Bismuth potassium titanate piezoelectric body, method for producing the same, piezoelectric element, and piezoelectric functional device

Abstract

The present invention produces an ideal potassium bismuth titanate piezoelectric body by preventing volatilization of Bi and K, and provides an excellent lead-free piezoelectric body, a piezoelectric element and a piezoelectric functional device. A piezoelectric body which is characterized by: containing potassium bismuth titanate that has a tetragonal perovskite structure; being uniaxially oriented in the polarization axis direction; and satisfying the requirement such that the lattice constant ratio c / a is larger than 1.040 and / or the requirement such that the Curie temperature is higher than 380°C. A method for producing a potassium bismuth titanate piezoelectric body, wherein a substrate is immersed in a water-containing solvent that contains potassium hydroxide, a bismuth starting material and a titanium starting material, and heat and pressure are sequentially applied thereto in a reaction container, thereby depositing potassium bismuth titanate having a tetragonal perovskite structure on the substrate.

Description

Technical Field

[0001] The present invention relates to a bismuth potassium titanate piezoelectric body, a method for manufacturing the same, a piezoelectric element, and a piezoelectric functional device.

Background Art

[0002] Piezoelectric bodies are widely used in actuator elements, pressure sensors, ultrasonic vibrators, noise filters, etc. Recently, attention has also been focused on their application to vibration power generation devices. As the currently used piezoelectric bodies, lead zirconate titanate (PZT)-based ones are the mainstream. However, from the perspective of environmental load, lead-free piezoelectric bodies are being explored. Among various lead-free piezoelectric bodies, the bismuth potassium titanate (BKT)-based piezoelectric body having a tetragonal crystal structure can theoretically have excellent piezoelectric properties, mechanical coupling properties, and a high Curie temperature, and thus can be a candidate for the above applications.

[0003] However, an ideal single crystal of bismuth potassium titanate has not been obtained. Bismuth potassium titanate is manufactured by sintering powders of bismuth potassium titanate. However, the obtained sintered body is polycrystalline, and there is a problem that bismuth and potassium volatilize because sintering is performed at a high temperature of 500 to 1200°C. The polarization value of the piezoelectric body is only about 30 μC / cm 2 and far from 75 μC / cm 2 of PZT, and even does not reach 35 μC / cm 2 of barium titanate (BT). Therefore, neither practical application nor development has been carried out.

[0004] It has been proposed to improve the piezoelectric properties by forming a film of bismuth potassium titanate by pulsed laser deposition (PLD) method (Patent Document 1). However, the PLD method is a method of depositing very slowly and is used for research purposes, but it is difficult to use for commercial production of piezoelectric bodies. Furthermore, even in the PLD method, the deposition temperature is a high temperature of 500°C or higher, the problem of volatilization of bismuth and potassium is not completely solved, the obtained piezoelectric body contains many secondary phases, and it is difficult to form a thick film, so the target piezoelectric body properties have not been obtained. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-79948 A Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, an object of the present invention is to solve the above problems, to provide a theoretical or near-theoretical bismuth potassium titanate piezoelectric material and a method for manufacturing the same, and to provide a lead-free piezoelectric material, piezoelectric element, and piezoelectric functional device having excellent piezoelectric properties. [Means for solving the problem]

[0007] As a result of extensive efforts to achieve the above object, the present inventors have found that the above object can be achieved by synthesizing a bismuth potassium titanate piezoelectric material using a hydrothermal synthesis method, and have thus completed the present invention.

[0008] The present invention includes the following aspects. (Aspect 1) A piezoelectric body comprising bismuth potassium titanate having a tetragonal perovskite structure, uniaxially oriented in the direction of the polarization axis, and satisfying the following requirements i) and / or ii): i) The lattice constant ratio c / a is greater than 1.040. ii) The Curie temperature is higher than 380°C. (Aspect 2) 2. The piezoelectric body according to embodiment 1, wherein the rocking curve half-width of the X-ray analysis peak of bismuth potassium titanate is 25° or less. (Aspect 3) iii) The piezoelectric body according to aspect 1 or 2, which contains HO or OH groups, and the content of the HO or OH groups is 10,000 ppm or less based on the total weight of the piezoelectric body. (Aspect 4) A piezoelectric body according to any one of aspects 1 to 3, which satisfies one or more of the following requirements: iv) Polarization value is 35 μC / cm 2 That's all. v) Remanent polarization of the PE loop (P r ) and saturation polarization (P sat ) ratio (squareness ratio) is 0.8 or more. vi) relative permittivity (ε r ) is less than or equal to 200. vii) Dielectric loss (tan δ) is 0.2 or less. viii) Piezoelectric strain constant (d 33 ) is 30 to 100 pm / V. ix) The withstand voltage is 300 kV / cm or more. (Aspect 5) 5. The piezoelectric body according to any one of aspects 1 to 4, wherein the piezoelectric body is self-polarized and the polarization direction is aligned without any polarization treatment.

[0009] (Aspect 6) A method for producing a bismuth potassium titanate piezoelectric body, comprising: immersing a substrate in a water-containing solvent containing potassium hydroxide, a bismuth raw material, and a titanium raw material in a reaction vessel; and applying heat and pressure to the substrate to form a film of bismuth potassium titanate having a tetragonal perovskite structure on the substrate. (Aspect 7) The molar ratio of potassium hydroxide to the bismuth raw material and the titanium raw material was 1:1.0×10 -5 ~1:1.0×10 2 The method of producing according to embodiment 6, wherein (Aspect 8) The molar ratio of the bismuth raw material to the titanium raw material is 1:1.0×10 -5 ~1:1.0×10 3 The method according to aspect 6 or 7, wherein (Aspect 9) A method according to any one of Aspects 6 to 8, wherein the concentration of potassium hydroxide in the water-containing solvent is 0.1 to 30 mol / L. (Aspect 10) The method according to any one of embodiments 6 to 9, wherein the substrate has a perovskite-based crystal structure. (Aspect 11) The method according to any one of aspects 6 to 10, wherein the substrate is made of a material selected from semiconductors, metals, plastics, and ceramics, and has a buffer layer with a perovskite-based crystal structure on its surface. (Aspect 12) 12. The method according to any one of aspects 6 to 11, wherein the substrate is a conductive substrate. (Aspect 13) The method according to any one of aspects 6 to 12, wherein the substrate has a surface that includes a flat surface and / or a curved surface. (Aspect 14) A method according to any one of Aspects 6 to 13, wherein the reaction vessel is a sealed vessel, and the temperature inside the reaction vessel is heated to a temperature of 50 to 300°C. (Aspect 15) The method according to any one of aspects 6 to 14, wherein the heating is carried out using microwaves. (Aspect 16) A method according to any one of Aspects 6 to 15, wherein the bismuth potassium titanate is annealed at a temperature of 100 to 750°C after being removed from the water-containing medium.

[0010] (Aspect 17) A piezoelectric element comprising a pair of electrodes sandwiching a piezoelectric body, the piezoelectric body comprising bismuth potassium titanate having a tetragonal perovskite structure, uniaxially oriented in the direction of the polarization axis, and satisfying the following requirements i) and / or ii): i) The lattice constant ratio c / a is greater than 1.040. ii) The Curie temperature is higher than 380°C. (Aspect 18) 18. The piezoelectric element according to embodiment 17, wherein the rocking curve half-width of the X-ray analysis peak of bismuth potassium titanate of the piezoelectric material is 25° or less. (Aspect 19) iii) The piezoelectric element according to aspect 17 or 18, wherein the piezoelectric body contains HO or OH groups, and the content of the HO or OH groups is 10,000 ppm or less based on the total weight of the piezoelectric body. (Aspect 20) 20. The piezoelectric element according to any one of aspects 17 to 19, wherein the piezoelectric body satisfies one or more of the following requirements: iv) Polarization value is 35 μC / cm 2 That's all. v) Remanent polarization of the PE loop (P r ) and saturation polarization (P sat ) ratio (squareness ratio) is 0.8 or more. vi) relative permittivity (ε r ) is less than or equal to 200. vii) Dielectric loss (tan δ) is 0.2 or less. viii) Piezoelectric strain constant (d 33 ) is 30 to 100 pm / V or more. ix) The withstand voltage is 300 kV / cm or more. (Aspect 21) 21. The piezoelectric element according to any one of aspects 17 to 20, wherein the piezoelectric body is self-polarized and the polarization direction is aligned without polarization treatment.

[0011] (Aspect 22) A piezoelectric functional device comprising the piezoelectric element according to any one of aspects 17 to 20, the piezoelectric functional device being selected from a noise filter, a medical ultrasonic probe, an ultrasonic transmitter, an ultrasonic sensor, a pyroelectric power generation device, a vibration power generation device, and an actuator. (Aspect 23) The piezoelectric function is a noise filter, and the noise filter has a polarization value of 35μC / cm 2 or more, squareness ratio 0.8 or more, relative permittivity (ε r ) is 200 or less, and the piezoelectric strain constant (d 33 23. The piezoelectric functional device according to aspect 22, comprising a piezoelectric body having a resistivity of 30 to 100 pm / V or more. [Effects of the Invention]

[0012] The potassium bismuth titanate provided by the present invention is an oriented crystal of potassium bismuth titanate closer to the theoretical potassium bismuth titanate, particularly a single crystal, compared with the conventionally obtained potassium bismuth titanate. The potassium bismuth titanate piezoelectric body provided by the present invention can have excellent piezoelectric characteristics, such as polarization value, rectangular ratio, or relative permittivity, compared with the conventionally obtained potassium bismuth titanate piezoelectric body. The potassium bismuth titanate piezoelectric body of the present invention is a lead-free piezoelectric body and has excellent piezoelectric characteristics, so a piezoelectric body, a piezoelectric element, and a piezoelectric device that can replace conventional lead zirconate titanate (PZT) can be provided. Since the potassium bismuth titanate piezoelectric body of the present invention can be manufactured at low temperature, it can be formed into a film on a substrate such as plastic that cannot be used at high temperature. In addition, the potassium bismuth titanate piezoelectric body of the present invention can be manufactured at low temperature but has excellent heat resistance in that its characteristics do not deteriorate even when annealed at high temperature.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0014] (Bismuth potassium titanate piezoelectric) In a first aspect, the present invention provides a piezoelectric body containing bismuth potassium titanate having a tetragonal perovskite structure, uniaxially oriented in the polarization axis direction, and satisfying the following requirements i) and / or ii). i) The lattice constant ratio c / a is greater than 1.020. ii) The Curie temperature is higher than 380°C.

[0015] In the sintering method, which is a conventionally known method for manufacturing bismuth potassium titanate piezoelectric, polycrystals are obtained. In both the sintering method and the PLD method, since synthesis is performed at a high temperature, bismuth and potassium volatilize, and it has not been possible to manufacture theoretical bismuth potassium titanate (crystals). However, according to the present invention, by using the hydrothermal method, it is possible to manufacture uniaxially oriented crystals, particularly single crystals, and since synthesis is performed at a low temperature of 300°C or lower, volatilization of bismuth and potassium can be prevented, and it has been found that it is possible to manufacture bismuth potassium titanate that is ideal or at least closer to an ideal one than those conventionally known.

[0016] The bismuth potassium titanate of the present invention is (Bi 0.5 K 0.5)It is represented by TiO3 and has a tetragonal perovskite structure. Perovskite-type oxides are represented by ABO3, and potassium bismuth titanate is a compound (crystal) in which Bi and K are present at the A site and Ti is present at the B site. In potassium bismuth titanate, the total number of Bi and K atoms is the same as the number of Ti atoms, and the number of Bi and K atoms is also the same. Oxygen defects may or may not be present. The potassium bismuth titanate of the present invention is produced by a hydrothermal method, so it may contain H2O or OH groups, but even if the H2O or OH groups are removed or reduced, compared with the prior art BKT, it has an ideal or nearly ideal BKT crystal structure.

[0017] When the OH group is present, it is considered to be mainly present at the A site. In that case, the total number of Bi and K atoms is smaller than the number of Ti atoms. Also, the OH group may be present at the oxygen site. In this case, the number of K atoms is larger than the number of Bi atoms, and the total number of Bi and K atoms is smaller than the number of Ti atoms. Although there is a possibility that it exists at the K site as an H2O molecule, it is considered to be low due to the relationship of charge compensation. The content of H2O or OH groups depends on the production conditions, but is 10,000 ppm or less, further 8,000 ppm or less, 7,000 ppm or less, 5,000 ppm or less, 4,000 ppm or less, 3,000 ppm or less based on the total weight of potassium bismuth titanate. The content of H2O or OH groups can be annealed to any content, 1,000 ppm or less, 100 ppm or less, particularly 0 ppm.

[0018] Hereinafter, potassium bismuth titanate is represented as (Bi 0.5 K 0.5 )TiO3 or (Bi,K)TiO3 or BKT.

[0019] Perovskite-type oxides are oxides represented by ABO3, and potassium bismuth titanate (Bi 0.5 K 0.5 )TiO3 can take a crystal structure other than tetragonal as a perovskite-type oxide, but the potassium bismuth titanate piezoelectric body of the present invention has a tetragonal perovskite structure.

