KR20160026186A - Lead-free piezoelectric ceramics and Actuator using the same - Google Patents

Abstract

The present invention relates to a lead-free piezoelectric ceramic for low cost and mass production, and an actuator using the same. More particularly, the present invention relates to a lead-free piezoelectric ceramic having a perovskite structure for applying to a smart actuator, and an actuator using the same. The lead-free piezoelectric ceramic according to the embodiment of the present invention includes a [Bi(Na,K)]TiO_3 based composite which forms a solid solution of (Bi,Na)TiO_3 and (Bi,K)TiO_3 and has a perovskite crystal structure. The Bi content of the [Bi(Na,K)]TiO_3 may be higher than a stoichiometry content. The total content of Na and K may be lower than a stoichiometry content.

Description

BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to a lead-free piezoelectric ceramics and an actuator including the same,

The present invention relates to a lead-free piezoelectric ceramics and an actuator including the same, and more particularly, to a lead-free piezoelectric ceramics having a perovskite structure for application to a smart actuator 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 possible material. 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.

Pb (Zr, Ti) O ₃ (hereinafter referred to as PZT) is generally used because it has excellent performance required in the industry, such as high electrical properties including high piezoelectric coefficient (d ₃₃ ) There is a problem that it is harmful to the human body and causes environmental problems. Therefore, in recent years, lead-free piezoelectric ceramics which do not use lead have been studied.

Substituents for the PZT system have been studied extensively with three materials, all of which have a perovskite structure, which can be represented by ABO ₃ . In order to have a stable perovskite structure, the sum of the atomic valences of the A site ion and the B site ion should be 6 ⁺ , and the size and mass of the ions are also important factors for determining the crystal structure and physical properties. The first candidate to replace PZT is BaTiO ₃ (BT). However, BT has a relatively low curie temperature of 130 < 0 > C and is difficult to apply. The second candidate is a perovskite structure based on Alkali Niobate, in which 5 ⁺ is the Nb element in the B site and 1 ⁺ is the Na ⁺ K in the A site. Na, K) NbO ₃ (hereinafter referred to as NKN) material. Korean Patent No. 10-0801477 discloses a method of using an NKN based material instead of a PZT based material. However, since Na ₂ O constituting NKN is highly volatile and K ₂ O is hygroscopic, In order to improve the piezoelectric characteristics, there is a difficult and complicated process to manufacture ceramics due to the use of methods such as hot pressing and RTGG (Reactive Templated Grain Growth). The third candidate is (Bi, Na), which contains 4 ⁺ element Ti in B site and Bi element which is heavy and 3 ^{+ in} P site as A site and Na which is 2 ⁺ TiO ₃ (hereinafter referred to as BNT).

BNT has the advantage of having a high phase transition temperature (T _c = 320 ° C), a remanent polarization (P _r = 38 μC / ㎠) and a coercive field (E _c = 73 kV / cm) as a ferroelectric substance at room temperature, As a result, bismuth (Bi) is volatilized. The biggest problem of BNT-based materials is that they have depolarization temperatures at temperatures lower than the phase transition temperature, making it difficult to apply them to the entire industry. In addition, there is a disadvantage that the coercive electric field is also too large and the piezoelectric characteristic (d ₃₃ ) and the electromechanical coupling coefficient (k _p ) are also low due to difficulty in poling the material due to high conductivity do.

Korean Patent Registration No. 10-0801477

The present invention provides a lead-free piezoelectric ceramics having a low anti-electric field value and a high electric field organic strain by a simple manufacturing process, thereby providing a lead-free piezoelectric ceramics which is inexpensive, easy to mass-produce and harmless to the human body, Thereby providing an actuator.

The lead-free piezoelectric ceramics according to the embodiments of the present invention are characterized in that [Bi (Na, K) TiO ₃ and Bi (Na, K) TiO ₃ form a solid solution and have a perovskite crystal structure. ] TiO ₃ based compound, the content of Bi in the [Bi (Na, K)] TiO ₃ based compound is more than the stoichiometric content, and the total content of Na and K may be less than the stoichiometric content.

The total content of Bi, Na and K in the [Bi (Na, K)] TiO ₃ based compound may be less than the stoichiometric content.

The [Bi (Na, K)] TiO ₃ -based compound [Bi (Na, K)] TiO ₃ and LaAlO ₃ form a solid solution and may have a perovskite crystal structure, and [Bi _{0 .5 + x (Na, K) 0.5-3x] TiO} 3 -LaAlO 3 ( where x is 0.0025 ≤ x ≤ 0.0075) can have the following composition of the.

