PIEZOELECTRIC MATERIAL AND METHOD OF MANUFACTURING THE SAME

Technical Field

[1] The present disclosure relates to a piezoelectric material and a method of manufacturing the piezoelectric material, and more particularly, to a lead- free piezoelectric material based on AgNbO ₃ -KNbO ₃ -NaNbO ₃ and a method of manufacturing the lead- free piezoelectric material. Background Art

[2] A piezoelectric material is a material capable of transforming mechanical energy applied thereto to electric energy, and vice versa. The piezoelectric effect is expressed by electromechanical coupling coefficient, K _p , which is defined as the ratio of mechanical energy generated in response to electric energy applied. A piezoelectric material of an excellent electromechanical coupling coefficient allows linear transformation between electric energy and mechanical energy. Accordingly, a mechanical transformation levelcan be controlled precisely, and an external vibration signal can be transformed precisely to a linear electric signal. Such a piezoelectric material is used as a material for ultrasonic vibrators, electromechanical ultrasonic transducers, actuators, and the like, which are widely used in the fields of ultrasonic apparatuses, display devices, audio devices, communication devices, sensors, etc.

[3] Most of the piezoelectric materials currently used are generally manufactured from

Pb(Zr, Ti)O ₃ (hereinafter referred to as "PZT") powder. The PZT powder is prepared through solid state synthesis which is performed by mixing principal compositions including PbO, ZrO ₂ and TiO ₂ , and impurities, such as MgO, Nb ₂ O ₅ , and the like, and sintering the mixture at high temperature. However, since the PZT powder is sintered at a relatively high temperature (l,200~l,350°C), a large quantity of PbO is volatilized during the sintering. Such volatilized PbO brings difficulties in the control of mi- crostructures and physical properties, and causes environmental problems such as acid rain when it is released into the atmosphere. In addition, when the PZT piezoelectric material is used for manufacturing a stacked piezoelectric device, such a high sintering temperature requires an internal electrode to be formed of a noble metal, such as Pt and Pd, which has high melting point.

[4] To address the above-mentioned concerns, there have been suggested various lead- free piezoelectric materials including a AgNbO ₃ -KNbO ₃ -NaNbO ₃ -based lead-free piezoelectric material. However, when a large amount of AgNbO ₃ is dissolved in (K ₅ Na)NbO ₃ , the formation of single phase perovskite is prevented. Thus, there has been little research on materials containing a large amount of AgNbO ₃ dissolved in (K ₅ Na)NbO ₃ . On the other hand, when a small amount of AgNbO ₃ is dissolved in (K ₅ Na)NbO ₃ , properties of the materials are not significantly improved. However, it was reported that, in an oxygen atmosphere, a single phase perovskite can besyn- thesized even when a relatively large amount of AgNbO ₃ is added. The reason for manufacturing AgNbO ₃ -KNbO ₃ -NaNbO ₃ in an oxygen atmosphere is that, when the sintering is performed in air, stability of AgNbO ₃ decreases remarkably, forming a large amount of second phases. However, the oxygenatmosphere increases manufacturing cost and decreases productivity. Disclosure of Invention Technical Problem

[5] The present disclosure provides a piezoelectric material that can replace PZT, and a method of manufacturing the piezoelectric material.

[6] The present disclosure also provides a piezoelectric material that forms XAgNbO ₃ - yKNb0 ₃ -zNaNb0 ₃ having a single phase perovskite crystal structure so that it can be sintered in air, and a method of manufacturing the piezoelectric material. Technical Solution

[7] In accordance with an exemplary embodiment, there is provided a method of manufacturing a piezoelectric material, including: preparing a raw material producing intermediate materials including AgNbO ₃ , KNbO ₃ , and NaNbO ₃ from the raw material; and synthesizing the intermediate materials to produce a piezoelectric material powder.

[8] The synthesizing of the intermediate materials may includesynthesizing two of the intermediate materials, and synthesizing the resultant material with the remaining intermediate material. That is, the synthesizing of the intermediate materials may include synthesizing AgNbO ₃ and NaNbO ₃ to produce AgNbO ₃ -NaNbO ₃ , and synthesizing AgNbO ₃ -NaNbO ₃ with KNbO ₃ .

[9] The synthesizing of the intermediate materials may be performed in air.

[10] The method may further include adding an oxide additive to the piezoelectric material powder, after the synthesizing of the intermediate materials. Here, the oxide additive may be one or more selected from Li ₂ O, Fe ₂ O ₃ , Bi ₂ O ₃ , V ₂ O ₅ , MnO ₂ , ZnO, Sb ₂ O ₃ , Sb ₂ O ₅ , and Ta ₂ O ₅ .

