JP2023032022A - Piezoelectric element - Google Patents

Abstract

To provide a piezoelectric element with excellent piezoelectric characteristics and reliability.SOLUTION: A piezoelectric element 1 includes a piezoelectric element body 10 composed of a lead-free piezoelectric porcelain composition and a pair of electrodes 21, 22 provided on a surface of the piezoelectric element body 10. The lead-free piezoelectric porcelain composition includes a main phase formed of a first crystalline phase composed of an alkali niobate perovskite oxide having piezoelectric properties and an auxiliary phase formed of a second crystalline phase composed of an M-Ti-O spinel compound (element M is a monovalent to tetravalent element). In the piezoelectric element body 10, the surface is formed of the main phase only, and the inside is formed by including the main phase and the auxiliary phase.SELECTED DRAWING: Figure 7A

Description

The present disclosure relates to piezoelectric elements.

Piezoelectric elements have a function of mutually converting electrical energy and mechanical energy, and are used in various applications. Conventionally, piezoelectric ceramic compositions containing lead, such as PZT (lead zirconate titanate), have been used for piezoelectric element bodies constituting piezoelectric elements. Lead-free piezoelectric ceramic compositions containing no lead have been proposed. For example, Patent Document 1 below discloses a secondary crystal phase containing a main phase formed of a first crystal phase composed of an alkaline niobate-based perovskite oxide having piezoelectric properties and a second crystal phase composed of an M—Ti—O-based spinel compound. A lead-free piezoelectric ceramic composition is disclosed having a phase. In this lead-free piezoelectric ceramic composition, the main phase and the subphase are mixed, and the voids formed in the main phase are filled with the subphase to stabilize the structure of the first crystal phase, thereby improving the piezoelectric characteristics. is planned.

However, as described above, in a lead-free piezoelectric ceramic composition in which a main phase and a subphase composed of different components are mixed, there is a possibility that cracks may occur starting from grain boundaries between the main phase and the subphase. . If cracks occur in the lead-free piezoelectric ceramic composition forming the piezoelectric element main body, the insulation resistance is lowered, and the function as the piezoelectric element is impaired.

JP 2019-31424 A

In view of the above situation, an object of the present technology is to provide a piezoelectric element having excellent piezoelectric characteristics and reliability by improving the crack resistance of a piezoelectric element body.

A piezoelectric element according to the present disclosure includes a piezoelectric element main body made of a lead-free piezoelectric ceramic composition, and a pair of electrodes provided on the surface of the piezoelectric element main body, wherein the lead-free piezoelectric ceramic composition is , a main phase formed of a first crystal phase composed of a niobate alkali perovskite oxide exhibiting piezoelectric properties, and a second crystal phase composed of an M--Ti--O system spinel compound (element M is a monovalent to tetravalent element). and a secondary phase formed of a crystalline phase, wherein in the piezoelectric element body, the surface is formed only of the primary phase, and the interior is formed of the primary phase and the secondary phase. , are piezoelectric elements.

According to the present disclosure, it is possible to provide a piezoelectric element having excellent piezoelectric characteristics and reliability.

FIG. 1 is a perspective view of a piezoelectric element according to an embodiment. FIG. 2 is a flow chart showing a method for manufacturing a piezoelectric element according to the embodiment. FIG. 3A is a photograph of the appearance of the piezoelectric element according to the example before electrodes are formed. FIG. 3B is a photograph of the appearance of the piezoelectric element according to the example after electrodes are formed (FIG. 3B). FIG. 4A is a laser microscopic photograph of the inside of a piezoelectric element body according to an example. FIG. 4B is an SEM photograph of the main phase portion inside the piezoelectric element body according to the example. FIG. 5 is a diagram showing the results of quantitative analysis of each element inside the piezoelectric element body according to the example by EPMA. FIG. 6 is a diagram showing the results of structural analysis of the main phase portion and the subphase portion inside the piezoelectric element body according to the example by XRD. FIG. 7A is a diagram showing the results of structural analysis of the inside and surface of the bulk piezoelectric element body according to the embodiment by XRD. as well as 7B and 7C are partial enlarged views of FIG. 7A. FIG. 8A is a diagram showing the results of structural analysis of the inside and surface of the piezoelectric element body according to the example formed in a sheet shape by XRD. FIG. 8B is a partially enlarged view of FIG. 8A. FIG. 9 is a graph showing the relationship between the bending strength of the piezoelectric element main body and the grain size of the second crystal phase according to the example. FIG. 10 is a graph showing the relationship between the grain size of the first crystal phase and the grain size of the second crystal phase in the piezoelectric element body according to the example. FIG. 11 is a table showing the crack generation rate in the reliability test of the piezoelectric element having the piezoelectric element main body according to the example, together with the grain size of the second crystal phase.

