Use of lead-free piezoelectric ceramics in devices requiring high permanent preload

Abstract

To provide a lead-free piezoelectric ceramic that can withstand a high permanent mechanical preload and does not generate heat during use. The lead-free piezoelectric ceramic is used in a device that requires a permanent preload of 20 MPa or more to be applied to the lead-free piezoelectric ceramic. The lead-free piezoelectric ceramic has the following composition: [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM)], where (0

Description

[Technical Field]

[0001] The present invention relates to the use of lead-free piezoelectric ceramics in devices that require the application of a high permanent preload to the lead-free piezoelectric ceramic. [Background technology]

[0002] Piezoelectric ceramics are capable of converting mechanical quantities such as pressure and acceleration into electrical quantities, and conversely, converting electrical signals into mechanical movement or vibration.

[0003] Piezoelectricity is based on the ability of certain crystals to generate an electric charge when subjected to a mechanical load, such as compression or tension (the forward piezoelectric effect). Conversely, these crystals undergo a controlled deformation when exposed to an electric field; this behavior is called the inverse piezoelectric effect. The polarity of the charge depends on the orientation of the crystal relative to the direction of the pressure.

[0004] Ceramics that exhibit piezoelectric properties belong to the group of ferroelectric materials. Conventional systems are mainly based on lead zirconate titanate (PZT), which consists of a mixed crystal of lead zirconate (PbZrO3) and lead titanate (PbTiO3).

[0005] Immediately after sintering, the domains (i.e., regions composed of elementary cells with uniform dipole orientation) of a ceramic body will exhibit random (statistically distributed) orientation. This means that the entire ceramic body is macroscopically isotropic and does not exhibit piezoelectric properties. These piezoelectric properties arise through "poling." In this poling process, the ceramic body is exposed to a strong direct current (DC) field, which aligns the electric dipoles along the direction of the field. The electric dipoles maintain this orientation (remanent polarization) even when the DC field is no longer applied. Remanent polarization is a necessary condition for ferroelectric ceramics to exhibit piezoelectric behavior.

[0006] The complete or partial loss of the domain alignment achieved by the poling process (depolarization) will degrade the piezoelectric properties of the material. Depolarization can be caused by three factors: thermal depolarization due to heat exposure, electrical depolarization due to an electric field acting in the opposite direction to the original polarization, and mechanical depolarization caused by high voltage loading, especially by shorting electrodes.

[0007] Piezoelectric ceramic components are used in a wide spectrum of electromechanical transducers covering a wide frequency range. In sensors, they are able to convert force, pressure, and acceleration into electrical signals. In acoustic generators and ultrasonic transducers, they convert electrical voltage into vibrations and deformations.

[0008] In ultrasonic applications, piezoelectric ceramic components generate high-power ultrasonic waves for ultrasonic cleaning, drilling, welding, and facilitating chemical processes. Piezoelectric ceramics are also used in many signal and information processing applications, including ultrasonic receivers and transmitters. They also play an important role in advanced sonar location and ranging, nondestructive material testing, and medical diagnostic equipment.

[0009] Piezoelectric ultrasonic transducers are used in various fields such as medical technology, food technology, process engineering, industrial production and automotive engineering. In the field of power ultrasound, the ultrasonic transducers used cover a power spectrum from a few watts (e.g. atomization) to several kilowatts (e.g. ultrasonic welding, ultrasonic cleaning) and a frequency range from approximately 20 kHz to 1 MHz.

[0010] In high-power applications, ultrasonic transducers used in this frequency range are typically constructed as metal-piezocomposite transducers (also known as bolted-on Langevin transducers, or BLT transducers). Piezoelectric ceramics use the inverse piezoelectric effect to convert alternating voltage into mechanical vibrations, which generate ultrasonic waves.

[0011] The piezoelectric ceramic, together with the electrode plates, is sandwiched between two metal parts by a screw bolt. The piezoelectric ceramic generates the mechanical vibrations, while the metal parts determine the frequency and amplitude distribution.

[0012] It has been shown that ceramics subjected to mechanical stress (such as in BLT transducers) usually fail prematurely due to tensile stress and crack initiation. To avoid tensile stress, piezoelectric ceramics must withstand a predefined prestress or preload (of a defined pressure). The optimal prestress depends on the compressive strength of the piezoelectric material and the static pressure distribution. The prestress or preload can be permanently applied within the device during operation, as in the case of BLT transducers.

[0013] The pretension (or preload) applied to the piezoelectric ceramic is crucial for the function and performance of the transducer. On the one hand, the applied mechanical compressive load prevents the piezoelectric ceramic from cracking due to mechanical tensile stress during operation. On the other hand, the preload minimizes friction losses at the joint. Furthermore, the preload also affects the performance of the ultrasonic transducer due to the nonlinearity of the material behavior.

