Piezoelectric ceramics

Abstract: A piezoelectric ceramic which maintains a high piezoelectric property in a composition having MPB structure and has an excellent ability for stabilizing the resonant frequency is provided. The piezoelectric ceramic has a solid solution expressed by the general expression xPbTiO3 - (1-x) PbZrO3 as its main component. The value of x in the above general expression causes the piezoelectric ceramic to have a composition having the morphotropic phase boundary structure. A ceramic having a spinel type structure is dispersed within the structure of the piezoelectric ceramic. The amount of the ceramic having the spinel type structure existing in the structure is set at about 5 volume % or less. Co2TiO4, Mn3O4, Fe3O4, Mg2TiO4 and a ceramic having a compound spinel type structure which is a solid solution of them can be used as the ceramic having the spinel type structure.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to piezoelectric ceramics and more particularly to piezoelectric ceramics used for ceramic filters, ceramic oscillators, discriminators, surface acoustic wave resonators or the like. DESCRIPTION OF PRIOR ART A piezoelectric ceramic having a lead titanate zirconate (PbTiO3-PbZrO3; hereinafter referred to as PZT") as a main component has been widely used as a ceramic material having piezoelectric properties (piezoelectric ceramic) employed, for instance, in ceramic filters and ceramic oscillators. Recently, a number of piezoelectric ceramics to which various third components or additives are added have been proposed in order to improve the piezoelectric characteristic. One such piezoelectric ceramic is where Pb(Mn 1/3 Nb 2/3 )O 3 , Pb(Mg 1/3 Nb 2/3 )O 3 or the like is solid-solubilized as the third component. The crystal structure of a piezoelectric ceramic containing such PZT as the main component changes depending on the ratio of PbTiO 3 /PbZrO 3 and takes either a rhombohedral crystal structure or a tetragonal crystal structure. The phase boundary between the rhombohedral structure and the tetragonal structure is called the Morphotropic Phase Boundary ("MPB"). It is known that excellent piezoelectric properties can be obtained in a composition having the ratio of PbTiO 3 /PbZrO 3 realizing a structure close to MPB ("MPB structure"). For example, when a piezoelectric ceramic having a composition realizing the MPB structure is used as a band-pass filter, the band width of the filter is broader. However, a PZT system piezoelectric ceramic having a composition realizing the MPB structure has a problem in that the temperature dependency of a resonant frequency and anti-resonant frequency is high. For example, the pass-band of a ceramic filter using such piezoelectric ceramic depends significantly on the temperature. Accordingly, even though the band-pass filter using the PZT system piezoelectric ceramic having the composition realizing the MPB structure has the aforementioned advantage, its use is limited because the temperature dependency of the frequency is significant. GB-A-1184888 discloses a piezoelectric ceramic comprising a solid solution expressed by the general expression xPbTiO 3 - (1-x)PbZrO 3 as its main component, said solid solution having morphotropic phase boundary structure. The composition of the ceramic body is made of the three components Pb(Mn 1/3 Ta 2/3 )O 3 , PbTiO 3 and PbZrO 3 , respectively. The crystal structures of these three components are all perovskite structure, and the composition of the ceramic body has pervoskite structure as well. DE-A-179 6 226 discloses a piezoelectric ceramic, comprising a solid solution expressed by the general expression xPbTiO 3 - (1-x)PbZrO 3 as its main component, said solid solution having morphotropic phase boundary structure. EP-A-033 8 799 discloses a method of making a ceramic material, selected from a titanate ceramic having perovskite-type structure and a ferrite ceramic having spinel-type structure. The purpose of this method is to obtain a titanate ceramic having a pervoskite-type structure or a ferrite ceramic having a spinel-type structure each of which has excellent homogeneity. This publication does not, however, disclose a ceramic having spinel-type structure dispersed in a piezoelectric ceramic having morphotropic phase boundary