Lead-free piezoelectric ceramic composition and lead-free piezoelectric element comprising same

Abstract

The disclosure of the present application relates to a lead-free piezoelectric ceramic composition and a lead-free piezoelectric element comprising same. The lead-free piezoelectric ceramic composition according to embodiments of the present disclosure can be utilized as a material of the lead-free piezoelectric element having improved piezoelectric characteristics and mechanical reliability. Even if manufactured in a thin shape, the lead-free piezoelectric element has excellent stretchability and high vibration amplitude, thereby being capable of widely playing back high-output sound and low-frequency band sound, and thus can be utilized as a material of a speaker for a thin display.

Description

Lead-free piezoelectric ceramic composition and lead-free piezoelectric element comprising the same

[0001] The present application disclosure relates to a lead-free piezoelectric ceramic composition and a lead-free piezoelectric element comprising the same.

[0002] As electronic devices like TVs and mobile devices continue to become slimmer, the adoption of piezoelectric speakers is increasing. This is because piezoelectric speakers are thinner, lighter, and consume less power than conventional dynamic speakers.

[0003] Meanwhile, Pb(Zr,Ti)O3 (hereinafter, PZT) flexible piezoelectric ceramic materials have excellent piezoelectric properties, are inexpensive, and have well-known manufacturing process technologies, so they have a wide range of applications in various sensors and actuators. However, PZT flexible piezoelectric ceramic materials have a problem in that they contain more than 50% of their total weight of lead. Recently, the use of lead-containing materials has been regulated worldwide, especially in the electronics industry, but PZT piezoelectric materials are excluded from the regulation because no lead-free material has been developed to replace them. However, if a lead-free piezoelectric ceramic material with excellent piezoelectric properties is developed, the use of PZT piezoelectric materials is expected to be limited. Accordingly, the development of a lead-free piezoelectric material that does not use lead is required.

[0004] As a lead-free piezoelectric ceramic material, the potassium sodium niobate ((K,Na)NbO3, hereinafter referred to as KNN) lead-free piezoelectric ceramic composition is a solid solution of KNbO3, an orthorhombic phase with ferroelectricity, and NaNbO3, an orthorhombic phase with antiferroelectricity, and has a high Curie temperature of 420 ℃ and a dielectric constant of 33 μC / cm. 2Due to its advantages of high remanent polarization and relatively large electromechanical coupling coefficient, KNN is attracting attention as a material to replace PZT-based piezoelectric materials in the future. However, KNN lead-free piezoelectric ceramic compositions have lower performance in terms of piezoelectric properties and temperature stability compared to existing PZT-based piezoelectric materials, and much research is being conducted to improve this.

[0005] In addition, when a piezoelectric element is manufactured in a thin shape according to conventional technology, there is a problem in that high-output sound cannot be reproduced because the elasticity is weak, and there is a problem in that low-frequency sound cannot be reproduced widely because the vibration amplitude is small.

[0006] Accordingly, the inventors of the present invention have developed Bi 1 / 2 (Na,K) 1 / 2 By adding calcium (Ca) to a TiO3 (hereinafter, BNKT) composition and compounding it with BaTiO3 (hereinafter, BCT), a new lead-free piezoelectric ceramic composition with improved piezoelectric properties and mechanical reliability was developed, and the present invention was completed.

[0007] An object of the present disclosure is to provide a lead-free piezoelectric ceramic composition having improved piezoelectric properties and mechanical reliability.

[0008] One object of the present invention is to provide a lead-free piezoelectric element having improved piezoelectric properties and mechanical reliability.

[0009] One object of the present invention is to provide a speaker including a lead-free piezoelectric element having improved piezoelectric properties and mechanical reliability.

[0010] The lead-free piezoelectric ceramic composition has a composition represented by the following chemical formula 1.

[0011] <Chemical Formula 1>

[0012] (1-x)Bi 0.5 (Na,K) 0.5 TiO3- x(M, Ca)TiO3

[0013] In the above formula, x is 0.01 < x < 0.2, and M is at least one of Ba and Sr.

