KR101125700B1 - Lead-free piezoelectric ceramics and method for the preparation thereof - Google Patents

Abstract

The present invention relates to a lead-free piezoelectric ceramics and a method for manufacturing the same, specifically, represented by the following Chemical Formula 1 having improved piezoelectric properties in a morphotropic phase boundary (MPB) and having a perovskite structure. Lead-free piezoelectric ceramics, and a method for producing the same to improve the sinterability, crystallinity and piezoelectric properties through particle size control of the powder using attrition miling.

(1-x) (Na _0.5 K _0.5 ) NbO ₃ -x (Ba _1-y Sr _y ) TiO ₃

Wherein x and y are 0 <x ≦ 0.1 and 0 <y <1, respectively.

Lead-free, Piezoelectric Ceramics, Piezoelectric Properties, Attrition Milling

Description

Lead-free piezoelectric ceramics and its manufacturing method {LEAD-FREE PIEZOELECTRIC CERAMICS AND METHOD FOR THE PREPARATION THEREOF}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to lead-free piezoelectric ceramics exhibiting improved piezoelectric properties in the phase coexistence region (MPB) and having a perovskite structure and a method of manufacturing the same.

Piezoelectric ceramics have piezoelectricity, in which mechanical deformation occurs when electricity is applied, and conversely, when mechanical deformation is applied, electricity is generated. Using such characteristics, piezoelectric ceramics are widely used as an ultrasonic vibrator such as an ultrasonic device, an imaging device, an audio device, a communication device, a sensor, and a component material such as a transducer or an actuator.

Pb (Zr, Ti) O ₃ (PZT) is the piezoelectric material having the best piezoelectric properties and is used in many applications. In the solid solution of PbTiO ₃ and PbZrO ₃ , a PZT solid solution having a strong piezoelectric characteristic and a Curie temperature of 390 ° C. was found in the morphotropic phase boundary (MPB) of the tetragonal-trigonal system. The use of ceramics for piezoelectric and piezoelectric actuators, piezoelectric transducers, sensors, and resonators using reverse piezoelectric effects has been actively conducted.

However, most ceramics with excellent piezoelectric properties have a composition containing lead (Pb), but since PbO is rapidly volatilized at 1000 ° C. or higher, it is difficult to reproducibly change the composition. It is prepared by adding. This is not only desirable in terms of price competitiveness, but also can cause serious environmental pollution. Therefore, the US and other countries regulate Pb usage. In particular, according to the Restriction of Hazardous Substance (RoHS), one of the regulatory requirements for the electronics industry in the European Union in February 2003, since July 2006, It has been announced that it will ban the use of heavy metals, including Pb, such as cadmium, mercury, hexavalent chromium and bromine-based flame retardants. Although Pb contained in electronic ceramic parts is an exception, the provision of prohibiting the use of Pb in electronic ceramic parts as soon as alternative materials are developed has led to the development of lead-free piezoelectric ceramic materials worldwide. It is becoming.

The research on lead-free piezoelectric ceramic materials is largely divided into bi-perovskite materials and (Na, K) NbO ₃ (NKN) -based materials. Among them, Bi-perovskite material has high electromechanical coupling coefficient (k _p ) and piezoelectric constant (d ₃₃ ), which is very likely to replace PZT, but it is anti-ferroelectric at 120 ° C. It has the disadvantage of changing to electric, so there is a limit to practical use.

On the other hand, NKN-based materials have high phase transition temperature, low electric field, and high residual polarization characteristics among lead-free piezoelectric ceramics. Therefore, NKN series materials are considered as one of the representative materials that can replace piezoelectric ceramics based on lead. In particular, when 5 mol% LiTaO ₃ is dissolved in the NKN composition, it is known that it forms a phase coexistence region (MPB) and shows excellent piezoelectric properties. However, due to the high hygroscopicity of raw materials such as Na ₂ CO ₃ and K ₂ CO ₃ and volatilization during sintering, it is known that it is difficult to produce alkaline nitrate-based (LiNbO) sintered bodies having high characteristics by the conventional sintering method. . Thus, up to now, sintering has been carried out using expensive special manufacturing processes such as cold-isostatic pressing (CIP) and reactive template grain growth (RTGG). In addition, when Li is not added to the NKN composition, most of the piezoelectric properties are not shown, whereas when Li is added, there is difficulty in mass production, such as difficulty in polarization and poor reproducibility due to low resistance.