[0014] Therefore, applying a targeted and reproducible mechanical preload is a core technology in building high-performance ultrasonic systems. If the preload is too low, there is a risk of damaging the piezoelectric ceramic. On the other hand, if the preload is too high, the piezoelectric ceramic will depolarize and lose its function (see Athena publication "Pressing and Bonding Piezoelectric Ultrasonic Converters").

[0015] For example, for ultrasonic transducers that operate at high temperatures or generate heat during operation, simply adjusting the preload to a specific value may not be sufficient because the preload may change due to temperature, which could result in reduced performance or even failure of the ultrasonic converter. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] International Publication No. 2022 / 2330984A1 [Patent Document 2] European Patent No. 3331840B1 [Non-patent literature]

[0017] [Non-Patent Document 1] "Lead-Free Piezoelectric Ceramics: Technology and Global Opportunities", Report Code: NAN063B, April 2022, BCC Publishing Summary of the Invention [Problem to be solved by the invention]

[0018] It was therefore an object of the present invention to provide a lead-free piezoelectric ceramic that can withstand high permanent mechanical preloads and does not heat up during use, in particular it must not depolarize or lose its piezoelectric properties.

[0019] This object has been solved by using a piezoelectric ceramic as claimed.

[0020] According to one aspect of the present invention, the lead-free piezoelectric ceramic is used in a device that requires a permanent mechanical preload to be applied to the lead-free piezoelectric ceramic, the applied mechanical preload being 20 MPa or greater, and the piezoelectric ceramic not losing its piezoelectric properties when used, for example, in an ultrasound transducer.

[0021] The lead-free piezoelectric ceramic has the following composition: [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM) (0 < X ≤ 1), (0 ≤ y < 1), (0 ≤ z < 1), (x + y + z = 1), (0.4 ≤ n ≤ 0.6), (0.4 ≤ m ≤ 0.6), and (0 ≤ a < 10) are applicable, M is any one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or a mixture thereof.

[0022] Surprisingly, when a preload of 20 MPa or more is applied to the lead-free piezoelectric ceramic, no warm-up or self-heating occurs. That is, depolarization accompanied by functional loss is not detected. Furthermore, surprisingly, the lead-free piezoelectric ceramic with a mechanical preload does not promote heat generation during use compared to conventional lead-containing piezoelectric ceramics. Usually, self-heating during use is correlated with a high value of dielectric loss tangent (tanδ). Therefore, it was surprising that the self-heating of the lead-free system used in the present invention was quite low. As will be detailed below, one of the reasons for this surprising behavior is the anisotropy coefficient.

[0023] As described above, due to a high mechanical preload, on the one hand, the frictional loss at the junction of the ceramic and the electrode of the ultrasonic transducer decreases, but on the other hand, the self-heating increases. The currently used lead-free ceramics show low self-heating under a high mechanical preload and vibration when used in an ultrasonic transducer. Therefore, a material having a depolarization temperature of less than 200 °C (such as less than 120 °C) may be suitable without functional loss.

[0024] Generally, when self-heating is suppressed low, the continuity of ultrasonic irradiation is improved. When less energy is lost by heating, most of the energy required for the operation of the ultrasonic transducer can be used for vibration. As a result, it is possible to operate the ultrasonic bath at room temperature, and cooling of the ultrasonic bath becomes unnecessary.

[0025] Furthermore, frictional heat is also reduced. For example, frictional heat is reduced by 40-50°C compared to conventional lead-containing piezoelectric ceramics. As frictional heat is reduced, electrical losses during use of ultrasonic transducers are also reduced, meaning that the efficiency of ultrasonic transducers using this lead-free piezoelectric ceramic is improved at mechanical preloads of 20 MPa or more.

[0026] Additionally, the operating time and overall lifespan of the ultrasonic transducer is improved due to reduced mechanical stress in the contact area between the piezoelectric ceramic and the metal electrode.

[0027] It has been noted that lead zirconate titanate (PZT) ceramics can be prestressed with preloads up to 35-45 MPa. When high power is applied, ceramic PZT components typically self-heat up to 50°C. However, surprisingly, lead-free ceramics can be prestressed with preloads in excess of 20 MPa with little self-heating (<10°C) during use.

[0028] The vibration or oscillation characteristics of piezoelectric ceramics are usually characterized by the electromechanical coupling coefficient k, which varies depending on the shape and vibration mode of the vibrator. The electromechanical coupling coefficient k is a measure of the efficiency of energy conversion from electrical energy to mechanical energy. Examples of coupling coefficients include the thickness coupling coefficient kt, which represents the vibration efficiency in the thickness direction of a thin disk, and the planar coupling coefficient kp, which represents the vibration efficiency in the radial direction of a thin disk.