[0014] In some embodiments, the Bi 0.5 (Na,K) 0.5 TiO3 may have a composition represented by the following chemical formula 2.

[0015] <Chemical Formula 2>

[0016] Bi 0.5 (Na a K 1-a ) 0.5 TiO3

[0017] In the above equation, a is 0.5 < a < 1.

[0018] In some embodiments, the (M, Ca)TiO3 may have a composition represented by the following chemical formula 3.

[0019] <Chemical Formula 3>

[0020] M b Ca 1-b TiO3

[0021] The lead-free piezoelectric element comprises a lead-free piezoelectric ceramic composition according to the embodiments described above.

[0022] In some embodiments, the lead-free piezoelectric element has a ratio c / a of the lattice constant c of the c-axis to the lattice constant a of the a-axis at room temperature of 1.008 ≤ c / a ≤ 1.190, and a lattice volume of 60.1 to 60.6 Å. 3 It could be.

[0023] In some embodiments, the lead-free piezoelectric element has a dielectric constant ( ε r ) can be between 950 and 1200.

[0024] In some embodiments, the lead-free piezoelectric element may have a field-induced strain of 250 to 330 pm / V.

[0025] The speaker comprises a lead-free piezoelectric element according to the embodiments described above.

[0026] The lead-free piezoelectric ceramic composition according to embodiments of the present disclosure can be utilized as a material for lead-free piezoelectric elements with improved piezoelectric properties and mechanical reliability. Even when manufactured in a thin form, the lead-free piezoelectric element exhibits excellent elasticity and high vibration amplitude, enabling it to reproduce high-power sound and a wide range of low-frequency sound, and thus can be utilized as a speaker material for thin displays.

[0027] Figure 1 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Ba 0.7 Ca 0.3 ) shows the X-ray diffraction characteristics of TiO3 (x = 0 ~ 0.100) in the range of 20-60° 2θ.

[0028] Figure 2 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Sr 0.7 Ca 0.3 ) shows the X-ray diffraction characteristics of TiO3 (x = 0 ~ 0.125) in the range of 20-60° 2θ.

[0029] Figure 3 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Ba 0.7 Ca 0.3 )This is a graph showing the results of measuring the piezoelectric constant of TiO3 (x = 0 ~ 0.100).

[0030] Figure 4 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Ba 0.7 Ca 0.3)This is a graph showing the results of measuring the lattice volume of TiO3 (x = 0 ~ 0.100).

[0031] Figure 5 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Sr 0.7 Ca 0.3 )This is a graph showing the results of measuring the piezoelectric constant of TiO3 (x = 0 ~ 0.125).

[0032] Figure 6 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Sr 0.7 Ca 0.3 )This is a graph showing the results of measuring the lattice volume of TiO3 (x = 0 ~ 0.125).

[0033] Figure 7 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Ba 0.7 Ca 0.3 ) Field-induced strain (S) of TiO3 (x = 0 ~ 0.100) max / E max ) is shown.

[0034] Figure 8 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Sr 0.7 Ca 0.3 ) Field-induced strain (S) of TiO3 (x = 0 ~ 0.125) max / E max ) is shown.

[0035] Figure 9 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na0.8 K 0.2 ) 0.5 TiO3- x(Ba 0.7 Ca 0.3 )This is a graph showing the dielectric constant measurement results of TiO3 (x = 0 ~ 0.100).

[0036] Figure 10 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Sr 0.7 Ca 0.3 )This is a graph showing the dielectric constant measurement results of TiO3 (x = 0 ~ 0.125).

[0037] Embodiments disclosed in the present application provide a lead-free piezoelectric ceramic composition that provides a lead-free piezoelectric element with improved piezoelectric properties and mechanical reliability. A speaker including the lead-free piezoelectric element is also provided.

