Piezoelectric element

Abstract

To provide a piezoelectric element that exhibits improved toughness while retaining necessary piezoelectric performance.SOLUTION: A piezoelectric element 10 comprises a piezoelectric body 11 made of a lead-free piezoelectric ceramic composition including a main phase formed of a first crystal phase composed of an alkali niobate-based perovskite oxide, and a subphase including a second crystal phase composed of at least one selected from an M-Nb-O-based tungsten bronze-type compound (where element M is an element having a valence of 1 to 4) and an N-Ti-O-based spinel-type compound (where element N is an element having a valence of 1 to 4), and electrodes 12, 13 in contact with the piezoelectric body 11, wherein the average pore diameter of pores formed in the piezoelectric body 11 is 8 μm or more and 50 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, most piezoelectric ceramics are made of PZT (lead zirconate titanate)-based materials. However, the lead contained in PZT is a cause of environmental burden and is therefore viewed as a problem. For this reason, in recent years, development of piezoelectric elements made of lead-free piezoelectric ceramics has been progressing. Hereinafter, lead-free piezoelectric ceramic materials are referred to as lead-free piezoelectric ceramic compositions.

[0003] One known example of a lead-free piezoelectric ceramic composition is the lead-free piezoelectric ceramic composition described in Patent Document 1. Patent Document 1 discloses a lead-free piezoelectric ceramic composition having a main phase formed of a first crystalline phase consisting of an alkali niobate-based perovskite-type oxide and a subphase including a second crystalline phase consisting of an M-Ti-O-based spinel-type compound (element M is a monovalent to tetravalent element). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5715309 Summary of the Invention [Problem to be solved by the invention]

[0005] From the viewpoint of handling the piezoelectric element, it is preferable that the piezoelectric body has a certain degree of toughness.

[0006] An object of the present disclosure is to provide a piezoelectric element that has improved toughness while maintaining necessary piezoelectric performance. [Means for solving the problem]

[0007] The piezoelectric element of the present disclosure comprises a piezoelectric body made of a lead-free piezoelectric ceramic composition including a main phase formed of a first crystalline phase consisting of an alkali niobate perovskite oxide and a subphase including a second crystalline phase consisting of at least one of an M-Nb-O tungsten bronze compound (element M is a monovalent to tetravalent element) and an N-Ti-O spinel compound (element N is a monovalent to tetravalent element), and an electrode in contact with the piezoelectric body, wherein the average pore size of pores formed in the piezoelectric body is 8 μm or more and 50 μm or less. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide a piezoelectric element that has improved toughness while maintaining the necessary piezoelectric performance. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view of a piezoelectric element according to an embodiment. [Figure 2] FIG. 2 shows the observation area of ​​the SEM image of the piezoelectric body of each of the piezoelectric elements according to Examples 1 to 4. [Figure 3] FIG. 3 is a stroke-time diagram showing the results of a four-point bending strength test on the piezoelectric bodies of the piezoelectric elements according to Examples 1 to 4. [Figure 4] FIG. 4 is a diagram showing the average pore diameter, stroke obtained by a four-point bending strength test, and piezoelectric constant d33m for the piezoelectric elements according to Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, embodiments of the present disclosure will be listed and described. <1> The piezoelectric element of the present disclosure comprises a piezoelectric body made of a lead-free piezoelectric ceramic composition including a main phase formed of a first crystalline phase consisting of an alkali niobate perovskite oxide and a subphase including a second crystalline phase consisting of at least one of an M-Nb-O tungsten bronze compound (element M is a monovalent to tetravalent element) and an N-Ti-O spinel compound (element N is a monovalent to tetravalent element), and an electrode in contact with the piezoelectric body, wherein the average pore size of pores formed in the piezoelectric body is 8 μm or more and 50 μm or less.