structure. JP-A-63-185080 discloses a piezoelectric ceramic which is obtained by sintering. A complex oxide of a perovskite structure as a basic composition is uniformely mixed with an oxide having a spinel structure which is integrated with the basic composition so that a complex oxide of perovskite structure is formed. SUMMARY OF THE INVENTION It is an object of the present invention to solve the aforementioned problem by providing a piezoelectric ceramic in which the dependency of pass-band to temperature change is reduced while having a composition realizing the MPB structure and an excellent piezoelectric property and which may be suitably used for a ceramic filter whose pass-band depends less on temperature and a ceramic oscillator having a stable oscillating frequency. In order to achieve the aforementioned object, according to the present invention, there is provided a piezoelectric ceramic having a solid solution expressed by a general expression: xPbTiO 3 (1-x) PbZrO 3 as its main component, wherein x in the above general expression has a value which causes the piezoelectric ceramic to have a composition in the vicinity of the MPB and in which a ceramic having a spinel type structure exists within the structure thereof. According to one aspect of the present invention, the content of the ceramic having the spinel type structure existing in the structure is set at about 5 volume % or less, preferably about 0.1 to 4.8 %. According to another aspect of the present invention, at least one of Co2TiO4, Mn3O4, Fe3O4, Mg2TiO4 and a spinel type structure solid solution of them is used as the ceramic having the spinel type structure. The reason why the amount of the ceramic having the spinel type structure (hereinafter referred simply as "spinel ceramic") existing within the piezoelectric ceramic is kept in a range of about 5 volume % or less is because the specific resistance of the obtained piezoelectric ceramic is lowered and it becomes difficult to implement a polarization process if the amount of the spinel ceramic exceeds 5 volume %. It is noted that although a detailed mechanism for bringing about a stability of temperature coefficient of the resonant frequency is not necessarily clear in the inventive piezoelectric ceramic, it is believed that the inventive piezoelectric ceramic can achieve the excellent thermal stability while maintaining a high piezoelectric property in a composition range in the vicinity of the MPB because a spinel phase which differs from a perovskite phase is formed during the sintering process of the piezoelectric ceramic and it cancels the instability of the crystal structure caused by phase change in a singular phase of the perovskite phase by some means. The above and other related objects and features of the invention will be apparent from a reading of the following description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGs. 1a and 1b are electron microscope (SEM) pictures of a polished plane of a piezoelectric ceramic showing the crystal structure thereof according to one embodiment of the present invention, which differ in that the FIG. 1b magnification is greater than that of FIG. 1a; FIG. 2 is an X-ray diffraction chart of the piezoelectric ceramic of one embodiment of the present invention; and FIG. 3 is a graph showing the relationship between temperature and rate of change of resonant frequency of radial oscillation of a sample (sample No. 2) of a comparative example and those of the invention (sample Nos. 3 and 5). DETAILED DESCRIPTION OF THE INVENTION The present invention will be explained below more concretely by showing a preferred embodiment thereof. It is noted that the present embodiment will be explained by exemplifying a piezoelectric ceramic containing, as a third component, Pb(Mn1/3Nb2/3)O3 expressed by the following general expression (1): 0.97{xPbTiO3 - (1-x)PbZrO3} - 0.03Pb(Mn1/3Nb2/3)O3 and in which a ceramic having a spinel type structure expressed by the following general expression (2) : Co 2 Tio 4 is dispersed. (1) Preparation of Piezoelectric Ceramic Powder Powders of PbO or Pb3O4, TiO2, ZrO2, MnO2 and Nb2O3 were prepared as starting raw materials. Those materials were weighed so that the value of x in the general expression (1) takes the value shown in Table 1 below, wet mixed using a ball