KR101635939B1 - Bismuth-based lead-free piezoelectric ceramics and Actuator using the same - Google Patents

Abstract

The present invention relates to a bismuth-based lead-free piezoelectric ceramics and an actuator including the same. More particularly, the present invention relates to a smart actuator, The present invention relates to a bismuth lead-free piezoelectric ceramics having a low coercive electric field and an actuator including the same.
The bismuth-based lead-free piezoelectric ceramics according to the embodiment of the present invention is characterized in that BiFeO ₃ is added to a [Bi (Na, K)] TiO ₃ base matrix containing Bi, Na, K, Ti and O as main components to form a solid solution, And may have a pseudo-cubic perovskite crystal structure.

Description

[0001] The present invention relates to a bismuth-based lead-free piezoelectric ceramics and an actuator including the same,

The present invention relates to a bismuth-based lead-free piezoelectric ceramics and an actuator including the same, and more particularly, to a bismuth-based lead-free piezoelectric ceramics having a low anti- To a bismuth-based lead-free piezoelectric ceramics having an electric field value and an actuator including the same.

Piezoelectric ceramics is a device in which mechanical deformation occurs when a voltage is applied and an external electric field is applied. It converts mechanical energy into electrical energy and electrical energy into mechanical vibration energy. It is a material with high conversion efficiency. It is applied to the acceleration sensor by using the principle that can convert vibration into electric energy. It is applied to pickup of record disc, microphone, speaker, buzzer etc. by using the principle of converting the sound of the audible area to electric energy. And is used as a device. In addition, since ultrasonic waves can be transmitted and received, it is used as a probe tip and a sonar of an ultrasonic sensor. If a force or pressure is applied from the outside, a high-voltage spark can be generated. It is applied to transformer. Other applications include ceramic filters, ultrasonic cleaners, ultrasonic machines, ultrasonic welding machines, ultrasonic humidifiers, etc. Piezoelectric materials, which are components used in smart information communication devices, Electro-mechanical system) and an actuator module mounted on a camera / microscope. (Pb, Zr) TiO ₃ (hereinafter referred to as PZT), (Na, K) NbO (PbTiO ₃ ), which are typical examples of the piezoelectric ceramics, have been developed for the development of piezoelectric ceramics. _And piezoelectric material of three-component type represented by BNT-ABO ₃ (for example, BNKT) structure and the like, which are non-lead type piezoelectric materials typified by ₃ (hereinafter referred to as NKN) and (Bi, Na) TiO ₃

In case of PZT piezoelectric material, high performance required in the industry such as excellent electric characteristic including high piezoelectric coefficient (d ₃₃ ) is excellent. However, since it is manufactured based on lead (Pb), it is harmful to human body and causes environmental problems . Therefore, in consideration of the fact that the regulations on the use of lead are strengthened day by day, non-linkage-based piezoelectric ceramics are being studied.

Lead-free piezoelectric ceramics such as NKN and BNT materials are harmless to the human body because they do not contain lead, but their electrical properties including piezoelectric characteristics are much lower than those of PZT-based piezoelectric materials. In order to improve the performance of the Pb-free ceramics, a material having an ABO ₃ structure is added to the NKN or BNT material to improve the characteristics of the material using a three-component system, and a material is manufactured by hydrothermal synthesis using a sol- Thereby improving the electrical characteristics of the material. Here, when the material is manufactured by the sol-gel method, although the electrical characteristics are improved, there is a disadvantage that the substrate deposition is not stable, the size of the particle size is not uniform, and a heat treatment process at a high temperature is required do. On the other hand, when a material having an ABO ₃ structure is added to an NKN or BNT material and a three-component Pb-free piezoelectric ceramics is produced by a uniaxial pressing method through a conventional-mixed method, It is simple and simple compared with the method, and it is easy to mass-produce the lead-free piezoelectric ceramics such that the uniform particle size can be formed through the calcination process. Although it is important to simplify the manufacturing process and reduce the cost through the solid state reaction method, it is also important to improve the piezoelectric constant (d ₃₃ ), the electromechanical coupling coefficient (K _p ), the electric field induced strain Strain (EFIS) characteristics, low dielectric loss tangent (tan δ), stable electrical characteristics, and compatibility with IC processes.

BaTiO ₃ (hereafter referred to as BT), (Ba, Zr) TiO ₃ (hereinafter referred to as BZT), (Ba, Ti) Ca) TiO3 (hereinafter referred to as BCT) and various other ceramic-based materials including BT-based materials. However, BT-based ferroelectric materials have a limit on the problem of heat generation in application to an actuator using a piezoelectric material because the Curie temperature (T _c ) is about 120 ° C. The NKN material, which is the most widely used material for lead-free piezoelectric ceramics, is disclosed in Korean Patent No. 10-0801477, and has a high piezoelectric constant (d ₃₃ ) of about 200 pC / N, (K _p ), but it has a disadvantage that the applied voltage required for application to an actuator module is high in an electric field induced strain (EFIS) characteristic applied to an actuator.

