Piezoelectric ceramic compositions

Abstract

The present invention provides a piezoelectric ceramic composition which contains no lead and has piezoelectric characteristics suitable for practical use. The piezo-electric ceramic composition has a complex perovskite structure represented by the formula: (1−x−y−z)BaTiO3—x(Bi½Na½) TiO3—yCaTiO3—zBa(Zn⅓Nb⅔)O3 and comprises Ba, Ti, O, Bi, Na, Ca, Zn and Nb in the proportion satisfying the following conditions of x, y and z:

Description

FIELD OF THE INVENTION

[0001] The present invention relates to piezoelectric ceramic compositions for use in piezoelectric ceramic elements, and particularly to piezoelectric ceramic compositions for use in materials for piezoelectric resonators and piezo vibrators, and piezoelectric elements for use in sensors, actuators, buzzers, etc.

BACKGROUND OF THE INVENTION

[0002] Ceramic compositions comprising lead zirconate titanate (PZT) or lead titanate (PT) as main components are widely used as piezoelectric ceramic compositions for use in piezoelectric ceramic elements of piezoelectric ceramic vibrators, piezoelectric ceramic filters, piezoelectric buzzers, piezoelectric sensors and actuators. To improve the ceramic characteristics of the above compositions, traces of additives such as Mn, etc. are added, or a part of Pb is substituted with Sr or a composite oxide such as Pb(Mg⅓Nb⅔)O3 so as to obtain a piezoelectric ceramic having desired piezoelectric characteristics.

[0003] On the other hand, as for piezoelectric ceramic compositions containing no lead, (Na,K)NbO3, Bi4Ti3O12, etc. are known as ferroelectrics capable of exhibiting piezoelectricity, which, however, have not achieved sufficient properties in view of performance in practical use, and thus, are examined dominantly for use in sensors and actuators to be used under high temperature conditions.

[0004] The typical piezoelectric ceramic compositions such as lead zirconate titanate, etc., mainly used at present, contain Pb in large amounts which have adverse influences on the global environment. As more and more public attentions recently have been attracted to the environmental problems, the dumping of the waste of these materials induces serious problems from a view point of environmental protection. Under these circumstances, development of a material which contains no lead and has piezoelectric characteristics suitable for practical use is desired. However, at the present, there has been provided no piezoelectric ceramic composition that has piezoelectric characteristics higher than those of PZT type ceramics and PT type ceramics both of which contain Pb. Especially, a ceramic composition comprising barium titanate as a main component has been used in a restricted application such as Langevin type ultrasonic resonator or the like. However, ceramic compositions of this type are not suitable for practical use in general-purpose sensors and actuators. This is because, in case where such a composition is used for the above sensors and actuators, heating is needed to bond and set hard for junction, and because the piezoelectric ceramic composition can not be heated at a temperature higher than a Curie temperature at which its ferroelectricity is lost.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is therefore to provide a piezoelectric ceramic composition which contains no lead and has piezoelectric characteristics suitable for practical use and thereby to solve the foregoing problems.

[0006] A piezoelectric ceramic composition according to the present invention has a complex perovskite structure represented by the formula:

(1−x−y−z)BaTiO3—x(Bi½Na½)TiO3—yCaTiO3—zBa(Zn⅓Nb⅔)O3  (1)

[0007] and the composition comprises Ba, Ti, O, Bi, Na, Ca, Zn and Nb in the proportion satisfying the following conditions of x, y and z in the formula:

0.0<x≦0.90,

0.0≦y≦0.20, and

0.0≦z≦0.05,  (2)

[0008] with the proviso that y and z are not zero at the same time (the condition of y=z=0 is excluded).