[0029] The anisotropy coefficient (AK) is calculated by the formula AK=k t / (k t +k p )(k t = thickness coupling coefficient and k p = planar coupling coefficient) according to k t and k p The determination is carried out in accordance with DIN EN 50324-2 "Piezoelectric properties of ceramic materials and components, Part 2: Measurement methods - small signals".

[0030] The lead-free ceramic used according to the present invention has a planar coupling coefficient k in the range of 0.1 to 0.35, preferably 0.15 to 0.3, for example 0.2 to 0.35, for example 0.28 to 0.32, or 0.1 to 0.25, for example 0.15 to 0.2. p may have

[0031] The lead-free ceramic used according to the present invention has a thickness coupling coefficient k in the range of 0.4 to 0.6, preferably 0.45 to 0.5, for example 0.45 to 0.55, for example 0.5 to 0.52. t may have

[0032] According to one embodiment, the lead-free piezoelectric ceramic has an anisotropy coefficient kt / (kt+kp)≧0.6, preferably ≧0.65, more preferably ≧0.7, preferably in the range of 0.6-0.8, more preferably 0.65-0.75, and k t is the thickness coupling coefficient, and k p is the planar coupling coefficient.

[0033] Anisotropy coefficients above 0.6 result in a lower planar coupling coefficient compared to the thickness coupling coefficient, which reduces friction between the end faces of lead-free piezoelectric ceramics and adjacent (non-piezoelectric) transducer materials compared to piezoelectric materials with anisotropy coefficients below 0.6, such as PZT ceramics (lead zirconate titanate). The reduced lateral contraction due to the anisotropy coefficient reduces heat generation due to the effects of planar friction during operation.

[0034] In one embodiment, the lead-free piezoelectric ceramic used has a depolarization temperature T of 50°C to 200°C, preferably 80°C to 180°C, more preferably 90°C to 160°C, and even more preferably 130°C to 145°C. D Therefore, lead-free piezoelectric ceramics are particularly suitable for use in devices requiring mechanical preloads of 20 MPa or more at operating temperatures between 20°C and 100°C.

[0035] Piezoelectric materials are further characterized by their piezoelectric charge constant, d, which quantifies the volume change when a piezoelectric material is exposed to an electric field. The piezoelectric charge constant indicates the ratio between the generated charge and the applied force (direct piezoelectric effect, units C / N).

[0036] Depending on the direction of the applied stress and induced polarization, three different piezoelectric charge constants, d33, d31, and d32, can be defined, where d33 refers to the induced polarization along the Z axis per unit stress applied along the Z axis, d31 refers to the induced polarization along the Z axis per unit stress applied along the X axis, and d32 refers to the induced polarization along the X axis per unit shear stress applied about the Y axis.

[0037] The lead-free ceramic used in the present invention may have a piezoelectric charge constant d33 (pC / N) of 100 to 200, preferably 110 to 195, and more preferably 120 to 180. A high piezoelectric charge constant d33 is advantageous.

[0038] As mentioned above, piezoelectric components are also characterized by their loss tangent (tan δ), which indicates the ratio of power loss to reactive power when the component is excited with a sinusoidal signal at a frequency much lower than its lowest resonant frequency.

[0039] The dielectric dissipation factor (DF) is an important material property of piezoelectric ceramics that controls the amount of self-heating under resonant conditions. Therefore, a low DF is advantageous. DF is defined as the ratio of the equivalent series resistance (ESR) to the magnitude of the capacitive reactance (Xc), i.e., DF = ESR / |Xc|. DF is also known as the loss tangent or tan(δ), where the angle δ is the deviation from 90° between the voltage and current of an ideal capacitor (i.e., lossless). DF is also the ratio of lost energy to stored energy, or Re / |Im| in impedance. DF is typically measured at 120 Hz (for AC power sources) or 1000 Hz (more commonly). The higher the DF, the greater the amount of heat generated by I2 ESR heating (QuadTech, 2003; Gebbia, 2001). DF is an important material property of piezoelectric ceramics. DF controls the amount of self-heating under resonance conditions to quantify the specific material type of either the actuator or the resonator (see Impact of the Dissipation Factor of Piezoelectric Ceramics on Ultrasonic Transducer Performance - ScienceDirect).

[0040] The lead-free ceramic used in the present invention may have a tan δ of 30-700, preferably 40-600, more preferably 40-400, for example, 40-200, for example, 40-100, or 200-600, for example, 200-400.

[0041] The lead-free ceramic used in this invention has a good piezoelectric coefficient d33 and an acceptable depolarization temperature T D It should be noted that it has a dielectric loss tangent of 50 to 600.

[0042] This is surprising because most lead-free ceramics have a high depolarization temperature T D is usually correlated with a low piezoelectric charge constant d33, and vice versa. However, the depolarization temperature T DSuch a negative correlation between the piezoelectric charge constant d33 does not promote the use of such lead-free ceramics under high preloading, which is the object of the present invention.