On the other hand, in the case of a specific composition of the lead-free piezoelectric material of the BNT series, a large displacement effect that is expected to be applied to an actuator requiring fast and large movement has been reported. The BNT material is a next generation lead- It is widely used as a component material for smart information devices. However, the biggest problem of the BNT-based materials is that they have a depolarization temperature at a temperature lower than the phase transition temperature, which makes it difficult to apply them to the entire industry. Further, there is a disadvantage that the piezoelectric characteristic d ₃₃ and the electromechanical coupling coefficient k _p are low because the coercive electric field is too large and the poling process of the material is difficult to proceed due to high conductivity .

Korean Patent Registration No. 10-0801477

The present invention relates to a bismuth-based lead-free piezoelectric ceramics having a high piezoelectric constant and a low dielectric loss value and a low dielectric constant value and a high remanent polarization value by a simple manufacturing process, The present invention provides a bismuth-based lead-free piezoelectric ceramics that is harmless to human body and satisfies excellent electrical characteristics, and is applied to an actuator to provide a bismuth lead-free piezoelectric ceramics having a high electric field induced strain (EFIS) Thereby providing an actuator.

Bismuth-based lead-free piezoelectric ceramics according to embodiments of the present invention are formed by adding BiFeO ₃ to a [Bi (Na, K)] TiO ₃ base matrix containing Bi, Na, K, Ti and O as main components to form a solid solution , Pseudo-cubic perovskite crystal structure.

The bismuth-based lead-free piezoelectric ceramics may have a composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ where x is 0.015 ≤ x ≤ 0.065.

The [Bi (Na, K)] TiO ₃ matrix may have a molar ratio of K to the total content of Na and K of 15 to 25 mol%.

The bismuth lead-free piezoelectric ceramics may have a positive maximum strain of 0.25% or more, an absolute value of a negative maximum strain of 0.01% or less, a smaller 1.4 ㎸ / ㎜ and a greater anti-field value than 0 ㎸ / And may be a relaxed ferroelectric.

The actuator according to embodiments of the present invention includes the bismuth-based lead-free piezoelectric ceramics; And an electrode for providing an electrical signal to the bismuth-based lead-free piezoelectric ceramics.

The method for producing a bismuth-based lead-free piezoelectric ceramics according to embodiments of the present invention includes mixing an oxide powder of Bi, Na, K, and Ti as main components of a Bi (Na, K) TiO ₃ matrix; First calcining the mixed powder; A second calcination step of adding an oxide powder of Bi and Fe constituting BiFeO ₃ to the first calcined powder; Pressing the second calcined powder to form a formed body; And sintering the shaped body to form a sintered body having a pseudo-cubic perovskite crystal structure.

The method may further include milling the first calcined powder or the second calcined powder.

In the step of mixing the oxide powders of Bi, Na, K and Ti, the molar ratio of K to the total content of Na and K may be 15 to 25 mol%.

The sintered body may have a composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ where x is 0.015 ≤ x ≤ 0.065.

The first calcination step and the second calcination step may be performed at a temperature of 750 to 850 DEG C for 1 to 3 hours.

The step of sintering the molded body to form a sintered body may be performed at 1,100 to 1,200 ° C for 1 hour to 3 hours.

The sintered body may be a relaxed ferroelectric.

Bismuth-based lead-free piezoelectric ceramics according to the present invention are produced by adding BiFeO ₃ containing bismuth (Bi) and ferrite (Fe) to a lead-free piezoelectric ceramics of [Bi (Na, K)] TiO ₃ -cubic) piezoelectric ceramics having a pervskite crystal structure, a high electric field induced strain (EFIS) characteristic and a low dielectric constant.

In addition, the present invention compensates the volatilization of Bi ions in the A-site of the perovskite crystal structure of BiFeO ₃ to reduce the change of the piezoelectric characteristics, and the Fe ions partially change the Ti ions having the same ionic radius The ferroelectric characteristics of the BNKT can be changed to the relaxed ferroelectric characteristics.

In addition, unlike PZT, which is harmful to human body and causes environmental pollution, the present invention provides a Bi-based piezoelectric ceramics material, so that environmentally friendly Pb-free piezoelectric ceramics can be obtained.

The bismuth-based piezoelectric ceramics manufacturing method according to the present invention can use the solid-phase reaction method, and the production process is simple and easy to produce, thereby mass production is possible and the production cost can be reduced. In addition, the calcination process according to the present invention can increase the homogeneity of the powder and obtain a dense sintered body.

1 is a perspective view showing a crystal structure of a BNKT lead-free piezoelectric ceramics according to an embodiment of the present invention.
FIG. 2 is a graph showing X-ray diffraction characteristics for each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention.
FIG. 3 is a graph showing a hysteresis loop showing electrical characteristics for each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention.
FIG. 4 is a graph showing the electric field induced strain of piezoelectric ceramics showing electrical characteristics for each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention. ; EFIS).
5 is a plan view showing the microstructure of each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention.
6 is a flowchart showing a method of manufacturing a bismuth-based lead-free piezoelectric ceramics according to another embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know. In the description, the same components are denoted by the same reference numerals, and the drawings are partially exaggerated in size to accurately describe the embodiments of the present invention, and the same reference numerals denote the same elements in the drawings.