[0011] The alkali niobate perovskite oxide that constitutes the main phase of a non-pressure ceramic composition is inherently a material prone to void formation. However, the voids in the main phase are filled with a subphase containing a second crystalline phase, stabilizing the structure of the main phase. By controlling the average pore size of the voids formed within the lead-free piezoelectric ceramic composition within the above-described range, the piezoelectric properties of a piezoelectric body made from the lead-free piezoelectric ceramic composition are adequately maintained. Furthermore, when voids are formed within the piezoelectric body, stress concentration can occur around the voids when stress is applied to the piezoelectric body. However, when the average pore size of the voids is within the above-described range, it is believed that the locations where stress concentration occurs within the piezoelectric body are more likely to be dispersed. As a result, the toughness of the piezoelectric body is presumably improved. In this way, a piezoelectric element with improved toughness while maintaining the required piezoelectric performance can be provided.

[0012] <2> <1> In the piezoelectric element described above, it is preferable that the porosity of the piezoelectric body is 1% or more and 3% or less.

[0013] By setting the porosity of the piezoelectric body within the above range, the piezoelectric element can be made to have a good balance between toughness and piezoelectric properties.

[0014] <Configuration of Piezoelectric Element 10> A piezoelectric element 10 according to this embodiment will be described with reference to FIG. 1. As shown in FIG. 1, the piezoelectric element 10 includes a piezoelectric body 11 and electrodes 12 and 13 in contact with the piezoelectric body 11. The piezoelectric body 11 is made of a lead-free piezoelectric ceramic composition, which will be described later, and is disk-shaped. One of the electrodes 12 and 13 is disposed on one side of the piezoelectric body 11, and the other is disposed on the other side of the piezoelectric body 11, with the piezoelectric body 11 sandwiched between the electrodes 12 and 13. The piezoelectric body 11 has piezoelectric properties. Therefore, when a voltage is applied to the electrodes 12 and 13, an electric field is generated in the piezoelectric body 11, causing the dimensions of the piezoelectric body 11 to expand and contract and the shape of the piezoelectric body 11 to change. Furthermore, when a force is applied to the piezoelectric body 11, the piezoelectric element 10 generates a voltage.

[0015] The piezoelectric body 11 is composed of a lead-free piezoelectric ceramic composition including a main phase formed of a first crystalline phase consisting of an alkali niobate perovskite oxide, and a subphase including a second crystalline phase consisting of at least one of an M-Nb-O tungsten bronze compound (element M is a monovalent to tetravalent element) and an N-Ti-O spinel compound (element N is a monovalent to tetravalent element).

[0016] In this embodiment, the ratio of the subphase in the lead-free piezoelectric ceramic composition is more than 0% by volume and 10% by volume or less, and the remainder is the first crystalline phase. In this specification, the first crystalline phase is also referred to as the "main phase," and crystalline phases other than the main phase are referred to as "subphases."

[0017] The alkali niobate perovskite oxide that forms the first crystal phase is represented by the following composition formula (1).

[0018] (K a Na b Li c C d ) e (D f E g )O h ···(1)

[0019] The element C is at least one of Ca (calcium), Sr (strontium), and Ba (barium). The element D is at least one of Nb (niobium), Ta (tantalum), Ti (titanium), Zr (zirconium), Hf (hafnium), Sn (tin), Sb (antimony), and Si (silicon), including at least Nb. The element E is at least one of Mg (magnesium), Al (aluminum), Sc (scandium), Mn (manganese), Fe (iron), Co (cobalt), Ni (nickel), Zn (zinc), Ga (gallium), and Y (yttrium). a + b + c + d = 1, a + b + c is not zero, e is arbitrary, f + g = 1, and h is an arbitrary value that constitutes a perovskite.

[0020] The above composition formula (1) can be rewritten as the following composition formula (1A).

[0021] (K a Na b Li c C1 d1 C2 d2 ) e (D1 f1 D2 f2 D3 f3 E1 g1 E2 g2 E3 g3 )O h (1A)

[0022] The above composition formula (1) and composition formula (1A) are equivalent, and the values ​​of the coefficients a to h in the above composition formula (1) are a+b+c+d1+d2=1, e is arbitrary, f1+f2+f3+g1+g2+g3=1, and h is an arbitrary value that constitutes a perovskite structure. When element C contains two types of metal elements, the value of the coefficient d of element C is expressed as the sum of the coefficients d1 and d2 of the two elements C1 and C2. When element D contains three types of metal elements, the value of the coefficient f of element D is expressed as the sum of the coefficients f1, f2, and f3 of the three elements D1, D2, and D3. The same applies when element D contains four or more types of metal elements.

