitably be lower than these values. For the reasons mentioned above, R3 is preferably 1% or more and 25% or less, and more preferably 2% or more and 20% or less.

[0044] The area ratio (hereinafter sometimes simply referred to as "R4") of lithium manganate 23 contained in the gap 22 relative to the total area of ​​the gap 22 is preferably 10% or more, and more preferably 20% or more. This significantly reduces the decrease in the electrical insulation performance of the multilayer piezoelectric element 100 during use. The upper limit of R4 is not limited, and it may be 100%, i.e., all of the gap 22 may be filled with lithium manganate 23.

[0045] Here, the presence of lithium manganate 23 in at least one of the gaps 22 within the internal electrode layer 20 is confirmed by the following procedure. First, the multilayer piezoelectric element 100 is cut by a plane perpendicular to the plane perpendicular to the stacking direction, passing near the center of gravity of the plane, and the cut surface is polished to expose the internal electrode layer 20. However, in the case of a multilayer element in which a connecting conductor 30 is formed to penetrate the piezoelectric ceramic layer 10, the cutting is performed while avoiding the connecting conductor 30. Next, the exposed internal electrode layer 20 is observed with an optical microscope to identify the conductive portion 21 having a metallic luster and the gap portion 22 where it is interrupted. Then, the location of the spotted portion in the gap portion 22, which is recognized by the difference in hue, lightness, or saturation with the piezoelectric ceramic layer 10, is identified. The spotted portion is often orange or black due to the coloration caused by manganese. Next, the content of Li, Mn, and Nb is measured in both the spotted areas and areas in the piezoelectric ceramic layer 10 where similar spots are not present, using a laser-irradiated inductively coupled plasma mass spectrometer (LA-ICP-MS). Next, from the obtained measurement results, the ratio of the amount of Li to the amount of Nb expressed in atomic percent (Li / Nb) and the ratio of the amount of Mn to the amount of Nb expressed in atomic percent (Mn / Nb) are calculated for each measurement location. Next, if the Li / Nb and Mn / Nb values ​​calculated for the spotted area are both greater than the values ​​calculated for the measurement locations in the piezoelectric ceramic layer 10, the spotted area is determined to be lithium manganese oxide 23. If there are many spotted areas with similar appearances, if the Li / Nb and Mn / Nb values ​​for three of these areas are greater than those for the measurement locations in the piezoelectric ceramic layer 10, the remaining spotted areas are also determined to be lithium manganese oxide 23.

[0046] Furthermore, the values ​​of R1, R2, R3, and R4 mentioned above are measured and calculated using the following procedure. First, using the procedure described above, three consecutive layers are arbitrarily selected from the exposed internal electrode layers 20 in an optical microscope image of a cross-section of a stacked piezoelectric element 100 in which lithium manganese oxide 23 is determined to be contained in the gaps 22 of the internal electrode layers 20. Next, for the selected internal electrode layer 20, as schematically shown in Figure 5, the total length (L) of the first layer t1 ), length of each gap 22 (L d11 , L d12 , ..., L d1a ) and the length (L) occupied by each lithium manganate 23 M11 , L M12 , ..., LM1p ), the total length of the second layer (L t2 ), length of each gap 22 (L d21 , L d22 , ..., L d2b ) and the length (L) occupied by each lithium manganate 23 M21 , L M22 , ..., L M2q ), and the total length of the third layer (L t3 ), length of each gap 22 (L d31 , L d32 , ..., L d3c ) and the length (L) occupied by each lithium manganate 23 M31 , L M32 , ..., L M3r The following values ​​are measured. The subscripts a, b, and c represent the number of interstitial spaces 22 present in each layer, and the subscripts p, q, and r represent the number of lithium manganate 23 present in each layer. Next, R1 is calculated from the measured length using equation (2) below, R2 using equation (3) below, R3 using equation (4) below, and R4 using equation (5) below.

[0047]

number

[0048] (Connecting conductor) The connecting conductor 30 electrically connects the internal electrode layers 20 every other layer. The material of the connecting conductor 30 is not particularly limited as long as it has high conductivity and is physically and chemically stable under the polarization conditions and the operating environment of the element described later, when it is formed on the surface of the multilayer piezoelectric element 100. Examples include silver (Ag), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), and alloys thereof. On the other hand, when the connecting conductor 30 is formed inside the multilayer piezoelectric element 100, penetrating the piezoelectric ceramic layer 10, it is preferable that it be a metal with a silver content of 50% by mass or more, similar to the conductive portion 21 of the internal electrode layer 20 described above.