The bismuth-based lead-free piezoelectric ceramics according to the embodiment of the present invention is characterized in that BiFeO ₃ is added to a [Bi (Na, K)] TiO ₃ base matrix containing Bi, Na, K, Ti and O as main components to form a solid solution, And may have a pseudo-cubic perovskite crystal structure.

(Bi, Na) TiO ₃ (hereinafter BNT) based lead-free (Pb-free) piezoelectric ceramic is not harmful to the human body, and as in the lead-based piezoelectric ceramic of a PZT-based ABO ₃ type perovskite (Perovskite) neungmyeon politics of structure ( Rhombohedral crystal structure, Ti ^{4 +} is located in B-site, and Bi ^{3 +} element and Na ^{1 +} element are mixed in A-site, resulting in a bivalent element. These materials cause electrical polarization in the Z-axis direction of the crystal structure due to the external electric field, and the Ti ^{4 +} ions located at the B-site cause mechanical polarization as a result of the polarization. In addition, the BNT-based material has two elements in the A-site and has the same shape as the relaxed ferroelectric. Most of the dielectrics have a low dielectric constant value, so that their degree of deformation is insignificant for practical application. In the case of crystals having no symmetrical center in the unit cell, they exhibit piezoelectric characteristics only when they are single crystals, In the case of a ferroelectric material which can change the direction of polarization by applying an electric field, a piezoelectric effect can be realized through a post-treatment process called poling in the form of polycrystals. In the case of electric field induced strain, only the expansion occurs irrespective of the direction of the electric field. However, in the case of piezoelectric deformation, the direction of the electric field varies depending on the direction of the electric field Expansion or contraction is possible and the degree of deformation thereof is sufficiently large, so that practical application is possible. In most industrial piezoelectric applications, ferroelectric materials are polarized and used.

The BNT material has a high Curie temperature (T _c ) and a large deformation at a specific composition, but it has a disadvantage that the antiferromagnetic system is too high. Therefore, a (Bi, K) TiO ₃ (BNKT) based on Bi (Na, K) TiO ₃ (hereinafter referred to as BKT).

1 is a perspective view illustrating a crystal structure of BNKT lead-free piezoelectric ceramics according to an embodiment of the present invention.

Referring to FIG. 1, the BNKT lead-free piezoelectric ceramics according to one embodiment of the present invention has a perovskite crystal structure having an ABO ₃ structure. In the A-site 100 of the perovskite structure, a trivalent element Bi and a monovalent element Na, K are mixed in half to form a divalent element, and a B-site 200 ) Is a tetravalent element, and when an external electric field is applied, the Ti element of the B-site displaces in the Z-axis and exhibits electric polarization.

The BNKT material is a bismuth (Pb) lead-free piezoelectric ceramics material having a perovskite structure of ABO ₃ and has a high piezoelectric property similar to that of a PZT material in a specific composition in which the relative molar ratio of sodium (Na) and potassium (K) And excellent ferroelectric properties. However, the sintering temperature of Bi, Na, K can not withstand the sintering temperature due to the volatilization property during sintering at high temperature, and the characteristics of the piezoelectric ceramics are not realized smoothly. Since the Bi element is located after the Pb element in the periodic table and the atomic weight is heavier than the Pb element, it is possible to replace the Pb ion with the Bi ion in the A-site and to form the trivalent ion in the valence state so that the BNT-based piezoelectric ceramics material And it is possible to realize high piezoelectric characteristics because it has a large polarizing property as it is located at the A-site. Therefore, it is expected that the piezoelectric characteristics and electric field induced strain (EFIS) characteristics can be improved by compensating the phenomenon of Bi ^{3 +} ions being volatilized by adding a Bi-based material. In the lead-free piezoelectric ceramics according to the present invention, BiFeO ₃ (BF) having a Rhombohedral crystal structure similar to that of a BNT-based piezoelectric ceramics was added to a [Bi (Na, K)] TiO ₃ matrix.

FIG. 2 is a graph showing X-ray diffraction characteristics for each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention, FIG. 2B is a graph showing X-ray diffraction characteristics at an angle of 39 to 48 DEG for crystal structure analysis. FIG.

In X-ray diffraction analysis, a specific X-ray beam is incident on the surface of ceramics while varying the angle, and the intensity of the X-ray beam diffracted according to the characteristics of the crystal plane is read to grasp the crystal structure. Referring to FIG. 2, the crystal structure of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ (hereinafter referred to as BNKT-BF) exists in the state of a pervskite structure of a typical ABO ₃ structure . In addition, it can be seen that the X-ray diffraction pattern of the ceramics specimen moves at a low angle up to the addition amount of BiFeO ₃ of 0.05 mol% (that is, x = 0.05). When Brag's law is used, -axis (30) / a-axis (10) ratio is higher. As the amount of BiFeO ₃ added increases, the c-axis (30) / a-axis (10), which is the distance between the plane and the plane of the crystal structure, increases and the tetragonality becomes stronger. However, when the addition amount of BiFeO ₃ is 0.07 mol% or more (that is, x = 0.07) or more, the X-ray diffraction pattern of the specimen does not move at a low angle any more and the addition amount of BiFeO ₃ is 0.09 mol% ) Or more, it is found that the BiFeO ₃ is excessively added to the second phase due to the excessive addition of BiFeO ₃ , and it is understood that the excessive addition of BiFeO ₃ deteriorates the electrical characteristics of the piezoelectric ceramics. When BiFeO ₃ is added too much, a secondary phase is generated in X-ray diffraction analysis due to excessive addition, resulting in deterioration of electrical characteristics. When too little BiFeO _{3 is} added, the effect due to addition of BiFeO ₃ is insufficient , The range of x can be limited to 0 ≤ x ≤ 0.09.