The _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 composition in the _{-LaAlO 3 [Bi 0 .5 + x} (Na, K) 0.5-3x] TiO 3 has a significant mole fraction than LaAlO ₃ Lt; / RTI >

The molar ratio of K to the total content of Na and K in the [Bi (Na, K)] TiO ₃ based compound may be 15 to 25 mol%.

The lead-free piezoelectric ceramics may have a positive maximum strain of 0.28% or more, an absolute value of negative maximum strain of 0.03% or less, a smaller value of 1.41 kV / mm and a greater anti-field value than 0 kV / , A depolarization temperature (T _d ) lower than 100 ° C and higher than room temperature.

The [Bi (Na, K)] TiO ₃ compound may be a relaxed ferroelectric.

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

The method for manufacturing a lead-free piezoelectric ceramics according to embodiments of the present invention includes the steps of: mixing oxide powders of Bi, Na, K, and Ti as main components of a [Bi (Na, K)] TiO ₃ solid solution; First calcining the mixed powder; A second calcination step of adding an oxide powder of La, Al to LaAlO ₃ to the first calcined powder; Pressing the second calcined powder to form a formed body; And sintering the formed body to form a sintered body having a 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 Bi content may be greater than the stoichiometric content and the total Na and K content may be less than the stoichiometric content.

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 [Bi (Na, K)] TiO ₃ solid solution has a composition of [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ wherein x is 0.0025 ≤ x ≤ 0.0075. .

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 [Bi (Na, K)] TiO ₃ solid solution may be a relaxed ferroelectric.

In order to compensate for the volatilization of Bi, the [Bi (Na, K)] TiO ₃ (hereinafter referred to as BNKT) lead-free piezoelectric ceramics of the present invention was produced by adding Bi to an amount exceeding a stoichiometric amount, It has high electric field organic strain and low anti-electric field characteristics by regulating amount less than stoichiometric amount.

In addition, the present invention [Bi (Na, K)] TiO 3 by the addition of LaAlO ₃ in the piezoelectric ceramic can be improved ferroelectric _{properties, [Bi 0 .5 + x (} Na, K) 0.5-3x] TiO 3 - It is also possible to manufacture piezoelectric ceramics which are relaxed ferroelectrics by adjusting x in the LaAlO ₃ composition. The amount of the three-component BNKT-LA added LaAlO ₃ (LA) in normal BNKT is the ferroelectric characteristic is improved than the regular BNKT a two-component system, Bi, and _{(Na, K) [Bi 0} .5 + x (Na , K) _0.5-3x ] TiO ₃ -LaAlO ₃ , the negative strain is decreased and the positive strain is increased as x increases, It becomes possible to manufacture ceramics having a high electric field organic strain.

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 manufacturing method of the piezoelectric ceramics according to the present invention is simple and easy to produce, and mass production is possible, thereby reducing the production cost. In addition, the calcination process may be carried out twice to increase the homogeneity of the powder and to obtain a dense sintered body.

Figure 1 shows each composition of the usual BNKT and _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 -LaAlO 3 (x = 0, 0.005, 0.01, 0.02) in accordance with one embodiment of the present invention ≪ / RTI >
FIG. 2 is a graph showing the relationship between the composition of normal BNKT and [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.005, 0.01, 0.02) according to an embodiment of the present invention And a hysteresis curve representing an electrical characteristic of the hysteresis loop.
FIG. 3 is a graph showing the relationship between the composition of normal BNKT and [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.005, 0.01, 0.02) according to an embodiment of the present invention A graph showing the electric field strain of a piezoelectric ceramics showing electrical characteristics with respect to an electric field.
4 is a specific temperature for each composition of _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 -LaAlO 3 (x = 0, 0.005, 0.01, 0.02) in accordance with one embodiment of the present invention Graph showing dielectric constant and dielectric loss.
5 is a flowchart illustrating a method of manufacturing a 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 lead-free piezoelectric ceramics according to the embodiment of the present invention is characterized in that (Bi, Na) TiO ₃ and (Bi, K) TiO ₃ form a solid solution and [Bi (Na, K)] TiO ₃ ≪ / RTI > compounds.