[0038]

[0039] Hereinafter, preferred embodiments of the present invention will be described with reference to the attached drawings. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided so that the present invention can be more completely described to a person having average knowledge in the relevant technical field. Therefore, the shape and size of the elements in the drawings may be exaggerated for clearer description, and elements indicated by the same reference numerals in the drawings are the same elements. In addition, the same reference numerals are used throughout the drawings for parts that have similar functions and actions. In addition, the term "including" a certain element throughout the specification does not exclude other elements, but rather means that other elements may be included, unless specifically stated otherwise.

[0040]

[0041] In one aspect of the present invention, a lead-free piezoelectric ceramic composition represented by chemical formula 1 is provided.

[0042] <Chemical Formula 1>

[0043] (1-x)Bi 0.5 (Na,K) 0.5 TiO3- x(M, Ca)TiO3

[0044] The above (M, Ca)TiO3 may be included in an amount of 1 to 20 mol% of the total mole of the composition of the above chemical formula 1, preferably 2 to 15 mol%, and more preferably 2.5 to 12.5 mol%. That is, x may be 0.01 < x < 0.2, preferably 0.02 < x < 0.15, and more preferably 0.025 < x < 0.125.

[0045] The above M may be at least one of Ba and Sr.

[0046] The lead-free piezoelectric ceramic composition of the present invention is environmentally friendly because it does not contain lead (Pt), and has the advantage of excellent piezoelectric properties, such as exhibiting relaxor ferroelectric properties in a ferroelectric phase.

[0047] The lead-free piezoelectric ceramic composition of the present invention is a lead-free piezoelectric ceramic composition represented by the above chemical formula 1, which is a composite composition of a composition of the following chemical formula 2 containing titanium oxide (TiO3) in a bismuth potassium sodium composition and a composition of the following chemical formula 3 containing titanium oxide (TiO3) in a barium calcium composition.

[0048] <Chemical Formula 2>

[0049] Bi 0.5 (Na a K 1-a ) 0.5 TiO3

[0050] <Chemical Formula 3>

[0051] M b Ca 1-b TiO3

[0052] At this time, in the chemical formula 2, a may be 0.5 < a < 1, preferably 0.6 < a < 0.9, and more preferably 0.75 < a < 0.85.

[0053] At this time, in the chemical formula 3, b may be 0.5 < b < 1, preferably 0.6 < b < 0.8, and more preferably 0.65 < b < 0.75.

[0054] In one aspect of the present invention, a lead-free piezoelectric element comprising the lead-free piezoelectric ceramic composition is provided.

[0055] The lead-free piezoelectric element of the present invention may have a ratio c / a of the lattice constant c of the c-axis to the lattice constant a of the a-axis of the piezoelectric element at room temperature of 1.008 ≤ c / a ≤ 1.190. When the ratio c / a at room temperature is in the above range, the piezoelectric ceramic has an excellent electromechanical coupling coefficient. Therefore, the driving power can be reduced by a piezoelectric element including the piezoelectric ceramic. When the ratio c / a is less than 1.008, the piezoelectric ceramic has a crystal structure similar to a cubic system. Therefore, in this case, the electromechanical coupling coefficient at room temperature decreases, the input voltage required to drive the piezoelectric element increases, and the driving power increases. When the ratio c / a is greater than 1.190, the voltage required for polarization processing increases.

[0056] Bi of the above chemical formula 2 0.5 (Na a K 1-a ) 0.5 M of the above chemical formula 3 in TiO3 b Ca 1-b When synthesizing by adding TiO3, the sizes of Ba, Sr, and Ca atoms located in the A-site are different, so M b Ca 1-b The lattice constant of TiO3 can be controlled by adjusting the content of Ba, Sr, and Ca.

[0057] Also, M b Ca 1-b The crystal lattice size of the final synthesized composition can be controlled depending on the content of TiO3.

[0058] The lattice volume of the lead-free piezoelectric element of the present invention is 60.1 to 60.6 Å. 3 It could be.

[0059] Dielectric constant of the lead-free piezoelectric element of the present invention ( ε r ) can be between 950 and 1200.

