Abstract:
A piezoelectric ceramic composition is provided that does not contain Pb, and yet has a large electromechanical coupling coefficient in comparison with a bismuth layered compound, as well as a piezoelectric ceramic element using the composition. This piezoelectric ceramic composition includes at least the elements Ag, Li, Nb, and O, and has an electromechanical coupling coefficient k 33  of not less than about 20%.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piezoelectric ceramic composition and a piezoelectric ceramic element using the composition. More particularly, the present invention relates to a piezoelectric ceramic composition useful as a material for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic resonators, and to a piezoelectric ceramic element using the composition. 
     2. Description of the Related Art 
     Conventionally, piezoelectric ceramic compositions comprising lead titanate zirconate (Pb(Ti x Zr 1−x )O 3 ) or lead titanate (PbTiO 3 ) as a main component are widely used for manufacturing piezoelectric elements such as piezoelectric ceramic filters. Recently, piezoelectric ceramic compositions comprising a bismuth layered compound such as CaBi 4 Ti 4 O 15  as a main component have also been developed. 
     Pb, however, is suspected as a material which might cause environmental problems and affects human bodies. On the other hand, piezoelectric ceramics comprising bismuth layered compounds as the main components are not widely used in practice since the electromechanical coupling coefficients k 33  are as small as less than 20%. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a piezoelectric ceramic composition which does not contain Pb and which has an electromechanical coupling coefficient larger than that of a bismuth layered compound, and to provide a piezoelectric ceramic element using the composition. 
     A piezoelectric ceramic composition according to the present invention is characterized in that it comprises at least the elements Ag, Li, Nb, and O, and has an electromechanical coupling coefficient k 33  of not less than about 20%. 
     It is also characterized in that it comprises a main component represented by the general formula (Ag 1−x Li x )(Nb 1−y Ta y )O 3 , wherein 0.075&lt;×&lt;0.40 and 0&lt;y&lt;0.20. In one embodiment, y=0 and the main component corresponds to (Ag 1−x Li x )NbO 3 . In another embodiment, y is greater than 0 and the main component corresponds to (Ag 1−x Li x )(Nb 1−y Ta y )O 3 . 
     Furthermore, it is preferable that the composition comprises at least one of a manganese oxide and a silicon oxide as an auxiliary component. It is more preferable that the manganese oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component, and that the silicon oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component. 
     Furthermore, a piezoelectric ceramic element according to the present invention comprises a piezoelectric ceramic made from the piezoelectric ceramic composition according to the present invention, and an electrode formed on the piezoelectric ceramic. 
     In the general formula for the piezoelectric ceramic composition according to the present invention, a range of x&lt;0.075 is not preferable since the transition temperature is decreased, across which a ferroelectric phase acting as a piezoelectric material turns into a paraelectric phase or an antiferroelectric phase which does not act as a piezoelectric material, with the result that there will be problems regarding the temperature stability of the elements composed of the piezoelectric ceramic composition. Also, when 0.40≦x, the resonance frequency constant will be smaller than 2,000 Hz/m, and polarization will not occur easily. When y is not less than 0.20, the transition temperature will be decreased, too. Accordingly, the ranges of 0.075≦x&lt;0.40 and 0≦y&lt;0.20 are required in the present invention. 
     Furthermore, in the present invention, the baking temperature can be decreased by adding a manganese oxide or a silicon oxide to the main component. It is to be noted that it is preferable that the manganese oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component, and that the silicon oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component, so as not to degrade the properties that are achieved when no such materials are added. 
     Furthermore, the piezoelectric ceramic composition according to the present invention may be a solid solution, a mixture, a polycrystal or a single crystal. 
     The above-described objects, other objects, features, and advantages of the present invention will become more apparent from the following detailed explanations of the embodiments when taken in conjunction with the accompanying drawings, tables, and examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing an example of a piezoelectric ceramic resonator according to the present invention; and 
     FIG. 2 is a cross-sectional illustrative view of the piezoelectric ceramic resonator shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the present invention will be described with reference to the following drawings, tables and examples, it is to be understood that the invention is not limited to the precise embodiments described below, and various changes and modifications may be made to various usages and conditions without departing from the spirit and scope of the invention as described in the claims that follow. 
     The piezoelectric ceramic composition according to the present invention can be manufactured in the same way as is applied to manufacturing of conventional ferroelectric materials and dielectric materials. For example, specific amounts of Ag 2 O, Nb 2 O 5 , Ta 2 O 5  and Li 2 CO 3  are first weighed out and blended together in a solvent such as water or ethanol, using zirconia balls or similar media for from 4 to 24 hours. A dispersing agent such as a sorbitan ester may be added for achieving more homogeneous blending. After that, the slurry thus blended is dried and then subjected to calcination in an oxidizing atmosphere at from 800 to 1,100° C. for from 1 to 24 hours using a common electric furnace. The calcined product is crushed and blended with a binder such as polyvinyl alcohol in a solvent such as water or ethanol, using zirconia balls or similar media and then dried. The powder thus obtained by drying is subjected to uniaxial pressing to form square plaque samples having a size of 12 mm in length, 12 mm in width and 3 mm in thickness. The samples are further subjected to baking in an oxidizing atmosphere at from 950 to 1,200° C. for from 3 to 10 hours. Piezoelectric ceramics from the piezoelectric ceramic composition according to the present invention can be manufactured by these operations. The present invention will be further explained, based on the examples below. 
     EXAMPLE 1 
     First, powders of Ag 2 O, Nb2O 5 , Ta 2 O 5  and Li 2 CO 3  were weighed out and blended together to prepare mixtures having specific compositions according to the values for x and y listed in Table 1. The mixtures were then subjected to calcination in an oxidizing atmosphere at from 850 to 1,100° C. for 10 hours, using an electric furnace. The powders thus obtained were crushed and blended with polyvinyl alcohol such that the polyvinyl alcohol was in an amount of 5 parts by weight based on 100 parts by weight of each powder. Then they were dried and subjected to uniaxial pressing (10 ton/cm 2 ) to form square plaque samples having a size of 12 mm in length, 12 mm in width, and about 2.5 mm in thickness. The samples thus obtained were baked in an oxidizing atmosphere at the temperatures listed in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Sample No. 
                 x 
                 y 
                 Baking temperature (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 *1 
                 0.000 
                 0.000 
                 1,040 
               
