PEROVSKITE-TYPE CERAMIC MATERIAL AND A PROCESS FOR PRODUCING THE SAME

Abstract: Two or more perovskite-type ceramic materials of different compositions that have been pulverized, are pre-­ baked separately to obtain two or more peroviskite-type ceramic materials having large dielectric constants and different Curie temperatues. These materials are then pulver­ ized and are mixed in a ratio which may be arbitrarily selected, provided that every material is included in said mixture, i.e., no material may be omitted, and are sintered to prepare a perovskite-type ceramic material without permit­ ting it to become a perfect solid solution. A ceramic material obtained by mixing and sintering a composition 0.1 of Pb (Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃ -type ceramic material and a composition 0.5 at an equal mol percent, exhibits a dielectric constant of about 6,800 at -20°C, about 12,000 at 20°C, and about 7,200 at 80°C, i.e., exhibits large dielectric constants over a wide range of temperature. If this perovskite-type ceramic material is used for a ceramic dielectric layer of a laminated capacitor, a laminated ceramic capacitor which is small yet which has a large capacity can be obtained, the electrostatic capacity thereof being little dependent upon the temperature and voltage.

Description: TECHNICAL FIELD The present invention relates to a perovskite ceramic having a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength, which is favorably employed in a ceramic dielectric layer of a small-size ceramic capacitor having a large capacity and small dependences of electrostatic capacity on the temperature and the voltage, and a fabrication method thereof. BACKGROUND ART A ceramic capacitor having a large capacity is desired to have small dependences of electrostatic capacity on the temperature and the voltage for facilitating designing of an electric power circuit and stabilizing the circuit characteristics. For this reason, a perovskite ceramic employed in a ceramic dielectric layer of a ceramic capacitor is desired to have a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength. On the other hand, in order to attain a large capacity, a sharp peak of dielectric constant accompanying a phase transition of the perovskite ceramic is generally utilized. Thus, the large capacity ceramic capacitor has such a disadvantage that the dependences of dielectric constant on the temperature and the electric field strength may be higher as the dielectric constant is higher. For the purpose of obviating the defect, intensive investigations have been being made for improving the temperature dependence of dielectric constant, while not so many studies have been being made for improving the electric field strength dependence of dielectric constant. Fig. 1 illustrates a process of preparing a ceramic which has heretofore been practiced for securing an improved dependence of dielectric constant on the temperature. Powders of raw materials are so weighed as to provide a composition ratio of a desired composition, to which a depressor agent is added for improving the temperature dependence of dielectric constant. After wet-mixing and drying, calcination is made to effect crystallization of the composition, followed by crushing. The crushed composition is subjected to pressing and sintering. Thus, a ceramic is prepared. The above-mentioned addition of the depressor agent to the composition in the course of weighing dulls the peak of dielectric constant accompanying the phase transition of the crystals, thus providing a small temperature dependence of dielectric constant. As an instance of an improvement of the temperature dependence of dielectric constant according to the above-mentioned method, Coffeen W.W. reported the results of addition of a metal stannate as the depressor agent to BaTiO₃ as a perovskite ceramic material (see J. Amer. Ceram. Soc., Vol. 37, pages 480-489, 1954). According to Coffeen, when a small amount of Bi₂(SnO₃)₃ is added to BaTiO₃ , the temperature dependence of dielectric constant is markedly improved, but the dielectric constant is decreased to about 1,000 or less. On the other hand, when 13 mol% of PbSnO₃ is added to BaTiO₃, the dielectric constant at room temperature is about 7,000, but the temperature dependence of dielectric constant is large. Okazaki et al. reported that, when NiSnO₃ or La₂O₃·3TiO₂ is added as the depressor agent to BaTiO₃, the ceramic turns to have a smaller temperature dependence of dielectric constant, and explained that the cause therefor may be attributed to formation of two phases in the ceramic (Journal of the Ceramic Society of Japan, Vol. 73, pages 106-112, 1965). However, in the case of the report of Okazaki et al., as low a dielectric constant as mere 3,000 or less was obtained. Jonker, G.H. reported that two phases, a Ca-rich phase containing a large amount of CaTiO₃ and a Ba-rich phase containing a large amount of BaTiO₃, are formed in a ceramic of a binary system of BaTiO₃ and CaTiO₃, resulting in a smaller temperature dependence of dielectric constant (see Philips Tech. Rev., Vol. 117, pages 129-137, 1955). However, since the dielectric constant of CaTiO₃ is as low as 130 at room temperature, the maximum value of dielectric constant in the binary ceramic after formation of the two phases is as low as 2,000 or less. Hennings, D. et al. reported that, when CdBi₂Nb₂O₉ is added to BaTiO₃, a ferroelectric phase and a paraelectric phase are formed in grain cores and grain shells, respectively, of crystalline grains, resulting in a smaller temperature dependence of dielectric constant (J. Amer. Ceram. Soc., Vol. 67, pages 249-254, 1984). In this case, however, the maximum value of dielectric constant is merely about 5,000. Particularly, in the case of a small temperature dependence of dielectric constant, the maximum value of dielectric constant is as low as about 3,000. US-A-4 078 938 discloses high dielectric ceramic compositions consisting essentially of Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃, wherein x ranges from about 0.2 to 0.5. This composition is obtained by a process which comprises the following steps: The raw material powders, for example, of PbO, Fe₂O₃, WO₃ and Nb₂O₅ are weighed so that the compositions of Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃, are obtained. These powders are homogeneously mixed and dried afterwards. The mixtures are calcined at a temperature in the range from 750°C to 850°C. Due to this calcination the mixtures become perovskite ceramic materials having a composition of Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃. These ceramic materials are complete solid solutions in which Pb(Fe 2/3 W 1/3 ) x and Pb(Fe 1/2 Nb 1/2 ) 1-x O₃ are dissolved mutually. The calcined ceramic materials are crushed and molded into a solid cylinder shape under pressure. The molded ceramic materials are sintered at a temperature in the range from 900°C to 950°C. The sintered perovskite ceramics are composed of crystals of a single composition of Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃. As described above, no perovskite ceramics having a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength have heretofore been obtained. Besides, the method comprising addition of a depressor agent requires a search for a specific depressor agent capable of providing a desired dependence of dielectric constant on the temperature in addition to a search for a perovskite ceramic material, and has disadvantages of liability to a decrease in resistivity, a shift of Curie temperature around which a high dielectric constant is exhibited, and/or a decrease in mechanical strength because of the addition of the depressor agent. Therefore, a difficulty has been encountered in controlling the temperature dependence of dielectric constant over a wide temperature range. DISCLOSURE OF THE INVENTION The first object of the present invention is to provide a perovskite ceramic; which obviates the above-mentioned disadvantages of conventional perovskite ceramic materials without spoiling the excellent characteristics essentially inherent in perovskite ceramic materials as the basic material; which has a high dielectric constant, a small temperature dependence of dielectric constant over a wide temperature range, an easy controllability of the above-mentioned dependence