KR20170042171A

KR20170042171A - Lead-free piezoelectric ceramics, manufacturing method thereof and actuator comprising thereof

Info

Publication number: KR20170042171A
Application number: KR1020150141801A
Authority: KR
Inventors: 김명호; 송태권; 박태곤; 김민수; 알리 후세인; 아메드 말리크 리즈완; 마크불 아단; 자만 아리프
Original assignee: 창원대학교 산학협력단
Priority date: 2015-10-08
Filing date: 2015-10-08
Publication date: 2017-04-18

Abstract

A lead-free piezoelectric ceramics according to the present invention is given the properties of relaxor ferroelectrics with reduced remanent polarization and antiferromagnetism by diffusing lithium (Li) and niobium (Nb) into a BNKT-ST lattice through a solid-state reaction in a perovskite-type lead-free piezoelectric ceramics. Thus, the lead-free piezoelectric ceramics exhibit a high strain, a high reverse piezoelectric constant (d_33^*) and a low temperature dependency even under a low electric field. In addition, a stack-type multi-layer actuator may be fabricated from the lead-free piezoelectric ceramics of the above composition, and the stack-type multi-layer actuator may exhibit excellent physical properties to have a high strain at a low electric field and may be effectively used in various fields.

Description

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a lead-free piezoelectric ceramics, a manufacturing method thereof, and an actuator including the same.

The present invention relates to a lead-free piezoelectric ceramics, a method of manufacturing the same, and an actuator including the same.

Conventionally, piezoelectric ceramics based on PZT-based materials have been widely used for manufacturing piezoelectric actuators because of their high field induced strain even in low electric fields.

However, as interest in the environment has increased and various regulatory laws have been introduced to prevent the spread of harmful substances, lead-free piezoelectric ceramics using eco-friendly materials have been produced as substitutes for lead-based piezoelectric ceramics and can be utilized as piezoelectric actuators There is an increasing demand for methods.

In particular, some BNT-based piezoelectric ceramics have been evaluated to have improved piezoelectric properties and can be used for manufacturing piezoelectric actuators. However, BNT-based ceramics exhibit high deformation at high electric fields of 6 kV / mm or more and are used for manufacturing piezoelectric actuators There is a difficult restriction, and the BNT-based ceramics have a problem that the piezoelectric reaction is significantly affected at a high temperature due to temperature.

Accordingly, in order to improve the physical properties of the piezoelectric ceramics, various studies have been conducted. In order to solve the problems described above, a compositional design, an orientation growth process, or a fabrication of a ceramic composite material ceramic-ceramic composites) have been studied for the production of ceramics.

Particularly, a variety of researches have been conducted to prepare BNT ceramics having a perovskite structure improved in stability through ion substitution by a solid-state reaction using a composition designing technique .

The composition of the perovskite structure can generally be expressed as ABO ₃ , and conventionally, a method of ion-exchanging only the A-site or B-site element of the same valence in the perovskite structure ceramic of the ABO ₃ composition The BNT ceramics produced by the present invention require a relatively high electric field to exhibit a high piezoelectric constant and are also known to exhibit poor piezoelectric properties at high temperatures. It was reported that there was a difficulty.

Therefore, it is necessary to study piezoelectric ceramics which exhibits high piezoelectric constant even at low electric field and is stable at high temperature.

1. Korean Registered Patent No. 10-1423418 (Published on July 18, 2014) 2. Korean Patent Laid-Open No. 10-2009-0088991 (Published on Aug. 21, 2009) 3. Korean Registered Patent No. 10-1333793 (Published on November 21, 2013) 4. Korean Registered Patent No. 10-1349335 (Published on January 02, 2014)

SUMMARY OF THE INVENTION An object of the present invention is to provide a lead-free piezoelectric ceramics having improved piezoelectric properties, a method for manufacturing the same, and an actuator made of a lead-free piezoelectric ceramics.

In order to accomplish the above object, the present invention provides a lead-free piezoelectric ceramics represented by the following Chemical Formula 1:

[Chemical Formula 1]

0.96 [{Bi _0.5 (Na _0.84 K _0.16 ) _0.5 } _1-x Li _x (Ti _1-y Nb _y ) O ₃ ] -0.04 SrTiO ₃

(Provided that 0 < x < 0.025 and 0 < y < 0.025).

In _{addition, 0.96 [{Bi 0 .5 (} Na 0 .84 K 0. 16) 0.5} 0.98 Li 0. ₀₂ (Ti _0.98 Nb _0.02 ) O ₃ ] -0.04 SrTiO ₃ .

Further, the 0.96 [{Bi ₀ _.5 (Na ₀ _.84 K _0. ₁₆ ) _0.5 } _0.98 Li ₀ _. ₀₂ (Ti _0.98 Nb _0.02 ) O ₃ ] -0.04 SrTiO ₃ is characterized by a dynamic piezoelectric constant (d ₃₃ ^* ) of 800 pm / V under an electric field of 5 kV / mm.

(A) pulverizing a mixed powder comprising Bi ₂ O ₃ , Na ₂ CO ₃ , TiO ₂ , K ₂ CO ₃ , SrCO ₃ , Li ₂ CO ₃ and Nb ₂ O ₅ ; (b) (C) calcining the shaped body using the mixed powder calcined in step (b) and then sintering the mixture; and (d) sintering the sintered body obtained in step (c) And a polarization aligning step.