[0049] (Side margin and cover portion) The side margin portion 40 and the cover portion 50 function as protective parts that protect the piezoelectric ceramic layer 10 and the internal electrode layer 20.

[0050] The side margin portion 40 and the cover portion 50 are preferably formed from a sintered body mainly composed of alkali niobate, similar to the piezoelectric ceramic layer 10, from the viewpoint of the shrinkage rate during firing of the multilayer piezoelectric element 100 and the relaxation of internal stress within the multilayer piezoelectric element 100. However, the material forming the side margin portion 40 and the cover portion 50 does not have to be mainly composed of alkali niobate, as long as it is a material with high insulating properties.

[0051] (external electrode) The external electrode has the function of electrically connecting the connecting conductor 30 and the drive circuit. Furthermore, if it is formed on a piezoelectric ceramic layer 10, it also has the function of applying voltage to it. The material of the external electrode is not particularly limited, as long as it has high conductivity and is physically and chemically stable under polarization conditions and in the operating environment of the piezoelectric element. Examples include silver (Ag), copper (Cu), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), and alloys thereof.

[0052] [Method for manufacturing multilayer piezoelectric ceramics] The multilayer piezoelectric element relating to this 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 an alkali niobate having a perovskite-type structure, mixing the calcined powder with a binder and a dispersion medium to prepare a slurry, forming the slurry into a sheet to obtain a green sheet, printing a paste containing a metal and a manganese compound with a silver content of 50% by mass or more onto the green sheet in the shape of the internal electrode layer, stacking a predetermined number of green sheets printed with the metal paste and pressing them together to obtain a manufactured form, removing the binder from the manufactured form and firing it to obtain a multilayer piezoelectric ceramic, applying a conductive paste to the surface of the multilayer piezoelectric ceramic where the internal electrode layer is exposed and then firing it to form connecting conductors, and applying a high voltage between the connecting conductors to perform polarization treatment of the piezoelectric ceramic layer. Each operation will be described in detail below.

[0053] (Preparation of raw material mixed powder) First, the raw material powders are mixed in a predetermined ratio to obtain a raw material mixed powder. Examples of raw material powders used include lithium carbonate (Li2CO3) as a lithium compound, sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) as sodium compounds, potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3) as potassium compounds, and niobium pentoxide (Nb2O5) as a niobium compound. In addition, although optional components, commonly used compounds include tantalum pentoxide (Ta2O5) as a tantalum compound and antimony trioxide (Sb2O3) as an antimony compound.

[0054] The method for mixing the raw material powders is not particularly limited as long as it ensures uniform mixing of each powder while minimizing the inclusion of impurities, and either dry mixing or wet mixing may be used. When using wet mixing with a ball mill, for example, partially stabilized zirconia (PSZ) balls can be used, and the mixture can be stirred for about 8 to 60 hours in a ball mill with an organic solvent such as ethanol as the dispersion medium, after which the organic solvent can be evaporated and dried.

[0055] (Preparation of calcined powder) Next, the raw material mixture is calcined to obtain calcined powder. Calcination is carried out under conditions in which the raw material powders react with each other to obtain an alkali niobate with a predetermined composition. For example, calcination can be performed in air at a temperature of 700 to 1000°C for 1 to 10 hours. The calcined powder may be used directly for slurry preparation, but it is preferable to crush it using a ball mill or stamp mill beforehand, as this will result in a uniform slurry and a smooth green sheet.

[0056] If commercially available alkali niobate powder is available, the preparation of the raw material mixture powder and the calcined powder described above may be omitted, and the subsequent operations may be performed on this powder instead.

[0057] (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 that volatilizes without leaving any carbon residue after calcination or a preceding binder removal treatment. Examples of binders that can be used include polyvinyl alcohol-based, polyvinyl butyral-based, cellulose-based, urethane-based, and vinyl acetate-based binders. The amount of binder used is not particularly limited, but since it will be removed in a later process, it is preferable to use as little as possible within the range where the desired moldability and shape retention can be obtained, in order to reduce raw material costs.

[0058] As the dispersion medium, one should be used that does not cause aggregation of the calcined powder and binder, and can be easily removed by volatilization or other means after the green sheet is formed, as described later. Examples of usable dispersion media include water and alcohol-based solvents.

[0059] Components that adjust the properties of the slurry, such as dispersants, plasticizers, and thickeners, may be added to the slurry.