even without any depressor agent, and a small dependence of dielectric constant on the electric field strength; and which can be suitably employed in a ceramic dielectric layer of a ceramic capacitor. The second object of this invention is to provide a fabrication method of a perovskite ceramic having a high dielectric constant, a small dependence of dielectric constant on the temperature over a wide temperature range, and a small dependence of dielectric constant on the electric field strength. The third object of this invention is to provide a multilayer ceramic capacitor having a small size, a large capacity, and small dependences of electrostatic capacity on the temperature and the voltage, which uses a perovskite ceramic having a high dielectric constant and small dependences of dielectric constant on the temperature and the electric field strength in a ceramic dielectric layer of the capacitor. In order to attain these objects, the perovskite ceramic of the present invention is characterized by comprising two kinds of perovskite ceramic materials having respective high dielectric constant and mutually different Curie temperatures coexisting therein without forming a complete solid solution thereof, a first kind of perovskite ceramic material having the formula wherein 0.1 ≦ x₁ ≦ 0.2, and a second other kind of perovskite ceramic material having the formula wherein 0.5 ≦x₂≦0.9, said two kinds of perovskite ceramic material coexisting in the mixing mole fraction ratio of 0.5 ≦ Y ≦ 0.8. Here, the perovskite ceramic of this invention comprising two kinds of perovskite ceramic materials conjointly present therein without complete solid solution thereof may be a ceramic material prepared by separately subjecting raw material powders of two kinds of perovskite ceramic materials having mutually different compositions to calcination to form two kinds of perovskite ceramic materials having mutually different Curie temperatures, crushing the materials thus formed, mixing the crushed materials at an adequate ratio, and sintering the mixture without complete solid solution of the materials. The temperature of calcination of at least one kind of the above-mentioned two kinds of perovskite ceramic materials may be the same as or higher than the temperature of sintering the mixture of the perovskite ceramic materials. The two kinds of perovskite ceramic materials may each have a dielectric constant of 1,000 or more at room temperature. The two perovskite ceramic materials may have respective compositions represented by a formula: wherein 0,1≦x₁≦0,2 for the first kind of Perovskite ceramic material, and wherein 0,5≦x₂≦0,9 for the second kind of Perovskite ceramic material. To the two kinds of perovskite ceramic materials may be added a minute amount of an oxide of a metal incapable of substituting for atoms occupying the body-centered positions of crystal lattices of the perovskite ceramic materials which oxide has a higher melting point than those of the perovskite ceramic materials. The oxide that may be added may be a kind selected from Dy₂O₃, Gd₂O₃ and Sm₂O₃. The process for preparing a perovskite ceramic according to the present invention is characterized by comprising the step of separately subjecting raw material powders of two kinds of perovskite ceramic materials having mutually different compositions to calcination to form two kinds of perovskite ceramic materials having mutually different Curie temperatures, the step of separately crushing and mixing the perovskite ceramic materials thus formed, and the step of sintering the resulting mixture of the perovskite ceramic materials without complete solid solution of the two kinds of perovskite ceramic materials. The temperature of calcination of at least one kind of the above-mentioned two kinds of perovskite ceramic materials may be the same as or higher than the temperature of sintering the mixture of the perovskite ceramic materials. The two kinds of perovskite ceramic materials may each independently have a dielectric constant of 1,000 or more. The multilayer ceramic capacitor of the present invention is characterized by comprising a ceramic dielectric layer made of a perovskite ceramic comprising coexisting two kinds of perovskite ceramic materials having mutually different compositions. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart illustrating a conventional fabrication method of a ceramic dielectric; Fig. 2 is a phase diagram showing a relationship between the composition ratio of perovskite ceramic and temperatures To and Ts of crystallization and melting decomposition, respectively, which may give a guideline on the calcining temperature in the present invention; Fig. 3 is a conceptional illustration of the state of crystals in the perovskite ceramic of this invention; Fig. 4 is a flow chart illustrating an embodiment of the fabrication method of a perovskite ceramic according to the present invention; Figs. 5 and 6 are X-ray diffraction charts of the perovskite ceramics according to the present invention; Fig. 7 is a chart showing a variation of the temperature dependence of dielectric constant with conditions of calcination of a perovskite ceramic composed of an equimolar mixture of compositions with x=0.1 and with x=0.5 in a formula: Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃, in an embodiment of the present invention; Fig. 8 is a chart showing a relationship between the composition ratio and the Curie temperature; Fig. 9 is a chart showing a variation of the temperature dependence of dielectric constant with different relative densities.; Fig. 10A is an illustration of the state of crystals in the perovskite ceramic of the present invention; Fig. 10B is an equivalent circuit diagram of Fig. 10A; Fig. 11 is a chart showing the temperature dependence of dielectric constant for unit compositions; Figs. 12 and 13 are charts showing the temperature dependences of dielectric constant based on the experimental values and the calculated values for the samples Nos. 4 and 4H with relative densities of 0.82 and 0.92, respectively; Fig. 14 is an SEM photograph showing a ceramic surface of an embodiment of the perovskite ceramic of the present invention; Fig. 15 is an SEM photograph showing a ceramic surface of a conventional perovskite ceramic; Fig. 16 is a chart showing a variation of the temperature dependence of dielectric constant with the mixing ratio of compositions in an embodiment of the perovskite ceramic of the present invention; Fig. 17 is a chart showing a variation of the temperature dependence of dielectric constant with the combination of composition ratios; Figs. 18 to 22 are concerned with embodiments of the present invention, wherein Figs. 18, 19 and 20 are charts showing variations of dielectric constant with the relative density for the samples Nos. 4, 8 and 10, respectively; Fig. 23 is a chart showing a comparison in the temperature dependence of dielectric constant between an embodiment of the perovskite ceramic of the present invention and a conventional perovskite ceramic; Fig. 24 is a chart showing a comparison in the rate of variation of dielectric constant with electric field strength between an embodiment of the perovskite ceramic of the present invention and a conventional perovskite ceramic; Fig. 25 is a flow chart illustrating another embodiment of the fabrication method of a perovskite ceramic according to the present invention; Fig. 26 is a chart showing a variation of crystal grain size with the amount of Dy₂O₃ added as to an embodiment of the perovskite ceramic of the present invention; Fig. 27 is a chart showing the influence of addition of Dy₂O₃ on the temperature dependence of dielectric constant; Fig. 28 is a flow chart showing still another embodiment of the fabrication method of a perovskite ceramic according to the present invention; Fig. 29 is a flow chart showing an embodiment of the fabrication method of a multilayer ceramic capacitor according to the present invention; Fig. 30 is a flow chart of a conventional fabrication method of a multilayer ceramic capacitor; Fig. 31 is a perspective view of an embodiment of the multilayer ceramic capacitor of the present invention; and Fig. 32 is a chart showing a comparison in the dependency of electrostatic capacity on the temperature between an embodiment of the multilayer ceramic capacitor