[0023] In the composition formula (1), K, Na, Li, and element C (Ca, Sr, Ba) are arranged in the so-called A site of the perovskite structure. Furthermore, element D (one or more of Nb, Ta, Ti, Zr, Hf, Sn, Sb, and Si, including at least Nb) and element E (one or more of Mg, Al, Sc, Mn, Fe, Co, Ni, Zn, Ga, and Y) are arranged in the so-called B site of the perovskite structure. Of the coefficients a, b, c, and d of the elements in the A site, it is preferable that the sum of a and b (a + b) is not zero, but coefficients c and d may be zero. Furthermore, of the coefficients f and g of elements D and E in the B site, it is preferable that the coefficient f of element D is not zero, but the coefficient g of element E may be zero. That is, the alkali niobate perovskite oxide of this embodiment is preferably a perovskite oxide that contains at least one or more alkali metals (K, Na, Li) and may also contain alkaline earth metals (Ca, Sr, Ba) at its A site, and that contains at least one of Nb, Ta, Ti, Zr, Hf, Sn, Sb, and Si, including at least Nb, and may also contain at least one other metal (Mg, Al, Sc, Mn, Fe, Co, Ni, Zn, Ga, Y) at its B site. Most preferably, the B site contains Nb.

[0024] The values ​​of the coefficients a to h in the composition formula (1) are selected from the combinations of values ​​that establish a perovskite structure, and are used to determine the electrical or piezoelectric properties (particularly the piezoelectric constant d 33Values can be selected preferably from the viewpoint of . Specifically, the coefficients a, b, and c are each values of 0 or more and less than 1, and it is preferable that a = b = c = 0 (that is, a lead-free piezoelectric ceramic composition containing neither K, Na, nor Li) does not hold. The coefficients a and b of K and Na are typically 0 < a ≤ 0.6 and 0 < b ≤ 0.6. The coefficient c of Li may be zero. The coefficient d of the element C (one or more of Ca, Sr, and Ba) may be zero, but 0 < d ≤ 0.2 is preferable, and 0 < d ≤ 0.1 is more preferable. The coefficient e for the entire A site is arbitrary, but 0.80 ≤ e ≤ 1.10 is preferable, 0.84 ≤ e ≤ 1.08 is more preferable, and 0.88 ≤ e ≤ 1.07 is most preferable. The coefficient h of oxygen can take any value such that the first crystal phase constitutes a perovskite-type oxide. A typical value of the coefficient h is about 3, and 3.0 ≤ h ≤ 3.1 is preferable. Note that the value of the coefficient h can be calculated from the electrical neutrality condition of the composition of the first crystal phase. However, as the composition of the first crystal phase, a composition slightly deviating from the electrical neutrality condition is also acceptable.

[0025] Among the alkali niobate perovskite-type oxides represented by the above composition formula (1), the oxides having K, Na, and Nb as main metal components are referred to as "KNN" or "KNN material" and are excellent in piezoelectric properties, electrical properties, etc.

[0026] The secondary phase includes a second crystal phase composed of at least one of an M-Nb-O-based tungsten bronze-type compound (element M is a monovalent to tetravalent element) and an N-Ti-O-based spinel-type compound (element N is a monovalent to tetravalent element). The M-Nb-O-based tungsten bronze-type compound has, as a main component, a compound represented by the following composition formula (2).

[0027] M x NbO 3-δ ···(2)

[0028] Element M is a monovalent to tetravalent element. Preferably, element M is at least one of K which is an alkali metal or Ba which is an alkaline earth metal.

[0029] In the above composition formula (2), the element M is located at the A site of the tungsten bronze structure, and Nb is located at the B site. It is generally known that compounds with a tungsten bronze structure have a structure in which oxygen octahedra, each consisting of six oxygen ions centered around the B site ion, are linearly connected in the c-axis direction. Therefore, the M-Nb-O tungsten bronze compound represented by composition formula (2) is considered to have a structure in which NbO6 octahedra are linearly connected.