[0060] Furthermore, various components to be included in the piezoelectric ceramic layer may be added to the slurry. Examples of such components include alkaline earth metal-containing compounds such as calcium carbonate (CaCO3), calcium metasilicate (CaSiO3), and calcium orthosilicate (Ca2SiO4), strontium carbonate (SrCO3), and barium carbonate (BaCO3); silver-containing compounds such as silver oxide (AgO); lithium-containing compounds such as lithium carbonate, lithium fluoride, and lithium manganate; manganese-containing compounds such as manganese oxide, manganese carbonate, manganese acetate, and lithium manganate; and silicon-containing compounds such as silicon dioxide (SiO2), calcium metasilicate (CaSiO3), and calcium orthosilicate (Ca2SiO4).

[0061] The method for mixing the calcined powder, binder, and dispersion medium is not particularly limited, as long as it ensures that each component is uniformly mixed while preventing the inclusion of impurities. One example is ball mill mixing.

[0062] (Preparation of a green sheet) Next, the obtained slurry is molded to obtain a green sheet. Conventional molding methods such as the doctor blade method can be used.

[0063] (Printing with metal paste) Next, an internal electrode paste containing a metal with a silver content of 50% by mass or more and a manganese compound is printed onto the obtained green sheet. Because the internal electrode paste contains a manganese compound, lithium that was not completely dissolved in the perovskite-type structure during the subsequent firing process reacts with the manganese compound in the internal electrode layer to produce lithium manganate, thereby forming gaps. To ensure that the area ratio R1 and the average area percentage R2 per gap are within a predetermined range, it is preferable to add 0.1% by mass or more and 3% by mass or less of the manganese compound to the internal electrode paste. Note that the values ​​of R1 and R2 are also affected by the composition of the piezoelectric ceramic layer, the amount of vehicle in the internal electrode paste, and the printing film thickness. Therefore, the amount of manganese compound to add to the paste should be determined by fabricating a multilayer piezoelectric element using internal electrode paste with various amounts of manganese compound under the actual manufacturing conditions, and ensuring that the values ​​of R1 and R2 are as specified.

[0064] To improve the adhesion strength to the piezoelectric ceramic layer after firing, glass frit or a powder having a similar composition to the alkali niobate powder contained in the green sheet may be added to the internal electrode paste.

[0065] When printing the internal electrode paste onto the green sheet, it is acceptable to leave a space that will become the side margin when the device is constructed as a multilayer piezoelectric element.

[0066] (Creation of the generated form) Next, a predetermined number of green sheets printed with internal electrode paste are stacked, and the resulting shape is created by pressing the green sheets together. The stacking and pressing can be carried out using conventional methods, such as pressing the stacked green sheets together in the stacking direction while heating them, and using a binder to heat-press them together.

[0067] During lamination and compression, green sheets that will serve as cover portions when the laminated piezoelectric element is formed may be added to both ends in the lamination direction. In this case, the added green sheets may have the same composition as the green sheet on which the internal electrode paste is printed, or a different composition. From the viewpoint of equalizing the shrinkage rate during firing, it is preferable that the composition of the added green sheets is the same as or similar to the green sheet on which the internal electrode paste is printed.

[0068] (Fabrication of multilayer piezoelectric ceramics) Next, the resulting molded structure is fired to obtain a multilayer piezoelectric ceramic. Prior to firing, the binder may be removed from the molded structure. In this case, binder removal and firing may be performed consecutively using the same firing apparatus. The conditions for binder removal and firing should be set appropriately considering the volatilization temperature and content of the binder, as well as the sinterability of the alkali niobate and the heat resistance of the metal contained in the internal electrode paste. An example of conditions for binder removal is firing at a temperature of 300 to 500°C in an atmospheric environment for 5 to 20 hours. An example of firing conditions is firing at a temperature of 800 to 1100°C in an atmospheric environment for 1 to 5 hours. When obtaining multiple multilayer piezoelectric ceramics from a single molded structure, the molded structure may be divided into several blocks prior to firing.

[0069] As described above, during the firing of the resulting form, lithium that was not completely dissolved in the perovskite-type structure reacts with manganese compounds in the internal electrode layer to produce lithium manganate, thereby forming voids. Additionally, during firing, silver in the internal electrode layer diffuses into the piezoelectric ceramic layer, which can also form voids.

[0070] If the green sheet forming the resulting structure contains at least one alkaline earth metal element selected from the group consisting of calcium, strontium, and barium, the interaction between the silver diffusing from the internal electrode layer during firing and the alkaline earth metal element results in a dense piezoelectric ceramic layer formed of fine sintered particles.