of the present invention and a conventional multilayer ceramic capacitor. BEST MODE FOR CARRYING OUT THE INVENTION In the Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃ system, the crystallization temperature To and the melting decomposition temperature Ts lower with an increase in the composition ratio x as shown in Fig. 2. Where a composition with x=x₁ (referred to as a "composition x₁") and a composition with x=x₂ (referred to as a "composition x₂") (x₁ < x₂) are used, it is believed that, when a mixture thereof is sintered, a composition with x=x₃ (referred to as a "composition x₃"), which is in the form of a solid solution of the composition x₁ and the composition x₂, may be formed in the crystal grain boundaries of both the compositions as shown in Fig. 3, and that the range of composition ratio for the composition x₃ may be represented by a formula: x₁<x₃<x₂. Accordingly, in order to obtain a perovskite ceramic having the composition x₁ and x₂ coexisting therein (mixed-sintering ceramic), it is necessary in Fig. 2, 1 ○ that the calcining temperatures T₁(x₁) and T₁(x₂) for the compositions x₁ and x₂, respectively, be in ranges of To(x₁)≦T₁(x₁)≦Ts(x₁) and To(x₂)≦T₁(x₂)≦Ts(x₂), respectively, wherein neither melting nor decomposition occurs, and 2 ○ that the sintering temperature T₂ never exceeds Ts(x₂) since the composition B begins to melt and decompose to increase the proportion of the composition x₃ when T₂>Ts(x₂) though as high a T₂ as possible is favorable for raising the ceramic density. Besides, when the T₁ of the composition x₂(=Tx₂), which has a lower Ts than that of the compositions x₁,is such as to be represented by a formula: Tx₂>To(x₂), the reactivity of crystal grains for a sintering reaction during the course of sintering is low, leading to a lowered ceramic density. Thus, calcination of Tx₂=To(x₂) is favorable. Since the temperature dependence of dielectric constant for a mixed-sintering ceramic of x₁-x₂-x₃ thus formed is expressed in the form of a compound of temperature dependences of dielectric constant for respective component compositions in accordance with the mixing ratios thereof, it is lowered. Example 1 Fig. 4 illustrates a fabrication method of a perovskite ceramic from two kinds of perovskite ceramic materials having mutually different Curie temperatures according to an embodiment of this invention. The only differences of the process according to this embodiment from the conventional process of calcination of a ceramic material as shown in Fig. 1 are that the steps of weighing raw material powders, calcining, and crushing are divided into two lines thereof, and that the step of mixing the compositions formed by calcination is added as a subsequent step. Thus, there is no need of a large change from the conventional process. An outstanding feature of the process according to this embodiment is that the existing equipment for conventional calcination of a ceramic material can be employed as such. A mixed-sintering ceramic prepared from a composition with x₁=0.1 (composition 0.1) and a composition with x₂=0.5 (composition 0.5) among compositions expressed by the formula: will now be explained. PbO, Fe₂O₃, WO₃ and Nb₂O₅ were used as raw material powders. They were so weighed as to provide values as shown in Table 1 in accordance with the process as shown in Fig. 4. 300 ml of pure water was added to each of two compositions thus provided, and the resulting mixture was subjected to wet-mixing in a ball mill for 24 hours, followed by drying in air of 150°C for two hours. Subsequently, the composition 0.1 and the composition 0.5 were respectively calcined in air, and crushed to an average grain size of 1µm or less by a ball mill. Thereafter, 50 mol% each of the two compositions were mixed, and about 1 ml of a 0.5% polyvinyl alcohol solution was added as the binder to 10g of the mixed composition, followed by pressing under a pressure of 3 tons/cm² to prepare pellets having a diameter of 12 mm and a thickness of 0.7 mm. Conditions to be checked in obtaining a mixed-sintering ceramic from the composition 0.1 and the composition 0.5 in reference to Fig. 2 are as follows. 1 ○ As to the calcining temperature T₁(=T 0.1 ) of the composition 0.1, 850°C≦T 0.1 ≦1,150°C. 2 ○ As to the calcining temperature T₁(=T 0.5 ) of the composition 0.5, T 0.5 =850°C 3 ○ As to the sintering temperature T₂(=Ts(0.5)), T₂=1,000°C Thus, calcination of the composition 0.1 and the composition 0.5 was carried out in air at respective temperature as shown in Table 2. X-ray diffraction patterns of the calcined compositions were measured with a CuKα radiation in a range of 2ϑ=20-90° to confirm crystallization. The pressed-formed pellets were sintered in air at 1,000°C. The relative densities (=apparent density d/theoretical density d o ) of the sintered ceramics were 0.75 to 0.87. In the same way as described above, X-ray diffraction patterns of these ceramics were measured. The results are partially shown in Figs. 5 and 6. Silver was electroded on both surfaces of the sintered ceramics for measurements of dielectric properties of the ceramics. Thereafter, the dielectric constant ε r and dielectric loss tangent tan δ of every ceramic were measured at 1kHz, 1 Vrms, in a measurement temperature range of -40 to 120°C. The resistivity was measured at 50V dc at room temperature. The results are shown in Fig. 7 and Table 2. Incidentally, each No. in Figs. 5, 6 and 7 corresponds to the sample No. in Table 2. (a) X-Ray Diffraction According to the results of X-ray diffraction, all the ceramics in Table 2 proved to have a perovskite structure, and, hence, not to be different in the crystal system itself. However, high-angle diffraction peaks proved to differ from sample to sample. As an instance of this, Fig. 5 shows a comparison in diffraction peaks of the (222) plane. As is apparent in the figure, in the samples Nos. 3, 4 and 5 prepared at T 0.1 ≧1,000°C, the number of diffraction peaks increased from 2 to 3, and the peak angles changed. For examining these changes in more detail, a comparison in diffraction peak of the (222) plane were made between the sample No. 1 prepared at T 0.1 <1,000°C, the sample No. 4 prepared at T 0.1 ≧1,000°C, and the composition 0.1 and the composition 0.5 as the basic components as shown in Fig. 6. As is apparent from the figure, the diffraction peaks of the sample No. 1 prepared at T 0.1 <1,000°C do not agree with those of either the composition 0.1 or the composition 0.5. On the other hand, when a comparison in diffraction peak angles was made between the sample No. 4 prepared at T 0.1 ≧1,000°C and the other compositions, the diffraction peaks of both the compositions 0.1 and 0.5 are observed in the sample No. 4. This may suggest formation of a mixed-sintering ceramic from the compositions 0.1 and 0.5. Additionally stated, the same results as described above were obtained as to the (221) plane and the (331) plane. (b) Dependence of Dielectric Constant on Temperature The dependences of dielectric constant on the temperature for the ceramics shown in Table 2 are shown in Fig. 7. As illustrated in the figure, the samples Nos. 1 and 2 prepared at T 0.1 < 1,000°C provide sharp peaks of single dielectric constant around 30°C. According to a relationship between the composition ratio x and the Curie temperature Tc as shown in Fig. 8 (see H. Takamizawa et al.: NEC Research and Development, 64, 10-17 (1982)), the peak temperature around 30°C corresponds to the composition with x=0.3 (composition 0.3), suggesting a complete solid solution composed of equimolar amounts of the compositions 0.1 and 0.5. The mixed-sintering ceramic of the samples Nos. 3, 4 and 5 prepared at T 0.1 ≧1,000°C provides a weak peak around 80°C in addition to a peak around 20°C. The collation of the results with Fig. 8 in the same way as described above suggests formation of a mixed-sintering ceramic of a composition with x≃0.3 (composition ≃0.3) and the composition 0.1. The dielectric constant of a mixed-sintering ceramic varies depending on the density of the ceramic as will be described later in detail. Fig. 9 shows temperature dependences of dielectric constant for the samples Nos. 4 and 4H having ratios d/d o of apparent density d to theoretical density d o (relative densities) of 0.82 and 0.92, respectively. The positions of the peaks around 20°C and 80°C