[0009] The piezoelectric ceramic composition of the present invention mentioned above contains no lead, and thus causes no environmental problem. In addition, it has a relatively large electromechanical coupling coefficient and therefore is suitably used for piezoelectric ceramic elements of piezoelectric ceramic filters, piezoelectric ceramic resonators, piezoelectric sensors, piezoelectric actuators, etc. Further, since the above piezoelectric ceramic composition has heat resistance (thermal stability) to heat of not lower than about 150° C., it becomes possible to construct a piezoelectric element using adhesive having a setting temperature of 150° C. In this regard, the above numerical range of x extrudes 0 and 1.0, because the single use of a known simple BatiO3 or (Bi½Na½)TiO3 is hard to provide a piezoelectric ceramic composition having heat resistance (thermal stability) sufficient for practical use. While the piezoelectric ceramic compositions of the present invention are represented by the formula (1), such compositions include not only compositions comprising independent phases of BaTiO3, (Bi½Na½)TiO3, CaTiO3 and Ba(Zn⅓Nb⅔)03, but also compositions of the above formula in which, at least, the respective atoms, Ba, Ti, O, Bi, Na, Ca, Zn and Nb are contained in the proportion satisfying the above conditions of numerical ranges (2) and in which these atoms constitutes a perovskite structure represented by ABO3.

[0010] It is preferable that the piezoelectric ceramic composition comprises MnO2 as a sub-component at a content of 0.01 to 1.0 wt. %. This is because, out of piezoelectric characteristics, particularly a mechanical quality factor (the acuteness of mechanical vibration at a resonance frequency) can be improved.

[0011] Also, it is preferable that the piezoelectric ceramic composition comprises a rare earth oxide such as Y2O3 as a sub-component at a content of 0.01 to 1.0 wt. %. This is because the time dependent change (a change with the passage of time) in capacity can be decreased, which leads to an improvement on the stability of piezoelectric characteristics with time.

[0012] The addition of the sub-components, that is, MnO2 and a rare earth oxide such as Y2O3 is effective to further improve the piezoelectric characteristics, and it is preferable that any of such sub-components should be added in an amount of not more than 1.0 wt. %, because the addition of not less than 1.0 wt. % of such a sub-component has a danger of degrading the characteristics as piezoelectric ceramics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a sectional view of an actuator (10) comprising a piezoelectric ceramic composition according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] With reference to FIG. 1, the actuator (10) comprises a piezoelectric ceramics (1) having electrodes (3a) and (3b) formed on both side thereof, and a piezoelectric ceramics (2) having electrodes (4a) and (4b) formed on both side thereof. Any of the piezoelectric ceramics (1) and (2) is formed from a piezoelectric ceramic composition according to the present invention. The electrode (3b) formed on the piezoelectric ceramics (1) is fixed to the electrode (4a) formed on the piezoelectric ceramics (2) with an adhesive to constitute an integrated body for forming the actuator (10). The piezoelectric ceramic composition of the present invention is described below.

[0015] The piezoelectric ceramic composition of the present invention has a complex perovskite structure (ABO3) based on BaTiO3. The piezoelectric ceramic composition of the present invention is represented by the formula:

(1−x−y−z) BaTiO3—x(Bi½Na½) TiO3—yCaTiO3—zBa(Zn⅓Nb⅔)O3  (1)

[0016] wherein the values of x, y and z are within the following ranges, respectively:

0.0<x≦0.90,

0.0≦y≦0.20, and

0.0≦z≦0.05  (2)

[0017] (with the proviso that y and z are not zero at the same time (the condition of y=z=0 is excluded)). In the perovskite structure represented by the formula of ABO31 the site A is a position on which Ba, Bi, Na and Ca can be present, and the site B is a position on which Ti, Zn and Nb can be present. The above piezoelectric ceramic composition of the present invention contains no lead and thus has no danger to cause the environmental problems.