[11] The method may further include sintering the piezoelectric material powder in air.

[12] In accordance with another exemplary embodiment, there is provided a piezoelectric materialhaving a composition of xAgNb0 ₃ -yKNb0 ₃ -zNaNb0 ₃ (where, 0<x<1.0, 0<y<1.0, and 0<z<1.0), and manufactured by the method in accordance with the exemplary embodiment. Here, the piezoelectric material may have a single phase perovskite crystal structure. Advantageous Effects

[13] According to the disclosure, a piezoelectric material powder of xAgNb0 ₃ -yKNb0 ₃ -

ZNaNbO ₃ is manufactured by preparing raw materials, producing intermediate materials (AgNbO ₃ , KNbO ₃ , and NaNbO ₃ ), respectively, and then synthesizing the intermediate materials in sequence.

[14] As such, xAgNb0 ₃ -yKNb0 ₃ -zNaNb0 ₃ having a single phase perovskite crystal structure can be manufactured. Accordingly, sintering can be performed in air, and thus the manufacturing cost can be reduced and the productivitycan be improved in comparison to the typical method performed in an oxygen atmosphere.

[15] In addition, oxide additives can be added during the production of the piezoelectric material powder, and thus the piezoelectric properties can be improved.

Brief Description of Drawings

[16] FIG. 1 is a flow diagram illustrating a method of manufacturing a piezoelectric material in accordance with an exemplary embodiment.

[17] FIG. 2 is a flow diagram illustrating a method of manufacturing a piezoelectric material in accordance with another exemplary embodiment.

[18] FIG. 3 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder manufactured through typical solid state synthesis.

[19] FIG. 4 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder having a composition of 0.2AgNbO ₃ -0.4KNbO ₃ -0.4NaNbO ₃ in accordance with still another exemplary embodiment.

[20] FIG. 5 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric materi- alpowder having a composition of 0.2AgNbO ₃ -0.8[xKNbO ₃ -(l-x)NaNbO ₃ ] in accordance with yet another exemplary embodiment.

[21] FIG. 6 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder containing an oxide additive in accordance with even another exemplary embodiment. Mode for the Invention

[22] Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. The present invention may, however, be embodied in 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, and will fully convey the scope of the present invention to those skilled in the art.

[23] FIG. 1 is a flow diagram illustrating a method of manufacturing a piezoelectric material in accordance with an exemplary embodiment.

[24] First, a raw material is prepared for producing a compound having a perovskite crystal structure (SlOO). That is, Ag, Nb and O for AgNbO ₃ are weighed and prepared according to their mole fractions, K, Nb and O for KNbO ₃ are weighed and prepared according to their mole fractions, and Na, Nb and O for NaNbO ₃ are weighed and prepared according to their mole fractions.

[25] The prepared raw materials are mixed and calcined to produce intermediate materials including AgNbO ₃ , KNbO ₃ , and NaNbO ₃ , respectively (S200). That is, to produce AgNbO ₃ , a mixture of Ag, Nb and O is prepared by weighing and mixing Ag, Nb and O according to their mole fractions, and the mixture is added with a dispersion solvent, pulverized through a first ball mill, and subjected to calcination, for example, at approximately 85O ⁰ C for approximately 5 hours. To produce KNbO ₃ , a mixture of K, Nb and O is prepared by weighing and mixing K, Nb and O according to their mole fractions, and the mixture is added with a dispersion solvent, pulverized through a first ball mill, and subjected to calcination, for example, at approximately 85O ⁰ C for approximately 5 hours. To produce NaNbO ₃ , a mixture of Na, Nb and O is prepared by weighing and mixing Na, Nb and O according to their mole fractions, and the mixture is added with a dispersion solvent, pulverized through a first ball mill, and subjected to calcination, for example, at approximately 85O ⁰ C for approximately 5 hours.

