Use of leadless piezoelectric ceramics in devices requiring high permanent preloading

Abstract

The invention relates to application of a lead-free piezoelectric ceramic in a device needing to apply a permanent preload to the lead-free piezoelectric ceramic.

Description

[0001] The present invention relates to the use of lead-free piezoelectric ceramics in devices requiring high permanent preloads on lead-free piezoelectric ceramics.

[0002] Description

[0003] Piezoelectric ceramics are capable of converting mechanical quantities such as pressure and acceleration into electrical quantities, or vice versa, of transforming electrical signals into mechanical motion or oscillations.

[0004] The piezoelectricity is based on the ability of certain crystals to generate an electric charge when subjected to mechanical stress or tension (direct piezoelectric effect). Conversely, these crystals undergo a controlled deformation when exposed to an electric field (behavior known as inverse piezoelectric effect). The polarity of the charge depends on the orientation of the crystal with respect to the direction of the stress.

[0005] Ceramics that exhibit piezoelectric properties belong to the group of ferroelectric materials. Conventional systems are mainly based on lead zirconate titanate (PZT), i.e. they consist of a mixed crystal of lead zirconate (PbZr03) and lead titanate (PbTi03).

[0006] Immediately after sintering, the domains of the ceramic body, i.e. regions consisting of unit cells with a uniform direction of the electric dipoles, will show arbitrary (statistically distributed) orientations, i.e. the macroscopic body is isotropic and does not exhibit piezoelectric properties. These piezoelectric properties must be derived from "polarization". In this process, the ceramic body is exposed to a strong DC electric field, which causes the electric dipoles to align in the direction of the field. Even when the DC field is no longer applied (a necessary condition for the piezoelectric behavior of ferroelectric ceramics), they will retain this orientation (remanent polarization).

[0007] A complete or partial elimination of the domain alignment achieved by the polarization process (depolarization) will reduce the piezoelectric properties of the material. Depolarization can be the result of three factors: thermal depolarization due to thermal exposure; electrical depolarization due to the action of an electric field opposite to the original polarization direction; mechanical depolarization caused by high compressive loads, especially in the case of short-circuited electrodes.

[0008] Piezoelectric ceramic components are used in a wide variety of electromechanical transducers covering a wide frequency range. In sensors, they are capable of converting forces, pressures and accelerations into electrical signals. In sound generators and ultrasonic transducers, they transform voltages into oscillations or deformations.

[0009] In ultrasonic applications, piezoelectric ceramic components generate high-power ultrasound for ultrasonic cleaning, drilling and welding, and for stimulating chemical processes. On the other hand, piezoelectric ceramics are found in many signal and information processing solutions in the form of ultrasonic receivers and transmitters. They also play a key role in advanced sonar positioning and ranging, non-destructive material testing and medical diagnostic equipment.

[0010] Piezoelectric ultrasonic transducers can be found in various fields of application 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 of about 20 kHz to 1 MHz.

[0011] Ultrasonic transducers for high-power applications used in this frequency range are usually constructed as metal-piezoelectric composite transducers (also referred to as bolted Langevin type transducers - or BLT transducers). The piezoceramics converts the alternating voltage into mechanical vibrations by the inverse piezoelectric effect and thus generates ultrasound.

[0012] The piezoceramics is clamped between two metal parts by means of threaded bolts together with the electrode plates. When the piezoceramics generates mechanical vibrations, the metal parts determine the frequency and amplitude distribution.

[0013] It has been shown that ceramics under mechanical stress (as in BLT transducers) usually fail early due to tensile stress and crack formation. In order to avoid tensile stress, the piezoceramics must be subjected to a predefined (defined stress) prestress or preload. The optimum prestress depends on the piezoelectric material's compressive strength and static pressure distribution. Said prestress or preload can also be permanently applied during device operation, as in BLT transducers.

[0014] The applied prestress (or preload) in the piezoceramics has a decisive importance for the functionality and performance of the transducer. On the one hand, the applied mechanical compression load protects the piezoceramics from cracking due to mechanical tensile stress during operation. On the other hand, the preload minimizes the frictional losses at the joints. Furthermore, due to the non-linearity of the material behavior, the preload influences the performance of the ultrasonic transducer.