[0018] It is difficult to raise the firing temperature of a pure BaTiO3 because its firing temperature range is narrow. As a result, the obtained ceramics can not have a sufficient sintered density, which causes a problem in that, when this ceramics is subjected to a polarization treatment so as to be functioned as a piezoelectric material, the yield thereof disadvantageously tends to decrease. This is because the ceramics can not withstand a high electric field applied in the polarization treatment. On the other hand, the piezoelectric ceramic composition of the present invention prepared by substituting the site A or the site B with an element which less degrades the characteristics can be fired within a wider temperature range, which makes it possible to fire the composition at a temperature which provides a highest sintered density. Thus, the resultant ceramics can have a higher sintered density. Accordingly, the use of the piezoelectric ceramic composition of the present invention makes it possible to achieve high voltage resistance. Therefore, the piezoelectric ceramic composition of the present invention is suitable as a material of piezo resonators and piezo vibrators for use in sensors, actuators and vibrators, in view of its wider firing temperature range, and the construction of piezoelectric elements becomes easy by using the piezoelectric ceramic composition of the present invention.

[0019] In addition, the conventional piezoelectric ceramic composition composed of BaTiO3 has a relatively low Curie temperature (127° C.) being a critical temperature at which a material loses piezoelectricity, and this composition has not sufficient heat resistance (thermal stability) in this view of point. On the other hand, the piezoelectric ceramic composition of the present invention is prepared by adding (Bi½Na½)TiO3 to BaTiO3, and therefore can have a higher Curie temperature and also can have heat resistance (thermal stability) to heat of not lower than about 150° C. Therefore, it becomes possible to construct a piezoelectric element using adhesive having a setting temperature of about 150° C., and even in such a case, the piezoelectric ceramic composition of the present invention does not degrade.

[0020] When the piezoelectric ceramic composition of the present invention further contains CaTiO3, a more stable perovskite crystalline structure can be formed. This is because, as described above, the lower limit of the firing temperature range is decreased by substituting the site A with Ca to thereby widen the firing temperature range. Also, by adding Ba(Zn⅓Nb⅔) O3, the dielectric constant of the composition can be raised, which contributes to miniatuarization of a piezoelectric element.

[0021] Since the piezoelectric ceramic composition, represented by the above formula, of the present invention has a relatively large electromechanical coupling coefficient (a square root of a rate of conversion of a given electrical input into a mechanical output, which indicates a capacity of electric-mechanical conversion), the piezoelectric ceramic composition of the present invention can be practically used for piezoelectric ceramic filters, piezoelectric ceramic vibrators, piezoelectric sensors, piezoelectric actuators, etc.

[0022] Further, in case where the above piezoelectric ceramic composition contains 0.01 to 1.0 wt. % of MnO2, paricularly the mechanical quality factor out of the piezoelectric characteristics can be improved, and thus, such a composition can provide excellent piezoelectric characteristics as a piezoelectric ceramics for use in sensors.

[0023] Furthermore, in case where the above piezoelectric ceramic composition contains 0.01 to 1.0 wt. % of a rare earth oxide such as Y2O3, the rate of change in capacity with time can be decreased, and thus, such a composition can provide improved stability of piezoelectric characteristics with time.

EXAMPLES

[0024] Hereinafter, Examples of the present invention will be described in more detail. Piezoelectric ceramic compositions according to the present invention, however, are not limited to the formulations of the samples described in the following Examples, and they can provide similar effects in so far as their formulations are changed within the above-mentioned ranges (2).

Example 1

[0025] A piezoelectric ceramic composition according to Example 1 was prepared as follows. First, BaCO3, Bi2O3, NaCO3, CaCO3, TiO2 and MnCO3 as starting raw materials were weighed so that a finally obtained piezoelectric ceramics could have a given formulation ratio indicated in Table 1, and they were homogeneously mixed in a ball mill. Next, the resultant composition was calcined at a temperature of 800 to 1,000° C. for 2 hours to form a compound. The compound was ground again in the ball mill, and polyvinyl alcohol was added to the particles of the compound, and the mixture was granulated. The granulated powder was subjected to uniaxial compression molding under a pressure of 70 MPa to form a disc with a diameter of 13 mm, which was then fired at a temperature of 1,000 to 1,400° C. for 2 hours.

[0026] This disc sample was polished to a thickness of 0.3 mm, and Ag was baked onto the disc sample at 700° C. to form an electrode thereon. After that, the disc sample was subjected to a polarization treatment by applying an electric field of 3 to 5 KV/mm thereto in a silicone oil of 80° C. for 30 minutes. Piezoelectric ceramics of Sample Nos. 1 to 7 and those of Comparative Examples 1 to 4 were obtained in this manner.

