Piezoelectric ceramic

Abstract

A piezoelectric ceramic having a basic composition expressed by a ternary PbTiO.sub.3 O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution, wherein part of the Pb is substituted by Sr, the Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount of more than 42 mol % of the ternary solid solution, and the ceramic contains Mn.sub.2 O.sub.3 in an amount of 0.01 wt % or more and less than 0.5 wt %. The piezoelectric ceramic provides a material for a piezoelectric device which operates without substantially varying in output, and thereby mitigates the load on the drive circuit, under a large temperature variation.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric ceramic suitable as a material for actuators.

2. Description of the Related Art

The piezoelectric ceramics having a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution in which part of the Pb is substituted by Ba, Sr, etc., provide good response and are utilized as piezoelectric elements in various actuators. The ternary solid solution has proportions of the three components and an amount of the substituted Pb suitable to provide the desired piezoelectric characteristics.

Japanese Unexamined Patent Publication (Kokai) No. 57-208183 discloses a piezoelectric ceramic having a composition modified from the basic composition by adding trace amounts of a metal oxide additive, such as MgO, Sb.sub.2 O.sub.3, etc., to provide improved mechanical properties.

U.S. Pat. No. 4,882,079 discloses another piezoelectric ceramic having a composition modified from the basic composition by adding trace amounts of a metal oxide additive such as Nb.sub.2 O.sub.5, WO.sub.3, La.sub.2 O.sub.3, etc., to provide improved piezoelectric characteristics.

The above-recited conventional piezoelectric ceramics have a problem in that, when they are used as actuator elements in automobile equipment and subjected to large temperature variations, the dielectric constants of the ceramic vary as the temperature varies, causing both the current supply to a drive circuit and the rise time of the applied voltage pulse to vary with the temperature. The drive circuit must be controlled to cope with this problem.

It is therefore desired that the temperature-caused (or temperature induced) change in the dielectric constant be reduced to mitigate the load on the drive circuit.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a piezoelectric ceramic having a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution and providing a reduced temperature-caused change in the dielectric constant.

To achieve the object according to the first aspect of the present invention, there is provided a piezoelectric ceramic having a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution, wherein part of the Pb is substituted by Sr, the PbTiO.sub.3 component is present in an amount of more than 42 mol % of the ternary solid solution, and the ceramic contains Mn.sub.2 O.sub.3 in an amount of 0.01 wt % or more and less than 0.5 wt %.

There is also provided, according to the second aspect of the present invention, a piezoelectric ceramic having a basic composition expressed by a ternary PbTiO.sub.3 component --PbTiO.sub.3 --PbZrO.sub.3 solid solution, wherein the Pb is partially substituted by Sr, the Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount of more than 42 mol % of the ternary solid solution, and the ceramic contains Nb.sub.2 O.sub.5 in an amount of less than 2 wt % and Mn.sub.2 O.sub.3 in an amount of 0.01 wt % or more and less than 0.5 wt %.

The present invention reduces the temperature-caused change in the electrostatic capacity by adding a trace amount of Mn.sub.2 O.sub.3 to a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution in which part of the Pb is substituted by Sr. The Mn.sub.2 O.sub.3 content of the ceramic must be 0.01 wt % or more and less than 0.5 wt %. Mn.sub.2 O.sub.3 must be present in an amount of 0.01 wt % or more to ensure the reduction of the temperature-caused change in the electrostatic capacity. The temperature-caused change in the electrostatic capacity becomes smaller as the Mn.sub.2 O.sub.3 content is increased. The Mn.sub.2 O.sub.3 content, however, must be less than 0.5 wt % because the temperature-caused change in the electrostatic capacity is increased again if the Mn.sub.2 O.sub.3 content is 0.5 wt % or more.

The basic composition of the present invention is a ternary solid solution defined by the formula:

xPb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --yPbTiO.sub.3 --zPbZrO.sub.3, in which the proportions of the three components, i.e., the values of x, y, and z, are adjusted in accordance with desired piezoelectric and mechanical properties. The y-value, i.e., the amount of PbTiO.sub.3, must be more than 42 mol % to ensure the effectiveness of the addition of Mn.sub.2 O.sub.3. To minimize the temperature-caused change in the electrostatic capacity, the ternary solid solution is preferably composed of Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 in an amount of 0.5 mol % or more and 3 mol % or less (i.e., 0.5.ltoreq..times..ltoreq.3 mol %), PbTiO.sub.3 in an amount of more than 42 mol % and less than 50 mol % (i.e., 42<y<50 mol %), and PbZrO.sub.3 in an amount of more than 47 mol % and 57.5 mol % or less (i.e., 47<z.ltoreq.57 mol %). More preferably, Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount of 0.5 mol % or more and less than 3 mol % (i.e., 0.5.ltoreq..times.<3 mol %). The preferred composition also provides improved piezoelectric properties such as the piezoelectric constant.

