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• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/80—Constructional details
  • H10N30/85—Piezoelectric or electrostrictive active materials
  • H10N30/853—Ceramic compositions
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/80—Constructional details
  • H10N30/85—Piezoelectric or electrostrictive active materials
  • H10N30/853—Ceramic compositions
  • H10N30/8536—Alkaline earth metal based oxides, e.g. barium titanates
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/80—Constructional details
  • H10N30/85—Piezoelectric or electrostrictive active materials
  • H10N30/853—Ceramic compositions
  • H10N30/8561—Bismuth-based oxides

Definitions

• 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.
• 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).
• PZT lead zirconate titanate
• PbZrOs mixed crystals of lead zirconate
• PbTiOs lead titanate
• the domains of a ceramic body i.e., the areas consisting of elementary cells of uniform dipole direction
• the macroscopic body is isotropic and shows no piezoelectric properties.
• These piezoelectric properties have to be originated by “polarization”.
• 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.
• Depolarization 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.
• piezo ceramic components In ultrasonic applications, piezo ceramic components generate high-power ultrasonic waves for ultrasonic cleaning, drilling and welding and for stimulating of chemical processes.
• 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.
• 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.
• the applied mechanical compressive load protects the piezo ceramics from cracking due to mechanical tensile stress during operation.
• the preload minimizes friction losses in the joints.
• the pre-load influences the performance of the ultrasonic transducer due to nonlinearity in the material behavior.
• 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.
• 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:
• M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.
• 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.
• the friction heat is reduced.
• 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.
• 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.
• AK anisotropy coefficient
• 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.
• 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.
• 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.
• T D depolarization temperature
• 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).
• d33, dsi and d32 three different piezoelectric charge constants may be defined: d33, dsi and d32, wherein d 33 - 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 d 33 is thereby of an advantage.
• 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 DF is also the ratio of (energy lost)/(energy stored) or Re/
• 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.
• 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.
• 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).
• BNT bismuth sodium titanate
• BNT-BT bismuth sodium titanate barium titanate
• ZnO zinc oxide
• the lead-free piezo ceramics has a density p of at least 5.0 g/cm 3 , preferably of at least 5.5 g/cm 3 , preferably in a range between 5.0 and 7.0 g/ cm 3 , more preferably in a range between 5.5 and 6.5 g/cm 3 , even more preferably in a range between 5. 5 and 6.0 g/cm 3 (measured in accordance to DIN EN 60672-2 (VDE 0335 Opera2):2000).
• the lead-free piezo ceramics used in the present invention has the following composition: [(100-a)(x(Bi n Na m )TiOs - y(Bi n K m )TiOs - zBaTiOs ) - aM]
• M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.
• the lead-free piezo ceramics used in the present case has the composition
• the lead-free piezo ceramics used in the present case has the composition
• 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 d 33 is achieved.
• 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.
• a lead-free piezo ceramic is used in the present case that has the following composition:
• M is one of the additives ZnO, MgO, TiO2, ZrO2, AI2O3 or a mixture thereof.
• the lead-free piezo ceramics used in the present case has the composition
• the lead-free piezo ceramics used in the present case has the composition
• 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 d 33 (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.
• the lead-free piezo ceramics used in the present case has the composition
• 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 d 33 (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.
• the lead-free piezo ceramics used according to the invention may contain some (minor) impurities.
• 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.
• KNN Sodium Potassium Niobate-based materials
• 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.
• the energy required to produce KNN components is higher than to manufacture lead containing PZT ceramics.
• KNN a material that is truly more environmentally friendly than conventional lead containing PZT ceramics.
• 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.
• an aqueous, homogenous suspension of Bi 2 O3, Na 2 CO3, TiO 2 , 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.
• one or more additional components which are introduced e.g. as oxidic powder, are added to the main system before or after calcination.
• the synthesis process of the lead free piezo ceramic comprises the following steps:
• the starting material such as Bi 2 Os, Na 2 CO3, TiO 2 , BaTiOs
• organic additives such as PVA, PEG,
• 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.
• Figure 2 temperature profile applied for a sample for determining depolarization temperature T D .
• 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:
• AK anisotropy coefficient
• p density
• 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.
• 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 .
• This effect is influenced by the lower anisotropy coefficient of PZT Sonox®P4 compared to the presently used BNT-based ceramics.
• the presently used BNT-based ceramic shows a reduced friction work when used between converter material in an ultrasound transducer.
• 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.
• 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 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.
• a breaking force of approx. 5N on average was measured.
• 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 )
• 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.
• 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).

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