EP4674239A1

EP4674239A1 - Use of a lead-free piezo ceramic in devices requiring high permanent preload

Info

Publication number: EP4674239A1
Application number: EP24711962.1A
Authority: EP
Inventors: Hans-Jürgen SCHREINER; Tanja Einhellinger-Müller; Friederike Assmann; Florian Schönhöfer
Original assignee: Ceramtec GmbH
Current assignee: Ceramtec GmbH
Priority date: 2023-03-02
Filing date: 2024-02-29
Publication date: 2026-01-07
Also published as: JP2026507707A; CN120814366A; WO2024180179A1

Abstract

The present invention is directed to the use of a lead-free piezo ceramic in devices that require the application of a permanent preload to the lead-free piezo ceramic.

Description

Use of a lead-free piezo ceramic in devices requiring high permanent preload

The present invention is related to the use of a lead-free piezo ceramic in devices that require the application of a high permanent preload to the lead-free piezo ceramic.

Description

Piezo ceramics are capable of converting mechanical quantities such as pressure and acceleration into electrical quantities or, conversely, of transforming electrical signals into mechanical movement or oscillations.

Piezoelectricity is based on the ability of certain crystals to generate electrical charge when mechanically loaded with pressure or tension (direct piezo effect). Conversely, these crystals undergo a controlled deformation when exposed to an electric field - a behavior referred to as the inverse piezo effect. The polarity of the charge depends on the orientation of the crystal relative to the direction of the pressure.

Ceramics exhibiting piezoelectric properties belong to the group of ferroelectric materials. Conventional systems are based mostly on lead zirconate titanate (PZT), i.e., they consist of mixed crystals of lead zirconate (PbZrOs) and lead titanate (PbTiOs).

Immediately after sintering, the domains of a ceramic body (i.e., the areas consisting of elementary cells of uniform dipole direction) will show an arbitrary (statistically distributed) orientation, i.e. the macroscopic body is isotropic and shows no piezoelectric properties. These piezoelectric properties have to be originated by “polarization". In this process, the ceramic body is exposed to a strong electric DC field that causes the electric dipoles to become aligned in the direction of field. They will maintain this orientation even when the DC field is no longer applied (remanent polarization) - a necessary condition for the piezoelectric behavior of ferroelectric ceramics.

A full or partial elimination of the domain alignment achieved by the polarizing process (depolarization) will degrade the piezoelectric properties of the material. Depolarization may be the result of three factors: Thermal depolarization due to heat exposure, electric depolarization due to electric fields acting against the original polarization direction, mechanical depolarization caused by high-pressure loads, especially with short-circuited electrodes. Piezo ceramic components are used in a broad spectrum of electromechanical transducers covering a wide frequency range. In sensors, they enable the conversion of forces, pressures and accelerations to electrical signals. In sound generators and ultrasonic transducers, they transform voltages into oscillations or deformations.

In ultrasonic applications, piezo ceramic components generate high-power ultrasonic waves for ultrasonic cleaning, drilling and welding and for stimulating of chemical processes. On the other hand, piezo 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 locating and ranging, non-destructive material testing, and medical diagnostic equipment.

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. atomisation) to several kilowatts (e.g. ultrasonic welding, ultrasonic cleaning) and a frequency range from about 20 kHz to 1 MHz.

The ultrasonic transducers used in this frequency range for high power applications are usually constructed as metal-piezo composite transducers (also known as bolt-tightening Langevin- type transducers - or BLT transducers). The piezo ceramics convert an alternating electrical voltage into mechanical vibrations by means of the inverse piezoelectric effect and thus generate the ultrasound.

The piezo ceramics together with the electrode plates are clamped between two metal parts by means of a screw bolt. While the piezo ceramics generate the mechanical vibrations, the metal parts determine the frequency and amplitude distribution.

It has been shown that ceramics under mechanical stress (like in a BLT transducer) typically fail early due to tensile stress and crack building. In order to avoid tensile stress, the piezo ceramics has to withstand a pre-defined pre-stress or pre-load (of a defined pressure). The optimum pre-stress depends on the piezoelectric material compression strength and static pressure distribution. Said pre-stress or pre-load can also be applied permanently in a device when in operation, like in a BLT transducer. The applied pre-tension (or preload) in the piezo ceramics is of decisive importance for the function and performance of the transducer. On the one hand, the applied mechanical compressive load protects the piezo ceramics from cracking due to mechanical tensile stress during operation. On the other hand, the preload minimizes friction losses in the joints. In addition, the pre-load influences the performance of the ultrasonic transducer due to nonlinearity in the material behavior.