[0043] In another embodiment, the lead-free piezoelectric ceramic used is characterized in that the breaking force (measured by the ball-on-ring method) exceeds 10 N, preferably 11 - 30 N for a sample with a diameter of 6 mm.

[0044] Examples of lead-free ceramics with an anisotropy coefficient exceeding 0.6 include bismuth sodium titanate (BNT) and bismuth sodium titanate barium titanate (BNT-BT). Adding one or more additional components such as ZnO increases the anisotropy coefficient compared to the reference without the additional components. General piezoelectric ceramics made of lead zirconate titanate (PZT) such as Sonox® P4 have an anisotropy coefficient of 0.6 or less.

[0045] In yet another embodiment, the density ρ of the lead-free piezoelectric ceramic is at least 5.0 g / cm 3 , preferably at least 5.5 g / cm 3 , preferably in the range of 5.0 - 7.0 g / cm 3 , more preferably in the range of 5.5 - 6.5 g / cm 3 , even more preferably in the range of 5.5 - 6.0 g / cm 3 (measured in accordance with DIN EN 60672-2(VDE 0335 Teil2):2000).

[0046] As described above, the lead-free piezoelectric ceramics used in the present invention have the following composition: [(100 - a)(x(Bi n Na m )TiO3 - zBaTiO3)-aM] (0 < X ≦ 1), (0 ≦ y < 1), (0 ≦ z < 1), (x + y + z = 1), (0.4 ≦ n ≦ 0.6), (0.4 ≦ m ≦ 0.6), and (0 ≦ a < 10) are applicable, M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or a mixture thereof.

[0047] In one embodiment, the lead-free piezoelectric ceramic used in the present invention has the following composition: [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM] a) where (0.8≦x≦1), (y=0), (0≦z≦0.2), (x+y+z=1), (n=m=0.5) and (0≦a<5) apply, and M is the additive ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof; b) (0.6≦x≦0.9), (0.1≦y≦0.4), (z=0), (x+y+z=1), (n=m=0.5), (0≦a<5) apply, where M is the additive ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof, or c) A combination of a) and b) in a three-phase system where y>0 and z>0.

[0048] In a further embodiment, the lead-free piezoelectric ceramic used in this example has the following composition: [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM)] a) where (0.92≦x≦0.95), (y=0), (0.05≦z≦0.08), (x+y+z=1), (n=m=0.5) and (0≦a<3) apply, and M is an additive ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof, or b) (0.77 ≤ x ≤ 0.81), (0.19 ≤ y ≤ 0.23), (z = 0), (x + y + z = 1), (n = m = 0.5) and (0 ≤ a < 3) are applicable, and M is either ZnO, MgO, TiO2, ZrO2, Al2O3, or a mixture thereof as an additive, or c) It is a combination of a) and b) in a three-phase system where y > 0 and z > 0.

[0049] In another preferred embodiment, (0 < a < 10); (0 < a < 5) and (0 < a < 3) are respectively applied to the described formulas; that is, a may not be zero.

[0050] In a more preferred embodiment, the lead-free piezoelectric ceramics are essentially free of calcium (Ca) and / or strontium (Sr). Being essentially free means that calcium-containing compounds and / or strontium-containing compounds are not added to the ceramics during the synthesis process. However, it should be understood that trace impurities at the ppm level may be contained in the piezoelectric ceramics. By removing calcium, high-temperature stability with excellent piezoelectric charge constant d33 is achieved.

[0051] It should be understood that the following two types of ceramics may be excluded from the above-mentioned lead-free ceramics: 0.79(Bi 0.5 Na 0.5 )TiO3 - 0.14(Bi 0.5 K 0.5 )TiO3 - 0.07 BaTiO3 and 0.88(Bi 0.5 Na 0.5 )TiO3 - 0.08(Bi 0.5 K 0.5 )TiO3 - 0.04 BaTiO3.

[0052] In a further preferred embodiment, the lead-free piezoelectric ceramics essentially do not contain potassium (K). "Essentially do not contain" means that potassium-containing compounds are not added to the ceramics during the synthesis process. However, it should be understood that trace amounts of impurities at the ppm level may be present in the piezoelectric ceramics.

[0053] Therefore, lead-free piezoelectric ceramics having the following composition are used in this example: [(100 - a)(x(Bi n Na m )TiO3 - zBaTiO3) - aM] (0 < x ≤ 1), (0 ≤ z < 1), (x + z = 1), (0.4 ≤ n ≤ 0.6), (0.4 ≤ m ≤ 0.6), (0 ≤ a < 10) are applicable, M is any one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or a mixture thereof.

