FERROELECTRIC CERAMICS

Abstract: The present invention relates to Perovskite ferroelectric ceramics comprising a Perovskite solid solution represented by the formula Pb 1-(3/2)a M a {(Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ wherein M is at least one element selected from the group consisting of La and Nd, x + y + z is 1, said Perovskite solution containing MnO₂. When compared with known ferroelectric ceramics, the ferroelectric ceramics according to the invention has a larger piezoelectric constant d and also a sufficiently high Curie temperature Tc, while having a mechanical quality factor Qm practically equal to that of the known ferroelectric ceramics. Accordingly, the ferroelectric ceramics according to the invention exhibit excellent characteristics when used in application such as a driving part of an actuator, for example, of an ultrasonic motor where mechanical resonance of the material used is utilized.

Description: FIELD OF THE INVENTION This invention relates to ferroelectric ceramics of Pb(Ni 1/3 Nb 2/3 )O₃-Pb(Zn 1/3 Nb 2/3 )O₃-PbTiO₃-PbZrO₃ series. More particularly, the invention relates to ferroelectric ceramics of Pb(Ni 1/3 Nb 2/3 )O₃-Pb(Zn 1/3 Nb 2/3 )O₃-PbTiO₃-PbZrO₃ series high in both piezoelectric constant d and mechanical quality factor Qm and also high in Curie temperature Tc. BACKGROUND OF THE INVENTION Piezoelectric materials comprising ferroelectric ceramics have heretofore been used in various applications such as piezoelectric filters, piezoelectric transducers, ultrasonic oscillators or piezoelectric buzzers. Known as the most typical ferroelectric ceramics used in such applications are solid solutions of PbTiO₃-PbZrO₃ series. Further, solid solutions of PbTiO₃-PbZrO₃ series combined with a solid solution of a complex Perovskite type compound such as Pb(Mg 1/3 Nb 2/3 )O₃, Pb(N 1/3 Nb 2/3 )O₃ or Pb(Zn 1/3 Nb 2/3 )O₃ are also known as ferroelectric ceramics having improved piezoelectric properties. On the other hand, the use of piezoelectric ceramic materials as an actuator has recently been studied. In this case, it is necessary to transform electric energy to mechanical energy by the piezoelectric ceramic material. In order to transform the energy piezoelectric ceramic materials having a large piezoelectric constant d are desired. Generally, the piezoelectric constant d in a piezoelectric ceramic material is related to an electromechanical coupling factor k and a relative dielectric constant ε of the piezoelectric ceramic material, and the relative equation is as follows: d∝k ε and therefore, in order to enlarge piezoelectric constant d, it must need to enlarge an electromechanical coupling factor k and/or a relative dielectric constant ε. Further, in applications of piezoelectric ceramic materials such as a driving part of an actuator, for example, of an ultrasonic motor, where mechanical resonance of the material is utilized, it is desired that both the piezoelectric constant d and the mechanical quality factor Qm are large. That is, when a piezoelectric ceramic material is used in an ultrasonic motor, i.e. in the resonant frequency region, if the mechanical quality factor Qm of the material is small, not only an amplitude of mechanical resonance of the material will not become large but also heat generated due to high frequency driving involved will become large, as the result, leading to undesirable reduction in spontaneous polarization of the material and to an undesirable decrease in piezoelectric constant d of the material. On the one hand, it is desired that piezoelectric ceramic materials have a high Curie temperature Tc in order that the undesirable reduction in spontaneous polarization or the undesirable decrease in piezoelectric d of the material is hard to occur even when heat generated due to high frequency driving of the material used as the actuator becomes large, and also in order that the temperature range at which the material is operable as the actuator is broadened. While the mechanical quality factor Qm of solid solutions of PbTiO₃-PbZrO₃ series or of PbTiO₃-PbZrO₃ series combined with a solid solution of a complex Perovskite such as Pb(Mg 1/3 Nb 2/3 )O₃, Pb(Ni 1/3 Nb 2/3 )O₃ or Pb(Zn 1/3 Nb 2/3 )O₃ can be improved by incorporation of such a hard compound as MnO₂ thereinto, there has been a problem in that the piezoelectric constant d of the material is drastically decreased as the amount of MnO₂ incorporated increases. While the piezoelectric constant d of the above-mentioned solid solutions can be improved, on the one hand, by incorporation of such a soft compound as Nb₂O₅, Ta₂O₅, La₂O₃, Nd₂O₃ or Bi₂O₃ thereinto, or by substituting parts of Pb of the above-mentioned solid solutions with Ba, Sr or Ca, there has been a problem in that the Curie temperature of the material is drastically decreased as the amount