[0020] Figure 1 shows the tetragonal perovskite structure (space group P4mm) of bismuth potassium titanate. In Figure 1, six oxygen atoms surround one titanium atom, forming a regular octahedron with oxygen atoms at the corners, and these regular octahedra are arranged vertically and horizontally in a three-dimensional crystal structure. Bismuth and potassium atoms are located in the spaces between the octahedra (space surrounded by eight regular octahedra), and the bismuth and potassium atoms form planes that are aligned parallel to the rows (planes) of oxygen and titanium atoms. It is a tetragonal crystal, and the angles between the a-axis, b-axis, and c-axis, which are the crystal lattice axes, are all right angles (90°), and the crystal lattice constant is a=b <cである。c軸方向に電荷が偏位していることによって、圧電特性が発揮される。したがって、一般的には、c / a比が大きい方が分極値もより大きくなり得る。

[0021] The piezoelectric bismuth potassium titanate of the present invention is uniaxially oriented in the direction of the polarization axis. When another crystalline film is grown on a crystalline substrate, a uniaxially oriented film is defined as a film in which one crystal axis of the crystal between the crystalline film and the crystalline substrate approximately coincides, while an epitaxial film is defined as a film in which two crystal axes of the crystal approximately coincide. In the present invention, uniaxial orientation encompasses epitaxial orientation. It is also possible to form uniaxially oriented films formed by "local epitaxial growth" in which each crystal grain grows epitaxially, or single-crystal epitaxial films in which the epitaxially grown crystal grains have a substantial size. The uniaxially oriented or epitaxially oriented film grown on the substrate may be separated from the substrate used for growth to form a single uniaxially oriented or epitaxially oriented film, or the separated uniaxially oriented or epitaxially oriented film may be bonded to another film or substrate. The uniaxially oriented or epitaxially oriented potassium titanate piezoelectric material exhibits excellent piezoelectric properties.

[0022] The orientation direction of the bismuth potassium titanate film of the piezoelectric material of the present invention may be any direction corresponding to the substrate, and may be, for example, (100), (110), (111), or the like.

[0023] The uniaxial orientation of the piezoelectric material in the polarization axis direction results in significantly superior polarization characteristics compared to polycrystalline sintered bodies. Even polycrystalline materials can be oriented in a certain direction as an average crystal orientation by polarization treatment (applying a predetermined voltage in a specific direction), but polycrystalline materials have limitations in orientation, which also restricts their piezoelectric characteristics. In contrast, the preferred piezoelectric materials of the present invention are uniaxially oriented in the polarization axis direction, particularly single crystals, which allows for the ideal polarization characteristics of bismuth potassium titanate to be achieved. The polarization direction is the c-axis direction described with reference to FIG. 1. The c-axis is the axis with a larger lattice constant than the other two axes (a-axis and b-axis) with the same lattice constant. Uniaxial orientation can be confirmed by analyzing the crystal orientation using X-ray diffraction. Preferably, the half-width of the rocking curve of the X-ray diffraction peak is 25° or less. More preferably, the half-width may be 20° or less, 15° or less, or 10° or less. The narrower the half-value width, the better, and may be, for example, 0.005° or more, 0.010° or more, or 0.03° or more. The piezoelectric body of the present invention is preferably uniaxially oriented in the polarization axis direction, but the piezoelectric body of the present invention may also be polycrystalline.

[0024] When the piezoelectric body of the present invention is a film formed on a substrate having an electrode, the orientation direction of the piezoelectric body is preferably perpendicular to the surface of the substrate. Since the piezoelectric body of the present invention is produced by a hydrothermal method, it is initially grown as a film on the surface of the substrate, but the film thickness can be increased, and the grown piezoelectric body can also be separated from the substrate for use. In the case of a piezoelectric body separated from a substrate, it is sufficient that the polarization axis is oriented in one direction of the bulk piezoelectric body, for example.

[0025] Conventionally, the lattice constant of bismuth potassium titanate is given by GOJones, et al., Powder Diffraction, 17. 301 (2002): a=3.9388Å α=90.00°, b=3.9388Å β=90.00°, c=3.9613Å γ=90.00° (c / a=1.006) It has been reported that, according to V.V. Ivanova, et al., Izv. Akad. Nauk SSSR, Ser. Fiz. 26, 354 (1962), a = 3.913 Å, α = 90.00°, b = 3.913 Å, β = 90.00°, c = 3.990 Å, γ = 90.00° (c / a = 1.020) It has been reported that Regarding the physical properties and characteristics of bismuth potassium titanate (bulk ceramics), according to Tadashi Takekusa, Special Lecture at the 36th Conference on Applications of Ferroelectrics (2019), Curie temperature: 380 °C, Depolarization temperature: 280 °C, Polarization value: 30 μC / cm 2 , d 33 100 pm / V, Kt: 0.42 It has been reported that

[0026] In a preferred embodiment, the bismuth potassium titanate piezoelectric of the present invention may have a lattice constant ratio c / a greater than 1.040. Since the lattice constant of the bismuth potassium titanate piezoelectric of the present invention is greater than the lattice constant ratio c / a of the conventionally reported bismuth potassium titanate, it is considered to be a crystallographically different crystal and a different substance from the conventionally known bismuth potassium titanate piezoelectric. Since the lattice constant of the bismuth potassium titanate piezoelectric of the present invention is greater than the lattice constant ratio c / a of the conventionally reported bismuth potassium titanate, its piezoelectric properties can also be superior to those of conventional piezoelectrics. The lattice constant ratio c / a of the bismuth potassium titanate of the present invention may be 1.040 or more, 1.044 or more, 1.050 or more, 1.053 or more. Also, it may be 1.070 or less, 1.065 or less. The lattice constant ratio c / a of bismuth potassium titanate is determined by analyzing the crystal structure by X-ray analysis, calculating the lattice constants a and c, and obtaining the ratio therefrom.

[0027] In a preferred embodiment, the bismuth potassium titanate piezoelectric material of the present invention may have a Curie temperature higher than 380 °C. Since the Curie temperature of the bismuth potassium titanate piezoelectric material of the present invention is higher than that of the conventionally reported bismuth potassium titanate, it is considered to be a substance different from the conventionally known bismuth potassium titanate. Since the Curie temperature of the bismuth potassium titanate of the present invention is higher than that of the conventionally reported bismuth potassium titanate, it can exhibit piezoelectric properties even at higher temperatures and is preferably excellent in heat resistance. The Curie temperature of the bismuth potassium titanate of the present invention may be 390 °C or higher, 400 °C or higher, 450 °C or higher, 500 °C or higher, 600 °C or higher, 700 °C or higher, and may also be 750 °C or lower, 700 °C or lower. The Curie temperature of bismuth potassium titanate is determined at the temperature at which the crystal phase becomes cubic with the change in crystal structure by high-temperature X-ray analysis, or at the temperature at which the slope of the decrease in lattice constant c with temperature change changes.

[0028] In a preferred embodiment, the bismuth potassium titanate piezoelectric material of the present invention contains H2O or OH groups in the crystal, and the content of H2O or OH groups may be 10,000 ppm or less based on the total weight of the piezoelectric material (or bismuth potassium titanate). Since the lattice constant ratio c / a and the Curie temperature of the bismuth potassium titanate of the present invention are basically maintained even when the H2O or OH groups in the crystal are removed, it is a substance different from the conventionally known bismuth potassium titanate. When bismuth potassium titanate contains H2O or OH groups, it has the effect of improving piezoelectric properties and is preferable. The content of H2O or OH groups in the bismuth potassium titanate of the present invention may be 10,000 ppm or less, 7,000 ppm or less, 4,000 ppm or less, 1,000 ppm or less, 500 ppm or less, 0 ppm. It may also be more than 0 ppm, 100 ppm or more, 1,000 ppm or more, 2,000 ppm or more, 3,000 ppm or more, 6,000 ppm or more. The content of H2O or OH groups is measured using the temperature-programmed desorption gas analysis method. 10 -4Heat up potassium bismuth titanate in a vacuum below, and measure, as a function of temperature, the intensity of chemical species desorbing from potassium bismuth titanate using a mass spectrometer, particularly the intensity of m / z = 18 derived from H2O or OH groups. Obtain the minimum value from the measured curve, and add up the differences from the intensity values at each temperature from the start to the end of the measurement to obtain the total. Multiply these total intensity values by the conversion factors for each component to calculate the quantitative values, and divide them by the mass of potassium bismuth titanate to determine the H2O or OH group content in potassium bismuth titanate.

[0029] The potassium bismuth titanate piezoelectric material of the present invention is, in a preferred embodiment, a novel and useful substance in that potassium bismuth titanate has the above i) lattice constant ratio c / a and / or ii) Curie temperature, and further iii) H2O or OH group content.

[0030] Furthermore, the potassium bismuth titanate of the present invention is a piezoelectric material. A piezoelectric material is a substance that exhibits polarization (surface charge) proportional to the applied pressure (force) when pressure is applied, and conversely deforms when an electric field is applied. Among piezoelectric materials, there are pyroelectric materials having pyroelectricity and ferroelectric materials having ferroelectricity. A pyroelectric material is a substance whose dielectric polarization (surface charge) changes due to a temperature change. A ferroelectric material refers to a substance in which electric dipoles are aligned even without an external electric field, and the direction of the dipoles can be changed by an electric field. Pyroelectric materials and ferroelectric materials are known to exhibit piezoelectricity.

[0031] The potassium bismuth titanate piezoelectric material of the present invention is, in a preferred embodiment, novel and useful in that it is a piezoelectric material that satisfies any one or more of the following iv) to ix). iv) The polarization value is 35 μC / cm 2 or more. v) The ratio (square ratio) of the remanent polarization (P r ) to the saturation polarization (P sat ) of the P-E loop is 0.80 or more. vi) The relative permittivity (ε r ) is 200 or less. vii) The dielectric loss (tanδ) is 0.2 or less. viii) The piezoelectric strain constant (d 33 ) is 30 to 100 pm / V or more. ix) The breakdown voltage is 300 kV / cm or more. Note that the requirements in iv) to ix) above are all values at room temperature (for example, 25 °C).

[0032] In a preferred embodiment, the bismuth potassium titanate piezoelectric body of the present invention has iv) a polarization value of 35 μC / cm 2 or more. The polarization value refers to the remanent polarization (P r ). The bismuth potassium titanate piezoelectric body of the present invention has a polarization value higher than the polarization value of 30 μC / cm of bismuth potassium titanate reported conventionally. 2 Therefore, it is different from the conventionally known bismuth potassium titanate piezoelectric body. Since the bismuth potassium titanate piezoelectric body of the present invention has a large polarization value, it can exhibit excellent piezoelectric characteristics, which is preferable. The polarization value of the bismuth potassium titanate piezoelectric body of the present invention is 40 μC / cm 2 or more, 50 μC / cm 2 or more, 60 μC / cm 2 or more, 70 μC / cm 2 or more, 80 μC / cm 2 or more, and may be 90 μC / cm 2 or more. Also, it may be 160 μC / cm 2 or less, 150 μC / cm 2 or less. The polarization value is measured by fabricating a structure in which the piezoelectric body is sandwiched between a pair of upper and lower electrodes, applying a voltage to the piezoelectric body, and calculating the polarization value and the electric field from the voltage and the capacitance. The curve representing the change in the polarization value and the electric field is called a polarization-electric field (P-E) loop. After the polarization value saturates with respect to the applied voltage, the polarization value obtained when the maximum voltage is applied is defined as the saturation polarization value. Then, when the applied electric field is applied in the reverse direction, the polarization value when the value of the electric field is 0 kV / cm is measured as the remanent polarization value.

[0033] In a preferred embodiment, the bismuth potassium titanate piezoelectric body of the present invention has v) the remanent polarization (P r ) and the saturation polarization (P sat) may be 0.80 or more. The bismuth potassium titanate piezoelectric body of the present invention is characterized in that the rectangular ratio is significantly larger than the rectangular ratio of the bismuth potassium titanate piezoelectric body reported in the past. It is different from the conventionally known bismuth potassium titanate piezoelectric body and can exhibit extremely excellent piezoelectric characteristics, so it is preferable. The rectangular ratio of the bismuth potassium titanate piezoelectric body of the present invention may be 0.85 or more, 0.90 or more, 0.95 or more. Such a high rectangular ratio belongs to a high value even compared with piezoelectric bodies other than bismuth potassium titanate. The rectangular ratio can be obtained by measuring the hysteresis loop of the electric field and polarization applied to the piezoelectric body and calculating from the P-E loop. The measurement method of the P-E loop was described in the measurement method of the polarization value. The rectangular ratio is defined as the ratio of the residual polarization value obtained when a voltage is applied after the polarization value has saturated with respect to the applied voltage and the saturation polarization value.

[0034] In a preferred embodiment, the bismuth potassium titanate piezoelectric body of the present invention has vi) relative permittivity (ε r ) may be 200 or less. The relative permittivity refers to the ratio of the permittivity of a substance to the permittivity of a vacuum. The bismuth potassium titanate piezoelectric body of the present invention has a relative permittivity (ε r ) smaller than the relative permittivity of bismuth potassium titanate reported in the past, so it is different from the conventionally known bismuth potassium titanate piezoelectric body. Since the bismuth potassium titanate piezoelectric body of the present invention has a small relative permittivity, it has a small dielectric loss and can exhibit excellent piezoelectric characteristics, which is preferable. The relative permittivity of the bismuth potassium titanate piezoelectric body of the present invention may be 150 or less, 100 or less, 80 or less. Also, it may be 10 or more, 30 or more. The measurement frequency of the relative permittivity may be from 2 Hz to 10 MHz, particularly 5 Hz to 1 MHz, 100 Hz to 10 kHz. The relative permittivity can be calculated by using an impedance analyzer to continuously change the measurement frequency on the piezoelectric body, applying a minute alternating voltage of 1 V or less, measuring the capacitance of the piezoelectric body, and using the film thickness, electrode area, and permittivity of a vacuum obtained in advance.