[0015] Therefore, the targeted and reproducible application of the mechanical preload is the core competence for building high-performance ultrasonic systems. If the preload is too low, there is a risk of damage to the piezoceramics; on the other hand, if the preload is too high, the piezoceramics will depolarize and thus lose its functionality (see publication "Pre-tensioning and bonding of piezoelectric ultrasonic converter" by Athena).

[0016] The specific adjustment of the preload is usually not sufficient for ultrasonic transducers that are operated at high temperatures, for example, or that heat up during operation, because the preload can change due to the temperature. This can lead to a reduction in the performance or failure of the ultrasonic transducer.

[0017] It is therefore an object of the present invention to provide a lead-free piezoelectric ceramic which can be used and withstand high permanent mechanical preloads and which does not heat up in use. In particular, depolarization and loss of piezoelectric properties should not occur.

[0018] This object is solved by using a piezoelectric ceramic as described in the claims.

[0019] According to one aspect of the present invention, a lead-free piezoelectric ceramic is used in a device where a permanent mechanical preload is applied to the lead-free piezoelectric ceramic, wherein the applied mechanical preload is > 20 MPa, and wherein the piezoelectric ceramic does not lose its piezoelectric properties in use, e.g. in an ultrasonic transducer.

[0020] The lead-free piezoelectric ceramic has the following composition:

[0021] [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM]

[0022] wherein (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, and

[0023] M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof.

[0024] Unexpectedly, no heating or self-heating occurs when a preload of > 20 MPa is applied to the lead-free piezoelectric ceramic; i.e. no depolarization with functional loss is detected. Furthermore, it is unexpected that the mechanically pre-stressed lead-free piezoelectric ceramic does not promote heating in use compared to conventional lead-containing piezoelectric ceramics. Self-heating during application is usually associated with a high dissipation factor (tan delta). Therefore, it is unexpected that the self-heating of the lead-free system used according to the present invention is rather low. One of the reasons for this unexpected behavior is the anisotropy factor, as will be discussed in more detail below.

[0025] As mentioned above, while on the one hand the high mechanical preload reduces the frictional losses at the junction of the ceramic and the electrode in the ultrasonic transducer, on the other hand the high mechanical preload increases the self-heating. Now, the lead-free ceramics currently used show a low self-heating under high mechanical preloads and oscillations when used in ultrasonic transducers. Therefore, a material with a reduced depolarization temperature of below 200°C, e.g. below 120°C, can be suitable and without functional loss.

[0026] Generally, the lower and reduced self-heating enables an improved continuity of the ultrasonic radiation. When less energy is lost due to the heating, a larger portion of the energy required for operating the ultrasonic transducer can be used for oscillation. The ultrasonic bath can be operated at room temperature, i.e. no cooling of the ultrasonic bath is required.

[0027] Furthermore, the frictional heat is reduced. For example, the frictional heat is reduced by 40°C to 50°C compared to conventional lead-containing piezoelectric ceramics. The reduced frictional heat is accompanied by a lower electrical loss when using the ultrasonic transducer, i.e. the efficiency of the ultrasonic transducer using the lead-free piezoelectric ceramic of the present application at a mechanical pre-load of 20 MPa or more is improved.

[0028] Furthermore, the mechanical stress of the contact area of the piezoelectric ceramic with the metal electrode is reduced and, thus, the operating time and the overall lifetime of the ultrasonic transducer are increased.

[0029] It is noted that zirconium titanate lead (PZT) ceramics can be pre-stressed with a pre-load of up to 35 MPa to 45 MPa. During high power applications, ceramic PZT components typically generate a self-heating of up to 50°C. However, it was unexpected that the lead-free ceramics can be pre-stressed with a pre-load of more than 20 MPa during the application without substantial self-heating (<10°C).

[0030] The vibration or oscillation properties of piezoelectric ceramics are generally characterized, inter alia, by their electromechanical coupling factor k, which differs depending on the oscillator geometry and the vibration mode. They are a measure of the energy conversion efficiency from electrical to mechanical energy. Examples of coupling factors are the thickness coupling factor k t (which describes the vibration efficiency of a thin disc in the thickness direction) or the planar coupling factor k p (which describes the vibration efficiency of a thin disc in the radial direction).

