WO2017203211A1

WO2017203211A1 - Temperature stable lead-free piezoelectric/electrostrictive materials with enhanced fatigue resistance

Info

Publication number: WO2017203211A1
Application number: PCT/GB2017/051383
Authority: WO
Inventors: Ian Michael Reaney; Amir KHESRO
Original assignee: Johnson Matthey Piezo Products Gmbh; Nunn, Andrew Dominic
Priority date: 2016-05-27
Filing date: 2017-05-18
Publication date: 2017-11-30
Also published as: GB2550887B; GB201609368D0; GB2550887A

Abstract

A lead-free piezoelectric and/or electrostrictive ceramic material having the general formula: (1-x) K_0.5Bi_0.5TiO₃ - x[Bi(RE)Fe(Ti)O₃]; wherein RE = non-radioactive rare earth elements as defined by IUPAC and wherein 0.01 < x < 0.25.

Description

TEMPERATURE STABLE LEAD-FREE PIEZOELECTRIC/ELECTROSTRICTIVE

MATERIALS WITH ENHANCED FATIGUE RESISTANCE

The present invention relates to temperature stable lead-free piezoelectric and/or electrostrictive materials with enhanced fatigue resistance based on compositions containing an end member of potassium bismuth titanate together with a rare earth and titanium doped bismuth ferrite.

Piezoelectric ceramic materials (also referred to as piezoelectric ceramics or piezoceramics) have been widely used in applications such as actuators, transducers, resonators, sensors, and random access memories. A piezoelectric actuator utilises the so- called inverse piezoelectric effect, in which an electrical signal is converted into a precisely controlled mechanical strain. The ability to control physical displacements with high precision makes piezoelectric actuators important for many applications such as cameras, phones, microscopes, fuel injectors, micro-pumps, ink cartridges and medical surgery instruments. A bending transducer can also be used as an energy harvester, whereby an electrical charge is generated within a piezoelectric ceramic material resulting from an applied mechanical force (the so-called piezoelectric effect).

Among commercial piezoelectric ceramics, lead zirconate titanate (PZT) and its related solid solutions are the most widely used, e.g. for actuators, due to their excellent piezoelectric properties including their large piezoelectric and electromechanical coupling coefficients, ease of fabrication and cost and the ease with which modifications by doping can be made during manufacturing. However, there are drawbacks to using PZT, which limit its desirability in many applications. One concern is its possible environmental effect due to the toxicity of highly volatile PbO which can evolve from PZT during fabrication. Another drawback of PZT piezoceramics is the strong fatigue behaviour. Fatigue is a phenomenon in which a piezoelectric material loses its switchable polarization and electromechanical strain during cyclic electrical loading. Many technological devices such as fuel injectors have harsh operating environment well above room temperature and hence the temperature stability of electromechanical strain is very important.

Electrostriction is a property of all dielectric materials, and is caused by a slight displacement of ions in the crystal lattice upon being exposed to an external electric field. Positive ions will be displaced in the direction of the field, while negative ions will be displaced in the opposite direction. This displacement will accumulate throughout the bulk material and result in an overall strain (elongation) in the direction of the field. All insulating materials consisting of more than one type of atoms will be ionic to some extent due to the difference of electronegativity of the atoms, and therefore exhibit electrostriction. The resulting strain (ratio of deformation to the original dimension) is proportional to the square of the polarization. The related piezoelectric effect occurs only in a particular class of dielectrics. Electrostriction applies to all crystal symmetries, while the piezoelectric effect only applies to the 20 piezoelectric point groups. Electrostriction is a quadratic effect, unlike piezoelectricity, which is a linear effect. Electrostrictors have application as actuators requiring relatively small displacements.

K_0.5Bo_.5Ti03 (KBT) was first fabricated in 1959 by G.A. Smolenskii et al (see Soviet Physics-Solid State 1 (10), 1429 (I960)). The piezoelectric properties of KBT are moderate (d₃₃ = 69 pc/N and strain 0.1 % at 80 kv/cm) for hot pressed ceramics (see Yuji Hiruma et al. Jpn. J. Appl. Phys. 44 (7A), 5040 (2005)). Subsequently, a number of researchers have investigated using KBT as an end member for developing lead-free piezoelectrics. These include potassium bismuth titanate-bismuth ferrite (KBT-BF) materials including 0.75Ko_.5Bio_.5Ti0₃-0.25BiFe0₃ (see M.I. Morozov et al, Applied Physics Letters 101 (25), 252904 (2012)) and 0.50K_0.5Bio_.5Ti0₃-0.50BiFe0₃ (see H. Matsuo et al, Journal of Applied Physics 108 (10), 104103 (2010)) having strain %, Smax/Emax and variation in strain with temperature (%) properties - where reported - as set out in Table 1.