               
                   
                 **2 
                 0.025 
                 0.000 
                 1,040 
               
               
                   
                 **3 
                 0.050 
                 0.000 
                 1,040 
               
               
                   
                 4 
                 0.075 
                 0.000 
                 1,040 
               
               
                   
                 5 
                 0.100 
                 0.000 
                 1,040 
               
               
                   
                 6 
                 0.125 
                 0.000 
                 1,040 
               
               
                   
                 7 
                 0.150 
                 0.000 
                 1,020 
               
               
                   
                 8 
                 0.200 
                 0.000 
                 1,020 
               
               
                   
                 9 
                 0.300 
                 0.000 
                 1,020 
               
               
                   
                 **10 
                 0.400 
                 0.000 
                 1,020 
               
               
                   
                 11 
                 0.100 
                 0.050 
                 1,020 
               
               
                   
                 12 
                 0.100 
                 0.100 
                 1,040 
               
               
                   
                 **13 
                 0.100 
                 0.200 
                 1,120 
               
               
                   
                 14 
                 0.075 
                 0.050 
                 1,020 
               
               
                   
                 15 
                 0.150 
                 0.100 
                 1,040 
               
               
                   
                 **16 
                 0.400 
                 0.100 
                 1,040 
               
               
                   
                   
               
             
          
         
       
     
     In all of the Tables herein, the asterisk * indicates that the compositions are out of the scope of the present invention and the mark ** indicates that the compositions are out of the scopes of some of the preferred embodiments of the present invention. 
     After that, a Ag paste was applied to the surfaces of the plaque samples, and the samples were baked at 800° C. Then, they were subjected to a polarization treatment by applying a direct-current voltage of from 50 kV/cm to 200 kV/cm in an insulating oil bath at from 100 to 150° C. for from 3 to 10 minutes. Next, square pillars having a size of 2 mm×2 mm×3 mm were cut out using a dicing machine. The samples thus obtained were subjected to measurements of dielectric constant, electromechanical coupling coefficient k 33  for the thickness vibration, piezoelectric constant d 33  for the thickness vibration, resonance frequency constant N for the thickness vibration, and transition temperature. The results are shown in Table 2. From Table 2, it is understood that the compositional range according to the present invention provides a good electromechanical coupling coefficient k 33  of not less than 20%, and a transition temperature of not less than 200° C. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Resonance 
                   
               
               
                   
                   
                 Coupling 
                 Piezoelectric 
                 frequency 
                 Transition 
               
               
                 Sample 
                 Dielectric 
                 coefficient k 33   
                 constant d 33   
                 constant 
                 temperature 
               
               
                 No. 
                 constant 
                 (%) 
                 (pC/N) 
                 (Hz/m) 
                 (° C.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 *1 
                 Measurement 
                 Measurement 
                 Measurement 
                 Measurement 
                 70 
               
               
                   
                 failed 
                 failed 
                 failed 
                 failed 
               
               
                 **2 
                 180 
                 21 
                 18 
                 1,873 
                 70 
               
               
                 **3 
                 194 
                 41 
                 52 
                 2,139 
                 150 
               
               
                 4 
                 253 
                 41 
                 55 
                 2,089 
                 210 
               
               
                 5 
                 304 
                 41 
                 55 
                 2,076 
                 290 
               