The targeted and reproducible application of mechanical preload is therefore a core competence in the construction of high-performance ultrasonic systems. If the preload is too low, there is a risk of damage to the piezo ceramics; if, on the other hand, the preload is too high, the piezo ceramics will depolarize and thus lose their function (see publication “Pretensioning and bonding of piezoelectric ultrasonic converter” by company Athena).

The specific adjustment of the preload is often not sufficient for ultrasonic transducers, for example, which are operated at high temperatures or heat up during operation, as the preload can change due to the temperature. This can lead to reduced performance or failure of the ultrasonic converter.

It was therefore an object of the invention to provide a lead-free piezo ceramic that can be used and withstand high permanent mechanical preloads and does not heat up when in use. In particular, no depolarization and loss of piezoelectric properties should occur.

This object was solved by using a piezo ceramic as described in the claims.

According to an aspect of the invention, a lead-free piezo ceramic is used in devices that require the application of a permanent mechanical preload to the lead-free piezo ceramic, wherein the applied mechanical preload is > 20 MPa and wherein the piezo ceramic does not lose its piezoelectric properties when in use, for example in an ultrasound transducer.

The lead-free piezo ceramic has the following composition:

[(100-a)(x(Bi_nNa_m )TiOs - y(Bi_nK_m )TiOs - zBaTiOs ) - aM]

Wherein (0 < x < 1 ), (0 < y < 1 ), (0 < z <1 ), (x + y-i- z = 1 ), (0.4 < n < 0.6), (0.4 < m < 0.6) and (0 < a < 10) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.

Surprisingly, when applying a preload of > 20 MPa to the lead-free piezo ceramic no warmup or self-heating occurs; i.e. no depolarization accompanied with a functional loss is detectable. It was furthermore surprising that the mechanically preloaded lead-free piezo ceramic does not promote a warmup when in use compared to conventional lead containing piezo ceramics. Self-heating during application normally correlates with high dissipation factors (tan 5). It was therefore surprising that the self-heating of the lead-free systems used according to this invention is rather low. One of the reasons for this surprising behavior is the anisotropy factor, as will be discussed in more detail below.

As indicated above, while a high mechanical preload reduces friction loss in the joints of ceramic and electrodes in an ultrasound transducer on one side, the high mechanical preload increases self-heating on the other side. The presently used lead-free ceramics show now a low self-heating under high mechanical preload and oscillation when in use in ultrasound transducers. Therefore materials with reduced depolarization temperatures of below 200°C, (such as below 120°C) may be suitable without a functional loss.

In general, a lower and reduced self-heating enables an improved continuity of ultra-sound irradiation. A larger part of the energy required for operating an ultrasound transducer can be used for oscillation, when less energy is lost by heating. The ultrasound bath can be operated at room temperature, i.e. a cooling of the ultrasound bath is not necessary.

Furthermore, the friction heat is reduced. For example, the friction heat is reduced by 40-50°C in comparison to conventional lead containing piezo ceramics. The reduced friction heat is accompanied by lower electrical losses when using an ultrasound transducer, i.e. the efficiency of ultrasound transducers using the present lead-free piezo ceramics at a mechanical preload of 20 MPa and more is improved.

Besides, the mechanical stress of the contact areas of the piezo ceramics with the metal electrodes is reduced and thus the operating time and overall lifetime of an ultrasound transducer is increased.

It is pointed out that lead zirconate titanate (PZT) ceramics may be prestressed with a pre-load of up to 35-45 MPa. During high power application usually a self-heating of up to 50°C of the ceramic PZT part occurs. However, it is surprising that lead free ceramics can be prestressed with a preload of more than 20 MPa almost without self-heating (< 10°C) during application

The vibration or oscillation properties of piezoceramics are usually characterized, among other things, by their electromechanical coupling factors k, which differ depending on the oscillator geometry and vibration mode. They are a measure of the efficiency of the energy conversion 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.