Table 1

Composition Strain % (E Smax Emax (pm/V) Variation in

kV/mm) strain with

temperature (%)

0.50K₀.₅Bio.₅Ti0₃- 0.23 (10) 230 (1 Hz) N/R

0.50BiFeO₃

prepared from nanosized

powders

For actuator applications, the Smax/Emax value is important, where S_maK is the positive strain at the applied field E_max-

The inventors have investigated KBT-BF materials and have discovered, very surprisingly, that by doping these materials with certain rare earth (RE) elements, the piezoelectric properties of the materials can be enhanced. In particular, the RE doped KBT- BF materials show particularly good fatigue behaviour and temperature stability.

Rare Earth elements, as defined by IUPAC, is one of a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides as well as scandium and yttrium. Promethium is radioactive and is not intended for use in the present invention. The non-radioactive RE elements are: scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysproprium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).

According to a first aspect the invention provides a lead-free piezoelectric and/or electrostrictive ceramic material having the general formula:

(1-x) K_0.5Bio_.5Ti0₃ - x[Bi(RE)Fe(Ti)0₃]; wherein RE = non-radioactive rare earth elements as defined by IUPAC and wherein 0.01 < x < 0.25.

Preferably, the non-radioactive rare earth element is La, Nd, Pr and/or Sm. The relative proportions of the components of the lead-free ceramic materials according to the invention may be varied during their production so that the products will have a specified operating temperature range. Depending upon the desired end use of the composition, the operating temperature of the ceramic product may differ from the peak maximum in permittivity (T_m; see Example 3 hereinbelow for an explanation) of the lead-free piezoelectric ceramic materials according to the invention. The relative amounts or proportions of the components in the lead-free piezoelectric materials according to the invention are expressed in terms of mole fraction or mole percent (mol%). These compositions represent the basic range of stability of the binary compositions with a stable perovskite phase at standard atmospheric conditions, i.e. cubic lattice structures with general formula of AB0₃, wherein A is a A-site ion located on the corners of the lattice and B is a B-site ion on the centre of the lattice and is a 3d, 4d or 5d transition metal.

Inventors designed the claimed series specifically to control the conductivity/di electric arising from BiFe0₃ concentration in the system. Hence the bismuth end member of the series is composed of BiFe0₃ - REFe0₃ and RE_2/3Ti0₃, which can be expressed e.g. as (1-x) Ko_.sBio_.sTiC - x[Bi(RE)Fe0.97(Ti0.03)O₃]. The intention of adding both RE³⁺ on the A-site and Ti⁴⁺ on the B-site was to decrease conductivity and loss and permit the materials to operate well at high temperature and field.

The inventors consider that the fatigue and temperature stability of the lead-free piezoelectric/electrostrictive ceramic materials according to the invention are particularly good. However, the d₃i and k_p of the claimed materials are considered less important because the high strain achieved in these materials is predominantly electrostrictive.

For many applications in which fatigue resistance of a piezoelectric ceramic is more important than the piezoelectric maximum strain performance, the lead-free piezoelectric ceramic material may be advantageous. Some of the lead-free piezo electric ceramic compositions according to the invention will meet or exceed the piezoelectric properties of doped PZT materials, and will provide a constant strain from room temperature to 300 °C with minimal or no degradation over the life of a device employing such material. Some of the lead-free piezo electric ceramic compositions according to the invention are predicted to have similar potential uses as PZT materials such as in actuators. Some of these applications will further benefit from the absence of lead in the piezoelectric ceramics. Preferably, the lead-free piezoelectric and/or electrostnctive ceramic material according to the invention has the general chemical formula (1-x) Ko_.sBio_.sTiC - x[Bi(RE)Fe0.97(Ti0.03)O₃], such as (1-x) K_0.5Bio_.5Ti0₃ - x[Bi(RE)Fe0.97(Ti0.03)O₃], wherein 0.01 < x < 0.25. The invention is exemplified herein by use of the rare earth elements lanthanum (La) - referred to herein as KBLFT - and neodymium (Nd) - referred to herein as KB FT - which are preferred. However, inventors believe that the following rare earth elements are applicable to the present invention because the ionic radii are comparable to La and Nd and so would also be expected to form stable perovskite phases. For this reason, praseodymium and samarium are predicted to have particular application in the present invention. Accordingly, the RE component of the piezoelectric ceramic material according to the invention comprises mixtures of one or more rare earth element.