Also, the step (b) is performed at 800 to 900 ° C.

Also, the step (c) is performed at a temperature of 1100 to 1200 ° C.

The present invention provides a lead-free piezoelectric actuator including the above-described lead-free piezoelectric ceramics.

In addition, the PQJ actuator is a multilayer actuator (MLA).

The stacked multilayer actuator includes ten layers made of the ceramic and has a dynamic piezoelectric constant (d ₃₃ ^* ) of 600 pm / V under an electric field of 4.5 kV / mm.

The lead-free piezoelectric ceramics according to the present invention are produced by simultaneously performing ion-exchange of A- and B-sites of lithium (Li) and niobium (Nb) through a solid-state reaction in a lead-free piezoelectric ceramics having a perovskite structure By increasing the content of lithium and niobium in the lattice to diffuse into the BNKT-ST lattice, conventional ferroelectric BNKT-ST ceramics with rhombohedral phase and tetragonal phase coexist with pseudocubic phase is induced by a perovskite-structured ceramic showing a reverse-piezoelectric constant and a non-ergodic relaxor state having a macroscopic domain showing ferroelectric behavior, , The domain becomes an ergodic relaxor state which is converted into a polar nano region, so that it becomes easy to move the polarization according to the electric field, The characteristics of the coercive boundaries reduce the relaxor ferroelectric (relaxor ferroelectrics) will exhibit. The prepared ceramic is stable down a change of the piezoelectric constant in a high electric field even at a low strain, 800pm / V represents the more qualified strain _{(Normalized strain, S max / E} max), temperature of less than 5kV / mm.

The lead-free piezoelectric ceramics described above has a composition of 0.96 [{Bi _0.5 (Na _0.84 K _0.16 ) _0.5 } _0.98 Li _0.02 (Ti _0.98 Nb _0.02 ) O ₃ ] -0.04 SrTiO ₃ , 0.40%, exhibits a high piezoelectric constant of 800 pm / V, maintains its piezoelectric properties even at a high temperature of 120 ° C, and exhibits excellent physical properties of a lead-free piezoelectric ceramics.

In addition, the lead-free piezoelectric ceramics as described above may be utilized to manufacture the actuator _{useful, 0.96 [{Bi 0 .5 (} Na 0 .84 K 0. 16) 0.5} 0.98 Li 0. _In the case of a stack-type multilayer actuator manufactured by stacking a plurality of lead-free piezoelectric ceramics having a composition of O ₂ O ₂ (Ti _0.98 Nb _0.02 ) O ₃ ] -0.04 SrTiO ₃ , a multilayer actuator of 0.27% at a low electric field of 4.5 kV / Exhibits excellent properties with high strain and can be effectively used in various fields.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the results of experiments of (a) specimens of actuators made of lead-free piezoelectric ceramics according to the present invention, (b) stacked multilayer actuators
Fig. 2 is a graph showing X-ray diffraction analysis (XRD) results of the lead-free piezoelectric ceramics according to the present invention in Examples 1 to 4 and Comparative Examples.
Fig. 3 is an image obtained by scanning electron microscopy (SEM) of the microstructure of the lead-free piezoelectric ceramics according to Examples 1, 3 and Comparative Example according to the present invention.
FIG. 4 is a graph showing dielectric constant and dielectric loss according to temperature of (a) a lead-free piezoelectric ceramics according to the present invention, and (b) a graph of γ of a modified Curie-Weiss rule.
Figure 5 Examples 1 to 3 and a graph showing the polarization of the electric field of the lead-free piezoelectric ceramic according to the comparative example, (e) in Example 1 to the remanent polarization of the lead-free piezoelectric ceramic according to 4 and Comparative Examples (P _r ), A maximum polarization (P _m ), and a coercive field (E _c ).
6 is a graph showing static charge constants of Pb-free piezoelectric ceramics according to Examples 1 to 4 and Comparative Examples.
7 is a graph showing a polarization hysteresis loop (PE hysteresis loop) according to the temperature of the lead-free piezoelectric ceramics according to Example 3 and Comparative Example.
8 is a graph showing the bipolar field-induced strain of the lead-free piezoelectric ceramics according to (a) Examples 1 to 3 and Comparative Example, (b) Examples 1 to 4 and Comparative (Negative strain) of the lead-free piezoelectric ceramics according to the example.
Fig. 9 is a graph showing the unipolar field-incuded strain of the lead-free piezoelectric ceramics of (a) Examples 2 to 4 and Comparative Example, (b) Example 1 to Example 4, (S _max ) and the inverse piezoelectric constant (d ₃₃ ^* ) of the lead-free piezoelectric ceramics.
10 is a graph showing the relationship between the piezoelectric ceramic composition of the present invention and the change rate of the inverse piezoelectric constant versus room temperature at the time of temperature rise of the lead-free piezoelectric ceramics according to Example 3 (a) Graph.
FIG. 11 is a graph showing the strain of an actuator and a specimen manufactured with the composition according to the present invention, and (b) an amount of actuator displacement according to an electric field. FIG.