[0071] Furthermore, if the green sheet forming the resulting structure contains silicon, it can act as a sintering aid, thereby lowering the firing temperature. In addition, silicon can react with elements contained in alkali niobate salts or separately added elements during firing to precipitate crystalline phases such as Li2SiO3, Li4SiO4, K3Nb3O6Si2O7, KNbSi2O7, K3LiSiO4, or KLi3SiO4, or amorphous phases containing these elements, thereby suppressing the volatilization of alkali metals and their deposition between sintered particles.

[0072] (Formation of connecting conductors) Next, a conductive paste is applied to the surface of the obtained multilayer piezoelectric ceramic where the internal electrode layer is exposed, and then baked to form a connecting conductor.

[0073] (Polarization treatment) Finally, a high voltage is applied between the connecting conductors to perform polarization treatment and obtain a multilayer piezoelectric element. The polarization treatment conditions are not particularly limited as long as they can align the direction of spontaneous polarization in each piezoelectric ceramic layer without causing damage such as cracks to the multilayer piezoelectric ceramic. As an example, an electric field of 4kV / mm to 6kV / mm can be applied at a temperature of 100°C to 150°C. [Examples]

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0075] (Example 1) As a powder of alkali niobate having a perovskite-type structure, the composition formula is Li 0.06 Na 0.52 K 0.42A calcined powder represented by NbO3 was prepared. To 100 mol% of this calcined powder, 0.65 mol% of Li2CO3, 1.3 mol% of SiO2, 0.5 mol% of CaCO3, 0.5 mol% of MnCO3, and a polyvinyl butyral-based binder were added and mixed using a wet ball mill. The resulting mixture slurry was formed using a doctor blade to obtain a green sheet with a thickness of 30 μm. On the other hand, an internal electrode paste was prepared by mixing 0.1 mass% of manganese oxide (MnO) with an Ag-Pd alloy paste (Ag / Pd mass ratio = 9 / 1). After forming an electrode pattern on the green sheet by screen printing the internal electrode paste, 26 layers of the green sheet were stacked and compressed under pressure of approximately 50 MPa while heating to obtain a laminate. After separating this laminate into individual pieces, a debinder treatment was performed in air, followed by firing in air at 980°C for 2 hours to obtain a fired body (layered piezoelectric ceramic). A conductive paste containing Ag was applied to the surface of this fired body, and the temperature was raised to 600°C to bake it, thereby forming a pair of connecting conductors and external electrodes. Finally, a polarization treatment was performed in an 80°C constant temperature bath by applying an electric field of 3.0 kV / mm between the pair of external electrodes for 3 minutes to obtain the multilayer piezoelectric element according to Example 1.

[0076] (Examples 2 and 3) Stacked piezoelectric elements according to Examples 2 and 3 were obtained in the same manner as in Example 1, except that the amount of manganese oxide (MnO) added to the Ag-Pd alloy paste was 0.5% by mass (Example 2) and 3% by mass (Example 3).

[0077] (Comparative Example 1) A multilayer piezoelectric element according to Comparative Example 1 was obtained using the same method as in Example 1, except that manganese oxide (MnO) was not added to the Ag-Pd alloy paste.

[0078] (Comparative Example 2) A multilayer piezoelectric element according to Comparative Example 2 was obtained using the same method as in Example 1, except that the amount of manganese oxide (MnO) added to the Ag-Pd alloy paste was 10% by mass.

[0079] <Rating> [Confirmation of gaps in the internal electrode layer and the presence or absence of lithium manganate] Each of the obtained multilayer piezoelectric elements was observed in cross-section using the method described above, and compositional analysis was performed on those in which spotted areas were observed. It was confirmed that gaps were found in the internal electrode layers of all multilayer piezoelectric elements, and that lithium manganese oxide was present in these gaps.

[0080] [Calculation of R1, R2, R3, and R4 in the internal electrode layer] For each of the obtained multilayer piezoelectric elements, cross-sectional observation and length measurements of the internal electrode layer, interstitial gap, and lithium manganate were performed using the method described above, and R1, R2, R3, and R4 of the internal electrode layer were calculated. The results are shown in Table 1.

[0081] [Measurement of electrical resistivity] Each obtained multilayer piezoelectric element was placed in an 80°C constant temperature bath, and the voltage and current values ​​were measured when a 5kV / mm electric field was applied for 5 minutes. The electrical resistivity of the multilayer piezoelectric element was then calculated based on the obtained measurements and element dimensions. The results are shown in Table 1.