[0054] In one embodiment, the lead-free piezoelectric ceramics used in this example have the following composition: [(100 - a)(x(Bi n Na m )TiO3 - zBaTiO3) - aM] a) Here, (0.8 ≤ x ≤ 1), (0 ≤ z ≤ 0.2), (x + z = 1), (n = m = 0.5) and (0 ≤ a < 5) are applicable, and M is any one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof, or b) (x = 1), (z = 0), (x + z = 1), (n = m = 0.5) and (0 ≤ a < 5) are applicable, and M is any one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof, or c) It is a combination of a) and b) in a three-phase system.

[0055] In a further embodiment, the lead-free piezoelectric ceramics used in this example have the following composition: [(100 - a)(x(Bi n Na m)TiO3-zBaTiO3)-aM] a) where (0.92≦x≦0.95), (0.05≦z≦0.08), (x+z=1), (n=m=0.5) and (0≦a<3) apply, and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof, or b) (x=1), (z=0), (x+z=1), (n=m=0.5) and (0≦a<3) apply, where M is the additive ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof, or c) A combination of a) and b) in a three-phase system.

[0056] In yet another preferred embodiment, the lead-free piezoelectric ceramic used in this example has the following composition: x(Bi n Na m )TiO3-zBaTiO3 (0.92≦x≦0.935), (0.065≦z≦0.08), (x+z=1), (n=m=0.5).

[0057] The lead-free ceramic according to the above preferred embodiment has a planar coupling coefficient k in the range of 0.15 to 0.35, for example, 0.28 to 0.32. p , thickness coupling coefficient k in the range of 0.45 to 0.55, e.g., 0.5 to 0.53 t a piezoelectric charge constant d33 (in pC / N) of 150 to 200, preferably 170 to 180; a depolarization temperature T of 80°C to 160°C, more preferably 80°C to 120°C; D and may have a tan δ in the range of 200 to 600, for example, 200 to 400.

[0058] In yet another preferred embodiment, the lead-free piezoelectric ceramic used in this example has the following composition: [(100-a)(x(Bi n Na m )TiO3-zBaTiO3)-aM] The following conditions apply: (0.92≦x≦0.935), (0.065≦z≦0.08), (x+z=1), (n=m=0.5), (0.5≦a<3), where M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof.

[0059] In another preferred embodiment, the ceramic may have the composition described above, for example, where a=0.75 (0.5≦a<1) or a=2.5 (2≦a<3) applies.

[0060] The lead-free ceramic used in accordance with the above preferred embodiment has a planar coupling coefficient k in the range of 0.1 to 0.25, for example, 0.15 to 0.2. p , thickness coupling coefficient k in the range of 0.43 to 0.55, e.g., 0.48 to 0.52 t a piezoelectric charge constant d33 (pC / N) of 100 to 150, preferably 110 to 140; a depolarization temperature T of 120°C to 200°C, preferably 130°C to 170°C; D and may have a tan δ of 30 to 700, for example, 40 to 100.

[0061] As mentioned above, the lead-free piezoelectric ceramics used in accordance with the present invention may contain some (trace) impurities. Therefore, the lead-free piezoelectric ceramics may further contain up to 1000 ppm of metal oxides. Possible metal oxides are oxides of any of the following metals: Fe, Ni, Ca, Si, K, Y, Sr, Nb, P, Sn, Sb, Hf, Mn, Li, Cl, Co, Ag, Mo, W, Pb, and Cd. These impurities may result from the manufacturing process and may have a minor effect on the properties of the piezoelectric ceramics.

[0062] Such ceramics are generally known from WO 2022 / 2330984 A1, in which the depolarization temperature, k p value, k tVarious properties of lead-free ceramics, such as the strength and density, have been described. However, the effect of subjecting lead-free ceramics to permanent mechanical preloads of more than 20 MPa has not been described or suggested.

[0063] It should be noted that another type of lead-free piezoelectric ceramic is known in the art: sodium potassium niobate (KNN)-based materials. KNN-based materials do not contain lead but do contain niobium. Recent environmental assessments have shown that niobium has a significant environmental impact, even early in its production cycle, due to the processes used to extract and refine the raw material. It is also important to consider that ores often contain heavy metals and radioactive materials that must be separated and disposed of. Furthermore, the energy required to manufacture KNN components (drying, firing, and sintering) is higher than that required to manufacture lead-containing PZT ceramics. Therefore, there is currently no consensus that KNN is truly a more environmentally friendly material than traditional lead-containing PZT ceramics. Furthermore, as shown in Table 2 below, KNN ceramics have a low anisotropy coefficient, making them unsuitable for use in the present invention.

[0064] The lead-free piezoelectric ceramics are preferably obtained according to EP 3 331 840 B1 "Production of lead-free piezoelectric ceramics in an aqueous environment" (Patent Document 2); other methods for the synthesis of lead-free piezoelectric ceramics are also possible.