[0027] The specific dielectric constants &egr;r, tan &dgr;, coupling coefficients kp in disc radial direction spreading vibration mode, coupling coefficients kt in disc thickness direction vertical vibration mode, mechanical quality factors QM and piezoelectric constants d31 of the piezoelectric ceramics of Sample Nos. 1 to 7 and Comparative Example 1 to 4 were determined, and Curie points Tc thereof at which the piezoelectricity of the piezoelectric ceramics was lost were determined based on a change in dielectric constant depending on temperature, by finding temperatures at which the dielectric constants became maximal values. The results are shown in Tables 1 and 2. In this connection, the values included in the parentheses on the column showing the Curie points Tc indicate transformation points into AF phases (antiferroelectricity). Since piezoelectricity is lost at this transformation point, the transformation point can be used as a criterion of heat resistance (thermal stability) as well as a Curie point Tc.

[0028] For comparison, a sample of a pure barium titanate used in a conventional piezoelectric ceramic composition (Comparative Example 1), samples of a pure sodium bismuthate titanate (Comparative Examples 2 and 3), and a sample of a composition of the formula in which x was zero (Comparative Example 4) were prepared, and the measurement results thereof are also shown in Tables 1 and 2. 1 TABLE 1 BaTiO3 (Bi1/2Na1/2) TiO3 CaTiO3 MnO2 1-x-y-z x y (wt. %) Sample 1 0.05 0.90 0.05 0.5 Sample 2 0.90 0.05 0.05 0.0 Sample 3 0.85 0.10 0.05 0.0 Sample 4 0.80 0.10 0.10 0.0 Sample 5 0.70 0.10 0.20 0.0 Sample 6 0.70 0.20 0.10 0.0 Sample 7 0.60 0.20 0.20 0.0 Com. Ex. 1 1.00 0.00 0.00 0.0 Com. Ex. 2 0.00 1.00 0.00 0.0 Com. Ex. 3 0.00 1.00 0.00 0.5 Com. Ex. 4 0.95 0.00 0.05 0.0

[0029] 2 TABLE 2 d31 (E-11 Tc &egr;r tan &xgr; kp kt QM m/V) (° C.) Sample 1 668 0.015 0.138 0.459 375 1.89 (180) Sample 2 481 0.027 0.217 0.451 124 3.22 154 Sample 3 505 0.023 0.211 0.327 110 3.45 183 Sample 4 512 0.021 0.205 0.343 132 3.24 181 Sample 5 455 0.023 0.205 0.351 149 2.2 157 Sample 6 442 0.025 0.188 0.357 137 1.69 165 Sample 7 406 0.025 0.171 0.366 130 1.6 167 Com. Ex. 1 1190 0.026 0.201 0.321 112 3.18 127 Com. Ex. 2 402 0.022 0.185 0.441 113 2.08 (190) Com. Ex. 3 335 0.02 0.130 0.438 816 1.4 (190) Com. Ex. 4 1065 0.048 0.197 0.445 111 3.5 139

[0030] It was known from Tables 1 and 2 that the piezoelectric ceramics of Sample Nos. 1 to 7 had Curie points, as criteria of heat resistance (thermal stability), of not lower than 150° C. and sufficiently large electromechanical coupling coefficients kp and kt. In contrast, the piezoelectric ceramics of Comparative Example 4 had a Curie point of lower than 150° C. Accordingly, the piezoelectric ceramic composition of Comparative Example 4 lost peizoelectricity at a temperature of lower than 150° C. and thus was found to have insufficient heat resistance (thermal stability) as well as the sample of the pure barium titanate (Comparative Example 1).

[0031] While the piezoelectric ceramics of Comparative Example 1 could have a relative sintered density of 96% only within a firing temperature range of 1,5000°±15° C., the piezoelectric ceramics of, for example, Sample No. 1 as a substituted solid solution containing a different element had a relative sintered density of not lower than 96.5% within a baking temperature range of 1,330°±30° C.

