Lead-free piezoelectric ceramic composition

Abstract

To provide a lead-free piezoelectric ceramic composition that exhibits superior piezoelectric characteristics.SOLUTION: The lead-free piezoelectric ceramic composition comprises 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 an M-Nb-O-based tungsten bronze-type compound (where element M is an element having a valence of 1 to 4) with Li added thereto.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to lead-free piezoelectric ceramic compositions. [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 lead-free piezoelectric ceramic materials has been progressing. Hereinafter, lead-free piezoelectric ceramic materials are referred to as lead-free piezoelectric ceramic compositions.

[0003] One 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 oxide and a subphase including a second crystalline phase consisting of an M-Ti-O-based spinel compound (wherein the element M is a monovalent to tetravalent element). Patent Document 1 discloses that the sinterability and piezoelectric properties of the lead-free piezoelectric ceramic composition are improved by incorporating Li into both the alkali niobate-based perovskite oxide forming the first crystalline phase and the M-Ti-O-based spinel compound forming the second crystalline phase. [Prior art documents] [Patent documents]

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

[0005] There is a need for the development of lead-free piezoelectric ceramic compositions having excellent piezoelectric properties and containing alkali niobate perovskite oxides as the main phase.

[0006] An object of the present disclosure is to provide a lead-free piezoelectric ceramic composition having excellent piezoelectric properties. [Means for solving the problem]

[0007] The lead-free piezoelectric ceramic composition of the present disclosure includes 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-Nb-O-based tungsten bronze-type compound (element M is a monovalent to tetravalent element) to which Li has been added. [Effects of the Invention]

[0008] According to the present disclosure, a lead-free piezoelectric ceramic composition having excellent piezoelectric properties can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows the radiation XRD patterns of the measurement samples of Examples 1 to 4 and Comparative Examples 1 and 2, with the diffraction angle 2θ ranging from 4° to 20°. [Figure 2] FIG. 2 shows the results of elemental analysis of the measurement samples of Examples 1 to 4 and Comparative Examples 1 and 2 by inductively coupled plasma mass spectrometry. [Figure 3] FIG. 3 shows the results of ion mapping measurements by time-of-flight secondary ion mass spectrometry on regions of the surfaces of measurement samples containing Li, where KNN phase and spinel phase mainly exist, corresponding to Examples 1 to 4. [Figure 4] FIG. 4 shows the results of ion mapping measurements by time-of-flight secondary ion mass spectrometry on regions of the surfaces of Li-containing measurement samples, each of which corresponds to Examples 1 to 4 and in which KNN phases and tungsten bronze phases are mainly present. [Figure 5] FIG. 5 is a diagram showing the results of a thermal cycle test carried out on each of the piezoelectric elements according to Examples 1 to 4 and Comparative Examples 1 and 2. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, embodiments of the present disclosure will be listed and described. <1> The lead-free piezoelectric ceramic composition of the present disclosure includes 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-Nb-O-based tungsten bronze-type compound (element M is a monovalent to tetravalent element) to which Li has been added.

[0011] According to this lead-free piezoelectric ceramic composition, the second crystalline phase composed of an M-Nb-O tungsten bronze type compound to which Li has been added stabilizes the structure of the first crystalline phase, thereby providing a lead-free piezoelectric ceramic composition with excellent piezoelectric properties.

[0012] <2> <1> In the lead-free piezoelectric ceramic composition described above, the element M is preferably at least one of K and Ba.

[0013] In this case, a lead-free piezoelectric ceramic composition that is inexpensive and has excellent piezoelectric properties can be obtained.

[0014] <3> <1> or <2> In the lead-free piezoelectric ceramic composition described in the above, the M-Nb-O based tungsten bronze compound to which Li is added has a composition formula M x NbO 3-δ (where δ is a value indicating oxygen deficiency or excess), and it is preferable that the main component is a compound where x satisfies 0≦x≦1.

[0015] In this case, the crystal structure of the Li-added M-Nb-O tungsten bronze compound becomes more similar to that of the alkali niobate perovskite oxide, which increases the affinity of the Li-added M-Nb-O tungsten bronze compound for the alkali niobate perovskite oxide, thereby providing a lead-free piezoelectric ceramic composition with excellent piezoelectric properties.