[0030] The A site of the tungsten bronze structure may or may not be filled with alkali ions. The coefficient x in the composition formula (2) satisfies 0≦x≦1.

[0031] In the M-Nb-O tungsten bronze type compound represented by the above composition formula (2), the coefficient x satisfies 0≦x≦1, for example, M3Nb5O 15 (x=0.6), M 5.75 Nb 10.8 O 30 (x=0.53), etc.

[0032] The oxygen coefficient 3-δ is a positive or negative value indicating an oxygen deficiency or excess, with the oxygen coefficient usually being 3. The oxygen coefficient 3-δ can take a value that indicates the second crystal phase constitutes a tungsten bronze-type compound. A typical value for the coefficient δ is δ=0, with 0≦δ≦0.1 being preferred. The value of the coefficient δ can be calculated from the electrical neutrality condition for the composition of the second crystal phase. However, compositions that slightly deviate from the electrical neutrality condition are also acceptable for the composition of the second crystal phase.

[0033] N-Ti-O spinel compounds are primarily composed of a compound represented by the following composition formula (3): In this specification, the term "spinel compound" includes both normal spinel compounds having a normal spinel crystal structure and inverse spinel compounds having an inverse spinel crystal structure. The element N in N-Ti-O spinel compounds is a monovalent to tetravalent element.

[0034] The N—Ti—O spinel compound is an oxide containing the elements N and Ti (titanium), and is represented, for example, by the following composition formula (3).

[0035] N y TiO z ·····(3)

[0036] Here, the element N is a monovalent to tetravalent element. Specifically, the element N is at least one element selected from the group consisting of Li, Mg, Al, Sc, Mn, Fe, Co, Ni, Zn, Ga, Y, and Zr.

[0037] The coefficients y and z are relative values ​​when the coefficient of Ti is set to 1. In order for the second crystal phase to form a spinel-type compound, the coefficient y preferably satisfies 0.5≦y≦5.0. The coefficient z may be any value that forms a spinel compound, but preferably satisfies, for example, 2≦z≦8.

[0038] The second crystal phase composed of a spinel-type compound stabilizes the structure of the first crystal phase, resulting in a lead-free piezoelectric ceramic composition with excellent piezoelectric properties. From the viewpoint of piezoelectric properties, the N-Ti-O spinel-type compound is preferably one represented by the composition formula N2TiO4 or (N1,N2)TiO4, which contains two divalent N atoms.

[0039] The spinel compound that forms the second crystal phase may be a normal spinel compound or an inverse spinel compound.

[0040] The second crystal phase may contain only the M-Nb-O tungsten bronze compound represented by the above composition formula (2) (element M is a monovalent to tetravalent element). The subphase may contain only the N-Ti-O spinel compound represented by the above composition formula (3) (element M is a monovalent to tetravalent element). Alternatively, the second crystal phase may contain both the M-Nb-O tungsten bronze compound represented by the above composition formula (2) (element M is a monovalent to tetravalent element) and the N-Ti-O spinel compound represented by the above composition formula (3) (element M is a monovalent to tetravalent element).

[0041] Although the alkali niobate perovskite oxide that constitutes the main phase is inherently a material that is prone to the formation of voids (void spaces), which are thought to be the cause of reduced piezoelectric properties, the structure of the main phase is stabilized by filling the voids in the main phase with a subphase containing a second crystalline phase, and therefore, piezoelectric bodies made from the lead-free piezoelectric ceramic composition have excellent piezoelectric properties.

[0042] The lead-free piezoelectric ceramic composition may contain a crystalline phase (third crystalline phase) other than the second crystalline phase as a subphase, as long as the object of the present invention is not impaired. Even when the subphase contains a crystalline phase (third crystalline phase, etc.) other than the second crystalline phase, the subphase fills voids formed between the fine crystals of the first crystalline phase.

[0043] In this embodiment, the proportion of the subphase in the lead-free piezoelectric ceramic composition is not particularly limited as long as it does not impair the object of the present invention, but for example, it is preferably more than 0 vol% and not more than 10 vol%, and more preferably 0.5 vol% or more and 5 vol% or less.