[0082] [Measurement of change in electrical insulation over time (average lifespan)] Each obtained multilayer piezoelectric element was placed in a constant temperature bath at 100°C, and a DC electric field of 8 kV / mm was applied between the external electrodes. The time until the current flowing between the external electrodes reached 1 mA or more was measured. The average value of this time for 10 elements was defined as the average lifetime. The obtained average lifetimes are shown in Table 1 as a ratio to the average lifetime of the multilayer piezoelectric element in Comparative Example 1, which is set to 100.

[0083] [Evaluation of piezoelectric properties] The piezoelectric properties of each obtained multilayer piezoelectric element are defined by the displacement performance d * 33The displacement performance per unit voltage (pm / V) was evaluated. First, a unipolar sine wave with a maximum electric field of 6 kV / mm at approximately 100 Hz was implanted into the multilayer piezoelectric ceramic, and the displacement of the multilayer piezoelectric element at that time was measured using a laser Doppler displacement meter. Then, the obtained displacement of the multilayer piezoelectric element was divided by the thickness of the piezoelectric ceramic layer (distance between electrodes), the maximum voltage calculated from the maximum electric field, and the number of piezoelectric ceramic layers constituting the multilayer piezoelectric element to determine the displacement performance per unit voltage d of one piezoelectric ceramic layer. * 33 The displacement performance d was calculated. * 33 The displacement performance d of the multilayer piezoelectric element according to Comparative Example 1 * 33 The ratios are shown in Table 1, with the value set to 100.

[0084] [Table 1]

[0085] Based on these results, it can be said that by including gaps in the internal electrode layer of a laminated piezoelectric element equipped with an alkali niobate-based piezoelectric ceramic layer in a predetermined proportion and size, and by including lithium manganese oxide in the gaps, the decrease in electrical insulation performance during use is reduced, and the displacement performance is also improved. [Industrial applicability]

[0086] According to the present invention, a multilayer piezoelectric element using alkali niobate-based piezoelectric ceramics can be made to exhibit minimal degradation of electrical insulation during use and large displacement when voltage is applied. Such a multilayer piezoelectric element is suitable for tactile module applications where large displacement and long lifespan are required. Furthermore, because the multilayer piezoelectric element does not contain lead as a component, it is also useful in that it can reduce the environmental burden throughout its lifecycle. [Explanation of Symbols]

[0087] 100 Multilayer Piezoelectric Elements 10 Piezoelectric ceramic layer 20, 20a, 20b internal electrode layer 21 Conductive part 22 Gap 23. Lithium manganese 30, 30a, 30b connecting conductors 40 Side margin section 50 Cover section

Claims

1. A piezoelectric ceramic layer containing silver, with an alkali niobate having a perovskite-type structure as the main component, and Displaced between the piezoelectric ceramic layers, It has a conductive part with a silver content of 50% by mass or more, Area percentage (R) of the conductive part １ ) is between 75% and 95%, The gap where the conductive part is absent is defined as the average area percentage per location (R ２ ) is 5% or less It has a size such that, An internal electrode layer containing lithium manganese at least one location in the gap. A multilayer piezoelectric element equipped with [a specific feature].

2. The multilayer piezoelectric element according to claim 1, wherein the piezoelectric ceramic layer further comprises at least one alkaline earth metal selected from the group consisting of calcium (Ca), strontium (Sr), and barium (Ba).

3. The multilayer piezoelectric element according to claim 1 or 2, wherein the alkali niobate is represented by the following composition formula. (A) ｕ M2 ｖ (K) １－ｗ－ｘ Na ｗ Li ｘ ) １－ｕ－ｖ ) ａ (Sb) ｙ That ｚ Nb １－ｙ－ｚ )O ３ …(1) (However, M2 in the formula represents at least one alkaline earth metal selected from the group consisting of calcium (Ca), strontium (Sr), and barium (Ba). Also, u, v, w, x, y, z, and a in the formula are numerical values ​​that satisfy the following inequalities, respectively: 0.005 < u ≤ 0.05, 0.002 < v ≤ 0.05, 0.007 < u + v ≤ 0.1, 0 ≤ w ≤ 1, 0.02 < x ≤ 0.1, 0.02 < w + x ≤ 1, 0 ≤ y ≤ 0.1, 0 ≤ z ≤ 0.4, and 1 < a ≤ 1.1.)

4. The area percentage (R) of the lithium manganate in the internal electrode layer ３ A multilayer piezoelectric element according to claim 1 or 2, wherein ) is 1% or more.

5. The area percentage (R) of lithium manganate in the gap ４ A multilayer piezoelectric element according to claim 1 or 2, wherein ) is 10% or more.

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