(Pb-free) piezoelectric ceramics based on (Bi, Na) TiO ₃ (hereinafter referred to as BNT) is a Rhombohedral crystal structure of a perovskite structure of ABO ₃ type like PZT- and has a, B-site is a material that is a Ti ^{4 +,} and a-site position, the Bi ^{3 + 1 +} Na element and elements are mixed as a result have the bivalent element half-and-half. These materials cause electrical polarization in the Z-axis direction of the crystal structure due to the external electric field and Ti ^{4 +} ions located at the B-site cause mechanical deformation as a result of the polarization. The BNT material has a high Curie temperature (T _c ) while the antioxidant system has a disadvantage that it is too high. Therefore, the addition of (Bi, K) TiO ₃ (hereinafter referred to as BKT) having a tetragonal perovskite crystal structure to BNT Solid solution (i.e., BNKT).

The [Bi (Na, K)] TiO ₃ (hereinafter referred to as BNKT) based compound is a bismuth (Bi) lead-free piezoelectric ceramics having a perovskite structure. The stoichiometric stoichiometry of the perovskite structure is ABO _{3. In} BNKT, Ti ^{4 +} is located at the B-site, and the Bi ³⁺ element and the Na ^{1 +} or K ^{1 +} element are mixed with each other in the A- As a material having an element, the relative molar ratio of Bi and (Na, K) is 1: 1 stoichiometrically.

(Bi (Na, K)) in order to improve low driving electric field, high electric field organic strain, temperature stability, frequency response and the like in order to meet practical electric characteristics required in the industry and to be applied to practical mobile piezoelectric actuators. ] TiO ₃ Fe lobe has a Sky bit structure a-site and B-site perovskite structure in the two-component BNKT material base for the purpose of replacement of the inside, the particle size is similar, the high LaAlO ₃ relative to ionic polarization . As a result, the ferroelectric characteristics were improved in the case of the three-component BNKT-LA in which LaAlO ₃ (LA) was added in a predetermined amount compared to the two-component system BNKT.

Figure 1 shows each composition of the usual BNKT and _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 -LaAlO 3 (x = 0, 0.005, 0.01, 0.02) in accordance with one embodiment of the present invention FIG. 1 (a) is a graph showing the X-ray diffraction characteristics at a position of 20 to 80 °, and FIG. 1 (b) is a graph showing X-ray diffraction characteristics at a position of 35 to 50 ° X-ray diffraction characteristics obtained by enlarging the graph of FIG.

Referring to Figure _{1, [Bi 0 .5 + x} (Na, K) 0.5-3x] TiO 3 -LaAlO 3 (x = 0, 0.005, 0.01, 0.02) The crystal structure of a typical page lobe ABO ₃ structure of the Sky It can be confirmed that the structure exists in a state of a structure. However, since it is confirmed that when the amount is controlled by x, the secondary image appears at x of 0.02 or more, the range of x can be limited to 0? X? 0.02.

FIG. 1 (b) shows that a binary BNKT composition without LaAlO ₃ has a tetragonal structure with a single peak of the (111) plane and peak separation of the (200) plane, Such X-ray diffraction characteristics can be confirmed also in a three-component system composition of BNKT-LA (x = 0) in which a predetermined amount (for example, 1 mol%) of LaAlO ₃ (LA) is added. This is shown by the relatively small ionic radius La ^{3 +} ion and Al ^{3 +} ions employed (solid solution) into the _{[Bi 0 .5 (Na, K} ) 0.5] TiO 3 grid (lattice) took place well. In the case of BNKT-LA (x = 0.005), BNKT-LA (x = 0.01) and BNKT-LA (x = 0.02) It can be seen that the tetragonal structure is well maintained even though the amount is controlled.

And the like _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 -LaAlO 3 (x = 0, 0.005, 0.01, 0.02) is controlled on the composition x is a bismuth (Bi) a stoichiometric amount of And the total content of sodium (Na) and potassium (K) is less than the stoichiometric content, the BNKT compound retains the perovskite structure, so that the content of bismuth (Bi) is larger than the stoichiometric content, (Na) and potassium (K) may be less than the stoichiometric content. BNKT-based compounds are required to satisfy the ABO ₃ stoichiometric stoichiometry of the perovskite structure, in which the Bi ^{3 +} element and the Na ^{1 +} or K ^{1 +} element are added to the A-site in half (ie, relative to Bi and (Na, The BNKT compound of the present invention must be added in excess of the stoichiometric amount (half of the A-site) to compensate for the volatilization of Bi, K retained the perovskite structure even when the total content was regulated to be less than the stoichiometric content (half of A-site).