[0035] In a preferred embodiment, the potassium bismuth titanate piezoelectric body of the present invention may have a dielectric loss (tanδ) of 0.2 or less. The measurement frequency of the dielectric loss (tanδ) may be from 1 Hz to 30 MHz. Dielectric loss refers to the degree of electrical energy loss that occurs within a dielectric when an alternating electric field is applied to the dielectric. The potassium bismuth titanate piezoelectric body of the present invention has a smaller dielectric loss than the dielectric loss of conventionally reported potassium bismuth titanate, so it is different from the conventionally known potassium bismuth titanate piezoelectric body. Since the potassium bismuth titanate piezoelectric body of the present invention has a small dielectric loss, it can be used in a relatively wide frequency band, which is preferable. The dielectric loss of the potassium bismuth titanate piezoelectric body of the present invention may be 0.15 or less, 0.1 or less, 0.05 or less. Also, it may be 0.001 or more. Dielectric loss is measured using an impedance analyzer by applying a minute alternating voltage of 1 V or less while continuously changing the measurement frequency to the piezoelectric body and measuring the current within the piezoelectric body. The measurement frequency of the dielectric loss may be from 2 Hz to 10 MHz, particularly from 5 Hz to 1 MHz, 100 Hz to 10 kHz. In the case of an ideal capacitor, the phase difference between the applied voltage and the current within the piezoelectric body is 90°, but in reality, it is (90 - δ)°, and the dielectric loss is measured as the dielectric loss tanδ from this value.

[0036] In a preferred embodiment, the potassium bismuth titanate piezoelectric body of the present invention, viii) the piezoelectric strain constant (d 33 ) may be from 30 to 100 pm / V. The piezoelectric strain constant refers to the constant obtained by dividing the amount of strain generated when a voltage is applied to the piezoelectric body by the voltage. Since the potassium bismuth titanate piezoelectric body of the present invention has a large piezoelectric constant, it can exhibit excellent piezoelectric characteristics, which is preferable. The piezoelectric strain constant of the potassium bismuth titanate piezoelectric body of the present invention may be 60 pm / V or more, 70 pm / V or more, 80 pm / V or more. Also, it may be 120 pm / V or less, 95 pm / V or less. The piezoelectric strain constant (d 33 ) can be obtained by measuring the curve of the voltage applied to the piezoelectric body and the displacement amount and calculating the slope of the applied voltage and the displacement amount. The measurement frequency is from 1 to 500 MHz, 10 to 100 kHz, particularly 10 kHz is preferable.

[0037] In a preferred embodiment, the bismuth potassium titanate piezoelectric material of the present invention may have an electric field resistance of ix) 300 kV / cm or more. The electric field resistance refers to the maximum electric field that can be applied. The bismuth potassium titanate piezoelectric material of the present invention has a large piezoelectric constant and therefore exhibits excellent piezoelectric properties, which is preferable. The electric field resistance of the bismuth potassium titanate piezoelectric material of the present invention may be 400 kV / cm or more, 500 kV / cm or more, or 600 kV / cm or more. It may also be 1500 kV / cm or more. The electric field resistance can be determined by performing PE measurements while changing the electric field.

[0038] The bismuth potassium titanate piezoelectric material of the present invention can satisfy one or more of the above physical properties iii) to ix), and is a useful lead-free piezoelectric material in that it satisfies one or more of the physical properties iii) to ix).

[0039] The bismuth potassium titanate piezoelectric material of the present invention is an excellent bismuth potassium titanate piezoelectric material in that it can satisfy one or more of the above physical properties iii) to ix). However, the physical properties of the bismuth potassium titanate piezoelectric material of the present invention, such as the polarization value, squareness ratio, relative permittivity, dielectric loss, piezoelectric strain constant, and withstand voltage, are not limited to the ranges of the above properties i) to ix).

[0040] In one embodiment, the bismuth potassium titanate piezoelectric material of the present invention may be formed as a film on a substrate. The substrate may be in the form of a plate, a block, or particles. By using bismuth potassium titanate produced by the production method of the present invention described below as a substrate and growing bismuth potassium titanate on its surface, it is possible to produce plate, block, or particle-shaped bismuth potassium titanate.

[0041] The substrate having the bismuth potassium titanate piezoelectric material of the present invention on its surface may be any of ceramics, metal, plastic, etc. When it is difficult to form a film of the bismuth potassium titanate piezoelectric material directly on the substrate, the film may be formed via a buffer layer. The bismuth potassium titanate piezoelectric material of the present invention can be formed into a film at a low temperature of 300°C or less, so that it can be used on a substrate that cannot be used at high temperatures.

[0042] The substrate preferably has a perovskite crystal structure, particularly a perovskite oxide. Perovskite oxides are oxides represented by the formula ABO3 (where A is selected from Li, Na, K, Rb, Mg, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, and B is selected from Mg, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, In, Sn, Hf, Ta, W, Ir, Pb, and Bi. A and B may be plural, and the oxide includes a solid solution.), such as BaTiO3, PbTiO3, KNbO3, and PbVO3.

[0043] Other substrates having a perovskite-based crystal structure include Cu3Au structure, ReO3 structure, K2NiF4 structure, Sr3Ti2O7 structure, and Sr4Ti3O 10 Structure, Bi4Ti3O 12 These include substrates with the NaCl structure, diamond structure, zinc blende structure, ZnS structure, high-temperature cristobalite structure, CaF2 structure, C-rare earth structure, and Y2O3 structure.

[0044] Furthermore, if the lattice constant of the substrate used in the present invention is the same as or approximate to the lattice constant of the potassium bismuth titanate film to be formed, it is preferable because the lattice matching is high. The difference between the lattice constant of the substrate and that of potassium bismuth titanate is preferably 10% or less, more preferably 5% or less. Since the lattice constant of potassium bismuth titanate is about 0.3890 to 0.3973 nm, a substrate having a lattice constant of about 0.3501 to 0.4370 nm, more preferably about 0.3696 to 0.4172 nm is preferable. High lattice matching between the substrate and the film is preferable because a uniaxially oriented film or even an epitaxial film can be formed. In addition, high lattice matching between the substrate and the film has the advantage of increasing the film thickness that can be formed and improving the film formation rate.

[0045] The substrate used in the present invention preferably exhibits conductivity. When the substrate exhibits conductivity, it becomes easier to form potassium bismuth titanate film. Particularly preferred is a substrate having a perovskite crystal structure that exhibits conductivity, such as Nb:SrTiO3, La:SrTiO3, etc.

[0046] Also, examples of materials having lattice constants and chemical species close to those of the film include, for example, KTaO 3や LaAlO3, etc. A substrate having lattice constants and chemical species close to those of the film and a conductive substrate (buffer layer) can be preferably combined.

[0047] The substrate used in the present invention may be a substrate having a perovskite crystal structure, or may be a substrate made of other ceramics, metal, plastic, etc. When using these substrates, it is preferable to include a buffer layer having a perovskite crystal structure and further having conductivity on the substrate surface. Examples of such buffer layers include SrRuO3, (Ba,Sr)RuO3, LaNiO3, La2NiO5, (La,Sr)CoO3, CaRuO3, and (Ca,Sr)RuO3.

[0048] (Method for manufacturing potassium bismuth titanate piezoelectric body: hydrothermal method) As a second aspect, the present invention provides a novel method for manufacturing a bismuth potassium titanate piezoelectric body. The manufacturing method according to the second aspect of the present invention is a hydrothermal method. The hydrothermal method is characterized in that bismuth potassium titanate can be produced at a low temperature, particularly 300 °C or lower, and is excellent as a manufacturing method for bismuth potassium titanate containing volatile bismuth and potassium.

[0049] The manufacturing method according to the second aspect of the present invention is a method for manufacturing a bismuth potassium titanate piezoelectric body, characterized in that a substrate is immersed in a water-containing solvent containing an alkali hydroxide, a bismuth raw material, and a titanium raw material in a reaction vessel, and heated and pressurized to form a film of bismuth potassium titanate on the substrate.

[0050] FIG. 2 schematically shows a cross-section of an example of a reaction apparatus used in the method for manufacturing a bismuth potassium titanate piezoelectric body of the present invention. In FIG. 2, a water-containing solvent, here water 2, is contained in a closed reaction vessel 1, and potassium hydroxide is dissolved in the water 2, and a bismuth raw material and a titanium raw material, for example, bismuth nitrate (Bi(NO3)3·5H2O) and titanium dioxide TiO2 are added as powders 3 and 4 in this example. Further, a substrate 5 is suspended from above into the reaction vessel 1 and immersed in water (alkali aqueous solution) 2.

[0051] In the reaction apparatus shown in FIG. 2, when the closed reaction vessel 1 is heated, the inside of the reaction vessel 1 is pressurized as it is heated, the solubility of the bismuth raw material 3 and the titanium raw material 4 in the alkali aqueous solution 2 increases, and they are gradually dissolved in the potassium hydroxide aqueous solution 2. At the same time, due to a heterogeneous reaction, heterogeneous nuclei of bismuth potassium titanate are generated on the surface of the substrate 5, and further film formation of bismuth potassium titanate thereon proceeds to form a film of bismuth potassium titanate on the surface of the substrate 5.

[0052] In the present invention, the reaction vessel is a sealed vessel that can heat and pressurize the inside of the vessel, which contains a water-containing solvent, an alkali hydroxide, and a bismuth raw material / titanium raw material in the water-containing solvent, and can be a vessel called an autoclave. The reaction vessel also has a structure capable of immersing one or more substrates in the water-containing solvent in the vessel, but the method of holding the substrates is not limited. For example, it can be a structure attached to a fixture 6 installed on the lid of the vessel. The direction of the substrate 5 immersed in the water-containing solvent can be appropriately set, such as vertical or horizontal.

[0053] The water-containing solvent used in the present invention (manufacturing method) is a solvent containing water, and in addition to water, it may be a mixed solvent of water and an organic solvent, but ion-exchanged water is particularly preferred. As the organic solvent used in mixture with water, an organic solvent that is soluble or miscible with water, such as alcohol, ketone, carboxylic acid, and ether, is preferred. By reducing the water content in the solvent, the amount of lattice OH ions, which are the most concerning impurities in the product in hydrothermal synthesis, can be reduced. Also, by using a water-containing solvent and changing the concentration of water in the solvent, the amount of water (OH ions) incorporated into potassium bismuth titanate produced can be changed, and the degree of self-polarization of potassium bismuth titanate can be controlled. Note that the amount of water (OH - ) incorporated can be reduced or controlled by increasing the film-forming temperature.

[0054] The concentration of potassium hydroxide in the water used in the present invention may be 0.1 to 30 mol / L, 0.1 to 20 mol / L, 2 to 8 mol / L, particularly 8 to 15 mol / L, and the upper and lower limit values within these ranges may be combined independently. When the concentration of potassium hydroxide is within these ranges, the reactivity with the bismuth raw material / potassium raw material increases, and it is considered that the nucleation and film-forming rate of potassium bismuth titanate are high, and the film thickness can also be increased.

[0055] In the present invention, potassium hydroxide may be dissolved in water (in the following description, the water may be a water-containing solvent) in the reaction vessel during the reaction (heating and pressurization), and may be previously dissolved in the water to be injected into the reaction vessel, or added to and dissolved in the water contained in the reaction vessel, or a combination thereof may be used.

[0056] The bismuth raw material used in the present invention refers to a single substance, a solid solution, or a mixture of substances capable of supplying bismuth ions in an aqueous potassium hydroxide solution, such as oxides, nitrates, hydroxides of bismuth, etc., and these may be hydrates.

[0057] The titanium raw material used in the present invention refers to a single substance, a solid solution, or a mixture of substances capable of supplying titanium ions in an aqueous potassium hydroxide solution, such as oxides, nitrates, titanium butoxide, titanium(IV) bis(ammonium lactate) dihydroxide of titanium, etc., and these may be hydrates.

[0058] Titanium oxide, particularly titanium dioxide, is preferably used as the titanium raw material, and the titanium dioxide may be of the anatase type or the rutile type. The anatase type is excellent in that a highly oriented film can be obtained, but the rutile type has the advantage of a higher film formation rate than the anatase type.

[0059] In the present invention, the addition amount of the bismuth raw material is such that the molar ratio of potassium hydroxide to the bismuth raw material may be 1:1.0×10 -5 ~1:1.0×10 2 This amount is preferable because when the addition amount of the bismuth raw material is more than the lower limit value of this range, a certain amount of bismuth dissolved in the solution can be present. When the amount is less than the upper limit value of this range, the dissolved bismuth is present in the solution without causing homogeneous nucleation, which is also preferable. The above molar ratio may be 1:1.0×10 -5 ~1:1.0×10 2 、1:1.0×10 -4 ~1:1.0×10 3 、and further may be 1:1.0×10 -2 ~1:1.0 -1

[0060] The molar ratio of the bismuth raw material to the titanium raw material may be from 1:1.0×10 -5 to 1:1.0×10 3 It may be. The above molar ratio is from 1:1.0×10 -3 to 1:1.0×10 2 or from 1:1.0×10 -1 to 1:1.0×10, or even from 1:0.2 to 1:8.