Therefore, the development of lead-free piezoelectric ceramics having excellent piezoelectric properties as a more economical and environmentally friendly process is urgently required.

Accordingly, an object of the present invention is to solve the above problems of the prior art, by employing (Ba _1-y Sr _y ) TiO ₃ of high dielectric constant in (Na _0.5 K _0.5 ) NbO ₃ piezoelectric ceramic composition having excellent piezoelectric properties. By greatly improving the piezoelectric properties, it is to provide a lead-free piezoelectric ceramics and a manufacturing method thereof that can replace PZT.

In order to achieve the above object, the present invention provides a lead-free piezoelectric ceramic represented by the following formula (1) exhibiting improved piezoelectric properties in the phase coexistence region (MPB) and having a perovskite structure.

<Formula 1>

(1-x) (Na _0.5 K _0.5 ) NbO ₃ -x (Ba _1-y Sr _y ) TiO ₃

Wherein x and y are 0 <x ≦ 0.1 and 0 <y <1, respectively.

In addition, the present invention provides a method of manufacturing the lead-free piezoelectric ceramics to improve the sinterability, crystallinity and piezoelectric properties through the control of the particle size of the powder using the attrition milling, when manufacturing the lead-free piezoelectric ceramics.

In addition, the present invention provides a piezoelectric element including the lead-free piezoelectric ceramics having excellent piezoelectric properties.

In the lead-free piezoelectric ceramics according to the present invention, a high dielectric constant of (Ba _1-y Sr _y ) TiO ₃ is dissolved in a (Na _0.5 K _0.5 ) NbO ₃ piezoelectric ceramic composition having excellent piezoelectric properties, thereby improving piezoelectric properties in the phase coexistence region (MPB). Properties and has a perovskite structure. Therefore, the lead-free piezoelectric ceramics according to the present invention can be applied to various piezoelectric elements such as actuators and ultrasonic sensors in place of PZT, and are useful as biomaterials such as human body implantable sensors as eco-friendly materials that do not contain Pb. Can be used.

According to the present invention, piezoelectric ceramics, which form a crystal phase according to a multicomponent composition for obtaining piezoelectric properties, that is, converting electrical energy into mechanical energy or converting mechanical energy into electrical energy, and which do not directly include environmental problems, are included. To provide.

The present invention for this purpose is (Na _0.5 K _0.5) NbO ₃ piezoelectric ceramic to the base composition, and a solid solution herein (Ba _1-y Sr _y) no associated piezoelectric TiO ₃ representing, improved piezoelectric properties, characterized in that the synthetic Provide ceramics.

The lead-free piezoelectric ceramics according to the present invention may be represented by the following Chemical Formula 1.

<Formula 1>

(1-x) (Na _0.5 K _0.5 ) NbO _3- x (Ba _1-y Sr _y ) TiO ₃

Wherein x and y are 0 <x ≦ 0.1 and 0 <y <1, respectively.

The lead-free piezoelectric ceramic of the present invention according to Chemical Formula 1 has a perovskite structure, provides a change in piezoelectric properties with respect to the phase change according to the molar ratio of the solid solution, and is improved in the phase coexistence region (MPB). It is characterized by exhibiting piezoelectric characteristics.

Lead-free piezoelectric ceramics according to the present invention

1) step of grinding and mixing Na ₂ CO ₃ , K ₂ CO ₃ , Nb ₂ O ₅ , BaCO ₃ , SrCO ₃ and TiO _{2 according} to the molar ratio composition

2) calcining the ground powder after drying;

3) regrind the calcined powder;

4) pressing and drying the regrind powder; And

5) it may be prepared by a method comprising the step of sintering the molded specimen.