Hereinafter, the present invention will be described in detail.

The present invention provides a lead-free piezoelectric ceramics represented by the following Formula 1:

[Chemical Formula 1]

(Provided that 0 < x < 0.025 and 0 < y < 0.025).

The above Pb-free piezoelectric ceramics can exhibit a pure ABO ₃ perovskite structure.

More specifically, the lead-free piezoelectric ceramics described above is composed of Bi (Na, K) Ti-SrTiO ₃ (BNKT-ST) having a composition of (Li) and niobium (Nb) are substituted for the A-site and the B-site of the piezoelectric ceramics which are the pure ABO ₃ perovskite structure, lithium and niobium are diffused into the lattice of the ceramics, and rhombohedral phase and a tetragonal phase coexist in a single phase of a pseudocubic phase, which results in a high piezoelectric constant due to the reduced residual polarization and antiferromagnetism.

For example, the lead-free piezoelectric ceramics according to the present invention may have a composition of 0.96 [{Bi _0.5 (Na _0.84 K _0.16 ) _0.5 } _0.98 Li _0.02 (Ti _0.98 Nb _0.02 ) O ₃ ] -0.04 SrTiO ₃ , The lead-free piezoelectric ceramics having such a composition exhibit a high strain at a low electric field of 5 kV / mm, and the normalized strain is excellent at 800 pm / V.

Further, even at a high temperature of 120 캜 or higher, the normalized strain change is less than 10%, which is suitable for manufacturing with a piezoelectric actuator.

The lead-free piezoelectric ceramics described above exhibits more excellent physical properties when having the ranges of x and y as described above, and when it exceeds the above range, an undesirable secondary phase in the lattice of the piezoelectric ceramics of a pure perovskite structure is produced So that the piezoelectric properties can be lowered.

The present invention also provides a method for manufacturing the lead-free piezoelectric ceramics described above.

A method of manufacturing a lead-free piezoelectric ceramics according to the present invention comprises the steps of: (a) mixing a mixed powder containing Bi ₂ O ₃ , Na ₂ CO ₃ , TiO ₂ , K ₂ CO ₃ , SrCO ₃ , Li ₂ CO ₃ and Nb ₂ O ₅ (B) calcining the mixed powder pulverized in the step (a); (c) sintering the formed body using the mixed powder calcined in the step (b); and (d) And a step of polarizing the sintered body obtained in step (c).

Hereinafter, a method for manufacturing a lead-free piezoelectric ceramics according to a preferred embodiment of the present invention will be described in more detail with respect to each step.

Step (a) of the present production method is a step of weighing the starting material to prepare the desired composition, and mixing and pulverizing the weighed starting material.

In this step, starting materials can be weighed and mixed according to the composition so that a lead-free piezoelectric ceramics having the following composition (1) can be produced:

[Chemical Formula 1]

(With the range of x being 0 < x < 0.025 and the range of y being 0 < y < 0.025).

To prepare lead-free piezoelectric ceramics of the above composition, a starting material comprising Bi ₂ O ₃ , Na ₂ CO ₃ , TiO ₂ , K ₂ CO ₃ , SrCO ₃ , Li ₂ CO ₃ and Nb ₂ O ₅ is used And the starting materials used may be commercially available grades of oxide reagents.

The starting materials described above can be configured to be dried using a variety of known methods so that they can be ground into particles of uniform size.

In this step, the starting material may be uniformly pulverized and mixed well by using various known methods, and zirconia (ZrO ₂ ) balls and ethanol ) Are mixed together into a ball of a nalgene material, and a ball mill is milled by a ball mill using a milling machine as a representative example.

In the step (b), the pulverized mixed powder may be dried and then calcined to remove organic substances, impurities, volatile gases, and the like contained in the mixed powder.

For this purpose, in this step, the mixed powder may be calcined by various known methods. For example, the calcined powder may be calcined in an alumina crucible covered with a lid. However, Deg.] C to 900 [deg.] C, preferably 850 [deg.] C, and the calcination time may be 2 hours or more.

In the step (c), the calcined powder is pressurized to form a compact, and the compact is sintered.

In this step, the calcined powder mixture is pulverized and homogenized as described above, and the pulverized powder mixture is sieved after adding a binder such as PVA (polyvinyl alcohol) and sieved so as to have a uniform particle size Powder can be obtained.

As described above, the powder obtained by sieving can be supplied to a forming mold having various shapes and sizes according to the purpose and characteristics, and the molded body can be produced by compressing. The molded body thus formed is sintered and compacted in powder form Can be configured to form grains.

The sintering of the above-mentioned shaped body can be preferably performed at 1100 to 1200 ° C, and more preferably at 1160 ° C.

In addition, the sintering can be configured so as to minimize the loss of volatile elements by spraying a powder of the same composition as above on the molded body in order to minimize the volatilization of volatile elements such as Bi, K or Na contained in the starting material have.

In the step (d), electrodes are formed on the sintered body sintered as described above, and poling is performed to manufacture the lead-free piezoelectric ceramics.

An electrode can be formed by coating a silver paste on the front and back surfaces of the sintered body and heating it at a high temperature of 600 ° C or more for a sufficient time.