[0061] In the present invention, the addition amount of the titanium raw material may be an amount such that the molar ratio of potassium hydroxide to the titanium raw material is from 1:1.0×10 -5 to 1:1.0×10 2 It is preferable that when the addition amount of the titanium raw material is more than the lower limit value of this range, a certain amount of titanium dissolved in the solution can exist. When the addition amount is less than the upper limit value of this range, the dissolved titanium exists in the solution without causing homogeneous nucleation, which is preferable. The above molar ratio is from 1:1.0×10 -5 to 1:1.0×10 2 or from 1:1.0×10 -4 to 1:1.0×10 1、 or even from 1:1.0×10 -3 to 1:1.0×10 -1 It may be.

[0062] In the present invention, it is preferable that the addition amounts of the bismuth raw material and the titanium raw material are the same number of moles or close to the same number of moles. However, if the molar ratio to potassium hydroxide is within the above range, they may be different numbers of moles.

[0063] It is preferable that the bismuth raw material and the titanium raw material are in powder form because their solubility (reactivity with the potassium hydroxide aqueous solution) in the potassium hydroxide aqueous solution during heating and pressurization is high, but they may be in other forms such as lumps.

[0064] The bismuth raw material and the titanium raw material may be placed in the reaction vessel in advance, and water (or an alkaline aqueous solution) may be added to the reaction vessel, or water (or a potassium hydroxide aqueous solution) may be placed in the reaction vessel, and then the bismuth raw material / titanium raw material may be added thereto.

[0065] In the manufacturing method of the present invention, the obtained bismuth potassium titanate is a crystal represented by the formula (Bi 0.5 K 0.5 )TiO3, but it can contain oxides such as CaO, CuO, MnO2, Sb2O3, BaO, ZrO2, and TiO2, and thereby characteristics such as piezoelectric characteristics can be improved. By adding these oxides into an alkaline aqueous solution, they are mainly incorporated as a solid solution and / or a mixture into the bismuth potassium titanate film after hydrothermal synthesis. A composite oxide may be formed. The addition amount of these oxides is not particularly limited as long as it is an amount that improves the characteristics of (Bi 0.5 K 0.5 )TiO3, and depends on the type of oxide. Based on (Bi 0.5 K 0.5 )TiO3, it may be 30% by weight or less, for example, 1 to 20% by weight, 1 to 10% by weight is preferable, but sometimes 2% by weight or more or 0.01 to 1% by weight is preferable. It is preferable that the addition amount of these oxides is an amount known to be soluble in (Bi 0.5 K 0.5 )TiO3 represented by the above formula. However, even if the addition amount is within the solid solubility limit, not all of it needs to be solid-solved and it may form a mixture.

[0066] In the manufacturing method of the present invention, the substrate on which (Bi 0.5 K 0.5 )TiO3 is formed into a film is not limited, but it is preferably a substrate having a perovskite crystal structure. When the substrate has a perovskite crystal structure, since the crystal structure is the same as that of bismuth potassium titanate, it is easy to form a bismuth potassium titanate film, and furthermore, a uniaxially oriented film or an epitaxial film can also be formed, which is preferable.

[0067] Examples of the substrate having a preferably used perovskite crystal structure include perovskite-type oxides, which are oxides represented by ABO3 (wherein A is selected from Li, Na, K, Rb, Mg, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, etc., and B is selected from Mg, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, In, Sn, Hf, Ta, W, Ir, Pb, Bi, etc. A and B can be plural, and the oxide includes a solid solution.), and examples thereof include BaTiO3, PbTiO3, KNbO3, PbVO3, etc.

[0068] In addition, examples of other substrates having a perovskite crystal structure include those having a Cu3Au structure, ReO3 structure, K2NiF4 structure, Sr3Ti2O7 structure, and Sr4Ti3O 10 structure, Bi4Ti3O 12 structure, those having a tungsten bronze structure, or substrates having an NaCl structure, diamond structure, sphalerite structure, ZnS structure, high-temperature cristobalite structure, CaF2 structure, C-rare earth structure, Y2O3 structure, etc.

[0069] Furthermore, it is preferable for the crystal lattice constant of the substrate used in the present invention to be the same as or similar to the crystal lattice constant of the bismuth potassium titanate to be produced, as this provides high lattice matching. The difference between the crystal lattice constant and the crystal lattice constant of bismuth potassium titanate is preferably 10% or less, and more preferably 5% or less. Since the crystal lattice constant of bismuth potassium titanate is approximately 0.3890 to 0.3973 nm, a substrate having a crystal lattice constant of approximately 0.3501 to 0.4370 nm, more preferably 3670 to 0.4172 nm, even more preferably 3773 to 0.4092 nm, and particularly preferably 0.3851 to 0.4012 nm is preferred. High lattice matching between the substrate and bismuth potassium titanate is preferred because it allows the formation of uniaxially oriented films and even epitaxial films. Furthermore, high lattice matching between the substrate and bismuth potassium titanate increases the amount of bismuth potassium titanate that can be formed into a film, thereby improving the film formation rate.

[0070] The substrate used in the present invention is preferably conductive. When the substrate is conductive, it becomes easier to form a bismuth potassium titanate film. In particular, a substrate having a conductive perovskite-based crystal structure is preferred, such as Nb:SrTiO3 or La:SrTiO3.

[0071] Furthermore, even if the substrate (or buffer layer) does not exhibit electrical conductivity, it is possible to increase the amount of bismuth potassium titanate and further improve the growth rate by using a substrate (or buffer layer) having a perovskite-based crystal structure such as SrTiO3 whose lattice constant and chemical species are similar to those of the film. Examples of materials whose lattice constant and chemical species are similar to those of bismuth potassium titanate include SrTiO3. Using SrTiO3 as the substrate increases the amount of bismuth potassium titanate obtained compared to materials such as LaAlO3 and KTaO3, which have the same perovskite-based crystal structure. A substrate whose lattice constant and chemical species are similar to those of bismuth potassium titanate can be preferably combined with a conductive substrate (buffer layer).

[0072] The substrate used in the present invention may be a substrate having a perovskite-based crystal structure, or may be a substrate made of other ceramics, metal, plastic, or the like. When using such a substrate, it is preferable that the substrate surface has a perovskite-based crystal structure and further includes a buffer layer that is electrically conductive. Examples of such a buffer layer include SrRuO3, (Ba,Sr)RuO3, LaNiO3, La2NiO5, (La,Sr)CoO3, CaRuO3, and (Ca,Sr)RuO3. In particular, the hydrothermal synthesis method of the present invention allows film formation at low temperatures below 300°C, even at 250°C or lower, so the substrate can be made of plastic, and the substrate can be curved and flexible.

[0073] As described above, the substrate can be immersed in the water (or potassium hydroxide aqueous solution) in the reaction vessel using an appropriate holding means. Since the present invention is a hydrothermal synthesis method, it is preferable that the surface of the substrate on which a film is to be formed is immersed in water (or potassium hydroxide aqueous solution). The surface of the substrate on which a film is not to be formed may be covered with a protective film or a masking agent.

[0074] Furthermore, by combining an area on the substrate surface where it is easy to form a film of bismuth potassium titanate with an area where it is difficult to form a film, selective growth of bismuth potassium titanate is possible. Furthermore, since the present invention uses a hydrothermal synthesis method, it is possible to form a film of bismuth potassium titanate even if the surface of the substrate is not flat but curved. In the present invention, it is possible to form a uniaxially oriented or epitaxially oriented potassium bismuth titanate film even if the surface of the substrate is not only flat but also curved.

[0075] In a reaction vessel, water (which may be a water-containing solvent as described above), potassium hydroxide dissolved in the water, the bismuth raw material / titanium raw material added to the water, and the substrate immersed in the water are prepared in any order, and the reaction vessel is then sealed and heated, thereby pressurizing the reaction vessel and causing bismuth potassium titanate to precipitate on the substrate, forming a film.

[0076] When potassium hydroxide reacts with bismuth raw material / titanium raw material, when using (Bi(NO3)3·5H2O) and TiO2 as the bismuth raw material / titanium raw material, 4KOH + Bi(NO3)3·5H2O + 2TiO2 ⇒2(Bi 0.5 K 0.5 )TiO3 + 3KNO3 + 7H2O Also, when expressing the bismuth raw material / titanium raw material as converted oxides, the following reaction formula: 2KOH + Bi2O3 + 4TiO2 ⇒ 4(Bi 0.5 K 0.5 )TiO3 + 2H2O is the reaction represented by.

[0077] Potassium bismuth titanate in powder form without forming a film on the substrate may be reused, for example, by regenerating it into the bismuth raw material / titanium raw material.

[0078] Potassium bismuth titanate for forming a film on the substrate can be a uniaxially oriented film, or even an epitaxial film.

[0079] The heating temperature of the reaction is not limited, but can be a low temperature of 300°C or lower. The lower limit may be room temperature (about 20°C) or higher, but generally a range of 50 - 300°C, and more preferably 100 - 250°C is preferred. In this way, according to the hydrothermal synthesis method, the reaction temperature can be made lower than the Curie temperature of potassium bismuth titanate, so there are no drawbacks such as cracks occurring during cooling after production, and high-quality potassium bismuth titanate (film) can be obtained.

[0080] When increasing the film-forming temperature, the amount of lattice OH ions, which are the most concerning impurities in the product in hydrothermal synthesis, can be reduced, and the amount of lattice OH - can be controlled by the film-forming temperature, and the piezoelectric properties can be adjusted.

[0081] In one aspect of the present invention, it was confirmed that even when the heating temperature of the reaction is less than 100 °C, high-quality potassium bismuth titanate (film) can be obtained as well as in the case of 100 °C or higher. If the temperature is less than 100 °C, there is a possibility that the hydrothermal synthesis method, which was previously at high pressure, can reduce the pressure to near atmospheric pressure, which is preferable in terms of productivity. It may be room temperature (about 20 °C) or higher and less than 100 °C, for example, 40 °C or higher and 80 °C or lower. When forming a film on a metal substrate or an organic device with a high coefficient of thermal expansion, it is preferable in terms of reducing the thermal strain applied to the film.

[0082] According to the manufacturing method of the present invention, although the film formation temperature is lower than the Curie temperature of potassium bismuth titanate, a potassium bismuth titanate oriented film that self-polarizes and exhibits excellent piezoelectric properties without polarization treatment can be obtained. However, the potassium bismuth titanate piezoelectric body of the present invention is characterized by a crystal structure having a specific lattice constant ratio c / a and / or a Curie temperature, and it does not necessarily have to be self-polarized.

[0083] Heating inside the reaction vessel may be performed by irradiating microwaves in addition to using an autoclave. According to microwave heating, the film formation rate is significantly improved compared to normal heating. For example, as a reaction device, an alkali-resistant and pressure-resistant reaction vessel (a double container of Teflon (registered trademark) / PEEK) is installed in a reaction device capable of microwave heating (for example, "flexiWAVE", Milestone General (registered trademark)), microwaves are irradiated onto the water placed in the reaction vessel, and it may be heated to the set temperature while observing the temperature inside the container with an optical fiber.

[0084] The pressure during the reaction may be a pressure that causes the pressure inside the reaction vessel to rise by heating a closed reaction vessel. Usually, it is considered to be about 3.0×10 3 ~9.0×10 6 Pa gauge, but it is not limited.

[0085] Thus, when the inside of the reaction vessel is heated in a state where potassium hydroxide, a bismuth raw material / titanium raw material, and a substrate are present in water inside the reaction vessel, the pressure inside the closed reaction vessel rises, the bismuth raw material / titanium raw material dissolves, and potassium bismuth titanate is formed into a film on the substrate surface by a heterogeneous reaction.

[0086] The reaction time may be any time as long as the raw materials can react and is not limited. Although it depends on the manufacturing equipment and raw material composition, in a single batch process, for example, it may be about 1 to 72 hours, more preferably about 1.5 to 36 hours, about 1.5 to 24 hours, but particularly about 5 to 30 minutes or about 5 to 10 minutes may also be acceptable.

[0087] Potassium bismuth titanate produced by the manufacturing method of the present invention can prevent bismuth and potassium from volatilizing during production compared to potassium bismuth titanate produced by conventional sintering methods or PLD methods. Therefore, the obtained potassium bismuth titanate can be closer to an ideal potassium bismuth titanate. Also, the generation of secondary phases such as bismuth titanate, potassium titanate, bismuth oxide, and titanium oxide can be prevented. Thus, its piezoelectric properties can be significantly superior to those of the conventional method and it is useful as a lead-free piezoelectric material to replace PZT.

[0088] In addition, the manufacturing method of the present invention can be carried out at a low temperature such as 300°C or lower, so there is an advantage that a substrate with low heat resistance can be used as the substrate.

[0089] Potassium bismuth titanate (film) produced by the manufacturing method of the present invention is an oriented film in which potassium bismuth titanate represented by the formula (Bi, K)TiO3 is uniaxially oriented or epitaxially oriented. Although the crystal of potassium bismuth titanate can preferably be a single crystal, it may also contain heterogeneous phases.

