Multilayer piezoelectric composite

Abstract: A multilayer piezoelectric composite comprising a bonded laminate each layer of which is composed of a green sheet of a low-temperature sintering lead zirconate titanate piezoelectric ceramic. The temperature coefficient of a frequency constant of at least one layer has a polarity opposite to the polarity of those of the other layers. The bonded laminate is sintered at a temperature which does not produce any or almost any interdiffusion between layers so that the absolute value of the temperatures coefficient of a frequency constant of the bonded laminate is smaller than the absolute value of the temperature coefficient of a frequency constant of any or one of the layers.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a multilayer piezoelectric composite and, more particularly, to a multilayer piezoelectric composite which is suitable for uses in which the temperature dependence of the piezoelectric characteristics is required to be low, especially as an electronic circuit element such as a resonance filter or a wave filter. Description of the Prior Art Piezoelectric materials which enable the mutual conversion between electric energy (electric field) and mechanical energy (strain) are not only utilized for capacitors and the like as ferroelectric materials but also used as piezoelectric vibrators for ultrasonic washers, tuning fork filters, ultrasonic wave oscillators, piezoelectric couplers and ceramic filters, piezoelectric igniting elements, piezoelectric elements for transformers, and delaying elements; in other words, they are used as a kind of functional materials. Such piezoelectric materials are classified into three types by their forms, namely, piezoelectric single crystals, piezoelectric thin films and piezoelectric ceramics. Among these, it is a lead zirconate titanate PbZrO₃ - PbTiO₃ (hereinunder referred to as "PZT") piezoelectric ceramic, which is a two-component solid solution of a ferroelectric material (zirconate) and an antiferroelectric material (titanate), that is most widely used. It is known that in the case of a PZT piezoelectric ceramic, when the molar ratio of zirconium to titanium (hereinunder referred to as "Z/T") is 53/47, a morphotropic phase transition from a rhombohedral phase to a tetragonal phase is caused and that piezoelectric constant and electromechanical coupling factor remarkably increase in the vicinity of the morphotropic phase boundary. However, in PZT piezoelectric ceramics having a composition approximate to the morphotropic phase boundary, the temperature dependence of the piezoelectric characteristic is high. Fig. 7 is a graph showing the temperature dependence of frequency constant Np (unit: Hz·m) of PZT. The numeral in parentheses represents temperature coefficient (TCNp) of frequency constant Np in a constant temperature range. The temperature coefficient in this specification means the value (unit: ppm) obtained by dividing the difference between the maximum value Np MAX and the minimum value Np MIN of the frequency constant Np in the temperature range of 20 to 100°C by the temperature difference ΔT and further dividing the quotient by the frequency constant Np₃₀ at a temperature 30°C, unless specified otherwise. With respect to PZT, however, since it is curved in a concave, the quotient is divided by Np₂₀ and N₇₀ and the temperature coefficients both in the positive direction and in the negative direction are shown. Additionally, the reason why the temperature range is restricted to below 100°C is that this range is considered to be a generally practical temperature range for a PZT piezoelectric ceramic. PZT 52 to PZT 55 in Fig. 7 represent single-layer PZT piezoelectric ceramics in which each numeral is equal respectively to 100 times the value of x in the general formula PbZr x Ti 1-x . For example, 100x = 53 represents a PZT piezoelectric ceramic correspond to the formula PbZr 0.53 Ti 0.47 O₃, herein referred to as "PZT 53". As shown in parentheses in the graph, the temperature coefficients of the conventional single-layer PZT piezoelectric ceramics are in the order of three figures (PZT 55: -160 × 10⁻⁶, PZT 54: -440 × 10⁻⁶, PZT 53: -345 × 10⁻⁶, +345 × 10⁻⁶, PZT 52: -460 × 10⁻⁶). A piezoelectric ceramic the piezoelectric characteristic of which greatly depends on temperature is not generally suitable for use as an electronic circuit element such as a resonance filter or a wave filter. As a result of