First, raw material powders having a purity of 99% to 99.999% are weighed according to the molar ratio of Na ₂ CO ₃ , K ₂ CO ₃ , Nb ₂ O ₅ , BaCO ₃ , SrCO ₃ and TiO ₂ , followed by a nylon jar. The mill is pulverized by ball milling for 24 to 72 hours at a speed of 160 to 180 rpm with a zirconia ball and a dispersion solvent. At this time, as a dispersion solvent, anhydrous alcoholic solvent, such as ethanol or methanol, is preferable. The pulverized powder is dried and then calcined at a temperature of 600 to 1100 ° C. for 1 to 10 hours to synthesize a powder having a phase of Formula 1.

The calcined powder is then mixed with zirconia balls and dispersing solvent in a nylon jar as described above, ball milled for 24 to 72 hours at a speed of 160 to 180 rpm, regrind and dried. At this time, attrition milling may be performed instead of conventional ball milling to make the powder easier to crush and to reduce the size of the pulverized particles to improve sinterability and crystallinity. In the case where regrinding is performed by attrition milling, the attrition milling is preferably performed for 1 to 3 hours at a speed of 180 to 220 rpm. It is preferred that the powder particles obtained by attrition milling have a diameter in the range of 0.2 to 0.3 μm.

The compacted powder is press-molded into a cylindrical shape, and then the molded specimen is sintered at 900 to 1200 ° C. for 1 to 8 hours. In order to prevent the change of the composition due to volatilization of the NaO having a low volatilization temperature during the sintering process, the present invention is to cover the specimen formed of the same powder and to perform the sintering process in the alumina crucible .

Subsequently, after sintering the sample, the electrode material is applied and then polarized by applying a 2.5 to 5 kV / mm direct current (DC) bias at 120 to 150 ° C. for 1 hour in a silicone oil.

As a result, the present invention can produce piezoelectric ceramics having a perovskite structure represented by Chemical Formula 1 having a composition different from that of conventional PZT ceramics.

The most characteristic step in the method of manufacturing the piezoelectric ceramics according to the present invention is that both ball milling and attrition milling may be employed when regrinding the calcined powder. Although piezoelectric ceramics are obtained with attrition milling, it is possible to obtain piezoelectric ceramics having improved sinterability, crystallinity and piezoelectric properties through particle size control of the powder by using attrition milling.

The powder milled by ball milling and the powder milled by attrition milling were sintered at the same temperature, resulting in electromechanical coupling factor (K _p ), piezoelectric constant d ₃₃ (pC / N, piezoelectric charge sensor constant). , The dielectric constant (ε ^T ₃₃ / ε ₀ ) and the piezoelectric properties of the mechanical quality factor (Q _m ) showed that the specimen sintered with the attrition milled powder was superior to the ball sintered specimen with the milled powder. (See Tables 1 and 2).

As described above, the present invention synthesizes piezoelectric ceramics having a perovskite structure containing no Pb by synthesizing (Na _0.5 K _0.5 ) NbO ₃ as a basic composition and (Ba _1-y Sr _y ) TiO ₃ as a solid solution. can do. Accordingly, the present invention can provide lead-free piezoelectric ceramics having excellent piezoelectric properties, in contrast to the conventional PZT ceramics, which have a large dispersion of properties due to volatilization of PbO during sintering, difficulty in securing reproducibility, and causing environmental pollution. In addition, it has the advantage of improving the stability and reproducibility of the piezoelectric ceramics and preventing environmental pollution due to the use of Pb. In addition, the present invention improves the sinterability and crystallinity through a simple and economical sintering process using attrition milling, compared to the conventional NKN ceramics can be significantly different piezoelectric properties depending on the process complexity and Li content. At the same time, excellent piezoelectric characteristics can be secured.

In addition, the present invention provides a piezoelectric element including the piezoelectric ceramics manufactured as described above. The piezoelectric element according to the present invention includes the piezoelectric ceramics and the electrodes formed on the piezoelectric ceramics.