[0090] The potassium bismuth titanate film formed by the manufacturing method of the present invention has excellent orientation and can be self-polarized (the polarization directions are aligned even without polarization treatment). Since it is self-polarized, it can be used as a piezoelectric element without poling polarization treatment. The orientation of the potassium bismuth titanate alignment film can also be improved by selecting a film formation substrate.

[0091] The obtained potassium bismuth titanate can be post-annealed, and according to the post-annealing, the crystallinity is improved. The temperature of the post-annealing may be, for example, 100°C to less than 800°C, 100°C to 750°C, particularly 500 to 700°C. Even if the potassium bismuth titanate of the present invention is annealed at a high temperature close to 800°C despite containing bismuth and potassium, it does not lose its excellent piezoelectric properties.

[0092] The potassium bismuth titanate obtained by the manufacturing method of the present invention is an extremely rare piezoelectric material in which self-polarization remains even when the temperature is raised to 700°C or higher, and has the characteristic that it is not necessary to perform polarization treatment for use as a piezoelectric.

[0093] According to the manufacturing method of the present invention, a potassium bismuth titanate piezoelectric body containing uniaxially oriented or epitaxially oriented crystals, a piezoelectric element using the same, and a piezoelectric functional device are provided.

[0094] The potassium bismuth titanate piezoelectric body obtained by the present invention is applied to various piezoelectric elements and the like. Typically, as shown in the example of the piezoelectric element 10 in FIG. 3, the substrate 11 has a lower electrode 12, and on it, a potassium bismuth titanate film 13 and further an upper electrode 14. However, a buffer layer 15 may be provided between the substrate 11 and the lower electrode 12 as necessary.

[0095] The potassium bismuth titanate piezoelectric body obtained by the manufacturing method of the present invention is widely applied to noise filters, piezoelectric actuator elements, pressure sensors, ultrasonic vibrators, vibration power generation devices, and the like.

[0096] (Alignment film, piezoelectric element, piezoelectric functional device) According to a third aspect of the present invention, there is provided a piezoelectric element and a piezoelectric functional device configured using a bismuth potassium titanate piezoelectric material represented by the formula (Bi,K)TiO3.

[0097] In a third aspect of the present invention (hereinafter also simply referred to as the present invention), the bismuth potassium titanate piezoelectric material may be one produced by the manufacturing method of the second aspect or the bismuth potassium titanate piezoelectric material described in the first aspect.

[0098] The bismuth potassium titanate piezoelectric material of the third aspect of the present invention may be a uniaxially or epitaxially oriented crystal (film). When a crystalline film is grown on a crystalline substrate, a uniaxially oriented film is defined as a film in which one crystal axis of the crystal between the crystalline film and the crystalline substrate substantially coincides with the growth axis of the crystal, while an epitaxial film is defined as a film in which two crystal axes of the crystal substantially coincide with the growth axis. It is also possible to form a uniaxially oriented film formed by "local epitaxial growth" in which each crystal grain grows epitaxially, or a single-crystal epitaxial film in which the epitaxially grown crystal grains have a substantial size. The uniaxially or epitaxially oriented film grown on a substrate may be separated from the substrate used for growth to form a uniaxially or epitaxially oriented film alone, or the separated uniaxially or epitaxially oriented film may be bonded to another film or substrate. By using a uniaxially or epitaxially oriented film as a bismuth potassium titanate film, excellent piezoelectric properties can be achieved.

[0099] The bismuth potassium titanate piezoelectric material provided in the second aspect of the present invention may be an oriented film having a thickness of, but not limited to, 10 nm to several mm, 30 nm to 100 μm, or even 50 nm to 20 μm. The bismuth potassium titanate piezoelectric material of the present invention may be in the form of a block having a minimum dimension of several mm to several tens of mm or more.

[0100] The bismuth potassium titanate oriented film may be formed on a substrate including a curved surface, and new applications (piezoelectric functional devices) that could not be realized with piezoelectric elements using conventional bismuth potassium titanate piezoelectric materials can be realized.

[0101] Further, in the third aspect of the present invention, as a new piezoelectric functional device using the novel potassium bismuth titanate and potassium bismuth titanate piezoelectric body of the present invention, for example, a noise filter, a medical ultrasonic probe, an ultrasonic transmitter, an ultrasonic sensor, a pyroelectric power generation device, a vibration power generation device, an actuator, etc. are provided.

[0102] Examples of the noise filter include a SAW (Surface Acoustic Wave) filter and a bulk acoustic wave filter. The bulk acoustic wave filter is a high-frequency filter that utilizes the resonance vibration of a piezoelectric film called a bulk acoustic wave. For example, there is a structure in which a cavity is provided below the resonator to facilitate the vibration of the piezoelectric film. Currently, SAW (surface acoustic wave) filters are often used for high-frequency filters.

[0103] A SAW filter is a passive component that converts a high-frequency signal input as an electrical signal into a surface wave with a wavelength of about several micrometers by the piezoelectric effect of a piezoelectric substrate, propagates the surface wave on the piezoelectric substrate, filters the desired frequency, and then extracts it as an electrical signal again. The frequency bands used in mobile communications such as mobile phones are centered around 100 MHz to 3 GHz, which are the VHF band and the UHF band. For various frequencies, the center frequency and bandwidth are determined and corresponding by changing the interval and length of the "comb-shaped electrodes" and the physical properties of the piezoelectric body and electrodes. As shown in Fig. 4(a), on the surface of the piezoelectric substrate 21, a comb-shaped electrode 22 on the excitation side with an electrode finger period λ and a comb-shaped electrode 23 on the reception side are patterned with a thin metal film such as Al, and SAW is excited by the comb-shaped electrode 22 and received by the comb-shaped electrode 23 using the piezoelectric effect, so that electrical input and output can be performed. Through this series of operations of exciting and receiving SAW by this comb-shaped electrode, only the frequency component fo in the relationship of fo = v / λ is selected, so that a filter function can be provided. Here, v is the vibration speed of the element. Fig. 4(b) shows a 1-port SAW filter, Fig. 4(c) shows a 2-port SAW filter, and it is an example in which reflectors 33 are provided on both sides of the SAW propagation direction of the comb-shaped electrode 32 in Fig. 4(b), the excitation electrode 34 and the reception electrode 35 in Fig. 4(c).

[0104] An ultrasonic probe is a component that transmits ultrasonic waves and receives the reflected ultrasonic waves for ultrasonic transmission and reception in an ultrasonic inspection device that displays them as images and blood flow information. The potassium bismuth titanate orientation film of the present invention has excellent piezoelectric properties such as polarization value, squareness ratio, heat resistance, relative permittivity, tanδ, etc. compared with the conventional potassium bismuth titanate orientation film, and there is a possibility that it can also be substituted for PZT as a lead-free piezoelectric body.

[0105] FIG. 5 schematically shows an example of the ultrasonic probe 40. The ultrasonic probe 40 includes a backing material 41, a vibrator (piezoelectric element) 42, an acoustic matching layer 43, and an acoustic lens 44. The backing material 41 is installed on the back surface of the vibrator, and serves to absorb the backward propagation of ultrasonic waves, suppress unnecessary vibrations, and shorten the pulse width of ultrasonic waves. The vibrator 42 is a component that transmits and receives ultrasonic waves. The acoustic matching layer 43 is a member that inserts a material having an intermediate acoustic impedance between the vibrator 42 and the living body because the acoustic impedance of the vibrator 42 is large compared to that of the living body, and ultrasonic waves would be reflected as they are, thereby minimizing reflections. The acoustic lens 44 has the role of focusing the ultrasonic beam, and silicone rubber is often used.

[0106] A medical ultrasonic probe is a medical ultrasonic diagnostic device that transmits ultrasonic waves to a living body such as the human body in a medical application, receives the reflected ultrasonic waves, and displays them as an image or blood flow information. The part that transmits and receives ultrasonic waves is the probe.

[0107] An ultrasonic transmitter is a transmitting electroacoustic transducer that converts an electrical signal into an acoustic vibration and radiates a sound wave into a medium. In many cases, it is also used as a receiving transducer and has the function of converting an acoustic vibration into an electrical signal.

[0108] An ultrasonic sensor is a sensor that emits ultrasonic waves, receives the ultrasonic waves reflected from an object and returning, detects the target object, measures the time until the waves return, and measures the distance to the target object. As a transmitter, a signal voltage is applied to a vibrator (piezoelectric element), and ultrasonic waves with the resonant vibration frequency of the vibrator are radiated into the air from a speaker (transmitter). As a receiver, the wave motion of ultrasonic waves from the air is received by a microphone (receiver), and the vibrator generates an electrical output. The transmitter and the receiver together are called an ultrasonic transducer (electroacoustic conversion element). In principle, one element serves as both the transmitter and the receiver for the electroacoustic conversion element. However, the vibration amplitude of the air is significantly different between transmission and reception, and it is more efficient to change the impedance. Therefore, it is normal to use separate transducers. A microcomputer is used to control the transmitter and the receiver and perform detection and distance measurement. The potassium bismuth titanate orientation film of the present invention has a higher output compared to the potassium bismuth titanate orientation film manufactured by the conventional hydrothermal synthesis method, and may have a high high-amplitude output comparable to PZT, and is promising as an ultrasonic sensor using a lead-free piezoelectric element. Since such an ultrasonic sensor has a high output, it can be advantageously used for detecting obstacles and measuring distances in automobiles.

[0109] Regarding a pyroelectric power generation device, referring to an example of the pyroelectric power generation device 50 schematically shown in FIG. 6, the pyroelectric element 51 is formed by sandwiching a ferroelectric 52 between electrodes 53. When a heat source 54 that changes with time acts on the pyroelectric element 51, a voltage that varies in the ferroelectric 52 is generated corresponding to the temperature change, and power generation is performed. Since the potassium bismuth titanate orientation film of the present invention has high piezoelectric characteristics compared to the potassium bismuth titanate orientation film manufactured by the conventional hydrothermal synthesis method, it is promising as a pyroelectric power generation device using a lead-free piezoelectric element.

[0110] A vibration-powered energy harvester is a power generator that converts the vibration of a vibrator (piezoelectric element) into electrical power when a mechanical external force or vibration is applied to the vibrator. The bismuth potassium titanate oriented film of the present invention has a higher output than oriented films of bismuth potassium titanate produced by conventional hydrothermal synthesis, and its output is closer to that of PZT, making it a promising vibration-powered energy harvester using a lead-free piezoelectric element.

[0111] An actuator is a device that generates mechanical force by displacing the piezoelectric body itself (inverse piezoelectric effect) when a voltage is applied to a piezoelectric element. The bismuth potassium titanate oriented film of the present invention has superior piezoelectric properties compared to oriented films of bismuth potassium titanate produced by conventional hydrothermal synthesis, making it a promising actuator using a lead-free piezoelectric element. [Example]

[0112] The present invention will be described below using examples, but the present invention is not limited to these examples.

[0113] (SEM observation of BKT, film thickness measurement) The film thickness was measured using a Hitachi High-Technologies S-4800 for SEM observation.

[0114] (BKT identification method, lattice constant measurement method) X-ray analysis, reciprocal lattice mapping (MRD hybrid), and wide-range 2θ-psi mapping were performed. The crystal structure was analyzed by X-ray analysis, and the lattice constant ratio c / a was calculated from the ratio of the lattice constants a and c.

[0115] (Method for measuring Curie temperature) The Curie temperature was determined as the temperature at which the crystal phase becomes cubic due to a change in crystal structure by high-temperature X-ray analysis, or as the temperature at which the slope of the decrease in lattice constant c with temperature changes.

[0116] (Method for measuring HO or OH group content) The H2O or OH group content of potassium bismuth titanate is measured using temperature-programmed desorption gas analysis. 10 -4 Potassium bismuth titanate was heated up in a vacuum as follows, and the intensity of chemical species desorbed from potassium bismuth titanate using a mass spectrometer, particularly the intensity of m / z = 18 derived from H2O or OH groups, was measured as a function of temperature. The minimum value was obtained from the measured curve, and the differences from the intensity values at each temperature were added up from the start to the end of the measurement to obtain a total. These total intensity values were multiplied by the conversion factors of each component to calculate quantitative values, which were then divided by the mass of potassium bismuth titanate to determine the H2O or OH group content in potassium bismuth titanate.

[0117] (P-E loop, S-E curve, measurement method of piezoelectric properties) For the polarization value, Pt, which is the upper electrode, was deposited on the potassium bismuth titanate piezoelectric body in advance to fabricate a structure of Pt / / (Bi,K)TiO3 / / SrRuO3. Probes were brought into contact with Pt and SrRuO3, a voltage was applied, and using a virtual ground measurement circuit, it was measured as a function of voltage and capacitance. The polarization value and electric field were changed using the electrode area and film thickness obtained in advance. The curve of the polarization value and electric field is called the P-E loop. After the polarization value saturated with respect to the applied voltage, the polarization value obtained when the maximum voltage was applied was taken as the saturation polarization value. Then, when the applied electric field was reversed, the polarization value when the electric field value was 0 kV / cm was measured as the remanent polarization value.