[0031] The anisotropy coefficient (AK) is calculated according to the formula AK = k t / (k t +k p ) (where k t = thickness coupling factor, and k p = planar coupling factor). k t and k p are determined according to DIN EN 50324-2 "Piezoelectric properties of ceramic materials and components, Part 2: Measuring method - small signal".

[0032] The planar coupling factor k pThe thickness coupling factor k can be 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.

[0033] The thickness coupling factor k of the lead-free ceramic used according to the application t The thickness coupling factor k can be 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.

[0034] The anisotropy factor k of the lead-free piezoelectric ceramic according to one embodiment t (k t +k p ) > 0.6, preferably > 0.65, more preferably > 0.7, preferably in the range of 0.6 to 0.8, more preferably 0.65 to 0.75, wherein k t is the thickness coupling factor and k p is the planar coupling factor.

[0035] The high anisotropy factor > 0.6 compared to piezoelectric materials with a low anisotropy factor of < 0.6, for example PZT ceramic (lead zirconate titanate), leads to a reduction of the friction between the end face of the lead-free piezoelectric ceramic and the adjacent (non-piezoelectric) transducer material due to the lower planar coupling factor compared to the thickness coupling factor. The reduction of the lateral contraction due to the anisotropy factor leads to a reduction of the heat generation due to the planar friction effects during operation.

[0036] In one embodiment, the depolarization temperature T D of the lead-free piezoelectric ceramic used is in the range of 50 °C to 200 °C, preferably 80 °C to 180 °C, more preferably 90 °C to 160 °C, even more preferably 130 °C to 145 °C. Thus, the lead-free piezoelectric ceramic is suitable for devices requiring a mechanical pre-load > 20 MPa, in particular at operating temperatures of 20 °C to 100 °C.

[0037] The piezoelectric material is further characterized by the piezoelectric charge constant d. The piezoelectric charge constant quantifies the volume change when the piezoelectric material is subjected to an electric field. It represents the ratio of the charge generated to the force applied (direct piezoelectric effect, unit C / N).

[0038] Depending on the direction of the applied stress and the induced polarization, three different piezoelectric charge constants can be defined: d 33 , d 31 and d 32 , wherein d 33 means induced polarization in z-axis direction / unit stress applied in z-axis, d 31 means induced polarization in z-axis / unit stress applied in x-axis, d 32means the polarization induced in the x-axis / unit shear stress applied about the y-axis.

[0039] The piezoelectric charge constant d33 of the lead-free ceramic used in the present invention 33 (in pC / N) can be 100 to 200, preferably 110 to 195, more preferably 120 to 180. Thereby, a high piezoelectric charge constant d33 33 has advantages.

[0040] As mentioned above, the piezoelectric component can also be characterized by its dissipation factor (tan delta). The dissipation factor represents the ratio of power loss to reactive power when the component is excited with a sinusoidal signal at a frequency well below its lowest resonant frequency.

[0041] The dissipation factor (DF) is an important material property of piezoelectric ceramics that determines the amount of self-heating under resonance conditions. Thereby, a low dissipation factor has advantages. The dissipation factor (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|. It is also known as the loss tangent or tan(delta), where the angle delta is the deviation from 90° between the voltage and current of an ideal capacitor (i.e. no loss). DF is also the ratio of (energy loss) / (energy storage) of the electrical impedance or Re / |Im|. It is usually measured at 120 Hz (for AC power supplies) or 1000 Hz (more common). The higher the DF, the more heat is generated via I 2ESR (QuadTech, 2003, Gebbia, 2001). DF is an important material property of piezoelectric ceramics; it determines the amount of self-heating under resonance conditions and thus quantifies the specific material type of an actuator or resonator (see: The Effects of Piezoelectric Ceramic Dissipation Factor on the Performance of Ultrasonic Transducers - ScienceDirect).

[0042] The tan delta of the lead-free ceramic used in the present invention can be 30 to 700, preferably 40 to 600, more preferably 40 to 400, for example 40 to 200, for example 40 to 100, or 200 to 600, for example 200 to 400.

[0043] It has to be noted that the lead-free ceramic used in the present invention combines a good piezoelectric coefficient d33 33 , an acceptable depolarization temperature Td D and a dissipation factor of 50 to 600.