of the soft compound incorporated or of the substitution amount increases. In light of the problems as mentioned above, the inventors conducted thorough investigations in metal ion substituents that may be used in Perovskite solid solutions of PbTiO₃-PbZrO₃ series, and as the result it has been found that solid solutions of Pb(Ni 1/3 Nb 2/3 )O₃-Pb(Zn 1/3 Nb 2/3 )O₃-PbTiO₃-PbZrO₃ series having a specific composition can be restrained from decreasing in piezoelectric constant d, can be improved in mechanical quality factor Qm and, in addition, can maintain a high Curie temperature Tc by substituting parts of Pb of the solid solutions with La or Nd in a specific amount and incorporating a specific amount of MnO₂ thereinto. The present invention has been accomplished on the basis of the above findings. OBJECT OF THE INVENTION The present invention is based on the above-mentioned findings, and accordingly an object of the invention is to provide ferroelectric ceramics large in both piezoelectric constant d and mechanical quality factor Qm and also high in Curie temperature and as a consequence, which are excellent in piezoelectric properties. SUMMARY OF THE INVENTION The ferroelectric ceramics according to the present invention is a Perovskite solid solution comprising a solid solution represented by the formula Pb 1-(3/2)a M a { (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ wherein M is at least one element selected from the group consisting of La and Nd, x + y + z is 1, a is from 0.005 to 0.03, b is from 0.5 to 0.95, x is from 0.1 to 0.4, y is from 0.3 to 0.5 and z is from 0.2 to 0.5, with the provision that MnO₂ is contained in an amount of from 0.3 to 1.0% by weight based on the Perovskite solid solution. The ferroelectric ceramics according to the invention comprising Perovskite crystals having the composition as mentioned above into which a specific amount of MnO₂ has been incorporated, have a larger piezoelectric constant d and also a sufficiently high Curie temperature compared with conventional ferroelectric ceramics, in spite of the fact that the present ferroelectric ceramics have a mechanical quality factor Qm practically equal to that of the conventional ferroelectric ceramics. Accordingly, the ferroelectric ceramics according to the invention exhibit excellent characteristics when used in applications as a driving part of an actuator, such as an ultrasonic motor wherein mechanical resonance of the material is utilized. DETAILED DESCRIPTION OF THE INVENTION The ferroelectric ceramics according to the present invention will now be described below in detail. The ferroelectric ceramics according to the invention is a Perovskite solid solution comprising a solid solution represented by the formula Pb 1-(3/2)a M a { (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ wherein M is at least one element selected from the group consisting of La and Nd, x + y + z is 1, a is from 0.005 to 0.03, preferably from 0.007 to 0.02 b is from 0.5 to 0.95, preferably from 0.6 to 0.9 x is from 0.1 to 0.4, preferably from 0.2 to 0.4 y is from 0.3 to 0.5, preferably from 0.33 to 0.4, and z is from 0.2 to 0.5, preferably from 0.27 to 0.4, and containing MnO₂ in an amount of from 0.3 to 1.0% by weight, preferably from 0.4 to 0.8 % by weight based on the Perovskite solid solution. When the Perovskite crystals constituting the ferroelectric ceramics according to the invention are represented by ABO₃, Pb and, La and/or Nd substituting the parts of Pb are present in the form of metal ions in the A sites of the crystals. In that case, a value "a" representing the amount of La and/or Nd substituting the parts of Pb is desirably not less than 0.005 in order to obtain a large piezoelectric constant d of the crystals, and is desirably not more than 0.03 in order to maintain the Curie temperature Tc of the crystals at a high level. Further, the amount of MnO₂ to be incorporated into the above-mentioned Perovskite crystals is desirably not less than 0.3% by weight based on the crystals in order to obtain a large mechanical quality factor Qm of the crystals, and is desirably not more than 1.0% by weight in order to maintain the piezoelectric constant d of the crystals at a high level. When parts of Pb of the Perovskite crystals having a specific composition range represented by the formula Pb{ (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ constituting the ferroelectric ceramics according to the invention is substituted with the specific amount of La and/or Nd and incorporated with the specific amount of MnO₂ in the manner now described, the mechanical quality factor Qm of the crystal can be improved and the Curie temperature Tc of the crystals can be maintained at a high level without decreasing so much the piezoelectric constant d of the crystals and, as a consequence, the