Further, the total content of Bi, Na and K in the [Bi (Na, K)] TiO ₃ based compound may be less than the stoichiometric content. BNKT-based compound has a molar ratio of Bi (A) and B (A) to Ti (B) in order that the molar ratio of A and B becomes 1: 1 in order to satisfy ABO _3, which is a stoichiometric stoichiometry of perovskite structure, However, the BNKT compound of the present invention retained the perovskite structure even when the molar ratio of Bi to Na (A) was less than that of Ti (B). According to one embodiment of the present invention _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 composition of Bi and Na, K molar ratio of (0.5 + x) of the total (A) + (0.5 - 3x) = 1 - 2x. When x is larger than 0, the molar ratio becomes smaller than Ti (B) of 1, but the perovskite structure is maintained as shown in Fig.

The [Bi (Na, K)] TiO 3 based compound is added to the LaAlO ₃ can be achieved [Bi (Na, K)] TiO 3 and LaAlO ₃ is a solid solution. In general, as a method for improving piezoelectric characteristics and sinterability of a lead-free piezoelectric ceramics based on bismuth (Bi), there are a method of adding a small amount of another element to a matrix and a method of replacing a part of constituent elements with another element. The ferroelectric and piezoelectric properties of [Bi (Na, K)] TiO ₃ -LaAlO ₃ lead-free piezoelectric ceramics were investigated by adding LaAlO ₃ in order to compensate for the drawbacks of bismuth (Pb) The ferroelectric characteristics were improved. And [Bi (Na, K)] a solid solution of TiO ₃ -LaAlO ₃ may have a perovskite crystal structure due to the LaAlO ₃ is a perovskite of the ABO ₃ structure.

Further, the [Bi (Na, K)] TiO ₃ compound may have a composition of [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ . The composition of [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ compensates for the volatilized amount of Bi by controlling the amount of excess of Bi and insufficient amounts of Na and K, Can form A-site vacancies.

And _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 composition of -LaAlO ₃ is _{(1-y) [Bi 0} .5 + x (Na, K) 0.5-3x] TiO 3 -yLaAlO ₃ (where, 0 <y <1) there formula by _{[Bi 0 .5 + x (Na} , K) 0.5-3x] to adjust the relative molar ratio of TiO ₃ and LaAlO ₃ a, [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ may be relatively larger than the molar ratio of _{LaAlO 3} . In the present invention, when y is increased to 0, 0.01, 0.03, and 0.05, y is the best performance at 0.01 and y is improved at 0.03, &Lt; 0.03.

The molar ratio of K to the total content of Na and K in the [Bi (Na, K)] TiO ₃ based compound may be 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. In one embodiment, in the present invention, the relative molar ratio of sodium (Na) to potassium (K) is selected to be 0.78: 0.22 in the [Bi (Na, K)] TiO ₃ based compound [Bi _{0 .5 + 0 .78} K _{0 .22} ) _0.5-3x ] TiO ₃ -LaAlO ₃ were prepared and dielectric and piezoelectric properties similar to those of PZT were obtained.

Therefore, when the molar ratio of K to the total content of Na and K in the above-described [Bi (Na, K)] TiO ₃ compound 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 and having excellent anti-electric field characteristics can be obtained.

FIG. 2 is a graph showing the relationship between the composition of normal BNKT and [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.005, 0.01, 0.02) according to an embodiment of the present invention Which is a graph showing hysteresis curves showing electrical characteristics with respect to the first embodiment.

Referring to FIG. 2, a hysteresis curve of a typical ferroelectric characteristic appears in an ordinary BNKT. In the three-component system of BNKT-LA (x = 0) in which LaAlO ₃ was added in a fixed amount, the remanent polarization remained almost constant, but the maximum polarization increased and the coercive field decreased. This is believed to be due to the enhancement of the maximum polarization in the BNKT perovskite structure by replacing the La ^{3 +} ion in the A-site and the Al ^{3 +} ion in the B-site. In addition, the amount of BNKT-LA (x = 0.005) controlled by BNKT-LA is excessively added to Bi and the amount of Na and K is insufficient to compensate for the amount of Bi to be volatilized, . The A-site vacancy generated in this way creates a defect field due to defects in the structure, which causes a long-range ferroelectric order of the oxygen octahedron (BO ₆ ) existing in the lattice to be interrupted Resulting in a short-range ferroelectric order. These results show that the remanent polarization in the hysteresis graph drops sharply. In this case, the relaxed ferroelectric characteristics are exhibited. When the anti-ferroelectric phase becomes ferroelectric phase under high electric field, transition, resulting in a large strain. In addition, in the BNKT-LA (x = 0.01) and BNKT-LA (x = 0.02) compositions, short-range ferroelectric order is more generated and low residual polarization and low maximum polarization are obtained.