[0016] <Details of the embodiment> The lead-free piezoelectric ceramic composition according to the embodiment will be described below with reference to the drawings.

[0017] <Configuration of lead-free piezoelectric ceramic composition> The lead-free piezoelectric ceramic composition of the present embodiment includes 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-Nb-O-based tungsten bronze-type compound (element M is a monovalent to tetravalent element) to which Li has been added.

[0018] 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."

[0019] The alkali niobate perovskite oxide that forms the first crystal phase is represented by the following composition formula (1).

[0020] (K a Na b Li c C d ) e (D f E g )O h ···(1)

[0021] 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.

[0022] The above composition formula (1) can be rewritten as the following composition formula (1A).

[0023] (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)

[0024] 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.

[0025] 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.

[0026] As the values of the coefficients a to h in the above compositional formula (1), among the combinations of values that form a perovskite structure, preferable values can be selected from the viewpoint of the electrical properties or piezoelectric properties (especially the piezoelectric constant d33) of the lead-free piezoelectric ceramic composition. Specifically, it is preferable that the coefficients a, b, and c are each values of 0 or more and less than 1, and 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, Ba) may be zero, but 0 < d ≤ 0.2 is preferable, and 0 < d ≤ 0.1 is more preferable. The coefficient e with respect to 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 deviated from the electrical neutrality condition is also acceptable.

[0027] Among the alkali niobate perovskite-type oxides represented by the above compositional formula (1), 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. In this specification, among the alkali niobate perovskite-type oxides represented by the above compositional formula (1), a first crystal phase composed of an oxide having K, Na, and Nb as main metal components is also referred to as a "KNN phase".

[0028] The secondary phase contains at least a second crystal phase composed of an M-Nb-O-based tungsten bronze-type compound to which Li is added. The M-Nb-O-based tungsten bronze-type compound has a compound represented by the following compositional formula (2) as a main component.

[0029] M x NbO 3-δ ···(2)

[0030] The element M is a monovalent to tetravalent element. Preferably, the element M is at least one of K, which is an alkali metal, and Ba, which is an alkaline earth metal.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] The inventors of the present application have newly discovered that in 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 represented by the above composition formula (1) and a subphase including a second crystalline phase consisting of an M-Nb-O-based tungsten bronze-type compound represented by the above composition formula (2), when Li element is added to the M-Nb-O-based tungsten bronze-type compound represented by the above composition formula (2), the piezoelectric properties of the lead-free piezoelectric ceramic composition are less likely to deteriorate.

[0036] Tungsten bronze compounds are generally represented as ABO3, where some of the A-site ions in the perovskite structure are missing. x The M-Nb-O tungsten bronze compound represented by composition formula (2) is based on the BO3 structure. When a trace amount of Li is added to the M-Nb-O tungsten bronze compound represented by composition formula (2), Li is incorporated into the A and B sites of the M-Nb-O tungsten bronze compound represented by composition formula (2), which makes the value of the coefficient x in composition formula (2) approach 1. In other words, the crystal structure of the M-Nb-O tungsten bronze compound represented by composition formula (2) to which Li has been added tends to resemble the crystal structure of the alkali niobate perovskite oxide represented by composition formula (1). This is presumably to increase the affinity of the M-Nb-O tungsten bronze compound represented by composition formula (2) to which Li has been added for the alkali niobate perovskite oxide represented by composition formula (1).

[0037] In addition to Li, other alkali metals such as Na and K can be added as elements to approximate the crystal structure of the M-Nb-O tungsten bronze compound represented by the above composition formula (2) to the crystal structure of the alkali niobate perovskite oxide represented by the above composition formula (1). On the other hand, Li, which has the smallest ionic radius among the alkali metals, is thought to be more likely to form a solid solution in the crystal of the M-Nb-O tungsten bronze compound represented by the above composition formula (2) than other alkali metals.