ferroelectric ceramics large in both piezoelectric constant d and mechanical quality factor Qm and also high in Curie temperature To can effectively be obtained. The ferroelectric ceramics having the above-mentioned composition as defined in the invention may be prepared by admixing of particulate metal compounds such as oxides and salts in such proportions which provide a desired composition when sintered, and sintering the admixture. Processes for preparing the starting particulate metal compounds are not particularly limited. They may be prepared by various known processes including liquid phase processes such as precipitation, coprecipitation, alkoxide, and sol-gel processes, and solid phase processes such as those based on decomposition of oxalates and blending oxides. The thus obtained admixture of particulate metal compounds in appropriate proportions may be calcined at a temperature of from 800°C to 1000°C, pulverized in a ball mill, dried, pressed to a sheet or disc under a pressure of from 490,5 · 10⁵ to 1471 10⁵ Pa (500 to 1500 kg/cm²), and finally sintered at a temperature of from 1000 to 1300°C, whereupon a desired ferroelectric ceramics may be obtained. EXAMPLES While the present invention will now be described below with reference to the following examples, it should be appreciated that the invention is in no way restricted to the examples. The radial electromechanical coupling factor (Kp), relative dielectric constant (ε T / 33/ε₀), piezoelectric constant (d₃₁), and mechanical quality factor (Qm) of the ferroelectric ceramics were measured in accordance with Japan Electronic Material Manufactures Association Standard (EMAS). Examples 1-2, Comparative Examples 1-4 PbO, ZrO₂, TiO₂, NiO, ZnO, Nb₂O₅, La₂O₃ and MnO₂ were weighed in proportions which provide each composition as indicated in Table 1, pulverized and admixed in a ball mill. The pulverized admixture was calcined at a temperature of 800° to 1000°C for a period of from 1 to 2 hours, pulverized in a ball mill and dried. The mixture was then pressed to a disc having a diameter of 25 mm under a pressure of 981· 10⁵ Pa (1000 kg/cm²) and finally sintered at a temperature of from 1050° to 1250°C for period of from 1 to 2 hours. The sintered disc thus prepared was polished to a thickness of 0.5 mm, electroded with silver paste on both surfaces and baked. It was then poled by application of a DC electric field of from 20 to 40 KV/cm in a silicone oil and thereafter aged for 12 hours. The specimen so prepared was tested for various electric properties. The results are shown in Table 1. In Table 1, a, b, x, y and z represent coefficients appearing in the formula: From comparison of Example 1 and of Comparative Example 1, it is understood that the ferroelectric ceramic having substituted parts of Pb with La and contained MnO₂ (Example 1) have a higher radial electromechanical coupling factor Kp and piezoelectric constant d₃₁ compared with the ferroelectric ceramic having contained MnO₂ but without substituting Pb with La (Comparative Example 1), while having a mechanical quality factor Qm practically equal to that of Comparative Example 1. From comparison of Example 1 and of Comparative Example 2, it is understood that the ferroelectric ceramics of Example 1 have a higher radial electromechanical coupling factor Kp and also a slightly lower piezoelectric constant d₃₁ compared with the ferroelectric ceramics having substituted parts of Pb with La but without containing MnO₂ (Comparative Example 2), in addition to such results as may sufficiently be predicted from the conventional information that the ferroelectric ceramics obtained in Example 1 is low in relative dielectric constant ε T / 33/ε₀ and high in mechanical quality factor Qm. It is also understood that there is observed a tendency similar to the above-mentioned when compared Example 2 with Comparative Examples 3 and 4, respectively. Examples 3-7, and Comparative Examples 5-7 Specimens of ferroelectric ceramics were prepared by the same procedure as described in Example 1 except that the values of coefficients a and b appearing in the formula Pb 1-(3/2)a La a { (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } 0.2 Ti 0.4 Zr 0.4 O₃ were changed as indicated in Table 2. The specimens so prepared were tested for various electric properties. The results are shown in Table 2. From comparison between Examples 4-5 and Comparative Example 1 and from comparison between Examples 6 - 7 and Comparative Example 5, it is understood that the effect obtainable by substitution of Pb with La is very great. From the results obtained in Comparative Examples 6 and 7, however, it is appreciated that when the value of coefficient b becomes not more than 0.5, the effect on substitution of Pb with La is not so great. Examples 8-9, and Comparative Examples 8-9 Specimens of ferroelectric