studies undertaken by the present inventors to solve this problem, it has been found that it is easy to obtain a piezoelectric ceramic having a low temperature dependence without the need for introducing a third component, by laminating not less than two piezoelectric ceramics at least one of which has a temperature coefficient having a polarity opposite to the polarity of that of the other layer(s) and which can be sintered at a temperature at which the ceramics form a solid solution, and sintering these ceramics at a comparatively low temperature which does not allow these ceramics to form a solid solution. The present invention has been achieved on the basis of this finding. The document Jap. J. Appl. Phys. 28 (1989), Suppl. 28-2, pp. 114-116, H. Komiya et al. discloses the production of multilayer piezoelectric composites by a tape casting technique. Mixed raw materials are calcined at 700°C for PZT (Pb-Zr-Ti) system and 800°C for PZNT (Pb-Zr-Nb-Ti) system materials respectively. The calcined powders are ground below 1µm. The particular sizes for the starting material and the ceramic in accordance with the present invention are not mentioned. The bonded laminate is sintered at 1050°C for PZT and at 1280°C for PZNT system materials. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a process for producing a multilayer piezoelectric composite having a low temperature dependence. A process for producing a multilayer piezoelectric composite according to the present invention comprises the steps as set forth in the claim. The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow sheet of a tape casting process; Figs. 2 to 5 are schematic sectional views of embodiments of multilayer piezoelectric composites according to the present invention; Fig. 6 is a graph showing the piezoelectric temperature characteristics of the embodiments of multilayer piezoelectric composite shown in Figs. 2 to 5, and Fig. 7 shows the piezoelectric temperature characteristics of various single-layer piezoelectric elements. DESCRIPTION OF THE PREFERRED EMBODIMENTS It is necessary in a multilayer piezoelectric composite produced in accordance with the present invention that the polarity of the temperature coefficient of the frequency constant of at least one layer of the bonded laminate is opposite to the polarity of those of the other layers. This is because the present invention aims at producing a piezoelectric element having a piezoelectric characteristic which has a low temperature dependence. The polarity of the temperature coefficient means a positive or negative polarity of the temperature coefficient of the frequency constant in a temperature range which is considered to be a practical range. It is also necessary that each layer of the bonded laminate is composed of a green sheet of a low-temperature sintering PZT piezoelectric ceramic. If a high-temperature sintering PZT piezoelectric ceramic is used, it is impossible to obtain a bonded laminate having a low temperature dependence because interdiffusion of ions is caused between layers during sintering the bonded laminate. In the process for producing a green sheet of a PZT piezoelectric ceramic, it is necessary to use a fine PZT powder having an excellent low-temperature sintering property. As an example of such a PZT powder having an excellent low-temperature property, lead zirconate titanate having a particle size of not more than 0.5 µm will be cited which is obtained by heat treating crystalline zirconium titanate represented by the general formula Zr x Ti 1-x O₂ and having a particle diameter of not more than 0.3 µm and lead oxide at a temperature of not higher than 800°C, and pulverizing the reaction product as disclosed in Japanese Patent Application No. 64695/1989, which was filed by the present applicant. Alternatively, it is possible to use a fine PZT powder obtained by an alkoxide method (sol-gel method), partial oxalic acid method, autoclaving method or fine material powder calcinating method. If any of the PZT piezoelectric ceramics has a high sintering temperature, the laminate is forced to be sintered at a high temperature, inconveniently resulting in interdiffusion between layers. This is the reason why each layer of the bonded laminate is composed of a green sheet of a low-temperature sintering PZT piezoelectric ceramic powder and the bonded laminate is sintered at a temperature which does not cause interdiffusion between layers. A process for producing