[0044] This is unexpected, since for most lead-free ceramics the high depolarization temperature T D is usually correlated with a low piezoelectric charge constant d 33 and vice versa. However, such a negative correlation between the depolarization temperature T D and the piezoelectric charge constant d 33 does not promote the use of such lead-free ceramics under high preloads, which is the object of the present invention.

[0045] In another embodiment, the lead-free piezoelectric ceramic used is characterized by a fracture force for a sample with a diameter of 6 mm > 10 N, preferably 11 N to 30 N, measured by the ball-on-ring method.

[0046] Lead-free ceramics with an anisotropy factor > 0.6 are, for example, sodium bismuth titanate (BNT) or sodium bismuth titanate-barium titanate (BNT-BT). The addition of one or more further components, such as ZnO, leads to an increase in the anisotropy factor compared to the reference without further components. Common piezoelectric ceramics made of lead zirconate titanate (PZT) have, for example, an anisotropy factor of P4 ≤ 0.6.

[0047] In yet another embodiment, the lead-free piezoelectric ceramic has a density p of at least 5.0 g / cm 3 , preferably at least 5.5 g / cm 3 , preferably in the range of 5.0 g / cm 3 to 7.0 g / cm 3 , more preferably in the range of 5.5 g / cm 3 to 6.5 g / cm 3 , even more preferably in the range of 5.5 g / cm 3 to 6.0 g / cm 3 , measured according to DIN EN 60672-2 (VDE 0335 Teil 2): 2000.

[0048] As mentioned above, the lead-free piezoelectric ceramic used in the present invention has the following composition:

[0049] [(100 - a)(x(Bi n Na m )TiO3 - y(Bi n K m )TiO3 - zBaTiO3) - aM]

[0050] wherein (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, and

[0051] M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or mixtures thereof. In one embodiment, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0052] [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM]

[0053] a) where applicable (0.8 < x < 1), (y = 0), (0 < z < 0.2), (x + y + z = 1), (n = m = 0.5) and (0 < a < 5), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or mixtures thereof, or

[0054] b) where applicable (0.6 < x < 0.9), (0.1 < y < 0.4), (z = 0), (x + y + z = 1), (n = m = 0.5) and (0 < a < 5), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or mixtures thereof, or

[0055] c) a combination of a) and b) in a three-phase system, where y > 0 and z > 0.

[0056] In another embodiment, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0057] [(100-a)(x(Bi n Na m )TiO3-y(Bi n K m )TiO3-zBaTiO3)-aM]

[0058] a) where applicable (0.92 < x < 0.95), (y = 0), (0.05 < z < 0.08), (x + y + z = 1), (n = m = 0.5) and (0 < a < 3), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or mixtures thereof, or

[0059] b) where applicable (0.77 < x < 0.81), (0.19 < y < 0.23), (z = 0), (x + y + z = 1), (n = m = 0.5) and (0 < a < 3), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3, or mixtures thereof, or

[0060] c) a combination of a) and b) in a three-phase system, wherein y > 0 and z > 0.

[0061] In another preferred embodiment, (0 < a < 10); (0 < a < 5) and (0 < a < 3) apply to the respective formulae described, respectively; i.e. a can not be zero.

[0062] In another preferred embodiment, the lead-free piezoelectric ceramic is essentially free of calcium (Ca) and / or strontium (Sr). Essentially free means that no calcium and / or strontium containing compounds are added to the ceramic during synthesis. However, it is understood that small amounts of impurities in the ppm range can be present in the piezoelectric ceramic. By omitting calcium, high temperature stability with good piezoelectric charge constant d 33 is achieved.

[0063] It is understood that the following two ceramics can be exempted from the above described lead-free ceramics: 0,79(Bi 0,5 Na 0,5 )TiO3-0,14(Bi 0,5 K 0,5 )TiO3-0,07BaTiO3 and 0,88(Bi 0,5 Na 0,5 )TiO3-0,08(Bi 0,5 K 0,5 )TiO3-0,04BaTiO3.