[Description of Embodiments of the Present Disclosure]
First, the embodiments of the present disclosure are listed and described.
<1> A piezoelectric element of the present disclosure includes a piezoelectric element main body made of a lead-free piezoelectric ceramic composition, and a pair of electrodes provided on the surface of the piezoelectric element main body, wherein the lead-free piezoelectric ceramic composition The object consists of a main phase formed of a first crystal phase consisting of an alkaline niobate perovskite oxide having piezoelectric properties, and an M-Ti-O-based spinel compound (element M is a monovalent to tetravalent element). and a sub-phase formed of a second crystal phase, wherein in the piezoelectric element body, the surface is formed only of the main phase, and the interior is formed of the main phase and the sub-phase. ing.

<2> In the piezoelectric element of <1> above, the piezoelectric element body can be formed in a flat plate shape.

<3> In the piezoelectric element of <1> or <2> above, the content of the second crystal phase in the interior of the piezoelectric element body is 0.5% by volume or more and 5.0% by volume or less. can be done.

<4> In the piezoelectric element according to <1> to <3> above, the average grain size of the first crystal phase may be 3.5 μm or less, and the average grain size of the second crystal phase may be 34 μm or less.

[Details of the embodiment of the present disclosure]
A specific example of the piezoelectric element of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these examples, but is indicated by the scope of the claims, and is intended to include all modifications within the meaning and scope of equivalents of the scope of the claims. It should be noted that the relative sizes and arrangement of the members in the drawings are not necessarily accurate, and the scale of some members has been changed in consideration of the convenience of explanation.

<Details of embodiment>
A piezoelectric element 1 according to this embodiment will be described with reference to FIGS. 1 and 2. FIG.
As shown in FIG. 1, the piezoelectric element 1 includes a piezoelectric element body 10 made of a lead-free piezoelectric ceramic composition, and a pair of electrodes 21 and 22 provided on the surface of the piezoelectric element body 10 . In the piezoelectric element 1 illustrated in FIG. 1, electrodes 21 and 22 are formed on two parallel main surfaces (plate surfaces) of a disk-shaped piezoelectric element body 10, respectively. The piezoelectric element main body 10 is polarized in the thickness direction.

Although the shape of the piezoelectric element main body 10 constituting the piezoelectric element 1 is not limited, it is preferable that the piezoelectric element main body 10 has a plate-like shape or a sheet-like shape with a sufficiently small thickness relative to the area of the main surface. Specifically, the thickness of the piezoelectric element main body 10 can be 300 μm or less, more specifically 200 μm or less. This is because by reducing the thickness, it is possible to more remarkably obtain the advantage of the present technology, which will be explained below, due to the absence of grain boundaries of different components on the surface, that is, the effect of improving the crack resistance and the like. In addition, the shape of a flat plate includes not only a shape in which the two main surfaces are planes parallel to each other, but also a shape that is slightly warped and the two main surfaces are not strictly parallel, or a shape that has a part that is not parallel. etc., which can be recognized as a flat plate as a whole. Further, the planar shape of the piezoelectric element main body 10 can be circular, rectangular, polygonal, elliptical, fan-shaped, or any other shape formed by combining straight lines and curved lines.

The piezoelectric element 1 can be widely used in devices using piezoelectric elements such as vibration detection, pressure detection, oscillation, and piezoelectric device applications. For example, sensors that detect various vibrations (knock sensors, combustion pressure sensors, etc.), vibrators, piezoelectric actuators, piezoelectric devices such as filters, high voltage generators, micro power sources, various drive devices, position control devices, vibration suppression devices , fluid discharge devices (paint discharge, fuel discharge, etc.), and various other devices. Piezoelectric elements according to embodiments of the present invention are particularly suitable for applications that require excellent thermal durability (for example, knock sensors, combustion pressure sensors, etc.).

The lead-free piezoelectric ceramic composition constituting the piezoelectric element main body 10 includes a main phase formed of a first crystal phase composed of an alkaline niobate perovskite oxide and an M—Ti—O based spinel compound (element M is monovalent to and a subphase formed of a second crystal phase consisting of a tetravalent element).

The alkali niobate-based perovskite oxide forming the first crystal phase has a composition formula (K _a Na _b L _c M1 _d ) _e (M2 _f M3 _g )O _3+h (wherein the metal element M1 is one or more of Ca and Ba, The metal element M2 is one or more of Nb, Ti, and Zr containing at least Nb, and the metal element M3 is one or more of Fe, Co, and Zn, where a+b+c+d=1, a+b+c is not zero, and e is 0.0. It satisfies 80<e<1.10, f+g=1, and h is a value indicating deficiency or excess of oxygen). The first crystal phase composed of such an alkali niobate-based perovskite oxide exhibits piezoelectric properties.