2 (b) in which the X-ray diffraction pattern was analyzed by adjusting the X-ray diffraction angle to 39 to 48 °, when the addition amount of BiFeO ₃ was 0 to 0.05 mol% (that is, 0 ≦ x ≦ 0.05) Since the X-ray diffraction pattern can clearly analyze the phenomenon of moving at low angles and the separation of the X-ray diffraction peaks occurring at a diffraction angle of about 46 ° does not occur, the specimen can have a pseudo-cubic structure . However, when the addition amount of BiFeO ₃ is 0.07 mol% (that is, x = 0.07) or more, slight X-ray diffraction peaks are observed, indicating that the crystal structure is changed due to the excessive addition of BiFeO ₃ . In addition, it can be seen that electrical characteristics change with the change of crystal structure.

The [Bi (Na, K)] TiO ₃ matrix may have a molar ratio of K to the total content of Na and K of about 15 to 25 mol%. BNKT has a morphotropic phase boundary (MPB) between the BNT and the tetragonal BKT, and it has been found that the BNT has dielectric and piezoelectric properties similar to those of the PZT system, The K content in BNKT is about 15 ~ 25 mol%, which shows high piezoelectric properties and excellent ferroelectric properties. The K content can be selected in this range. Among them, the temperature stability and T _d (depolarization temperature) in about 22 mol% content of K, electroluminescent organic strain (electric field induced strain) appears higher in the relative gatneunde a low coercive field (E _c) value, the Na and K The molar ratio of K to the total content is not limited thereto. Further, the addition of BiFeO ₃ (BF) having a surface crystal structure similar to BNT reduces the molar ratio of K, so that the content of K is preferably about 22 to 25 mol%. In one embodiment of the present invention, sodium (Na ) and the relative molar ratio of 0.78 of potassium (K): 0.22 of [Bi (Na, K)] TiO ₃ system as the matrix _{[Bi 0 .5 (Na 0 .78} K 0 .22) 0.5] TiO 3 -BiFeO 3 composition And dielectric and piezoelectric properties similar to those of PZT ceramics were obtained.

Therefore, assuming that the molar ratio of K to the total content of Na and K in the [Bi (Na, K)] TiO ₃ system is about 15 to 25 mol%, it corresponds to the vicinity of the morphotropic crystalline region between BNT and BKT (Bi) lead-free piezoelectric ceramics having dielectric and piezoelectric characteristics similar to those of PZT ceramics and having excellent anti-electric field characteristics can be obtained.

FIG. 3 is a graph showing a hysteresis loop showing electrical characteristics for each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention.

3, when the amount of BiFeO ₃ added was 0.03 to 0.05 mol% (that is, 0.03 ≦ x ≦ 0.05), the maximum polarization value was decreased compared to the pure BNKT specimen, but the residual polarization value and anti- The characteristics of the relaxed ferroelectric material can be confirmed. This relaxation ferroelectric property appears because the Bi ^{3 +} ion of BiFeO ₃ (BF) enters the A-site of BNKT-BF and compensates for the volatilization of Bi during the sintering process, the number of coordination according to the theory of the radius size of 6 individual Fe ^{3 +} ions in the ionic radius of about 0.65 Å in the similar ionic radii of Ti ^{4 +} (0.61 Å) in the B-site Fe ^{3 +} ions can six coordination Can be partially substituted in place of the Ti ^& lt ^{; 4 + &} gt ^; ions in the B-site of the perovskite structure. Since the Fe ³⁺ ion has a valence state lower than that of the Ti ^{4 +} ion, the mixing of the Fe ^{3 +} ions in the octahedral site of the perovskite structure generates an excessive negative charge, vacancy. The clamping effect due to the oxygen deficiency of the crystal structure inhibits the rearrangement of the ferroelectric domains during the poling process, resulting in a decrease in the maximum polarization of the hysteresis curve. The reason why the difference between the maximum polarization value and the residual polarization value is large is that the addition of BiFeO ₃ (BF) changes the ferroelectric characteristic when an external electric field is applied and the anti-ferroelectric state when an external electric field is not applied (That is, ferroelectric characteristics become unstable). The decrease of the maximum polarization value and the decrease of the residual polarization value and the anti-electric field value by the addition of BiFeO ₃ (BF) are the long-range ferroelectric order phenomenon which occurs mainly in the BNKT-based piezoelectric ceramics. And the relaxed ferroelectric characteristics are exhibited by the addition of BiFeO ₃ (BF). However, when the addition amount of BiFeO ₃ is 0.07 mol% or more (that is, x = 0.07) or more, the maximum polarization value sharply decreases, which indicates that electrical characteristic deterioration occurs. The pseudo-cubic structure is BiFeO _{3 &lt}; / RTI >

Table 1 shows anti-electric field values according to the composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ (x = 0, 0.03, 0.05, 0.07, 0.09) of the present invention.