[0064] In another preferred embodiment, the lead-free piezoelectric ceramic is essentially free of potassium (K). Essentially free means that no potassium containing compounds are added to the ceramic during synthesis. However, it is understood that small amounts of impurities in the ppm range can be present in the piezoelectric ceramic.

[0065] Thus, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0066] [(100-a)(x(Bi n Na m )TiO3-zBaTiO3)-aM]

[0067] wherein (0 < x < 1), (0 < z < 1), (x + z = 1), (0.4 < n < 0.6), (0.4 < m < 0.6) and (0 < a < 10) apply, and

[0068] M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof. In one embodiment, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0069] [(100-a)(x(Bi n Na m)TiO3-zBaTiO3)-aM]

[0070] a) where applicable (0.8 < x < 1), (0 < z < 0.2), (x + z = 1), (n = m = 0.5) and (0 < a < 5), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof, or

[0071] b) where applicable (x = 1), (z = 0), (x + z = 1), (n = m = 0.5) and (0 < a < 5), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof, or

[0072] c) a combination of a) and b) in a three-phase system.

[0073] In another embodiment, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0074] [(100-a)(x(Bi n Na m )TiO3-zBaTiO3)-aM]

[0075] a) where applicable (0.92 < x < 0.95), (0.05 < z < 0.08), (x + z = 1), (n = m = 0.5) and (0 < a < 3), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof, or

[0076] b) where applicable (x = 1), (z = 0), (x + z = 1), (n = m = 0.5) and (0 < a < 3), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or mixtures thereof, or

[0077] c) a combination of a) and b) in a three-phase system.

[0078] In yet another preferred embodiment, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0079] x(Bi n Na m )TiO3-zBaTiO3

[0080] where (0.92 < x < 0.935), (0.065 < z < 0.08), (x + z = 1), (n = m = 0.5).

[0081] The lead-free ceramic according to the above preferred embodiments can have a planar coupling factor k in the range of 0.15 to 0.35, for example 0.28 to 0.32.p a thickness coupling factor k of 0.45 to 0.55, for example 0.5 to 0.53 t a piezoelectric charge constant d of 150 to 200, preferably 170 to 180 33 in pC / N, a depolarization temperature T of 80°C to 160°C, more preferably 80°C to 120°C D and a tan delta of 200 to 600, for example 200 to 400.

[0082] In yet another preferred embodiment, the lead-free piezoelectric ceramic used in the present case has the following composition:

[0083] [(100-a)(x(Bi n Na m )TiO3-zBaTiO3)-aM]

[0084] wherein (0.92 < x < 0.935), (0.065 < z < 0.08), (x + z = 1), (n = m = 0.5) and (0,5 < a < 3) apply, and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof.

[0085] In another preferred embodiment, the ceramic can have the above composition, wherein (0,5 < a < 1) applies, for example a = 0.75, or (2 < a < 3) applies, for example a = 2.5.

[0086] The lead-free ceramic used according to the above preferred embodiments can have a planar coupling factor k in the range of 0.1 to 0.25, for example 0.15 to 0.2 p a thickness coupling factor k of 0.43 to 0.55, for example 0.48 to 0.52 t a piezoelectric charge constant d of 100 to 150, preferably 110 to 140 33 in pC / N, a depolarization temperature T of 120°C to 200°C, more preferably 130°C to 170°C D and a tan delta of 30 to 700, for example 40 to 100.

[0087] As mentioned before, the lead-free piezoelectric ceramic used according to the present application can comprise some (small amounts) of impurities. Thus, the lead-free piezoelectric ceramic can also comprise < 1000 ppm of additional metal oxides. Possible metal oxides are oxides 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. These impurities can be introduced due to the manufacturing process, but can have a slight influence on the properties of the piezoelectric ceramic.

[0088] Such ceramics are generally known from WO 2022 / 2330984 A1. In this document different properties of lead-free ceramics are described, e.g. depolarization temperature, k p value, k t value and density. However, the effect of applying a permanent mechanical pre-load of 20 MPa or more to the lead-free ceramic is not described or suggested.