The sintered body having the electrode can be manufactured by using a variety of known methods to induce polarization and to align the ferroelectric domains in one direction to produce a Pb-free piezoelectric ceramics exhibiting permanent piezoelectric characteristics. In order to prevent dielectric breakdown of ceramics, A method of polarizing by supplying a direct current voltage in a silicone oil having a high dielectric constant is exemplified.

A lead-free piezoelectric ceramics exhibiting a high strain and an inverse piezoelectric constant in a low electric field and having a low change in strain even at a high temperature can be manufactured by using the manufacturing method of the lead-free piezoelectric ceramics according to the present invention.

The lead-free piezoelectric actuator including the lead-free piezoelectric ceramics of the present invention can be provided. The lead-free piezoelectric ceramics may be in the form of an actuator formed of a single layer of the lead-free piezoelectric ceramics, multilayer actuators (MLA), and the lead-free piezoelectric actuators described above exhibit excellent piezoelectric properties.

For example, in the case of a lead-free piezoelectric actuator in which a lead-free piezoelectric ceramics is formed as a single layer, it exhibits a high strain of 0.34% at a low electric field of 4.5 kV / mm and has excellent piezoelectric properties. As another example, Layer multilayer actuator has a strain of 0.27% at an electric field of 4.5 kV / mm and can be effectively used for manufacturing various types of devices that generate mechanical motion by using piezoelectric ceramics as a driving force source.

Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples.

The examples and experimental examples presented are only a concrete example of the present invention and are not intended to limit the scope of the present invention.

< Example 1>

Using a solid-phase reaction method (solid state reaction) 0.96 [{ Bi 0 .5 (Na 0 .84 K 0. 16) 0.5} 1-x Li x (Ti 1-y Nb y) O 3] -0.04SrTiO 3 (x, y = 0.010).

For this purpose, commercially available grades of oxide reagents and starting materials Bi ₂ O ₃ , Na ₂ CO ₃ , TiO ₂ , K ₂ CO ₃ , SrCO ₃ , Li ₂ CO ₃ and Nb ₂ O ₅ were weighed and weighed Powder.

The prepared mixed powder was added to ethanol and ball milled for 24 hours using a zirconia ball. The ball milled mixed powder was dried and then calcined at 850 ° C for 2 hours. The calcined powder mixture was ball milled for 24 hours and then dried. The dried mixture was mixed with a PVA aqueous solution to prepare a 150 mesh, . &Lt; / RTI >

The thus-filtered mixture was put into a disk mold having a diameter of 10 mm and pressed at 98 MPa to form a mixed powder to prepare a disk-shaped specimen.

The prepared specimens were placed in an alumina crucible and sintered at 1160 ° C for 2 hours. Sintered disc specimens were placed in silicone oil and polished by direct voltage of 4 kV / mm for 15 minutes to produce lead - free piezoelectric ceramics.

< Example 2>

Example 1 in the same manner as _{0.96 [{Bi 0 .5 (Na} 0 .84 K 0. 16) 0.5} 1- x Li x (Ti 1-y Nb y) O 3] -0.04SrTiO 3 (x, y = 0.015) was prepared.

< Example 3>

Example 1 in the same manner as _{0.96 [{Bi 0 .5 (Na} 0 .84 K 0. 16) 0.5} 1- x Li x (Ti 1-y Nb y) O 3] -0.04SrTiO 3 (x, y = 0.020) was prepared.

< Example 4>

Example 1 in the same manner as _{0.96 [{Bi 0 .5 (Na} 0 .84 K 0. 16) 0.5} 1- x Li x (Ti 1-y Nb y) O 3] -0.04SrTiO 3 (x, y = 0.025) was prepared.

< Example 5>

The silver paste was coated on both sides of the disk-shaped specimen manufactured according to Example 3, and the disk specimen coated with silver paste was sintered at 650 ° C for 30 minutes to form an electrode. As shown in FIG. 1 (a) An actuator was manufactured.

< Example 6>

Twelve disc specimens prepared according to Example 3 were prepared. Ten specimens were coated with silver paste on both sides, and the remaining two specimens were coated on either the top or bottom, respectively. The disk specimens coated with silver paste were heat treated at 650 ℃ for 30 minutes to form electrodes. The disk specimens were laminated by drying in an oven at 80 DEG C in an oven using an insulating epoxy and attached to the piezoelectric ceramic layer using silver wire and conductive epoxy to produce a multilayered multilayer actuator as shown in Fig. 1 (b) .

< Comparative Example >

The starting material in Li ₂ CO _3, Nb ₂ O _5, and carried out, except that did not include the Example 1 in the same manner as _{0.96 [{Bi 0 .5 (Na} 0 .84 K 0. 16) 0.5} TiO 3] -0.04 SrTiO ₃ (x, y = 0).

< Experimental Example 1> Analysis of crystal characteristics

In order to confirm the crystal phase of the produced ceramics, XRD diffraction analysis of the disk specimens of the ceramics according to Examples 1 to 4 and Comparative Example was performed to analyze the crystal phase, and the analysis results are shown in FIG. As shown in FIG. 2, in the case of the ceramics according to Examples 1 to 4, the XRD pattern appeared in the range of the scattering angle within the range of 20 to 70 degrees. In the case of the ceramics according to Examples 1 to 4, The perovskite phase was observed.