are not different, but the mixed-sintering ceramic having the higher density is higher in dielectric constant. (c) Analysis of Mode of Coexistence By Equivalent Circuit The mixed-sintering ceramics proved to be a mixture of the composition 0.1 and the composition 0.5 according to the X-ray diffraction but a mixture of the composition 0.1 and the composition ≃0.3 according to the temperature dependence of dielectric constant. In view of this, in order to presume a proportion of each component, an equivalent circuit as shown in Fig. 10B was conceived with a hypothetical model in which the composition 0.3 might be formed around crystal grain boundaries of the compositions 0.1 and 0.5 as shown in Fig. 10A on the assumption that the compositions 0.1, 0.3 and 0.5 may coexist in a mixed-sintering ceramic. From Fig. 10B, the dielectric constant ε rmix (T) of the mixed-sintering ceramic at a temperature T can be expressed by the following equation. wherein d 0.1 , d 0.3 and d 0.5 are thicknesses of the compositions with x=0.1, 0.3 and 0.5, respectively; and ε r0.1 (T), ε r0.3 (T) and ε r0.5 (T) are ε r values of the compositions with x=0.1, 0.3 and 0.5 at a temperature T, respectively. The d mix in the equation (1) was set to be 15µm since the average grain size of the sample No. 4 in Table 2 was 7.5µm (Martin diameter). On the other hand, the ε r0.1 (T), ε r0.3 (T) and ε r0.5 (T) was determined from the temperature dependences of dielectric constant for the respective compositions as shown in Fig. 11. The values of ε rmix in a range of -30 to 90°C were found using the thickness d 0.1 , d 0.3 and d 0.5 as the parameters, and compared with those of the sample No. 4 having a relative density of 0.82 as shown in Fig. 12. It has been clarified from the figure that, when d 0.1 is approximately equal to d 0.3 , and when d 0.5 is about three times as large as d 0.1 or d 0.3 , the calculated values substantially agree with the experimental values with respect to the sample No. 4 having a relative density of 0.82. On the other hand, the mixed-sintering ceramic highly densified to have a relative density of 0.92 (sample No. 4H) had a smaller average grain diameter of 3.5µm. Therefore, it has been clarified that, when calculation is made using d 0.1 , d 0.3 and d 0.5 as parameters with d mix of 7µm in the equation (1), the calculated values substantially agree with the experimental values in the case of d 0.1 : d 0.3 : d 0.5 = 1:1:5 as shown in Fig. 13. The Lichtenecker's formula is well known as an empirical formula giving the dielectric constant of a mixture, and expressed by the following equation. In the equation (2), a, b and c are respective volume ratios of the composition 0.1, the composition 0.3 and the composition 0.5; and a+b+c=1 . As a result of calculation of the value of ε rmix which was made using ε r0.1 (T), ε r0.3 (T) and ε r0.5 (T) found from Fig. 11 as well as a, b and c as the parameters in the same way as in the case of the equation of the equivalent circuit, it has been clarified that the calculated values substantially agree with the experimental values in the case of a:b:c = 0.15:0.25:0.60. These results may prove good approximation to the experimental values, when consideration is given to the facts that pellets of the sample are disks having a very small thickness in comparison with diameter thereof, that the volume ratio of a:b:c can be regarded as being substantially equivalent to the thickness ratio of d 0.1 :d 0.3 :d 0.5 in the equation of the equivalent circuit, and that the above-mentioned formula of a:b:c = 0.15:0.25:0.60 can be rewritten to read a:b:c: = 1:1.7:4.0 which is considerably close to 1:1:5.0 in the equation of the equivalent circuit. (d) Observation of Crystal by Scanning Electron Microscope (SEM) The crystal structure of the mixed-sintering ceramic (sample No. 4H in Table 5) having a relative density of 0.92 and the same composition as that of the sample No. 4 in Table 2 and prepared under the same calcination conditions as in the case of the sample No. 4 was observed by SEM. The SEM photograph (magnification : x 5,400) of the ceramic surface, which was prepared by polishing the surface of the ceramic, etching the resulting surface with an etching solution composed of 95ml of pure water, 5ml of HCl, and about 0.3ml of HF, and subjecting the etched surface to Au coating, is shown in Fig. 14. The crystal grains included two kinds of grains, large crystal grains and small crystal grains. The peripheral portion of each crystal grain is etched deeper than the central portion thereof to provide an appearance like stairs. This may suggest that the two kinds of crystal grains having mutually different compositions each have regions of a phase of solid solution with the other component in addition to respective regions of essential components. An SEM photograph of a conventional ceramic (composition with x= 1/3) etched in the same manner as described above is shown in Fig. 15. It will be understood that the conventional ceramic consists of uniform crystal grains. As will be understood from the above, when the calcining temperature of the composition 0.1 is the same as or higher than the sintering temperature (1,000°C), the resulting mixed-sintering ceramic is not a complete solid solution but a mixture of the composition 0.1, the composition 0.3, and the composition 0.5. Therefore, the temperature dependence of dielectric constant is reduced. Thus, it has been demonstrated that, when the calcining temperature of a composition to be calcined at the higher temperature, namely the composition 0.1, is set higher than the sintering temperature of a mixture of two compositions, the mixture does not turn to complete solid solution. The ceramics of the samples Nos. 3, 4 and 5 are not only reduced in the variation of dielectric constant ε r with the temperature to about 1/2 those of the samples Nos. 1 and 2, but also comparably small in tan δ to those of the samples Nos. 1 and 2 as shown in Table 2 and relatively high in resistivity. Although the sintered temperature of the composition 0.1 exceeds 1,000°C, a mixture thereof with the composition 0.5 can be calcined at a temperature of 1,000°C below the above-mentioned temperature. Since the lattice constants of the composition 0.1 and the composition 0.5 are 4.016 Å and 4.006 Å, respectively, which are close to each other, the ceramic characteristically does not have any distortion formed and is not reduced in mechanical strength. Example 2 The influence of a mixing mole fraction ratio on dielectric properties was examined. Compositions prepared by so weighing as to provide respective composition ratios of x₁ =0.1 and x₂ =0.5 and separately calcining at 1,050°C and 850°C, respectively were mixed at varied mixing ratios: Y =0.5/(0.1+0.5)(mol%) as shown in Table 3 and sintered in air at 1,000°C. The relative densities of the sintered ceramics were 0.82 to 0.88. Table 3, shows values of dielectric constant, tan δ, and resistivity with respect to the sintered ceramics, too. The table demonstrates that ε r increases as the mixing ratio Y increases. Fig. 16 shows a relationship between the mixing ratio Y and the temperature dependence of dielectric constant. Each No. in Fig. 16 corresponds to the sample No. in Table 3. The results of the sample No. 4 in Table 2 above, which corresponds to Y=50 mol%, were added to the figure. As shown in the figure, with an increase in the mixing ratio Y, the peak around 80°C, which in corresponds to the peak of the composition 0.1, sequentially lowers whereas the peak on the low temperature side rises with a slight shift thereof from 20°C, to 0°C. Therefore, the temperature dependence of dielectric constant in a range of -25°C to 85°C (stipulated as Y characteristic in JIS) increases. It has been demonstrated from the above that the temperature dependence of dielectric constant in a given temperature range (25°C to 85°C in the above case) can be controlled by the mixing ratio Y. Example 3 The influence of a combination of composition ratios on dielectric properties was examined. When the composition ratios x₁ and x₂ are set to be 0.1 and 0.6, 0.7 or 0.9, respectively, the calcination and sintering conditions derived from Fig. 2 are as shown in Table 4. Mixtures having a mixing ratio Y [x₂/(0.1+x₂)] of 50 mol% were sintered in air. The relative densities of the