[0032] As has been understood from the above facts, it becomes easy to form the piezoelectric ceramics of Sample Nos. 1 to 7 which were the solid solutions of (Bi½Na½)TiO3 and CaTiO3 in BaTiO3 and which contained no lead, within wider firing temperature ranges. In addition, it was known that the piezoelectric ceramics of Sample Nos. 1 to 7 of the present Example had sufficient heat resistance (thermal stability) and electromechanical coupling coefficients kp and kt. Accordingly, as in the piezoelectric ceramics of Sample Nos. 1 to 7, the piezoelectric ceramic compositions of the formula (1) in which the values of x and y are 0.0<x≦0.90 and 0.0≦y≦0.20, respectively, have sufficient heat resistance (thermal stability) and sufficient electromechanical coupling coefficients kp and kt and therefore can be suitably used for piezoelectric elements in a variety of applications. Furthermore, the piezoelectric ceramics of Sample No. 1 which contained MnO2 had an increased mechanical quality factor QM as shown in Tables 1 and 2 and therefore can be suitably used for particularly sensors and actuators which are required to be constructed using a material having a large mechanical quality factor QM.

Example 2

[0033] A piezoelectric ceramic composition according to Example 2 was prepared as follows. First, BaCO3, Bi2O3, NaCo3, CaCO3, TiO2, ZnO, Nb2O5 and MnCO3 as starting raw materials were weighed so that a finally obtained piezoelectric ceramics could have a formulation ratio indicated in Table 3, and they were homogeneously mixed in a ball mill. Next, the resultant composition was calcined at a temperature of 800 to 1,000° C. for 2 hours to form a compound. The compound was ground again in the ball mill, and polyvinyl alcohol was added to the grains of the compound, and the mixture was granulated. The resultant powder was subjected to uniaxial compression molding under a pressure of 70 MPa to form a disc with a diameter of 13 mm, which was then baked at a temperature of 1,200 to 1,4000° C. for 2 hours. This disc sample was polished to a thickness of 0.3 mm, and Ag was baked onto the disc sample at 700° C. to form an electrode thereon. After that, the disc sample was subjected to a polarization treatment by applying an electric field of 3 to 5 KV/mm thereto in a silicone oil of 80° C. for 30 minutes. Piezoelectric ceramics of Sample Nos. 8 to 14 were obtained in this manner.

[0034] The specific dielectric constants &egr;r, tan &dgr;, coupling coefficients kp in disc radial direction spreading vibration mode, coupling coefficients kt in disc thickness direction vertical vibration mode, mechanical quality factors QM, piezoelectric constants d31, and Curie points Tc of the piezoelectric ceramics of Sample Nos. 8 to 14 were determined in the same manner as in Example 1. The measurement results thereof are shown in Table 4. 3 TABLE 3 (Bi½Na½) Ba (Zn⅓Nb⅔) BaTiO3 TiO3 CaTiO3 O3 MnO2 1-x-y-z x y z (wt. %) Sample 8 0.84 0.10 0.05 0.01 0.0 Sample 9 0.49 0.50 0.00 0.01 0.0 Sample 10 0.14 0.80 0.05 0.01 0.0 Sample 11 0.14 0.85 0.00 0.01 0.0 Sample 12 0.14 0.85 0.00 0.01 1.0 Sample 13 0.73 0.15 0.10 0.02 0.0 Sample 14 0.78 0.20 0.00 0.02 0.0 Com. Ex. 5 0.98 0.00 0.00 0.02 0.0 Com. Ex. 6 0.93 0.00 0.05 0.02 0.0 Com. Ex. 7 0.78 0.00 0.20 0.02 0.0 Com. Ex. 8 0.60 0.20 0.15 0.05 0.0