Preferably, RE in the lead-free piezoelectric and/or electrostrictive ceramic material according to the invention is Nd and wherein 0.03 < x < 0.15, particularly preferably wherein 0.06 < x < 0.12 and most preferably wherein x is 0.09. Where RE in the lead-free piezoelectric and/or electrostrictive ceramic material according to the invention is La, preferably 0.02 < x < 0.15, particularly preferably 0.06 < x < 0.12 and most preferably x is 0.10.

According to a second aspect, the invention provides a piezoelectric and/or electrostrictive element comprising a lead-free piezoelectric and/or electrostrictive ceramic material according to the invention disposed on a substrate and having an electrode attached to the piezoelectric ceramic material.

According to third aspect, the invention provides a multilayer stack of two or more piezoelectric and/or electrostrictive elements for use as multi-layered actuator devices, each element comprising a layer of lead-free piezoelectric and/or electrostrictive ceramic material according to the first aspect of the invention and an electrode layer applied to the lead-free piezoelectric and/or electrostrictive ceramic material.

The piezoelectric and/or electrostrictive element or multilayer stack of two or more piezoelectric and/or electrostrictive elements according to the second or third aspects of the invention can be used as a transducer, a sensor, a loudspeaker, an audio amplifier, an energy harvester in a hand held device, such as a medical dispenser, e.g. an, an actuator in a camera, mobile phone, microscope, fuel injector, micro-pump, ink cartridge, valve, medical surgery instrument, braille reader, Jacquard knitting machine, cool mist generator etc.

In some applications according to the second aspect of the invention, the substrate is a flexible substrate, preferably a fatigue resistant flexible substrate such as a carbon-fibre- reinforced plastic (CRP). In other applications, such as the mist generator, the substrate is a solid, e.g. a ceramic disc.

According to a fourth aspect, the invention provides the use of a piezoelectric and/or electrostrictive ceramic element according to the second aspect of the invention or a multilayer stack of two or more piezoelectric and/or electrostrictive elements according to the third aspect of the invention as an actuator or a bending transducer.

In a fifth aspect, the invention provides the use of a piezoelectric and/or electrostrictive ceramic material according to the first aspect of the invention in the manufacture of a piezoelectric and/or electrostrictive element according to the second aspect of the invention or a multilayer stack of two or more piezoelectric and/or electrostrictive elements according to the third aspect of the invention.

According to a sixth aspect, the invention provides a method of making a lead-free piezoelectric and/or electrostrictive ceramic material according to the first aspect of the invention comprising the steps of batch mixing reactant powders of Bi₂0₃, K₂C0₃, Ti0₂, Fe₂0₃ and RE₂0₃, wherein RE = non-radioactive rare earth elements as defined by IUPAC in the appropriate stoichiometric proportions to achieve the desired value of x in the product; grinding the reactant powders together; and calcining the ground reactant powders at a temperature of >500°C.

According to seventh aspect, the invention provides a method of making a multilayer stack of two or more piezoelectric and/or electrostrictive elements according to the fourth aspect of the invention by casting, e.g. from a slurry, a layer of the lead-free piezoelectric and/or electrostrictive ceramic material according to the first aspect of the invention onto a carrier film, applying an electrode material, e.g. platinum, onto the cast layer of lead-free piezoelectric and/or electrostrictive ceramic material, co-firing the cast lead-free piezoelectric and/or electrostrictive ceramic material and applied electrode material to generate a piezoelectric and/or electrostrictive element and constructing a multiplayer stack comprising two or more piezoelectric and/or electrostrictive elements by disposing one piezoelectric and/or electrostrictive element on top of another piezoelectric and/or electrostrictive element.