resulting mixed-sintering ceramics were 0.72 to 0.82. Fig. 17 shows the temperature dependence of dielectric constant for the pelletized ceramics. Each No. in Fig. 17 corresponds to the sample No. in Table 4. The results of the sample No. 4 with x₂=0.5 in Table 2 above were added to the figure. Table 4 shows the values of dielectric constant ε r , tan δ, and resistivity at 20°C with respect to the sintered ceramics. As can be seen in the table, there is a tendency that, with an increase in the composition ratio x₂, the resistivity rises, but the ε r lowers, and the tan δ increases. As shown in Fig. 17, in this case where the combination of composition ratios was varied, the mixed-sintering ceramics obtained shows two peaks in the temperature dependence of dielectric constant. As the value of the composition ratio x₂ increases, the peak on the low temperature side sequentially shifts toward the low temperature side though the peak around 80°C, which corresponds to the peak of the composition 0.1, does not shift. With an increase in the value of x₂, the dielectric constant lowers but the temperature dependence thereof becomes smaller in a wide temperature range. Thus, it has been demonstrated that the temperature range wherein the temperature dependence of dielectric constant is small can be controlled by the combination of composition ratios of compositions mixed. Example 4 The influence of the density of a mixed-sintering ceramic on the dielectric constant thereof will be discussed. The relative density d/d o of a mixed-sintering ceramic varies depending on the grain sizes of ceramic materials calcined and crushed, the pressure of pressing into a pellet, etc. As the grain size of the materials to be pressed is smaller, and/or as the pressure at the time of pressing is higher, the relative density is more increased. Mixed-sintering ceramics having varied densities were prepared using the same composition, and calcining and sintering conditions as in the case of the sample No. 4 in Table 2 in Example 1. More specifically, a composition 0.1 in the system was calcined at 1,050°C while a composition 0.5 in the system was calcined at 850°C. After crushing, equimolar amounts of the compositions were mixed and sintered at 1,000°C. The varied densities were provided by varying the crushing time after the calcination and the pressure of press forming. The dielectric constants of the mixed-sintering ceramics having the same composition but varied densities are shown in Fig. 18. In the figure, ε max represents maximum values of dielectric constant appearing 20°C in mixed-sintering ceramics. Fig. 19 shows a relationship between the dielectric constant and the relative density in mixed-sintering ceramics having a composition of the sample No. 8 as in Table 3 in Example 2. In the figure, ε max represents a maximum values of dielectric constant appearing around 0°C, and ε at 20°C represents values of dielectric constant at 20°C. Fig. 20 shows a relationship between the dielectric constant and the relative density in mixed-sintering ceramics having a composition of the sample No. 10 as in Table 4 in Example 3. In the figure, ε max and ε at 20°C represents maximum values of dielectric constant appearing around -20° and values of dielectric constant at 20°C, respectively. As can be seen in Figs. 18 to 20, the dielectric constant of the mixed-sintering ceramic increases with an increase in the relative density. A composition 0.1 of 0.8µm or less in average grain size and a composition 0.5 of 0.5µm or less in average grain size were mixed at a mixing ratio of Y=50 mol% (sample No. 4H) and Y=80 mol% (sample No. 8H), subjected to pressing, and sintered to obtain mixed-sintering ceramics having relative densities of 0.92 and 0.93, respectively. The sintering conditions were the same as in Examples 1 and 2. The temperature dependences of dielectric constant and dielectric properties for the mixed-sintering ceramics thus highly densified (samples Nos. 4H and 8H) are shown in Fig. 21 and Table 5, respectively. These ceramics show marked increases in dielectric constant particularly at lower temperatures than 100°C, as compared with the ceramics of the samples Nos. 4 and 8 having lower densities as shown in Figs. 7 and 16, respectively. Further, the highly densified ceramics (the samples Nos. 4H and 8H) are smaller in tan δ and higher in resistivity as shown in Table 5. The temperature dependences of dielectric constant and dielectric properties for mixed-sintering ceramics having varied combinations of composition ratios and increased densities are shown in Fig. 22 and Table 6, respectively. In the figure, the sample No. 10H is a mixed-sintering ceramic prepared from an equimolar mixture of the composition 0.1 and the composition 0.7, and having a relative density of 0.87. The sintering conditions were the same as in Example 3. The sample No. 12H is a mixed-sintering ceramic prepared by mixing equimolar amounts of the composition 0.2 calcined at 1,050°C and crushed and the the composition 0.5 calcined at 850°C and crushed, pressing the mixture, and sintering the pressed mixture at 1,000°C, and having a relative density of 0.92. The curve for the sample No. 4H in Fig. 21 is inserted here, too. Fig. 23 shows a comparison in temperature dependence of dielectric constant ε r between a ceramic according to the present invention and the conventional ceramics. In Fig. 23, the solid line represents the sample No. 4H highly densified as an embodiment of this invention, and the dot-and-bar line and the broken line represent a Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃ ceramic (wherein x=1/3) and a {(Ba 1-x Cax)O}·m{(Ti 1-y Zry)O₂} ceramic (1.005≦m≦1.03, 0.02≦x≦0.22, 0.10≦y≦0.16), respectively, which were calcined according to the conventional production process as shown in Fig. 1, and which are used as commercially available capacitor materials. In a comparison of the ceramic according to the present invention with the Pb(Fe 2/3 W 1/3 ) 1/3 (Fe 1/2 Nb 1/2 ) 2/3 O₃ ceramic, the ceramic according to the present invention has higher values of dielectric constant ε r at or above 50°C and a much smaller temperature dependence of dielectric constant though it has a lower peak value of dielectric constant ε r . Thus, the ceramic according to the present invention is advantageous in securing high dielectric constants over a wide temperature range. In a comparison of the ceramic according to present invention with the {(Ba 1-x Cax)O}·m{(Ti 1-y Zry)O₂} ceramic, the ceramic according to the present invention shows higher dielectric constants over all temperature range. This will suggest that the ceramic according to the present invention may be very advantageous in securing a small temperature dependence of dielectric constant ε r over a wide temperature range without addition of any depressor agent, as compared with the conventional ceramics. Example 5 Discussion will be made about dependence of dielectric constant on the electric field strength for a perovskite mixed-sintering ceramic according to the present invention. Fig. 24 shows a comparison in electric field strength dependence of dielectric constant at 20°C between a Pb(Fe 2/3 W 1/3 ) x (Fe 1/2 Nb 1/2 ) 1-x O₃ ceramic (x=1/3) calcined according to the conventional production process as shown in Fig. 1, which ceramic is used as a commercially available capacitor material, and the mixed-sintering ceramics according to the present invention (sample No. 4H in Table 5 and samples Nos. 9H and 10H in Table 6). Electric field dependence of dielectric constant are expressed in terms of the rate of ε r change (|Δεr/εr0kV/mm|) based on the dielectric constant ε r (ε r0kV/mm) in the case of no application of electric field (0kv/mm). As shown in the figure, the mixed-sintering ceramics according to the present invention all have smaller electric field strength dependences of dielectric constant than that of the conventional ceramic. Also there is shown a tendency that the electric field strength dependence of dielectric constant decreases with an increase in the difference in the value of x between compositions mixed. For example, the value of |Δε r /ε r0kV /mm | when an electric field of 1kV/mm was applied was 80% in the case of the conventional ceramic, 57% in the case of the mixed-sintering ceramic (sample No. 4H) prepared from the compositions