[0118] The piezoelectric strain was measured from the voltage applied to the piezoelectric body and the displacement amount. The piezoelectric strain constant (d 33 ) was determined by calculation from the slope of the applied voltage and displacement amount.

[0119] (Dielectric properties: measurement method of relative permittivity) The relative permittivity was measured using an impedance analyzer. While continuously changing the measurement frequency on the piezoelectric body, a tiny AC voltage of 1 V or less was applied, the capacitance of the piezoelectric body was measured, and using the film thickness, electrode area, and permittivity of vacuum obtained in advance, the relative permittivity of the piezoelectric body was measured.

[0120] (Dielectric properties: Method for measuring dielectric loss) The dielectric loss was measured by applying a small AC voltage of 1 V or less while continuously changing the measurement frequency to the piezoelectric body using an impedance analyzer and measuring the current in the piezoelectric body. In the case of an ideal capacitor, the phase difference between the applied voltage and the current in the piezoelectric body is 90°, but in reality, it is (90 - δ)°, and the dielectric loss was measured as the dielectric loss tanδ from this value.

[0121] (Example 1: Fabrication of BKT) (Bi,K)TiO3 (BKT) was fabricated by the hydrothermal synthesis method as follows.

[0122] Into an autoclave (internal volume 70 mL) with a Teflon (registered trademark) - coated interior as shown in Figure 2, 10 mol / L potassium hydroxide prepared using deionized water, 0.25 g of bismuth nitrate (Bi(NO3)3·5H2O), and 0.15 g of titanium dioxide (TiO2) were placed. Further, (001)SrRuO3 / / (001)SrTiO3 substrates with a length of 7.5 mm, width of 5 mm, thickness of 0.5 mm and a length of 15 mm, width of 7.5 mm, thickness of 0.5 mm (SrRuO3 thickness 70 nm) suspended from the lid were immersed in the potassium hydroxide aqueous solution, the lid of the autoclave was closed, and the autoclave was sealed.

[0123] The sealed autoclave was heated to 240°C and heated at 240°C for 3 hours to carry out the hydrothermal synthesis reaction. Then, the removed substrate was washed multiple times with deionized water and dried at 140°C to obtain a (Bi,K)TiO3 (BKT) film.

[0124] (Example 2: Identification of BKT) The cross - sectional photograph of the obtained BKT film observed by SEM (Hitachi High - Technologies S - 4800) is shown in Figure 7. The BKT film was uniform and had a film thickness of about 70 nm.

[0125] The X-ray diffraction (XRD) chart of the BKT film is shown in Fig. 8. From the θ-2θ and pole figure measurement results, it was shown that it is a (100)-oriented (Bi,K)TiO3 film having a tetragonal perovskite structure and following the orientation of the substrate. Since a four-fold symmetric peak corresponding to the (101) plane was confirmed, it was an epitaxially grown film. The full width at half maximum (FWHM) of the rocking curve of the X-ray analysis peak was 0.048°. Also, there were no peaks associated with typical Bi4Ti3O 12 or K 0.5 Bi 4.5 Ti4O 15 etc.

[0126] The reciprocal lattice mapping measurement (MRD hybrid) of the BKT film is shown in Fig. 9. It was an epitaxial film that was matched with the SrTiO3 single crystal substrate and SrRuO3 which is a buffer layer. The lattice constants calculated from the symmetric and asymmetric planes were a = 0.3905 nm and c = 0.4081 nm.

[0127] The wide range 2θ-psi mapping measurement results of the BKT film are shown in Fig. 10. Measurements were made when rotating Phi and when fixed to (111), but no heterophase etc. was detected. Also, the splitting of the peak in the psi direction on the high angle side is due to the accuracy of the tilt correction when attaching the sample to the measuring device.

[0128] Figure 11 shows the TEM measurement results for the BKT film. Figure 11(a) shows a longitudinal cross section of the BKT film, which is, from bottom to top, the SrTiO3 film, SrRuO3 film, (Bi,K)TiO3 film, and carbon film. The carbon film is a protective film for TEM measurement. Figures 11(b) and 11(c) show SAD region 1, which is near the protective carbon film / (Bi,K)TiO3 film interface; SAD region 2, which is near the (Bi,K)TiO3 / SrRuO3 film interface; and SAD region 3, which is near the SrRuO3 / SrTiO3 film interface, as shown in Figure 11(a). These lattice images and selected-area diffraction (SAD) patterns confirm that each crystalline film is epitaxially grown. No significant amorphous regions are observed. Figure 12 shows the results of HAADF-STEM measurement of the BKT film. Figure 12(a) shows the interface between the BKT film and the SrRuO3 film, and Figure 12(b) shows the interface between the SrRuO3 film and the SrTiO3 film. The BKT film is an epitaxial film that is perfectly matched with the SrTiO3 single crystal substrate and the SrRuO3 buffer layer, and a clear junction interface is observed, suggesting that the effects of etching and diffusion are minimal.

[0129] (Curie temperature measurement) Figure 13 shows the lattice constant c of BKT as a function of temperature obtained from the high-temperature X-ray diffraction (HT-XRD) chart of the (003) plane of BKT prepared in Example 1. From this high-temperature X-ray diffraction result, the Curie temperature of BKT is above 600°C, where the slope changes.

[0130] The BKT film was heated in a vacuum, and the heating temperature and the amount of HO released from the film were measured (TDS measurement) as shown in Figure 14(a)(b). The intensity corresponding to m / z = 18 increased below 100°C, with a relatively low-intensity peak around 100°C and a relatively large peak observed around 200–300°C. The release of physisorbed HO corresponds to a peak around 100°C, the release of chemisorbed HO corresponds to a peak around 200°C, and the release of OH incorporated within the crystal corresponds to a peak around 250–300°C. In Figure 14(a)(b), the increase in intensity around 800°C is due to O2, as it is also present in the blank, and is not due to the BKT film sample. In quantitative analysis, only selected m / z values are measured and quantitative values are calculated using concentration conversion coefficients. This allows for detection of low intensities and allows for highly accurate detection of the desorption of selected elements. The SRO / STO substrate was separately confirmed to be identical to the blank. Qualitative and quantitative analysis detected desorbed gas originating from HO at m / z=18, confirming the presence of water within the film. The total content of HO or OH groups was 0.57 ng for the 80 nm film and 181 ng for the 1300 nm film. The masses of these samples were calculated from the sample area and density to be 15 μg and 27.1 μg, respectively. The resulting HO and OH groups contained within the film were 3750 and 6670 ppm, respectively.

[0131] (Example 3: Creation of piezoelectric element and piezoelectric characteristics) In the same manner as in Example 1, a BKT film with a tetragonal perovskite structure was formed on an SrTiO substrate on which a SrRuO film had been formed, and then a platinum electrode was formed by electron beam evaporation on the as-deposited BKT film (not annealed), and the BKT film was sandwiched between the upper and lower electrodes to produce the piezoelectric element of Example 3.

[0132] The piezoelectric properties of the piezoelectric element were measured using a Polytec NLV-2500 compact laser Doppler vibrometer and a Toyo Corporation FCE ferroelectric evaluation system. 33 The frequency of the AC voltage in the measurement was 10 kHz.

[0133] The results of measuring the polarization - electric field loop (P - E hysteresis) of the piezoelectric element of Example 3 are shown in Fig. 15(a). Fig. 15(a) is the P - E loop, and Figs. 15(b) and (c) are graphs showing the remnant electric field (P r ) and the coercive electric field (E c ) as functions of the electric field (E) applied when measuring the P - E loop. The applied electric field was in the range of about - 800 kV / cm to 800 kV / cm, and the frequency was 10 kHz.

[0134] According to Fig. 15(a), a P - E hysteresis loop with an excellent squareness ratio (remnant polarization (P r ) / saturation polarization (P sat ) = 0.94) is obtained. Referring to Figs. 15(b) and (c), as the measuring electric field (P) is increased, hysteresis suddenly appears from a certain electric field (P r around 150 - 200 kV / cm), and moreover, both the remnant electric field (P r ) and the coercive electric field (E c ) increase rapidly. After that (when P r is around 250 kV / cm or higher), even if the electric field (E) is further increased, both the remnant electric field (P r ) and the coercive electric field (E c ) increase, but the increase amplitude is very small. From this, it is confirmed that the BKT film of the piezoelectric element of Example 3 has a spontaneous polarization before applying an electric field, that is, it is self - polarized.

[0135] Moreover, referring to the shape of the hysteresis, the polarization increases rapidly at a certain electric field and almost reaches saturation, and moreover, even when the electric field is decreased from there, the polarization hardly decreases until the electric field becomes zero. It is recognized that this is an extremely excellent rectangular hysteresis. The squareness ratio of this P - E loop ((remnant polarization (P r ) / saturation polarization (P sat )) shows 0.90 or more up to an applied electric field of 500 kV / cm. When an electric field below that is applied, since it shows a minor loop instead of the saturation polarization, it gradually decreases.

[0136] Figure 15(d) shows the results of measuring the relative permittivity (ε r ) and dielectric loss (tanδ) by applying an AC voltage of 200 mV to the piezoelectric element before and after this P-E loop measurement. Before and after this P-E loop measurement, the relative permittivity (ε r ) is very small, and although the dielectric loss (tanδ) fluctuates in the low-frequency region, it is small overall. The relative permittivity (ε r ) is 100 or less in a wide frequency range, and the dielectric loss (tanδ) is also 0.1 or less, and further 0.5 or less in a wide frequency range. In Fig. 15(d), it is indicated in the drawing which of the two types of curves is the relative permittivity and tanδ. In other similar figures (Fig. 21, Fig. 34A, Fig. 34B, Fig. 37, Fig. 47, Fig. 48, etc.) hereafter, it is not always indicated which of the two types of curves is the relative permittivity and tanδ, but from the similarity with the figure of Fig. 15(d), it is clear which curve is the relative permittivity and tanδ.

[0137] Fig. 16 shows the results of examining the frequency dependence of the strain - electric field (S-E) of the piezoelectric element of Example 3 as an evaluation of the piezoelectric characteristics of the piezoelectric element of Example 3. The S-E curve is shown together with the P-E loop when an electric field is applied at 10 kHz in the graph of Fig. 16(a). It shows a piezoelectric strain (electric-field-induced strain) that is approximately proportional to the electric field, and the piezoelectric coefficient d 33 is 62 pm / V. In Fig. 16(a), it is indicated in the drawing which of the two types of curves is the electric-field-induced strain and polarization. In other similar figures (Fig. 22, etc.) hereafter, it is not always indicated which of the two types of curves is the electric-field-induced strain and polarization, but from the similarity with the figure of Fig. 16(a), it is clear which curve is the electric-field-induced strain and polarization.

[0138] Also, Figs. 16(b) and (c) show the P-E loop and S-E curve when voltages of 2 V, 5 V, and 7 V are applied to the piezoelectric element (piezoelectric element after electrode formation) of Example 3 without applying a poling treatment (a treatment for aligning the polarization of the piezoelectric element by applying a high voltage in advance before use). Fig. 16(d) shows the frequency dependence of the piezoelectric strain constant d 33 . The piezoelectric strain constant (d 33) were 61, 62, and 62 pm / V when the applied electric fields were 290 kV / cm, 710 kV / cm, and 1000 kV / cm, respectively. From these results, it can be seen that the piezoelectric element of Example 3 is self-polarized and exhibits relatively high piezoelectricity at a low applied voltage without the need to apply a high voltage to align the direction of spontaneous polarization, that is, without "poling treatment". Therefore, it is recognized that the BKT film is "self-polarized". In addition, it is recognized that the piezoelectric element has excellent piezoelectric properties even at a low applied voltage.

[0139] The frequency dependence of the P-E loop was measured by changing the frequency of the applied electric field. The results are shown in FIGS. 17 to 18. It is noted that in a wide frequency range from low frequency to high frequency, especially at low frequencies, a very excellent rectangular ratio hysteresis is exhibited.

[0140] FIG. 19 shows the frequency dependence of the remanent polarization (P r ) and the coercive electric field (E c ) obtained from these measurements. Regarding the remanent polarization (P r ), in a wide frequency range from low frequency to high frequency, especially at low frequencies, a high remanent polarization (P 2 ) of 70 to 80 μC / cm r ) is shown. Regarding the coercive electric field (E c ), although there is frequency dependence, a high coercive electric field (E c ) of 200 to 700 kV / cm in a wide frequency range from low frequency to high frequency and 200 kV / cm or more especially at low frequencies is obtained.

[0141] (Example 4: Annealing Temperature) The BKT films formed on SrTiO3 substrates on which SrRuO3 was deposited in the same manner as in Example 1 were annealed at 600 °C, 700 °C, and 800 °C, respectively, to observe the changes in the BKT films of Example 4. The annealing was carried out in an oxygen atmosphere, the temperature was raised at a rate of 10 °C / min, held at the target temperature for 10 minutes, and then cooled at a cooling rate (set value) of 10 °C / min.

[0142] X-ray diffraction (XRD) charts of these annealed BKT films and the unannealed BKT film of Example 1 are shown in Figure 20(a). The intensity ratios of Bi and K obtained by X-ray fluorescence analysis of these BKT films are shown in Figure 20(b). Up until annealing at 700°C, there was no change in the XRD chart, and the X-ray fluorescence peak intensities of Bi and K did not decrease compared to the as-depo film. However, for the film annealed at 800°C, a TiO peak appeared, and the X-ray fluorescence peak intensities of Bi and K decreased, indicating that some of the Bi and K had evaporated. The c / a ratio also remained almost unchanged compared to the as-depo film up to annealing at 700°C.