[0060] The field-induced strain of the lead-free piezoelectric element of the present invention may be 250 to 330 pm / V. When the field-induced strain is less than 250 pm / V, the input voltage required to drive the piezoelectric element increases, and the driving power increases. The piezoelectric element has a field-induced strain coefficient (S max / E max ) is important, M b Ca 1-b TiO3 to Bi 0.5 (Na a K 1-a ) 0.5 When synthesized by adding to TiO3, there is a high possibility that various atoms will be randomly arranged in the A-site, such as +1-valent Na, K, +2-valent Ba, Sr, Ca, and +3-valent Bi, so M b Ca 1-b As the amount of TiO3 added increases, the relaxed ferroelectric properties are exhibited.

[0061] The lead-free piezoelectric ceramic composition provided in one aspect of the present invention can be manufactured by the following method.

[0062] A method for manufacturing a lead-free piezoelectric ceramic composition provided in one aspect of the present invention includes a step of weighing bismuth precursor powder, sodium precursor powder, potassium precursor powder, and titanium precursor powder to form a composition of the above chemical formula 2, and then mixing and calcining the same to form a first ceramic powder having the composition of the above chemical formula 2.

[0063] The above step may be a step of solid-phase synthesis of the first ceramic powder having the composition of the above chemical formula 2.

[0064] At this time, each of the bismuth precursor powder, sodium precursor powder, potassium precursor powder and titanium precursor powder is Bi2O3, It may be a ceramic powder of Na2CO3, K2CO3 and TiO2, but is not limited thereto, and other types of powders used in solid-state synthesis of the first ceramic powder having the composition of the above chemical formula 2 may be used.

[0065] The above-mentioned weighed bismuth precursor powder, sodium precursor powder, potassium precursor powder and titanium precursor powder can be mixed by a ball milling method, preferably wet mixed by a ball milling method, and more preferably wet mixed for 22 to 26 hours.

[0066] The above wet-mixed mixed powder can be calcined at 800°C to 1000°C after drying, and preferably, the first ceramic powder having the composition of the above chemical formula 2 can be manufactured by calcining in the above temperature range for 1 to 6 hours.

[0067] In another aspect of the present invention, a method for manufacturing a lead-free piezoelectric ceramic composition is provided, which comprises the steps of weighing barium precursor powder, strontium precursor powder, calcium precursor powder, and titanium precursor powder so as to form a composition of the following chemical formula 3, and then mixing and calcining them to form a second ceramic powder having the composition of the above chemical formula 3.

[0068] The above step may be a step of solid-phase synthesis of a second ceramic powder having the composition of the above chemical formula 3.

[0069] At this time, each of the barium precursor powder, strontium precursor powder, calcium precursor powder and titanium precursor powder may be ceramic powders of BaCO3, SrCO3, CaCO3 and TiO2, but is not limited thereto, and other types of powders used for solid-phase synthesis of the second ceramic powder having the composition of the above chemical formula 3 may be used.

[0070] The above-mentioned weighed barium precursor powder, calcium precursor powder and titanium precursor powder can be mixed by a ball milling method, preferably wet mixed by a ball milling method, and more preferably wet mixed for 22 to 26 hours.

[0071] The above wet-mixed mixed powder can be calcined at 800°C to 1000°C after drying, and preferably calcined in the above temperature range for 1 to 6 hours to produce a second ceramic powder having the composition of the above chemical formula 3.

[0072] In another aspect of the present invention, a method for manufacturing a lead-free piezoelectric ceramic composition is provided, which includes a step of forming a composition of the following chemical formula 1 by sintering a mixed powder obtained by mixing the first ceramic powder and the second ceramic powder.

[0073] The above mixing may be performed by mixing the first ceramic powder and the second ceramic powder so that x in the above chemical formula 1 is 0.01 < x < 0.2, preferably 0.02 < x < 0.15, and more preferably 0.025 < x < 0.125, in order to manufacture a lead-free piezoelectric ceramic composition having improved piezoelectric properties and temperature stability of the piezoelectric properties at room temperature.