In the ternary solid solution, the Pb amount substituted by Sr is not necessarily limited. The substituted Pb amount is preferably 15 mol % or less, because the Curie temperature sharply drops when more than 15 mol % of Pb is substituted by Sr. On the other hand, when the Pb amount substituted by Sr is less than 5 mol %, the Curie temperature is too high to provide good piezoelectric properties.

According to the second aspect of the present invention, Nb.sub.2 O.sub.5 is present to provide an increased displacement in a piezoelectric device and an increased mechanical strength. The Nb.sub.2 O.sub.5 content must be less than 2 wt % to ensure good piezoelectric properties. The Nb.sub.2 O.sub.5 content is preferably 1 wt % or less.

The piezoelectric ceramic of the present invention is produced by a conventional powder metallurgy process. The starting powder materials are weighed, mixed, and pre-fired to provide a desired chemical composition of the final product ceramic. The pre-fired material is then ground and granulated to form a powder, which is then compacted and fired. The preferred starting powder materials are provided in the form of a metal oxide such as PbO, Y.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2, Nb.sub.2 O.sub.5, Mn.sub.2 O.sub.3, etc.

The present invention is based on a finding that a piezoelectric ceramic having a basic composition expressed. by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution, when containing a specific amount of Mn.sub.2 O.sub.3, exhibits a reduced temperature-caused change in the electrostatic capacity, although the reason for the reduction has not been clarified. The ceramic according to the present invention is advantageously used to produce a piezoelectric device exhibiting a reduced output change and improved reliability even under a large temperature change.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the temperature-caused change in the electrostatic capacity as a function of the Mn.sub.2 O.sub.3 content.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

EXAMPLE 1: SAMPLES 1-4 & COMPARATIVE SAMPLES C1-C6

Piezoelectric ceramic samples having a basic composition expressed by a ternary xPb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --yPbTiO.sub.3 --zPbZrO.sub.3 solid solution (x, y, z in mol %) in which part of the Pb is substituted by Sr and containing Mn.sub.2 O.sub.3 in an amount of W wt % shown in Table 1 were produced by the following process. The produced samples have the compositions shown in Table 1, in which the values of x, y and z and the substituted Pb amount are varied and each variation has the Mn.sub.2 O.sub.3 contents of 0 wt % for the comparative samples C1-C5 and 0.1 wt % for the inventive samples and comparative example C6.

To provide the shown compositions, the starting powder materials of PbO, TiO.sub.2, Y.sub.2 O.sub.3, ZrO.sub.2, Nb.sub.2 O.sub.5, and Mn.sub.2 O.sub.3 were weighed and mixed in a wet type ball mill. The mixture was pre-fired at 700.degree. to 1050.degree. C. for 3 to 10 hours. The pre-fired materials were then ground and milled in a ball mill, and after adding water, polyvinyl alcohol or other adhesive agents, the materials were further milled and granulated in a spray dryer or the like. The granulated powders were compacted under a pressure of from 300 to 1000 kgf/cm.sup.2 and the compacts were fired at 1050.degree. to 1300.degree. C. for 0.5 to 4 hours to provide fired products.

The fired products were press-formed to a disc having a diameter of 17 mm and a thickness of 0.5 mm, on both sides of which electrodes of silver or other conductive materials were then formed by a conventional method. The discs were polarized by application of a voltage of 20 to 30 kV/cm for 0.1 to 2 hours at 20 to 150.degree. C. After 24 hours were elapsed, the discs were subjected to measurement of the properties. The electrostatic capacity was measured at a frequency of 1 kHz and an amplitude of 1 V by an impedance analyzer. The temperature-caused change in the electrostatic capacity is determined as a ratio of the value measured at 150.degree. C. to the value measured at -30.degree. C. The dielectric constant d.sub.31 and the electromechanical coupling factor were measured and determined by a resonance/anti-resonance method.

Table 1 demonstrates that the samples containing Mn.sub.2 O.sub.3 exhibited a reduction in the temperature-caused change in the electrostatic capacity with respect to the samples not containing Mn.sub.2 O.sub.3 under the various values of x, y, and z and the various substituted Pb content. The effect of the addition of Mn.sub.2 O.sub.3, however, is not observed when the y value is 42 mol % as can be seen from Comparative Samples C5 and C6, and therefore, the y value must be more than 42 mol % of the ternary solid solution.