[0143] (Example 5: Annealing Temperature) The BKT film was annealed at 600°C, 700°C, and 800°C in the same manner as in Example 4, and then sandwiched between upper and lower platinum electrodes in the same manner as in Example 2 to produce the piezoelectric element of Example 5.

[0144] The piezoelectric characteristics of the piezoelectric element of Example 5 were measured in the same manner as in Examples 3 and 4. The lower graphs in Figures 21(a) to 21(c) show the PE loops (measurement conditions were the same as in Example 3, 10 kHz) of the unannealed BKT film and the BKT film annealed at 600°C and 700°C. The upper graphs in Figures 21(a) to 21(c) show the relative dielectric constant (ε) measured before and after the PE loop measurement by applying a voltage of 200 mV to the piezoelectric element. r These results show that the ferroelectric properties are excellent even when annealed at 700°C. However, when annealed at 800°C, the leakage current was high, making it difficult to measure the ferroelectric properties.

[0145] Figure 22 shows the PE loops and SE curves of an unannealed BKT film and BKT films annealed at 600°C and 700°C. Figure 22 confirms that even when annealed at 700°C, the film exhibits piezoelectricity at low applied voltages without the so-called "polarization treatment," and that it exhibits "self-polarization."

[0146] (Example 6: Film-forming time / film-forming temperature) In the same manner as in Example 1, except that the film deposition temperature and time were changed to 70°C, 166 hours; 270°C, 3 hours; 240°C, 3, 4, 12 hours; 200°C, 3 hours, 66 hours; 150°C, 20 hours; 100°C, 66 hours, a BKT film was deposited.

[0147] The XRD chart shown in Fig. 23(a) is for the case where the film deposition time was changed to 3 hours, 4 hours, and 12 hours at a film deposition temperature of 240°C. Only the peaks due to (00l)(Bi,K)TiO3 were observed, indicating that a (100)-oriented (Bi,K)TiO3 film was obtained following the (100)cSrRuO3 / / SrTiO3 orientation even when the film deposition time was changed. Also, from the pole figure measurement in Fig. 23(b), a four-fold symmetric peak corresponding to (101)(Bi,K)TiO3 was observed, indicating that it is an epitaxial film. The values of the out-of-plane and in-plane lattice constants calculated from the reciprocal lattice mapping measurement were 0.3905 nm for the a-axis and 0.4081 nm for the c-axis.

[0148] The XRD chart of the BKT film with a film deposition temperature of 200°C and a film deposition time of 66 hours is shown in the XRD chart in Fig. 23(c). Similarly, even when the film deposition temperature was decreased, only the peaks due to (00l)(Bi,K)TiO3 were observed. This indicates that a (100)-oriented (Bi,K)TiO3 film was obtained following the (100)cSrRuO3 / / SrTiO3 orientation even when the film deposition time was changed. Focusing on (003)(Bi,K)TiO3, it can be seen that the peak has shifted to the low-angle side compared to when deposited at 240°C. From this, it can be seen that the out-of-plane lattice constant has increased.

[0149] The results of measuring the time dependence of the film thickness of the BKT films obtained by varying the film deposition time at each film deposition temperature are summarized in Fig. 24. The film was deposited in 3 hours, and this was the same even when the temperature was lowered to 200°C. As the film deposition time increased, the film thickness increased.

[0150] In the same manner as described above, except that the film formation temperature was set to 240 °C and the film formation time was varied, BKT films having various film thicknesses were formed. Fig. 25(a) shows the XRD charts of the obtained BKT films (film thicknesses: 33 nm, 65 nm, 98 nm, 331 nm, 616 nm, 1530 nm), and (Bi,K)TiO3 films oriented in the (100) direction according to the (100)cSrRuO3 / / SrTiO3 orientation were obtained at any film thickness. Fig. 25(b) shows the relationship between the film thickness and the film formation time, indicating that the film thickness increases with the film formation time.

[0151] Fig. 26 shows the pole figure measurement results of BKT films with different film thicknesses (film thicknesses: 33 nm, 68 nm, 140 nm, 1200 nm) prepared in the same manner as described above. Since four-fold symmetric peaks corresponding to (101)(Bi,K)TiO3 are observed, it is shown that the films are epitaxial films.

[0152] Fig. 27 shows the results of measuring the relative permittivity (ε r ) and the dielectric loss (tanδ) for BKT films with different film thicknesses (film thicknesses: 33 nm, 98 nm, 140 nm, 1530 nm) prepared in the same manner as described above. All of them show excellent dielectric properties.

[0153] Fig. 28 shows the polarization-electric field loops (P-E hysteresis) measured for BKT films with different film thicknesses (film thicknesses: 33 nm, 140 nm, 120 nm, 1530 nm) prepared in the same manner as described above. All of them show excellent square P-E hysteresis.

[0154] Fig. 29 is a graph showing the film thickness dependence of the remanent polarization (P r ) and the coercive field (E c ) measured for BKT films with different film thicknesses prepared in the same manner as described above. If the film thickness is above a certain level, the dielectric properties of the remanent polarization (P r ) and the coercive field (E c ) are stable and independent of the film thickness.

[0155] Fig. 30 shows the remanent polarization (P r) and coercive field (E c ) as a function of the electric field. The BKT films fabricated at each deposition temperature show a clear saturation, which correlates with the c-axis orientation of the resulting BKT films.

[0156] Figure 31 shows the X-ray diffraction chart (Figure 31(a)) and lattice constant (Figure 31(b)) of the BKT film obtained when the deposition temperature was changed. Figure 31(a) shows that even when the deposition temperature was changed, only peaks due to the (001)(Bi,K)TiO3 film were observed, indicating that an oriented film was synthesized in accordance with the substrate orientation. Figure 31(b) also shows that as the temperature decreased, the c-axis length, i.e., the out-of-plane lattice constant, shifted to a lower angle, indicating that c / a increased. Furthermore, even when the deposition temperature was lowered, no peaks due to impurity phases such as heterogeneous phases were observed. The in-plane lattice constant was measured using reciprocal lattice mapping.

[0157] In particular, even at a deposition temperature of 70°C, only peaks attributable to the (00l)(Bi,K)TiO3 film were observed, and it was noteworthy that an oriented film was synthesized in accordance with the orientation of the substrate. This indicates the possibility of reducing the pressure in the hydrothermal synthesis method, which was previously performed under high pressure, to near atmospheric pressure.

[0158] Figure 32 shows the results of reciprocal lattice mapping of BKT films obtained at different deposition temperatures. The results of the reciprocal lattice mapping of the (003) and (-103) planes show that the peak position of BKT decreases with decreasing deposition temperature, indicating that the lattice constant c increases with decreasing deposition temperature. However, at all deposition temperatures, the peak positions of the SrTiO3 substrate and the SrRuO3 and (Bi,K)TiO3 buffer layers are vertically aligned, meaning there is almost no misalignment on the horizontal axis. This indicates that the lattice constant a is consistent across the substrate, buffer layer, and BKT film.

[0159] Figure 33(a) shows the vapor pressure of the Bi precursor solution and the saturated water vapor pressure of HO when the temperature is changed. Figure 33(b) shows the change in the film thickness and film deposition rate of the BKT film obtained when the film deposition temperature is changed. The film deposition rate peaks at a film deposition temperature of around 200°C. The BKT film obtained at 200°C also had the smoothest surface.

[0160] 34A and 34B show the dielectric properties, PE loops, and SE curves of the BKT film obtained when the film deposition temperature was changed. The piezoelectric properties were observed over a wide range of temperatures, and were excellent at all temperatures, but were particularly excellent between 150 and 240°C.

[0161] Relative permittivity (ε r ) were 111, 74, 77, 77, and 171 for the BKT films deposited at 70, 100, 150, 200, 240, and 270°C, respectively. These values correspond to the relative dielectric constant (ε r ) was shown.

[0162] The dielectric loss (tan δ) was 0.08, 0.04, 0.02, 0.04, and 0.08 for the BKT films deposited at 70, 100, 150, 200, 240, and 270° C. These values indicated the dielectric loss (tan δ) at 10 kHz.

[0163] The coercive electric fields were 430, 420, 410, 460, and 460 kV / cm for the BKT films deposited at 70, 100, 150, 200, 240, and 270° C. These values corresponded to the values for an applied electric field of 700 kV / cm.

[0164] The remanent polarization values for the BKT films deposited at 70, 100, 150, 200, 240, and 270°C were 137, 119, 100, 87, and 92 μC / cm, respectively. 2 These values were obtained when the applied electric field was 700 kV / cm.

[0165] The angular ratio was 0.93, 0.95, 0.93, 0.93, and 0.88 for the BKT films formed at 70, 100, 150, 200, 240, and 270 °C, respectively. These values represent those at an applied electric field of 700 kV / cm.

[0166] Piezoelectric strain constant (d 33 ) was 61, 58, 63, 62, and 65 pm / V for the BKT films formed at 70, 100, 150, 200, 240, and 270 °C, respectively. These values represent those at an applied electric field of 700 kV / cm.

[0167] The breakdown voltage here is defined as the maximum value of the electric field applied when a P-E hysteresis loop was obtained, and was 687, 709, 869, 1420, and 1000 kV / cm for the BKT films formed at 70, 100, 150, 200, 240, and 270 °C, respectively. Here, 240 °C shows the maximum value, but this is due to the maximum applied voltage value and has no relation to the temperature dependence.

[0168] Figures 35A and 35B show the applied voltage dependence of the anti-electric field (E c ), remanent polarization (P r ), and saturation polarization (P sat ) of the BKT films obtained when the film formation temperature was changed. The BKT films fabricated at each film formation temperature show distinct saturation, which correlates with the obtained BKT films being c-axis oriented.

[0169] Figure 36 shows the angular ratio (P r / P sat ) of the BKT films obtained when the film formation temperature was changed. The angular ratio (P r / P sat ) shows distinct saturation for each film formation temperature, and there was a point where P r / P sat ≥ 0.9 was shown in all films by applying a voltage of 800 kV / cm or higher.

[0170] Figure 37(a) shows the frequency dependence of the dielectric constant and dielectric loss of a typical BKT film. This BKT film is a 70 nm sample obtained at 240°C. Figure 37(b) shows the film deposition temperature dependence of the dielectric constant and dielectric loss obtained at 10 kHz. At all film deposition temperatures, the dielectric constant was 200 or less, and in particular, the dielectric constant was 100 or less between 150 and 240°C. Furthermore, at all film deposition temperatures, the dielectric loss was 0.1 or less, and in particular, the dielectric loss was 0.05 or less between 150 and 240°C.

[0171] Figure 38 shows the relationship between the deposition temperature and the coercive field (E c ), remnant polarization (P r ), saturated polarization (P sat ) and squareness ratio (P r / P sat ) shows the film deposition temperature dependence. c ) is independent of the deposition temperature and is 300 kV / cm or more, and the remanent polarization (P r ) decreases with decreasing film deposition temperature. r ) and saturation polarization (P sat ) increased to a maximum of 128 μC / cm 2 and 137 μC / cm 2 These results are correlated with the increase in lattice constant c due to the decrease in film formation temperature. r / P sat ) was 0.8 or more at all film formation temperatures, and in particular, was 0.9 or more at film formation temperatures of 100 to 240°C.

[0172] Figure 39 shows the relationship between the deposition temperature and the piezoelectric strain constant (d 33 ) was shown. Values of 50 pm / V or more were observed for all film deposition temperatures, and stable characteristics were obtained.

[0173] Example 7: KOH Concentration BKT membranes were formed in the same manner as in Example 1, except that the KOH concentration was changed to 6 mol / L, 10 mol / L, 12 mol / L, and 13.6 mol / L.

[0174] Figure 40(a) shows the X-ray analysis chart of the obtained BKT film. When the solution concentration is changed, it is confirmed that the BKT film is obtained. Even when the film is formed using KOH solutions with different concentrations, only the peaks attributed to the (00l)(Bi,K)TiO3 film are observed, indicating that an oriented film following the substrate orientation is synthesized. Also, in the case of a low concentration of 6 mol / L, since the c-axis length, i.e., the out-of-plane lattice constant, is shifted to the low-angle side, it is shown that c / a increases. Moreover, even when the KOH solution concentration is decreased, no peaks attributed to impurity phases such as heterogeneous phases are observed.

[0175] Figure 40(b) shows the film thickness of the BKT films obtained by changing the concentration of KOH (in Example 7, the film formation time is constant, and the film thickness in Figure 40(b) also represents the film formation rate as a result). Currently, in the investigated concentration range, it is suggested that the vicinity of 10 mol / L is the region where the highest film formation rate is obtained.

[0176] (Example 8: Bi raw material and Ti raw material with different concentrations) The X-ray analysis charts of the BKT films formed with different concentrations of Bi raw material and Ti raw material shown in Figure 41(a) are shown in Figure 41(b). When the concentrations of the Bi raw material and the Ti raw material are changed, the lattice constant c does not change significantly. Even when the concentrations of the Bi raw material and the Ti raw material are increased or decreased, no peaks attributed to impurity phases such as heterogeneous phases are observed.