Through this, it was found that lithium and niobium were successfully diffused into the lattice of the ceramics and formed a complete solid solution phase.

In the comparative example, dispersed peaks of (111) / (1i1) and (002) / (200) coinciding with the rhombohedral phase and the tetragonal phase were observed at 40 ° and 46 ° , Indicating that rhombohedral and tetragonal phases coexist.

On the other hand, in the ceramics of Examples 1 to 4, the (111) / (1 1 1) dispersion peak at 40 ° shown in the comparative example was combined toward the single (111) peak, (200) peaks, indicating that phase change to the pseudocubic phase occurs due to the substitution of lithium and niobium ions in the phase where the rhombohedral phase and the tetragonal phase coexist, and a pure perovskite It can be confirmed that it has a trailing image.

< Experimental Example 2> Microstructure and particle size analysis

In order to analyze the microstructure and average particle size of the prepared ceramics, the surface of the disk-shaped specimen of the piezoelectric ceramics prepared according to Examples 1, 3 and Comparative Example was photographed using a scanning electron microscope (SEM) Is shown in Fig.

As shown in FIG. 3, the ceramic formed bodies according to Examples 1, 3, and Comparative Example were found to have a high density due to a good sintering property. In addition to having regular grain shapes and boundaries, , Which is a relatively high density of 95%.

In order to analyze the particle characteristics of the piezoelectric ceramics, the average particle size of the ceramics prepared by the linear intercept method was analyzed.

The average particle size of the ceramics according to the comparative example was found to be 1.8 탆, and the average particle size of the ceramics according to Example 4 was found to be 1.1 탆.

As a result, it can be seen that there is no significant change except that the average particle size is slightly reduced depending on the substitution amounts of lithium and niobium.

< Experimental Example 3> Analysis of dielectric properties

In order to analyze the temperature-dependent dielectric constant and dielectric loss (tan δ) of the ceramics at various frequencies, an impedance meter (1 kHz, 10 kHz and 100 kHz) were used to analyze the dielectric constant and dielectric loss of the ceramics according to Examples 1, 3 and Comparative Example, and the analysis results are shown in FIG. 4 (a).

Corresponding to the depolarization temperature (T _d ) and the temperature (T _max ) in the maximum dielectric constant (ε _m ) in the specimens of ceramics according to Examples 1, 3 and Comparative Example, Anomalies were shown and the piezoelectric curves of the ceramics according to the comparative examples exhibited a relatively sharp depolarization temperature (T _d ) curve peak as compared with the ceramics according to Examples 1 and 3, In the case of the ceramics according to Example 3, dispersion due to frequency was observed due to the reduction of the dielectric constant, and it was confirmed that the ceramic showed a gentle peak.

At a temperature slightly lower than the depolarization temperature, the dielectric constant of the ceramics according to the comparative example sharply decreases without dispersion according to the frequency. It can be confirmed that the polar nano area having polar reorientability at a high temperature The non-ergodic relaxor state (NR), in which polar nano-domains frozen at low temperatures are observed in the ergodic relaxor state (ER) of the nano-scale regions (PNRs) Is induced and converted to a stable ferroelectric phase having a domain structure in the presence of an external electric field.

In the ceramics according to Examples 1 and 3, frequency dispersion appears at a low temperature near the depolarization temperature (T _d ), and piezoelectric ceramics simultaneously substituted with lithium and niobium exhibit dynamical fluctuations in extreme nano-domains.

These fluctuations in the extreme nano-domains do not result in ordering due to cooling, and this phenomenon is called the ergot mitigator.

In this Ergodic relaxed body, it was confirmed that the extreme nano - domains can be aligned by external electric field and converted into ferroelectric state, and the behavior of ferroelectric converted when external electric field is removed disappears.

Therefore, the phase transition to the ferroelectric phase induced by the external electric field is dependent on the polarization structure type of the non-eroded relaxation body peculiar to the BNT ceramics and the substitution amount of the substituted lithium and niobium dopant in the non-eroded relaxed body, It is caused by the change of sieve.

In the graph of the comparative example shown in Fig. 4 (a), a sharp increase near 100 deg. C indicates that the polar nano-region which was frozen at a low temperature is polar reorientable by a high temperature, It is predicted that it will be converted into a mitigating body.

However, when the LN was simultaneously substituted as in Example 1 and Example 3, the frequency dispersion assumed to be the variation of the extreme nano region at a low temperature was confirmed.

It was confirmed that the flexible nano-domains of the erbodiment can be arranged by an external electric field to be transformed into a ferroelectric state, and the ferroelectric behavior, which has been converted when the external electric field is removed, disappears.

The diffuse phase transition of the ceramics was analyzed by analyzing the slope (log (1 /? _R _- 1 /? _M ) versus log (TT _m ) of the dielectric constant curve of the prepared ceramics) (b).

In order to measure the diffusion phase of the ceramics according to Examples 1, 3 and Comparative Example, a crystal modification VI shown in the following Chemical Formula 1 was used.