[26] The produced intermediate materials are synthesized (S300). In more specific, any two intermediate materials selected from AgNbO ₃ , KNbO ₃ , and NaNbO ₃ are synthesized first, and the resultant material is then synthesized with the remaining intermediate material. For example, AgNbO ₃ and NaNbO ₃ are synthesized to produce AgNbO ₃ -NaNbO ₃ , and then AgNbO ₃ -NaNbO ₃ is synthesized with KNbO ₃ to produce AgNbO ₃ -NaNbO ₃ -KNbO ₃ . That is, AgNbO ₃ and NaNbO ₃ are mixed according to the desired mole fractions, and the mixture is calcined in air at a temperature ranging from approximately 800 ⁰ C to approximately 1,100 ⁰ C to produce AgNbO ₃ -NaNbO ₃ . The synthesized AgNbO ₃ -NaNbO ₃ is very stable in air even at a high temperature. Then, the synthesized AgNbO ₃ -NaNbO ₃ is added with a desired mole fraction of KNbO ₃ , and the mixture is calcined in air, for example, at a temperature ranging from approximately 800 ⁰ C to approximately 1,100 ⁰ C to produce a piezoelectric material powder of xAgNb0 ₃ -yKNb0 ₃ -zNaNb0 ₃ (where, 0<x<1.0, 0<y<1.0, and 0<z<1.0).

[27] The piezoelectric materialpowder is added with a small amount of organic material, such as polyvinyl alcohol (PVA) and polyvinyl butyral (PVB), and then a final material is manufactured (S400).

[28] The piezoelectric material powder is formed into an approximately 1 cm diameter disk-shape under a pressure of approximately 1 ton/cm ² , to produce a specimen (S500).

[29] Adsorbed water and adhesive water areremoved from the specimen, for example, at approximately 25O ⁰ C by heating the specimen at a heating rate of approximately 1 °C/min, and combined water and binders are burned away, for example, at ap- proximately 600 ⁰ C for approximately 2 hours, respectively. Then, the specimen is heat- treated in airat approximately 900 ⁰ C to approximately 1,100 ⁰ C for approximately 2 hours, to produce a sintered specimen (S600).

[30] The sintered specimen is subjected to polishing and washing. After screen-printing a silver paste on both side of the sintered specimen, the sintered specimen is baked at approximately 700 ⁰ C for approximately 10 minutes to form a silver electrode, and applied with a voltage of approximately 3 kV/mm to approximately 5 kV/mm for approximately 30 minutes in silicon oil for polarization treatment (S700).

[31] According to the method of manufacturing a piezoelectric material in accordance with theexemplary embodiment, a piezoelectric material powder of xAgNb0 ₃ -yKNb0 ₃ -ZNaNbO ₃ is manufactured by synthesizing raw materials to produce intermediate materials including AgNbO ₃ , KNbO ₃ , and NaNbO ₃ , respectively, and then synthesizing the intermediate materials in sequence. Here, oxide additives may be added thereto. A method of manufacturing a piezoelectric material added with oxide additives will be described below with reference to FIG. 2. Descriptions of FIG. 2 similar to that of FIG. 1 will be made briefly.

[32] FIG. 2 is a flow diagram illustrating a method of manufacturing a piezoelectric material in accordance with another exemplary embodiment.

[33] First, a raw material is prepared for producing a compound having a perovskite crystal structure (SlOO).

[34] The prepared raw materials are mixed and calcined to produce intermediate materials including AgNbO ₃ , KNbO ₃ , and NaNbO ₃ , respectively (S200).

[35] The produced intermediate materials are synthesized to produce a primary powder

(S300). In more specific, any two intermediate materials selected from AgNbO ₃ , KNbO ₃ , and NaNbO ₃ are synthesized first, and the resultant material is then synthesized with the remaining intermediate material to produce a primary powder of xAgNb0 ₃ -yKNb0 ₃ -zNaNb0 ₃ (where, 0<x<1.0, 0<y<1.0, and 0<z<1.0).

[36] The primary powder is added with one or more oxide additives, such as Li ₂ O, Fe ₂ O ₃ ,

Bi ₂ O ₃ , V ₂ O ₅ , MnO ₂ , ZnO, Sb ₂ O ₃ , Sb ₂ O ₅ , Ta ₂ O ₅ and the like. Then, the mixture is subjected to second ball-mill, drying, pulverizing,and screening to produce a secondary powder (S350).

[37] The secondary powder is added with a small amount of organic material, such as polyvinyl alcohol (PVA) and polyvinyl butyral (PVB), and then a final material is manufactured (S400).

[38] The secondary powder is formed to produce a specimen (S500).

[39] Adsorbed water and adhesive water are removed from the specimen, and combined water and binders are burned away. Then, the specimen is heat-treated in airto produce a sintered specimen (S 600). [40] The sintered specimen is subjected to polishing and washing. After screen-printing silver paste on both side of the sintered specimen, and forming a silver electrode, the specimen is applied with a voltage in silicon oil for polarization treatment (S700).