[0089] It should be noted that another class of lead-free piezoelectric ceramics is known in the art: materials based on sodium potassium niobate (KNN). While KNN-based materials do not contain lead, they do contain niobium. Recent environmental assessments have shown that niobium also has a large environmental impact during the early stages of its production cycle, due to the procedures used in the extraction and purification of the raw material, taking into account that the ore usually contains heavy materials and radioactive materials that need to be separated and disposed of. Furthermore, the energy required to produce KNN components (drying, calcination and sintering) is higher than the energy required to manufacture lead-containing PZT ceramics. Therefore, there is currently no consensus that KNN is a truly more environmentally friendly material than conventional lead-containing PZT ceramics. Moreover, as shown in Table 2 below, KNN ceramics have a low anisotropy coefficient and are therefore not suitable for the use according to the present application.

[0090] The lead-free piezoelectric ceramic is preferably obtained according to EP 3 331 840 B1 “Production of lead-free piezoceramics in an aqueous environment”. Other methods for the synthesis of lead-free piezoelectric ceramics are also possible.

[0091] In a typical process, an aqueous homogeneous suspension of Bi2O3, Na2CO3, TiO2, BaTiO3 and any additional ingredients is provided in a first step. The aqueous suspension is freeze-dried or spray-dried and subsequently calcined, e.g. at 800 °C to 900 °C. In order to produce doped lead-free materials or lead-free composites, one or more additional components are added to the main system before or after calcination, e.g. introduced as oxide powders.

[0092] In a more specific embodiment, the synthesis process of the lead-free piezoelectric ceramic comprises the following steps:

[0093] - providing the required amount of starting materials (e.g. Bi2O3, Na2CO3, TiO2, BaTiO3);

[0094] - mixing the starting materials and freezing;

[0095] - calcination, e.g. at a temperature above 800 °C,

[0096] - milling / de-agglomeration / doping, in particular with ZnO,

[0097] - addition of organic additives, such as PVA, PEG,

[0098] - granulation, such as by spray drying,

[0099] - shaping of the granulated material, such as under pressure

[0100] - heating and sintering,

[0101] - final processing, such as metallization and polarization.

[0102] The present lead-free piezoceramics are preferably used in drive units for ultrasonic transducers.

[0103] Such ultrasonic transducers, such as BLT-transducers (bolt fastened Langevin type transducers) comprise a drive unit, wherein the drive unit is formed by laminating together a plurality of piezoceramics and electrode plates as described previously. The drive unit is clamped between a front block and a back block, wherein the front block and the back block are fastened together by applying a preload of > 20 MPa using fastening bolts. The front block emits ultrasonic waves from its surface.

[0104] The present application will now be explained in more detail with reference to the examples and the accompanying drawings. The drawings show:

[0105] Figure 1 : Strength of the lead-free piezoceramics compared to a variety of PZT based materials,

[0106] Figure 2 : Temperature profile applied to the sample for determining the depolarization temperature T D . Examples

[0107] The following examples are included to illustrate certain aspects and embodiments of the application as described in the claims. However, it should be understood by those skilled in the art that the following description is merely illustrative and should not be considered to be limiting in any way to the present application.

[0108] Embodiments of the lead-free piezoceramics according to the present application are provided in Table 1.

[0109] The lead-free piezoceramics shown in Table 1 are synthesized similar to the following example:

[0110] - Providing the required amount of starting materials Bi2O3, Na2CO3, TiO2, BaTiO3;

[0111] - Wet mixing and de-agglomeration of the starting materials,

[0112] - Homogeneous distribution by freezing

[0113] - Calcination above 800 °C

[0114] - the grinding process is carried out in a mill together with the addition of ZnO

[0115] - addition of PVA and PEG,

[0116] - granulation by spray drying;

[0117] - shaping of the granulated material under pressure,

[0118] - debindering at temperatures above 600°C

[0119] - sintering at different temperatures above 1000°C,

[0120] - hard machining

[0121] - metallization,

[0122] - polarization at 5 kV / mm

[0123] - measurement 24 hours after polarization

[0124] PZT P4 and KNN are comparative examples. Whereas PZT P4 is a commercially available lead-containing piezoelectric ceramic, KNN is a sodium potassium niobate-based ceramic (“Lead-free Piezoelectric Ceramics: Technologies and Global Opportunities”, Report Code: NAN063B, April 2022, BCC Publishing).