The piezoelectric element according to the present invention may be manufactured by various methods commonly known using the piezoelectric ceramics, and various modifications and changes may be made depending on the characteristics and process convenience of the desired element. As a result, the present invention does not include Pb, and has an excellent piezoelectric property and an environmentally friendly piezoelectric device having improved reliability.

Hereinafter, the present invention will be described in more detail with reference to Examples. These examples are only for illustrating the present invention more specifically, it will be apparent to those skilled in the art that the scope of the present invention is not limited by these examples in accordance with the gist of the present invention.

Example 1

Raw material powders having a purity of 99% to 99.999% were weighed according to the molar ratio of Na ₂ CO ₃ , K ₂ CO ₃ , Nb ₂ O ₅ , BaCO ₃ , SrCO ₃ and TiO ₂ , and then zirconia balls, ethanol and Together by milling ball milling for 72 hours at a speed of 180 rpm. The pulverized powder was dried and then calcined at 950 ° C. for 3 hours to synthesize a powder having a phase of Formula 1.

The calcined powder was then ball milled for 72 hours at a speed of 180 rpm by mixing zirconia balls and ethanol in a nylon jar, or regrind by attrition milling for 2 hours at a speed of 200 rpm and then dried. Each regrind powder was molded into a cylindrical shape by applying a pressure of 100 MPa, and then the molded specimen was sintered at 1060 ° C. for 3 hours. In order to prevent degradation of the piezoelectric properties due to volatilization of NaO having a low volatilization temperature during sintering, the specimen was covered with a homogeneous powder and then sintered in an alumina crucible.

After the sintered body produced by the above process was polished to a thickness of 1 mm using an abrasive, silver (Ag) electrodes were applied to both surfaces, and then silvered at 600 ° C. for 15 minutes to form a silver electrode. The polarization treatment was performed by applying a 3.5 kV / mm direct current (DC) bias for 1 hour in silicon oil at about 120 ° C., and then measuring piezoelectric properties after 24 hours. The electromechanical coupling coefficient (K _p ), piezoelectric constant d ₃₃ (pC / N), dielectric constant (ε ^T ₃₃ / ε ₀ ) and mechanical quality factor (Q _m ) were measured as piezoelectric properties of the prepared piezoelectric ceramics. Here, the electromechanical coupling coefficient and the mechanical quality coefficient were measured by the resonance anti-resonance method using an impedance analyzer, the dielectric constant was measured at 1 kHz using an LCR meter, and the piezoelectric constant was measured using a D ₃₃ meter.

Tables 1 and 2 show piezoelectric property evaluation results of the piezoelectric ceramics prepared in Examples, and the piezoelectric properties were compared according to the molar ratio of the solid solution (Ba _0.96 Sr _0.04 ) TiO ₃ and the type of regrinding method.

First, Table 1 is the _{(1-x) (Na 0.5} K 0.5) prepared in _{NbO 3 -x (Ba 1-y} Sr y) TiO 3 solid solution (Ba _1-y Sr _y) TiO ₃ in the piezoelectric ceramic of the following composition The molar ratio y of Ba and Sr to 0.04 is fixed and the molar ratio x of solid solution to 0.01, 0.02, 0.03, 0.04, 0.05 and 0.07 The piezoelectric properties were compared according to the contents.At this time, the powders of each composition were regrind by ball milling and sintered at 1075 ℃, and the samples sintered at 1060 and 1075 ℃ after regrind by attrition milling. It was.