The A site of the alkali niobate-based perovskite oxide contains at least one of the alkali metals K, Na, and Li (a + b + c is not zero), and the alkaline earth metal Ca and Ba as the metal element M1. (a+b+c+d=1). However, the coefficient d may be zero, that is, the metal element M1 may not be included. In addition, the B site of perovskite contains at least one of metal elements M1 and M2 (f+g=1). The metal element M2 is one or more of Nb, Ti, and Zr containing at least Nb. An alkali niobate-based perovskite oxide containing Nb can provide a lead-free piezoelectric ceramic composition having a higher Curie temperature (Tc) than an alkali tantalate-based perovskite oxide containing tantalum but not containing Nb. preferable. Furthermore, the metal element M3 is one or more of Fe, Co, and Zn. Of the coefficient of oxygen 3+h, the coefficient h is a positive or negative value that indicates the deficiency or excess of oxygen with respect to the coefficient of oxygen, which is usually 3.

Examples of the above niobate alkali perovskite _oxides include compositions _{of ( K0.5Na0.5 ) NbO3} , ( _{K0.45Na0.45Li0.1} ) _{( Nb0.9Ti0.1} ) _O2.95 , ( _{K0.5Na0.4Ba 0.1} ) _NbO3.05 .

As the values of the coefficients a to h in the composition formula of the alkali niobate perovskite oxide, the electrical characteristics or piezoelectric characteristics of the lead-free piezoelectric ceramic composition (especially the piezoelectric constant d ₃₃ ) can be selected.

Specifically, the A site of the alkali niobate-based perovskite oxide contains at least one of the alkali metals K, Na, and Li (a + b + c is not zero), and 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, but preferably 0<c≦0.2, more preferably 0<c≦0.1. The coefficient d of the metal element M1 (specifically, one or more of Ca and Ba) may be zero, but preferably 0<d≦0.2, more preferably 0<d≦0.1. The coefficient e for the entire A site is 0.80≦e≦1.10, preferably 0.84≦e≦1.08, more preferably 0.88≦e≦1.07.
The coefficient of oxygen (3+h) can take a value such that the first crystal phase constitutes a perovskite oxide. A typical value for the factor h is h=0, preferably 0≤h≤0.1. The value of coefficient h can be calculated from the electrically neutral condition of the composition of the first crystal phase. However, as the composition of the first crystal phase, a composition slightly deviating from the electrically neutral condition is also acceptable.

In particular, an alkali niobate system having a composition formula ( _{KaNabLicCad1Bad2} ) _e ( _Nbf1 , _Tif2 , _Zrf3 ) _{O3+h and having} K, Na and _Nb as main _metal components The first crystal phase formed of perovskite oxide has excellent piezoelectric properties, electrical properties, insulating properties, and high-temperature durability, and there is no sudden change in properties between -50°C and +150°C. A lead-free piezoelectric ceramic composition can be provided.

The M—Ti—O-based spinel compound (the element M is a monovalent to tetravalent element) forming the second crystal phase has a composition formula (M4, Ti)O ₄ (wherein the metal element M4 is Fe, Co, Zn one or more of). The second crystal phase composed of such an M--Ti--O system spinel compound does not exhibit piezoelectric properties, but is dispersed in the main phase formed of the first crystal phase in the form of scattered dots. to fill the voids within.

The present lead-free piezoelectric ceramic composition containing a main phase formed of the first crystal phase and a subphase formed of the second crystal phase contains both Ca and Ba as the metal element M1, and the metal element M2 It is preferable that the composition contains Nb, Ti, and Zr as the metal element M3 and does not contain any of Fe, Co, and Zn as the metal element M3. This lead-free piezoelectric ceramic composition becomes a lead-free piezoelectric ceramic composition particularly excellent in piezoelectric properties such as piezoelectric constant _d33 and electromechanical coupling coefficient Kr.

The interior of the piezoelectric element main body 10 according to the present disclosure is formed including a main phase formed of the first crystal phase and a subphase formed of the second crystal phase. The structure of the first crystal phase is stabilized by filling the vacancies in the main phase of the first crystal phase with the secondary phase formed of the second crystal phase. As a result, the lead-free piezoelectric ceramic composition inside the piezoelectric element main body 10 becomes a dense composition with a low porosity, exhibiting excellent piezoelectric properties such as a piezoelectric constant _d33 and an electromechanical coupling coefficient Kr.