[0035] 4 TABLE 4 d31 (E-11 Tc &egr;r tan &xgr; kp kt QM m/V) (° C.) Sample 8 468 0.023 0.176 0.345 117 2.14 150 Sample 9 453 0.024 0.153 0.353 89 1.58 223 Sample 10 793 0.025 0.195 0.428 83 2.42 238 Sample 11 836 0.023 0.198 0.435 87 2.44 244 Sample 12 634 0.013 0.126 0.442 376 1.82 245 Sample 13 461 0.023 0.216 0.332 120 2.28 155 Sample 14 436 0.024 0.207 0.321 132 2.03 166 Com. Ex. 5 2700 0.025 — — — — 96 Com. Ex. 6 2414 0.024 0.165 0.238 189 4.6 102 Com. Ex. 7 1579 0.103 0.182 0.279 136 3.6 115 Com. Ex. 8 1075 0.033 0.188 0.329 134 3.27 123

[0036] It was known from Tables 3 and 4 that the piezoelectric ceramics of Sample Nos. 8 to 14 had Curie points of not lower than 150° C. and higher heat resistance (thermal stability) than those of the piezoelectric ceramics of Comparative Examples 5 to 8, and also had relatively large electromechanical coupling coefficents kp and kt. The piezoelectric ceramics of Comparative Example 8{z (content of Ba(Zn⅓Nb⅔)O3) is set 0.05} had an excessively lowered Curie point, and therefore was unsuitable for use.

[0037] As is understood from the above results, the piezoelectric ceramics, containing no lead, of Sample Nos. 8 to 14, which were formed as solid solutions of (Bi½Na½)TiO3, CaTiO3 and Ba(Zn⅓Nb⅔)O3 in BaTiO3, had sufficient heat resistance (thermal stability) and sufficient electromechanical coupling coefficients kp and kt. Therefore, the piezoelectric ceramics of Sample Nos. 8 to 14 of the present Example can be suitably used for various piezoelectric elements because the piezoelectric ceramic compositions of the formula (1) in which the values of x, y and z are 0.0<x≦0.90, 0.0≦y≦0.20 and 0.0≦z<0.05, respectively, have sufficient heat resistance (thermal stability) and sufficient electromechanical coupling coefficients kp and kt.

[0038] In addition, the piezoelectric ceramics of Sample No. 12 containing 1.0 wt. % of MnO2 had an increased mechanical quality factor QM as shown in Table 4 and therefore can be suitably used for particularly sensors and actuators which are required to be constructed using a material having a large mechanical quality factor QM.

Example 3

[0039] A piezoelectric ceramic composition according to Example 3 was prepared as follows. First, BaCO3, Bi2O3, NaCo3, CaCO3, TiO2, MnCO3, Y2O3, Dy2O3, Ho2O3 and Yb2O3 as starting raw materials were weighed so that a finally obtained piezoelectric ceramics could have a formulation ratio indicated in Table 5, and they were homogeneously mixed in a ball mill. Next, the resultant composition was calcined at a temperature of 800 to 1,000° C. for 2 hours to form a compound. The compound was ground again in the ball mill, and polyvinyl alcohol was added to the grains of the compound, and the mixture was granulated. The resultant powder was subjected to uniaxial compression molding under a pressure of 70 MPa to form a disc with a diameter of 13 mm, which was then baked at a temperature of 1,200 to 1,400° C. for 2 hours.

[0040] This disc sample was polished to a thickness of 0.3 mm, and Ag was baked onto the disc sample at 700° C. to form an electrode thereon. After that, the disc sample was subjected to a polarization treatment by applying an electric field of 3 to 5 KV/mm thereto in a silicone oil of 80° C. for 30 minutes. Piezoelectric ceramics of Sample Nos. 15 to 20 were obtained in this manner.