with x₁=0.1 and x₂=0.5, 48% in the case of the mixed-sintering ceramic (sample No. 9H) prepared from the compositions with x₁=0.1 and x₂=0.6, and 37% in the case of the mixed-sintering ceramic (sample No. 10H) prepared from the compositions with x₁=0.1 and x₂=0.7, thus demonstrating a decreasing trend. Thus, the electric field strength dependence of dielectric constant can be arbitrarily decreased by a combination of compositions mixed. Needless to say, the electric field strength dependence of dielectric constant can be controlled by varying the mixing ratio. Example 6 The perovskite ceramics (mixed-sintering ceramics) according to the present invention as described in Examples 1 to 4 are suitable for use in ceramic dielectric layers of capacitors. In the case of a multilayer ceramic capacitor comprising laminated thin ceramic dielectric layers, crystal grains in the sintered ceramic are desired to be small and uniform. For satisfying such a demand, investigations were made to find a mixed-sintering ceramic having small and non-scattered sizes of crystal grains. The perovskite is a double oxide represented by a chemical formula: ABO₃, in which A constitutes a primitive cubic lattice, B is located in the body-centered position thereof, and O is located in the face-centered position thereof. In order to suppress the growth of crystal grains, a chemically stable metal oxide was added in effecting mixed-sintering of two kinds of perovskite ceramic materials having mutually different Curie temperatures without complete solid solution thereof. Such metal oxide is the oxide of an element incapable of occupying the B position of a perovskite ceramic material ABO₃ and has a higher melting point than that of the following perovskite ceramic materials. The influence of the addition was examined. As an instance, a composition with x₁=0.1 (composition 0.1) and a composition with x₂=0.5 (composition 0.5) according to the present invention were used as the two kinds of perovskite ceramic materials, and a metal oxide, Dy₂O₃, was used for suppressing the growth of grains. The results will be explained. Fig. 25 shows the production process. Raw material powders, PbO, Fe₂O₃, WO₃ and Nb₂O₃, were so weighted as to provide values Of x₁ of 0.1 and x₂ of 0.5 as mentioned above to form two compositions followed by separately mixing the compositions together with pure water in a ball mill. After dehydration and drying, the mixtures were respectively calcined in air at 1,050°C and 850°C for one hour, followed by crushing in a ball mill. The compositions after the calcination were subjected to a measurement of X-ray diffraction patterns in a range of 2ϑ=20 to 90° by using a CuKα radiation to confirm formation of the desired perovskite ceramic materials. Equimolar amounts of these two kinds of perovskite ceramic materials were mixed, followed by addition of Dy₂O₃. The resulting mixture was pressed into a pellet of about 12 mm in diameter and about 0.7 mm in thickness by using a polyvinyl alcohol solution as the binder. The pellet was sintered in air at 1,000°C for one hour. The sintered surface of the sintering ceramic was observed by an electron microscope (magnification: x 1,000) to measure the average grain size (Martin diameter) of crystal grains. Silver was electroded on both surfaces of the sintering surface. As to the dielectric properties, the dielectric constant ε r was measured at 1kHz in a temperature range of -40 to 120°C. Fig. 26 shows a relationship between the amount of Dy₂O₃ added and the average grain size and the grain size deviation of crystal grains of the ceramic. The amount of Dy₂O₃ added is expressed in terms of mol% based on the mixture of the materials, the composition 0.1 and the composition 0.5. The number of crystal grains measured was 20. As shown in the figure, as the amount of Dy₂O₃ added is increased to 0.1 mol%, the average grain size of crystal grains decreases to 2.6µm, which is about 1/2 an average grain size of 5.0µm in the case of the ceramic having no Dy₂O₃ added, and the grain size deviation considerably dwindles to 1 to 4µm as against 1.5 to 10µm as the grain size deviation of the ceramic having no Dy₂O₃ added. However, as the amount of Dy₂O₃ added exceeds 0.2 mol%, the average grain size and the grain size deviation increases. Thus, it is understood that the effect is remarkable in suppression of the growth of grains when D₂O₃ is added in an amount within a range of 0.1 to 0.2 mol%. Fig. 27 shows the temperature dependences of dielectric constant for the mixed-sintering ceramics without Dy₂O₃ addition and with 0.1 mol% of Dy₂O₃ added. In the figure, the curve (a) is for the ceramic having no Dy₂O₃ added and the curve (b) is for the ceramic having 0.1 mol% of Dy₂O₃ added. If substitution of other metal ions for B in the positions of B occurs in the perovskite ceramic material (ABO₃), the Curie temperature changes and the peak of dielectric constant observed in the temperature dependence of dielectric constant shifts. However, in a comparison in the temperature dependence of dielectric constant between the mixed-sintering ceramic having no Dy₂O₃ added and the mixed-sintering ceramic having 0.1 mol% of Dy₂O₃ added as shown in Fig. 27, weak peaks of dielectric constant appeared at 20°C and 80°C in either case of the ceramics, and no shift of peaks could be observed. Furthermore, the addition of Dy₂O₃ did not spoil the dielectric constant and the flatness of temperature dependence of dielectric constant. Therefore, it is believed that Dy₂O₃ added may not be diffused into crystal grains of the composition 0.1 and 0.5 as the perovskite ceramic materials but exist mainly in the grain boundaries to exert only a grain growth-suppressing effect. In this Example, the method was explained which comprises separately calcining ceramic materials to be mixed, crushing the calcined materials, mixing the crushed materials at a desired molar ratio while, at the same time, adding a metal oxide capable of suppressing the growth of crystal grains (for example, Dy₂O₃), and sintering the mixture. Alternatively as shown in Fig. 28, however, the same effect as mentioned above can be obtained even according to a method comprising adding a metal oxide capable of suppressing the growth of crystal grains (for example, Dy₂O₃) in the course of weighing raw materials, followed by calcining, crushing, mixing at a desired molar ratio, and sintering. Further alternatively, the same effect can be obtained by adding a metal oxide capable of suppressing the growth of crystal grains to only the perovskite ceramic material showing a high rate of crystal growth during sintering (composition 0.5 in this Example). The oxide to be added is required, for avoiding spoiling the dielectric properties of the ceramic material, to be incapable of solid soluting into the perovskite ceramic material during sintering, particularly to be an oxide of a metal element incapable of substituting for an atom in the position of B (body-centered position) in a general formula: ABO₃. Further, the presence of the oxide in the grain boundaries without substantial solid solution thereof into the perovskite ceramic material is effective for suppressing the growth of crystal grains. In this respect, a chemically stable oxide having a higher melting point than that of the perovskite ceramic materials is effective. As an example of such an oxide, Dy₂O₃ (melting point: 2,375°C) has been mentioned. Gd₂O₃(melting point: 2,395°C) and Sm₂O₃ (melting point: 2,250°C) can exert the same effect, too. Example 7 A multilayer ceramic capacitor was produced which uses a ceramic comprising two or more kinds of perovskite ceramic material having mutually different Curie temperatures and coexisting without complete solid solution thereof in a ceramic dielectric layer of the capacitor. A case will now be described where a multilayer ceramic capacitor having a capacity of 10µF was produced using a ceramic composed of an equimolar mixture of a composition with x₁=0.2 (composition 0.2) and a composition with x₂=0.5 (composition 0.5). The production process was as shown in Fig. 29. The steps ranging from weighing to crushing in the process were divided into two lines of the corresponding steps and a step of mixing the crushed materials was added, as different from a conventional process of producing a multilayer ceramic