NOTATION AND NOMENCLATURE

The term "Curie temperature" (T_c) refers to the temperature above which a piezoelectric material loses its spontaneous polarization and piezoelectric characteristics. For high temperature applications a high T_c material is desired, however a high T_c does not guarantee a stable output and generally electromechanical strains increase with increasing temperature. In real world application, to avoid a loss in spontaneous polarization and piezoelectric characteristics, a piezoelectric ceramic material is generally limited in use to approximately 80-100°C below its Curie temperature. Accordingly, preferred Curie temperatures are >200°C. However, Curie temperatures that are too high result in d₃₃ that is impractically low.

The temperature corresponding to the maximum of the dielectric constant is called the "temperature T_m". See also the explanation in Example 3 hereinbelow.

The term "polarization hysteresis" refers to lead-free piezoelectric ceramic materials that display non-linear polarization characteristics indicative of a polar state.

The term "k" refers to the electromechanical coupling factor and is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or converts mechanical energy into electrical energy. For a thin disc of piezoelectric ceramic the planar coupling factor, k_p, expresses radial coupling - the coupling between an electric field parallel to the direction in which the ceramic element is polarized (direction 3) and mechanical effects that produce radial vibrations, relative to the direction of polarization (direction 1 and direction 2).

The term "electromechanical strain" refers to an electric field induced strain and is commonly expressed in terms of one or more piezoelectric coefficients (d₃₃ and d₃i, for example), where d (units pm/V) is the tensor property that relates the strain to the applied electric field (V/m). The d₃₃ coefficient can be measured in many different ways, such a piezoelectric resonance, the direct piezoelectric effect, the indirect piezoelectric effect, and others. In the context of this disclosure, the d₃₃ coefficient is calculated as the ratio between the maximum electromechanical strain and the applied electric field (d₃₃ = S_max/E_max). Sometimes this is described as the effective piezoelectric coefficient or the normalized strain or d₃₃*. An example of its use is given in Y. Hiruma et al, J. Appl. Phys. 103:084121 (2008).

The term "d₃₃" refers to the induced polarization in direction 3 (parallel to direction in which ceramic element is polarized) per unit stress applied in direction 3 or induced strain in direction 3 per unit electric field applied in direction 3. The term "d₃₁" refers to induced polarization in direction 3 (parallel to direction in which ceramic element is polarized) per unit stress applied in direction 1 (perpendicular to direction in which ceramic element is polarized) or induced strain in direction 1 per unit electric field applied in direction 3. In the context of piezoelectric ceramic materials, the term "fatigue" refers to the observed loss of polarization and electromechanical strain after the application of a cyclic electric field.

The term "polarization remanence" refers to the polarization measured at zero field during a polarization hysteresis measurement. It is a unique characteristic of polar, nonlinear dielectrics.

In order that the invention may be more fully understood, the following Examples are provided by way of illustration only and with reference to the accompanying drawings, in which:

Fig. 1 is a graph showing electromechanical strain values at applied electrical fields ranging from -6.0 kV/mm to 6.0 kV/mm (i.e. "bipolar strain loops") of a representative KBLFT (x=0.04, 0.10 and x =0.15) material according to the invention;

Fig. 2 is a graph showing electromechanical strain values at applied electrical fields ranging from -6.0 kV/mm to 6.0 kV/mm (i.e. "bipolar strain loops") of a representative KB FT (x=0.03, 0.09 and 0.15) material according to the invention; and Fig 3 is a graph illustrating the fatigue behaviour of KB FT (x=0.09) shown in Figure 2. A bipolar strain loop for data recorded on the first test cycle is overlaid with data for the same material after 10⁶ cycles.

EXAMPLES Properties of the piezoelectric and/or electrostrictive ceramic materials according to the invention may be evaluated by piezoelectric resonance measurements following the customary IEEE standard, poling studies to measure the low-field electromechanical strain coefficient d₃₃, fatigue measurements, and studies of the temperature dependence of these piezoelectric properties. Example 1 : Production of Rare Earth-containing and Ti doped Compositions.