[0065] In a typical process, a homogeneous aqueous suspension containing Bi2O3, Na2CO3, TiO2, BaTiO3, and other optional components is first prepared. The aqueous suspension is freeze-dried or spray-dried, and then calcined, for example, at 800-900°C. To produce doped lead-free materials or lead-free composites, one or more additional components, introduced as oxide powders, are added to the base system either before or after calcination.

[0066] In a more specific embodiment, the synthesis process for the lead-free piezoelectric ceramic comprises the following steps: - preparing the required amount of starting materials (Bi2O3, Na2CO3, TiO2, BaTiO3, etc.); - mixing and freezing the starting materials; - calcination, for example at a temperature of 800°C or higher, - a milling / deagglomeration / doping step, especially with ZnO, - adding organic additives such as PVA, PEG, etc.; - granulating, for example by spray drying, - shaping the granulation, for example under pressure, - heating and sintering, - Final processing steps such as metallization and poling.

[0067] The lead-free piezoelectric ceramic of the present invention is suitable for use in a drive unit for an ultrasonic transducer.

[0068] Such ultrasonic transducers, for example, BLT (bolt-locked Langevin) transducers, include a drive unit, which is formed by stacking multiple piezoelectric ceramics and electrode plates as described above. The drive unit is sandwiched between a front mass and a rear mass, which are fixed together using fastening bolts with a preload of 20 MPa or more applied. The front mass emits ultrasonic waves from its surface. [Brief explanation of the drawings]

[0069] The invention will now be explained in more detail with reference to examples and drawings. [Figure 1] The strength of lead-free piezoelectric ceramics is shown in comparison with various PZT-based materials. [Figure 2] 1 shows the temperature profile applied to the sample to determine the depolarization temperature T D . DETAILED DESCRIPTION OF THE INVENTION

[0070] (Example) The following examples are included to demonstrate certain aspects and embodiments of the claimed invention, although those of ordinary skill in the art should understand that the following descriptions are illustrative only and are not intended to limit the invention in any way.

[0071] Table 1 shows embodiments of the lead-free piezoelectric ceramic according to the present invention.

[0072] The lead-free piezoelectric ceramics shown in Table 1 are synthesized according to the following examples: -Prepare the required amounts of starting materials Bi2O3, Na2CO3, TiO2, BaTiO3; -Wet mixing and deagglomeration of the starting materials; -Provide a uniformly distributed freezing; Firing at temperatures above -800°C; -The grinding process is carried out in a mill containing ZnO; -Add PVA and PEG; -Perform granulation by spray drying; -forming the granules under pressure; Debinding at temperatures above -600°C; -Sintering at different temperatures above 1000°C; -Perform hard machining; - carrying out metallization, -5kV / mm polarization treatment -Measurements are taken 24 hours after polarization.

[0073] PZT Sonox® P4 and KNN are comparative examples. PZT Sonox® P4 is a commercially available lead-containing piezoelectric ceramic, while KNN is a sodium potassium niobate-based ceramic ("Lead-Free Piezoelectric Ceramics: Technology and Global Opportunities," Report Code: NAN063B, April 2022, BCC Publishing) (Non-Patent Document 1).

[0074] [Table 1]

[0075] k t = thickness coupling coefficient k p = Planar coupling coefficient AK = anisotropy coefficient ρ=density d33 = piezoelectric constant d31 = piezoelectric constant (perpendicular to the polarization direction) T D = Depolarization temperature (linked to phase transition) tanδ = dielectric tangent (dielectric loss coefficient) Curie temperature (°C): The temperature at which the dielectric constant of a ferroelectric ceramic reaches its maximum value. At this temperature, the piezoelectric ceramic loses its polarization. For this reason, the operating temperature should generally not exceed half the Curie temperature.

[0076] As shown in Table 1, the lead-free ceramics used in the present invention are PZT Sonox® P4 and KNN, and their k p The depolarization temperature T is the temperature at which the ceramics depolarize. D When the temperature is set to correspond to the temperature of the material, half the Curie temperature can be used.

[0077] Furthermore, experimental data (not shown) reveal that PZT Sonox® P4 and BNT-based ceramics exhibit different temperature profiles in oscillator configurations. These data support the theory that BNT-based ceramics do not promote self-heating or warm-up during use compared to traditional lead-containing piezoelectric ceramics.

[0078] This effect is due to the lower anisotropy coefficient of PZT Sonox® P4 compared to currently used BNT-based ceramics.

[0079] As previously mentioned, currently used BNT-based ceramics exhibit reduced frictional work when used between converter materials in ultrasonic transducers.

[0080] Friction work W R is the formula W R =F * μ * s (F = force, μ = coefficient of friction, s = displacement). Aniso (lead-free ceramic with high anisotropy coefficient >0.6) and s PZT (PZT ceramic, e.g. Sonox® P4) has a charge constant d 31Aniso and d 31PZT The ratio of frictional power W to frictional power is 0.15 to 0.35 for the materials listed in Table 1. Taking into account different densities, the frictional power W of the materials listed in Table 1 can be calculated as follows: RAniso / W RPZT (as a measure of self-heating) ranges from 0.11 to 0.27, i.e., the work of friction is reduced by 11 to 27% for lead-free ceramics with anisotropy coefficients above 0.6 compared to PZT ceramics.