[0044] <Method of manufacturing the piezoelectric element 10> An example of a method for manufacturing the piezoelectric element 10 having the above configuration will be described below.

[0045] First, multiple raw material powders necessary for forming the main phase are prepared and weighed to obtain the desired composition. The raw material powders may be oxides, carbonates, or hydroxides of the elements contained in the main phase. Ethanol is added to a mixture of the weighed raw material powders, and the mixture is wet-mixed in a ball mill for preferably 15 hours or more to obtain a slurry. The resulting slurry is dried, and the mixed powder obtained after drying is calcined in air at 600 to 1200°C for 1 to 10 hours to obtain a powdered calcined main phase.

[0046] Furthermore, multiple raw material powders necessary for forming the subphase are prepared and weighed to obtain the desired composition. The raw material powders may be oxides, carbonates, or hydroxides of the elements contained in the subphase. Ethanol is added to the weighed raw material powders, and the mixture is wet-mixed in a ball mill for preferably 15 hours or more to obtain a slurry. The resulting slurry is dried, and the mixed powder obtained after drying is calcined in air at 600 to 1200°C for 1 to 10 hours to obtain a powdered calcined subphase.

[0047] Next, the obtained main phase calcined product and subphase calcined product are each weighed, and a dispersant, binder, and ethanol are added in a ball mill, followed by pulverization and mixing to form a slurry. The obtained slurry is dried to obtain a granulated powder. The obtained granulated powder is molded into a disk shape, for example, by uniaxial pressing at a pressure of 150 MPa to obtain a compact.

[0048] The obtained compact is subjected to a debinding step in which the binder is debound by holding the compact in an air atmosphere at 500 to 800°C for 2 to 10 hours. The debound compact is then fired, for example, in an air atmosphere at 900 to 1400°C for 1 to 100 hours, to obtain a piezoelectric body as a sintered body of the lead-free piezoelectric ceramic composition.

[0049] After polishing both the front and back surfaces of the resulting piezoelectric body, electrodes made of Au are formed by, for example, sputtering. The laminate with the electrodes formed on the piezoelectric body is subjected to a polarization process in which a DC voltage of several kV / mm is applied in silicone oil at several tens of degrees Celsius, thereby developing the piezoelectric properties of the piezoelectric body. In this way, piezoelectric element 10 is obtained.

[0050] The above-described manufacturing method is merely an example, and various other steps and processing conditions can be used to manufacture the piezoelectric element 10. For example, instead of separately producing calcined products of the main phase and subphase in advance and then mixing and firing the powders of both, a piezoelectric body made of a lead-free piezoelectric ceramic composition may be manufactured by mixing raw materials in a quantitative ratio corresponding to the final composition of the lead-free piezoelectric ceramic composition and firing the mixture. [Example]

[0051] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0052] <Examples 1 to 4> (Preparation of main phase calcined product) The raw material powders were K2CO3 powder, Na2CO3 powder, Nb2O5 powder, CaCO3 powder, TiO2 powder, ZrO2 powder, and BaCO3 powder. Each raw material powder was weighed according to the coefficients a, b, c, d, e, f, and g in the composition formula (1). An appropriate amount of ethanol was added to the weighed mixture of raw material powders, and the mixture was wet-mixed in a ball mill for 15 hours to obtain a slurry. The resulting slurry was dried, and the mixed powder obtained after drying was calcined in an air atmosphere at 600 to 1200°C for 1 to 10 hours to obtain a powdered calcined main phase product.

[0053] (Preparation of second crystal phase calcined product) K2CO3 powder, Nb2O5 powder, and BaCO3 powder were prepared as raw material powders for the M-Nb-O tungsten bronze compound represented by the above composition formula (2), and each of these raw material powders was weighed according to the value of the coefficient x in the above composition formula (2). An appropriate amount of ethanol was added to the mixture of the weighed raw material powders, and the mixture was wet-mixed using a ball mill for 15 hours to obtain a slurry. The obtained slurry was dried, and the mixed powder obtained after drying was calcined in an air atmosphere at 600 to 1200°C for 1 to 10 hours to obtain a powdery calcined product of the second crystalline phase.