EXAMPLE 2: SAMPLES 8-10 & COMPARATIVE SAMPLE C7

Piezoelectric ceramic samples having a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 -45.5 PbTiO.sub.3 -53.5 PbZrO.sub.3 solid solution (in mol %) in which 9 mol % Pb is substituted by Sr and containing Mn.sub.2 O.sub.3 in an amount of W wt % shown in Table 2 were produced by the process similar to that used in Example 1. The properties of the produced ceramic discs were measured in the same manner as in Example 1 and the measured data are summarized in Table 2.

The data of the temperature-caused change in the electrostatic capacity in Table 2 are also plotted in FIG. 1 as a function of the Mn.sub.2 O.sub.3 content. FIG. 1 shows that the temperature-caused change in the electrostatic capacity is reduced as the Mn.sub.2 O.sub.3 content is increased.

EXAMPLE 3: SAMPLES 11-13 & COMPARATIVE SAMPLES 8-11

Piezoelectric ceramic samples having a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 -45.5 PbTiO.sub.3 -53.5 PbZrO.sub.3 solid solution (in mol %) in which 9 mol % Pb is substituted by Sr and containing Mn.sub.2 O.sub.3 in an amount of W wt % and Nb.sub.2 O.sub.5 in an amount of U wt % shown in Table 3 were produced by the process similar to that used in Example 1. The properties of the produced ceramic discs were measured in the same manner as in Example 1 and the measured data are summarized in Table 3.

Table 3 also shows that the temperature-caused change in the electrostatic capacity is reduced as the Mn.sub.2 O.sub.3 content is increased under the presence of the added Nb.sub.2 O.sub.5. It should be noted, however, that the temperature-caused change in the electrostatic capacity is increased in Comparative Sample 11 in which the Mn.sub.2 O.sub.3 content is increased to 0.5 wt %.

It is concluded from Tables 2 and 3 that the Mn.sub.2 O.sub.3 content must be 0.01 wt % or more and less than 0.5 wt %.

As herein described above, the present invention provides a piezoelectric ceramic based on a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 with part of the Pb substituted by Sr, in which the temperature-caused change in the electrostatic capacity is reduced by the addition of Mn.sub.2 O.sub.3. The temperature-caused change in the electrostatic capacity is further reduced when the amounts of the three components of the ternary solid solution and the substituted Pb amount are limited in the specified ranges.

The piezoelectric ceramic of the present invention is advantageous in industry because it provides a material for a piezoelectric device which operates without substantially varying in output and thereby mitigating the load on the drive circuit, under a large temperature variation.

TABLE 1__________________________________________________________________________ Temperature-caused Electrostatic change in DielectricComposition capacity at room electrostatic constantSample Pb substituted Fired temperature capacity d31 .times.No. x y z by Sr W density (nF) (C150.degree. C./C-30.degree. C.) 10.sup.-12 m/V Kp__________________________________________________________________________C1 1 48 51 9 0 7.58 2.46 2.92 103 0.541 1 48 51 9 0.1 7.56 7.28 1.64 185 0.57C2 1 45.5 53.5 11.5 0 7.51 7.92 1.60 189 0.542 1 45.5 53.5 11.5 0.1 7.51 8.09 1.47 214 0.60C3 1 45.5 53.5 5 0 7.62 4.43 3.49 168 0.623 1 45.5 53.5 5 0.1 7.50 8.13 1.60 221 0.61C4 3 44 53 9 0 7.52 8.76 2.63 266 0.654 3 44 53 9 0.1 7.56 2.20 2.56 87 0.50C5 1 42 57 9 0 7.52 5.15 1.92 123 0.47C6 1 42 57 9 0.1 7.56 2.22 2.64 88 0.51__________________________________________________________________________ (Note) Samples 1-4: Inventive Samples Samples C1-C6: Comparative Samples __________________________________________________________________________ Temperature-caused Electrostatic change in DielectricComposition capacity at room electrostatic constantSample Pb substituted Fired temperature capacity d31 .times.No. x y z by Sr W density (nF) (C150.degree. C./C-30.degree. C.) 10.sup.-12 m/V Kp__________________________________________________________________________c7 1 45.5 53.5 9 0 7.58 7.85 1.54 164 0.47 8 1 45.5 53.5 9 0.025 7.48 7.83 1.45 256 0.63 9 1 45.5 53.5 9 0.05 7.56 7.81 1.40 212 0.5810 1 45.5 53.5 9 0.12 7.50 6.69 1.31 192 0.57__________________________________________________________________________ (Note) Samples 8-10: Inventive Samples Samples C7: Comparative Samples TABLE 3__________________________________________________________________________ Temperature-caused Electrostatic change in DielectricComposition capacity at room electrostatic constantSample Pb substituted Fired temperature capacity d31 .times.No. x y z by Sr W U density (nF) (C150.degree. C./C-30.degree. 10.sup.-12 m/V Kp__________________________________________________________________________C8 1 45.5 53.5 9 0 1 7.55 9.20 2.05 286 0.6811 1 45.5 53.5 9 0.1 1 7.50 4.49 1.50 100 0.43C9 1 45.5 53.5 9 0 0.3 7.50 9.14 1.78 308 0.7112 1 45.5 53.5 9 0.1 0.3 7.48 7.33 1.41 170 0.53C10 1 45.5 53.5 9 0 0.5 7.55 9.20 2.04 77 0.6213 1 45.5 53.5 9 0.25 0.5 7.45 7.73 1.46 213 0.60C11 3 45.5 53.5 9 0.5 0.5 7.55 5.84 2.13 188 0.60__________________________________________________________________________ (Note) Samples 11-13: Inventive Samples Samples C8-C11: Comparative Samples