The anisotropy coefficient (AK) is calculated according to the formula AK = k_t/(k_t+k_p) (with k_t = thickness coupling factor and k_p = planar coupling factor). The determination of k_t and k_p is carried out according to DIN EN 50324-2 "Piezoelectric properties of ceramic materials and components, part 2: Measuring method - small signal".

The lead-free ceramic used according to the invention may have a planar coupling factor k_p in a range between 0.1 and 0.35, preferably between 0.15 and 0.3, for example between 0.2 and 0.35, such as 0.28-0.32, or between 0.1 and 0.25, such as 0.15 - 0.2.

The lead-free ceramic used according to the invention may have a thickness coupling factor k_t in a range between 0.4 and 0.6, preferably between 0.45 and 0.5, for example between 0.45 and 0.55, such as 0.5-0.52.

According to an embodiment, the lead-free piezo ceramic has an anisotropy coefficient of k_t/(k_t + k_p) > 0.6, preferably > 0.65, more preferably > 0.7, preferably in a range between 0.6 and 0.8, more preferably between 0.65 and 0.75 with k_t being the thickness coupling factor and k_p being the planar coupling factor.

A high anisotropy coefficient > 0.6 results in reduced friction between the end faces of the lead- free piezo ceramic and the adjacent (non-piezoelectric) transducer material compared to piezoelectric materials with low anisotropy coefficients < 0.6, such as PZT ceramics (lead zirconium titanate), due to the lower planar coupling factor compared to the thickness coupling factor. The reduced transverse contraction due to the anisotropy coefficient leads to reduced heat generation due to planar friction effects during operation. In an embodiment, the lead-free piezo ceramic used has a depolarization temperature T_D of 50°C to 200°C, preferably of 80°C to 180°C, more preferably of 90°C to 160°C, even more preferably of 130°C to 145°C. Thus, the lead-free piezo ceramic is suitable for use in devices requiring mechanical pre-loads of > 20 MPa, in particular at working temperatures between 20°C to 100°C.

Piezoelectric materials are further characterized by piezoelectric charge constant d. The piezoelectric charge constant quantifies the volume change when a piezoelectric material is subject to an electric field. It denotes the ratio between charge generated and force applied (direct piezo effect, unit C/N).

Depending on the direction of applied stress and induced polarization three different piezoelectric charge constants may be defined: d33, dsi and d32, wherein d₃₃ - means induced polarization in direction z axis per unit stress applied in z axis, dsi - means induced polarization in z axis per unit stress applied in x axis, d32 - means induced polarization in x axis per unit shear stress applied about y axis.

The lead-free ceramic used in the present invention may have a piezoelectric charge constant d33 (in pC/N) between 100 and 200, preferably between 1 10 and 195, more preferably between 120 and 180. A high piezoelectric charge constant d₃₃ is thereby of an advantage.

As mentioned above, piezoelectric components may also be characterized by their dissipation factor (tan 5). The dissipation factor denotes the ratio between power loss and reactive power when the component is excited with a sine-wave signal at a frequency far below its lowest resonant frequency.

The dissipation factor (DF) is an important material property of piezoceramics that governs the amount of self-heating under resonant conditions. A low dissipation factor is thereby of an advantage. The dissipation factor (DF) is defined as the ratio of the equivalent series resistance (ESR) and the magnitude of the capacitance reactance (Xc), i.e., DF = ESR/|Xc| . It is also known as the loss tangent or tan(b), where the angle 5 is the deviation from 90° between voltage and current for an ideal capacitor (i.e., no losses). The DF is also the ratio of (energy lost)/(energy stored) or Re/|lm| of the impedance. It is typically measured at 120 Hz (for AC power) or 1000 Hz (more common). The higher the DF the more heat is generated via I 2 ESR heating (QuadTech, 2003, Gebbia, 2001 ). The DF is an important material property of the piezoceramics; it governs the amount of self-heating under resonant conditions, and thus quantifies a particular material type for either an actuator or resonator (see:The Effects of Piezoelectric Ceramic Dissipation Factor on the Performance of Ultrasonic Transducers - ScienceDirect).

The lead-free ceramic used in the present invention may have a tan 5 between 30 and 700, preferably between 40 and 600, more preferably between 40 and 400, for example between 40 and 200, such as 40-100 or between 200 and 600, such as 200 and 400.