Furtherance BNKT
(x = 0) BNKT-BF
(x = 0.03) BNKT-BF
(x = 0.05) BNKT-BF
(x = 0.07) BNKT-BF
(x = 0.09) Anti-electric field value
(KV / mm) 2.75 1.335 1.295 1.253 1.409

As shown in Table 1, the BNKT-BF lead-free piezoelectric ceramics has a coercive field value of less than 1.4 kV / mm and an increase in the amount of BiFeO ₃ to x of 0.07. , The anti-electric field value rose again. On the other hand, it is considered that x having the lowest anti - electric field value may have an electric field value lower than 1.253 kV / mm at around 0.07.

Thus, BNKT-BF lead-free piezoelectric ceramic of the present invention is BiFeO ₃ (BF) to BNKT is added by giving compensate for volatilization of Bi _{ions, BNKT (E c = 2.75 ㎸} / ㎜) a low coercive field than (E _c = 1.4 KV / mm or less).

FIG. 4 is a graph showing the electric field induced strain of piezoelectric ceramics showing electrical characteristics for each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention. ; EFIS).

4 showing the electric field induced strain (EFIS) characteristic curve of the BNKT-BF material according to the addition amount of BiFeO _3, the material of the BNKT without the addition of BiFeO ₃ (BF) exhibited a large negative strain negative strain, and forms a curve of a typical ferroelectric material. However, the BNKT material with BF added changes the electric field induced strain (EFIS) characteristic curves according to the external electric field. As shown in the graph, the electric field organic strain characteristics are further improved, and the negative strain negative strain) is significantly reduced. These changes are caused by partial compensation and replacement of the crystal structure A-site and B-site ions of BNKT ceramics.

As described above, the Bi ions of A-site reduce the change of piezoelectric characteristics by compensating the volatilization generated by Bi ions of BiFeO ₃ during sintering, and the Fe ^{3 +} ions have ion radius of about 0.65 Å and are located at the B-site It is similar to the ion radius of Ti ^{4 +} (0.61 Å), so the Ti ^{4 +} ion can be partially replaced. Due to the ionic substitution phenomenon of BiFeO ₃ , a structural twist phenomenon occurs in the crystal structure of the BNKT material, so that the ferroelectric characteristic of the BNKT changes to the relaxed ferroelectric characteristic.

Table 2 shows the maximum strains of the amounts according to the composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ (x = 0, 0.015, 0.03, 0.05, 0.065, 0.07, Table.

Furtherance BNKT
(x = 0) BNKT-BF
(x = 0.015) BNKT-BF
(x = 0.03) BNKT-BF
(x = 0.05) BNKT-BF
(x = 0.065) BNKT-BF
(x = 0.07) BNKT-BF
(x = 0.09) Strain
(%) 0.187 0.285 0.393 0.367 0.275 0.242 0.239

As shown in Table 2, BNKT-BF lead-free piezoelectric ceramics can have a positive maximum strain of 0.25% or more. BNKT-BF shows that the maximum strain of positive is about 0.28% when x is 0.015, and the maximum strain of positive increases as the amount of BiFeO ₃ is increased up to x 0.03, while the maximum strain at x is about 0.03 About 0.4%), and when x is larger than about 0.03, the maximum strain of the positive is lowered again. The range of x is preferably limited to 0.015 ≤ x ≤ 0.065 where the positive maximum strain is about 0.28% or more. On the other hand, when x is around 0.03, it is considered that it may have a maximum strain higher than about 0.4%.

Table 3 shows the negative maximum strain according to the composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ (x = 0, 0.015, 0.03, 0.05, 0.065, 0.07, 0.09) Table.

Furtherance BNKT
(x = 0) BNKT-BF
(x = 0.015) BNKT-BF
(x = 0.03) BNKT-BF
(x = 0.05) BNKT-BF
(x = 0.065) BNKT-BF
(x = 0.07) BNKT-BF
(x = 0.09) Strain
(%) -0.09 -0.007 -0.005 -0.004 -0.006 -0.007 -0.003

As shown in Table 3, the BNKT-BF lead-free piezoelectric ceramics may have an absolute value of the maximum negative strain (i.e., negative strain with the largest absolute value) of 0.01% or less. BNKT-BF has a negative maximum strain of about -0.007% when x is 0.015, and a negative maximum strain of about -0.003% with an absolute value of about 0.03 to 0.09, where the positive maximum strain The absolute value of the negative maximum strain is 0.01% or less even when 0.015 ≤ x ≤ 0.065, which is about 0.28% or more. On the other hand, in the vicinity of 0.03 to 0.09 x, the absolute value may be considered to have a negative maximum strain lower than 0.003%.