[Chemical Formula 1]

(Where T _m is the temperature at which the dielectric constant reaches its maximum value,? _M is the maximum dielectric constant,? Is the phase transition degree indicating the dielectric relaxation degree of the dielectric constant curve, and C ₂ is the Curie constant).

Both γ and C ₂ are material constants which are influenced by the composition and the structure of the material. γ has a value between 1 and 2. When γ is 1, it is called a general ferroelectric. When γ is 2 Relaxed ferroelectric.

The γ values measured at the frequency of 1 kHz were 1.42 for the ceramics according to the comparative example, 1.51 for the example 1, and 1.68 for the example 3, Respectively.

< Experimental Example 4> Polarization - Electric field hysteresis curve (P-E hysteresis loop Analysis of dielectric characteristics using

In order to analyze the dielectric characteristics of the prepared ceramics, the polarization hysteresis loop of the ceramics according to Examples 1 to 3 and Comparative Example was calculated at room temperature, and the calculation results were shown in Figs. 5 (a) to 5 5 (d).

As shown in FIGS. 5 (a) to 5 (d), ceramics of all compositions exhibited hysteresis curves of a general ferroelectric although the hysteresis curves differ depending on the substitution amounts of lithium and niobium under an electric field.

In the case of the ceramics according to the comparative example, a typical ferroelectric substance having a relatively large remnant polarization (P _r ) and a coercive field (E _c ) exhibiting the coexistence of the rhombohedral and tetragonal phases and the non- Respectively.

On the other hand, in the ceramics according to Examples 1 to 3, as the substitution of lithium and niobium continued, the shape of the hysteresis curve and the polarization value greatly changed.

In the ceramics according to Example 3 with pseudocubic symmetry, the structural phase transition occurs at room temperature.

This structural phase transition resulted in a rapid decrease of both the remanent polarization and the coercive field. The result is that when the electric field is applied, the polarization state appears, but when the electric field is zero, the polarization state is not aligned the incipient presence of the non-polar state appears. Based on these results, a hysteresis curve appears in the polarization-electric field hysteresis curve,

As a result, the external electric field induces the phase transition from the ergodic relaxation body to the metastable ferroelectric arrangement state and returns to the original eroded relaxed body when the electric field is removed. This is because the domain structure is temporarily And has a short-range nano-domain structure that is rearranged.

In addition, the decrease in remanent polarization due to the increase of lithium and niobium content is due to the compositional disorder of composition due to the substitution of lithium and niobium, resulting in the disappearance of the macroscopic ferroelectric domain and the growth of the extreme nano- May be caused by.

Also, a clear deformation of the hysteresis loop shape through the instability of the induced ferroelectric array indicates that the depolarization temperature is closely related at room temperature.

FIG. 5 (f) is a graph showing changes in residual polarization, maximum polarization and anti-electric field in Examples 1 to 4 and Comparative Examples.

As shown in FIG. 5 (f), it can be seen that the higher the substituted content of lithium and niobium in the ceramics according to Examples 1 to 4, the smaller the remanent polarization and the antiferromagnetic field. This is because the ferroelectric And the residual polarization is reduced due to the polarization of the polarization structure due to the growth of the nano-domain of the domain. In the ceramics according to Examples 1 to 4, lithium and niobium are substituted for pure BNKT-ST ceramics, And that the hysteresis curves become narrower due to the structural phase change at the pseudopubic phase in the coexistence of the tetragonal phase.

The ceramics according to Examples 1 and 2 exhibiting the characteristics of the non-eroded relaxed body having a small amount of substitution of ceramics, lithium and niobium according to the comparative example at a low temperature showed a very high residual polarization value after the external electric field was removed On the other hand, in the case of the ceramics according to Example 3, the eroded relaxation property was shown to have a small residual polarization value close to zero.

< Experimental Example 5> Piezoelectric characteristics analysis

The piezoelectric constant d ₃₃ of the ceramics according to Examples 1 to 4 and Comparative Example was analyzed in order to analyze the piezoelectric characteristics according to the lithium and niobium contents of the prepared ceramics. Respectively.

As shown in FIG. 6, the ceramics according to Examples 1 and 2 exhibited a high static charge constant (d ₃₃ ) value, while in Examples 3 and 4 in which the lithium and niobium contents were relatively high, Respectively.

Such reduction of the pre-static constant value can be explained by the thermodynamic theory of the ferroelectric according to the following formula 2:

(2)

(Where Q ₁₁ is a constant representing the perovskite material, ie, an electrostrictive coefficient, P _r is the remanent polarization, and ε ^T ₃₃ is the dielectric constant of the material).

It can also be seen that d ₃₃ of the prepared ceramics shows a sharp decrease from the ceramics according to Example 3 and Example 4 in proportion to P _r .

This is because, in the comparative example in which the ceramics according to Example 3 and Example 4 were substituted by lithium and niobium, both the rhombohedral and tetragonal phases were induced, the structural phase transition was induced to the pseudocubic phase, (S _max / E _max = d ₃₃ ^* ) is increased. As shown in FIG. 5, in the ceramics according to the third and fourth embodiments, a sharp reduction in the total static pressure constant appear.