It has to be noted that the lead-free ceramic used in the present invention combines a good piezoelectric coefficient d33, an acceptable depolarization temperature TD as well as a dissipation factor between 50-600.

This is surprising since for most of the lead-free ceramics a high depolarisation temperature TD typically correlates with a low piezoelectric charge constant d₃₃ and vice versa. However, such a negative correlation between depolarisation temperature TD and piezoelectric charge constant d₃₃ does not promote the use of such lead free ceramics under a high preload as it is the object of the present invention.

In another embodiment, the lead-free piezo ceramics used is characterized by fracture forces (measured by the ball-on-ring method) of > 10N, preferably between 1 1 and 30N for samples with 6mm diameter.

Lead-free ceramics with an anisotropy coefficient > 0.6 are e.g. bismuth sodium titanate (BNT) or bismuth sodium titanate barium titanate (BNT-BT). The addition of one or more additional components such as ZnO leads to an increase in the anisotropy coefficient compared to the reference without additional component. Common piezoceramics made of lead zirconium titanate (PZT), such as Sonox®P4, have anisotropy coefficients of _s_0.6.

In yet another embodiment, the lead-free piezo ceramics has a density p of at least 5.0 g/cm³, preferably of at least 5.5 g/cm³, preferably in a range between 5.0 and 7.0 g/ cm³, more preferably in a range between 5.5 and 6.5 g/cm³, even more preferably in a range between 5. 5 and 6.0 g/cm³ (measured in accordance to DIN EN 60672-2 (VDE 0335 Teil2):2000).

As mentioned above, the lead-free piezo ceramics used in the present invention has the following composition: [(100-a)(x(Bi_nNa_m )TiOs - y(Bi_nK_m )TiOs - zBaTiOs ) - aM]

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) applies, and

M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.

In an embodiment, the lead-free piezo ceramics used in the present case has the composition

[(100-a)(x(Bi_nNa_m)TiO3 - y(Bi_n K_m )TiOs - zBaTiOs ) - aM] a) wherein (0.8 < x < 1 ), (y = 0), (0 < z < 0.2), (x + y + z = 1 ), (n = m = 0.5) and (0 < a < 5) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or b) wherein (0.6 < x < 0.9), (0.1 < y < 0.4), (z = 0), (x + y + z = 1 ), (n = m = 0.5) and (0 < a < 5) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or c) a combination of a) and b) in the three-phase system wherein y > 0 and z > 0.

In a further embodiment, the lead-free piezo ceramics used in the present case has the composition

[(100-a)(x(Bi_nNa_m)TiO3 - y(Bi_n K_m )TiOs - zBaTiOs ) - aM] a) wherein (0.92 < x < 0.95), (y = 0), (0.05 < z < 0.08), (x + y + z = 1 ), (n = m = 0.5) and (0 < a < 3) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or b) wherein (0.77 < x < 0.81 ), (0.19 < y < 0.23), (z = 0), (x + y + z = 1 ), (n = m = 0.5) and (0 < a < 3) applies and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or c) a combination of a) and b) in the three-phase system wherein y > 0 and z >0.

In another preferred embodiment, (0 < a < 10) ; (0 < a < 5) and (0 < a < 3) applies for the respective described respective formulas; i.e. a may be not Zero. In a further preferred embodiment, the lead-free piezo ceramics is essentially free of Calcium (Ca) and/or Strontium (Sr). Essentially free means that no Calcium containing and/or Strontium containing compound is added to the ceramics during synthesis. However, it is to be understood that minor impurities in the ppm range may be present in the piezo ceramics. By omitting Calcium a high temperature stability with a good piezoelectric charge constant d₃₃ is achieved.

It is to be understood that the following two ceramics may be exempt from the above described lead free ceramics: 0,79 (Bio,₅Na₀,5)Ti03 - 0,14 (Bio,₅K₀,5)Ti03 - 0,07 BaTiOs and 0,88 (Bio,sNao,5)Ti03 - 0,08 (Bio,sKo,5)Ti03 - 0,04 BaTiOs

In a further preferred embodiment, the lead-free piezo ceramics is essentially free of Potassium (K). Essentially free means that no Potassium containing compound is added to the ceramics during synthesis. However, it is to be understood that minor impurities in the ppm range may be present in the piezo ceramics.