[41] Comparative Example

[42] FIG. 3 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder manufactured through typical solid state synthesis in air, the diagram being taken after calcination. Here, the typical solid state synthesis indicates the method of manufacturing a piezoelectric material powder by mixing Ag ₂ O, K ₂ CO ₃ , Na ₂ CO ₃ , and Nb ₂ O ₅ according to a predetermined mole ratio and performing a typical manufacturing method. From FIG. 3, it can be seen that, in the piezoelectric material powder manufactured by such typical solid state synthesis, a large amount of second phases formed though the calcination was performed at approximately 900 ⁰ C. In the piezoelectric material powder manufactured by the typical sold state synthesis, the formation of the second phase increases with temperature. Accordingly, the process is preferably performed in an oxygen atmosphere to prevent the formation of second phases.

[43] Example

[44] FIG. 4 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder of 0.2AgNbO ₃ -0.4KNbO ₃ -0.4NaNbO ₃ in accordance with an exemplary embodiment, the diagram being taken after calcination at 1,000 ⁰ C. That is, the X-ray diffraction pattern was taken after calcining the piezoelectric material powder of xAgNb0 ₃ -yKNb0 ₃ -zNaNb0 ₃ , which was manufactured by producing raw material powders including AgNbO ₃ , KNbO ₃ , and NaNbO ₃ , respectively, and then synthesizing the raw material powders in sequence according to an exemplary embodiment. From FIG. 4, it can be seen that second phases did not form and a single phase perovskite compound was synthesized well. This compound is stable up to its melting temperature.

[45] FIG. 5 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder of xAgNb0 ₃ -yKNb0 ₃ -zNaNb0 ₃ in accordance with an exemplary embodiment. Here, the diagram shows X-ray diffraction patterns of compositions where x=0.2, (y+z)=0.8, and y/z is 48/52, 50/50, 52/48, 54/46, 56/44, respectively, and the diagram was taken after calcining the compositions in air. From FIG. 5, it can be seen that, in all of the compounds,a complete single phase perovskite structure was synthesized.

[46] FIG. 6 is a diagram illustrating an X-ray diffraction pattern of a piezoelectric material powder of 0.2AgNbO ₃ -0.8[0.5KNbO ₃ -0.5NaNbO ₃ ] containing 0.5wt% ZnO as an oxide additive in accordance with an exemplary embodiment, the diagram being taken after calcination in air. From FIG. 6, it can be seen that a compound having a structure of a complete single phase perovskite was synthesized even when an oxide additive was added thereto.

[47] Properties of the piezoelectric material of KNbO ₃ -NaNbO ₃ manufactured by the typical solid state synthesis, and the piezoelectric material of xAgNb0 ₃ -yKNb0 ₃ - ZNaNbO ₃ manufactured by exemplary embodiments are given in Table 1. Properties shown in Table 1 include measured values of permittivity, electromechanical coupling coefficient (K _p ), and mechanical quality factor (Q _m ). The measured values of the electromechanical coupling coefficient show examples of a planar mode of a piezoelec- tricceramic vibrator in a disk-shape. However, the effect of the present invention is not limited to the planar mode of a piezoelectric ceramic vibrator in a disk-shape, and the present invention is also available in other vibration modes, such as thickness-longitudinal vibration and thickness sliding vibration, used in other piezoelectric ceramic vibrators, particularly, for example, a resonator.

[48] Table 1 [Table 1] [Table ]

[49] As shown in Table 1, whereas the piezoelectric material of pure (K ₅ Na)NbO ₃ manufactured by the typical solid state synthesis was measured to have a permittivity of 402 and an electromechanical coupling coefficient, K _p of 29.5%, the piezoelectric material manufactured in accordance with the exemplary embodiment was measured to have a permittivity of 549 to 575 and an electrochemical coupling coefficient, K _p of 38.7% to 39.0%. In addition, the piezoelectric materialmanufactured in accordance with the exemplary embodiment was measured to have a mechanical quality factor, Q _m of 97 to 99. Therefore, it can be seen that the piezoelectric materialin accordance with the exemplary embodiment is improved in all of permittivity, electromechanical coupling coefficient (Kp), and mechanical quality factor (Qm) in comparison with the piezoelectric material of pure (K ₅ Na)NbO ₃ manufactured by the typical solid state synthesis, and thus has very excellent properties.

[50] Although a piezoelectric materialand a method of manufacturing the piezoelectric material has been described with reference to the specific embodiments, itis not limited thereto. Therefore, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the present invention defined by the appended claims, and that the above- described exemplary embodiments are merely illustrative and not restrictive.