Solid Solutions in the series (1-x) K_0.5Bio_.5Ti0₃ - x [0.80 BiFe0₃- 0.15 LaFe0₃- 0.05 La_2/3Ti0₃] x = 0 (not according to the invention), 0.02, 0.04, 0.06, 0.08, 0.10, 0.12 and 0.15 and (1-x) K_0.5Bio_.5Ti0₃ - x [0.82 BiFe0₃- 0.15 NdFe0₃- 0.03 Nd_2/3Ti0₃] for x = 0 (not according to the invention), 0.03, 0.06, 0.09, 0.12 and 0.15 were prepared by conventional solid state oxide route using Bi₂0₃ (99.9 %, Across Organics) K₂C0₃ (99 %, Fisher Scientific), Ti0₂ (99.9 %, Sigma Aldrich) Nd₂0₃, (99.9 % Acros Organics) Fe₂0₃ (99+ %, Sigma Aldrich) and La₂0₃ (99.99 % Stanford Materials) as the starting materials. The raw materials were dried and batched up in stoichiometric amounts. The batches were attrition milled for 1 hour in Union Processes Attritor (Szegvari

Attritor System, Union Process, Arkon, Ohio, USA) using 3 mm diameter yttria-stabilised zirconia media in isopropanol at 300 rpm. The slurry was separated from media and dried overnight at 80 °C in an oven under extraction. The mixed dried powders were then sieved through 300 micron mesh and reacted 800 °C for 4 hours in a muffle furnace

The reacted powders were attrition milled and dried as above. - 3 wt. % PVA was mixed with the calcined powders using a mortar and pestle. The powders were then pressed to form pellets of ~ 1.5 mm applying 1 ton force for 1 minute using a uniaxial press. The pellets were sintered between 1070 - 1090 °C for 2 hours after a binder burn out at 550°C for 3 h. Optimum sintering temperatures were decided on the basis of phase purity and measured density. Exemplary compositions B-F of the KBNFT (x) series are defined in Table 2; and exemplary compositions G-M of the KBLFT (x) series are also defined in Table 3. The unary perovskite composition KBFT, i.e. KBNFT or KBLFT, wherein x=0.00 (composition A) is included for comparison purposes only. According to the nomenclature used herein, the materials of the KBNFT series, wherein x = 0.09 is referred to as KBNFT9; and wherein x = 0.12, KBNFT12 etc. Similarly, the compositions of the KBLFT series, whereon x = 0.10 is referred to herein as KBLFT 10 etc. Table 2 - KBNFT(x) series

Table 3 - KBLFTfx) series

Identifier Value of "x" K₀.₅Bio.₅Ti0₃ [0.80BiFeO₃- (Mol%) 0.15LaFeO₃- 0.05La₂/₃TiO₃]

(Mol%)

KBLFT15 0.15 85 14

When the intended use of the lead-free piezoelectric and/or electrostrictive ceramic materials according to the invention requires a thin film product, the production method may be modified to include chemical solution deposition using chemical precursors such bismuth nitrate, titanium isopropoxide etc., or sputtering using solid state sintered or hot-pressed ceramic targets. Any suitable sputtering or chemical deposition method may be used for this purpose. The resulting thin film ceramic may have a thickness in the range of about 50 nm to about 10 μπι, in some cases. For end uses such as sensors or transducers, which require the use of piezoelectric composites, the lead-free piezoelectric ceramic materials according to the invention can be modified for this purpose. The ceramic powder is ground or milled to the desired particle size and loaded into polymer matrix to create a 0-3 piezoelectric composite. The ceramic powder can be formed into sintered rods or fibres using injection moulding or similar technique and loaded into a polymer matrix to create a 1-3 piezoelectric composite. The polymer may be piezoelectric, such as PVDF, or non-piezoelectric such as epoxy depending on the final application.

Example 2: XRD (X-ray diffraction) In the X-ray diffraction method, sample is exposed to monochromatic incident X-rays at different angles and intensities of diffracted peaks from randomly oriented crystallites are recorded. For XRD, sintered pellets were polished and then annealed at 500°C overnight to reduce the mechanical stresses produced during polishing. Room temperature XRD data was collected using Bruker D2 Phaser with CuKa source in the range 20 to 70 degrees 2Θ at a step size of 0.02°.