x molar ratio
milling 0.01 0.02 0.03 0.04 0.05 0.07 Ball milling
(1075 ℃) density 4.26 4.29 4.24 4.22 3.74 3.43 ε ^T ₃₃ / ε ₀ 355.69 533.42 669.15 739.27 890.7 925.6 d ₃₃ 114 120 134 152 128 103 k _P 34.51 33.14 30.09 28.69 22.02 18.24 Q _m 120.6 121.9 108.6 100.7 95.7 65.3 Attrition Milling
(1075 ℃) density 4.3 4.27 4.26 4.31 4.18 3.93 ε ^T ₃₃ / ε ₀ 347.8 536.8 667.44 894.45 993.45 1064.3 d ₃₃ 109 131 143 166 140 121 k _P 35.92 34.8 31.7 31.01 27.79 26.2 Q _m 112.85 109.59 99.59 98.52 80.92 78.3 Attrition Milling
(1060 ℃) density 4.28 4.22 4.27 4.24 4.19 3.49 ε ^T ₃₃ / ε ₀ 549.03 560.27 632.8 635.33 1235.4 703.66 d ₃₃ 122 155 123 157 213 72 k _P 36.51 31.34 29.44 24.13 38.78 14.25 Q _m 123.06 98.36 106.74 90.09 103.69 88.94

As shown in Table 1, it was confirmed that the piezoelectric properties of the specimen sintered with the attrition milled powder were much superior to the piezoelectric properties of the specimen sintered with the ball milled powder according to the present invention. This is because the particle size of the attrition milled powder is smaller and the crystallinity and sinterability of the finally obtained piezoelectric ceramics is increased. In addition, the piezoelectric ceramics according to the present invention showed the best piezoelectric properties in the range of the molar ratio x of the solid solution in the range of 0.03 ≦ x ≦ 0.06, and formed the phase coexistence region (MPB) within the above range. In particular, when the attrition milled powder having a composition of x of 0.05 was sintered at 1060 ° C., piezoelectric ceramics having the best piezoelectric properties with a piezoelectric constant of d ₃₃ = 213 pC / N were obtained. 1 is a (1-x) (Na _0.5 K _0.5 ) NbO ₃ -x (Ba _0.96 Sr _0.04 ) TiO ₃ composition according to the present invention In the lead-free piezoelectric ceramics, the graph shows the change of the piezoelectric constant (d ₃₃ ) as the x value is changed.

Table 2 shows the molar ratio of solid solution (Ba _1-y Sr _y ) TiO ₃ in piezoelectric ceramics of (1-x) (Na _0.5 K _0.5 ) NbO ₃ -x (Ba _1-y Sr _y ) TiO ₃ composition prepared above. x was fixed at 0.04 and the molar ratio y of Ba and Sr was changed to 0, 0.03, 0.04, 0.05, 0.06 and 0.07, and the piezoelectric properties were compared. The powder of each composition was regrind by ball milling and then 1075. Specimens sintered at &lt; RTI ID = 0.0 &gt; C &lt; / RTI &gt;

y molar ratio
milling 0 0.03 0.04 0.05 0.06 0.07 Ball milling
(1075 ℃) density 4.31 4.29 4.3 4.22 4.19 3.75 ε ^T ₃₃ / ε ₀ 551.2 632.8 712.9 775 852.9 - d ₃₃ 118 121 152 136 109 - k _P 31.98 32.19 28.69 27.61 23.99 - Q _m 125.6 111.9 100.7 101.3 62.1 Attrition Milling
(1075 ℃) density 4.33 4.35 4.36 4.29 4.18 4.01 ε ^T ₃₃ / ε ₀ 556.3 732.5 694.3 913.7 966.5 1038.6 d ₃₃ 121 143 166 152 119 108 k _P 31.18 30.99 31.01 29.53 26.52 23.6 Q _m 121.3 116.4 98.52 100.9 87.2 63.8

As shown in Table 2, the piezoelectric ceramics according to the present invention showed the best piezoelectric properties in the range of the molar ratio y value of Ba and Sr of the solid solution in the range of 0.03≤y≤0.05, and the phase coexistence region (MPB) ) Was formed. In particular, when the attrition milled powder having a composition of y 0.04 was sintered at 1075 ° C., piezoelectric ceramics having the best piezoelectric properties with a piezoelectric constant of d ₃₃ = 166 pC / N were obtained. 2 is 0.96 according to the invention _{(Na 0.5 K 0.5) NbO 3-} 0.04 (Ba 1-y Sr y) TiO ₃ composition of It is a graph showing the change of the piezoelectric constant (d ₃₃ ) as the y value of the lead-free piezoelectric ceramics is changed.