[0081] The reduced friction between the end faces has the advantage of reducing frictional heat generation and electrical loss. Until now, the typical application temperature was above 120-130°C due to the low anisotropy coefficient of 0.6 or less and the associated high heat generation. This allows frictional heat to be reduced to less than 50°C in high preload applications exceeding 20 MPa. Another benefit resulting from the reduced heat generation is that when the anisotropy coefficient exceeds 0.6 and the depolarization temperature T D The lead-free material with a temperature of less than 140°C is suitable for high preload applications exceeding 20 MPa.

[0082] Lead-free piezoelectric ceramics with an anisotropy coefficient greater than 0.6, such as BNT-BT or BNT-BT composites, have fracture forces ranging from 11 to 29 N, averaging more than 15 N, measured by the ball-on-ring method on 6 mm diameter samples. Therefore, they possess fracture strengths more than twice those of conventional soft PZT-based ceramics, such as Sonox® P5. In the case of Sonox® P5, an average fracture force of approximately 5 N was measured. Within the range of dispersion, lead-free piezoelectric ceramics with an anisotropy coefficient greater than 0.6 can achieve strengths up to five times greater. Hard PZT ceramics, such as Sonox® P4, have fracture strengths averaging approximately 12 N, which corresponds to the lower end of the strength range for lead-free piezoelectric ceramics (see also the diagram in Figure 1).

[0083] (Explanation of breaking force measurement using the ball-on-ring method (for a sample with a diameter of 6 mm)) Required equipment: -Measuring frame (1) with built-in actuator and load cell, prepared for destructive testing -Homemade Charge Amplifier (2) - Voltage source TTI PLH250-P(3) - Agilent DSO-X 2024A oscilloscope (4) -Force measuring multimeters(5)

[0084] (procedure) The piezoelectric disc is positioned over the load cell so that only the edge areas are in contact with the ring. Then, a ball attached to the actuator is positioned over the sample so that the applied voltage is zero (no force is applied to the sample). By slowly increasing the voltage applied to the actuator, the actuator will extend and apply a defined force to the sample. The increase in applied voltage can be tracked on an oscilloscope. As the applied voltage increases, the force acting on the disc also increases. Fracture appears as a sudden drop in the voltage curve, as the actuator no longer has any resistance to pressing down on the disc. The maximum voltage before the drop reflects the maximum force that can be withstood. This value is calculated by multiplying the voltage by the charge scale of the charge amplifier.

[0085] (depolarization temperature T D measurement routine) The sample (preferably a thin disk) is mounted in a temperature kiln and the temperature profile shown in Figure 2 is applied.

[0086] The process starts at 20°C. Heating continues up to 180°C, and the small signal impedance is measured continuously every 2K. As the material is depolarized, k t The value drops to zero. Take a measurement at the point where the curve drops sharply and subtract 10°C (to ensure piezoelectric operation in environments close to this point). This calculated temperature is T D (depolarization temperature).

Claims

1. Use of a lead-free piezoelectric ceramic in a device that requires a permanent preload of 20 MPa or more to be applied to the lead-free piezoelectric ceramic, wherein the lead-free piezoelectric ceramic has the following composition: (1000)(g(3). ｎ 90 ｍ THIS IS ３ 9(D) ｎ ( 1999 ). ｍ THIS IS ３ THIS THIS ３ )NIGHT (0<X≦1), (0≦y<1), (0≦z<1), (x+y+z=1), (0.4≦n≦0.6), (0.4≦m≦0.6), and (0≦a<10) apply; M is additive ZnO, MgO, TiO ２ , ZrO ２ , Al ２ O ３ or a mixture of these.

2. The lead-free piezoelectric ceramic has the following composition: (1000)(g(3). ｎ 90 ｍ THIS IS ３ THIS THIS ３ EXPERIENCE (0<x≦1), (0≦z<1), (x+z=1), (0.4≦n≦0.6), (0.4≦m≦0.6), and (0≦a<10) apply; M is additive ZnO, MgO, TiO ２ , ZrO ２ , Al ２ O ３ 2. Use of the lead-free piezoelectric ceramic according to claim 1, characterized in that it is any one of the following or a mixture thereof.

3. The lead-free piezoelectric ceramic has the following composition: x() ｎ ! ｍ )) ３ : !)ﯯ ３ 3. Use of the lead-free piezoelectric ceramic according to claim 1 or 2, characterized in that (0.92≦x≦0.935), (0.065≦z≦0.08), (x+z=1), and (n=m=0.5).