mill, dehydrated, dried, calcined for two hours in 850 to 1000°C and then ground to obtain calcined piezoelectric ceramic powder A. Sample No. Composition *ε kp (%) Cfr.p (ppm/°C) Spinel Type Structure Ceramic (volume %) X Spinel Type Structure Ceramic (weight %) *1 0.46 0.0 1390 56.3 -169 0.0 *2 0.47 0.0 1580 53.7 265 0.0 3 0.47 0.1 1230 58.1 62 0.1 4 0.48 0.2 1430 58.7 36 0.3 5 0.48 0.4 1330 58.6 31 0.6 6 0.48 0.8 1360 56.2 -9 0.9 7 0.49 0.8 1410 57.3 20 1.1 8 0.49 1.5 1110 56.3 -10 2.0 9 0.50 1.5 1450 57.4 8 2.1 10 0.51 3.0 1360 55.9 -3 4.8 11 0.52 3.0 1190 54.0 12 4.8 *12 0.52 4.0 680 - - 5.8 *13 0.53 4.0 700 - - 5.6 *14 0.53 5.0 490 - - 7.2 Remarks: For ε of sample Nos. 12, 13 and 14 from which no piezoelectric property could be observed, ε of the samples before polarization is shown. It is to be noted that although the above-mentioned oxides have been used as the starting materials of the calcined powder A in the embodiment described above, the starting materials are not limited only to those and various materials such as carbonates which become the above-mentioned oxide may be used. (2) Preparation of Spinel Ceramic Powder CoO and TO 2 powders were prepared as starting materials of the spinel ceramic powder to be mixed with the above-mentioned calcined piezoelectric ceramic powder A, were weighed so as to meet the composition of the above-mentioned general expression (2), wet mixed using a ball mill, dehydrated, dried, calcined for two hours in 850 to 1000°C and ground to obtain calcined spinel ceramic powder B. (3) Creation of Piezoelectric Ceramic Containing Spinel Ceramic The piezoelectric ceramic powder A and the spinel ceramic power B prepared as described above were mixed in the ratio as shown in Table 1 above, ground together with an organic binder such as vinyl acetate emulsion, mixed and dried to form a granulate. Then, the granular powder thus obtained was molded into a disc having a 12 mm diameter and 1 mm thickness under a pressure of about 1000 kg/cm 2 . A piezoelectric ceramic was obtained by firing the molded disc for two hours in 1150 to 1300°C. Silver electrodes were baked on the both sides of the piezoelectric ceramic and a polarization process was implemented under the conditions of 60 to 200°C of temperature and 1.5 to 5 kV/mm for five to 30 minutes. FIGs. 1a and 1b are SEM pictures of a polished plane showing the crystal structure of the piezoelectric ceramic sample No. 7 thus obtained. In FIGs. 1a and 1b, dark portions 1 through 5 and others are ceramics having the spinel type structure. FIG. 2 is an X-ray diffraction chart of the piezoelectric ceramic sample No. 7. In FIG. 2, peaks marked with O are those of PZT and peaks marked with X are those of the ceramic having the spinel type structure. It can be seen from FIGs. 1a and 1b and FIG. 2 described above that a ceramic having a spinel type structure exists within the obtained piezoelectric ceramic. In this embodiment, each composition has the MBP structure. In a pure PZT, it is known that a composition whose x value is about 0.52 can be defined as one having the MPB structure. When additives such as the third component or the spinel structure are present, the x value which realizes the MPB structure may vary in accordance with the amount of the additives. The x value realizing the MPB structure is determined by a X-ray analysis in which structures of a series of compositions having various x values are observed. (4) Measurement of Characteristics Characteristics of the piezoelectric ceramic to which the polarization process was implemented as described above were measured with respect to dielectric constant (ε), an electromechanical coupling factor (kp) of radial oscillation and a temperature coefficient (Cfr.p) of the resonant frequency of the radial oscillation. Table 1 above shows the results. It is noted that in Table 1, those sample Nos. marked with *" are comparative examples which are outside of the scope of the present invention and the other samples are embodiments within the scope of the present invention. FIG. 3 shows the relation between temperature and percentage of resonant frequency change of radial oscillation of a sample (sample No. 2) of the comparative example and samples (sample Nos. 3 and 5) of the invention. Samples Nos. 12, 13 and 14 could not be polarized