[0054] Furthermore, TiO2 powder, Fe2O3 powder, CoO powder, and ZnO powder were prepared as raw material powders for the N-Ti-O spinel compound represented by the above composition formula (3), and each of these raw material powders was weighed according to the values ​​of the coefficients y and z in the above composition formula (3). An appropriate amount of ethanol was added to the mixture of the weighed raw material powders, and the mixture was wet-mixed using a ball mill for 15 hours to obtain a slurry. The obtained slurry was dried, and the mixed powder obtained after drying was calcined in an air atmosphere at 600 to 1200°C for 1 to 10 hours to obtain a powdery calcined product of the second crystalline phase.

[0055] Next, the obtained main phase calcined product and the subphase calcined product, i.e., the second crystal phase calcined product, were each weighed, and a dispersant, a binder, and ethanol were added in a ball mill, followed by pulverization and mixing to obtain a slurry. The obtained slurry was dried, and the obtained dried product was appropriately granulated, and then molded into a disk shape by uniaxial pressing at a pressure of 150 MPa using a mold with a diameter of 30 mm to obtain a green body.

[0056] The obtained compact was subjected to a debinding step by being held in an air atmosphere at 500°C to 800°C for 2 to 10 hours. The debound compact was then fired by being held in an air atmosphere at 900°C to 1400°C for 1 to 100 hours, thereby obtaining piezoelectric body 11.

[0057] After polishing both the front and back surfaces of the piezoelectric body 11, electrodes 12 and 13 made of Au were formed by sputtering. The laminate in which the electrodes 12 and 13 were formed on the piezoelectric body 11 was subjected to a polarization process by applying a DC voltage of 2.5 kv / mm in silicone oil at 80°C, thereby obtaining the piezoelectric element 10.

[0058] In each step of obtaining the piezoelectric body 11 as a sintered body of the lead-free piezoelectric ceramic composition, if the voids formed in the main phase are not filled with the subphase, voids where neither the main phase nor the subphase exists remain, and voids are formed in the piezoelectric body 11. In this embodiment, the size of the voids formed in the piezoelectric body 11 could be controlled by appropriately setting the firing temperature of the green body after the debinding step. Specifically, the higher the firing temperature of the green body after the debinding step, the smaller the average pore diameter of the voids formed in the piezoelectric body 11 tended to be. By changing the firing temperature of the green body after the debinding step within the range of 900°C to 1400°C, piezoelectric elements 10 according to Examples 1 to 4 were obtained, each having a different average pore diameter of the voids formed in the piezoelectric body 11.

[0059] <Evaluation> The piezoelectric elements 10 according to Examples 1 to 4 were subjected to the following tests.

[0060] <Measurement of average pore diameter (D50)> The polished surface of the piezoelectric body 11 of each of the piezoelectric elements 10 according to Examples 1 to 4 was photographed at a magnification of 1000x or 10000x using a scanning electron microscope (SEM). A rectangular observation area of ​​3000µm x 2200µm was set for the obtained SEM image (backscattered electron image). The scanning electron microscope used was a digital microscope SEM VHX-D510 manufactured by Keyence Corporation. The observation area of ​​the SEM image corresponding to each measurement sample according to Examples 1 to 4 is shown in FIG. 2. The black areas present in the observation area correspond to areas where voids are formed in the piezoelectric body 11. The areas other than the black areas present in the observation area correspond to areas where the main phase or subphase is present and no voids are formed in the piezoelectric body 11. FIG. 2(A) corresponds to Example 1. FIG. 2(B) corresponds to Example 2. FIG. 2(C) corresponds to Example 3. FIG. 2(D) corresponds to Example 4.

[0061] Using the automatic area measurement function of the image analysis software installed in the SEM VHX-D510, the average diameter of the black areas present in the observation area was calculated as the average pore diameter. Specifically, the observation area of ​​the SEM image was binarized using the median brightness of each pixel to generate a binarized image, and the maximum diameter of each of the black areas scattered within the generated binarized image was calculated as the diameter of each pore scattered within the observation area. A histogram was created sorting the calculated pore diameters in order from smallest to largest, and the cumulative 50% diameter (D50) of the histogram based on the number of pores was calculated as the average pore diameter. The results are shown in Figure 4.