Accordingly, a lead-free piezo ceramic is used in the present case that has the following composition:

[(100-a)(x(Bi_nNa_m )TiOs - zBaTiOs ) - aM]

Wherein (0 < x < 1 ), (0 < z <1), (x + z = 1 ), (0.4 < n < 0.6), (0.4 < m < 0.6) and (0 < a < 10) applies, and

M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.

[ (100-a)(x(Bi_nNa_m)TiO3 - zBaTiOs ) - aM] a) wherein (0.8 < x < 1 ), (0 < z < 0.2), (x + z = 1 ), (n = m = 0.5) and (0 < a < 5) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or b) wherein (x = 1 ), (z = 0), (x + z = 1 ), (n = m = 0.5) and (0 < a < 5) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or c) a combination of a) and b) in the three-phase system. In a further embodiment, the lead-free piezo ceramics used in the present case has the composition

[ (100-a)(x(Bi_nNa_m)TiO3 - zBaTiOs ) - aM] a) wherein (0.92 < x < 0.95), (0.05 < z < 0.08), (x + z = 1 ), (n = m = 0.5) and (0 < a < 3) applies, and M is one of the additives ZnO, MgO, TiO2, ZrC>2, AI2O3 or a mixture thereof, or b) wherein (x = 1 ), (z = 0), (x + z = 1 ), (n = m = 0.5) and (0 < a < 3) applies and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof, or c) a combination of a) and b) in the three-phase system.

In still a further preferred embodiment, the lead-free piezo ceramics used in the present case has the composition x(Bi_nNa_m)TiO3 - zBaTiOs wherein (0.92 < x < 0.935), (0.065 < z < 0.08), (x + z = 1 ), (n = m = 0.5).

The lead-free ceramic according to above preferred embodiment may have a planar coupling factor k_p in a range between 0.15 and 0.35, such as 0.28-0.32, a thickness coupling factor k_t between 0.45 and 0.55, such as 0.5-0.53, a piezoelectric charge constant d₃₃ (in pC/N) between 150 and 200, preferably between 170 and 180, a depolarization temperature T_D of 80°C to 160°C, more preferably of 80 to 120°C, and a tan 5 between 200 and 600, such as 200 and 400.

In yet another preferred embodiment, the lead-free piezo ceramics used in the present case has the composition

[ (100-a)(x(Bi_nNa_m)TiO3 - zBaTiOs ) - aM] wherein (0.92 < x < 0.935), (0.065 < z < 0.08), (x + z = 1 ), (n = m = 0.5) and (0,5 < a < 3,) applies, and M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof. In another preferred embodiment, the ceramic may have the above composition wherein (0,5 < a < 1 ) for example a = 0.75, or (2 < a < 3,) for example a = 2.5, applies.

The lead-free ceramic used according to above preferred embodiment may have a planar coupling factor k_p in a range between 0.1 and 0.25, such as 0.15 - 0.2, a thickness coupling factor k_t in a range between 0.43 and 0.55, such as 0.48 - 0.52, a piezoelectric charge constant d₃₃ (in pC/N) between 100 and 150, preferably between 110 and 140, a depolarization temperature T_D of 120°C to 200°C, preferably of 130°C to 170°C, and a tan 5 between 30 and 700, such as 40-100.

As mentioned previously, the lead-free piezo ceramics used according to the invention may contain some (minor) impurities. Thus, it is also possible that the lead-free piezo ceramics contains further metal oxides < l OOOppm. 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 may be due to the manufacturing process but may have minor effects on the properties of the piezo ceramic.

Such ceramics are in generally known from WO2022/2330984 A1. Different properties of the lead-free ceramics are described in this document, such as depolarization temperature, k_p, k_t - values and density. However, the effect of a permanent mechanical preload of 20 MPa or higher applied to the lead-free ceramics is not described nor suggested.

It is to be noted that another class of lead-free piezo ceramics is known in the art: Sodium Potassium Niobate-based materials (KNN). Although KNN-based materials do not contain lead, they comprise niobium. A recent environmental assessment has indicated that niobium also has a large environmental impact during the early stage of its production cycle due to the procedures used in raw material extraction and purification, considering also that the ore often includes heavy and radioactive materials that need to be separated and disposed of. In addition, the energy required to produce KNN components (drying, calcination, and sintering) is higher than to manufacture lead containing PZT ceramics. Consequently, at the present time, there is no consensus in considering KNN a material that is truly more environmentally friendly than conventional lead containing PZT ceramics. Besides, as shown in Table 2 further below, KNN ceramic has a low anisotropy coefficient, and is thus not suited for the use according to the invention. The lead-free piezo ceramics are preferably obtained according to EP3 331 840 B1 "Production of lead-free piezoceramics in an aqueous environment": Other methods for the synthesis of lead-free piezoceramics are also possible.