Table 1 shows the relationship between the coercive electric field value of BNKT and the anti-electric field value according to the composition of [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.005, 0.01, .

Furtherance BNKT BNKT-LA
(x = 0) BNKT-LA
(x = 0.005) BNKT-LA
(x = 0.01) BNKT-LA
(x = 0.02) Anti-electric field value
(KV / mm) 2.3 1.41 0.75 0.43 0.33

As shown in Table 1, the BNKT-LA lead-free piezoelectric ceramics has a coercive field of less than 1.41 ㎸ / ㎜ and is closer to 0 ㎸ / ㎜ as x increases to 0.005, 0.01 and 0.02. Accordingly, the anti - electric field value can be adjusted to the value of x, and the anti - electric field value can be made as close as possible to 0 kV / mm.

Thus, the BNKT-LA lead-free piezoelectric ceramics of the present invention has a coercive field (E _c = 1.41 kV / mm or less) characteristic lower than that of BNKT (E _c = 2.3 kV / mm).

FIG. 3 is a graph showing the relationship between the composition of normal BNKT and [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.005, 0.01, 0.02) according to an embodiment of the present invention Of the piezoelectric ceramics showing electrical characteristics with respect to the electric field.

Referring to FIG. 3, typical BNKTs exhibit a typical ferroelectric shape in a strain characteristic as well as a polarization characteristic. It can be confirmed that the ferroelectric characteristics are further improved in the three-component system of BNKT-LA (x = 0) in which LaAlO ₃ is added in a predetermined amount. However, as mentioned in FIG. 2, in the amorphous BNKT-LA (x = 0.005) composition, the long-range ferroelectric is collapsed due to the influence of A-site vacancy and the ferroelectric domain characteristics are deteriorated And the negative strain, which is the switching effect of the domain, is lost. It is a characteristic that occurs when the ferroelectric characteristic changes to the relaxed ferroelectric characteristic by controlling the amount of Bi and (Na, K). As a technical effect, the positive strain value increases and the maximum strain (S _max ) The value of the normalized strain (d ₃₃ = S _max / E _max ) is increased and the value of d ₃₃ is increased (that is, excellent piezoelectric property). As shown in the graph, the negative strain was changed to a positive strain while the maximum strain was increased to about 0.34%. This large strain was observed in an anti-ferroelectric phase under a high electric field, Is transformed into a ferroelectric phase, and when the electric field disappears, it is transformed into an anti-ferroelectric phase. However, in the case of BNKT-LA (x = 0.01) and BNKT-LA (x = 0.02) compositions in which Bi is excessively added, the electric field for transformation from an anti- ferroelectric phase to a ferroelectric phase Is somewhat high, so that it does not appear that large deformation occurs.

Table 2 shows the relationship between the positive maximum strain of BNKT and the [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.0025, 0.005, 0.0075, 0.01, Of the maximum strain.

Furtherance BNKT BNKT-LA
(x = 0) BNKT-LA
(x = 0.0025) BNKT-LA
(x = 0.005) BNKT-LA
(x = 0.0075) BNKT-LA
(x = 0.01) BNKT-LA
(x = 0.02) Strain
(%) 0.18 0.28 0.31 0.34 0.29 0.24 0.12

Table 2 shows that the BNKT-LA lead-free piezoelectric ceramics has a maximum strain of 0.28% or more in the positive strain. BNKT-LA has a maximum strain of positive 0.28%, a positive maximum strain of x up to 0.005, and a maximum strain of about 0.34% at x = 0.005, it is preferable that the range of x is limited to 0.0025? x? 0.0075 where the positive maximum strain is about 0.3% or more. On the other hand, if x is greater than about 0.005 with the highest amount of maximum strain, then the positive maximum strain is again lowered, where 0 ≤ x ≤ 0.005, where the positive maximum strain is lower than the increasing width. On the other hand, it seems that x may have a maximum strain higher than 0.34% in the vicinity of 0.005.

Table 3 shows the relationship between the negative maximum strain of BNKT and the [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.0025, 0.005, 0.0075, 0.01, And the maximum strain of the sound according to the following equation.