[0074] If, in the chemical formula 1, x is outside the range of 0.01 ≤ x ≤ 0.2, a problem of deterioration of piezoelectric properties may occur. In addition, since the composition of the chemical formula 1 has a range of 0.01 ≤ x ≤ 0.2, it can be utilized as a material for a lead-free piezoelectric element having improved piezoelectric properties and mechanical reliability.

[0075] The above mixture may be mixed by a ball milling method, preferably wet mixed and ground by a ball milling method, and more preferably wet mixed and ground for 22 to 26 hours.

[0076]

[0077] Thereafter, the mixed powder of the first ceramic powder and the second ceramic powder that have been mixed and ground can be molded after drying.

[0078] The above molding can be performed using various molding methods used in solid-state synthesis, and for example, the molding can be performed using a compression molding method to produce a plate-shaped molded body.

[0079] The molded mixed powder can be sintered at a temperature of 1100°C to 1250°C, preferably at a temperature of 1110°C to 1220°C, more preferably at a temperature of 1130°C to 1200°C.

[0080] The lead-free piezoelectric ceramic composition represented by Chemical Formula 1 manufactured by the manufacturing method of the present invention is environmentally friendly as it does not contain lead (Pb) and can be used as a material for a lead-free piezoelectric element with improved piezoelectric properties and mechanical reliability. In particular, the lead-free piezoelectric element has excellent elasticity and a high vibration amplitude even when manufactured in a thin shape, enabling it to reproduce high-output sound and a wide range of low-frequency sound, and thus can be usefully used as a speaker material for a thin display.

[0081]

[0082] Hereinafter, the present invention will be described in detail through examples.

[0083]

[0084] However, the following examples are only illustrative of the present invention, and the content of the present invention is not limited by the following examples.

[0085]

[0086] Example

[0087] In one embodiment of the present invention, Bi 0.5 (Na,K) 0.5 The dielectric and piezoelectric properties were investigated by changing the amount of (Ba, Sr Ca)TiO3 added to TiO3.

[0088] Bi2O3 of the purity commonly used for industrial purposes, Ceramic powders of Na2CO3, K2CO3, BaCO3, SrCO3, CaCO3, and TiO2 were weighed and then wet-mixed for 24 hours using a ball milling method to prepare a slurry. Afterwards, for solid-state reaction, the slurry was dried and calcined at 850°C for 3 hours or 950°C for 5 hours.

[0089] The above mixed powder was molded at a pressure of about 100 MPa using a mold with a diameter of 12 mm, and then placed in an electric furnace and sintered at about 1110°C to 1220°C for 3 hours to produce a lead-free piezoelectric ceramic composition.

[0090] At this time, when (Ba, Ca)TiO3 is added in an amount less than 0.01, the effect of obtaining the expected excellent piezoelectric properties is minimal, and when it is added in an amount greater than 0.1, there is a problem of deterioration of the electric field-induced deformation properties. Therefore, in the present invention, it is preferable that (Ba, Ca)TiO3 be added in a small amount in the range of 0.01 to 0.1 to be added to the composition.

[0091] Meanwhile, the most important property in piezoelectric materials is the field-induced strain (S max / E max, the unit is pm / V). In terms of practicality, it is desirable for the electric field-induced strain of the piezoelectric material for speakers to be 200 pm / V or higher. Bi according to the present invention 0.5 (Na,K) 0.5 The composition with (Ba, Ca)TiO3 added to TiO3 has an electric field-induced strain (S) of 200 pm / V or more. max / E max ) has.

[0092] Through this, it was confirmed that the lead-free piezoelectric ceramic composition according to the present invention can improve piezoelectric properties by exhibiting superior electric field-induced strain compared to conventional lead-free piezoelectric materials for speakers.

[0093] According to one embodiment of the present invention, the effects of structural, dielectric, ferroelectric, and piezoelectric properties according to the (Ba, Sr, Ca)TiO3 content were confirmed.