The content of the second crystal phase inside the piezoelectric element main body 10 is preferably 0.5% by volume or more and 5.0% by volume or less. If the content of the second crystal phase is low in the lead-free piezoelectric ceramic composition forming the piezoelectric element main body 10, the amount of the secondary phase that fills the holes formed in the main phase decreases, resulting in a high porosity. As a result, the strength decreases and leaks are likely to occur. Conversely, if the content of the second crystalline phase is too high, the content of the first crystalline phase having piezoelectric properties will be too low, failing to exhibit sufficient functions as a piezoelectric body. By setting the content of the second crystal phase within the above range, it is possible to obtain the piezoelectric element 1 having well-balanced strength and piezoelectric characteristics.

Further, in the piezoelectric element main body 10, the smaller the grain size of the first crystal phase and the second crystal phase, the better. Specifically, the average grain size _D50 of the first crystal phase is preferably 3.5 μm or less, and the average grain size _D50 of the second crystal phase is preferably 34 μm or less. By setting the average grain size _D50 of the first crystal phase and the second crystal phase in the above range for the entire piezoelectric element body 10, the crack resistance of the piezoelectric element body 10 can be improved. It is presumed that when the average grain size _D50 of the first and second crystal phases is small and the subphase is finely dispersed in the main phase, cracks are less likely to occur at the grain boundaries between the two phases. It should be noted that the crack resistance property is a property evaluated by the crack occurrence rate when a reliability test of driving the piezoelectric element under constant temperature and humidity is carried out. If a crack occurs in the lead-free piezoelectric ceramic composition forming the piezoelectric element main body 10, the insulation resistance is lowered and the function of the piezoelectric element 1 is impaired. It is an important index for obtaining

The surface of the piezoelectric element main body 10 according to the present disclosure is formed only by the main phase consisting of the first crystal phase. If grain boundaries of crystals made of different components exist on the surface of the piezoelectric element main body, cracks are likely to occur starting from these grain boundaries. In the piezoelectric element main body 10 according to the present disclosure, the surface is formed only of the main phase consisting of the first crystal phase, and grain boundaries of crystals consisting of different components do not exist on the surface. Therefore, compared to a configuration in which different components are mixed over the entire piezoelectric element body, excellent crack resistance can be exhibited. In addition, since there is no grain boundary between phases made up of different components on the surface of the piezoelectric element body 10, compared to a configuration in which the main phase and the sub-phase are mixed over the entire piezoelectric element body, the transverse bending Strength is also improved. Therefore, a piezoelectric element that is hard to break and has high strength can be obtained. The flexural strength refers to the internal stress value at breaking load when a flexural test is performed in which a load is applied to the piezoelectric element by a load cell.

An example of a method for manufacturing the piezoelectric element 1 according to this embodiment will be described with reference to FIG.
As shown in FIG. 2, first, a first calcined powder is produced in steps S1 and S2. Specifically, in step S1, K ₂ CO ₃ powder, Na ₂ CO ₃ powder, Li ₂ CO ₃ powder, CaCO ₃ powder, and BaCO ₃ are used as raw materials (first raw materials) for the first crystal phase forming the main phase. Powder, Nb ₂ O ₅ powder, TiO ₂ powder, ZrO ₂ powder, Fe ₂ O ₃ powder, CoO powder, and ZnO powder are prepared, and the composition formula of the first crystal phase to be provided (( _Ka Na _b Li _c M1 _d ) _e (M2 _f M3 _g )O _3+h ) are weighed according to the values of coefficients a to g. Next, ethanol is added to these powders of the first raw material, and wet-mixed using a ball mill for 15 hours or more to obtain a slurry of the first raw material.

In step S2, the mixed powder obtained by drying the slurry of the first raw material is calcined (at 600° C. to 1200° C. for 1 to 10 hours in an air atmosphere) to produce a first calcined powder.

On the other hand, the second calcined powder is produced in steps S3 and S4. Specifically, in step S3, as the raw material (second raw material) for the second crystal phase that forms the main part of the subphase, TiO ₂ powder, Fe ₂ O ₃ powder, CoO powder, and ZnO powder are selected. are selected and weighed according to the composition formula ((M4, Ti)O ₄ ) of the second crystal phase. Next, ethanol is added to the powder of the second raw material, and the mixture is wet-mixed using a ball mill for 15 hours or more to obtain a slurry of the second raw material.

In step S4, the mixed powder obtained by drying the slurry of the second raw material is calcined (at 600° C. to 1200° C. for 1 to 10 hours in an air atmosphere) to produce a second calcined powder.

In step S5, the first calcined powder (the calcined powder of the first raw material) and the second calcined powder (the calcined powder of the second raw material) are weighed, and a ball mill is used to add a dispersant, a binder, and ethanol. pulverize and mix to make a slurry. Further, this slurry is dried to obtain a mixed powder. The mixing ratio of the first calcined powder and the second calcined powder in step S5 is determined by considering the ratio of the subphase in the lead-free piezoelectric ceramic composition (piezoelectric element main body) to be finally obtained. good.