[0062] <Porosity measurement> The porosity of the piezoelectric body 11 of each of the piezoelectric elements 10 according to Examples 1 to 4 was measured using an automatic area measurement function of image analysis software installed in the SEM VHX-D510. Specifically, for the binarized images of the observation regions of the SEM images obtained as Figures 2(A), 2(B), 2(C), and 2(D), the porosity was measured based on the ratio of the area of ​​the portion corresponding to pores to the total area of ​​the observation region. As a result, it was confirmed that the porosity of each of the piezoelectric bodies 11 of the piezoelectric elements 10 according to Examples 1 to 4 was in the range of 1% or more and 3% or less.

[0063] <4-point bending strength test> A four-point bending strength test conforming to Japanese Industrial Standard JIS R1601 was conducted on the piezoelectric body 11 of each of the piezoelectric elements 10 according to Examples 1 to 4. Specifically, a test specimen for the four-point bending strength test was cut out from the piezoelectric body 11 of each of the piezoelectric elements 10 according to Examples 1 to 4, and the stroke, which is the amount of indentation of the tester into the test specimen from the start of application of a load to the test specimen until the test specimen breaks, was measured. As a result of the test, the stroke-time diagram shown in FIG. 3 was obtained. The test specimen broke at the stroke and time when the right end of each stroke-time diagram shown in FIG. 3 began to extend vertically. The stroke at which the test specimens according to Examples 1, 2, 3, and 4 broke is shown in FIG. 4.

[0064] <Piezoelectric constant> The piezoelectric constant d33m was measured using a d33 meter for each of the piezoelectric elements 10 according to Examples 1 to 4. The results are shown in FIG.

[0065] As shown in FIG. 4, the average pore diameters of the pores formed in the piezoelectric bodies 11 according to Examples 1, 2, 3 and 4 were 50 μm, 12 μm, 8 μm and 9 μm, respectively.

[0066] As shown in FIG. 3 , the stroke and time from the start of application of load to the test specimen until the test specimen broke were greatest in Example 1, followed by Examples 2, 3, and 4, in that order. As shown in FIG. 3 , the strokes at which the test specimens of Examples 1, 2, 3, and 4 broke were 8.79 mm, 7.77 mm, 6.47 mm, and 5.68 mm, respectively. That is, it was confirmed that the toughness of the piezoelectric body 11 tends to increase as the average pore size of the pores formed in the piezoelectric body 11 increases within the range of 8 μm to 50 μm. Furthermore, the results of this four-point bending strength test confirmed that the piezoelectric body 11 of each of the piezoelectric elements 10 of Examples 1 to 4 has sufficient material strength for use as a piezoelectric element 10. That is, it was confirmed that when the average pore size of the pores formed in the piezoelectric body 11 is within the range of 8 μm to 50 μm, the piezoelectric body 11 has sufficient material strength for use as a piezoelectric element 10. From the viewpoint of ensuring sufficient toughness of the piezoelectric body 11, it is more preferable that the average pore size of the pores formed in the piezoelectric body 11 be 12 μm or more and 50 μm or less.

[0067] When stress is applied to piezoelectric body 11, it is thought that stress concentration occurs around the pores formed in piezoelectric body 11. However, when the average pore size of the pores is in the range of 8 μm or more and 50 μm or less, it is thought that the locations where stress concentration occurs tend to be dispersed in piezoelectric body 11. As a result, it is presumed that the toughness of piezoelectric body 11 is improved.