               
                 6 
                 284 
                 35 
                 50 
                 2,065 
                 310 
               
               
                 7 
                 263 
                 27 
                 41 
                 2,073 
                 310 
               
               
                 8 
                 242 
                 21 
                 27 
                 2,122 
                 310 
               
               
                 9 
                 232 
                 25 
                 35 
                 2,239 
                 320 
               
               
                 **10 
                 179 
                 22 
                 31 
                 1,956 
                 320 
               
               
                 11 
                 159 
                 44 
                 53 
                 2,141 
                 260 
               
               
                 12 
                 154 
                 45 
                 53 
                 2,149 
                 250 
               
               
                 **13 
                 178 
                 42 
                 50 
                 2,188 
                 160 
               
               
                 14 
                 153 
                 39 
                 43 
                 2,075 
                 230 
               
               
                 15 
                 154 
                 45 
                 53 
                 2,149 
                 210 
               
               
                 **16 
                 132 
                 28 
                 38 
                 2,052 
                 80 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 2 
     In a similar way to that applied to Example 1, powders of Ag 2 O, Nb 2 O 5 , Ta 2 O 5  and Li 2 CO 3  were first weighed out and blended together to prepare mixtures having specific compositions according to the values for x and y listed in Table 3. The mixtures were then subjected to calcination in an oxidizing atmosphere at from 900 to 1,200° C. for 10 hours using an electric furnace. powders of MnCO 3  and/or SiO 2  in an amount shown in Table 3 was added to the powders, and then the powders were blended with polyvinyl alcohol such that the polyvinyl alcohol was in an amount of 5 parts by weight based on 100 parts by weight of each powder. Then they were dried and subjected to uniaxial pressing (10 ton/cm 2 ) to form square plaque samples having a size of 12 mm in length, 12 mm in width and about 2.5 mm in thickness. The samples thus obtained were baked in an oxidizing atmosphere at the temperatures listed in Table 3. After that, a Ag paste was applied to the surfaces of the plaque samples, and the samples were baked at 800° C. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Sam- 
                   
                   
                 Baking 
                 Amount of 
                 Amount of 
               
               
                 ple 
                   
                   
                 temperature 
                 MnCO 3  added 
                 SiO 2  added 
               
               
                 No. 
                 x 
                 y 
                 (° C.) 
                 (parts by weight) 
                 (parts by weight) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  17 
                 0.075 
                 0.000 
                 1,020 
                 0.2 
                 0.0 
               
               
                  18 
                 0.075 
                 0.000 
                 1,000 
                 2.0 
                 0.0 
               
               
                  19 
                 0.100 
                 0.000 
                 1,009 
                 3.0 
                 0.0 
               
               
                  20 
                 0.100 
                 0.000 
                 980 
                 5.0 
                 0.0 
               
               
                 **21 
                 0.150 
                 0.000 
                 980 
                 6.0 
                 0.0 
               
               
                  22 
                 0.075 
                 0.000 
                 1,020 
                 0.0 
                 0.2 
               
               
                  23 
                 0.075 
                 0.000 
                 1,020 
                 0.0 
                 2.0 
               
               
                  24 
                 0.100 
                 0.000 
                 1,000 
                 0.0 
                 3.0 
               
               
                  25 
                 0.100 
                 0.000 
                 980 
                 0.0 
                 5.0 
               
               
                 **26 
                 0.150 
                 0.000 
                 980 
                 0.0 
                 6.0 
               
               
                  27 
                 0.125 
                 0.000 
                 980 
                 0.2 
                 0.2 
               
               
                  28 
                 0.125 
                 0.000 
                 980 
                 3.0 
                 2.0 
               
               
                  29 
                 0.100 
                 0.050 
                 980 
                 3.0 
                 0.0 
               
               
                  30 
                 0.100 
                 0.100 
                 1,000 
                 0.0 
                 2.0 
               
               
                 **31 
                 0.100 
                 0.050 
                 980 
                 6.0 
                 0.0 
               
               
                 **32 
                 0.100 
                 0.100 
                 1,000 
                 0.0 
                 6.0 
               
               
                   
               
             
          
         
       
     
     After that, they were subjected to a polarization treatment by applying a direct-current voltage of from 50 kV/cm to 200 kV/cm in an insulating oil bath at from 100 to 150° C. for from 3 to 10 minutes. Next, square pillars having a size of 2 mm×2 mm×3 mm were cut out, using a dicing machine. The samples thus obtained were subjected to measurements of dielectric constant, electromechanical coupling coefficient k 33  for the thickness vibration, piezoelectric constant d 33  for the thickness vibration, resonance frequency constant N for the thickness vibration, and transition temperature. The results are shown in Table 4. As shown in Table 4, by adding MnCO 3  and/or SiO 2 , it is possible to obtain a piezoelectric ceramic composition having a good electromechanical coupling coefficient k 33  of not less than 20%, which is equivalent to that of a sample without the addition of such a material, having a transition temperature of not less than 200° C. and having a low baking temperature. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                   
                 Resonance 
                   