[0125]

[0126] Table 1

[0127] k t = thickness coupling factor

[0128] K P = planar coupling factor

[0129] AK = anisotropy coefficient

[0130] p = density

[0131] d 33 = piezoelectric constant

[0132] d 31 = piezoelectric constant transverse to the polarization direction

[0133] T D = depolarization temperature (associated with a phase transition)

[0134] tan d = dielectric dissipation factor

[0135] *T C (°C) Curie temperature: This is the temperature at which the dielectric constant of a ferroelectric ceramic will reach its maximum value. At this temperature, a piezoelectric ceramic will lose its polarization state. For this reason, the operating temperature should not exceed half of the Curie temperature.

[0136] As can be seen in Table 1, the lead-free ceramics used in the present application differ from PZT P4 and KNN differ in their respective k p values and anisotropy coefficients AK. Both ceramics do not show a phase transition and are therefore preferably characterized by a Curie temperature. Half of the Curie temperature can be used for analogy to the depolarization temperature T D .

[0137] Furthermore, experimental data (not shown) show that PZT P4 and BNT-based ceramics show different temperature profiles in an oscillator setup. These data support the theory that BNT-based ceramics do not promote self-heating or warming up in use compared to conventional lead-containing piezoelectric ceramics.

[0138] This effect is influenced by the lower anisotropy coefficient of PZT P4 compared to currently used BNT-based ceramics.

[0139] As mentioned, currently used BNT-based ceramics show a reduced friction work when used between conversion materials in an ultrasonic transducer.

[0140] The friction work W R can be calculated with the formula W R = F * μ * s (with F = force, μ = friction coefficient, s = displacement). For the materials mentioned in Table 1, s Aniso (lead-free ceramics with high anisotropy coefficients of > 0.6) to s PZT (PZT ceramics such as P4) can be calculated by the ratio of the charge constants d 31Aniso and d 31PZT and is 0.15 to 0.35. Considering the different densities, this leads to a ratio of the friction work W RAniso / W RPZT (as a measure for self-heating) of values of 0.11 to 0.27, i.e. the friction work of lead-free ceramics with high anisotropy coefficients of > 0.6 is reduced by 11 to 27% compared to PZT ceramics.

[0141] The reduction in friction between the end faces is beneficial because the development of frictional heat and therefore electrical losses is also reduced. Due to the low anisotropy coefficient of ≤ 0.6 and the associated high heat development, common application temperatures to date have been > 120°C to 130°C. In applications with high preloads of > 20 MPa, the frictional heat can thus be reduced to < 50°C. Another advantage resulting from the reduced heat generation is the high anisotropy coefficient of > 0.6 and the depolarization temperature T < 140°C. D The lead-free material is suitable for applications with high preloads >20MPa.

[0142] Lead-free piezoelectric ceramics with an anisotropy coefficient > 0.6, such as BNT-BT or BNT-BT composites, have a fracture force of 11 N to 29 N and an average of > 15 N for samples with a diameter of 6 mm, as measured by the ball-and-ring method. Therefore, their fracture strength is higher than that of conventional soft PZT-based ceramics such as More than twice that of P5. In the case of P5, the average breaking force was measured to be about 5 N. Within the distribution range, lead-free piezoelectric ceramics with anisotropy coefficient > 0.6 can achieve up to 5 times higher strength. Hard PZT ceramics such as The fracture strength values ​​of P4 are on average about 12 N and are therefore in the lower strength range of lead-free piezoelectric ceramics (see also Figure 1 (Figure ).

[0143] Explanation of the measurement of the breaking force with the ball-on-ring method (for samples with a diameter of 6 mm)

[0144] Equipment needed:

[0145] - Measuring frame with actuator and load cell for fracture testing (1)

[0146] - Homemade charge amplifier (2)

[0147] -Voltage source TTIPLH250-P(3)

[0148] -Oscilloscope Agilent DSO-X 2024A(4)

[0149] -Multimeter for force measurement (5)

[0150] program:

[0151] The piezoelectric disc is placed on the force sensor such that only the edge region is on the ring. Then the ball attached to the actuator is positioned on the sample such that the applied voltage is 0 (no force is applied to the sample). By slowly increasing the voltage applied to the actuator, the ball expands and applies a defined force to the actuator. The increase in applied voltage can be seen on an oscilloscope. As the applied voltage increases, the force on the disc also increases. The breakage is manifested as a sudden drop in the voltage curve as the resistive body (piezoelectric disc) against which the actuator presses fails. The maximum voltage before the drop reflects the maximum force that can be withstood. This is calculated by multiplying the voltage by the charge scale of the charge amplifier.