In step S6, calcination is performed (at 600° C. to 1200° C. for 1 hour to 10 hours in an air atmosphere) to produce mixed calcined powder.

In step S7, the calcined powder obtained in step S6 is pulverized and mixed with a dispersant, a binder, and ethanol to form a slurry, and the slurry is dried by a spray dryer to granulate granulated powder. The granulated powder is put into a press mold and press-molded, or put into an extruder, extrusion-molded, and punched out with a mold to obtain a molded body of a predetermined shape. Examples of the shape of this molded body include a disk shape, a columnar shape, a rectangular flat plate shape, and the like. After that, CIP treatment (cold isostatic pressing treatment) is performed at a pressure of 150 MPa, for example, to obtain a compacted compact.

In step S8, the compact obtained in step 7 is fired. Specifically, the molded body is held at, for example, 500° C. to 800° C. in an air atmosphere for 2 to 10 hours to remove the binder (degreasing). Furthermore, the obtained molded body after degreasing is held at a specific temperature (for example, 1150 ° C.) selected from 900 ° C. to 1400 ° C. in an air atmosphere for 1 hour to 10 hours. A piezoelectric ceramic composition (piezoelectric element body) is obtained. The sintering in step S8 is preferably sealed sintering performed in a state where the compact is sealed in a closed container. This is to prevent metal elements such as alkali metals (Li, Na, K) contained in the compact from volatilizing to the outside during firing.

In step S9, electrodes of a predetermined shape are formed at predetermined positions of the piezoelectric element body obtained by firing. For example, a pair of circular electrodes 21 and 22 are formed on both sides of a disk-shaped piezoelectric element body 10 (see FIG. 1). At this time, the electrodes 21 and 22 are formed in a state in which the fired surface is maintained on the surface without subjecting the piezoelectric element body to polishing or the like.

In step S10, a DC high voltage is applied to the formed electrodes to polarize the piezoelectric element main body (lead-free piezoelectric ceramic composition). It should be noted that the polarization can be performed in the air or in an inert gas as well as in the insulating oil.

As described above, the piezoelectric element 1 having the polarized piezoelectric element body 10 is completed.
According to this manufacturing method, since the first and second calcined powders are produced in steps S1 to S4, there is an advantage that the composition and ratio of the first crystal phase and the second crystal phase can be easily controlled.

A sample relating to the piezoelectric element 1 described above was produced, and analyzed and evaluated. The following description will be made with reference to FIGS. 3A to 12. FIG.

A disk-shaped piezoelectric element body 10 according to the example as shown in FIG. 3A was manufactured according to steps S1 to S8 described in the manufacturing method described above. Subsequently, step S9 is performed to form electrodes 21 and 22 by baking electrode layers made of Ag on substantially the entire surfaces of the two circular plate surfaces of the piezoelectric element main body 10, respectively. , 22 was formed, and the piezoelectric element 1 as shown in FIG. 3B was manufactured by polarizing the piezoelectric element main body 10 in the thickness direction.

In the above, in step S1, K ₂ CO ₃ powder, Na ₂ CO ₃ powder, Li ₂ CO ₃ powder, CaCO ₃ powder, BaCO ₃ powder, Nb ₂ O ₅ powder, TiO ₂ powder, and ZrO ₂ powder are prepared, For the first crystal phase represented by the composition formula ((K _a Na _b Li _c Ca _{d1 Bad2} ) _e (Nb _f1 , Ti _f2 , Zr _f3 )O _3+h ), a=0.35, b=0. 53, c = 0.02, d1 = 0.04, d2 = 0.06, e = 1.00, f1 = 0.91, f2 = 0.03, f3 = 0.06 , the first raw material was weighed.
Further, in step S3, Fe ₂ O ₃ powder, CoO powder, ZnO powder, and TiO ₂ powder are prepared, and the second crystal phase represented by the composition formula (Fe, Co, Zn, Ti) O ₄ is mixed with Fe:Co :Zn:Ti=0.30:0.30:0.40:1.00.
In step S8, when obtaining the disk-shaped piezoelectric element body 10 made of the lead-free piezoelectric ceramic composition, the piezoelectric element body 10 was formed into a bulk shape with a diameter of 30 mm and a thickness of 4 mm by a press molding machine. Example 2 was obtained by forming a sheet having a thickness of 100 μm with an extruder and then punching out a sheet having a diameter of 30 mm.