In a typical process, an aqueous, homogenous suspension of Bi₂O3, Na₂CO3, TiO₂, BaTiOs and any further ingredients is provided in a first step. The aqueous suspension is freeze dried or spray-dried and subsequently calcined, for example at 800-900°C. To produce a doped lead-free material or a lead-free composite material, one or more additional components, which are introduced e.g. as oxidic powder, are added to the main system before or after calcination.

In a more specific embodiment, the synthesis process of the lead free piezo ceramic comprises the following steps:

- providing the starting material (such as Bi₂Os, Na₂CO3, TiO₂, BaTiOs) in the required amounts;

- mixing the starting material and freezing;

- calcination, for example at temperatures over 800°C,

- grinding / deagglomeration / doping, in particular with ZnO,

- adding organic additives, such as PVA, PEG,

- granulation, for example by spray drying,

- shaping the granulated material, for example under pressure,

- heating and sintering,

- final processing, for example metallization, and polarization.

The present lead-free piezo ceramic is preferably used in a drive unit for ultrasound transducers.

Such an ultrasound transducer, for example a BLT-transducer (bolt-tightening Langevin-type transducer), comprises a drive unit, wherein the drive unit is formed by laminating together a plurality of piezo-ceramics as described previously and electrode plates. The drive unit is sandwiched between a front mass and a rear mass, wherein front mass and rear mass are fastened together using a fastening bolt by applying a pre-load of > 20 MPa. The front mass emits ultrasonic waves from the surface thereof.

The invention is now explained in more detail with reference to the examples and figures. It shows. Figure 1 Strengths of lead-free piezoceramics compared to various PZT-based materials,

Figure 2: temperature profile applied for a sample for determining depolarization temperature T_D.

Examples

The following examples are included to demonstrate certain aspects and embodiments of the invention as described in the claims. It should be appreciated by those of skill in the art, however, that the following description is illustrative only and should not be taken in any way as a restriction of the invention.

Embodiments of the lead-free piezo electric ceramic according to the invention are provided in Table 1.

The lead-free piezo ceramic illustrated in Table 1 are synthesized in analogy to the following example:

- providing the starting material Bi₂O3, Na₂CO3, TiO₂, BaTiOs in the required amounts;

- wet mixing and deagglomeration of the starting material,

- freezing of homogeneous distribution

- calcination over 800°C

- grinding process in a mill with added ZnO

- adding PVA and PEG,

- granulation by spray drying,

- shaping the granulated material under pressure,

- debindering at temperature over 600°C

- sintering at different temperatures over 1000°C, hard machining

- metallization, polarization with5 kV/mm,

- measuring 24h after polarization

PZT Sonox®P4 and KNN are comparative examples. While PZT Sonox®P4 is a commercially available lead containing piezoceramic, KNN is a sodium potassium niobate based ceramic (“Lead-free Piezoelectric Ceramics: Technologies and Global Opportunities”, Report Code: NAN063B, April 2022, BCC Publishing). Table 1 k_t = thickness coupling factor k_p = planar coupling factor

AK = anisotropy coefficient p = density d₃₃ = piezoelectric constant dsi = piezoelectric constant transverse to polarization direction TD = depolarization temperature (coupled to phase transition) tan 5 = dielectric dissipation factor *T_c(°C) Curie Temperature: This is the temperature at which the dielectric permittivity of ferroelectric ceramics will reach its maximum. At this temperature, a piezoelectric ceramic will lose its polarized state. For this reason, operating temperatures normally should not exceed half the Curie temperature.

As can be seen in Table 1 the lead-free ceramics used in the present invention differ from PZT Sonox®P4 and KNN in their respective k_p value and anisotropy coefficient AK. Both ceramics do not show phase transition and are thus preferably characterized by the Curie temperature. Half of the Curie temperature may be used in analogy to the depolarization temperature T_D.