As such, the BNKT-BF lead-free piezoelectric ceramics of the present invention can have a high strain maximum strain of 0.25% or more. More specifically, it may have a maximum strain amount of 0.25 to 0.4%. In addition, the BNKT-BF lead-free piezoelectric ceramics can have a negative maximum strain of about 0 (about -0.003%), and a high maximum strain and a negative maximum strain of zero exhibit a relaxed ferroelectric characteristic. Therefore, the lead-free piezoelectric ceramics of the present invention can be a relaxation type ferroelectric.

5 (a) is a plan view showing a microstructure of each composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ according to an embodiment of the present invention, , FIG. 5 (b) shows x = 0.03, FIG. 5 (c) shows x = 0.05, FIG. 5 (d) shows x = 0.07 and FIG. 5 (e) shows x = 0.09.

Referring to FIG. 5, it can be seen that the microstructure of the BNKT is not excessively sintered and the grain growth is performed at an appropriate sintering temperature. Pure BNKT specimens have the same grain size and form a rectangular morphology. However, since the addition of BiFeO ₃ (BF) affects the formation of BNKT crystal grains, the overall crystal grain size increases as the amount of BiFeO ₃ added increases, and the crystal form increases from rectangular to round shape. The increase in the grain size is caused by sintered metal oxides, which are caused by the reaction between ferrite (Fe ₂ O ₃ ), sodium carbonate (Na ₂ CO ₃ ) and potassium carbonate (K ₂ CO ₃ ) during sintering. In addition, FIG. 7 (b) shows that the structure having the most excellent electrical characteristics and containing 3 mol% of BiFeO ₃ (that is, x = 0.03) exhibits the most dense structure.

Meanwhile, the BNKT-BF lead-free piezoelectric ceramics according to the present invention can be manufactured by a solid-state process. The solid-phase reaction method can simplify the production process and reduce the manufacturing cost.

As described above, the present invention relates to a method for producing a bismuth (Bi) based piezoelectric ceramics in which BiFeO ₃ is added to BNKT lead-free piezoelectric ceramics, wherein (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ Bi-based lead-free piezoelectric ceramics were produced by varying the amount of BiFeO ₃ added with x being the relative molar ratio of [Bi (Na, K)] TiO ₃ and BiFeO _{3 in} the composition of 0.015 ≤ x ≤ 0.065, (Bi) -based lead-free piezoelectric ceramics, which have high electric field organic strain (S _max = 0.25% or more) and low piezoelectric constant (E _c = 1.4 kV / Thereby making it possible to manufacture a lead-free piezoelectric ceramics.

The actuator according to another embodiment of the present invention may include the bismuth-based lead-free piezoelectric ceramics of the present invention and the electrode for providing an electrical signal to the bismuth lead-free piezoelectric ceramics.

The material of the electrode is not particularly limited, and a material generally used for a piezoelectric element is sufficient. Examples of the electrode material may include metals of Ti, Pt, Ta, Ir, Sr, In, Sn, Au, Al, Fe, Cr and Ni and oxides of these metals, May be stacked.

The actuator may be a smart actuator, and may be mounted on a micro-electro-mechanical system (MEMS), a camera, a microscope, or the like. Further, excellent performance can be obtained by using the bismuth-based lead-free piezoelectric ceramics of the present invention exhibiting a high strain rate under a low electric field driving condition.

6 is a flowchart showing a method of manufacturing a bismuth-based lead-free piezoelectric ceramics according to another embodiment of the present invention.

Referring to FIG. 6, a method for manufacturing a bismuth-based lead-free piezoelectric ceramics according to another embodiment of the present invention will be described in detail.

A method of manufacturing a bismuth-based lead-free piezoelectric ceramic according to still another embodiment of the present invention includes the steps of: (S100) mixing oxide powders of Bi, Na, K, and Ti which are the main components of a Bi (Na, K) TiO ₃ matrix; First calcining the mixed powder (S200); A second calcination step (S300) of adding an oxide powder of Bi and Fe constituting BiFeO ₃ to the first calcined powder; Forming a formed body by pressing the second calcined powder (S400); And sintering the formed body to form a sintered body having a pseudo-cubic perovskite crystal structure (S500).

First, oxide powders of Bi, Na, K, and Ti which are the main components of the [Bi (Na, K)] TiO ₃ matrix are mixed (S100). At this _{time, [Bi 0 .5 (Na,} K) 0.5] TiO 3 composition for Bi, Na, K, to mix weighed along the oxide powder of Ti in the mole ratios of the powder, Bi, Na, K satisfaction, Ti Of the oxide powder may include Bi ₂ O ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , and TiO ₂ . In addition, the molar ratio of K to the total content of Na and K can be about 15 to 25 mol% so that the BNKT exhibits high piezoelectric properties and excellent ferroelectric properties.

On the other hand, since the oxide powder of Na and K is hygroscopic, the weight can be increased by absorbing moisture from the surrounding environment during storage. As a result, the composition is different by the amount of moisture contained in weighing, It is preferable to dry the oxide powders of Na and K and weigh them. The drying method can be carried out in a drying oven, and it is enough to make a completely dried state in which the weight loss due to the drying of the contained moisture no longer occurs, and there is no particular limitation on the drying method. The mixing method of powders can be wet-mixed by a ball-milling method. As the solvent, an organic solvent such as ethyl alcohol or methyl alcohol can be used. In wet mixing, it can be dried in a micro-oven for about 24 hours to completely remove moisture.