a multilayer piezielectric composite in accordance with the present invention will be explained hereinunder with reference to the flow sheet shown Fig. 1 while citing a tape casting process as an example. Starting materials are first weighed so that each layer has a desired composition, and mixed together with acetone by a wet process in a ball mill containing zirconia balls for about 10 hours. Thereafter, the mixture is calcinated at a temperature of about 700°C for about 1 hour. The thus-obtained calcinated PZT powder is pulverized in a ball mill and a planetary mill into particles about 1 µm in diameter, and appropriate amounts of solvent and binder are added thereto followed by thorough agitation. The resulting mixture is defoamed under a reduced pressure to obtain a slurry. The slurry obtained is molded into a green sheet of a piezoelectric ceramic powder of about 100 µm thick by a tape casting process while using a doctor blade. In the same way, plural kinds of green sheets which have different polarities and absolute values of temperature coefficient are produced in an appropriate number for each in accordance with the compositions of composite piezoelectric elements required. Finally, an appropriate number of sheets are selected for each kind of PZT piezoelectric ceramic having a different polarity so as to produce a multilayer piezoelectric composite having a temperature coefficient corresponding to the purpose of use, and these selected green sheets are laminated. After the thermocompression bonding of these laminated sheets at a temperature of about 80°C under an applied pressure of about 300 kg/cm², the bonded laminate is sintered at a temperature of about 800 to 1,100°C, preferably no higher than 1,100°C. Additionally, although the ordinary sintering temperature of a piezoelectric ceramic is considered to be not lower than 1,200°C ( Ceramics Dielectric Engineering by Kiyoshi Okazaki, p. 347, published by Gakken-sha (1983)), it is impossible to prevent unification between layers at such a high sintering temperature. For example, in the case of a bonded laminate obtained by laminating green sheets having a plurality of compositions in the above-described manner which are obtained by thoroughly mixing 2 mol of commercially available titanium oxide (A-110, reference code produced by Sakai Chemical Industry Co., Ltd.) and 2 mol of lead oxide (Special L, produced by Dai-Nippon Toryo Co., Ltd.) in a ball mill, calcinating the mixture at 1,000°C and molding the mixture, since the sintering temperature of the bonded laminate becomes not lower than 1,200°C, it becomes a complete solid solution. The temperature characteristic of this bonded laminate therefore is that the temperature coefficient has a peak of an intermediate value of the respective layers, and it is impossible to obtain a flat Np temperature characteristic coefficient like that of a laminated composite piezoelectric element according to the present invention. In the above-described manner, a multilayer piezoelectric composite is produced in accordance with the present invention. What is important in producing it is that it is necessary to sinter a bonded laminate at a temperature which hardly allows a solid solution to be formed between the layers having different compositions and yet makes each layer sufficiently dense to prevent the unification of the compositions (namely, the formation of a solid solution between layers) by diffusion of the compositions between the layers having different compositions. This is because if a solid solution is formed between layers, it is impossible to obtain the target piezoelectric ceramic having a low temperature dependency in a temperature range (ordinarily 0 to 100°C). It is possible to obtain a piezoelectric element having a further lower temperature tendency without changing the total thickness of the multilayer piezoelectric composite by deliberately selecting the compositions of piezoelectric ceramics to be laminated and appropriately selecting the thickness of each of the layers having thc respective compositions in the present invention. It is also possible to add an appropriate amount of relaxor such as a rare earth element, oxide of Cr, Mn, Fe, Nb or the like, perovskite of Ba(Mg 1/3 Nb 2/3)O3 , barium titanate or the like, Pb(Mg 1/3 Nb 2/3)O3 , Pb(Ni 1/3 Nb 2/3)O3 , Pb(Co 1/3 Nb 2/3)O3 and Pb(Fe 1/2 Nb 1/2)O3 to a PZT powder. The order of lamination of layers of different compositions is