The inside of the piezoelectric element main body 10 according to Example 1 manufactured as described above was observed with a microscope, and the particle size distribution was measured.
Specifically, the piezoelectric element main body 10 according to Example 1 was polished to remove the surface and tested for measurement. Using a laser microscope VK-X100 manufactured by Keyence Corporation, a region containing the main phase and the subphase was observed. Further, using a reflective microscope JSM-6390LA manufactured by JEOL, the main phase portion was observed in a field of view of 65 μm×45 μm. A laser microscopic photograph is shown in FIG. 4A, and a SEM (scanning electron microscope) photograph of the main phase portion is shown in FIG. 4B.

As shown in FIG. 4A, inside the piezoelectric element body 10, the second crystalline phase, which looks black, is scattered and arranged in a light-colored main phase formed of the first crystalline phase. ing. Further, from FIG. 4B, it can be seen that although vacancies are observed in the first crystal phase, the size of the vacancies is relatively small and the vacancy rate is not high.

Subsequently, composition analysis was performed on the main phase and subphase portions inside the piezoelectric element main body 10 according to Example 1. FIG.
Specifically, the piezoelectric element main body 10 according to Example 1 was polished to remove the surface, and the element distribution of the region containing the main phase and the subphase was investigated using an EPMA (electron probe microanalyzer). Results are shown in FIG.
Among the 12 images, the images in the first row are, from the left, the backscattered electron image of the investigation area and the elemental distribution images of O, Na, and K. The images on the second row are elemental distribution images of Ca, Ti, Fe, and Co from the left. The images on the third row are elemental distribution images of Zn, Zr, Nb, and Ba from the left. Li is not investigated because it is difficult to measure with EPMA. As can be understood from the backscattered electron image and the like, most of the investigation area (image) is the main phase. (also shown in black) are located. Small black dots distributed in the main phase are vacancies or pits.

As can be understood from the images in FIG. 5, Na, K, and Nb are present in large amounts in the main phase, but the amount of Na, K, and Nb in the subphase is smaller than that in the main phase. I understand. On the other hand, with respect to Ca, Ba, and Zr, since the amount of addition itself is small, it is difficult to understand from the image, but these elements are present in the main phase, but these elements are almost absent in the subphase. It turns out. On the other hand, it can be seen that Ti is contained in the main phase in a small amount, but is contained in the subphase in a relatively large amount. Also, it can be seen that Fe, Co, and Zn are contained in the subphase but not in the main phase.
From these, it is considered that the main phase is formed of perovskite crystals (first crystal phase) represented by the composition formula (K, Na, Li, Ca, Ba)(Nb, Ti, Zr)O ₃ . Also, the subphase is considered to be formed of spinel crystals (second crystal phase) represented by the composition formula (Fe, Co, Zn, Ti)O ₄ .

Subsequently, each sample was subjected to structural analysis by XRD (X-ray diffraction method).
First, structural analysis of the main phase portion and the sub-phase portion inside the piezoelectric element main body 10 according to Example 1 was performed.
Specifically, the piezoelectric element main body 10 according to Example 1 was polished to remove the surface and tested for analysis. A micro X-ray diffractometer was used as an XRD measuring instrument, and analysis was performed in the range of 2θ = 20° to 90° using CuKα rays. Results are shown in FIG.

Two peak profiles are also displayed in FIG. 6 for reference. Of these, the peak profile in the upper row is for potassium sodium niobate K _0.48 Na _0.52 NbO ₃ which is a perovskite crystal, and the peak profile in the lower row is for iron titanium oxide Fe _2.50 Ti _0.50 O ₄ which is a spinel crystal.
The observed X-ray _intensity distribution _shows almost the same waveform for both the main phase portion and _the subphase portion. They are almost identical. From this, it can be understood that the main phase portion contains potassium sodium niobate K _0.48 Na _0.52 NbO ₃ or perovskite-type crystal grains having a composition similar thereto.

On the other hand, in the observed X-ray intensity distribution, the portion indicated by the black arrow is a peak that does not exist in the X-ray intensity distribution of the main phase portion but exists in the X-ray intensity distribution of the subphase portion. Since this peak almost coincides with the peak of the iron-titanium oxide Fe _2.50 Ti _0.50 O ₄ for reference (lower row), the subphase portion contains iron-titanium oxide Fe _2.50 Ti _0.50 O ₄ or a composition similar thereto. of spinel-type crystal grains are included.

From these points as well, the main phase is formed of the first crystal phase composed of perovskite crystals represented by the composition formula (K, Na, Li, Ca, Ba) (Nb, Ti, Zr) _O3 . Conceivable. Also, the sub-phase is considered to be formed of a second crystal phase composed of spinel-type crystals represented by the composition formula (Fe, Co, Zn, Ti)O ₄ .