Furthermore, experimental data (not shown) reveal a different temperature curve for PZT Sonox®P4 and BNT-based ceramics in an oscillator setup. These data support the theory that the BNT-based ceramics do not promote a self heating or warm up when in use compared to conventional lead containing piezo ceramics.

This effect is influenced by the lower anisotropy coefficient of PZT Sonox®P4 compared to the presently used BNT-based ceramics.

As mentioned, the presently used BNT-based ceramic shows a reduced friction work when used between converter material in an ultrasound transducer.

The friction work W_R can be calculated with the formula W_R = F * p * s (with F = force, p = friction coefficient, s = displacement). The ratio of SAniso (lead-free ceramics with high anisotropy coefficients > 0.6) to SPZT (PZT ceramics e.g. Sonox®P4) can be calculated via the ratio of the charge constants dsiAniso to dsipzT and is between 0.15 and 0.35 for the materials mentioned in Table 1. Taking into account the different densities, this results in a value between 0.11 and 0.27 for the ratio of the friction work W_RAniso/W_RPzT (as a measure for self-heating) for the materials mentioned in Table 1 , i.e. a reduced friction work of 11 to 27% for lead-free ceramics with high anisotropy coefficients > 0.6 compared to PZT ceramics.

Reduced friction between the end faces is beneficial in that the development of frictional heat and thus electrical losses are also reduced. Common application temperatures have so far been > 120-130°C due to a low anisotropy coefficient < 0.6 and the associated high heat development. The frictional heat can thus be reduced to < 50°C in applications with high preloads > 20 MPa. Another advantage resulting from the reduced heat generation is that lead- free materials with high anisotropy coefficients > 0.6 and depolarization temperatures T_D < 140°C are suitable for use in applications with high preloads > 20MPa.

The fracture forces of lead-free piezoceramics with anisotropy coefficients > 0.6, such as BNT- BT or BNT-BT composites, are 11 to 29N for samples with 6 mm diameter measured by the ball-on-ring method and > 15N on average. They thus have more than twice the breaking strength of a conventional soft PZT-based ceramic, such as Sonox® P5. In the case of Sonox®P5, a breaking force of approx. 5N on average was measured. Within the range of dispersion, lead-free piezoceramics with an anisotropy coefficient > 0.6 can achieve strengths up to 5 times higher. Hard PZT ceramics, such as Sonox® P4, have fracture strength values of approx. 12N on average and are thus in the lower strength range of lead-free piezoceramics (see also diagram of Fig. 1 )

Description of fracture force measurement with the ball-on-rino method (for samples with a diameter of 6mm)

Equipment needed:

- Measuring frame prepared for fracture test with housed actuator and load cell (1 )

- Charge amplifier self-made (2)

- Voltage source TTI PLH250-P (3)

- Oscilloscope Agilent DSO-X 2024A (4)

- Multimeter for force measurement (5)

Procedure:

The piezoelectric disc is positioned over the load cell so that only the edge area rests on a ring. Then the ball attached to an actuator is positioned on the sample so that the applied voltage is 0 (no force is applied to the sample). By slowly increasing the voltage applied to the actuator, it expands and applies a defined force to the actuator. The increase in the applied voltage can be followed on the oscilloscope. As the applied voltage increases, so does the force acting on the disc. The rupture is expressed by an abrupt drop in the voltage curve, as the resistance against which the actuator is pressing (the piezoelectric disc) gives way. The maximum of the 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. Measurement-routine for Depolarisation temperature T_D:

A sample (preferable a thin disc) is mounted in a temperature kiln and following temperature profile is applied as illustrated in the Diagram of Figure 2.

The process is started at 20°C. The heating is continued up to 180°C and the small signal impedance is continuously measured every 2K. When the material depolarizes, the k_t value falls down to zero. The value is taken where the curve drops and is reduced by 10 °C (to guarantee piezoelectric behavior in the application close to this point). This resulting temperature is determined as T_D (depolarization temperature).

Claims

1 . Use of a lead-free piezo ceramic in devices that require the application of a permanent preload of > 20 MPa to the lead-free piezo ceramic, wherein the lead-free piezo ceramic has the following composition:

[ (100-a)(x(Bi_nNa_m )TiOs - y(Bi_nK_m )TiOs - zBaTiOs ) - aM]

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) applies, and

M is one of the additives ZnO, MgO, TiO2, ZrC>2, AI2O3 or a mixture thereof.