[0038] In this way, the second crystalline phase, which is composed of the M-Nb-O tungsten bronze-type compound represented by the above composition formula (2) to which Li has been added, can stabilize the structure of the first crystalline phase. Specifically, the second crystalline phase fills the voids formed between the fine crystals of the first crystalline phase. As a result, the fine crystals of the first crystalline phase are bonded together by the second crystalline phase, which is presumably why the structural stability of the piezoelectric ceramic composition is improved and the piezoelectric properties are improved. This allows for the production of a lead-free piezoelectric ceramic composition with excellent piezoelectric properties. In this specification, the second crystalline phase, which is composed of the M-Nb-O tungsten bronze-type compound represented by the above composition formula (2), is also referred to as the "tungsten bronze phase."

[0039] 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 (such as a third crystalline phase) other than the second crystalline phase, the subphase fills voids formed between the fine crystals of the first crystalline phase. In this embodiment, the subphase of the lead-free piezoelectric ceramic composition contains a third crystalline phase consisting of an N-Ti-O-based spinel-type compound represented by the following composition formula (3):

[0040] 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.

[0041] The N—Ti—O spinel compound of the third crystal phase is an oxide containing the elements N and Ti (titanium), and is represented by, for example, the following composition formula (3).

[0042] N y TiO z ·····(3)

[0043] Here, the element N is a monovalent to tetravalent element. Specifically, the element N is at least one selected from the group consisting of Li, Mg, Al, Sc, Mn, Fe, Co, Ni, Zn, Ga, Y, and Zr. When Li is contained as the element N, it is preferable that one or more metal elements other than Li among the above metal elements be contained together with Li, so that the third crystal phase forms a spinel compound.

[0044] The coefficients y and z are relative values ​​when the coefficient of Ti is set to 1. In order for the third 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.

[0045] The third crystalline phase, which is composed of a spinel-type compound, stabilizes the structure of the first crystalline phase, similar to the second crystalline phase, resulting in a lead-free piezoelectric ceramic composition with excellent piezoelectric properties. From the viewpoint of piezoelectric properties, it is preferable to use an N-Ti-O spinel-type compound represented by the composition formula N2TiO4 containing two divalent N atoms or (N1,N2)TiO4 as the third crystalline phase.

[0046] The spinel compound forming the third crystal phase may be a normal spinel compound or an inverse spinel compound. In this specification, the third crystal phase consisting of the N-Ti-O spinel compound represented by the above composition formula (3) is also referred to as a "spinel phase."

[0047] Whether the first crystalline phase is composed of an alkali niobate perovskite-type oxide, whether the second crystalline phase is composed of a tungsten bronze-type compound, and whether the third crystalline phase is composed of a spinel-type compound can be determined by X-ray diffraction (XRD). Elemental analysis of lead-free piezoelectric ceramic compositions can be performed using inductively coupled plasma mass spectrometry (ICP-MS). Inductively coupled plasma mass spectrometry uses argon plasma as an ion source to ionize elements contained in a sample, and the ions are separated and detected based on their mass-to-charge ratio. Elements can be identified from the mass-to-charge ratio of the detected ions, and metal ions such as alkali metals and alkaline earth metals, including Li, can be quantified by counting the detected ions. From the quantified metal ion content, the elemental ratio, which is the proportion of elements contained in the sample, can be calculated in at% (atomic ratio). Furthermore, the presence or absence of Li in a lead-free piezoelectric ceramic composition can be determined by performing elemental mapping measurements on the surface of the lead-free piezoelectric ceramic composition using time-of-flight secondary ion mass spectrometry (TOF-SIMS).

[0048] 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.

[0049] <Method for producing lead-free piezoelectric ceramic composition> An example of a method for producing the lead-free piezoelectric ceramic composition having the above configuration will be described below.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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 sintered body of the lead-free piezoelectric ceramic composition.

[0054] The above-described manufacturing method is merely an example, and various other steps and processing conditions for manufacturing a lead-free piezoelectric ceramic composition can be used. For example, instead of separately producing calcined products of the main phase and the subphase and then mixing and firing the powders of the two, the 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]

[0055] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0056] <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.