because the specific resistance of the samples were small. The ε set forth is the value prior to polarization. As it is apparent from Table 1 and FIG. 3, while the percentage of the resonant frequency change increases in the piezoelectric ceramic of the comparative example, the change is reduced in the piezoelectric ceramic of the present invention without lowering the electromechanical coupling factor. It is noted that although the ceramic having PZT-Pb(Mn1/3Nb2/3)O3 as the main and third components has been explained in the embodiment described above, the piezoelectric ceramic of the present invention is not confined only to that. That is, it is possible to apply the present invention to various piezoelectric ceramics including the MPB composition such as a two component PZT system or two component PZT system to which Pb(Mg1/3Nb2/3)O3 or Pb(Ni1/3Nb2/3)O3 has been solid-solubilized. Further, the type of the ceramic having the spinel type structure which exists in the piezoelectric ceramic of the present invention is also not confined only to the Co2TiO4 described above. That is, it is possible to use Mn3O4, Fe3O4, Mg2TiO4 or the like and it is also possible to use a ceramic having a compound spinel type structure which is a solid solution of two or more of them. Further, according to the present invention, not only the temperature dependence of the resonant frequency of the radial oscillation of the disc piezoelectric ceramic described above, but also the temperature dependence of the resonant frequency of perpendicular oscillation and thickness sliding oscillation may be reduced. The present invention is not limited to the embodiment described above in other aspects. That is, it may be applied or modified in various ways in terms of the manufacturing conditions such as the sintering temperature and time and the shape and dimensions of the piezoelectric ceramic within the scope of the spirit of the present invention. As described above, because a ceramic having the spinel type structure exists within the structure of the PZT system piezoelectric ceramic which is expressed by xPbTiO 3 - (1-x) PbZrO3 and in which x, i.e. the ratio of Ti/Zr, has a value which realizes a structure including MPB structure, the temperature dependence of the resonant frequency in the piezoelectric ceramic of the present invention is reduced while maintaining the high piezoelectric property of the composition. While a preferred embodiment has been described, variations thereto will occur to those skilled in the art within the scope of the present invention which are defined by the following claims.

Claims: A piezoelectric ceramic, comprising a solid solution expressed by the general expression xPbTiO 3 - (1-x)PbZrO 3 as its main component, said solid solution having morphotropic phase boundary structure, characterized in that a ceramic having spinel type structure is dispersed within said solid solution having morphotropic phase boundary structure. The piezoelectric ceramic according to Claim 1, wherein said ceramic having the spinel type structure is about 5 volume % or less. The piezoelectric ceramic according to Claim 2, wherein said ceramic having the spinel type structure is at least one member of the group consisting of Co2TiO4, Mn3O4, Fe3O4, Mg2TiO4 and a spinel type structure ceramic which is a solid solution of two or more of the group members. The piezoelectric ceramic according to Claim 3, wherein said ceramic having the spinel type structure is about 0.1 to 4.8 volume %. The piezoelectric ceramic according to Claim 1, wherein said ceramic having the spinel type structure is about 0.1 to 4.8 volume %. The piezoelectric ceramic according to Claim 5, wherein said ceramic having the spinel type structure is Co 2 TiO 4 . The piezoelectric ceramic according to Claim 6, wherein x is about 0.47 to 0.52. The piezoelectric ceramic according to Claim 1, wherein said ceramic having the spinel type structure includes at least Co 2 TiO 4 .

Inventors: HAYASHI KOHICHI, NADA KENICHI, SHIRATSUYU KOSUKE, HAYASHI, KOHICHI, NADA, KENICHI, SHIRATSUYU, KOSUKE, Hayashi, Kohichi, c/o Murata Manufact. Co., Ltd., Nada, Kenichi, c/o Murata Manufact. Co., Ltd., Shiratsuyu, Kosuke, c/o Murata Manufact. Co., Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: C04B35
IPC codes: C04B35, H01L41