Samples prepared according to Example 1 were subjected to x-ray diffraction analysis which confirmed that the material consisted of a single-phase perovskite structure. The resulting X-ray diffraction pattern showed no evidence of any secondary phases in these compositions.

Example 3 : LCR Meter (L=Inductance, C=Capacitance, R= Resistance)

For LCR measurements an AC signal at fixed frequency is applied on the sample and current passing through the sample is recorded. These processed voltages and current signals from the sample under test are fed to the digital integrator unit which displays the capacitance values on screen.

For LCR, samples were ground to ~1 mm thickness and then silver paste was applied as an electrode contact. The silver paste was dried at 200 °C. A LCR meter (Model 4284A, Hewlett Packard) connected to computer through CP-IB interface was used to measure capacitance at fixed frequencies 1 kHz, 10 kHz, 100 kHz, 250 kHz and 1 MHz The sample was loaded into a conductivity jig and placed in a tube furnace. An extra thermocouple was inserted near the sample for accurate measurement of temperature. Data readings were taken every 60 seconds to a total data population of 550 individual readings at from room temperature to 500°C. Dielectric constant was calculated from capacitance using the following formula

C. t

Where C is capacitance (F), t is thickness of the sample (m), A is area of face of the pellet (m²), ε₀ is permittivity of free space, 8.854* 10^"12(F/m) and ε_Γ is relative permittivity or dielectric constant. KBLFT and KB FT are relaxor ferroelectrics and do not have a T_c like ferroelectric materials. The peak maximum in permittivity for a relaxor is represented by T_m and is not equivalent of a T_c. The strain induced in these materials are predominantly electrostrictive and do not require a ferroelectric order. However it depends on polarisation and a significant decrease in polarisation after T_mcan result in a decreased electromechanical strain. The peak maximum in permittivity (T_m) at 1 MHz of the KB FT(x) series and

KBLFT (x) series prepared according to Example 1 are set out in Tables 4 and 5.

Table 4 Identifier Peak maximum in

permittivity(Tm)

KBFT 370 °C

KBNFT3 333 °C

KBNFT6 322 °C

KBNFT9 300 °C

KBNFT12 291 °C

KBNFT15 280 °C

Table 5

Identifier Peak maximum in

permittivity(Tm)

KBFT 370 °C

KBLFT2 352 °C

KBLFT4 333 °C

KBLFT6 333 °C

KBLFT8 311 °C

KBLFT10 307 °C

KBLFT12 299 °C

KBLFT15 291 °C

Note that above the transition at 300°C the material has its lowest point of loss which means that the material is unlikely to suffer from significant time dependent degradation when operated at it maximum temperature of 300 °C.

By way of comparison, conventional PZT piezoelectric ceramics typically demonstrate an operating temperature < 200 °C.

In conclusion, the improved losses and reduced conductivity of the RE and Ti doped systems permit the application of a large field which permits a strong electrostrictive response which is maintained until high temperature. Example 4: Direct Piezoelectricity

Piezoelectric charge constant (d₃₃) was measured using (Piezotest. Model PM300, London, UK) piezoelectric meter after DC poling in silicon oil at 5kV/mm and 100°C. A dynamic force of 0.25 N with frequency of 110 Hz was applied to take these measurements.

The d₃₃ values of the KB FT(x) series and KBLFT (x) series prepared according to Example 1 are set out in Tables 6 and 7.

Table 6

These data show that the lead-free piezoelectric and/or electrostrictive ceramic materials according to the invention do not have significant piezoelectric behaviour as compared with known PZT materials: the peak d₃₃ for either the KBLFT or the KB FT series is 125 pC/N. However, the materials show high strain with excellent temperature stability and fatigue resistant (see Examples 6 and 7) combined with an upper operating limit of at least 300°C, which is exceptional in both lead and lead-free materials.

Example 5: Polarisation and Strain Measurements

For polarization (P) and bipolar strain (S) vs. electric field (E), the piezoelectric samples were thinned to ~ 0.7 mm and then silver paste was applied to both sides of the sample as an electrode contact. The measurements were taken using an aix-ACCT TF2000FE- HV ferroelectric test unit (aix-ACCT Inc., Germany).