[0068] It is empirically known that, depending on the firing conditions, such as firing temperature and firing time, in the process of obtaining a piezoelectric body as a sintered body of a lead-free piezoelectric ceramic composition, warping and waviness may easily occur in the resulting sintered body. In this embodiment, deformation of piezoelectric body 11, such as warping and waviness, occurred more significantly in Example 2 than in Example 1, in Example 3 than in Example 2, and in Example 4 than in Example 3. The degree of deformation of piezoelectric body 11 in Examples 1 to 4 is within the allowable range for manufacturing piezoelectric element 10; however, exceeding this range can cause variations in the thickness of the piezoelectric element, variations in the porcelain density, and variations in the piezoelectric characteristics of the deformed portion. For this reason, piezoelectric body 11 is preferably obtained under firing conditions such that the average pore diameter of pores formed in piezoelectric body 11 is 8 μm or greater.

[0069] As shown in FIG. 4 , the piezoelectric constants d33m of the piezoelectric elements 10 according to Examples 1, 2, 3, and 4 were 152.3 pC / N, 154.4 pC / N, 157.0 pC / N, and 157.6 pC / N, respectively. That is, it was confirmed that the piezoelectric constant d33m of the piezoelectric element 10 tends to decrease as the average pore diameter of the pores formed in the piezoelectric body 11 increases within the range of 8 μm to 50 μm. On the other hand, it was also confirmed that the piezoelectric element 10 exhibits a sufficient piezoelectric constant d33m even when the average pore diameter of the pores formed in the piezoelectric body 11 is 50 μm. From the viewpoint of ensuring higher piezoelectric properties, it was confirmed that the average pore diameter of the pores formed in the piezoelectric body 11 is preferably 50 μm or less. From the viewpoint of ensuring higher piezoelectric properties, it is more preferable that the average pore diameter of the pores formed in the piezoelectric body 11 be 8 μm or more and 12 μm or less, and even more preferably 8 μm or more and 9 μm or less.

[0070] It has been empirically confirmed that when the porosity of the piezoelectric body 11 is 10% or more, and a DC voltage of about 4 kV / mm is applied to a laminate in which electrodes 12 and 13 are formed on the piezoelectric body 11 in silicone oil at several tens of degrees Celsius as a polarization treatment, leakage occurs in the piezoelectric body 11. Therefore, it is preferable that the porosity of the piezoelectric body 11 is less than 10%. If the porosity of the piezoelectric body 11 is 3% or less, such leakage does not occur.

[0071] It has been traditionally believed that the fewer voids formed in a piezoelectric body, the better for improving the piezoelectric properties of a piezoelectric element. It has also been traditionally believed that filling the voids formed between the crystals of the main phase with a subphase results in a dense structure of a piezoelectric body made of a lead-free piezoelectric ceramic composition, thereby improving the material strength of the piezoelectric body. In this regard, the inventors of the present application have newly discovered that when voids are formed in a piezoelectric body, if the average pore size of the voids is 8 μm or more and 50 μm or less, the toughness of the piezoelectric body can be improved while maintaining sufficient piezoelectric properties for use as a piezoelectric element. The inventors have also newly discovered that if the porosity of a piezoelectric body in which such voids are formed is in the range of 1% or more and 3% or less, a piezoelectric element with a good balance between toughness and piezoelectric properties can be provided.

[0072] In this embodiment, the piezoelectric element 10 has a single-layer structure including a single layer of piezoelectric body 11 and electrodes 12 and 13 arranged on the surface of the piezoelectric body. The piezoelectric element may have a multi-layer structure in which piezoelectric bodies and electrodes are alternately stacked. [Explanation of symbols]

[0073] 10: Piezoelectric element 11: Piezoelectric body 12, 13: Electrodes

Claims

1. a main phase formed of a first crystalline phase made of an alkali niobate perovskite oxide; a subphase including a second crystalline phase consisting of at least one of an M-Nb-O based tungsten bronze type compound (element M is a monovalent to tetravalent element) and an N-Ti-O based spinel type compound (element N is a monovalent to tetravalent element); a piezoelectric body made of a lead-free piezoelectric ceramic composition containing an electrode in contact with the piezoelectric body; A piezoelectric element comprising: A piezoelectric element, wherein the average pore size of the pores formed in the piezoelectric body is 8 μm or more and 50 μm or less.

2. 2. The piezoelectric element according to claim 1, wherein the porosity of the piezoelectric body is 1% or more and 3% or less.

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