Next, in order to grow the particles of the mixed powders, the mixture is put into a high-temperature sintering furnace and subjected to a first calcination (S200). At this time, the first calcination can be carried out at a temperature of 750 to 850 캜 for 1 to 3 hours, and the acceleration / deceleration rate can be 3 to 7 캜 / min. If the calcination is carried out at a temperature of 750 ° C or lower, the reaction between the raw powders becomes insufficient. If the calcination proceeds at a temperature higher than 850 ° C, difficulty of pulverization may occur. In addition, if the rate of increase and decrease is too fast, the temperature distribution of the raw material powders becomes uneven, and if it is too slow, the process time becomes long. On the other hand, in order to increase the homogeneity of the mixed powder, it is possible to perform calcination again at a temperature higher than the calcined temperature after repeating the milling and drying. Thereafter, the first calcined powder may be ball-milled and pulverized with a dispersion solvent.

Next, an oxide powder of Bi, Fe constituting BiFeO ₃ is added to the first calcined powder to perform a second calcination (S300). In this case, the second calcination can be performed at a temperature of 750 to 850 ° C for 1 hour to 3 hours, and the second calcination can be performed after ball milling so that the first calcined powder and the oxide powder of Bi and Fe are well mixed And the oxide powders of Bi and Fe may include Bi ₂ O ₃ and Fe ₂ O ₃ . Here, the reason why the BNKT powder is calcined once, and then the BiFeO ₃ (BF) is added and calcined again is to increase the homogeneity of the powder and to obtain a dense sintered body. More specifically, calcination of the BNKT powder sufficiently reacted with BNKT to obtain more homogeneous BNKT powder. After sufficient reaction of BNKT, BiFeO ₃ (BF) was added to form A-site and B-BNKT in the BNKT perovskite structure -site, so that the BNKT-BF compound becomes dense, and by calcining it again, a homogeneous and dense BNKT-BF compound can be obtained. As a result, BNKT-BF can have a perovskite crystal structure without a secondary phase. The entire oxide powder may be calcined at one time, but it is preferable to calcine twice to improve the performance of the BNKT-BF lead-free piezoelectric ceramics. The powder calcined after the second calcination can be ball milled and pulverized with a dispersion solvent.

Subsequently, about 0.5 g of the calcined powder is put into a molder having a diameter of 10 Ø and uniaxially pressed on a compression molding machine to form a molded body (S400). At this time, a small amount (for example, 1 wt%) of a binder (for example, PVA (Polyvinyl Alcohol)) may be added to the second calcined powder to facilitate the formation of the powder. Thereafter, in order to evaporate both the binder and a small amount of moisture (for example, adsorbed water [H ₂ O] and water [OH]), a burn-out process is performed in a high-temperature sintering furnace at 600 to 700 ° C. for 4 hours . Here, the rate of acceleration / deceleration can be 3 to 7 占 폚 / min and can be increased or decreased for about 2 hours.

Next, the formed body is put into a high-temperature sintering furnace and sintered to form a sintered body (S500). The sintered body may have a pseudo-cubic perovskite crystal structure and may be sintered at a temperature of 1,100 to 1,200 ° C for 1 hour to 3 hours. In the sintering, the sintering is not sufficient at a temperature of 1,100 ° C or lower, and the perovskite crystallinity is insufficient. At a temperature of 1,200 ° C or higher, the particle size becomes too large and defects in the structure may occur due to the volatilization of bismuth have. The BNKT Pb-free piezoelectric ceramics of the present invention is advantageous in that it can be sintered at a relatively lower temperature than the conventional PZT piezoelectric material.

Wherein the sintered body has a composition of [Bi (Na, K)] TiO ₃ -xBiFeO ₃ (where x is 0.015 ≤ x ≤ 0.065) as the [Bi (Na, K)] TiO ₃ solid solution solid And may be a relaxed ferroelectric.

Next, after the sintered body is polished and cleaned, a silver paste is screen-printed on both sides, dried in a drying oven at 100 ° C, and then heat-treated at 600 ° C for 10 minutes to form an electrode have. The material of the electrode is not limited to this, and a material generally used for a piezoelectric element is sufficient. Examples of the electrode material may include metals of Ti, Pt, Ta, Ir, Sr, In, Sn, Au, Al, Fe, Cr and Ni and oxides of these metals, May be stacked. After the electrode is formed, a piezoelectric element can be manufactured by performing polarization treatment by applying a voltage to the silicon oil. It is preferable that the silicone oil is maintained at a room temperature (25 캜) to 120 캜, and the voltage is preferably 1 to 7 kV / mm.