The sample holder had an integrated heating unit and was coupled with a laser beam interferometer. Hence, the system was able to perform simultaneous acquisition of polarization and electromechanical strain data over a wide range of temperatures. The sample holder was filled with silicon oil to increase the range of applied voltage without any electric arcing. All measurements were taken at a fixed frequency of 1 Hz.

As shown in Figs. 1 and 2, the electromechanical strain of a specimen of composition of KB FT9 and KBLFTIO were measured at an applied electrical field of 6 kV/mm and appears as the expected butterfly loops. The results shown in Figs. 2 and 5 and in Tables 8 and 9 demonstrate moderate electromechanical strains with an approximate d₃₃* value of X pm/V for KB FT9 and Y pm/V for KBLFTIO. The piezoelectric strain coefficient d₃₃ of the lead-free piezoelectric ceramic material are generally below those of PZT. By way of comparison, conventional PZT piezoelectric ceramics typically demonstrate a low-field d₃₃ of 200 pm/V - 600 pm/V.

Table 8: Electric Field Induced Strain (KB FTfx) Compositions)

Identifier Maximum Electric Field Normalised

Strain (%) (kV/mm) strain (d33*

(pm V))

KB FT15 0.11 6 183

Table 9: Electric Field Induced Strain (KBLFT(x) Compositions)

The polarization hysteresis data for representative compositions of KB FT(x) and KBLFT(x) shown in Figures 1 and 4 indicate ferroelectric behaviour.

Example 6: Fatigue

In real world applications, an actuator is exposed to long-term cyclic loading. Continuous electric cycling can cause severe degradation in strain evident from many studies on PZT. Bipolar loading is reported to cause more degradation of polarisation and electromechanical strain as compared to unipolar cycling. To study the fatigue behaviour, KB FT9 was exposed to a bipolar loading of 5 kV/mm (~3E_C) for 10⁶ cycles at a frequency of 10 Hz. A fatigue resistant behaviour is observed as shown in Fig 3. The bipolar strain reduces by <6 % and P_max by < 4 % after 10⁶ bipolar cycles. PZT have generally poor piezoelectric fatigue properties, e.g. PIC 151, (PI Ceramic Lederhose Germany) has been shown to lose about 50 % of 2P_r after 3 x 10⁶ bipolar cycles (~ 2E_C ) accompanied by severe asymmetric degradation in bipolar strain (see J. Nuffer et al., Acta mater. 48 3783-3794 (2000)).

Example 7: Temperature Stability Example 4 was repeated over a range of temperatures for the KB FT9 and KBLFTIO samples. Tables 10 and 11 include data for normalised bipolar strain as a function of temperature.

Table 10: KB FT9

Table 1

Example 8: Multilayer

For multilayers, a ceramic slurry was prepared by mixing calcined powders of KB FT9 with binder (Butar B-98 Sigma), plasticizers (Kollisolve PEG E 400 and benzyl butyl phthalate), dispersant (Hypermer KD-1) and solvents (ethanol and methanol) using a speed mixer at a speed of 1200 rpm for 15 minutes. Green ceramic tapes were prepared by casting the mixture onto moving silicon coated PET carrier film using Mistier TCC-1200 with a single doctor blade. The gap between carrier and blade was 400 μπι. A DEK 247 screen printer was used to print Pt electrodes (C60903D5; Gwent

Electronic Materials) onto the green tape. The green tape with Pt electrodes was peeled off the carrier film and stacked into a multilayer structure. Each stack consisted of 10 electrode layers with a buffer layer both on top and bottom, i.e. the stack is sandwiched between buffer layers. The stacks were then sintered at the same temperature as bulk ceramics following a two-step binder burn-out at 350 °C and 600 °C each for two hours.

Table 11 : KB FT9 Multilayer

These results show that the lead-free piezoelectric ceramic materials according to the invention show particularly stable performance across a wide temperature range.

A comparison of the temperature range of use and variation in strain as compared with commercially available materials is summarised in Table 12. Table 12

References for Table 12.

1. C. Galassi, M. Dinescu, K. Uchno, M. Sayer, Piezoelectric Materials; Advances in Science, Technology and Application, Vol. 76, Kluwer Academic Publishers, 2000.