[0094] Figure 1 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Ba 0.7 Ca 0.3 ) shows the X-ray diffraction characteristics of TiO3 (x = 0 ~ 0.100) in the range of 20-60° 2θ.

[0095] Figure 2 is a lead-free piezoelectric ceramic (1-x)Bi manufactured according to one embodiment of the present invention. 0.5 (Na 0.8 K 0.2 ) 0.5 TiO3- x(Sr 0.7 Ca 0.3 ) shows the X-ray diffraction characteristics of TiO3 (x = 0 ~ 0.125) in the range of 20-60° 2θ.

[0096] According to one embodiment of the present invention, it was found that the lattice constant and lattice volume changed depending on the change in the amount of (Ba, Sr, Ca)TiO3 added. As the amount of (Ba, Ca)TiO3 added increased, the lattice constant, the lattice constant ratio c / a, and the lattice volume increased (see FIGS. 3 and 4), and as the amount of (Sr, Ca)TiO3 added increased, the lattice constant and the lattice volume decreased (see FIGS. 5 and 6).

[0097] In addition, as the amount of (M, Ca)TiO3 added increases, the field-induced strain increases, and in particular, a high field-induced strain was observed when x=0.050 to 0.075 was added (see Figs. 7 and 8). When (Ba, Ca)TiO3 was added at x=0.075 and (Sr, Ca)TiO3 was added at x=0.050, the field-induced strain according to the electric field change was confirmed, and high field-induced strains of 285 pm / V and 343 pm / V were observed, respectively (see Figs. 7 and 8).

[0098] Additionally, the dielectric constant increases with the addition of (M, Ca)TiO3 and Bi 0.5 (Na,K) 0.5 It was observed that the addition of x = 0.025 compared to the single component TiO3 increased by about 25% or more (see Figs. 9 and 10).

[0099] From these results, (1-x)Bi according to the present invention 0.5 (Na,K) 0.5 It can be confirmed that the TiO3- x(M, Ca)TiO3 ceramic composition is a promising lead-free piezoelectric material.

[0100] The lead-free piezoelectric ceramic composition according to embodiments of the present disclosure can be utilized as a material for lead-free piezoelectric elements with improved piezoelectric properties and mechanical reliability. Even when manufactured in a thin form, the lead-free piezoelectric element exhibits excellent elasticity and high vibration amplitude, enabling it to reproduce high-power sound and a wide range of low-frequency sound, and thus can be utilized as a speaker material for thin displays.

Claims

1. A lead-free piezoelectric ceramic composition having a composition of the following chemical formula 1: <Chemical Formula 1> (1-x)Bi 0.5 (Na,K) 0.5 TiO3- x(M, Ca)TiO3 In the above equation, x is 0.01 < x < 0.2, M is either Ba or Sr.

2. In claim 1, Bi above 0.5 (Na,K) 0.5 TiO3 is a lead-free piezoelectric ceramic composition having the following chemical formula 2: <Chemical Formula 2> Bi 0.5 (Na a K 1-a ) 0.5 TiO3 In the above equation, a is 0.5 < a < 1.

3. In claim 1, The above (M, Ca)TiO3 is a lead-free piezoelectric ceramic composition having a composition of the following chemical formula 3: <Chemical Formula 3> M b Ca 1-b TiO3 In the above equation, b is 0.5 < b < 1.

4. A lead-free piezoelectric element comprising the lead-free piezoelectric ceramic composition of claim 1.

5. In claim 4, The ratio of the lattice constant c of the c-axis to the lattice constant a of the a-axis at room temperature, c / a, is 1.008 ≤ c / a ≤ 1.190, Lattice volume is 60.1 to 60.6 Å 3 In, lead-free piezoelectric element 6. In claim 4, Genetic constant ( ε r ) is 950 to 1200, lead-free piezoelectric element.

7. In claim 4, A lead-free piezoelectric element having a field-induced strain of 250 to 330 pm / V.

8. A speaker comprising the lead-free piezoelectric element of claim 4.

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