[0057] (Preparation of second crystal phase calcined product) K2CO3 powder, Nb2O5 powder, and BaCO3 powder were prepared as raw material powders, and each of these raw material powders was weighed out according to the value of coefficient x in the above composition formula (2). Li2CO3 powder was also weighed out so that the elemental proportion of Li in the entire lead-free piezoelectric ceramic composition was 0.25 at% in Example 1, 0.50 at% in Example 2, 0.75 at% in Example 3, and 1.00 at% in Example 4. 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 air at 600 to 1200°C for 1 to 10 hours to obtain a powdered calcined product of the second crystalline phase.

[0058] (Preparation of the third crystal phase calcined product) TiO2 powder, Fe2O3 powder, CoO powder, and ZnO powder were prepared as raw material powders, 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 third crystalline phase.

[0059] Next, the obtained main phase calcined product and the subphase calcined products, the second crystal phase calcined product and the third crystal phase calcined product, were each weighed, and a dispersant, 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 molded into a disk shape by uniaxial pressing at a pressure of 150 MPa using a 30 mm diameter mold to obtain a green body. A debinding step was performed by holding the obtained green body in an air atmosphere at 500°C to 800°C for 2 to 10 hours. The debinding green body was then fired in an air atmosphere at 900°C to 1400°C for 1 to 100 hours to obtain a sintered body of the lead-free piezoelectric ceramic composition.

[0060] In order to evaluate the piezoelectric properties of the obtained lead-free piezoelectric ceramic composition, a piezoelectric element was manufactured using the obtained lead-free piezoelectric ceramic composition. The piezoelectric element was manufactured as follows. After polishing both the front and back surfaces of the obtained molded body of the lead-free piezoelectric ceramic composition, external electrodes made of Au were formed by sputtering. After the electrodes were formed, the lead-free piezoelectric ceramic composition was subjected to a polarization treatment by applying a DC voltage of 2.5 kv / mm in silicone oil at 80°C, and a piezoelectric element was obtained.

[0061] <Comparative Example 1> In Comparative Example 1, no Li2CO3 powder was added to the raw material powder in producing the second crystal phase calcined product. Other steps for producing the measurement sample in Comparative Example 1 were the same as those in Examples 1-4.

[0062] <Comparative Example 2> In Comparative Example 2, a second crystal phase calcined product was not produced, but a main phase calcined product and a third crystal phase calcined product were produced as a subphase calcined product. When producing the third crystal phase calcined product, Li2CO3 powder was weighed out as the raw material powder so that the elemental proportion of Li in the lead-free piezoelectric ceramic composition was 1.00 at%. The main phase calcined product and the Li-added third crystal phase calcined product were each weighed, and a dispersant, 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 resulting dried product was appropriately granulated. The resulting dried product was then uniaxially pressed at a pressure of 150 MPa using a 30 mm diameter mold to form a disk-shaped compact.

[0063] <Evaluation> The measurement samples according to Examples 1 to 4 and Comparative Examples 1 and 2 were subjected to the following tests.

[0064] <Powder X-ray diffraction> For each measurement sample of Examples 1 to 4 and Comparative Examples 1 and 2, the composition of the compounds contained in each measurement sample was identified using XRD. A sintered body of the lead-free piezoelectric ceramic composition serving as the measurement sample was pulverized using a pestle and mortar, and the pulverized fine powder was filled into the tip of a Lindemann glass capillary with a diameter of 0.3 mm. This was used as the measurement object and measured by the Debye-Scherrer method using a powder X-ray diffractometer. The measurement conditions were monochromatic X-rays with a wavelength of 0.7 Å, an X-ray exposure time of 12 minutes, and a diffraction angle 2θ range of 0° to 70°. The radiation XRD pattern obtained for each measurement sample is shown in Figure 1.

[0065] Figure 1 shows the radiation XRD patterns of each measurement sample, measured in 0.01° steps over the diffraction angle 2θ range of 4° to 20°. The horizontal axis of Figure 1 represents the diffraction angle 2θ (°), and the vertical axis represents the intensity (arbitrary unit).