Claims

1. A piezoelectric ceramic having a basic composition expressed as a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution, wherein a portion of the Pb is substituted by Sr, wherein the PbTiO.sub.3 is present in an amount of more than 42 mol % of the ternary solid solution, and wherein the ceramic contains Mn.sub.2 O.sub.3 in an amount in a range from at least 0.01 wt % to less than 0.5 wt %.

2. A piezoelectric ceramic according to claim 1, wherein the Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount in a range from 0.5 mol % to 3 mol % of the ternary solid solution, wherein the PbTiO.sub.3 is present in an amount in a range from more than 42 mol % to less than 50 mol % of the ternary solid solution, and wherein the PbZrO.sub.3 is present in an amount in a range between 47 mol % and 57.5 mol % of the ternary solid solution.

3. A piezoelectric ceramic according to claim 1, wherein the portion of the Pb substituted by the Sr is in a range from more than 5 mol % to at most 15 mol % of the ternary solid solution.

4. A piezoelectric ceramic according to claim 2, wherein the portion of the Pb substituted by the Sr is in a range from more than 5 mol % to at most 15 mol % of the ternary solid solution.

5. A piezoelectric ceramic according to claim 1, wherein the Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount in a range from at least 0.5 mol % to less than 3 mol %.

6. A piezoelectric ceramic according to claim 1, wherein the ceramic contains is free of any metal oxide additives other than Mn.sub.2 O.sub.3.

7. A piezoelectric ceramic having a basic composition expressed as a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution, wherein a portion of the Pb is substituted by Sr, wherein the PbTiO.sub.3 is present in an amount of more than 42 mol % of the ternary solid solution, and wherein the ceramic contains Nb.sub.2 O.sub.5 in an amount of less than 2 wt % and Mn.sub.2 O.sub.3 in an amount in a range from at least 0.01 wt % to less than 0.5 wt %.

8. A piezoelectric ceramic according to claim 7, wherein the Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount in a range from 0.5 mol % to 3 mol % of the ternary solid solution, wherein the PbTiO.sub.3 is present in an amount in a range from more than 42 mol % to less than 50 mol % of the ternary solid solution, and wherein the PbZrO.sub.3 is present in an amount in a range between 47 mol % and 57.5 mol % of the ternary solid solution.

9. A piezoelectric ceramic according to claim 7, wherein the portion of the Pb substituted by the Sr is in a range from more than 5 mol % to at most 15 mol % of the ternary solid solution.

10. A piezoelectric ceramic according to claim 8, wherein the portion of the Pb substituted by the Sr is in a range from more than 5 mol % to at most 15 mol % of the ternary solid solution.

11. A piezoelectric ceramic according to claim 7, wherein the Nb.sub.2 O.sub.5 is present in an amount not greater than 1 wt % of the ceramic.

12. A piezoelectric ceramic according to claim 8, wherein the Nb.sub.2 O.sub.5 is present in an amount not greater than 1 wt % of the ceramic.

13. A piezoelectric ceramic according to claim 7, wherein the Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 is present in an amount in a range from at least 0.5 mol % to less than 3 mol %.