[0152] Depolarization temperature T D Measurement procedure:

[0153] The sample (preferably a thin disc) is mounted in a temperature furnace and the following temperature profile is applied, as shown in the graph of Figure 2

[0154] The procedure starts at 20 °C. The heating continues up to 180 °C, with small signal impedance measured continuously every 2 K. When the material depolarizes, the k t value drops to zero. The value at the point of the drop and 10 °C lower (to ensure the piezoelectric behavior when applied close to this point) is taken. This resulting temperature is determined as T D (depolarization temperature).​

Claims

1. Use of a lead-free piezoelectric ceramic in a device in which a permanent preload of ≥20 MPa is to be applied to the lead-free piezoelectric ceramic, wherein the lead-free piezoelectric ceramic has the following composition: [(100-a)(x(Bi n And m )TiO3–y(Bi n K m )TiO3-zBaTiO3)-aM] where applicable (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), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof.

2. The use of the lead-free piezoelectric ceramic according to claim 1, characterized in that The lead-free piezoelectric ceramic has the following composition: [(100-a)(x(Bi n And m )TiO3-zBaTiO3)-aM] where applicable (0 < x ≤ 1), (0 ≤ z < 1), (x + z = 1), (0.4 ≤ n ≤ 0.6), (0.4 ≤ m ≤ 0.6) and (0 ≤ a < 10), and M is one of the additives ZnO, MgO, TiO2, ZrO2, Al2O3 or a mixture thereof.

3. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that The lead-free piezoelectric ceramic has the following composition: x(With n And m )TiO3-zBaTiO3 where (0.92 ≤ x ≤ 0.935), (0.065 ≤ z ≤ 0.08), (x + z = 1), (n = m = 0.5).

4. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that The lead-free piezoelectric ceramic has the following composition: [(100-a)(x(Bi n And m )TiO3-zBaTiO3)-aM] where applicable (0.92 ≤ x ≤ 0.935), (0.065 ≤ z ≤ 0.08), (x + z = 1), (n = m = 0.

5. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that The planar coupling factor k of the lead-free piezoelectric ceramic p In the range of 0.1 to 0.35, preferably 0.15 to 0.32, such as 0.15 to 0.35, for example 0.28 to 0.32, or 0.1 to 0.25, such as 0.15 to 0.

2.

6. Use of the lead-free piezoelectric ceramic according to claims 1 and 2, characterized in that The thickness coupling factor k of the lead-free piezoelectric ceramic t In the range of 0.4 to 0.6, preferably 0.48 to 0.52, such as 0.45 to 0.55, such as 0.5 to 0.52, or 0.43 to 0.55, such as 0.48 to 0.

52.

7. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that The anisotropy coefficient k of the lead-free piezoelectric ceramic t / (k t +k p )≥0.6, preferably ≥0.65, more preferably ≥0.7, preferably in the range of 0.6 to 0.8, more preferably 0.65 to 0.79, more preferably 0.7 to 0.79, wherein k t is the thickness coupling factor and k p is the planar coupling factor.

8. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that Depolarization temperature T of lead-free piezoelectric ceramics D It is 50°C to 200°C, preferably 80°C to 180°C, more preferably 90°C to 160°C, even more preferably 130°C to 145°C.

9. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that Piezoelectric charge constant d of lead-free piezoelectric ceramics 33 (in terms of pC / N) is 100 to 200, preferably 110 to 190, more preferably 120 to 180.

10. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that ​ ​ 12. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that ​ 13. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that 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 at 5.0 g / cm 3 Up to 7.0g / cm 3 within the range of 5.5 g / cm 3 Up to 6.5g / cm 3 in the range of 5.5 g / cm 3 Up to 6.0g / cm 3 within the range.

14. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that ​ 15. Use of a lead-free piezoelectric ceramic according to one of the preceding claims, characterized in that ​ ​ ​

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