[0066] Analysis of the obtained XRD patterns confirmed the presence of a crystalline phase (KNN phase) formed by an alkali niobate-based perovskite-type oxide, a crystalline phase (tungsten bronze phase) formed by a tungsten bronze-type compound, and a crystalline phase (spinel phase) formed by a spinel-type compound in each of the measurement samples of Examples 1 to 4 and Comparative Example 1. Furthermore, the peak intensities of the XRD patterns corresponding to the KNN phase, tungsten bronze phase, and spinel phase confirmed that the KNN phase was the main phase, and the tungsten bronze phase and spinel phase were subphases. On the other hand, the measurement sample of Comparative Example 2 confirmed the presence of a KNN phase as the main phase and a spinel phase as the subphase, but no tungsten bronze phase was confirmed. That is, Comparative Example 2 was confirmed to be composed of a KNN phase as the main phase and a spinel phase as the subphase, and did not contain a tungsten bronze phase.

[0067] <Inductively Coupled Plasma Mass Spectrometry> Elemental analysis of each measurement sample of Examples 1 to 4 and Comparative Examples 1 and 2 was carried out using inductively coupled plasma mass spectrometry (ICP-MS). The analysis results of each measurement sample of Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Figure 2. In Figure 2, the element ratio, which is the proportion of each element contained in each measurement sample, is expressed in at%.

[0068] As shown in FIG. 2, the Li content of Comparative Example 1 was 0.00 at%. The Li content of Example 1 was 0.25 at%. The Li content of Example 2 was 0.50 at%. The Li content of Example 3 was 0.75 at%. The Li content of Example 4 was 1.00 at%. The Li content of Comparative Example 2 was 1.00 at%. The proportions of other elements in Examples 1 to 4 and Comparative Examples 1 and 2 were as shown in FIG. 2.

[0069] <Time-of-flight secondary ion mass spectrometry> Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on the surface of the Li-containing measurement sample corresponding to Examples 1 to 4 using a time-of-flight secondary ion mass spectrometer, and mapping measurements of each of the ions Li, Co, Na, K, Fe, Ti, Ba, Nb, Zr, Zn, and Ca were performed. The measurement area was 200 μm × 200 μm. The distribution images of each ion obtained by the measurement are shown in Figures 3 and 4. Figures 3 and 4 show distribution images of each ion corresponding to the results of mapping measurements of different areas on the surface of the measurement sample. In each distribution image, the higher the brightness, the more strongly the ions are detected.

[0070] Figures 3(A) to 3(L) show the results of mapping measurements using TOF-SIMS, showing the distribution images of Li, Co, Na, K, ZnO, Fe, Ti, Ba, Nb, Zr, Zn, and Ca. In Figure 3(A), which shows the distribution image of Li, region A1 in the upper left corner of the distribution image shows higher brightness than the surrounding areas. In response to this, the character Li in Figure 3(A) is underlined.

[0071] As in Figure 3(A), in Figures 3(B), 3(F), 3(G), and 3(K), region A1 exhibits higher brightness than the surrounding regions. In Figure 3, the characters Co, Fe, Ti, and Zn are underlined in Figures 3(B), 3(F), 3(G), and 3(K), where region A1 exhibits higher brightness than the other regions. These elements Co, Fe, Ti, and Zn are components of the spinel phase. Furthermore, the brightness of the region corresponding to region A1 in the distribution maps corresponding to Figures 3(C), 3(D), 3(E), 3(H), 3(I), 3(J), and 3(L) is lower than the brightness of the region corresponding to region A1 in the distribution maps corresponding to Figures 3(B), 3(F), 3(G), and 3(K). Furthermore, in Figures 3(C), 3(D), and 3(I), which correspond to the distribution diagrams of Na, K, and Nb, the brightness of the region corresponding to region A1 is lower than the brightness of the regions other than region A1. Na, K, and Nb are the main elements that constitute the KNN phase, and are elements that do not constitute the spinel phase of this embodiment. Therefore, it is estimated that the spinel phase is present in region A1 of the measurement region. It is also estimated that the KNN phase is present in the region surrounding region A1, where the spinel phase is estimated to exist.