Next, structural analysis of the inside and surface of the bulk piezoelectric element main body 10 according to Example 1 was performed.
Specifically, for the "inside", the piezoelectric element main body 10 according to Example 1 was polished to remove the surface and tested for analysis in the same manner as in the above analysis. As for the "surface", the surface of the piezoelectric element main body 10 according to Example 1 was analyzed without being polished or the like, but in the state of being sintered in step S8. The results are shown in Figures 7A-7C.
7B and 7C are partial enlarged views of FIG. 7A, and FIGS. 7A to 7C also show the peak profile of cobalt titanium oxide Co ₂ TiO ₄ which is a spinel crystal for reference. .

As shown in FIGS. 7B and 7C, the profile for the interior of Example 1 clearly shows a peak at the same position as the reference spinel crystal, whereas the profile for the surface of Example 1 , no peak was observed at the same position. From this, the inside of the piezoelectric element main body 10 contains spinel crystals having a composition similar to Co ₂ TiO ₄ , whereas the surface of the piezoelectric element main body 10 does not contain spinel crystals. I understand.

Next, structural analysis of the inside and surface of the sheet-like piezoelectric element main body 10 according to Example 2 was performed.
For the analysis, the surface of the piezoelectric element main body 10 according to Example 2 was removed and the "inside" was analyzed in the same manner as described above, and the "surface" was analyzed with the fired surface left. The results are shown in Figures 8A and 8B. Note that FIG. 8B is a partially enlarged view of FIG. 8A.

As shown in FIG. 8B, the profile for the interior of Example 2 has a peak at the same position as the reference spinel crystal, whereas the profile for the surface of Example 2 has a peak at the same position. No peak was observed. This result also shows that the surface of the piezoelectric element main body 10 does not contain spinel crystals. 7A to 7C relating to Example 1 rather than FIGS. It is presumed that in the case of Example 1, which has a bulk shape, the analysis of the interior was performed in a state in which the effects of the surface were more eliminated than in Example 2.

Subsequently, the bending strength of the sheet-like piezoelectric element main body 10 according to Example 2 was measured using a load cell.
For this measurement, a plurality of samples were prepared and tested in which the average grain size _D50 of the second crystal phase was varied in three steps by changing the temperature conditions during the preparation of the piezoelectric element main body 10 . The average particle diameter _D50 of the second crystal phase is a value obtained by calculating the volume cumulative median diameter using a laser diffraction/scattering particle size distribution analyzer LA-950V2 manufactured by Horiba, Ltd. Results are shown in FIG.

As can be seen from FIG. 9, the piezoelectric element body 10 of Example 2 all exhibited a high bending strength of ₅₀ MPa or more. was found to show In particular, by setting the average grain size _D50 of the second crystal phase to 50 μm or less, an excellent bending strength of 80 MPa or more is exhibited. It is presumed that the smaller the average grain size _D50 of the second crystal phase, the easier it is for the holes in the first crystal phase to be filled, the denser the structure inside the piezoelectric element body 10, and the higher the strength. be done.

Subsequently, for the samples in which the average grain size _D50 of the second crystal phase was varied in three stages, the average grain size of the first crystal phase existing inside and the second crystal phase was investigated for the relationship between the average particle size of
Specifically, the piezoelectric element body 10 according to Example 2, in which the average grain size _D50 of the second crystal phase was varied in three steps, was polished to remove the surface, and then subjected to measurement. For the first crystal phase, a reflective microscope JSM-6390LA manufactured by JEOL was used to observe in a field of view of 65 μm × 45 μm, and for the second crystal phase, a laser microscope VK-X100 manufactured by Keyence Corporation was used. Observation was carried out in a field range in which about 100 particles of the second crystal phase appearing black were observed. Then, using the image analysis software PhotoRuler, the average particle size _D50 of the first crystal phase and the average particle size (black particle size) _D50 of the second crystal phase were calculated. Results are shown in FIG.

From FIG. 10, it can be seen that in these samples, the average grain size _D50 of the first crystal phase is represented by a positive linear function with respect to the average grain size _D50 of the second crystal phase. The average grain size _D50 of the first crystal phase of the samples that showed good results in the bending test was 4.5 μm or less, and the average grain size _D50 of the first crystal phases of the samples that showed better results. is 3.8 μm or less.

Subsequently, the reliability test of the piezoelectric element 1 having the piezoelectric element main body 10 according to Example 2 was performed. Specifically, electrodes 21 and 22 each made of Ag are formed on the sheet-like piezoelectric element main body 10 according to the second embodiment in step S9, and polarization is performed in step S10 to fabricate the piezoelectric element 1. A vibration plate was bonded to this piezoelectric element 1 to prepare a buzzer element, which was subjected to a reliability test. In the reliability test, the buzzer element was driven for 504 hours in a constant temperature and humidity chamber at a temperature of 85° C. and a humidity of 85%, and the crack occurrence rate after the test was investigated. The results are shown in the table of FIG.