2. Use of a lead-free piezo ceramic according to claim 1 , characterized in that the lead- free piezo ceramics has the following composition:

[ (100-a)(x(Bi_nNa_m )TiOs - zBaTiOs ) - aM]

Wherein (0 < x < 1 ), (0 < z <1 ), (x + z = 1 ), (0.4 < n < 0.6), (0.4 < m < 0.6) and (0 < a < 10) applies, and

M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.

3. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that the lead-free piezo ceramics has the following composition: x(Bi_nNa_m)TiO3 - zBaTiOs wherein (0.92 < x < 0.935), (0.065 < z < 0.08), (x + z = 1 ), (n = m = 0.5).

4. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that the lead-free piezo ceramics has the following composition:

[ (100-a)(x(Bi_nNa_m)Ti03 - zBaTiOs ) - aM] wherein (0.92 < x < 0.935), (0.065 < z < 0.08), (x + z = 1 ), (n = m = 0.5) and (0.5 < a < 3,) applies, and M is one of the additives ZnO, MgO, TiO2, ZrC>2, AI2O3 or a mixture thereof.

5. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that the lead-free piezo ceramic has a planar coupling factor k_p in a range between 0.1 and 0.35, preferably between 0.15 and 0.32, for example between 0.15 and 0.35, such as 0.28-0.32, or between 0.1 and 0.25, such as 0.15 - 0.2.

6. Use of a lead-free piezo ceramic according to claim 1 and 2, characterized in that the lead-free piezo ceramic has a thickness coupling factor k_t in a range between 0.4 and 0.6, preferably between 0.48 and 0.52, for example between 0.45 and 0.55, such as 0.5-0.52 or between 0.43-0.55, such as 0.48-0.52

7. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that the lead-free piezo ceramic has an anisotropy coeffiecient of k_t/(k_t + k_p) > 0.6, preferably > 0.65, more preferably > 0.7, preferably in a range between 0.6 and 0.8, more preferably between 0.65 and 0.79, more preferably between 0.7 and 0.79 with k_t being the thickness coupling factor and k_p being the planar coupling factor.

8. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that lead-free piezo ceramics has a depolarisation temperature T_D of 50°C to 200°C, preferably of 80°C to 180°C, more preferably of 90°C to 160°C, even more preferably of 130°C to 145°C.

9. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that lead-free piezo ceramics has a piezoelectric charge constant d₃₃ (in pC/N) between 100 and 200, preferably between 1 10 and 190, more preferably between 120 and 180.

10. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that lead-free piezo ceramics has a tan 6 between 30 and 700, preferably between 40 and 600, more preferably between 40 and 400, for example between 40 and 700, such as 40-100 or between 200 and 600, such as 200 and 400.

11 . Use of a lead-free piezo ceramics according to one of the preceding claims, in devices requiring a permanent preload to the piezo ceramic of > 20 MPa at working temperatures between 20 and 100°C.

12. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that lead-free piezo ceramics has fracture forces (measured by the ball-on-ring method) of > 10N, preferably between 11 and 30N.

13. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that lead-free piezo ceramics has a density p of at least 5.0 g/cm³, preferably of at least 5.5 g/cm³, preferably in a range between 5.0 and 7.0 g/ cm³, more preferably in a range between 5.5 and 6.5 g/cm³, even more preferably in a range between 5. 5 and 6.0 g/cm³.

14. Use of a lead-free piezo ceramic according to one of the preceding claims, characterized in that the lead-free piezo ceramics is essentially free of Calcium (Ca) and/or Strontium (Sr), except for minor impurities.

15. Use of a lead-free piezo ceramic according to one of the preceding claims characterized in that the lead-free piezo ceramics contains further metal oxides, the metal oxides being 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.

16. Use of a lead-free piezo ceramic according to one of the preceding claims in a drive unit for ultrasound transducers.

17. An ultrasound transducer comprising a drive unit, wherein the drive unit is formed by laminating together a plurality of piezo-ceramics as described in one of the preceding claims and electrode plates, wherein the drive unit is sandwiched between a front mass and a rear mass, wherein front mass and rear mass are fastened together by a fastening bolt, wherein the pre-load applied by the fastening bolt is > 20 MPa.