               
               
                   
                   
                 Coupling 
                 Piezoelectric 
                 frequency 
                 Transition 
               
               
                 Sample 
                 Dielectric 
                 coefficient k 33   
                 constant d 33   
                 constant 
                 temperature 
               
               
                 No. 
                 constant 
                 (%) 
                 (pC/N) 
                 (Hz/m) 
                 (° C.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 17 
                 259 
                 40 
                 53 
                 2,053 
                 220 
               
               
                 18 
                 263 
                 42 
                 57 
                 2,073 
                 215 
               
               
                 19 
                 310 
                 40 
                 54 
                 2,081 
                 290 
               
               
                 20 
                 313 
                 43 
                 55 
                 2,075 
                 295 
               
               
                 **21 
                 292 
                 22 
                 34 
                 1,756 
                 305 
               
               
                 22 
                 255 
                 43 
                 57 
                 2,016 
                 210 
               
               
                 23 
                 258 
                 41 
                 58 
                 2,063 
                 210 
               
               
                 24 
                 316 
                 45 
                 56 
                 2,066 
                 295 
               
               
                 25 
                 314 
                 42 
                 53 
                 2,032 
                 290 
               
               
                 **26 
                 285 
                 23 
                 36 
                 1,873 
                 300 
               
               
                 27 
                 294 
                 38 
                 49 
                 2,073 
                 315 
               
               
                 28 
                 296 
                 39 
                 53 
                 2,072 
                 310 
               
               
                 29 
                 175 
                 42 
                 47 
                 2,103 
                 255 
               
               
                 30 
                 163 
                 41 
                 46 
                 2,089 
                 245 
               
               
                 **31 
                 186 
                 32 
                 42 
                 1,898 
                 255 
               
               
                 **32 
                 177 
                 34 
                 40 
                 1,946 
                 250 
               
               
                   
               
             
          
         
       
     
     FIG. 1 is a perspective view showing an example of a piezoelectric ceramic element according to the present invention. FIG. 2 is a cross-sectional view thereof. The piezoelectric ceramic element shown in FIGS. 1 and 2 is a piezoelectric ceramic resonatortor  10 . The piezoelectric ceramic resonator  10  comprises a piezoelectric ceramic  12  in a cubic shape, for example. The piezoelectric ceramic  12  comprises two sheets of piezoelectric ceramic layers  12   a  and  12   b . These piezoelectric ceramic layers  12   a  and  12   b  are composed of a piezoelectric ceramic composition according to the above-described present invention, and are laminated and consolidated to form a unitary structure. These piezoelectric ceramic layers  12   a  and  12   b  are polarized along the same thickness direction as shown by the arrows in FIG.  2 . 
     A vibrating electrode  14   a  in a circular shape, for example, is placed in between the piezoelectric ceramic layers  12   a  and  12   b , and at the center of the interface. A lead electrode  16   a  in a T shape, for example, is formed connecting the vibrating electrode  14   a  and one edge surface of the piezoelectric ceramic  12 . A vibrating electrode  14   b  in a circular shape, for example, is placed at the center of the surface of the piezoelectric ceramic layer  12   a . A lead electrode  16   b  in a T shape, for example, is formed connecting the vibrating electrode  14   b  and the other edge surface of the piezoelectric ceramic  12 . Furthermore, a vibrating electrode  14   c  in a circular shape, for example, is placed at the center of the surface of the piezoelectric ceramic layer  12   b . A lead electrode  16   c  in a T shape, for example, is formed connecting the vibrating electrode  14   c  and the other edge surface of the piezoelectric ceramic  12 . 
     Thereafter, the lead electrode  16   a  is connected to an external electrode  20   a  via a lead wire  18   a , and the lead electrode  16   b  and  16   c  are connected to another external electrode  20   b  via another lead wire  18   b.    
     Hereupon, it is to be noted that the present invention is also applicable to various piezoelectric ceramic resonators other than the above-described piezoelectric ceramic resonator  10  as well as other piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators. 
     According to the present invention, a piezoelectric ceramic composition can be obtained, which provides a large (not less than about 20%) electromechanical coupling coefficient k 33  in comparison with a bismuth layered compound, without containing Pb, and can therefore be of practical use. Furthermore, by adding at least one of a manganese oxide and a silicon oxide as an auxiliary component to the main component of the piezoelectric ceramic composition according to the present invention, a lower baking temperature can be realized, without degrading various properties such as dielectric constant, electromechanical coupling coefficient k 33  for the thickness vibration, piezoelectric constant d 33  for the thickness vibration, resonance frequency constant N for the thickness vibration and transition temperature.