4. The lead-free piezoelectric ceramic has the following composition: (1000)(g(3). ｎ 90 ｍ THIS IS ３ THIS THIS ３ EXPERIENCE (0.92≦x≦0.935), (0.065≦z≦0.08), (x+z=1), (n=m=0.5), and (0.5≦a<3) are applied, and M is an additive such as ZnO, MgO, TiO ２ , ZrO ２ , Al ２ O ３ 4. Use of the lead-free piezoelectric ceramic according to claim 1, wherein the piezoelectric ceramic is any one of the following, or a mixture thereof.

5. The lead-free piezoelectric ceramic has a planar coupling coefficient k in the range of 0.1 to 0.35, preferably 0.15 to 0.32, for example, 0.15 to 0.35, for example, 0.28 to 0.32, or 0.1 to 0.25, for example, 0.15 to 0.

2. ｐ Use of the lead-free piezoelectric ceramic according to any one of claims 1 to 4, characterized in that it has

6. The lead-free piezoelectric ceramic has a thickness coupling coefficient k in the range of 0.4 to 0.6, preferably 0.48 to 0.52, for example 0.45 to 0.55, for example 0.5 to 0.52, or 0.43 to 0.55, for example 0.48 to 0.

52. ｔ The present invention is characterized by having the following. Use of the lead-free piezoelectric ceramic according to claims 1 and 2.

7. The lead-free piezoelectric ceramic has an anisotropy coefficient of k ｔ / (k ｔ +k ｐ ) ≧0.6, preferably ≧0.65, more preferably ≧0.7, preferably 0.6 to 0.8, more preferably 0.65 to 0.79, more preferably 0.7 to 0.79; k ｔ is the thickness coupling coefficient, and k ｐ Use of the lead-free piezoelectric ceramic according to any one of claims 1 to 6, characterized in that is the planar coupling coefficient.

8. The lead-free piezoelectric ceramic has a depolarization temperature T of 50°C to 200°C, preferably 80°C to 180°C, more preferably 90°C to 160°C, and even more preferably 130°C to 145°C. Ｄ Use of the lead-free piezoelectric ceramic according to any one of claims 1 to 7, characterized in that it has

9. The lead-free piezoelectric ceramic has a piezoelectric charge constant d of 100 to 200, preferably 110 to 190, and more preferably 120 to 180. ３３ Use of the lead-free piezoelectric ceramic according to any one of claims 1 to 8, characterized in that it has a pC / N ratio of 0.01 to 0.

01.

10. 10. Use of lead-free piezoelectric ceramics according to any one of claims 1 to 9, characterized in that the lead-free piezoelectric ceramics have a tan δ of 30 to 700, preferably 40 to 600, more preferably 40 to 400, for example 40 to 700, for example 40 to 100 or 200 to 600, for example 200 to 400.

11. Use of the lead-free piezoelectric ceramic according to any one of claims 1 to 10 in a device requiring a permanent preload of 20 MPa or more on the piezoelectric ceramic at an operating temperature of 20 to 100°C.

12. 11. Use of lead-free piezoelectric ceramics according to any one of claims 1 to 10, characterized in that the lead-free piezoelectric ceramics have a breaking force (measured by the ball-on-ring method) of more than 10 N, preferably between 11 and 30 N.

13. The lead-free piezoelectric ceramic has a density of at least 5.0 g / cm ３ , preferably at least 5.5 g / cm ３ , preferably 5.0 to 7.0 g / cm ３ More preferably, in the range of 5.5 to 6.5 g / cm ３ and even more preferably in the range of 5.5 to 6.0 g / cm ３ Use of lead-free piezoelectric ceramics according to any one of claims 1 to 10, characterized in that they have a density ρ in the range of

14. 11. Use of lead-free piezoelectric ceramics according to any one of claims 1 to 10, characterized in that the lead-free piezoelectric ceramics are essentially free of calcium (Ca) and / or strontium (Sr) except for trace impurities.

15. 15. Use of a lead-free piezoelectric ceramic according to any one of the preceding claims, characterized in that the lead-free piezoelectric ceramic comprises a further metal oxide, which is an oxide of one of the following metals: Fe, Ni, Ca, Si, K, Y, Sr, Nb, P, Sn, Sb, Hf, Mn, Li, Cl, Co, Ag, Mo, W, Pb, Cd.

16. Use of the lead-free piezoelectric ceramic according to any one of claims 1 to 15 in a drive unit for an ultrasonic transducer.

17. 17. An ultrasonic transducer comprising a drive unit, the drive unit being formed by stacking a plurality of piezoelectric ceramics and electrode plates according to any one of claims 1 to 16, the drive unit being sandwiched between a front mass and a rear mass, the front mass and the rear mass being fastened together by fastening bolts, and a preload applied by the fastening bolts being 20 MPa or more.

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