Furtherance BNKT BNKT-LA
(x = 0) BNKT-LA
(x = 0.0025) BNKT-LA
(x = 0.005) BNKT-LA
(x = 0.0075) BNKT-LA
(x = 0.01) BNKT-LA
(x = 0.02) Strain
(%) -0.15 -0.14 -0.025 -0.01 -0.005 0 0

In Table 3, the absolute value of the negative maximum strain (that is, the negative strain having the largest absolute value) of the BNKT-LA Pb-free piezoelectric ceramics may be 0.03% or less. BNKT-LA has a negative maximum strain of -0.025% at x = 0.0025 and a negative maximum strain of -0.005% at x = 0.0075. When x is greater than 0.01, the negative maximum strain 0%), the absolute value of the negative maximum strain is 0.03% or less even when 0.0025 ≤ x ≤ 0.0075 where the positive maximum strain is about 0.3% or more.

Thus, the BNKT-LA lead-free piezoelectric ceramics of the present invention can have a high strain maximum strain of 0.28% or more. More specifically, it may have a maximum strain amount in the range of 0.28 to 0.4%. In addition, BNKT-LA lead-free piezoelectric ceramics can have a negative maximum strain of 0 (about 0%), with a high maximum strain and a negative maximum strain of 0 showing relaxed ferroelectric properties.

4 is a specific temperature for each composition of _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 -LaAlO 3 (x = 0, 0.005, 0.01, 0.02) in accordance with one embodiment of the present invention Dielectric constant and dielectric loss are data measured after poling under an electric field of 6 kV / mm in 80 ° C silicone oil.

4, the depolarization temperature (T _d ) is determined to be the first peak. In a normal BNKT, the depolarization temperature T _d is about 170 ° C. and the depolarization temperature T it is confirmed that the dielectric constant increases sharply in the vicinity of _d ). However, in BNKT-LA (x = 0) in which LaAlO ₃ is added in a fixed amount, the depolarization temperature (T _d ) is 120 ° C. and the change of the permittivity is changed into the form of diffuse phase transition. That is, it can be seen that the LaAlO ₃ is added as _{1 mol% 0.99 [Bi 0 .5} (Na, K) 0.5] TiO 3 -0.01LaAlO 3 composition is modified to a relaxor ferroelectric (relaxor) type. The depolarization temperature (T _d ) is generally known as the deformation temperature which changes from the ferroelectric phase to the anti-ferroelectric phase. The depolarization temperature (T _d ) is 0.99 [Bi _{0 .5 + x} (Ie, as the A-site vacancy increases) in the (Na, K) _0.5-3x ] TiO ₃ -0.01LaAlO ₃ , x increases to 0, 0.005, 0.01 and 0.02. This is in the same context as the long-range ferroelectric order is disturbed by A-site vacancy as described above. That is, the anti-ferroelectric phase and the ferroelectric phase coexist at a room temperature (about 25 ° C.) as the depolarization temperature (T _d ) falls to a low temperature (for example, normal temperature) The phase is reversibly changed to produce a high strain. Therefore, the [Bi (Na, K)] TiO ₃ compound of the present invention can be a relaxed ferroelectric.

Table 4 shows the relationship between the depolarization temperature (T _d ) of the BNKT and [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (x = 0, 0.0025, 0.005, 0.0075, ) is a table showing the depolarization temperature (T _d) according to the composition.

Furtherance BNKT BNKT-LA
(x = 0) BNKT-LA
(x = 0.005) BNKT-LA
(x = 0.01) BNKT-LA
(x = 0.02) T _d
(° C) 168 104 51 30 Measurement area (30 to 300 ° C) or less

Table 4 shows that the depolarization temperature (T _d ) of BNKT-LA lead-free piezoelectric ceramics may be lower than 100 ° C. BNKT-LA is x is at zero, with the depolarization temperature (T _d) of 104 ℃, when x is 0.01 days and has a depolarization temperature (T _d) of 30 ℃, when x is 0.02 days, of below 30 ℃ And has a depolarization temperature (T _d ). At this time, since the antiferroelectric phase and the ferroelectric phase must coexist at room temperature (about 25 ° C), a value below room temperature (about 25 ° C) is meaningless.

Meanwhile, the BNKT-LA 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 such, the present invention provides a bismuth (Bi) lead-free piezoelectric ceramics to which LaAlO ₃ is added to BNKT lead-free piezoelectric ceramics, wherein [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ (Bi) -based lead-free piezoelectric ceramics by changing the content of Bi, Na and K to lead-free piezoelectric ceramics having a composition of 0.0025? X? 0.0075, (Bi) lead-free piezoelectric ceramics having the characteristics of low dielectric constant (E _c = 1.41 kV / mm or less) and excellent piezoelectric properties while having characteristics of higher electric field organic strain (S _max = 0.28% or more) do.