As can be seen from the table of FIG. 11, the crack generation rate of the buzzer element according to the example was relatively low, 55% or less, in spite of the severe test. In particular, no cracks were observed in the samples in which the average grain size _D50 of the second crystal phase was 34 μm or less, confirming that the use of such piezoelectric elements can provide devices with extremely high reliability. was done. Regarding these, referring to the relationship between the average grain size _D50 of the first crystal phase and the average grain size _D50 of the second crystal phase shown in FIG. The average grain size _D50 of the first crystalline phase is 4.0 μm or less, and the average grain size _D50 of the first crystalline phase of the sample showing better results is 3.5 μm or less. As described above, the piezoelectric element 1 in which the average grain size _D50 of the first crystal phase in the piezoelectric element main body 10 is 3.5 μm or less and the average grain size _D50 of the second crystal phase is 34 μm or less is reliable. It can be seen that a particularly excellent device can be provided for

As described above, the piezoelectric element 1 according to this embodiment includes the piezoelectric element main body 10 made of a lead-free piezoelectric ceramic composition, and the pair of electrodes 21 and 22 provided on the surface of the piezoelectric element main body 10 . The lead-free piezoelectric ceramic composition of element 1 includes a main phase formed of a first crystal phase composed of an alkaline niobate perovskite oxide having piezoelectric properties, and an M--Ti--O-based spinel compound (element M is and a secondary phase formed of a second crystal phase composed of a monovalent to tetravalent element), and in the piezoelectric element body 10, the surface is formed only by the main phase, and the interior is composed of the main phase and the secondary phase. It is formed including phases.
In such a configuration, since grain boundaries of crystals made of different components do not exist on the surface of the piezoelectric element main body 10, the durability is lower than that of a configuration in which different components are mixed over the entire piezoelectric element main body 10. Crack resistance is improved. Moreover, the main phase and the sub-phase are mixed inside the piezoelectric element main body 10, so that the piezoelectric element main body 10 can exhibit good piezoelectric characteristics as compared with a configuration in which the entire piezoelectric element main body 10 is composed only of the main phase. As a result, it is possible to obtain the piezoelectric element 1 having excellent reliability by improving the crack resistance of the piezoelectric element main body 10 while exhibiting good piezoelectric properties. Moreover, in the piezoelectric element 1 having such a configuration, the bending strength is improved as compared with a configuration in which the main phase and the sub-phase are mixed over the entire piezoelectric element main body 10 . Therefore, it is possible to obtain the piezoelectric element 1 that is hard to break and has high strength.

Further, in the piezoelectric element 1 according to this embodiment, the piezoelectric element main body 10 is preferably formed in a flat plate shape. With this configuration, the advantage of not having grain boundaries of different components on the surface of the piezoelectric element main body 10, that is, the effect of improving the above-described crack resistance and the like can be made even more remarkable.

In addition, in the piezoelectric element 1 according to this embodiment, the content of the second crystal phase inside the piezoelectric element body is preferably 0.5% by volume or more and 5.0% by volume or less. With such a configuration, it is possible to obtain the piezoelectric element 1 having well-balanced strength and piezoelectric characteristics.

Moreover, in the piezoelectric element 1 according to the present embodiment, it is preferable that the average grain size _D50 of the first crystal phase is 3.5 μm or less, and the average grain size _D50 of the second crystal phase is 34 μm or less. With this configuration, the possibility of cracks occurring in the piezoelectric element main body 10 is extremely reduced, and a highly reliable piezoelectric element 1 can be obtained.

DESCRIPTION OF SYMBOLS 1... Piezoelectric element 10... Piezoelectric element main body 21, 22... Electrode

Claims (4)

a piezoelectric element main body made of a lead-free piezoelectric ceramic composition;
A piezoelectric element comprising a pair of electrodes provided on the surface of the piezoelectric element main body,
The lead-free piezoelectric ceramic composition is
a main phase formed of a first crystal phase composed of an alkaline niobate-based perovskite oxide having piezoelectric properties;
a subphase formed of a second crystal phase made of an M-Ti-O-based spinel compound (element M is a monovalent to tetravalent element),
In the piezoelectric element main body,
The surface is formed only by the main phase,
A piezoelectric element, the inside of which is formed to contain the main phase and the sub-phase. 2. The piezoelectric element according to claim 1, wherein said piezoelectric element main body is formed in a flat plate shape. 3. The piezoelectric element according to claim 1, wherein the content of said second crystal phase in said interior of said piezoelectric element main body is 0.5% by volume or more and 5.0% by volume or less. 4. The piezoelectric element body according to claim 1, wherein the first crystal phase has an average grain size of 3.5 μm or less, and the second crystal phase has an average grain size of 34 μm or less. The piezoelectric element according to .

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