[0041] The specific dielectric constants &egr;r, tan &dgr;, coupling coefficients kp in disc radial direction spreading vibration mode, coupling coefficients kt in disc thickness direction vertical vibration mode, mechanical quality factors QM and rates of change in capacity found after 1,000 hours had passed, of the piezoelectric ceramics of Sample Nos. 15 to 20 were determined in the same manner as in Example 1. The measurement results thereof are shown in Table 6. 5 TABLE 5 (Bi½Na½) Rare earth oxide BaTiO3 TiO3 CaTiO3 MnO2 Amount 1-x-y-z x y (wt. %) Type (wt. %) Sample 15 0.10 0.85 0.05 0.50 — 0.0 Sample 16 0.10 0.85 0.05 0.50 Y2O3 0.5 Sample 17 0.10 0.85 0.05 0.50 Y2O3 1.0 Sample 18 0.10 0.85 0.05 0.50 Dy2O3 0.5 Sample 19 0.10 0.85 0.05 0.50 Ho2O3 0.5 Sample 20 0.10 0.85 0.05 0.50 Yb2O3 0.5 Com. Ex. 0.10 0.85 0.05 0.50 Y2O3 1.5 9

[0042] 6 TABLE 6 Rate of change in capacity after &egr;r tan &xgr; kp kt QM 1000 hr. (%) Sample 15 617 0.012 0.143 0.446 398 −0.98 Sample 16 634 0.012 0.142 0.435 427 −0.53 Sample 17 637 0.015 0.126 0.436 404 −0.48 Sample 18 627 0.012 0.138 0.433 425 −0.50 Sample 19 627 0.012 0.139 0.436 415 −0.53 Sample 20 624 0.012 0.140 0.435 410 −0.50 Com. Ex. 9 654 0.045 0.050 0.165 40 −0.58

[0043] It was known from Tables 5 and 6 that, when the piezoelectric ceramics of Sample Nos. 16 to 20 and that of Sample No. 15 were compared in view of the rate of change in capacity after 1,000 hours had passed, the addition of the rare earth oxides were effective to lower the rate of change in capacity with time, as seen in the piezoelectric ceramics of Sample Nos. 16 to 20. However, as seen in the piezoelectric ceramics of Comparative Example 9 which contained the rare earth oxide, if the amount of the rare earth oxide was 1.5 wt. %, the resultant piezoelectric ceramics had decreased coupling coefficients kp and kt and a decreased mechanical quality factor QM. In contrast, since the piezoelectric ceramics of Sample Nos. 16 to 20 contained not more than 1.0 wt. % of rare earth oxides, the mechanical quality factors QM improved by the addition of MnO2 were not lowered. In this connection, although not shown in Table 6, any of the Curie points of the piezoelectric ceramics of Sample Nos. 15 to 20 was not lower than 150° C.

[0044] Some electric materials for use in sensors are required to have large mechanical quality factors QM and also have stability against changes with time. In view of these requirements, the use of the piezoelectric ceramics of Sample Nos. 16 to 20 of the present Example 3 which contained 1.0 wt. % or less of the rare earth oxides in addition to 0.5 wt. % of MnO2 is effective to provide especially practical sensors and actuators.

[0045] As has been described above, the piezoelectric ceramic compositions according to the present invention contain no lead which has induced the environmental problems so far, and have relatively large electromechanical coupling coefficients and sufficient heat resistance (thermal stability). Therefore, these piezoelectric ceramic compositions are useful, for example, as materials for piezoelectric elements of sensors, actuators, piezoelectric vibrators, filters, etc.

Claims

1. A piezoelectric ceramic composition having a complex perovskite structure represented by the formula: (1−x−y−z) BaTiO3—x(Bi½Na½) TiO3—yCaTiO3—zBa(Zn⅓Nb⅔)O3 wherein said composition comprises Ba, Ti, O, Bi, Na, Ca, Zn and Nb in the proportion satisfying the following conditions of x, y and z:

0.0<x≦0.90, 0.0≦y≦0.20, and 0.0≦z≦0.05,

with the proviso that y and z are not zero at the same time (the condition of y=z=0 is excluded).

2. A piezoelectric ceramic composition according to claim 1, which comprises 0.01 wt. % to 1.0 wt. % of MnO2.

3. A piezoelectric ceramic composition according to claim 1 or 2, which comprises 0.01 wt. % to 1.0 wt. % of a rare earth oxide.