capacitor as shown in Fig. 30. More specifically, two kinds of raw material powders of perovskite ceramic materials having mutually different compositions are separately calcined at mutually different temperatures to form two kinds of perovskite ceramic materials. These materials are then crushed and mixed at an adequate ratio. Specifically, powders of PbO, Fe₂O₃, WO₃ and Nb₂O₃ are weighed in such amounts as to provide compositions 0.2 and 0.5, and subjected to wet-mixing, dehydration, and drying. These steps may be practiced according to the conventional process. The material of the composition 0.2 is calcined in air at 1,050°C for one hour, while the material of the composition 0.5 is calcined in air at 850°C for one hour. The calcined materials are crushed and mixed together in equimolar amounts. If needed, a usual binder and/or a dispersant may be added. The resulting mixture is formed into a green sheet having a thickness of 25µm in the same manner as in the case of a commercially available multilayer ceramic capacitor. For example, an internal electrode consisting of an Ag 80%-Pd 20% alloy is printed on the sheet, followed by lamination and pressing and cutting at a certain chip size. Thereafter, the resulting laminate chip is so sintered in air at 1,000°C for one hour that two or more kinds of perovskite ceramic materials can coexist. The layer thickness after the sintering is 20µm. Thereafter, an external electrode is attached to the laminate chip to produce a multilayer ceramic capacitor as shown in Fig. 31, which includes a ceramic dielectric layer 1, the external electrode 2, the internal electrode 3, a lead wire 4, a solder 5 connecting the external electrode with the lead wire. Fig. 32 shows a comparison in dielectric properties between the capacitor according to the above-mentioned embodiment of the present invention and a commercially available ceramic capacitor (10µF) using a ceramic of x=1/3 produced according to a conventional process. The dielectric properties of the ceramic used in the capacitor according to the present invention are flattened in the form of overlapped properties of the perovskite ceramic materials prepared from the respective compositions 0.2 and 0.5 to decrease the temperature dependence of electrostatic capacity. For example, in the case of a service involving 3µF or more, the service temperature range of the capacitor according to the present invention expands to about two times (from -25∼65°C to - 65∼100°C) that of the commercially available ceramic capacitor in the case of no application of voltage, and to about three times (from 5∼55°C to -45∼100°) that of the commercially available ceramic capacitor in the case of application of 50Vdc. On the other hand, in the case of a service involving a service temperature range of -25 to 80°C and a voltage of 50 Vdc, the capacitor according to the present invention can provide an electrostatic capacity of three times or more (3µF or more) that of the commercially available ceramic capacitor, and decrease the rate of change in the electrostatic capacity with the temperature (ΔC/C 20°C) to 1/2 or less that of the commercially available conventional ceramic capacitor (from 79% to 36%). Therefore, the multilayer ceramic capacitor according to the present invention can keep large capacities over a wider temperature range than that of the conventional capacitor, and diminish that dependences of electrostatic capacity on the temperature and the electric field strength. Example 8 The same two kinds of perovskite ceramic materials as in Example 7 were calcined crushed, and mixed with addition of 0.1 mol% of Dy₂O₃, based on the whole amount of the mixed perovskite ceramic material. Thereafter, a multilayer ceramic capacitor was prepared in the same manner as in Example 7. As shown in Fig. 26, the addition of Dy₂O₃ reduced the average grain size of ceramic crystal grains after sintering to as small a size as 2.6 m, and made the scattering of grain sizes smaller, which are favorable for formation of a multilayer ceramic capacitor. However, when the amount of Dy₂O₃ added exceeds 0.2 mol%, the scattering of grain sizes becomes unfavorably large. Addition of 0.1 to 0.2 mol% of Dy₂O₃ did not cause changes, in electrostatic capacity, temperature dependence of electrostatic capacity, and voltage dependence of electrostatic capacity, of the multilayer ceramic capacitor. The oxide to be added is not limited to Dy₂O₃, and may be a chemically stable oxide of a high melting point, the metal atoms of which are incapable of substituting for atoms located in the body-centered positions in a perovskite ceramic material, and which is to be present in the crystal grain boundaries of a perovskite ceramic. The case where the compositions 0.2 and 0.5 were mixed has been illustrated as an example. A multilayer ceramic capacitor of a lead-attached type is shown as an embodiment of the present invention in Fig. 31. Besides the lead-attached type, however, not only a chip type but also a structure of a multilayer ceramic capacitor covered with a resin (a mold type and a dip type) and a structure having a changed form of a lead (a plug-in type and a ribbon lead type) can be, needless to say, adopted as the type of the multilayer ceramic capacitor of the present invention. POSSIBLE APPLICATIONS IN THE INDUSTRY As explained in detail, the perovskite ceramic according to the present invention can enjoy not only an effective combination of temperature dependences of dielectric constant for respective mixed materials but also an effective reduction of dependence of dielectric constant on the electric field strength by virtue of a mixture of the perovskite ceramic materials having respective high dielectric constants and mutually different Curie temperatures. Furthermore, the ceramic of the present invention has a high dielectric constant, a relatively small dielectric loss tangent, and a high resistivity. Accordingly, it is advantageous in that the dependences of dielectric constant on the temperature and the electric field strength are small in a wide temperature range even without containment of any depressor agent added. Moreover, since the ceramic of this invention can be prepared by calcination at 1,000°C or a lower temperature, a cheap furnace material can be used in a sintering furnace, and the energy for calcination and sintering may be small. Thus, the ceramic of this invention has large industrial merits. Addition of a metal oxide capable of suppressing the growth of crystal grains, such as Dy₂O₃, is advantageous in that the crystal grain size can be reduced with a small deviation of grain size without spoiling the dielectric properties characteristic of the ceramic comprising two or more kinds of perovskite ceramic materials coexisting therein. Therefore, when a ceramic containing such a metal oxide added thereto is used as a material for a multilayer ceramic capacitor, a ceramic dielectric layer advantageously can be thin with few defects such as pinholes. Use of the ceramic of the present invention comprising two kinds of perovskite ceramic materials coexisting therein and having respective high dielectric constants and mutually different Curie temperatures as the ceramic dielectric layer of a multilayer ceramic capacitor is advantageous in that large capacities can be secured over a wider temperature range with small dependences of electrostatic capacity on the temperature and the voltage than those of conventional multilayer ceramic capacitors. Therefore, use of the capacitor according to the present invention in an environment involving fluctuation of temperature and/or application of voltage is advantageous. For example, a power source device for mounting on a stationary artificial satellite is exposed to periodically fluctuating temperatures between -20 and 50°C in the service environment and subjected to voltage application. Use of the capacitor according to the present invention in a filter circuit in such a device is advantageous in that a high stability can be materialized in the power source devices, and that the number of capacitors to be used can be decreased by a number which can be compensated by a decrease in the dependences of electrostatic capacity on the temperature and the voltage, leading to reduction of the mounting area and reduction of the weight as well as economizing.