< Experimental Example 6> Temperature dependent Polarization - Electric Field Hysteresis Curve Analysis

In order to analyze the change of the polarization according to the temperature of the prepared ceramics, the polarization change according to the temperature under the electric field of the ceramics according to the comparative example and the example 3 was measured and the result was shown as a PE hysteresis loop according to the temperature The results are shown in Fig.

As shown in FIG. 7, in the ceramics according to the comparative example, the residual polarization and the antioxidant decrease as the temperature increases from 25 ° C to 75 ° C in the polarization-electric field hysteresis curve, and the hysteresis curve becomes narrower when the temperature exceeds 100 ° C Which causes polarization to be affected by the temperature increase.

This is because when the temperature approaches 100 ° C, the domain structure appearing in the long-range region of the ferroelectric is changed into a short-range nano-nano-region, which causes the ferroelectric phase to become a metastable phase, BNKT-ST ceramics, which are not substituted with lithium and niobium as in the ceramics according to the comparative examples, exhibit an irreversible ferroelectric state stable in an external electric field. When the temperature exceeds 100 ° C. to 150 ° C., To a stable reversible ferroelectric phase.

On the other hand, the ceramics according to Example 3 showed a gradual decrease in the maximum polarization as well as the remanent polarization at room temperature in the polarization-electric field hysteresis curve. This phenomenon indicates that the ceramics in which lithium and niobium are substituted exhibit a ferroelectric region Is destroyed and converted into a nano-domain structure, which causes distortion of the lattice and interferes with ferroelectric alignment in the long range, resulting in a decrease in remanent polarization and anti-electric field.

As described above, in the ceramics according to Example 3, lithium and niobium are substituted and reversed from the ergot relaxation body to the ferroelectric phase under an external electric field, resulting in an electric field induced phase transition and a high strain.

< Experimental Example 7> Bipolar Electric field organic Strain analysis

The bipolar electric field-induced strain and negative strain of the prepared ceramics under an electric field of a frequency of 100 mHz were analyzed and the results of the analysis are shown in FIG.

As shown in Fig. 8 (a), the ceramics according to the comparative example exhibits typical characteristics of a ferroelectric body, showing a typical butterfly hysteresis curve represented by pure BNKT-ST ceramics.

As shown in Figs. 8 (a) and 8 (b), in the ceramics according to Examples 1 and 2, the substitution amount of lithium and niobium was small and the negative strain was increased.

On the other hand, in the ceramics according to Examples 3 and 4, it was confirmed that the negative strain rapidly decreased and the maximum strain increased sharply as the substitution amount of lithium and niobium increased, and in the case where the negative strain was close to 0 3 and Example 4, it was confirmed that the ceramics according to Examples 3 and 4 can be ideally used as an actuator ceramics.

Further, in the ceramics according to Example 3, the negative ferroelectricity abruptly changed from the typical ferroelectric behavior, and most of the negative strain was lost. The reason for this phenomenon is that a change in the domain structure occurs during the circulation of the electric field of the bipolar It was predictable.

Further, in the case of the ceramics according to Example 3, at the boundary between the non-eroded relaxed body and the eroded relaxed body, the quasi-cubic normal of the eroded relaxed body was induced in the long range order state of the ferroelectric metastable state It is found that bipolar cyclic strain of bipolar electric field increases rapidly at room temperature due to electric field-induced phase transition, and most of negative strain is lost. It was predicted that a change in the domain structure occurred during the rise.

< Experimental Example 8> Unipolar Field Organic strain and By composition Field Organic strain analysis

Unipolar field-induced strain (unipolar field-induced strain, 5 kV / mm electric field) of ceramics according to Examples 2 to 4 and Comparative Example was analyzed in order to analyze the influence of ceramics by lithium and niobium substitution, 1 to Experimental Example 4 and Comparative Example were analyzed and the results of the analysis are shown in FIG.

As shown in FIG. 9 (a), the ceramic according to the comparative example exhibited 0.09% electric field induced deformation, and the strain increased as the lithium and niobium substitution amount increased. In Example 3, the maximum strain was 0.40% .

9 (b), the inverse piezoelectric constant (S _max / E _{max =} d ₃₃ ^* , dynamic piezoelectric constant) of the ceramics according to the comparative example was found to be 180 pm / V, The inverse piezoelectric constant in the ceramics according to the present invention is 800 pm / V, and it can be confirmed that the ceramics produced by lithium and niobium substitution have a high inverse piezoelectric constant.

These results show that the abrupt increase of the field organic strain and the inverse piezoelectric constant is due to the chemical inequality of the A-site and the B-site in pure BNKLiTN-ST ceramics, It is predicted that inducing nano-sized polarization formation induces a phase transition from a non-ergodic relaxant similar to a ferroelectric phase to an ergot relaxant.

< Experimental Example 9> Temperature dependent Field Organic strain

In order to analyze the change of the electric field organic strain according to the temperature of the produced ceramics, the electric field organic strain according to the temperature of the ceramics according to Example 3 and Comparative Example was measured, and the measurement result is shown in FIG. 10 (a).

As shown in FIG. 10 (a), in the case of the ceramics according to the comparative example, a typical ferroelectric state is exhibited at 25 to 100 ° C., and the strain is rapidly increased to 0.40% at 120 ° C. or higher. To 0.32%.