not specified so long as the bonded laminate produced does not cause strain and diffusion between layers at the time of sintering. The present invention is not restricted to a process for producing a multilayer piezoelectric composite in which the absolute value of the temperature coefficient of the frequency constant of the bonded laminate is smaller than the absolute value of the temperature coefficient of the frequency constant of any of the layers, but includes a process for producing a multilayer piezoelectric composite in which the absolute value of the temperature coefficient of the frequency constant of the bonded laminate is smaller than the absolute value of the temperature coefficient of the frequency constant of one of the layers. For example, a multilayer piezoelectric composite having a temperature coefficient of +20 which is produced by laminating an appropriate number of green sheets having a temperature coefficient of + 100 and an appropriate number of green sheets having a temperature coefficient of -10, and a process for producing the same, are included in the scope of the present invention. The present invention will be explained in more detail with reference to the following examples. A. Production of green sheet of piezoelectric ceramic Green sheets of piezoelectric ceramics having various compositions were produced by a tape casting process by using equimolar quantities of zirconium titanate (Zr x Ti 1-x )O₂ powder having an average particle diameter of 0.1 µm and lead oxide powder (PbO) as starting materials. (Process Example 1) A green sheet of Pb(Zr 0.52 Ti 0.43 )O₃ was produced. The starting materials were weighed so as to have the above composition and mixed together with acetone by a wet process in a ball mill containing zirconia balls for about 10 hours. Thereafter, the mixture was calcinated at a temperature of 700°C for 1 hour. The thus-obtained calcinated PZT powder was pulverized in a ball mill or planetary mill or the like into a powder having an average particle diameter of 0.2 µm. To 100 g of the powder obtained were added 30 g of a solvent and 7 g of a binder and the mixture was thoroughly mixed in a ball mill. The resulting mixture was defoamed under reduced pressure to obtain a slurry. The compositions of the solvent and the binder used in this example are shown in the following. "Part" means "part by weight". <Solvent> <Binder> The slurry obtained in this way was molded by a tape casting process while using a doctor blade and the molded product was naturally dried for 24 hours to obtain a green sheet of a piezoelectric ceramic powder 100 µm thick. (Process Example 2) A green sheet of Pb(Zr 0.53 Ti 0.47 )O₃ was produced. A green sheet of a piezoelectric ceramic powder 100 µm thick was produced in the same way as in Process Example 1 except for using equimolar quantities of zirconium titanate (Zr 0.53 Ti 0.47 )O₂ powder and lead oxide powder (PbO) as the starting materials so as to have the above-described composition. (Process Example 3) A green sheet of Pb(Zr 0.54 Ti 0.46 )O₃ was produced. A green sheet of a piezoelectric ceramic powder 100 µm thick was produced in the same way as in Process Example 1 except for using equimolar quantities of zirconium titanate (Zr 0.54 Ti 0.46 )O₂ powder and lead oxide powder (PbO) as the starting materials so as to have the above-described composition. (Process Example 4) A green sheet of Pb(Zr 0.55 Ti 0.45 )O₃ was produced. A green sheet of a piezoelectric ceramic powder 100 µm thick was produced in the same way as in Process Example 1 except for using equimolar quantities of zirconium titanate (Zr 0.55 Ti 0.45 )O₂ powder and lead oxide powder (PbO) as the starting materials so as to have the above-described composition. B. Production of multilayer piezoelectric composite (Example 1) The green sheets of Pb(Zr 0.52 Ti 0.48 )O₃ obtained in Process Example 1 and the green sheets of Pb(Zr 0.54 Ti 0.46 )O₃ obtained in Process Example 3 were laminated in a ratio of numbers of sheets of 1 : 1 in the manner shown by the section in Fig. 2. The laminate was subjected to thermocompression bonding at a temperature of 80°C and an applied pressure of 300 kg/cm², and the bonded laminate was sintered at 1,000°C for 2 hours to obtain a sample PZT52-54 (1 : 1) of a multilayer piezoelectric composite by the process of the present invention. (Example 2) A sample PZT52-55 (1 : 2) of a multilayer piezoelectric composite was obtained according to the