Thus, the present invention is [Bi (Na, K)] TiO 3 Lead BiFeO ₃ lead-free ceramic material is a (1-x) employed in the ceramic material [Bi (Na, K)] TiO 3 -xBiFeO 3 ( where, x (Bi) -based lead-free piezoelectric ceramics having a dielectric and piezoelectric characteristics similar to those of PZT at the vicinity of the ceramics system can be obtained by providing a bismuth (Bi) -free lead-free piezoelectric ceramics manufacturing method represented by the composition of And BNKT piezoelectric ceramics to which BaTiO ₃ , CeO ₂ , SrCO ₃ , Al ₂ O ₃ and the like are added together with the solidification of the conventional pure BNT or BKT material, Lead-free piezoelectric ceramics having strain can be obtained. The BNKT-BF Pb-free piezoelectric ceramics production method of the present invention is a simple production method of a solid-phase reaction method, and its production cost is low and mass production is easy.

As described above, the present invention provides a lead-free piezoelectric ceramics having a composition of [Bi (Na, K)] TiO ₃ -BiFeO ₃ , to which BiFeO ₃ is added to BNKT lead-free piezoelectric ceramics, and [Bi (Na, K)] TiO ₃ and BiFeO (E _c = 1.4 kV / mm or less) as compared with the conventional bismuth (Bi) based lead-free piezoelectric ceramics by changing the addition amount of BiFeO ₃ by controlling the molar ratio x of ₃ to 0.015 ≤ x ≤ 0.065 (Bi) lead-free piezoelectric ceramics excellent in the electric field organic strain (S _max = 0.25% or more) can be produced. As a result, it is possible to provide a bismuth (Bi) piezoelectric ceramics excellent in piezoelectric characteristics. In other words, it is possible to provide a Bi-based lead-free piezoelectric ceramics in which a crystalline region having a dielectric and piezoelectric properties similar to those of PZT is present, and is superior to a conventional bismuth-based Pb- (Bi) lead-free piezoelectric ceramics excellent in piezoelectric characteristics having electric field organic strain and low coercive field characteristics can be obtained. In addition, a smart actuator having excellent performance can be manufactured using such a lead-free piezoelectric ceramics. The BNKT-BF lead-free piezoelectric ceramics of the present invention can be manufactured by a simple production method of a solid-phase reaction method, and thus the production cost is low, mass production is easy, and a bismuth (Bi) to be.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be construed as limited to the embodiments set forth herein. Those skilled in the art will appreciate that various modifications and equivalent embodiments may be possible. Accordingly, the technical scope of the present invention should be defined by the following claims.

10: a-axis 20: b-axis
30: c-axis 100: A-site (Bi, Na, K)
200: B-site (Ti) 300: O-site (O)

Claims (15)

BiFeO ₃ is added to the [Bi (Na, K)] TiO ₃ base matrix containing Bi, Na, K, Ti and O as main components to form a solid solution,
Has a pseudo-cubic perovskite crystal structure,
A relaxed ferroelectric bismuth lead-free piezoelectric ceramics.
The method according to claim 1,
Wherein the bismuth-based lead-free piezoelectric ceramics has a composition of (1-x) [Bi (Na, K)] TiO ₃ -xBiFeO ₃ wherein x is 0.015 ≤ x ≤ 0.065.
The method according to claim 1,
Wherein the [Bi (Na, K)] TiO ₃ matrix is a bismuth-based lead-free piezoelectric ceramics having a molar ratio of K to the total content of Na and K of 15 to 25 mol%.
The method according to claim 1,
Wherein the bismuth-based lead-free piezoelectric ceramics has a positive maximum strain of 0.25 to 0.4%.
The method of claim 4,
Wherein the bismuth-based lead-free piezoelectric ceramics has an absolute value of a maximum negative strain of 0.01% or less.
The method according to claim 1,
Wherein the bismuth-based lead-free piezoelectric ceramics has a coercive field value smaller than 1.4 kV / mm and larger than 0 kV / mm.
delete A bismuth-based lead-free piezoelectric ceramics according to any one of claims 1 to 6; And
And an electrode for providing an electrical signal to the bismuth-based lead-free piezoelectric ceramics.
Mixing an oxide powder of Bi, Na, K, and Ti which is a main component of the [Bi (Na, K)] TiO ₃ matrix;
First calcining the mixed powder;
A second calcination step of adding an oxide powder of Bi and Fe constituting BiFeO ₃ to the first calcined powder;
Pressing the second calcined powder to form a formed body; And
And sintering the formed body to form a sintered body having a pseudo-cubic perovskite crystal structure.
The method of claim 9,
Further comprising the step of milling the first calcined powder or the second calcined powder.
The method of claim 9,
Wherein the molar ratio of K to the total content of Na and K is 15 to 25 mol% in the step of mixing the oxide powder of Bi, Na, K, and Ti.
The method of claim 9,
The sintered body is (1-x) [Bi ( Na, K)] TiO 3 -xBiFeO 3 ( where, x is 0.015 ≤ x ≤ 0.065) process for producing a bismuth-based lead-free piezoelectric ceramic having a composition of.
The method of claim 9,
Wherein the first calcining step and the second calcining step are performed at a temperature of 750 to 850 占 폚 for 1 hour to 3 hours.
The method of claim 9,
And sintering the molded body to form a sintered body, the sintered body is sintered at 1,100 to 1,200 ° C for 1 hour to 3 hours.
The method of claim 9,
Wherein the sintered body is a relaxed ferroelectric body.

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