As described above, the present invention can produce piezoelectric ceramics having excellent piezoelectric properties by synthesizing (Ba _1-y Sr _y ) TiO ₃ into a solid solution in a specific ratio based on (Na _0.5 K _0.5 ) NbO ₃ piezoelectric ceramics. . The piezoelectric properties are different depending on the molar ratio of the solid solution, and by controlling them, desired electromechanical coupling coefficients, dielectric constants, piezoelectric constants and mechanical quality coefficients can be obtained. The piezoelectric ceramics of the present invention also exhibit improved sinterability and crystallinity by controlling the particle size of the powder by attrition milling. Moreover, since the piezoelectric ceramics of the present invention do not contain Pb, the reliability of the properties due to volatilization of PbO is prevented from sintering and the stability and reproducibility of the synthesized piezoelectric ceramics are secured, thereby improving the reliability of the piezoelectric element. have. In particular, there is an environmentally friendly advantage because it does not contain Pb, a material that causes environmental problems and harmful to the human body.

The specific parts of the present invention have been described in detail, and it is apparent to those skilled in the art that such specific descriptions are merely preferred embodiments, and thus the scope of the present invention is not limited thereto. something to do. Therefore, the substantial scope of the present invention will be defined by the appended claims and equivalents thereof.

1 is a (1-x) (Na _0.5 K _0.5 ) NbO ₃ -x (Ba _0.96 Sr _0.04 ) TiO ₃ composition according to the present invention This graph shows the change of piezoelectric constant (d ₃₃ ) according to the change in molar ratio x value in lead-free piezoelectric ceramics.

Figure 2 is 0.96 (Na _0.5 K _0.5 ) NbO ₃ -0.04 (Ba _1-y Sr _y ) TiO ₃ composition of the present invention This graph shows the change of piezoelectric constant (d ₃₃ ) according to the change of molar ratio y in lead-free piezoelectric ceramics.

Claims (10)

Lead-free piezoelectric ceramics having a perovskite structure of Formula 1: <Formula 1> (1-x) (Na _0.5 K _0.5 ) NbO ₃ -x (Ba _1-y Sr _y ) TiO ₃ Wherein x and y are molar ratios of 0.02 ≦ x ≦ 0.06 and 0.03 ≦ y ≦ 0.05, respectively. In claim 1, The lead-free piezoelectric ceramics 1) weighing and mixing Na ₂ CO ₃ , K ₂ CO ₃ , Nb ₂ O ₅ , BaCO ₃ , SrCO ₃ and TiO _{2 according} to the molar ratio composition, followed by grinding; 2) calcining the ground powder after drying; 3) regrind the calcined powder; 4) pressing and drying the regrind powder; And 5) lead-free piezoelectric ceramics, characterized in that prepared from the step of sintering the shaped specimen. 3. The method of claim 2, The grinding in step 1) is performed by ball milling the mixture at a speed of 160 to 180 rpm for 24 to 72 hours. 3. The method of claim 2, The calcination in step 2) is performed for 1 to 10 hours at a temperature of 600 to 1100 ℃ lead-free piezoelectric ceramics. 3. The method of claim 2, Lead-free piezoelectric ceramics, characterized in that the regrinding in step 3) is performed by ball milling or attrition milling. The method of claim 5, The regrinding is lead-free piezoelectric ceramics, characterized in that the calcined powder is carried out by ball milling for 24 to 72 hours at a speed of 160 to 180 rpm. The method of claim 5, The regrind is a lead-free piezoelectric ceramics, characterized in that the calcined powder is performed by attrition milling for 1 to 3 hours at a speed of 180 to 220 rpm. 8. The method of claim 7, The powder regrind by the attrition milling has a particle diameter in the range of 0.2 to 0.3 μm. 3. The method of claim 2, Lead-free piezoelectric ceramics, characterized in that the sintering in step 5) is carried out at 900 to 1200 ℃ for 1 to 8 hours. A piezoelectric element comprising the lead-free piezoelectric ceramic according to any one of claims 1 to 9.

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