Claims: A perovskite ceramic characterized by comprising two kinds of perovskite ceramic materials having respective high dielectric constant and mutually different Curie temperatures coexisting therein without forming a complete solid solution thereof, a first kind of perovskite ceramic material having the formula wherein 0.1 ≦ x₁ ≦ 0.2, and a second other kind of perovskite ceramic material having the formula wherein 0.5 ≦ x₂ ≦ 0.9, said two kinds of perovskite ceramic material coexisting in the mixing mole fraction ratio of 0.5 ≦ Y ≦ 0.8. A perovskite ceramic as claimed in claim 1 characterized in that said two kinds of perovskite ceramic materials coexisting in the ceramic without forming a complete solid solution thereof constitute a ceramic prepared by separately preliminarily calcining raw material powders of two kinds of perovskite ceramic materials having mutually different compositions to form two kinds of perovskite ceramic materials having mutually different Curie temperatures, crushing said materials formed, mixing the crushed materials at an adequate ratio, and sintering the mixture without forming a complete solid solution thereof. A perovskite ceramic as claimed in claim 2 characterized in that the calcining temperature of at least one kind of said two kinds of perovskite ceramic materials is the same as or higher than the sintering temperature of said mixture of the perovskite ceramic materials. A perovskite ceramic as claimed in claim 1 characterized in that the dielectric constants of said two kinds of perovskite ceramic materials at normal temperatures each independently is 1,000 or more. A perovskite ceramic as claimed in claim 1, further containing a minute amount of an oxide which is an oxide of a metal element incapable of substituting for atoms occupying the body-centered positions of crystal lattices of said two kinds of perovskite ceramic materials, and which has a higher melting point than the melting points of said perovskite ceramic materials. A perovskite ceramic as claimed in claim 5 characterized in that said oxide is selected from the group consisting of Dy₂O₃, Gd₂O₃ and Sm₂O₃. A process for preparing a perovskite ceramic wherein two kinds of perovskite ceramic materials coexist, characterized by comprising the steps of: weighing and mixing raw material powders including PbO, Fe₂O₃ , WO₃, and Nb₂O₅ to provide a first mixture having the composition wherein 0.1 ≦ x₁ ≦ 0.2; calcinating said first mixture; crushing the resultant first perovskite ceramic material; weighing and mixing raw material powders including PbO, Fe₂O₃, WO₃, and Nb₂O₅ to provide a second mixture having the composition wherein 0.5 ≦ x₂ ≦ 0.9; calcinating said second mixture; crushing the resultant second perovskite ceramic material; mixing said first perovskite ceramic material and said second perovskite ceramic material in the ratio 0.5 ≦ Y ≦ 0.8; pressing the mixed first and second perovskite ceramic materials into pellets; and sintering said pellets without forming a complete solid solution of said first and second perovskite ceramic materials. A process for preparing a perovskite ceramic as claimed in claim 7 characterized in that the calcining temperature of at least one kind of said two kinds of perovskite ceramic materials is the same as or higher than the sintering temperature of said mixed perovskite ceramic materials. A process for preparing a perovskite ceramic as claimed in claim 7 characterized in that the dielectric constants of said two kinds of perovskite ceramic materials at normal temperatures each independently are 1,000 or more. A process for preparing a perovskite ceramic as claimed in claim 7 characterized in that a minute amount of an oxide which is an oxide of a metal element incapable of substituting for atoms occupying the body-centered positions of crystal lattices of said two kinds of perovskite ceramic materials, and which has a higher melting point than the melting points of said perovskite ceramic materials is added to said perovskite ceramic materials. A process for preparing a perovskite ceramic as claimed in claim 10 characterized in that said oxide is selected from the group consisting of Dy₂O₃, Gd₂O₃ and Sm₂O₃. A multi layer perovskite ceramic capacitor characterized by comprising two kinds of perovskite ceramic materials having respective high dielectric constant and mutually different Curie temperatures coexisting therein without forming a complete solid solution thereof, a first kind of perovskite ceramic material having the formula wherein 0.1 ≦ x₁ ≦ 0.2, and a second kind of ceramic perovskite material having the formula wherein 0.5 ≦ x₂ ≦ 0.9, said two kinds of perovskite ceramic material coexisting in the mixing mole fraction ratio of 0.5 ≦ Y ≦ 0.8. A multi layer perovskite ceramic capacitor as claimed in claim 12 characterized in that the dielectric constants of said two kinds of perovskite ceramic materials at normal temperatures each independently is 1.000 or more. A multi layer perovskite ceramic capacitor as claimed in claim 12, further containing a minute amount of an oxide which is an oxide of a metal element incapable of substituting for atoms occupying the body-centered positions of crystal lattices of said two kinds of perovskite ceramic materials, and which has a higher melting point than the melting points of said perovskite ceramic materials. A multi layer perovskite ceramic capacitor as claimed in claim 14 characterized in that said oxide is selected from the group consisting of Dy₂O₃, Gd₂O₃ and Sm₂O₃.

Inventors: KIUCHI, KAZUO, TAKAHARA, HIDEYUKI
Applicants: NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1985
Main classification: C04B35
IPC codes: C04B35, H01G4