[0072] Therefore, it can be inferred from FIG. 3 that Li is present in the spinel phase, and that the amount of Li present in the KNN phase is either minute compared to the amount of Li present in the spinel phase, or that no Li is present in the KNN phase.

[0073] Figures 4(A) to 4(L) show the results of TOF-SIMS mapping measurements of a measurement area on the surface of the measurement sample that is different from the measurement area corresponding to Figure 3, and show the distribution images of Li, Co, Na, K, ZnO, Fe, Ti, Ba, Nb, Zr, Zn, and Ca. In Figure 4(A), which shows the distribution image of Li, region A2 in the upper left of the distribution image exhibits higher brightness than the surrounding area of ​​region A2. In response to this, the character Li in Figure 4(A) is underlined.

[0074] As in FIG. 4(A), FIG. 4(H) also shows that region A2 exhibits a higher brightness than the surrounding region. In FIG. 4, the "Ba" character corresponding to FIG. 4(H), where region A2 exhibits a higher brightness than the other regions, is underlined. Ba is an element that constitutes the tungsten bronze phase. K and Nb are also elements that constitute the tungsten bronze phase, and therefore the "K" and "Nb" characters are underlined in FIG. 4(D) and FIG. 4(I), which show the distribution images of K and Nb. Since K and Nb are contained in both the KNN phase and the tungsten bronze phase, in FIG. 4(D) and FIG. 4(I), there is almost no difference in brightness between the region corresponding to region A2 and the other regions, and both regions corresponding to region A2 exhibit relatively high brightness. Furthermore, the brightness of the region corresponding to region A2 in each of the other distribution maps corresponding to FIGS. 4(B), 4(C), 4(E), 4(F), 4(G), 4(J), 4(K), and 4(L) is lower than the brightness of the region corresponding to region A2 in each of the distribution maps corresponding to FIGS. 4(D), 4(H), and 4(I). Furthermore, in FIG. 4(C), which corresponds to the distribution map of Na, the brightness of the region corresponding to region A2 is lower than the brightness of the regions other than region A2. Na is a major element constituting the KNN phase and is not an element constituting the tungsten bronze phase of this embodiment. Therefore, it is estimated that the tungsten bronze phase is present in region A2 of the measurement region. It is also estimated that the KNN phase is present in the region surrounding region A2, where the tungsten bronze phase is estimated to be present.

[0075] Therefore, it can be inferred from FIG. 4 that Li is present in the tungsten bronze phase, and that the amount of Li present in the KNN phase is either minute compared to the amount of Li present in the tungsten bronze phase, or that no Li is present in the KNN phase.

[0076] It has been previously believed that when Li is added to a lead-free piezoelectric ceramic composition having a KNN first crystalline phase as the main phase and a tungsten bronze second crystalline phase and a spinel third crystalline phase as subphases, the added Li is primarily dissolved in the main phase. However, as shown in Figures 3 and 4, when Li is added to a lead-free piezoelectric ceramic composition having a KNN first crystalline phase as the main phase and a tungsten bronze second crystalline phase and a spinel third crystalline phase as subphases, depending on the preparation conditions of the lead-free piezoelectric ceramic composition, the added Li may be selectively dissolved in the subphase rather than the KNN main phase. Preparation conditions for the lead-free piezoelectric ceramic composition include the selection of a Li source, the conditions for adding the Li source, the firing conditions for preparing the calcined products of the main phase and subphase, the temperature and other conditions for the debinding process for the compact obtained by combining the calcined main phase and subphase, and the firing conditions for obtaining the final sintered body.

[0077] <Thermal cycle test> A thermal cycle test was carried out on each of the piezoelectric elements according to Examples 1 to 4 and Comparative Examples 1 and 2. The thermal cycle test was carried out according to the following procedure. (1) The relative dielectric constant ε at room temperature for the piezoelectric elements according to Examples 1 to 4 and Comparative Examples 1 and 2 33 T / ε0 was measured as the initial value. (2) Next, a thermal cycle in which the temperature was changed between 0° C. and 100° C. was repeated 100 times. The holding times at 0° C. and 100° C. were each 30 minutes. (3) After that, the relative dielectric constant ε 33 T / ε0 was measured as a characteristic value after the thermal cycle test.