2. D. Wang, Y. Fotinich, G. P. Carman, Journal of Applied Physics 1998, 83, 5342.

3. M. Acosta, Strain mechanisms in lead-free ferooelectrics actuators, Springer International Publishing, 2015.

4. S. Yasuyoshi, T. Hisaaki, T. Toshihiko, N. Tatsuhinko, T. Kazumasa, H. Takahiko, N. Toshiatsu, N. Masaya, Nature 2004, 434, 84.

Claims

A lead-free piezoelectric and/or electrostrictive ceramic material having the general formula:

(1-x) K₀.₅Bio.₅Ti0₃ - x[Bi(RE)Fe(Ti)0₃]; wherein RE = non-radioactive rare earth elements as defined by IUPAC and wherein 0.01 < x < 0.25.

A lead-free piezoelectric and/or electrostrictive ceramic material according to claim 1, wherein RE = La, Nd, Pr and/or Sm

A lead-free piezoelectric and/or electrostrictive ceramic material according to claim 1 comprising a solid solution having a stable perovskite structure at standard atmospheric conditions.

A lead-free piezoelectric and/or electrostrictive ceramic material according to claim 1, 2 or 3, having the general chemical formula (1-x) Ko_.sBio_.sTiC - x[Bi(RE)Fe0.97(Ti0.03)O₃], such as (1-x) K_0.5Bio_.5Ti0₃ - x[0.80BiFeO₃- 0.15(RE)FeO₃-0.05(RE)_2/3TiO₃], wherein 0.05 < x < 0.15

A lead-free piezoelectric and/or electrostrictive ceramic material according to any preceding claim, wherein RE is La or Nd.

A lead-free piezoelectric and/or electrostrictive ceramic material according to any preceding claim, wherein RE is Nd and wherein 0.05 < x < 0.15.

A lead-free piezoelectric and/or electrostrictive ceramic material according to any claim 6, wherein RE is Nd and wherein 0.06 < x < 0.12.

A lead-free piezoelectric and/or electrostrictive ceramic material according to any of claims 1 to 5, wherein RE is La and wherein 0.05 < x < 0.15.

A lead-free piezoelectric and/or electrostrictive ceramic material according to claim 8, wherein RE is La and wherein 0.06 < x < 0.12.

10. A piezoelectric and/or electrostrictive element comprising a lead-free piezoelectric and/or electrostrictive ceramic material according to any preceding claim disposed on a substrate and having an electrode attached to the piezoelectric ceramic material.

11. A piezoelectric element according to claim 10, wherein the substrate is a flexible substrate.

12. A multilayer stack of two or more piezoelectric and/or electrostrictive elements, each element comprising a layer of lead-free piezoelectric and/or electrostrictive ceramic material according to any of claims 1 to 9 and an electrode layer applied to the lead- free piezoelectric and/or electrostrictive ceramic material.

13. A multilayer stack according to claim 12, wherein the elements are sandwiched between buffer layers.

14. The use of a piezoelectric ceramic element or a multilayer stack of piezoelectric elements according to claim 10, 11, 12 or 13, as an actuator or a bending transducer.

15. The use of a piezoelectric and/or electrostrictive ceramic material according to any of claims 1 to 9 in the manufacture of a piezoelectric element according to any of claims 10 to 14.

16. A method of making a lead-free piezoelectric and/or electrostrictive ceramic material according to any of claims 1 to 9 comprising the steps of batch mixing reactant powders of Bi₂0₃, K₂C0₃, Ti0₂, Fe₂0₃ and RE₂0₃, wherein RE = non-radioactive rare earth elements as defined by IUPAC in appropriate stoichiometric proportions to achieve the desired value of x in the product; grinding the reactant powders together; and calcining the ground reactant powders at a temperature of >500°C.

17. A method of making a multilayer stack of two or more piezoelectric and/or electrostrictive elements according to claim 12 or 13 comprising the steps of casting a layer of the lead-free piezoelectric and/or electrostrictive ceramic material onto a carrier film, applying an electrode material onto the cast layer of lead-free piezoelectric and/or electrostrictive ceramic material, co-firing the cast lead-free piezoelectric and/or electrostrictive ceramic material and applied electrode material to generate a piezoelectric and/or electrostrictive element and constructing a multiplayer stack comprising two or more piezoelectric and/or electrostrictive elements by disposing one piezoelectric and/or electrostrictive element on top of another piezoelectric and/or electrostrictive element.