The actuator according to another embodiment of the present invention may include the lead-free piezoelectric ceramics of the present invention and an electrode for providing an electrical signal to the 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 lead-free piezoelectric ceramics of the present invention exhibiting a high strain rate under low-field driving conditions.

5 is a flowchart illustrating a method of manufacturing a lead-free piezoelectric ceramics according to another embodiment of the present invention.

Referring to FIG. 5, a method of manufacturing a lead-free piezoelectric ceramics according to another embodiment of the present invention will be described in detail. However, duplicate elements of the lead-free piezoelectric ceramics described above will be omitted.

A method of manufacturing a lead-free piezoelectric ceramics according to another embodiment of the present invention includes the steps of: (S100) mixing oxide powders of Bi, Na, K, and Ti as main components of a [Bi (Na, K)] TiO ₃ solid solution; First calcining the mixed powder (S200); A second calcination step (S300) of adding an oxide powder of La, Al constituting LaAlO ₃ to the first calcined powder; Forming a formed body by pressing the second calcined powder (S400); And forming a sintered body having a perovskite crystal structure by sintering the formed body (S500).

First, oxide powders of Bi, Na, K and Ti which are the main components of the [Bi (Na, K)] TiO ₃ solid solution are mixed (S100). At this _{time, [Bi 0 .5 + x (} Na, K) 0.5-3x] Bi so as to satisfy a composition of TiO _3, Na, K, to mix weighed along the oxide powder of Ti in the mole ratios of the powder, Bi, Na , K, and Ti oxide powders may include Bi ₂ O ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , and TiO ₂ . Also, the relative molar ratio of Bi and (Na, K) can be adjusted by adjusting x, and it is preferable that the range of x is limited to 0.0025? X? 0.0075 where the positive maximum strain is about 0.3% or more. Here, the oxide powder of Bi may be mixed so that the Bi content is greater than the stoichiometric content, and the Na and K oxide powders may be mixed so that the total content of Na and K is less than the stoichiometric content. This can compensate for the volatilization of Bi and form A-site vacancies due to the lack of Na and K. 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 La, Al constituting LaAlO ₃ is added to the first calcined powder to perform a second calcination (S300). At this time, 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 La and Al are mixed well And the oxide powder of La, Al may include La ₂ O ₃ and Al ₂ O ₃ . Here, the reason for calcining the BNKT powder once and then adding LaAlO ₃ (LA) to the calcined powder 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 a more homogeneous BNKT powder. After sufficient reaction of the BNKT, LaAlO ₃ (LA) was added to form A-site in the BNKT perovskite structure and B -site, the BNKT-LA compound becomes dense, and by calcining it again, a homogeneous and dense BNKT-LA compound can be obtained. As a result, BNKT-LA can have a perovskite crystal structure without a secondary phase. The whole oxide powder may be calcined at one time, but it is preferable to calcine twice to improve the performance of the BNKT-LA Pb-free piezoelectric ceramics. The powder calcined after the second calcination can be ball milled and pulverized with a dispersion solvent.

Subsequently, about 0.3 g of the calcined powder is put into a molder having a diameter of 10 Ø and uniaxially pressed on the 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 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.

The sintered body [Bi (Na, K)] as TiO ₃ based solid _{solution, [Bi 0 .5 + x (} Na, K) 0.5-3x] TiO 3 -LaAlO 3 ( where x is 0.0025 ≤ x ≤ 0.0075) 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-free ceramic materials in the LaAlO ₃ The lead-free ceramic material employment _{[Bi 0.5 + x (Na,} K) 0.5-3x] TiO 3 -LaAlO 3 ( here, the (Bi) -based lead-free piezoelectric ceramics having dielectric and piezoelectric properties similar to those of PZT in the vicinity of the ceramics system can be obtained by providing a bismuth (Bi) -free lead- Ceramic and BNKT piezoelectric ceramics to which BaTiO ₃ , CeO ₂ , Bi ₂ O ₃ , SrCO ₃ and the like are added together with the solidification of the conventional pure BNT or BKT material, Lead-free piezoelectric ceramics having an electric field strain can be obtained. The BNKT-LA Pb-free piezoelectric ceramics manufacturing method of the present invention is a simple manufacturing method of a solid-phase reaction method, and its manufacturing 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 _{0.5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO _{3 in which LaAlO 3} is added to BNKT lead-free piezoelectric ceramics and x satisfies 0.0025 ≤ x ≤ (E _c = 1.41 kV / mm or less) as compared with conventional bismuth-based Pb-free piezoelectric ceramics by changing the contents of Bi and (Na, K) S _max = 0.28% or more) can be also produced excellent bismuth (Bi) based lead-free piezoelectric ceramic, and thereby allow the piezoelectric properties can provide excellent bismuth (Bi) based piezoelectric ceramic. In other words, it is possible to provide a Bi-based lead-free piezoelectric ceramics in which a crystalline region having a dielectric property and a piezoelectric characteristic similar to those of PZT is present, and to provide a high-purity ceramics (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-LA lead-free piezoelectric ceramics according to 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 harmless to the human body and environmentally friendly by using bismuth- 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.