[0177] The graph in Figure 42 shows the concentrations of the Ti raw material and the Bi raw material with respect to the KOH solution measured in Examples 7 and 8, expressed as molar ratios.

[0178] (Example 9: Different Ti raw materials, anatase type and rutile type) In other examples, anatase-type titanium dioxide (TiO2) was used as the raw material. However, in Example 9, a BKT film was formed using both anatase-type and rutile-type titanium dioxide as the raw material titanium dioxide (TiO2). Fig. 43(a) shows the XRD chart of the obtained BKT film, and Fig. 43(b) shows the dependence of the film thickness of the BKT film on the film formation time. According to the XRD chart in Fig. 43(a), regardless of whether anatase-type or rutile-type titanium dioxide was used, only the peaks attributed to the (00l)(Bi,K)TiO3 film were observed, indicating that an oriented film was synthesized according to the orientation of the substrate. At the same film formation time of 12 hours, the maximum film thickness was 330 nm for the anatase type, while it was 990 nm for the rutile type, which was about three times that of the anatase type. Fig. 43(b) shows the dependence of the film thickness of the BKT film obtained using anatase-type and rutile-type titanium dioxide on the film formation.

[0179] Fig. 44 shows the polarization-electric field loop, remnant polarization (P r ) and coercive electric field (E c ) measured for the BKT film obtained using rutile-type titanium dioxide.

[0180] Similar to other examples, a BKT film was formed using rutile-type titanium dioxide and substrates with different crystal orientations. The substrates used were (100)cSRO / / (100)STO, (110)cSRO / / (110)STO, and (111)cSRO / / (111)STO (SRO is SrRuO3 and STO is SrTiO3). Fig. 45(a) shows the XRD chart of the obtained BKT film, and Fig. 45(b) shows the Wide range 2θ-psi mapping measurement results of the obtained BKT film. It can be confirmed that a single-phase BKT film without misorientation was obtained regardless of whether the crystal orientation of the substrate was (100), (110), or (111). Fig. 45(b) shows the relationship between the film thickness and the film formation time.

[0181] (Example 10: Buffer layer SrRuO3 with different film formation conditions) A BKT film was formed in the same manner as in Example 1, except that SrRuO3 (SRO), which is a buffer layer on the substrate, was fabricated under different film-forming conditions to use SrRuO3 (STO) with different lattice constants, half-value widths, and conductivity (resistivity). Two types of substrates (A) and (B) with different film-forming conditions were used: SrRuO3 / / SrTiO3 (SRO / / STO).

[0182] The XRD charts of substrates (A) and (B) are shown in Figs. 47(a) and (b). The half-value width of the peak of substrate (B) is narrower and the peak is sharper (higher degree of orientation).

[0183] Fig. 47(c) shows the XRD charts of the BKT films formed on these substrates (A) and (B). Even when formed on SrRuO3 fabricated under different conditions, (Bi,K)TiO3 films were formed according to the orientation of the substrate. It can be seen that when substrate (B), that is, when the buffer layer has a high degree of orientation, the degree of orientation of the upper film is improved. The film thicknesses of these films are about the same. Fig. 47(d) shows the P-E loops of the BKT films formed on these substrates (A) and (B). When substrate (B), that is, when the buffer layer has a high degree of orientation, in the region where the film thickness is thin, the piezoelectric properties and squareness ratio are excellent.

[0184] Using the above-mentioned substrate (A) with a low degree of orientation, which is the same as in Example 1, the film-forming temperature was changed from 240 °C to 200 °C, and the film-forming time was changed from 3 hours to 13.5 hours to form a BKT film. The XRD chart of the obtained BKT film is shown in Fig. 48(a), the dielectric properties of the BKT film are shown in Fig. 48(b), the P-E loop is shown in Fig. 36(c), and the S-E curve is shown in Fig. 48(d). Comparing Fig. 48(c) with Fig. 48(d), by making the film thicker, the ferroelectric properties were improved and clear piezoelectric properties were shown. It is considered that the degree of orientation increased compared to when the film thickness was thin. Also, no heterogeneous phases of crystallinity were observed by XRD even when the conditions were changed. The full width at half maximum (FWHM) of the rocking curve of the X-ray analysis peak in Fig. 48(a) was 0.513°.

[0185] (Example 11: Different substrates; KTaO3 single crystal substrate) In the same manner as in Example 1, except that a KTaO3 single crystal was used as the substrate, and an SrRuO3 buffer layer was formed thereon, a BKT film was formed at a film deposition temperature of 200 °C and a film deposition time of 15 hours. Also, a LaAlO3 single crystal was used as the substrate, and an SrRuO3 buffer layer was formed thereon, and a BKT film was formed at a film deposition temperature of 200 °C and a film deposition time of 15 hours. Figures 49(a) and (b) show XRD charts of SrRuO3 films deposited on SrTiO3 and KTaO3 substrates, and further BKT films deposited on these substrates. Since the lattice constant of KTaO3 is larger than that of SrTiO3, the peak position of the deposited SrRuO3 is shifted to the high-angle side, and it is considered that the lattice constant a has increased. Also, peaks of the BKT film on SrRuO3 / / KTaO3 were observed, and no peaks due to impurities were confirmed. Figure 49(c) shows the Wide range 2θ-psi mapping measurement results of the BKT film deposited on the SRO / / KTO substrate. It was shown that there were no peaks due to impurities in the wide-range X-ray measurement results either.

[0186] Figure 50(a) shows XRD charts of BKT films deposited on SrRuO3 / / SrTiO3 and SrRuO3 / / LaAlO3 substrates. Similar to the BKT films deposited on other substrates, there were no peaks due to impurities. The dielectric properties of the BKT film deposited on the SrRuO3 / / LaAlO3 substrate are shown in Figure 50(b), the P-E loop in Figure 38(c), the applied electric field dependence of the breakdown field and the remanent polarization value in Figures 50(d) and (e), and the S-E curve in Figure 38(f). Compared with the BKT film deposited on SrTiO3, it showed comparable relative permittivity, dielectric loss, and remanent polarization value.

[0187] (Example 12: Different substrates; plastic substrate) In the same manner as in Example 1, except that a polysulfone plate was used as the substrate, Ti metal and Pt metal were deposited thereon by sputtering, and further LaNiO3 was deposited by sputtering to form a buffer layer, and a BKT film was formed at a film deposition temperature of 150 °C and a film deposition time of 72 hours.

[0188] Fig. 51(a) is a schematic diagram of the obtained film, and Fig. 51(b) shows the Wide range 2θ-psi mapping measurement results of the BKT film formed on a LaNiO3 / Pt / Ti / polysulfone substrate. A polycrystalline BKT film was obtained, and no hetero-phase peaks other than BKT were confirmed.

Explanation of symbols

[0189] 1: Reaction vessel 2: Water (alkaline aqueous solution) 3: Niobium raw material 4: Tantalum raw material 5: Substrate 6: Fixture 10: Piezoelectric element 11: Substrate 12: Lower electrode 13: Bismuth potassium titanate film 14: Upper electrode 15: Buffer layer 21: Piezoelectric substrate 22: Comb-shaped electrode (excitation side) 23: Comb-shaped electrode (reception side) 31: Piezoelectric substrate 32: Comb-shaped electrode 33: Reflector 34: Excitation electrode 35: Reception electrode 40: Ultrasonic probe 41: Backing material 42: Vibrator (piezoelectric element) 43: Acoustic matcher 44: Acoustic lens 50: Pyroelectric power generation device 51: Pyroelectric element 52: Ferroelectric 53: Electrode 54: Heat source

Claims

1. A piezoelectric body composed of potassium bismuth titanate having a tetragonal perovskite structure, uniaxially oriented in the direction of the polarization axis, and satisfying the following requirement i) and / or ii). i) The lattice constant ratio c / a is greater than 1.

040. ii) The Curie temperature is higher than 380 °C.

2. The piezoelectric body according to claim 1, further containing an oxide other than potassium bismuth titanate within a range of 30% by weight or less based on potassium bismuth titanate.

3. The piezoelectric body according to claim 1 or 2, wherein the half-width of the rocking curve of the X-ray analysis peak of potassium bismuth titanate is 25° or less.

4. iii) H ２ O or an OH group, and the content of the H ２ O or OH group is 10,000 ppm or less based on the total weight of the piezoelectric body. The piezoelectric body according to any one of claims 1 to 3.

5. The piezoelectric body according to any one of claims 1 to 4, wherein the piezoelectric body satisfies any one or more of the following requirements. iv) The polarization value is 35 μC / cm ２ or more. v) The ratio (square ratio) of the residual polarization (P ｒ ),) to the saturation polarization (P sat ) is 0.8 or more. vi) The relative permittivity (ε r ) is 200 or less. vii) The dielectric loss (tanδ) is 0.2 or less. viii) The piezoelectric strain constant (d ３３ ) is 30 to 100 pm / V. ix) The withstand voltage is 300 kV / cm or more.

6. The piezoelectric body according to any one of claims 1 to 5, wherein the piezoelectric body is self-polarized and the polarization directions are aligned without polarization treatment.

7. A method for manufacturing a potassium bismuth titanate piezoelectric body according to any one of claims 1 to 6, characterized in that in a reaction vessel, a substrate is immersed in a water-containing solvent containing potassium hydroxide, a bismuth raw material, and a titanium raw material, and heated and pressurized to form a film of potassium bismuth titanate having a tetragonal perovskite structure on the substrate.

8. The molar ratio of potassium hydroxide to each of the bismuth raw material and the titanium raw material is 1:1.0×10 －５ to 1:1.0×10 ２ The manufacturing method according to claim 7, wherein the ratio is as defined above.

9. The molar ratio of the bismuth raw material to the titanium raw material is from 1:1.0×10 －５ to 1:1.0×10 ３ The production method according to claim 7 or 8, wherein the molar ratio is as defined above.

10. The manufacturing method according to any one of claims 7 to 9, wherein the concentration of potassium hydroxide in the water-containing solvent is 0.1 to 30 mol / L.

11. The manufacturing method according to any one of claims 7 to 10, wherein the substrate has a perovskite-based crystal structure.

12. The manufacturing method according to any one of claims 7 to 11, wherein the substrate is made of a material selected from semiconductors, metals, plastics, and ceramics, and has a buffer layer of a perovskite-based crystal structure on its surface.

13. The manufacturing method according to any one of claims 7 to 12, wherein the substrate is a conductive substrate.

14. The manufacturing method according to any one of claims 7 to 13, wherein the substrate has a surface including a flat surface and / or a curved surface.

15. The manufacturing method according to any one of claims 7 to 14, wherein the reaction vessel is a sealed vessel, and the temperature inside the reaction vessel is heated to a temperature of 50 to 300 °C.

16. The manufacturing method according to any one of claims 7 to 15, wherein heating is performed using microwaves.

17. The manufacturing method according to any one of claims 7 to 16, wherein after taking out bismuth potassium titanate from a water-containing medium, annealing is performed at a temperature of 100 to 750 °C.

18. A piezoelectric element in which a pair of electrodes sandwich a piezoelectric body, the piezoelectric body being composed of bismuth potassium titanate having a tetragonal perovskite structure, uniaxially oriented in the direction of the polarization axis, and satisfying the following requirements i) and / or ii): Piezoelectric element. i) The lattice constant ratio c / a is greater than 1.

040. ii) The Curie temperature is higher than 380 °C.

19. The piezoelectric element according to claim 18, wherein the piezoelectric body further contains an oxide other than bismuth potassium titanate within a range of 30% by weight or less based on bismuth potassium titanate.

20. The piezoelectric element according to claim 18 or 19, wherein the half-width of the rocking curve of the X-ray analysis peak of bismuth potassium titanate of the piezoelectric body is 25° or less.

21. iii) the piezoelectric body contains an H ２ O or OH group, and the content of the H ２ O or OH group is 10,000 ppm or less based on the total weight of the piezoelectric body, the piezoelectric element according to any one of claims 18 to 20.

22. The piezoelectric element according to any one of claims 18 to 21, wherein the piezoelectric body satisfies any one or more of the following requirements. iv) The polarization value is 35 μC / cm ２ or more. v) The ratio of the remanent polarization (P ｒ ) to the saturation polarization (P sat ), i.e., the rectangularity ratio, is 0.8 or more. vi) The relative permittivity (ε r ) is 200 or less. vii) The dielectric loss (tanδ) is 0.2 or less. viii) The piezoelectric strain constant (d ３３ ) is 30 to 100 pm / V or more. ix) The breakdown voltage is 300 kV / cm or more.

23. The piezoelectric element according to any one of claims 18 to 22, wherein the piezoelectric body is self-polarized and the polarization directions are aligned without polarization treatment.

24. A piezoelectric functional device including the piezoelectric element according to any one of claims 18 to 23, which is a piezoelectric functional device selected from a noise filter, a medical ultrasonic probe, an ultrasonic transmitter, an ultrasonic sensor, a pyroelectric power generation device, a vibration power generation device, and an actuator.

25. The piezoelectric functional means is a noise filter, and the noise filter has a polarization value of 35 μC / cm ２ or more, a rectangular ratio of 0.8 or more, a relative permittivity (ε r ), and a piezoelectric strain constant (d ３３ ) of 30 to 100 pm / N or more, the piezoelectric functional device according to claim 24, comprising a piezoelectric body.

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