However, in Example 3, the electric field strain was maintained at 0.35% or more at 25 to 120 ° C, and the strain was found to have a temperature stability in the range of 25 to 120 ° C.

In order to analyze the change of the inverse piezoelectric constant according to the temperature of the produced ceramics, the inverse piezoelectric constant according to the temperature of the ceramics according to Example 3 was measured, and the measurement result was compared with the conventionally known ceramics (PZT5H 10 (b), together with the values of the inverse piezoelectric constant according to the temperatures of PZT4, BNT-BT-KNN, KNN-LF4, CZ5 and BNT-BKT-ST.

As shown in Fig. 10 (b), the ceramics according to Example 3 had a strain hardly exceed ± 10% even in the temperature range of 25 to 125 deg. C, and temperature dependency was low unlike the conventionally known ferroelectric ceramics. It is predicted that it has suitable physical properties to be utilized as a stable actuator at a range of temperatures.

In order to analyze the temperature stability of the prepared ceramics, the inverse piezoelectric constant value and the electric field at the elevated temperature of the ceramics according to Example 3 were measured. The measurement results were compared with the results of a conventional known piezoelectric ceramics (BMT- The inverse piezoelectric constant values and electric fields of PMN-PT, CZ5, BNK-BA and BNT-BKT-KNN are shown together in FIG. 10 (c).

As shown in Fig. 10 (c), it was confirmed that the ceramics according to Example 3 had a high inverse piezoelectric constant value and thus had a stable physical property even when the temperature rises.

< Experimental Example 10> Characteristic Analysis of Actuator

In order to analyze the physical properties of the actuator manufactured using the manufactured ceramics, the unipolar electric field organic strain, the maximum strain ( _Smax ), the maximum electric field ( _Emax ) and the normalized strain S of the actuator according to Example 5 and Example 6 _max / E _max ) was measured. The measurement results are shown in Fig.

As shown in FIG. 11 (a), in the actuator according to the fifth embodiment, the maximum strain of 0.34% and the inverse piezoelectric constant of 755 pm / V were confirmed. However, in the actuator according to Example 6, %, And an inverse piezoelectric constant of 600 pm / V. The maximum strain and the inverse piezoelectric constant were found to be very high as compared with a conventional actuator made of a piezoelectric ceramic containing no lead.

As a result, it can be seen that the actuator manufactured using the prepared ceramics exhibits a relatively high maximum strain even in a low electric field, high inverse piezoelectric constant value, and low in temperature dependency, so that it can be effectively used as an actuator in various fields.

In order to analyze the physical properties of the actuator manufactured using the produced ceramics, the displacement amount was analyzed under unipolar electric fields with time according to the actuator according to the embodiment 5 and the embodiment 6. The analysis result is shown in FIG. 11 (b) .

As shown in FIG. 11 (b), the vertical dynamic variation exhibited a high value of 16 μm under a low electric field of 4.5 kV / mm, and the actuator according to Example 6 exhibits a relatively low history loss .

Accordingly, it was determined that the piezoelectric actuator manufactured from the ceramics as described above could replace the piezoelectric actuator manufactured using the conventional PZT piezoelectric ceramics.

Claims

A lead-free piezoelectric ceramics represented by the following formula (1)
[Chemical Formula 1]
0.96 [{Bi _0.5 (Na _0.84 K _0.16 ) _0.5 } _1-x Li _x (Ti _1-y Nb _y ) O ₃ ] -0.04 SrTiO ₃
(Provided that 0 < x < 0.025 and 0 < y < 0.025).

The method according to claim 1,
_{0.96 [{Bi 0 .5 (Na} 0 .84 K 0. 16) 0.5} 0.98 Li 0. ₀₂ (Ti _0.98 Nb _0.02 ) O ₃ ] -0.04 SrTiO ₃ .

3. The method of claim 2,
Wherein the dynamic piezoelectric constant (d ₃₃ ^* ) is 800 pm / V under an electric field of 5 kV / mm.

(a) pulverizing a mixed powder comprising Bi ₂ O ₃ , Na ₂ CO ₃ , TiO ₂ , K ₂ CO ₃ , SrCO ₃ , Li ₂ CO ₃ and Nb ₂ O ₅ ;
(b) calcining the pulverized mixed powder in the step (a);
(c) sintering the formed body using the mixed powder calcined in the step (b); And
(d) polarizing and aligning the sintered body obtained in the step (c).

5. The method of claim 4,
Wherein the step (b) is performed at a temperature of 800 to 900 占 폚.

5. The method of claim 4,
Wherein the step (c) is performed at a temperature of 1100 to 1200 占 폚.

A lead-free piezoelectric actuator including the ceramics according to any one of claims 1 to 3.

8. The method of claim 7,
Wherein the multilayer actuator is a multilayer actuator (MLA).

9. The method of claim 8,
The multilayer actuator (MLA) includes ten layers made of the above ceramics and has a dynamic piezoelectric constant (d ₃₃ ^* ) of 600 pm / V under an electric field of 4.5 kV / mm. Pb free piezoelectric actuators.