S100: powder mixing S200: 1st calcination
S300: Powder addition and second calcination S400: Formation
S500: Sintered body formation

Claims (20)

Includes a (Bi, Na) TiO ₃ and (Bi, K) TiO ₃ that forms a solid solution, perovskite (Perovskite) having a crystal structure of [Bi (Na, K)] TiO 3 type compound,
A lead-free piezoelectric ceramics wherein the content of Bi in the [Bi (Na, K)] TiO ₃ based compound is greater than the stoichiometric content and the total content of Na and K is less than the stoichiometric content.
The method according to claim 1,
Wherein the total content of Bi, Na and K in the [Bi (Na, K)] TiO ₃ based compound is less than the stoichiometric content.
The method according to claim 1,
The above-mentioned [Bi (Na, K)] TiO ₃ based compound is a solid solution of [Bi (Na, K)] TiO ₃ and LaAlO _{3 and} has a perovskite crystal structure.
The method of claim 3,
The [Bi (Na, K)] TiO ₃ -based compound has a composition of [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ where x is 0.0025 ≤ x ≤ 0.0075. Pb free piezoelectric ceramics.
The method of claim 4,
The _{[Bi 0 .5 + x (Na} , K) 0.5-3x] TiO 3 composition in the _{-LaAlO 3 [Bi 0 .5 + x} (Na, K) 0.5-3x] TiO 3 has a significant mole fraction than LaAlO ₃ Lead - free Piezoelectric Ceramics.
The method according to claim 1,
Wherein the molar ratio of K to the total content of Na and K in said [Bi (Na, K)] TiO ₃ based compound is 15 to 25 mol%.
The method according to claim 1,
The lead-free piezoelectric ceramics has a positive maximum strain of 0.28% or more.
The method of claim 7,
Wherein the lead-free piezoelectric ceramics has an absolute value of a negative maximum strain of 0.03% or less.
The method according to claim 1,
The lead-free piezoelectric ceramics have a coercive field value smaller than 1.41 kV / mm and larger than 0 kV / mm.
The method according to claim 1,
The lead-free piezoelectric ceramics have a depolarization temperature (T _d ) lower than 100 캜 and higher than room temperature.
The method according to claim 1,
The [Bi (Na, K)] TiO ₃ -based compound is a relaxed ferroelectric, lead-free piezoelectric ceramics.
A lead-free piezoelectric ceramics according to any one of claims 1 to 11; And
And an electrode for providing an electrical signal to the lead-free piezoelectric ceramics.
Mixing an oxide powder of Bi, Na, K, and Ti as main components of a [Bi (Na, K)] TiO ₃ solid solution;
First calcining the mixed powder;
A second calcination step of adding an oxide powder of La, Al to LaAlO 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 perovskite crystal structure.
14. The method of claim 13,
Further comprising milling the first calcined powder or the second calcined powder. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
14. The method of claim 13,
Wherein a Bi content is greater than a stoichiometric content and a total content of Na and K is less than a stoichiometric content in the step of mixing oxide powders of Bi, Na, K, and Ti.
14. The method of claim 13,
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 powders of Bi, Na, K and Ti.
14. The method of claim 13,
The above-mentioned [Bi (Na, K)] TiO ₃ solid solution has a composition of [Bi _{0 .5 + x} (Na, K) _0.5-3x ] TiO ₃ -LaAlO ₃ where x is 0.0025 ≤ x ≤ 0.0075 (Method for manufacturing lead - free piezoelectric ceramics).
14. The method of claim 13,
Wherein the first calcining step and the second calcining step are performed at a temperature of 750 to 850 DEG C for 1 to 3 hours.
14. The method of claim 13,
And sintering the formed body to form a sintered body is performed at 1,100 to 1,200 ° C for 1 hour to 3 hours.
14. The method of claim 13,
Wherein the [Bi (Na, K)] TiO ₃ solid solution is a relaxed ferroelectric.

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