[0078] The relative dielectric constant ε of the piezoelectric elements according to Examples 1 to 4 and Comparative Examples 1 and 2 33 Figure 5 shows the rate of change of T / ε0 from the initial value to the characteristic value after the thermal cycle test.

[0079] In the piezoelectric element according to Comparative Example 2, which does not contain a tungsten bronze phase as a subphase, the relative dielectric constant ε 33 T / ε0 decreased by 33.5%. 33 The rate of decrease in T / ε0 was 1 / 100 of the relative dielectric constant ε of the piezoelectric elements according to Comparative Example 1 and Examples 1 to 4, which contain a tungsten bronze phase as a subphase. 33 In other words, it was confirmed that when a lead-free piezoelectric ceramic composition having a KNN phase as the main phase contains a tungsten bronze phase and a spinel phase as subphases, it exhibits higher thermal durability than a composition containing a spinel phase as the subphase but not a tungsten bronze phase.

[0080] In addition, in the piezoelectric elements according to Comparative Example 1, which includes a tungsten bronze phase in the subphase, and Comparative Example 1 among Examples 1 to 4, in which Li is not added to the subphase, the relative dielectric constant ε 33 T / ε0 decreased by 32.6%. 33 The rate of decrease in T / ε0 is the same as that of the piezoelectric elements according to Examples 1 to 4 in which Li is added to the subphase. 33 In other words, it was confirmed that when a lead-free piezoelectric ceramic composition having a KNN phase as the main phase contains a tungsten bronze phase and a spinel phase as subphases, and Li is added to the subphase, the composition exhibits superior thermal durability compared to when Li is not added to the subphase.

[0081] Furthermore, the relative dielectric constant ε of the piezoelectric element according to Example 1, in which the Li content in the lead-free piezoelectric ceramic composition is 0.25 at % 33 The decrease rate of the relative dielectric constant ε of the piezoelectric element according to Example 2, in which the lead-free piezoelectric ceramic composition contained 0.05 at % Li, was 19.8%. 33 The decrease rate of the relative dielectric constant ε of the piezoelectric element according to Example 3, in which the lead-free piezoelectric ceramic composition contained 0.75 at % Li, was 9.4%. 33 The decrease rate of the relative dielectric constant ε of the piezoelectric element according to Example 4, in which the lead-free piezoelectric ceramic composition contained 1.00 at % Li, was 7.4%. 33The decrease rate of the relative permittivity ε was 3.3%. Thus, in the lead-free piezoelectric ceramic composition in which the atomic ratio of Li is in the range of 0.00 at % to 1.00 at %, the greater the atomic ratio of Li, the lower the relative permittivity ε 33 It was confirmed that the rate of decrease in T / ε0 was small.

[0082] As mentioned above, it is presumed that the Li added to the lead-free piezoelectric ceramic composition is selectively dissolved in the subphase consisting of the tungsten bronze phase and the spinel phase, rather than in the main phase consisting of the KNN phase. Furthermore, when a lead-free piezoelectric ceramic composition having the KNN phase as the main phase contains the tungsten bronze phase and the spinel phase as subphases, it exhibits superior thermal durability compared to a composition having the spinel phase as the subphase but not the tungsten bronze phase. Therefore, it has been confirmed that when a lead-free piezoelectric ceramic composition having the KNN phase as the main phase contains the tungsten bronze phase and the spinel phase as subphases, the addition of Li to the tungsten bronze phase improves the piezoelectric properties. [Explanation of symbols]

[0083] A1,A2:Area

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 made of an M-Nb-O based tungsten bronze type compound to which Li has been added (element M is a monovalent to tetravalent element); A lead-free piezoelectric ceramic composition comprising:

2. 2. The lead-free piezoelectric ceramic composition according to claim 1, wherein said element M is at least one of K and Ba.

3. The M-Nb-O tungsten bronze compound to which Li has been added is Composition formula M ｘ NbO ３－δ 3. The lead-free piezoelectric ceramic composition according to claim 1, wherein δ is a value indicating an oxygen deficiency or excess, and x satisfies 0≦x≦1.

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