ceramics were prepared by the same procedures as described in Examples 1 and 7 and Comparative Example 1 except that parts of Pb were substituted with Nd in place of La. The specimens so prepared were tested for various electric properties. The results are shown in Table 3. From comparison between Example 8 and Comparative Example 8 and from comparison between Example 9 and Comparative Example 9, it is understood that the specimens obtained by containing MnO₂ therein (Examples 8 and 9) are high in mechanical quality factor Qm and, in addition thereto, are higher in radial electromechanical coupling factor Kp compared with the specimens obtained by substituting parts of Pb with Nd but without containing MnO₂ therein (Comparative Examples 8 and 9), and also have values of piezoelectric constant d₃₁ practically equal to those of the specimens of Comparative Examples 8 and 9. EFFECT OF THE INVENTION As illustrated hereinbefore, the ferroelectric ceramics according to the present invention are Perovskite solid solutions comprising solid solutions having a specific composition represented by the following formula. Pb{ (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ In the Perovskite solid solutions mentioned above, parts of Pb have been substituted with a specific amount of La or Nd, and also a specific amount of MnO₂ is contained. Therefore, when compared with known ferroelectric ceramics, the ferroelectric ceramic according to the invention have a larger piezoelectric constant d and maintain a high Curie temperature Tc, while having a mechanical quality factor Qm practically equal to that of the known ferroelectric ceramics. Accordingly, the ferroelectric ceramics according to the invention exhibit excellent characteristics when used in applications such as a driving part of an actuator, for example, of an ultrasonic motor, where mechanical resonance of the material used is utilized.

Claims: Claims for the following Contracting states : AT, BE, CH, DE, DK, FR, GB, GR, IT, LI, LU, NL and SE A ferroelectric ceramic which is a Perovskite solid solution, which comprises a solid solution represented by the formula: Pb 1-(3/2)a M a { (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ wherein M is at least one element selected from the group consisting of La and Nd, x + y + z is 1, a is from 0.005 to 0.03, b is from 0.5 to 0.95, x is from 0.1 to 0.4, y is from 0.3 to 0.5 and z is from 0.2 to 0.5, and said Perovskite solid solution further contains MnO₂ in an amount of from 0.3 to 1.0% by weight based on the Perovskite solid solution. The ferroelectric ceramic as claimed in claim 1 wherein M in the formula is La. The ferroelectric ceramic as claimed in claim 1 wherein M in the formula is Nd. Use of a ferroelectric ceramic according to any of claims 1 to 3 as an actuator. Claims for the following Contracting state : ES A ferroelectric ceramic which is a Perovskite solid solution, which comprises a solid solution represented by the formula: Pb 1-(3/2)a M a { (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ wherein M is at least one element selected from the group consisting of La and Nd, x + y + z is 1, a is from 0.005 to 0.03, b is from 0.5 to 0.95, x is from 0.1 to 0.4, y is from 0.3 to 0.5 and z is from 0.2 to 0.5, and said Perovskite solid solution further contains MnO₂ in an amount of from 0.3 to 1.0% by weight based on the Perovskite solid solution. The ferroelectric ceramic as claimed in claim 1 wherein M in the formula is La. The ferroelectric ceramic as claimed in claim 1 wherein M in the formula is Nd. A process for preparing a ferroelectric ceramic which is a perovskite solid solution, which comprises a solid solution represented by the formula: Pb 1-(3/2)a M a { (Ni 1/3 Nb 2/3 ) 1-b (Zn 1/3 Nb 2/3 ) b } x Ti y Zr z O₃ wherein M is at least one element selected from the group consisting of La and Nd, x + y + z is 1, a is from 0.005 to 0.03, b is from 0.5 to 0.95, x is from 0.1 to 0.4, y is from 0.3 to 0.5 and z is from 0.2 to 0.5, and said Perovskite solid solution further contains MnO₂ in an amount of from 0.3 to 1.0% by weight based on the Perovskite solid solution. by admixing the corresponding particulate metal compounds such as orides and salts in such proportions which provide the desired composition when sintered and sintering the admixture. A method for preparing a ferroelectric ceramic as claimed in claim 4 wherein M in the formula is La. A method for preparing a ferroelectric ceramic as claimed in claim 4 wherein M in the formula is Nd. Use of a ferroelectric ceramic according to any of claims 4 to 6 as an actuator.

Inventors: MASE, HIROSHI, MITSUI PETROCHEMICAL IND. LTD., USHIDA, YOSHIHISA, MITSUI PETROCHEMICAL IND. LTD.
Applicants: MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1990
Main classification: C04B35
IPC codes: H01L41, C04B35