present invention in the same way as in Example 1 except that green sheets of Pb(Zr 0.52 Ti 0.48 )O₃ obtained in Process Example 1 and green sheets of Pb(Zr 0.55 Ti 0.45 )O₃ obtained in Process Example 4 were laminated in a ratio of numbers of sheets of 1 : 2 in the manner shown by the section in Fig. 3. (Example 3) A sample PZT52-55 (1 : 3) of a multilayer piezoelectric composite was obtained according to the present invention in the same way as in Example 1 except that green sheets of Pb(Zr 0.52 Ti 0.48 )O₃ obtained in Process Example 1 and green sheets of Pb(Zr 0.55 Ti 0.45 )O₃ obtained in Process Example 4 were laminated in a ratio of numbers of sheets of 1 : 3 in the manner shown by the section in Fig. 4. (Example 4) A sample PZT52-55 (1 : 4) of a multilayer piezoelectric composite was obtained according to the present invention in the same way as in Example 1 except that green sheets of Pb(Zr 0.52 Ti 0.48 )O₃ obtained in Process Example 1 and green sheets of Pb(Zr 0.55 Ti 0.45 )O₃ obtained in Process Example 4 were laminated in a ratio of numbers of sheets of 1 : 4 in the manner shown by the section in Fig. 5. C. Measurement of piezoelectric temperature characteristics Silver was sputtered on both side surfaces of each of the samples produced in Examples 1 to 4 to form electrodes. The temperature characteristic of the frequency constant Np, which is a typical index of piezoelectric characteristic, of each sample was measured by a resonance - anti-resonance method by using an LF impedance analyzer (Model HP4192A, produced by Hewlett Packard). Fig. 6 is a graph showing the measured temperature characteristic of the frequency constant Np of each sample wherein the ordinate represents Np and the abscissa represents temperature (°C). The piezoelectric temperature characteristics of the conventional single-layer piezoelectric elements shown in Fig. 7 and explained under the subtitle of Description of the Prior Art in this specification are also shown in Fig. 6 for the sake of comparison. The temperature coefficient of each sample at 30°C calculated on the basis of Figs. 6 and 7 is shown in the following table. It is observed from Fig. 6 and the above table that while the conventional single-layer piezoelectric elements have a high temperature dependence as represented by a temperature coefficient as large as a three-figure ppm, any of the multilayer piezoelectric composites produced in accordance with the present invention obtained in Examples 1 to 4 has a very low temperature dependence as represented by the temperature coefficient as small as a two-figure ppm in a practical temperature range. As described above, since a multilayer piezielectric composite produced in accordance with the present invention has a piezoelectric characteristic having a low temperature dependence, it is suitable for uses in which such a characteristic is required, especially as an electronic circuit element such as a resonance filter or a wave filter.

Claims: A process of producing a multilayer piezoelectric composite comprising the steps of: (a) obtaining lead zirconate titanate having a particle diameter of not more than 0.5 µm by heat treating equimolar quantities of crystalline zirconium titanate represented by the general formula Zr x Ti 1-x O₂ and having a particle diameter of not more than 0.3 µm and lead oxide at a temperature of not more than 800°C and pulverising the resultant product; (b) molding plural kinds of said pulverised product from (a) to prepare respective plural green sheets of lead zirconate titanate piezoelectric ceramic, (c) laminating and bonding together under pressure said green sheets from (b) to prepare a bonded laminate at least one layer of which has a temperature coefficient of frequency constant whose polarity is opposite to the polarity of that of the other layer or layers; and (d) sintering the bonded laminate at a temperature not higher than 1100°C so that substantially no interdiffusion takes place between the layers, thereby providing a multilayer piezoelectric composite whose absolute temperature coefficient of frequency constant is smaller than that of any or one of the layers.

Inventors: ABE KAZUNOBU, FUKAI KIYOSHI, OAKI MASASHI, SAKATA KOICHIRO, TAKENAKA TADASHI, ABE, KAZUNOBU, FUKAI, KIYOSHI, OAKI, MASASHI, SAKATA, KOICHIRO, TAKENAKA, TADASHI
Applicants: SAKAI CHEMICAL INDUSTRY CO, SAKAI CHEMICAL INDUSTRY CO., LTD.,
Application year: 1990
Main classification: H01L41
IPC codes: H01L41